Observations, modeling and optimization of yield, selectivity and activity during dehydrogenation of isobutane and propane in a Pd membrane reactor

Dehydrogenation of isobutane and propane was carried out in a membrane reactor made of a Pd/Ru (or Pd/Ag) tube packed with a supported Pt catalyst. The shell side was swept by a stream of nitrogen or its mixture with hydrogen. Significant gains in yield were achieved by separating the hydrogen through the selective Pd membrane: up to 76% butene at 500 degrees C (compared with 32% in equilibrium) and 70% propene at 550 degrees C (23% at equ.). The attained yields, however, were limited at low feed rates by suppressed catalyst activity in the absence of hydrogen. To avoid low activity and fast aging, hydrogen concentration should be kept at about 2% by adjusting the shell or tube dow rates. Fast deactivation was observed with high ratios of shell to tube flow rates. The degree of cracking and of isomerisation increases with conversion. Temperature should be kept below 500 degrees C, during butane dehydrogenation, to avoid cracking and fast aging. Yields under high pressures (18 psi for isobutane and 100 psi for propane) were similar to those obtained under atmospheric conditions. Operation under pressure may be advantageous as high purity hydrogen can be produced. The yield dependence on feed rate and on hydrogen shell-side pressure were adequately described (at 500 degrees C) by a simple model, that incorporates a three-parameter rate expression, that accounts for the accelerating role of hydrogen pressure. The degree of cracking and isomerisation were adequately described by a single-parameter rate expression which assumes that the main and side reactions occur on the same sites. The model was optimized to determine the feed and shell flow rates which maximize the yield. The optimization suggests that, in the present design, the yield cannot be improved significantly beyond 90%, but that almost complete conversion could be achieved when the reactor profile of hydrogen pressure is optimized.