Ind. Eng. Chem. Res. 1993,32, 2201-2207
2201
Adsorptive Separation of Propylene-Propane Mixtures H a r r i Jarvelint and J a m e s
R. Fair’
Separations Research Program, The University of Texas at Austin, Austin, Texas 78712
The separation of propylene-propane mixtures is of great commercial importance and is carried out by fractional distillation. It is claimed to be the most energy-intensive distillation practiced in the United States. The purpose of this paper is to describe experimental work that suggests a practical alternative to distillation for separating the C3 hydrocarbons: adsorption. As studied, the process involves three adsorptive steps: initial separation with molecular sieves with heavy dilution with an inert gas; separation of propylene and propane separately from the inert gas, using activated carbon; and drying of the product streams with any of several available desiccants. The research information presented here deals with the initial step and includes both equilibrium and kinetic data. Isotherms are provided for propylene and propane adsorbed on three zeolites, activated alumina, silica gel, and coconut-based activated carbon. Breakthrough data are provided for both adsorption and regeneration steps for the zeolites, which were found to be superior to the other adsorbents for breakthrough separations. A flow diagram for the complete proposed process is included. The separation of propylene-propane mixtures is one of the most important operations in the petrochemical industry. Such mixtures usually result from the thermal or catalytic cracking of hydrocarbons, and the majority of them represent coproducts with ethylene. Their separation is of great economic consequence, since the separated propylene has many uses, one of the most important being as monomer feedstock for polypropylene elastomer production. For most end uses the propylene must have a purity of at least 99.5 mol % . The propane fraction can be recycled to the cracking step or used separately, e.g., as liquefied petroleum gas (LPG) for household heating. The conventional method for separating the propylene propane mixture is fractional distillation. The relative volatility for the mixture is in the range of 1.09-1.15 (Laurance and Swift, 1972), depending on composition and pressure of operation. A large number of contacting stages are required (over 1001, and the associated high reflux ratio requires a large input of energy. The U.S. Department of Energy has reported that the propylene/ propane separation is the most energy-intensive single distillation practiced commercially (Wiley, 1992). While the distillation separation can be carried out at aboveambient temperatures, where water can be used as the coolant for the condenser, it is more economical to operate at subambient temperatures with a refrigerated overhead vapor, often utilizing heat pumping techniques. The ratio of propylene to propane in the mixture varies according to cracking conditions, but for research purposes the composition can be assumed to be equimolar. The separation operation is often called “C3 splitting”. The present paper describes an alternate approach to distillation for making the C3 separation: selective adsorption in the gas phase. Studies of the kinetics and equilibria for propane and propylene, singly and in admixture, have been carried out at The University of Texas at Austin and will be described. Although preliminary evaluations of the total adsorptive separation process have been conducted, the emphasis here will be on the research results of the adsorption steps. These results should enable others to make their own evaluations according to their specific cost conditions. In essence, in the proposed process the propylene/ propane mixture is diluted with nitrogen and separated + Neete
Oy,Porvoo, Finland.
using molecular sieves. Downstream adsorption steps separate the propylene and propane from nitrogen and remove water vapor introduced in the process. The work in our laboratories has been done with research grade chemicals, and no hydrocarbon diluents (e.g., ethane and Cq compounds) have been used. However, allowance for diluents is straightforward and will be discussed. A flow diagram of the envisioned process is shown in Figure 1. Previous Work The adsorptive separation of propylene-propane mixtures has been of interest for some years, but very little of a nature useful for design has been published. In most cases, individual component isotherm data and breakthrough kinetics have been measured, but little success with mixtures has been found. Lewis et al. (1950)included propylene and propane in a series of adsorption studies using silica gel and Columbia G activated carbon adsorbents. Their work was limited to isotherm measurements, with the indication that silica gel would be the more selective adsorbent if mixtures were to be used. In 1968 Peterson et al. proposed separating the Cs mixture with zeolite molecular sieves, finding the 5A type to give preferential adsorption of propylene. Their scheme was to use an eluent such as butane or pentane for regeneration, followed by recovery of propylene by distillation. Their experiments were made at temperatures of 105-175 “C and a pressure of 1450 kPa, with bulk mixtures of propane and propylene, under which conditions the polymerization of propylene on the zeolite was a problem. Their work emphasized the regeneration step; in fact, no breakthrough data for the initial propanepropylene adsorption were given. Smith and Burnet (1971) explored the individual component adsorptions on Columbia LC carbon, and reported both equilibrium and kinetic data. They also deduced effective mass-transfer coefficients for the adsorption step. Friederich (1970) measured mixture isotherms for propylene-propane, and his data were used by Brown et al. (1978)to model breakthrough results for each of the components. Costaet al. (1981) measured individual and mixture isotherms for propylene and propane as well as other binary mixtures on Witco L-1667 carbon, and modeled their results using a real adsorption theory; excellent results were obtained using Wilson and UNI-
0888-5885/93/2632-2201$04.00/00 1993 American Chemical Society
2202 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
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Figure 1. Block flow diagram for the proposed process to separate propylene-propane mixtures by three-stage adsorption.
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I
I
I
P BIA
Figure 2. Flow diagram of the experimental equipment for adsorption and desorption breakthrough measurements. Key symbole: AV = automatic valve;FIC = flow indicating controller;FRN = furnace;PI = pressure indicator;PIC = pressure indicating controller;TI = temperatutre indicator.
QUAC models for the ternary (including adsorbent) systems. Glanz and Findenegg (1984)studied equilibrium adsorption on graphitized carbon black and found only a weak preferential adsorption of propane from propylenepropane mixtures. Shu et al. (1990) studied a variable temperature stepwise desorption of propylenelpropane mixtures from 13X molecular sieves, showing that with careful control of temperature, and without the use of diluents, a relatively concentrated stream of propylene could be removed during a portion of the regeneration cycle.
More recently the technology and economics of the Ca separation have been discussed by Kumar et al. (1992). They emphasized the commercial importance of the separation and proposed a hybrid adsorptionldistillation scheme to provide high-purity streams of propane and propylene. Entering propanelpropylene would be given an adsorptive pretreatment to remove contaminants, and then the bulk mixture would be separated by an appropriate adsorbent. The total process scheme is "suggested", but may have some backup experimental information that has been kept confidential.
Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 2203 Table I. Properties of Adsorbents
molecular sieves type &aP+ particle size bulk density, kg/m8 nowce (1
4A
E 1/8 in. 690 Union Carbide
activated silica gel
5A E
E
13X
1/8 in.
1/8 in.
690 Union Carbide
620
Union Carbide
41 G 3/9me& 690 Davienn
activated alumina ST-4191-1
S 3.35mm I40 ALCOA
activated carbon ST-AWA-13 G 4/8 me& 430 Sorb-Tech
E = extrudates; G = granular; S = npherea.
Supporting the second step of the process herein proposed are the kinetics of the propanelnitrogen separation reported by Schork and Fair (1988) and Huang and Fair (1988);in these studies Witco JXC carbon was used. The present work includes the propylene1 nitrogen s e p aration over activated carbon. Removal of water vapor from propylene and propane by adsorption on activated alumina or molecular sieves has been treated widely in the journal and trade literature and does not merit further experimental verification. Guides to other literature adsorption equilibria for propane and propylene have been published by Ray (1983) and Valenzuela and Myers (1989). Experimental Work The propylenelpropane breakthrough measurements were made in a continuous-flow pilot system, a flow diagram of which is shown in Figure 2. The system can be operated up to 427 "C and 2169 kPa, is highly instrumented, and in operation is controlled by an Intel SYP-310computer with extensive data logging capability. Thiscomputer accepts allof the processsignals,maintains a digital log of all variables, and updates the signals every 10 s. The inlet gas mixture is handled by blending valves and checked by mass flowmeters and chromatographic analyses. Samples can be taken at a number of places, and for the present work the samples were analyzed by a Varian 3700 gas chromatograph with a flame ionization detector (FID). No attempt was made to recycle the effluent gas. The heart of the svstem is the adsorntion bed. shown in Figure 3. It has & inside diameter i f 84.7 mm and a bed length of 1219 mm. Intermediate bed thermocouples are provided, for following temperature gradients. The bed can be run essentially adiabatically through the use of insulation and feedback shell heaters. The adsorbent is supported on a perforated plate as shown in the figure. All metal parta of the system are fabricated from type 316 stainless steel. Breakthrough runs were made in the conventional manner. Inlet concentrations of hydrocarbons were in the range of 1.C-3.5 mol %, with nitrogen as the carrier gas. For most cases, breakthrough was continued to full bed saturation, as will be presented later in appropriate plots. Regeneration runs were made with hot nitrogen at 150-200 "C, with adsorbate removal being monitored by chromatographic analyses. Six adsorbenta were included in the study: types 4A, 5A,and 13X zeolite molecular sieves; silica gel; type ST4191-1 activated alumina; and Sorb-Tech AWX-13 activated carbon. Sources and essential properties of these materials are shown in Table I. The adsorbates were all research grade in purity. Nitrogen, propane, and propylene were obtained from Union Carbide Corporation. For the propane, the sum of impurities was less than 1.0%by volume; these impurities were stated by the manufacturer to be ethane, propylene, isobutane, and n-butane. Propylene was also CP grade
D E T U 'I'
Figure 3. Details of the adsorber. hPssure nansdueer-,
and pump
Cold
w
Figure 4. Equilibrium call for isotherm measurements.
with the same impurities. One analysis of the propylene showed the following: propane, 5580 ppmw; ethane, 97 ppmw; isobutane,
