Adiabatic Reactor Research Articles

Evaluation of alternative processes of CO2 methanation: Design, optimization, control, techno-economic and environmental analysis

Converting carbon dioxide (CO2) into substitute natural gas (SNG) has received renewed interests recently. It not only realizes large scale CO2 conversion, but also provides a synthetic source for those countries lacking natural gas. This study firstly attempts to explore and evaluate alternative processes for CO2 methanation, in which four schemes (six processes overall) based on adiabatic and/or non-adiabatic fixed-bed reactors are focused. Rigorous process design, optimization, waste heat recovery, economic and environmental evaluation, and control are covered. From this work, the most promising configuration (i.e. Scheme 4B) uses only two reactors. A counter-current cooling, non-adiabatic reactor equipped with internal recycle goes first, and an adiabatic reactor follows. This process reveals great CO2 reduction potential (i.e. CO2-e: −3.338 kg/kg), as compared with other previously developed processes converting CO2 to value-added chemicals (i.e. CO2-e: −0.154 to 2.242 kg/kg). More importantly, better operability of the first reactor is indicated by the generated temperature profile. Finally, a suitable control strategy is developed for this scheme, which results in satisfactory closed-loop responses under throughput and composition disturbances. For future work, clarifying how the feed impurities influence the reaction kinetics, and developing a less expensive hydrogen source, will be recommended.

Effect of Mixing and Reaction on a fast Step Growth Polymerization**

Abstract A polyurethane reaction under different mixing rates and reaction rates are studied in an adiabatic batch reactor. Results are explained by using a slab model and dimensional analysis. Four characteristic times are used to describe the interactions of mixing, diffusion and chemical reaction, which may serve as conceptual guidelines in determining the proper reaction rate and mixing power for a specific reaction system.

Kinetics and Reactor Design Principles of Volatile Fatty Acid Ketonization for Sustainable Aviation Fuel Production

Ketonization of wet waste-derived carboxylic acids (volatile fatty acids, VFAs) constitutes the first step of a process to catalytically upgrade VFAs to an alkane sustainable aviation fuel blendstock. VFA ketonization has been demonstrated at near-theoretical yields at the lab scale, and robust operation of industrial-scale ketonization reactors is essential for the commercialization of VFA upgrading to sustainable aviation fuel. We present a ketonization kinetic study of hexanoic acid, a VFA model compound, over commercial ZrO2 and use the kinetic parameters derived from the study in an adiabatic packed-bed reactor simulation of hexanoic acid ketonization running to near-complete (98%) conversion. A key findings from the kinetic study is that ketonization rate is positive order in acid pressure at low (<10 kPa) pressures and transitions to zero order at higher pressures, conforming to a Langmuir–Hinshelwood surface coupling mechanism. Rates are inhibited by ketonization coproduct water but not by ketones themselves or coproduct CO2. Reactor simulations using these kinetics show that rate inhibition by water controls reactor size and that size requirements can be lessened by employing designs that allow for the removal of water from the partially converted acid stream.

Sorption enhanced carbon dioxide hydrogenation to methanol: Process design and optimization

Sorption enhanced synthesis has been previously shown to improve carbon dioxide hydrogenation to methanol by mitigating the thermodynamic limitations. This work investigates the efficiency of methanol synthesis via sorption enhanced carbon dioxide hydrogenation focusing on determining the optimal process parameters. The study is based upon a fully dynamic experimentally validated model of the process which is extended to account for adsorbent regeneration, downstream product separation and recirculation of the unreacted gases. An additional reactor configuration with a guard adsorbent layer is proposed for production of high purity methanol product. A multi-objective optimization study is performed to investigate the tradeoff between methanol production rate and product purity. The obtained results indicate that for synthesis of high purity methanol product, the optimal values of reactor temperature and catalyst mass fraction in the bed are 215 °C/0.65 and 235 °C/0.50 for the adiabatic and quasi-isothermal reactors, respectively.

Comparative Kinetic Analysis and Process Optimization for the Production of Dimethyl Ether via Methanol Dehydration over a γ‐Alumina Catalyst

AbstractVarious kinetic models of methanol dehydration to dimethyl ether over a commercial γ‐alumina catalyst were compared with a view to selecting the most appropriate model as a basis for process optimization. To achieve significant improvements in the conventional design, the Berčič‐and‐Levec kinetic model was employed and process intensification was applied to develop a more energy‐efficient process, by enhancing the adiabatic reactor performance and maximizing the heat recovery from the highly exothermic reactor. The single‐pass conversion of methanol was increased to 83 %, with an inlet temperature of 217 °C to the adiabatic reactor. Application of process intensification resulted in an improved flowsheet, which reduced the total energy requirements by 59.3 % and cut the CO2 emissions by 60.8 %.

Thermodynamic modeling of a class of distributed systems with diffusion

In this work, we aim at addressing thermodynamic modeling and passivity properties of a class of distributed parameter systems (DPSs) with dispersion that are described by partial differential equations (PDEs). The class of distributed parameter systems account for a general class of thermodynamic models with spatial-temporal characteristics. For this class of distributed systems, various contributions on stability analysis, control and estimator designs have been made in the existing literature. On a different note, this contribution aims at providing a thermodynamic perspective on the modeling of the transport processes and passivity using flux expressions based on thermodynamical driving forces. A linearized model is derived and an invertible linear transformation is applied to further simplify the model by eliminating the transport terms. A case study on an adiabatic tubular reactor with dispersion is used as an example.

Dissipative Boundary Control for an Adiabatic Plug Flow Reactor With Mass Recycle

In this contribution the stability properties and regulation of a class of convective systems described by first order hyperbolic partial differential equations with boundary recycle is studied. The system’s setting is consistent with the first and second laws of thermodynamics, allowing to use the entropy functional as a storage function and the internal entropy production as the dissipation similarly to Hamiltonian systems, usually not well defined for systems with mass flows. It is found that the difference of the entropy evaluated at the boundaries is directly proportional to the supply rate, fulfilling the dissipation inequality. Furthermore, the dynamics of the entropy balance allow to define a saturated Proportional-Integral controller with a cascade structure: The inner loop tracks an entropy reference, while the outer loop regulates a process variable. The regulation is achieved with a lumped actuator, using continuous measurements at the boundaries. The controller is applied to an adiabatic plug flow reactor with a recycle of the output stream, a configuration known to be potentially unstable with dissociation reactions. Finally, the controller is tested to track a set point facing several disturbances using the recycle rate as the control variable.

The effect of catalyst formulation and Rh dispersion on the performance of a CPO fuel processor investigated by operando sampling technique and predictive modelling analysis

The catalytic partial oxidation (CPO) of methane and iso-octane on Rh-coated monoliths is studied in an adiabatic reactor where axial temperature and concentration profiles are collected by a spatially resolved sampling technique. In CH4-CPO, the Rh/MgAl2O4 outperforms the Rh/α-Al2O3 formulation, due to a significant improvement of Rh dispersion. In iso-octane CPO, the beneficial effect of the improved Rh surface is less important due to the intrinsic lower sensitivity of the system. However, a non-negligible impact of Rh dispersion on the extent of hydrocarbon side-products is observed. This factor, together with the lower acidity of the spinel support, contributes to limit the C build-up. Reactor model and kinetic schemes allow to rationalize the measurements and explore the more general effect of Rh specific surface on the key performance indicators of the CPO reformer, that is syngas productivity and hot-spot temperature. Gas-solid diffusion rate makes such indicators strictly fuel-specific.

Comparison of NSGA-III with NSGA-II for multi objective optimization of adiabatic styrene reactor

Nature is continuously evolving itself by the different mechanisms so that every aspect of it becomes optimized for different objectives around it. Evolutionary algorithms are developed by mimicking these different mechanisms of evolution. Genetic algorithm is one such evolutionary algorithm. The family of non-dominated sorting genetic algorithms (NSGA) due to its simplicity and efficiency is the most widely used method for solving industrial multi-objective optimization problems. The consideration of elitism in NSGA-II has made it much computationally efficient than NSGA. Recently developed NSGA-III is reported to be more efficient for many-objective (more than two) optimization problems.In this work, the efficacy of NSGA-III is compared with NSGA-II for a three-objective optimization problem of a Styrene reactor. Productivity, Yield and Selectivity of styrene is considered as the three objective functions for the adiabatic styrene reactor. Pareto optimal sets are obtained from both NSGA-II and NSGA-III. The results obtained from NSGA-III shows to provide a more diverse range of optimal operating conditions than NSGA-II.

A mass-temperature decoupled discretization strategy for large-scale molecular-level kinetic model

The molecular conversion of complex mixture involves a large number of species and reactions. The corresponding kinetic model consists of a series of ordinary differential equations with severe stiffness, leading to an exponentially growing computational time. To reduce the computational time, we proposed a mass-temperature decoupled discretization strategy for a large-scale molecular-level kinetic model. The method separates the mass balance and heat balance calculations in the rigorous adiabatic reactor model and divided the reactor into several isothermal segments. After discretization, the differential equations for heat balance can be replaced by algebraic equations between nodes. We used a molecular-level diesel hydrotreating kinetic model as the case to validate the proposed method. A good agreement between the discretization model and rigorous model was observed while the computational time was significantly shortened. Moreover, the method was also extended to heavy oil fluid catalytic cracking and accurately calculated the product composition and temperature distribution.

Kinetic model for the esterification of free fatty acids from palm oil with methanol using sulfuric acid as catalyst and sensitivity analysis in tubular reactors

AbstractIndustrial biodiesel production from crude palm oil (CPO) by homogeneous transesterification requires some conditioning stages. One is deodorization, where free fatty acids (FFA) are stripped out from the CPO. The FFA from the deodorizer is esterified using a homogeneous acid catalyst to produce more biodiesel and improve process profitability. This work studied the sulfuric acid‐catalyzed esterification of FFA with methanol. The factors evaluated were temperature (between 40 and 60°C) and catalyst concentration (between 0.15 and 1.5 wt% based on the mixture). The parameters of a reversible second‐order kinetic model were adjusted from experimental data using a genetic algorithm. The kinetic model, which adequately represents the esterification reaction, according to the Fisher–Snedecor test, was used to perform a sensitivity analysis in isothermal, adiabatic, and non‐isolated continuous tubular esterification reactors using ASPEN HYSYS V10. The results showed that the highest conversion (~96%) was predicted using an isothermal reactor. However, its installation and operational costs could also be the highest. An adiabatic reactor was preferred, which optimal conversion of 94.5% was predicted at temperature, catalyst concentration, residence time, and methanol‐to‐FFA molar ratio of 140°C, 0.3 wt%, 47 min, and 6.7, respectively, its predicted operational cost was 0.63 dollars per biodiesel kilogram. Therefore, the adjusted and validated model has a relevant importance in the biofuel sector, not only in Colombia, but also worldwide.

Autoignition of Reacting Hydrocarbon Mixture With Negative Temperature Coefficient Due to the Cold-Spot and Cold Chamber Wall

AbstractAutoignition of an n-heptane/air mixture was simulated in nonuniform temperature environments of a rapid compression machine (RCM) and shock-tube (ST) with and without the presence of a cold-spot. The simulations were performed to investigate how the presence of a cold-spot and the cold boundary layer of the chamber wall may affect the ignition delay of the hydrocarbon mixture with negative temperature coefficient (NTC) behavior. The simulations were performed using three models: (1) three-dimensional (3D) computational fluid dynamics (CFD) model, (2) zero-dimensional (0D) homogenous batch reactor model by including the heat transfer model, and (3) 0D adiabatic homogenous batch reactor model. A detailed n-heptane mechanism was reduced in this work and used for 3D combustion modeling. A cold-spot critical radius of 7 mm was determined, which affects the ignition delay by more than 9%. In addition, two combustion modes were observed in the combustion chamber with a nonuniform temperature environment. With the first combustion mode, combustion starts at the high gas temperature region of the combustion chamber and quickly propagates toward the periphery of the chamber. In this combustion mode, the location of the maximum concentration of hydroxyl radical and the maximum temperature are the same. With the second combustion mode, the combustion starts at the periphery of the chamber, where the temperature is lower than the center of the chamber due to heat transfer to the cold chamber wall. The location of maximum concentration of the hydroxyl radical and maximum temperature is different with this combustion mode. The two observed combustion modes are due to the NTC behavior of the n-heptane mixture. The 0D homogenous batch reactor model (with and without heat transfer models) failed to mimic the ignition delay accurately when the second combustion mode was present. In addition, a propagating combustion has been observed in the simulation which is in agreement with some of the optical autoignition diagnostics of these hydrocarbons. This propagating combustion leads to a gradual pressure rise during autoignition, rather than a sharp pressure rise. The results of this work show that 0D homogenous batch reactor models are unable to simulate autoignition of mixtures with NTC behavior.

Dynamic behaviour of integrated chemical looping process with pressure swing adsorption in small scale on-site H2 and pure CO2 production

The design of a fully integrated Chemical looping reforming (CLR), single adiabatic water gas shift reactor (WGSR) and Pressure swing adsorption (PSA) operated under dynamic conditions for small scale H2 generation with inherent pure CO2 production is carried out. The dynamically operated packed bed reactors taking part in the chemical looping process have been modelled, designed and simulated to operate with transient feeds from an integrated PSA unit used for the production of 130 Nm3/h of pure H2 (99.9999% purity). As by-product, 51 Nm3/h of pure CO2 (>98.8% purity) is also produced. A rapid cycle 8-bed configuration increases the H2 recovery by 4% whilst reducing the tail gas buffer tank volume requirement by 44%. The effect of the PSA dynamic tail gas composition used as fuel for the CLR reduction reactor stage was found negligible regarding the continuity of the process and the performance of the plant, as it affected only the reduction outlet gas composition profile but had little effect on the reactor bed temperature profile. With respect to the design of the chemical looping reactor beds, an analysis has been performed on the effect of heat losses showing that at higher heat transfer coefficient (U = 5.0 W∙m−2∙K−1) CH4 conversion decreased significantly (≈90% compared to adiabatic operation), therefore a different strategy was implemented. The overall study demonstrates the process design feasibility for producing blue H2 or renewable H2 from methane/bio-methane in decentralised and modular units.

Synthesis of Si nanoparticle chains and nanowhiskers by the monosilane decomposition in an adiabatic process during cyclic compression

A cyclic adiabatic compression reactor is used for the Si powder fabrication by the monosilane decomposition in a gas mixture with argon. Along with arbitrary agglomerations of Si nanoparticles, this method allows synthesizing nanoparticle chains and nanowhiskers, as an outlet gas pressure in the reactor increases from about 2.0 to 5.0 MPa. The increase in pressure corresponds to an increase in temperature from ~450 to 1050 °C, which leads to a change in the particle structure from amorphous to crystalline and reduces the particle size distribution. The particle chain and nanowhisker formation is associated with an increase in the duration of the synthesis process. The optical properties of the synthesized Si powders were determined by Si/SiO2 interfaces appeared by the oxidation in the air. The method is characterized by a relatively high production rate of Si powders of various morphologies, which may be of interest for various applications.

A Hybrid Modeling Approach for Catalyst Monitoring and Lifetime Prediction

In this work, we present a hybrid fundamental-empirical model to monitor and predict the catalyst lifetime of an operating industrial reactor. The hybrid model combines a fundamental adiabatic reactor model to calculate the activity of the catalyst bed with an empirical partial least-squares model to predict the catalyst activity at different operating conditions. A baseline model was trained on process data and validated separately using analytical data, showing good agreement between the measured reactant breakthrough of the reactor train and the predicted values from the model over 18 years of continuous operation of four industrial production reactors. To implement the model for catalyst activity monitoring, the model must closely match the current catalyst charge performance. Therefore, the baseline model parameters are updated automatically with new plant data using a filter algorithm.

Productivity Enhancement for the Oxidative Coupling of Methane in Adiabatic Layered-Bed Reactors

Ethylene production via oxidative coupling of methane (OCM) in adiabatic packed-bed reactors is a promising route within the context of natural gas valorization. However, the high exothermicity of OCM imposes low inlet temperatures and, hence, low space-time yields. Layered-bed reactors are proposed in the present work as a potential alternative to mitigate this issue. In this modeling investigation, a first layer of active Sr/La2O3 catalyst is followed by a second layer of more selective NaMnW/SiO2 catalyst. Optimized bed compositions and operating conditions are shown to lead to up to a 10 times higher space-time yield compared to a single-catalyst bed. The achieved values of ca. 15 molC2+ s–1 mr–3 are comparable to conventional values from steam cracking of ethane, suggesting layered-bed adiabatic configurations as a promising way forward for intensified ethylene production via OCM.

An enhanced composting process with bioaugmentation: Mathematical modelling and process optimization

In this paper, two different types of biowaste composting processes were carried out - composting without and with bioaugmentation. All experiments were performed in an adiabatic reactor for 14 days. Composting enhanced with bioaugmentation was the better choice because the thermophilic phase was achieved earlier, making the composting time shorter. Additionally, a higher conversion of substrate (amount of substrate consumed) was also noticed in the process enhanced by bioaugmentation. A mathematical model was developed and process parameters were estimated in order to optimize the composting process. Based on good agreement between experimental data and the mathematical model simulation results, a three-level-four-factor Box-Behnken experimental design was employed to define the optimal process conditions for further studies. It was found that the air flow rate and the mass fraction of the substrate have the most significant effect on the composting process. An improvement of the composting process was achieved after altering the mentioned variables, resulting in shorter composting time and higher conversion of the substrate.

Numerical simulation of the thermal decomposition of tert-butyl peroxyacetate in adiabatic tests

Adiabatic calorimeters (ARCs) are critical in thermal analysis and thermal hazard assessment. As testing equipment continually improves, analyzing changes in physical fields in sample pools during thermal decomposition reactions is increasingly essential. Therefore, this study analyzed the thermal decomposition of tert-butyl peroxyacetate (TBPA). On the basis of the computational fluid dynamics (CFD) numerical simulation method and the kinetic model of TBPA thermal decomposition, a full-scale model of an adiabatic reactor for the thermal decomposition of TBPA was constructed. The temperature rise curve obtained after monitoring the temperature of the system during thermal decomposition was compared with that obtained during the experiment; thus, the rationality of the CFD model was verified. Accordingly, the temperature field, temperature rate, and velocity field in the reactor were analyzed. The temperature distribution of the system was relatively uniform during thermal decomposition under completely adiabatic conditions, resulting in an effectively nonexistent temperature gradient. At the same time, the self-heat rate (dT/dt) of the system in the process of thermal decomposition was analyzed. It was found that self-heat rate (dT/dt) of the system in the full sensing state was much larger than that in the experimental process, maximum self-heat rate ((dT/dt)max) reaching 15℃/min, while in the experimental process was only 1.074℃/min.

Analysis of methanol synthesis using CO2 hydrogenation and syngas produced from biogas-based reforming processes

Utilizing the greenhouse gas CO2 as a feedstock in chemical processing can offer alternative solutions to long-term storage. In this study, a systematic analysis of methanol synthesis performance was analyzed based on both thermodynamic equilibrium and kinetic models using captured CO2 and syngas produced from biogas as feedstock. Using reactor inlet temperature as a parameter, it was found that methanol yield can be enhanced by increasing residential time from increased reactor diameter. The longer reactor can increase the residential time but a large pressure drop caused a decrease in methanol yield. Due to the exothermic reaction nature, methanol yield from an adiabatic reactor is lower than that from the isothermal reactor due to temperature rise. From the results obtained for CO2 hydrogenation, methanol yield can be enhanced by water removal. The CO2 conversion was found to increase with increased reaction temperature due to methanol and carbon monoxide productions. Using CO and CO2 as limiting species, high combined CO and CO2 conversion can be obtained from syngas with low CO2/H2 and high CO/H2 ratios. However, methanol production per mole of H2 depends on the H2 utility instead of combined CO and CO2 conversion. Finally, syngas produced from biogas by using combined dry and steam reforming reactions was used as the feedstock for the methanol synthesis. To obtained the syngas composition suggested from industrial applications, CH4 and H2O were added in the combined reforming process. With higher CH4 content in the biogas, higher methanol production and lower water production can be obtained. With an increased recycle ratio for unreacted syngas, methanol production can be enhanced.

Autothermal Reforming of Diesel Oil for PdCeCrFeCu/Al2O3-Catalyzed Hydrogen Production

Abstract To make clear the feasibility and influence factors of diesel fuel autothermal reforming to hydrogen, PdCeCr- FeCu/Al2O3 catalyst was prepared by equivalent-volume impregnation method. Experimental facility based on an adiabatic tubular reactor with preheating section was designed and set up, the behaviors of diesel reforming to hydrogen with straight-run diesel as a raw material according to the analysis of the components were studied. Diesel oil reforming over a catalyst for hydrogen production was analyzed using an adiabatic tubular reactor with a preheating section that was designed and built in-house. The operating conditions were optimized. Under the suitable operating conditions, viz., catalyst bed inlet temperature of 700 °C, diesel liquid space velocity of 0.24 h−1, water-carbon ratio of 20, and oxygen-carbon ratio of 0.6, the hydrogen yield reached 28.3 (mol/mol).