Complex retrofitting

The rest of this chapter is structured as follows. The next section describes HENs and the structural representation used in this study for HEN retrofitting. Section 7.3 discusses the improvement techniques that can be used to retrofit HENs. Section 7.4 describes the methodology to retrofit HENs, which includes MOO, HEN model and exchanger reassignment strategy (ERS). The case study solved by the methodology is described in Section 7.5. Section 7.6 discusses the results obtained for simple, moderate and complex retrofitting. Finally, conclusions of this study are presented in Section 7.7. 

In the following sub-sections, MOO results for simple, moderate and complex retrofitting of HEN in the case study are presented and discussed. The objectives used in MOO are cost of retrofit and utility cost, and the approach temperature on each heat exchanger in HEN... 

Figure 7.12 Comparison of Pareto fronts for complex retrofitting at various generations with a population size of2000 pluses, circles, triangles and diamonds are the solutions at 500, 1000, 1500 and 2000 generations. The solution selected for discussion is in the oval.

Figure 7.13 HEN after complex retrofitting according to the selected solution in Figure 7.12.

Figure 7.14 Comparison of Pareto fronts for simple, moderating and complex retrofitting of HEN in the case study diamonds and pluses are the solutions for simple retrofitting with 15% and 30% area addition, respectively and circles and triangles are the solutions for moderate and complex retrofitting, respectively.

A minimum approach temperature of 15 °C is used for optimizing simple, moderate and complex HEN retrofitting in the case study in this chapter. Study the effect of changing the minimum approach temperature from 15 °C to 10 °C, 20 °C and 30 °C for simple, moderate and/or complex retrofitting. Note that this requires meth-ods/programs similar to those described in this chapter. 

Due to the importance of getting the design basis and data right for a complex retrofit project, the following guidelines could be used ... 

The retrofit of more complex distillation sequences can also be considered. Within a larger, more complex sequence, any pair of columns that are together in a sequence can be considered as candidates for the same retrofit modifications as those discussed for two-column retrofit. 

This approach to heat exchanger network retrofit allows modifications to be introduced one at a time. In this way, the designer has control over the complexity of the network retrofit. At each stage, a suggested modification can be... 

The approach leads to simple and practical retrofit designs and has the major advantage that it allows the designer to assess modifications one at a time and to keep control over the complexity of the retrofit. Its disadvantage is that different combinations of modifications can be taken and there is no guarantee that this will lead to an optimum network retrofit. However, it is almost impossible to say that any retrofit is optimal or nonoptimal. The features of each retrofit are unique and it is difficult to formulate all the constraints for a retrofit in order to guarantee the very best retrofit. 

Complex steam systems usually feature many important degrees of freedom to be optimized. To establish the steam costs for retrofit of site processes requires an optimization model to be developed. This allows the steam loads for process heating to be gradually decreased and the steam system reoptimized at each setting. The result in cost... 

The converging-diverging steam jet is a startlingly complex device. Not only is the theory of operation rather weird, but the jets are subject to a wide range of odd, poorly understood, and never reported malfunctions. For all these reasons, I dearly love to retrofit and troubleshoot steam jet systems. 

The nonlinear nature of these mixed-integer optimization problems may arise from (i) nonlinear relations in the integer domain exclusively (e.g., products of binary variables in the quadratic assignment model), (ii) nonlinear relations in the continuous domain only (e.g., complex nonlinear input-output model in a distillation column or reactor unit), (iii) nonlinear relations in the joint integer-continuous domain (e.g., products of continuous and binary variables in the schedul-ing/planning of batch processes, and retrofit of heat recovery systems). In this chapter, we will focus on nonlinearities due to relations (ii) and (iii). An excellent book that studies mixed-integer linear optimization, and nonlinear integer relationships in combinatorial optimization is the one by Nemhauser and Wolsey (1988). 

The internal modifications to retrofit the distillation system consider the distillation column and its internals. In this level of retrofit, some changes are made in the internals or nozzles of the main column and the side strippers. Moreover, the reboilers and pump around loops are examined and adjusted as required. Therefore, more complex modifications are required at this level that lead to higher investments. The objective of these modifications is to find out the ideal distribution of stages for each section in the main column [4]. This ideal design is applied to the existing column by modilying its internal to the niinimrim extent to reduce the costs. 

Halasz, L. Nagy, A.B. Ivicz, T. Friedler, F. Fan, L.T. Optimal retrofit design and operation of the steam-supply system of a chemical complex. Appl. Therm. Eng. 2000, 22 (8), 939-947. 

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