Semi-batch Stirred Tank Reactor Research Articles

Multi-scale modeling of a mixing-precipitation process in a semibatch stirred tank

In this work the influence of turbulent mixing on the course of barium sulfate precipitation is investigated for a process carried out in a semibatch stirred tank reactor. A time-scale analysis for the controlling process mechanisms is presented to highlight the multi-scale nature of the process. A detailed CFD based process model is presented which accounts for all relevant phenomena from the micro- to the macro-scale and interactions between mechanisms on different scales. Computational results of the models for various operating conditions, i.e., different agitation rates, reactant concentrations and feed addition modes, are compared with experimental data and simulation results obtained using a simpler mechanistic model, thus highlighting the strengths of the complex model.

Data mining macrokinetic approach based on ANN and its application to model industrial oxidation of p-xylene to terephthalic acid

Considering that kinetics and thermodynamics are coupled with heat and mass transfer effects being present in the liquid-phase catalytic oxidation of p-xylene to terephthalic acid (OXTA) in an industrial type of continuous stirred tank reactor (CSTR) and that the time evolution of the concentration of all the interest intermediate and final products of OXTA in the industrial CSTR cannot be obtained to estimate macrokinetic parameters, a novel data mining macrokinetic approach based on artificial neural network (ANN) was proposed to develop the macrokinetic model of OXTA in the industrial CSTR, which mines intrinsic kinetics and transport phenomena information from the sample data collected from OXTA in the industrial CSTR. Firstly, the reaction orders of OXTA are estimated by the mass transfer-free experiment data in the laboratory semi-batch stirred tank reactor. The kinetics of OXTA in the industrial CSTR is assumed to be zeroth-order with respect to gaseous reactants, 0.65-order with respect to p-xylene, and first-order with respect to the other liquid reactants, respectively. Secondly, ANN is employed to model the influence of the reaction parameters on the rate constants of OXTA in the industrial CSTR. Based on the sample data collected from OXTA in the industrial CSTR, heuristic differential evolution algorithm is employed to adjust the weights and thresholds of the rate constant ANN in such a way that it minimizes the prediction error of the macrokinetic model, and thus the optimal weights and thresholds are obtained and the macrokinetic model of OXTA in the industrial CSTR is developed. The reliability of the macrokinetic model was investigated and the satisfactory results were obtained. Further, a generalized macrokinetic approach for multi-phase reaction in industrial reactor was suggested too.

Double-Feed Semibatch Precipitation Effects of Mixing

The way in which reagents are mixed can affect significantly the size distribution and morphology of the solid product. The process of precipitation can be so fast that even the micromixing process that is usually the fastest of the turbulent mixing sequence can affect the course of precipitation and the quality of precipitation product. Effects of mixing are particularly high in the case of the double-feed precipitation process. The process of BaSO 4 precipitation in a double-feed semibatch stirred tank reactor is modelled using a non-equilibrium multiple-time-scale mixing model and a beta distribution of the mixture fraction in combination with a simple conditional moment closure based on linear interpolation of local instantaneous reactant concentration values. Mixing, nucleation and crystal growth are modeled in the feed zone using CFD, whereas nucleation and growth of crystals in the bulk are simulated using a simple concept of the semibatch well-mixed tank. Modelling predicts all trends observed in experiments.

Comparison of Vinyl Acetate ‐ Butyl Acrylate Emulsion Copolymerizations Conducted in a Continuous Pulsed Sieve Plate Column Reactor and in a Batch Stirred Tank Reactor

AbstractIn this work, vinyl acetate/butyl acrylate emulsion copolymerizations carried out in a continuous tubular reactor (pulsed sieve plate column, PSPC) were compared to those conducted in a semibatch stirred tank reactor under similar operational conditions. In order to minimize the compositional drift along the PSPC, reactions were carried out with different numbers (2, 3 and 4) of lateral feed streams of the more reactive monomer (butyl acrylate). For comparison, fed batch reactions were conducted with the same number of intermittent shot additions of butyl acrylate, at the corresponding batch times. Both systems (tubular and semibatch) with distributed feeding of more reactive monomer are able to reduce composition drift thus providing more uniformity in copolymer composition. In addition, the tubular reactor presents much better control of temperature than the tank reactor, which is important to achieve higher productivity.

Interaction between Mixing, Chemical Reactions, and Precipitation

The way in which reagents are mixed can have a large influence on the product distribution of chemical reactions and the size distribution of particles of the solid product. To model effects of mixing on various scales on the course of chemical reactions and precipitation processes, a nonequilibrium multiple-time-scale mixing model and a beta distribution of the mixture fraction are applied in combination with a simple conditional moment closure based on linear interpolation of local instantaneous reactant concentration values. The mixing model is linked to CFD (standard k−e model) and the model predictions are compared with experimental data for fast parallel chemical reactions and barium sulfate precipitation both carried out in the single-feed semibatch stirred-tank reactor.

Determination of the intrinsic kinetics of CO [formula omitted] gas hydrate formation using in situ particle size analysis

The intrinsic kinetics of CO 2 hydrate formation were investigated using a semi-batch stirred tank reactor equipped with a focused beam reflectance method (FBRM) probe for in situ particle size analysis. Experiments were carried out at temperatures ranging from 274 to 279 K and pressures ranging from 1.6 to 3.0 MPa. The experiments were designed to obtain the intrinsic rate constant using a modified version of the model of Englezos et al. (Chem. Eng. Sci. 42(1987a)2647) that incorporates the measured particle size data. It was found that the intrinsic rate constant for CO 2 hydrate formation varied from 3.214 × 10 - 3 to 6.423 × 10 - 3 mol/m 2 Pa s, with the minimum value occurring at roughly 277 K.

Determination of the intrinsic rate constant and activation energy of CO 2 gas hydrate decomposition using in-situ particle size analysis

Experimental data on the rate of decomposition of CO 2 gas hydrates has been obtained using a semi-batch stirred tank reactor, with an in-situ particle size analyser, at temperatures ranging from 274 to 281 K and pressures ranging from 1.4 to 3.3 MPa . A method for calculating the moments of the particle size distribution has been presented. The experimental data was analysed using the kinetic model of Clarke and Bishnoi (Determination of the intrinsic kinetics of gas hydrate decomposition kinetics using particle size analysis, Presented at the Third International Conference on Gas Hydrates, Salt Lake City, Utah, July 18–22, 1999; Chem. Eng. Sci. 55 (2000) 4869) in its differential form in order to account for the slight change in temperature during the decomposition of CO 2 hydrates. The applicability of the new instrument for measuring gas hydrate decomposition kinetics was examined by conducting experiments with ethane at conditions similar to those encountered by Clarke and Bishnoi (2000). It was seen that the previously obtained rate constants for ethane hydrate decomposition were able to predict the new obtained data. A new procedure for regressing the intrinsic rate constant and activation energy has also been described and it is seen that the activation energy is 102.88 kJ/ mol and the intrinsic rate constant is 1.83×10 8 mol/( m 2 Pa s ) for CO 2 gas hydrate decomposition.

Multi-scale modeling of a reactive mixing process in a semibatch stirred tank

In this work the influence of turbulent mixing on the course of two parallel chemical reactions (neutralization of sodium hydroxide and hydrolysis of ethyl chloroacetate) is investigated for a process carried out in a semibatch stirred tank reactor. The multi-scale nature of the process is highlighted by a time-scale analysis of controlling process mechanisms. Two reactive mixing models of different complexity, both accounting for all relevant phenomena from the micro- to the macroscale, are applied for the simulation of the process. Computational results of both models for various operating conditions are compared with experimental data, thus highlighting the strengths of the two models.

CFD Modelling of Mixing Effects on the Course of Parallel Chemical Reactions Carried out in a Stirred Tank

AbstractEffects of turbulent mixing on the course of two fast parallel chemical reactions (neutralization of sodium hydroxide and hydrolysis of ethyl chloroacetate) carried out in a semibatch stirred tank reactor are experimentally investigated and numerically simulated. The flow pattern in the stirred tank is predicted using CFD and experimentally validated using Laser Doppler Anemometry. Mixing effects are modelled using three CFD based models. In the first and the second model the Beta probability distribution and the spiked distribution are used respectively; in the third model concentration fluctuations are neglected.

Micromixing in Boiling and Hot Sparged Systems: Development of a New Reaction Pair

The relevance of micromixing in fast reactions can be characterized by measuring the product distribution of a known test reaction. Although several test reaction systems have been published none is suitable for boiling or nearly boiling conditions. This paper presents details of a new test reaction system suitable for sparged aqueous systems at temperatures up to 100°C. This system, which is easy to use, provides the basis for experiments carried out in a standard semi-batch stirred tank reactor (STR) agitated with a Chemineer CD-6 hollow-blade impeller. The quantitative characterization provided by liquid micromixing sheds light on the effect of temperature, and hence vapour pressure, in the operation of gas–liquid reactors.

Effect of mixing on product quality in semibatch stirred tank reactors

AbstractSemibatch stirred tank reactors with a turbulent flow field are used frequently in the chemical process industry to accomplish mixing tasks. Mixing in a turbulent flow consists of several processes. Descriptions for these processes from literature are used to construct a model to calculate the selectivity of a mixing‐sensitive reaction in a semibatch stirred tank reactor. The model is validated by determining the effects of various process parameters on the selectivity of the third Bourne reaction. Calculation of the selectivity requires information on the hydrodynamic parameters of the stirred vessel studied. The essential detailed description of these hydrodynamic parameters to obtain a good agreement between measured and calculated selectivities was determined. The model proved to be successful in predicting the product distribution of a competitive reaction, making it useful for the design and scale‐up of stirred‐tank reactors.

Measuring and modelling the rate of decomposition of gas hydrates formed from mixtures of methane and ethane

The rate of decomposition of gas hydrates formed from mixtures of methane and ethane was measured in a semi-batch stirred-tank reactor. The experimental apparatus was the same as that used by Clarke and Bishnoi (Chem. Eng. Sci. 55 (2000) 4869; Can. J. Chem. Eng. (2000) accepted for publication) to measure the kinetics of decomposition of hydrates formed from pure ethane and methane. Experiments were conducted with methane/ethane mixtures ranging from 25% methane to 75% methane and at temperatures and pressures between 274 and 278 K and 6.39 to 14.88 bar , respectively. The model of Clarke and Bishnoi was extended to hydrates that are formed from gas mixtures. For hydrates that were known to form structure I, no new parameters were needed and the model was found to accurately predict the data. For the structure II hydrates, it was assumed that the rate constant of ethane was independent of structure because ethane only occupies the large cavities. The intrinsic rate constant and activation energy for methane in structure II were determined to be 8.06×10 3 mol/ m 2 Pa s and 77.33 kJ/ mol , respectively.

Determination of the intrinsic rate of ethane gas hydrate decomposition

The kinetics of ethane hydrate decomposition was studied using an isothermal/isobaric semibatch stirred-tank reactor. The experimental set-up and procedure used was similar to that used by Kim, Bishnoi, Heidemann and Rizvi (1987), Chemical Engineering Science, 42, (1645–1653) with one major modification; the addition of an on-line particle size analyser. The experiments were conducted at temperatures ranging from 274.15 to 281.15 K and at pressures between 5 and 11 bar. A new mathematical model is developed to determine the rate constant of decomposition. The model accounts for the distribution of particle sizes in the hydrate phase when determining the intrinsic rate constant of decomposition. The effect of the rate of stirring on the rate constant was also investigated to establish the regime where mass and heat transfer effects were negligible. The data were then obtained to determine the intrinsic rate constant. It was found that the activation energy for ethane hydrate decomposition is 104 kJ/mol and the intrinsic rate constant of decomposition is 2.56×10 8 mol/m 2 Pa s.

Hybrid first-principle–neural-network approach to modelling of the liquid–liquid reacting system

Detailed investigations have been carried out to check the ability of multilayer neural networks to model the simultaneous mass transfer and chemical reaction in the liquid–liquid reacting system. In this approach the intrinsic reaction kinetics and diffusive mass transfer are represented by a black-box and only the input–output signals are analysed. The data for training of the net have been taken from the experiments performed in a RC1 Mettler Toledo reaction calorimeter. The hydrolysis of propionic anhydrite catalysed with sulphuric acid has been chosen as a testing reaction. The hybrid, first-principle–neural-network model has been defined to describe batch and semibatch stirred tank reactors operating at different conditions. Good accuracy and flexibility of the proposed approach have been obtained for a properly defined experimental programme supplying data for learning.

Interaction between chemical reactions and mixing on various scales

The way in which reagents are mixed can have a large influence on the product distribution of a chemical reaction. This has been analysed earlier when micromixing is the limiting mixing step. Additional segregation at a larger scale has only been treated in detail when the local turbulent dispersion of a feed stream was relevant. Here additional segregation due to the finite disintegration rate of large concentration eddies is represented by a feasible structure to describe the environment within which micromixing and chemical reaction take place. The resulting model contains one time constant each for micro- and mesomixing. Their estimation is discussed and applied to predicting the yields of fast complex reactions in plug-flow static mixers and in a semibatch stirred tank reactor under conditions where neither macromixing nor turbulent dispersion were limiting. The comparison with measured yields is good for micromixing and fairly satisfactory for inertial-convective mesomixing. Further research on this step is needed.

Kinetics of formation of carbon dioxide clathrate hydrates

The new experimental apparatus capable of observing the clathrate hydrate formation kinetics was developed in this study. Experimental data on the kinetics of carbon dioxide hydrate formation were carefully measured. The experiments were carried out in a semi-batch stirred tank reactor with stirring rate of 500 rpm at three different temperatures between 275.2 and 279.2 K and at pressures ranging from 2.0 to 3.5 MPa. The kinetic model was adopted to predict the growth of hydrates with only one adjustable parameter which represented the rate constant for the hydrate particle growth. The model was based on the crystallization theory coupled with the two-film theory for gas absorption into the liquid phase. The model predictions matched the experimental data very well with the largest deviation of 7.18%, which is within experimental error range. This study is the first for the kinetic data of carbon dioxide hydrate formation and important in developing carbon dioxide fixation process using clathrate hydrate phenomenon.

Interaction of macro- and micromixing on particle size distribution in reactive precipitation

The effects of mixing factors (stirrer speed, feed time, feed location and viscosity) on particle size distribution (PSD) in the precipitation of BaSO 4 conducted in a semi-batch stirred tank reactor were investigated. It was found that there exist critical points on the curves of mean size and dimensionless variance versus the mixing factors, at which the mean size or the dimensionless variance is minimal. These results originate from the interaction of macromixing and micromixing. A new mixing-precipitation model including micromixing, turbulent dispersion (mesomixing) and backmixing (macromixing) was proposed to predict PSD. Critical effects of mixing factors, stirrer speed and feed time on PSD were quantitatively predicted by the present model. The results coincided satisfactorily with those of the experiments.

An approximate method for predicting the product distribution of fast reactions in stirred-tank reactors

The product distribution of two sets of multiple reactions were observed to depend upon the impeller speed and the feed location in a semibatch stirred-tank reactor as well as upon the viscosity. A new method to predict the product distribution of one reaction set based upon measurements for the other and a micromixing model is developed, subject to approximations which are explained. Examples of how to apply the new method are given.

What Does the Modelling of Reactive Flows Ask of Cfd ?

This contribution emphasises physically based models for the mixing-reaction coupling. The fluid mechanical information needed to implement such models can then be identified. A semi-batch stirred tank reactor carrying out the competitive-consecutive reactions A + B = R; R + B = S is considered. Reagent A is in the tank and B is added so slowly that it mixes with the tank's average contents. Only micromixing then determines the product distribution between R and S, whereby segregation promotes S. The second topic is micromixing when the concentrations in the surroudings of the reaction zones are determined by mesomixing i. e. mixing which is intermediate in scale between that of micromixing and that of the whole vessel. Fresh B, emerging from a feed pipe, is transported by the local velocity field and dispersed transverse to its local streamline by turbulent diffusion. Dispersion gives an intermingling of A (from surroundings) and B (from feed pipe) which is coarse relative to the dissipation scales.

Light scattering kinetics study of cyclopropane hydrate growth

Experimental formation data for cyclopropane type I hydrate were obtained in a semi-batch stirred tank reactor. Kinetics experiments were conducted at temperatures of 278.1 to 283.1 K and pressures of 0.2 to 0.38 Mpa. The two experimental techniques, described in this paper, are respectively based on the measurement of the gas consumption rate during hydrate formation and on the laser diffraction of particles. The study reveals that the formation rate is proportional to the difference in the fugacity of the dissolved gas and the three phase equilibrium fugacity at the experimental pressure. The last experimental method is used to measure size distribution of particles in view of the determination of the second moment of particle size distribution (PSD). These two experimental methods show that it is possible to quantify the main kinetics parameters in view of the future modelling of the kinetics phenomena.