Equilibrium-limited Reactions Research Articles

Zeolite Membrane-Based Low-Temperature Dehydrogenation of a Liquid Organic Hydrogen Carrier: A Key Step in the Development of a Hydrogen Economy.

Methylcyclohexane (MCH) dehydrogenation is an equilibrium-limited reaction that requires high temperatures (>300°C) for complete conversion. However, high-temperature operation can degrade catalytic activity and produce unwanted side products. Thus, a hybrid zeolite membrane (Z) is prepared on the inner surface of a tubular support and used it as a wall in a membrane reactor (MR) configuration. Pt/C catalysts is packed diluted with quartz sand inside the Z-coated tube and applied the MR for MCH dehydrogenation at low temperatures (190-250°C). Z showed a remarkable H2-permselectivity in the presence of both toluene and MCH, yielding separation factors over 350. The Z-based MR achieved higher MCH conversion (75.3%±0.8% at 220°C) than the conventional packed-bed reactor (56.4%±0.3%) and the equilibrium state (53.2%), owing to the selective removal of H2 through Z. In summary, the hybrid zeolite MR enhances MCH dehydrogenation at low temperatures by overcoming thermodynamic limitations and improves the catalytic performance and product selectivity of the reaction.

Periodic wet milling as a solution to size-based separation of crystal products from biocatalyst for continuous reactive crystallization

Combining the synthesis and crystallization of a compound into a single step may be beneficial for certain processes. Motivations range from process intensification to improving reaction network yield by shifting an equilibrium limited reaction or protecting an intermediate, as is the case in the biocatalytic synthesis of beta-lactam antibiotics. Adapting such a reactive crystallization process to continuous manufacturing faces a challenge: the continuous separation of crystals from biocatalyst carriers. In this study, a size-based method for this separation problem is proposed. Separation is performed using a filter placed on the reactive crystallization vessel outlet. The primary factors for filter design are carrier and crystal size dictated by the separation condition Lcrystal<Lfilter<Lcarrier. Enzymatic assays confirmed an increasing loss of activity and selectivity for the biocatalyst, Penicillin G Acylase, when large carrier sizes are used for immobilization, limiting the maximum Lcarrier that can be used in the process. Intermittent wet milling, however, is shown to be effective in reducing the crystal size and allowing the use of sufficiently small carriers while still satisfying the criterion for solid–solid separation. Slurry transfer tests were performed to check the possibility of isokinetic withdrawal when the separator filter was used. Filter size of 300μm, a carrier size of 300–425μm, and intermittent wet milling of 2–3 reactor volumes per residence time at 5000RPM were found to provide an almost complete separation, acceptable biocatalyst kinetics, and an isokinetic crystal product withdrawal.

Potential of Pervaporation-Based Dehydration Processes as an Equilibrium-Limited Reactions Enhancer: Proof-of-Concept and Process Scale-up for an Acrylic Ester

Chemicals derived from equilibrium-limited reactions represent a huge challenge mainly from the process sustainability point of view. Usually, high temperatures are required to reach reasonable kin...

Inter‐integration reactive distillation with vapor permeation for ethyl levulinate production: Equipment development and experimental validating

AbstractEthyl levulinate, one of the main derivatives of levulinic acid (LA), is of significant potential as platform chemicals for bio‐based materials. The esterification of LA was generally carried out in a conventional batch reactor or in a conventional reactive distillation column. However, traditional methods are hard to deal with equilibrium limited reactions and azeotropic issues. Therefore, the inter‐integration reactive distillation with vapor permeation (R‐VP‐D) process, which integrated reaction, vapor permeation, and distillation into one single unit, is proposed in this paper and validated in the pilot‐scale experiments. A comparative study is made between a pilot‐scale RD column with and without VP module. Owing to the water‐selective VP membrane and the ingenious design of related apparatuses, the R‐VP‐D process reveal a superiority in LA conversion of 21.9% maximum higher than RD without VP process and removing of product water about 53.6% from VP module, which indicates its promising industrial application in process intensification field.

Steady-State and Dynamic Simulation Study of Reactive Distillation for FFA Esterification in Biodiesel Synthesis

Reactive distillation (RD) holds promise for process intensification in biodiesel production since it integrates reaction and separation. It simplifies the process and enhances the conversion of the equilibrium limited reactions. To ensure the stability in RD operation, sensitivity study and process control simulation are necessary. In this work, RD was employed for free fatty acid (FFA) esterification of mixed non edible oils in biodiesel synthesis. Non edible oils used were waste cooking oil, crude jatropha oil, and crude nyamplung oil (Calophyllum inophyllum L). Simulation was conducted using ASPEN Plus V8.8. Sensitivity study was carried out to determine the effects of the operating condition alteration. A dynamic simulation was performed as a Proportional-Integral-Derivative (PID) controller tuning. It was revealed that the highest FFA conversion was 85%, achieved at the feed stage of 7, distillate rate of 0.22 kmol/hr, and oil to methanol molar ratio of 1:5. Level, pressure and temperature controls were installed in RD. Then, a dynamic simulation was applied as a PID controller tuning. Three different controller tuning methods, viz. Ziegler-Nichols, Cohen-Coon, and Internal Model Control, were studied. The best PID parameter was obtained by using Cohen-Coon method which provided fastest rise time, lowest settling time and lowest overshoot.

Modeling and simulation of a reactive separation system for carbon capture and utilization in biogas streams

An effective energy storage approach, such as Power-to-Gas, is essential to provide the grid with a steady supply of power generated by renewable sources. In a Power-to-Gas concept, excess renewable energy is stored in the form of H2 which, in turn, can react with CO2 from sources such as biogas to be converted into CH4 and generate Renewable Natural Gas (RNG). Methanation however, is an exothermic and equilibrium-limited reaction which suffers from poor heat management when carried-out in conventional packed-bed reactors. Additionally, unreacted H2 mixed with CH4 has to be removed before the RNG is injected into the pipeline. To improve the methanation of biogas, two commercial high temperature reactive separation systems have been studied numerically in this work: (i) an extractor/distributor membrane reactor employing a carbon molecular sieve membrane; (ii) a distributor membrane reactor using a palladium membrane. Both configurations have been shown to considerably reduce the content of CO2 and H2 in the resulting RNG and, hence, the post-processing efforts needed as compared to a packed-bed reactor operating under the same conditions.

High-Pressure Phase Equilibrium Studies of Multicomponent (Alcohol-Water-Ionic Liquid-CO2) Systems

Selective water (by-product) separation from reaction mixtures stands as an important process intensification strategy for equilibrium-limited reactions. In this work, the possibility of using a high-pressure biphasic reaction media composed of a hydrophobic ionic liquid, 1-hexy-3-methylimidazolium tetracyanoborate, and carbon dioxide was explored for levulinic acid production from 1,4-butanediol. Vapour-liquid equilibrium measurements were performed for the binary (diol+CO2), ternary (diol+CO2+IL), and quaternary systems (diol+CO2+IL+water), at 313.2 K and pressures up to 18 MPa. The static analytical method was used in a high-pressure phase equilibrium apparatus equipped with a visual sapphire cell. The capability of the quaternary system to perform physical water separation is discussed in this paper.

Derivation of rate equations for equilibrium limited gas-solid reactions

In this work the differences between the formulation of reaction rate equations for equilibrium limited reactions in homogeneous and heterogeneous systems are investigated. A rigorous derivation for equilibrium limited gas-solid reactions is proposed based on the core ideas of homogeneous gas phase reactions. The resulting reaction rate expression allows for two distinct reaction rate mechanisms for the forward and reverse reaction direction and is, therefore, valid in both reaction directions. The thermodynamic consistency and the validity of the proposed reaction rate expression is shown using a simple reactor model. The results are compared to traditional reaction rate expressions and it is explained why they are not able to appropriately describe the true reaction behavior in some cases. The implications and use cases for process modeling are investigated and suggestions are proposed for solving numerical challenges associated with the proposed reaction rate equations.

Multi-scale modeling and simulation of a novel membrane reactor (MR)/adsorptive reactor (AR) process

The objective of this study is the modeling and simulation of a novel process, consisting of a membrane reactor (MR)/adsorptive reactor (AR) sequence that intensifies equilibrium-limited reactions and associated separation processes. A comprehensive, multi-scale, multiphase, steady-state/dynamic, computational fluid dynamics (CFD)-based process model is developed, that quantifies the many underlying complex physicochemical phenomena occurring at the pellet, and reactor scales. Model simulations are then carried-out, to study the influence of the individual units’ operating and design parameters on overall system performance, and to better understand this simultaneous reaction/separation, steady-state/cyclic process. Simulation-based optimization studies are then carried out to identify acceptable ranges for both operating and design parameters, so as to meet pre-defined process performance specifications.

“Pushing and pulling” the equilibrium through bubble mediated reactive separation for ethyl acetate production

The alcohol is fed as vapor within the bubbles which means alcohol is always in deficit, providing an opportunity to convert an equilibrium limited reaction to nearly irreversible one.

A novel method for determining the optimal operating points of reactive distillation processes

Reactive distillation (RD) allows reaction and separation to take place simultaneously in the same unit, thus giving major benefits especially to equilibrium limited reactions. Although the application of RD in chemical industries is attractive, it is considerably challenging. Unlike classic distillation, the optimal configuration of RD from an economical perspective is hardly identified quickly. Usually, any specific reaction system may need extensive studies and rigorous simulations to develop a RD model. This study aims to determine the optimal operating points of a RD application in a quick and reliable way. A novel method is employed for a clear visualization of the RD applicability area (i.e. a plot of reflux ratio vs number of stages). Using this method, an economic analysis can be performed resulting in essential insights into the optimal configurations. The production of amyl acetate by esterification of amyl alcohol and acetic acid is selected as case study, since this reaction sufficiently represents non-ideal behaviours in real systems. The outcome of the analysis reveals that the boundary line of its RD applicability graph consists of the optimal points of RD configurations which generate the lowest total annual cost in the RD operation. Furthermore, it is observed that the additional cost for the reactive section (relative to a separation section) is marginal, which means that the rules of thumb for the optimal configurations in classic distillation could also be applied.

Modeling of reactive batch distillation processes for control

Reactive batch distillation (RBD) is a preferred process intensification technology to carry out equilibrium-limited reactions. It is a multicomponent, multiphase system. Appropriate process description requires dynamic modeling of coupled thermodynamics and transport phenomena including the chemical reactions. Such models are barely applicable to online model based operation technology such as model predictive control, real-time optimization and online process monitoring. Therefore, in this paper, the rigorous dynamic model of an RBD is transformed into a set of decoupled ordinary differential equations using linear transformation matrices, called extent transformation, that preserve the physical meaning of the transformed variables. The resulting model has a state space representation with a diagonal state matrix. This representation is suitable for control purposes and can be considered as a linear parameter varying system. Based on the final structure of the model, controllability conditions are stated, and model reduction scenarios are proposed. Finally, the model based on extent transformations is compared with the rigorous nonlinear model via the simulation of a polyesterification process.

Scale-Up of a Continuous Extraction Process for Driving an Equilibrium-Limited Reaction to Completion

We report a strategy for implementing a scalable continuous extraction to drive an equilibrium-limited reaction to completion. Our approach is simple, requiring only standard equipment and glassware, and can be designed to meet the needs of the process (e.g., reduced cycle time or reduced solvent use) with minimal development time. We investigated the process parameters, such as the flow rate of fresh n-heptane (extracting solvent) into the system, at a 0.5–5 g scale and reduced the overall time and total amount of n-heptane needed to drive the reaction to completion. The improved conditions for the extraction were demonstrated at the 50 g scale, leading to a similar yield of the desired product and total reaction time as at the 5 g scale, verifying that our reactions and extraction conditions were scalable. In particular, our fit-for-purpose approach is customizable to a number of systems and can be implemented in a short period of time with a limited amount of material.

Dynamic Performance Optimization of a Pilot-Scale Reactive Distillation Process by Economics Optimizing Control

The most prominent example of process intensification by process integration is reactive distillation (RD), which can realize a high reactant conversion and a high purity of the target product in equilibrium-limited reaction systems. Reactive distillation is mostly used for single reactions, but realizations for multiple-reaction systems leading to more than one target product can also be found. In multipurpose chemical processes, the target product of a multiple-reaction system can be switched during the production. When combining fluid separation and multiple-reaction systems, e.g., in a RD column, it is challenging to find operating conditions that lead to the most profitable operation of the process. Economics optimizing control where an economic objective is employed in a model-predictive control framework has the potential to drive the process to the economic optimum during operation based upon a process model and online measurements. However, so far only few applications of such advanced process co...

Catalysis for efficient low-temperature hydrogen production and storage

Hydrogen, an energy-intensive and clean power source, has been treated as an ideal energy carrier to replace the coal and oil based global energy system for the more sustainable economic development. Especially, with the increasing finical support from worldwide in the researching of renewable energy both for the public and military using. Generally, hydrogen cycling includes three steps: (1) the continuously large scale hydrogen generation; (2) the safe and efficient hydrogen storage and transportation; (3) the efficient conversion of hydrogen to electricity or other forms of energy. For the using of hydrogen, a representative example is polymer electrolyte membrane fuel cells (PEMFCs) which has been widely studied. However, the bottle-neck for the bursting of hydrogen economy besides the effective using of hydrogen lies in the efficient generation and safe storage of hydrogen. Recently, our research group developed α-molybdenum carbide (α-MoC) supported Au and Pt catalysts for the low temperature water-gas shift (WGS) reaction and aqueous-phase reforming of methanol, respectively, both were essential reactions for industrial hydrogen generation. The WGS reaction (where carbon monoxide plus water yields hydrogen and carbon dioxide) is an important process for hydrogen generation and carbon monoxide removal in various energy-related chemical operations. This equilibrium-limited reaction is favored at a low working temperature. However, the reported catalysts by now always show unsatisfied performance under low operating temperatures and no catalysts ever reported reaching the activity of 0.1 molCO molmetal−1 s−1 below 150°C. Meanwhile, potential application in fuel cells also requires a WGS catalyst to be highly active, stable, and energy-efficient to match the working temperature of on-site hydrogen generation and consumption units. Concerning this problem, we synthesized layered gold clusters on a molybdenum carbide (Au/α-MoC) substrate to create an interfacial catalyst system for the ultralow-temperature WGS reaction. Water was activated over α-MoC at 30°C, whereas carbon monoxide adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting, leading to a high WGS activity reaching 1.05 molCO molmetal−1 s−1 under 120°C which is one order of magnitude higher than the early reported catalysts. Beside the Au/α-MoC for robust WGS reaction, we also developed highly distributed single atom Pt supported by molybdenum carbide (Pt/α-MoC) for aqueous-phase reforming of methanol. With the reforming of methanol and water, hydrogen with a high gravimetric density of 18.8% by weight can be in situ released. The average turnover frequency of Pt/α-MoC can reach 18046 mol of hydrogen per mole of platinum per hour at 190°C, which is two order of magnitude higher than the traditional catalysts. Based on the X-ray absorption fine structure (XAFS) and single atom resolution electron microscopy characterization results, Pt was determined to be atomic dispersed over α-MoC support at the low metal loadings, which maximized the exposure of Pt atom. Based on reaction mechanism investigation and DFT calculation, Pt/α-MoC was proved to be a bifunctional catalyst, with α-MoC support dissociating the O−H bond of H2O and methanol at low temperature, atomic dispersion of Pt scissoring C−H bond of CH3OH. The effectively reforming of intermediates with surface hydroxyls generate CO2 at the interface of Pt/α-MoC. The excellent performance of our catalytic system provide a new strategy for the efficient low-temperature hydrogen production and storage. In this short perspective, we introduce the new findings by our group and also summarize the recently developed representative catalysts involving the low temperature water-gas shift reaction and aqueous-phase reforming of methanol. We hope it can provide a reference for the future designing of catalysts in the field of hydrogen production and storage.

Novel method for mapping the applicability of reactive distillation

Reactive distillation (RD) is a great process intensification concept applicable to equilibrium limited reaction systems, but how can anyone decide quickly if RD is indeed worth applying? To answer this question, this study proposes a mapping method for checking the applicability of reactive distillation (RD). The initial development is for one of the most relevant subset of quaternary reversible reactions (A + B ⇄ C + D, with boiling points Tb,C < Tb,A < Tb,B < Tb,D), by using only basic chemical (equilibrium and kinetics) and physical (relative volatilities) parameters. Generic cases, assuming ideal thermodynamics and constant parameters, are used to obtain a set of RD applicability graphs that provide broad insights into the RD operation. In addition, the new mapping method provides reasonable estimates of the RD applicability to real (non-ideal) chemical systems based on the available pre-defined maps (which are actually applicability graphs of the generic ideal cases). This new approach leads to a straightforward estimation of the applicability of RD to real systems, prior to performing any rigorous process simulations and without any clear-cut decision making (as used in previous studies).

Effects of H2O in the Feed of Sulfur Recovery Unit on Sulfur Production and Aromatics Emission from Claus Furnace

An increase in the exploitation of sour reservoirs with a high content of acid gas (mainly H2S and CO2) and the enforcement of stringent emission standards have triggered demand for high sulfur recovery efficiency in sulfur recovery units (SRUs). The high amount of H2O that is present in feed gases to the SRU can act as a diluent in the thermal section of the SRU and can also pose operational difficulties and have substantial chemical effect on the equilibrium-limited reactions of sulfur production and the destruction of impurities. This paper explores the role of H2O (0–12% in the furnace feed) in sulfur recovery and in the fate of undesirable aromatics, COS, and CS2 in the thermal section using a detailed reaction mechanism and industrial SRU operating parameters. An increasing H2O concentration in the feed decreased the sulfur recovery efficiency in the thermal section significantly, as the equilibrium conversion of H2S to sulfur was affected. The higher H2O concentrations increased the concentrations ...

Thermodynamics of enzyme-catalyzed esterifications: II. Levulinic acid esterification with short-chain alcohols.

Levulinic acid was esterified with methanol, ethanol, and 1-butanol with the final goal to predict the maximum yield of these equilibrium-limited reactions as function of medium composition. In a first step, standard reaction data (standard Gibbs energy of reaction Δ R g 0 ) were determined from experimental formation properties. Unexpectedly, these Δ R g 0 values strongly deviated from data obtained with classical group contribution methods that are typically used if experimental standard data is not available. In a second step, reaction equilibrium concentrations obtained from esterification catalyzed by Novozym 435 at 323.15 K were measured, and the corresponding activity coefficients of the reacting agents were predicted with perturbed-chain statistical associating fluid theory (PC-SAFT). The so-obtained thermodynamic activities were used to determine Δ R g 0 at 323.15 K. These results could be used to cross-validate Δ R g 0 from experimental formation data. In a third step, reaction-equilibrium experiments showed that equilibrium position of the reactions under consideration depends strongly on the concentration of water and on the ratio of levulinic acid: alcohol in the initial reaction mixtures. The maximum yield of the esters was calculated using Δ R g 0 data from this work and activity coefficients of the reacting agents predicted with PC-SAFT for varying feed composition of the reaction mixtures. The use of the new Δ R g 0 data combined with PC-SAFT allowed good agreement to the measured yields, while predictions based on Δ R g 0 values obtained with group contribution methods showed high deviations to experimental yields.

Performance Evaluation of Pervaporation Technology for Process Intensification of Butyl Acrylate Synthesis

Pervaporation-based hybrid processes have been investigated to overcome the drawbacks of equilibrium-limited reactions. Pervaporation processes are strongly recommended for heat-sensitive products and azeotropic mixtures as in the butyl acrylate system case, since pervaporation can operate at lower temperatures than distillation. In this work, experimental pervaporation data for multicomponent mixtures in the absence of reaction were measured for the compounds involved in the esterification reaction of acrylic acid with n-butanol at different temperatures: 323, 353, and 363 K. A commercial tubular microporous silica membrane from Pervatech was used which is highly selective to water, and its performance was evaluated by studying several parameters, like the selectivity, permeate fluxes, driving force of species, and separation factor. The effects of temperature and feed composition were assessed for binary, ternary, and quaternary mixtures. Increasing the temperature increases significantly the total perm...

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.