Batch operation economics

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Reichsteia and Grbssner s second L-ascorbic acid synthesis became the basis for the iadustrial vitamin C production. Many chemical and technical modifications have improved the efficiency of each step, enabling this multistep synthesis to remain the principal, most economical process up to the present (ca 1997) (46). L-Ascorbic acid is produced ia large, iategrated, automated faciUties, involving both continuous and batch operations. The process steps are outlined ia Figure 7. Procedures require ca 1.7-kg L-sorbose/kg of L-ascorbic acid with ca 66% overall yield ia 1977 (55). Siace 1977, further continuous improvement of each vitamin C production step has taken place. Today s overall ascorbic acid yield from L-sorbose is ca 75%. In the mid-1930s, the overall yield from L-sorbose was ca 30%. 

If the product in no way adheres to the dryer parts and simple cyclone collectors are sufficient for gas-sohds separation, batch operation of a spray dryer may be considered. Otherwise, the time and costs for cleaning the large equipment parts make them rarely economical for other than continuous processing of a single material. 

The batch handling of wet or semidry crystalline materials is substantially more difficult than the storing and handling of dry crystalline materials. A batch operation has economic application only on a relatively small scale or when temperature or product characteristics require unusual precautions. 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

Storage and receiving are activities that can greatly contribute to a safe and economic operation. It is here that quality control can be achieved at minimal cost. Label verification and other quality assurance measures can increase the confidence level that the correct chemicals have arrived, thereby potentially circumventing the use of wrong chemicals. Wrongly shipped chemicals can be returned to the manufacturer with minimal or no cost to the batch operation owner. As with all processes and activities it is of great importance to apply the principles of inherent safety, in particular the minimization and attenuation principles (CCPS G- 41). 

Often pilot plant or research data for developing a process are obtained on a batch operation. Later, a continuous process will usually prove that smaller equipment can be used and that the operation. vill be more economical. Normally batch mixing requires 10%-25% more power than continuous [29] for stable conditions how ev-er, the reaction time for continuous flow is always longer than the reaction time for batch flow, but the practical result may show batch time cycle is increased by filling,... 

When the specified production capacities are low, processes based on batch reactors will usually have lower capital investment requirements than processes calling for continuous operation, so batch reactors are often preferred for new and untried processes during the initial stages of development. As production requirements increase in response to market demands, it may become more economic to shift to continuous processing but, even in these cases, there are many industrial situations where batch operation is preferable. This is particularly true when the operating expenses associated with the reactor are a minor fraction of total product cost. At low production capacities, construction and instrumentation requirements for batch reactors are usually cheaper than for continuous process equipment. Moreover, it is generally easier to start up, shut down, and control a batch reactor than a comparable capacity continuous flow reactor. 

Batch reactors are widely used in the chemical industry for producing materials that are needed in limited quantity, particularly in those cases where the processing cost represents only a small fraction of the total value of the product. Since modern industry stresses the use of continuous processes because they lend themselves most readily to mass production, chemical engineers may, in some instances, tend to overlook the economic superiority of batch operations. One should not become so fascinated with the continuous process, or the more complex and interesting design analysis associated therewith, as to lose sight of the economic penalty exacted by this degree of technical sophistication. 

The anthraquinone vat dyes provide, in certain instances, the ultimate in fastness properties although the shades are often rather dull. The products are, however, frequently difficult to prepare commercially, requiring multistage small-batch operations and are thus less economically viable than related dye classes. Thus, they are used almost exclusively in high quality outlets of long life expectancy. An example of historic interest is the dye formed by condensation of two molecules of 1-aminoanthraquinone and one of cyanuric chloride (9) which was discovered in 1921 (Cl Vat Orange 18). The significance of the reactive... 

Semibatch reactors can be employed when parallel reactions of different orders occur. In such cases, it may be economical to use a semibatch rather than a batch operation. 

In the case of large batch-operated production plants, extensive CO2 recovery from the extractor during pressure reduction is necessary for economic reasons. Recovery of CO2 from the extractor after completion of the extraction phase is made in three steps ... 

A batch operation usually has economic application only on small scale, or when multiple products are produced in common facilities. 

Continuous and steady-state operation is usually most economical. However, when smaller quantities of material are processed, it is often more advantageous to charge the entire batch at once. In fact, in many cases this is the only way the process can be done. This is called batch operation and is a transient operation from start-up to shut-down. A batch operation presents a more difficult design problem. 

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