The paper presents the experimental study of a novel unsteady-statemembrane gas separation approach for recovery of a slow-permeant component in the membrane module with periodical retentate withdrawals. The case study consisted in the separation of binary test mixtures based on the fast-permeant main component (N2O, C2H2) and the slow-permeant impurity (1%vol. of N2) using a radial countercurrent membrane module. The novel semi-batch withdrawal technique was shown to intensify the separation process and provide up to 40% increase in separation efficiency compared to a steady-state operation of the same productivity.
Comparative calculations with a mathematical model designed by the authors, which takes into consideration energy transfer from gas flowing through a given channel to gas which penetrates this channel from an adjacent channel, as well as a model which omits this phenomenon, respectively, were made for the process of separating gas mixtures carried out with an inert sweep gas in the fourend capillary membrane module. Calculations were made for the process of biogas separation using a PMSP polymer membrane, relative to helium as the sweep gas. It was demonstrated that omitting the energy transfer in the mathematical model might lead to obtaining results which indicate that the capacity of the process expressed by the value of feed flux subjected to separation is by several percent higher than in reality.
This paper analyses the real behaviour of the fluid in the channels of a three-end membrane module. The commonly accepted mathematical model of membrane separation of gas mixtures in such modules assumes a plug flow of fluid through the feed channel and perfect mixing in the permeate channel. This article discusses the admissibility of accepting such an assumption regarding the fluid behaviour in the permeate channel. Throughout analysis of the values of the Péclet number criterion, it has been demonstrated that in the industrial processes of membrane gas separation, the necessary conditions for the perfect mixing in the permeate channel are not met. Then, CFD simulations were performed in order to establish the real fluid behaviour in this channel. It was proved that in the permeate channel the fluid movement corresponds to the plug flow, with the concentration differences at both ends of the module being insignificant. In view of the observations made, the admissibility of concentration stability assumptions in the mathematical models for the permeate channel was discussed.
A novel absorbing pervaporation hybrid technique has been evaluated experimentally for the recovery of ammonia from the gas mixture in a recycle loop of synthesis plants. This process of hybridization brings together the combination of energy-efficient membrane gas separation based on poly(dimethylsiloxane) poly(diphenylsilsesquioxane) with a high selective sorption technique where a water solution with polyethylene glycol 400 (PEG-400) was used as the liquid absorbent. Process efficiency was studied using the pure and mixed gases. The influence of PEG-400 content in aqueous solutions on process selectivity and separation efficiency was studied. The ammonia recovery efficiency evaluation of an absorbing pervaporation technique was performed and compared with the conventional membrane gas separation. It was shown that the absorbing pervaporation technique outperforms the conventional membrane method in the whole range of productivity, producing the ammonia with a purity of 99.93 vol.% using the PEG 80 wt.% solution. The proposed method may be considered as an attractive solution in the optimization of the Haber process.