A cyclic adsorption process simulator was used to study novel high-temperature pressure swing adsorption (PSA) cycles. Based on the use of a K-promoted hydrotalcite-like (HTlc) adsorbent and six different (vacuum swing) stripping PSA cycles, these cycles were designed to process a typical stack gas effluent at 575 K containing 15 vol % CO 2 , 75 vol % N 2 , and 10 vol % H 2 O into CO 2 -depleted and CO 2 -enriched streams. More than a thousand (1260) simulations were conducted to the periodic state to study and interpret the effects of the light-product purge-to-feed ratio, the cycle step time, the high-to-low pressure ratio, the heavyproduct recycle ratio, and the feed throughput (θ) on the process performance. The cycle configuration was changed from 4-bed 4-step, 4-bed 5-step, and 5-bed 5-step designs that utilized combinations of light-reflux (LR) and/or heavy-reflux (HR) steps, and cocurrent depressurization (CoD) and/or countercurrent depressurization (CnD) steps. The process performance was judged in terms of the CO 2 purity in the heavy product (y CO 2 ,HP ), with the CO 2 recovery (R CO 2 ) and θ both being secondary process performance indicators. Any PSA process with a HR step outperformed any PSA process with only a LR step, regardless of whether a CoD step was added or not. The best performance was obtained from the 4-bed 4-step stripping PSA cycle with HR obtained from the CnD step, with y CO 2 ,HP ) 82.7 vol %, R CO 2 ) 17.4%, and θ ) 14.4 L STP h -1 kg -1 . The next best performance was obtained from the 5-bed 5-step stripping PSA cycle with LR and HR obtained from LR purge, with y CO 2 ,HP ) 75.5 vol %, R CO 2 ) 48.8%, and θ ) 23.1 L STP h -1 kg -1 . Overall, this study further substantiated the feasibility of a high-temperature stripping PSA cycle for CO 2 concentration from flue gas using an HTlc adsorbent. It also disclosed the importance of the PSA cycle configuration to the process performance, by gaining an understanding of and appreciation for the use of HR, and it exposed the rigor involved in determining the best PSA cycle sequence for a given application.

A five-step physiochemical pathway for the cyclic dehydrogenation and rehydrogenation of LiAlH4 from Li3AlH6, LiH, and Al was developed. The LiAlH4 produced by this physiochemical route exhibited excellent dehydrogenation kinetics in the 80-100 degrees C range, providing about 4 wt % hydrogen. The decomposed LiAlH4 was also fully rehydrogenated through the physiochemical pathway using tetrahydrofuran (THF). The enthalpy change associated with the formation of a LiAlH4.4THF adduct in THF played the essential role in fostering this rehydrogenation from the Li3AlH6, LiH, and Al dehydrogenation products. The kinetics of rehydrogenation was also significantly improved by adding Ti as a catalyst and by mechanochemical treatment, with the decomposition products readily converting into LiAlH4 at ambient temperature and pressures of 4.5-97.5 bar.

The effects of SWNTs, MWNTs, AC, C(60), and G when used as a cocatalyst with Ti on the dehydrogenation and hydrogenation kinetics of NaAlH(4) were investigated for the first time in the important temperature range of 90 to 250 degrees C. All five carbons exhibited significant, sustaining, and synergistic cocatalytic effects on the dehydrogenation and hydrogenation kinetics of Ti-doped NaAlH(4) that persisted through charge and discharge cycling. SWNTs were the best cocatalyst, G was the worst cocatalyst, and all five carbons were inactive as a catalyst unless Ti was present. The carbon most likely was imparting an electronic contribution through the interaction of its facile pi-electrons with Ti through a hydrogen spillover mechanism, which explained why one carbon was better than another one in terms of optimal aromatic character, out-of-plane exposure of pi-electrons, and interaction of pi-bonds with neighboring sheets.

A new procedure for the direct synthesis of metal complex hydrides, NaAlH 4 and LiAlH 4 , has been developed through hydrogenation of their respective metal hydrides, NaH and LiH, with Al, both in the presence of a catalyst and in the presence of a liquid complexing agent. This procedure involves three steps: blending the TiCl 3 catalyst precursor with reactants using dry high-pressure ball milling (HPBM) in hydrogen, hydrogenation using HPBM in hydrogen and tetrahydrofuran (THF), and vacuum filtration and drying. Essentially, pure NaAlH 4 and LiAlH 4 products were produced in high yield at ambient temperature and near ambient pressure. For the Na and Li systems, the Ti nanoparticles, stabilized in THF, together with the formation of a MAlH 4 ‚ xTHF adduct (M ) Na or Li), both facilitated the hydrogenation of MH at such mild conditions. For the Na system, x was equal to 4 or 8, but the interaction was very weak, and for the Li system, x was equal to 4, but the interaction was very strong. In general, THF, the Ti catalyst, and wet HPBM all played essential roles in promoting the hydrogenation reactions, which proceeded through either a direct pathway (2MH + 2Al + 3H 2 f 2MAlH 4 ), or a sequential pathway (6MH + 2Al + 3H 2 f 2M 3 AlH 6 followed by M 3 AlH 6 + 2Al + 3H 2 f 3MAlH 4 ), with the formation of sodium hexahydridoaluminate limiting in the latter case. The same procedure was tried unsuccessfully for the direct synthesis of Mg(AlH 4 ) 2 from MgH 2 and for AlH 3 from Al.

A novel 2-bed 4-step enriching pressure vacuum swing adsorption (PVSA) cycle was developed for the production of N2 from air, using an X-type zeolite, with the separation being equilibrium-driven. The four steps were low-pressure feed, countercurrent heavy product pressurization, countercurrent high-pressure heavy reflux, and cocurrent depressurization. The enriched heavy and light products (N2 and O2, respectively) were withdrawn from the system during the feed and heavy reflux steps, with both products being produced at the high pressure. In a series of 20 PVSA experiments, which were performed at a constant high-to-low-pressure ratio (π = 4.1), the effects of the feed and heavy product flow rates on the periodic state process performance were studied in terms of the O2 impurity, N2 recovery, N2 productivity, and feed throughput. The observed trends were as expected, with higher N2 recoveries (and corresponding N2 productivities) and O2 impurities generally being obtained at higher feed throughputs and heavy product flow rates. With regard to performance, in terms of N2 purity, one run produced a relatively high-purity N2 product that contained 0.8 vol % O2 at a N2 recovery of 23.7%, N2 productivity of 7.0 L STP h-1 kg-1, and feed throughput of 38.2 L STP h-1 kg-1. In terms of N2 recovery, another run produced a relatively high N2 recovery of 69.4% at a N2 productivity of 38.5 L STP h-1 kg-1 and feed throughput of 60.6 L STP h-1 kg-1, with the N2 product containing 10.6 vol % O2. This run also resulted in the highest N2 productivity. The overall performance of this relatively simple 2-bed 4-step enriching PVSA cycle compared quite favorably with the performances of the more-familiar N2 PSA processes that are either equilibrium-driven or kinetically driven and based exclusively on the stripping PSA cycle concept, especially because π was reasonably small.