Enhanced pressure swing membrane absorption process for CO₂ removal from shifted syngas with dendrimer-ionic liquid mixtures as absorbent

Using pure ionic liquid [bmim][DCA] or its dendrimer-containing solutions as absorbents, studies of a novel five-valve pressure swing membrane absorption (PSMAB) process for CO₂ removal from lower temperature shifted syngas were carried out with porous hydrophobized polyether ether ketone (PEEK) hollow fiber based membrane modules. At first how the preferred absorption step duration in each cycle was determined was presented. Then we investigated the effects of the following aspects of the membrane module: reducing the tube-side dead volume in the hollow fiber module; decreasing the fiber packing density leading to increased absorbent liquid layer thickness around any fiber; increasing the tube-side gas volume. All three design features in the membrane module greatly enhanced the gas absorption behavior and improved the product qualities. Increasing temperature reduced the product qualities when pure [bmim][DCA] was used. Adding polyamidoamine (PAMAM) dendrimer Gen 0 to the ionic liquid greatly improved its CO₂ absorption capability especially at higher temperatures. Among all absorbents tested, 20.0 wt % dendrimer in [bmim][DCA] showed the best absorption enhancing effect at 100 °C. For a feed gas pressure of 250 psig, He-rich product having a CO₂ concentration around 24.7% (actual value should be around 16.7% based on dead volume influence elimination) and CO₂-rich product with CO ₂ concentration around 85.7% (actual value should be higher) have been achieved. We have also carried out a second stage PSMAB test with a 14.0% CO₂ in helium as feed gas, and a He-rich product with CO₂ concentration around 8.55% (actual value should be around 5.50%) could be achieved at 100 °C. A new module having a larger tube-side gas volume yielded a CO₂-rich stream having 90.7% CO₂. The newly developed five-valve PSMAB process shows great application prospects for CO ₂ removal from precombustion shifted syngas; significant membrane module redesign is, however, needed.