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To produce O2 with a high purity of 99+% and a high productivity from various oxygen-rich feeds, a parametric study was done on a six-step pressure swing adsorption (PSA) purifier using carbon molecular sieve (CMS). The cyclic performances of the PSA process such as purity, recovery, and productivity were compared under nonisothermal conditions. To study the effects of N2 amount on the PSA purifier, various feeds with 90% O2 or more were experimentally and theoretically applied for the PSA process. Since N2 plays a key role in product purity, the maximum purity of the PSA was 99% O2 with 51.5% recovery from a higher nitrogen feed (O2:Ar:N2; 90:4:6 vol %) and 99.8% O2 with 56.9% recovery from a lower nitrogen feed (O2:Ar:N2; 95:4:1 vol %) within the experimental range. The adsorption step time and feed flow rate served as key operating variables in the purification of the oxygen-rich feeds because the concentration wave fronts of minor impurities such as N2 and Ar were controlled by kinetic selectivity. To...

The adsorption dynamics of N2, O2, and Ar in kinetic separation bed with CMS and equilibrium separation bed with zeolite 13X were investigated by using dried air as a feed. In the CMS bed initially saturated with He, Ar was the first breakthrough component showing a small roll-up and N2 followed at a very close interval. Then, the breakthrough of O2 occurred with a broad roll-up due to its fast diffusion rate and the relatively slow diffusion rate of N2. In the zeolite 13X bed initially saturated with O2, the breakthrough of Ar first occurred with roll-up owing to the strong adsorption of N2, then the breakthrough of N2 followed after a very short interval. Based upon these results, the cyclic adsorption dynamics of the zeolite 13X VSA for air bulk separation and CMS PSA for oxygen purification were studied. The five-step two-bed O2 VSA with zeolite 13X produced O2 of over 90% purity with high recovery. The MTZ variation of N2 during the adsorption and vacuum steps was described in detail. In the case of the six-step two-bed PSA process for O2 purification, O2 of 99.8+% purity could be produced from the binary mixture (O2/Ar—95:5 vol.%). The dynamic adsorption behaviors were investigated by using a concentration-dependent rate model incorporated with mass, energy, and momentum balances. The model reasonably predicted the adsorption dynamics at the equilibrium and kinetic separation beds.

Four different pressure swing adsorption (PSA) cycles using CMS for oxygen purification were developed to produce high-purity oxygen of over 99% with a high level of productivity. The cyclic performances such as purity, recovery, and productivity of the four different PSA cycles were experimentally and theoretically compared under non-isothermal conditions. In addition, one binary (O 2 /Ar; 95:5 vol%) and two ternary (O 2 /Ar/N 2 ; 95:4:1 and 90:4:6 vol%) mixtures were used to study the effects of feed composition. The PSA cycles with two consecutive blowdown steps produced oxygen with 98.0–99.9% purity and 56–66% recovery. The PSA cycle with oxygen generation in the second blowdown step produced oxygen with a higher level of purity and productivity. Also, the cycle introducing a pressure equalization step instead of a pure step produced oxygen with about 99.8% purity and 78% recovery. The period for the cyclic steady state of the ternary feed with 1% N 2 was slightly longer than that of the binary feed, while the PSA performance of the ternary feed was similar to that of the binary feed without nitrogen. However, in the ternary feed with 6% N 2 , the purity of the O 2 in the purification cycles decreased by up to 97.3%. Therefore, nitrogen played a key role in producing high-purity oxygen in the CMS PSA instead of argon.

A three-bed PVSA (pressure vacuum swing adsorption) process, combining equilibrium separation with kinetic separation, was developed to overcome the 94% O2 purity restriction inherent to air separation in the adsorption process. To produce 97+% and/or 99+% purity O2 directly from air, the PVSA process with two zeolite 10X beds and one CMS bed was executed at 33.44–45.60 to 253.31 kPa. In addition, the effluent gas from the CMS bed to be used for O2 purification was backfilled to the zeolite 10X bed to improve its purity, recovery, and productivity in bulk separation of the air. PVSA I, which made use of a single blowdown/backfill step, produced an O2 product with a purity of 95.4–97.4% and a recovery of 43.4–84.8%, whereas PVSA II, which used two consecutive blowdown/backfill steps, produced O2 with a purity of 98.2–99.2% and a recovery of 47.2–63.6%. Because the primary impurity in the O2 product was Ar, the amounts of N2 contained in the product were in the range of 4000–5000 ppm at PVSA I and several tens of ppm at PVSA II. A nonisothermal dynamic model incorporating mass, energy, and momentum balances was applied to predict the process dynamics. Using the linear driving force (LDF) model with constant diffusivity for the equilibrium separation bed and a modified LDF model with concentration dependency of the diffusion rate for the kinetic separation bed, the dynamic model was able to accurately predict the results of the experiment. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Adsorption and desorption in zeolite 5A and CMS beds were compared by using a ternary mixture (N 2/ O2/Ar; 78 : 21 : 1 vol%). Because the breakthrough curves for both beds show a tail by temperature variance, a non- isothermal mathematical model was applied to the simulation of adsorption dynamics. The LDF model with a constant rate parameter was enough to predict the experimental brea kthrough and temperature curves of an equilibrium separa- tion bed, while the modified LDF model with a concentrati on-dependent parameter should be applied to a kinetic sep- aration bed. In the CMS bed initially saturated with He, Ar was the first breakthrough component with N 2 following after a short interval. Then, after a long interval, the breakthrough of O 2 occurred with a broad roll-up due to its fast diffusion rate and the relative ly slow diffusion rate of N 2. In the CMS bed initially saturated with O 2, the breakthrough curves of O2 and N2 showed a very broad shape because of the slow diffusion of N 2 into CMS. In the zeolite 5A bed, the breakthrough time sequence was Ar, O2, and N2 at very close time intervals. After the sharp roll-ups of O 2 and Ar, the variation of the breakthrough curves was negligible. The in flection of the temperature profile in the zeolite 5A bed was caused by the crossover of the O 2 and N2 MTZs, while in the CMS bed it was caused by the difference in the diffu- sion rates of O2 and N2.

Adsorption and desorption experiments for the binary mixture (N2/O2; 79:21 vol%) on zeolite 5A, 10X, and 13X beds were performed to study the dynamic characteristics of air separation adsorption processes. Because the breakthrough and desorption curves showed a tail by temperature variance in the beds, a nonisothermal dynamic model incorporating mass and energy balances was applied to the simulation of adsorption dynamics using the Langmuir–Freundlich model and the LDF approximation. The breakthrough and desorption results were compared among three different beds with respect to the breakthrough and desorption times, tailing effect, and temperature variation with the effects of pressure and flow rate. On the basis of the similar bed density, the order of breakthrough time and desorption time was zeolite 10X, 13X, and 5A beds. Also, the O2 MTZ of the zeolite 10X bed was slightly sharper than those of the zeolite 5A and 13X beds due to more favorable N2 isotherm of zeolite 10X. Furthermore, the breakthrough...

Adsorption characteristics in a layered bed packed with activated carbon and zeolite 5A were studied experimentally and theoretically through the breakthrough experiments of binary (H2/CO, H2/CH4, H2/CO2), ternary (H2/CH4/CO), and five-component (H2/CH4/CO/N2/CO2) systems. The effects of adsorption pressure, feed flow rate, and carbon-to-zeolite ratio on the breakthrough curve in a layered bed were observed. Breakthrough curves in all of the mixtures showed tailing due to temperature variance in the bed by the heat of adsorption. In the layered bed, each mixture showed the specific characteristics of concentration and temperature wave fronts in each layer. Although the amount of impurity in a ternary mixture is larger than that in a binary mixture, the breakthrough time and elongation depending on the adsorption pressure and flow rate showed an intermediate value between the results of the H2/CO and H2/CH4 mixtures, and the tailing was not much different from those in these binary systems. Also, the roll-...