Robust autothermal microchannel reactors

Autothermal microchannel reactors are intensified process units that bring significant energy efficiency benefits over their conventional counterparts. Efficiency gains are obtained, however, at the cost of operational challenges. These stem from the loss of control handles that is inherent to combining several unit operations in a single physical device. In this paper, we investigate the impact of two recently proposed reactor design concepts (a segmented catalyst macromorphology and an embedded layer of phase change material) aimed at improving the steady state energy distribution and, respectively, preventing the advent of hotspots during transient operation, on reactor dynamics and control. Using an autothermal microchannel reactor coupling steam methane reforming with methane catalytic combustion as a prototype system, we demonstrate through rigorous simulations that these design innovations have a synergistic effect, resulting in superior steady-state performance and excellent disturbance rejection ability.     

吸附反应器动态特性的研究

针对反应速率方程为γ_A=k_1c_A~m-k_2c_B~n的非瞬时可逆均相反应A(?)vB、液相反应或浓度很稀的气相反应和反应物A或产物B瞬时非线性Langmuir吸附的情况,研究了反应和吸附相结合的吸附反应器之动态特性.建立了平推流固定床吸附反应器的动态模型,通过数值解法获得了反应物阶跃和方波输入时,吸附反应器出口的动态响应曲线.研究结果表明,吸附反应器无需附加净化步骤就能获得高纯度的产物;吸附反应物时,反应转化率降低,吸附产物时,反应转化率提高.并讨论了吸附平衡常数和反应速率常数对吸附反应器性能的影响.     

Analysis of plug flow reactors with variable mass density

The design of plug flow reactors with variable mass density is examined. Equations which include a two‐term constitutive equation for the reaction rate are derived for the flow of liquids and for the flow of ideal gases in steady plug flow reactors. It is shown that the addition of the second term in the constitutive equation can have a significant effect on the calculation of the reactor volume needed to carry out a specific conversion of the reactant. Published experimental plug flow reactor data support the observation that a reaction rate constitutive equation with two terms can provide a good representation of the experimental data for variable mass density reactors. © 2014 American Institute of Chemical Engineers AIChE J 60: 4185–4189, 2014     

Forced temperature cycling of a catalyst layer and its application to propylene oxidation

In this study, a new reaction device suited for forced temperature cycling was developed. This device has a heating element in the reaction tube, and forced temperature cycling was realized by operating this element intermittently. The energy needed for the operation was about 20 W, which is much less than that required to operate reactors used in previous studies of temperature cycling. This reactor was used to examine the effect of periodic operation on the oxidation of propylene. It was found that the conversion under periodic conditions was higher than that observed under steady state. In addition, the reaction system approached a relaxed steady state as the cycle time was reduced to 1 s. The effect of forced temperature cycling on propylene oxidation was successfully demonstrated.     

Dynamics of adsorptive reactors I — Instantaneous nonlinear adsorption and finite zero order irreversible reaction

The dynamics of adsorptive reactors, in which the coupling of nonlinear Langmuir adsorption with zero order reaction A → B occurs, is considered. The method of characteristics is used to develop, analytically, the responses of fixed bed (plug flow) adsorber/reactors to step, pulse or Dirac inputs of reactant A. An increase in the breakthrough time of the column is observed as a consequence of the coupling of both phenomena. The responses of CSTR adsorber/reactors to step or Dirac inputs of reactant A are derived, as well. A method for the determination of equilibrium and kinetic parameters is proposed.     

Control of periodically operated reactors

Control of periodically operated reactors has in common with control of reactors operating at steady state the objective of minimizing the effect of disturbances on an objective function such as the cost of a product or the deviation of an outlet concentration of a pollutant from a statuary target. Simple feedback control employing feedback PID regulators, however, is not “adequate for most disturbances because of problems with tracking a time-varying output and the necessarily non-linear character of the reactors with respect to controlled as well as uncontrolled inputs. This contribution is a review of the literature and a discussion of research needs. The literature on the control of periodically operated reactors is not voluminous. Nevertheless this literature clearly indicates that model based predictive controllers can be used for this type of reactor”. Further research on the limitations, maintenance and implementation costs of model based controllers in this application would be worthwhile. Experimental studies are wholly absent. Unique regulators for other periodic operations, such as adaptive control or forcing the output toward a reference trajectory using an open loop model based control strategy, certainly warrant study of their application to periodically operated reactors. Additionally, proper design of the reactor may lead to configurations that are simpler to control and that may not require complex control strategies for efficient operation.     

Optimal heating profiles in tubular reactors with solid-phase axial wall conduction for isothermal operation

Optimal heat input profiles to maintain near isothermal operation in endothermic tubular reactors with axial solid-phase conduction were derived using a one-dimensional steady state plug flow model capturing the main transport phenomena in flow reactors. A cost criterion to minimize the deviation from isothermality was defined and optimal control theory is applied to calculate the optimal heating profiles as a function of position. It is shown the perfect isothermality is mathematically possible only if the wall conduction term is infinitesimally small. Using singular optimal control theory, analytical expressions of the input profiles are derived including a simple criterion for near-isothermal operation in terms of dimensionless reactor parameters.     

Design of tubular reactors in recycle systems

The article presents an approach to design tubular reactors in recycle systems, based on non-linear analysis. A pseudo-homogeneous plug-flow reactor model is used. It is assumed that the separation unit delivers product and recycle streams with fixed composition. The stand-alone reactor has a unique stable steady state. The coupled reactor–separation–recycle system shows four types of conversion versus plant Damköhler number bifurcation diagrams. A feasible steady state exists only if the reactor volume exceeds a critical value. For isothermal reactor, the steady state is unique and stable. For non-isothermal reactor, one or two steady states are possible. In the second situation the low-conversion state is unstable. In some parameter regions, the unique state is unstable. The design should ensure state unicity and stability, which are favoured by large heat-transfer capacity, low coolant temperature and high reactor-inlet temperature. A case study demonstrates that these phenomena can be easily found in real plants.     

Comparison between simulated moving bed and reverse flow chromatographic reactors for equilibrium limited reactions

A staged linear model, containing five parameters, is developed to compare equivalent simulated moving bed chromatographic reactors (SMBCR) and reverse flow chromatographic reactors (RFCR). A first order reversible reaction and linear adsorption equilibrium, with preferential adsorption of the reactant is assumed. The analysis uses simple, easily computable analytical solutions that rigorously represents the transients in the cyclic steady state for both the RFCR and the SMBR. A comparison between the two types of reactors is carried out to determine the maximum conversion attainable and the range of operation where these systems have advantages over conventional steady state reactors. It is found that the maximum conversion of both reactors is similar. The range of operation in terms of amount of catalyst and range of switching times favors the RFCR, while the conversion at low separation factors favors the SMBCR.     

Optimal design and control of non-adiabatic loop reactors

Loop reactors may compete with other heat recuperating technologies, like reverse-flow reactors, for catalytic abatement of low-concentration volatile organic compounds (VOC). Their main drawback, the narrow domain of operating parameters, can be resolved with a proper control system.The main goal of this research is to extend the optimal loop-reactor design conditions derived earlier to non-adiabatic units and to reactions with non-monotonic reaction kinetics and to suggest a systematic design of control. By optimal design we imply design that expands the envelope of operation in the parameters space, and specifically one that extends the operation into lower feed concentrations. A simple analytical approximation for pulse velocity in the non-adiabatic one-dimensional (1-D) model has been derived and compared with the numerical results     

Design and optimisation of batch and semi-batch reactors

This paper addresses a systematic methodology for batch and semi-batch reactor design and optimisation for both ideal and non-ideal mixing. It can be applied to non-isothermal and multiphase systems. The method starts from a general representation in the form of a temporal superstructure based on the similarity of between plug flow reactors and ideal batch reactors. The temporal superstructure of a batch reactor exists in both the space and time dimensions. For non-ideal mixing, this paper addresses a mixing compartment network model to represent mixing inside reactors. The mixing compartment network is then included into the temporal superstructure to model non-ideally mixed batch reactors and the mixing pattern optimised with the other variables. Besides the operation variables for batch reactors, this method can also suggest the optimum mixing pattern and promising reactor configurations for mechanical design. A profile-based approach is proposed to make a search of the profiles for temperature, pressure and feed addition. This approach starts from a set of initial profiles of temperature, pressure and feed addition. Then the performance of the batch reactor is evaluated against the objective function under different profiles. An optimal set of profiles is then found by this profile searching process. A stochastic optimisation technique based on simulated annealing is employed to obtain optimal solutions. This method is also extended to multiphase reaction systems based on the concept of shadow reactor compartments. A number of case studies are presented to illustrate the use of the proposed methodology.     

Equilibrium theory and nonlinear waves for reactive distillation columns and chromatographic reactors

A general framework for analyzing and understanding the dynamics of reactive separation processes is developed. The theory is based on the assumption of simultaneous phase and reaction equilibrium. It makes use of transformed concentration variables, which were first introduced by Doherty and co-workers for the steady state design of reactive distillation processes (Proc. Roy. Soc. London A 413 (1987a) 459). It is shown that these transformed variables can be directly generalized to the dynamic problem considered here. Further, they can also be applied to other reactive separation processes like fixed bed as well as countercurrent chromatographic reactors, for example. They provide profound insight into the dynamic behavior of these processes and reveal bounds of feasible operation caused by reactive azeotropy. It is shown that reactive azeotropy, which is a well-known phenomenon in reactive distillation (Proc. Roy. Soc. London A 413 (1987b) 443) may also rise under very similar conditions in other reactive separation processes like chromatographic reactors for example.     

Steady‐State and Dynamic Reactor Models for Hydrotreatment of Oil Fractions: A Review

The modeling of catalytic hydrotreatment reactors for petroleum fractions have been classified in several ways, for example steady‐state and dynamic, pseudohomogeneous, and heterogeneous, and so on. Depending on the system to be modeled, operating conditions and type of feedstock, these approaches could exhibit some advantages and disadvantages, wide scopes and limitations. In this review, the discussion about those modeling aspects, already published, is used to develop a generalized reactor model, which can be simplified in order to derive each single model previously reported. Some guides to estimate model parameters are also given.     

Bifurcation control and eigenstructure assignment in continuous solution polymerization of vinyl acetate☆

The major difficulty in achieving good performance of industrial polymerization reactors lies in the lack of under-standing of their nonlinear dynamics and the lack of wel-developed techniques for the control of nonlinear pro-cesses, which are usually accompanied with bifurcation phenomenon. This work aims at investigating the nonlinear behavior of the parameterized nonlinear system of vinyl acetate polymerization and further modifying the bifurcation characteristics of this process via a washout filter-aid control er, with all the original steady state equilibria preserved. Advantages and possible extensions of the proposed methodology are discussed to provide scientific guide for further controller design and operation improvement.     

Syngas chemical looping process: Dynamic modeling of a moving‐bed reducer

The syngas chemical looping process coproduces hydrogen and electricity with iron oxide based oxygen carriers in a circulating moving bed system. In this article, a one‐dimensional (1‐D) dynamic model is developed to simulate the countercurrent gas–solid reactive flow in the moving‐bed reducer. This model is validated by TGA and bench‐scale experiments. Both the steady state and dynamic composition profiles are obtained to help understand the reaction and reactor behaviors. Numerical simulation on the effects of reactor length is conducted to optimize the moving‐bed reducer design. It is also found that minor variations in the feed rate ratio near a critical point that is represented by the reaction equilibrium could yield a significant difference in the time required for the reactions to reach a steady‐state operation. Such a difference has an important practical implication in that the moving‐bed reducer should be designed and operated to circumvent the critical point. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3432–3443, 2013     

基于改进状态转移算法的串级平推流反应器动力学参数估计

实际化工过程反应系统通常由若干个相互关联的平推流反应器串级构成，要建立其机理模型，并估计其动力学参数，就必须反复求解大规模非线性微分方程组，计算代价较高。针对智能优化算法在求解昂贵优化问题时计算成本过高，而工业过程模型往往只需要找到一组满意解即可的特点，提出了一种基于改进状态转移算法的串级平推流反应器动力学参数估计方法。该算法在保留标准状态转移算法全局寻优能力和快速收敛能力的同时，利用对立算子优化初始种群，并根据误差阈值判定满意解中止条件，可有效节约计算时间。以实际炼油企业的加氢裂化装置为研究对象，采用所提方法对其反应系统的动力学参数进行估计，仿真结果表明了所提方法的有效性。     

Advancements in droplet reactor systems represent new opportunities in chemical reactor engineering: A perspective

Traditional chemical reactors, such as batch reactors, continuous reactors, and semi-batch reactors, have been extensively studied and frequently act as central components of modern chemical plants. Recently, various advances in reaction times, surface-to-volume ratios, required amounts of reagents, and throughput have led to new directions in the design of miniaturized chemical reactors. In this Perspective, we provide an overview of the progress from traditional to miniaturized chemical reactors by summarizing the characteristics and applications of different types of reactors. Furthermore, we compare classical chemical reactors and miniaturized droplet reactors to highlight advancements in the design of droplet reactor systems based on open functional surfaces. Finally, we provide an outlook on the research directions of miniaturized droplet reactors.     

Effective approach to cyclic steady state in the catalytic reverse-flow combustion of methane

Computer-based simulations of a reverse-flow reactor should be carried out till the attainment of the so-called cyclic steady state. Usually this state is achieved by the method of direct dynamic simulations. In the paper of Unger et al. (Comput. Chem. Eng. 21 (1997) 5167.) special approaches, making use of various minimization algorithms based most often on Newton algorithms, are proposed. In the present paper one deals with a comparison and an appraisal of these methods, applied to the reverse-flow catalytic combustion of methane that occurs in coal-mine ventilation air.     

Integrated control and process design during optimal polymer grade transition operations

In this work we address the simultaneous process control and design problem of polymerization reactors during dynamic grade transition operation. The problem is cast as a Mixed-Integer Dynamic Optimization (MIDO) formulation and, by using the full discretization approach for solving dynamic optimization problems [Kameswaran, S., & Biegler, L. T. (2006). Simultaneous dynamic optimization strategies: Recent advances and challenges. Computers & Chemical Engineering, 30 (10–12), 1560–1575], is transformed into a Mixed-Integer Nonlinear Program (MINLP). The resulting MINLP is solved using a full space nonconvex optimization formulation [Flores-Tlacuahuac, A., & Biegler, L. T. (2007). Simultaneous Mixed-Integer Dynamic Optimization for integrated design and control. Computers & Chemical Engineering, 31, 588–600]. The control and design formulation has been applied to two polymerization reactors featuring highly nonlinear behavior. In both cases, the proposed MIDO formulation was capable of finding optimal solutions. This amounts to finding optimal steady states, reactor designs, as well as open-loop and closed-loop dynamic optimal trajectories, control structures and controller parameters by specifying either the polymer molecular weight distribution or monomer conversion. Because CPU solution time tends to increase with system complexity, some strategies for lowering CPU time are discussed.     

OPERATION POLICIES FOR A GAS—LIQUID STIRRED TANK REACTOR

The operation of a gas-liquid stirred tank reactor is simulated, including batch operation, steady and unsteady state, continuous operation and cyclic variable volume operation. A pseudo first order reaction is considered. Analytical expressions for the enhancement factor of gas absorption rate are derived for each case, and productivity obtainable under different operation policies is evaluated. Results obtained are shown graphically for specific cases for the purpose of demonstrating the influence of some of the parameters on the system

The equations as presented can be easily extended to other kinetics, and serve as a useful guide for optimum operational policies in a gas-liquid stirred tank reactor.