Stirred tank external

A reduced reaction rate may result from external diffusional restrictions on the surface of carrier materials. In stirred tanks external diffusion plays a minor role as long as the reaction liquid is stirred sufficiently. Further, partition effects can lead to different solubilities inside and outside the carriers. Partition has to be taken into account when ionic or adsorptive forces of low concentrated solutes interact with carrier materials [81 - 83]. The most crucial effects are observed in porous particles due to internal or porous diffusion as outlined below. 

Vegetable oils can be extracted from their parent seeds by contacting them with solvent in a well-stirred tank. External film resistance can in these cases often be neglected and the principal events are confined to the interior of the seeds. Figure 4.4 can then be used to carry out pertinent calculations. 

In many types of contactors, such as stirred tanks, rotary agitated columns, and pulsed columns, mechanical energy is appHed externally in order to reduce the drop si2e far below the values estimated from equations 36 and 37 and thereby increase the rate of mass transfer. The theory of local isotropic turbulence can be appHed to the breakup of a large drop into smaller ones (66), resulting in an expression of the form... 

The leached solids must be separated from the extract by settling and decantation or by external filters, centrifuges, or thickeners, all of which are treated elsewhere in Sec. 18. The difficulty of solids-extract separation and the fact that a batch stirred tank provides only a single equilibrium stage are its major disadvantages. 

FIG. 23-1 Heat transfer to stirred tank reactors, a) Jacket, (h) Internal coils, (c) Internal tubes, (d) External heat exchanger, (e) External reflux condenser. if) Fired heater. (Walas, Reaction Kinetics for Chemical Engineers, McGraw-Hill, 1959). 

Various experimental methods to evaluate the kinetics of flow processes existed even in the last centuty. They developed gradually with the expansion of the petrochemical industry. In the 1940s, conversion versus residence time measurement in tubular reactors was the basic tool for rate evaluations. In the 1950s, differential reactor experiments became popular. Only in the 1960s did the use of Continuous-flow Stirred Tank Reactors (CSTRs) start to spread for kinetic studies. A large variety of CSTRs was used to study heterogeneous (contact) catalytic reactions. These included spinning basket CSTRs as well as many kinds of fixed bed reactors with external or internal recycle pumps (Jankowski 1978, Berty 1984.)... 

Reactors may be operated batchwise or continuously, e.g. in tubular, tubes in shell (with or without internal catalyst beds), continuous stirred tank or fluidized bed reactors. Continuous reactors generally offer the advantage of low materials inventory and reduced variation of operating parameters. Recycle of reactants, products or of diluent is often used with continuous reactors, possibly in conjunction with an external heat exchanger. 

Recycling of partially reacted feed streams is usually carried out after the product is separated and recovered. Unreacted feedstock can be separated and recycled to (ultimate) extinction. Figure 4.2 shows a different situation. It is a loop reactor where some of the reaction mass is returned to the inlet without separation. Internal recycle exists in every stirred tank reactor. An external recycle loop as shown in Figure 4.2 is less common, but is used, particularly in large plants where a conventional stirred tank would have heat transfer limitations. The net throughput for the system is Q = but an amount q is recycled back to the reactor inlet so that the flow through the reactor is Qin + q- Performance of this loop reactor system depends on the recycle ratio qlQin and on the type of reactor that is in the loop. Fast external recycle has... 

The existence of three steady states, two stable and one metastable, is common for exothermic reactions in stirred tanks. Also common is the existence of only one steady state. For the styrene polymerization example, three steady states exist for a limited range of the process variables. For example, if Ti is sufficiently low, no reaction occurs, and only the lower steady state is possible. If Tin is sufficiently high, only the upper, runaway condition can be realized. The external heat transfer term, UAextiTout — Text in Equation (5.28) can also be used to vary the location and number of steady states. 

Example 5.8 Suppose that, to achieve a desired molecular weight, the styrene polymerization must be conducted at 413 K. Use external heat transfer to achieve this temperature as the single steady state in a stirred tank. 

Therefore, many traditional designs, such as stirred tank reactors, incorporate heat transfer in the process (jacket, external or internal coil, etc.). However, in these devices, there is a significant distance between the heat transfer site and the site of the chemical reaction where heat is released. As a consequence semibatch mode is implemented while batch mode and/or systems are diluted. 

As will be shown later the equation above is identical to the mass balance equation for a continuous stirred-tank reactor. The recycle can be provided either by an external pump as shown in Fig. 5.4-18 or by an impeller installed within the reaction chamber. The latter design was proposed by Weychert and Trela (1968). A commercial and advantageously modified version of such a reactor has been developed by Berty (1974, 1979), see Fig. 5.4-19. In these reactors, the relative velocity between the catalyst particles and the fluid phases is incretised without increasing the overall feed and outlet flow rates. 

Heat can be added to or removed from stirred-tank reactors via external jackets (Figure 7.5a), internal coils (Figure 7.5b) or separate heat exchangers by means of a flow loop (Figure 7.5c). Figure 7.5d shows vaporization of the contents being condensed and refluxed to remove heat. A variation on Figure 7.5d would not reflux the evaporated... 

All reactor modes sometimes can be operated advantageously with recycle of part of the product or intermediate streams. When the recycle is heated or cooled appropriately it can serve to moderate undesirable temperature travel. This function is well served with pumparound from a stirred tank through an external heat exchanger. Recycle streams also can be processed for changes in composition before return. 

The standard esterification reactor is a stirred tank reactor. Due to the required latent heat for the evaporation of EG and water, heating coils are installed in addition to the heating jacket. In some cases, an external heat exchanger, together with a recirculation pump, is necessary to ensure sufficient heat transfer. During esterification, the melt viscosity is low to moderate (ca. 20 to 800mPas) and no special stirrer design is required. 

Isolated rat hepatocytes were immobilized in cellulose multiporous microcarriers by Kino et al. [24]. The microcarriers had a pore size of 100 pm and protected the cells from external shear stress. A newly developed stirred tank reactor contained the microcarrier-immobilized hepatocytes. The 02-supply was improved by using an oxygenator. The performance of microcarrier-im-mobihzed hepatocytes in the reactor was as good as that in floating culture and they demonstrated good ammonia metaboUsm. 

The rates of heat transfer between the fermentation broth and the heat-transfer fluid (such as steam or cooling water flowing through the external jacket or the coil) can be estimated from the data provided in Chapter 5. For example, the film coefficient of heat transfer to or from the broth contained in a jacketed or coiled stirred-tank fermentor can be estimated using Equation 5.13. In the case of non-Newtonian liquids, the apparent viscosity, as defined by Equation 2.6, should be used. 

There are two particularly common, and technically very important, manifestations of open systems and which have become increasingly used in the study of oscillatory processes. These are (i) the chemical engineers continuously fed well-stirred tank reactor (CSTR) and (ii) cells in which there is transport across a boundary or membrane from an external reservoir and in which diffusional processes are usually important. 

Figure 17.9. Stirred tank reactors, batch and continuous, (a) With agitator and internal heat transfer surface, batch or continuous, (b) With pumparound mixing and external heat transfer surface, batch or continuous, (c) Three-stage continuous stirred tank reactor battery, (d) Three-stage continuous stirred tank battery in a single shell.

Figure 17.33. Heat transfer to stirred-tank reactors (a) jacket (b) internal coils (c) internal tubes (d) external heat exchanger (e) external reflux condenser (f) fired heater (Walas, 1959).

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