Reversible Exothermic Reaction Research Articles

"Impact of hydride composite materials on thermochemical hydrogen compression"

The formation of a hydride from a metal or intermetallic alloy is an exothermic reversible chemical reaction. The thermodynamic properties governing hydride formation facilitate their thermochemical applications, such as hydrogen compression. Thermal transport inside the materials is of highest importance for system dynamics and its productivity. In this contribution, we demonstrate a two-stage thermochemical hydrogen compressor using the AB5 alloy LaNi5 in the first stage and an AB2 alloy TiMn2 (Hydralloy®) in the second stage. To evaluate the impact of the thermal conductivity of the materials, the productivity of the compressor was compared for pure metal hydride compacts (MH) and metal hydride composite materials (MHC). Compared to pure MH pellets, the MHC used in this study contain expanded natural graphite (ENG), a secondary phase with highest thermal conductivity. As the radial thermal conductivity of the MHC increased, the time required for the loading, temperature change and unloading steps was successfully reduced by 400%. Productivity was increased by over 320 % from 14 NlH2/(kgMaterial*h) to 44 NlH2/(kgMaterial*h). Overall, MHC have the potential to simplify handling, reactor design and reduce investment costs for thermochemical compression systems. Thus, MHC have highest impact and potential for thermochemical applications.

Optimization of an improved calcium-looping process for thermochemical energy storage in concentrating solar power plants

The calcium-looping (CaL) process comprises an endothermic calcination reaction, where CaO and CO2 are generated from CaCO3, and its reverse exothermic carbonation reaction. CaL is promising for thermochemical energy storage (TCES) in concentrating solar power plants. The CaL-TCES process includes: a calciner where solar energy is transformed into thermochemical energy; a carbonator where the stored energy is released; turbines for electrical power generation; and tanks where the reaction products are stored. In this work, the CaL-TCES process is simulated and optimized using gPROMS Process. The innovation lies in identifying process improvements: make-up and purge streams to reduce CaO deactivation after cycling; a water separation process at the calciner outlet to allow calcination with water vapor as fluidizing gas; and the use of several degrees of freedom for process optimization. The goal is to maximize the thermal-to-electrical efficiency. By optimizing the carbonator and main turbine outlet pressures, the efficiency is improved from 38.1 % in the literature to 39.2 %. When make-up and purge streams are considered, the savings in power supply owing to the purged CaO allow improving the efficiency to 43.0 %. The water separation process reduces the thermal-to-electrical efficiency to 34.7 %, but allows a higher solar-to-thermal efficiency or a smaller calcination reactor.

Experimental studies on LaNi4.25Al0.75 alloy for hydrogen and thermal energy storage applications

Reversible exothermic and endothermic reactions between metals/alloys and hydrogen gas provide great opportunity to utilize various thermal energy sources such as waste heat, industrial exhaust, and solar thermal energy. Metal hydrides with favourable properties to operate at medium temperature heat (about 150 °C) are limited, and studies on hydrides in this temperature range are scarce. Hence, the present study aims at experimental investigations on LaNi4.25Al0.75 alloy in the temperature range of 150 °C–200 °C. A novel cartridge type of reactor is employed to investigate the hydrogen storage characteristics and thermal storage performance of this alloy. LaNi4.25Al0.75 is found to have a hydrogen storage capacity of about 1.20 wt% at 10 bar and 25 °C. In addition, it can store a total thermal energy of 285.7 kJ.kgMH−1 and can deliver heat at an average rate of 287.5 W.kgMH−1 at an efficiency of 64.1%.

A review on high‐temperature thermochemical heat storage: Particle reactors and materials based on solid–gas reactions

AbstractIn order to produce electricity beyond insolation hours and supply to the electrical grid, thermal energy storage (TES) system plays a major role in CSP (concentrated solar power) plants. Current CSP plants use molten salts as both sensible heat storage media and heat transfer fluid, to operate up to 560°C. To meet the future high operating temperature and efficiency, thermochemical storage (TCS) emerged as an attractive alternatives for next generation CSP plants. In these systems, the solar thermal energy is stored by endothermic reaction and subsequently released when the energy is needed by exothermic reversible reaction. This review compares and summarizes different thermochemical storage systems that are currently being investigated, especially TCS based on metal oxides. Various experimental, numerical, and technological studies on the development of particle reactors and materials for high‐temperature TCS applications are presented. Advantages and disadvantages of different types heat storage systems (sensible, latent, and thermochemical), and particle receivers (stacked, fluidized, and entrained), have been discussed and reported.This article is categorized under: Sustainable Energy > Solar Energy Emerging Technologies > Energy Storage Emerging Technologies > Materials

Modeling and simulation of a steam-selective membrane reactor for power-to-methanol

The hydrogenation of CO2 to methanol is currently receiving increased attention following the deployment of Power-to-X processes. In this exothermic reversible reaction, the CO2 conversion is strongly limited by the thermodynamic equilibrium and the offer of commercial catalysts is mostly limited to Cu-based materials. Therefore, available options for improving methanol production consider either process design and optimization and/or using multifunctional reactors that provide advantageous features compared to conventional reactors, despite additional complexity. In this work we have modeled and simulated a membrane reactor (MR) featuring steam removal for shifting the reaction equilibrium to the products side and thus not only improving the CO2 conversion but also contributing to the increase of catalyst life span and representing a reliable alternative for heat removal. For this purpose, the proposed model for the MR considered the properties of a hydrophilic sodalite (SOD) membrane and the kinetics of a Cu/ZnO/Al2O3 catalyst. A split feed strategy has been devised to tackle poor membrane selectivity in the temperature range of interest. At 270 °C and 50 atm, the split feed allowed increasing the methanol yield up to 18%, while an ideal membrane (i.e., infinitely selective to steam) up to 31% (both in relative terms), when compared to a traditional reactor. Moreover, using the MR with the SOD membrane in the referred conditions it was obtained a permeate outlet stream containing only reactants and steam, therefore easily recyclable.

ANALYSIS OF STUDENTS' ABILITY TO IDENTIFY SYMBOLIC REPRESENTATIONS IN CHEMISTRY

To describe the behavior of matter and energy, chemists classify them in three distinct domains: macroscopic, microscopic, and symbolic. The ability to use these three representations is the basis for understanding concepts in chemistry. This study aims to analyze students' ability to identify symbolic representations in chemistry. The research design uses a one shot case study. The subjects of this study were students of prospective science teachers as many as 85 students. Data collection techniques using tests and rubrics. The results showed that of the ten symbolic representation statements, only three statements achieved the highest percentage of correct answers, namely statements about writing ionization reactions and writing electron symbols. There are two statements where almost 90 percent of students answered incorrectly. The statement is about reversible or irreversible reaction equations and exothermic reaction equations. From this research, it can be concluded that students' ability to identify symbolic representations in chemistry still needs to be improved, because the average score is still low.

A novel fluidized bed “thermochemical battery” for energy storage in concentrated solar thermal technologies

Thermochemical energy storage is gaining widespread consideration to increase energy dispatchability in concentrating solar thermal power plants. Accordingly, excess solar energy input drives an endothermic reaction, accomplishing high energy densities and virtually unlimited storage times. As gas–solid reactions are usually involved, multiphase reactor design is essential for the success of this technology. A novel concept of directly-irradiated fluidized bed autothermal reactor is investigated for applications in concentrated solar thermal technologies. The device can be operated as a rechargeable battery, alternating a charge phase, during which solar energy is collected and stored by an endothermal gas–solid reaction, and a discharge phase, during which the stored chemical energy is released by the reverse exothermic reaction. The autothermal operation, during the charge process, consists in the recovery of the sensible heat of the reaction products to preheat the reactants by means of an internal double-pipe countercurrent heat exchanger. This operation allows to increase the overall efficiency, reducing the required solar energy input. A compartmental model to simulate the operation of the thermochemical battery is developed and closed with constitutive equations and parameters obtained by previous experimental studies on lab-scale test facilities. Limestone calcination/carbonation has been considered as model reversible reaction. Both the charge and the discharge steps were assessed investigating the effect of the design and operational variables. For the charge operation, an optimal temperature was found around 900 °C with thermal efficiencies close to 90%. For the discharge operation, thermal efficiency was found to depend almost solely on the reactor temperature, reaching values as high as 80%, whereas the gas flowrate can be set independently. The upper limit for reaction temperature is set to the reaction equilibrium condition corresponding to the inlet concentration of carbon dioxide. The obtained results represent the basis for the realization of a new prototype, that will serve for the complete proof-of-concept of the directly-irradiated fluidized bed autothermal reactor.

Maximization of the profit and reactant conversion considering partial pressures in an ammonia synthesis reactor using a derivative‐free method

AbstractIn the present paper, an optimization model is proposed for the ammonia synthesis reactor. The reversible exothermic reaction has nitrogen and hydrogen as reactants, at high temperatures, and high pressures with iron catalyst. Two different single‐objective optimization problems were considered, the maximization of the economic return and the maximization of the nitrogen conversion. The reaction rate model was defined as a function of partial pressures of the components. The problem was coded and solved in MATLAB using a derivative‐free method after reformulating a constrained optimization problem into an unconstrained one, by penalizing the infeasibilities of the constraints in the objective functions (barrier function). The main contributions of this paper are the combination of direct‐search methods in solving the optimization problem, the evaluation of the maximum ammonia production and the temperature profile. The optimal values found were better than the ones published in the literature.

Model Predictive Control of Jacket Tubular Reactors with a Reversible Exothermic Reaction

This article addresses the model predictive controller design for a jacket tubular reactor with a simple reversible exothermic reaction (A ⇌ B). Using energy and mass balance laws, four nonlinear h...

Experiment and kinetic model study on modified potassium-based CO2 adsorbent

In this paper, aluminium chloride crystal was used as precursor to prepare silica-alumina aerogel support by sol-gel method, and the active component K2CO3 was supported on the aerogel support by incipient-wetness impregnation method to obtain a modified potassium-based adsorbent. CO2 adsorption experiment was performed on the adsorbent under different reaction conditions using a small fixed-bed reactor self designed. Based on the characterization results of low temperature N2 adsorption-desorption experiments and scanning electron microscopy, CO2 adsorption characteristics of modified potassium-based adsorbents were studied. The prepared silica-alumina aerogel support has good microstructure and uniform pores with specific surface area and pore volume are 445.7517 m2/g and 2.3270 cm3/g, respectively, and the mesopores account for more than 99%. After loading, the active component K2CO3 is uniformly dispersed. Through model comparison, it was determined that the shrinking core model and Avrami fractional kinetic model were used to fit the experimental results in order to study the reaction kinetics and adsorption kinetics. The results show that as the temperature rises, the amount of CO2 adsorbed by the adsorbent increases first and then decreases. The increase in temperature can improve the activity of the active site of the adsorbent and make the reversible exothermic reaction occur while the adsorbent adsorbs CO2 cannot proceed in the forward direction at the same time. According to the kinetic analysis, the control period of the surface chemical reaction is too short due to the high temperature, which caused low carbonation conversion. The optimal designed loading of modified potassium-based adsorbent, reaction temperature, water vapor pretreatment time, CO2 concentration and total gas volume were 30%, 60 °C, 30 min, 12.5%, and 500 mL/min, respectively.

Role of particle size on the cohesive behavior of limestone powders at high temperature

Thermal Energy Storage (TES) using granular solids is gaining momentum in the last years. With no degradation up to very high temperatures and very low price the use of some granular materials such as sand or SiC would be feasible for storing sensible heat at large scale. A further step beyond TES is thermochemical energy storage (TCES) wherein the granular solids undergo a highly endothermic reaction at high temperature. Energy can be in this way more efficiently stored in the long term and released on demand by means of the reverse exothermic reaction. The Calcium Looping process, based on the calcination/carbonation of CaCO3, is being actively investigated for this purpose. However, a caveat of using granular solids for energy storage is the possible increase of interparticle adhesive forces with temperature which would severely hamper the flowability of the solids in the process. The cohesiveness of granular materials is essentially determined by particle size. In this paper we investigate the dependence of the tensile yield strength and compressibility of CaCO3 powders on temperature and consolidation stress using samples of narrow particle size distribution in the relevant range between ∼30 and ∼80μm particle size and temperatures up to 500°C. Our experimental results show that powder cohesiveness is greatly increased with temperature especially in the case of the finest powders whose tensile yield strength can be increased by up 2 orders of magnitude. The increase of cohesiveness with temperature is further enhanced with a previously applied consolidation stress, which is particularly relevant for applications wherein large amounts of solids are to be stored at high temperature. Experimental data are consistent with the predictions by a contact mechanics model assuming that the solids deform plastically at interparticle contacts. A main conclusion from our work is that some mechanical properties of the solids, specially the mechanical hardness, and how they change with temperature, play a critical role on the flowability of the solids as affected by an increase of temperature.

Real-Time Optimization and Control of Nonlinear Processes Using Machine Learning

Machine learning has attracted extensive interest in the process engineering field, due to the capability of modeling complex nonlinear process behavior. This work presents a method for combining neural network models with first-principles models in real-time optimization (RTO) and model predictive control (MPC) and demonstrates the application to two chemical process examples. First, the proposed methodology that integrates a neural network model and a first-principles model in the optimization problems of RTO and MPC is discussed. Then, two chemical process examples are presented. In the first example, a continuous stirred tank reactor (CSTR) with a reversible exothermic reaction is studied. A feed-forward neural network model is used to approximate the nonlinear reaction rate and is combined with a first-principles model in RTO and MPC. An RTO is designed to find the optimal reactor operating condition balancing energy cost and reactant conversion, and an MPC is designed to drive the process to the optimal operating condition. A variation in energy price is introduced to demonstrate that the developed RTO scheme is able to minimize operation cost and yields a closed-loop performance that is very close to the one attained by RTO/MPC using the first-principles model. In the second example, a distillation column is used to demonstrate an industrial application of the use of machine learning to model nonlinearities in RTO. A feed-forward neural network is first built to obtain the phase equilibrium properties and then combined with a first-principles model in RTO, which is designed to maximize the operation profit and calculate optimal set-points for the controllers. A variation in feed concentration is introduced to demonstrate that the developed RTO scheme can increase operation profit for all considered conditions.

Progress in thermochemical energy storage for concentrated solar power: A review

Energy plays an important role in a fast-paced modern society. With the depletion of fossil energy, effective utilization of solar energy is getting increasingly urgent. Thermal energy storage is an inevitable choice for effective utilization of renewable energy sources. As one of the most promising renewable energy sources, solar energy is inexhaustible. But it has some shortcomings such as instability and intermittency, affected by time, climate, and geographical location. Thermal energy storage technology, which can effectively reduce the cost of concentrated solar power generation, plays a crucial role in bridging the gap between energy supply and demand. In addition, thermal energy storage subsystem can improve performance and reliability of the whole energy system. According to different principles, thermal storage technology is generally classified as sensible heat storage, latent heat storage, and thermochemical energy storage. Most solar thermal power generation systems, currently demonstrated and operated in the world, adopt the method of sensible thermal energy storage. In contrast, thermochemical energy storage is a relatively new concept, which is still in the stage of basic test and verification. Thermochemical energy storage technology stores and releases energy through endothermic and exothermic reversible reactions. A closed system with separated reactants and products, in theory, can store energy indefinitely. The main thermochemical energy storage systems include redox system, metal hydride system, carbonate decomposition system, ammonia decomposition system, methane reforming system, and inorganic hydroxide system.

Design of a MW-scale thermo-chemical energy storage reactor

The reversible exothermic reaction of CaO with water is considered one of the most promising reactions for high temperature thermal energy storage. In this paper, a novel technical design of a MW-scale thermochemical energy storage reactor for this reaction is presented. The aim is to provide an easy, modular and scalable reactor, suitable for industrial scale application. The reactor concept features a bubbling fluidized bed with a continuous, guided solid flow and immersed heat exchanger tubes. To investigate the reactor design, a model is build using clustered CSTRs. The technical feasibility of the concept is proven in experimental tests, which are also used to identify key parameters of the model. Fluidization of the fine CaO/Ca(OH)2 powder was found to be challenging, but problems were overcome using mild calcination conditions and a special gas distributor plate. Using the model, it is found, that a thermal power of 15 MW can be expected from a reactor volume of 100 m3. To study influences of different parameters on the reactor model performance, a sensitivity analysis is carried out and heat transfer between the reactor and the immersed heat exchangers is found to have by far the largest influence and the reaction system performance. Future research should therefore focus more on heat transfer.

The ammonia looping system for mid-temperature thermochemical energy storage

Thermochemical reactions have a great potential for energy storage and transport. Their application to solar energy is of utmost interest because the possibility of reaching high energy densities and seasonal storage capacity. In this work, thermochemical energy storage of Concentrated Solar Power (CSP) based on an ammonia looping (AL) system is analysed. The AL process for energy storage is based on the reversible reaction of ammonia to produce hydrogen and nitrogen. Concentrating solar energy is used to carry out the decomposition endothermic reaction at temperatures around 650 oC, which fits in the range of currently commercial CSP plants with tower technology. The stored energy is released through the reverse exothermic reaction. Our work is focused on energy integration in the system modelled by pinch analysis to optimize the process performance and competitiveness. As result a novel configuration is derived which is able to recover high-temperature heat for electricity production with a thermal-to-electric efficiency up to 27 %. The current study shows a clear interest of the system from an energy integration perspective. Further research should be conducted to access the potential for commercial applications.

Optimal control policy for tracking optimal progression of temperature in a batch reactor – Some insights into the choice of objective function

This paper deals with the development of an optimal control strategy for control of a batch reactor in which an exothermic reversible reaction takes place. A new optimal control policy is proposed in this work to force the reactor operation track the Optimal Progression of Temperature and achieve a maximum end point conversion. This is done by including a term in the Objective function that puts a weight on the overall error between the reactor temperature and the optimal temperature. Further, by proper choice of weight assigned to the control moves in the objective function, the optimal control policy is forced to bypass Singular Arc . Control vector iteration method is used to solve the optimal control problem. The control policy is tested using the batch reactor data reported in the literature. The results show that the optimal control policy proposed in this work is able to track the optimal progression of temperature and achieve maximum end point conversion in the batch reactor.

Ce promoted V2O5 catalyst in oxidation of SO2 reaction

Harmful environmental effects of SO2 on the start-up of the sulfuric acid plant process motivated studying ceria as a promoter for V2O5 catalyst that is employed in SO2 oxidation. Effects of SO2(%), ceria percentage, temperature and O2/SO2 were investigated to optimize ceria loading on V2O5 catalyst. To determine the main and interaction effects, a full factorial design of experiments including 315 experiments was used. The results showed that the catalyst with 7wt% ceria (VaCe7) possessed the highest catalytic performance. To study the effect of more parameters on this catalyst, other 135 post-experiments with full factorial designs were performed after the optimization of Ce contents. The results revealed that mean SO2 conversion increased with increasing SO2 (%) and O2/SO2 ratio, and decreasing GHSV. Also, the maximum mean conversion was found at the reaction temperature of 450°C by the trade-off of kinetic and thermodynamic factors of the reversible exothermic reaction. VaCe7 was found to have more catalyst activities than that the without-ceria (VaCe0) case. The synthesized catalysts were characterized by XPS, BET, FTIR, SEM, XRD and ICP. The SEM mapping revealed that the cerium oxide had better distribution on VaCe7 than VaCe9&11. FTIR, XRD and XPS showed the presence of CeVO4 in the Ce-promoted catalyst and confirmed that ceria promotes V2O5 by the formation of CeVO4 network, reduces the activation energy of reduction–oxidation and increases the rate of SO2 oxidation reaction. The stability tests demonstrated good catalytic performance stability of the synthesized catalyst up to 50h in the reaction. Results at the initial times of the stability tests showed that the catalyst with ceria promoter (VaCe7) had higher activity than VaCe0 at the start-up time. Also, VaCe7 had higher activity at the start-up than its normal conditions, which could be related to the good oxygen storage capacity of ceria because the catalyst at the start-up is faced to the oxygen of air. This high startup activity caused to use it in starting up of sulfuric acid plants that decreases SO2 emission to the environment.

Experimental Simulation of Three-Dimensional Attainable Region for the Synthesis of Exothermic Reversible Reaction: Ethyl Acetate Synthesis Case Study

Exothermic reversible reactions are an industrially important class of processes, and many have complex kinetics associated with them. This article shows how experimentally measured variables such as residence time, temperature, and conversion can be used to construct a three-dimensional attainable region for a typical exothermic reversible reaction without taking into consideration the kinetics associated with the reaction. The experiments were conducted using an adiabatic batch reactor fitted with a thermistor for temperature measurement. The article also shows how the two-dimensional conversion–temperature plot was obtained from the three-dimensional residence time–temperature–conversion plot. The two-dimensional plot was then used to propose the optimal process configuration for the process where preheating, reaction, and mixing are allowed. Examination of the boundary of the two-dimensional attainable region of the conversion–temperature plot provided the optimal combination of reaction and mixing an...

Control of the Hot Spot Temperature in an Industrial SO2 Converter

This study addresses the problem of controlling by simulation the magnitude of the maximal catalyst temperature, or hot spot, in a four catalyst beds SO2 converter by manipulating the reaction mixture volumetric flow rate. The control of the maximal catalyst temperature is carried out in order to avoid the occurrence of a hot spot inside the catalyst mass and to keep high catalyst efficiency. Command algorithm used is the generalised predictive control (GPC) with off line process identification. The performance and robustness of the GPC controller are evaluated for the case of a kinetic complex and reversible exothermic reaction. The results obtained by numerical simulation show the possibility of the regulation of the hot spot temperature below a pre-specified value despite the occurrence of strong perturbations.

Reaction Laws and Macro-Kinetics of 4-Carboxybenzaldehyde Hydrogenation over Pd/TiO<sub>2</sub> Catalyst

The effects of temperature and hydrogen partial pressure on 4-Carboxybenzaldehyde (4-CBA) hydrogenation over Pd/TiO2 catalyst were investigated in a batch reactor. The results show that 4-CBA hydrogenation is a consecutive reaction. And 4-CBA hydrogenation to 4-hydroxymethylbenzoic (4-HMBA) is a reversible exothermic reaction. Meanwhile, a parallel reaction of 4-CBA decarbonylation occurs during the process. The hydrogenation rate of 4-HMBA is more sensitive to temperature and hydrogen partial pressure than that of 4-CBA. Macro-kinetics of 4-CBA reaction over Pd/TiO2 was obtained from the experimental data using the power-law kinetics model. The apparent activation energies of 4-CBA hydrogenation, 4-HMBA hydrogenation, 4-CBA decarbonylation and 4-HMBA dehydrogenation are 29.65 kJ/mol, 42.55 kJ/mol, 74.32 kJ/mol and 69.34 kJ/mol, respectively. The reaction orders with respect to 4-CBA and 4-HMBA are both first-order.