Equilibrium-limited Reactions Research Articles

Coupling of in-situ pervaporation for the enhanced esterification of propionic acid with isobutyl alcohol over cenosphere based catalyst

In the present work, pervaporation assisted esterification of propionic acid and isobutyl alcohol, to produce isobutyl propionate and water was investigated in an in-situ type pervaporation reactor. A commercial polymeric hydrophilic PVA-PES membrane was used to shift the conversion of equilibrium limited esterification reaction by the removal of water from the reaction mixture. Catalyst developed from cenosphere (waste material) was used in the study. The effects of process parameters viz; reaction temperature, catalyst loading, alcohol/acid molar ratio and a ratio of effective membrane area to initial reaction volume (S/V) were studied.Catalyst characterization elucidated that the catalyst exhibited significant silica content, surface acidity with a considerable surface area. The membrane showed a significant selectivity towards the water removal which subsequently enhanced the pervaporation performance. A substantial enhancement in the conversion of esterification reaction from 67% to 88% was observed by the incorporation of pervaporation at a temperature of 353K. The membrane and catalyst revealed a significant stability up to four and three reaction runs respectively. Results revealed that all process parameters linearly affected the conversion and permeate flux of pervaporation assisted esterification. The maximum esterification conversion was obtained to be 92% at a reaction temperature of 353K, catalyst loading of 20g/L, alcohol/acid molar ratio of 2.5 and S/V ratio of 19.63m−1.

In Situ Separation of the Chiral Target Compound (S)-2-Pentanol in Biocatalytic Reactive Distillation

In situ separation of a chiral target compound is realized for the first time as distillate on top of an integrated biocatalytic batch reactive distillation column. The applied reaction system is the racemic resolution of (R/S)-2-pentanol in a transesterification with propyl butyrate, which is catalyzed by immobilized Candida antarctica lipase B (Novozym435). Biocatalyst integration is realized in baskets of wire gauze catalytic packings as column internals. The equilibrium limited reaction is shifted to the product side by fractional distillation of low boiling propanol and the target compound (S)-2-pentanol. Increased molar fractions up to x(S)-2-PeOH = 65 ± 4% with an enantiomeric excess of 90 ± 4% and 51 ± 3% overall conversion are reached for (S)-2-pentanol. Feasibility of the reaction system is investigated in our preselection tool. This tool is developed to evaluate predefined operation conditions in the column setup for transesterification reactions based on criteria regarding biocatalyst stabilit...

Overcoming pressure drop losses in membrane reactors by semi-batch operation

Abstract The feasibility of harnessing the increase in moles in certain equilibrium-limited reactions to alleviate the pressure driving force requirement for permeation through membranes is demonstrated. The studies are conducted in a packed-bed membrane reactor operated as a semi-batch membrane reactor (SBMR) to capture the pressure generating potential of the reaction. The actual system investigated is the steam methane reforming, a reaction in which the number of moles doubles, carried out at various temperatures (873 K and 923 K) and pressures (0.5–1.5 MPa) with a Ni/MgAl2O4 catalyst. A silica-alumina membrane with a hydrogen permeance of 2.2 × 10−7 mol m−2 s−1 Pa−1 at 923 K prepared by chemical vapor deposition was used in the membrane reactor. The hydrogen productivities obtained in the SBMR were compared with the productivities obtained in a membrane reactor operated at the same conditions. At low pressures (0.5, 1.0 MPa) the hydrogen productivities of the SBMR were comparable to those obtained with the continuous membrane reactor, but at high pressure (1.5 MPa) the SBMR showed superior performance. One-dimensional modeling studies gave good agreement between simulated and experimental results obtained from both reactor types. Based on these calculations, further estimations on hydrogen productivities were made at high pressures. These results indicated that the SBMR improved the hydrogen production when the steam methane reforming was conducted above 1.5 MPa. The SBMR described here could be utilized in a system where multiple units would be arranged to undertake parallel filling and discharge operations, much akin to the arrangement used in a pressure swing adsorption system.

Multi-objective optimization of industrial membrane SMR to produce syngas for Fischer-Tropsch production using NSGA-II and decision makings

Membrane reactors are an advanced technology with vast application capacities for equilibrium limited endothermic reactions. The main propose of this study is to offer an optimized packed-bed membrane steam methane reforming (SMR) tubular reactor for sustainable CH4 conversion by implementing triple-objective optimization model based on optimum H2/CO ratio for low temperature Fischer-Tropsch (F-T) process. In this study a one dimensional pseudo-homogeneous model based on mass, energy, and momentum conservation laws is used to simulate the behavior of a packed-bed membrane reactor for production of syngas by SMR. In the optimization section, the proposed work explores optimal values of various decision variables that simultaneously maximize CH4 conversion, H2 selectivity, and CO selectivity by applying elitist non-dominated sorting genetic algorithm (NSGA-II). Pareto optimal frontier between triple objectives is obtained in three spaces and best optimal value is selected by using LINMAP, TOPSIS, Shannon's entropy and Fuzzy Bellman-Zadeh decision making methods. The final optimal solutions illustrate that the membrane reactor presents higher CH4 conversion which can be operated under milder conditions than the conventional reactor.

Dynamic Modeling of CATOFIN® Fixed-Bed Iso-Butane Dehydrogenation Reactor for Operational Optimization

Abstract The catalytic dehydrogenation of iso-butane to iso-butylene is an equilibrium limited endothermic reaction and requires high temperature. The catalyst deactivates quickly, due to deposition of carbonaceous species and countered by periodic regeneration. The reaction-engineering constraints are tied up with operation and/or technology design features. CATOFIN® is a sophisticated commercialized technology for propane/iso-butane dehydrogenation using multiple adiabatic fixed-bed reactors having Cr2O3/Al2O3 as catalyst, that undergo cyclic operations (~18–30m); dehydrogenation, regeneration, evacuation, purging and reduction. It is always a concern, how to maintain CATOFIN® reactor at an optimum production, while overcoming gradual decrease of heat in catalyst bed and deactivation. A homogeneous one-dimensional dynamic reactor model for a commercial CATOFIN® fixed-bed iso-butane dehydrogenation reactor is developed in an equation oriented (EO) platform Aspen Custom Modeler (ACM), for operational optimization and process intensification. Both reaction and regeneration steps were modeled and results were validated. The model predicts the dynamic behavior and demonstrates the extent of catalyst utilization with operating conditions and time, coke formation and removal, etc. The model computes optimum catalyst bed temperature profiles, feed rate, pre-heating, rates for reaction and regeneration, fuel gas requirement, optimum catalyst amount, overall cycle time optimization, and suggest best operational philosophy.

Modeling study of a pervaporation membrane reactor for improving oxime hydrolysis reaction

The conversion of equilibrium-limited reactions can be efficiently improved by selective removal of product using a Pervaporation Membrane Reactor (PVMR), which attracts substantial attentions due to its excellent performance. For modeling the PVMR process, a pseudo-binary system was often assumed, nevertheless, it is inadequate to predict the performance when both the reactant and product have the similar properties and can simultaneous permeate through the membrane. A detailed model was established by coupling multi-component pervaporation and reaction kinetics with the butanoxime hydrolysis reaction employed as the experiment system. A modified Flory–Huggins (FH) approach and the Maxwell–Stefan (MS) formulation were used to describe the pervaporation model. The mass transfer fluxes and the concentration profiles of permeants within the membrane were modeled by a finite difference approach. Two coexisted behaviors, i.e. competitive adsorption and plasticizing effects were considered to influence mass transfer. The PVMR performance was predicted and a good agreement between experiment and calculation was obtained. The mechanism that how the simultaneous permeation of reactant and product influence the PVMR performance were further clarified in different operating conditions, including initial reactant concentration, downstream pressure, membrane thickness, ratio of membrane area to reaction volume, and temperature.

The driving force as key element to evaluate the pervaporation performance of multicomponent mixtures

Pervaporation presents an efficient alternative to other high energy demanding technologies, such as distillation, in the separation of challenging mixtures in the chemical industry (e.g., azeotropic mixtures). In this work, the transesterification reaction between butanol and methyl acetate to produce methanol and butyl acetate is considered as case of study of an equilibrium-limited reaction. The objective is to emphasize the importance of a prior evaluation of the driving force of each compound in order to determine when pervaporation is an appropriate technique to perform the separation of multicomponent mixtures. Three commercial membranes from PolyAn GmbH (Germany) and two membranes from Sulzer Chemtech (Switzerland), selected from a previous study, are used for the separation of equimolar mixtures of methanol–methyl acetate–butanol–butyl acetate, at 30, 40 and 50°C (±2°C). Results show that all the studied membranes present a preference for the permeance of butanol, reaching permeances of butanol until 140,000 GPU. However, methanol and methyl acetate are the compounds with a higher driving force due to their higher volatility, leading to higher concentration in the permeate and preferential separation instead of butanol. Thus, the study of permeances/permeabilities and selectivities is required but not sufficient to evaluate the real performance of pervaporation. The driving force has to be evaluated, and membranes that enhance the effect of the driving force by permeating the species with the highest driving force should be preferably selected. Then, pervaporation is used under conditions of maximum performance.

Optimizing packing heterogeneity for sorption enhanced metathesis reaction

An equilibrium-limited heterogeneous catalytic reaction, propene metathesis is suitable for process intensification via sorption enhanced reaction. In this work, we investigate the effect of catalyst/adsorbent configuration for propene metathesis in conjunction with pressure swing reaction. The catalyst and adsorbent configuration variations were defined in terms of packing heterogeneity index (PHI) and their effects were investigated experimentally and theoretically. Model predictions were tested against experimental data with variable PHI and adsorption/reaction conditions, including the absence of heterogeneity and of separation process. The product 2-butene was strongly adsorbed and retained by the intermediate adsorbent layers, thereby increasing reactant concentration in the reaction zone and enhancing conversion and rate of reaction in the subsequent layer. Model predictions were found to agree reasonably with experimental data and were used to elucidate the mechanism and optimizing principle for such reactors.

Kinetic enhancement of ammonia decomposition as a chemical hydrogen carrier in palladium membrane reactor

Decomposition of ammonia as a promising chemical hydrogen carrier is carried out at low temperatures using a ruthenium supported catalyst, where a highly efficient hydrogen recovery from ammonia is being desired. The equilibrium conversion is almost 100% in the range of 623–723K but the decomposition remains in a low conversion, so that insufficient activity of present catalysts should be improved. This study attempts to prove that this kinetically limited decomposition can be enhanced in the palladium membrane reactor, while in most of membrane reactor applications the equilibrium limited reactions can be shifted to the product side by selective hydrogen separation.The Langmuir-Hinshelwood types of rate equations, seven reaction models, for ammonia decomposition using a Ru/SiO2 catalyst were derived and compared with experimental results. As the result, the combinative desorption of nitrogen atom was found to be the rate determining step in the range of 623–723K. One dimensional model for a palladium membrane reactor could show that the kinetic enhancement differed considerably according to the reaction model (rate expression).In the ammonia decomposition employing a membrane reactor with a 200μm-thick palladium tube, 15% increase in conversion compared with the conventional packed reactor and 60% of the hydrogen recovery at 723K could be obtained. Simulation showed that further enhancement would be achievable by using thinner palladium membrane.

Highly Stable External Short-Circuit-Assisted Oxygen Ionic Transport Membrane Reactor for Carbon Dioxide Reduction Coupled with Methane Partial Oxidation

A membrane reactor allows for simultaneous separation and reaction, and thus, can play a good role to produce value-added chemicals. In this work, we demonstrated such a membrane reactor based on fluorite oxide samarium-doped ceria (SDC) using an external short-circuit concept for oxygen permeation. The fluorite phase was employed to impart its high structural stability, while its limited electronic conductivity was overcome by the application of an external short circuit to function the SDC membrane for oxygen transport. On one side of the membrane, i.e., feed side, carbon dioxide decomposition into carbon monoxide and oxygen was carried out with the aid of a Pt or Ag catalyst. The resultant oxygen was concurrently depleted on the membrane surface and transported to the other side of the membrane, favorably shifting this equilibrium-limited reaction to the product side. The transported oxygen on the permeate side with the aid of a GdNi/Al2O3 catalyst was then consumed by the reaction with methane to form syngas, i.e., carbon monoxide and hydrogen. As such, the required driving force for gas transport through the membrane can be sustained by coupling two different reactions in one membrane reactor, whose stability to withstand these different gases at high temperatures is attained in this paper. We also examined the effect of the membrane thickness, oxygen ionic transport rate, and CO2 and CH4 flow rates to the membrane reactor performance. More importantly, here, we proved the feasibility of a highly stable membrane reactor based on an external short circuit as evidenced by achieving the constant performance in CO selectivity, CH4 conversion, CO2 conversion, and O2 flux during 100 h of operation and unaltered membrane structure after this operation together with the coking resistance.

Methane membrane steam reforming: Heat duty assessment

Membrane reactors are an innovative technology with huge application potentialities for equilibrium limited endothermic reactions. Assembling a membrane selective to a reaction product avoids the equilibrium conditions to be achieved, supporting the reactions at lower operating temperatures. Taking as an example the natural gas steam reforming, a methane conversion around 98% can be reached imposing an operating temperature of 823 K, much lower than that of the traditional process. In the present paper, a stringent analysis of heat power requirement needed to carry out the natural gas steam reforming process by applying a membrane reactor is made. The simulations allows to understand how the main operating parameters (inlet temperature, inlet methane flow-rate, steam to carbon ratio, ratio between sweeping steam and inlet methane, operating reaction pressure) influence the total heat power required by the process, divided among power contributions for the reaction heat duty, reactant steam and permeation steam generation and preheating. Moreover, the specific thermal energy per mole of pure H2 is computed and assessed. Optimizing the operating conditions set, a specific thermal energy per mole of pure hydrogen of 92.3 kWh kmol−1 is obtained corresponding to a total thermal power of 687.4 kW required to convert, in a single membrane reactor, a methane flow-rate of 2 kmol h−1 (GHSV = 9.590 h−1) with a conversion around 98%.

Reactive Stripping for the Catalytic Exchange of Hydrogen Isotopes

A pilot-scale countercurrent trickle bed reactor (TBR), packed with a water-repellent catalyst, is investigated for deuterium exchange between water and hydrogen, to be used for the production of heavy water. The reactant, i.e. heavy water, from liquid phase is stripped by the gas phase wherein, it undergoes an equilibrium limited exchange reaction with hydrogen over a solid catalyst. A two phase mathematical model is developed incorporating reaction kinetics, mass transfer resistances, and dispersion in liquid phase flow to simulate the reactor performance. Model parameters such as kinetic rate constant, mass transfer coefficient etc. were determined by independent experiments. The reactor model is then experimentally validated by performing reactions in pilot scale column. The validated model is further used to arrive at a suitable design for the desired performance.

Process intensification: New technologies (SMBR and PermSMBR) for the synthesis of acetals

Acetals, as 1,1-diethoxyethane (DEE) and 1,1-dibutoxyethane (DBE), can be used as green oxygenates for diesel blending due to their capacity to reduce particulate matter emissions and to enhance the diesel cetane number. The use of multifunctional technologies, where reaction and separation occur in the same unit, provides the most feasible engineering solution for their synthesis, which involves equilibrium limited reactions. Motivated by that, two innovative technologies for the production of DEE and DBE acetals were explored: the Simulated Moving Bed Reactor and the recently developed Simulated Moving Bed Membrane Reactor – PermSMBR. In this work, the main results and characteristics of each process are presented and discussed.

Development of Dehydrogenation Catalysts and Processes

Catalytic dehydrogenation plays an important role in production of light (C3–C4 carbon range), detergent range (C10–C13 carbon range) olefins and for ethylbenzene dehydrogenation to styrene. During the World War II, catalytic dehydrogenation of butane over a chromia–alumina catalyst was practiced for the production of butenes that were dimerized to octenes and hydrogenated to octanes to yield high-octane aviation fuels. The earlier catalyst development employed chromia–alumina catalyst and more recent catalytic developments use platinum or modified platinum catalysts. Dehydrogenation is a highly endothermic process and as such is an equilibrium limited reaction. Thus important aspects in dehydrogenation entail approaching equilibrium or near-equilibrium conversion while minimizing side reactions and coke formation.

Formation of Thermal Wave Trains in Loop Reactors: Stability Limits and Spatiotemporal Structure for Reversible Reactions

Networks of fixed bed reactors with periodically switched inlet and outlet sections are studied with reference to equilibrium limited reactions. The methanol synthesis is selected as representative reaction example and the mechanisms governing the emergence of periodic regimes corresponding to trains of traveling thermal waves are analyzed. Analytical approximations, accounting for the influence of the implemented switching strategy and in satisfactory agreement with numerical simulation, are derived for the switch time stability limits in terms of temperature fronts velocities. Large and sudden variations in the switch time stability range of any wave train solution are demonstrated to occur when the sum of the velocities of declining temperature fronts exceeds the sum of the velocities of rising temperature fronts. The illustrated results provide indications on how to design and operate the network so as to generate thermal wave trains with desired number of waves and stability limits.

CFD Simulation of Water Gas Shift Membrane Reactor--Pressure Effects on the Performance of the Reactor

Membrane reactor is a process intensified equipment that carries out both the reaction and separation in a single vessel. The equilibrium limited water gas shift reaction is an ideal reaction to be carried out in a membrane reactor as it improves the conversion of the reaction and reduces the space requirement for the reactor. Computational fluid dynamics offers a virtual prototyping of the reactor and helps in design, optimization, and scale-up of the reactor. To obtain pure hydrogen from the membrane reactor, the pressure of the reactor needs to be optimized. Hence the water gas shift membrane reactor is subjected to computational fluid dynamic analysis to understand the role played by pressure on the performance of the reactor using three different gas mixtures. The CO conversion and H2 recovery for the different operating pressures are simulated and the effects of pressure are discussed in this paper.

Reactive distillation for biodiesel production from soybean oil

Biodiesel, which is regarded as a promising alternative to a conventional petroleum-based diesel fuel, can be produced from transesterification of vegetable oils and alcohol in conventional batch and continuous reactors. Since the transesterification is an equilibrium-limited reaction, a large excess of reactants is usually used to increase the production of biodiesel, thereby requiring more expensive separation of unreacted raw materials. This study proposed the use of a reactive distillation for transesterification of soybean oil and methanol catalyzed by sodium hydroxide to produce biodiesel. The simulation results showed that a suitable configuration of the reactive distillation column consists of three reactive stages. The optimal conditions for the reactive distillation operation are at the molar feed ratio of methanol and oil at 4.5: 1, reflux ratio of 3, and reboiler duty of 1.6×107 kJ h−1. Methanol and soybean oil should be fed into the column at the first stage. The effect of important operating and design parameters on the performance of reactive distillation was also presented.

Steam reforming of heptane in a fluidized bed membrane reactor

n-Heptane served as a model compound to study steam reforming of naphtha as an alternative feedstock to natural gas for production of pure hydrogen in a fluidized bed membrane reactor. Selective removal of hydrogen using Pd 77Ag 23 membrane panels shifted the equilibrium-limited reactions to greater conversion of the hydrocarbons and lower yields of methane, an intermediate product. Experiments were conducted with no membranes, with one membrane panel, and with six panels along the height of the reactor to understand the performance improvement due to hydrogen removal in a reactor where catalyst particles were fluidized. Results indicate that a fluidized bed membrane reactor (FBMR) can provide a compact reformer for pure hydrogen production from a liquid hydrocarbon feedstock at moderate temperatures (475–550 °C). Under the experimental conditions investigated, the maximum achieved yield of pure hydrogen was 14.7 moles of pure hydrogen per mole of heptane fed.

Steady state and dynamic simulation of a hybrid reactive separation process

AbstractModelling and simulation of hybrid reactive separation system in steady state and in dynamic regime was carried out. The investigated hybrid process consisted of a reactive distillation column and a pervaporation membrane located in the distillate stream to remove water from the process. Heterogeneously catalysed esterification of propionic acid with propan-1-ol to propyl propionate and water was chosen as the model chemical reaction. Esterification reactions are a typical example of equilibrium-limited reactions producing water as a by-product. Using just a pervaporation membrane brings the biggest benefit in increasing the yield of one of the reactants due to the removal of water. To study reactive separation processes, a model of the hybrid system in steady state and in dynamic regime was developed. Steady state behaviour of the model was studied for different hybrid system configurations. The effect of catalyst amount doubling was also investigated. Dynamic behaviour of the system during the step changes of propionic acid feed flow rate and during the membrane module failure was investigated. For this reason, the conversion of propionic acid, purity of the product stream, mole fraction of water, and the temperature in three different parts of the reactive distillation column were monitored.

A simplified method for limit conversion calculation in membrane reactors

AbstractMembrane reactors (MRs) are often used to carry out equilibrium limited reactions. This is because the thermodynamic equilibrium is a strong constrain for traditional systems. Even with very active catalysts, traditional reactors (TRs) cannot give conversions higher than those allowed by the thermodynamic equilibrium. On the contrary, MRs are able to shift the equilibrium of a traditional system owing to the removal of at least one reaction product that takes place simultaneously to the reaction. In this work, a simplified method for the calculation of limit conversion in MR is discussed and compared with literature methods. The typical method for calculating equilibrium conversions in TR is also discussed. It has been demonstrated that the simplified method applied to two reaction systems gives fast predictions of the limit conversion for MR. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.