Batch product-removal process

Figure 12.1 Continuous and batch processes (a) continuous process (b) batch process (c) fed-batch process (d) batch-product removal process.

Figure 12.1c. In batch-product removal, the chemicals are fed to the process before processing begins and steps 2 and 3 are combined that is, the product is removed continuously as the processing occurs, as shown in Figure 12.Id. In effect, fed-batch and batch-product removal processes are semicontinuous processes.

The challenge in designing a batch, fed-batch, or batch-product removal process is in deciding on the size of the vessel and the processing time. This is complicated for the latter two processes where the flow rate and concentration of the feed stream or the flow rate of the product stream as a function of time strongly influence the performance of the process. Note that the determination of optimal operating profiles is referred to as the solution to the optimal control problem. This subject is introduced in the next section. 

Usually, for the production of small quantities of high-priced chemicals, such as in th manufacture of pharmaceuticals, foods, electronic materials, and specialty chemicals, batch fed-batch, and batch-product removal processes are preferred. This is often the case in bio processing, for example, when drugs are synthesized in a series of chemical reactions, eacl having small yields, and requiring difficult separations to recover small amounts of product This is also the case for banquet facilities in hotels, which prepare foods in batches, and fo many unit operations in the manufacture of semiconductors. As discussed in Chapters 3 am 4, these processes usually involve a recipe, that is, a sequence of tasks, to be carried out h various items of equipment. In the latter sections of this chapter, variations on batch proces schedules are discussed, as well as methods for optimizing the schedules. 

Solution Polymerization. In this process an inert solvent is added to the reaction mass. The solvent adds its heat capacity and reduces the viscosity, faciUtating convective heat transfer. The solvent can also be refluxed to remove heat. On the other hand, the solvent wastes reactor space and reduces both rate and molecular weight as compared to bulk polymerisation. Additional technology is needed to separate the polymer product and to recover and store the solvent. Both batch and continuous processes are used. 

Biocatalysts in nature tend to be optimized to perform best in aqueous environments, at neutral pH, temperatures below 40 °C, and at low osmotic pressure. These conditions are sometimes in conflict with the need of the chemist or process engineer to optimize a reaction with respect to space-time yield or high product concentration in order to facilitate downstream processing. Furthermore, enzymes and whole cells are often inhibited by products or substrates. This might be overcome by the use of continuously operated stirred tank reactors, fed-batch reactors, or reactors with in situ product removal [14, 15]. The addition of organic solvents to increase the solubility of substrates and/or products is a common practice [16]. 

All processes may be classified as batch, continuous, or semibatch depending on how materials are transferred into and out of the system. Also, the process operation may be characterized as unsteady state (i.e., transient) or steady state, depending on whether the process variables (e.g., pressure, temperature, compositions, flowrate, etc.) are changing with time or not, respectively. In a batch process, the entire feed material (i.e., charge) is added instantaneously to the system marking the beginning of the process, and all the contents of the system including the products are removed at a later time, at the end of the process. In a continuous process, the materials enter and leave the system as continuous streams, but not necessarily at the same rate. In a semibalch process, the feed may be added at once but the products removed continuously, or vice versa. It is evident that batch and semibatch processes are inherently unsteady state, whereas continuous processes may be operated in a steady or unsteady-state mode. Start-up and shut-down procedures of a steady continuous production process are examples of transient operation. 

In many cases, problems cannot be overcome by biological means. This is especially true for those related to inhibition by substrate or product. There may, however, be technical solutions to these problems. Nowadays, complicated feed strategies with different substrates can be achieved through the use of flow injection analysis, on-line sensors, mass flow meters and sophisticated computer control. Such control coupled to a fed-batch mode of operation (Figure 2.5) can often eleviate problems caused by substrate inhibition. For some processes, continuous product removal can avoid the problems associated with product inhibition the various options include ... 

Due to the fact that it is assumed that a washout occurs directly after product is removed, two consecutive batches of product will be separated by a washout. In terms of time points, a unit can only start processing a batch two time points after the first batch starts. This is due to the fact that three time points are used to describe the batch processing and unit cleaning operations. Furthermore, the assumption that a cleaning operation follows product removal negates the need for a separate binary variable to represent the cleaning operation in a unit. Therefore, constraint (8.20) is included to ensure that two product producing tasks do not occur in consecutive time points. 

Figure 4.14 Chemical adducts for by-product removal synthesis of isopropyl-cis-Ag-hexadecanoate from isopropylpalmitate applying a repetitive batch process using a sequence of stirred-tank reactor, extraction module, filtration step and chromatographic downstream processing...

A type of processing equipment in which the entire amount of material to be used is put into the mixer and mixed for a definite period, with multiple recirculation of material through the mixing zone, in contrast to what happens in a continuous mixer. After the mixing period the whole amount of material is removed from the mixer. In oil production or processing, the process in which emulsion is collected in a tank and then broken in a batch. This process is used as opposed to continuous or flow-line treating of emulsions. 

A gasifier is a continuous gas process in conjunction with either a batch of solids or continuous solids feed and product removal. The gas phase passing up through the bed obeys plug flow behavior. In continuous solids handling, the bed is fed from the top and emptied from the bottom. These solids also obey plug flow assumptions with flow countercurrent to the gas phase. 

Catalytic reactions can take place in either the liquid or vapor phase. Liquid phase reactions can be run in either a continuous manner or as a batch process while vapor phase reactions are run only in a continuous mode. In a batch reaction the catalyst, reactants, and other components of the reaction mixture are placed in an appropriate reaction vessel, the reaction is run and the products removed from the vessel and separated from the catalyst. In a continuous system the reactants are passed through the catalyst and the products removed at the same rate as the reactants are added. The applicability of vapor phase processes is limited by the volatility and thermal stability of the reactants and products so such processes are not commonly involved in the preparation of even moderately complex molecules. Because of this, primary attention will be placed here on liquid phase processes with vapor phase systems of secondary importance. A discussion of the different types of reactors used for each of these processes is found in the following chapter. The present discussion is concerned with the effect that the different reaction parameters can have on the outcome of a catalytic reaction. 

Since the processes dealt with are batch processes the inherent discontinuous nature has to be taken into account. Scheduling constraints are formulated to ensure that the processing of raw material occurs after a washout has taken place, i.e. once a vessel has been cleaned, and after a previous batch has finished processing. Further constraints ensure a washout begins directly after the product has been removed from a processing vessel. Duration constraints are formulated for the production and washouts. 

Crystal growth is a layer-by-layer process, and the retention time required in most commercial equipment to produce crystals of the size normally desired is often on the order of 2 to 6 h. Growth rates are usually limited to less than 1 to 2 pm/min. On the other hand, nucle-ation in a supersaturated solution can be generated in a fraction of a second. The influence of any upsets in operating conditions, in terms of the excess nuclei produced, is very short-term in comparison with the total growth period of the product removed from the crystallizer. A worst-case scenario for batch or continuous operation occurs when the explosion of nuclei is so severe that it is impossible to grow an acceptable crystal size distribution, requiring redesolution or washout of the system. In a practical sense, this means that steadiness of operation is much more important in crystallization equipment than it is in many other types of process equipment. 

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. 

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