| Summary: | 博士 === 臺灣大學 === 化學工程學研究所 === 96 === The methanol steam reforming on double-jacketed palladium membrane reactors was studied by numerical simulation. A catalytic oxidation of methanol and/or hydrogen was used to provide the necessary heat for steam reforming, and hydrogen permeation by palladium membrane takes place simultaneously. The molar fractions of species and reformer temperatures were analyzed under co-current and counter-current operation between oxidation and reformer sides. The results indicated that the co-current operation outperforms the counter-current operation because of higher methanol conversion and reformer temperature than those under counter-current operation. In the transient study, the gaseous compositions and reformer temperatures were simulated under two conditions: (1) starting a cold catalyst bed at 433K and a hot inlet at 543K; (2) starting a hot catalyst bed at 543K and a cold inlet at 433K. The results revealed that both operating conditions reached thermal equilibrium within 150 dimensionless time. The operating condition (1) yielded higher methanol conversion and reformer temperature than those under condition (2) after reaching steady state. The operating condition (2) had higher methanol conversion than those of condition (1) in the beginning but low conversion at final steady state. Using hot steam to heat reformer can decrease the required time during the preheating period. In addition, two strategies were compared to analyze the reformer response when a temporary extra hydrogen demand is required. The results showed that increasing methanol mass flow rate outperforms increasing reformer temperature.
The methanol conversion, hydrogen recovery yield and production rate were further analyzed at steady state. The simulation results exhibited that increasing the volumetric flow rate of hydrogen in permeation side could enhance hydrogen permeation rate cross
the membrane. The favorable velocity ratio from permeator to reformer was 10. However, hydrogen removal could lower the temperature in the reformer. The hydrogen production rate increases as temperature increased at a given Damköhler number, but the methanol conversion and hydrogen recovery yield decreased. In addition, a suitable molar ratio of air to methanol was 1.3 under three air inlet temperatures. The performance of a double-jacketed membrane reactor was compared with an autothermal reactor based on methanol conversion, hydrogen recovery yield and production rate. Under the same feeding conditions, the double-jacketed reactor could convert more methanol at a given reactor volume than the autothermal reactor.
The transport limitations of double-jacketed membrane reactors were studied by the ratios of reformer diameter to catalyst particle diameter, ratios of reformer diameter to length, void fractions, and ratios of exit permeator to reformer pressure. From the analysis of pressure drop and radial temperature gradient profiles, we suggested a set of favorable ratio of catalyst particle diameter to reformer diameter, ratio of reformer diameter to length, void fraction, and ratio of exit permeator to reformer pressure to be 10, 0.1, 0.3~0.5 and 10, respectively. The minimization of temperature gradients was explored by either decreasing the inlet temperature or applying an oxidation tube inside the permeator. Although decreasing the inlet temperature reduced the heat transfer limitations due to decreasing the reforming rate, the methanol conversion and hydrogen yield were also decreased. A triple-jacketed membrane reactor was proposed. The heat transfer limitations were significantly reduced due to an effective heat transfer.
|
|---|
