Generic Modeling Framework for Gas Separations Using Multibed Pressure Swing Adsorption Processes

This report summarizes the work performed under the Department of Energy’s National Energy Technology Laboratory (DOE-NETL) Advanced Energy Systems Program award number DE-FE0026163 Improving Energy Efficiency of Air Separation via Hollow Fiber Sorbents. The overall objective of the project was to develop technology to produce oxygen for use in coal gasification processes at a significantly lower cost than that of the commercial state-of-the-art technology. The focus of the project was development and optimization of hollow fiber sorbents for use in a pressure swing adsorption (PSA) process for producing oxygen from air. The key technical advancement under this program was the development of a hollow fiber sorbent capable of producing oxygen in a PSA process. PSAs produce oxygen by using a beaded zeolite material (typically lithium exchanged zeolites or LiX) that selectively adsorb nitrogen over oxygen. For a fiber to adsorb nitrogen, LiX needed to be integrated into a polymer fiber and then activated at over 350C to remove residual moisture. After screening multiple materials and process techniques, Matrimid/LiX hollow fibers were produced that were able to withstand the activation and had crystalline nitrogen capacity close to pure LiX crystals. The fibers were dual layer fibers with a barrier layer isolating the hollow bore from the LiX embedded layer. The barrier layer was formed from an Ultem® based solution. The hollow bore was intended for encapsulating phase change materials to ensure isothermal PSA operation. Modeling work from the project suggested that the benefits of this additional complexity were marginal, thus the team fabricated non-hollow fibers comprised of Matrimid and LiX. The Matrimid/LiX fibers were loaded into a module and tested on a bench scale PSA unit. The fibers were able to separate oxygen from air, but the purity and recovery were lower than expected and lower than traditional beaded LiX zeolites, although modeling results suggested that the bed sizes for fiber sorbents could be half the size of a beaded LiX bed. Additional work will be needed to understand the shortfall and to produce fiber sorbent modules with performance equal to the targets set for this program. In addition to producing fibers, modeling work was carried out at both the fiber level and process level to guide both fiber sorbent development and to determine the optimal process conditions for a PSA using fiber sorbents. A 2D hollow fiber model was developed in gPROMS Model Builder. This model was based on literature data for mass transfer and other parameters. This model used a basic single bed PSA process to determine important fiber properties and process conditions for achieving high oxygen recovery. The preliminary 2D fiber model was a valuable tool, however, for step-out technologies, such as a PSA process with fiber sorbents, intelligent optimization and plant control was needed to effectively determine minimum power designs. Therefore, a gPROMS Process Builder-based optimization tool was developed with an “Automated Cycle Optimizer” model that was able to determine optimal cycle conditions for a fiber sorbent PSA. The results from the optimizer were used in the final power analysis. Flow sheet analysis and techno-economic analysis (TEA) was carried out to determine a lower power solution versus the baseline case. The original project scope suggested that the fiber sorbent PSA could be used to debottleneck a cryogenic air separation plant (cryo-ASU) with the combined plant having lower power than a standalone cryo-ASU. However, early flowsheet analysis showed that the PSA/cryo-ASU concept would not lead to a lower power solution. However, further flowsheet analysis suggested that an oxygen PSA combined with a nitrogen PSA could have lower power versus state-of-the-art air separation technology. Work concluded with the TEA showing a fiber sorbent oxygen PSA combined with an advanced nitrogen PSA could be up to 8% lower power versus the base case cryo-ASU. Overall, there is significant potential remaining for the zeolite hollow fiber sorbent concept, and this project successfully identified -- and ultimately remedied -- many technical challenges associated with scale-up.

Adsorptive separations have gained considerable importance in process industry. A generic simulator has been developed in this work for Pressure Swing Adsorption (PSA) processes. Several simple and complex, conventional and unconventional PSA cycles have been studied using this simulator. Distinction has been made between PSA processes where a raffinate stream richer in the weakly adsorbed component as compared to the feed is the desired product as against processes where extract stream richer in the strongly adsorbed component is the desired product. These are termed as raffinate PSA and extract PSA respectively. Extract PSA is an unconventional process variation. The studies include several simple and complex PSA cycles for raffinate and extract PSA of industrial importance. The studies are aided by a generic simulator for all PSA process variations developed for the purpose. The simulator is equally applicable to Vacuum Swing Adsorption (VSA), Pressure Vacuum Swing Adsorption (PVSA), rapid cycle PSA processes, etc.

Heuristic design of pressure swing adsorption: a preliminary study

The enrichment of useful trace components in air was carried out using a new two-bed pressure swing adsorption (PSA) process that utilized an enriching reflux (ER) cycle designed specifically for enriching the heavy component. Another new feature of this PSA process was the use of parallel equalization (PEQ), which not only saved recompression energy, but also favorably exchanged the axial concentration distribution between the two columns. These new PSA concepts were demonstrated by experimentally enriching Xe and CO2 in air using 13X MS zeolite. ER PSA substantially enriched both gases to values as high as 80 times the feed concentration with 90% recovery using a pressure ratio of only 12.5. This enrichment was far superior to that achieved with a conventional stripping reflux (SR) PSA cycle, which was limited thermodynamically to the pressure ratio and was typically less than half of it. A parametric study revealed the effects of the pressure ratio, half-cycle time, feed flow rate, flow rate ratio (enriched gas/feed), and several different PEQ schemes on the enrichment of each gas. Increases in the flow rate and pressure ratios resulted in increases in the enrichments of both gases, whereas increases in the feed flow rate and half-cycle time revealed maxima in the enrichments. Partial PEQ through the upper half of the column nearer to the feed (lean) end was also determined to be superior to other PEQ schemes.

The performance of multi-bed pressure swing adsorption (PSA) process for producing high purity hydrogen from synthesis gas was studied experimentally and theoretically using layered beds of activated carbon and zeolite 5A. Nonisothermal and nonadiabatic models, considering linear driving force model and Dual-site Langmuir adsorption isotherm model, were used. The effects of the following PSA variables on separation process were investigated: linear velocity of feed, adsorption time and purge gas quantity. As a result, we recovered a high purity H2 product (99.999%) with a recovery of 66% from synthesis gas when the pressure was cycled between 1 and 8 atm at ambient temperature.

An equilibrium model for pressure swing adsorption (PSA) process with equalization step is developed by using the method of characteristics. The concentration profiles at the cyclic steady state are obtained by tracking the movement of wave fronts derived from the characteristic lines during the transient cycles. The results of this model compare quite well with the conventional finite difference method, but with less computational effort. Multiple waves in PSA processes are clearly demonstrated by this model. Effects of equalization step on PSA process performances are also analyzed using this model.

Global warming has become more and more serious, which is caused by greenhouse gases. Cutting down the emission of CO2 has already become one of the major research target in the world. This study utilized a pressure swing adsorption (PSA) process to separate high-purity hydrogen and to capture CO2 from synthesis gas, which is the effluent stream of water-gas-shift reactor. The purified H2 can be sent to gas turbine for generating electrical power or can be used for other energy source, whilst the CO2 can be recovered and sequestrated to reduce greenhouse-gas effects. The PSA process studied is a two stage dual-bed eight-step process at room temperature using adsorbents: modified activated carbon AC5-KS and zeolite 13X-Ca. It is assumed that the gas mixture from which water has been removed enters the PSA process. The feed gas entering the PSA process consists of 1.3 % CO, 41.4 % CO2 and 57.3 % H2. It uses the method of lines combined with upwind differences, cubic spline approximation and LSODE of ODEPACK software to solve the equations. The optimal operating condition is obtained by varying the operating variables, such as feed pressure, bed length, step time, etc. Furthermore, the first stage H2-PSA could achieve 99.98 % purity and 79 % recovery of H2 as the top product and the second stage CO2-PSA could obtain about 92 % purity and 98 % recovery of CO2 as the bottom product. By PSA process, the goal of energy generation and environmental protection could be achieved at the same time.

Combination of adsorption followed by ozone oxidation with pressure swing adsorption technology for the removal of VOCs from contaminated air streams

Carbon molecular sieves for gas separation processes

Besides adsorption equilibrium and adsorption kinetics, nonisothermal temperature effects have a high impact on the simulation results and hence on the process design of pressure swing adsorption (PSA) processes. Due to moderate pressures of PSA processes, the gas phase is usually treated as ideal for the ease of solving the underlying model equations. As a consequence, only the ideal gas enthalpy is considered for the energy balance of the gas phase of the adsorber column. Real gas effects like the Joule-Thomson effect or the real gas mixing enthalpy effects are usually neglected. With the simulation package ADLIN, it is possible to model the gas phase in general as real phase in both the material and energy balance while the accuracy is depending on the applied underlying real gas model. The modelling of the adsorbed phase is not subject to this investigation and is modelled in the same manner as usual. On an example of detailed dynamic modelling of high pressure swing adsorption for the purification of hydrogen, the impact of nonideal gas phase vs. ideal gas phase towards the simulation results is shown. Furthermore, additional results of PSA processes are discussed based on the overall material and energy balance of the gas separation process. It turns out that for the simulation of typical hydrogen purification processes by PSA the ideal gas model is usually sufficient to get reasonable results. For high adsorption pressure or methane purification PSA, the nonideal gas simulation results deviate significantly from the ideal gas behavior, and real gas models should be applied for detailed dynamic PSA simulation. Although the capacity of the PSA was generally increased due to the nonideality of the gas phase, the product recovery was generally reduced.

Integrated gasification combined cycle (IGCC) with CO2 capture and sequestration (CCS) is a promising technology to efficiently mitigate the emission of CO2. Warm CO2 removal has been predicted to make the CO2 capture process more efficient. Here, we investigate the efficiency penalties associated with CO2 removal via a pressure swing adsorption (PSA) process using metal hydroxide sorbents at elevated temperature. We use numerical models constructed in MATLAB and integrate these with Aspen Plus process simulations. We apply these models to both general metal hydroxides of variable enthalpy of adsorption and real metal hydroxides identified using density functional theory (DFT) calculations. We show that having an enthalpy of adsorption between 15 and 20 kJ/mol results in a PSA process that gives an overall IGCC–CCS efficiency that is competitive with the conventional IGCC–CCS process using (cold) Selexol. An enthalpy of adsorption of 20 kJ/mol is predicted to be the most favorable because it yielded a pro...

Dynamic adsorption–desorption measurements of CO2 and CH4 in amino-MIL-53(Al) were carried out in an adsorption breakthrough setup at different temperatures (303, 318, and 333 K) and pressures (1, 5, and 30 bar) to study the desorption dynamics of CO2 in amino-MIL(Al) as it plays an important role in the design of pressure swing adsorption (PSA) process for the upgrading of biogas. 13X zeolite was used as a reference material. The dynamic adsorption selectivity as well as the desorption efficiency of CO2 in both amino-MIL-53(Al) and 13X zeolite were calculated to evaluate the potential of amino-MIL-53(Al) for the upgrading of biogas by PSA 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.

Approximation of a PSA process based on an equivalent continuous countercurrent flow process: Blow-up and blow-down representation

To recover 99% CO2 from flue gas containing 10−15% CO2, a two-stage pressure swing adsorption (PSA) process is usually employed. At the first stage of the two-stage PSA, CO2 is concentrated to 40−6...