- Email: [email protected]
Reduction of CO2 emissions by a membrane contacting process☆
Fuel 82 (2003) 2153–2159 www.fuelfirst.com
Reduction of CO2 emissions by a membrane contacting processq M. Mavroudi, S.P. Kaldis, G.P. Sakellaropoulos* Chemical Process Engineering Research Institute, Aristotle University of Thessaloniki, P.O. Box 1520, Thessaloniki 54006, Greece Received 6 November 2002; revised 30 April 2003; accepted 8 May 2003; available online 12 June 2003
Abstract A membrane-based gas– liquid contacting process was evaluated in this work for CO2 removal from flue gases. The absorption of CO2 from a CO2 – N2 mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO2 removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO2 removal was nearly complete (, 99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal. q 2003 Elsevier Ltd. All rights reserved. Keywords: Membrane contactor; CO2 removal; Modeling
1. Introduction Although carbon dioxide is only one of the many greenhouse gases and certainly a non-toxic one, it contributes significantly to the greenhouse effect by anthropogenic sources [1]. Without major policy or technology changes, future concentrations of CO2 will continue to increase, mainly as a result of fossil fuel uses in transport, heating and power generation [2]. Given this high degree of dependence on fossil fuels and the difficulties in their large scale replacement by alternative options, such as nuclear and renewables, it is important to devise techniques which would reduce greenhouse gas emissions arising from the use of fossil fuels. Conventional capture technologies based on a variety of physical and chemical processes including absorption, adsorption, cryogenics and membranes, involve problems that adversely affect the energy efficiency and the economics of power stations [3 – 5]. Efficient and flexible technologies, capable to remove greenhouse gases are needed, operating over a wide range of concentration levels and a wide range of volumetric flow rates. To meet these needs hybrid processes are introduced * Corresponding author. Tel.: þ 30-2310-996271; fax: þ 30-2310996168. E-mail address: [email protected] (G.P. Sakellaropoulos). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00154-6
by combining conventional techniques. In a membrane contactor, separation of a pollutant through the membrane is completely integrated with the absorption operation in order to exploit the benefits of both technologies. The membrane offers a flexible, modular, energy efficient device with a high specific surface area. The absorption process can offer a high selectivity and a high driving force for transport even at low concentrations. The membrane-based gas absorption technique operates efficiently and it can be adapted easily to the specific demands of an individual plant. These advantages paved the way for the application of membrane contacting technology in the recovery/removal of acid gases from flue gases, natural gas and various industrial process gas streams as reviewed by Ho and Sirkar [6] and Gabelman and Hwang [7]. Membrane-based absorption may be applied to most systems treated by conventional gas absorption technology. The absorption of a variety of gases in an acidic or alkaline medium using a membrane hollow fiber module was studied initially by Cussler and his coworkers [8 – 10]. Since then, several investigators have studied membranebased absorption of acid gases [11 – 19], mostly in lab-made and bench scale membrane contactors. The TNO Institute research group [20,21] worked on a larger scale, using a patented membrane module. A membrane gas absorption pilot plant with a capacity of 100 N m3/h was also built and it was tested for two six-month seasons. Over 95% SO2
2154
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
recovery was obtained and no problems with dust, particle condensation, or other flue gas components (e.g. NOx, CO2, HCl) were encountered. In the present work the performance of a commercially available membrane contactor is assessed for carbon dioxide removal from a power plant flue gas. The commercial Liqui-Cel Extra Flow membrane module (Hoechst Celanese Corp., Charlotte, NC, USA) was used to obtain reliable data and to predict separation performance for process scale-up. Absorption experiments were carried out using a simulated typical power plant flue gas as feed gas, and water or aqueous diethanolamine (DEA) solutions, as absorbents. The influence of tube side and shell side dynamics were studied and a mathematical model was used to predict mass transfer coefficients. Experimental results were compared with those of conventional technologies and discussed in terms of future developments of the membrane contacting process for acid gas capture. 1.1. Theory Membrane gas absorption is based on a gas – liquid contact across a hydrophobic microporous membrane. This membrane forms a permeable barrier between the liquid and the gas phase, which permits mass transfer between the two phases without dispersing one phase into the other. The gas preferentially fills the hydrophobic membrane pores and meets the liquid at the opposite side of the membrane. The liquid phase pressure should be slightly higher than that of the gas phase to prevent dispersion of gas bubbles into the liquid. As long as the excess aqueous solution pressure is less than the breakthrough pressure of the membrane [22] the solution does not penetrate the pores and the gas/liquid interface is immobilized at the pore mouth of the membrane on the solution side. Operation of gas – liquid membrane contactors differs from that of other membrane processes, such as filtration, since there is no convective flow through the pores, but only diffusive transport of certain components. This is the main reason that membrane contactors are less sensitive to fouling than conventional membranes. As the membrane is non-selective, the chemistry of the separation is the same as for conventional equipment. The choice of a suitable combination of absorption liquid, membrane characteristics and operation mode determines the selectivity of the process. Mass transfer is determined by the consecutive steps in the three phases shown schematically in Fig. 1, i.e. diffusion of gaseous component i from the bulk gas to the membrane wall, diffusion through the pores of the membrane to the membrane – liquid interface, and dissolution into the liquid absorbent, followed by liquid phase diffusion/chemical reaction. Hence, the overall liquid phase mass transfer resistance, 1=Koverall (s/cm), can be expressed by
Fig. 1. Mass transfer regions and dominant resistances in a membrane contactor.
a resistance-in-series model 1 Koverall
¼
Hi Hi 1 þ þ Eki;liquid ki;gas ki;membrane
ð1Þ
where ki;gas is the gas side mass transfer coefficient (cm/s), ki;membrane is the membrane mass transfer coefficient (cm/s), ki;liquid is the liquid phase mass transfer coefficient (cm/s), Hi is the dimensionless Henry’s constant, and E is an enhancement factor due to the chemical reaction. The overall liquid phase mass transfer coefficient, Koverall ; is calculated by Eq. (2): Koverall ¼
Ql cout il Am ðDCÞlm
ð2Þ
where Ql is the liquid flow rate (cm3/s), cout il is the liquid phase outlet concentration of CO2 (mol/cm3) and ðDCÞlm is the logarithmic mean driving force based on liquid phase concentrations: ðDCÞlm ¼
out outp ðcinp il "2 cil Þ 2 ðcil# Þ out ðcinp il 2 cil Þ ln outp cil
ð3Þ
outp The cinp are hypothetical liquid phase concenil and cil trations in equilibrium with the corresponding gas phase CO2 concentrations, cig ; expressed by Henry’s law as:
cpil ¼ Hi cig
ð4Þ
2. Model development A mathematical model was developed using the resistance-in-series concept, combining process conditions, membrane properties and module geometric characteristics. The following assumptions are made: (a) single component (CO2) absorption from a CO2 –N2 gas mixture flowing through the fiber lumen into an aqueous solution flowing in the shell side, as described schematically in Fig. 2; (b) steady state and isothermal operation; (c) Newtonian fluids with constant physical properties; (d) fully developed laminar flow in the lumen; (e) negligible radial convection and axial diffusion; and (f) applicability of Henry’s law.
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
2155
Fig. 2. Flow configuration through a typical hollow fiber.
With these assumptions, the CO2 concentration profile in the lumen can be estimated from the differential mass balance ›Cig ›Cig 1 › ¼ Dig r uz ð5Þ r ›r ›z ›r where Dig is the diffusion coefficient in the lumen fluid (cm2/s), considered here independent of concentration and derived from Chapman –Enskog theory [23], and uz the gas velocity inside the lumen (cm/s). This velocity profile is given by " 2 # r uz ¼ u 1 2 ð6Þ ri where u is the maximum velocity inside the fiber lumen (cm/ s) and ri the inner fiber radius (cm). The initial condition for CO2 in the lumen is specified as in ; Cig ¼ Cig
for z ¼ 0 and all r
ð7Þ
in where Cig is the inlet concentration of CO2 in the gas phase (mol/cm3). In the radial direction, symmetry is assumed at the axis of the cylindrical fiber:
›Cig ¼ 0; ›r
for r ¼ 0 and all z
ð8Þ
At the gas –liquid interface, the conservation of mass with respect to CO2 is expressed as Dig
›Cig ¼ 2Kext ðCig 2 ms Cil Þ; ›r
for r ¼ ri and all z
ð9Þ
where ms is the distribution coefficient between gas and liquid phase and Kext the external (membrane wall and shell side) mass transfer coefficient (cm/s), calculated by Eq. (10): 1 Hi 1 ¼ þ Kext ki;membrane Eki;liquid
ð10Þ
The shell side mass transfer coefficient, ki;liquid ; is calculated by an engineering correlation developed for this type of cross flow membrane contactors [24]. The membrane mass transfer coefficient, ki;membrane depends on the diffusivity of absorbed gas in the gas-filled, Dig ; or liquid-filled pores, Dil ; and the thickness lm ; porosity 1; and tortuosity t of the membrane wall. When partial wetting of the membrane
is considered, the effective mass transfer coefficient is reduced by a factor, which denotes the fraction of the pores wetted by the liquid phase, x; according to: 1 ð1 2 xÞtlm xtlm þ ¼ ki;membrane Dig 1 Dil 1
ð11Þ
In pure water, absorption of CO2 is accompanied by the following reaction: þ CO2 þ 2H2 O $ HCO2 3 þ H3 O
Because the value of equilibrium constant of the reaction is very small, the formation of bicarbonate is very small [12]. Therefore, CO2 is absorbed physically. For absorption in an aqueous DEA solution, chemical reaction should be considered. The zwitterions-mechanism [25] was adopted for this theoretical study: k1 =k21
CO2 þ R2 NH $ R2 NHþ COO2 k2
R2 NHþ COO2 þ R2 NH ! R2 NCOO2 þ R2 NHþ 2 The mechanism consists of two steps, the formation of the zwitterions followed by the removal of a proton by the amine. The physicochemical properties, such as diffusivities of CO2 in gas phase and in aqueous solutions, and solubilities were obtained from the literature [23,25,26]. In the case of chemical reaction, the enhancement factor E in Eq. (10), was evaluated from the classical solution of gas absorption accompanied by a chemical reaction [27]. The set of partial differential Eqs. (5) –(11) was solved using the method of lines. A finite difference scheme in dimensionless r direction was applied to convert the set of partial differential equations into a system of ordinary differential equations (ODE). The latter gives an ODE initial value problem, which was solved by the explicit ‘hybrid’ method [28].
3. Experimental A gas mixture containing 15% CO2 – 85% N2 (procured and certified by Air Liquid Hellas S.A.G.I.) was selected as the feed gas. This mixture simulates a typical composition (in major components) of flue gases from a coal combustion
2156
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
Table 1 Specifications for the hollow fiber membrane module Effective membrane area (cm2) Effective area/volume (cm21) Fiber diameter inside/outside (cm) Effective fiber length (cm) Fiber porosity (%) Fiber pore size (mm) Number of fibers
14,000 29.3 0.024/0.030 15.24 40 0.03 10,000
power plant. Pure water and aqueous solutions of DEA (0.5 –2 M) were chosen as absorption liquids. Water used through out the study was reverse osmosis grade and in contact with ambient air in order to be saturated with N2. Therefore, no transfer of N2 was involved in this study. A Liqui-Cel Extra Flow membrane contactor (Hoechst Celanese Corp., Charlotte, NC, USA) was used, 8 cm dia. £ 28 cm long (2.5 in. £ 8 in.). Detailed design and configuration features of this type of cross flow membrane module have been presented in the literature [29]. Specifications of the hollow fiber membrane module are given in Table 1. 4Fig. 3 is a schematic representation of the experimental set-up. The feed gas was introduced directly from a gas cylinder to the fiber lumen of the module at a pressure between 1 and 1.5 atm and the liquid flowed through the shell side at a pressure of 0.2 up to 0.5 atm higher than that of the gas. The feed gas flow was controlled and measured with a precision rotameter. Pressure gauges at the contactor inlet and outlet indicated the gas pressure, while a back pressure regulator was installed in the exit gas line in order to maintain the desired pressure, if necessary. A check valve was used before the contactor inlet to prevent accidental injection of liquid into the feed gas line. The liquid absorbent was pumped from the liquid storage tank. The instrumentation in the liquid line for the control and measurement of flow and pressure was similar to that in the gas line. The composition of the gas streams was monitored by sampling a slip stream from the feed or exit line and diverting it into a Perkin– Elmer
8500 gas chromatograph with a thermal conductivity detector. Before sending the gas sample to the chromatograph, liquid droplets and moisture were removed by a glass liquid trap and a moisture trap. The module was kept in an isothermal condition, using a temperature control water bath and experiments were carried out at 23 ^ 0.5 8C. Gas flow rates were varied from 50 to 240 N cm3/s and the liquid flow rates from 10 to 160 cm3/s. All data were obtained at steady state, after 30 min of operating time, which allowed for system stabilization. Steady state was indicated by a constant CO2 concentration in the exiting gas stream. Three samples were taken under the same operating conditions and the average value was calculated.
4. Results and discussion Carbon dioxide absorption experiments in pure water were conducted initially to evaluate the performance of the hollow fiber membrane contactor. The dependence of CO2 gas outlet concentration, expressed in a dimensionless form in ðcout ig =cig Þ; on the gas and absorbent flow rates is shown in Fig. 4. The amount of CO2 in the gas outlet decreases as the liquid flow rate increases. Obviously, an increase in the liquid flow rate results in a lower liquid phase mass transfer resistance and, hence, in a more efficient gas removal. This effect is more pronounced when the gas flow rate is low. For higher gas flow rates, an equivalent gas removal can be achieved if a higher volume module is used providing more gas –liquid contact area. The membrane contacting process shows an excellent performance in terms of mass transfer. out in The depletion of CO2 ððcin ig 2 cig Þ=cig Þ was as high as 75%, for gas and liquid flow rates equal to 50 N cm3/s and 160 cm3/s, respectively. Theoretical estimations obtained from the numerical solution of Eqs. (5) – (11) are plotted in the same diagram. Model estimates of outlet CO2 concentration calculated for
Fig. 3. Schematic representation of the membrane module contactor unit.
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
Fig. 4. Membrane-based CO2 absorption in water.
a completely hydrophobic membrane, i.e. for non-wetted pores, are lower than those experimentally measured, especially for high gas and liquid flow rates. An additional mass transfer resistance may be considered as a probable reason of this deviation. The lumen side and the shell side resistances to mass transfer become negligible for high gas and liquid flow rates, respectively, and the membrane resistance seems to dominate the process. Considering, nonwetted membrane pores, the membrane resistance, and consequently the overall resistance to mass transfer, is underestimated and the model predicts higher CO2 removal. Trace impurities and long term operation of the membrane are presumed to modify the surface hydrophobicity and, thus, to allow some penetration of liquid into the pores, thereby decreasing the membrane mass transfer coefficient [16,30,31]. The theoretical predictions of CO2 depletion were recalculated taking into account some possible penetration of the liquid into the membrane pores. The membrane mass transfer coefficient was calculated from Eq. (11) assuming partially wetted membrane pores. The fraction of wetted pores, x; was adjusted to fit the experimental data and ranged from 2.5 to 8%, increasing with liquid flow rate. Although the fraction of wetted pores
Fig. 5. Effect of chemical reaction on membrane-based CO2 absorption. Feed gas: 100 N cm3/s. Simulation: partially wetted mode (solid curves).
2157
is small, the membrane resistance to mass transfer increases significantly. Simulation results for partially wetted mode are plotted as solid curves in the same diagram. In the presence of DEA as absorbent, absorption is facilitated by chemical reaction. Fig. 5 shows the overall liquid phase mass transfer coefficient, Koverall ; as a function of liquid flow rate. In this case, mass transfer increases appreciably even for low DEA concentrations. By increasing DEA concentration in the liquid phase from 0 to 2 M, the mass transfer of CO2 increases because of an increase in the reaction rate. At low liquid flow rates, the solution becomes saturated with CO2, and the mass transfer rate decreases. At high liquid flow rates, the absorbent does not become saturated with CO2, and the gas absorption rate increases. Theoretical estimations assuming partially wetted pores are also plotted in Fig. 5 and seem to be in good agreement with the experimental results. The membrane mass transfer coefficient was calculated from Eq. (11), assuming that a fraction of membrane pores, x; was liquid filled. The adjustable values of x ranged from 1.5 to 7%, similar to those estimated for the CO2 – water system. The separation performance of the hollow fiber membrane contactor, expressed as percentage of CO2 removal, is given in Fig. 6 versus liquid flow rate for a constant gas flow rate of 100 N cm3/s. Compared to water, performance improves considerably, reaching up to 99% CO2 removal, by using aqueous DEA solutions as absorbent under the same experimental conditions. Theoretical predictions for partially wetted pores, plotted in the same diagram, are in good agreement with the experimental results. The adjustable values of x were similar to those used in Fig. 5. The use of a chemical aqueous solution instead of water enhances mass transfer of CO2 and, therefore, the scrubbing capacity of the liquid absorbent improves. Consequently, for an equivalent gas removal the required absorbent flow rate decreases and it can be achieved in a smaller contactor. Against these considerable advantages, the main disadvantage is the cost of chemical solvents. Since organic amines
Fig. 6. Removal of CO2 in various absorbents. Feed gas: 100 N cm3/s. Simulation: partially wetted mode (solid curves).
2158
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
can be regenerated, their use is generally more economic than other liquid absorbents, which are discarded after use. This indicates that DEA is suitable as an absorbent for CO2 removal in a membrane contacting process. However, comparison of theoretical and experimental results for all gas/liquid systems studied indicate that the membrane is operated somewhere between the limits of gas-filled and liquid-filled pores. The penetration of the liquid into the pores, as explained by the necking phenomenon, depends on the gas – liquid system and on the characteristics of the fiber used. In order to prevent this wetting, research efforts should focus on new membrane materials having a high degree of hydrophobicity [16,32], and/or on new liquid absorbents with improved surface tension properties [31]. In order to evaluate the membrane-based technology efficiency, the absorption performance of the hollow fiber membrane contactor needs to be compared with that of a conventional packed absorber. For this, the classical measure of absorption technology efficiency was used, i.e. the height of a transfer unit (HTU) required for CO2 absorption in an absorbent (here pure water and DEA). In Fig. 7, the required HTUs for physical and chemical absorption of CO2 under the same operating conditions are plotted as a function of superficial velocity for the case of the tested membrane contactor and for packed contactors
with Raschig rings of diameter dr ¼ 25 and 50 mm. The height of transfer unit based on liquid phase, HTUL, was plotted in Fig. 7(a) for the case of CO2 absorption in water, while the height of transfer unit based on gas phase, HTUG, demanded for a gas phase controlled process, was plotted in Fig. 7(b) for the case of CO2 absorption in DEA. The values of HTU for packed contactors were calculated with the use of empirical correlations [33]. The HTUs for membrane technology were based on the experimental results presented in this work. Under the same operating conditions, the experimentally obtained HTU values for the membrane contactor were significantly lower than those of a conventional packed contactor, for CO2 absorption both in water and in DEA. This leads to lower specific area requirements and, hence, to a reduction in absorber investment costs compared to packed towers. Thus, a significantly higher amount of CO2 removal can be achieved with a relatively small module and low flow velocities. Another benefit of using membrane technology instead of conventional absorption equipment is that membrane operation can be scaled-up linearly, so that an increase in capacity is achieved simply by adding membrane modules.
5. Conclusions Gas – liquid contactors based on hollow fiber membranes can be used to reduce acid gas emissions such as CO2 from flue gases. The present experimental results with a simulated flue gas mixture (CO2, N2) show that high levels of CO2 removal (, 99%) can be accomplished with chemical solution absorbents. Physical absorbents like water can also be used albeit not with such high levels of CO2 absorption. To simulate the behavior of gas –liquid membrane contactors, a mathematical model was developed. The model takes into account mass transfer at the gas, liquid and membrane interfaces, chemical reaction and possible membrane wetting. Comparison of model estimations with experimental results indicates partial wetting of the membrane, resulting to significant membrane resistance to mass transfer. Compared to conventional technologies, hollow fiber membrane modules offer high specific surface area and, therefore, the calculated HTU values for similar operating conditions are significantly smaller. This should lead to more compact, efficient and easily scaled-up absorption units.
Acknowledgements
Fig. 7. HTU for a membrane contactor and for packed towers for CO2 absorption in (a) water and (b) DEA.
The authors wish to thank the European Coal and Steel Community for support of this work. Maria Mavroudi wishes to thank the State Scholarship’s Foundation for economical support of her PhD research, which includes part of this work.
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
References [1] Freund P. Abatement and mitigation of carbon dioxide emissions from power generation. IEA GHG Programme. Available at: http://www. ieagreen.org.uk/; 1998. [2] Wuebbles DJ, Jain AK. Concerns about climate change and the role of fossil fuel use. Fuel Process Technol 2001;71:99–119. [3] Herzog H, Drake E, Tester J. A research needs assessment for the capture, utilization and disposal of carbon dioxide from fossil fuelfired power plants. MIT Report No. DE-FG02-92ER30194.A000, Cambridge, MA; 1993 [4] IEA Greenhouse Gas R&D Programme. Carbon dioxide capture from power stations. Final report, London, http://www.ieagreen.org.uk [5] Rao AB, Rubin ES. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. Environ Sci Technol 2002;36(20):4467–75. [6] Ho WWS, Sirkar KK. Membrane handbook. New York: Van Nostrand Reinhold; 1992. [7] Gabelman A, Hwang ST. Hollow fiber membrane contactors. J Membr Sci 1999;159:61–106. [8] Zhang Q, Cussler EL. Microporous hollow fibers for gas absorption: I. Mass transfer in the liquid. J Membr Sci 1985;23: 321– 32. [9] Zhang Q, Cussler EL. Microporous hollow fibers for gas absorption: I. Mass transfer across the membrane. J Membr Sci 1985;23:333–45. [10] Yang MC, Cussler EL. Designing hollow-fiber contactors. AIChE J 1986;32(7):1910–6. [11] Cooney DO, Jackson CC. Gas absorption in a hollow fiber device. Chem Engng Commun 1989;79:153–63. [12] Karoor S, Sirkar KK. Gas absorption studies in microporous hollow fiber membrane modules. Ind Engng Chem Res 1993;32: 674–84. [13] Kreulen H, Smolders CA, Versteeg GF, van Swaaij WPM. Microporous hollow fibre membrane modules as gas-liquid contactors; Part 1. Physical mass transfer processes. A specific application: mass transfer in highly viscous liquids. J Membr Sci 1993;78:197–216. [14] Kreulen H, Smolders CA, Versteeg GF, van Swaaij WPM. Microporous hollow fibre membrane modules as gas–liquid contactors; Part 2. Mass transfer with chemical reaction. J Membr Sci 1993;78:217–38. [15] Rangwala HA. Absorption of carbon dioxide into aqueous solutions using hollow fiber membrane contactors. J Membr Sci 1996;112: 229–40. [16] Nishikawa N, Ishibashi M, Ohta H, Akutsu N, Matsumoto H, Kamata T, Kitamura H. CO2 removal by hollow-fiber gas–liquid contactor. Energy Convers Mgmt 1995;36(6– 9):415–8.
2159
[17] Al-Saffar HB, Ozturk B, Hughes R. A comparison of porous and nonporous gas–liquid membrane contactors for gas separation. Trans IchemE 1997;75(A):685–92. [18] Chun MS, Lee KH. Analysis on a hydrophobic hollow-fiber membrane absorber and experimental observations of CO2 removal by enhanced absorption. Sep Sci Techol 1997;32(15):2445– 66. [19] Li K, Teo WK. Use of permeation and absorption methods for CO2 removal in hollow fibre membrane modules. Sep Purif Tech 1998;13: 79–88. [20] Feron PHM, Jansen AE, Klaasen R. Membrane technology in carbon dioxide removal. Energy Convers Mgmt 1992;33(5 –8):421–8. [21] Jansen AE, Klaasen R, Feron PHM, Hanemaaijer JH, ter Meulen BPh. Membrane gas absorption processes in environmental applications. In: Crespo JG, Bo¨ddecker KW, editors. Membrane processes in separation and purification. The Netherlands: Kluwer Academic; 1994. [22] Kim BS, Harriott P. Critical entry pressure for liquids in hydrophobic membranes. J Colloid Interface Sci 1987;115(1):1 –8. [23] Reid RC, Prausnitz JM, Sherwood TK. The properties of gases and liquids. New York: McGraw-Hill; 1977. [24] Scho¨ner P, Plucinski P, Nitsch W, Daiminger U. Mass transfer in the shell side of cross flow hollow fiber modules. Chem Engng Sci 1998; 53(13):2319 –26. [25] Versteeg GF, van Dijck LAJ, van Swaaij WPM. On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions. An overview. Chem Engng Commun 1996;144:113–58. [26] Versteeg GF, van Swaaij WPM. Solubility and diffusivity of acid gases (CO2, N2O) in aqueous alkanolamine solution. J Chem Engng Data 1988;33:29 –34. [27] Danckwerts PV. Gas– liquid reactions. New York: McGraw-Hill; 1970. [28] Ashour SS, Hanna OT. A new very simple explicit method for the integration of mildly stiff ordinary differential equations. Comput Chem Engng 1990;14(3):267–72. [29] Sengupta A, Peterson PA, Miller BD, Schneider J, Fulk Jr CW. Largescale application of membrane contactors for gas transfer from or to ultrapure water. Sep Purif Tech 1998;14:189 –200. [30] Mahmud H, Kumar A, Narbaitz RM, Matsuura T. A study of mass transfer in the membrane air-stripping process using microporous polypropylene hollow fibers. J Membr Sci 2000;179:29– 41. [31] Kumar PS, Hogendoorn JA, Feron PHM, Versteeg GF. New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors. Chem Engng Sci 2002;57:1639 –51. [32] Feron PHM, Jansen AE. CO2 separation with polyolefin membrane contactors and dedicated absorption liquids: performances and prospects. Sep Purif Tech 2002;27:231–42. [33] Perry RH, Green DW. Perry’s chemical engineers’ handbook. New York: McGraw-Hill; 1984.
Reduction of CO2 emissions by a membrane contacting processq M. Mavroudi, S.P. Kaldis, G.P. Sakellaropoulos* Chemical Process Engineering Research Institute, Aristotle University of Thessaloniki, P.O. Box 1520, Thessaloniki 54006, Greece Received 6 November 2002; revised 30 April 2003; accepted 8 May 2003; available online 12 June 2003
Abstract A membrane-based gas– liquid contacting process was evaluated in this work for CO2 removal from flue gases. The absorption of CO2 from a CO2 – N2 mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO2 removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO2 removal was nearly complete (, 99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal. q 2003 Elsevier Ltd. All rights reserved. Keywords: Membrane contactor; CO2 removal; Modeling
1. Introduction Although carbon dioxide is only one of the many greenhouse gases and certainly a non-toxic one, it contributes significantly to the greenhouse effect by anthropogenic sources [1]. Without major policy or technology changes, future concentrations of CO2 will continue to increase, mainly as a result of fossil fuel uses in transport, heating and power generation [2]. Given this high degree of dependence on fossil fuels and the difficulties in their large scale replacement by alternative options, such as nuclear and renewables, it is important to devise techniques which would reduce greenhouse gas emissions arising from the use of fossil fuels. Conventional capture technologies based on a variety of physical and chemical processes including absorption, adsorption, cryogenics and membranes, involve problems that adversely affect the energy efficiency and the economics of power stations [3 – 5]. Efficient and flexible technologies, capable to remove greenhouse gases are needed, operating over a wide range of concentration levels and a wide range of volumetric flow rates. To meet these needs hybrid processes are introduced * Corresponding author. Tel.: þ 30-2310-996271; fax: þ 30-2310996168. E-mail address: [email protected] (G.P. Sakellaropoulos). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00154-6
by combining conventional techniques. In a membrane contactor, separation of a pollutant through the membrane is completely integrated with the absorption operation in order to exploit the benefits of both technologies. The membrane offers a flexible, modular, energy efficient device with a high specific surface area. The absorption process can offer a high selectivity and a high driving force for transport even at low concentrations. The membrane-based gas absorption technique operates efficiently and it can be adapted easily to the specific demands of an individual plant. These advantages paved the way for the application of membrane contacting technology in the recovery/removal of acid gases from flue gases, natural gas and various industrial process gas streams as reviewed by Ho and Sirkar [6] and Gabelman and Hwang [7]. Membrane-based absorption may be applied to most systems treated by conventional gas absorption technology. The absorption of a variety of gases in an acidic or alkaline medium using a membrane hollow fiber module was studied initially by Cussler and his coworkers [8 – 10]. Since then, several investigators have studied membranebased absorption of acid gases [11 – 19], mostly in lab-made and bench scale membrane contactors. The TNO Institute research group [20,21] worked on a larger scale, using a patented membrane module. A membrane gas absorption pilot plant with a capacity of 100 N m3/h was also built and it was tested for two six-month seasons. Over 95% SO2
2154
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
recovery was obtained and no problems with dust, particle condensation, or other flue gas components (e.g. NOx, CO2, HCl) were encountered. In the present work the performance of a commercially available membrane contactor is assessed for carbon dioxide removal from a power plant flue gas. The commercial Liqui-Cel Extra Flow membrane module (Hoechst Celanese Corp., Charlotte, NC, USA) was used to obtain reliable data and to predict separation performance for process scale-up. Absorption experiments were carried out using a simulated typical power plant flue gas as feed gas, and water or aqueous diethanolamine (DEA) solutions, as absorbents. The influence of tube side and shell side dynamics were studied and a mathematical model was used to predict mass transfer coefficients. Experimental results were compared with those of conventional technologies and discussed in terms of future developments of the membrane contacting process for acid gas capture. 1.1. Theory Membrane gas absorption is based on a gas – liquid contact across a hydrophobic microporous membrane. This membrane forms a permeable barrier between the liquid and the gas phase, which permits mass transfer between the two phases without dispersing one phase into the other. The gas preferentially fills the hydrophobic membrane pores and meets the liquid at the opposite side of the membrane. The liquid phase pressure should be slightly higher than that of the gas phase to prevent dispersion of gas bubbles into the liquid. As long as the excess aqueous solution pressure is less than the breakthrough pressure of the membrane [22] the solution does not penetrate the pores and the gas/liquid interface is immobilized at the pore mouth of the membrane on the solution side. Operation of gas – liquid membrane contactors differs from that of other membrane processes, such as filtration, since there is no convective flow through the pores, but only diffusive transport of certain components. This is the main reason that membrane contactors are less sensitive to fouling than conventional membranes. As the membrane is non-selective, the chemistry of the separation is the same as for conventional equipment. The choice of a suitable combination of absorption liquid, membrane characteristics and operation mode determines the selectivity of the process. Mass transfer is determined by the consecutive steps in the three phases shown schematically in Fig. 1, i.e. diffusion of gaseous component i from the bulk gas to the membrane wall, diffusion through the pores of the membrane to the membrane – liquid interface, and dissolution into the liquid absorbent, followed by liquid phase diffusion/chemical reaction. Hence, the overall liquid phase mass transfer resistance, 1=Koverall (s/cm), can be expressed by
Fig. 1. Mass transfer regions and dominant resistances in a membrane contactor.
a resistance-in-series model 1 Koverall
¼
Hi Hi 1 þ þ Eki;liquid ki;gas ki;membrane
ð1Þ
where ki;gas is the gas side mass transfer coefficient (cm/s), ki;membrane is the membrane mass transfer coefficient (cm/s), ki;liquid is the liquid phase mass transfer coefficient (cm/s), Hi is the dimensionless Henry’s constant, and E is an enhancement factor due to the chemical reaction. The overall liquid phase mass transfer coefficient, Koverall ; is calculated by Eq. (2): Koverall ¼
Ql cout il Am ðDCÞlm
ð2Þ
where Ql is the liquid flow rate (cm3/s), cout il is the liquid phase outlet concentration of CO2 (mol/cm3) and ðDCÞlm is the logarithmic mean driving force based on liquid phase concentrations: ðDCÞlm ¼
out outp ðcinp il "2 cil Þ 2 ðcil# Þ out ðcinp il 2 cil Þ ln outp cil
ð3Þ
outp The cinp are hypothetical liquid phase concenil and cil trations in equilibrium with the corresponding gas phase CO2 concentrations, cig ; expressed by Henry’s law as:
cpil ¼ Hi cig
ð4Þ
2. Model development A mathematical model was developed using the resistance-in-series concept, combining process conditions, membrane properties and module geometric characteristics. The following assumptions are made: (a) single component (CO2) absorption from a CO2 –N2 gas mixture flowing through the fiber lumen into an aqueous solution flowing in the shell side, as described schematically in Fig. 2; (b) steady state and isothermal operation; (c) Newtonian fluids with constant physical properties; (d) fully developed laminar flow in the lumen; (e) negligible radial convection and axial diffusion; and (f) applicability of Henry’s law.
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
2155
Fig. 2. Flow configuration through a typical hollow fiber.
With these assumptions, the CO2 concentration profile in the lumen can be estimated from the differential mass balance ›Cig ›Cig 1 › ¼ Dig r uz ð5Þ r ›r ›z ›r where Dig is the diffusion coefficient in the lumen fluid (cm2/s), considered here independent of concentration and derived from Chapman –Enskog theory [23], and uz the gas velocity inside the lumen (cm/s). This velocity profile is given by " 2 # r uz ¼ u 1 2 ð6Þ ri where u is the maximum velocity inside the fiber lumen (cm/ s) and ri the inner fiber radius (cm). The initial condition for CO2 in the lumen is specified as in ; Cig ¼ Cig
for z ¼ 0 and all r
ð7Þ
in where Cig is the inlet concentration of CO2 in the gas phase (mol/cm3). In the radial direction, symmetry is assumed at the axis of the cylindrical fiber:
›Cig ¼ 0; ›r
for r ¼ 0 and all z
ð8Þ
At the gas –liquid interface, the conservation of mass with respect to CO2 is expressed as Dig
›Cig ¼ 2Kext ðCig 2 ms Cil Þ; ›r
for r ¼ ri and all z
ð9Þ
where ms is the distribution coefficient between gas and liquid phase and Kext the external (membrane wall and shell side) mass transfer coefficient (cm/s), calculated by Eq. (10): 1 Hi 1 ¼ þ Kext ki;membrane Eki;liquid
ð10Þ
The shell side mass transfer coefficient, ki;liquid ; is calculated by an engineering correlation developed for this type of cross flow membrane contactors [24]. The membrane mass transfer coefficient, ki;membrane depends on the diffusivity of absorbed gas in the gas-filled, Dig ; or liquid-filled pores, Dil ; and the thickness lm ; porosity 1; and tortuosity t of the membrane wall. When partial wetting of the membrane
is considered, the effective mass transfer coefficient is reduced by a factor, which denotes the fraction of the pores wetted by the liquid phase, x; according to: 1 ð1 2 xÞtlm xtlm þ ¼ ki;membrane Dig 1 Dil 1
ð11Þ
In pure water, absorption of CO2 is accompanied by the following reaction: þ CO2 þ 2H2 O $ HCO2 3 þ H3 O
Because the value of equilibrium constant of the reaction is very small, the formation of bicarbonate is very small [12]. Therefore, CO2 is absorbed physically. For absorption in an aqueous DEA solution, chemical reaction should be considered. The zwitterions-mechanism [25] was adopted for this theoretical study: k1 =k21
CO2 þ R2 NH $ R2 NHþ COO2 k2
R2 NHþ COO2 þ R2 NH ! R2 NCOO2 þ R2 NHþ 2 The mechanism consists of two steps, the formation of the zwitterions followed by the removal of a proton by the amine. The physicochemical properties, such as diffusivities of CO2 in gas phase and in aqueous solutions, and solubilities were obtained from the literature [23,25,26]. In the case of chemical reaction, the enhancement factor E in Eq. (10), was evaluated from the classical solution of gas absorption accompanied by a chemical reaction [27]. The set of partial differential Eqs. (5) –(11) was solved using the method of lines. A finite difference scheme in dimensionless r direction was applied to convert the set of partial differential equations into a system of ordinary differential equations (ODE). The latter gives an ODE initial value problem, which was solved by the explicit ‘hybrid’ method [28].
3. Experimental A gas mixture containing 15% CO2 – 85% N2 (procured and certified by Air Liquid Hellas S.A.G.I.) was selected as the feed gas. This mixture simulates a typical composition (in major components) of flue gases from a coal combustion
2156
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
Table 1 Specifications for the hollow fiber membrane module Effective membrane area (cm2) Effective area/volume (cm21) Fiber diameter inside/outside (cm) Effective fiber length (cm) Fiber porosity (%) Fiber pore size (mm) Number of fibers
14,000 29.3 0.024/0.030 15.24 40 0.03 10,000
power plant. Pure water and aqueous solutions of DEA (0.5 –2 M) were chosen as absorption liquids. Water used through out the study was reverse osmosis grade and in contact with ambient air in order to be saturated with N2. Therefore, no transfer of N2 was involved in this study. A Liqui-Cel Extra Flow membrane contactor (Hoechst Celanese Corp., Charlotte, NC, USA) was used, 8 cm dia. £ 28 cm long (2.5 in. £ 8 in.). Detailed design and configuration features of this type of cross flow membrane module have been presented in the literature [29]. Specifications of the hollow fiber membrane module are given in Table 1. 4Fig. 3 is a schematic representation of the experimental set-up. The feed gas was introduced directly from a gas cylinder to the fiber lumen of the module at a pressure between 1 and 1.5 atm and the liquid flowed through the shell side at a pressure of 0.2 up to 0.5 atm higher than that of the gas. The feed gas flow was controlled and measured with a precision rotameter. Pressure gauges at the contactor inlet and outlet indicated the gas pressure, while a back pressure regulator was installed in the exit gas line in order to maintain the desired pressure, if necessary. A check valve was used before the contactor inlet to prevent accidental injection of liquid into the feed gas line. The liquid absorbent was pumped from the liquid storage tank. The instrumentation in the liquid line for the control and measurement of flow and pressure was similar to that in the gas line. The composition of the gas streams was monitored by sampling a slip stream from the feed or exit line and diverting it into a Perkin– Elmer
8500 gas chromatograph with a thermal conductivity detector. Before sending the gas sample to the chromatograph, liquid droplets and moisture were removed by a glass liquid trap and a moisture trap. The module was kept in an isothermal condition, using a temperature control water bath and experiments were carried out at 23 ^ 0.5 8C. Gas flow rates were varied from 50 to 240 N cm3/s and the liquid flow rates from 10 to 160 cm3/s. All data were obtained at steady state, after 30 min of operating time, which allowed for system stabilization. Steady state was indicated by a constant CO2 concentration in the exiting gas stream. Three samples were taken under the same operating conditions and the average value was calculated.
4. Results and discussion Carbon dioxide absorption experiments in pure water were conducted initially to evaluate the performance of the hollow fiber membrane contactor. The dependence of CO2 gas outlet concentration, expressed in a dimensionless form in ðcout ig =cig Þ; on the gas and absorbent flow rates is shown in Fig. 4. The amount of CO2 in the gas outlet decreases as the liquid flow rate increases. Obviously, an increase in the liquid flow rate results in a lower liquid phase mass transfer resistance and, hence, in a more efficient gas removal. This effect is more pronounced when the gas flow rate is low. For higher gas flow rates, an equivalent gas removal can be achieved if a higher volume module is used providing more gas –liquid contact area. The membrane contacting process shows an excellent performance in terms of mass transfer. out in The depletion of CO2 ððcin ig 2 cig Þ=cig Þ was as high as 75%, for gas and liquid flow rates equal to 50 N cm3/s and 160 cm3/s, respectively. Theoretical estimations obtained from the numerical solution of Eqs. (5) – (11) are plotted in the same diagram. Model estimates of outlet CO2 concentration calculated for
Fig. 3. Schematic representation of the membrane module contactor unit.
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
Fig. 4. Membrane-based CO2 absorption in water.
a completely hydrophobic membrane, i.e. for non-wetted pores, are lower than those experimentally measured, especially for high gas and liquid flow rates. An additional mass transfer resistance may be considered as a probable reason of this deviation. The lumen side and the shell side resistances to mass transfer become negligible for high gas and liquid flow rates, respectively, and the membrane resistance seems to dominate the process. Considering, nonwetted membrane pores, the membrane resistance, and consequently the overall resistance to mass transfer, is underestimated and the model predicts higher CO2 removal. Trace impurities and long term operation of the membrane are presumed to modify the surface hydrophobicity and, thus, to allow some penetration of liquid into the pores, thereby decreasing the membrane mass transfer coefficient [16,30,31]. The theoretical predictions of CO2 depletion were recalculated taking into account some possible penetration of the liquid into the membrane pores. The membrane mass transfer coefficient was calculated from Eq. (11) assuming partially wetted membrane pores. The fraction of wetted pores, x; was adjusted to fit the experimental data and ranged from 2.5 to 8%, increasing with liquid flow rate. Although the fraction of wetted pores
Fig. 5. Effect of chemical reaction on membrane-based CO2 absorption. Feed gas: 100 N cm3/s. Simulation: partially wetted mode (solid curves).
2157
is small, the membrane resistance to mass transfer increases significantly. Simulation results for partially wetted mode are plotted as solid curves in the same diagram. In the presence of DEA as absorbent, absorption is facilitated by chemical reaction. Fig. 5 shows the overall liquid phase mass transfer coefficient, Koverall ; as a function of liquid flow rate. In this case, mass transfer increases appreciably even for low DEA concentrations. By increasing DEA concentration in the liquid phase from 0 to 2 M, the mass transfer of CO2 increases because of an increase in the reaction rate. At low liquid flow rates, the solution becomes saturated with CO2, and the mass transfer rate decreases. At high liquid flow rates, the absorbent does not become saturated with CO2, and the gas absorption rate increases. Theoretical estimations assuming partially wetted pores are also plotted in Fig. 5 and seem to be in good agreement with the experimental results. The membrane mass transfer coefficient was calculated from Eq. (11), assuming that a fraction of membrane pores, x; was liquid filled. The adjustable values of x ranged from 1.5 to 7%, similar to those estimated for the CO2 – water system. The separation performance of the hollow fiber membrane contactor, expressed as percentage of CO2 removal, is given in Fig. 6 versus liquid flow rate for a constant gas flow rate of 100 N cm3/s. Compared to water, performance improves considerably, reaching up to 99% CO2 removal, by using aqueous DEA solutions as absorbent under the same experimental conditions. Theoretical predictions for partially wetted pores, plotted in the same diagram, are in good agreement with the experimental results. The adjustable values of x were similar to those used in Fig. 5. The use of a chemical aqueous solution instead of water enhances mass transfer of CO2 and, therefore, the scrubbing capacity of the liquid absorbent improves. Consequently, for an equivalent gas removal the required absorbent flow rate decreases and it can be achieved in a smaller contactor. Against these considerable advantages, the main disadvantage is the cost of chemical solvents. Since organic amines
Fig. 6. Removal of CO2 in various absorbents. Feed gas: 100 N cm3/s. Simulation: partially wetted mode (solid curves).
2158
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
can be regenerated, their use is generally more economic than other liquid absorbents, which are discarded after use. This indicates that DEA is suitable as an absorbent for CO2 removal in a membrane contacting process. However, comparison of theoretical and experimental results for all gas/liquid systems studied indicate that the membrane is operated somewhere between the limits of gas-filled and liquid-filled pores. The penetration of the liquid into the pores, as explained by the necking phenomenon, depends on the gas – liquid system and on the characteristics of the fiber used. In order to prevent this wetting, research efforts should focus on new membrane materials having a high degree of hydrophobicity [16,32], and/or on new liquid absorbents with improved surface tension properties [31]. In order to evaluate the membrane-based technology efficiency, the absorption performance of the hollow fiber membrane contactor needs to be compared with that of a conventional packed absorber. For this, the classical measure of absorption technology efficiency was used, i.e. the height of a transfer unit (HTU) required for CO2 absorption in an absorbent (here pure water and DEA). In Fig. 7, the required HTUs for physical and chemical absorption of CO2 under the same operating conditions are plotted as a function of superficial velocity for the case of the tested membrane contactor and for packed contactors
with Raschig rings of diameter dr ¼ 25 and 50 mm. The height of transfer unit based on liquid phase, HTUL, was plotted in Fig. 7(a) for the case of CO2 absorption in water, while the height of transfer unit based on gas phase, HTUG, demanded for a gas phase controlled process, was plotted in Fig. 7(b) for the case of CO2 absorption in DEA. The values of HTU for packed contactors were calculated with the use of empirical correlations [33]. The HTUs for membrane technology were based on the experimental results presented in this work. Under the same operating conditions, the experimentally obtained HTU values for the membrane contactor were significantly lower than those of a conventional packed contactor, for CO2 absorption both in water and in DEA. This leads to lower specific area requirements and, hence, to a reduction in absorber investment costs compared to packed towers. Thus, a significantly higher amount of CO2 removal can be achieved with a relatively small module and low flow velocities. Another benefit of using membrane technology instead of conventional absorption equipment is that membrane operation can be scaled-up linearly, so that an increase in capacity is achieved simply by adding membrane modules.
5. Conclusions Gas – liquid contactors based on hollow fiber membranes can be used to reduce acid gas emissions such as CO2 from flue gases. The present experimental results with a simulated flue gas mixture (CO2, N2) show that high levels of CO2 removal (, 99%) can be accomplished with chemical solution absorbents. Physical absorbents like water can also be used albeit not with such high levels of CO2 absorption. To simulate the behavior of gas –liquid membrane contactors, a mathematical model was developed. The model takes into account mass transfer at the gas, liquid and membrane interfaces, chemical reaction and possible membrane wetting. Comparison of model estimations with experimental results indicates partial wetting of the membrane, resulting to significant membrane resistance to mass transfer. Compared to conventional technologies, hollow fiber membrane modules offer high specific surface area and, therefore, the calculated HTU values for similar operating conditions are significantly smaller. This should lead to more compact, efficient and easily scaled-up absorption units.
Acknowledgements
Fig. 7. HTU for a membrane contactor and for packed towers for CO2 absorption in (a) water and (b) DEA.
The authors wish to thank the European Coal and Steel Community for support of this work. Maria Mavroudi wishes to thank the State Scholarship’s Foundation for economical support of her PhD research, which includes part of this work.
M. Mavroudi et al. / Fuel 82 (2003) 2153–2159
References [1] Freund P. Abatement and mitigation of carbon dioxide emissions from power generation. IEA GHG Programme. Available at: http://www. ieagreen.org.uk/; 1998. [2] Wuebbles DJ, Jain AK. Concerns about climate change and the role of fossil fuel use. Fuel Process Technol 2001;71:99–119. [3] Herzog H, Drake E, Tester J. A research needs assessment for the capture, utilization and disposal of carbon dioxide from fossil fuelfired power plants. MIT Report No. DE-FG02-92ER30194.A000, Cambridge, MA; 1993 [4] IEA Greenhouse Gas R&D Programme. Carbon dioxide capture from power stations. Final report, London, http://www.ieagreen.org.uk [5] Rao AB, Rubin ES. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. Environ Sci Technol 2002;36(20):4467–75. [6] Ho WWS, Sirkar KK. Membrane handbook. New York: Van Nostrand Reinhold; 1992. [7] Gabelman A, Hwang ST. Hollow fiber membrane contactors. J Membr Sci 1999;159:61–106. [8] Zhang Q, Cussler EL. Microporous hollow fibers for gas absorption: I. Mass transfer in the liquid. J Membr Sci 1985;23: 321– 32. [9] Zhang Q, Cussler EL. Microporous hollow fibers for gas absorption: I. Mass transfer across the membrane. J Membr Sci 1985;23:333–45. [10] Yang MC, Cussler EL. Designing hollow-fiber contactors. AIChE J 1986;32(7):1910–6. [11] Cooney DO, Jackson CC. Gas absorption in a hollow fiber device. Chem Engng Commun 1989;79:153–63. [12] Karoor S, Sirkar KK. Gas absorption studies in microporous hollow fiber membrane modules. Ind Engng Chem Res 1993;32: 674–84. [13] Kreulen H, Smolders CA, Versteeg GF, van Swaaij WPM. Microporous hollow fibre membrane modules as gas-liquid contactors; Part 1. Physical mass transfer processes. A specific application: mass transfer in highly viscous liquids. J Membr Sci 1993;78:197–216. [14] Kreulen H, Smolders CA, Versteeg GF, van Swaaij WPM. Microporous hollow fibre membrane modules as gas–liquid contactors; Part 2. Mass transfer with chemical reaction. J Membr Sci 1993;78:217–38. [15] Rangwala HA. Absorption of carbon dioxide into aqueous solutions using hollow fiber membrane contactors. J Membr Sci 1996;112: 229–40. [16] Nishikawa N, Ishibashi M, Ohta H, Akutsu N, Matsumoto H, Kamata T, Kitamura H. CO2 removal by hollow-fiber gas–liquid contactor. Energy Convers Mgmt 1995;36(6– 9):415–8.
2159
[17] Al-Saffar HB, Ozturk B, Hughes R. A comparison of porous and nonporous gas–liquid membrane contactors for gas separation. Trans IchemE 1997;75(A):685–92. [18] Chun MS, Lee KH. Analysis on a hydrophobic hollow-fiber membrane absorber and experimental observations of CO2 removal by enhanced absorption. Sep Sci Techol 1997;32(15):2445– 66. [19] Li K, Teo WK. Use of permeation and absorption methods for CO2 removal in hollow fibre membrane modules. Sep Purif Tech 1998;13: 79–88. [20] Feron PHM, Jansen AE, Klaasen R. Membrane technology in carbon dioxide removal. Energy Convers Mgmt 1992;33(5 –8):421–8. [21] Jansen AE, Klaasen R, Feron PHM, Hanemaaijer JH, ter Meulen BPh. Membrane gas absorption processes in environmental applications. In: Crespo JG, Bo¨ddecker KW, editors. Membrane processes in separation and purification. The Netherlands: Kluwer Academic; 1994. [22] Kim BS, Harriott P. Critical entry pressure for liquids in hydrophobic membranes. J Colloid Interface Sci 1987;115(1):1 –8. [23] Reid RC, Prausnitz JM, Sherwood TK. The properties of gases and liquids. New York: McGraw-Hill; 1977. [24] Scho¨ner P, Plucinski P, Nitsch W, Daiminger U. Mass transfer in the shell side of cross flow hollow fiber modules. Chem Engng Sci 1998; 53(13):2319 –26. [25] Versteeg GF, van Dijck LAJ, van Swaaij WPM. On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions. An overview. Chem Engng Commun 1996;144:113–58. [26] Versteeg GF, van Swaaij WPM. Solubility and diffusivity of acid gases (CO2, N2O) in aqueous alkanolamine solution. J Chem Engng Data 1988;33:29 –34. [27] Danckwerts PV. Gas– liquid reactions. New York: McGraw-Hill; 1970. [28] Ashour SS, Hanna OT. A new very simple explicit method for the integration of mildly stiff ordinary differential equations. Comput Chem Engng 1990;14(3):267–72. [29] Sengupta A, Peterson PA, Miller BD, Schneider J, Fulk Jr CW. Largescale application of membrane contactors for gas transfer from or to ultrapure water. Sep Purif Tech 1998;14:189 –200. [30] Mahmud H, Kumar A, Narbaitz RM, Matsuura T. A study of mass transfer in the membrane air-stripping process using microporous polypropylene hollow fibers. J Membr Sci 2000;179:29– 41. [31] Kumar PS, Hogendoorn JA, Feron PHM, Versteeg GF. New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors. Chem Engng Sci 2002;57:1639 –51. [32] Feron PHM, Jansen AE. CO2 separation with polyolefin membrane contactors and dedicated absorption liquids: performances and prospects. Sep Purif Tech 2002;27:231–42. [33] Perry RH, Green DW. Perry’s chemical engineers’ handbook. New York: McGraw-Hill; 1984.