Simulation study on the effect of gas permeation on the hydrodynamic characteristics of membrane-assisted micro fluidized beds☆
Simulation study on the effect of gas permeation on the hydrodynamic characteristics of membrane-assisted micro fluidized beds☆
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DOI：
10.1016/j.apm.2014.04.044
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发表时间：
2014-09
影响因子：
5
通讯作者：
Lianghui Tan;I. Roghair;M. van Sint Annaland
中科院分区：
文献类型：
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作者：
Lianghui Tan;I. Roghair;M. van Sint Annaland
最近的研究表明，膜辅助流化床反应器具有各种应用的潜力，特别是在超纯氢生产方面。由于流化床优良的传质特性，可以克服浓度极化（即传质限制），通过最大化单位体积安装膜面积进一步提高膜辅助流化床反应器的生产能力，从而产生了微结构膜辅助流化床反应器的概念。本研究采用离散颗粒模型系统地进行了数值模拟，详细研究了气体通过封闭多孔膜壁的加入和抽提对单个膜辅助微流化床隔室流体动力特性的影响。特别地，研究了渗透比（通过膜的气体量相对于进给量）和安装膜面积对流体动力学的影响。通过向内或向外的气相通量模拟了通过限制乳液相的多孔膜壁的气体添加或萃取，发现这对床层流体力学有非常显著的影响。详细讨论了气体渗透对膜辅助微流化床中固体循环模式、固体含率分布和孔隙率概率密度函数的影响。已经发现，气体渗透会对床层膨胀产生不利的影响，这是由于气体在抽气时穿过床层中心或在加气时靠近膜壁而引起的。此外，在气体抽取的情况下，在膜壁附近形成致密区（固体含率增加）可能会增加床层到膜的传质阻力。这些影响会大大降低气固接触和气体停留时间，从而使反应器性能恶化。另一方面，通过适当调整气体通过膜的速度以及其他设计参数和操作条件，可以避免由气体渗透引起的这些问题。
Recent research has shown the potential of membrane-assisted fluidized bed reactors for various applications, and for ultra-pure hydrogen production in particular. Due to the excellent mass transfer characteristics of fluidized beds, concentration polarization (i.e. mass transfer limitation) can be overcome and the production capacity of membrane-assisted fluidized bed reactors could be further improved by maximizing the installed membrane area per unit volume, leading to the concept of a micro-structured membrane-assisted fluidized bed reactor. In this study, numerical simulations have been systematically carried out with a discrete particle model to investigate in detail the effects of gas addition and extraction through the confining porous membrane walls on the hydrodynamic characteristics of a single membrane-assisted micro fluidized bed compartment. In particular, the effect of the permeation ratio (amount of gas permeated through the membrane relative to the amount fed) and the installed membrane area on the hydrodynamics was investigated. Gas addition or extraction via the porous membrane walls confining the emulsion phase was simulated via inward or outward directed fluxes of the gas phase, which was found to have a very pronounced influence on the bed hydrodynamics. The effects of gas permeation on the solids circulation pattern, solids holdup distribution and porosity probability density function in membrane-assisted micro fluidized beds have been discussed in great detail. It has been found that gas permeation can have an adverse effect on the bed expansion caused by gas by-passing either through the bed center for the case of gas extraction or close to the membrane walls for the case of gas addition. In addition, the formation of densified zones (increased solids holdup) close to the membrane wall that was observed in case of gas extraction may increase the bed-to-membrane mass transfer resistance. These effects may strongly decrease the gas–solid contacting and the gas residence time, which may deteriorate the reactor performance. On the other hand, it is shown that these problems caused by gas permeation may be avoided by properly tuning the gas velocity through the membrane via membrane area and other design parameters and operating conditions.