Batch reactors disadvantages

A semi-batch reactor has the same disadvantages as the batch reactor. However, it has the advantages of good temperature control and the capability of minimizing unwanted side reactions by maintaining a low concentration of one of the reactants. Semi-batch reactors are also of value when parallel reactions of different orders occur, where it may be more profitable to use semi-batch rather than batch operations. In many applications semi-batch reactors involve a substantial increase in the volume of reaction mixture during a processing cycle (i.e., emulsion polymerization). 

The disadvantages associated with the use of a batch reactor include the high labor and... 

In this chapter, we first consider uses of batch reactors, and their advantages and disadvantages compared with continuous-flow reactors. After considering what the essential features of process design are, we then develop design or performance equations for both isothermal and nonisothermal operation. The latter requires the energy balance, in addition to the material balance. We continue with an example of optimal performance of a batch reactor, and conclude with a discussion of semibatch and semi-continuous operation. We restrict attention to simple systems, deferring treatment of complex systems to Chapter 18. 

We focus mainly on the advantages and disadvantages of semibatch reactors. A semicontinuous reactor may be treated in many cases as either a batch reactor or a continuous reactor, depending on the overall kinetics and/or the phase of interest. 

In terms of cost and versatility the batch reactor is the unit of choice. One disadvantage is the need for frequent sampling or monitoring of the performance, although instrumentation can be provided at moderate cost nowadays. The residence time can be varied over a wide range and many different reactions can be handled at different times. The quality of mixing and heat transfer may not be easy to relate to those in an eventual commercial unit. 

First of all, before we compare flow reactors, let us mention the batch reactor briefly. The batch reactor has the advantage of small instrumentation cost and flexibility of operation (may be shut down easily and quickly). It has the disadvantage of high labor and handling cost, often considerable shutdown time to empty, clean out, and refill, and poorer quality control of the product. Hence we may generalize to state that the batch reactor is well suited to produce small amounts of material and to produce many different products from one piece of equipment. On the other hand, for the chemical treatment of materials in large amounts the continuous process is nearly always found to be more economical. 

The major disadvantage of batch reaction now is the hold-up time between batches. Although the actual reaction time necessary to process a given amount of feed may be substantially less than for a time-averaged reactor such as a CSTR, when the hold-up time is added, the total process time may be greater. Other disadvantages of the batch reactor are dependent on the particular type of reaction being considered, such as whether the reaction is in parallel or series. 

Perhaps the biggest disadvantage of a batch reactor is the difficulty encountered for isolation of intermediates. For series reactions such as A B C, where B is the desired product, it is difficult to stop the reaction (quench) without overshooting. 

For this purpose an analytical thin-layer cell and a laboratory-scale batch reactor were successfully used. The coupled system was controlled by the NADH concentration level. The major disadvantage of the process is the poor stability of the hydrogenase. 

Compared with batch reactors, tubular reactors have the advantage of easier heat removal or supply Heat release or consumption at the entry of a tube is as great as in a batch reactor at start, but the surface-to-volume ratio is more favorable, and the entering fluid can help to cool or heat. A disadvantage compared with a batch reactor is that a tube at steady state, like a CSTR, gives information only on the conversion achieved at the conditions of the respective experiment, whereas one single batch experiments with samples taken at frequent intervals scans the entire conversion range. 

Most work has focused on the use of fixed-and fluidized-bed reactors. Fixed-bed reactors are commonly used as batch reactors with the disadvantage of long ramp times. The choice of temperature and heating rate leads to strong differences in the product distribution. 

Batch polymerization reactors are ideal to manufacture small volume polymers, specialty polymers, and polymers that are difficult to make in continuous reactors. Emulsion polymers, suspension polymers, and precipitation polymers are mostly made by batch polymerization processes. One of the disadvantages of a batch reactor is that the ratio of heat transfer surface area to reactor volume decreases as the reactor size is increased. For many polymer products made in batch reactors, the process economy improves with an increase in reactor size. Therefore, effective heat removal becomes a critical factor in designing and controlling a large-scale batch polymerization reactor. 

Liquid-Phase Reactions. Semibateh reactors and CSTRs are used primarily for liquid-phase reactions. A semibatch reactor (Figure 1-15) has essentially Units the same disadvantages as the batch reactor. However, it has the advantages of... 

Bench-scale kinetic experiments can be conducted in batch, continuous stirred-tank (CSTR), tubular plug-flow, or differential reactors. The last of these can be operated with once-through flow or recycle. Advantages and disadvantages of the various types are discussed. In particular Batch reactors are inexpensive, but require attention to rapid attainment of reaction conditions at start CSTRs are excellent for gas-liquid, but less so for gas-phase reactions tubular reactors are especially suited for reactions of heterogeneous catalysis and differential reactors operated "once through" are best for measurement of initial rates. 

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