Ind. Eng. Chem. Res. 1997, 36, 2749-2756
2749
SEPARATIONS Modification of Resin-Type Adsorbents for Ethane/Ethylene Separation Zhongbiao Wu,† Sang-Sup Han,‡ Soon-Haeng Cho,*,‡ Jong-Nam Kim,‡ Kuck-Tack Chue,‡ and Ralph T. Yang§ Korea Institute of Energy Research, 71-2, Jangdong, Yusongku, Taejon 305-343, Korea, Chemical Engineering Department, Zhejiang University, Hangzhou 310027, China, and Chemical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136
Modified adsorbents, Ag+-exchanged resins, have been prepared and studied for ethane/ethylene separation by adsorption. On Ag+-exchanged Amberlyst 35 (36.5% exchange) at 25 °C and 1 atm, the equilibrium adsorbed amount for C2H4 is 1.48 mmol/g, and the equilibrium adsorption ratio for C2H4/C2H6 is 6.4. The adsorption capacity is completely restored at 100-105 °C, although small residual amounts exist after desorption at 25 °C and 75 °C. For the adsorption encompassing both physical adsorption and π-complexation with energy heterogeneity, the equilibrium data are correlated with an equilibrium isotherm equation employing two fitting parameters. The fitted results agree well with the experimental data. Furthermore, the isosteric heats of adsorption and the diffusion time constants are calculated from experimental data. Considering all adsorption characteristics, these adsorbents show potential for application employing cyclic adsorption processes. Introduction Olefin/paraffin separations comprise a group of the most important separations in the petrochemical and chemical industry. However, currently there are a very limited number of promising alternatives for the process of distillation (Eldridge, 1993). Cryogenic distillation has been applied to these separations (Keller et al., 1992) although they are a class of costly operations (Wiley, 1992). For example, ethylene/ethane separation is performed at around -25 °C and 320 psig in a column containing over 100 trays because of the close relative volatilities, and the associated high reflux demands a high energy requirement. Consequently the olefin/ paraffin separations account for 6.1% of the total energy for all distillations (Humphrey et al., 1991). This large requirement provides the incentive for ongoing olefin/ paraffin separation technology research (Eldridge, 1993). Therefore, the adsorptive olefin/paraffin separation has been of interest for some years. However, it should also be noted that the commercially available adsorbents do not have significant selectivities for olefins (over corresponding paraffins), and the use of these adsorbents would require additional, substantial operations (Kulvaranon et al., 1990; Kumar et al., 1992; Jarvelin and Fair, 1993; Ghosh et al., 1993). The most promising adsorption technology approach appears to be that of complexation agents for olefin/ paraffin separations. Chemical complexation bonds are usually weak (4-15 kcal/mol) and are reversible, and they represent large opportunities for applications in difficult separations (King, 1987). The complexation * To whom correspondence should be addressed. E-mail:
[email protected]. † Zhejiang University. ‡ Korea Institute of Energy Research. § University of Michigan. S0888-5885(97)00185-1 CCC: $14.00
agents can be classified into three forms: solution, solid adsorbent, and membrane. The solid adsorbents are attractive in comparison with the other two because gas/ solid operations can be simpler as well as more efficient. There have been some reports on the use of solid adsorbents containing complexing agents. Crystalline CuCl has been considered for olefin/paraffin separations (Gilliland et al., 1941; Gilliland, 1945; Long, 1972). Some solid adsorbents also have been investigated for olefin/paraffin separation, such as Cu(I)-exchanged type-Y zeolites (Rosback, 1973; Yang, 1995), dehydrated porous silver- and copper-containing cation-exchange resins (Dielacher et al., 1976), polystyrene-supported aluminum silver chloride, AgAlCl4 (Hirai et al., 1985, Hirai, 1992), and CuCl-polystyrene resin containing amino groups (Hirai et al., 1986, Hirai, 1992). Recently, Yang and Kikkinides (1995) reported the most promising results among the previous attempts on the sorbents for olefin/paraffin separation by adsorption via π-complexation. They showed that on the Ag+ resin at 25 °C and 1 atm, C2H4 adsorption capacity was 1.15 mmol/g, and the equilibrium adsorption ratio for C2H4/C2H6 was 9.2. Cheng and Yang (1995) dispersed cuprous chloride on pillared interlayered clays (PILC). Among them, CuCl/TiO2-PILC adsorbent was the best which showed a C2H4/C2H6 adsorption ratio ) 5.3 and C3H6/C3H8 ) 2.9 at 1 atm and 25 °C. Although the olefin/paraffin adsorption ratios on the PILC sorbent were not as high as those on the Ag-resin, they yielded a steeper portion above the knee of the isotherm and more rapid adsorption rates. In this paper, three basic types of cation-exchange polymeric resins were selected as the substrates for adsorbent modifications, i.e., Amberlyst 15, Amberlyst 35, and DOWEX 88. The adsorbents were synthesized by exchanging the Ag+ cation on the above substrates, and then their adsorption characteristics for C2H4/C2H6 separation were investigated. © 1997 American Chemical Society
2750 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 Table 1. Physical Properties of Cation Exchange Resins substrate
acidic type
functional avg pore particle max. operating water surface particle capacity group diameter (Å) size (mm) temp (°C) content (%) area (m2/g) density (kg/m3) (mequiv/g of dry resin)
Amberlyst 35 (wet) very strongly SO3-H+ Amberlyst 15 (dry) strongly SO3-H+ DOWEX 88 strongly SO3-H+
300 250 -
0.3-1.2 0.2-1.2 0.4-1.2
Experimental Section Modification of Adsorbents. Table 1 shows the physical properties of three types of cation exchange resins. These resins are functionalized, macroreticular polystyrene cross-linked with some divinyl benzene. Amberlyst 35 and Amberlyst 15 were obtained from Rohm & Haas Co., and DOWEX 88 from Dow Chemical Co. The functional groups for cation exchange are SO3-H+ for Amberlyst 15 and Amberlyst 35, and SO3-Na+ for DOWEX 88 where H+ and Na+ can be exchanged by a more affinitive cation such as Ag+. The procedure for adsorbent modification included three stages as follows: (1) Pretreatments of the Substrates. The sample was washed successively with deionized water and methanol, followed by drying in air at 105 °C for 8-10 h. Prior to ion exchange, the sample was treated in water vaporizer for 15-30 min, followed by cooling at room temperature. Then the sample was ready for ion exchange. (2) Ion Exchange. For each exchange, 5 g of dry resin was added to 250 mL of aqueous solution of AgNO3 (0.014-0.028 N). The ion exchange was carried out at room temperature for 1.0-3.0 h. (3) Posttreatments of the Ag+-Exchanged Resins. After a number of exchanges, the sample was again washed by water and methanol successively. It was then dried in air at 105 °C for 8-10 h and weighed. A magnetic stirrer was used in all washings and ion exchanges. The degree of Ag+-ion exchange (DIE) was determined by the weight increase after ion exchange. The weight gain for complete ion exchange of H+ (or Na+) by Ag+ was equivalent to 5.2, 4.7, and 4.3 mequiv/g for Amberlyst 35, Amberlyst 15, and DOWEX 88, respectively. The accuracy for determining DIE by this procedure was within 0.1%. Since the DIE was measured by the net weight gain of the same sample under the same gas phase conditions, swelling was not a factor. Adsorbates. The hydrocarbons used as the adsorbates were the following: C2H4 with a minimum purity of 99.7% (from Air Products & Chemicals Co.); C2H6 with minimum purity 99.3% (from Air Products & Chemicals Co.). These gases were used without further purification. Helium with a minimum purity of 99.995% was used as the inert carrier gas and the gas environment for regeneration. Measurements of Adsorption/Desorption Rates and Equilibrium. A Cahn 1100 microbalance system, involving data acquisition with the interface control system, was used to measure gravimetrically the adsorption/desorption rates and equilibrium amounts. At the end of the adsorption experiment, a desorption experiment was carried out to check the reversibility of the adsorption isotherm. Furthermore, the experiments were performed at two temperatures (25 °C and 75 °C), from which the isosteric heats of adsorption were calculated.
140 120 150
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