Mixing batch tank

Ideally, the oxygen scavenger is added continuously rather than periodically shot-dosed where batch tanks are employed, there should not be more than one day s supply of oxygen scavenger in the tank. The tank contents should not be agitated any longer than necessary, to avoid aerating the mix. 

Reactor design usually begins in the laboratory with a kinetic study. Data are taken in small-scale, specially designed equipment that hopefully (but not inevitably) approximates an ideal, isothermal reactor batch, perfectly mixed stirred tank, or piston flow. The laboratory data are fit to a kinetic model using the methods of Chapter 7. The kinetic model is then combined with a transport model to give the overall design. 

In this chapter the simulation examples are described. As seen from the Table of Contents, the examples are organised according to twelve application areas Batch Reactors, Continuous Tank Reactors, Tubular Reactors, Semi-Continuous Reactors, Mixing Models, Tank Flow Examples, Process Control, Mass Transfer Processes, Distillation Processes, Heat Transfer, and Dynamic Numerical Examples. There are aspects of some examples which relate them to more than one application area, which is usually apparent from the titles of the examples. Within each section, the examples are listed in order of their degree of difficulty. 

Suppose that a well-mixed stirred tank is being used as a fed-batch fermentor at a constant feed rate F (m h ), substrate concentration in the feed C j (kg m ), and at a dilution rate D equal to the specific cell growth rate p. Ihe cell concentration Cjj (kgrn ) and the substrate concentration (kgm ) in the fermentor do not... 

If the compositions vary with position in the reactor, which is the case with a tubular reactor, a differential element of volume SV, must be used, and the equation integrated at a later stage. Otherwise, if the compositions are uniform, e.g. a well-mixed batch reactor or a continuous stirred-tank reactor, then the size of the volume element is immaterial it may conveniently be unit volume (1 m3) or it may be the whole reactor. Similarly, if the compositions are changing with time as in a batch reactor, the material balance must be made over a differential element of time. Otherwise for a tubular or a continuous stirred-tank reactor operating in a steady state, where compositions do not vary with time, the time interval used is immaterial and may conveniently be unit time (1 s). Bearing in mind these considerations the general material balance may be written ... 

Powders are dissolved either directly in the main mix tank or premix tank or indirectly using a vortex-type mixer (Figure 8.1) where powder is dropped into the vortex of a horizontally mounted pump head recirculating the fluid from and to the batch tank. Some specialised versions of this mixer can handle very viscous blending applications (50,000 cP or more). 

One measure of the amount of liquid motion in an agitated tank is velocity. However, by the very nature of mixing requirements, liquid velocities must be somewhat random in both direction and magnitude. Since actual velocity is difficult to measure and depends on location in the tank, an artificial, defined velocity called bulk velocity has been found to be a more practical measure of agitation intensity. Bulk velocity is defined as the impeller pumping capacity (volumetric flow rate) divided by the cross-sectional area of the tank. For consistency, the cross-sectional area is based on an equivalent square batch tank diameter. A square batch is one in which the liquid level is equal to the tank diameter. 

The reactors treated in the book thus far—the perfectly mixed batch, the plug-flow tubular, and the perfectly mixed continuous tank reactors—have been modeled as ideal reactors. Unfortunately, in the real world we often observe behavior very different from that expected from the exemplar this behavior is tme of students, engineers, college professors, and chemical reactors. Just as we must learn to work with people who are not perfect, so the reactor analyst must learn to diagnose and handle chemical reactors whose performance deviates from the ideal. Nonideal reactors and the principles behind their analysis form the subject of this chapter and the next. 

Reactor designs are characterized as either homogeneous or heterogeneous. Typically, homogeneous reactors are well mixed stirred tanks (either batch or continuous), but can also be tubular reactors. They are widely used in the chemical industry from pilot plant to full-scale production. Examples include decomposition of azomethane, production of ethylene glycol, and the copolymerization of styrene and butadiene. 

Many specialty chemicals are produced in semibatch reactors where a reactant is added gradually into a batch reactor. This problem concerns the governing equations of such operations without considering chemical reactions. A well-mixed batch reactor initially contains 200 L of pure water. At time t = 0, we start feeding a brine stream with a salt concentration of 180 g/L into the tank at a constant rate of 50 L/min. Calculate ... 

The size and niunber of batching tanks depend upon whether the plant uses continuous sterilizers or batch sterilization. The difference is that in the latter case, the tanks can be large (5 0 to 80% of the size of the fermenter), and usually all the materials are mixed together. For continuous sterilizers, there is usually a minimum of four smaller tanks so that proteins, carbohydrates and salts can be batched and sterilized separately. In this case, the tanks are considerably smaller than the fermenter. 

In ihe three idealized types of reactors just discussed (the perfectly mixed batch reactor, the plug-fiow tubular reactor (PFR). and the perfectly mixed con-tinuous-siirred tank reactor (CSTR), the design equations (i.e.. mole balances) were dei doped based on reactor volume. The deris ation of the design equation for a packed-bed catalytic reactor (PBR) will be carried out in a manner analogous to the development of the tubular design equation. To accomplish this derivation. we simply replace the volume coordinate in Equation (MO) with (he catalyst weight coordinate H (Figure - 4). 

The biggest disadvantage of batch operations is their fixed scale. Once a batch tank size is chosen, it is difficult to run smaller campaigns. Each weighing/metering approach has a minimum amount that can be added accurately and there is a minimum level that can be properly mixed. There is also some hold up in the batch that cannot be pumped out at the end. This is fixed based on the size of the mixer as well as the hold up volume in the transfer lines to the bottle-filling lines, and therefore, making smaller batches in the same mixer increases the scrap or contamination as a percent of the production. 

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