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•Multi-objective optimization determines reactor structure and conditions.•Enhanced water-gas shift (WGS) reaction gives more uniform temperature profiles.•Sorption-membrane-enhanced ...WGS provides higher H2 yield and lower catalyst mass.•Two or more units with optimized co-feeding further lower reactor costs.•Proposed optimal reactor has two units with trickling CaO sorbent particles.
In this feasibility study, a novel industrial-scale reactor structure for continuous hydrogen production via intensified water-gas shift (WGS) reaction is proposed. It considers both trickling calcium-oxide sorbent for carbon dioxide removal (SOR) and Pd-based membrane for hydrogen separation (MEM). It is shown that WGS, SOR, MEM, and cooling can be decoupled with a special reactor superstructure mathematically represented with the pseudo-homogenous one-dimensional model. The final reactor structure and operating conditions are determined by using rigorous multi-objective optimization. Two objective functions take all main costs into account (total reactor volume and respective volumetric fractions for the catalyst, sorbent, and membrane) and the main benefit (hydrogen yield). The results show that the best cost-benefit relation can be achieved with the two-module reactor and combined WGS and SOR processes, with 95% carbon monoxide conversion (64% higher than the equilibrium conversion at the same conditions) and the outlet-stream containing only 0.7% of carbon dioxide.
A multiphase fixed-bed reactor (FBR) model for Fischer–Tropsch Synthesis has been developed. A high level of details is considered for description of the phenomena on the reactor and particle scale. ...Detailed kinetics is used, with parameters estimated from experiments with a cobalt-based catalyst. Model robustness has been validated using literature data. Performance analysis was made for a conventional scale FBR with egg-shell distribution of catalyst and a millimeter-scale FBR with small particles and uniform distribution. In both cases, diffusion limitations are almost eliminated due to use of small diffusion lengths. For similar qualitative results, a milli-scaled design would result in a significantly lower reactor volume, but the capital costs could be high due to large wall area and a vast number of tubes. Heat removal is efficient in both cases, and pressure drop in the milli-scale reactor is low due to the use of a shorter bed and lower velocity.
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•Sorption-enhanced water–gas shift allows for the production of highly pure H2.•A dynamic, non-ideal shrinking core model was applied for the carbonation reaction.•A sorbent ...conversion-dependent effective diffusion coefficient described the process.•The model perfectly reproduced the experimental results and gave valuable insight.•Cyclic simulations showed how the process can be operated continuously.
Hydrogen, an important energy carrier of the future, produces no pollution and has a high content of energy. It is formed as a direct product of the water–gas shift (WGS) reaction, which occurs in various processes for the production of hydrogen, ammonia, methanol and different hydrocarbons, and is also a side reaction during the steam reforming of hydrocarbons and Fisher–Tropsch synthesis. Since it is an equilibrium reaction, it may be intensified by the selective removal of the products, which can lead to higher yields and energy savings. In this study, carbon dioxide was removed through chemisorption on CaO particles. In the first part, the WGS reaction kinetics were obtained on an industrial iron-chromium catalyst in a packed-bed reactor. In the second part, the CO2 chemisorption kinetics on CaO sorbent particles were examined, simultaneously with the WGS reaction. A modified dynamic shrinking-core model was used to describe the carbonation reaction, which accounted for the non-ideal core shrinkage. With the introduction of a sorbent conversion-dependent effective diffusion coefficient, the model perfectly reproduced the obtained experimental results. Valuable insight into the sorption-enhanced process was obtained with the full concentration profiles of the species involved (CO, H2O, CO2, H2) in time and space, as well as the conversion of the sorbent particles, also in the radial dimension. The developed model was used to simulate a cyclic sorption-enhanced water–gas shift operation in a revolver-type manner which allows for continuous sorbent regeneration and a much higher-than-equilibrium hydrogen production for various operational parameters. The significance of the model lies in the precise replication of the experimental results and its applicability to the vast area of the newly-emerged industrial sorption-enhanced technologies.
Our previously developed mathematical model is used for parametric sensitivity and optimization study of conventional and milliscale fixed-bed reactors (FBRs) for Fischer–Tropsch synthesis (FTS). ...Five indicators are chosen to analyze the influence of eight parameters on the FBRs’ performance. The results show the scale of the effects caused by changing single parameter values and highlight the most important ones. Subsequently, the model is used to perform a rigorous multivariable optimization of the FBRs’ performance in the steady state. Three optimization functions are used, depicting different reactor costs. Four design parameters (tube length and diameter, particle diameter, and catalyst layer thickness) and five operating parameters (inlet and wall temperature, inlet pressure, H2/CO ratio, velocity) are optimized simultaneously. The results indicate that optimal results, in terms of reactor design and operating parameters and FBR performance, highly depend on the selected objective function and values of constrained parameters (especially methane selectivity and the partial pressure of water).
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•Milli fixed bed Fischer-Tropsch reactors exhibit fast and highly nonlinear dynamics.•Thermal runaway may occur with disturbance of coolant temperature and feed flowrate.•Forced ...periodic operation may enhance Fischer-Tropsch reactor performance.•Temperature control is crucial – advanced control systems are recommended.
Dynamic performance of a milli-scale fixed bed reactor for Fischer-Tropsch synthesis (FTS) was studied using a dynamic pseudo-homogeneous 1D reactor model. The model uses detailed kinetics to describe the rates of FTS product formation. Dynamic responses of the process variables and main performance indicators, including productivity of C5+ hydrocarbons and CH4 selectivity, to input step changes, were analyzed. Total of 7 inlet variables were used in step-change-response analysis, with different magnitudes of change and for two initial steady-state conditions. Reactor simulations show highly nonlinear behavior due to phenomena coupling and fast dynamics due to system small scale and intensified rates within. In addition, reactor model shows instability related to thermal runaway with certain magnitudes of step change of coolant temperature and feed flow rate. The analysis outlines the effects of potential process disturbances on operation of milli-scale fixed bed reactor for FTS and provides general guidelines for control systems.
► PI rises challenges for control due to less DOF, tight interactions and fast dynamics. ► Model-based control has to be adapted for intensified processes and equipment. ► Alternative driving forces ...and energy sources can be used as new actuation options. ► Actuation possibilities should be examined systematically within process synthesis. ► Process synthesis should exploit PI and integrate design, operation and control.
This is a review and position article discussing the role and prospective for process control in process intensification. Firstly, the article outlines the classical role of control in process systems, presenting an overview of control systems’ development, from basic PID control to the advanced model based hierarchical structures. Further on, the paper reviews the research articles discussing control issues of intensified process equipment, specifically of reactive distillation, divided wall distillation, simulated moving bed reactors and micro-scale systems. In the next section, the focus is on more fundamental, dynamic characteristics of selected intensified process categories, which are elucidated in several examples. The goal of this analysis is to stress to the potential challenges for control of intensified processes. More importantly, the aim of this part is to emphasize to the opportunities for control, which are associated with new actuation possibilities arising from process intensification. Finally, a new concept of process synthesis is elaborated, which is based on process intensification and actuation improvement. The concept enables integration of process operation, design and control through dynamic optimization. This simultaneous synthesis approach should provide optimal operation and more efficient control of complex intensified systems. It may also suggest innovative process solutions which are more economically and environmentally efficient and agile.
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A three-stage method for reactor synthesis is proposed.Method exploits PI principles, which are systematically included in optimization.Reaction superstructure consisting of ...phenomenological modules is optimized.Method displays a theoretical potential for innovative solutions.Method is demonstrated on a general liquid phase reaction system.
A novel method for reaction synthesis is proposed. The method establishes strong interconnection between process intensification (PI) principles and process system engineering (PSE) techniques, which are used for problem formulation and optimization. The general aim is to demonstrate a potential for innovative solutions that combine both optimal reactor structure and its operational regime. The method consists of three stages: (I) Reaction Screening, in which phenomenological modules are defined; (II) Reaction System Superstructure and Mathematical Modeling in which modules (building blocks) are connected in a generic reactor superstructure; and (III) Optimization in which optimal structure and operational regime is derived, using techno-economical objective function and different optimization methods. The proposed method is demonstrated on a general example of two parallel endothermic reactions in liquid phase. The optimization results show that continuous steady-state reactor system outperforms fed-batch reactor and has similar performance as more complex periodically operated continuous reactor, and thus presents the optimal solution.
We investigate effects of catalyst activity, catalyst particle shape (sphere, slab, and hollow cylinder), size (i.e., diffusion length), catalyst distribution (uniform vs eggshell type distribution ...for a spherical particle), and process conditions (temperature, pressure, syngas composition, and conversion level) on catalyst effectiveness factor and methane selectivity inside the catalyst pellet. In numerical simulations we utilize kinetic parameters for CO consumption rate and CH4 formation rate determined from experiments with a highly active Co/Re/γ-Al2O3 catalyst. It is found that the use of small spherical particles (0.2–0.5 mm) or eggshell distribution for larger spherical particles with catalyst layer thickness less than approximately 0.13 mm is needed to avoid negative impact of diffusional limitations on CH4 selectivity under typical Fischer–Tropsch synthesis operating conditions. For monolith reactors with wash-coated catalyst, diffusional limitations can be avoided by using a catalyst layer thickness less than 0.11 mm at base case conditions (473 K, 25 bar, and H2/CO molar ratio of 2).
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•Methane selectivity increases sharply beyond a critical value of Thiele modulus.•Kinetic region extends with increase in pressure or decrease in H2/CO feed ratio.•Methane selectivity ...decreases with increase in conversion and/or decrease in H2/CO.•Multiple steady-state solutions are possible for some values of Thiele modulus.•Hybrid kinetic model was used to predict hydrocarbon product distribution.
In this study we investigate performance characteristics (catalyst effectiveness, CH4 selectivity, and hydrocarbon product distribution) of with a highly active Co/Re/Al2O3 catalyst particle for Fischer-Tropsch synthesis. In numerical simulations we utilize kinetic parameters for CO consumption rate, CH4 formation rate and hydrocarbon formation rates (C2+ hydrocarbons) determined from experiments with this catalyst to study effects of catalyst activity, catalyst particle shape (sphere, slab, solid and hollow cylinder), size (i.e. diffusion length), catalyst distribution (uniform vs. eggshell type distribution for a spherical particle) and process conditions (temperature, pressure, syngas composition and conversion level) on the catalyst performance. With increase in Thiele modulus (i.e. particle size at a fixed set of process conditions) we observe increasing H2/CO ratio profile towards the center of the particle resulting in increase of local and average CH4 selectivity. The goal is to find conditions which allow one to use sufficiently large particles to reduce pressure drop, while avoiding negative influence of diffusional limitations on selectivity and activity. For each catalyst particle shape we determined values of Thiele modulus, i.e. characteristic length of diffusion, corresponding to the upper limit of the kinetic region, and investigated how it changes with operating conditions. We found that simultaneous increase of pressure and the use of syngas with H2/CO feed ratio of 1.4–1.7 is the best strategy for mitigating the negative impact of intraparticle diffusional limitations on CH4 selectivity. For a spherical particle of 1 mm in diameter, one can achieve CH4 selectivity of 5.6% with catalyst effectiveness factor of 1.07 at the reactor inlet by operating at 50 bar, 473 K and H2/CO = 1.4.
