CARBON MOLECULAR SIEVE HOLLOW FIBER MEMBRANE REACTORS FOR PROPANE DEHYDROGENATION

Abstract

Propylene (C3H6) is a crucial petrochemical feedstock for a number of bulk chemicals and polymers. While steam cracking currently dominates C3H6 production, propane (C3H8) dehydrogenation (PDH) has been increasingly practiced addressing the gap between C3H6 demand and production. The C3H8 conversion of PDH reaction is challenged by thermodynamic limitation and catalyst deactivation at elevated reaction temperature. Membrane reactors can address both challenges and hence enhance the energy efficiency of PDH by achieving attractive and stable C3H8 conversion at low reaction temperature via selective removal of the hydrogen (H2) product to shift the reaction equilibrium. Large-scale practice of PDH membrane reactors has not occurred due to the lack of scalable membranes that can provide attractive H2/C3H8 separation performance at PDH conditions. Carbon molecular sieve (CMS) hollow fiber membranes are a class of tunable and scalable inorganic membranes that are stable under non-oxidative high-temperature conditions, and therefore are potentially promising for PDH membrane reactors.This PhD dissertation aims to investigate low-temperature propane dehydrogenation in novel membrane reactors comprising asymmetric CMS hollow fiber membranes. First, asymmetric polyimide-derived CMS hollow fiber membranes were fabricated and their high-temperature H2/C3H8 performance was assessed. The roles of membrane pyrolysis temperature, permeation temperature, and feed composition on high-temperature H2/C3H8 separation performance were systematically investigated. The effects of high-temperature H2 and C3H6 exposure on CMS pore structure and transport properties were also examined. Under a continuous permeation test (~130 hours) of H2/C3H8 feed mixture at 600 oC, the asymmetric CMS hollow fiber membranes showed stable separation performance with outstanding H2 permeance of 430 GPU and H2/C3H8 separation factor of 511 exceeding those of microporous oxide membranes. H2-permeable CMS hollow fiber membrane reactors were created using the asymmetric CMS hollow fiber membranes and platinum-based catalysts. The effects of reactor operating conditions (i.e., reaction temperature, feed space velocity, sweep flow rate, C3H8 partial pressure, number of CMS hollow fibers) on PDH performance of the CMS hollow fiber membrane reactor were studied. Due to selective removal of H2 product, the H2-permeable CMS hollow fiber membrane reactor showed up to 300% higher C3H8 conversion than equilibrium conversion. Stable performance with commercially attractive C3H8 conversion (above 30%) and high C3H6 selectivity (above 98%) were obtained in the CMS hollow fiber membrane reactor at 450 °C for over 110 hours. The CMS hollow fiber membrane reactors developed in this dissertation outperform PDH membrane reactors reported in literature by having higher conversion enhancement, lower reaction temperature, and the lowest deactivation rate. These experimental results demonstrated the attractiveness of the novel CMS hollow fiber membrane reactors for energy efficient C3H6 production. One-dimensional isothermal models were further developed by material balance to understand the cooperative reaction and separation in the CMS hollow fiber membrane reactors. The modeling results of H2-permeable CMS hollow fiber membrane reactor showed overall good agreement with experimental results. The models also demonstrated the viability of C3H6-permeable CMS hollow fiber membrane reactor and catalytic CMS hollow fiber membrane reactor, which provide valuable guidance to future development of CMS hollow fiber membrane reactors following this PhD research.