Preparation of Facilitated Transport Membranes Using Ag+ - Chitosan Complex and Their Propylene/Propane Separation

The separation of light olefin/paraffin is an important process in petrochemical refining industries. Traditionally, cryogenic distillation process has been used for a long time. But due to their similar size and condensability (or volatility), this process has been carried out under the energy-consuming conditions - low temperature, high pressure and large distillation column in separation trains. Many researchers have investigated the alternative processes [1,2]. Among them, the most attractive technology is a facilitated or carrier-mediated transport membrane using the reversible -complex formation between transition metal ions and double bond of olefin compounds. In this paper, Ag⁺-containing chitosan chelate membrane was prepared as a new water-swollen membrane material with facilitated transport property. AgNO₃ is chosen as a facilitation agent because Ag⁺ is known to be the most non-toxic among transition metal ions. Chitosan is also chosen because it can form easily water-swollen chitosan chelate membranes containing high Ag⁺ content due to its abundant amino groups to make the coordinated sites toward the transition metal ions. Ag⁺ containing chitosan complex dense/composite membranes were prepared from chitosan and AgNO₃ solution and their facilitated properties of  propylene/propane  were investigated against preparation and operation conditions. In the case of dense membranes,  with increasing AgNO₃ conc., and propylene permeability increased highly and propane permeability decreases steeply and thus, high propylene/propane selectivity is observed.  As the operation time increased, both propylene decreased slightly but propane permeability decreased highly and thus, propylene/propane selectivity increased highly.  As the temperature increased or pressure decreased, propylene permeability increased but propane permeability was too small to measure exactly.   Long-term test showed that high permeability of above 10 barrer and high selectivity of above 1,700 could be maintained with the membranes through periodic regeneration 3M AgNO₃ aqueous solution. In the case of composite membranes, with increasing pressure, propylene permeance decreased highly and  propane permeance increased slightly and thus, propylene/propane  selectivity decreased.  As the temperature or pressure increased, the permeance of propylene and propane increased and the propylene/propane selectivity decreased.  The long-term stability test showed the membrane performance could be maintained using humidified feed gas or periodic regeneration - propylene permeance( <<3 GPU), propane permeance(<< 0T.03 GPU) and high propylene/propane selectivity(>> 100) were observed.