Non-isobaric Model Research Articles

Hydrogen production from coke oven gas using pressure swing adsorption process − a mathematical modelling approach

Coal is playing a major role as a reductant and as an energy source in the present world steel production due to its low cost and widespread distribution around the world. At the same time, being the largest contributor to global CO2 emissions, coal faces significant environmental challenges in terms of air pollution and global warming. Hydrogen is a promising alternative for coal in lowering the steel industry’s CO2 footprint, but the availability of green hydrogen is currently limited by its high production cost. This research study focuses on developing a pressure swing adsorption (PSA) technology that will allow for continued use of coal for a smooth transition towards green hydrogen-based steel production, by better utilisation of its by-product coke oven gas to produce high purity hydrogen. A generic, fast and robust simulation tool for simulating a variety of PSA processes considering both equilibrium and kinetic effects using a detailed non-isothermal and non-isobaric model is developed in the study. The adsorption equilibrium data required for the model are calculated from experimental results using the non-linear regression data fitting method. A series of rigorous parametric studies and breakthrough tests are performed using the developed mathematical model for better understanding of the effects of different factors on the PSA process performance. With the better understanding obtained from the above-mentioned parametric studies, the model is optimised by performing several simulation tests to achieve a high process performance in terms of purity and recovery of the H2 product, productivity of the adsorbents and energy consumption for compression of gases. The optimised 14-step multi-bed PSA cycle developed in this study allows for an improved energy efficiency of coal usage by better utilisation of its by-product coke oven gas by converting it into valuable high purity (>99.999%) hydrogen product with a recovery of over 75%.

An Investigation of Effective Parameters in Deuterium-Tritium Fuel Gains in Shock Ignition Method Driven by Laser Beams

Shock ignition (SI) is one of the methods considered in the concept of inertial confinement fusion (ICF). This two-step ICF process separates fuel assembly and ignition, relaxing the driver requirements and promising high gains. In SI scheme, a strong spherical shock wave converging at the end of the initial laser pulse ignites the pre-compressed fuel. In shock ignition, when the hot spot pressure is much higher than the surrounding cold fuel pressure, the fuel structure is considered non-isobaric. In this research, ignition conditions and fuel efficiency in shock ignition method were investigated. Then, the fuel efficiency correlations of total fuel energy, fuel gain and hot-spot radius in a non-isobaric model of fuel assembly were improved and compared with the numerical results of deuterium-tritium (DT) homogeneous fuel in SI scenario. Calculations showed that the hot spot formation conditions depend on the hot spot density and the surrounding cold fuel. Furthermore, using the improved energy efficiency equation, the role of physical parameters such as fuel mass and different ratios of hot spot pressure to the surrounding cold fuel pressure in energy efficiency were investigated.

GRAMS: A general framework describing adsorption, reaction and sorption-enhanced reaction processes

We present a generalized reaction-adsorption modeling and simulation (GRAMS) framework that captures both reaction and adsorption dynamics in columns with solid catalysts, porous adsorbents or both. It is based on a 1-dimensional, pseudo-homogeneous, non-isothermal, non-adiabatic and non-isobaric model that is extensively validated using experimental data from literature for different adsorption-reaction systems. For demonstrating the capabilities of the high-fidelity framework, simulations are performed for several configurations of a packed-column system containing (i) pure catalyst as in a fixed bed reactor, (ii) pure adsorbent as in a multi-step pressure swing adsorption (PSA) process, (iii) homogeneously-distributed uniform mixture of adsorbent and catalyst as in a cyclic sorption enhanced reaction process (SERP), and (iv) heterogeneously-compartmentalized adsorbent and catalyst as in a layered SERP. The model predictions show good agreement with experimental observations for sorption-enhanced steam methane reforming (SE-SMR), sorption-enhanced water gas shift reaction (SE-WGSR), PSA processes and conventional SMR. The framework is also used for evaluating the performance of a novel process with integrated SE-SMR and reverse water gas shift reaction (RWGSR) for direct natural-gas-to-syngas conversion. GRAMS is implemented such that it can be used for optimal design and intensification of novel, modular and multi-functional processes by combining multiple phenomena within a single unit.

CO2 capture in cation-exchanged metal–organic frameworks: Holistic modeling from molecular simulation to process optimization

While CO2 capture has been extensively investigated in different metal–organic frameworks (MOFs), their performance under practical process conditions is scarcely examined. In this study, a multi-scale modeling study is reported to examine CO2 capture from flue gas by vacuum swing adsorption (VSA) process using cation-exchanged rho zeolite-like MOFs (rho-ZMOFs) as adsorbents. Three rho-ZMOFs namely Na-, Mg- and Al-rho-ZMOFs are considered and compared with 13X Zeolite, which is the current benchmark for post-combustion capture from dry flue gas. First, Monte Carlo (MC) simulations are conducted to estimate the adsorption isotherms of pure CO2 and N2 in the range from 0 to 100kPa and their mixtures of varying composition at a total pressure of 100kPa. The pure gas isotherms are then fitted by the dual-site Langmuir model. Subsequently, the extended dual-site Langmuir model is used to satisfactorily predict the equilibrium behavior of CO2/N2 binary mixtures over a wide range of composition. In addition, micropore diffusivities are calculated from molecular dynamics (MD) simulations and compared with the estimated macropore diffusivities in order to determine the controlling mechanism of mass transfer. Macropore is expected when these adsorbents are pelletized before using in a separation process. For each rho-ZMOF adsorbent, a four-step VSA process with light product pressurization for CO2 capture and concentration (CCC) from dry flue gas containing 15% CO2 in balance N2 is then simulated using a non-isothermal, non-isobaric model. The process is optimized using multi-objective optimization based on a genetic algorithm for minimizing the energy penalty and maximizing the process productivity, subject to the purity and recovery constraints of 95 and 90%, respectively. The operating spaces of the three rho-ZMOFs are similar to that of 13X zeolite. However, the minimum energy penalty in Al-rho-ZMOF (156kWh/t CO2) is lower than in 13X (165kWh/t CO2). While identifying the cation-exchanged rho-ZMOFs as interesting candidates for CO2 capture, this study also demonstrates that the multi-scale modeling approach adopted here is an effective methodology to screen and design novel MOFs for CCC and other separation applications.

Improvement of non-isobaric model for shock ignition

In this paper, improved relations of total fuel energy, fuel gain, hot-spot radius and total areal density in a non-isobaric model of fuel assembly have been derived and compared with the numerical results of [J. Schmitt, J.W. Bates, S.P. Obenschain, S.T. Zalesak, D.E. Fyfe, Phys. Plasmas 17, 042701 (2010); S. Atzeni, A. Marocchino, A. Schiavi, G. Schurtz, New J. Phys. 15, 045004 (2013)] and several simulations performed by MULTI-1D radiation hydrodynamic code for shock ignition scenario. Our calculations indicate that the approximations made by [M.D. Rosen, J.D. Lindl, A.R. Thiessen, LLNL Laser Program Annual Report, UCRL-50021-83, pp. 3–5 (1983); J. Schmitt, J.W. Bates, S.P. Obenschain, S.T. Zalesak, D.E. Fyfe, Phys. Plasmas 17, 042701 (2010)] for the calculation of burn-up fraction are not accurate enough to give results consistent with simulations. Therefore, we have introduced more appropriate approximations for the burn-up fraction and total areal density of the fuel that are in more agreement with simulation results of shock ignition. Meanwhile, it is shown that the related formulas of the non-isobaric model for total fuel energy, fuel gain and also hot-spot radius cannot determine the model parameters independently, but improved model choose a better selection and less restrictions on determination of the parameters for the non-isobaric model. Such derivations can be used in theoretical studies of the ignition conditions and burn-up fraction of the fuel in shock ignition scenario.

CO2 capture from dry flue gas by vacuum swing adsorption: A pilot plant study

The capture and concentration of CO2 from a dry flue gas by vacuum swing adsorption (VSA) has been experimentally demonstrated in a pilot plant. The pilot plant has the provision for using two coupled columns that are each packed with approximately 41 kg of Zeochem zeolite 13X. Breakthrough experiments were first carried out by perturbing a N2 saturated bed with 15% CO2 and 85% N2 feed, which is representative of a dry flue gas from coal‐fired power plants. The breakthrough results showed long plateaus in temperature profiles confirming a near adiabatic behavior. In the process study, a basic four‐step vacuum swing adsorption (VSA) cycle comprising the following steps: pressurization with feed, adsorption, forward blowdown, and reverse evacuation was investigated first. In the absence of any coupling among the steps, a single bed was used. With this cycle configuration, CO2 was concentrated to 95.9 ± 1% with a recovery of 86.4 ± 5.6%. To improve the process performance, a four‐step cycle with light product pressurization (LPP) using two beds was investigated. This cycle was able to achieve 94.8 ± 1% purity and 89.7 ± 5.6% recovery. The Department of Energy requirements are 95% purity and 90% recovery. The proposed underlying physics of performance improvement of the four‐step cycle with LPP has also been experimentally validated. The pilot plant results were then used for detailed validation of a one‐dimensional, nonisothermal, and nonisobaric model. Both transient profiles of various measured variables and cyclic steady state performance results were compared with the model predictions, and they were in good agreement. The energy consumptions in the pilot plant experiments were 339–583 ± 36.7 kWh tonne−1 CO2 captured and they were significantly different from the theoretical power consumptions obtained from isentropic compression calculations. The productivities were 0.87–1.4 ± 0.07 tonne CO2 m−3 adsorbent day−1. The results from our pilot plant were also compared with available results from other pilot plant studies on CO2 capture from flue gas. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1830–1842, 2014

Efficiency of Nitrogen Desorption from LiX Zeolite by Rapid Oxygen Purge in a Pancake Adsorber

SignificanceA nonequilibrium, nonisothermal, nonisobaric model was used for numerical simulation of the efficiency of N2 desorption from a LiX zeolite column by rapid purge with O2 in a pancake adsorber. The key parameters included desorption time, adsorbent particle size, and the adsorber length to diameter ratio. The efficiency was found to be a complex function of these variables.

Pd-based membrane steam reformers: A simulation study of reactor performance

Steam reforming of natural gas is the main process for the production of hydrogen and synthesis gas needed in the chemical industry as reactants for the manufacturing of important products, mainly ammonia and methanol. Moreover, the development of fuel cell technology in many application fields (stationary and automotive) will increase in the next years the need of industrial quantities of pure hydrogen.This increasing demand is stimulating the development of new technologies for producing large amounts of hydrogen at low cost. The most promising process is based on the integration of ultra-selective membranes in steam reforming reactor, removing continuously hydrogen from reaction environment and preventing equilibrium conditions to be achieved.In this work the integrated membrane methane steam reformer performance is evaluated by a bi-dimensional non-isothermal non-isobaric model and a comparison with the traditional reformer varying the gas mixture residence time and the wall temperature is made. Results obtained attest the remarkable improvement in the steam reforming reactor by integrating the selective membrane: if a long residence time is imposed (50kgcats/mol), the methane conversion is more than doubled in respect to the traditional technology. On the other hand, the increase of the wall temperature improves the performance of both the traditional and the membrane reactor. However, selective membranes have a stringent technological threshold (T<823K) which limits the thermal level accepted inside the reactor.

A versatile process simulator for adsorptive separations

Simulation of adsorptive separation processes is one of the most challenging problems in chemical engineering. We have recently developed and validated a nonisothermal, nonlinear, nonequilibrium and nonisobaric model to simulate accurately these processes. Using this simulator and fundamental adsorption data, we can predict the performance of a variety of adsorption cycles with relative ease. Features of the simulator include multiple adsorbents, multicomponent gas mixtures, choice of equilibrium and mass transfer models and multiple process steps. The transport equations, some of the thermodynamic models and other unique features of this simulator are discussed. Use of the simulator is illustrated for equilibrium (zeolite) and kinetic (carbon molecular sieve) based gas separation processes. Results from these simulations are compared against experimental data. Future challenges of adsorption modeling are also discussed.