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Adsorption: Journal of the International Adsorption Society (v.4, #2)
Mathematical Modeling of Multicomponent Nonisothermal Adsorption in Sorbent Particles Under Pressure Swing Conditions by Atanas Serbezov; Stratis V. Sotirchos (pp. 93-111).
A detailed model for nonisothermal sorption of multicomponent mixtures in a single sorbent particle (monodisperse or bidisperse with negligible intracrystalline mass transport limitations) under pressure swing conditions is developed in this study. The dusty-gas model is used to describe the coupling of the molar fluxes, the temperature, the partial pressures and the partial pressure gradients of the components in the pore space of the particle. The variations of the temperature are described by an energy equation in which both convective and conductive modes of heat transport are accounted for. No limitations are imposed on the number of the components in the mixture and on the type of the adsorption isotherm. The model is applied in the investigation of the industrially important air-zeolite 5A system. Two cases with respect to the surrounding gas phase are examined: infinite environment, which is representative for single particle experiments, and finite environment, which is representative for the situation in packed bed adsorbers. It is found that in an infinite environment the external and internal temperature gradients are equally important while in a finite environment the external heat transport limitations are negligible. It is concluded that in modeling the nonisothermal operation of adsorption processes occurring in packed beds it is not necessary to allow for the temperature differences between the gas phase and the surface of the adsorbing particles. Furthermore, if the temperature gradients within the particles can be neglected, only a single temperature equation is needed to describe the energy transport in the bed.
Keywords: heat effects; pressure swing adsorption; temperature swing adsorption; multicomponent mass transport; dusty-gas model; boundary conditions
On the Optimisation of Periodic Adsorption Processes by S. Nilchan; C.C. Pantelides (pp. 113-147).
A rigorous mathematical programming based approach to the optimisation of general periodic adsorption processes is presented. Detailed dynamic models taking account of the spatial variation of properties within the adsorption bed(s) are used. The resulting systems of partial differential and algebraic equations are reduced to sets of algebraic constraints by discretisation with respect to both spatial and temporal dimensions. Periodic boundary conditions are imposed to represent directly the “cyclic steady-state” of the system. Additional constraints are introduced to characterise the interactions between multiple beds in the process as well as any relevant design specifications and operating restrictions. The optimal operating and/or design decisions can be determined by solving an optimisation problem with constraints representing a single bed over a single cycle of operation, irrespective of the number of adsorption beds in the process.
Keywords: pressure-swing adsorption; periodic processes; process optimisation; dynamic optimisation
Effect of Operation Symmetry on Pressure Swing Adsorption Process by Shain J. Doong; Paul Propsner (pp. 149-158).
In a multi-bed pressure swing adsorption (PSA) process, cycle steps with gas flow transferring from one bed to another such as equalization, purge, etc. are generally practiced to enhance the product recovery. However, if the flows for the connected beds in these steps are not balanced, the PSA process may not operate in a symmetrical manner. In the modeling of the PSA process, most of the simulations consider only one bed and assume that the rest of the beds would behave in a same way. In order to assess the impact of bed symmetry on the PSA performance, a new PSA model capable of studying bed symmetry in a two-bed system is developed. Experimental results from this paper show that uneven equalization flow can result in a lower product purity and a peculiar purity curve at different equalization levels. This phenomenon can be successfully predicted by this model. Simulation results also show that in large-scale PSA units, asymmetrical operation can cause drastically different temperature profiles in different adsorbers and hence a much lower performance. This paper demonstrates the importance of maintaining operation symmetry in PSA processes.
Keywords: pressure swing adsorption; gas separation; simulation
Periodic State Heat Effects in Pressure Swing Adsorption-Solvent Vapor Recovery by Yujun Liu; James A. Ritter (pp. 159-172).
Heat effects in the pressure swing adsorption (PSA)-n-butane vapor recovery process were investigated at the periodic state by computer simulation. The PSA process utilized a two-bed, four-step, vacuum swing cycle and BAX activated carbon as the adsorbent. The heat effects were manifested by varying the heat transfer coefficient (h) from isothermal to adiabatic, while simultaneously varying the adsorbed phase heat capacity (Cpa) from zero to that of the saturated liquid. In terms of the bed capacity factor (BCF), isothermal operation always resulted in the best performance, whereas adiabatic operation was not the worst; independent of Cpa, the worst performance occurred at an intermediate h. Cpa also had a significant effect on the BCF, where a larger Cpa (i.e., a larger heat sink) always decreased the BCF and thus improved the process performance. A factorial analysis showed that the effect of Cpa on the BCF became even more pronounced as the cycle time increased. h and Cpa had essentially no effect on the solvent vapor enrichment under the conditions investigated. Overall, this study demonstrated that the effects of h and Cpa are uniquely coupled; thus knowing their magnitudes is paramount to obtaining accurate predictions from a PSA-solvent vapor recovery model.
Keywords: computer simulation; butane; activated carbon; environmental; adsorbed phase heat capacity
R and D Note: Separation of a Nitrogen-Carbon Dioxide Mixture by Rapid Pressure Swing Adsorption by Zhixiong Zhang; Jianyu Guan; Zhenhua Ye (pp. 173-177).
A N2-CO2 mixture is separated in a rapid pressure swing adsorption apparatus, which consists of single or double adsorbent beds filled with silica gel and operates in the sequence of adsorption, backflow and desorption. Nitrogen-rich gas is produced at the top of the bed, and carbon dioxide-rich gas at the bottom. Carbon dioxide purity of 89.5% and recovery of 70% were obtained in the single-bed apparatus, while purity of 93.5% and recovery of 72.3% were obtained in the double-bed apparatus. The feed in both cases consisted of 81% N2 and 19% CO2.
Keywords: rapid pressure swing adsorption; parametric pumping; carbon dioxide recovery