Flowsheet Optimization—Case Studies

In this section, two case studies are presented. The first is a relatively simple example involving one decision variable and one constraint, in which the venture profit for a process to manufacture ethyl chloride is maximized. In the second example, it is desired to optimize the operation of a multidraw distillation column, in which a mixture of normal paraffins is separated into four product streams, two of which are sidestreams. This involves four decision variables and a number of constraints. Although neither of the examples involves the optimization of accurate measures of plant profitability, detailed costing could be included. 

EXAMPLE 18.4 Maximizing the Venture Profit in Ethyl Chloride Manufacture 

Ethyl Chloride Manufacture and ASPEN — Tutorials — Material and Energy Balances Ethyl Chloride Manufacture) is maximized by adjusting the purge (W) flow rate. To estimate the venture profit, the following information is supplied 

EXAMPLE JS.S Optimization of a Distillation Tower with Sidedraws 

As shown in the multimedia CD-ROM that accompanies this book (see HYSYS Tutorials Process Design — Multi-draw Tower Optimization and ASPEN — Tutorials — Process Design — Multi-draw Tower Optimization), the solution of this NLP requires care, since the gradient-based SQP method is sen- 

---

Fig, 2. Ammonia process case study. Reprinted with permission from Comp. Chem. Eng., 11, 143-152, Y.-D. Lang and L. T. Biegler, A Unified Algorithm for flowsheet optimization, Copyright 1987, Pergamon Press PLC. 

Figure 2.13 illustrates the variation of the economic potential during flowsheet synthesis at different stages as a function of the dominant variable, reactor conversion. EPmin is necessary to ensure the economic viability of the process. At the input/output level EP2 sets the upper limit of the reactor conversion. On the other hand, the lower bound is set at the reactor/separation/recycle level by EP3, which accounts for the cost of reactor and recycles, and eventually of the separations. In this way, the range of optimal conversion can be determined. This problem may be handled conveniently by means of standard optimization capabilities of simulation packages, as demonstrated by the case study of a HDA plant [3]. 

Finally, the process simulators have the ability to optimize a process flowsheet by adjusting the continuous variables such as the purge/recycle ratio and the reflux ratio. Chapter 18 presents a general discussion of optimization methods, followed by two case studies. 

With the feasible path approach the optimization algorithm automatically performs case studies by variing input data. There are several drawbacks the process equations (32c) have to be solved every time the performance index is evaluated. Efficient gradient-based optimization techniques can only be used with great difficulties because derivatives can only be evaluated by perturbing the entire flowsheet with respect to the decision variables. This is very time consuming. Second, process units are often described by discrete and discontinuous relations or by functions that may be nondifferentiable at certain points. To overcome these problems quadratic module models can be... 

---