2390
Ind. Eng. Chem. Res. 1993,32, 2390-2399
Hybrid Adsorption-Distillation Process for Separating Propane and Propylene Tushar K. Ghosh, Hon-Da Lin, and Anthony L. Hines**t College of Engineering, University of Missouri-Columbia, Columbia, Missouri 65211
The separation of propylene from a propane-propylene mixture by distillation is a energy-intensive process. A hybrid adsorption-distillation system has a great potential in reducing the energy consumption. A significant amount of energy can be saved relative to a process using only distillation, if a typical separation is carried out by distillation up to a propylene concentration of approximately 80% and then continuing the separation of propane from propylene by adsorption. A volumetric adsorption apparatus was designed to obtain the data a t high pressures. The pure component data of propane and propylene were obtained on silica gel, molecular sieve 13X, and activated carbon. Although activated carbon has a greater capacity for both propane and propylene than either of the two adsorbenta, it was only slightly selective for propylene. Silica gel has the greatest selectivity for propylene, which ranged from 2 to 4. None of the adsorbenta was found to be selective for propane. The propane-propylene mixture behaved nonideally on the solid surface as indicated by the negative deviations of activity coefficients. The nonideality of the mixture can be attributed primarily to surface effects rather than to interactions between adsorbate molecules. A binary model has been proposed to predict mole fractions in the adsorbed phase and the total amount adsorbed from the pure component data. The pure component isotherm model of Hines et al. was extended to binary mixtures when the binary model was developed. Excellent agreement was obtained between experimental data and predicted values for mole fractions in the adsorbed phase, the total amount adsorbed, and adsorbed-phase activity coefficients. Introduction Propylene is produced in large quantities in the petrochemical industry for use as a chemical intermediate. It can be produced by the catalytic dehydrogenation of propane, which results in a mixture that contains approximately 45 7% propylene and 557% propane. Currently, the two gases are separated by distillation, which is very energy intensive because of the small difference in their relative volatilities. A distillation column used to separate propane and propylene typically contains more than 100 theoretical stages and can be up to 300 f t tall. The separation of propylene from propane by distillation consumes about 50 million BTU per year (Barron et al., 1987) and is considered as one of the largest consumers of energy in the petrochemicalindustry. Since the demand for propylene is expected to grow from 3.1 5% to 3.6 7% a year during the 19909,new separation strategies that have the potential to reduce energy consumptionare attracting more attention. One of the more promising hybrid processes combines adsorption with distillation. The design of such a process requires a good understanding of the adsorption characteristics of propane and propylene as pure components and as a mixture. Since distillation is typically carried out at pressures greater than 175 psia, it is advantageous if the adsorption portion of the hybrid process is also carried out at the same pressure. However, only limited equilibrium adsorption data at high pressures for either pure propane or propylene or their mixtures are available in the literature. In fact, there is very little adsorption data available in the literature that addresses the separation of propane and propylene at any pressure. In 1950, Lewis et al. studied the binary adsorption of propane and propylene on an activated carbon and on
* To whom correspondence should be addressed. t
Current address: Honda of America Mfg. Inc., Marysville,
OH 43040.
silica gel at a constant pressure (atmospheric). They also obtained adsorption isotherms of pure propane and pure propylene on the same two adsorbents from very low pressure to atmospheric conditions. From the pure component studies, Lewis et al. found that the adsorption capacity of propane and propylene was considerablyhigher on activated carbon than on silica gel. Their binary studies showed that the selectivity of propylene on activated carbon was only slightly greater than 1.0 (less than 1.1). For silica gel the selectivity (separationfactor) of propylene varied from about 2 at low propylene concentrations up to 4 at higher concentrations. To compare the feasibility of replacing distillation by adsorption, one must keep in mind that the relative volatility (separation factor) of propylene and propane is only about 1.1,which necessitates the use of great deal of energy to separate the two by distillation. Obviously, adsorption is superior in this respect. Althoughthe activated carbon used in their study does not appear promising as an adsorbent for separating propane and propylene, the high selectivity of silica gel offers a significant improvement over distillation. Friederich and Mullins (1972)obtained pure component adsorption data for propane and propylene on carbon black at 298 K from low pressure up to 700 Torr and binary data at 298 K and 700 Torr. They found the selectivity of propane on carbon black to be only about 1.1.This is in contrast to Lewis et al.’s data, which showed the selectivity of propylene to be slightly greater than 1. The pressure at which Frederich and Mullins took their data is also too low to be of significant use in the design of a hybrid process. However, their work does demonstrate that certain types of activated carbon can be used to adsorb propane from propylene. Kulvaranon (1988) studied the binary adsorption and desorption of propane and propylene on Linde 5A and 13X molecular sieves. He found a propylene selectivity greater than 5 on 13X. From an economic analysis in which adsorption and distillation were compared, Kulvaranon concluded that adsorption was a feasible alter-
0888-5885/93/2632-239Q$04.00/0@ 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2391 Table I. Prowrtiee of Solid Abrrorbente
effectivelyin many chemical processes, but the separation of propane and propylene by combining adsorption with distillation has received little attention. Such a hybrid process may be particularly attractive if a high-purity product in either the top or bottom section of a separation column is desired. The minimum number of theoretical trays in a distillation column increases rapidly if the propylene concentration in the top of the column is 80 % or greater. Thus, carrying out a typical separation by distillation up to a propylene concentration of approximately 80%,and then continuing the separation of propane from propylene by adsorption, can conceptually save a great deal of energy relative to a process using only distillation. An adsorber also can be used to treat the stream at the bottom of the distillation column to recover much of the propylene present in the propane-rich product. Adsorption is a very selective separation process and is generally used to remove a small quantity of an adsorbate from a fluid stream. As a result, data are usually obtained at very low partial pressures and low system pressures. In the case of propane and propylene, mixture data are required at high concentrations for each component and at high pressures. The aim of the present work is to evaluate three types of commerciallyavailable adsorbents and determine their capacity and selectivity for propane and propylene. These adsorbents were silica gel, molecular sieve 13X, and activated carbon. The adsorbent that had the best selectivity for either component was used to obtain the binary adsorption data. A volumetric adsorption apparatus was designed to obtain the data at pressures above atmospheric. The low pressure data were obtained gravimetrically by using an electrobalance housed in an all glass apparatus. A binary model has been proposed to predict mole fractions in the adsorbed phase and the total amount adsorbed from the pure component data. The
BPL molecular activated silica gel sieve carbon (Grade401 13X 6 X 16 mesh 6 X 12 mesh 8 X 12 mesh
property
particle size" surface area) S (mVg) micropores 823 meso- and macropores 50 total 874 pore volume,b V (cmg/g) microporee 0.47 meso- and macropores 0.10 total 0.57 particle density (g/cmg) 0.80 average pore dim,' 26 4 v i s (A) bulk density (g/cma) 0.60 equilib water capacity (wt%)
moisture content as shipped0 ( w t % )
663 9 672
294 101 395
0.38 0.02 0.40 1.13 24
0.14 0.27 0.41 1.13 41.7'
0.72
0.72 29.5
--
0
g 0.2 0.4 0.6 0.8 Mole Fraction of Propane in Adsorbed Phase, x
3.3
-
-
Gbab and Hincs Metbod
1
Figure 12. Comparisonof binary equilibriumdata and totalamount adsorbed for propane-propylene mixtures on silica gel at 45 psia.
calculated from the Wilson-Redlich-Kwong equations. The input parameters used in the simulation were similar to those of Gottschlich and Roberts (1990). The feed was assumed to be a saturated liquid at 250 psia and contained 54% propane and 46% propylene. The pressure drop across the tower was assumed to be 20 psia. The top and bottom products left the column as saturated liquids. A value for the recovery of propylene was assumed, and the number of equilibrium stages and the condenser and reboiler duties were calculated. A reflux ratio of 1.2 times of the minimum was used in the column. Since adsorbers can be used in both the bottom and top of the column, it was not necessary to have a high level of recovery of propylene in the top stream. This reduced the number of equilibrium stages significantly. The number of equilibrium stages and the condenser and reboiler duties at various recovery levels for propylene Table V. Energy Consumption during Separation of Propane and Propylene by Distillation. mole fraction of propylene case no. recovery ( % ) bP bottom no. of theor trays condenser duty (BTU/h) reboiler duty (BTU/h) 0.9950 0.0251 121 -4 907 430 4 911 040 1 97.0 0.0372 2 95.66 0.9500 85 -4 642 220 4 645 290 3 95.00 0.8001 0.0507 59 -3 788 600 3 789 340 4 94.99 0.9OOO 0.0448 72 -4 367 070 4 369 460 6 90.00 0.8OoO 0.0954 50 -3 584 970 3 586 120 6 90.00 0.9OOO 0.0852 62 -4 132 840 4 135 550 7 90.00 0.9499 0.0816 73 -4 361 940 4 365 330 a Feed composition: propane, 54%; propylene, 46%. Feed rate: 100 mol/h. Feed conditions: saturated liquid at 250 psia.
2398 Ind. Eng. Chem. Res., Vol. 32, No. 10,1993
having high selectivities for both propane and propylene would be needed. Nomenclature A = surface area AI,Az, A3 = parameters of the pure component isotherm n = amount adsorbed on the adsorbent no = amount adsorbed at pressure P P = system pressure P = equilibrium pressure as defined in eq 1
-1
/
q = adsorption capacity at saturation R = gas constant T = system temperature x = mole fraction in the adsorbed phase y = mole fraction in the gas phase
Experimental Data
-
Chosh and H i m Method
0.0 0.2
0
0.6
0.4
1
0.8
Mole Fraction of Propane in Adsorbed Phase, x
Greek Letters
activity coefficient = spreading pressure = fugacity coefficients
y = ?r
cp
4.5
5
Subscripts i = component i m = refers to mixture
4.3
1 9
Superscripts s = standard state
iij
ca
-Y
4.1
C C
Literature Cited Arnold, J. R. Adsorption of Gas Mixtures, Nitrogen-Oxygen on
3.9
9,
3
Experimental Data Ghosb and Hines Method
3.5
0
0.2
0.6
0.4
0.8
1
Mole Fraction of Propane in Adsorbed Phase, x
Figure 14. Compariaon of binary equilibrium data and total amount adsorbed for propane-propylene mixtures on silica gel at 105 psia.
h
t
0.8
t
i T 298 K
t
1
0.6
0.2
.
m
1
0.0'
0
Propme Plop~,~nr
- Propane - Prop,lmr "
"
0.2
"
'
"
0.4
} }
Experimentla Data Ghosh and Hines Model
'
"
0.6
'
"
0.8
" 1
Mole Fraction of Propane in Adsorbed Phase, x
Figure18. Comparison of activitycoefficientsfor propane-propylene mixtures on silica gel at 15 psia.
system, most of the energy used is consumed when the adsorbent is regenerated for repeated use. The amount depends on the adsorption-regeneration scheme. If a pressure swing adsorption-desorption scheme is chosen for the present system, the energy requirement will be even less since the feed to the adsorber is already a t a high pressure. Although a hybrid adsorption-distillation process has the potential for reducing energy consumption, either an innovative adsorption/desorption process would be required to produce high-purity propylene or adsorbents
Anatase. J. Am. Chem. SOC.1949,71,104. Barron, T. S.; Heist, J. A,; Hunt, K. M.; Wrobel, P. J. 'Industrial Applicationsof Freeze Technology";Final report to Electric Power Research Institute; Report No. EM-5232: 1987. Cook, W. H.; Basmadjian, D. The Prediction of Binary Adsorption Equilibrium From Pure Component Isotherm. Can.J. Chem.Eng. 1965,43, 78. Costa, E.; Sotelo, J. L.; Calleja, G.; Marron, C. Adsorption of Binary and Ternary Hydrocarbon Gas Mixtures on Activated Carbon: Experimental Determination and Theoretical Prediction of the Ternary Equilibrium Data. MChE J. 1981,27(l),5. Friederich, R. 0.; Mullins, J. C. Adsorption Equilibriums of Binary Hydrocarbon Mixtures on Homogeneous Carbon Black at 25 OC. Znd. Eng. Chem. Fundam. 1972,ll (4), 439. Ghosh, T. K.; Hines, A. L. A New Approach for Predicting Binary Equilibrium Data from Pure Component Isotherms. Proceedings Second China-Japan- USA Conference on Fundamentals of Adsorption, Hangzhou, China, May 12-15; 1991. Gottschlich,D. E.;Roberta, D. L. 'Energy Minimization of Separation Process Using Conventional/Membrane Hybrid Systems.Topical Report for Case 1: Separation of Propylene and Propane;" SRI Project No. 6519, 1990. Hasean, N. M.; Ghosh, T. K.; Hines, A. L.; Loyalka, S. K. Water Vapor Adsorptionon Molecular Sieve-13X. Chem. Eng. Commun. 1991,105, 1. Haydel, J. J.;Kobayashi, R. Adsorption Equilibria in the MethanePropane-Silica Gel System a t High Pressures. Znd. Eng. Chem. Fundam. 1967,6 (4), 646. Hines, A. L.; Kuo, S. L.; Dural, N. A New Isotherm for Adsorption on Heterogeneous Adsorbenta. Sep. Sci. Technol. 1990,25 (7), 869. Jaroniec, M. Adsorption of Gas Mixtures on Heterogeneous Solid Surfaces. Analytical Solution of Integral Equation for Jovanovic Adsorption Isotherm. J. Colloid Interface Sci. 1978,53(3), 422. Jaroniec, M. Adsorption of Gas Mixtures on Heterogeneous Solid Surfaces. 11. Adsorption Isotherms for Gaseous Mixtures Whose Pure Component Isotherma Show the Freundlich, Toth, and Langmuir Behaviors. Colloid Polym. Sci. 1977,255,32. Jaroniec, M. Description of Kinetics and Equilibrium State of Adsorptionfrom MulticomponentGas Mixtures on Solid Surfaces. Thin Solid Films 1980, 71, 273. Jaroniec, M.; Toth, J. Adsorption of Gas Mixtures on Heterogeneous Solid Surfaces: I. Extension of Toth Isotherm on Adsorptionfrom Gas Mixtures. Colloid Polym. Sci. 1976,254,643.
Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 2399 Jaroniec, M.; Toth, J. Adsorption of Gas Mixtures on Heterogeneous Solid Surfaces. III. Extension of Toth Isotherm on Multilayer Adsorption of Gas Mixtures. Colloid Polym. Sci. 1978,256,690. Kulvaranon, S.An Experimental Study of Adsorption and Variable Temperature Desorption of Ethanol-Water, Propane-Propylene, and Hydrogen Sulfide-Carbon Dioxide-Propane. M.S. Thesis, University of Missouri-Rolla, 1988. Kulvaranon, S.;Findley, M. E.; Liapis, A. I. Increased Separation by Variable-Temperature Stepwise Desorption in Multicomponent Adsorption Processes. Znd. Eng. Chem. Res. 1990,29,106. Lewis,W. K.; Gilliland, C. R.;Chertow,B.; Cadogan,W. P. Adsorption Equilibria of Hydrocarbon Mixtures. Znd. Eng. Chem. 1950a,42, 1319. Lewis, W. K.; Gilliland, C. R.;Chertow, B.; Hoffman, W. H. VaporAdsorbate Equilibrium I. PropanaPropylene on Activated Carbon and on Silica Gel. Znd. Eng. Chem. 1950b,42,1153.
Myers, A. L.; Prausnitz, J. M. Thermodynamic of Mixed-Gas Adsorption. AZChE J. 1965,11 (l),121. Paludetto, R.;Storti, G.; Gamba, G.; Carra, S.; M. Morbidelli, M. On Multicomponent Adsorption Equilibria of Xylene Mixtures on Zeolite. Znd. Eng. Chem. Res. 1987,26,2250. Ruthven, D. M.Principles of Adsorption and Adsorption Processes; Wiley: New York, 1974. Soave, G. Equilibrium Constanta from a Modified Redlich-Kwong Equation of State. Chem. Eng. Sci. 1972,27,1197. Yang, R.T. Gas Separation by Adsorption Processes; Butterworth Stoneham, MA, 1987.
Received for review January 21, 1993 Revised munuscript received August 3, 1993 Accepted August 5, 1993.
Lewis,W.K.;Gilliland,C.R.;Chertow,B.;Cadogan,W.P.Adaorption Equilibria Pure Gas Isotherm. Znd. Eng. Chem. 195Oc,42,1326. Lin, H.-D. Adsorption of Propylene and Propane on Various Adsorbenta a t High Pressures. M.S. Thesis, University of Missouri-Columbia, 1993.
* Abstract published in Advance ACS Abstracts, September 15, 1993.
