Development and modelling of compact dual membrane contactor for gas treatment

Cai, Jingjing (2016) Development and modelling of compact dual membrane contactor for gas treatment. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this work a novel dual hollow fibre membrane contactor (HFMC) has been designed and modeled for offshore gas processing when there are stringent limitations on space and weight allocated to the purification process. In the dual membrane module, the membrane core is composed of porous polypropylene fibre membrane and nonporous silicone rubber fibre membrane. The target gas component diffuses through the porous membrane and is absorbed by solvent circulated in the shell side. The dissolved gas is then stripped out of the solvent, driven by differential pressure into the nonporous membrane. As a result, the solvent is partially regenerated and the concentration driving force can be maintained. A preliminary study of the performance of the dual HFMC has been conducted through a numerical approach. A mathematical model was developed to describe the mass transfer process in the proposed dual membrane contactor. Simulation results from the model demonstrate that the proposed novel dual HFMC could improve the gas removal efficiency by 36%, compared to a single membrane system. The proposed model was validated through lab scale experiments. Based on the experimental results, the mass transfer resistance in the fabricated dual membrane module was evaluated and an empirical correlation was developed to predict the mass transfer coefficient in the dual membrane contactor. The experiments also showed that the dual membrane module increased the permeation flux by 12-78%, compared to conventional modules containing only one type of porous membrane. The enhancement could be further improved by optimizing the membrane design and operational conditions. The mass transfer resistance in the dual membrane module existed mainly in the liquid phase and the membrane’s contribution to the total mass transfer resistance was approximately 7%. By reducing the gas flow rates, increasing the liquid flow rates and lowering the vacuum pressure in nonporous fibers, the absorption performance of the proposed dual membrane contactor could be further enhanced. To further reveal the fundamentals of the mass transfer process and module performance, the impact of module design (module geometry etc.) on absorption performance was investigated using computational fluid dynamics (CFD). The CFD model was first validated with experimental data. Design factors, such as the shell configuration, inlet geometry, header shape, fibre bed height and packing density, and their impact on shell-side flow and overall contactor performance were studied. The simulation results showed the shell-side flow dynamics played a critical role in determining the module performance which was strongly influenced by module geometries. Under the same operational conditions, shell-side velocity and shell-side flow uniformity were found different in modules with various configurations. Compared to the average shell-side velocity, the shell side flow distribution was proven to play a less significant role on the overall mass transfer performance. Combined with empirical correlations derived from experiments, CFD modeling approach could potentially decrease the number of experiments required, thus reducing costs and time required. The CFD model was then expanded to study the concentration profiles within the HFMC and simulate the impact of factors such as baffled modules and modules containing unevenly distributed fibre bundle. The research demonstrated that the CFD simulation could be used as an effective design tool in the development and optimization of cross flow membrane module. Compared to the conventional absorption column, the lab scale dual membrane contactor designed and fabricated in this study could reduce the footprint by 80% and solvent consumption by 70%. This result was obtained by comparing the experimental results from a lab scale packed column to the modeled results of dual membrane module containing the equivalent amount of interfacial area. This difference could be further enhanced by increasing the fiber packing density in the membrane core of dual membrane module. The compatibility, capability in reducing size and solvent consumption and ease in operation makes the dual membrane contactor ideal candidate for offshore gas processing where space has stringent specifications.

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