A superstructure‐based optimal synthesis of PSA cycles for post‐combustion CO2 capture

Recent developments have shown pressure/vacuum swing adsorption (PSA/VSA) to be a promising option to effectively capture CO₂ from flue gas streams. In most commercial PSA cycles, the weakly adsorbed component in the mixture is the desired product, and enriching the strongly adsorbed CO₂ is not a concern. On the other hand, it is necessary to concentrate CO₂ to high purity to reduce CO₂ sequestration costs and minimize safety and environmental risks. Thus, it is necessary to develop PSA processes specifically targeted to obtain pure strongly adsorbed component. A multitude of PSA/VSA cycles have been developed in the literature for CO₂ capture from feedstocks low in CO₂ concentration. However, no systematic methodology has been suggested to develop, evaluate, and optimize PSA cycles for high purity CO₂ capture. This study presents a systematic optimization‐based formulation to synthesize novel PSA cycles for a given application. In particular, a novel PSA superstructure is presented to design optimal PSA cycle configurations and evaluate CO₂ capture strategies. The superstructure is rich enough to predict a number of different PSA operating steps. The bed connections in the superstructure are governed by time‐dependent control variables, which can be varied to realize most PSA operating steps. An optimal sequence of operating steps is achieved through the formulation of an optimal control problem with the partial differential and algebraic equations of the PSA system and the cyclic steady state condition. Large‐scale optimization capabilities have enabled us to adopt a complete discretization methodology to solve the optimal control problem as a large‐scale nonlinear program, using the nonlinear optimization solver IPOPT. The superstructure approach is demonstrated for case studies related to post‐combustion CO₂ capture. In particular, optimal PSA cycles were synthesized, which maximize CO₂ recovery for a given purity, and minimize overall power consumption. The results show the potential of the superstructure to predict PSA cycles with up to 98% purity and recovery of CO₂. Moreover, for recovery of around 85% and purity of over 90%, these cycles can recover CO₂ from atmospheric flue gas with a low power consumption of 465 k Wh tonne^?1 CO₂. The approach presented is, therefore, very promising and quite useful for evaluating the suitability of different adsorbents, feedstocks, and operating strategies for PSA, and assessing its usefulness for CO₂ capture. Published 2009 American Institute of Chemical Engineers AIChE J, 2010     

Efficient pressure swing adsorption for improving H<Subscript>2</Subscript> recovery in precombustion CO<Subscript>2</Subscript> capture

An efficient design for pressure swing adsorption (PSA) operations is introduced for CO₂ capture in the pre-combustion process to improve H₂ recovery and CO₂ purity at a low energy consumption. The proposed PSA sequence increases the H₂ recovery by introducing a purge step which uses a recycle of CO₂-rich stream and a pressure equalizing step. The H₂ recovery from the syngas can be increased over 98% by providing a sufficient purge flow of 48.8% of the initial syngas feeding rate. The bed size (375m³/(kmol CO₂/s)) and the energy consumption for the compression of recycled CO₂-rich gas (6 kW/(mol CO₂/s)) are much smaller than those of other PSA processes that have a CO₂ compression system to increase the product purity and recovery.     

Optimization and Control of Pressure Swing Adsorption Processes Under Uncertainty

The real‐time periodic performance of a pressure swing adsorption (PSA) system strongly depends on the choice of key decision variables and operational considerations such as processing steps and column pressure temporal profiles, making its design and operation a challenging task. This work presents a detailed optimization‐based approach for simultaneously incorporating PSA design, operational, and control aspects under the effect of time variant and invariant disturbances. It is applied to a two‐bed, six‐step PSA system represented by a rigorous mathematical model, where the key optimization objective is to maximize the expected H₂ recovery while achieving a closed loop product H₂ purity of 99.99%, for separating 70% H₂, 30% CH₄ feed. The benefits over sequential design and control approach are shown in terms of closed‐loop recovery improvement of more than 3%, while the incorporation of explicit/multiparametric model predictive controllers improves the closed loop performance. © 2012 American Institute of Chemical Engineers AIChE J, 59: 120–131, 2013     

Integrated adsorbent‐process optimization for carbon capture and concentration using vacuum swing adsorption cycles

A novel approach for integrated adsorbent and process design is proposed. The traditional pressure or vacuum swing adsorption (PSA) / vacuum swing adsorption (VSA) process optimization for chosen objectives, where operating conditions are the decision variables, and CO₂ purity and recovery are constraints, is expanded to include adsorbent isotherm characteristics as additional decision variables. Two VSA cycles, namely a four‐step process¹, currently known to have the lowest energy consumption for CO₂ capture and concentration (CCC), and a six‐step process², recently proven to have a wider operating window for the evacuation pressure, have been investigated in the current study. The integrated optimization results simultaneously provide the lower bound of minimum energy and upper bound of maximum productivity for CCC achievable from the two VSA processes along with the operating conditions and the corresponding isotherm shapes necessary to achieve them. It may be viewed as an enabler for adsorbent design or expedient adsorbent search by process inversion. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2987–2995, 2017     

Technico‐economical assessment of MFI‐type zeolite membranes for CO2 capture from postcombustion flue gases

A detailed survey of the effect of moisture on the CO₂/N₂ permeation and separation performance of Mobile Five (MFI) zeolite membranes in view of downstream postcombustion CO₂ capture applications in power plants and incinerators is presented. The membranes, displaying a nanocomposite architecture, have been prepared on α‐alumina tubes by pore‐plugging hydrothermal synthesis at 443 K for 89 h using a precursor clear solution with molar composition 1 SiO₂:0.45 tetrapropylammonium hydroxide:27.8 H₂O. The synthesized membranes present reasonable permeation and CO₂/N₂ separation properties even in the presence of high water concentrations in the gas stream. A critical discussion is also provided on the technico‐economical feasibility (i.e., CO₂ recovery, CO₂ purity in the permeate, module volume, and energy consumption) of a membrane cascade unit for CO₂ capture and liquefaction/supercritical storage from standard flue gases emitted from an incinerator. Our results suggest that the permeate pressure should be kept under primary vacuum to promote the CO₂ driving force within the membrane. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3183–3194, 2012     

Analysis of energetics and economics of sub-ambient hybrid post-combustion carbon dioxide capture

Adsorption of CO₂ from post-combustion flue gas is one of the leading candidates for globally impactful carbon capture systems. This work focused on understanding the opportunities and limitations of sub-ambient CO₂ capture processes utilizing a multistage separation process. A hybrid process design using a combination of pressure-driven separation of CO₂ from flue gas (e.g., adsorption- or membrane-based separation) followed by CO₂-rich product liquefaction to produce high-purity (>99%) CO₂ at pipeline conditions is considered. The operating pressure of the separation unit is a key cost parameter and also an important process variable that regulates the available heat removal necessary to reach the sub-ambient operating conditions. The economic viability of applying pressure swing adsorption (PSA) processes using fiber sorbent contactors with internal heat management was found to be most influenced by the productivity of the adsorption system, with productivities as high as 0.015 /kg_sorb⁻¹ sec⁻¹ being required to reduce costs of capture below $60/ton CO₂ captured. This analysis was carried out using a simplified two-bed process, and thus there is opportunity for further cost reduction with exploration of more complex cycle designs. Three exemplar fiber sorbents (MIL-101(Cr), UiO-66, and zeolite 13X) were considered for application in the sub-ambient process of PSA unit. Among the considered sorbents, zeolite 13X fiber composites were found to perform better at ambient temperatures as compared to sub-ambient. MIL-101(Cr) and UiO-66 fiber composites had improved purity, recovery, and productivity at colder temperatures reducing costs of capture as low as $61/ton CO₂. Future economic improvement could be achieved by reducing the required operating pressure of the PSA unit and pushing the Pareto frontier closer to the final pipeline requirement via a combination of PSA cycle design and material selection.     

Separation of a Five-Component Gas Mixture by Pressure Swing Adsorption

Abstract

Bulk separation of a five-component mixture simulating coal gasification products was performed by pressure swing adsorption (PSA) using activated carbon. The PSA cycle consisted of four commercially used steps: (I) pressurization with H₂, (II) adsorption, (III) blowdown, and (IV) evacuation. Using this cycle, four products were obtained with a single PSA unit: H₂ (over 99.7% purity), CO, CH₄, and acid gas (CO₂ + H₂S). The first three products contained less than 0.001% H₂S, and the acid gas was suitable for sulfur recovery. A mathematical model incorporating equilibrium adsorption of mixture and mass transfer resistance (of CO₂) was found capable of simulating all steps of the PSA cycle. The model simulation results were in fair agreement with the experimental data. A fundamental understanding of the dynamics of the cyclic process was gained through the model.     

Optimal operation of the pressure swing adsorption (PSA) process for CO<Subscript>2</Subscript> recovery

The operation of PSA (Pressure Swing Adsorption) processes is a highly nonlinear and challenging problem. We propose a systematic procedure to achieve the optimal operation of a PSA process. The model of the PSA process for CO₂ separation and recovery is developed first and optimization is performed to identify optimal operating conditions based on the model. The effectiveness of the model developed is demonstrated by numerical simulations and experiments using CO₂ and N₂ gases and zeolite 13X. Breakthrough curves and temperature changes in the bed are computed from the model and the results are compared with those of experiments. The effects of the adsorption time and reflux ratio on the product purity and the recovery are identified through numerical simulations. The optimization problem is formulated based on nonlinear equations obtained from simulations. The optimal operating conditions identified are applied to experiments. The results show higher recovery of CO₂ under optimal operating conditions.     

Simultaneous Production of Hydrogen and Carbon Dioxide from Steam Reformer Off-Gas by Pressure Swing Adsorption

Abstract

Pressure swing adsorption (PSA) processes are used for the production of ultrapure hydrogen from a crude hydrogen stream containing H₂O, CO₂, CO, CH₄, and N₂ impurities which is produced by steam reformation of natural gas or naphtha. Two commercial PSA processes designed for this purpose are reviewed and a new commercial PSA process which simultaneously produces ultrapure hydrogen and high purity carbon dioxide products from the crude hydrogen with high recoveries of both components is described. Performance data for the new process are reported.     

Experimental and theoretical study on H2/CO2 separation by a five-step one-column psa process

An experimental and theoretical study is performed for bulk separation of H₂/CO₂ mixture (70/30 volume %) by PSA process with zeolite 5A, a process widely used commercially in conjunction with the catalytic steam reforming of natural gas or naphtha. For the optimized adsorption conditions of PSA, the characteristics of adsorption/desorption characteristics have been studied through breakthrough and desorption experiments under various conditions. The purge-to-feed ratio is important to the H₂ product purity only at a long adsorption step time. H₂ could be concentrated from 70% in the feed to 99.99% at H₂ recovery of 67.5%. The results of all five steps in PSA are successfully predicted by the LDF model considering an energy balance and nonlinear isotherm. For the model, the effective diffusivities (D,) are obtained separately from the uptake curves of H₂ and CO₂. The Langmuir-Freundlich isotherm is used to correlate the experimental equilibrium data and is very well fitted to the results.     

Layered two- and four-bed PSA processes for H2 recovery from coal gas

Huge amounts of global warming gas emissions have prompted interest in the recovery of H₂ from off-gases in the iron and steel industries. Pressure swing adsorption (PSA) processes with layered beds packed with zeolite 5A and activated carbon were applied for H₂ recovery from coal gas with relatively low H₂ concentrations (H₂/CO₂/CH₄/CO/N₂; 38/50/1/1/10 vol.%). Breakthrough curves in the layered bed showed behavior results between the zeolite 5A bed and the activated carbon bed. The bed with the higher zeolite ratio produced H₂ of higher purity in the PSA operation, but recovery loss became more significant with its increasing ratio. The variation of purity and recovery by operating variables were more significant in the two-bed PSA process than they were in the four-bed PSA process. The purity in the two-bed PSA varied asymptotically according to P/F ratio in the range of 0.1–0.3, while purity variation in the four-bed PSA process was almost linear. The zeolite layer in the two-bed PSA process worked as a separator of N₂, while that in the four-bed PSA process worked as a purifier of N₂. The four-bed PSA process could produce H₂ with a purity of 96–99.5% and a recovery of 71–85% with N₂ as the major impurity. The dynamics of the breakthrough and H₂ PSA processes were studied using a non-isothermal dynamic model.     

Adsorptive cyclic purification process for CO2 mixtures captured from coal power plants

CO₂ capture technology combined with bulk separation and purification processes has become an attractive alternative to reduce capture costs. Furthermore, the required purity in the application for CO₂ conversion and utilization is more stringent than that required from a captured CO₂ mixture for geological storage. In this study, an adsorptive cyclic purification process was developed to upgrade a CO₂/N₂ mixture captured from greenhouse gas emission plants as a feasibility study for a second capture unit or captured CO₂ purifier. To purify 90% CO₂ with balance N₂ as a captured gas mixture, two‐bed pressure swing adsorption and pressure vacuum swing adsorption (PVSA) processes using activated carbon were experimentally and theoretically studied at adsorption pressures of 250–650 kPa and a fixed vacuum pressure of 50 kPa. CO₂ with higher than 95% purity was produced with more than 89% recovery. However, a four‐bed PVSA process could successfully produce CO₂ with greater than 98% purity and 90% recovery. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1051–1063, 2017     

Carbon Di-Oxide Removal with Mesoporous Adsorbents in a Single Column Pressure Swing Adsorber

Abstract

A five-step PSA cycle was studied for CO₂ separation from CO₂-N₂ gas mixture in a single column at elevated temperatures using Poly-ethyleneimine (PEI) impregnated mesoporous silica SBA-15 as adsorbent. The PSA cycle study included a strong adsorptive rinse step in which the strongly adsorbed component, i.e., CO₂ was used for rinsing the adsorbent bed in order to increase the purity of CO₂ product. The study indicates that the adsorbent is regenerable under typical PSA conditions. The productivity of the adsorbent studied for CO₂ separation was found to be comparable with commercial zeolite adsorbents as reported in literature.     

High-performance oxygen transport membrane reactors integrated with IGCC for carbon capture

Precombustion carbon capture is an effective strategy to reduce large-scale CO₂ emissions, which is mainly used in the area of integrated gasification combined cycle (IGCC) power plants. Oxygen transport membranes (OTMs) were suggested as the air separation unit to produce high purity oxygen for the gasifier. However, the improvement in efficiency was limited. Here, a new IGCC process is reported based on a robust OTM reactor, where the OTM reactor is used behind the coal gasifier. This IGCC-OTM process fulfills syngas oxidation, H₂ production, and carbon capture in one unit, thus a significant decrease of the energy penalty is expectable. The membrane reactor does not use noble metal components, and exhibits high hydrogen production rates, high hydrogen separation factor (10³–10⁴), and stable performance in a gas mixture mimicking real syngas compositions from a coal gasifier with H₂S concentrations up to 1,000 ppm.     

Comparative assessment of sub-critical versus advanced super-critical oxyfuel fired PF boilers with CO2 sequestration facilities

This work focuses on the techno-economic assessment of bituminous coal fired sub- and super-critical pulverised fuel boilers from an oxyfuel based CO₂ capture point of view. At the initial stage, two conventional power plants with a nominal power output of above 600 MWe based on the above steam cycles are designed, simulated and optimised. Built upon these technologies, CO₂ capture facilities are incorporated within the base plants resulting in a nominal power output of 500 MWe. In this manner, some sensible heat generated in the air separation unit and the CO₂ capture train can be redirected to the steam cycle resulting in a higher plant efficiency. The simulation results of conventional sub- and super-critical plants are compared with their CO₂ capture counterparts to disclose the effect of sequestration on the overall system performance attributes. This systematic approach allows the investigation of the effects of the CO₂ capture on both cycles. In the literature, super-critical plants are often considered for a CO₂ capture option. These, however, are not based on a systematic evaluation of these technologies and concentrate mainly on one or two key features. In this work several techno-economic plant attributes such as the fuel consumptions, the utility usages, the plant performance parameters as well as the specific CO₂ generation and capture rates are calculated and weighed against each other. Finally, an economic evaluation of the system is conducted along with sensitivity analyses in connection with some key features such as discounted cash flow rates, capital investments and plant efficiencies as well as fuel and operating costs.     

Integrated Adsorbent Process Optimization for Minimum Cost of Electricity Including Carbon Capture by a VSA Process

Assessing vacuum swing adsorption (VSA) technology for postcombustion CO₂ capture and concentration (CCC) using energy and productivity indicators are useful, but its ultimate test must be the cost of electricity from a power plant including CCC. Here, our integrated optimization platform (Khurana and Farooq, AlChE J. 2017;63:2987–2995) developed earlier to simultaneously obtain the optimum adsorbent and process conditions is extended to include a comprehensive costing framework. The framework is complete with scale-up design and column scheduling, and compliant with National Energy Technology Laboratory costing guidelines for carbon capture. This is the ultimate tool that enables integrated optimization to minimize the cost of electricity. The Shell Cansolv CO₂ capture system is used as the benchmark for evaluating the best performance of two VSA cycles for two adsorbents. The operating conditions and isotherm shapes necessary to achieve the lowest possible cost of electricity for the two VSA cycles are also presented to facilitate designing or searching the best adsorbent for CCC. © 2018 American Institute of Chemical Engineers AIChE J, 65: 184–195, 2019     

Cycle synthesis and optimization of a VSA process for postcombustion CO2 capture

A systematic analysis of several vacuum swing adsorption (VSA) cycles with Zeochem zeolite 13X as the adsorbent to capture CO₂ from dry, flue gas containing 15% CO₂ in N₂ is reported. Full optimization of the analyzed VSA cycles using genetic algorithm has been performed to obtain purity‐recovery and energy‐productivity Pareto fronts. These cycles are assessed for their ability to produce high‐purity CO₂ at high recovery. Configurations satisfying 90% purity‐recovery constraints are ranked according to their energy‐productivity Pareto fronts. It is shown that a 4‐step VSA cycle with light product pressurization gives the minimum energy penalty of 131 kWh/tonne CO₂ captured at a productivity of 0.57 mol CO₂/m³ adsorbent/s. The minimum energy consumption required to achieve 95 and 97% purities, both at 90% recoveries, are 154 and 186 kWh/tonne CO₂ captured, respectively. For the proposed cycle, it is shown that significant increase in productivity can be achieved with a marginal increase in energy consumption. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4735–4748, 2013     

Zeolite Apgiia for Adsorption Based Carbon Dioxide Capture

Adsorption of synthetic flue gas on a commercial zeolite 13X (APGIIA) with targeted Si/Al ratio has been studied aiming to design an adsorption process for CO₂ capture from post-combustion power plants. Adsorption equilibrium of pure gases (CO₂ and N₂) has been measured in a wide range of temperatures: 303, 333, 363, 393, 423, 473 K. Adsorption equilibrium was fitted with the multisite Langmuir model. The adsorption capacity of the zeolite pellets for CO₂ is 4.54 mol/kg and 0.26 mol/kg for N₂ at 303 K and 100 kPa. The dynamic behavior of the pellets in a fixed bed was also studied by measuring breakthrough curves. Adsorption and desorption was analyzed in order to understand the regeneration of the adsorbent.

Based on equilibrium and kinetic data, two different adsorption technologies were simulated: Vacuum Pressure Swing Adsorption (VPSA) and Temperature Swing Adsorption (TSA). A CO₂ recovery of 63.0% with 72.1% purity was obtained using a five-step PSA cycle included rinse step. In a 5-step TSA process, however, a CO₂ purity of 78.7% and recovery of 76.6% can be achieved under a heating temperature of 423 K.     

Pressure swing adsorption/membrane hybrid processes for hydrogen purification with a high recovery

Hydrogen was recovered and purified from coal gasification-produced syngas using two kinds of hybrid processes: a pressure swing adsorption (PSA)-membrane system (a PSA unit followed by a membrane separation unit) and a membrane-PSA system (a membrane separation unit followed by a PSA unit). The PSA operational parameters were adjusted to control the product purity and the membrane operational parameters were adjusted to control the hydrogen recovery so that both a pure hydrogen product (>99.9%) and a high recovery (>90%) were obtained simultaneously. The hybrid hydrogen purification processes were simulated using HYSYS and the processes were evaluated in terms of hydrogen product purity and hydrogen recovery. For comparison, a PSA process and a membrane separation process were also used individually for hydrogen purification. Neither process alone produced high purity hydrogen with a high recovery. The PSA-membrane hybrid process produced hydrogen that was 99.98% pure with a recovery of 91.71%, whereas the membrane-PSA hybrid process produced hydrogen that was 99.99% pure with a recovery of 91.71%. The PSA-membrane hybrid process achieved higher total H₂ recoveries than the membrane-PSA hybrid process under the same H₂ recovery of membrane separation unit. Meanwhile, the membrane-PSA hybrid process achieved a higher total H₂ recovery (97.06%) than PSA-membrane hybrid process (94.35%) at the same H₂ concentration of PSA feed gas (62.57%).

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Multiobjective optimization of microalgae (Chlorella sp.) growth in a photobioreactor using Box‐Behnken design approach

This study investigates a parameter optimization approach to maximize the specific growth rate of the Chlorella vulgaris microalgae species, its biomass productivity, and CO₂ capture rate. For this purpose, the Box‐Behnken experimental design technique is applied with temperature, nitrogen to phosphorus ratio, and light‐dark cycle per day, as the growth controlling parameters. For each response, a quadratic model is developed separately describing the algal specific growth rate, biomass productivity, and CO₂ capture rate, respectively. The maximum specific growth rate of 0.84 d^?1 is obtained at 25 °C, with a nitrogen to phosphorus ratio of 3.4:1, and light‐dark cycles of 24/0 h. Maximum biomass productivity of 147.3 mg L^?1 d^?1 is found at 30 °C, with a nitrogen to phosphorus ratio of 3:1, and light‐dark cycles of 12/12 h. In addition, the maximum CO₂ capture rate of 159.5 mg L^?1 d^?1 is also obtained at 30 °C, with a nitrogen to phosphorus ratio of 4:1, and light‐dark cycles of 23/1 h. Finally, a multi‐response optimization method is applied to maximize the specific growth rate, biomass productivity, and CO₂ capture rate, simultaneously. The optimal set of 30 °C, a nitrogen to phosphorus ratio 3:1, and light‐dark cycles 16/8 h, provide the maximum specific growth rate of 0.66 per day, biomass productivity of 147.6 mg L^?1 d^?1, and CO₂ capture rate of 141.7 mg L^?1 d^?1.