Removal of benzene from nitrogen by using polypropylene hollow fiber gas–liquid membrane contactor
Introduction
Benzene is a known carcinogen which is commonly found in crude petroleum and petroleum products. The benzene-contaminated N2 or air stream emitted during various petroleum and nature gas operations activities can create a hazard to public health and environment. Several technologies have been developed to prevent benzene emission from vented gas, such as activated carbon adsorption, biological treatment, condensation, conventional gas–liquid absorption, membrane based vapor separation, and so on. All these technologies mentioned above have considerable limitations. In practical operation, the flammability of activated carbon adsorption on hydrophobic brings an additional risk [1], [2]. Biological treatment is not flexible enough to deal with wide range changes of gas concentration and component. Condensation needs additional safety measures like inertization or explosion proof design. Membrane based vapor separation is not economic to reduce gas concentration to a very low level [3]. Conventional gas–liquid absorption suffers from many drawbacks such as flooding, foaming, entraining, channeling, high capital and operating costs.
Gas–liquid membrane contactor is emerging as an alternative technology for selective separation of gaseous components. The membrane used in this contactor forms a permeable barrier between the gas phase and the liquid phase, which permits mass transfer between the two phases without dispersing one phase into the other. Consequently, gas–liquid membrane contactor provides some practical advantages, such as high surface area per unit contactor volume, known gas–liquid interfacial area, operational flexibility, independent control of gas and liquid flow rates, easiness in scale up, and so on.
Qi and Clussler [4] first explored the possibility of using microporous polypropylene membrane contactor to remove CO2 from air by NaOH solution. Following their original work, many theoretical and experimental studies have been carried out in order to understand the mass transfer mechanism in gas–liquid membrane absorption processes. Most of the researching works were focused on the removal of CO2 [5], H2S and SO2 from N2, CH4, flue and natural gas [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], or the separation of olefin/paraffin with silver nitrate acting as an absorbent [18], [19]. However, there were only few reports about the application of gas–liquid membrane contactor on the treatment of N2 or air stream containing volatile organic compounds. Poddar et al. [20] studied the removal of toluene, xylene, and acetone from air. In his study, two different kinds of inert liquids, silicone oil and heat transfer oil, were employed as absorbent, respectively. The effects of operating conditions, such as gas/liquid flow rate, gas concentration on the mass transfer efficiency were investigated. A differential mass transfer model based on the diffusion theory was developed to simulate the behavior of absorption process.
The aim of this paper was to evaluate the performance of gas–liquid membrane contactor in benzene/N2 mixture separation process. The aqueous N-Formylmorpholine (NFM) solution was first selected as absorbent because of its good compatibility with the PP membrane [21] and high absorption capacity for aromatic hydrocarbon at ambient temperature [22]. A numerical model was established to simulate the mass transfer process in order to gain an insight into the gas–liquid membrane absorption process. The operating conditions that affected the system performance were also been investigated.
Section snippets
Model development
A numerical model was developed to describe benzene capture from the gas mixture by the aqueous NFM solution by using the resistance-in-series concept, combining process conditions, membrane properties and module geometric characteristics. The model is developed for a segment of a hollow fiber. In order to obtain high mass transfer efficiency, the gas–liquid membrane contactor is operated in “non-wetted mode”. The gas flows inside the lumen and the liquid flows countercurrent in the shell side.
Experimental unit and procedures
The experimental setup is schematically depicted in Fig. 2. The absorbent was prepared in the feed tank with deionized water to a given concentration. The N2 gas from N2 cylinder was divided into two streams, one of them was blown into a bubble tank in which temperature was carefully maintained by an electric-heated thermostatic water bath (SY-3, Shanghai shenli instrument Co. Ltd., Shanghai, China). The N2 gas was bubbled in pure benzene with a very low speed and became a gas mixture
Gas concentration profile in radial direction
Fig. 3 shows the gas concentration profile in radial direction. As shown in this figure, the gas concentration changes very small along r direction in gas phase (including lumen side and membrane phase), which agrees with the results presented by Al-Marzouqi and El-Nass [28]. This may be due to the fact that the diffusion coefficient in the lumen side (9.32 × 10−6 m2/s) and membrane phase (9.27 × 10−6 m2/s) are almost four orders of magnitude larger than that in the shell side (5.31 × 10−10 m2/s). Thus,
Conclusion
In this work, the potential of removal benzene from N2 by using PP hollow fiber gas–liquid membrane contactor was evaluated via a series of theoretical modeling and experimental studies, where a new aqueous solution based on the NFM was selected as absorbent. It was found that the benzene removal from the benzene/N2 mixture was approximately 99% when the membrane contactor was operated in a comparatively high liquid flow rate (>100 mL/min). The benzene flux is influenced significantly by the gas
Acknowledgments
The authors acknowledge the research fund from Environmental Protection Office of Jiangsu Province of China. The authors also wish to thank Dr. Yongan Gu at the University of Regina for correcting the grammar errors in this paper.
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