Carbon Molecular Sieves for Hydrocarbon Separations by Adsorption

Abstract

The use of vacuum pressure swing adsorption (VSA-PSA) with carbon molecular sieves (CMS) as selective sorbents is evaluated as an alternative technology for methane−nitrogen and propane−propylene separations. The larger molecules, methane and propane, are very low-diffusing species, resulting in processes where nitrogen and propylene are retained in the bed. For the methane−nitrogen separation, a Skarstrom cycle (pressurization, feed, blowdown, and purge) was used with the advantage of recovering methane in the feed step with a low-pressure drop. At ambient temperature, from a mixture with 20% of nitrogen balanced by methane, a purity >93% was obtained. For the propane−propylene mixture, a five-step cycle was usedpressurization, feed, rinse, intermediate depressurization, and countercurrent blowdownwhere purified propylene is obtained as a low-pressure product. Starting with an equimolar mixture at 373 K, the purity of propylene was 83% with a product recovery of 84%.

High CO2/CH4 and C2 Hydrocarbons/CH4 Selectivity in a Chemically Robust Porous Coordination Polymer

“…[ 10 ] Thus, we proceeded to conduct breakthrough experiments, because they can evaluate the gas separation ability of adsorbents under kinetic fl owing gas conditions, which are pertinent to the pressure swing adsorption (PSA) process used in industry [ 24 ] . The breakthrough results for the CH 4 mixtures (CH 4 /C 2 H 6 : 80/20; CH 4 /CO 2 : 60/40) in Figure 5 show that 1 has good capacity to purify CH 4 . The concentration of outfl owing gas by gas chromatography (GC) was almost 0% for C 2 H 6 or CO 2 and 100% for CH 4 , indicating that compound 1 provides complete separation of C 2 H 6 or CO 2 from CH 4 under fl owing conditions, which is consistent with equilibrium adsorption behaviors and the IAST results.…”

“…The breakthrough results for the CH 4 mixtures (CH 4 /C 2 H 6 : 80/20; CH 4 /CO 2 : 60/40) in Figure 5 show that 1 has good capacity to purify CH 4 . The concentration of outfl owing gas by gas chromatography (GC) was almost 0% for C 2 H 6 or CO 2 and 100% for CH 4 , indicating that compound 1 provides complete separation of C 2 H 6 or CO 2 from CH 4 under fl owing conditions, which is consistent with equilibrium adsorption behaviors and the IAST results.…”

“…[ 20 ] The uniform 1D channel with an open metal chain suggests that 1 might be a promising candidate for gas separation. Singlecomponent adsorption isotherms for CH 4 , CO 2 , C 2 H 4 and C 2 H 6 were measured to 100 kPa at 195 K. As shown in Figure 3 a, C 2 H 4 and C 2 H 6 adsorption isotherms in 1 display quick saturation around 10 kPa, however, the uptake of CH 4 increases slowly following this pressure. Importantly, the differences in the uptake amounts at P = 10 kPa (CO 2 : 8.9 mmol/g; C 2 H 6 : 6.2 mmol/g; C 2 H 4 : 6.6 mmol/g; CH 4 : 0.38 mmol/g) indicate that 1 is a potential candidate for the purifi cation of CH 4 from C1 and C2 hydrocarbon mixtures.…”

“…[ 1 ] However, geochemical gas streams commonly contain high concentrations of CO 2 and C2 (C 2 H 4 and C 2 H 6 ) hydrocarbons that will result in the corrosion of gas pipelines [ 2 ] and also impede the conversion of methane to value-added products. [ 3 ] Thus, extensive efforts are being made worldwide for the development of effi cient and economical separation; [ 4 ] however, untill now, this is still a signifi cant challenge. [ 5 ] Traditionally, light hydrocarbon separations are performed by cryogenic distillation, which entails large energy costs.…”

“…[ 7 ] Because of the theoretically limitless possibilities in assembling organic linkers and metal or cluster connectors, a large number of structures have been reported, but only a few of them show outstanding selectivity for CO 2 over CH 4 , [ 5 ] which is mainly attributed to the large quadruple moment of CO 2 whereas CH 4 has none. [ 8 ] In addition, the separation of C2 (C 2 H 4 and C 2 H 6 ) from CH 4 mixtures has rarely been studied despite these light hydrocarbons being widely utilized as gas energy sources and raw materials. [ 9 ] This is mainly due to their similar physical and chemical properties (kinetic parameters for C 2 H 6 : 3.9 Å; C 2 H 4 : 3.9 Å and CH 4 : 3.8 Å), which lead to the diffi culty for adsorption-based separation.…”