Kinetics Of Water-gas Shift Reaction Research Articles

Modeling analysis of water-gas-shift reaction on catalyst-packed ceramic-carbonate dual-phase membrane reactor for hydrogen production

Water-gas shift (WGS) reaction for hydrogen production and CO2 removal/capture in a catalyst-packed CO2-permselective membrane reactor is studied experimentally and simulated with a computational model. The reactor utilizes a tubular dense ceramic-carbonate dual-phase membrane with a commercial high-temperature WGS catalyst. Equations governing CO2 permeation through the membrane and WGS reaction kinetics on the catalyst were obtained through independent CO2 permeation experiments and a fixed-bed reaction kinetic study. The simulation results using the validated model are presented to examine the effects of feed gas composition, steam/CO ratio, gas hourly space velocity, temperature, reactor pressure, sweep to feed molar flow rate ratio, and the membrane area to catalyst volume ratio on the performance of the membrane reactor for WGS with CO2 removal. Under optimized conditions, the catalyst-packed membrane reactor for WGS demonstrates significantly enhanced CO conversion and product H2 purity by effectively removing/capturing CO2 from the WGS reaction side.

Hydrogen production: Perspectives, separation with special emphasis on kinetics of WGS reaction: A state-of-the-art review

Fossil fuel depletion, global warming, energy security, and climate change spur interest in commercial and environmentally friendly alternative fuels. Palladium-based catalytic membrane technology currently produces ultrapure hydrogen from fossil fuels. Palladium exhibits high permeability, selectivity for hydrogen, and good surface properties. Properties of some palladium alloys enable the industrial production of hydrogen. Reaction rates and conversion depend on several parameters. This document reviews kinetic expressions for the water–gas shift (WGS) reactions and WGS combined with Fischer–Tropsch synthesis (WGS–FTS) reactions at high and low temperatures along with the details of the catalysts and operating conditions. The discussion includes mathematical reactor modeling.

Kinetics and reactor modeling for CaO sorption-enhanced high-temperature water–gas shift (SE–WGS) reaction for hydrogen production

Hydrogen, an important energy carrier of the future, produces no pollution and has a high content of energy. It is formed as a direct product of the water–gas shift (WGS) reaction, which occurs in various processes for the production of hydrogen, ammonia, methanol and different hydrocarbons, and is also a side reaction during the steam reforming of hydrocarbons and Fisher–Tropsch synthesis. Since it is an equilibrium reaction, it may be intensified by the selective removal of the products, which can lead to higher yields and energy savings. In this study, carbon dioxide was removed through chemisorption on CaO particles. In the first part, the WGS reaction kinetics were obtained on an industrial iron-chromium catalyst in a packed-bed reactor. In the second part, the CO2 chemisorption kinetics on CaO sorbent particles were examined, simultaneously with the WGS reaction. A modified dynamic shrinking-core model was used to describe the carbonation reaction, which accounted for the non-ideal core shrinkage. With the introduction of a sorbent conversion-dependent effective diffusion coefficient, the model perfectly reproduced the obtained experimental results. Valuable insight into the sorption-enhanced process was obtained with the full concentration profiles of the species involved (CO, H2O, CO2, H2) in time and space, as well as the conversion of the sorbent particles, also in the radial dimension. The developed model was used to simulate a cyclic sorption-enhanced water–gas shift operation in a revolver-type manner which allows for continuous sorbent regeneration and a much higher-than-equilibrium hydrogen production for various operational parameters. The significance of the model lies in the precise replication of the experimental results and its applicability to the vast area of the newly-emerged industrial sorption-enhanced technologies.

Assessing the performance of an industrial SBCR for Fischer–Tropsch synthesis: Experimental and modeling

The main objective of this study is to predict the performance of an industrial‐scale (ID = 5.8 m) slurry bubble column reactor (SBCR) operating with iron‐based catalyst for Fischer–Tropsch (FT) synthesis, with emphasis on catalyst deactivation. To achieve this objective, a comprehensive reactor model, incorporating the hydrodynamic and mass‐transfer parameters (gas holdup, εG, Sauter‐mean diameter of gas bubbles, d32, and volumetric liquid‐side mass‐transfer coefficients, kLa), and FT as well as water gas shift reaction kinetics, was developed. The hydrodynamic and mass‐transfer parameters for He/N2 gaseous mixtures, as surrogates for H2/CO, were obtained in an actual molten FT reactor wax produced from the same reactor. The data were measured in a pilot‐scale (0.29 m) SBCR under different pressures (4–31 bar), temperatures (380–500 K), superficial gas velocities (0.1–0.3 m/s), and iron‐based catalyst concentrations (0–45 wt %). The data were modeled and predictive correlations were incorporated into the reactor model. The reactor model was then used to study the effects of catalyst concentration and reactor length‐to‐diameter ratio (L/D) on the water partial pressure, which is mainly responsible for iron catalyst deactivation, the H2 and CO conversions and the C5+ product yields. The modeling results of the industrial SBCR investigated in this study showed that (1) the water partial pressure should be maintained under 3 bars to minimize deactivation of the iron‐based catalyst used; (2) the catalyst concentration has much more impact on the gas holdup and reactor performance than the reactor height; and (3) the reactor should be operated in the kinetically controlled regime with an L/D of 4.48 and a catalyst concentration of 22 wt % to maximize C5+ products yield, while minimizing the iron catalyst deactivation. Under such conditions, the H2 and CO conversions were 49.4% and 69.3%, respectively, and the C5+ products yield was 435.6 ton/day. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3838–3857, 2015

Validation of a dynamic model of hydrogen permeation through Pd-based membranes

Pd-based membranes have been studied for pure hydrogen separation from syngas: in particular, a mathematical model of a Pd membrane for hydrogen separation has been developed. This model can be used in process and assessment studies of the parameters which characterize the mass transfer phenomena (such as: hydrogen permeability, surface coverage and limiting step). By coupling the permeation and water gas shift reaction kinetics, it can also be used to evaluate the performances of the membrane reactor. Further, it can be helpful to evaluate the best assembly and sizing of a H 2/CO 2 separation system. The model takes into account the kinetics of H 2 adsorption/desorption on Pd surface, the H 2 permeation into the palladium bulk and in the porous layer, and the kinetics of CO, CO 2, H 2O, O 2, H 2S competitive adsorption/desorption on Pd surface. It is also comprehensive of flux equations and bulk mass, momentum and energy balance. The results released by the model were compared to the experimental data during both the transient phase and the steady state conditions. A satisfactory agreement between model and experimental data was found.

A Review of the Water Gas Shift Reaction Kinetics

The world’s progression towards the Hydrogen economy is facilitating the production of hydrogen from various resources. In the carbon based hydrogen production, Water gas shift reaction is the intermediate step used for hydrogen enrichment and CO reduction in the synthesis gas. This paper makes a critical review of the developments in the modeling approaches of the reaction for use in designing and simulating the water gas shift reactor. Considering the fact that the rate of the reaction is dependent on various parameters including the composition of the catalyst, the active surface and structure of the catalyst, the size of the catalyst, age of the catalyst, its operating temperature and pressure and the composition of the gases, it is difficult to narrow down the expression for the shift reaction. With different authors conducting experiments still to validate the kinetic expressions for the shift reaction, continuous research on different composition and new catalysts are also reported periodically. Moreover the commercial catalyst manufacturers seldom provide information on the catalyst. This makes the task of designers difficult to model the shift reaction. This review provides a consolidated listing of the various important kinetic expressions published for both the high temperature and the low temperature water gas shift reaction along with the details of the catalysts and the operating conditions at which they have been validated.

Vanadium-promoted Pt/CeO2 catalyst for water–gas shift reaction

Water–gas shift reaction has found new purpose and challenges due to its application in clean power generation systems based on fuel cells. It has been shown that the catalysts performance is dependent on both the metal phase and the nature of the support. In this contribution the modification of Pt/CeO2 catalyst with vanadium was exploited. Spectroscopic techniques revealed that distinct dispersed VOx species were generated onto the catalysts by using different vanadium loadings. The reduction of the catalysts and CO adsorption were monitored by infrared spectroscopy. It was found that the formation/decomposition of surface carbonates is affected by vanadium. Generation of multicoordinated carbonates is inhibited due to the decrease of the exposed cerium surface cations. The catalytic activity increased up to a vanadium surface density of 6 V atoms/nm2. Improvement in water–gas shift reaction kinetics is associated with the VOCe bonds.

Uncatalyzed and wall‐catalyzed forward water–gas shift reaction kinetics

AbstractThe kinetics of the high‐temperature (1070–1134 K), low‐ and high‐pressure gas‐phase forward water–gas shift reaction (fWGSR) were evaluated in an empty quartz reactor and a quartz reactor packed with quartz particles. The power‐law expression for the reaction rate was consistent with the Bradford mechanism and was invariant with respect to pressure. The experimental rate constant was lower than that published by Graven and Long, and slightly higher than estimates obtained using the reaction rate expression derived from the Bradford mechanism in conjunction with values of reaction rate constants obtained from the GRI database. Similar experiments conducted using a reactor composed of Inconel® 600, a representative reactor shell material, exhibited substantially enhanced rates of reaction. A simple power‐law rate expression was incorporated into a surface‐catalyzed plug flow reactor (PFR) model to correlate the results between 600 and 900 K. Palladium and palladium–copper alloy surfaces, representative of hydrogen membranes, were also shown to enhance the fWGSR rate, but not as much as the Inconel® 600 surfaces. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Water gas shift reaction kinetics in Fischer–Tropsch synthesis over an industrial Fe–Mn catalyst

The kinetics of water gas shift (WGS) reaction over an Fe–Mn catalyst under Fischer–Tropsch synthesis (FTS) reaction conditions is studied in a spinning basket reactor. Experimental conditions are varied as follows: temperature of 533–573 K, reactor pressure of 10.0–26.5 bar, H 2/CO feed ratio of 0.66–2.0 and space velocity of 0.66–2.65×10 −3 Nm 3 kg cat −1 s −1. By separately fitting WGS kinetics parameters with experimental data, which is possible in the spinning basket reactor with neglecting concentration and temperature gradients, different kinetics models of WGS are derived and discriminated on the basis of four sets of WGS elementary reactions. Kinetics experimental results show that the WGS reaction under FTS reaction conditions is far from equilibrium. Two types of WGS mechanisms are investigated. One is the formate mechanism, and the other is the direct oxidation mechanism. It is found that the formate mechanism is better in fitting experimental data than the direct oxidation mechanism over the Fe–Mn catalyst under the FTS reaction conditions. The optimized kinetics model with formate intermediate dissociation as the rate-determining step (RDS) can fit the WGS experimental results well. The simplified WGS kinetics model can easily be used for industrial modeling applications.

Kinetics of Water-Gas Shift Reaction in Supercritical Water

The kinetics of the water gas shift reaction was investigated in supercritical water using the Raman spectroscopic method for in situ quantitative measurement of reactants and relevant products in a stopped flow type reactor. The reaction proceeded according to the stoichiometric reaction as CO + H2O → CO2 + H2, whose rate was first order on the concentration of CO. The dependence on the reactant water was 1.5 ± 0.1 in the water density range from 4 to 25 mol dm–3 at a temperature of 380°C. The steady state concentration of formic acid was in the order of 10–4 against the initial concentration of CO and dependent on a 1.2 ± 0.3 order on the water density. These findings evidence that the rate determining step of the water gas shift reaction is the formation process of formic acid, which subsequently dissociates to CO2 + H2 at a much larger reaction rate. The slightly larger dependence than unity on the water density in the reaction rate as well as in the steady state concentration of the intermediate formic acid suggests that the transition state structure of the formic acid formation process resembles a certain kind of complex formed between a CO molecule and several water molecules such as suggested by Melius et al. (1990) by ab initio calculations.

Water gas shift reaction kinetics and reactor modeling for fuel cell grade hydrogen

The kinetics of the water gas shift reaction was studied to evaluate existing reaction mechanisms, test various rate expressions and simulate the performance in a methanol fuel processor for fuel cell applications. The reaction was carried out in a micro reactor testing unit using a commercial Sud-Chemie Cu/ZnO/Al2O3 catalyst between 120 and 250 °C with a range of feed rates and compositions. Using non-linear least squares optimization, the parameters in five rate expressions were fit to the experimental data. Based on a review of published work on the WGS reaction mechanism, our study found that a rate expression derived from a regenerative mechanism and another rate expression derived from adsorptive mechanism fit the experimental data equally well. Numerical integration of a one-dimensional PFR model was used for this parameter fitting. An empirical rate expression, rCO=kPCOPH2O(1−β) with activation energy of 47.4 kJ/mol was also obtained from the experimental data. Reactor performance was simulated to determine catalyst loadings required to achieve specific CO conversions as a function of temperature and water feed rate. These results are useful in studying the design trade offs available to reformer systems.

Water gas shift reaction kinetics under noncatalytic conditions in supercritical water

The kinetics of the water gas shift reaction was studied under noncatalytic conditions in supercritical water at CO/H 2O ratios of 0.03 and at temperatures from 653 to 713 K. The selectivities of CO 2 and hydrogen were almost equal and did not change with pressure at 673 K. The increase of pressure and water density sensitively promoted the reaction at 653 K from 25 to 30 MPa whereas the pressure and water density did not seem to affect the rate constant at 673 K from 10 to 30 MPa. The first order rate constant for CO conversion was k=10 5.58±1.38 exp(−1.16±0.19×10 5/ RT) /s at 10–59.6 MPa and 653–866 K.