Activated carbon for hydrogen purification by pressure swing adsorption: Multicomponent breakthrough curves and PSA performance

In this work, multicomponent breakthrough experiments (binary H₂-CO₂, ternary H₂-CO₂-CO and five-component H₂-CO₂-CO-CH₄-N₂) were performed under different operating conditions in activated carbon extrudates to validate the mathematical model. A 10 steps one-column VPSA experiment was also performed. These experiments allow experimental validation of adsorption equilibrium predicted by the multicomponent extension of the Virial isotherm and a fixed-bed mathematical model. In the VPSA experiment, a 99.981% hydrogen purity stream (with 63 ppm of CO contamination) was obtained with a hydrogen recovery of 81.6% and an adsorbent productivity of .The mathematical model was also employed to assess the effect of operating conditions and the influence of step times and pressure equalizations in the PSA unit. It was verified that high-purity hydrogen (>99.99%) can be obtained using this adsorbent with recoveries higher than 75% and unit productivities of .     

The effect of reliable prediction of final pressure during pressure equalization steps on the performance of PSA cycles

The present work aims at modeling the performance of isothermal PSA cycles for the production of H₂ from refinery fuel gas by introducing a more reliable calculation of the final pressure during the pressure equalization steps. The latter calculation is performed based on the law of conservation of mass in a system comprehending the depressurizing and pressurizing beds in contact. Single adsorbent (zeolite 5A) dual and six-bed PSA processes have been considered. The PSA cycle performance is compared with a conventional model considering an arithmetic mean for the final pressure during the pressure equalization steps (old model). It is shown that the new model predicts lower values for product purity and recovery when compared with the old model. The error in the estimation of the product recovery is larger than the corresponding value for product purity and may exceed 9%. It is shown that the error in the calculation of purity and recovery strongly depends on the number of beds. The error in the calculation of product recovery increases approximately two-fold increasing the number of beds from 2 to 6. Therefore, the present study shows that implementation of a more robust method for the evaluation of final pressure during the equalization steps is imperative for the development of new models of industrial PSA processes, especially for the number of beds exceeding 2.     

Analysis of a piston PSA process for air separation

The piston driven PSA process offers the potential for achieving productivity improvement by rapid piston action. In the present work, experiments were performed on a laboratory scale piston driven PSA test rig with provisions to vary all the important operating variables, namely, phase angle configuration, stroke length, cycling speed duration and angles of feed introduction and product withdrawal. Air separation on 13X zeolite was chosen as the model experimental system. Experiments with adsorbent particles of two different sizes confirmed that mass transfer resistance is important and may significantly affect the separation performance. A mathematical model was developed to simulate the process. The numerical solution was verified by simulating limiting conditions that had analytical solutions. Some basic model assumptions related to piston motion were verified by comparing with experiments conducted at well-defined limiting conditions like empty column and total recycle. Flow resistance in the connecting tubes seemed to explain the observed difference in both phase and amplitude of the pressure profiles measured at the two ends of the column. The model predictions were generally in good agreement with the experimental observations. The model was used to perform a parametric study in the operating regions that were not covered in the experiments. General inferences are made regarding the operating configurations that are expected to improve system performance.     

Analysis of the boundary conditions for the simulation of the pressure equalization step in PSA cycles

In this work, the physical validity of Danckwerts boundary conditions (original and modified) for simulating the connection of two beds in the pressure equalization step of a PSA cycle with a dynamic model including axial dispersion is analyzed. A model of this kind has been employed to simulate the separation of a carbon dioxide/methane mixture with silicalite as the adsorbent using a Skarstrom PSA cycle including a pressure equalization step before the pressurization step of each bed. It is demonstrated that both kinds of boundary conditions can lead to unrealistic results (either mass and heat flux are not conserved or molar fraction and temperature are not continuous in both beds) if the contribution of dispersion to axial mass flow is important, which occurs for long equalization times. To overcome these problems, the continuity of both dependent variables and fluxes as boundary conditions is proposed, which lead to the expected results for very long equalization times (the flux of each component is conserved and flat and continuous spatial profiles of all the dependent variables are obtained in both beds). These boundary conditions, unlike the ones proposed in the literature, can be the same regardless of the direction of flow. The impact of the different kind of boundary conditions on the performance results of the selected PSA process at the cyclic steady state is also analyzed.     

Adsorptive reactor technology for VOC abatement

The use of the monolith as an adsorptive reactor (M_AR) is proposed as a viable and novel alternative for VOC disposal. The M_AR combines adsorptive separation and catalytic combustion of the VOC in a single reactor unit and is thought to make effective utilisation of energy due to efficient heat integration. Theoretical studies on the feasibility and application of the adsorptive reactor concept for VOC oxidation is presented in this paper. Thus unlike previous work, present studies focus on an exothermic reaction system and the ability of the M_AR to control thermal runaway. A two dimensional mathematical model accounting for non isothermal adsorption and reaction, mass transfer limited adsorption kinetics and non linear (Tóth) adsorption equilibria, has been developed. The process is operated cyclically in two steps: adsorption and desorption/reaction. The VOC is fed into the reactor in the adsorption step and captured to produce a pure carrier gas effluent. Concentration and thermal swing is induced in the second step by means of an air feed. The most outstanding feature of the M_AR is its ability to prevent thermal runaway whilst maintaining a high VOC conversion. Simulation results indicate that the careful selection of step times for adsorption and desorption, feed temperatures and inlet velocities lead to stability and energy requirements which outperform equivalent conventional designs. The M_AR is thermally more stable due to the controlled release of the reactant from the adsorbed phase into the reaction zone, and also the heat integration of endothermic desorption and exothermic reaction.     

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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A novel graphical method for the integration of hydrogen distribution systems with purification reuse

Increase in refining demand and tighter environmental regulations have led to sharp increases in hydrogen consumption of oil refineries. Hydrogen conservation and effective use are of interest to refineries whose operations and profitability are constrained by hydrogen. Purification is widely used in hydrogen networks of refineries to reduce hydrogen production load. To minimize hydrogen utility consumption, it is necessary to optimize the hydrogen network with purification as a whole. In this paper, for hydrogen purification process, a triangle rule (which can be generalized to polygon rule) is proposed for graphical representation of its mass balance. The proposed procedure treats the product concentration and recovery rate of the purification process as adjustable parameters. An ensuing graphical method is developed for targeting the pinch point and minimum utility consumption of the hydrogen system with purification reuse. This graphical method can be used for any purification devices and in systems with any utility concentration. A refinery case is studied to demonstrate the optimization method.     

Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors: A simulation study

An important decision in the design of fluidized bed reactors is which of several flow regimes to choose. Almost all fluidized bed reactor models are restricted to a single flow regime, making comparison difficult, especially near the regime boundaries. This paper examines the performance of fluidized bed methane reformers with three models—a simple equilibrium model and two kinetic distributed models, based on different assumptions of varying sophistication. Membranes are incorporated to improve reactor performance. Eighteen cases are simulated for different flow regimes and membrane configurations. Predictions for the fast fluidization and turbulent flow regimes show that the rate-controlling step is permeation through the membranes. Bubbling regime simulations predict somewhat less hydrogen production than for turbulent and fast fluidization, due to the effects of interphase crossflow and mass transfer. Overall reactor performance is predicted to be best under turbulent fluidization operation. Practical considerations also affect the advantages, shortcomings and ultimate choice of flow regime.     

A bubbling fluidized bed solar reactor model of biomass char high temperature steam-only gasification

A two phase biomass char (biochar) steam gasification model based on the systems kinetics is developed in a bubbling fluidized bed with concentrated solar heat as source of energy. The model calculates the dynamic and steady state profiles, as well as the complex parameters of fluidized beds. This robust model is capable of predicting the temperature and concentration profiles of gases in the bubble, emulsion gas and solid phases. The Rosseland equation is used to calculate the radiative transfer within the bed. Due to the nature of the fluidized bed, the small bed thermal conductivity and bigger void between particles, there is a large temperature gradient throughout the bed, indicating that the system is highly non-isothermal. The set-up of a fluidized bed with solar irradiation in the upper side of the reactor is found to be a less efficient gasifying system in comparison with a packed bed, but could be optimized if the source of heat is changed to the bottom of the reactor. The trends and responses of the model are in good agreement with the experimental trends reported in the literature. Hydrogen is the principal product followed by carbon monoxide, the carbon dioxide production is small and the methane production is negligible.     

Numerical analysis of hydrogen absorption in a P/M metal bed

In a porous metal bed, two-dimensional heat and mass transfer are analyzed for hydrogen absorption in the cylindrical coordinates. For high cooling rates an annular geometry is selected for the porous metal bed, and the bed is cooled by a fluid on both the internal and the external surfaces. The absorption process is analyzed numerically for the porous LaNi₅ P/M metal bed. Variations of the metal hydride temperature and hydrogen/metal atomic ratio are calculated in the radial and axial directions using a computational fluid dynamics (CFD) program. It is observed that the hydride formation takes places near the cold boundaries. The results are compared with the numerical results given in the literature for different geometries.     

Adsorption and regeneration dynamic characteristics of methane and hydrogen binary system

In order to optimize the performance of an adsorption column, the adsorption and regeneration dynamic characteristics were studied for 20% methane and 80% hydrogen binary system on nonisothermal and nonadiabatic conditions. The adsorption dynamic characteristics were studied at various flow rates, 7.2 LPM to15.8 LPM, and at various adsorption pressures, 6 to 12 atm. Also, regeneration dynamic characteristics were studied at various purge rates, 1.5 to 3.5 LPM, and constant pressure, 1.2 atm. Nonisothermal and nonadiabatic models, considering linear driving force model and Langmuir-Freundlich adsorption isotherm model, were considered to compare between prediction and experimental data.     

Lattice Boltzmann modelling of unsteady-state 2D concentration profiles in adsorption bed

This study describes a lattice Boltzmann model (LBM) developed to simulate two-dimensional (2D) unsteady-state concentration profiles, including breakthrough curves, in a tubular column packed with adsorbents. The model using d3q19 (three dimensions and 19 speeds) lattice solves the 3D time-dependent convection-diffusion-adsorption equation for an ideal binary gaseous mixture assuming different velocity profiles in the column, including radially flat (plug flow) and non-uniform across the column's cross-section. The simulation results show significant concentration gradient across the cross-section depending upon the d/d_p ratio. The model results corroborate the experimental measurements made in the adsorption bed that the concentration due to breakthrough may be larger near the wall than at the core of the column due to the relatively larger local velocity in the vicinity of the wall. The LBM results have significance from the perspective of the physical understanding of the concentration profiles prevalent in the adsorption bed as well as effective design of a large-scale column. The model results are validated with the analytical solution to 1D axial dispersion problem, and to a few simple flow problems, such as Poiseuille and Couette flows.     

简捷法确定提纯回用氢网络目标值

与只考虑直接回用的氢网络相比,具有提纯单元的氢网络能显著减少新鲜氢气的消耗量,但其设计及求解提纯目标值过程均更为复杂。对于单杂质、提纯单元采用固定浓度模型的提纯回用氢网络,结合此类网络的特点,提出了一种简捷法确定网络目标值。首先假设提纯后氢物流量足够大,由此得出初始提纯夹点。当初始夹点估算正确时,由夹点之下的需求物流和源物流的流量与杂质质量衡算即可得出提纯回用氢网络的目标值;当初始夹点估算不正确时,可以第一次计算结果为基础判断得出正确夹点,再增加一步简单计算,也可得到提纯回用目标值。计算实例表明本文方法计算简单且有效。     

A network theory for the structured modelling of chemical processes

A structuring methodology for dynamic models of chemical engineering processes is presented. The main ideas of the methodology were outlined in a previous publication for the class of well-mixed systems. In this contribution, the methodology is extended to spatially distributed systems and to particulate processes. Furthermore, the structuring principle is used to make a conceptual link between the macroscopic world of process simulation and the microscopic world of molecular simulation. It is shown that a uniform structuring principle can be applied to the modularisation of most classes of chemical engineering models. The structuring principle can be used as a theoretical framework for the implementation of modular families of chemical engineering models in modern computer aided modelling tools.     

Synthesis and electrochemical study of nanoporous Pd-Ag alloys for hydrogen sorption

We report on the synthesis of novel nanoporous Pd-Ag electrocatalysts using a facile hydrothermal method where the portion of Ag was varied from 0 to 40%. Scanning electron microscopy (SEM) was used to examine the morphologies of the prepared nanoporous materials. Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) were used to directly and indirectly characterize the composition of the formed Pd-Ag nanostructures. X-ray diffraction (XRD) analysis confirmed that the formed Pd-Ag nanomaterials were alloys with a face-centered cubic structure. Electrochemical methods were used to study the capacity and kinetics of hydrogen sorption into the nanoporous Pd and Pd-Ag alloys. The nanoporous Pd-Ag alloy with 20% silver possesses the highest capacity for the α phase hydrogen sorption, which is over 4 times higher than the pure nanoporous Pd. The combination of the enhanced α phase hydrogen sorption capacity and diminishing of the α- and β-phase transition makes the nanoporous Pd-Ag alloys promising for hydrogen selective membranes and hydrogen dissociation catalysts.     

Mathematical modelling of Sec pathway mechanism in Escherichia coli: a case study for periplasmic translocation of maltose binding protein-glucose isomerase fusion protein

Within the framework of reported information on the Sec pathway mechanism, a mathematical model for the periplasmic translocation of fusion proteins in bacteria was developed. The mathematical model includes all stages of the targeting stage and assume that the ATP-driven translocation stage is completed in a single step. The equations for the targeting stage involved cytoplasmic folding rate and SecB binding kinetics. Rate equations for the translocation stage were derived using King-Altman and network reduction techniques. Experimental data for maltose binding protein-glucose isomerase fusion protein (MBP-GI) translocation and reported data for MBP translocation were used to estimate the parameters. The simulation results show that the model fits well to the experimental data of cytoplasmic and periplasmic MBP-GI distributions. When the values of the targeting stage parameters, k₁ and k₃ or the concentration of SecB are changed, the model correctly predicts the expected changes in the MBP-GI distribution to the cytoplasmic and periplasmic spaces. The SecB complex and the preprotein concentrations predicted attain steady state immediately, within seconds, and their amounts are very low when compared to MBP-GI in either compartment. The model can be made applicable to any protein that uses the Sec pathway, and with ATP and Sec pathway protein limitations.     

Experimental studies of a hybrid adsorbent-membrane reactor (HAMR) system for hydrogen production

An experimental study of the performance of a novel reactor system—termed the hybrid adsorbent-membrane reactor (HAMR)—is described. In the HAMR the reaction and membrane separation steps are coupled with adsorption. It was shown previously by our group for esterification reactions that this system results in significantly improved performance. The focus in this paper is on the use of the HAMR for hydrogen production. We present experimental investigations of the HAMR for the water-gas-shift (WGS) reaction using layered double hydroxides as adsorbents for CO₂ and nanoporous H₂-selective carbon molecular sieve membranes. The reactor characteristics are investigated for a range of temperatures and pressures relevant to the WGS application, and are compared with the predictions of a mathematical model previously developed by our group.     

Synthesis and characterization of poly (ethylene oxide) containing copolyimides for hydrogen purification

Reverse selective membranes comprising poly(ethylene oxide) (PEO) containing copolyimides (PEO-PI) with variations of acid dianhydrides and diamines have been synthesized for hydrogen purification. The reverse selectivity of the membranes decimate the energy required for hydrogen recompression process. Factors including PEO content, PEO molecular weight, and fractional free volume (FFV) that would affect the gas transport performance have been investigated and elucidated in terms of degree of crystallinity, phase separation in the PEO domain as well as inter-penetration between the hard and soft segments. In mixed gas tests of CO₂ and H₂ mixtures, a highly condensable CO₂ out compete H₂ for the sorption sites in hard segment and diminishes H₂ permeability. Thus the CO₂/H₂ selectivity in the mixed gas tests is much higher than that in pure gas tests. Mixed gas permeation tests at 35 °C and 2atm show that the best reverse selective membranes have a CO₂ permeability of 179.3 Barrers and a CO₂/H₂ permselectivity of 22.7. The physical properties of PEO-PIs have also been characterized by FTIR, DSC, GPC, WAXS, AFM and tensile strain tests.     

A revised mono-dimensional particle bed model for fluidized beds

The formulation of the equations of change proposed by Foscolo and Gibilaro in their original mono-dimensional particle bed model (PBM) for the prediction of the fluid-bed stability of Geldart's group A powders has been revisited along with the relevant closure relationships. The buoyancy has been expressed in accordance with its “classical” definition, which regards it as being equal to the weight of the fluidizing fluid displaced by the particle phase. A new constitutive equation has been developed for the drag force; this proves more accurate than the expression used in the original PMB particularly in the intermediate flow regimes comprised between the viscous and inertial ones. The “elastic” force has been estimated by employing a rigorous approach which needs not resort to equilibrium-based relations. The result, enhanced in accuracy and breadth of validity, considers “elastic” force and drag force proportional. The equations of change themselves have been partly revised. The pressure gradient is no longer shared by the two phases in proportion to their volume fractions, but has been accounted for only in the continuous one. Conversely, the “elastic” force has been included, with opposite signs, in the linear momentum equations pertaining to both phases, so that the principle of action and reaction, to which the force is subjected, is fulfilled. The revised model has been validated by performing a fluid-bed stability analysis on a wide range of Geldart's group A powders at different operating temperatures. Predicted values for the minimum bubbling voidage estimated by means of the revised model have been compared with experimental values and with predictions from both the original Foscolo and Gibilaro model and that previously revised by Jean and Fan. The latter has been found to be always in good agreement with the model proposed here, whereas the former has seemed to somewhat underestimate the bed minimum bubbling voidage thus anticipating the transition between homogeneous and bubbling fluidization. All of the models have proved to yield predictions whose validity is strongly dependent on the particular powder in hand and on the operating conditions considered.