Numerical Study on the Sweep Gas Flow Effects on Water-Gas Shift Reaction in Membrane Reactors

Abstract

A numerical model is built to study the effect of sweep gas flow field on the hydrogen production by water-gas shift reaction (WGSR) carried out in membrane reactor (MR). Different flow types of sweep gas flow patterns were created using various inlet/outlet port designs and various baffle plates for the permeation side of the MR. Using CO conversion and H2 recovery to characterize the reactor performance, the results showed that counter flow mode in which the reactant (CO+H2O) flows in opposite direction relative to the sweep gas flow has best performance as compared with other flow modes studied including co-flow, parallel flow, central flow, and sided flow. It was also found that the reactor performance was not improved by the introduction of baffles in the permeation side design. One of the reasons may be due to the sacrifice of membrane surface when the baffles were added. The other reason may be attributed to the enhanced convective heat transfer that reduces the WGSR reaction temperature. By varying the inlet temperature, the sweep gas flow can serve as cooling or heating fluid in the reactor. For both cooling or heating cases reactor performance could be degraded due to lower catalyst activity or reversed WGSR, respectively. The numerical results also indicated that the size of permeation side had insignificant effect on the MR performance.

本研究為探討膜反應器掃氣端流場的影響，建立水氣轉化反應三維數學模型，模擬水氣轉化反應在膜反應器中，能量、動量、物質傳輸現象，以找出最佳掃氣端設計。為探討膜反應器的性能，本研究改變掃氣端流場型態、掃氣流量大小、掃氣端管徑大小、掃氣入口溫度，操作條件在反應端壓力15atm，水碳比1。 由數值模擬結果發現，在掃氣端流動性不佳下，導致氫氣濃度增加，減少膜氫氣滲透通量，氫氣回收率與CO轉化率降低。比較不同掃氣流場型態，發現逆向為最佳掃氣流場設計。比較順向流與逆向流模型中，逆向流表面氫氣擴散通量較平均，有助於膜氫氣擴散。本研究加入隔板則增加擾動的設計，並未提升膜反應器性能，其性能甚至低於順向流模型。 對於掃氣端管徑大小的影響，其對膜反應器性能影響不大。改變掃氣端入口溫度，主要影響反應端溫度升溫降溫變化，在掃氣入口溫度480℃時，有最佳的CO轉化率。