Elsevier

Journal of Membrane Science

Volume 181, Issue 2, 30 January 2001, Pages 289-293
Journal of Membrane Science

Short communication
Effect of water on the facilitated transport of olefins through solid polymer electrolyte membranes

https://doi.org/10.1016/S0376-7388(00)00628-1Get rights and content

Abstract

Polymer electrolyte membranes composed of poly(2-ethyl-2-oxazoline) (POZ) and AgBF4 are prepared for olefin/paraffin separations. The stability and effect of relative humidity on permeance and selectivity are investigated using propylene/propane mixed gases. The selectivity of propylene over propane remains high on the long time operation with a dry feed gas, indicating that membranes are effective for propylene/propane separation in the absence of water. As humidity of the feed gas increases, both propylene and propane permeances increase, which resulting in the slight decrease of selectivity.

Introduction

Olefin/paraffin separation is one of the most important processes in petrochemical industries and is currently carried out by the energy intensive cryogenic distillation process. According to the recent report by DOE, 0.12 Quads of energy (1 Quad=1015 BTU) is used yearly for olefin/paraffin distillation [1]. This large energy and capital investment requirement provides the incentive for olefin/paraffin separation research.

The membrane process using the concept of facilitated transport has been considered to be an energy-saving process. In the facilitated transport (FT) membranes, carrier-mediated transport occurs, in addition to a normal Fickian transport, due to the reversible reaction of carrier with a specific solute, and separation efficiency can be improved remarkably. Therefore, a substantial body of studies has been performed to separate olefin/paraffin mixtures using the concept of facilitated transport in liquid [2], [3], [4], [5], [6], [7], [8], [9] as well as solid membranes containing silver [10], [11], [12], [13], [14] or copper cation [15]. Because evaporation of liquid media in liquid membranes gives a serious drawback and problems for practical applications, the development of solid FT membranes is demanding.

Recently, we have reported solid FT membranes containing AgBF4 or AgCF3SO3 dissolved either in poly(2-ethyl-2-oxazoline) or poly(N-vinylpyrrolidone) for the separation of olefin/paraffin mixtures [12], [13]. Although these membranes did not contain any kind of plasticizers, both olefin permeance and its selectivity over paraffin were very high on the operation with a dry feed stream. In polymer electrolyte membranes, silver salts are dissolved in polymer solvents to make solid solution and consequently to produce free silver cations, which are a carrier for olefin [10], [12], [16]. This implies that polymer solvents in polymer electrolyte membranes can act as a replacement of water in liquid membranes. Therefore, olefin facilitation occurs without water in the polymer electrolyte membranes. However, polymer electrolyte membranes are extremely hygroscopic and their properties may be affected by water content. Therefore, in this study, the effect of water on the facilitated transport of olefin is investigated to verify that these membranes are effective even in the absence of water.

Section snippets

Sample preparation

Poly(2-ethyl-2-oxazoline) (POZ) (molecular weight=500,000 kg/mole), and silver tetrafluoroborate (AgBF4) (98%) were purchased from Aldrich Chemical Co. (Milwaukee, WI) and used without further purification. An appropriate amount of the silver salt was dissolved in the 20 wt.% POZ solution in water and stirred for several hours at room temperature. The mole ratio of a carbonyl oxygen to a silver ion in the solution was 1:1. The solution was then coated onto a microporous membrane support (from

Results and discussion

A stability test was first carried out using the dry feed gas, for the POZ/AgBF4 membrane with [OZ]:[Ag]=1:1 mole ratio, at 23°C and Δp=276 kPa. The membrane was mounted in a permeation cell shortly after taken out from the vacuum oven. A plot of total permeance versus time is shown in Fig. 2. As shown in Fig. 2, about 12–15 h have passed before reaching a steady state and the total permeance of mixed gas at the steady state was 11.3 GPU (1 GPU=1×10−6 cm3(STP)/cm2 s cmHg). The stage cut, the ratio of

Conclusions

The results indicate that the membrane prepared in this study shows facilitation effect of olefin without the aid of water and is insensitive to the humidity of the feed stream at low RH values. When the relative humidity (RH) is higher than 10%, the permeances of both propylene and propane increase gradually owing to the plasticization of the membrane by water, which resulting in the slight decrease of propylene selectivity over propane.

Acknowledgements

The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Korea through the Creative Research Initiatives Program.

References (16)

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