Multiple Separation Duties

However, 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 (Logsdon et al., 1990 Mujtaba and Macchietto, 1996 Sharif et al., 1998). 

Figure 3.9. Multiple Separation Duties and Multiple Operational Alternatives...

Mujtaba and Macchietto (1996) extended the work or Mujtaba and Macchietto (1993) to include multiple separation duties. Logsdon et al. (1990) also considered multiple separation duties using short-cut model. Some of these works are presented in Chapter 7. 

Mujtaba and Macchietto (1996) presented a more general formulation for optimal design and operation, dealing in particular with multiple separation duties, multicomponent mixtures, more complex operations (involving off-cuts) and more general objective functions. The method utilises a dynamic model (Type IV, Chapter 4) of the column in the form of a generic system of DAEs. Models of various rigor (type III and V, etc. of Chapter 4) can therefore be used. 

Design and Operation Optimisation for Single and Multiple Separation Duties Problem Formulation and Solution... 

Mujtaba and Macchietto (1996) presented a general design and operation optimisation problem formulation and solution for single and multiple separation duties. 

Figure 7.3. Batch distillation Column with Multiple Separation Duty and Multiple Operational Alternatives. [Mujtaba and Macchietto, 1996]c...

The multiple separation duty specifications can be made in several ways. Two of which are ... 

Reprinted from Journal of Process Control, 6, Mujtaba, I.M. and Macchietto, S., Simultaneous optimisation of design and operation of multicomponent batch distillation column-single and multiple separation duties, 27-36, Copyright (1996), with permission from Elsevier Science. ... 

Multiple Separation Duties, Two Mixtures (M=2) Effect of Different Allocation Time... 

Table 7.4. Summary of the Results for Multiple Separation Duties [Mujtaba and Macchietto, 1996]k...

Logsdon et al. (1990) formulated multiperiod multiple separation duties design and operation optimisation problem for two binary mixtures. Only one distillate product per mixture was considered according to the operation shown in Figure 7.7. 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

Using the problem formulation and solution given in Logsdon et al. (1990) the optimal design, operation and profit are shown in Table 7.6. The optimal reflux ratio profiles are shown in Figure 7.8. Period 1 refers to Task 1 and Period 2 refers to Task 2. The results in Table 7.6 and Figure 7.8 clearly show the benefit of simultaneous design and operation optimisation for multiple separation duties. The benefit has been obtained due to reduction in batch time. 

This example is taken from Mujtaba and Macchietto (1996). Here, the profit function (Equation 7.27) used by Logsdon et al. (1990) is considered for the multiple separation duties presented in section 7.3.4.2. Using the input data presented in Table 7.3 with (0, = 02 = 0 and tsul = tm2 - 0 hr) the optimal design and operation policies are obtained and the results are presented in Table 7.7. The profit per batch for each separation is calculated by multiplying the profit per hr and the batch time. 

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 ... 

A two-level optimisation solution technique as presented in Chapter 6 and 7 can be used for a similar optimisation problem. For a given product specifications (in terms of purity of key component in each Task) and considering ReTi as the only outer level optimisation variable, the above MDO problem (OP) can be decomposed into a series of independent minimum time problem (Single-period Dynamic Optimisation (SDO) problem) in the inner level. For each iteration of the outer level optimisation, the inner-level problems are to be solved. As mentioned in the earlier chapters, the method is efficient for simultaneous design and operation optimisation especially with multiple separation duties. 

Separation of multiple mixtures using a single column is a very common feature in CBD columns (Mujtaba and Macchietto, 1996). If a continuous column were to be used for batch distillation then it would be interesting to evaluate the performance of such column undergoing multiple separation duties. 

Mujtaba (1997) used the minimum time to evaluate the performance of continuous column operation under multiple separation duties. However, time does not explicitly appear in continuous column model equations but the feed rate is a measure of the batch time (t = BJF). Note, maximisation of the feed rate will therefore ensure minimisation of the batch time. 

Table 11.8. Performance of Continuous Columns Undergoing Multiple Separation Duties. [Adopted from Mujtaba, 1997]...

The performance of continuous columns undergoing multiple separation duties is evaluated in terms of minimum batch time and yearly profit. By choosing an... 

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