Two-stage PSA/VSA to produce H2 with CO2 capture via steam methane reforming (SMR)

A two-stage pressure/vacuum swing adsorption (PSA/VSA) process was proposed to produce high purity H₂ from steam methane reforming (SMR) gas and capture CO₂ from the tail gas of the SMR-H₂-PSA unit. Notably, a ten-bed PSA process with activated carbon and 5A zeolite was designed to produce 99.99+% H₂ with over 85% recovery from the SMR gas (CH₄/CO/CO₂/H₂ = 3.5/0.5/20/76 vol%). Moreover, a three-bed VSA system was constructed to recover CO₂ from the tail gas using silica gel as the adsorbent. CO₂ product with 95% purity and over 90% recovery could be attained. Additionally, the effects of various operating parameters on the process performances were investigated in detail.     

Carbon nanoflake hybrid for biohydrogen CO2 capture: Breakthrough adsorption test

Biohydrogen gas is a hot topic for H₂ fuel at present. However, removal of the unwanted CO₂ through adsorption is required before any system is supplied with high-purity H₂ gas. Herein, we prepared a novel carbon nanoflake hybrid for efficient biohydrogen CO₂ capture by combining the advantages of carbon, metal oxide, and amine. Among the samples, SH800 showed a remarkable high CO₂ adsorption capacity of 29.8 wt.% (6.77 mmol/g) at 25°C and 1 atm, the highest ever reported at low pressure and temperature. The regeneration experiment also demonstrated robust reversibility over five cycles in the absence of heat treatment. Moreover, it displayed a highly accessible adsorption site with a Brunauer-Emmett-Teller (BET) surface area of 600 m²/g and an optimal 6.6-nm average mesopore structure. Another hybrid named SH500 was also developed. This hybrid showed a comparable CO₂ uptake of 27.8 wt.%, being competitive to SH800 but with entirely different chemical properties. Both samples were analyzed by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy, (XPS) and were tested for CO₂ capture through a breakthrough experiment. A highly porous solid adsorbent was also produced via soft-template synthesis. In summary, the correct amount of dynamic factors, such as high surface area, mesopore-micropore morphology, activation temperature, metal hybridization, and N moieties, played a major role in the carbon engineering of CO₂ adsorbent.     

Artificial neural network based optimization for hydrogen purification performance of pressure swing adsorption

A pressure swing adsorption (PSA) cycle model is implemented on Aspen Adsorption platform and is applied for simulating the PSA procedures of ternary-component gas mixture with molar fraction of H₂/CO₂/CO = 0.68/0.27/0.05 on Cu-BTC adsorbent bed. The simulation results of breakthrough curves and PSA cycle performance fit well with the experimental data from literature. The effects of adsorption pressure, product flow rate and adsorption time on the PSA system performance are further studied. Increasing adsorption pressure increases hydrogen purity and decreases hydrogen recovery, while prolonging adsorption time and reducing product flow rate raise hydrogen recovery and lower hydrogen purity. Then an artificial neural network (ANN) model is built for predicting PSA system performance and further optimizing the operation parameters of the PSA cycle. The performance data obtained from the Aspen model is used to train ANN model. The trained ANN model has good capability to predict the hydrogen purification performance of PSA cycle with reasonable accuracy and considerable speed. Based on the ANN model, an optimization is realized for finding optimal parameters of PSA cycle. This research shows that it is feasible to find optimal operation parameters of PSA cycle by the optimization algorithm based on the ANN model which was trained on the data produced from Aspen model.     

Efficient recovery of syngas from dry methane reforming product by a dual pressure swing adsorption process

In recent years, there has been growing interest in the dry reforming of methane (using CO₂ instead of H₂O) to obtain syngas, due to its economic and environmental advantages. In this reaction, to achieve conversions close to 100%, it is necessary to work at temperatures higher than 1000 °C. However, to attenuate the catalyst deactivation by sintering is convenient to work at lower temperatures, so that normally the syngas (mixture CO + H₂) is obtained mixed with unreacted CO₂ and CH₄. In this work a process has been simulated to recover the syngas from the product of a dry reforming reaction of methane at 700 °C by means of a Dual PSA (Dual Pressure Swing Adsorption) process with heavy reflux using BPL activated carbon as an adsorbent, operating at 25 °C. The process can recover syngas with purity and recovery higher than 99%. Unreacted CO₂ and CH₄ can be recycled to the reactor, leading to effective CO₂ and CH₄ conversions close to 100%. The process specific energy input (SEI) is 4.7 thermal kJ per L (STP) of syngas. The process can also be used to recover the syngas contained in the tail gas of a H₂ purification PSA from SMR-off gas.     

Simulation of 12-bed vacuum pressure-swing adsorption for hydrogen separation from methanol-steam reforming off-gas

This study focuses on analysis of a 12-bed vacuum pressure-swing adsorption (VPSA) process capable of purifying hydrogen from a ternary mixture (H₂/CO₂/CO 75/24/1 mol%) derived from methanol-steam reforming. The process produces 9 kmol H₂/h with less than 2 ppm and 0.2 ppm of CO2 and CO, respectively, to supply a polymer electrolyte membrane fuel cell. The process model is developed in Aspen Adsorption® using the “uni-bed” approach. A parametric study of H₂ purity and recovery with respect to adsorption pressure, adsorbent height, activated carbon:zeolite ratio, feed composition, and number of beds is performed. Results show 12-bed VPSA can meet the H₂ purity goals, with H₂ recovery as high as 75.75%. Adsorption occurs at 7 bar, the column height is 1.2 m, and the adsorbent ratio is 70%:30%. A 4-bed VPSA can achieve the same purity goals as the 12-bed process, but H₂ recovery decreases to 61.34%.     

Elevated temperature pressure swing adsorption process for reactive separation of CO/CO2 in H2-rich gas

Purification of CO and CO₂ to the ppm level in H₂-rich gas without losing H₂ is one of the technical difficulties for fuel cell power systems. In this work, a two-column seven-step elevated temperature pressure swing system with high purification performance was proposed. The concept of reactive separation by adding water gas shift catalysts into the columns filled with elevated temperature CO₂ adsorbents was adopted. The H₂ recovery ratio and H₂ purity were greatly improved by the introduction of steam rinse and steam purge, which could be realized due to the increasing operating temperature (200–450 °C). An optimized operating region to both achieve high efficiency and low energy consumption was proposed. The optimized case with 0.09 purge-to-feed ratio and 0.15 rinse-to-feed ratio could achieve 99.6% H₂ recovery ratio and 99.9991% H₂ purity at a stable state for a feed gas containing 1% CO, 1% CO₂, 10% H₂O, and 88% H_2. No performance degradation was observed for at least 1000 cycles. The proposed (ET-PSA) system possessed self-purification ability while the columns were penetrated by CO₂. It is however suggested that periodical heat regeneration should be adopted to accelerate performance recovery during long-term operation.     

Hydrogen purification using compact pressure swing adsorption system for fuel cell

Adsorption of CO and CO₂ in mixtures of H₂/CO/CO₂ was achieved using compact pressure swing adsorption (CPSA) system to produce purified hydrogen for use in fuel cell. A CPSA system was designed by combining four adsorption beds that simultaneously operate at different processes in the pressure swing adsorption (PSA) process cycle. The overall diameter of the cylindrical shell of the CPSA is 35 cm and its height is 40 cm. Several suitable adsorbent materials for CO and CO₂ adsorption in a hydrogen stream were identified and their adsorption properties were tested. Activated carbon from Sigma–Aldrich was the adsorbent chosen. It has a surface area of 695.07 m²/g. CO adsorption capacity (STP) of 0.55 mmol/g and CO₂ at 2.05 mmol/g were obtained. The CPSA system has a rapid process cycle that can supply hydrogen continuously without disruption by the regeneration process of the adsorbent. The process cycle in each column of the CPSA consists of pressurization, adsorption, blowdown and purging processes. CPSA is capable of reducing the CO concentration in a H₂/CO/CO₂ mixture from 4000 ppm to 1.4 ppm and the CO₂ concentration from 5% to 7.0 ppm CO₂ in 60 cycles and 3600 s. Based on the mixture used in the experimental work, the H₂ purity obtained was 99.999%, product throughput of 0.04 kg H₂/kg adsorbent with purge/feed ratio was 0.001 and vent loss/feed ratio was 0.02. It is therefore concluded that the CPSA system met the required specifications of hydrogen purity for fuel cell applications.     

Comparative thermodynamic analysis of adsorption,membrane and adsorption-membrane hybrid reactor systems for methanol steam reforming

The thermodynamic analysis of steam reforming of methanol without and with fractional removal of H₂ and CO₂ in adsorption, membrane and adsorption-membrane hybrid reactor systems to produce fuel cell grade H₂ with minimal carbon formation is investigated. The results indicate that the removal of undesired CO₂ by CO₂ adsorbent is most effective process for the production of high purity H₂ than H₂ removal by membrane. However, the membrane is effective only above 30% H₂ removal. It is possible to obtain H₂ yield of 2.6 with negligibly small amount of CO and carbon formation at T = 405 K, P = 1 atm, 80% removal of CO₂ and 100% methanol conversion. Identical results are achieved even at lower temperature of 345 K in adsorption-membrane hybrid reactor system at 80% removal of H₂ and CO₂. Thus high grade H₂ can be produced by single step process and further processing to reduce CO by PROX reactor is not necessary.     

Thermodynamic analysis of hydrogen production via glycerol steam reforming with CO2 adsorption

Thermodynamic equilibrium for glycerol steam reforming to hydrogen with carbon dioxide capture was investigated using Gibbs free energy minimization method. Potential advantage of using CaO as CO₂ adsorbent is to generate hydrogen-rich gas without a water gas shift (WGS) reactor for proton exchange membrane fuel cell (PEMFC) application. The optimal operation conditions are at 900 K, the water-to-glycerol molar ratio of 4, the CaO-to-glycerol molar ratio of 10 and atmospheric pressure. Under the optimal conditions, complete glycerol conversion and 96.80% H₂ and 0.73% CO concentration could be achieved with no coke. In addition, reaction conditions for coke-free and coke-formed regions are also discussed in glycerol steam reforming with or without CO₂ separation. Glycerol steam reforming with CO₂ adsorption has the higher energy efficiency than that without adsorption under the same reaction conditions.     

Integrated catalytic adsorption (ICA) steam gasification system for enhanced hydrogen production using palm kernel shell

This paper investigates the integrated catalytic adsorption (ICA) steam gasification of palm kernel shell for hydrogen rich gas production using pilot scale fluidized bed gasifier under atmospheric condition. The effect of temperature (600–750 °C) and steam to biomass ratio (1.5–2.5 wt/wt) on hydrogen (H₂) yield, product gas composition, gas yield, char yield, gasification and carbon conversion efficiency, and lower heating values are studied. The results show that H₂ hydrogen composition of 82.11 vol% is achieved at temperature of 675 °C, and negligible carbon dioxide (CO₂) composition is observed at 600 °C and 675 °C at a constant steam to biomass ratio of 2.0 wt/wt. In addition, maximum H₂ yield of 150 g/kg biomass is observed at 750 °C and at steam to biomass ratio of 2.0 wt/wt. A good heating value of product gas which is 14.37 MJ/Nm³ is obtained at 600 °C and steam to biomass ratio of 2.0 wt/wt. Temperature and steam to biomass ratio both enhanced H₂ yield but temperature is the most influential factor. Utilization of adsorbent and catalyst produced higher H₂ composition, yield and gas heating values as demonstrated by biomass catalytic steam gasification and steam gasification with in situ CO₂ adsorbent.     

Simulation and analysis of dual-reflux pressure swing adsorption using silica gel for blue coal gas initial separation

A dual-reflux pressure swing adsorption (DR-PSA) process was proposed and simulated to initially separate the blue coal gas, aiming to capture carbon dioxide (CO₂) and enrich hydrogen (H₂), simultaneously. With a feed flow rate of 7.290 slpm, a light product reflux flow rate of 0.505 slpm and the heavy product reflux flow rate of 3.68 slpm, the developed DR-PSA process could capture CO₂ up to 64.01% with a recovery of 99.60% and enrich H₂ up to 34.66% with a recovery of 97.63% from the blue coal gas (36.2% N₂/28.5% H₂/13.9% CO/12.7% CO₂/8.7% CH₄). In addition, in order to optimize the process, the effects of various operating parameters on the DR-PSA process performance in terms of product purity and recovery were discussed in detail, including the feed position, the light product reflux ratio and the heavy product reflux ratio. Moreover, the dynamic distribution behaviors of pressure, temperature and gas-solid concentration were presented to explain and evaluate the process separation performance in depth under different operating conditions.     

Optimization of pressure swing adsorption for hydrogen purification based on Box-Behnken design method

Pressure swing adsorption (PSA) is an important technology for mixture gas separation and purification. In this work, a dynamic model for a layered adsorption bed packed with activated carbon and zeolite 5A was developed and validated to study the PSA process. The model was validated by calculating breakthrough curves of a five-component gas mixture (H₂/CH₄/CO/N₂/CO₂ = 56.4/26.6/8.4/5.5/3.1 mol%) and comparing the results with available experimental data. The purification performance of six-step layered bed PSA cycle was studied using the model. In order to optimize the cycle, the Box-Behnken design (BBD) method was used, as implemented in Design Expert?. The parametric study showed that, for adsorption step durations ranging from 160 to 200 s, as the adsorption time increased, the purity decreased, whereas the recovery and productivity increased. During the pressure equalization step, the purity increased as the pressure equalization time increased, but the recovery and productivity decreased for step durations ranging from 10 to 30 s. As the P/F ratio (hydrogen used in purge step to hydrogen fed in adsorption step) increased from 0.05 to 0.125, the purity increased, whereas the recovery and productivity decreased. The optimization of the layered bed PSA process by the BBD method was then performed. In addition to the adsorption time, the pressure equalization time and the P/F ratio were considered as independent optimization parameters. Quadratic regression equations were then obtained for three responses of the system, namely purity, recovery, and productivity. When purity is set as the main performance indicator, the following values were obtained for the optimization parameters: an adsorption time of 168 s, a pressure equalization time of 14 s, and a P/F ratio of 0.11. Under those conditions, the system achieved a purity of 99.99%, a recovery of 57.76%, and a productivity of 6.41 mol/(kg·h).     

Long term and performance testing of NaMg double salts for H2/CO2 separation

This work investigates the synthesis and performance of double salts for H₂/CO₂ separation. A series of NaMg double salts were prepared based on xMg(NO₃)₂: yNa₂CO₃: zH₂O and characterised. The best sorbents reached CO₂ uptake of 17.9 wt% at 0.62 MPa and 375 °C. The NaMg double salts preferentially sorbed CO₂ as determined by breakthrough tests. The NaMg double salts were packed in a sorbent bed and tested for H₂/CO₂ separation at the back end of a water gas shift reactor. The space velocity had the largest impact on the performance of the sorbent bed, as increasing the space velocity from 2.16 × 10^?3 to 9.51 × 10^?3 s^?1 sped up the breakthrough time by 84%. Increasing the feed gas pressure from 0.3 to 0.6 MPa reduced the breakthrough time by ～45%. The NaMg double salt sorbents were exposed for over 1000 h of continuous temperature including 28 cycles of sorption and desorption, and proved to be stable during changes of operating conditions such as flow rates and pressures.     

Tailored Ce- and Zr-doped Ni/hydrotalcite materials for superior sorption-enhanced steam methane reforming

Multi-functional hybrid materials are attractive for producing high-purity hydrogen (H₂) via catalytic steam reforming coupled with low temperature adsorptive separation of CO₂. In this work, modified Ni/hydrotalcite-like (HTlc) hybrid materials promoted with Ce and Zr species were synthesized and applied for the sorption-enhanced steam methane reforming process (or SESMR). The promotion with Ce and Zr resulted in strongly basic sites for CO₂ adsorption, and hence, improved H₂ production. Especially, the Ce-promoted hybrid material (Ce-HM1) exhibited the highest adsorption capacity (1.41 mol CO₂/kg sorbent), producing 97.1 mol% H₂ at T = 723 K, P = 0.1 MPa, S/C = 4.5 mol/mol and gas hourly space velocity or GHSV = 3600 mL/(g h); the breakthrough time was 1 h. High surface area and basicity of the promoted materials inhibited coke formation and undesired reactions. In addition to the improved catalytic activity and adsorption characteristics, these materials were stable and easily regenerable. Multi-cycle durability tests revealed that both the promoted materials Ce-HM1 and Zr-HM1 remained stable for up to 13 and 17 cycles. In contrast, the unpromoted hybrid material (HM1) was stable for 9 cycles only. Thus, promotion with Ce and Zr was beneficial for producing pure H₂.     

Numerical study of heat transfer in process of gas adsorption in a Cu-benzene- 1, 3, 5-tricarboxylic acid particle adsorption bed

The heat transfer mechanism in the gas adsorption of a Cu-benzene- 1, 3, 5-tricarboxylic acid particle adsorption bed is investigated by a model combining the lattice Boltzmann method with the grand canonical Monte Carlo method. The effects of distribution and thermal conductivity of the adsorption particle and types of adsorbates (CO₂, CH₄, and H₂) on gas adsorption are discussed. The results indicate that in the fluid region, the temperature peak in the particle random range, low particle thermal conductivity, and CH₄ adsorption are higher than those in other cases. In the solid region, the temperature peak is independent of the particle range, whereas the temperature peak in low particle thermal conductivity and CO₂ adsorption are higher than those in other cases. In the case of high thermal conductivity of the adsorbate and particle, the adsorbent with low adsorption heat is recommended to improve the adsorption performance of the adsorption bed.     

Augmented hydrogen adsorption on metal (Mg,Mn) doped α-phase TeO2: A DFT investigation

TeO₂, as a promising gas sensor material, has been extensively studied for its capacity to detect hydrogen with high sensitivity. First-principles calculations were applied to explore the adsorption properties of hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), and hydrogen sulfide (H₂S) on TeO₂ doped with either Mg or Mn to explore this compound's potential as hydrogen sensors. Hydrogen is more readily adsorbed on pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ than CO₂, CH₄ and H₂S molecules by calculating their adsorption energy and charge transfer; the sequence of adsorption strength is H₂>H₂S > CO₂>CH₄. The hydrogen molecules and pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ form H–O bonds with lengths of 0.98, 0.98 and 0.99 Å, respectively, indicating that chemical adsorption is dominant between them. The adsorption of hydrogen leads to significant changes in the density of states (DOSs) of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂, which may lead to changes in their electrical conductivity. Moreover, the larger diffusion coefficients for hydrogen on the surfaces of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ relative to other gases indicates that hydrogen diffuses readily in TeO₂-based sensing materials, and the higher gas concentration contributes to improvements in response performance. This finding offers a theoretical basis for experimental explorations of the influence of metal dopants on TeO₂ hydrogen sensing performance.     

Thermally moderated hollow fiber sorbent modules in rapidly cycled pressure swing adsorption mode for hydrogen purification

We describe thermally moderated multi-layered pseudo-monolithic hollow fiber sorbents entities, which can be packed into compact modules to provide small-footprint, efficient H₂ purification/CO₂ removal systems for use in on-site steam methane reformer product gas separations. Dual-layer hollow fibers are created via dry-jet, wet-quench spinning with an inner “active” core of cellulose acetate (porous binder) and zeolite NaY (69 wt% zeolite NaY) and an external sheath layer of pure cellulose acetate. The co-spun sheath layer reduces the surface porosity of the fiber and was used as a smooth coating surface for a poly(vinyl-alcohol) post-treatment, which reduced the gas permeance through the fiber sorbent by at least 7 orders of magnitude, essentially creating an impermeable sheath layer. The interstitial volume between the individual fibers was filled with a thermally-moderating paraffin wax. CO₂ breakthrough experiments on the hollow fiber sorbent modules with and without paraffin wax revealed that the “passively” cooled paraffin wax module had 12.5% longer breakthrough times than the “non-isothermal” module. The latent heat of fusion/melting of the wax offsets the released latent heat of sorption/desorption of the zeolites. One-hundred rapidly cycled pressure swing adsorption cycles were performed on the “passively” cooled hollow fiber sorbents using 25 vol% CO₂/75 vol% He (H₂ surrogate) at 60 °C and 113 psia, resulting in a product purity of 99.2% and a product recovery of 88.1% thus achieving process conditions and product quality comparable to conventional pellet processes. Isothermal and non-isothermal dynamic modeling of the hollow fiber sorbent module and a traditional packed bed using gPROMS^® indicated that the fiber sorbents have sharper fronts (232% sharper) and longer adsorbate breakthrough times (66% longer), further confirming the applicability of the new fiber sorbent approach for H₂ purification.     

H2S removal from biogas for fuelling MCFCs: New adsorbing materials

Cu and Zn modified 13X zeolites prepared by ion exchange or impregnation and activated carbons (ACs) treated with KOH, NaOH or Na₂CO₃ solutions were studied as H₂S sorbents for biogas purification for fuelling molten carbonate fuel cells. H₂S sorption was studied in a new experimental set-up equipped with a high sensitivity potentiometric system for the analysis of H₂S. Breakthrough curves were obtained at 40 °C with a fixed bed of 20 mg of the samples under a stream (6 L h⁻¹) of 8 ppm H₂S/He mixture. The adsorption properties of 13X zeolite improved with addition of Cu or Zn:Cu exchanged zeolite showed the best performances with a breakthrough time of 580 min at 0.5 ppm H₂S, that is 12 times longer than the parent zeolite. In general, unmodified and modified ACs were more effective H₂S sorbents than zeolites. Treating ACs with NaOH, KOH, or Na₂CO₃ solutions improved the H₂S adsorption properties: AC treated with Na₂CO₃ was the most effective sorbent, showing a breakthrough time of 1222 min at 0.5 ppm, that is twice the time of the parent AC.     

Methane steam reforming using a membrane reactor equipped with a Pd-based composite membrane for effective hydrogen production

Herein, a methane steam reforming (MSR) reaction was carried out using a Pd composite membrane reactor packed with a commercial Ru/Al₂O₃ catalyst under mild operating conditions, to produce hydrogen with CO₂ capture. The Pd composite membrane was fabricated on a tubular stainless steel support by the electroless plating (ELP) method. The membrane exhibited a hydrogen permeance of 2.26 × 10^?3 mol m² s^?1 Pa^?0.5, H₂/N₂ selectivity of 145 at 773 K, and pressure difference of 20.3 kPa. The MSR reaction, which was carried out at steam to carbon ratio (S/C) = 3.0, gas hourly space velocity (GHSV) = 1700 h^?1, and 773 K, showed that methane conversion increased with the pressure difference and reached 79.5% at ΔP = 506 kPa. This value was ～1.9 time higher than the equilibrium value at 773 K and 101 kPa. Comparing with the previous studies which introduced sweeping gas for low hydrogen partial pressure in the permeate stream, very high pressure difference (2500–2900 kPa) for increase of hydrogen recovery and very low GHSV (<150) for increase hydraulic retention time (HRT), our result was worthy of notice. The gas composition monitored during the long-term stability test showed that the permeate side was composed of 97.8 vol% H₂, and the retentate side contained 67.8 vol% CO₂ with 22.2 vol% CH₄. When energy was recovered by CH₄ combustion in the retentate streams, pre-combustion carbon capture was accomplished using the Pd-based composite membrane reactor.     

Carbon dioxide adsorption on mesoporous silica surfaces containing amine-like motifs

The postcombustion separation of CO₂ from a flue gas mixture is a unit operation in carbon capture. Today, CO₂ is normally separated with alkanolamines in aqueous solutions. These absorption processes are energy intensive and costly. Increased environmental considerations and the significant footprints of many energy sources warrant the development of new gas separation techniques for the competitive implementation of carbon capture and storage technologies. Improved adsorbent-mediated separation processes are candidates for such new low-energy low-cost processes. In this study, porous silica-based adsorbents with amine-like motifs were synthesized. The temperature- and pressure-dependent adsorption of CO₂ and CO₂/H₂O mixtures were determined and compared for these materials. The experimental uptake capacities of the materials modified with primary propyl amine moieties were significantly higher than those of materials modified with bis-ethanol amine or amidine. The propyl-amine-modified samples also showed good selectivity for CO₂ over nitrogen gas.