Distillation operating policy

Sorensen (1999) has suggested a cyclic operation policy for batch distillation with repeated filling and dumping of the reflux drum. This configuration achieves the maximum attainable separation and requires minimal control. Furthermore, such a column can be operated very safely. 

Mujtaba, I.M., Optimal Operational Policies in Batch Distillation. PhD Thesis, (Imperial College, University of London, 1989). 

All the optimisation problem formulations presented above were aimed to achieve optimal operation policies for a variety of objective functions but for a single period operation (i.e. single distillation task). In single period operation only one product cut is made from both binary and multicomponent mixtures and optimal operation policy is restricted only to that period. 

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... 

Bemot, C., Doherty, M.F. and Malone, M.F., Design and operating policies for multicomponent batch distillation. AIChE Annual Meeting, Los Angeles, USA (1991). 

This example is taken from Mujtaba and Macchietto (1996). The problem is to design a column for 2 binary separation duties. One of the separations is very easy compared to the other one. The fraction of production time for each duty is specified together with the still capacity (B0) and the vapour load (V). Each binary mixture produces only one main distillate product and a bottom residue (states MPf= Dl, Bfl] and MP2=[D2, Bf2]) from feed states EFt= Fl and EF2= F2], respectively, with only one distillation task in each separation duty. Desired purities are specified for the two main-cuts (x Di and xID2). Also obtain the optimal operating policies in terms of reflux ratio for the separations. 

As presented in the earlier chapters, the operating policy for a batch distillation column can be determined in terms of reflux ratio, product recoveries and vapour boilup rate as a function of time (open-loop control). Under nominal conditions, the optimal operating policy may be specified equivalently in terms of a set-point trajectory for controllers manipulating these inputs. In the presence of uncertainty, these specifications for the optimal operating policy are no longer equivalent and it is important to evaluate and compare their performance. 

Also Mujtaba (1997) considered the separation of binary mixtures into one distillate product of specified purity. The objectives were to find out whether it was possible to replace conventional dynamic operation of batch columns by steady state operation using continuous columns for a comparable recovery, energy consumption, operation time, productivity, etc. and to obtain optimal operating policy in terms of reflux ratio. The following strategy was considered to compare the performances of the two types of operations ... 

The operating policy for constant reflux operation would be lo allow movement of the distillate purity until die composite purity from time 0 to time t, meets specification. It might be desirable at this point to shift to variable reflux as described below. 

In this paper we introduce a novel operational policy for the purification of an intermediate boiling component via batch distillation. The novel operational policy is based on feasibility studies of a cyclic distillation column provided with a side withdrawal. The process is validated via computer based simulations. Furthermore, the effects of the most important process parameters are investigated. 

Sprensen, E., Prenzler, M., 1997, A cyclic operating policy for batch distillation -Theory and practice. Comp. Chem. Engng., Vol. 21, Suppl., S1215-S1220. 

This paper focuses on the modelling and optimisation of an industrial semibatch distillation process with a polymerisation reaction taking place in the reboiler. The dynamic model presented here is implemented in CHEMCAD and validated with experimental data from the industrial plant. An approach to optimise the economical performance of the semibatch process is discussed. As a result of the work so far the control structures and operation policies were improved and it was shown that further optimisation is worthwhile. 

Optimizing batch distillation operations can have a significant economic impact, especially when the separation of high-value chemicals is involved. The control variable for optimizing a batch distillation product is the reflux ratio policy, i.e., the variation of the reflux ratio with time. As... 

Shinji Hasebe, Masaru Noda, and lori Hashimoto. Optimal operation policy for total reflux and multi-batch distillation systems. Comp. Chem. Eng., 23 523-532, 1999. 

Sandro Macchietto and Iqbal M. Mujtaba. Design of operation policies for batch distillation. In G. V. Reklaitis, editor, NATO Advanced Study Institute Series F14.3, pages 174-215. Springer-Verlag, Berlin, 1996. 

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