PROPYLENE SEPARATION FROM GASEOUS STREAMS BY PRESSURE SWING ADSORPTION

Abstract

Propylene (or Propene) is the earliest raw material used for industrial-scale production of a petrochemical. It was used for the production of isopropanol over 60 years back. In modern petrochemical complexes, Propylene finds multiple applications for the production of various major petrochemical derivatives like polypropylene, acrylonitrile, cumene, etc. as well as value additions of fuel like gasoline to improve its octane rating. Propylene has registered steady growth in terms of production volume and is the second largest petrochemical produced by volume. The importance of propylene as a feed stock is evident from its usage as a precursor to numerous petrochemicals. A concise summary of the propylene production process including basic principles, brief process description and details of the main process licensors are included in the introductory chapter. This chapter also gives a brief account of conventional propylene-propane separation techniques as well as the alternative approaches reported in the literature. Despite being such an important petrochemical, propylene is mostly produced as a byproduct as it is generated in sufficient amounts in the production of ethylene by steam cracking and Fluid Catalytic Cracking (FCC) processes. Steam cracking and FCC accounts for 56 and 20% of the world’s propylene production. The C3 (Propane-Propylene) fraction from the off-gases of these units are separated in propylene and propane rich streams by distillation. Separation of propylene and propane is a very energy intensive process in petroleum refineries and petrochemical industries, requiring large cryogenic distillation columns with 75-90 m height and 2-6 m diameter operating at a temperature of 240 K and a pressure of ~310 kPa with over 200 theoretical stages and a very high reflux ratio (R~10) due to low relative volatility of propane with respect to propylene. This separation becomes challenging because propylene and propane molecules have very similar properties e.g. Close boiling point (-42 and -47.6 °C), molecular weight (44 and 42 g/mol respectively), polarizability (6.29 and 6.26 10-24 cm3), kinetic diameter (0.40 and 0.43 nm) respectively. Propylene recovery is also desirable from the propane rich bottom product of propane-propylene splitter (distillation column). This product is used for fuel application and may contain 5-15% propylene. The recovery of propylene from this stream is desirable as propylene may cause problems of engine and injector deposits and also due regulatory ABSTRACT III specifications on maximum propylene content in fuel grade propane in countries like USA. E.g., the maximum allowable propylene content in HD-5 and HD-10 grades of propane is 5 and 10% respectively in the USA. Another instance of need for propylene recovery is from gas-phase polypropylene production units. Polymerization of propylene results in solid polymer product containing inter particle monomer. Before pelletization, the solid polymer phase is purged with nitrogen to recover the monomer. The propylene content of this product purge gas may reach up to 50%. Recovery and recycling of propylene is economically beneficial. There is a need for an economically viable method for recovering valuable olefin monomer from nitrogen purge gas and recovering nitrogen with satisfactory recovery and purity. A number of alternative separation processes have been proposed including membrane separation using highly selective membranes, absorption, adsorption and its variants like adsorption-distillation hybrid systems and pressure swing adsorption (PSA) or Vacuum Swing Adsorption (VSA). The overall objective of the present work is to pursue and describe the adsorptive studies on separation of propylene from propylene bearing streams by experimental Pressure Swing Adsorption (PSA) for binary mixtures arising from: (i) Steam Cracking of hydrocarbons (~50% propylene + balance propane) (ii) Fluid catalytic cracking (~75% propylene + balance propane) (iii) Propane-propylene splitter bottom (5-15% propylene + balance propane) (iv) Polypropylene reactor purge gas (10-50% propylene + balance Nitrogen) The scope of experimental studies undertaken include:  Comprehensive literature survey to identify the adsorbents, process operating conditions and PSA process configuration and to know the experimental results for benchmarking purpose  To characterize and evaluate different adsorbents by:  Structural properties  Single component equilibrium isotherm data ABSTRACT IV  To develop an Ideal Adsorbed Solution Theory (IAST) based solver to predict the binary isotherm data and solver validation and  To evaluate binary adsorption equilibrium data based on IAST solver  To conduct experimental dynamic PSA studies to explore the applicability of recovery of propylene from above-identified feed streams An exhaustive literature survey reveals that many adsorbents including metal organic frameworks (MOFs), zeolites, Na-ETS-10 and π- complexation adsorbents have been reported in the literature for propylene-propane separation. The Experimental adsorption data for propane-propylene binary adsorption is scantily available in open literature due to requirement of measurement set up, precise analytical facility and the considerable time taken in these measurements. The available experimental or predicted binary data for commercial zeolites (13X and 5A) from different manufacturers is limited to 100kPa pressure. Practical applications isotherm data for higher partial pressure will be required. A comprehensive evaluation of the applicability of these adsorbents as an adsorbent for PSA/VSA process on the basis of such scant data is difficult as the operating pressures of PSA/VSA process are higher than this pressure range. Ideal Adsorbed Solution Theory (IAST) is a powerful tool to predict the performance of an adsorbent on the basis of competitive multicomponent adsorption prediction. An IAST based solver was developed and validated against the literature reported data for prediction of binary adsorption of propylene-propane and propylene-nitrogen mixtures. The binary adsorption data was predicted for a temperature and pressure range 303-423 K and 0-500 kPa. The IAST predicts high propylene selectivity over both propane and nitrogen. Three adsorbents including laboratory synthesized adsorbent Na-ETS-10 and commercial adsorbents namely zeolite 13X (Z10-04), zeolite 5A (UOP) were tested for their applicability for selective propylene adsorption from propylene containing streams (propylene-propane and propylene-nitrogen mixture). The screening was conducted by equilibrium adsorption isotherm, and binary adsorption predicted based on the IAST theory. Among the commercial adsorbents, Zeolite 13 X showed the highest capacity for propylene adsorption and also the propylene-propane and propylene-nitrogen selectivity calculated on the basis IAST. In the case of laboratory synthesized adsorbents Na-ETS-10, the results obtained over different batches of adsorbent was not consistent and reproducible. During initial attempts, ABSTRACT V the adsorbent was co-formed with impurities of Quartz phase. The synthesis procedure was modified by different synthesis procedures reported in the literature and ultimately the synthesis procedure optimized to get consistent pure Na-ETS-10 phase formation. It was however felt that further scale-up of this adsorbent for adsorptive process development might itself take significant time. The experimental data generated for propylene-propane and propylene-propane generated has been compared on the basis of key process performance indicators like purity, recovery and productivity of propylene with a single column six step VSA cycle. Effect of operating parameters like feed flow rate, adsorption temperature, adsorption pressure and adsorption time etc have been studied to identify favorable operating conditions. The experimental results of this thesis suggest that propylene can be recovered in refinery grade purity from a mixture containing 15 mol % propylene in propane. This type of propylene recovery work from dilute streams has not been reported elsewhere. Simultaneous propylene and nitrogen recovery from polypropylene reactor purge gas which may contain up to 50% propylene was experimentally studied. A very high purity nitrogen product was recovered at recovery values of over 60 mol%. A preliminary economic analysis indicates that the recovery of high purity nitrogen presents attractive monitory benefits. The recovered propylene can also be used as a feed stock to alkylation process within refinery and petrochemical complex.