The effect of mixing on reactor performance: an application of perturbation technique

The effect of local mixing on the performance of a reactor system is derived by the perturbation technique. It is found that the sign of the inner product of the adjoint vector λ and the acceleration vector ? is the only indicator as to whether or not local mixing at a certain point of a reaction path is desirable. A complex reaction system with competing side reactions of different order is treated as an example. The possibility of applying the results in this work to the selection of the optimal type of reactor is also discussed.     

CFD simulation of mixing in tall gas-liquid stirred vessel: Role of local flow patterns

In this work, we have used the computational fluid dynamics (CFD)-based models to investigate the gas-liquid flows generated by three down-pumping pitched blade turbines. A two-fluid model along with the standard k-ε turbulence model was used to simulate the dispersed gas-liquid flow in a stirred vessel. Appropriate drag corrections to account for bulk turbulence [Khopkar and Ranade, 2005. CFD simulation of gas-liquid flow in a stirred vessel: VC, S33 and L33 flow regimes. A.I.Ch.E. Journal, accepted for publication] were developed to correctly simulate different flow regimes. The computational snapshot approach was used to simulate impeller rotation and was implemented in the commercial CFD code, FLUENT4.5 (of Fluent. Inc., USA). The computational model has successfully captured the flow regimes as observed during experiments. The particle trajectory simulations were then carried out to examine the influence of the different flow regimes on the circulation time distribution. The model predictions were verified by comparing the predicted results with the experimental data of [Shewale and Pandit, 2006. Studies in multiple impeller agitated gas-liquid contactors. Chemical Engineering Science 61, 489-504]. The computational model and results discussed in this study would be useful for explaining the implications local flow patterns on the mixing process and extending the applications of CFD models for simulating large multiphase stirred reactors.     

Modeling and simulation of an industrial slurry-phase catalytic olefin polymerization reactor series

In the present study, a comprehensive mathematical model is developed to simulate the dynamic behaviour of an industrial slurry-phase olefin catalytic polymerization loop reactor series. More specifically, the effects of various operating conditions on the dynamic reactor behaviour (i.e., reaction temperature and pressure, inflow rates of catalyst, monomers and diluent, etc.) as well as the on the molecular and rheological polyolefin properties (i.e., M_n, M_w, MWD, complex viscosity, etc.) are fully assessed. According to the proposed modeling approach, each loop reactor (i.e., consisting of the loop reactor and the settling legs) is modeled as an ideal CSTR in series with a semi-continuous product removal unit. Dynamic macroscopic mass species and energy balances are derived to calculate the dynamic evolution of the concentrations of the various molecular species as well as of temperature profiles and heat removal in the two loop reactors. The polymer molecular properties (i.e., number- and weight-average molecular weights and molecular weight distribution) are determined by employing a generalized multi-site, Ziegler–Natta (Z–N) kinetic scheme in conjunction with the well-known method of moments. All the thermodynamic calculations, regarding the equilibrium species concentration in the various phases (i.e., solids and liquid), are carried out using the Sanchez–Lacombe Equation of State (S–L EOS). It is shown that the proposed comprehensive model is capable of simulating the dynamic operation of an industrial slurry-phase cascade-loop reactor series under different plant operating policies (i.e., start-up, grade transition, etc.).     

Two-phase flow simulation of reactor clarifiers

In this work, hydrodynamic behavior of flow in three different reactor clarifiers was simulated by three-dimensional, multiphase flow model. The primary construction of reactor clarifier was based on the Bansin Water Treatment Plant, Taiwan. This is the traditional construction, and we call it Type A. The other two were designed in such a way as to improve effluent water quality. The traditional clarifier construction was varied in these to make a large well angle (Type B) and a gradually larger inlet pipe (Type C). Solid effluent flux can be calculated directly from this model. The simulation results showed that under the same daily throughput, the Type C construction of clarifier could decrease upflow fluid velocity in the clarifier and, therefore, reduce effluent water turbidity.     

Downer reactor: From fundamental study to industrial application

Downer reactor, in which gas and solids move downward co-currently, has unique features such as the plug-flow reactor performance and relatively uniform flow structure compared to other gas-solids fluidized bed reactors, e.g., bubbling bed, turbulent bed and riser. Downer is therefore acknowledged as a novel multiphase flow reactor with great potential in high-severity operated processes, such as the high temperature, ultra-short contact time reactions with the intermediates as the desired products. Typical process developments in industry have directed to (1) the new-generation refinery process for cracking of heavier feedstock to gasoline and light olefins (e.g., propylene) as by-products; and (2) coal pyrolysis in hydrogen plasma which opens up a direct means for producing acetylene, i.e., a new route to synthesize chemicals from a clean coal utilization process. This paper is to give a comprehensive review on the development of fundamental researches on downer reactors as well as the particular industrial demonstrations for the fluid catalytic cracking (FCC) of heavy oils and coal pyrolysis in thermal plasma.     

CFD simulation of liquid-phase mixing in solid-liquid stirred reactor

A comprehensive CFD model was developed to gain an insight into solid suspension and its implications on the liquid-phase mixing process in a solid-liquid stirred reactor. The turbulent solid-liquid flow in a stirred reactor was simulated using a two-fluid model with the standard k-ε turbulence model with mixture properties. The multiple reference frames (MRFs) approach was used to simulate impeller rotation in a fully baffled reactor. The computational model with necessary sub-models was mapped on to a commercial solver FLUENT 6.2 (of Fluent Inc., USA). The predicted solid concentration distribution was compared with the experimental data of Yamazaki et al. [1986. Concentration profiles of solids suspended in a stirred tank. Powder Technology 48, 205-216]. The computational model was then further extended to simulate and understand the implications of the suspension quality on liquid-phase mixing process. The computational model and the predicted results discussed here will be useful for understanding the liquid-phase mixing process in stirred slurry reactors in various stages of solid suspension.     

Modeling and simulation of an industrial slurry phase ethylene polymerization reactor: effect of reactor operating variables

A hierarchical and computationally efficient mathematical model was developed to explain the polymerization of high-density polyethylene (HDPE) in an isothermal, industrial, continuous stirred tank slurry reactor (CSTR). A modified polymeric multi-grain model (PMGM) was used. Steady-state macroscopic mass balance equations were derived for all species (namely, monomer, solvent, catalyst and polymer) to obtain the final particle size and the required monomer and solvent input rates for a given catalyst input and the reactor residence time. The interphase mass transfer coefficients were calculated for the industrial CSTR using the operating data on the reactor. The present model was tuned with some data on an isothermal industrial reactor and the simulation results were compared with data on another set of industrial reactor. The comparison revealed that the present tuned model is capable of predicting the productivity and the polymer yield at various catalyst feed rates and the mean residence times. The effects of variation of two operating variables (catalyst feed rate and mean residence time) on the productivity, the polymer yield, the polydispersity index (PDI) and the operational safety were analyzed. The present study indicated that an optimal value of the reactor residence time (for maximum productivity per catalyst particle) exists at any catalyst feed rate.

     

Modelling, simulation and control of an industrial, semi-batch, emulsion-polymerization reactor

This paper presents an emulsion-polymerization model that is designed for an industrial, semi-batch reactor. The model consists of a reaction model and a calorimetry model, and as such enables us to predict the reactor temperature and the batch-output parameters, i.e., the conversion, the solids content and the viscosity. The model was validated on real-plant data and used in the analysis and design of the reactants dosing control. The control strategy proposed is valid for cases where evaporative cooling is either the only or an additional way to remove the heat of the reaction. It consists of an initiator and monomer dosing control, using the reactor temperature as a controlled variable. The simulation results and the real-plant testing show that the proposed reactants dosing control significantly reduces the variations in the reactor temperature and at the same time results in more uniform batch results.     

The influence of mixing on lysozyme renaturation during refolding in an oscillatory flow and a stirred-tank reactor

Protein refolding is a key unit operation in many processes that produce recombinant biopharmaceuticals using Escherichia coli. Yield in this step generally controls overall process yield, and at industrially relevant protein concentrations is limited by aggregation. While most refolding operations are optimised with respect to chemical environment, the physical processes affecting yield have been neglected. In this study, we demonstrate that refolding yield for the model protein lysozyme is dependent on mixing intensity during dilution refolding. This is shown for two different reactor configurations: a standard stirred-tank reactor and a novel oscillatory flow reactor. We further show that the effect of mixing is dependent on the type of chaotrope employed for denaturation. Yield falls significantly when mixing intensity is decreased following urea denaturation, while the effect of mixing is not apparent when guanidine hydrochloride is employed as the denaturant. In batch tests we further confirm that, for urea, the “path” of dilution affects yield, and hence the observed sensitivity to mixing is not unexpected. We conclude that mixing is a critical parameter that must be optimised in industrial reactors, along with the usual chemical and protein-specific parameters.     

A mass-transfer study of two-phase flow in an electrochemical reactor

The electrodeposition of Cu²⁺ ions from a solution of 1 M H₂SO₄ has been studied in a parallel plate cell with two-phase (liquidgas) flow. The results are interpreted in terms of the LevichLandau and TaylorPrandtl models. The conclusion is reached that the mass-transfer enhancement observed results mainly from increase in linear velocity of the liquid phase due to a reduction of the effective cross-sectional area of the cell caused by the bubbles. The industrial prospects for such a system are discussed.     

Modeling, simulation and control of dimethyl ether synthesis in an industrial fixed-bed reactor

Dimethyl ether (DME) as a clean fuel has attracted the interest of many researchers from both industrial communities and academia. The commercially proven process for large scale production of dimethyl ether consists of catalytic dehydration of methanol in an adiabatic fixed-bed reactor. In this study, the industrial reactor of DME synthesis with the accompanying feed preheater has been simulated and controlled in dynamic conditions. The proposed model, consisting of a set of algebraic and partial differential equations, is based on a heterogeneous one-dimensional unsteady state formulation. To verify the proposed model, the simulation results have been compared to available data from an industrial reactor at steady state conditions. A good agreement has been found between the simulation and plant data. A sensitivity analysis has been carried out to evaluate the influence of different possible disturbances on the process. Also, the controllability of the process has been investigated through dynamic simulation of the process under a conventional feedback PID controller. The responses of the system to disturbance and setpoint changes have shown that the control structure can maintain the process at the desired conditions with an appropriate dynamic behavior.     

Chemical reactor network application to predict the emission of nitrogen oxides in an industrial combustion chamber

A new chemical reactor network model is developed to predict the emission of nitrogen oxides in an industrial combustion chamber operating on liquefied petroleum gas. The boundary conditions and operating parameters used for this model are typical operating conditions of an industrial combustion chamber. The global mechanism is developed by GRI-MECH 3.0 in the UW code. The model predictions are compared with experimental data. The chemical reactor network model provides an accurate estimation of nitrogen oxide emission.     

Startup of an industrial adiabatic tubular reactor

The dynamic behaviour of an adiabatic tubular plant reactor during the startup is demonstrated, together with the impact of a feed-pump failure of one of the reactants. A dynamic model of the reactor system is presented, and the system response is calculated as a function of experimentally-determined, time-dependent, manipulated variables. The values of model parameters are estimated by using the SimuSolv (1991) computer program. The data set collected during the reactor start-up is used for the parameter estimation procedure. An excellent agreement is obtained between the experimental and the calculated system response. Many continuously-operated commercial reactors require a complete conversion of one of the main reactants at the reactor exit. It is shown that for an industrial tubular reactor a much higher initial reactor temperature is required during the startup, compared to the reactor inlet temperature during normal steady-state operation, to ensure a complete reactant conversion. Much more research is necessary to determine whether this is a generally valid rule.     

Large-eddy simulation of the turbulent free-surface flow in an unbaffled stirred tank reactor

This article deals with the large-eddy simulation (LES) of a complex turbulent free-surface flow in an unbaffled mixing tank reactor. The free-surface vortex generated in such a configuration is captured using a front-tracking method, while the stirrer is modelled with an immersed boundary condition technique. Comparisons of mean and fluctuating velocities show good agreement with both theory and experimental laser Doppler velocimetry measurements. The study of mean and instantaneous hydrodynamics points out several interesting features, especially coherent structures, which may have a strong impact on mixing in the reactor. Finally, Reynolds stresses analysis confirms the high anisotropy of turbulence throughout the tank.     

The simulation of an existing ethane/dehydrogenation reactor

This paper is concerned with the mathematical representation of an existing box-type furnace for the thermal dehydrogenation of ethane. The furnace is composed of a preheating convection section and a radiant heated section. The two models considering (i) the tube-side calculation and (ii) the radiant heat transfer in the combustion chamber are independent of each other. A study is made of the effects of pressure, heat flux level and “steam/hydrocarbon” ratio on the effluent composition. A significant improvement in reactor operation if a more uniform heat flux could be achieved is predicted. From the radiant heat transfer computations an unexpected variation in flame temperature in the furnace under study was detected.     

自适应时间步长在化学驱油藏模拟中的应用

目前国内大多化学驱数值模拟软件中应用了固定时间步长或者通过组分浓度控制时间步长的方法,在油藏模拟计算过程中,算法的稳定性受限于时间步长的设置及模型对其他参数的敏感性。通过在软件中加入压力-组分浓度联控时间步长,以及在方程不收敛时自动修正当前时间步长重新计算的功能,改进了化学驱数值模拟软件的计算稳定性,并通过概念模型和矿场模型的应用对改进的功能进行了验证。     

Development of a non-contact micro-liquid mixing method and application to chemical auto-analyzers

A new mixing method, which can mix some hundred microliters of liquid in a vessel without any physical contact, was developed. This mixing method used free surface waves in a vessel generated by ultrasound irradiation from outside the vessel. First, feasibility of circulation flow induced by waves was discussed with a numerical simulation. The mixing mechanism was then experimentally verified by flow visualization using a dye. Next, the method was applied to mixing serums and reagents. The serum-reagent mixture was visualized by using the Schlieren method, and the mixing performance was quantified from the visualized images. The experimental results show that the new mixing method is suitable for volumes of serum-reagent mixtures ranging up to . Finally, by implementing the new mixing method in a standard chemical auto-analyzer, a new kind of chemical auto-analyzer that does not suffer cross-contamination between samples was produced.     

The application of flow dispersion models to the FM01-LC laboratory filter-press reactor

The flow distribution in the rectangular channel of a laboratory filter-press electrochemical reactor was evaluated using three flow models namely: (a) axial dispersion, (b) sum of two phases and (c) fast and stagnant zones. In the case of the axial-dispersion model, several methods have been used to calculate the Peclet number; the moment method, the non-linear least-squares and the Laplace transform technique. Several boundary conditions, involving different physical and experimental assumptions of the flow were used to solve the partial differential equation that describes the flow behaviour. A total of nine expressions to examine flow dispersion has been used. The comparison of experimental and predicted response signals was made by evaluating the root mean squared error. A data fit in real time has been found to be a better choice as solutions based on the evaluation of moments are prone to error due the overweight of the signal at long times. Data fitting in the Laplace plane is very accurate but it does not guarantee a good fit in real time. Models based on the sum of a fast and a slow or stagnant phase resulted in solutions having very low values of the extension of the slow and stagnant phases, the assumption of a single phase with some degree of dispersion was considered more appropriate.     

Simulation of an industrial nylon 6 tubular reactor

A commonly used energy-efficient nylon 6 reactor is simulated under steady-state conditions. The effects of various operating conditions and parameters, e.g., feed composition, temperature and flow rate, heat transfer coefficients, and reactor dimensions, on the temperature and molecular weight profiles are studied. A temperature maximum is observed in the reactor under usual conditions of operation. The maximum value of the temperature is sensitive to the feed conditions, and one has to ensure that degradation reactions speeded up at high temperatures do not affect product characteristics. The model and the numerical technique used are fairly simple and account for most of the important features of industrial reactors. Hence, these can be used in the development of digital-control algorithms in the future.     

Operation of a flow reactor under chemical equilibrium conditions

The conditions of reaching a state of chemical equilibrium in a perfect-mixing flow reactor are analyzed. Using the reaction A ai B as an example, the dependences of the composition in the reactor, the rate of the reaction, and the production rate of the reactor on the values of volume and rate constants are studied. It is shown that a state of chemical equilibrium can be reached only under conditions of an unrestricted increase in the residence time of the reaction mixture in the reactor.