a review of separation technologies in current and future bio refineries

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Available online at www.sciencedirect.com

Separation and Purification Technology 62 (2008) 1–21

Review

A review of separation technologies in current and future biorefineries

Hua-Jiang Huang a, Shri Ramaswamy a,∗, U.W. Tschirner a, B.V. Ramarao b

a Department of Bioproducts and Biosystems Engineering, Kaufert Lab, University of Minnesota, Saint Paul, MN 55108, USAb Department of Paper and Bioprocess Engineering, SUNY-CESF, Syracuse, NY 13210, USA

Received 5 December 2007; accepted 14 December 2007

bstract

Biorefineries process bioresources such as agriculture or forest biomass to produce energy and a wide variety of precursor chemicals andio-based materials, similar to the modern petroleum refineries. Industrial platform chemicals such as acetic acid, liquid fuels such as bioethanolnd biodegradable plastics such as polyhydroxyalkanoates can be produced from wood and other lignocellulosic biomass. Biorefineries use aariety of separation methods often to produce high value co-products from the various feed streams. In this paper, a critical review of separationethods and technologies related to biorefining including pre-extraction of hemicellulose and other value-added chemicals, detoxification of

ermentation hydrolyzates, and ethanol product separation and dehydration is presented. For future biorefineries, extractive distillation with ionic
iquids and hyperbranched polymers, adsorption with molecular sieve and bio-based adsorbents, nanofiltration, extractive-fermentation, membraneervaporation in bioreactors, and vacuum membrane distillation (VMD) hold significant potential and great promise for further investigation,evelopment and application.
2008 Elsevier B.V. All rights reserved.

eywords: Biorefinery; Separation technologies; Ethanol; Biofuels; Bioprocess engineering; Detoxification

ontents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1. Corn-to-ethanol biorefinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2. Lignocellulosic biomass-to-ethanol biorefinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3. Integrated lignocellulose/forest biorefinery (ILCB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Pre-extraction of hemicellulose and other value-added chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1. Extraction of value-added co-products from corn-to-ethanol process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1. Extraction of corn germ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2. Extraction of corn fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.3. Extraction of zein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2. Pre-extraction of value-added chemicals in integrated forest biorefinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.1. Pre-extraction of hemicellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.2. Pre-extraction of antioxidants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3. Removal of inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84. Recovery of ethanol and ethanol dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.1. Ordinary distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.2. Azeotropic distillation (AD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3. Extractive distillation (ED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3.1. Extractive distillation with liquid solvent . . . . . . . . . . .4.3.2. Extractive distillation with dissolved salt . . . . . . . . . . .

∗ Corresponding author. Tel.: +1 612 624 8797; fax: +1 612 625 6286.E-mail address: [email protected] (S. Ramaswamy).

383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2007.12.011

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mailto:[email protected]

dx.doi.org/10.1016/j.seppur.2007.12.011

2 H.-J. Huang et al. / Separation and Purification Technology 62 (2008) 1–21

4.3.3. Extractive distillation with the mixture of liquid extractant and dissolved salt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.3.4. Extractive distillation with ionic liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.3.5. Extractive distillation with hyperbranched polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.3.6. Summary of extractive distillation with different separating agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.4. Liquid–liquid extraction-fermentation hybrid (extractive fermentation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.5. Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.5.1. Vapor-phase adsorption of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.5.2. Liquid-phase adsorption of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.5.3. Advantages and disadvantages of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.6. Membrane separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.6.1. Hydrophilic membrane for removal of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.6.2. Hydrophobic membrane for removal of ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.6.3. Membrane pervaporation-bioreactor hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.6.4. Vacuum Membrane Distillation (VMD) – bioreactor hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18. . . . .

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In the wet mill process (Fig. 2), corn is cleaned, steeped,de-germed to obtain germ from which corn oil is extracted,defibered to obtain fiber, and subjected to separation of gluten

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Among the variety of possible products from the biorefin-ry, liquid transportation fuels in the form of ethanol (or whats now referred to as bioethanol) is rapidly gaining significance.ioethanol is likely to be a prominent product for future biore-neries; hence this review will focus on separation technologies

ncorporating bioethanol as a principal product.There has been an increasing interest in biomass derived

thanol due to the rapid increase in the price of crude oil and theerceived strength of the global demand of petroleum. Therere many advantages in using bioresource derived ethanol asliquid transportation fuel. Bioethanol blended with gasoline

xtends crude oil utilization, reduces reliance on oil imports andelp mitigate increasing oil prices. The higher oxygen contentf ethanol results in relatively cleaner combustion and has longeen used as an additive in gasoline to reduce urban smog andther environmental pollution problems. Agricultural sourcesuch as corn or sugar cane are annual crops which sequesterarbon from the atmosphere in annual cycles. Forest resourcesuch as woody biomass capture and release atmospheric carbonver a few decades. Fossil fuels release ancient carbon and otherreenhouse gases into the atmosphere significantly contributingo global climate change processes whereas bioresource basedransportation fuels can be carbon neutral [1].

Basically, there are three kinds of biomass-to-ethanoliorefineries: corn-to-ethanol, basic lignocellulosic biomass-o-ethanol biorefinery and integrated lignocellulosic biomass-o-ethanol and other co-products including the concept ofntegrated forest biorefinery.

.1. Corn-to-ethanol biorefinery

At present, there are two major processes to produce fuelthanol from corn: the dry-grind (67%) and the wet mill (33%).n general, the wet milling process produces high-value co-
roducts such as fiber, germ and gluten by pre-processing prioro fermentation to ethanol, thus it is more capital- and energy-ntensive [2]. The conventional dry mill consists of grinding,ooking, liquefaction, saccharification of the starch to sugars
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

ith enzymes, fermentation of the sugars to ethanol with yeast,ollowed by distillation and dehydration processes of ethanol.he solids from the distillation bottom are dried to obtainistillers’ dried grains with protein (DDG) as an animal feed-tuff [2]. Based on the conventional dry mill process, a fewodified dry-grind processes have been developed by recov-

ring germ or both germ and fiber before fermentation. As anxample, the “Quick Germ” process recovers germ prior to fer-entation (Fig. 1) [3]. Recently, another modified dry-grind

rocess, which allows separation of non-fermentable corn com-onents such as germ and fiber for further reduction of cost,as developed by Taylor and Singh [4]. In their process, cornernels are treated with anhydrous ammonia gas, so the ker-el components become loose and thus germ and fiber cane readily recovered as value-added co-products for food or

Fig. 1. Modified corn dry-grind (“Quick Germ”) process [3].

H.-J. Huang et al. / Separation and Purifi

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nd starch. This is followed by the same steps as those oforn-grind process including saccharification, fermentation, dis-illation and dehydration of ethanol, etc. [3].

Therefore, in general, the common separation steps in theorn-to-ethanol biorefinery are as follows:

pre-separation of non-fermentable germ and fiber, for themodified dry-grind process;pre-separation of starch and other high-value co-productssuch as fiber, germ and gluten, for the wet mill process;separation of ethanol from dilute beer liquors and dehydrationof ethanol.

.2. Lignocellulosic biomass-to-ethanol biorefinery

While significant progress has been made in the conversion ofignocellulosic or cellulosic biomass to fuel ethanol, it has not yeteen commercialized due to existing technical, economic, andommercial barriers. However, cellulosic ethanol can be moreffective and promising as an alternative renewable bio-fuel thanorn ethanol in the long run because it could greatly reduce theet greenhouse gas (GHG) emissions as well as higher net fossiluel displacement potential [5].

The largest potential feedstock for ethanol is lignocellulosiciomass such as agricultural residues (e.g., corn stover, croptraws, sugar cane bagasse), herbaceous crops (e.g., alfalfa,witchgrass), forestry wastes, wood (hardwoods, softwoods),astepaper, and other wastes such as municipal waste [6].The basic process for conversion of cellulosic biomass to fuel

thanol mainly consists of eight steps: feedstock handling, pre-reatment and conditioning/detoxification, saccharification ando-fermentation, product separation and purification, wastewa-er treatment, product storage, lignin combustion for productionf electricity and steam, and all other utilities (Fig. 3) [7]. This
verall process involves the following separation steps:
removal of inhibitors in fermentor;ethanol recovery from beer and its dehydration;

tvid

cation Technology 62 (2008) 1–21 3

wastewater treatment;pre-extraction of hemicellulose and separation of hemicellu-lose from other components in the extract when consideringseparate fermentation of pentoses and hexoses.

.3. Integrated lignocellulose/forest biorefinery (ILCB)

Obviously, forests are an enormous source of lignocellulosicaterial. At present, pulp mills represent an excellent existing

latform for retrofitting in forest products industry. In recentears, however, the U.S. forest products industry has encoun-ered severe competition from overseas, leading to mergers andignificant downsizing [8]. Thus, a new concept—the forestiorefinery based on the existing pulp mills was proposed to pro-uce added fuel and chemicals, together with pulp and paper, inrder to increase the overall revenue streams and profitability.ig. 4 below is the general process block diagram of integratedorest biorefinery. This process involves careful pre-extractionf hemicellulosic sugars prior to pulping and isolation of longnd short fiber after pulping, hemicellulose conversion to ethanoln a bioreactor; the short fiber (cellulose) can be converted intothanol in another bioreactor, and the long fiber (cellulose) usedor the production of paper and other fiber based materials suchs bio-composites. Besides, lignin dissolved into black liquorfter pulping can be further gasified to produce syngas. Theesulting syngas can be further synthesized to produce fuels andhemicals, and electricity and process steam.

In brief, therefore, the integrated forest biorefinery can makeull use of all the feedstock components to produce value-addedultiple co-products including energy (electricity and steam),

nd various chemicals, along with the major products such asaper and fuel grade ethanol. This scenario of optimized systemf multiple co-products similar to today’s petroleum refineryffers enormous opportunity for the future renewable resourcesased integrated biorefinery.

In addition to the same separation tasks as those of the basicignocellulosic biomass-to-ethanol biorefinery, the integratedorest biorefinery includes the following additional separationasks:

pre-extraction of hemicellulose, and separation of hemicellu-lose from other components in the extract;separation of short fiber and long fiber;syngas cleaning and conversion.

The purpose of the present paper is to explore a large varietyf potential separation approaches and technologies which mightelp reduce the overall ethanol production cost, and improve theverall techno-economic feasibility of the biorefinery based onlarge number of published separation technology literature.hus, the following sections will focus on variety of separation

echnologies in the pre-extraction of hemicellulose and otheralue-added chemicals, removal of fermentation inhibitors forncreasing product yield, and recovery of ethanol and ethanolehydration.

4 H.-J. Huang et al. / Separation and Purification Technology 62 (2008) 1–21

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. Pre-extraction of hemicellulose and otheralue-added chemicals

.1. Extraction of value-added co-products fromorn-to-ethanol process

.1.1. Extraction of corn germ
It is important to extract corn germ since it can bring addi-
ional value. The defatted germ from dry-grind or wet millrocesses can be saccharified efficiently into glucose, xylose

fiig

Fig. 4. Process block diagram of an

c lignocellulose to ethanol biorefinery [7].

nd arabinose using enzymes such as enzymes Aureobasidiump. [9]. These sugars can later be fermented to ethanol. Thealue-added co-products represent a potentially considerablerofit from germ extraction.

In the conventional wet-milling degermination (refer toig. 2), after soaking of corn, the germ is recovered by meansf hydrocyclones [10]. Singh and Eckhoff proposed a modi-
ed dry-grind, called “Quick Germ” ethanol process (Fig. 1),
nvolving soaking whole corn in water and then recoveringerm by conventional wet-milling degermination, with the germ

integrated forest biorefinery.

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ield of 6.76% under the optimal soaking conditions of 12 hrnd 59 ◦C [11]. They also performed the economic assessmentf the “Quick Germ” process, and showed it to be profitable.he corn oil and the germ meal recovered from the germave a much higher price (US$ 0.53–0.66/kg) than that ofDG (US$ 0.13–0.20/kg) [3]. This means that the “Quickerm” modified process produces more value from co-products

han the conventional dry-grind process with only DDG as theo-product. In addition, the removal of non-fermentables cannhance the subsequent fermentation. The combination of theerm recovery as a co-product and the increase in the fer-entation capacity leads to ethanol cost reduction by 2.69 ¢/L

10.19 ¢/gal), compared with the conventional dry-grind ethanolrocess [3].

.1.2. Extraction of corn fiberAt present, most corn fiber is not recovered, and it finally

ixed into low-value animal feeds, which will have a narrowarket in the future due to the growing ethanol production [2].orn fiber can be used to produce ethanol [12,13] and higheralue co-products such as corn fiber oil [14,15] and corn fiberum [15,16]. Corn fiber, i.e. bran from the corn hull, is abun-ant in hemicellulose (xylose and other pentose sugars). Bothexose and pentose can be hydrolyzed by dilute acid hydroly-is of corn fiber, which can then be fermented to ethanol with. coli strain [12,13]. The additional fermentation of the fiber

raction of corn kernel could increase the ethanol yield morehan 10% through appropriate pretreatment of the corn fiber17]. In addition, corn fiber can also be utilized to manufac-ure corn fiber oil by extraction, which has low cholesterolue to its ferulate esters content, particularly sitostanyl ester.hus, it is a healthy dietary additive. Therefore, corn fiber andorn fiber oil have potential added values in the corn-to-ethanolrocess.

Dried, milled corn fiber can be separated by either hot liq-id water or steam. Allen made an experimental comparisonetween hot liquid water and steam fractionation of corn fibery using the same equipment [17]. The results show that thereatment with hot liquid water at 215 ◦C and 5–10% solids load-ngs obtained much higher pentosan recovery than with steamt 210–220 ◦C and >50% solids loadings. In addition, the liq-id product from the former method did not have inhibition tohe final glucose fermentation by S. cerivisiae while the later

ethod did [17].The fiber oil can be extracted by a novel process which

ncludes a pre-grinding step followed by extraction with hex-ne [18]. The so-called “Quick Fiber” process was developed toecover corn fiber from the mash after degermination and beforeermentation in the dry-grind corn–ethanol process. This processainly consists of the following steps: soaking corn in water,

erm recovery, fiber separation where dry starch is added to theemaining slurry after degermination to increase its density sohat the fiber is floated and separated due to the density differ-
nce, and washing the recovered fiber, etc. [15,19]. Experimentalesults show that the quick fiber yields are 6–7%, correspondingo 46–60% of the total fiber recovered by the wet mill process15].
ilaa


.1.3. Extraction of zeinZein which is prolamine (gluten)-rich, water insoluble pro-

ein is a value-added co-product for a dry-grind ethanol plant.ein or its resins have many potential uses, for instances, in fiber,dhesive, coating, cosmetic, textile, and biodegradable plastics20]. Zein takes up 45–50% protein in corn. At present, onlybout 500 tons/year of zein was produced from corn gluteneal, with price in the range of US$ 10–40/kg, depending on

urity [20]. Thus, to minimize the overall cost of ethanol plant,t is necessary to extract zein from milled maize [21]. Recenttudies showed that extractive separation of zein from maizeould be commercially feasible in an ethanol plant [22]. In 2002,ickey et al. [21] investigated the low-cost extraction of zein indry-grind ethanol plant. They explored three different methods

or displacing extracted liquid from the extracted corn particle.esults show that centrifugation with ethanol rinsing, was morefficient and feasible in recovering zein (protein) in the extract,nd settling the extracted corn in water, compared to the otherwo methods (packed bed displacement and gravitational set-ling into water). Most recently in 2006, Cheryan [23] receivedpatent for extracting zein and/or oil from dry-milled corn withthanol, separating the liquid phase containing ethanol, oil andein and the solid phase containing corn solids. Then, the liquidhase is ultra-filtered with a membrane to retain zein and passhe oil and ethanol mixture.

.2. Pre-extraction of value-added chemicals in integratedorest biorefinery

In conventional kraft pulping processes, most of the hemicel-ulose from wood is degraded into oligomers or mono sugars,tc., which are dissolved in black liquor along with dissolvedignin and the pulping chemicals (inorganic substance). Thelack liquor is usually combusted for steam and electricityeneration. However, since hemicellulose has a considerablyower heating value than lignin, the combustion of hemicel-ulose represents uneconomical use of the feedstock resource.n an integrated lignocellulose biorefinery (ILCB), therefore,re-extraction of hemicelluloses followed by the production ofalue-added products such as ethanol, sugar-based polyesters orther chemicals offers a tremendous valued-added opportunity24].

In addition to the pre-extraction of hemicellulose, pre-eparation of naturally occurring food antioxidants (phenolics)rior to pulping can also be considered, in order to make fullse of feedstock to get added value and thus reduce the overallroduction cost and improve the overall profitability [25,26].

.2.1. Pre-extraction of hemicelluloseLignocellulosic biomass consists of three major fractions:

ellulose (35–50 of dry weight), hemicellulose (20–35%) andignin (10–25%). Conversion of lignocellulosic materials toigher value products requires fractionation of the material into
ts components: lignin, cellulose, and hemicellulose. Hemicellu-ose is a heterogeneous polymer comprising of pentoses (xylose,rabinose), hexoses (mannose, glucose, galactose), and sugarcids [27]. The removal and recovery of hemicellulose is an

6 Purifi

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ssential feature of pretreatment processes for biological con-ersion to ethanol or other products [28]. To date, a varietyf effective pretreatment methods to hydrolyze and fraction-te hemicellulose components have been investigated, includingilute acid pretreatments [29–32], liquid hot water extraction33–35], steam explosion-based extraction [36–42], dilute acid-team explosion [43] and alkaline extraction [44–46]. Ammoniaber/freeze explosion (AFEX) is another well-known pretreat-ent. However, it is hard to remove hemicellulose by AFEX,

ven though it has been shown to remove lignin very well [28].osier et al. also pointed out that dilute acid pretreatment,

iquid hot water extraction, steam explosion-based extraction,ilute acid-steam explosion are effective in removing hemicel-ulose, though they considered alkaline extraction to have only

inor effect for removal of hemicellulose [28]. On the otherand, alkaline extraction has been shown to very effective foremicellulose removal [44–46]. In the following, dilute acid pre-reatment, liquid hot water extraction, steam explosion-basedxtraction, dilute acid–steam explosion and alkaline extractionill be discussed.Dilute sulfuric acid pretreatment, with pH control by ammo-

ia or/and lime is one of the most promising approaches, becausef its lower costs and higher hemicellulose yields (up to 90%)30]. Dilute-acid (0.5–1.0% sulfuric) at moderate temperatures140–190 C) effectively can recover most of the hemicelluloses dissolved sugars [29].

Higher temperatures (200–230 ◦C) water extraction can com-letely recover hemicellulose from hardwoods and herbaceousaterials, without significant degradation [33]. Saska and Ozer

34] showed that hemicellulose from sugar cane bagasse can beuccessfully extracted with water as the extractant. Under theperating conditions of the solid/liquid ratio at 1:5, the extrac-ion temperature at 150–170 ◦C, and the extraction time with5–30 min, 89% the original amount of xylose was recovered.he major advantages of the water extraction method over theilute acid pretreatment are: lower corrosion to equipment, lessylose degradation and thus less byproducts including inhibitoryompounds in the extracts, and more easier recovery of acid fromhe hydrolyzate. With particle size reduction prior to extrac-ion, the aqueous process gave almost 90% recovery of xylose,uperior to steam explosion-based extraction [34].

Steam explosion is an effective pretreatment for hemicellu-ose hydrolysis [39–42]. In this process, biomass is pretreatedy pressurized steam followed by rapid relieving of pressure,hich breaks down the lignocellulosic structure so that the

ignin is readily depolymerized and thus the hemicellulose isasily hydrolyzed [42]. The steam explosion process can resultn around 50% insoluble residue of the wood, consisting mainlyf cellulose. The remainder which chiefly contains hemicellu-ose and lignin, can be recovered with alkali extraction [41].himizu et al. [39] performed steam-explosion of various speciesf hardwood chips at 180–308 ◦C for 1–20 min, leading to par-ial hydrolysis of hemicellulose, and the resulting sugars can
hen be extracted with water. The xylose yield was 10–20% ofhe starting materials [39]. Ibrahim and Glasser [40] used steamreatment to break down and separate the red oak wood chipsnto fibers and polymer products, resulting in nearly complete
cation Technology 62 (2008) 1–21

ecovery of xylan and obtainment of almost hemicellulose-freeulps. Recently, Josefsson et al. [41] utilized the steam explo-ion process to fractionate the aspen wood components, for theurpose of obtaining a high cellulose yield and an appropriateW distribution, while recovering hemicelluloses. Pulps with

ifferent xylan contents ranging from less than 1% to 7% and dif-erent MW of cellulose ranging from less than 40,000 to 900,000ere prepared at varying time and temperature conditions [41].

n comparison with alternative methods, the steam explosion isore environmental friendly and it requires lower capital invest-ent [42]. The process, however, has a disadvantage in that it is

ifficult to restrain fibers from fragmentation [41].Tucker et al. [43] investigated the combined dilute acid–steam

xplosion method for biomass treatment. Corn stover was sub-ected to 1 wt% H2SO4 for 70–840 s in a steam explosion reactort 160, 180, and 190 ◦C. The obtained yields of xylose were3–77% of theoretical at 160–180 ◦C, and more than 90% at90 ◦C.

N’Diaye et al. [44] extracted hemicelluloses from poplarPopulus tremuloides) using a modified twin-screw extruderith a 5% NaOH solution as extracting solvent. This extruder or

xtrusion reactor, called a thermo-mechanico-chemical fraction-tion system [47], allows the integration of extrusion, cooking,iquid–solid extraction, and liquid/solid separation (filtration) insingle step, and operates in a continuous mode. With such a

eactor, alkaline extraction can be operated at a lower L/S ratiosix times less than a batch reactor) and lower residence time, and0% of the initial hemicelluloses (pentosans) can be recovered.he whole process for extracting the hemicelluloses is shown inig. 5.

Recently, as illustrated in Fig. 6., hemicellulose was separatedrom aspen (Populus tremula) by alkali extraction combinedith hydrogen peroxide treatment, ultrafiltration and recoveryy spray drying [45]. Specifically, aspen wood were first cut andefined. The resulting fiber suspension was treated with a dilute

Fig. 5. Process block diagram for extracting hemicelluloses [44].


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as subjected to a NaOH (1%)–ethanol (70%) solution to solu-ilize lignin. After centrifugation, hemicellulose was extractedith 4% NaOH. The combined two filtrates were bleached witheroxide to minimize the residual lignin content. The suspensionas finally ultrafiltered and spray-dried to obtain hemicellulose.Sun et al. [46] investigated the extraction of hemicelluloses

rom fast-growing poplar wood. In their process shown in Fig. 7,oplar wood chips were firstly dried, and dewaxed by extrac-ion with toluene-ethanol mixture (2:1, v/v), followed by partialelignification with an acidic NaCl solution, hemicellulosesxaction with 1.5–8.5% NaOH leading to 65.6–89.3% solubi-ization of the original hemicelluloses, filtration of the slurrybtained into solid (cellulose) and filtrate (hydrolyzates), neu-ralization of the hydrolyzates to pH 5.5, and precipitation ofemicelluloses in 95% EtOH, and filtration, washing and dryingf hemicelluloses.

From Figs. 5–7, it is found that alkali extraction with NaOHs used in all these three processes, and this method is effec-ive in extraction of hemicelluloses from hardwood. Since the
emicelluloses concentration of filtered hydrolyzates is veryow, e.g., 2–3% depending on the solid/solvent ratio used, its very important to concentrate hemicelluloses in order for effi-ient subsequent fermentation into ethanol or xylitol, or to obtain
ig. 7. Separation of hemicellulose and cellulose from fast-growing poplar wood46].

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ure hemicelluloses for other use, for example, in making newio-based materials. It is also found that the method of pre-ipitation with ethanol is used for separation of hemicellulosesn the two processes of Figs. 5 and 7, while the ultrafiltrations used in the process of Fig. 6. First two methods mentionedbove involve acidification or neutralization with acid to adjustH to 5–5.5 suitable for precipitation of hemicelluloses withthanol. In this process, ethanol and acid are additional chemi-als required and additional equipments for recovery of ethanols necessary, thus leading to increase in production and capi-al cost. In addition, some ethanol is probably included in therecipitated solid phase representing ethanol loss or additionaleparation cost. The latter process is simplified by employingltrafiltration, leading to possible reduction in operation costnd capital cost. As to reactors, a twin-screw extruder repre-enting a highly efficient continuous operation is used in therocess of Fig. 5. Though the types of reactors are not shown inhe other two processes, they are likely to be the batch reactorsn the lab-scale. Basically, the water extraction method, whichepresents a mild pretreatment, brings about a higher molecu-ar weight hemicellulose [48]. Taking this into comprehensiveonsideration, twin-screw extruder combined with ultrafiltrationight be a good choice for isolation of hemicelluloses by water

xtraction.Most extraction procedures, other than water extraction, are

ften operated in more severe pretreatment conditions, produc-ng hemicelluloses with smaller molecular weights. Thus, withltrafiltration for these hydrolyzates, the hemicelluloses reten-ion was not high enough to obtain efficient isolation [49,50].n these cases, however, nanofiltration can be used instead ofltrafiltration. Most recently, Schlesinger et al. [50] proved thatanofiltration, which has been commercially used in industry forround fifteen years, is much better than ultrafiltration for sepa-ating hemicelluloses from hydrolyzates by alkaline procedure.hey investigated the performance of four polymeric nanofil-

ration and one tight ultrafiltration membranes for isolatingemicelluloses from alkaline process liquors containing 200 g/laOH. The experimental results showed that the hemicellulosesf molar mass over 1000 g/mol are almost retained. In addition,wo of the membranes with the nominal molecular weight cut-ff (MWCO) of 200–300 and 200–250 g/mol, respectively, areost efficient in retention of up to 90% of hemicelluloses, while

he tight ultrafiltration membrane with MWCO of 2000 g/molxhibited less than 70% retention of hemicelluloses. Ali et al.atented an alkaline treatment system for recovering hemicel-uloses where pre-filtration units with a screen size of 400–650

esh, followed by one nanofiltration membrane was able toetain compounds with a molecular weight of about 200 andigher [51].

Therefore, nanofiltration is an excellent separation proce-ure for recovery of hemicelluloses from hydrolyzates, and theombination of a twin-screw extruder and nanofiltration can beonsidered to be the best selection for extracting hemicelluloses
rom hardwood chips.
The above referenced research can be directly related to theasic lignocellulosic biomass-to-biorefinery without the pulpingrocess. However, these researches did not evaluate the effect

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f pre-extraction on the downstream pulping yield and paperuality, which is very important for ILCB. Taking this intoonsideration, more recently, van Heiningen [24] pre-extractedemicellulose from mixed hardwood chips with two methodsf different extractants: pure water and 10% alkaline solutions.he extracted chips were then subjected to Kraft cooking at stan-ard conditions. Results showed that with these two approacheshe pulp yield decreased 5–7%, though 10% organics could bextracted. To avoid this problem, they employed a new methodhich can increase the pulp yield to the same level or even 1%igher than that of a Kraft cook control (no extraction). Thisethod, which is still in the process of a patent application, was

hown to result in other benefits: (1) a 3% reduction of effectivelkali (EA) charge in the digester, (2) a 40% increase in deligni-cation rate, (3) rejects reduction at higher kappa numbers, and4) an 8% decrease in organic load to the recovery boiler, basedn o.d. (oven-dried) wood.

.2.2. Pre-extraction of antioxidantsIn addition to hemicelluloses, naturally occurring antioxi-

ants (phenolics or polyphenolics) which could be used as aheap, renewable food additive, can also be produced from lig-ocellulosic wood materials [25,26]. The process for productionf both hemicelluloses and antioxidants by mild acid hydrolysiss seen in Fig. 8. [26]. In this process, Eucalyptus globules woodhips were subjected to acid hydrolysis with 2.5–5% H2SO4t a liquid/solid ratio of 8:1 g/g and 100–130 ◦C. The resultinglurry was vacuum-filtered into a hydrolyzates and a solid con-
isting of cellulose and lignin. Then antioxidants were extractedrom hydrolyzate with ethyl acetate as solvent. The resultingrganic phase was vacuum-evaporated to remove and recyclethyl acetate to the extractor, leaving the antioxidants-containing
ig. 8. Solvent extraction of hemicelluloses and antioxidant from wood [26].

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xtracts behind. The aqueous phase from extraction containsylose (representing hemicellulose) which can be fermented toield ethanol and/or xylitol with different yeasts.

. Removal of inhibitors

It is well-known that thermo-chemical pretreatment of lig-ocelluloses, e.g. dilute acid hydrolysis and steam explosion,an release not only the fermentable pentose and hexose sugars,ut also various compounds which are inhibitory to microorgan-sms and lead to apparent reduction in fermentation yield androductivity. Since the detoxification process can be expensivend take a large portion of the whole ethanol production cost,etoxification is a key step and selection of the proper detoxifi-ation method becomes very important. For example, one studyhowed that detoxification process comprised 22% of the ethanolroduction cost with Willow as feedstock [52].

In general, there are three major groups of inhibitors:liphatic acids (acetic, formic and levulinic acid), furan deriva-ives furfural and 5-hydroxymethylfurfural (HMF), and phenolicompounds (phenol, vanillin, p-hydroxybenzoic acid) [53,54].he mechanisms of inhibition for these three groups of inhibitorsas reviewed by Palmqvist and Hahn-Hagerdal [53]. In addition,se of different biomass as feedstock, pretreatment methods,nd fermentation organisms results in different inhibitory com-ounds and different concentration of inhibitors. As an example,ore than 35 potential inhibitors to S. cerevisiae fermentation in

ilute nitric acid hydrolyzates of hybrid poplar were identified55].

In order to enhance the efficiency of hydrolyzate fermen-ation, in addition to optimization of the pretreatment andydrolysis process for minimizing formation of the hydroly-is byproducts (inhibitors), it is necessary to remove inhibitorsdetoxify hydrolyzates) prior to fermentation or in situ detoxi-cation. The detoxification can be either chemical, physical, oriological [56]. The most commonly used methods for detoxifi-ation of hydrolyzates before fermentation are: evaporation [57],olvent extraction [25,26,58], overliming with calcium hydrox-de [54,59], activated charcoal [54,60–62], ion exchange resins54,62], and enzymatic detoxification [63,64].

Evaporation is a simple procedure to remove acetic acid, fur-ural and other volatile components in the hydrolyzates. Fornstance, Converti et al. [57] hydrolyzed the E. globules woody steam explosion and dilute acid treatment at 100 ◦C, followedy boiling or evaporating the obtained hydrolyzate for 160 mino decrease the concentration of acetic acid and furfural from1.2 to 1.0 g/l and from 1.2 to 0.5 g/l, respectively. These areelow their inhibitory levels for the fermentation of xylose toylitol by Pachysolen tannophilus strain, showing that in thisase the simple evaporation method is sufficient to eliminate thenhibition of acetic acid and furfural.

Solvent extraction with ethyl acetate is effective to remove all
f the inhibitory compounds except for the residual acetic acid58], e.g. ethyl acetate extraction can be used to remove 56%cetic acid and all of furfural, vanillin, and 4-hydroxybenzoiccid [56].

Purification Technology 62 (2008) 1–21 9

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In the overliming process, the hydrolyzate is detoxified byddition of Ca(OH)2 to adjust the pH to 9–10, leading to pre-ipitation of inhibitory compounds. After filtration, the resultingydrolyzate is then readjusted to 5.5 with dilute H2SO4 readyor fermentation [56,57]. This is a very effective process, but itroduces a large amount of gypsum [7].

In the activated charcoal adsorption process, lignin-derivednhibitory compounds (phenolics) are adsorbed on activatedharcoal, and after its adsorption saturation, the charcoal iseactivated or regenerated by heating, e.g., by boiling it inistilled water for 3 h [57]. Parajo et al. [60] performed experi-ents to observe the effect of different operating variables, i.e.,

ydrolyzate concentration, adsorbent charge (hydrolyzate: char-oal ratio) and adsorption time, on the subsequent fermentationf xylose to xylitol by the yeast Debaryomyces hansenii. Resultshow that a hydrolyzate:charcoal ratio of 205 g/g was sufficientor improving subsequent fermentation.

Ion exchange resin (IER) is a well-known detoxificationethod. For illustration, Van Zyi et al. [65] studied the elimina-

ion of acetic acid inhibition of d-xylose fermentation by Pichiatipitis with ion exchange resin. The inhibition degree was foundo be dependent on the acetic acid concentration, the oxygenvailability, and the pH value. A comparison was done betweenhe fermentation of an untreated acid hydrolyzate of sugar caneagasse and the fermentation of the hydrolyzate treated by annion exchange resin with the removal of 84% of the acetic acid.he results show that the former ethanol yield is 0.27 g/g sugar,hile the latter ethanol yield increased by 0.36 g/g sugar.Enzymatic treatment is usually effective to remove phenolic

ompounds, e.g. removal of phenolics from Willow hydrolyzatesreated with steam and SO2 by applying laccase [63]. Basically,accase treatment can remove most of the phenolics, but notcetic acid, furfural and HMF. For example, around 80% ofhe phenolic compounds was removed from sugarcane bagasseydrolyzates obtained by steam explosion with the phenoloxi-ase laccase [66]. In addition, reductive detoxification of furfuralo less toxic furfuryl alcohol can be performed by using thethanologenic bacterium Escherichia coli strain LYO1 [64].

In order to select an efficient detoxification approach, someesearchers have made comparisons between different detoxi-cation procedures. For instance, Cantarella et al. [59] madecomparison between Ca(OH)2 overliming, water rinsing,

ater–ethyl acetate two-phase contacting, and in situ detox-fication with high level yeast inocula, for the purpose ofliminating the inhibition problem in saccharification of cel-ulose from steam-exploded (SE) poplar wood to glucose byellulases and fermentation of glucose to ethanol by S. cere-isiae (Baker’s yeast). The water-rinsing treatment removedater-soluble inhibitors, thus enhanced the enzymatic hydroly-

is of SE-substrate. Obviously, however, this method is suitableor washing away the inhibitors in cellulose hydrolyzates, butot for removal of inhibitors in hemicellulosic pre-hydrolyzatesince most pentose sugars are also water-soluble. This compar-
son showed that overliming with Ca(OH)2 is the most efficientethod among those investigated.Most recently, Chandel et al. [54] investigated the detoxi-
cation of sugarcane bagasse hydrolyzate to improve ethanol

(ciO

ig. 9. Block diagram of the hydrolyzate detoxification with IER [62]: (1) cationxchanger in H+ form; (2) anion exchanger in Cl− form; (3) cation exchangern H+ form; (4) anion exchanger in OH− form.

roduction by Candida shehatae NCIM 3501. In their researchork, five detoxification methods were compared, includingeutralization, overliming, activated charcoal, ion exchangeesins (IER), and enzymatic detoxification using laccase. Itas demonstrated that ion exchange treatment was most effi-

ient in removing furans (63.4%), total phenolics (75.8%)nd acetic acid (85.2%). Activated charcoal could remove8.7, 57 and 46.8% of furans, phenolics and acetic acid,espectively. Laccase treatment could remove 77.5% of totalhenolics, but it could not reduce the furans and aceticcid contents in its hydrolyzates. Overliming could reduce5.8% furans and 35.8% phenolics in its treated hydrolyzate,ut not acetic acid. Fermentation of hydrolyzates detoxifiedy different methods with Candida shehatae NCIM 3501howed that the ethanol yields obtained by different detox-fication treatments are in the following decreasing order:on exchange (0.48 g/g) > activated charcoal (0.42 g/g) > laccase0.37 g/g) > overliming (0.30 g/g) > neutralization (0.22 g/g).

Villarreal et al. [62] studied on detoxification of Eucalyptusemicellulose hydrolyzate, i.e., removal of acetic acid, furfural,MF, and phenolics, for xylitol production by Candida guil-

iermondii with active charcoal and a series of IER columnsFig. 9) composed of four different resins (alternate cationic andnionic) in sequence. Their results showed that IER can removell inhibitory components (aliphatic acids, furan derivatives andhenols) without significant loss of sugar, superior to activatedharcoal.

As will be discussed in Sections 4.4, 4.6.3 and 4.6.4, initu detoxification by using extractive-fermentation, membraneervaporation-bioreactor hybrid, and vacuum membrane dis-illation (VMD)-bioreactor hybrid processes can effectivelyeparate product ethanol and remove inhibitors simultaneously.

Briefly, in summary, evaporation is a simple way to removecetic acid, furfural and other volatile components, but it is dif-cult to remove the heavier components with higher boilingoints. Extraction with solvent (e.g., ethyl acetate) is efficientn removing all the inhibitors, but it needs additional solvent
ethyl acetate) and solvent recovery for recycle use. Activatedharcoal adsorption can remove the phenolic compounds, butt is not so efficient in removal of acetic acid and furfural.verliming and IER are more efficient procedures for removal

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dhealth and safety concerns with the storage of either carcinogenic(benzene) or highly flammable (cyclohexane) solvent. For thisreasons, AD method is less applied in the ethanol production.

0 H.-J. Huang et al. / Separation and

f different inhibitors from hydrolyzates, but the former leadso a large amount of gypsum. It could be concluded that IER

ethod is the current best choice for detoxification becausef its high detoxification efficiency, easy (continuous) oper-tion and flexible combination of different anion and cationxchangers, while the enzymatic treatment can possibly be theuture choice. In addition, extractive-fermentation, membraneervaporation-bioreactor hybrid, and VMD-bioreactor hybridre very promising processes to remove inhibitory compoundsn addition to increasing ethanol yield.

. Recovery of ethanol and ethanol dehydration

Downstream from the fermentor, the so-called beer, is usu-lly dilute aqueous solution containing about 5–12 wt% ethanol.eparation of ethanol from beer is an energy-intensive process.t usually takes up a large fraction of the total energy requirementor the whole biorefinery.

There is a common problem in the dehydration of ethanol,ecause ethanol forms a minimum boiling mixture, so calledzeotropic mixture or azeotrope, at 95.6% by weight (97.2% byolume) with water at a temperature of 78.15 ◦C, which makes itmpossible to separate ethanol–water in a single distillation col-mn. In general, for the solution containing 10–85 wt% ethanol,istillation is effective, while for the mixture containing morehan 85 wt% ethanol, distillation becomes expensive because theeed ethanol concentration is near the azeotropic point (95.6%),equiring high reflux ratios and additional equipment, especiallyhen anhydrous ethanol is required [67]. Recently, the separa-

ion of dilute ethanol–water mixture is usually divided into twoarge steps: approximately 92.4 wt% ethanol is firstly obtainedrom the dilute aqueous solution by using the ordinary distil-ation, then the resulting ethanol is further dehydrated in ordero achieve anhydrous ethanol by employing azeotropic distilla-ion, extractive distillation, liquid–liquid extraction, adsorption,r some complex hybrid separation methods. To help select theest or suitable separation method from these alternatives, theetailed description of all these methods is given in the follow-ng.

.1. Ordinary distillation

Ordinary distillation (OD) is a commonly used process foreparation of two or more components in a solution based onheir relative volatilities or the difference in their boiling temper-tures. The ethanol–water azeotrope can be eliminated or brokenown to produce anhydrous ethanol by only lowering the opera-ion pressure to a vacuum condition like 0.11 atm, but this is notconomical [68]. In a biorefinery, therefore, an OD column, alsoalled beer column or pre-concentrator column, is often used tooncentrate dilute ethanol to 92.4 wt%, as mentioned above.

.2. Azeotropic distillation (AD)

Azeotropic distillation involves adding a third volatile com-onent, called entrainer, which forms a ternary azeotrope withhe two components to be separated and thus changes their rela- F


ive volatilities and finally alters their separation factor (activityoefficients) in the distillation system. The two componentso be separated are generally close boiling components or anzeotropic mixture [69]. So, AD can be used to separate close-oiling mixtures or azeotrope.

The AD system typically consists of two distillation columnsor dehydration of 92.4 wt% ethanol solution from the OD col-mn:

1) A dehydration column (azeotropic column) for further con-centration in the presence of entrainer.

2) An entrainer recovery column (stripping column) for sepa-ration of entrainer from the product stream.

n the dehydration column, ethanol (>99 wt%) exits from the bot-oms, while water vapor, solvent, and small amounts of ethanolxit from the tops. The top stream enters a separator, calledecanter, and splits into ethanol-entrainer (organic phase) andater-entrainer (aqueous phase) streams. The former is refluxedack into the first column, while the latter is processed in thentrainer recovery column [70–73]. The process flow sheet ishown in Fig. 10.

The commonly used entrainers for breaking binarythanol–water azeoptropes by heterogeneous azeoptropic dis-illation are benzene [72,74], toluene [75,76] and cyclohexane77]. A mixed solvent, e.g., a mixture of benzene and n-octanean also be used [72]. Benzene is a traditional entrainer in het-rogeneous azeotropic distillation for ethanol dehydration. Forany years, however, benzene has been substituted by other sol-

ents because of its carcinogenic effect. Currently, cyclohexanes one of the most used entrainers for this separation [77]. How-ver, cyclohexane also has the disadvantage of flammability.

The two-columns azeotropic system mentioned above has theisadvantage of high energy requirement, large capital cost, and

ig. 10. Flow sheet of two-column process for ethanol dehydration [72,73].


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.3. Extractive distillation (ED)

Like AD, ED is a vapor–liquid separation process with theddition of a third component to increase the relative volatilityf the components to be separated. In the ED process, a selectiveigh boiling solvent is utilized to alter the activity coefficientsnd hence increase the separation factor. This method is com-only employed in chemical industry to separate close boiling

oint or azeotropic mixtures. The third component added as sep-rating agent can be liquid solvent, ionic liquid, dissolved salt,mixture of volatile liquid solvent and dissolved salt, or hyper-ranched polymer, leading to corresponding five categories ofxtractive distillations, which will be discussed in the following.

.3.1. Extractive distillation with liquid solventThe conventional liquid solvents used as extractants (extrac-

ive agents) in extractive distillation are usually of high boilingoints. The typical extractive distillation for ethanol dehydrations illustrated in Fig. 11 where a suitable amount of high-boilingon-ideal solvent is introduced in the upper part above the feed.

One of the most commonly used extractive solvents in extrac-ive distillation for ethanol dehydration is ethylene glycol, withhich anhydrous ethanol could be produced from the fermen-

ation broth in a column with only 18 theoretical stages, a loweflux ratio of 1.5 and a low solvent/feed ratio of 0.27 [70].

eirelles et al. (1992) had verified that this process could beompetitive with azeotropic distillation, under specific operatingonditions [78].

Gasoline is a good solvent for extractive distillation of beerolution to produce motor fuel ethanol, i.e., gasohol. Fig. 12hows the simplified process flow diagram (PFD) for gasoholroduction where the ethanol–water volatility is reversed by theddition of gasoline consisting of high percent of C7 and C8ractions, causing water to be withdrawn overhead with residual
thanol and lighter hydrocarbons, and the ethanol–solvent mix-ure to exit at the bottom. The bottom stream is then mixed withhe organic phase of the decanter to provide the final gasoholroduct [72].
ig. 11. Extractive distillation (C1: extractive distillation column, C2: solventecovery column).

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ig. 12. Simplified PFD of ethanol dehydration by extractive distillation withasoline [72]. (HH: heavier hydrocarbon; LH: lighter hydrocarbon).

.3.2. Extractive distillation with dissolved saltFor some systems, e.g. ethanol–water system, a dissolved

alt can be added as a separating agent into extractive distilla-ion where the salt dissolved into the liquid so as to enhanceonsiderably the relative volatility of the more volatile compo-ent of the mixture to be separated, due to the so-called “saltffect” [79]. Fig. 13 is a typical simplified PFD of this process.

The most commonly tested dissolved salts in extractive dis-illation for ethanol dehydration are potassium acetate [80–82],odium acetate [83] and calcium chloride [68,79,84]. Cook andurter [81] studied the extractive distillation process with potas-ium acetate as separating agent in a pilot-scale bubble-cap trayolumn, and found that the ethanol–water azeotrope could beliminated with relatively small amount of salt. They also made aomparison of advantages and disadvantages between dissolvedalts and conventional liquid extractants, and found that extrac-ive distillation with salts is more efficient in ethanol–water

2equirement, with azeotropic distillation (using benzene, pen-ane and diethyl ester), extractive distillation (using ethylene

ig. 13. Typical simplified PFD of the extractive distillation with dissolved salt80].

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lycol or gasoline), solvent extraction, and membrane pervapo-ation. It was found that the extractive distillation with CaCl2onsumes almost the same energy as membrane pervaporation,nd these two methods are superior to the other mentionedn energy-saving. In addition, a mixture of two or more saltsan also be employed in extractive distillation. For instance, a0/30 mixture of potassium and sodium acetate was utilizedn the HIAG (Holz Industrie Acetien Geselleschaft) extractiveistillation process, which could produce more than 99.8 wt%thanol, with lower capital and operating costs (energy con-umption) compared to conventional azeotropic distillation withenzene or extractive distillation with ethylene glycol [79,83].ecently, Pinto et al. [85] simulated both conventional (usingthylene glycol) and saline extractive distillation with differentalts such as NaCl, KCl, KI and CaCl2 for ethanol dehydrationy use of Aspen Plus, and made comparison between them. Itas shown that CaCl2 provides the largest salting out effect on

thanol among the four salts mentioned, and that saline extrac-ive distillation with CaCl2 has the lower energy consumption asompared with conventional extractive distillation with ethylenelycol. It was demonstrated that the saline extractive distillations a better process to obtain anhydrous ethanol from the fermen-ation broth, due to the use of only one column, which requireshe lowest energy consumption, and uses non-toxic solvent.

Recently, Ligero et al. [86] proposed and compared twoifferent process flowsheets of extractive distillation with potas-ium acetate as separating agent by simulation. In the first flowheet, dilute ethanol solution is fed to the extractive distillationolumn, followed by salt recovery in a multiple effect evapora-or and a spray dryer, and then recycled to the column. In theecond, dilute ethanol solution is concentrated firstly by con-entional distillation, and the resulting concentrated ethanol ishen fed to the extractive distillation column followed by a sin-le spray dryer for recovery of salt, which is also recycled in theolumn. It is shown that the second flowsheet has less energyonsumption than the first.

In order to reduce energy consumption, heat integration isften considered and included in the ethanol separation pro-ess. For example, Lynd and Grethlein [82] optimized a processowsheet by heat integration for anhydrous ethanol productionrom beer liquors. The process consists of a pre-concentrationolumn with intermediate heat pumps and optimal side-streameturn, a saline extractive distillation column with potassiumcetate as an agent, a salt-concentrating evaporator, and a sprayryer. It was shown that the process proposed requires lowerapital costs and much less energy consumption in comparisonith conventional separation procedures for making anhydrous

thanol.

.3.3. Extractive distillation with the mixture of liquidxtractant and dissolved salt

Similar to the liquid extractant or dissolved salt, the combi-ation of both liquid extractant and dissolved salt can be used
s separating agent in extractive distillation for ethanol purifi-ation, with the same process flowsheet. In general, only a littlemount of salt was required in the mixture of liquid extractantnd dissolved salt.
sotf


Some scholars [87–89] had also investigated the ED processor ethanol dehydration with mixtures of solvent liquid and vari-us dissolved salts (NaCl, CaCl2, SrCl2, AlCl3, KNO3, Cu(NO3), Al(CO3)3, CH3COOK and K2CO3). The relative volatilitiesith mixtures of ethylene glycol and different salts are found

n the range of 1.9–4.15, with the following order of salt effect:lCl3 > CaCl2 > NaCl; Al(CO3)3 > Cu(NO3)2 > KNO3, and therder of the effect of acidic roots: Ac− > Cl− > NO3

−, wherehe ethanol solution : separating agent ratio (v/v) is 1.0, and thealt concentration is 0.2 g salt per ml of solvent. More recently,ei et al. [90] measured the vapor–liquid equilibria of three sys-

ems including ethanol–water, ethanol–water–ethylene glycol,nd ethanol–water–ethylene glycol–CaCl2, at finite concentra-ion and normal pressure. The results proved that the extractiveistillation with combined ethylene glycol and dissolved saltas more efficient in separating ethanol and water than with

thylene glycol only.

.3.4. Extractive distillation with ionic liquidExtractive distillation with ionic liquids (IL) as separating

gent is a novel method for separation of ethanol–water mixture91]. This process has the advantages of high separation ability,asy operation, and no problem of entrainment of the solventnto the top product of the column as compared to extractiveistillation with the mixture of liquid solvent and solid salt [92].onic liquid as separating agent can greatly enhance the relativeolatility of ethanol over water, due to the similar salt effect tohe solid salt.

Ionic liquids (IL), or room-temperature ionic liquids (usuallymixture of organic cation and an inorganic anion), are promis-

ng separating agents for extractive distillation of ethanol–waterixture, due to their favorable properties such as low viscosity,

hermal stability, good solubility and lower corrosiveness thanrdinary high melting salts. The commercially available ioniciquids suitable for use as separating agent for the extractiveistillation, are 1-butyl-3-methylimidazolium tetrafluoroborate[BMIM]+[BF4]−), 1-ethyl-3-methylimidazolium tetrafluorob-rate ([EMIM]+[BF4]−) and 1-butyl-3-methylimidazoliumhloride ([BMIM]+[Cl]−). Seiler et al. [93] investigated theossibility of using these ionic liquids as separating agent inhe extractive distillation for ethanol dehydration, the processiagram of which is illustrated in Fig. 14. It was shown thathese ionic liquids remarkably increase the relative volatil-ty of ethanol to water, in the following order: [BMIM][Cl]− > [EMIM]+[BF4]− > [BMIM]+[BF4]−. It was also foundhat the influence of [BMIM] +[Cl]− and [EMIM]+[BF4]− onhe relative volatility is greater than that of the conventionaleparating agent 1,2-ethanediol. In addition, it was shown byrocess simulation that the overall heat duty can be saved upo 24% for the [EMIM]+[BF4]− process as compared with theonventional ED process.

.3.5. Extractive distillation with hyperbranched polymersLike ionic liquids, hyperbranched polymers are also novel

eparating agents used in extractive distillation for dehydrationf ethanol from aqueous solutions. Hyperbranched polymers,he highly branched macromolecules with a large number ofunctional groups, can be readily manufactured by one-step reac-


Fig. 14. Extractive distillation using ionic liquid as non-volatile entrainer (A:m

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ain column, B: flash drum, C: stripping column) [93].

ions, representing economically favorable agents for large-scalendustrial applications [93]. Unlike linear polymers, hyper-ranched polymers have the features of remarkable selectivitynd capacity, low viscosity and thermal stability. Therefore,hey were recently suggested as entrainers in extractive dis-illation for separating azeotropic mixtures [91]. For instance,m [94] employed non-volatile polymeric entrainers such asoly(ethylene glycol) and poly(acrylic acid) for dehydratingthanol. In his research, the experimental test of solubility androup contribution model calculations were used to guide inhe initial selection of possible polymers. The VLE data mea-
ured show that polymeric entrainers, e.g., Poly(ethylene glycol)t 10 wt% and poly(acrylic acid) at 0.45 wt%, can break thethanol–water azeotrope for ethanol dehydration. f
able 1ummary of dehydration technologies of extractive distillation with different agents

echnologies Advantages

xtractive distillation with liquid solvent Less energy consumption than athe high boiling point of the addthe possible solvents.

xtractive distillation with dissolved salt High production capacity and losmaller solvent ratio; does not cproduct due to its non-volatilityany safety and health hazards.

xtractive distillation with the mixture ofliquid extractant and dissolved salt

Integrates the advantages of botand dissolved salt (high separat

xtractive distillation with ionic liquid (IL) (1) IL cannot pollute the distillaconsiderable reduction of requinon-volatility, high selectivitieslarger variety of feasible IL regproperties (solubility, capacity,thermal stability) can be tailorecolumn required, representing l

xtractive distillation with hyperbranchedpolymers

(1) Excellent separation efficiencan not contaminate the top pro


Seiler et al. [95] studied the phase equilibria of ternarythanol–water–polymers systems, including vapor–liquid,iquid–liquid and solid–liquid–liquid equilibria. It is foundhat the commercially available hyperbranched polyesters andyperbranched polyesteramides can break the ethanol–waterzeotrope. The most tested hyperbranched polymer as entraineror extractive distillation of the ethanol–water mixture is hyper-ranched polyglycerol (PG). It was found that the effect of PGn the relative volatility of ethanol to water was the same orders that of the conventional entrainer 1,2-ethanediol. It was alsoound by process simulation that the overall heat duty can beaved up to 19% for the PG process, compared to the conven-ional ED process [93].

.3.6. Summary of extractive distillation with differenteparating agents

The advantages and disadvantages of extractive distillationith different separating agents are summarized in Table 1.riefly, compared to extractive distillation with liquid solvent,issolved salt or the mixture of liquid solvent and dissolvedalt, extractive distillation with IL or hyperbranched polymersas excellent separation efficiency and selectivity without pollu-ion of distillate by separating agents, thus requires less energyonsumption. In addition, the recent development of halogen-ree and hydrolysis-stable IL such as [BMIM][octylsulfate]ECOENGTM 418) brings some promise [93]. Therefore, extrac-ive distillation with IL and hyperbranched polymers representwo most promising novel separations.

.4. Liquid–liquid extraction-fermentation hybrid

Liquid–liquid extraction is a particularly promising approachor the recovery of anhydrous ethanol from the aque-

Disadvantages

zeotropic distillation because ofed solvent; flexible selection of

Very high solvent/feed mass ratio, upto 5–8, leading to much consumptionof energy.

w energy assumption due to itsontaminate the overhead; environment-friendly and no

Potential problems in dissolution,transport and recycle of salt; potentialjam and erosion to equipment.

h liquid solvent (easy operation)ion ability).

Less availability of suitable salts;potential corrosion of salts to theequipment; possible contaminant ofthe overhead product by liquidextractants.

te due to their non-volatility; (2)red heat duties because of theirand capacities, especially a

eneration options; (3) IL’sselectivity, viscosity andd; (4) Only one distillationow energy consumption.

IL containing halogen anions isexpensive and has insufficientstability to hydrolysis for long-termapplications; small amounts ofcorrosive and toxic substance (HF)forms during the hydrolysis.

cy and selectivity; (2) entrainersduct.

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us fermentation broth with low energy requirement [68].iquid–liquid extraction is generally combined with fermen-

ation, called extractive-fermentation, where in situ extractions carried out to remove product ethanol and other inhibitoryompounds, hence inhibitions caused by ethanol and othernhibitors is eliminated, causing an increase in the ethanolield.

It is very important to select a high efficient solvent forxtracting ethanol from beer liquors. The criteria of solventelection are [96]:

non-toxic to microorganism,high distribution coefficient,high selectivity with respect to the product,low solubility in the aqueous phase,density different from that of the broth to ensure phase sepa-ration by gravity,low viscosity, large interfacial tension and low tendency toemulsify in the broth,high stability,not expensive.

ome potential biocompatible solvents for extraction of ethanolrom beer liquor reported by several investigators include oleyllcohol [96,97], n-dodecanol [98,99], isoamyl acetate and iso-ctyl alcohol [100], nonanoic acid [101], etc. In the continuousermentation of ethanol with the thermophilic, anaerobic bac-erium Clostridium thermohydrosulfuricum, as shown in Fig. 15,leyl alcohol was used as extractant for simultaneously in situxtraction of ethanol in order to eliminate the ethanol productnhibition. Results showed that the ethanol yield of in situ extrac-ion is two times that of fermentations without in situ extraction96]. Oleyl alcohol was also utilized in a simultaneous sac-harification and extractive fermentation (SSEF) process whereellulose hydrolyzate was fermented to ethanol, and the ethanolroduct was removed by extraction with oleyl alcohol. With
SEF, ethanol productivity increased by 65% and the amount ofater required was greatly reduced, compared to non-extractive
ed batch simultaneous saccharification and fermentation (SSF)97]. The combination of increasing ethanol yield and decreas-

Fig. 15. Continuous fermentation with in situ extraction [96].

4

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ng water consumption results in obvious reduction of overallthanol production cost.

Gyamerah and Glover [98] developed a pilot-scale extractiveermentation for producing ethanol with n-dodecanol as extrac-ant to remove the product and with recycle of the fermentedroth raffinate. It was found that the fresh water consumptioneduced by 78%, due to the successful recycle of the fermen-ation water. Koullas et al. [100] experimentally screened andxamined several organic solvents such as isoamyl acetate,so-octyl alcohol, n-butyl acetate, dibutyl ether and dibutylxalate as potential extractants for the liquid–liquid extrac-ion of ethanol from aqueous solutions. Both isoamyl acetatend iso-octyl alcohol were found to be very good agents withthanol distribution coefficients of above 1, and separationactors in Bancroft coordinates of the order of 70 and 2000,espectively. Most recently, Boudreau and Hill [101] extractedthanol from fermentation broth by use of three fatty acidsvaleric acid, oleic acid and nonanoic acid) as solvents, fol-owed by a flash process. It was found that the combinedonanoic acid extraction with the flash process consumed 38%ess thermal energy, compared with the conventional distillationrocess.

In short, extractive fermentation, which combines solventxtraction and fermentation together, results increase in ethanolield and decrease in fresh water consumption. Coupling extrac-ive fermentation with flash separation can also bring aboutignificant reduction in energy consumption. However, thisrocess requires careful selection of biocompatible extractinggents.

.5. Adsorption

There are two categories of adsorption in the ethanol–watereparation: the liquid-phase adsorption of water from the fer-entation broth and the vapor-phase adsorption of water from

he process stream out of distillation column [102].

.5.1. Vapor-phase adsorption of waterThe most potential adsorbents applied for vapor-phase

dsorption of water from ethanol–water mixtures include inor-anic adsorbents such as molecular sieves [103,104], lithiumhloride [105], silica gel [105], and activated alumina [106],nd bio-based adsorbents such as corn grits [67,102,105].

.5.1.1. Inorganic adsorbents. Zeolites molecular sieve (typeA and 4A) are widely employed in separating ethanol–waterixture [107]. 3A zeolite molecular sieves which has a nominal

ore size of 3 Angstroms (0.30 nm) can be used for dehydra-ion of polar liquids such as ethanol. Water molecules, with anpproximate molecular diameter of 0.28 nm, can easily pene-rate the pores of the molecular sieve adsorbent, while ethanol,ith an approximate molecular diameter of 0.44 nm, is simul-

aneously retained [103]. Recently, Al-Asheh et al. [104] also
tudied ethanol–water separation using molecular sieves (3A,A and 5A).
In terms of application, inorganic adsorbents such as molecu-ar sieves, lithium chloride, and silica gel have been successfully

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pplied as dehydration desiccants in fermentation ethanol plants105].

.5.1.2. Bio-based adsorbents. The potential bio-based adsor-ents include cornmeal, cracked corn, starch, corn cobs, wheattraw, bagasse, cellulose, hemicellulose, wood chips, otherrains, etc. [108]. Basically, bio-based adsorbents can be clas-ified into starch-based (e.g., cornmeal, corn crite) [109], andignocellulosic adsorbents (e.g., rice straw, bagasse [110]).

Ladisch and Dyck first investigated the biomass adsorptionf water for ethanol dehydration and demonstrated that starchynd cellulosic biomass can be employed as an adsorbent to selec-ively adsorb water in the vapor mixture to obtain more than9.5 wt% ethanol [111]. Since then, much attention has beenaid on this field and a number of related papers have beenublished [67,102,104,109,110,112–116].

The most potential starch-based adsorbents have been inves-igated. The adsorption of water from ethanol–water vapor

ixture on a variety of starchy materials, such as cooked corn,orn grits and starch, which have different mean particle diame-ers and different relative amounts of amylose and amylopectin,ad been experimentally measured at 90 ◦C. The results demon-trate that water selectivity over ethanol can be increased withhe amylopectin/amylose ratio in starches [67]. Recently, theapors of 92.4 wt% ethanol from distillation were passed overfixed bed of corn grits, after which almost all the water

s adsorbed on corn grits and anhydrous ethanol is obtained102]. This approach of water vapor adsorption had alreadyeen applied in many fermentation ethanol plants [112]. Hu andie [113] experimentally studied on fixed-bed adsorption withhinese cornmeal as adsorbent and fluidized-bed regeneration

or breaking the ethanol–water azeotrope to obtain anhydrousthanol. It was found that the factors influencing the adsorp-ion capacity of water include the vapor superficial velocityowing through the fixed bed, the bed temperature, and thearticle size distribution of cornmeal. The adsorption capacityf water was determined to be 0.14–0.025 g water g/g adsor-ent. The desorption (regeneration) operation was improvedy employing a fluidized bed instead of the general fixed-ed, in order to efficiently control the bed channel and fasterhe operation. The regeneration temperature was 105 ◦C. Mostecently, Chang et al. [114] investigated the adsorption capac-ty and selectivity of cornmeal for ethanol dehydration on ailot-scale fixed-bed adsorber at temperatures of 82–100 ◦C.esults show that for the vapor containing 93.8 wt% ethanol,

he water selectivity over ethanol on the adsorbent for athe breakthrough point is about 0.5–0.6 under the tempera-ure of 91 ◦C. They [109] also fit the experimental data intohe adsorption equilibrium models, including those based onolanyi adsorption potential theory and Sircar’s model, andound that the models are in good agreement with the exper-mental data.

With respect to ligno-cellulosic materials as adsorbents, the
ignocellulosic adsorbents – bagasse, rice straw, and microcrys-alline cellulose powder had been investigated for adsorptionf water in the vapor mixture with 80–90% ethanol to producenhydrous ethanol [110]. It is reported that the adsorption on
arfe


ignocellulosic materials is primarily dependent on the hydroxylroups of the carbohydrates and the lignin [115]. Most recently,l-Asheh et al. [104] studied the separation of ethanol–waterixtures using natural corncobs, natural and activated palm

tone and oak. The other three lignocellulose-based adsorbentsbleached wood pulp, oak sawdust, and kenaf core) have alsoeen explored for dehydrating the concentrated ethanol solutionontaining 90, 95, and 97 wt% ethanol in a thermal swing adsorp-ion column. It was shown that water is selectively adsorbed andnhydrous ethanol was obtained [116].

.5.2. Liquid-phase adsorption of waterNearly twenty years ago, A-type zeolites were shown to

ave a high capacity and selectivity in separating water fromthanol–water mixtures [117]. Recently, several combinations oftarch-based and cellulosic materials, including white corn grits,-amylase-modified yellow corn grits, polysaccharide-basedynthesized adsorbent, and slightly gelled polysaccharide-basedynthesized adsorbent, have also been tested and screened foriquid-phase adsorption of water. It was shown that starch-baseddsorbents can remove liquid-phase water between 1 and 20 wt%rom ethanol without the adsorbent being dissolved. The adsorp-ion capacity of water increases with increasing water contentn the ethanol–water solution. Compared with silica gel and

olecular sieves, these starch-based adsorbents have lower non-quilibrium adsorption capacity at water concentration below0 wt%. At the concentrations of above 10 wt%, however, thetarch-based adsorbents have similar non-equilibrium adsorp-ion capacity to that of the inorganic adsorbents, under theame adsorption and regeneration conditions. The starch-baseddsorbents adsorb water by forming hydrogen bonds betweenhe hydroxyl groups on the surface of the adsorbent and theater molecules [102]. The use of �-amylase to modify poros-

ty and surface properties of starch resulted in materials withnhanced water sorption properties compared to the native mate-ial [105].

More recently, a thermodynamic and kinetic study on liquidhase adsorption of water from ethanol–water mixtures usingtarch as the adsorbent has also been published [107]. Amongvariety of bio-based adsorbents, corn grits are reported as thenly bio-based adsorbents which have been successfully appliedn industry, though the other bio-based material such as cellulosend hemicellulose also have adsorptive properties [105].

.5.3. Advantages and disadvantages of adsorptionThe vapor phase adsorption consumes lower energy than dis-

illation, because of only one-time vaporization required [113].eolite molecular sieves are highly selective, but water is verytrongly adsorbed and high temperatures and/or low pressuresre required to regenerate them [67]. Bio-based adsorbents haveower separation capacity than molecular sieves, but their regen-ration temperature is much lower than molecular sieves. Inddition, molecular sieves are more expensive than bio-based
dsorbents. In some cases of using bio-based adsorbents foremoval of water, the saturated adsorbents can be used directly aseedstock, and simply fresh adsorbents are used without regen-ration step.

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.6. Membrane separation

For a few decades, membrane pervaporation (PV) has beenonsidered as one of the most effective and energy-saving pro-ess for separation of azeotropic mixtures. So far, over 100 plantsn the world use PV technique for alcohol dehydration [118]. Inrinciple, pervaporation is based on the solution-diffusion mech-nism. Its driving force is the gradient of the chemical potentialetween the feed and the permeate sides of the membrane. Ineneral, there are two different pervaporation processes: vacuumnd sweep gas pervaporation. In the vacuum PV process, solu-ion to be separated contacts the membrane at the feed side, i.e.etentate or upstream side, where the retained retentate leaveshe unit. On the permeate (downstream) side, the partial pressuref pervaporated permeate is lowered by using a vacuum pump.he sweep gas pervaporation uses an inert sweep gas such as N2n the permeate side to reduce the permeate partial pressure.

Membranes can be either hydrophilic or hydrophobic. In gen-ral, most membranes are hydrophilic or water permselectiveue to water’s smaller molecular size, while few membranesre hydrophobic or ethanol permselective. Based on materialssed for membrane production, there are three categories ofembranes: inorganic, polymeric and composite membrane.

.6.1. Hydrophilic membrane for removal of water

.6.1.1. Inorganic membrane. Inorganic pervaporation mem-ranes have recently become commercially available inhemical reaction engineering, because of their superior tem-erature stability and mechanical strength [119]. For example,ubular zeolite and silica membranes are still stable to temper-tures of above 300 ◦C and feed pressures of above 100 bar.he tabular Zeolite NaA membrane module, having the perva-oration flux of ca. 2.35 kg m−2 h−1 and the separation factorf above 5000 for the solution of 95 wt% ethanol at 95 ◦C,an be available at a low price [106]. Shah et al. [120] alsotudied the pervaporation separation of ethanol–water with aaA-zeolite membrane for a wide range of operation condi-

ions. It was demonstrated that the ionic Na+ sites in the zeoliteatrix play an important role in the water transport through theembrane.The first commercial large-scale PV plant, composed of

6 membrane modules, with each containing 125 NaA-zeolite
embrane tubes, could produce 530 L/h of more than 99.8 wt%
thanol from 90 wt% solvent at 120 ◦C. The NaA-zeoliteembrane showed high water-selective permeation and high

ermeation flux [121].

aawp

able 2V performance of zeolite membranes for separating ethanol–water mixtures

embrane xw (wt% water) T (K)

eolite T 10 348eolite T 10 348aA 5 368aA 10 348aX 10 348aY 10 348

here αw/e is the separator factor of water over ethanol.


Table 2. lists some PV separation factors for the hydrophiliceolite membranes.

.6.1.2. Polymeric membrane. So far, a large number of poly-eric pervaporation membranes, for example cellulose acetate

utyrate membrane [126], PDMS (polydimethylsiloxane) mem-rane [127], PDMS-PS IPN supported membranes [128],romatic polyetherimide membranes [129] have been inves-igated. O’Brien and Craig [127] utilized the commerciallyvailable PDMS membrane module in a continuous fermen-ation/membrane pervaporation system to produce ethanol,esulting in permeate of 20–23 wt% while 4–6 wt% level wasetained in stirred tank fermentor. The selectivity ranged from.8 to 4.1. Ruckenstein et al. utilized polydimethylsiloxane-olystyrene interpenetrating polymer network (PDMS-PS IPN)upported membranes for pervaporation separation of ethanolrom aqueous solutions. As PS is more hydrophobic and hasigher tensile strength than PDMS, the mechanical and film-orming properties of PDMS-PS are better than those of PDMS.he selectivity of these PDMS-PS membranes varied with the

eed composition. For the feed having low ethanol concentration,he membrane was more selective for ethanol, while for the feedith high ethanol concentration it was more selective for water

128]. Schue et al. [129] investigated the sorption, diffusion andervaporation of ethanol solution in homogeneous and compos-te aromatic polyetherimide membranes. The performance ofhese membranes was found dependent on the permeate diffu-ivity rather than its solubility.

.6.1.3. Composite or mixed membrane. To combine thedvantages of inorganic membrane and polymeric membrane forbtaining high ratio of membrane performance/cost, recently,arious inorganic-polymer or polymer–polymer compositeembranes, such as polystyrenesulfonate/alumina [130], poly-

lectrolytes multi-layer [131], KA zeolite-incorporated cross-inked PVA multilayer mixed matrix membranes (MMMMs)132], and poly(vinyl alcohol) (PVA)-sodium alginate (SA)lend membranes [118], have been studied for pervaporationeparation of ethanol/water mixtures. It is demonstrated by Mar-in that the separation factor of polystyrenesulfonate/aluminaomposite membranes was up to 400 [130]. Tieke et al. preparedulti-layer membranes by alternate adsorption of cationic and

nionic polyelectrolytes onto porous support membranes, andchieved highest separation capability when polyelectrolytesith high charge density such as polyetherimide (PEI) andolyvinylsulfate (PVS) are used [131].

Flux (kg m−2 h−1) αw/e Ref.

1.25 2,200 [122]1.10 900 [123]2.35 5,000 [106]2.15 10,000 [124]1.91 170 [125]1.59 130 [125]


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In 2006, Guan et al. fabricated MMMMs (multilayer mixedatrix membranes) consisting of a selective mixed matrixembrane (MMM) top layer, a porous poly(acrylonitrile-co-ethyl acrylate) [poly(AN-co-MA)] intermediate layer and a

olyphenylene sulfide (PPS) nonwoven fabrics substrate. Its found that the separation performance of the MMMM isuperior to that of multi-ply homogenous membranes (MHM)ontaining no zeolite [133]. In addition, a series of three-ayer zeolite-embedded PVA composite membranes have beenuccessfully fabricated using different zeolites with a load-ng of 20 wt%, including 3A, 4A, 5A, NaX, NaY, silicalitend beta. Results showed that the addition of zeolite resultedn decrease in activation energies for water and ethanol, andence increase in separation selectivity [132]. The hydrophilicVA is chosen as the polymeric material because it is theost attractive and economical polymer material for ethanol

ehydration [134].Dong et al. prepared a hollow-fiber composite membrane,

VA-SA blend, supported by a polysulfone (PS) hollow-fiberltrafiltration membrane for pervaporation ethanol dehydration.ith ethanol concentration at 90 wt% in the feed at 45 ◦C,

he separation factors and permeation fluxes were 384 and84 g/m2 h, respectively [118].

.6.2. Hydrophobic membrane for removal of ethanolSo far, many researchers have investigated a number of

ydrophobic membranes, including (1) the most potentialydrophobic polymeric membranes poly(1-trimethylsilyl-1-ropyne) (PTMSP) [135,136] and poly(dimethyl silox-ne)[PDMS] [137–139] membranes, (2) hydrophobic zeoliteembranes [140–142], and (3) the potential composite mem-

ranes, i.e., silicalite-PDMS membranes which consists ofilicatlite-1 particles dispersed in PDMS [143–145]. In Vane’secent review [146], the ethanol–water separation factors ofTMSP and PDMS membranes (polymeric), hydrophobic azeo-

ite membranes, and composite silicate-silicone rubber (PDMS)embranes are tabulated from a large number of published work

n the literature. Based on literature data, the ethanol–watereparation factors of PDMS, PTMSP, composite membranes,nd zeolite are reported to be in the range of 4.4–10.8,–26, 7–59, 12–106, respectively. However, the ethanol–watereparation factors in some other cases might exceed theseanges. For instance, the separation factor of ethanol overater was 218, when using a silicate zeolite membrane where

thanol (98.2 wt%) at permeate was continuously obtainedrom the fermentation broth of 20 wt% ethanol [147]. In gen-ral, the ethanol–water separation factors are largely rankedn the following order: PDMS < PTMSP < composite mem-ranes < zeolite membranes.

To date, hydrophobic zeolite membranes are commerciallyvailable, while polymeric membranes (PDMS, PTMSP) andomposite membranes are still under investigation. Zeoliteembranes are more expensive than polymer membranes,

ut zeolite membranes have higher separation factors andux than polymer membranes. Therefore, zeolite membranesay be more cost effective on per unit ethanol basis

146].Fe

ig. 16. A simplified membrane pervaporation-bioreactor hybrid process [148].

.6.3. Membrane pervaporation-bioreactor hybridFermentation broth generally contains inhibiting substances

ncluding ethanol product, flavors (phenolics), and other chemi-als. This problem can be overcome by combining fermentationith hydrophobic membrane pervaporation for removal of the

nhibitors from the fermentation broth, as seen in Fig. 16. Hence,he process can be carried out continuously and the recoveredrganic VOCs (ethanol, acetone, butanol, 2-propanol) can beeused within other processes.

In the real application, a microfiltration membrane is addedefore pervaporation to avoid fouling of the hydrophobic mem-rane. Besides, the ethanol-enriched solution, i.e., the permeatef the hydrophobic membrane, can be further dehydrated toroduce anhydrous ethanol. The complete process diagram isllustrated in Fig. 17.

.6.4. Vacuum Membrane Distillation (VMD) – bioreactorybrid

Membrane distillation (MD) is an appealing process suitableor separation of aqueous mixtures. There are four types of MD:irect contact membrane distillation (DCMD), air gap mem-rane distillation (AGMD), sweeping gas membrane distillationSGMD) and vacuum membrane distillation (VMD). Actually,MD is quite similar to pervaporation, the only difference being

hat the separation factor here is established by vapor–liquidquilibrium of the feed solution which is not affected by the

ig. 17. A complete membrane pervaporation-bioreactor hybrid process forthanol fermentation and dehydration [146].

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f ethanol and the other inhibitory compounds from fermen-ation broths. Till now, there has been much reported on thisopic [150–152]. As an example, Gryta et al. produced ethanoln a membrane distillation bioreactor where porous capillaryolypropylene membranes were applied for separating volatileompounds, including ethanol and other inhibitors, from theeed (broth), leading to increase in the productivity and theugar-to-ethanol conversion rate [151]. VMD is commerciallyompetitive because of its high selectivity of ethanol over water,arge flux, high thermal efficiency and low energy cost [92].

. Conclusions

This paper attempts to provide a critical review of the variouseparation technologies used in today’s corn–ethanol biorefinerys well as the challenges and opportunities in future cellulosicthanol and integrated lignocellulosic biorefinery producing liq-id fuels and other co-products. As shown in this review, therere two key separation steps in the biorefinery that offers chal-enges and opportunities. First is the separation of fermentationnhibitors after the pre-extraction of hemicelluloses from ligno-ellulosic biomass. Here the promising separation technologiesre the three in situ detoxification hybrid processes includingxtractive-fermentation, membrane pervaporation-bioreactor,nd VMD–bioreactor, which can eliminate the inhibition ofroducts and inhibitory compounds, increase the fermentationield and productivity, and reduce (fresh) water consumptionue to recycle. Currently, ion exchange resin (IER) method ishe preferred choice for detoxification and will be still used in theuture biorefinery because of its high detoxification efficiency,asy (continuous) operation and flexible combination of differ-nt anion and cation exchangers, while the enzymatic treatmentill grow in the future.The second key separation challenge in biorefinery is the

zeotropic nature of ethanol–water mixture posing challengeso remove the last amounts of water producing fuel gradethanol. For ethanol–water azeotropic separation, promisingechnologies are the extractive distillation with ionic liquid andyperbranched polymers, adsorption with molecular sieve andio-based adsorbents, representing low energy consumption.

In addition to the above key steps, other separation steps iniorefining are pre-extraction of hemicellulose (for ILCB) andther value-added chemicals (corn germ, fiber, zein or glutenrom corn–ethanol plants). Key separation technologies for theseteps include “Quick Germ” process for corn germ recovery,Quick Fiber” process for corn fiber recovery, extraction ofein from corn with ethanol followed by ultrafiltration, andhe effective pretreatments for hemicellulose extraction suchs dilute acid pretreatments, liquid hot water extraction, steamxplosion-based extraction, dilute acid-steam explosion andlkaline extraction. Membrane separation, especially nanofiltra-ion represents a promising separation procedure for recovery ofemicelluloses from hydrolyzates because of its low energy con-
umption and excellent separation efficiency. In addition, theombination of a twin-screw extruder and nanofiltration mayecome the preferred choice for extracting hemicelluloses fromardwood chips in the future.

eferences

[1] W.R. Morrow, W.M. Griffin, H.S. Matthews, Modeling switchgrassderived cellulosic ethanol distribution in the united states, Environ. Sci.Technol. 40 (9) (2006) 2877–2886.

[2] R.J. Bothast, M.A. Schlicher, Biotechnological processes for conversionof corn into ethanol, Appl. Microbiol. Biotechnol. 67 (2005) 19–25.

[3] V. Singh, S.R. Eckhoff, Economics of germ preseparation for dry-grindethanol facilities, Cereal Chem. 74 (4) (1997) 462–466.

[4] New Milling Methods Improve Corn Ethanol Production, website:http://www.agclassroom.org/teen/ars pdf/tech/2004/07milling.pdf (Nov.29, 2006).

[5] L.A. Kszos, Bioenergy from switchgrass: reducing production costsby improving yield and optimizing crop management, website:http://www.ornl.gov/∼webworks/cppr/y2001/pres/114121.pdf (Nov. 29,2006).

[6] C.E. Wyman, Ethanol production from lignocellulosic biomass: overview,in: C.E. Wyman (Ed.), Handbook on Bioethanol: Product Ion and Utiliza-tion, Taylor & Francis, Washington, DC, 1996, pp. 1–18.

[7] A. Aden, M. Ruth, K. Ibsen, J. Jechura, K. Neeves, J. Sheehan, et al., Lig-nocellulosic biomass to ethanol process design and economics utilizingco-current dilute acid prehydrolysis and enzymatic hydrolysis for cornstover, NREL report TP-510-32438, 2002.

[8] A.V. Heiningen, Converting a kraft pulp mill into an IntegratedForest BioRefinery (IFBR), http://www.forestbioproducts.umaine.edu/ForestBiorefinery.pdf (Nov. 29, 2006).

[9] D.T. Leathers, Enzymatic saccharification of defatted corn germ, Biotech-nol. Lett. 26 (3) (2004) 203–207.

[10] P. Blanchard, Wet milling, in: Technology of Corn Wet Milling and Asso-ciated Processes, Elsevier Science, Amsterdam, 1992, pp. 92–93.

[11] V. Singh, S.R. Eckhoff, Effect of soak time, soak temperature, and lacticacid on germ recovery, Cereal Chem. 73 (1996) 716–720.

[12] B.S. Dien, R.B. Hespell, L.O. Ingram, R.J. Bothast, Conversion of cornmilling fibrous co-products into ethanol by recombinant Escherichia colistrains KO 11 and SL 40, World J. Microbiol. Biotechnol. 13 (6) (1997)619–625.

[13] D.J. O’Brien, G.E. Senske, M.J. Kurantz, J.C. Craig Jr., Ethanol recoveryfrom corn fiber hydrolyzate fermentations by pervaporation, Bioresour.Technol. 92 (2004) 15–19.

[14] R.A. Moreau, M.J. Powell, K.B. Hicks, Extraction and quantitative anal-ysis of oil from commercial corn, J. Agric. Food Chem. 44 (1996)2149–2154.

[15] V. Singh, R.A. Moreau, L.W. Doner, S.R. Eckhoff, K.B. Hicks, Recoveryof fiber in the corn dry-grind ethanol process: a feedstock for valuablecoproducts, Cereal Chem. 76 (6) (1999) 868–872.

[16] L.W. Doner, K.B. Hicks, Isolation of hemicellulose from corn fiber byalkaline hydrogen peroxide extraction, Cereal Chem. 74 (1997) 176–181.

[17] S.G. Allen, D. Schulman, J. Lichwa, M.J. Antal, J.M. Laser, L.R. Lynd,A Comparison between hot liquid water and steam fractionation of cornfiber, Ind. Eng. Chem. Res. 40 (2001) 2934–2941.

[18] R.A. Moreau, K.B. Hicks, R.J. Nicolosi, R.A. Norton, Corn fiber oil itspreparation and use, United States Patent 5843499, 1998.

[19] J. Wahjudi, L. Xu, P. Wang, V. Singh, P. Buriak, K.D. Rausch, A.J.McAloo, M.E. Tumbleson, S.R. Eckhoff, Quick fiber process: effect ofmash temperature, dry solids, and residual germ on fiber yield and purity,Cereal Chem. 77 (5) (2000) 640–644.

[20] R. Shukla, M. Cheryan, Zein: the industrial protein from corn, Ind. CropsProd. 13 (3) (2001) 171–192.

[21] L.C. Dickey, N. Parris, J.C. Craig, M.J. Kurantz, Separation of maizeparticles from alcohol extracts with minimal losses, Ind. Crops Prod. 16(2002) 145–154.

[22] L.C. Dickey, A. McAloon, J.C. Craig, N. Parris, Estimating the cost ofextracting cereal protein with ethanol, Ind. Crops Prod. 10 (2) (1999)
137–143.
[23] M. Cheryan. Method and system for extraction of zein from corn, USPatent 7,045,607, 2006.

[24] V. Heiningen, Hemicellulose extraction and its integration inpulp production, in: Forest Products Industry of the Future, Quarterly

http://www.agclassroom.org/teen/ars_pdf/tech/2004/07milling.pdf

http://www.ornl.gov/~webworks/cppr/y2001/pres/114121.pdf

http://www.forestbioproducts.umaine.edu/ForestBiorefinery.pdf

http://www.forestbioproducts.umaine.edu/ForestBiorefinery.pdf

Purifi
Status Reports, September 30, 2005, U.S. Department of Energy website:http://www.eere.energy.gov/industry/forest/pdfs/quarterlyhighlights.pdf.

[25] J.M. Cruz, J.M. Dominguez, H. Dominguez, J.C. Parajo, Solventextraction of hemicellulosic wood hydrolyzates: a procedure useful forobtaining both detoxified fermentation media and polyphenols withantioxidant activity, Food Chem. 67 (1999) 147–153.

[26] J. Gonzalez, J.M. Cruz, H. Dominguez, J.C. Parajo, Production ofantioxidants from Eucalyptus globulus wood by solvent extraction ofhemicellulose hydrolyzates, Food Chem. 84 (2004) 243–251.

[27] B.C. Saha, Hemicellulose bioconversion, J. Ind Microbiol. Biotechnol.30 (2003) 279–291.

[28] N. Mosier, C. Wyman, B. Dale, R. Elander, Y.Y. Lee, M. Holtzapple, M.Ladisch, Features of promising technologies for pretreatment of ligno-cellulosic biomass, Bioresour. Technol. 96 (2005) 673–686.

[29] D.R. Knappert, H.E. Grethlein, A.O. Converse, Partial acid hydrolysisof poplar wood as a pretreatment for enzymatic hydrolysis, Biotechnol.Bioeng. 11 (1981) 67–77.

[30] Pretreatment of biomass, in: T.-A. Hsu, C.E. Wyman (Eds.), Handbookon Bioethanol Production and Utilization, Applied Energy TechnologySeries, Taylor & Francis, Washington DC, 1996 (Chapter 10).

[31] Y.Y. Lee, P. Iyer, R.W. Torget, Dilute-acid hydrolysis of lignocellulosicbiomass 65 (1999) 93–115.

[32] Q.A. Nguyen, M.P. Tucker, F.A. Keller, F.P. Eddy, Twostage dilute-acidpretreatment of softwoods, Appl. Biochem. Biotechnol. 84/86 (2000)561–576.

[33] W.S.L. Mok, M.J.J. Antal, Uncatalyzed solvolysis of whole biomasshemicellulose by hot compressed liquid water, Ind. Eng. Chem. Res. 31(1992) 1157.

[34] M. Saska, E. Ozer, Aqueous extraction of sugarcane bagasse hemicel-lulose and production of xylose syrup, Biotechnol. Bioeng. 45 (1995)517–523.

[35] J.R. Weil, M. Brewer, R. Hendrickson, A. Sarikaya, M.R. Ladisch,Continuous pH monitoring during pretreatment of yellow poplar woodsawdust by pressure cooking in water, Appl. Biochem. Biotechnol. 70–72(1998) 99–111.

[36] H.H. Brownell, J.N. Saddler, Steam pretreatment of lignocellulosic mate-rial for enhanced enzymatic hydrolysis, Biotechnol. Bioeng. 29 (1987)228–235.

[37] M. Heitz, E. Capek-Menard, P.G. Koeberle, J. Gagne, E. Chornet, R.P.Overend, J.D. Taylor, E. Yu, Fractionation of Populus tremuloides in thepilot plant scale: optimization of steam pretreatment conditions usingSTAKE II technology, Bioresour. Technol. 35 (1991) 23–32.

[38] J.R. Weil, A. Sarikaya, S.-L. Rau, J. Goetz, C.M. Ladisch, M. Brewer, R.Hendrickson, M.R. Ladisch, Pretreatment of yellow poplar sawdust bypressure cooking in water, Appl. Biochem. Biotechnol. 68 (1/2) (1997)21–40.

[39] K. Shimizu, K. Sudo, H. Ono, M. Ishihara, T. Fujii, S. Hishiyama,Integrated process for total utilization of wood components by steam-explosion pretreatment, Biomass Bioenergy 14 (3) (1998) 195–203.

[40] M. Ibrahim, W.G. Glasser, Steam-assisted biomass fractionation. Part III:a quantitative evaluation of the “clean fractionation” concept, Bioresour.Technol. 70 (1999) 181–192.

[41] T. Josefsson, H. Lennholm, G. Gellerstedt, Steam explosion of aspenwood. Characterisation of reaction products, Holzforschung 56 (2002)289–297.

[42] C. Cara, E. Ruiz, I. Ballesteros, M.J. Negro, E. Castro, Enhanced enzy-matic hydrolysis of olive tree wood by steam explosion and alkalineperoxide delignification, Process Biochem. 41 (2006) 423–429.

[43] M.P. Tucker, K.H. Kim, M.M. Newman, Q.A. Nguyen, Effects of temper-ature and moisture on dilute-acid steam explosion pretreatment of cornstover and cellulase enzyme digestibility, Appl. Biochem. Biotechnol.105 (2003) 165–177.

[44] S. N’Diaye, L. Rigal, P. Larocque, P.F. Vidal, Extraction of hemicellu-
loses from poplar, Populus tremuloides, using an extruder-type twin-screwreactor: a feasibility study, Bioresour. Technol. 57 (1996) 61–67.
[45] I. Gabrielii, P. Gatenholm, W.G. Glasser, R.K. Jain, L. Kenne, Sepa-ration, characterization and hydrogel-formation of hemicellulose fromaspen wood, Carbohydr. Polym. 43 (2000) 367–374.


[46] R.C. Sun, J.M. Fang, J. Tomkinson, Z.C. Geng, J.C. Liu, Fractional isola-tion, physico-chemical characterization and homogeneous esterificationof hemicelluloses from fast-growing poplar wood, Carbohydr. Polym. 44(2001) 29–39.

[47] S. N’Diaye, L. Rigal, Factors influencing the alkaline extraction of poplarhemicelluloses in a twin-screw reactor: correlation with specific mechan-ical energy and residence time distribution of the liquid phase, Bioresour.Technol. 75 (2000) 13–18.

[48] A.J. Ragauskas, M. Nagy, D.H. Kim, C.A. Echert, J.P. Hallett, C.L.Liotta, From wood to fuels: integrating biofuels and pulp production,Ind. Biotechnol. 2 (2006) 55–65.

[49] S. Hurlen, A. Olsen, Removal of hemicellulose from steeping lye byultrafiltration, in: Proc. 5th International Dissolving Pulps Conference,1980, pp. 54–58.

[50] R. Schlesinger, G. Gotzinger, H. Sixta, A. Friedl, M. Harasek, Evaluationof alkali resistant nanofiltration membranes for the separation of hemi-cellulose from concentrated alkaline process liquors, Desalination 192(2006) 303–314.

[51] O.F. Ali, J.T. Cenicola, J. Li, J.D. Taylor, Process for producing alkalinetreated cellulosic fibers, United States Patent 6,896,810 (2005).

[52] M. von Sivers, G. Zacchi, L. Olsson, B. Hahn-Hagerdal, Cost analysisof ethanol production from willow using recombinant Escherichia coli,Biotechnol. Prog. 10 (1994) 555–560.

[53] E. Palmqvist, B. Hahn-Hagerdal, Fermentation of lignocellulosichydrolyzates II: inhibitors and mechanisms of inhibition, Bioresour. Tech-nol. 74 (2000) 25–33.

[54] A.K. Chandel, R.K. Kapoor, A. Singh, R.C. Kuhad, Detoxification ofsugarcane bagasse hydrolyzate improves ethanol production by Candidashehatae NCIM 3501, Bioresour. Technol. 98 (10) (2007) 1947–1950.

[55] C. Luo, D.L. Brink, H.W. Blanch, Identification of potential fermentationinhibitors in conversion of hybrid poplar hydrolyzate to ethanol, BiomassBioenergy 22 (2002) 125–138.

[56] E. Palmqvist, B. Hahn-Hagerdal, Fermentation of lignocellulosichydrolyzates I: inhibition and detoxification, Bioresour. Technol. 74(2000) 17–24.

[57] A. Converti, J.M. Dominguez, P. Perego, S.S. Silva, M. Zilli, Woodhydrolysis and hydrolyzate detoxification for subsequent xylitol produc-tion, Chem. Eng. Technol. 23 (2000) 1013–1020.

[58] J.J. Wilson, L. Deschatelets, N.K. Nishikawa, Comparative fermentabil-ity of enzymatic and acid hydrolyzates of steam-pretreated aspenwoodhemicellulose by Pichia stipitis CBS 5776, Appl. Microbiol. Biotechnol.31 (1989) 592–596.

[59] M. Cantarella, L. Cantarella, A. Gallifuoco, A. Spera, F. Alfani, Compar-ison of different detoxification methods for steam-exploded poplar woodas a substrate for the bioproduction of ethanol in SHF and SSF, ProcessBiochem. 39 (2004) 1533–1542.

[60] J.C. Parajo, H. Dominguez &, J.M. Dominguez, Charcoal adsorption ofwood hydrolyzates for improving their fermentability: influence, Biore-sour. Technol. 57 (1996) 179–185.

[61] L. Canilha, J.B. de, A. e Silva, A.I.N. Solenzal, Eucalyptus hydrolyzatedetoxification with activated charcoal adsorption or ion-exchange resinsfor xylitol production, Process Biochem. 39 (2004) 1909–1912.

[62] M.L.M. Villarreal, A.M.R. Prata, M.G.A. Felipe, J.B. Almeida E Silva,Detoxification procedures of eucalyptus hemicellulose hydrolyzate forxylitol production by Candida guilliermondii, Enzyme Microb. Technol.40 (1) (2006) 17–24.

[63] L.J. Jonsson, E. Palmqvist, N.-O. Nilvebrant, B. Hahn-Hagerdal, Detox-ification of wood hydrolyzates with laccase and peroxidase from thewhite-rot fungus Trametes versicolor, Appl. Microbiol. Biotechnol. 49(1998) 691–697.

[64] T. Gutierrez, L.O. Ingram, J.F. Preston, Purification and characterizationof a furfural reductase (FFR) from Escherichia coli strain LYO1—anenzyme important in the detoxification of furfural during ethanol produc-
tion, J. Biotechnol. 121 (2006) 154–164.
[65] C. van Zyi, B.A. Prior, J.C. du Preez, Acetic acid inhibition of d-xylose fer-mentation by Pichia stipitis, Enzyme Microb. Technol. 13 (1991) 82–86.

[66] C. Martin, M. Galbe, C.F. Wahlbom, B. Hahn-Hagerdal, L.J. Jonsson,Ethanol production from enzymatic hydrolyzates of sugarcane bagasse

http://www.eere.energy.gov/industry/forest/pdfs/quarterlyhighlights.pdf

2 Purifi
using recombinant xylose-utilising Saccharomyces cerevisiae, EnzymeMicrob. Technol. 31 (2002) 274–282.

[67] J.P. Crawshaw, J.H. Hills, Sorption of ethanol and water by starchy mate-rials, Ind. Eng. Chem. Res. 29 (1990) 307–309.

[68] C. Black, Distillation modeling of ethanol recovery and dehydration pro-cesses for ethanol and gasohol, Chem. Eng. Prog. 76 (1980) 78–85.

[69] C. Black, Azeotropic disitllation results from automatic computer cal-culations, in: R.F. Gould (Ed.), Extractive and Azeotropic Distillation,1972.

[70] F.-M. Lee, R.H. Pahl, Solvent screening study and conceptual extrac-tive distillation process to produce anhydrous ethanol from fermentationbroth, Ind. Eng. Chem. Process Des. Dev. 24 (1985) 168–172.

[71] J.W. Kovach III, W.D. Seider, Heterogenous azeotropic distillaion-homotopy-continuation methods, Comput. Chem. Engng. 11 (6) (1987)593–605.

[72] A. Chianese, F. Zinnamosca, Ethanol dehydration by azeotropic distilla-tion with a mixed-solvent entrainer, Chem. Eng. J. 43 (1990) 59–65.

[73] W.L. Luyben, Control of a multiunit heterogeneous azeotropic distillationprocess, AIChE J. 52 (2) (2006) 623–637.

[74] S.K. Wasylkiewicz, L.C. Kobylka, F.J.L. Castillo, Synthesis and designof heterogeneous separation systems with recycle streams, Chem. Eng. J.92 (2003) 201–208.

[75] L.R. Partin, A case study in applying pc software: Fowsheet synthesis, in:Proceedings of Chemputers 4: Computer Tools for the Chemical ProcessIndustries, Houston, Texas, 11–13 March, 1996.

[76] G. Feng, L.T. Fan, F. Friedler, Synthesizing alternative sequences via aP-graph-based approach in azeotropic distillation systems, Waste Manag.20 (2000) 639–643.

[77] V. Gomis, A. Font, R. Pedraza, M.D. Saquete, Isobaric vapor–liquidand vapor–liquid–liquid equilibrium data for the system water +ethanol + cyclohexane, Fluid Phase Equilibria 235 (2005) 7–10.

[78] S. Meirelles, H. Weiss, Herfurth, Ethanol dehydration by extractive dis-tillation, J. Chem. Tech. Biotechnol. 53 (1992) 268.

[79] M. Llano-Restrepo, J. Aguilar-Arias, Modeling and simulation of salineextractive distillation columns for the production of absolute ethanol,Comput. Chem. Eng 27 (2003) 527–549.

[80] W.F. Furter, Extractive distillation by salt effect, in: R.F. Gould (Ed.),Extractive and Azeotropic Distillation, Advances in Chemistry Series,vol. 115, 1972, pp. 35–45.

[81] R.A. Cook, W.F. Furter, Extractive distillation employing a dissolved saltas separating agent, Can. J. Chem. Eng. 46 (1968) 119–123.

[82] L.R. Lynd, H.E. Grethlein, IHOSR/Extractive distillation for ethanol sep-aration, Chem. Eng. Prog. 80 (1984) 59–62.

[83] W.F. Furter, Extractive distillation by salt effect, Chem. Eng. Commun.116 (1992) 35–40.

[84] D. Barba, V. Brandani, G. Giacomo, Hyperazeotropic ethanol salted-outby extractive distillation, Theoretical evaluation and experimental check,Chem. Eng. Sci. 40 (1985) 2287–2292.

[85] R.T.P. Pinto, M.R. Wolf-Maciel, L. Lintomen, Saline extractive distil-lation process for ethanol purification, Comput. Chem. Eng. 24 (2000)1689–1694.

[86] E.L. Ligero, T.M.K. Ravagnani, Dehydration of ethanol with salt extrac-tive distillation-a comparative analysis between processes with saltrecovery, Chem. Eng. Processing 42 (7) (2003) 543–552.

[87] Z.T. Duan, L.H. Lei, R.Q. Zhou, Study on extractive distillation withsalt(I), Petrochem. Technol. (China) 9 (1980) 350–353.

[88] L.H. Lei, Z.T. Duan, Y.F. Xu, Study on extractive distillation with salt(II),Petrochem. Technol. (China) 11 (1982) 404–409.

[89] Q.K. Zhang, W.C. Qian, W.J. Jian, Study on extractive distillation withsalt(III), Petrochem. Technol. (China) 13 (1984) 1–9.

[90] Z. Lei, H. Wang, R. Zhou, Z. Duan, Influence of salt added to solvent onextractive distillation, Chem. Eng. J. 87 (2002) 149–156.

[91] M. Arlt, M. Seiler, C. Jork, T. Schneider, DE Patent No. 10114734 (2001).
[92] Z. Lei, B. Chen, Z. Ding, Special Distillation Processes, first ed., Elsevier,
Amsterdam, 2005.[93] M. Seiler, C. Jork, A. Kavarnou, W. Arlt, R. Hirsch, Separation of

azeotropic mixtures using hyperbranched polymers or ionic liquids,AIChE J. 50 (10) (2004) 2439–2454.


[94] Al-Amer Am, Investigating polymeric entrainers for azeotropic distilla-tion of the ethanol/water and mtbe/methanol systems, Ind. Eng. Chem.Res. 39 (10) (2000) 3901–3906.

[95] M. Seiler, D. Kohler, W. Arlt, Hyperbranched polymers: new selectivesolvents for extractive distillation and solvent extraction, Sep. Purif.nTechnol. 29 (3) (2002) 245–263.

[96] C. Weilnhammer, E. Blass, Continuous fermentation with product recov-ery by in-situ extraction, Chem. Eng. Technol. 17 (1994) 365–373.

[97] J.W. Moritz, S.J.B. Duff, Simultaneous saccharification and extractivefermentation of cellulosic substrates, Biotechnol. Bioeng. 49 (1996)504–511.

[98] M. Gyamerah, J. Glover, Production of ethanol by continuous fermenta-tion and liquid–liquid extraction, J. Chem. Tech. Biotechnol. 66 (1996)145–152.

[99] N. Boluda, V. Gomis, F. Ruiz, H. Bailador, The influence oftemperature on the liquid–liquid–solid equilibria of the ternary sys-tem water + ethanol + 1-dodecanol, Fluid Phase Equilibria 235 (2005)99–103.

[100] D.P. Koullas, O.S. Umealu, E.G. Koukios, Solvent selection for the extrac-tion of ethanol from aqueous solutions, Sep. Sci. Technol. 34 (11) (1999)2153–2163.

[101] T.M. Boudreau, G.A. Hill, Improved ethanol–water separation using fattyacids, Process Biochem. 41 (2006) 980–983.

[102] K.E. Beery, M.R. Ladisch, Adsorption of water from liquid-phaseethanol–water mixtures at room temperature using starch-based adsor-bents, Ind. Eng. Chem. Res. 40 (2001) 2112–2115.

[103] M.J. Carmo, J.C. Gubulin, Ethanol–water adsorption on commercial 3azeolites: kinetic and thermodynamic data, Braz. J. Chem. Eng. 14 (3)(1997) 1–10.

[104] S. Al-Asheh, F. Ganat, N. Al-Lagtah, Separation of ethanol–water mix-tures using molecular sieves and biobased adsorbents, Chem. Eng. Res.Des. 82 (A7) (2004) 855–864.

[105] K.E. Beery, M.R. Ladisch, Chemistry and properties of starch baseddesiccants, Enzyme Microb. Technol. 28 (2001) 573–581.

[106] M. Kondo, M. Komori, H. Kita, K. Okamoto, Tubular type pervaporationmodule with zeolite NaA membrane, J. Membr. Sci. 133 (1997) 133–141.

[107] M.J. Carmo, M.G. Adeodato, A.M. Moreira, E.J.S. Parente Jr., R.S. Vieira,Kinetic and Thermodynamic study on the liquid phase adsorption bystarchy materials in the alcohol–water system, Adsorption 10 (3) (2004)211–218.

[108] M.R. Ladisch, G.T. Tsao, Vapor phase dehydration of aqueous alcoholmixtures, U.S. Patent 4 345 973, 1982.

[109] H. Chang, X.-G. Yuan, H.T.A.-W. Zeng, Experimental investigationand modeling of adsorption of water and ethanol on cornmeal in anethanol–water binary vapor system, Chem. Eng. Technol. 29 (4) (2006)454–461.

[110] S.K. Rakshit, P. Ghosh, V.S. Bisaria, Ethanol separation by selectiveadsorption of water, Bioprocess Biosyst. Eng. 8 (5/6) (1993) 279–282.

[111] M.R. Ladisch, K. Dyck, Dehydration of ethanol: new approach givespositive energy balance, Science 205 (1979) 898–900.

[112] M.R. Ladisch, M. Voloch, J. Hong, P. Bienkowski, G.T. Tsao, Cornmealadsorber for dehydrating ethanol vapors, Ind. Eng. Chem. Process Des.Dev. 23 (1984) 437–443.

[113] X. Hu, W. Xie, Fixed-Bed adsorption and fluidized-bed regeneration forbreaking the azeotrope of ethanol and water, Sep. Sci. Technol. 36 (1)(2001) 125–136.

[114] H. Chang, X.-G. Yuan, H. Tian, A.-W. Zeng, Experimentalstudy on the adsorption of water and ethanol by cornmeal forethanol dehydration, Ind. Eng. Chem. Res. 45 (2006) 3916–3921.

[115] J. Berthold, M. Rinaudo, L. Salmen, Association of water to polar groups;estimations by an adsorption model for ligno-cellulosic materials, Col-loids Surf. A: Physicochem. Eng. Aspects 112 (2) (1996) 117–129.

[116] T.J. Benson, C.E. George, Cellulose based adsorbent materials for thedehydration of ethanol using thermal swing adsorption, Chem. Mater.Sci. 11 (1) (2005) 697–701.

[117] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, JohnWiley, New York, 1984.

Purifi
[118] Y.Q. Dong, L. Zhang, J.N. Shen, M.Y. Song, H.L. Chen, Preparation ofpoly(vinyl alcohol)-sodium alginate hollow-fiber composite membranesand pervaporation dehydration characterization of aqueous alcohol mix-tures, Desalination 193 (2006) 202–210.

[119] S. Sommer, T. Melin, Zeolite membranes in chemical industry, in: The2000 18th Annual Membrane Technology/Separations Planning Confer-ence, Boston, Massachusetts, USA, 2000.

[120] D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, Per-vaporation of alcohol–water and dimethylformamide–water mixturesusing hydrophilic zeolite NaA membranes: mechanisms and experimentalresults, J. Membr. Sci. 179 (2000) 185–205.

[121] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaAmembrane, Sep. Purif. Technol. 25 (1–3) (2001) 251–260.

[122] S. Takaki, H. Kita, K.I. Okamoto, Symp. Ser. Soc. Chem. Eng. Jpn. 66(1998) 90.

[123] Y. Cui, H. Kita, K.-I. Okamoto, Zeolite T membrane: preparation, char-acterization, pervaporation of water/organic liquid mixtures and acidstability, J. Membr. Sci. 236 (2004) 17–27.

[124] K.I. Okamoto, H. Kita, K. Horii, K. Tanaka, Zeolite NaA membrane:preparation, single-gas permeation, and pervaporation and vapor perme-ation of water/organic liquid mixtures, Ind. Eng. Chem. Res. 40 (2001)163–175.

[125] H. Kita, K. Fuchida, T. Horita, H. Asamura, K.I. Okamoto, Preparation ofFaujasite membranes and their permeation properties, Sep. Purif. Technol.25 (2001) 261–268.

[126] W.S Wu, et al., Pervaporation of water and ethanol using a celluloseacetate butyrate membrane, J. Colloid Interface Sci. 160 (1993) 502–504.

[127] D.J. O’Brien, J.C. Craig, Ethanol production in a continuous fermenta-tion/membrane pervaporation system, Appl. Microbiol. Biotechnol. 44(1996) 699–704.

[128] E. Ruckenstein, L. Liang, Pervaporation of ethanol–water mixturesthrough polydimethylsiloxane-polysstyrene interpenetrating polymernetwork supported membranes, J. Membr. Sci. 114 (1996) 227–234.

[129] R. Schue, et al., Sorption, diffusion and pervaporation of water/ethanolmixtures in polyetherimide membranes, Polym. Int. 48 (1999) 171–180.

[130] C.R. Martin, P. Aranda, W.-J. Chen, Pervaporation separation ofethanol/water mixtures by polystyrenesulfonate/alumina compositemembranes, J. Membr. Sci. 107 (1995) 199–207.

[131] B. Tieke, L. Krasemann, Highly efficient composite membranes forethanol–water pervaporation, Chem. Eng. Technol. 23 (2000) 211–213.

[132] Z. Huang, H.-M. Guan, W.L. Tan, X.-Y. Qiao, S. Kulprathipanja, Perva-poration study of aqueous ethanol solution through zeolite-incorporatedmultilayer poly(vinyl alcohol) membranes: effect of zeolites, J. Membr.Sci. 276 (2006) 260–271.

[133] H.-M. Guan, T.-S. Chung, Z. Huang, M.L. Chng, S. Kulprathipanja,Poly(vinyl alcohol) multilayer mixed matrix membranes for thedehydration of ethanol–water mixture, J. Membr. Sci. 268 (2006)113–122.

[134] H. Ohya, K. Matsumoto, Y. Negishi, T. Hino, H.S. Choi, The separationof water–alcohol separation by pervaporation with PVA–PAN compositemembranes, J. Membr. Sci. 68 (1992) 141–148.

[135] Y. Nagase, Y. Takamura, K. Matsui, Chemical modification ofpoly(substituted-acetylene): V. Alkylsilylation of poly(1-trimethylsilyl-


1-propyne) and improved liquid separating property at pervaporation, J.Appl. Polym. Sci. 42 (1991) 185–190.

[136] J.R. Gonzalez-Velasco, et al., Pervaporation of ethanol–water mixturesthrough poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes, Desali-nation 149 (2002) 61–65.

[137] C.S. Slater, P.J. Hickey, F.P. Juricic, Pervaporation of aqueous ethanolmixtures through poly(dimethyl siloxane) membranes, Sep. Sci. Technol.25 (1990) 1063–1077.

[138] S. Takegami, H. Yamada, S. Tsujii, Pervaporation of ethanol/watermixtures using novel hydrophobicmembranes containing polydimethyl-siloxane, J. Membr. Sci. 75 (1992) 93–105.

[139] X. Chen, Z.H. Ping, Y.C. Long, Separation properties of alcohol-watermixture through silicalite-I-filled silicone rubber membranes by perva-poration, J. Appl. Polym. Sci. 67 (1998) 629–636.

[140] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami, K. Haraya, Separa-tion of ethanol/water mixture by silicalite membrane on pervaporation, J.Membr. Sci. 95 (1994) 221–228.

[141] T. Ikegami, H. Yanagishita, D. Kitamoto, K. Haraya, T. Nakane, H. Mat-suda, T. Koura Nand Sano, Production of highly concentrated ethanol in acoupled fermentation/pervaporation process using silicalite membranes,Biotechnol. Tech. 11 (1997) 921–924.

[142] T. Ikegami, H. Yanagishita, D. Kitamoto, K. Haraya, T. Nakane, H. Mat-suda, N. Koura, T. Sano, Highly concentrated aqueous ethanol solutionsby pervaporation using silicalite membrane—improvement of ethanolselectivity by addition of sugars to ethanol solution, Biotechnol. Lett.21 (1999) 1037–1041.

[143] I.F.J. Vankelecom, D. Depre, S. De Beukelaer, J.B. Uytterhoeven, Influ-ence of zeolites in PDMS membranes: pervaporation of water/alcoholmixtures, J. Phys. Chem. 99 (13) (1995) 193–197.

[144] I.F.J. Vankelecom, S. De Beukelaer, J.B. Uytterhoeven, Sorption and per-vaporation of aroma compounds using zeolitefilled PDMS membranes,J. Phys. Chem B 101 (1997) 5186–5190.

[145] B. Moermans, W. De Beuckelaer, I.F.J. Vankelecom, R. Ravishankar, J.A.Martens, P.A. Jacobs, Incorporation of nano-sized zeolites in membranes,Chem. Commun. (2000) 2467–2468.

[146] M.V. Leland, A review of pervaporation for product recovery frombiomass fermentation processes, J. Chem. Technol. Biotechnol. 80 (2005)603–629.

[147] M. Nomura, T. Bin, S.I. Nakao, Selective ethanol extraction from fermen-tation broth using a silicalite membrane, Sep. Purif. Technol. 27 (2002)59–66.

[148] F. Lipnizki, S. Hausmanns, G. Laufenberg, R. Field, B. Kunz, Use ofpervaporation-bioreactor hybrid processes in biotechnology, Chem. Eng.Technol. 23 (7) (2000) 569–577.

[149] C. Gostoli, G.C. Sarti, Separation of liquid mixtures by membrane distil-lation, J. Membr. Sci. 41 (1989) 211–224.

[150] J. Bausa, W. Marquardt, Shortcut design methods for hybrid membrane-distillation processes for the separation of nonideal multicomponentmixtures, Ind. Eng. Chem. Res. 39 (2000) 1658–1672.

[151] M. Gryta, et al., Ethanol Production in membrane distillation bioreactor,Catal. Today 56 (2000) 159–165.

[152] M.A. Izquierdo-Gil, G. Jonsson, Factors affecting flux and ethanol sepa-ration performance in vacuum membrane distillation (VMD), J. Membr.Sci. 214 (2003) 113–130.

a review of separation technologies in current and future bio refineries

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