Performance of silicone-coated polymeric membrane in separation of hydrocarbons and nitrogen mixtures☆
Introduction
The applications of rubbery polymer-coated membranes for separating volatile organic components (VOCs) from a gas stream have been reported. Various industries are using more than 100 units supplied by Membrane Technology and Research Inc. (MTR), US; GKSS, Germany; and Dalian Institute of Chemical Physics (DICP), China. An average market growth of 8–10% per year has been mentioned for these applications. It has been realized that the membranes are often key separation units resulting in superior high-value products, substantial savings in energy and raw materials. There are possibilities of numerous applications of such processes for recovering hydrocarbons and recycling nitrogen in the petroleum and polymer synthesis industries. More fundamental research has been done to understand the sorption, diffusion and permeation effects of VOCs on the membranes coated with rubbery polymers [1], [2], [3], [4], [5], [6]. Among the rubbery polymers, poly(dimethylsiloxane) (PDMS) is a solubility-selective polymer that is more permeable to vapors (condensable) than to gases (non-condensable). Its glass transition temperature is among the lowest values recorded for polymers (−129 °C) indicating a very flexible polymer backbone with long-range segmental motion active event at very low temperatures [7], [8]. Many studies of chlorinated and other VOCs/N2 as well as CO2/N2 mixtures through PDMS membranes have been performed [9], [10], [11], [12], however, most of the researchers have concentrated on pure component permeations. Merkel et al. [13] reported the permeabilities of pure hydrogen, oxygen, nitrogen, carbon dioxide, methane, ethane, propane and their perflourocarbons through PDMS. They found that there was strong plasticization as propane penetrated the PDMS membrane. An increase in penetrant diffusivity was believed to occur from increased polymer local segmental motion caused by the presence of penetrant molecules in the polymer matrix. As penetrant pressure and, therefore, the penetrant concentration in the polymer increases, the tendency to plasticize a polymer matrix increases particularly for strongly sorbing penetrants. For designing a membrane process to separate a gas mixture, the fundamental permeation parameters of the gases present in the mixture will provide more meaningful data. However, there is a general lack of such data in literature on separation of hydrocarbons from nitrogen. The present work reports the permeation of lower hydrocarbons such as ethylene, ethane, propylene and propane from lean binary, ternary and quaternary mixtures in nitrogen through composite PDMS–polysulfone membrane at an ambient temperature of 22 °C and a total pressure of 650 kPa (g).
Section snippets
Experimental apparatus
The gas permeation apparatus fabricated in our laboratory is a standard constant-pressure permeation design. It comprised of three sections namely feed preparation, membrane cell and data collection. A desired composition and flow rate of a hydrocarbons and nitrogen mixture were produced in the feed preparation section before the membrane cell. There were four mass flow controllers and a readout, which had controls to set a fixed ratio of hydrocarbon/nitrogen mixtures. Three mass flow
Materials
Two filler-free composite poly(dimethylsiloxane) membranes labeled as A and B, supplied by different laboratories were used for pure and mixed gas permeation experiments. Membrane A consisted of a highly microporous polysulfone support coated with a 0.20 μm thick PDMS layer. Membrane B consisted of dense homogenous polysulfone support coated with a 0.45 μm thick PDMS layer. They were cast from a polysulfone N-methyl pyrrolidone (NMP) solution by gelation in cold water. Nitrogen, oxygen, ethylene,
Procedure
The membrane performances were characterized in terms of permeance (pressure normalized flux) and selectivity [15], [16]. For pure gas experiments, both upstream and downstream lines of the membrane cell were purged with penetrant gas prior to a permeation experiment. A steady state was deemed to be achieved when variation in the permeate gas flow rate was less than 2%. At steady state, the permeance could be evaluated at a given feed pressure difference, which could be raised up to 730 kPa (g)
Physical properties of PDMS composite membrane
A variation in the morphology of composite membranes made from same materials could impact the permselectivity of gas and hydrocarbons remarkably. Membranes’ properties and selected reported data [13] for PDMS membranes are listed in Table 2. It is seen that the permeances of membrane A are much larger than those of membrane B and literature data [13] owing to either extremely thin coating layer and/or highly microporous support.
Considering that same polymer materials were utilized in
Conclusions
It was observed that propane and propylene significantly plasticized the PDMS coating layer both as pure hydrocarbons and as a component of gas mixtures. At applied pressures that were lower than the plasticization pressures, the observed permeance order was , which concluded that propylene was more permeable than propane. As the pressure was increased to higher than the plasticization pressure, the order was changed to , which matches the
Acknowledgements
Authors are thankful to Mr. Brad Stimson for his help with the experimental system and Dr. Jamal Kurdi for helpful discussions. Financial support from Natural Resources Canada, under PERD project number 11402 is gratefully acknowledged.
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NRCC No. 46482.