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수소분리

정경열, 소재현, 박승빈, 양승만

대전광역시 유성구 373-1, 한국과학기술원 화학공학과

Separation of hydrogen from gas mixture by using the inorganic membrane prepared by the multistep structure modification

Kyeong Youl Jung, Jae-Hyun So, Seung Bin Park, Seugn-Man Yang

Department of Chemical Engineering, KAIST

373-1, Kusong-dong, Yusong-gu, Taejon 305-701, Korea

1. Introduction

The inorganic membraen can be used for the separation of hydrogen from the coal gasification mixture which are composed of hydrogne, carbon dioxide, methane, nitrogen because of its durability at high temperature and pressure[1]. Successful application of the inorganic membrane depend upon the ability to prepare membranes with desired pore size and pore structure. In oder to obtain the hydrogen with a high purity from the goal gasfication mixture, we need to prepare the membrane with the high separability of hydrogen with respect to not only the nitrogen but also the carbon dioxide.

Newly improved techniques including sol-gel processing and CVD provide an opportunity to synthisize the inorganic membrane with a high quality. Lee et. al. showed that the palladium impregnation into the pore of the alumina support lead to a high selectivity of hydrogen[2]. Lee et al. prepared the sol-gel deriven alumina membrane by soaking and vapor-deposition method[3]. The membrane has the separation factor of above 5 at 673K even when the transmembrane pressure was 18 kPa.

In this research, the porous alumina membrane was prepared by the multistep structure modification(Sol-gel, Hydrolysis, SVD etc.) to increase its separability for the hydrogen from the H2/N2 and H2/CO2 mixture. The effects of the temperature and the inlet composition of hydrogen on the performance of the prepared membrane were investigated.

2. Experimental

(1) Preparation of Inorganic Membrane

The sol-gel process, in-situ hydrolysis, and soaking vapor deposition(SVD) technique were applied to modify the pore of the inorganic membrane[2,3,4,5]. The porous support used in this work are □-alumina tube which has the length of 500 mm, the outer diameter of 10 mm, and the inner diameter of 7mm. The pore size of this membrane was reduced by above techniques to enhance the separability of hydrogen from the gas mixture; H2/N2, H2/CO2. In the first, the pore of the membrane was modified by the sol-gel process. Secondly, the in-situ hydrolysis was applied by using TEOS(Tetra Ethyl Ortho Silicate, Adrich chemical Co.). Thirdly, the palladium was impregnated into the pore of the membrane by using the SVD method[5].

(2) Separation Experiment

The separation experiment was carried out for the pure gas and the gas mixture at 298 and 673 K, respectively. Fig.1 is the schematic diagram for the hydrogen separation from the H2/N2 and H2/CO2 mixture. The flow rate of each gases, which are mixed and flow to the outside of the membrane, was controlled by using the mass flow controller(MFC, UFC-1500A, UNIT instrument Inc.). The flow rate of the permeated gases was measured by the MFC. The sweeping gas for the permeate was not used. The composition of each components in the permeate stream was analyzed by using the GC(HP 5890 Series II) with thermal conductivity detector(TCD) and the Porapark Q column. The inlet compositions of H2/N2 and H2/CO2 were varied from 20 to 50 %(volume percent of H2). This hydrogen composition is similar to that of the coal gasfication products. The separation factors are defined as

[eqn: {Y_H_2 / {(1- Y_H_2 )}} over {X_H_2 / { (1-X_H_2 ) }}]                              (1)

where [eqn: P_i] is the permeability of the i component, [eqn: Y_H_2 , X_H_2] is the molar fraction of hydrogen at the inlet and permeate stream, respectively.

3. Result and Discussion

Fig.2 is permeabilities of the pure H2, N2, and CO2 gases as a function of the pressure difference. The separation through the membrane prepared by the multistep pore structure modification is achieved mainly by Knudsen flow mechanism. The separation factor of hydrogen for pure gases was above the Knudsen limit up to the pressure difference of 69 kPa and 400 oC.

Fig.3. is the separation factors of hydrogen as a function of the pressure difference for the H2/N2 and H2/CO2 mixture at the room temprature and 400 oC when the inlet composition of hydrogen was 20 vol.%. The separation factor defined as the equation (1) was increased with increasing the pressure difference and the temperature. When the inlet composition of hydrogen was increased, the separation factor was slightly increased both of  H2/N2 and H2/CO2 mixture as shown in Fig.4. The maximum separation factor, which can be obtained by the prepared membrane, was 1.9 and 2.1 for the H2/N2 and H2/CO2 mixture respectively at 400 oC and the pressure difference of 276 kPa when the inlet composition of hydrogen was 50 vol%.

The higher separation factor at 400 oC can be explained by that impregnated palladium particles enhance the adsorption of hydrogen on its surface. The separability of the prepared membrane for the H2/CO2 mixture was better than that for the H2/N2 mixture as shown in Fig.3 and Fig.4.

The separability of the prepared membrane was low to apply to the separation of hydrogen from the coal gasification mixture although its separation factor for pure gases was above the Knudsen limit at the low pressure difference and 400 oC. This result showed that we need to prepare the inorganic membrane which has much higher separation factor than the Knudsen limit in order to use the inorganic membrane for the separation of hydrogen from the goal gasification mixture.

4. Conclusions

The separation through the membrane prepared by the multistep pore structure modification such as sol-gel, in-situ hydrolysis, SVD was achieved by Knudsen flow mechanism.

Although the separation factor of hydrogen for pure gases was above the Knudsen limit at the low pressure difference and 400 oC, the separation factors for the H2/N2 and H2/CO2 mixture were not above 2.1.

When the inlet composition of hydrogen was increased, the separation factors were increased for both the H2/N2 and the H2/CO2 mixture.

The separability of the membrane prepared by the multistep pore structure modification for the hydrogen in case of the H2/CO2 mixture was better than that for the H2/N2 mixture.

References

1.D. Li and S. T. Hwang, J. Mem. Sci., 66, 119(1992)

2.S.-J. Lee, S.-M. Yang, and S. B. Park, J. Mem. Sci., 96,223(1994)

3.S.-Y. Lee, S._J. Lee, S. J. Kwon, S.-M. Yang, and S. B. Park, J. Mem. Sci., 108, 97(1995)

4.J.-H. So, K.-Y. Yoon, S.-M. Yang, and S. B. Park, The 8th Int. Joint Symp. on Chem. Eng. Taejeon/Chungnam-Kyushu 1995, Taejeon, Korea

5.Keon-Yeong Yoon,"Preparation of Alumina Membrane via Multistep Pore Modification and Hydrogen Separation Characteristics",M.S. Thesis, KAIST, Korea(1996)

Fig.1. Schematic diagram of the apparatus for the separation of hydrogen from the gas mixture

Fig2. The permeability of pure gases(H2, N2, CO2) at 25 and 400 oC

Fig.3. The separation factor of hydrogen for H2/N2 and H2/CO2 mixture in case of XH2=0.2

Fig.4. The separation factor of hydrogen for H2/N2 and H2/CO2 mixture in case of XH2=0.5