Stirred reactors, industrial applications

Houcine I, Plasari E, David R, Villermaux J. Feedstream jet intermittency phenomenon in a continuous stirred tank reactor. Chem Eng J 1999 72 19-29. Zlokarnik M. Dimensional analysis and scale-up in theory and industrial application. In Levin M, ed. Process Scale-Up in the Pharmaceutical Industry. New York Marcel Dekker, 2001. 

The two-phase (gas-solid) continuous stirred-tank reactors are represented by laboratory reactors as, for instance, one-pass differential reactors, reactors with forced recirculation, one-pellet reactors, etc. The industrial applications are the fluidized beds.2 Table V presents a list of experimental studies along with a very brief description of each system studied. 

TABLE 10.1 Some Industrial Applications of Stirred Reactors... 

Figure 3 shows a technical modification of the general principle illustrated in Figure 1 while in Figure 1 the two liquid phases are separated during the reaction, but in Figure 3 these steps are divided into two different units. As in the Ruhrche-mie/Rhone-Poulenc process of propene hydroformylation, in this industrial application the reaction steps and the separation step are divided. The reaction takes place in a continuously stirred tank reactor, while the phase separation is carried out in a decanter. 

There are of course a number of well established alternatives to the stirred tank used in the large scale chemical industries, and strategies have been proposed for selection of the best reactor for a given multiphase reaction The nature of flow and contacting in these reactors differs significantly. Industrial applications are largely dominated however by a small number of these ... 

Emulsion polymerization as a continuous operation has been described in the patent literature an has found industrial application. One technical process mentioned uses a coiled pipe of such dimensions that at a certain temperature the monomer emulsion polymerizes completely during its pa sage. If necessaiy, a pipe system can be built that is heated to different temperatures at its various sections. Other continuous processes can be carried out in polymerization towers if the monomer has a lower density than water and the polymer has a higher one. The monomer emulsion is added at the top of the tower and agitated by stirring devices, with turbulence limited to the upper parts of the tower. As the polymerization proceeds and the polymer sinks to the bottom of the tower, new monomer is introduced at the top and latex removed at the base. Instead of one tower, a battery of interconnected reactors can be advantageously used in a similar procedure. 

During polymerization with a CSTR, the monomer and the other components of the polymerization recipe are fed continuously into the reactor while the polymerization product mixture is continually withdrawn from the reactor. The application of the CSTR in suitable polymerization processes reduces, to some extent, the heat removal problems encountered in batch and tubular reactors due to the cooling effect from the addition of cold feed and the removal of the heat of reaction with the effluent. Even though the supporting equipment requirements may be relatively substantial, continuous stirred tank reactors are economically attractive for industrial production and consistent product quality. 

Industrial applications of PTC are concentrated upon the manufacture of Fine Chemicals and Organic Intermediates. Recent reviews identify the potential of PTC in these fields as almost unlimited . The reasons for such an endorsement are the advantages of cost and environmental friendliness that PTC enjoys over existing technologies. In this prqier, we examined the effects of oscillation amplitude and fi-equency on the reaction rate and conversion of the PTC reaction, and compared the results with those carried out in a Continuously Stirred Tank Reactor (STR). Our preliminary results show that the OBR promotes improved reaction rates and conversion at lower power requirements. 

In many industrial applications such as bubble columns and stirred tank reactors, it is of interest to know the local concentration of bubbles. In general, this is an extremely difficult problem because the bubbles modify the flow and one must compute shape and velocity of the bubbles simultaneously with the motion of the liquid phase. However, for dilute flows, one may be able to obtain some progress with the so-called one-way coupling approximation. In this approach, one ignores the effect of the bubbles on the motion of the liquid. This is reasonable provided that the gas volume fraction is very small and if the bubbles are smaller than the energy-containing eddies so that the turbulence created by the bubbles is unimportant. One then integrates an approximate equation of motion for the bubble. 

Fluidized beds find numerous industrial applications, for example, in chemical reactors employing catalysts, in waste incineration furnaces, or for the desiccation of sohds. Fluidization makes it possible to provide a veiy good contact between the fluid and solid. The exchange surface is at a maximum and the fluid is replenished. Transfers are therefore facilitated. The stirring of particles also allows for good homogenization within the bed. This is sometimes useful from a thermal viewpoint temperature is more homogeneous and the occurrence of hot spots, a classic drawback of fixed beds, is thus avoided. 

Many industrially important chemical reactions occur in liquid-liquid systems since heat and mass transfer can be very efficient in agitated heterogeneous stirred reactors. The reaction usually takes place in the dispersed phase. Transport rates depend on the slip velocity between the phases as shown in eqs. (12-63) and (12-64). They are applicable only to single drops that are larger than the turbulent macroscale and are presented for illustrative purposes only. A tank-specific correlation is given later. The heat transfer coefficient, hx, for a single sphere is given by... 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

Many wastewater flows in industry can not be treated by standard aerobic or anaerobic treatment methods due to the presence of relatively low concentration of toxic pollutants. Ozone can be used as a pretreatment step for the selective oxidation of these toxic pollutants. Due to the high costs of ozone it is important to minimise the loss of ozone due to reaction of ozone with non-toxic easily biodegradable compounds, ozone decay and discharge of ozone with the effluent from the ozone reactor. By means of a mathematical model, set up for a plug flow reactor and a continuos flow stirred tank reactor, it is possible to calculate more quantitatively the efficiency of the ozone use, independent of reaction kinetics, mass transfer rates of ozone and reactor type. The model predicts that the oxidation process is most efficiently realised by application of a plug flow reactor instead of a continuous flow stirred tank reactor. 

Applications of kinetic principles to industrial reactions are often useful. Initial kinetic studies of the esterification reaction are usually conducted on a small scale in a well stirred batch reactor. In many cases, results front batch studies can be used in the evaluation of the esterification reaction in a continuous operating configuration. 

The term fermentation is used to describe the biological transformation of chemicals. In its most generic application, a fermentor may be batch, continuous-stirred tank (chemostat), or continuous plug flow (immobilized cell). Most industrial fermentors are batch. Several configurations exist for these batch reactors to facilitate aeration. These include sparged tanks, horizontal fermentors, and biological towers. 

Knowledge of these types of reactors is important because some industrial reactors approach the idealized types or may be simulated by a number of ideal reactors. In this chapter, we will review the above reactors and their applications in the chemical process industries. Additionally, multiphase reactors such as the fixed and fluidized beds are reviewed. In Chapter 5, the numerical method of analysis will be used to model the concentration-time profiles of various reactions in a batch reactor, and provide sizing of the batch, semi-batch, continuous flow stirred tank, and plug flow reactors for both isothermal and adiabatic conditions. 

The rational design of a reaction system to produce a desired polymer is more feasible today by virtue of mathematical tools which permit one to predict product distribution as affected by reactor type and conditions. New analytical tools such as gel permeation chromatography are beginning to be used to check technical predictions and to aid in defining molecular parameters as they affect product properties. The vast majority of work concerns bulk or solution polymerization in isothermal batch or continuous stirred tank reactors. There is a clear need to develop techniques to permit fuller application of reaction engineering to realistic nonisothermal systems, emulsion systems, and systems at high conversion found industrially. A mathematical framework is also needed which will start with carefully planned experimental data and efficiently indicate a polymerization mechanism and statistical estimates of kinetic constants rather than vice-versa. 

Monolithic supports are commonly used for environmental applications and will be discussed in more detail later.-5 Batch reactors are used mostly for small-scale production such as the hydrogenation of intermediates in the production of medicines in the pharmaceutical industry. The catalyst powder is mixed in a precise amount of reactant in a pressurized-stirred autoclave. A gaseous reactant, usually H2, is introduced at elevated pressures and the reaction proceeds with continuous monitoring of the H2 consumed. The catalyst is separated from the product via filtration and is often used again depending on its retained activity and selectivity. 

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