Chemical Engineering Journal 195–196 (2012) 188–197

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Chemical Engineering Journal

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Membrane gas–solvent contactor trials of CO2 absorption from syngas

Colin A. Scholes, Michael Simioni, Abdul Qader, Geoff W. Stevens, Sandra E. Kentish ⇑Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC 3010, Australia

h i g h l i g h t s

" Membrane gas absorption tested with PP and PTFE membranes using carbonate and amine solvents." Solvent in shell side led to higher overall mass transfer coefficients than solvent in lumen." Pilot scale trials with syngas showed reduced performance due to membrane wetting.

a r t i c l e i n f o

Article history:Received 9 January 2012Received in revised form 10 April 2012Accepted 10 April 2012Available online 27 April 2012

Keywords:Membrane ContactorPolypropylenePotassium carbonateMonoethanolaminePolytetrafluoroethyleneSyngas

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.04.034

⇑ Corresponding author. Tel.: +61 3 8344 6682; faxE-mail address: sandraek@unimelb.edu.au (S.E. Ke

a b s t r a c t

Membrane gas–solvent contactors incorporate the advantages of both solvent absorption and membranegas separation technologies. Here, gas–solvent contactors are applied to the separation of carbon dioxidefrom syngas in a coal fired pilot plant. Two contactors, based on polypropylene (PP) and polytetrafluoro-ethylene (PTFE), are trialed with two solvents, 30 wt.% monoethanolamine (MEA) and 30 wt.% potassiumcarbonate (K2CO3) solutions. To validate performance, results were also obtained with a mixture of 10%CO2 in N2 in the laboratory. All contactor–solvent systems tested in the laboratory behaved in accordancewith membrane contactor models with only minor pore wetting observed. Mass transfer coefficientswere improved when solvent flowed on the shell side of the contactor due to increased turbulenceand reduced pore wetting relative to the lumen side. In contrast, for the pilot plant trials with syngas,only the PP–K2CO3 and PTFE–MEA systems provided mass transfer coefficients similar to those deter-mined in the laboratory. For the PTFE–K2CO3 system, additional pore wetting resulted in reduced overallmass transfer coefficients. The PTFE–MEA system retained the best overall mass transfer performance,due to reduced pore wetting and greater reaction enhancement.

� 2012 Elsevier B.V. All rights reserved.

2 3

1. Introduction

The demonstration of carbon capture technologies is becomingincreasingly important, as solutions to reduce anthropogenic car-bon emissions are sought. Two viable carbon capture technologiesare membrane gas separation and reversible solvent absorption[1], both of which are currently commercialized in natural gas pro-cessing. Hybrid membrane–solvent systems, known as membranegas absorption, seek to exploit the advantages of both membranegas separation and solvent absorption technologies [2]. The processinvolves the transfer of CO2 from the process gas through a non-selective porous hollow-fiber membrane where it is chemically ab-sorbed into a solvent. This takes advantage of the highly selectivenature of solvent technology, while incorporating the benefits ofmembrane technology in terms of reduced equipment size, themodular nature of the equipment, and flexibility in orientation[3]. A membrane contactor can achieve much greater mass transferarea per unit volume than conventional solvent absorption column

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: +61 3 8344 4153.ntish).

technology. Reed et al. [4] indicate that 500–600 m /m can beachieved in a membrane contactor compared to 100–250 m2/m3

in a traditional column. Similarly, Falk-Pedersen et al. [3] indicatethat the reduced specific area of a membrane contactor allows fora 65–75% reduction in weight and size compared to conventionaltowers. Further, the membrane acts to physically separate theliquid and gas flows, which eliminates foaming and reduces liquidchanneling, two major operating issues in solvent absorptioncolumns [2].

There are three main strategies for carbon capture from com-bustion processes, post-combustion capture, pre-combustion cap-ture and oxy-fired combustion [1]. In pre-combustion capture,fossil fuels are reformed into synthesis gas (syngas) comprisedmainly of hydrogen and carbon monoxide [5]. More hydrogen isproduced by further converting CO through the water gas-shiftreaction, resulting in high pressure CO2 and H2 [6]. Separation ofthese two components allows for the storage of CO2, while H2

can be used for a number of purposes, such as power generation[1]. Pre-combustion processes can be further classified into thosethat use oxygen-blown gasification and those that use an air-blown gasifier [7]. In the former case, the shifted syngas is a simple

Nomenclature

d membrane thickness (m)dnw membrane pore thickness that is not wetted (m)dw membrane pore thickness that is wetted (m)e membrane porositys tortuosityAi mass transfer area based on inner fiber diameter (m2)Ao mass transfer area based on outer fiber diameter (m2)C� equilibrium concentration of the bulk liquid (M)CLM log mean average of the inlet and outlet concentrations

in the bulk gas phase (M)CMEA concentration of the MEA solvent (M)din inner diameter of the tube (m)dh diameter of contactor shell side (m)DCO2 diffusion coefficient of CO2 in the liquid phase (m/s)DG diffusivity in the gas phase (m/s)Dl diffusivity of CO2 in the lumen side (m/s)Ds diffusivity of CO2 in the shell side (m/s)E enhancement factorG inert gas flowrate (mol/s)Gz Graetz dimensionless numberKe equilibrium constant for the reaction

kg gas phase mass transfer coefficient (m/s)kl liquid phase mass transfer coefficient (m/s)km membrane mass transfer coefficient (m/s)km,nw mass transfer coefficients within the non-wetted mem-

brane pore (m/s)km,w mass transfer coefficient with the wetted membrane

pore (m/s)kr;CO2 reaction rate coefficient between CO2 and the solvent

(L3/mol s)kt mass transfer coefficient for the tube (lumen) side (m/s)K overall mass transfer coefficient based on the internal

diameter (m/s)l length of contactor (m)m partition coefficientN molar flux (mol/m2 s)Re Reynolds dimensionless numberSc Schmidt dimensionless numberSh Sherwood dimensionless numberxo solvent loadingY mole ratio of CO2 in the gas phase

C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197 189

mixture of CO2 and H2. In air-blown gasification the syngas is di-luted by a significant stream of nitrogen, complicating the CO2 cap-ture operation. In particular there is need for separationtechnologies that will remove CO2 from the syngas stream whileleaving H2 and N2 in the process gas.

A range of porous contactors and solvents have been trialed forCO2 removal in such applications on the laboratory scale. The mostcommon membrane materials are polypropylene (PP), polyethyl-ene (PE) and polytetrafluoroethylene (PTFE) while water, aminessuch as monoethanolamine, amino acid salts and NaOH have allbeen tested as solvents [8–19]. Many of these studies have shownthat the potential for membrane gas contactors is limited by thewetting of the membrane pores, which reduces the overall masstransfer coefficient [8,20,21].

To date, the only significant pilot plant trials of porous mem-brane contactors were those undertaken by Kvaerner for naturalgas sweetening in 1998–1999 [3,22]. Initial pilot plant trials useda PTFE contactor with activated MDEA, while later trials employeda physical solvent (Morphysorb�). These pilot scale trials also iden-tified membrane pore wetting as a major issue. Accurate pressureregulation across the contactor was also essential to protect themembrane hollow fibers from rupture and collapse.

In this work, we report the performance of two porous hollowfiber membrane contactors for the separation of CO2 from air-blown syngas in a pilot capture plant. These trials were conductedas part of the CO2CRC Mulgrave capture project [23,24]. In supportof the pilot capture plant findings, laboratory measurements of CO2

separation with similar contactors from a N2–CO2 gas mixture arealso reported. The two contactors are made from PP and PTFE; bothof which have been reported in the literature as contactors for CO2

separation. Two solvent systems are studied; 30 wt.% monoetha-nolamine (MEA) and 30 wt.% potassium carbonate (K2CO3) solu-tion. The first solvent is considered a standard amine approachfor CO2 separation, while the second solvent has been widely ap-plied in the Benfield process for CO2 separation [25].

2. Theory

The CO2 molar flux (N) through a membrane contactor into thesolvent is given by:

N ¼ Yin � YoutGAi

¼ KiCLM ð1Þ

where G is the inert gas flowrate, Ai is the mass transfer area basedon the internal fiber diameter, K the overall mass transfer coeffi-cient based on the internal diameter and Y is the mole ratio ofCO2 in the gas phase:

Y ¼ PCO2

P � PCO2

ð2Þ

CLM represents the log mean average of the concentration driv-ing force between the bulk gas phase and the liquid. As the reactionof CO2 with the solvent is rapid, the equilibrium concentration ofCO2 that would be in equilibrium with the bulk liquid (C⁄) can beassumed to be zero [2]. This means that the log mean driving forcecan be approximated simply by the log mean average of the inletand outlet concentrations of CO2 in the bulk gas phase:

CLM ¼Cin � Cout

ln CinCout

ð3Þ

The overall mass transfer across the membrane consists of threemass transfer stages, the transfer of CO2 across the gas boundarylayer, the transfer of CO2 through the membrane pore and finallythe transfer of CO2 across the solvent boundary layer. Each of thesestages acts as a resistance to mass transfer, and is usually ex-pressed as a series [25]. For solvent flow through the lumen side:

1ki¼ Ai

Aokgþ Ai

ALMkmþ 1

mEklð4Þ

where kg, km and kl are the gas, membrane and liquid side physicalmass transfer coefficients respectively, Ai is the inner diameter area,ALM the log mean area, m is the partition coefficient (defined as theratio of liquid to gas concentrations at equilibrium) and E is theenhancement factor due to the chemical reaction in the solvent.The partition coefficient for MEA is 0.76 at 25 �C and K2CO3 at35 �C is 1.75 [26]. For non-wetted pores, kg and km � kl, and theoverall mass transfer coefficient can be approximated by:

Ki ¼ mEkl ð5Þ

For the MEA system, the Enhancement Factor can be modeledby a pseudo first order system [27]:

190 C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197

ECO2 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffikr;CO2 � CMEA � DCO2

pkl

ð6Þ

where kr;CO2 is the reaction rate coefficient of the reaction betweenCO2 and the solvent, CMEA is the concentration of the MEA solvent,and DCO2 is the diffusion coefficient of CO2 in the liquid phase. Bothkr;CO2 and DCO2 are functions of temperature, pressure and concen-tration. For K2CO3, the enhancement factor is given by [28]:

E ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDCO2 kr;CO2 Keð1� xoÞ=2xo

pkl

ð7Þ

where Ke is the equilibrium constant for the reaction and xo is theloading defined by:

X0 ¼molCO2

molsolvent¼ HCO�3

Kþð8Þ

and a function of CO2 partial pressure and temperature.The mass transfer across the lumen side boundary layer is well-

known to be modeled by the Graetz–Leveque correlation [29]:

Sht ¼ktdin

Dl¼ 1:62 �ScRe

din

l

� �� �1=3

Gz > 6 ð9Þ

Or alternatively,

Sht ¼ktdin

Dl¼ 0:5 � ScRe

din

l

� �Gz < 6 ð10Þ

where Sh, Re, Sc and Gz are the dimensionless Sherwood, Reynolds,Schmidt and Graetz numbers respectively [30]. The Graetz numberis:

Gz ¼ Sc � Re � din

1

� �ð11Þ

kt is the mass transfer coefficient for the tube (lumen) side, din theinner diameter of the tube, Dl the diffusivity of CO2 in the lumenphase, and l the length of the contactor.

A number of empirical correlations have been proposed formodeling mass transfer through the shell side of membrane cont-actors [31]. The correlations that are most appropriate for theexperimental conditions experienced here is that developed byYang and Cussler [29,31] for randomly packed modules:

ShS ¼ksdh

Ds¼ 1:25

Re � dh

l

� �0:93

Sc0:33 ð12Þ

where ks is the shell side mass transfer coefficient and dh is the shellside diameter. The Sherwood number is an effective ratio of theconvective to diffusive mass transport processes. The Reynoldsnumber is based on the randomly packed definition [32,33].

For gas-filled pores, the membrane pore resistance is the in-verse of the mass transfer coefficient, given by [20]:

1km;nw

¼ dsDGe

ð13Þ

Where d is the thickness, s is the tortuosity, e is the porosity ofthe membrane and DG is the diffusivity in the gas phase. When thepores become wetted, a more complex expression is required:

1km¼ 1

km;nwþ 1

mEkm;wð14Þ

where km,nw and km,w represent the mass-transfer coefficients withinthe non-wetted and wetted lengths of the pores (dnw + dw = d) [34]:

1km;w

¼ dwsDLe

ð15Þ

1km;nw

¼ dnwsDGe

ð16Þ

Given all other parameters are known, the wetted length (dw)can be evaluated from the experimentally determined overall masstransfer coefficient. The pore wetting fraction is then give by:

Pore wetting ¼ dw

dð17Þ

The overall accuracy of this approach is ±20% [31].

3. Experimental

The PP contactor used in all syngas capture pilot plant studieswas a LiquiCel MiniModule with 7400 fibers (Membrana). Labora-tory studies with K2CO3 used a similar LiquiCel MiniModule with2300 fibers. It was not possible to complete laboratory studies withMEA, as the seals and plastic components of the smaller LiquiCelmodule failed upon extended exposure to this solvent. Similarly,the larger PP contactor could not be operated at 65 �C in the pilotplant trials because of a failure of the contactor seals at this tem-perature under the syngas conditions. The PTFE contactor for bothlaboratory and syngas pilot plant studies was in-house built usingPTFE fibers obtained from Markel Corporation. Details of the cont-actor dimensions and fiber details are provided in Table 1 and aschematic showing the arrangement in these contactors for a sol-vent-in-shell flow arrangement is provided in Fig. 2. MEA was sup-plied by Orica Chemicals Australia and made up to a 30 wt.%solution, while potassium carbonate was supplied by Sigma–Al-drich and made up to a 30 wt.% solution. Details of these solventsystems are provided in Table 2.

For the laboratory studies, a 90% N2–10% CO2 gas mixture wassupplied by BOC Gas Ltd. For the pilot capture plant studies, syngaswas supplied from an air-blown research gasifier as part of theCO2CRC Mulgrave Capture project [23,24]. The feed gas composi-tion was provided by the gasification operator, with the averagecomposition across the full operational time in Table 3. However,this composition varied considerably during and between days.The feed syngas was cooled to ambient temperature through unin-sulated pipework, before a separator vessel was used to removecondensed water and heavy hydrocarbons. A filter was also presentto prevent any dust entering the process lines to the membranemodules. The syngas was then reheated to either 35 or 65 �C as re-quired before entering the contactor module.

Both syngas capture pilot plant trials and laboratory measure-ments used the same membrane pilot plant (Fig. 1). This pilot cap-ture plant was housed within a custom built cabinet. All valves andfittings were made from stainless steel (Swagelok). Pressuregauges (Swagelok) ranged between 0 at 12 bar and pressure trans-mitters (GEMS sensors and control – Basingstoke England) withdisplays (PR electronics) ranged between 0 and 12 bar for all lines.A back-pressure regulator (Porter 08011) controlled the pressurewithin the modules, with feed gas flowrate measured by gasrotameters (0–10 L/min Kytala (Muurame Finland)) and the reten-tate flowrate measured by a gas meter (Ampy Email meteringModel 750) or universal flowmeter (Agilent TechnologiesADM3000). All temperature measurements were by K-type ther-mocouples (ECE Fasil), with electronic displays (PR electronics).

The solvent (30 wt.% K2CO3 or 30 wt.% MEA), was stored in a20 L tank and pumped (Micropump with Ismatec controller)through the solvent circuit. Flowrate was measured on a rotameter(Kyala) and controlled by the pump speed, with the differentialpressure across the module controlled manually on the solventside by a needle valve. To trap any solvent breakthrough on thecontactor and prevent damage to downstream instrumentation, aseparator was positioned on the retentate output of the module.The exit gas from all lines was recombined before exhaust, withnon-return valves to prevent back flow. During syngas capture pi-lot plant studies, to prevent hazardous conditions downstream,

Table 1Specifications of the four porous hollow-fiber contactors.

PP contactorpilot plant

PP contactor laboratoryK2CO3 measurements

PTFEcontactor

Supplier Membrana Membrana MarkelShell diameter (m) 0.0425 0.018 0.045Length (m) 0.14 0.10 0.147Outer fiber diameter (m) 0.0003 0.0003 0.002Inner fiber diameter (m) 0.00022 0.00022 0.0016No. of fibers 7400 2300 19Average pore size (lm) 0.1 0.1 0.16Porosity (%) 40 40 22.5Mass transfer area (m2) 0.716 0.159 0.014

Table 2Characteristics of K2CO3 and MEA.

K2CO3 (30 wt.%) MEA (30 wt.% at 35 �C)

35 �C 65 �C

Density (g/L) 1286 1269 [43] 999 [44]Viscosity (cP) 3.43 1.80 [45] 1.90 [44]Diffusivity (m/s) 2.62 � 10�9 4.47 � 10�9 [46] 2.44 � 10�9 [26]Interfacial tension 81 mN/m [47,32] 56 mN/m [48]

63 mN/m [25]Enhancement factor at 10% loading 1.2 1.6 82Sc No. 1017 318 9.6

Table 3Average unshifted syngas feed composition (mol %) to the membrane pilot captureplant.

Mol% Error

CO2 14.0 ±1.0H2 11.7 ±1.8CO 11.5 ±3.3N2 60.0 ±4.3CH4 2.5 ±0.2Heavy hydrocarbons 0.21 ±0.04H2O 0.11 ±0.04

C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197 191

this exit gas was diluted with N2, and sent to the onsite air extrac-tion system.

For syngas capture pilot plant studies, gas sampling wasachieved by connection of 10 mL sampling bombs at the analysispoints. The sample bomb was flushed three times with the processgas before the sample was taken. All gas compositions were mea-sured by gas chromatography (Hewett Packard, with a MolecularSieve and HP-PLOT Q column in series and carrier gas Helium).For laboratory measurements, a dedicated CO2 analyzer (HoribaVA3000) was used. Solvent sampling was achieved by collectingthe loaded solvent (�10 mL) in sampling vials from the solventsample analysis point with analysis through standard methodolo-gies [35].

The syngas feed to the module was at �1.3 bar(a) while the N2–CO2 gas mixture in the laboratory was at �1.05 bar(a). The pres-sure of the syngas feed was dictated by the gasifier operator andis considerably lower than the standard gasification pressure of�30 bar. Operating the membrane contactor at this lower pressureis expected to have little influence on the mass transfer throughthe solvent and gas phases, but could limit the amount of pore wet-ting observed. The solvent pressure was held greater than the gaspressure to prevent bubble formation in the solvent line. For eachflowrate and temperature condition, the contactor was operatedfor 60–90 min before sampling was undertaken, to ensure stea-dy-state conditions had been achieved. For the laboratory mea-surements, consecutive experiments were undertaken with thesolvent on both the lumen or shell side of the contactors. For the

capture pilot plant measurements under syngas, the solvent waspassed only on the lumen side of both contactors due to opera-tional constraints. The average differential pressure across thecontactor through the pilot plant measurements was kept between0.04 and 0.12 bar, though upon start-up a differential pressurespike of 0.5 bar was often observed.

4. Results and discussion

The laboratory measurements under N2–CO2 mixed gas condi-tions (post-combustion) are presented first for both contactorsand solvents, followed by the performance of both contactorsand solvents in the syngas (pre-combustion) capture pilot planttrials.

4.1. Laboratory results – polypropylene contactor

The overall mass transfer coefficient for CO2 sorption throughthe polypropylene contactor into K2CO3 is provided in Fig. 3 for sol-vent in the lumen side and Fig. 4 for solvent in the shell side. In allcases there is a trend of increasing mass transfer coefficient withincreasing solvent flowrate. This is associated with the higher sol-vent flowrate increasing turbulence in the solvent side boundarylayer, reducing the resistance to CO2 transfer [2]. Operating at ahigher temperature is expected to lead to an increase in the masstransfer coefficient because of an increase in the rate of reaction be-tween carbonate and CO2 in the solvent boundary layer and areduction in solution viscosity, as well as an increase in diffusioncoefficient. However, for the solvent-in-lumen measurement nodifference in the mass transfer coefficient is observed withtemperature, while for the solvent-in-shell measurement, only aslight increase is observed as temperature increases. This behaviormay reflect the minimal change in the Enhancement factor betweenthese two temperatures [28] due to slow reaction kinetics (seeTable 2). The other reason for this behavior may relate to the reduc-tion in interfacial tension that occurs as temperature increases. Thisreduction causes a coincident reduction in the pore breakthroughpressure and hence greater pore wetting [25]. The pore wettingfraction can indeed be calculated from the overall mass transfer

Fig. 1. Schematic of shell side solvent flow and lumen side gas flow.

Fig. 2. Schematic and photo of membrane gas–solvent contactor pilot captureplant. FIC = flow indicator and control, FI = flow indicator, TI = temperature indica-tor, PI = pressure indicator and PIC = pressure indicator and control.

Fig. 3. The CO2 overall mass transfer coefficient for a polypropylene contactor with30 wt.% K2CO3 (at 35 and 65 �C) on the lumen side upon exposure to 10% CO2 in N2

in the laboratory.

192 C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197

coefficient using Eqs. (1)–(17). The results reflect this phenomenon,with pore wetting greater at higher temperatures (Table 4).

Greater pore wetting is also observed for solvent-in-lumen flowcompared to solvent-in-shell flow (Table 4). This is due to a greatersolvent pressure drop along the length of the contactor. Solvent

flow through the lumen side generates a pressure drop of around0.3 kPa at 35 �C and 0.15 kPa at 65 �C in this commercial module.The pressure drop across the shell side for the solvent was�0.02 kPa irrespective of the solvent or the contactor. Hence, forsolvent-in-lumen flow, the higher feed pressure at the entry intothe lumen is more likely to force solvent into the pores, than whensolvent flow is on the shell side. For this lumen solvent flow, thepore wetting increases as solvent flowrate increases (Table 4),reflecting the increasing pressure drop and hence increasing feedpressure as this flowrate changes.

For K2CO3, the solvent-in-shell overall mass transfer coefficient(Fig. 4) is an order of magnitude greater than that observed for thesolvent-in-lumen side (Fig. 3). This behavior reflects both thereduced pore wetting discussed above and greater fluid mixing[36]. Chun and Lee [11] report very similar mass transfercoefficients (2 � 10�4 to 1 � 10�3 cm/s) at comparable solvent

Fig. 4. The CO2 overall mass transfer coefficient for a polypropylene contactor with30 wt.% K2CO3 (at 35 and 65 �C) on the shell side upon exposure to 10% CO2 in N2 inthe laboratory.

Table 4Pore wetting percentage (%) of PP–K2CO3 determined from laboratory results.

Average flowrate (L/min) Lumen Shell

35 �C 65 �C 35 �C 65 �C

Polypropylene and K2CO3 – pore wetting (%)0.005 3 5 0.6 0.60.008 4 6 0.3 0.80.012 5 7 0.1 0.80.017 5 9 0 1.50.018 6 11 0 0.80.022 9 14 0 1.60.024 13 12 0 1.50.028 19 18 0 1.0

Fig. 5. The CO2 loading in 30 wt.% K2CO3 as a function of the ratio of solvent to gasflowrate for PP contactor, solvent on the lumen side at 35 (d) and 65 �C (j), and onthe shell side at 35 (d) upon exposure to 10% CO2 in N2 in the laboratory.

Fig. 6. The CO2 overall mass transfer coefficient for a PTFE contactor with 30 wt.%MEA, on both lumen and shell side at 35 �C upon exposure to 10% CO2 in N2 in thelaboratory.

C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197 193

flowrates for 15% K2CO3 and solvent in the shell. In contrast, bothDindore et al. [37] as well as Karoor and Sirkar [14] report highermass transfer coefficients for PP–H2O systems of 2.3 � 10�3 and

2.6 � 10�3 cm/s respectively with solvent on the lumen side. Thesehigher coefficients reflect considerably higher solvent flowrates, 18and 6 cm/s respectively, compared to the maximum solvent flow-rate of 0.44 cm/s reported here. Operating in more turbulent flowconditions produces greater mass transfer. However, the shorterresidence time that results from a greater solvent flow results inlower solvent loadings (Fig. 5). At an industrial scale this must becompensated for by multiple passes through membrane modulesin series, to ensure that a full solvent loading is achieved. Differen-tial pressures would also need to be carefully managed to avoid sol-vent breakthrough.

4.2. Laboratory results – PTFE contactor

The overall mass transfer coefficients for the PTFE contactorwith MEA in both the lumen and shell sides are provided inFig. 6. Again, the overall mass transfer coefficient increases withsolvent flowrate because of increased turbulence in the solventboundary layer. There is also again clear evidence of increasedmass transfer when the solvent is on the shell side. The magnitudeof the mass transfer coefficient for the PTFE–MEA system is largerthan that reported in the literature; Yeon et al. [38] reports a valueof 1.34 � 10�3 cm/s with 5 wt.% MEA and Kim and Yang [39] have amaximum transfer coefficient of 2.5 � 10�3 cm/s for 4 wt.% MEA, ata comparable solvent flowrate. The differences probably reflect themore concentrated MEA used in the current work (30 wt.%) whichwill increase reaction enhancement.

For both the PTFE and PP contactor, the pore wetting fractions(Tables 4 and 5) are generally lower than those observed in ourprior work, which used flat sheet membranes with MEA solvent.In this case, pore wetting fractions from 24% to over 100% were ob-served [34]. The higher values in this case may reflect the flat sheetformat, which used overhead stirring, rather than a crossflowarrangement.

The overall mass transfer coefficients for PTFE with K2CO3 at 35and 65 �C are provided in Fig. 7 for solvent on the lumen side andFig. 8 for solvent on the shell side. The mass transfer coefficientsobserved are an order of magnitude lower than for the MEA sys-tem, reflecting the low rate of reaction for the carbonate–CO2 reac-tion at these temperatures and hence the lower reactionenhancement (see Table 2). In these systems there is a clear dis-tinction that the higher temperature does enhance the mass trans-

Fig. 7. The CO2 overall mass transfer coefficient for a PTFE contactor with 30 wt.%K2CO3 (at 35 and 65 �C) on lumen side upon exposure to 10% CO2 in N2 in thelaboratory.

Fig. 8. The CO2 overall mass transfer coefficient for a PTFE contactor with 30 wt.%K2CO3 (at 35 and 65 �C) on the shell side upon exposure to 10% CO2 in N2 in thelaboratory.

Table 5Pore wetting percentage (%) of PTFE–MEA and K2CO3 determined from laboratoryresults.

Average flowrate (L/min) Lumen Shell

MEA K2CO3 K2CO3 MEA K2CO3 K2CO3

35 �C 65 �C 35 �C 65 �C

PTFE with MEA and K2CO3 – pore wetting (%)0.005 0 13 16 2 20.007 25 17 2 30.011 1 21 18 2 2 20.017 1 26 19 3 2 20.020 3 21 20.025 22 21 3 2 20.026 3 250.031 3 21 60.034 60.046 3 6

Fig. 9. The CO2 overall mass transfer coefficient for a polypropylene contactor with30 wt.% K2CO3, at 35 �C on lumen side, for laboratory (d) and pilot plantmeasurements (j).

194 C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197

fer rate, for both lumen and shell solvent flow. Indeed for the shellside, there is an enhancement of �1.7 times for the 65 �C systemcompared to the 35 �C. The mass transfer coefficients are in goodagreement with the literature, Nii and Takeuchi [40] report forPTFE with 2 M K2CO3 solution a maximum mass transfer coeffi-cient of 5.6 � 10�4 cm/s at ambient temperature and with solventon the lumen side, which is of similar magnitude to that measuredin Fig. 7.

The solvent-in-lumen result for the mass transfer coefficient forK2CO3 in the PTFE contactor is an order of magnitude greater thanthat in the PP contactor. This is believed to be associated with thesignificantly smaller number of fibers in the PTFE contactor. For acomparable solvent flowrate, much higher solvent velocities areobtained in the PTFE contactor leading to higher Reynolds num-bers. This contactor has a maximum Reynolds number of 9 onthe lumen side, while for PP this value is 0.14. Again, greater porewetting is observed on the lumen side than on the shell side(Table 5) and this is attributed to the greater pressure required

for solvent flow through the fibers, on average 0.47 kPa comparedto �0.02 kPa for the shell, For the solvent-in shell arrangement, theoverall mass transfer coefficient for PTFE–K2CO3 is of similar mag-nitude to the PP–K2CO3 system at 35 �C.

In this laboratory work, pore wetting in the PTFE contactor issimilar to that in the PP contactor when the solvent is on the lumenside. While PTFE is more hydrophobic [41], the pore diameter inthese membrane fibers is larger (0.16 lm) than in the PP contactor(0.1 lm) and this larger pore size offsets the changes in hydropho-bicity. There is a clear difference in the overall mass transfer coef-ficient, with MEA providing an order of magnitude faster masstransfer than K2CO3. This relates to the much faster reaction rateof the MEA solvent with CO2. The criteria for solvent selection mustinclude consideration of this reaction rate, as it strongly dictatesthe observed overall mass transfer coefficient. However, solventresistance is also important, especially for corrosive amine solventssuch as MEA.

4.3. Pilot capture plant with syngas

For the PP–K2CO3 system there is good agreement between thepilot capture plant results and those determined under laboratoryconditions (Fig. 9), with a consistent trend as the Reynolds numberincreases.

Fig. 10. Overall CO2 mass transfer coefficient for the polypropylene contactor with30 wt.% MEA, at 35 �C on the lumen side for pilot plant measurements (j), as afunction of the solvent Reynolds Number.

Fig. 11. Overall CO2 mass transfer coefficient for the PTFE contactor with 30 wt.%K2CO3, at 35 �C on the lumen side, for laboratory (d) and pilot plant measurements(j) as a function of the solvent Reynolds number.

Fig. 12. Overall CO2 mass transfer coefficient for the PTFE contactor with 30 wt.%K2CO3, at 65 �C on the lumen side, for laboratory (d) and pilot plant measurements(j) as a function of the solvent Reynolds number.

C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197 195

For the PP–MEA system (Fig. 10), the pilot capture plant resultsindicate reduced performance under syngas conditions, with theoverall mass transfer coefficient falling by around an order of mag-nitude compared to that expected from the literature. Yeon et al.[38] report a value for a polyvinylidenefluoride contactor with 5%MEA of 2.1 � 10�3 cm/s, while Kosaraju et al. [42] for a poly(4-methyl-1-pentene) contactor with 11.6% MEA reports a slightlylower mass transfer coefficient of 0.96 � 10�3 cm/s. In our ownpublished work with PP and 30 wt.% MEA, a value of 0.05 m/s is ob-tained at slightly higher Reynolds numbers [25]. Indeed, the valuesfor this system under pilot capture plant conditions are compara-ble with K2CO3 even though this solvent has a much smaller reac-tion enhancement (Table 2).

This reduction in mass transfer coefficient is associated withgreater pore wetting by the MEA solvent (Table 6). The cause ofthis wetting may be associated with operational issues on start-up, where it was difficult to control the differential pressure acrossthe membrane as initial syngas entered the system. Furthermore,the syngas supply pressure varied considerably during operation(±0.2 bar). The response time lag in changing solvent pressurewas slow compared to these gas pressure fluctuations making dif-ferential pressure control extremely difficult. This will be a criticalissue in operating membrane gas–solvent contactors on a largescale.

The CO2 loading over the Reynolds number range was a rela-tively constant low value of 0.07–0.10 mol CO2 per mol of MEA.Again, much longer flow paths will be required in a full scale oper-ation to ensure greater solvent loadings.

Figs. 11 and 12 compare pilot capture plant and laboratory re-sults for the PTFE–K2CO3 system at 35 and 65 �C respectively. Itis clear that the pilot capture plant performance of the PTFE cont-actor is again reduced compared to the laboratory case. This can

Table 6Pore wetting percentage (%) for PP and PTFE pilot plant trials.

Polypropylene Poly tetrafluoroethylenePilot plant Pilot plant

Solvent in Lumen Lumen

K2CO3, 35 �C 33 12K2CO3, 65 �C – 9MEA, 35 �C 41 32 Fig. 13. Solvent breakthrough on the PTFE contactor under syngas conditions, as

evidenced by the beads of solvent appearing at the left (feed entry) end.

Fig. 14. Overall CO2 mass transfer coefficient for the PTFE contactor with 30 wt.%MEA, at 35 �C on the lumen side, for laboratory (d) and pilot plant measurements(j) as a function of the solvent Reynolds number.

196 C.A. Scholes et al. / Chemical Engineering Journal 195–196 (2012) 188–197

again be attributed to additional pore wetting of the membrane(Table 6). Indeed, solvent breakthrough was visually observed atthe feed end of this contactor (Fig. 13) where the solvent pressurewas highest. The corresponding solvent loading at both tempera-tures was also low because of the lower packing density and smallcontactor area, with values between 0.01 and 0.05 mol CO2/molK2CO3 observed.

For the PTFE–MEA system, the overall mass transfer coefficientobserved under syngas conditions is equivalent to that measuredin the laboratory (Fig. 14). This implies that there is no additionalpore wetting in this system and optimal mass transfer is occurring.This result is somewhat surprising as MEA would be expected towet the PTFE pores more readily than K2CO3 due to its lower sur-face tension (Table 2). The results in this case may represent a bet-ter control of transmembrane pressure drop during startup. TheCO2 loading in MEA for the syngas experiments was very low at0.02–0.05 over the Reynolds number range, mostly due to thelow mass transfer area for this contactor.

5. Conclusion

CO2 separation from syngas through membrane gas–solventcontactors has been demonstrated. Pilot capture plant results showthat a PTFE contactor with 30 wt.% MEA solution provided the bestoverall mass transfer coefficient, consistent with literature expec-tations. The presence of pore wetting in the PTFE contactor withK2CO3 solvent and the PP contactor with MEA solvent decreasedthe performance of these contactor–solvent systems. In addition,the low rate of reaction for the K2CO3 solvent at the temperaturesused meant that lower overall mass transfer coefficients were ob-served, even when pore wetting was minor.

The application of membrane gas–solvent contactors to carboncapture from syngas will rely on the development of membranematerials and solvent systems that resist pore wetting. However,this work has also shown the importance of developing processcontrol systems that can minimize pressure differential spikes be-tween the solvent and gas sides of the contactor, which force sol-vent into the pores. This is consistent with the findings from theoriginal Kvaerner work [3,22]. The work has also shown the impor-tance of careful materials selection in module design, with degra-dation to the module housing and seals by the MEA solvent

limiting the experiments that could be conducted with thissolvent.

Acknowledgements

The authors would like to thank HRL for access to equipment, aswell as the contributions of Joannelle Bacus, Wen Tao, George Chenand Gang Li. Funding for this project is provided by the CRC forGreenhouse Gas Technologies (CO2CRC) through the AustralianGovernment Cooperative Research Centre program and facilitiesfrom the Particulate Fluids Processing Centre of the University ofMelbourne.

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