Mechanical Strength and Bubble Point of Hollow Fiber Supported Ionic Liquid Membranes

Membranes are an interesting option for carbon capture from coal gasification processes because the partial pressure driving force for CO₂ is high.  A typical post-water gas shift fuel gas stream consists of 40% H₂, 30% CO₂, and 20% H₂O, plus contaminants, at a pressure of 25 bar and temperature of 260 °C.  Many researchers have attempted to improve the gas transport characteristics of traditional polymeric membranes by suspending ionic liquids within the polymer pores.  While this does improve permeability and selectivity, it can compromise the mechanical integrity of the membrane.  The liquid is only held in place by capillary forces and can be expelled by high transmembrane pressures such as those found in a post-water gas shift stream.  Furthermore, ionic liquids can also act as plasticizing agents that weaken the polymer support.  In this work the thermal and mechanical stability of hollow fiber supported ionic liquid membranes was evaluated to determine if it is possible to design a membrane of this type that can survive in a gasification application.

Preliminary studies were conducted by measuring the bubble point of various types of flat sheet supports in combination with a common ionic liquid, 1-hexyl-3-methylimidalzolium bis(trifluoromethyl sulfonyl) imide ([hmim][Tf₂N]).  The bubble point is a measure of the maximum pressure that a supported liquid membrane can withstand before expelling fluid from the membrane pores.  Using standard flat sheet supports, it was possible to study the effects of pore structure, pore size, and temperature on the bubble point of a simple supported ionic liquid membrane.  Later, hollow fiber supports were fabricated and tested more rigorously to simulate the materials and conditions that would be used in a practical application.  Fibers were fabricated via nonsolvent-induced phase separation/extraction, employing a single layer spinneret extrusion system.  Two high strength glassy polymers known by the trade names Matrimid® and Torlon® were used as the polymeric hollow fiber support materials, and 1-hexyl-3-methylimidalzolium bis(trifluoromethyl sulfonyl) imide ([hmim][Tf₂N]) was selected as the ionic liquid.  Fibers were tested for their glass transition temperature, tensile strength, and Young’s modulus both with and without ionic liquid.  Also, using a custom apparatus, the bubble point and rupture pressure of each fiber were measured.  It was found that adding ionic liquid has a plasticizing effect on these polymers, which decreased the thermal and mechanical stability somewhat.  However, very high bubble and rupture pressures can still be achieved in a well designed fiber.