Multiperiod Operation

Iyer RR and Grossmann IE (1998) Synthesis and Operational Planning of Utility Systems for Multiperiod Operation, Comp Chem Eng, 22 979. 

Floudas, C. A., and Grossmann, I. E., Synthesis of flexible heat exchanger networks for multiperiod operation. Comp. Chem. Eng. 10, 153 (1986). 

In batch distillation, as the overhead composition varies during operation, a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (see Chapter 3). Purities of the main-cuts are usually determined by the market or downstream process requirements but the amounts recovered must be selected based on the economic trade off between longer distillation times (hence productivity), reflux ratio levels (hence energy costs), product values, etc. Increasing the recovery of a particular species in a particular cut may have strong effects on the recovery of other species in subsequent cuts or, in fact, on the ability to achieve at all the required purity specifications in subsequent cuts. The profitable operation of such processes therefore requires consideration of the whole (multiperiod) operation. 

Recall from Chapter 1 that, a single mixture (binary or multicomponent) can be separated into several products (single separation duty) and multiple mixtures (binary or multicomponent) can be processed, each producing a number of products (multiple separation duties) using only one CBD column thus leading to multiperiod operation in both cases. 

For single separation duty, Mujtaba and Macchietto (1993) proposed a method, based on extensions of the techniques of Mujtaba (1989) and Mujtaba and Macchietto (1988, 1989, 1991, 1992), to determine the optimal multiperiod operation policies for binary and general multicomponent batch distillation of a given feed mixture, with several main-cuts and off-cuts. A two level dynamic optimisation formulation was presented so as to maximise a general profit function for the multiperiod operation, subject to general constraints. The solution of this problem determines the optimal amount of each main and off cut, the optimal duration of each distillation task and the optimal reflux ratio profiles during each production period. The outer level optimisation maximises the profit function by... 

The dynamic optimisation problem formulation is illustrated for representative multiperiod operations. The STNs in Figures 6.1 and 6.2 for binary and ternary mixtures undergoing single separation duty describe the multiperiod operations (see Chapter 3). For other networks, mixtures with larger number of components and other constraints the problem formulation requires only simple modifications of that presented in this section. 

The optimisation problem formulation for the multiperiod operation given in Figure 6.1 can now be written as follows ... 

The reflux ratio is discretised into two time intervals for task 1 and one time interval for task 2. Thus a total of 3 reflux ratio levels and 3 switching times are optimised for the whole multiperiod operation. Three cases are considered, corresponding to different values of the main-cut 1 product. For each case the... 

Figure 7.7. Multiperiod Operating Sequences used by Logsdon et al.

For a fixed column design, Bonny et al. (1996) considered multiperiod operation optimisation but with multiple separation duties. That is why this is presented in this chapter rather than in Chapter 6. The optimisation problem can be stated as ... 

This example is taken from Mujtaba (1989) and Mujtaba and Macchietto (1992) where the same ternary mixture as in example 1 was considered for the whole multiperiod operation which includes 2 main-cuts and 2 intermediate off-cuts. The column consists of 5 (NT) intermediate plates, a total condenser and a reboiler. The column is charged with the same amount and composition of the fresh feed as was the case in example 1. Column initialisation, holdup distribution and condenser vapour load are also same as those in example 1. 

The minimum batch times for the individual cuts and for the whole multiperiod operation are presented in Table 8.8 together with the optimal amount of recycle and its composition for each cut. The percentage time savings using recycle policies are also shown for the individual cuts and also for the whole operation. Figure 8.18 shows the accumulated distillate and composition profile with and without recycle case for the operation. These also show the optimal reflux ratio profiles. Please see Mujtaba (1989) for the solution statistics for this example problem. 

Wajge and Reklaitis (1999) considered a simple single duty multiperiod operation using the hydrolysis reaction of acetic anhydride (Ullmann, 1985) to demonstrate the RBDOPT framework. Acetic anhydride is manufactured by dehydration of acetic acid. The by-product of the process is a mixture of acetic anhydride, water and acetic acid with boiling points 412 K, 373.2 K and 391.1 K respectively for each component in the mixture. The main concern is that the acetic anhydride and water in the mixture may hydrolyse quite quickly to produce back the acetic acid. 

Iyer R.R. and Grossmann I.E. 1998b. Synthesis of operational planning of utility systems for multiperiod operation, Comput. Chem. Eng., 22, 979-993. 

Several authors were addressed the synthesis and design of utility plants. Among these authors, Papoulias and Grossmann (1983) described a MILP model for the synthesis and design of utility systems, for fixed demands. Iyer and Grossmann described models for multiperiod operational planning (1997) and synthesis and operational planning (1998) of utility systems. 

Oliveira Francisco (2002) described a methodology for the synthesis and multiperiod operational planning of utility systems in a heat integrated industrial complex. This comprises a multiperiod model for utility systems including environmental concerns. The purpose of the present paper is to show the structure of this modified model and the resolution algorithm, applied to a simple example problem. 

Iyer, R. and Grossmann, I.E., 1997, Optimal Multiperiod Operational Planning for Utility Systems. Comput. Chem. Eng., 21(8), 787-800. 

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