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Ethyl lactate as a solvent: Properties, applications and productionprocesses – a review

Carla S. M. Pereira, Viviana M. T. M. Silva* and Alırio E. Rodrigues

Received 6th May 2011, Accepted 11th July 2011DOI: 10.1039/c1gc15523g

Ethyl lactate is an environmentally benign solvent with effectiveness comparable topetroleum-based solvents. The worldwide solvent market is about 30 million pounds per year, whereethyl lactate can have an important share. It is considered a chemical commodity and has attractedmuch attention in recent years, since it is formed by the esterification reaction of ethanol and lacticacid, which can be generated from biomass raw materials through fermentation. In this work, anoverview regarding the main properties and applications of ethyl lactate, as well as its synthesis andproduction processes, with a particular emphasis on reactive/separation processes, is presented.

1. Introduction

Organic esters are a very important class of chemicals havingapplications in a variety of areas in the chemical industry, suchas perfumes, flavours, pharmaceuticals, plasticizers, solvents andintermediates.1 Ethyl lactate is an important monobasic ester,also known as lactic acid ethyl ester (IUPAC name: Ethyl (S)-2-hydroxypropanoate), with molecular formula C5H10O3 (seeFig. 1). It is a clear to slightly yellow liquid, and it is foundnaturally in small quantities in a wide variety of foods, includingwine, chicken, and some fruits.

Ethyl lactate can be either in the levo (S) or dextro (R) forms,and it is industrially produced as a racemic mixture througha reversible reaction between ethanol and lactic acid, whereinwater is a by-product. Lactic acid itself is a very importantchemical with applications in the food, pharmaceutical andcosmetic industries. It is also used as a monomer for the manu-facture of biodegradable polymers, as substitutes for traditionalpetrochemical polymers. Lactic acid has been produced throughboth chemical synthesis or fermentation routes. This last pro-cess is the most common,2 however, lactic acid derived fromfermentation broths requires extensive purification processesthat should be inexpensive and environmentally friendly. Ethyllactate hydrolysis is one of the methods used to obtain pure lacticacid.3,4

Ethyl lactate has attracted much attention in recent years,which is reflected in the number of articles published, as shownin Fig. 2. This is probably due to the environmental movementthat emerged in the 1970s (United Nations Conference on theHuman Environment at Stockholm in 1972). Chemical legisla-

Laboratory of Separation and Reaction Engineering (LSRE), AssociateLaboratory LSRE/LCM, Department of Chemical Engineering, Facultyof Engineering, University of Porto, Rua Dr Roberto Frias s/n,4200-465, Porto, Portugal. E-mail: viviana.silva@fe.up.pt; Fax: +351225081674; Tel: +351 225081489

Fig. 1 The molecular structure of ethyl lactate.

tion and environmental regulation were mainly nation based,until the 1980s. Since then, with the increase of internationalchemical trade and the knowledge of the impact of globaltrade on the environment, international environmental treaties,directives, and conventions have emerged.5 Moreover, it was inthe 1980s that environmental protection was connected witheconomic development, leading to dissemination of “sustainabledevelopment” as a means to encourage the integration ofpolicies across sectors (Rio Declaration on Environment andDevelopment and Johannesburg Declaration on SustainableDevelopment, informally known as Earth Summit 1992 and2002, respectively). Furthermore, with the Yom Kippur War(1973) and the subsequent Arab Oil Embargo, the IranianRevolution (between November 1978 and June 1979), andthe Iraq–Iran War (1980), crude oil production declined, and,consequently, the crude oil price increased drastically, from 3USD per barrel in 1972 up to 35 USD in 1981. More recently,the PDVSA strike (Venezuela, 2002–2003) and the Iraq War

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Carla Pereira was born in 1980 in Coimbra, Portugal. She obtained her Ph.D. degree in Chemical and Biological Engineering fromPorto University (FEUP) in 2009 working on process intensification for the sustainable production of ethyl lactate based on membraneand chromatographic reactors under the supervision of Professor Alırio Rodrigues and Dr Viviana Silva. This work was developed inthe Laboratory of Separation and Reaction Engineering, where she is currently working as a post-doctoral fellow. Her research interestsfocus on integrated reaction and separation processes, particularly, simulated moving bed reactor and pervaporation membrane reactors,for diesel additives and green solvents production.

Viviana Silva (1974) received her Ph.D. in Chemical Engineering from University of Porto (Portugal) in 2003. In 2001, she was AssistantProfessor at Instituto Politecnico de Braganca (Portugal) and, in 2006, she joined Fluidinova (Portugal) as Group Leader. In November2007, she returned to the Associated Laboratory LSRE/LCM (University of Porto) as Assistant Researcher. Her primary interest is theprocess intensification of green fuels and solvents, being awarded with the 1st prize “IChemE Awards for Innovation & Excellence 2008”,in the category “ABB Global Consulting Award for Sustainable Technology”. Since 2006, she is also developing continuous processes forthe production of nanoparticles and microcapsules based on the NETmix R© technology.

Alırio E. Rodrigues is Professor of Chemical Engineering at the University of Porto (Portugal). He graduated in Chemical Engineeringfrom the University of Porto in 1968 and received his Dr-Ing degree at the University of Nancy (France) in 1973. He is Director ofthe Associate Laboratory LSRE/LCM (http://lsre.fe.up.pt). His main research interests are in cyclic separation/reaction processes(simulated moving bed, pressure swing adsorption, and parametric pumping), chemical reaction engineering and product engineering(perfume engineering and microencapsulation).

From left to right: Viviana Silva, Alırio E. Rodrigues, Carla Pereira

Fig. 2 Publications related to ethyl lactate since 1980 using alldatabases. Total number of publications with “ethyl lactate” in thetopic = 1566. Total number of publications with “ethyl lactate” inthe title = 304. Total number of publications with “ethyl lactate” +“synthesis” = 51. Data were obtained from ISI Web of Knowledge on5th of July 2011.

(2003), together with a weak dollar and the continued rapidgrowth in Asian economies and their petroleum consumption,

the crude oil price reached a new maximum of 126 USD perbarrel in June 2008, followed by a drastic drop due to thefinancial crisis, being currently at about 94 USD.6 Consideringboth the environmental regulations, as well as the increase incrude oil prices, the search for greener chemicals obtained fromrenewable sources is imperative for sustainable development.The most popular green solvents are water (aqueous biphasic),supercritical carbon dioxide and ionic liquids.7 In spite of theincreasing interest, less attention is being given to bio-basedsolvents, such as the lactate ester family solvents,8 in which ethyllactate is the most important member.

Currently, a great challenge is the design and implementa-tion of completely green products and processes. There is nosystematic and reliable method to ensure that the chemistryimplemented is green, since the number of chemical synthesispathways is vast, and in general it is only possible to verifyif a proposed manufacturing process is “greener” than otheralternatives. Anastas and Warner have developed the “TwelvePrinciples of Green Chemistry”, which are a list of suggestionson how to design a greener process and/or a greener product,9

which are:1. It is better to prevent waste than to treat or clean up waste

after it is formed.

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2. Synthetic methods should be designed to maximize theincorporation of all materials used in the process into the finalproduct.

3. Wherever practicable, synthetic methodologies should bedesigned to use and generate substances that possess little or notoxicity to human health and the environment.

4. Chemical products should be designed to preserve efficacyof function while reducing toxicity.

5. The use of auxiliary substances (e.g. solvents, separationagents, etc.) should be made unnecessary whenever possible and,innocuous when used.

6. Energy requirements should be recognized for their en-vironmental and economic impacts and should be minimized.Synthetic methods should be conducted at ambient temperatureand pressure.

7. A raw material or feedstock should be renewable rather thandepleting whenever technically and economically practical.

8. Unnecessary derivatization (blocking group, protec-tion/deprotection, and temporary modification of physi-cal/chemical processes) should be avoided whenever possible.

9. Catalytic reagents (as selective as possible) are superior tostoichiometric reagents.

10. Chemical products should be designed so that at the end oftheir function they do not persist in the environment and breakdown into innocuous degradation products.

11. Analytical methodologies need to be further developed toallow for real-time in-process monitoring and control prior tothe formation of hazardous substances.

12. Substances and the form of a substance used in a chemicalprocess should be chosen so as to minimize the potentialfor chemical accidents, including releases, explosions, andfires.

The 2nd principle can be quantitatively evaluated throughSheldon’s E-factor, which is a very simple green chemistrymetric, defined by the ratio of the mass of waste per unit ofdesired product.10,11 To evaluate how green a product is, theenvironmental impact associated with all the stages of its life(production, use, and disposal) should be quantified using, forexample, the life cycle assessment (LCA) methodology.12 Forthose who do not have the data and/or the skills required to do aproper LCA of a solvent, Jessop13 suggests a simple methodologyto do a quick “green-ness test”, that involves the analysis ofthe solvent’s synthesis tree. In that work it is reported that theethyl lactate synthesis involves more chemical steps than thesynthesis of hexane or methanol, but has significantly fewer stepsthan some ionic liquids, such as 1-butyl-3-methyli-midazoliumacetate ([Bmim]OAc).

Anyway, ethyl lactate is in accordance with at least eight ofthe “Twelve Principles of Green Chemistry”:

1) Ethyl lactate can be produced from renewable raw materialsthat can be a more environmentally friendly alternative topetrochemical solvent: 7th principle.

2) Ethyl lactate is 100% biodegradable, easy to recycle, non-corrosive, non-carcinogenic and non-ozone depleting.14 Indeed,it is so benign that the U.S. Food and Drug Administrationapproved its use in food products: 3rd, 4th and 10th principles.

3) Ethyl lactate can be produced through heterogeneouscatalysis without using an excess of any of the reactants; theelimination of homogenous catalysts (usually mineral acids)

avoids the use of corrosive catalysts and, thus, eliminates afurther step of their neutralization: 1st and 9th principles.

4) Ethyl lactate can be produced by using hybrid technologieswhere reaction and separation of at least one product take placein a single unity eliminating the use of solvents, reducing thecapital cost (less separation units are needed) and requiring lessenergy consumption: 5th and 6th principles.

In summary, as shown in Fig. 3, ethyl lactate can be easilyobtained by carbohydrate feedstocks, since it is produced fromethanol and lactic acid that are obtained by fermentation ofbiomass, as corn starch crops; and it is a biodegradable com-pound with good properties to be applied as a green solvent inseveral applications, such as organic synthesis, pharmaceuticalpreparations, fragrances, for inks and coatings industries, andfood additives.

Fig. 3 The life cycle of ethyl lactate.

Following these four topics, this review will address: (i) the useof renewable resources, where the concept of a biorefinery willbe briefly introduced and the platform of the chemicals involvedin the ethyl lactate synthesis will be presented; (ii) ethyl lactate’smain properties and applications; (iii) its synthesis through theesterification reaction between ethanol and lactic acid; and (iv)ethyl lactate production processes with particular emphasis onprocess intensification.

2. Renewable resources

In recent years, an increasing demand for using biorenewablematerials instead of petroleum based feedstocks for chemicalsproduction, driven by environmental concerns and by theconcept of sustainability, has been noticed. Biobased productsare one of the main pillars of a sustainable economy. Natureproduces 170 billion tons of biomass per year by photosynthesis,75% of which belong to the class of carbohydrates; however, just3–4% of these compounds are used by humans for food and non-food purposes.15 Carbohydrates are very abundant renewableresources and they are currently considered as an important

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Fig. 4 A schematic diagram of a biorefinery for precursor-contained biomass.19–20

feedstock for the Green Chemistry of the future.16–18 Industrialplants, called biorefineries, have been proposed, where biomassis converted economically and ecologically, into chemicals,materials, fuels and energy (see Fig. 4). Biorefineries couldbe the basis of the new bioindustry. Its concept is similar tothe petroleum refinery; the difference is that the biorefineryis based on conversion of biomass feedstocks instead ofcrude oil.

Ethanol platform

Ethanol, an important raw material in the chemical industryand also used as transportation fuel, can be produced froma variety of biomass crops, including sugar crops (e.g., sugarcane and sugar beet), starch crops (e.g., corn and cassava),or cellulosic feedstocks (e.g., wood, grasses and agriculturalresidues). Ethanol production from starch crops involves, asthe main steps: liquefaction and saccharification (conversionto sugar), milling, pressing, fermentation and distillation. Theproduction from cellulosic feedstocks is similar; however, itis significantly more difficult and costly to convert celluloseand hemicellulose into their component sugars (glucose andxylose, respectively) than is the case for starches.21 In 2006,about 37 billion liters of ethanol were produced worldwidefrom starch and sugar crops.22 In 2008, the cellulosic ethanolindustry developed some new commercial-scale plants: in theUnited States, plants with a 12 million liter capacity per yearwere operational, and an additional 80 million liters per year ofcapacity (26 new plants) were under construction; in Canada,a capacity of 6 million liters per year was operational; inEurope, several plants were operational in Germany, Spain,and Sweden, and a capacity of 10 million liters per year wasunder construction.23 Ethanol derived from cellulosic crops isappealing since it broadens the scope of potential feedstocksbeyond starch and sugar-based food crops. Moreover, cellulosicethanol can be more effective and promising as an alternativerenewable biofuel than corn ethanol because it reduces the netgreenhouse gas (GHG) emissions even more when comparedwith the petroleum fuel24,25 (see Fig. 5).

Fig. 5 The percent change in greenhouse gas emissions.24

Lactic acid platform

Lactic acid (2-hydroxypropionic acid) is an important chemicalplatform for the biorenewable economy. It is an a-hydroxy acidcontaining a hydroxyl group adjacent to the carboxylic acidfunctional group; a review on lactic acid chemistry can be foundin the literature.26

Lactic acid can be produced through chemical synthesis orthrough the fermentation of different carbohydrates, such as,glucose (from starch), maltose (produced by specific enzymaticstarch conversion), sucrose (from syrups, juices, and molasses),or lactose (from whey).27,28 Alternative feedstocks, particularlyfrom wastes, are being sought. In recent years, lactic acidfermentation and separation from fresh grass juice have alsobeen investigated.29–31 In Brandenburg, Germany, a pilot plantis being operated to ferment fresh grass juice in combinationwith hydrolyzed cereals to lactic acid.32 In Utzenaich, Austria,a pilot scale biorefinery was implemented in 2008, whichseparates lactic acid and amino acids from grass silage pressjuice.29 Currently, lactic acid is commercially produced byfermentation of glucose. One of the most important stepsin lactic acid production is the recovery from fermentationbroths. The separation and purification stages represent about50% of the total production cost. However, current advances

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in membrane-based separation and purification technologies,particularly in microfiltration, ultrafiltration and electrodialysis,have originated new processes that should reduce lactic acidproduction costs.33 Lactic acid production is around 350 000tons per year and the predicted worldwide growth per year is12–15%.33 Many products are derived from lactic acid; some ofthem are new chemical products and others are just biobasedroutes to chemicals currently produced from petroleum; themost important ones are shown in Fig. 6. Among these, perhapsthe most interesting are: acrylic acid used as a raw materialfor the production of polymeric products applied in surfacecoatings, textiles, adhesives, paper treatment, detergents, super-absorbent materials, among others;34 and polylactic acid used inpackaging, agricultural products and disposable material, andwith great potential for applications in medicine, surgery, andpharmaceuticals.35 Another product that can be derived fromlactic acid is propylene glycol; however, at the present time,lactic acid prices (€1.2–€1.5/kg) are of the same order as that ofpetrochemical based propylene glycol.36 Nevertheless, with theincrease of crude oil prices and with the further technologicaldevelopment of lactic acid production, it is expected that thiscompound will become cheap enough in order to offer abiomass-based route to produce propylene glycol.

Fig. 6 Some potential derivatives of lactic acid.28,35

Ethyl lactate properties and applications

Ethyl lactate can be used as a food additive, in perfumery,as flavour chemicals and solvent, which can dissolve aceticacid cellulose and many resins.37 Its main application is as asolvent, being particularly attractive for the coatings industryas a result of its high solvency power, high boiling point, lowvapour pressure and low surface tension. Some physical andthermodynamic properties of ethyl lactate as well of the speciesinvolved in its synthesis are presented in the Appendix.

Almost all manufacturing and processing industries dependon the use of solvents (see Fig. 7). Some industry experts suggestthat ethyl lactate could replace the traditional solvents in morethan 80% of their applications.38 However, this is probablyhighly inflated, since ethyl lactate is a high boiling polar proticsolvent13 and there are applications where non-polar, aproticand/or lower boiling point solvents are required. Annually, justin the U.S.A, the solvents demand is about 10 billion poundsat prices from $0.90 to $1.70 per pound. Selling prices forethyl lactate have ranged from $1.50 to $2.00 per pound, but

Fig. 7 Solvent demand by market segment.40

processing advances could drive the price as low as $0.85 to$1.00 per pound, enabling ethyl lactate to compete directly withthe petroleum-derived solvents currently used.38,39 Moreover, thecrude oil prices have risen sharply, making ethyl lactate as agreen solvent more commercially attractive, and due to the risingenvironmental consciousness, some consumers are willing to paymore for products that are less detrimental to the environment.

Weis and Visco,41 developed a study using computer-aidedmolecular design to identify potential new environmentallyfriendly solvents as a supplement to GlaxoSmithKline’s solventselection guide,42 and ethyl lactate was presented as one of thepotential solvents. This evidence is supported by many referencesdocumenting ethyl lactate applications as an environmentallyfriendly solvent, presented below.

Ethyl lactate is a desirable coating for wood, polystyrene andmetals and also acts as a very effective paint stripper and graffitiremover. It can also be used in magnetic tape coatings replacingthe hazardous air pollutants (MEK, MIBK and toluene).43

Ethyl lactate is replacing solvents, such as N-methyl pyrrolidone(NMP),44 toluene, acetone and xylene, which has resulted in asafer workplace. In Table 1, the solvating properties of ethyllactate and NMP are presented.

Other applications of ethyl lactate are as a cleaning agentfor the polyurethane industry and for metal surfaces, efficientlyremoving greases, oils, adhesives and solid fuels. Recently, italso been shown to be effective on the removal of copper fromcontaminated soils.45 Ethyl lactate can also be applied in thepharmaceutical industry as a dissolving/dispersing excipient forvarious biologically active compounds without destroying thepharmacological activity of the active ingredient. It is a veryeffective agent for solubilising biologically active compoundsthat are difficult to solubilise in usual excipients.46,47 Ethyl lactatecan also be used as a more environmentally friendly alternative

Table 1 The solvating properties of ethyl lactate and N-methylpyrrolidone

Ethyl lactateN-Methylpyrrolidone

Kauri Butanol(KB) Value >1000 350

Solubilityparameters

Hildebrand parameter 21.3 23.1

Disperse Hansen parameter 7.8 8.8Polar Hansen parameter 3.7 6.0

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Table 2 The major advantages of ethyl lactate

Ethyl Lactate Benefits

100% Biodegradable Renewable – made from corn andother carbohydrates

FDA approved as a flavouradditive

EPA approved SNAP solvent

Non carcinogenic Non corrosiveGreat penetrationcharacteristics

Stable in solvent formulations untilexposed to water

Rinses easily with water High solvency power for resins,polymers and dyes

High boiling point Easy and inexpensive to recycleLow VOC Not an ozone depleting chemicalLow vapor pressure Not a hazardous air pollutant

route to produce 1,2-propanediol, which is normally producedby the hydration of propylene oxide derived from petrochemicalresources.48

A significant amount of solvents are used in pharmaceuticalmanufacturing processes; indeed, it was estimated that around80% of the total mass of chemicals involved in the pharmaceuti-cal manufacture comprises solvents.49 The redesign of syntheticprocesses in order to reduce the amount of solvent appliedand the use of nontoxic and nonhazardous solvents that areeasy to recover and re-use are of major importance. Fromthis perspective, ethyl lactate is currently being advertised asan environmentally attractive solvent for chemical reactions.7,11

Ethyl lactate has been used in the greener synthesis of arylaldimines,50 synparvolide B,51 varitriol,52 among others. It wasalso used as a green solvent to extract phytosterols from wet cornfiber, which provides an oil product with free phytosterols andfree fatty acids,53 and to extract carotenoid, an effective solventfor both the cis and trans lycopene isomers from dried tomatopowder.54

The major advantages of ethyl lactate are summarized inTable 2.

4. Synthesis of ethyl lactate

The conventional way to produce ethyl lactate is the esterificationof lactic acid with ethanol catalyzed by an acid catalyst,according to the reaction:

Ethanol Eth Lactic Acid La

Ethyl Lactate EL Water W

H( ) ( )

( ) ( )

+ ← →⎯⎯+

+

The use of lactic acid and ethanol as reactants for the ethyllactate synthesis has the advantage of both being producedfrom renewable resources (by glucose or sugar fermentationprocesses).

Lactic acid has a bifunctional nature (contains a hydroxylgroup adjacent to the carboxylic acid) and thus it undergoesintermolecular esterification in aqueous solutions above 20 wt%to form a linear dimer, and higher oligomer acids.55,56 An 88wt% lactic acid solution comprises 43.5 mol% of monomer(La1), 9.2 mol% of dimer (La2), 1.8 mol% of trimer (La3) andabout 45 mol% of water. On the other hand a 20 wt% aqueoussolution of lactic acid is constituted only by monomer andwater, with a monomer molar percentage of about 5.6 mol%.57

The self-esterification degree increases with increasing acidconcentration. This makes the use of lactic acid difficult asa reactant for ethyl lactate synthesis, since the use of a highlactic acid concentration implies the presence of lactic acidoligomers during the esterification, which will be converted intothe corresponding esters that simultaneously undergo hydrolysisand transesterification, leading to a mixture of acid and estermonomer and oligomers:

2La1 ¤ La2 + W (lactic acid dimer formation)

La1 + La2 ¤ La3 + W (lactic acid trimer formation)

. . .

La1 + Lan-1 ¤ Lan + W (lactic acid oligomer formation)

with n ≥ 2

La1 + Eth ¤ EL1 + W (ethyl lactate formation)

La2 + Eth ¤ EL2 + W (ethyl lactate dimer formation)

La3 + Eth ¤ EL3 + W (ethyl lactate trimer formation)

. . .

Lan + Eth ¤ ELn + W (ethyl lactate oligomer formation)

where:

Some of the kinetic studies regarding ethyl lactate syn-thesis are performed using a 20 wt% lactic acid solution inorder to avoid the formation of oligomers, but even whenhigher lactic acid concentrations are used, the presence ofoligomers is normally neglected. Just two works take intoaccount the oligomers; however, their amount is less than 5%

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Table 3 Summary of the kinetic studies of the esterification reaction between ethanol and lactic acid

Refs. CatalystTemperaturerange

Lactic acidsolution(wt.%)

Oligomerspresence Kinetic model

Expression ofthe components

ActivationEnergy(kJ mol-1)

Troupe and DiMilla, 195761 Sulfuric acid 25–100 ◦C 85; 44 neglected empirical equation concentrations 62.47Tanaka et al., 200237 Amberlyst 15 90–92 ◦C 91 considered simple nth-order

reversible rateexpressions

concentrations 47.00c

Benedict et al., 200362 Without catalyst 95 ◦C 88 neglected homogeneous concentrations —Amberlyst XN-1010 75–95 ◦C 88 neglected based on single-site

mechanismsconcentrations 30.54

Engin et al., 200363 Heteropoly acidsupported onLewatit R© S100

70 ◦C 92 neglected simple nth-orderreversible rateexpressions

concentrations —

Zhang et al., 200464 002 60–88 ◦C 20 neglected Langmuir–Hinshelwood

activitiesa 51.58

NKC Langmuir–Hinshelwood

activitiesa 52.26

Asthana et al., 200657 Amberlyst 15 62–90 ◦C 20; 50; 88 considered simple nth-orderreversible rateexpressions

concentrations 48.00c

Delgado et al., 200760 Amberlyst 15 55–86 ◦C 20 neglected Langmuir–Hinshelwood

activitiesb 52.29

Without catalyst 55–85 ◦C 20 neglected homogeneous activitiesb 62.50Pereira et al., 200858 Amberlyst 15 50–90 ◦C 88 neglected Langmuir–

Hinshelwoodactivitiesb 49.98

Bamoharram et al., 201065 Preyssler acid 70–85 ◦C 20 neglected simple nth-orderreversible rateexpressions

concentrations 47.11

a activity coefficients calculated by the UNIFAC model. b activity coefficients calculated by the UNIQUAC model. c activation energy of the lacticacid monomer esterification.

at equilibrium.37,57 The extent of oligomer formation dependson the water concentration, but also on the ethanol contents;for example, for a lactic acid solution of 88 w.%, the oligomerscomposition at equilibrium is 2.4 molar% when using a molarratio between ethanol and lactic acid of 1 and of 0.4 molar%when using an ethanol : lactic acid molar ratio of 3.58

Esterifications are self-catalyzed reactions, since the H+ cationreleased from the partial dissociation of the carboxylic acidused as a reactant catalyses the reaction. However, the use ofa catalyst is favourable for the reaction rate as the kinetics of theself-catalyzed reaction are extremely slow, since its rate dependson the autoprotolysis of the carboxylic acid. For example, thelactic acid acidity constant is pKa = 3.86 at 25 ◦C, and thereforean aqueous solution with 85% of lactic acid (about 10.8 M)has a pH = 1.4. Typically, the catalytic production of lactatesis performed with homogeneous catalysts using acids, suchas sulphuric acid, phosphoric acid and anhydrous hydrogenchloride. However, the use of a heterogeneous catalyst (as forexample, zeolites, ion-echange resins like Amberlyst 15-wet,Nafion NR50, among others) has clear advantages, i.e.: easyto separate from the reaction mixture; long life time; higherpurity of products (side reactions can be eliminated or areless significant); and elimination of the corrosive environmentcaused by the discharge of acid containing waste. A summaryof the kinetic studies performed for lactic acid esterificationwith ethanol is presented in Table 3. As can be seen, the kineticmodel is usually expressed in terms of species concentration;few authors take into account the non-ideality of the reactionmixture using activities instead of concentration. There are areasonable number of kinetic studies for this system availablein the literature; however, only one presents the thermodynamic

equilibrium constant defined as function of the species liquidactivities, described by the following expression: ln(K) = 2.9625- 515.13/T(K).58 There is another work regarding the vapour–liquid reactive equilibrium for the ethyl lactate synthesis,59 wherethe thermodynamic equilibrium constant is also determined,but the values predicted by the proposed equilibrium constantexpression present high deviations from the experimental ones.

As mentioned previously, the hydrolysis of ethyl lactate is alsoan important reaction since it can be used to obtain highly purelactic acid. However, just one work presents a study regardingthis subject.60

5. Production processes

There are a considerable number of patented processes for ethyllactate production; in most of them, the esterification reactionbetween ethanol and lactic acid is carried out until equilibriumand then the ethyl lactate is separated from the reaction mixtureby distillation. In order to overcome the equilibrium limitation,an excess of ethanol is applied and a strong acid is used asa catalyst, the most common being sulphuric acid.66–69 Thereare similar processes, but using ammonium lactate instead oflactic acid.70–75 A recent work76 presents the synthesis of ethyllactate from ammonium lactate solution by the coupling ofsolvent extraction with esterification. Dimethyl sulfoxide, N-methyl pyrrolidine and triethyl phosphate were evaluated assolvents, but the last proved to be the most efficient.

Arkema (France) has two patented processes related tocontinuous ethyl lactate production from the esterificationreaction between lactic acid and ethanol using sulphuric acidas catalyst. One consists of extracting a mixture comprising

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ethyl lactate, ethanol, water and different heavy products fromthe reaction medium at partial lactic acid conversion rateand, then, feeding the mixture to a reduced-pressure flashseparation, producing an overhead stream containing a mixtureof ethyl lactate, ethanol and water, that is subjected to afractional distillation column.77,78 The other is characterized bythe continuous extraction of a near-azeotropic water/ethanolgas mixture from the reaction medium, followed by dehydrationof this mixture using molecular sieves, where two streams areobtained: an ethanol gas stream that is recycled to the reactionmedium; and a water/ethanol stream that is fed to a distillationcolumn.79,80 An ethyl lactate purity higher than 95% is reported inthe first process (initial ethanol/lactic acid molar ratio equal to2.5; esterification carried out at 80 ◦C; flash separation at 85 ◦Cand 50 mbar, and fractional distillation at a column bottomtemperature of 155 ◦C and top temperature of 77.2 ◦C), whilefor the second an ethyl lactate purity higher than 97% is claimed.

The use of this kind of process (reactor followed by separa-tion units in order to recover the ethyl lactate, to remove theby-product (water) and to recycle the unconverted reactants tothe reactor) represents high costs, either capital or operating,and the use of equipment and techniques that are more compactand energy efficient are preferable for ethyl lactate production.

5.1 Process intensification

Ethyl lactate is formed through a thermodynamic equilibriumlimited reaction between lactic acid and ethanol with water asa by-product, thus its production is improved using multifunc-tional reactors, where reaction and separation are integrated intoa single unit, since at least one of the products is continuouslyremoved from the reaction mixture to lead to depletion of thelimiting reactant, increasing the ethyl lactate yield and purity.Some reactive separation processes have already been studiedfor ethyl lactate production, as membrane reactors, reactivedistillation and chromatographic reactors. These studies arepresented next.

5.1.1. Membrane based processes. The Argonne NationalLaboratory patented a process that consists of a reactor coupledwith a pervaporation membrane unit for water removal and

this is followed by separation of the reaction mixture in twoconsecutive distillation columns or, alternatively, by severalpervaporation steps (see Fig. 8).81 It is reported that a lacticacid conversion of 99% and an ethyl lactate purity of 76% wasobtained, for an initial ethanol/lactic acid molar ratio of 2 : 1, areaction mixture temperature of 95 ◦C, a permeate-side vacuumpressure less than 0.5 mbar and, as the catalyst, an ion-exchangeresin, Amberlyst XN-1010, at 10% of lactic acid weight.

Fig. 8 A schematic representation of ethyl lactate production using areactor coupled with a pervaporation membrane unit and followed by:a) pervaporation unit; b) two distillation columns.

The ethyl lactate synthesis on pervaporation and vapour-permeation membrane reactors was also studied by otherauthors. Three different configurations were adopted (Fig. 9):(i) batch reactor, where the esterification reaction takes place,followed by a membrane for water removal, and reflux of theretentate to the reactor;62,81–85 (ii) membrane inside a batchreactor37,86,87 and (iii) continuous integrated membrane reactor,that consists of a tubular membrane packed with a heteroge-neous catalyst (Amberlyst 15-wet).88 The type of hydrophilicmembranes tested were polymeric,62,81,82 ceramic88 and organic–inorganic hybrid membranes84,87 for pervaporation and zeolitesfor vapour permeation.37,86 A summary of the results obtainedfor ethyl lactate synthesis by means of membrane reactors ispresented in Table 4.

Table 4 Ethyl lactate production by means of membrane reactors

Refs. Membrane TemperatureMembrane waterflux (kg m-2 h-1)a Catalyst

Eth/La molarratio

LaConversion EL Purity

Rathin andShih-Perng,199881

GFT PerVap 1005 95 ◦C 1.20 Amberlyst XN-1010 2.0 99% 76%

Jafar et al., 200286 zeolite A 70 ◦C 0.18 p-toluene sulphonic acid 2.0 95% —Tanaka et al.,200237

zeolite T 120 ◦C 0.33 Amberlyst 15 2.4 99% —

Benedict et al.2003, 200662,82

GFT PerVap 1005 95 ◦C — Amberlyst XN-1010 1.2 71%d —

Budd, et al.,200487

zeolite/polyelectrolytemultilayer

100 ◦C 0.60b p-toluenesulfonic acid 2.0 90% —

0.19c

Ma et al., 200984 chitosan–TEOS 80 ◦C 0.27 Amberlyst 15 3.0 80%e —Pereira et al.,201088

Microporous silica 70 ◦C 2.55 Amberlyst 15 1 98% 96%

a for water/ethanol liquid mixture (ª10/90 wt%); b sheet membrane (70 ◦C); c tube membrane (70 ◦C); d after 8 h; e after 9 h.

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Fig. 9 The layout of a pervaporation membrane reactor: (i) batch reactor with an external pervaporation unit; (ii) membrane and reactor in thesame unit; (iii) continuous integrated membrane reactor.

5.1.2 Reactive distillation. The reactive distillation (RD)technology was successfully implemented by Asthana andcollaborators,89 where a lactic acid conversion of 95% and anethyl lactate purity of 95% were obtained, when a mixture ofethanol and 88 wt% lactic acid solution (ethanol/lactic acidmolar ratio of 3.6) was fed and using a bottom temperature of128 ◦C. A schematic representation of the RD unit is shown inFig. 10. This process was also studied by Gao and co-workers,90

where an ethyl lactate yield of just 53% is reported, using abottom temperature of about 115 ◦C and a feed ratio of ethanolto lactic acid of 4 : 1.

Fig. 10 A schematic representation of a reactive distillation unit forthe production of ethyl lactate.

5.1.3 Distillation and membrane based process. A novelsemicontinuous distillation (SD) process was proposed byAdams and Seider,91 where a distillation column interacting witha middle vessel, a continuous stirred tank reactor (CSTR) anda pervaporation unit are used. The reaction takes place in theCSTR until equilibrium is reached, then the reaction mixture ismoved to the middle vessel that is integrated with the distillationcolumn, and the CSTR is recharged. The middle vessel feeds areaction mixture to the distillation column, which recycles backa side stream. During the cycle, ethanol and water are collectedin the distillate and are separated in a pervaporation unit usinga hydrophilic membrane, which allows the breaking of theethanol/water azeotrope. The retentate stream rich in ethanol isrecycled back to the CSTR. Lactic acid is collected at the bottomof the distillation column and it is also recycled to the CSTRat decreasing flow rates, eventually approaching zero flow, sinceit is being consumed in the reaction. During this process water,ethanol, and lactic acid are removed from the middle vessel, and,therefore, at the end of the cycle, when the products are collected,the middle vessel contains highly concentrated ethyl lactate (seeFig. 11). It was shown that the SD process is more feasible,

Fig. 11 The semicontinuous process for the ethyl lactate production.91

from an economic point of view, than a traditional batch or acontinuous process, for an intermediate range of ethyl lactateproduction, however, for high production rates, the continuousprocess remains the most suitable.

5.1.4 Chromatographic reactors. The application of a sim-ulated moving bed reactor (SMBR) for ethyl lactate productionwas studied by Pereira et al.,92 where it is reported an ethyl lactateproductivity of 18.06 kgEL/(Lads.day)), a desorbent consumptionof 4.75 LEth/kgEL and an ethyl lactate purity of 95% (withoutethanol), when using this technology at 50 ◦C, ethanol asa desorbent and Amberlyst-15 wet resin as a catalyst andselective adsorbent for water (see Fig. 12). The maximum ethyllactate productivity attained was 31.7 kgEL/(Lads.day)), but it wasaccompanied by a high consumption of ethanol (7.6 LEth/kgEL).When using SMBR technology, two diluted streams are ob-tained: the extract and the raffinate (Fig. 12), requiring two

Fig. 12 A schematic diagram of the SMBR unit the with configuration3-3-4-2 applied to ethyl lactate synthesis. Each box represents a SMBRcolumn packed with the resin Amberlyst-15 wet.

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Table 5 Basic properties of lactic acid, ethanol, ethyl lactate and water95

Properties Lactic Acid Ethanol Ethyl Lactate Water

Molecular weight – M/g mol-1 90.079 46.069 118.133 18.015Density – r/g cm-3 1.209 0.789 1.031 1.027Melting temperature – T f (K) 289.95–291.15 159.15 248.25 273.15Normal boiling temperature – Tb (K) 395.15 351.45 426.15–427.15 373.15Critical temperature – T c (K) 616.00 516.25 588.00 647.13Critical pressure – Pc (bar) 59.65 63.84 38.60 221.20Critical volume – V c (cm3 mol-1) 216.9 166.9 354.0 57.1Acentric factor – w 1.035 0.637 0.793 0.344

additional separation steps. In order to achieve high ethyl lactateproductivity without the penalty on the consumption of ethanol,a new technology was recently developed: the simulated movingbed membrane reactor (PermSMBR), which comprises theintegration of hydrophilic membranes into the SMBR columns93

(Fig. 13). These membranes enhance the water removal, allowingthe elimination of the extract stream and leading to a betterperformance for ethyl lactate production, as shown in the workof Silva and collaborators.94 For ethyl lactate synthesis, the RDand SMBR processes require more than 152% and 165% ofthe ethanol consumption, respectively, of the new PermSMBRtechnology.

Fig. 13 A schematic diagram of a PermSMBR unit with 4 sections(configuration 3-3-4-2) for ethyl lactate synthesis. Each box representsa PermSMBR column that consists of a set of membranes packed withthe Amberlyst 15-wet resin.

6. Conclusions

A literature survey regarding ethyl lactate production processeswas presented in this review with special focus on reac-tive/separation processes, since they provide the most feasibleengineering solution to the sustainable synthesis of ethyl lactate,as at least one of the products is continuously removed from thereaction mixture, leading to depletion of the limiting reactant.

Membrane reactors, reactive distillation and chromatographicreactors, namely, SMBR and PermSMBR technologies, havebeen successfully implemented for ethyl lactate synthesis. Inall these processes, it was possible to attain nearly completelactic acid conversion and high ethyl lactate purity (≥95%),although under different operating conditions. The RD processis the one that requires more heat supply, since it operates athigh temperatures (128 ◦C bottom temperature); however, thecapital costs are lower and it is a more industrially consolidatedtechnology. The membrane reactor requires more moderatetemperatures (around 70 ◦C) and an extra separation step isnot necessary; however, ethyl lactate productivity is low, themembranes’ durability and/or stability under acidic conditionsmight be a problem and it has the additional associated vacuumcosts. Using the SMBR it is possible to attain high ethyl

lactate productivity, but with the penalty of the consumptionof desorbent, which should be further separated and recycledin the SMBR unit, two subsequent separation units beingnecessary. PermSMBR seems the most promising technologyfor ethyl lactate production, since it operates at moderatetemperatures (70 ◦C), and results in high ethyl lactate productionrates, low desorbent consumption and just one additionalseparation step to separate ethyl lactate from ethanol. However,it requires vacuum costs and the limitations associated with themembranes’ stability/durability are also an issue. Moreover, theproof-of-concept of this technology has not yet been performed.A fair comparison of technologies must be done in terms ofeconomic and environmental impact assessment, in order todetermine the most viable process for the efficient production ofethyl lactate.

Appendix: Physical and Thermodynamic Properties

In Table 5 some physical and thermodynamic properties of lacticacid, ethanol, ethyl lactate and water are presented.95

Density and molar volume

The modified form of the Rackett equation was selected forthe correlation of saturated liquid density as a function oftemperature,

rLc=

− −

ABT

Tn( )1 (A.1)

with rL(g cm-3) and T/K. The constants used are presented inTable 6.

The molar volumes for all components were estimated withthe Gunn–Yamada method:96

V Tf T

f TV( ) =

( )( )R

R(A.2)

where

f (T) = H1(1 - wH2) (A.3)

H1 = 0.33593 - 0.33953T r + 1.51941T 2r - 2.02512T 3

r +1.11422T 4

r

(A.4)

H2 = 0.29607 - 0.9045T r - 0.04842T 2r (A.5)

TT

Tor

T

Trc

R

c

= (A.6)

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Table 6 Constants used for density calculation95

Constants Lactic Acid Ethanol Water Ethyl Lactate

A 0.39816 0.26570 0.34710 0.33372B 0.26350 0.26395 0.27400 0.21190n 0.28570 0.23670 0.28571 0.45530Tmin (K) 291.15 159.05 273.16 247.15Tmax (K) T c T c T c T c

Table 7 Molar volumes at the reference temperature

VR (cm3 mol-1)

Reference TemperatureTR (K) Lactic Acid Ethanol Ethyl Lactate Water

293.15 74.640 58.174 113.986 18.082

V R is the molar volume at the reference temperature TR

(cm3 mol-1), w is the acentric factor and T c is the critical T/K.The Gunn–Yamada method could be used just in the case

when the liquid molar volume is known at some temperature(reference temperature). The reference molar volumes are pre-sented at Table 7.

Viscosity

The correlation for liquid viscosity as a function of temperatureis given by eqn (A.7) (see constants in Table 8).

log102hL = + + +A

B

TCT DT (A.7)

with hL(cP) and T/K.Ethyl lactate viscosity data are very scarce in the literature.

Some works just present the viscosity of this compound at asingle temperature.97 However, in the works of Aparicio andAlcalde the values of ethyl lactate viscosity for a wide rangeof pressures and temperatures are reported.98,99 In Table 9,the viscosities of ethyl lactate estimated by the correlationmentioned above (eqn (A.7)) and experimentally measured arepresented. As can be seen, the correlation estimates smallerviscosity values than the ones obtained experimentally. At278.15 K, the correlation leads to a value 4% smaller than thevalue reported in Aparicio and Alcalde.98,99

The lactic acid viscosity might be estimated by the followingexpression:100

hLA = − + − ×⎛⎝⎜⎜⎜

⎞⎠⎟⎟⎟⎟exp .

.. ln( )14 403

4097 90 4407

TT (A.8)

Table 8 Constants used for viscosity calculation95

Constants Ethanol Water Ethyl Lactate

A -6.4406 -10.2158 -20.0105B 1.1176 ¥ 103 1.7925 ¥ 103 3.2123 ¥ 103

C 1.3721 ¥ 10-2 1.7730 ¥ 10-2 4.1891 ¥ 10-2

D -1.5465 ¥ 10-5 -1.2631 ¥ 10-5 -3.2733 ¥ 10-5

Tmin (K) 240 273 247Tmax (K) T c 643 T c

Table 9 Ethyl lactate viscosity for different temperatures at atmo-spheric pressure

h (cP) 278.15 K 298.15 K 318.15 K

Estimated by Eq. A.7 (Ref. 95) 4.55 2.21 1.26Experimental (Ref. 97) — 2.40 —Experimental (Ref. 98,99) 4.75 2.53 1.6

Table 10 Viscosity of lactic acid

T/K 293.15 323.15h (cP) 53.67 13.99

Table 11 Constants used for vapour pressure calculation95

Constants Lactic Acid Ethanol Water Ethyl Lactate

A -27.0836 23.8442 29.8605 32.0863B -3.9661 ¥ 103 -2.8642 ¥ 103 -3.1522 ¥ 103 -2.9164 ¥ 103

C 2.0233 ¥ 101 -5.0474 -7.3037 -9.5666D -4.2176 ¥ 10-2 3.7448 ¥ 10-11 2.4247 ¥ 10-9 6.5114 ¥ 10-3

E 2.0310 ¥ 10-5 2.7361 ¥ 10-7 1.8090 ¥ 10-6 4.5645 ¥ 10-13

Tmin (K) 291.15 159.05 273.16 247.15Tmax (K) T c T c T c T c

Table 12 Constants used for heat capacity calculation95

Constants Ethanola Watera Ethyl Lactate Lactic acida

A 1.0264 ¥ 105 2.7637 ¥ 105 -46.239 6.1082 ¥ 104

B -1.3963 ¥ 102 -2.0901 ¥ 103 2.1823 5.0343 ¥ 102

C -3.0341 ¥ 10-2 8.1250 -5.9832 ¥ 10-3 —D 2.0386 ¥ 10-3 -1.4116 ¥ 10-2 6.8683 ¥ 10-6 —E — 9.3701 ¥ 10-6 — —Tmin (K) 159.05 273.16 248.00 289.90Tmax (K) 390.00 533.15 529.00 675.00Error <3% <1% — <10%

a parameters taken from ref. 100.

Table 13 Constants used for heat vaporization calculation100

Constants Ethanol Water Ethyl Lactate Lactic acid

A 5.5789 ¥ 107 5.2053 ¥ 107 8.0260E ¥ 107 1.0436 ¥ 108

B 3.1245 ¥ 10-1 3.1990 ¥ 10-1 4.0930 ¥ 10-1 3.8548 ¥ 10-1

C — -2.1200 ¥ 10-1 — —D — 2.5795 ¥ 10-1 — —Tmin (K) 159.05 273.16 247.15 289.90Tmax (K) 514.00 647.13 588.00 675.00Error <1% <1% <10% <25%

with hLA (Pa s)and T/K. The lactic acid viscosity at twotemperatures is presented in Table 10.

Vapour pressure

The Antoine-type equation with extended term was selected forcorrelation of vapour pressure as a function of temperature:

log log10 102P A

B

TC T DT ETvp = + + + + (A.9)

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Table 14 Constants used for thermal conductivity calculation100

Constants Ethanol Water Ethyl Lactate Lactic acid

A 2.4680 ¥ 10-1 -4.3200 ¥ 10-1 2.8358 ¥ 10-1 3.4850 ¥ 10-1

B -2.6400 ¥10-4

5.7255 ¥ 10-3 -3.5110 ¥ 10-4 -3.7085 ¥ 10-4

C — -8.0780 ¥ 10-6 — —D — 1.8610 ¥ 10-9 — —Tmin (K) 159.05 273.16 247.15 289.90Tmax (K) 353.15 633.15 427.65 490.00Error <5% <1% <25% <10%

with Pvp (mmHg) and T/K.95 The constants used in the vapourpressure calculations are presented in Table 11 and its graphicalrepresentation as a function of temperature is shown in Fig. 14.

Fig. 14 Variation of the vapour pressure of the compounds with thetemperature.

Liquid heat capacity

The correlation for the heat capacity of a liquid is a seriesexpansion in temperature, given by eqn (A.10).

Cp = A + BT + CT 2 + DT 3 + ET 4 (A.10)

with Cp (J kmol-1 K-1) and T/K, with exception for the ethyllactate species where Cp is given in J mol-1 K-1.

The liquid heat capacity as a function of temperature wasestimated by the above equation (Fig. 15), using the constantspresented in Table 12.

Heat of vaporization

The correlation selected for the calculation of the heat ofvaporization as a function of temperature is given by eqn(A.11).100

DH A TB CT DTV

rr r

2= −⎡⎣ ⎤⎦

+ +( )1 (A.11)

with T r = T/T c and DHV in J K-1mol-1.The heat of vaporization was estimated by the correlation

given by eqn (A.11) (see Fig. 16) using the constants presentedin Table 13.

Fig. 15 Variation of the liquid heat capacity of the different specieswith the temperature.

Fig. 16 Variation of the heat of vaporization of the different specieswith the temperature.

Thermal conductivity

The thermal conductivity was calculated by eqn (A.12) using theconstants presented in Table 14.100

l = A + BT + CT 2 + DT 3 + ET 4 (A.12)

with l (W m-1 K-1) and T/K.The coefficients for each chemical compound for all the cor-

relations presented in this appendix were based on appropriateexperimental data available in the literature, with the exceptionof the Gunn–Yamada method to estimate the molar volume,since this is based on the corresponding-states principle.

Acknowledgements

The authors acknowledge financial support provided byFundacao para a Ciencia e a Tecnologia (Portugal) throughthe project PPCDT/EQU/61580/2004 and post-doctoral grantSFRH/BPD/71358/2010 of C.S.M. Pereira.

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