Study on Pressure Swing Adsorption Removing C+2 from Natural Gas as Raw Material for Thermal Chlorination

The experimental investigation demonstrates that a satisfactory result can be expected for pressure swing adsorption (PSA) purification of natural gas as raw material for thermal chlorination process. Using hh-4 molecular sieve as adsorbent for removing C2 components, the suitable adsorption pressure is 0.4-0.45 MPa, desorption vacuum is 0.08-0.09 MPa and circulation time is 20-21 min.     

Parametric Study of a Pressure Swing Adsorption Process

The performance of a pressure swing adsorption (PSA) process for production of high purity hydrogen from a binary methane-hydrogen mixture is simulated using a detailed, adiabatic PSA model. An activated carbon is used for selective adsorption of methane over hydrogen. The effects of various independent process variables (feed gas pressure and composition, purge gas pressure and quantity, configuration of process steps) on the key dependent process variables (hydrogen recovery at high purity, hydrogen production capacity) are evaluated. It is demonstrated that many different combinations of PSA process steps, their operating conditions, and the feed gas conditions can be chosen to produce an identical product gas with different hydrogen recovery and productivity.     

Effect of Operation Symmetry on Pressure Swing Adsorption Process

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.     

捕集高湿烟气中CO2的变压吸附技术

变压吸附法在捕集烟气中的CO₂这一领域中显示出强大的优越性,但实际电厂烟气含有少量的水蒸气,这对利用常规吸附剂捕集CO₂造成很大挑战。为解决上述瓶颈问题,改进变压吸附工艺以及开发对湿度不敏感、高效的吸附剂成为最主要的途径。本文详细介绍了两种常用的变压吸附工艺,即多层变压吸附及微波辅助真空再生法分离高湿烟气的研究进展,综述了近年来研发的适于捕集高湿烟气的高效吸附剂,系统阐述了各种吸附剂的物理化学特性及其吸附CO₂、H₂O的机制,并在此基础上讨论了变压吸附技术捕集高湿烟气时存在的问题,提出了研究展望。相信随着人们对变压吸附工艺的改进以及对新型高效吸附剂的进一步研发,必将显著降低捕集高湿烟气中CO₂的成本,这将对燃煤电厂等高湿CO₂排放源的温室气体减排具有重要意义。     

Pressure Swing Adsorption Technologies for Carbon Dioxide Capture

The principles of pressure swing adsorption (PSA) for carbon dioxide capture are reviewed. Previous work on PSA, relevant modeling and experimental investigation for specifically carbon dioxide separation are also presented and significant findings highlighted. Simple rules for PSA process design based on analysis of the inherent properties of adsorbate-adsorbent systems encompassing equilibrium isotherm, adsorption kinetics, shape of breakthrough curves, screening and selection of adsorbent, bed porosity, adsorption time, purge to feed ratio, residence time, pressure equalization and rinse steps are provided to promote better understanding of the technology so that it gains wider acceptance in the future to address the global environmental concern, particularly in the removal of carbon dioxide as a greenhouse gas.     

Comparison of Finite Difference Techniques for Simulating Pressure Swing Adsorption

Three different finite-difference routines were compared for solving the nonlinear, coupled, partial differential and algebraic equations that describe pressure swing adsorption processes. A successive substitution method (SS), a block LU decomposition procedure (BLUD), and the method of lines approach with adaptive time stepping (DASSL) were used to simulate and compare the computation times required to reach the periodic state for two different PSA systems: PSA-air drying and PSA-solvent vapor recovery. For both systems, the results showed that DASSL was nearly twice as fast as BLUD, whereas SS was nearly an order of magnitude slower than BLUD. DASSL and BLUD were also very robust and accurate, as nearly identical bed profiles were obtained from both methods under both transient and periodic state conditions.     

Simulation of a Combined Isomerization Reactor & Pressure Swing Adsorption Unit

The Total Isomerization Process developed by Union Carbide in 1970 (Gary, 1987) for the conversion of normal paraffin's to their isomers consists of a reactor followed by a PSA unit each operating at similar pressures and temperatures. The combination of these two operations in one unit in a Pressure Swing Adsorption Reactor (PSAR) process may provide an increased throughput and a significant cost saving in ancillary equipment.The simulation of a mathematical model linking the catalyst packed-bed and the adsorbent packed-bed is reported. The catalyst is a Pd/Y-zeolite and the adsorbent is 5A zeolite. The simulated feed consists of 17% each of n- and isopentane with the remainder being hydrogen. The mathematical model assumes dispersed plug-flow in both sections, constant velocity in the reactor section but varying in the adsorber, with mass transfer in the adsorber section due to external fluid film resistance and macropore diffusion in series. The fraction of the total column length occupied by the catalyst (denoted by ) is accounted for in the model by solving numerically using orthogonal collocation on finite elements. Parameters varied are the ratio of catalyst/column length (), temperature range (506–533 K), high pressure (15–20 bars), with the low pressure held constant at 2 bars. The catalyst/column ratio has a strong effect at low temperatures. The optimum catalyst/column length ratio appears to be controlled by the low pressure step and occurs at = 0.7 for the assumptions used in this work.     

Mathematical Modeling of Multicomponent Nonisothermal Adsorption in Sorbent Particles Under Pressure Swing Conditions

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.     

An Equilibrium Relation for Gas Adsorption Applications

A new multicomponent equilibrium relation is proposed for engineering applications. This relation is based on a lattice model for mixtures of unequally sized molecules. An approximation is introduced for loading-dependent enthalies of adsorption that simplifies the energy balance in fixed-bed models and allows efficient solution of the equation set in process simulations. Comparisons are made with nonideal binary data in the literature.     

R and D Note: Separation of a Nitrogen-Carbon Dioxide Mixture by Rapid Pressure Swing Adsorption

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.