Reactors, continuous backmix batch

If either the batch or the tubular type of reactor is chosen, the reactor size and product distribution can be calculated by using the batch or longitudinal-flow equations. For a stirred-tank continuous reactor, the backmixing equations can be used. If a packed or baffled tower is used, then the calculations must be made for both the longitudinal and back-mixing cases. Proper extrapolation must then be made from empirical data or previous experience. 

In section 3.4 the selectivities of competitive and consecutive reactions were calculated for two reactor types the batch or plug flow reactor (no backmixing), and the perfectly mixed continuous reactor. The following general conclusions can be formulated in terms of backmixing ... 

Example 9.11 Which type of isothermal reactor would produce the narrowest possible distribution of chain lengths in a free-radical addition polymerization continuous stirred tank reactor (CSTR, or backmix), batch (assume perfect stirring in each of the previous), plug-flow tubular, or laminar-flow tubular ... 

A continuous flow stirred tank reactor (CFSTR) differs from the batch reactor in that the feed mixture continuously enters and the outlet mixture is continuously withdrawn. There is intense mixing in the reactor to destroy any concentration and temperature differences. Heat transfer must be extremely efficient to keep the temperature of the reaction mixture equal to the temperature of the heat transfer medium. The CFSTR can either be used alone or as part of a series of battery CFSTRs as shown in Figure 4-5. If several vessels are used in series, the net effect is partial backmixing. 

The yield that can be attained by a semibatch process is generally higher because the semibatch run starts from scratch, with maximum values of both variables Cg (o) = Cg and k] (o) = k . However, the yield from a continuous run in which t equals the batch time is governed by the product of Cg (t) and kj (t), so > and k (t) = k °. Because neither of these conditions is likely to be fulfilled completely, a continuous polymerization in a backmix reactor will probably always fail to attain the Y attainable by a semibatch reactor at the same t. However, several backmix reactors in series will approach the behavior of a plug flow continuous reactor, which is equivalent to a semibatch reactor. 

In this chapter, we describe several ideal types of reactors based on two modes of operation (batch and continuous), and ideal flow patterns (backmix and tubular) for the continuous mode. From a kinetics point of view, these reactor types illustrate different ways in which rate of reaction can be measured experimentally and interpreted operationally. From a reactor point of view, the treatment also serves to introduce important concepts and terminology of CRE (developed further in Chapters 12 to 18). Such ideal reactor models serve as points of departure or first approximations for actual reactors. For illustration at this stage, we use only simple systems. 

Finally, some remarks on the operation of mechanically agitated gas-liquid reactors are worth mentioning. The mode of operation (i.e., batch, semibatch, continuous, periodic, etc.) depends on the specific need of the system. For example, the level of liquid-phase backmixing can be controlled to any desired level by operating the gas-liquid reactor in a periodic or semibatch manner. This provides an alternative to the tanks in series or plug flow with recycle system and provides a potential method of increasing the yield of the desired intermediate in complex reaction schemes. In some cases of industrial importance, the mode of operation needs to be such that the concentration of the solute gas (such as Cl2, H2S, etc.) as the reactor outlet is kept at a specific value. As shown by Joshi et al. (1982), this can be achieved by a number of different operational and control strategies. 

When a monomer split-feed operation based on the experimental result shown in Fig. 32 was applied, for example, to a continuous tubular pre-reactor with some backmixing, the number of polymer particles increased by about 30% at Mpi=0.02 g/cm -water, compared to the number produced in a batch reactor, as shown in Fig. 33. 

Compared to batch processes, continuous processes often show a higher space-time yield. Reaction conditions may be kept within certain limits more easily. For easier scale-up of some enzyme-catalyzed reactions, the Enzyme Membrane Reactor (EMR) has been developed. The principle is shown in Fig. 7-26 A. The difference in size between a biocatalyst and the reactants enables continuous homogeneous catalysis to be achieved while retaining the catalyst in the vessel. For this purpose, commercially available ultrafiltration membranes are used. When continuously operated, the EMR behaves as a continuous stirred tank reactor (CSTR) with complete backmixing. For large-scale membrane reactors, hollow-fiber membranes or stacked flat membranes are used 129. To prevent concentration polarization on the membrane, the reaction mixture is circulated along the membrane surface by a low-shear recirculation pump (Fig. 7-26 B). 

To understand the underlying principles of these reactors, which are treated in Parts III and IV, we provide the groundwork in this section by considering the basic reactor types. Thus we first describe the batch reactor (BR) most commonly used in organic synthesis and its continuous counterpart, the plug-flow reactor (PFR). These represent one extreme characterized by total absence of backmix-ing from fluid elements downstream in time or space. On the other extreme, we have the fully (or perfectly) mixed-flow reactor (MFR), also called the... 

Compare and contrast batch, continuous, and semi-batch reactors from the point of view of backmixing. 

Very high degrees of conversion (of end groups) are required, which means that either a batch reactor has to be used, or a continuous reactor with extremely low backmixing. 

It was shown that the residence time distribution in the reactor may have a considerable effect, since this influences the concentration profiles of the reactants in time, TTiere are significant differences between batch (or plug flow) reactors and continuous mixed reactors. When the undesired reaction is of a higher order, the CSTR has the highest selectivity. For reactors with other residence time distributions, the qualitative effect of backmixing was discussed in section 7,2.1 A, Quantitative effects can be computed by numerical methods. 

For batch systems a stirred vessel or loop reactor with an in-line mixer is used. Where plug flow is required, for long residence times a cascade of stirred vessels or loop reactors is commonly used, and for short residence times the choice will often be a static mixer or ejectors. For continuous flow systems requiring an approach to backmixed flow, stirred vessels or loop reactors are indicated. 

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