Ind. Eng. Chem. Res. 2005, 44, 1557-1564
1557
Four-Bed Vacuum Pressure Swing Adsorption Process for Propylene/Propane Separation Masoud Mofarahi* Chemical Engineering Department, Persian Gulf University, Bushehr, Iran
Mojtaba Sadrameli and Jafar Towfighi Chemical Engineering Department, Tarbiat Modarres University, P.O. Box 14115-111, Tehran, Iran
A vacuum pressure swing adsorption (VPSA) using zeolite 5A was studied experimentally and theoretically. An equimolar mixture of propylene/propane was used as a feed gas for an eightstep, four-bed VPSA process. Isotherm data of zeolite 5A at various temperatures and breakthrough curves of propylene and propane for both adsorption and regeneration steps were obtained to acquire the input parameters for the test runs with the pilot plant. Experimental measurements by the pilot plant have been obtained between a maximum pressure of PH ) 5.8-6.4 bar and a minimum pressure of PL ) 0.1-0.05 bar at a constant temperature of 70 °C. The effects of feed rate and dilution with H2, purge rate, and vacuum pressure on the product purity and recovery were also investigated via experimentation. Using a feed rate of 320 NL/h of a mixture of 50% propylene/propane at 70 °C, a purity of 92% propylene and a recovery of 29% were obtained. Comparison of the results from the experiments and the simulation gave a reasonable agreement. 1. Introduction Propylene/propane gas separation is an important issue in the petrochemical industry, because it is one of the most demanding energetic separation processes. The conventional method for propylene/propane separation is fractional distillation, which is an extensive, energy-consuming process. The relative volatility of this system is 1.0-1.1 at temperatures in the range of 244327 K and a total pressure of 1.7-22 bar. Consequently, if traditional distillation is used, more than 100 theoretical plates are needed when polymer-grade propylene is required as a product. This makes the propylene/ propane separation one of the most energy-expensive separations in the petrochemical industry. The adsorption process seems to be the most attractive alternative, because of the maturity of technology, the available adsorbents, and the low-cost, low-energy (but highly efficient) gas separation system. Hybrid methods combining traditional distillation and adsorption process have been proposed as economical alternatives. A propylene/propane gas mixture usually results from the thermal or catalytic cracking of hydrocarbons.1 The ratio of propylene to propane in the mixture varies according to cracking conditions; however, for research purposes, the composition can be assumed to be equimolar. Recent efforts to find and characterize the available sorbents to perform propylene/propane separation via vacuum pressure swing adsorption (VPSA) have appeared in the literature.2-9 Generally, the available sorbents have a higher selectivity for propylene. The pressure swing adsorption operation is then constrained to produce propylene during the desorption step. According to the highly favorable character of the propylene adsorption isotherm, it seems to be sensible to * To whom correspondence should be addressed. Fax: +98 771 4545188. E-mail:
[email protected].
operate the process at elevated temperatures and to reduce the pressure in the regeneration step to vacuum conditions. Da Silva and Rodrigues10 proposed a fivestep VPSA with the following steps: (i) pressurization with feed flow from 0.1 bar to 5 bar with a total volumetric flow of 120 NL/h (25% propylene, 25% propane, and 50% nitrogen) at 150 °C; (ii) high-pressure adsorption; (iii) medium-pressure purge with product; (iv) blowdown; and (v) evacuation. Using 4A zeolite pellets as an sorbent and a propylene-enriched stream of 98 mol % (relative to the propylene/propane mixture), with 4.4% of nitrogen, a recovery of 19% was obtained. Zeolite 5A is commonly used for the separation of iso-/ normal paraffins by size exclusion. The size cage of calcium zeolite 5A has an internal volume of 776 Å, formed by a cubic lattice of sodalites. The free aperture of the pore is 4.2 Å, allowing for the passage of molecules with a kinetic diameter of
