A COMPARISION OF POROUS AND NON-POROUS GAS- LIQUID MEMBRANE CONTACTORS FOR GAS SEPARATION

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1 A COMPARISION OF POROUS AND NON-POROUS GAS- LIQUID MEMBRANE CONTACTORS FOR GAS SEPARATION Abdullah A. Elamar 1 Al H. Elhudhry 2 1 Head, Department of Chemcal Engneerng, Faculty of Engneerng, Al-Tahad Unversty, P.O. Box 633, Srte, Lbya (correspondng author). E-mal: abdullahal2007@yahoo.com 2 Professor, Department of Chemcal Engneerng, Faculty of Engneerng, Al-Tahad Unversty, P.O. Box 633, Srte, Lbya. E-mal: elhudayr@yahoo.com Abstract A slcone rubber and porous polyvnyldene fluorde (PVDF) hollow fbre membranes wth hgh porosty coupled wth lqud absorbent were assessed for removal of CO 2 from a gas stream. The effects of dfferent operatng condtons on the permeaton process have been nvestgated.e. co-current and counter-current flow, lqud flow rate, gas flow rate, and dfferent pressure ratos. In addton, to mprove slcone rubber hollow fbre membrane performance, baffles were nstalled wthn the shell of the permeator to ncrease lqud fbre contact. Results obtaned showed that the use of an absorbent lqud n the permeate sde of the permeator enhances CO 2 separaton and reduces the requrements for a hgh pressure rato across the membrane. A mathematcal model based on the effectve permeablty of the gaseous mxtures has been used to assess the performance of both hollow fbre membranes, and a correlaton for the shell sde mass transfer has been developed. Numercal smulatons agreed well wth the experment. Keywords: carbon doxde absorpton; non-porous slcone rubber and porous polyvnyldene fluorde (PVDF) hollow fbre membranes. 1

2 1. Introducton Gas absorpton processes are of major ndustral mportance. Wth ncreasng concern about gaseous emssons nto atmosphere, the demand for mproved and economcal gas purfcaton devces s expected to grow n the near future. The stack gas emsson known to cause global warmng, consstng mostly of CO2 s comng under ncreased regulaton. The problem of removng one or more of such gaseous components from a gas stream has gven rse, over the years, to a number of methods of achevng ths. Selectve lqud absorpton has proved to be a versatle method and s the one most commonly employed. However, such mass transfer systems do have a number of dsadvantages, ncludng the requrement for perodc regeneraton of the absorbng lqud, and n backed bed absorbers the dsadvantages of floodng and channellng under certan operatng condtons. In recent years, alternatves to the conventonal modes of gas contactng are beng developed; one such devce, whch shows a lot of promse, s the membrane-based contactng devce. Hstorcally, polymerc membranes, whch are usually nonporous, have been used for separaton of gases and lquds. Commercal utlzaton of separaton of gases through sold membranes s stll somewhat lmted due to two reasons: low permeablty and low separaton factors. To overcome ths problem membranes are now beng developed whch are thnner, more selectve and capable of wthstandng hgher temperature. Durng the same perod several alternatve methods to mprove the permeablty of the membranes were explored. Snce gas dffusvty through lquds s several orders of magntude hgher than those through sold materals, one of the frst alternatves explored was the use of lqud membranes. The mmoblzed lqud membrane technque, frst developed by Ward and Robb (1967), was used successfully n the separaton of CO2 and N2 mxtures. In ths method, a lqud s mpregnated n the pores of a mcroporous flm or n the pores of a hollow fbre (Bhave and Srkar (1967)). By ths method the permeablty and selectvty can be drastcally ncreased. However, ths method has several drawbacks, the most mportant of whch s the evaporaton of lqud durng extended perods of operaton. Thus, unless proper humdfcaton was adopted, the membranes were nherently unstable and were not used for commercal applcaton. But the dea of usng a lqud porous membrane was hghly attractve and led to several other lqud-based membrane separaton technques. One such technque s hollow fbre contaned lqud membrane, whch was successfully used for the separaton of CO2/N2 (Majumdar et al. (1988)) and CO2/CH4 (Guha et al. (1992)). An alternatve method of separatng of a gas mxture wth the lqud mcroporous hydrophobc hollow fbre membranes. Here the mcroporous membrane-based devce acts as a gas absorber, a gas-lqud contactor wth the gas flowng on one sde and absorbent lqud flowng on the other sde of membrane. Hollow fbre membrane contactors are a promsng way to accomplsh separaton processes such as gas absorpton and lqud-lqud extracton. Membrane-based gas separaton devces offer several advantages over conventonal contactng devces. 2

3 These contactors can supply twenty to one hundred tmes more surface area per unt volume than conventonal equpment, and furthermore ths area can be mantaned even at small flow rates where conventonal packed towers are napproprate. Moreover, membrane contactors can reduce or avod gas and lqud entranment, whch can lmt both packed tower and mxer-settler performance (Yang and Cussler 1986, Majumdar, Guha and Srkar 1988, Cooney and Jackson 1989). The membrane performance can be mproved by ncreasng the lqud flow rate. Mass transfer resstances are smaller n the turbulent than n the lamnar flow regme. Ths turbulence can be created by nstallng baffles wthn the shell. Therefore, the membrane performance can be mproved wthout the need of ncreasng the lqud flow rate, whch leads to an ncrease n lqud consumpton and energy used n pumpng. Wang and Cussler (1993) examned the effect of the number of baffles on mass transfer performance and they found baffles ncrease the velocty of the shell flud and lead to mprove mass transfer and therefore the effcency performance of the membrane ncreases. 2. Mathematcal Model The model conssts of a set of dfferental equatons to analyse gas permeaton and absorpton n a bnary and multcomponent gas mxture n co-current and counter current flow patterns wth absorbng lqud n the shell sde of the permeator. The followng assumptons wear made 1. The permeablty of each gas component s effectve permeablty n the system. The effectve permeablty s a functon of the total mass transfer coeffcent, kt and gven as follows: Peff.= kt where s the membrane thckness 2. The axal pressure drop n the permeator s neglgble for both the shell and tube sde. 3. Neglgble gas phase concentraton gradents exst n the permeaton drecton,.e., no concentraton polarzaton occurs. 4. End effects nsde the permeator are neglgble. 5. The deformaton of the hollow fbres under nternal pressure s neglgble under the low pressures used n ths work. 6. Plug flow has been assumed on both sdes of the membrane. For a bnary gas mxture, mass conservaton over a dfferental length of the module dl gves the followng equatons for the gas flow rates n the non permeated stream (Gu), non permeate mol fracton (x) and concentraton n the lqud stream (c): d ( G' u x' ) dl' P ND lm p u C' x' S (1) 3

4 dx ' 1 dg' u P NDlm C' x' pu x' dl' G' u dl' S (2) dx' j dx' dl' dl' (3) dg ' u NDlm C' C' j P pu x' Pj pu x' j dl' S (4) S j dc' 1 dg' u x' (5) dl' Q1 dl' where the prme symbol denotes the local value n the approprate stream (all symbols are defned n the nomenclature). Ths system of dfferental equatons was solved usng the Runge Kutta algorthm for co-current flow wth ntal condton: l=0; x=x, Gu=Gf, C=0 and an teratve method was devsed for the counter current flow wth the followng boundary condtons: l=0; x=x, l=l, C=0 3. Expermental The equpment employed conssted of a gas flow secton, dehumdfer, permeator cell, and analyzng system. A schematc dagram of the expermental apparatus s shown n Fgure 1. Two types of hollow fbre membranes modules were employed n ths study. Module one contaned porous polyvnyldene fluorde (PVDF) hollow fbre membranes. Ths module was made up of 21 hollow fbres wth an affectve length of 0.30 m and an effectve surface area of m 2. Membranes n module two comprsed non-porous polydmethylsloxane (slcone rubber) conssted of a mm dameter tube actng as a shell contanng 21 hollow fbres wth an effectve surface area m 2. Fve baffle barrers (100mm apart) were nstalled nsde the shell to create turbulence n the lqud flow to mprove slcone rubber hollow fbre membrane performance (Fgure 2). Specfcatons of the modules are gven n Table 1. The gas mxture used n the work was composed of 9.5% CO2 and 90.5% N2, whch was ntroduced nto the hollow fbre lumen, whle the absorbng lqud (water) was fed nto the shell sde. Ths mode of operaton was based on the fndngs of Cooney and Jackson 7. They explored whether havng the gas flow on the tube sde or on the shell sde of the hollow fbre would have a sgnfcant effect on gas absorpton n the hollow fbre module. They concluded that havng the gas flow through the tubes gave much better absorpton. Ths behavour was attrbuted to the hgh probablty that a sgnfcant fracton of the gas stream may flow along the outsde of the tube bundles, 4

5 whle channellng of the gas through other parts of the tube bundles may also occur. Experments were carred out at dfferent operatng condtons, ncludng feed and purge flow rates, feed pressure and two flow patterns co-current and counter current. The experments were run untl steady state operaton was attaned (no change n flow rates and composton wth tme). Analyses of the retentate gas streams were made usng an nfrared detector for CO2 determnaton. Analyss of the lqud phase after absorpton of CO2 was acheved usng a standard ttraton method usng alkal soluton. The amounts of CO2 absorbed determned from both methods were n good agreement wth a maxmum error of 4%. Fgure 1. A schematc dagram of the expermental apparatus Fgure 2 Hollow fbre permeator. 5

6 Geometrcal characterstcs of hollow fbre modules Geometrcal characterstc PVDF Slcone Rubber HF Number of Fbres, N Outsde fbre dameter, do,.m Insde fbre dameter, d, m Thckness,, m Length, Z, m Effectve surface area, m Shell tube nsde dameter, ds, mm Effectve pore sze, m non-porous Packng fracton, Packng fracton = n do 2 / ds 2, where ds s the nsde dameter of the shell; the surface area of fbres s gven by A = do Z n, and the hydraulc dameter s gven by de = (4 flow area) / (total fbre crcumference). 4. Result and dscusson The capablty of the hollow fbre membrane modules for the removal of CO2 from a bnary gas mxture has been nvestgated expermentally. The effect of a number of mportant parameters on CO2 depleton has been explored. The separaton performances of the membrane have been evaluated n terms of the mass transfer coeffcent, permeaton rate, and fractonal removal of the gas consttuents n the mxture. The experments dscussed below were performed wth lqud absorbent flowng n the shell sde and the gas flowng ether countercurrently or co-currently n the tube sde of the hollow fbre modules. Fgure 5.50 shows separaton performances of membrane modules n terms of the fractonal removal of CO2. Comparson was made between module (1) non-porous slcone rubber hollow fbre membrane and (polyvnyldenefluorde) asymmetrc hollow fbre membrane module (4) coupled wth water as absorbent lqud. The Fgure shows PVDF module s more adequate as a contactor for removal of CO2 even at low lqud flow rates. 6

7 Fractonal Removal of CO2, %/cm 2 Fractonal Removal of CO2 % Counter current flow gas space tme = 2.11 s PVDF membrane module 21 fbre Effectve surface area m non-porous membrane module 21 fbre Effectve surface area m Lqud Reynods number, Re L Fgure 4.50 Comparson of performance of membrane modules as fractonal removal of CO 2 based on area 3.9 Counter current flow ts = 2.11 s fbre non-porous slcone rubber (wth baffles) 161-fbre non-porous slcone rubber (wthout baffles) Lqud Reynolds number, Re L Fgure 5.51 Comparson of the proformance of modules as permeaton rate of CO 2 based on area The separaton performance of non-porous slcone rubber hollow fbre membranes n term of the fractonal removal of CO2 has been compared between two modules to show the effect of baffles. In the frst module Ozturk et al. (1997) used 161 hollow fbres wth an effectve surface area of m 2. In the second module, whch has been used durng the course of ths study, only 21 hollow fbres wth an effectve surface area of m 2 have been used and 5 baffles have been nstalled. Both modules were compared at the same followng condtons: gas space tme (ts =2.11s), 7

8 % CO2 concentraton n retentate stream pressure rato=1 and counter current flow pattern at varous shell sde Reynolds numbers. Ths comparson s shown n Fgure As expected, the turbulence created by nstallng baffles wthn the shell leads to mproved performance of the membranes n term of the fractonal removal of CO2. The reason for such results s manly due to the baffles creatng a hgh turbulence on the lqud sde of the fbre, whch reduces the lqud resstance to mass transfer. Consequently the membrane performance mproves. Fgure 4.21 shows the effect lqud flow rates on the concentraton of CO2 n the retentate stream. The result was obtaned expermentally under counter current flow mode at pressure rato of 1. The gas flow rate was held constant at 27.2 ml/mn for all the runs shown n the fgure whle varyng the lqud flow rate. As can be seen from the fgure that the CO2 concentraton n the lqud outlet decreased as the lqud flow rate was ncreased Counter current flow Pressure rato = Qg = 4.5E-7 m3/s Water flow rates, Q L E7 m 3 /s Fgure 4.21 Effect of water flow rates on CO 2 concentraton n the PVDF membrane module The varaton n permeaton rates and selectvtes wth ncreasng pressure ratos s shown n the Fgure 4.9. The results shown n the fgure were obtaned at a gas flow rate of m 3 /s correspondng to gas resdence tme of 2.11s and at constant lqud flow rate. The results showed that the permeaton rate of CO2 was ncreased by 47 % when the pressure rato across the membrane was double, but the selectvty between CO2-N2 decreased by 50 %. Usng an extra pressure casng ncreases n the permeaton rate of N2 as well. Consequently, selectvty of the membrane decreased. 8

9 CO2-N2 Selectvty Permeaton rate of CO2 JE8, Kmol/m 2.s Counter current flow Reynolds number =86.49 ts =2.11 s Pressure rato, (P tube /P shell ) Fgure 4.9 Effect of pressure on CO 2 permeaton rate and selectvty n the non-porous membrane module Fgure 5.10 shows the effect of pressure rato on fractonal removal of CO2; the experments were performed for the counter current flow pattern. The gas flow rate was held constant at m 3 /s for all the runs shown n ths fgure whle varyng the lqud flow rate. As can be seen from the fgure, as the lqud flow rate ncreases the fractonal removal of CO2 ncreases and ncreases n feed pressure result n consequent ncreases n the fractonal removal of CO2. From Fgure 5.10 t s observed that, a hgh removal can be ganed by pressursng the gas sde of the fbres. The separaton process usng the non-porous membrane s a pressure drven process; ncreasng the drvng force across the membrane would ncrease the permeaton rate. 9

10 Fractonal removal of CO2 % Counter current flow ts =2.11 s pressure rato =1.0 pressure rato =1.5 pressure rato = Water flow rate, Q L E7 m 3 /s Fgure 4.10 Effect of pressure on fractonal removal of CO 2 n non-porous membrane module 5. Conclusons The removal of carbon doxde from a gas mxture usng hollow fbre membranes has been studed. The mproved performance has been shown to be attrbuted to the nstalled of baffles wthn the shell of the permeator of the non-porous membrane. For membrane gas absorpton, a very thn non-porous membrane can mprove the performance of a porous membrane support. The selectvty of the membrane tself s not so mportant, because hgh selectvty can be acheved usng a sutable absorbent lqud for removal of the gaseous component, and controllng the operatng condtons. It has been demonstrated that the membrane performance was mproved by ncreasng the lqud flow rate. In the non-porous slcone rubber membrane, whch has been used durng the course of ths study, only 21 hollow fbres wth an effectve surface area of m 2 have been used and 5 baffles have been nstalled wthn the shell n order to ncrease lqud contact wth the tube. Moreover, the contact area n a hollow fbre devce can be made more effcent by ncreasng the number of fbres wthout any substantal ncrease n the physcal sze of the contactng devce. Nomenclature C concentraton, kmol/m 3 D dffuson coeffcent, m 2 / s 10

11 Dlm dameter of fbre, m kl lqud phase mass transfer coeffcent, m/s km mass transfer coeffcent n the membrane, m/s KT total mass transfer coeffcent, m 2 /s l length of fbre, m N number of fbres p pressure, kpa, Peff effectve gas permeablty, Kmol m/m 2 s kpa ) Qg gas flow rate, m 3 / s QL lqud flow rate, m 3 / s R unversal gas constant, m 3.kPa/ kmol.k ReL lqud Reynolds number S solublty, kmol/m 3 kpa Sh Sherwood number T temperature, K UL velocty, m / s X mole fracton n feed gas, dmensonless x mole fracton on non-permeate sde, dmensonless Greek symbols membrane thckness, m packng fracton of fbres Subscrpts lm u j f w logarthmc mean value non-permeate sde frst components second component feed water Reference: 1. Al-saffar, H. B., Oklany, J. S., Ozturk, B., and Hughes, R., Removal of Carbon Doxde from Gas Streams Usng A Gas/Lqud Hollow Fbre Module. Trans IChemE, Vol. 73, Part B, May, PP , Al-saffar, H. B., Ozturk, B., and Hughes, R., A Comparson of Porous and Non-porous Gas-Lqud Membrane Contactors for Gas Separaton. Trans IChemE, Vol. 75, Part A, October, PP , Cooney, D. O., and Jackson, C. C., Gas Absorpton n a Hollow Fber Devce. Chem. Eng. Comm. Vol. 79, pp , Elamar, A., Separaon of Acdc Gases Usng Hollow Fbre Membrane Contactors, ph. D. thess, Department of Chemcal Engneerng Unversty of Salford, Karoor, S., and Srkar, K. K., Gas Absorpton Studes n Mcroporous Hollow Fber Membrane Modules. Ind. Eng. Chem. Res Vol. 32, pp ,

12 6. Kreulen, H., Smolders, C. A., Versteeg, G. F., and Van swaaj, W. P. M., Determnaton of Mass Transfer Rates n Wetted and Non-wetted Mcroporous Membranes. Chem. Eng. Sc. Vol. 48, No. 11, PP , Kreulen, H., Smolders, C. A., Versteeg, G. F., and Van swaaj, W. P. M., Mcroporous Hollow Fbre Membrane Modules as gas lqud Contactors. Part 1. Physcal Mass Transfer Processes. A specfc applcaton: Mass transfer n Hghly Vscous Lquds. J. Membr. Sc., 78, pp , Kreulen, H., Smolders, C. A., Versteeg, G. F., and Van swaaj, W. P. M., Mcroporous Hollow Fbre Membrane Modules as gas lqud Contactors. Part 2. Mass Transfer wth Chemcal Reacton. J. Membr. Sc., 78, pp , L, K., Gas separaton Usng Membranes, ph. D. thess, Department of Chemcal and Gas Engneerng Unversty of Salford, L, K., and Teo, W. K., Use of Permeaton and Absorpton methods for CO2 Removal n Hollow Fbre Membrane Modules. Sep. and Purf. Tech. 13. PP , L, K., Mona, S.L., and Teo, W. K. Desgn of a CO2 Scrubber for Self-contaned Breathng Systems Usng a Mcroporous Membrane. J. Membr. Sc.,, 86, PP , Matson, S. L.,Lopez, J., and Qunn, J. A., Separaton of Gases wth Synthetc Membranes. Chem. Eng. Sc. Vol. 38, No. 4, PP , Yang, M. C. and Cussler, E. L. Desgnng Hollow Fber Contactors. AIChE Journal. Vol. 32, No. 11. PP ,