Chapter 19 Separation of Ethylene from Ethane Using Perfluorosulfonic Acid Ion-Exchange Membranes
Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch019
1
Anawat Sungpet, Paul M.Thoen ,J. Douglas Way, and John R. Dorgan Chemical Engineering and Petroleum Refining Department, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401-1887 +
Facilitated transport of ethylene through Ag -containing perfluorosulfonic acid ion-exchange membranes results in high separation factors for ethylene over ethane. Ethylene of higher than 99 percent purity was obtainedfroma 50:50 mixture of ethylene and ethane at 25 °C and feed and permeate pressures of 1 atmosphere. An ethylene permeability of over 2000 Barrer was obtained. Two different types of perfluorosulfonic acid membranes were studied to determine the importance of ionic site density and water content on membrane performance. The effect of temperature on the transport mechanism was also studied over the range of 5-35 °C. In addition, the transport data obtained at high pressure show carrier-saturation and membrane compaction phenomena. Separation processes in petroleum and petrochemical industries are highly energy intensive. Among these processes, olefin separations consume the most energy, 0.12 Quad/year (1 Quad = a million billion BTU) (7). Ethylene purification alone required 9 trillion BTU in 1993 (2). Consequently, there is an economic incentive to develop alternative separation processes with lower energy consumption. Membrane technology is a promising process for olefin/paraffin separation. It could be used in a hybrid process with existing cryogenic distillation processes or possibly replace distillation processes altogether to reduce energy consumption. Despite commercial applications in hydrogen separation, acid gas removal, and production of nitrogen (3), polymer membranes have not been applied to the separation of hydrocarbon mixtures, such as olefinsfromparaffins. The primary problems are lack of selectivity and low permeabilities. Facilitated transport membranes are capable of providing very high separation factors while achieving reasonablefluxes(4). Unfortunately, this kind of 1
Current Address: Golden Technologies Company, Inc. 4545 McIntyre Street, Golden, CO 80403 0097-6156/96/0642-0270$15.00/0 © 1996 American Chemical Society
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
19. SUNGPET ET AL.
Separation of Ethylene from Ethane
271
membrane has problems with deactivation of the carrier and loss of the solvent when used for gas separation. These two major problems must be solved before facilitated transport membrane processes can be applied to industrial separations. The separation of ethylenefromethane by facilitated transport membranes containing Ag has been studied by many investigators. Hughes et al. (5,6) were the first to demonstrate facilitated transport of olefins by immobilized Ag solutions in porous supports. LeBlanc et al. (7) used sulfonated polyphenylene oxide membranes in the Ag form and obtained a high separation factor for ethylene over ethane. Teramoto et al. (8) used an aqueous silver nitrate solution supported by a cellulosefilterfor the separation of ethylenefromethane. Kraus (9) described a water-free Nafion 415 (DuPont) treated with silver nitrate solution and glycerol. Kawakami et al. (JO) studied the permeation of ethylene and propylene thorough supported liquid membranes of Rh -poly(ethylene glycol) in glass microfiberfilters.Teramoto et al. (77) proposed aflowingliquid membrane in which silver nitrate was used as a carrier for ethylene. Kanno et al. (12) used Nafion 417 (DuPont), equivalent weight 1100, incorporated with Ag . Davis et al. (75) developed a facilitated transport-distillation hybrid system for olefin separations. Yurkovetsky et al. (14) investigated the permeation of ethylene and ethane through Ag exchanged sulfonated polysulfones and sulfonated poly(phenylene oxide) ion-exchange membranes. Hinich et al. (75) discussed the transient increase of ethylene-ethane selectivity of poly(phenylene oxides) membranes. Eriksen et al. (16) studied facilitated transport of ethylene through Ag form Nafion 117 as a function of the ethylene partial pressure up to 760 mm Hg. at 25 °C. The same authors (77) also studied glycerine-treated, water-swollen Ag -Nafion 117 for ethylene/ethane separation. Tsou et al. (18) developed a liquid membrane contactor system using aqueous AgN(>3 solution for ethylene/ethane separation. Finally, Richter et al. (79) described facilitated transport of ethylene across polyelectrolyte gels loaded with Ag . In this present work, DuPont's Nafion 117 polymer and the experimental perfluorosulfonic acid (PFSA) membrane developed by the Dow Chemical Company (Product XUS-13204.10) were investigated for the separation of ethylene from ethane. The differences in chemical structures between the membranes strongly affects the membrane properties. The Dow membrane has a shorter repeat unit compared with Nafion 117 (Figure 1). Consequently, the Dow membrane has a higher ionic site density when dry. In addition, the Dow membrane is a more amorphous polymer because of the shorter repeat unit while Nafion 117 is more crystalline (20). This difference in the morphologies of these membranes leads to some very interesting variations in properties, such as water content and mechanical strength. The effect of water content on the facilitated transport of ethylene through the membranes was studied. In addition, the effect of run conditions, such as temperature and pressure, on the membrane performance have been evaluated. +
+
Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch019
+
3+
+
+
+
+
+
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
272
CHEMICAL SEPARATIONS WITH LIQUID MEMBRANES
-[(CF CF ) (CF ÇF)] -
-[(CT CT ) (CF ÇF)] 2
2
n
2
2
x
I
n
2
x
OCF CF S0 H
OCF-CFCF, 2
2
2
2
3
OCF CF S0 H 2
2
3
Dow Membrane Structure, n = 5-6
Nafion 117 Structure, n = 6-7
Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch019
Figure 1. Nafion 117 and Dow Membrane Structures
Experimental. Materials. Nafion 117 membrane with an equivalent weight of 1100 (Nafion 117) was purchasedfromC.G. Processing, Inc. (Rockland, DE). The membrane thickness is 170 μπι when dry and 210 μπι when fully hydrated. The experimental Dow membrane, with an equivalent weight of 803, was kindly provided by the Dow Chemical Company (Midland, MI). It is 95 microns thick when dry and 116 microns when hydrated. Silver nitrate, 99+%, A.C.S. reagent and sodium hydroxide, 97+ %, A.C.S. reagent were used. Purified and deionized water was used in all experiments. The ethylene and ethane purities were 99.5% and 99.0%, respectively. Helium of 99.999% purity was used as theflowsystem sweep gas and the carrier gas for gas chromatography. The compressed gases were used without further purification. Membrane Preparation. The as-received membranes were cleaned by boiling at 113 °C in concentrated nitric acid for 2 hours. After slowly cooling to room temperature, then membrane wasrinsedthoroughly with water and then exchanged into the Na form by boiling at 95 °C in 1M NaOH for 1 hour. The membrane was allowed to cool to room temperature and then rinsed thoroughly with water. To obtain the Ag form membrane, a Na -form membrane was immersed in a 1M AgN03 solution for 12 hours and subsequentlyrinsedwith water. The hydration temperature of the membranes prepared by this procedure is 113 °C. To study the effects of hydration temperature, some of the membranes were prepared by cleaning and ion exchange at room temperature. The hydration temperature of the membranes prepared by this procedure was 22 °C. +
+
+
Transport Measurements. The permeability test apparatus and membrane cell are similar to those used by Way et al. (27). Pure ethylene and ethane were mixed in a gas flow system to obtain the desired feed composition. Helium was used as the sweep gas and both the feed and sweep gases were saturated with water before entering the membrane cell. The gas flow configuration in the membrane cell was crossflow.The humidifiers and membrane cell were placed in a water bath for temperature control. The temperature was kept at 25 °C, except for the temperature effect study. Permeate and retentatefromthe membrane cell were
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
19.
SUNGPETETAL.
Separation of Ethylene from Ethane
273
dried by passage over anhydrous calcium sulfate before being vented or injected into the gas chromatograph. The feed and sweep gas pressures were controlled by back pressure regulators.
Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch019
Results and Discussion. Water Content and Hydration Temperature Effects. As mentioned earlier, the Dow membrane is more amorphous than Nafion 117. This allows the Dow membrane polymer matrix to adsorb more water than Nafion 117. The temperature at which the membrane is hydrated also influences the swelling of the ionomer matrix. The highest temperature used during the membrane preparation procedure controls the water content of the membrane. The water content of the membrane was calculated by dividing the weight of water absorbed by the total weight of the hydrated membrane. The water content of a membrane is primarily controlled by the inherent structure of the ionic polymer (Table I).
Hydration Temperature i(°C) 22 113
+
Ag Form Nafion 117 12.9 19.6
+
Ag Form Dow 28.3 34.9
+
Na Form Nafion 117 15.1 23.5
+
Na Form Dow 36.3 44.7
Both the water content and swelling of the polymer matrix have a large effect on the local ionic concentration, i.e. moles of Ag per volume of water (Table Π). The local Ag concentrations in the polar ionic cluster regions of the PFSA membranes were calculated by assuming complete Ag exchange. The molar ion-exchange capacity and the volume of water in the hydrated membrane were determined and the local Ag concentration was calculated. +
+
+
+
Table Π. Local Ag* Concentration (M) Hydration Temperature ( °C) Nafion 117 Dow Membrane 22 7.89 4.14 113 4.45 3.83
+
Despite a lower local Ag concentration, the Dow membrane showed a higher ethylene permeability (Figure 2). The ethane permeability of the Dow membrane, 13.1 ± 0.5 Barrer, was also higher than that of Nafion 117, 5.8 ± 0.3 Barrer.
In Chemical Separations with Liquid Membranes; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
274
CHEMICAL SEPARATIONS WITH LIQUID MEMBRANES
8000 ADow,T =113 °C A Dow, T = 22 °C ONafîon,T =113 °C 6000 Η • Nafion, T = 22 °C H
H
H
β
1 Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0642.ch019
