Optimization of Batch Processes

Whether parallel operations, larger or smaller items of equipment and intermediate storage should be used can only be judged based on economic trade-offs. However, this is still not the complete picture as far as the batch process trade-offs are concerned. So far the batch size has not been varied. Batch size can be varied as a function of cycle time. Overall, the variables are  

In addition to these variables, which result from the batch nature of the process, there are still the variables considered earlier for continuous processes  

All of these variables must be varied in order to minimize the total cost or maximize the economic potential (see Chapter 2). This is a complex optimization problem involving both continuous variables (e.g. batch size) and integer variables (e.g. number of units in parallel) and is outside the scope of the present text9. 

However, the designer should not loose sight of the fact that, whilst schedule planning and optimization provides the basis for the design of batch processes, production schedules are often disrupted once production has commenced. For example, what happens if a key item of equipment breaks down and needs to be repaired What happens if 

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Section 3 covers the synthesis of a chemical process. The minimum information required to simulate a process is given, as are the basics of using a process simulator. The choice of the appropriate thermodynamic model to use in a simulation is covered, and the choice of separation operations is covered. In addition, process optimization (including an introduction to optimization of batch processes) and heat integration techniques are covered in this section. 

Unlike continuous systems, batch operations do not run under steady-state conditions, and their performance varies with time. As discussed in Chapter 2, the important issues with batch systems are the optimal scheduling of different equipment to produce a variety of products and the determination of optimal cycle times for batch processes. Therefore, the optimization of batch operations often involves determining the best processing time for a certain operation, the best time at which a certain action should take place, or the best distribution of actions over a period of time. The optimization of batch processes is, in itself, a very broad topic and certainly beyond the scope of this section of this chapter. Rather than try to address the many interesting problems in this field, the approach here is to illustrate several inportant concepts through the use of exanples. The interested reader is encouraged to read further into this subject [7-101. 

Finally, an introduction to the optimization of batch processes was givem Specifically, methods to determine the optimal mix of products in a multiproduct batch facility and the determination of optimum cycle time were covered. 

B. Srinivasan, S. Palanki, and D. Bonvin. Dynamic optimization of batch processes i. characterization of the nominal solution. Comp. Chem. Eng, 27 1,... 

The purification of value-added pharmaceuticals in the past required multiple chromatographic steps for batch purification processes. The design and optimization of these processes were often cumbersome and the operations were fundamentally complex. Individual batch processes requires optimization between chromatographic efficiency and enantioselectivity, which results in major economic ramifications. An additional problem was the extremely short time for development of the purification process. Commercial constraints demand that the time interval between non-optimized laboratory bench purification and the first process-scale production for clinical trials are kept to a minimum. Therefore, rapid process design and optimization methods based on computer aided simulation of an SMB process will assist at this stage. 

Stefanis, S.K., Livingston, A.G., Pistikopoulos, E.N. (1997) Environmental Impact Considerations in the Optimal Design and Scheduling of Batch Processes. Computers in Chemical Engineering, 21(10), 1073-1094. 

In the analysis, synthesis and optimization of batch plants complexity arises from the various operational philosophies that are inherent in time dependent processes. In a situation where the intermediate is allowed to wait in the same unit from which it is produced until the next unit is available, the operational philosophy is commonly known as no intermediate storage (NIS) operational philosophy. This philosophy is depicted in Fig. 1.3. NIS operational philosophy is usually adopted if operational space is of essence, since intermediate storage tanks can occupy considerable area. 

Gouws, J., Majozi, T., 2008. Impact of multiple storage in wastewater minimisation for multicontaminant batch plants towards zero effluent. Ind. Eng. Chem. Res., 47 369-379 Majozi, T., Zhu, X., 2001. A novel continuous-time MILP formulation for multipurpose batch plants. 1. Short-term scheduling. Ind. Eng. Chem. Res. 40(23) 5935-5949 Quesada, I., Grossmann, I. E., 1995. Global optimization of bilinear process networks with multicomponent flows. Comput. Chem. Eng. 19 1219-1242... 

Kemp, I.C., Deakin, A.W., 1989. The cascade analysis for energy and process integration of batch processes, Part 1, Chem. Eng. Res. Des., 67 495-509 Kiperstok, A., Sharratt, P.N., 1995. On the optimization of mass exchange networks for the removal of pollutants. Trans. IChemE, 73b 271-277... 

Yao, Z.L., Yuan, X.G., 2000. An approach to optimal design of batch processes with waste minimization. Comput. Chem. Eng., 24 1437-1444. 

Similarly, whole-cell Lactobacillus kefir DSM 20587, which possesses two alcohol dehydrogenases for both asymmetric reduction steps, was applied in the reduction of tert-butyl 6-chloro-3,5-dioxohexanoate for asymmetric synthesis of ft rf-butyl-(31 ,5S)-6-chloro-dihydroxyhexanoate (Figure 7.5), a chiral building block for the HMG-CoA reductase inhibitor [ 17]. A final product concentration of 120 him and a specific product capacity of 2.4 mmol per gram dry cell were achieved in an optimized fed-batch process. Ado 99% was obtained for (3R,5S)- and (3.S, 55)-te/ f-butyl-6-chloro-dihydroxyhexanoate with the space-time yield being 4.7 mmolL-1 h-1. 

MILP Optimization Models for Short-term Scheduling of Batch Processes... 

It is the objective of this paper to provide a comprehensive review of the state-of-the art of short-term batch scheduling. Our aim is to provide answers to the questions posed in the above paragraph. The paper is organized as follows. We first present a classification for scheduling problems of batch processes, as well as of the features that characterize the optimization models for scheduling. We then discuss representative MILP optimization approaches for general network and sequential batch plants, focusing on discrete and continuous-time models. Computational... 

State-of-the-art review of optimization methods for short-term scheduling of batch processes. 

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