Polyimide와 Polysulfone막에 의한 순수 CO2, N2 및 혼합기체의 투과특성

문기호, 민준호, 오세헌, 장대준

현대중공업 산업기술연구소 환경연구실

Permeation Characteristics of Pure CO2 and N2, and Their Mixtures through Polyimide and Polysulfone membranes

K. Moon, J. Min, S. Oh, D. Chang

Environment Research Department, Industrial Research Institute

Hyundai Heavy Industries

1. Introduction

Membrane-based gas separation processes are rapidly growing because of their simplicity, flexibility, and economic reasons. Application of membrane to separation of CO2 from various gas mixtures may be allowable due to its  high permselectivity with respect to other ordinary gases such as oxygen, nitrogen, and hydrocarbon molecules. Currently, polymeric membranes have been widely used for CO2 removal in the natural gas, CO2 enrichment from  enhanced oil recovery, separation of CO2 and CH4 from biogas, etc.

In the present study, the performance of CO2 separation for CO2-N2       mixtures with polyimide and polysulfone membranes was investigated. The geometric configuration of the two modules is designed so similarly that they have the same effective length and area. The permeation flow rates of pure CO2 and N2 are measured increasing applied pressure. The permeation characteristics of their mixtures are investigated by changing cut ratio and composition as well as the applied pressure. Whether the permeation characteristics of mixtures could be predicted from those of pure components is testified by a simulation of the modules.

2. Experimental

In permeation experiment are used two hollow-fiber modules, of which fibers are made up with asymmetric polyimide and polysulfone membranes. Commonly, each hollow-fiber has a thin outer dense layer supported by a microporous structure. Since the two modules are made as identical as possible, their geometric characteristics including input and output locations, number of fibers (800), length of a fiber (28cm), and consequently the effective area (0.15m2) are effectively the same.

The schematic diagram of the experimental apparatus is shown in Figure 1. The apparatus consists of three main parts of pretreatment, membrane module, and analyzer. In pretreatment section, the feed gas stream, whose flow rate is precisely regulated by mass flow controller, is preheated and passed through molecular sieves to remove impurities harmful to membrane performance. The membrane module is placed in a water bath whose temperature is set to the preheater's one. There is a long helical pipe line in front of the module to prevent imperfect heating in the preheater. The feed stream is introduced into the shell side, and the permeate side is opened to atmosphere. The shell- and tube-side streams are made to flow counter-currently. The pressure of reject stream is manipulated by a back-pressure controller, and the flow rate of feed, reject, and permeate are measured by mass flow rate measurement kits. The analyzer of a gas chromatography equipped with TCD is prepared for detecting the composition of reject and permeate streams in case of permeation of mixtures.

3. Experimental Results and Discussion

The steady state permeation rates of pure CO2 and N2 through the membranes were measured at 303K, 313K, 323K and in the range of a pressure difference up to 1000cmHg. Figures 2 and 3 present the permeance for CO2 and N2  are presented as a function of the pressure difference across the membrane. To good agreement with dual-mode transport theory, the permeance decreases with the applied pressure. It is noted that the decreasing rate is mitigated as temperature is increased. The permeances of two membranes are effectively of the same order.

Based on the results of pure gas permeation, the separation factor, the ratio of CO2 to N2 permeances, is shown in Figure 4 with temperature. Since the activation energy of CO2 is less than that of N2, the separation factor decreases with temperature.

In order to simulate the permeation of gas mixture, the governing equations of mass conservation and pressure drop are solved by finite difference algorithm. To overcome the divergence problem often encountered in forward integral methods, the implicit algorithm is employed in the present study. Analysis of numerical error shows that this method guarantees sufficient precision.

Figure 5 shows the dependence of mole fraction in reject and permeate streams on the cut ratio or the ratio of permeate to feed streams. For small applied pressure, the performance of polyimide membrane for mixture permeation is well predicted from the numerical simulation(dotted line) based on the results of pure gas permeation. The performance of polysulfone is, to some degree, lowered in comparison with pure gas permeation. It is the plasticization effect by CO2 that causes the deterioration. As the applied pressure is increased, both membranes are found to be subject to the effect, and their permselectivity is considerably decreased. Though it is apparent that CO2 mainly causes the effect, the decrease in the permselectivity seems complicatedly dependent on the total applied pressure, partial N2 pressure, and cut ratio as well as CO2 partial pressure.

Acknowledgement

The authors gratefully acknowledge Daelim Poly of Daelim Industrial Co. for providing membrane modules.

References

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Figure 1. Schematic diagram of experimental apparatus