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Langmuir 2002, 18, 7456-7461
Adsorption Equilibria and Kinetics of Propylene and Propane on Natural Erionite and on Erionite Exchanged with K+ and Ag+ Gelacio Aguilar-Armenta* and Marı´a Eugenia Patin˜o-Iglesias Centro de Investigacio´ n de la Facultad de Ciencias Quı´micas, Beneme´ rita Universidad Auto´ noma de Puebla. Boul. 14 Sur y Av. San Claudio, Ciudad Universitaria, C.P. 72570, Puebla, Pue., Me´ xico Received November 7, 2001. In Final Form: July 3, 2002 In this study, the adsorption equilibria and kinetics of pure propane (C3H8) and propylene (C3H6) on natural erionite (ERI), from Agua Prieta (Sonora, Mexico), and on cation-exchanged erionite samples (K-ERI and Ag-ERI) have been measured at different temperatures using a glass high-vacuum volumetric device. All of the samples showed selectivity toward propylene. The ion-exchange treatment influenced, remarkably, the C3H8 and C3H6 adsorption behaviors. The adsorbed amounts of both hydrocarbons on K-ERI and Ag-ERI samples were lower than those on ERI, a result that can be ascribed to a decrease in micropore volume. It was established that the adsorptive separation of these gases on K-ERI could be effected most efficiently at 111 °C for gas-adsorbent contact times in the range 20-100 s, whereas the Ag-ERI sample could be recommended for this separation for the temperature range 50-80 °C and for short gas-adsorbent contact times in the interval 10-30 s.
Introduction Light olefin/paraffin mixtures have been predominantly separated by cryogenic distillation that is both expensive and energy intensive. In general, the separation of a low relative volatility mixture, such as propane/propylene, by using distillation alone is a critical task. The enormous costs associated with these separations are the major motivation of continued research in alternative separation techniques for olefin purification. Selective adsorption is a potential alternative method, which is based on the specific interaction between the olefin and chemically modified adsorbents. However, adsorbents based on kinetic or steric effects1 for the separations of a single olefin from its corresponding paraffin could be used. Adsorption studies of pure hydrocarbons using microporous adsorbents, such as zeolites, are very important because the information about equilibrium isotherms and uptake rates of pure components on these adsorbents permits us to predict how they would behave in any given multicomponent system. The use of these microporous materials for separation of olefin/paraffin, such as propane and propylene, has the advantage of substituting cryogenic distillation for this gas mixture, which has been used for over 60 years. More than 30 years ago, Peterson et al.2 found that the selective adsorption of propylene on a type 5A zeolite appears to be a commercially practicable process for the recovery of propylene from mixtures with propane. It has been reported3 that 5A and 13X zeolites can be used to make the propylene/propane separation, so long as significant amounts of diluent nitrogen are used. Da Silva * To whom correspondence should be addressed. Telephone: (52 222) 2 295 500 ext. 7383. Fax: (52 222) 2 295 584. E-mail:
[email protected]. (1) Yang, R. T. In Gas Separation by Adsorption Processes; Imperial College Press and World Scientific Publishers: River Edge, NJ, 1997; Vol. 1, Chapter 1. (2) Peterson, D. L.; Helfferich, F.; Griep, R. K. Proc.-Soc. Chem. Ind., Chem. Eng. Group 1968, 217-230. (3) Ja¨rvelin, H.; Fair, J. R. Ind. Eng. Chem. Res. 1993, 32, 22012207.
and Rodrigues4 have measured the single-adsorption equilibrium isotherms and adsorption kinetics of propylene and propane over commercial 13X and 4A zeolites at temperatures between 373 and 473 K. It was established4 that the 13X zeolite shows a higher loading capacity and lower mass-transfer resistance, while the 4A zeolite shows the highest selectivity for propylene. Padin and Yang5 reported that new adsorbents for ethane/ethylene and propane/propylene separations were prepared by dispersing AgNO3 over γ-Al2O3, SiO2, and MCM-41 substrates. The authors5 also reported that the adsorption rates for these adsorbents were fast, with 100% completion achieved within 300 s, for which reason they state that these adsorbents are good candidates for use with pressure swing adsorption (PSA) processes. Padin et al.6 noted that the 4A zeolite with 95% Li+ cations (NaLiA) had the optimal characteristics for the kinetic separation of a propane/ propylene mixture. In the same work, they also reported that an aluminophosphate (AlPO4-14) is capable of sterically excluding propane. Grande and Rodrigues7 reported equilibrium and kinetic data of propane and propylene over narrow and wide pore silica gel samples, showing that the loadings of propylene are 1.5 times higher than those for propane with both adsorbents. More recently, Rege and Yang8 through computer simulation compared the performance of AgNO3/SiO2 and AlPO4-14 as sorbents for propane/propylene separation using a PSA process. These authors found that the π-complexation sorbent (AgNO3/SiO2) outperformed the molecular sieve (steric) sorbent (AlPO4-14), especially for a propylene-rich feed. The objective of this work is to measure the adsorption equilibrium isotherms of pure propane (C3H8) and propylene (C3H6) on natural erionite (ERI), from Agua Prieta (4) Da Silva, F. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 1999, 38, 2051-2057. (5) Padin, J.; Yang, R. T. Chem. Eng. Sci. 2000, 55, 2607-2616. (6) Padin, J.; Rege, S. U.; Yang, R. T.; Cheng, L. S. Chem. Eng. Sci. 2000, 55, 4525-4535. (7) Grande, C. A.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2001, 40, 1686-1693. (8) Rege, S. U.; Yang, R. T. Chem. Eng. Sci. 2002, 57, 1139-1149.
10.1021/la011656o CCC: $22.00 © 2002 American Chemical Society Published on Web 08/29/2002
Propylene/Propane on Natural/Cation-Exchanged Erionite
(Sonora, Mexico), and on cation-exchanged erionite samples (K-ERI and Ag-ERI) at different temperatures using a conventional high-vacuum volumetric apparatus. In addition to the study of the adsorption equilibria, the corresponding adsorption kinetics were also measured in order to evaluate the influence of cation exchange of natural erionite on the adsorption kinetics of these hydrocarbons. On the basis of the results obtained, we assess their possible use in the separation of propylene/ propane mixtures. To our knowledge, no work on adsorption of these hydrocarbons over any natural zeolites has been published.
Langmuir, Vol. 18, No. 20, 2002 7457 Table 1. Chemical Composition (atom %) of Untreated and Cation-Exchanged Erionite from Agua Prieta, Sonora (Mexico) samplea
Na
ERI K-ERI Ag-ERI
0.81 1.13 1.51 0.00 19.24 5.56 0.69 0.60 0.00 0.00 3.73 0.00 17.08 5.13 0.46 0.34 0.00 0.00 1.26 3.70 16.08 4.76 0.64 0.80
Ca
K
Ag
Si
Al
Mg
Fe
Si/Al 3.46 3.33 3.38
a ERI denotes the natural untreated erionite. The K-ERI and Ag-ERI samples were prepared by aqueous exchange of the ERI sample with KCl and AgNO3 solutions, respectively.
Experimental Section The gas adsorption equilibrium isotherms and kinetic uptake of gases were measured at different temperatures in a conventional high-vacuum volumetric device, totally made of Pyrex glass and equipped with grease-free valves. The high vacuum (
