Ethylene/ethane separation and concentration by hollow fiber facilitated transport membrane module with permeation of silver nitrate solution

Novel hollow fiber facilitated transport membrane modules were used for ethylene/ethane separation using an aqueous silver nitrate solution as a carrier solution. In this membrane system, a feed gas consisting of 80 mol% C2H4 and 20 mol% C2H6 and an aqueous 4 M silver nitrate solution are supplied to the lumen side (high pressure side, feed side) of a hollow fiber ultrafiltration membrane module and flow upward. In the lumen, the solution absorbs C2H4 selectively to become a rich or laden solution. Some part of the rich solution permeates the membrane to the permeate side (low pressure side, shell side) and other part of the solution leaves the module from its top and then it is supplied to the top of the shell side, where the absorbed gas is stripped from the rich solution and recovered as enriched ethylene. The lean solution is circulated to the lumen of the hollow fiber module by a pump. The feed side pressure was from 200 to 500 kPa and the permeate side pressure was atmospheric. Three modules with different liquid permeability and design were used in experiments. The module, in which the carrier solution permeated the membrane at the upper part of the module, had higher performance than the module in which the solution permeated at the lower part. The module, in which the solution did not permeate the membrane, also had high performance although a large gas-liquid separator was needed downstream of the module. The module efficiency, which is defined as the ratio of the experimentally observed ethylene recovery to the recovery calculated by assuming gas-liquid phase equilibria at both sides of the membrane, was from 61 to almost 100%. Ethylene in the feed gas was concentrated from 80 to 99.8 mol%. The maximum C2H4 permeance was 1.1 × 10−4 mol m−2 s−1 kPa−1 (3.3 × 10−4 cm3 cm−2 s−1 cm Hg−1) and the selectivity of C2H4 over C2H6 was about 375 at the ethylene partial pressure of 164 kPa. The maximum ethylene recovery was 87% when the mean residence time of the feed gas in the module was 4.9 s. The membrane was confirmed to be stable during a series of experiments.