Types of Reactors

The selection of a bioreactor and operation mode starts with the type of biocatalysts (enzymes or whole cells) that will be used to produce the desired product, and if the biocalalysts will be suspended or immobilized with a recycle option. Each of these possibihties should be examined while taking into consideration the reactions stoichiometry and kinetics, mixing needs, gas exchange, heat removal and capital and operating costs (Merchuk and Asenjo, 1995). 

For example, different fermentation schemes have been developed for the production of ethanol. Conventional batch, continuous, cell recycle and immobilized cell processes, as well as membrane, extraction and vacuum processes, which selectively remove ethanol from the fermentation medium as it is formed, were compared on identical bases using a consistent model for yeast metabolism (Maiorella et al., 1984). The continuous flow stirred tank reactor (CSTR) with cell recycle, tower and plug flow reactors all showed similar cost savings of about 10% compared to batch fermentation. Cell recycle increases cell density inside the fermentor, which is important in reducing fermentation cost. 

Several bioreactors of interest are discussed in the next section, including the stirred tank reactor (STR), bubble column, hollow fiber and monolithic reactors. 

The temperature inside a large bioreactor is maintained by heating and cooling coils placed in the vessel. However, a heating jacket maintains the operating temperature for small bioreactors. 

High agitation and aeration rates can result in foam formation inside the bioreactor, which can block the exhaust gas line and cause loss of cells and media and possible contamination when the foam is washed out from the bioreactor. Mechanical foam breakers and antifoam chemicals are used to control foam formation inside the bioreactor. The working volume of the bioreactor is usually 70-80% of the vessels physical volume. 

When a high liquid hold-up is required in the reactor, one of the types shown in Fig. 4.1 d, e and / may be used. The bubble column d is simply a vessel filled with liquid, with a sparger ring at the base for dispersing the gas. In some cases a draught 

Equations for Mast Transfer with Chamical Reaction 

In designing a gas-liquid reactor, there is the need not only to provide for the required temperature and pressure for the reaction, but also to ensure adequate interfacial area of contact between the two phases. Although the reactor as such is 

The absorption of a gas by a liquid with simultaneous reaction in the liquid phase is the most important case. There are several theories of mass transfer between two fluid phases (see Volume 1, Chapter 10 Volume 2, Chapter 12), but for the purpose of illustration the film theory will be used here. Results from the possibly more realistic penetration theory are similar numerically, although more complicated in their mathematical form0,4.  

Consider a second order reaction in the liquid phase between a substance A which is transferred from the gas phase and reactant B which is in the liquid phase only. The gas will be taken as consisting of pure A so that complications arising from gas film resistance are avoided. The stoichiometry of the reaction is represented by  

Tubular reactor The contact time is the same for all molecules or fluid elements along the reactor when the velocity is uniform in the cross section of the tube, satisfying the plug flow. All molecules have the same velocity. Therefore, the concentration is uniform in a cross section of the tube and varies only along the reactor. In the isothermal case, the temperature remains constant in the longitudinal and radial directions. In the nonisothermal case, the temperature varies along the reactor. This reactor will be denominated ideal PFR (plug flow reactor). 

Tank reactor The molecules should have the same mean residence time in the tank. Therefore, the concentration inside the tank should be equal to the concentration at the reactor outlet, implying in a uniform and perfect mixture. To reach a perfect mixture, dead volume must be avoided so that the mean residence time is uniform. A reactor in these conditions will be an ideal CSTR (continuous stirred-tank reactor). 

Batch reactor The mixture must be perfect leading to a homogeneous concentration throughout the reactor volume. The reactor should be well stirred and dead volume cannot be present. The temperature is also uniform. 

For purposes of kinetic modeling, it is important to collect reaction rate data that are free from experimental artifacts. Various types of reactors can be used to acquire these data, and the first portion of this chapter discusses these reactors. The second half of the chapter describes models which introduce the effect of mass and heat transfer gradients on the observed reaction rate, and it then provides different methods to evaluate the presence or absence of such artifacts in both gas-phase and liquid-phase reactions involving porous catalysts. 

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Before we can explore how reactor conditions can be chosen, we require some measure of reactor performance. For polymerization reactors, the most important measure of performance is the distribution of molecular weights in the polymer product. The distribution of molecular weights dictates the mechanical properties of the polymer. For other types of reactors, three important parameters are used to describe their performance ... 

In the preceding section, the choice of reactor type was made on the basis of which gave the most appropriate concentration profile as the reaction progressed in order to minimize volume for single reactions or maximize selectivity for multiple reactions for a given conversion. However, after making the decision to choose one type of reactor or another, there are still important concentration effects to be considered. 

Even after the type of reactor is chosen, excess of FEED 1 or FEED 2 can be used ... 

The unit Kureha operated at Nakoso to process 120,000 metric tons per year of naphtha produces a mix of acetylene and ethylene at a 1 1 ratio. Kureha s development work was directed toward producing ethylene from cmde oil. Their work showed that at extreme operating conditions, 2000°C and short residence time, appreciable acetylene production was possible. In the process, cmde oil or naphtha is sprayed with superheated steam into the specially designed reactor. The steam is superheated to 2000°C in refractory lined, pebble bed regenerative-type heaters. A pair of the heaters are used with countercurrent flows of combustion gas and steam to alternately heat the refractory and produce the superheated steam. In addition to the acetylene and ethylene products, the process produces a variety of by-products including pitch, tars, and oils rich in naphthalene. One of the important attributes of this type of reactor is its abiUty to produce variable quantities of ethylene as a coproduct by dropping the reaction temperature (20—22). 

Wet Oxidation Reactor Design. Several types of reactor designs have been employed for wet oxidation processes. Zimpro, the largest manufacturer of wet oxidation systems, typically uses a tower reactor system. The reactor is a bubble tower where air is introduced at the bottom to achieve plug flow with controlled back-mixing. Residence time is typically under one hour. A horizontal, stirred tank reactor system, known as the Wetox process, was initially developed by Barber-Cohnan, and is also offered by Zimpro. 

The feature that is most usefiil in distinguishing commercial methanol processes from one another is the type of reactor used. The four basic types in use ate shown in Figure 7. There are a variety of proprietary reactor designs commercially available from Hcensors, all of which are either one of these four types or a combination of two among them (17—22). 

Herein reactors are described in their most prominent appHcation, that of electric power. Eive distinctly different reactors, ie, pressurized water reactors, boiling water reactors, heavy water reactors, graphite reactors, and fast breeder reactors, are emphasized. A variety of other appHcations and types of reactors also exist. Whereas space does not permit identification of all of the reactors that have been built over the years, each contributed experience of processes and knowledge about the performance of materials, components, and systems. 

Among continuous reactors, the dominant system used to produce parasubstituted alkylphenols is a fixed-bed reactor holding a soHd acid catalyst. Figure 3 shows an example of this type of reactor. The phenol and alkene are premixed and heated or cooled to the desired feed temperature. This mix is fed to the reactor where it contacts the porous soHd, acid-impregnated catalyst. A key design consideration for this type of reactor is the removal of the heat of reaction. 

Most reactors have evolved from concentrated efforts focused on one type of reactor. Some processes have emerged from parallel developments using markedly different reactor types. In most cases, the reactor selected for laboratory study has become the reactor type used industrially because further development usually favors extending this technology. Descriptions of some industrially important petrochemical processes and their reactors are available (74—76). Following are illustrative examples of reactor usage, classified according to reactor type. 

Catalysts intended for different appHcations may require their own unique types of reactor and operating conditions, but the key to designing a successful system is to use the same feedstock composition that is expected in the ultimate commercial installation and to impose so far as is possible the same operating conditions as will be used commercially (35). This usually means a reactor design involving a tubular or smaH-bed reactor of one type or another that can simulate either commercial multitubular reactors or commercial-size catalyst beds, including radial flow reactors. 

FIG. 12-85 Perforated-tray type of reactor-discharge control. 

Type of Reactor The specific type of reac tor that is most compatible (or least incompatible) with the CTiosen combination of the preceding parameters seldom is clearly and unequivocally perceived without difficulty, if at all. In the end, however, that remains the objective. As is always true, the ultimate criteria are rehabihty and profitability. 

Many successful types of reactors are illustrated throughout this section. Additional sketches may be found in other books on this topic, particularly in Walas Chemical Process Equipment Selection and Design, Butterworths, 1990) and Ullmann Encyclopedia of Chemical Technology (in German), vol. 3, Verlag Chemie, 1973, pp. 321-518). 

Material and energy balances of common types of reactors are summarized in several tables of Sec. 7. For review purposes some material balances are restated here. For the /ith stage of a CSTR batteiy,... 

The three main types of reactors shown in Fig. 27-6 are in aclual commercial use the moving bed, the fluidized bed, and the entrained bed. The moving bed is often referred to as a. fixed bed because the coal bed is kept at a constant height. These differ in size, coal feed, reactant and product flows, residence time, and reaction temperature. 

I Shunt reactors These are provided as shown in Figure 24.23 to compensate for the distributed lumped capacitances, C , on EHV networks and also to limit temporary overvoltages caused during a load rejection, followed by a ground fault or a phase fault within the prescribed steady-state voltage limits, as noted in Table 24.3. They ab.sorb reactive power to offset the charging power demand of EHV lines (Table 24.2, column 9). The selection of a reactor can be made on the basis of the duty it has to perform and the compensation required. Some of the different types of reactors and their characteristics are described in Chapter 27. 

Design criterion and l-iji characteristics of different types of reactor 27/848... 

These types of reactors can now be used as current limiting reactors and also as harmonic suppressors. They are also recommended for capacitor application due to their linear characteristic which will not disturb the tuning of the filter circuit. 

These are the most successful types of reactors presently available. The Internal reciprocating plunger types, for example, that of Nelles in Jankowski et al (1978), do not provide a steady uniform flow. Of those operating with rotating blowers or turbines, the best known are those of Garanin et al (1967), Brown and Bennett (1972), Livbjerg and Villadsen (1971). These and that of Rbmer and Luft (1974) are shown on Figures 2.4.2 a-d. 

Hinton, Sir C., The place of the Calder Hall type of reactor in nuclear power generation, J. Brit. Nucl. Energy Conf, 1957, 2, 43 46. 

In the case of thermodynamics, the designer can investigate the nature of the reaction heat and whether the reaction is reversible. If these exothermic reactions are irreversible, attention may be focused on the influence of reactor design on conversion and with heat transfer control. An objective of reactor design is to determine the size and type of reactor and mode of operation for the required job. The choice... 

In general, the optimum conditions cannot be precisely attained in real reactors. Therefore, the selection of the reactor type is made to approximate the optimum conditions as closely as possible. For this purpose, mathematical models of the process in several different types of reactors are derived. The optimum condition for selected parameters (e.g., temperature profile) is then compared with those obtained from the mathematical expressions for different reactors. Consequently, selection is based on the reactor type that most closely approaches the optimum. 

The name continuous flow-stirred tank reactor is nicely descriptive of a type of reactor that frequently for both production and fundamental kinetic studies. Unfortunately, this name, abbreviated as CSTR, misses the essence of the idealization completely. The ideality arises from the assumption in the analysis that the reactor is perfectly mixed, and that it is homogeneous. A better name for this model might be continuous perfectly mixed reactor (CPMR). 

Although the Westinghouse s PWR Shippingport reactor was the first LW R to generate electricity in the U.S., GE s BWR Dresden 1 reactor followed within a year. Operating power reactors range from 600 to 1,200 MWe (million watts of electric power). Since the thermodynamic efficiency is -33%, the thermal heat production is 1,800 to 3,600 MWt. Both types of reactor operate at about the same temperature (-bOOT),... 

Its unique design suggests several accident scenarios that could not occur at other reactors. For example, failure to supply ECC to 1/16 of the core due to the failure of an ECC inlet valve. On the other hand, some phenomena of concern to other types of reactors seem impossible (e.g., core-concrete interactions). The list of phenomena for consideration came from previous studies, comments of an external review group and from literature review. From this, came the issues selected for the accident progression event tree (APET) according to uncertainty and point estimates. 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

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