Optimal Retrofit Strategy of Heat Exchanger Networks Applied in Crude Oil Distillation Units

Abstract

A novel mathematical programming method was applied to acquire the optimal heat exchanger network (HEN) retrofit in the crude oil distillation unit. The mathematical model was based on the stagewise superstructure model with splits of streams from the work of Yee et al.[Chin. J. Chem. Eng. 2007, 15 (2), 296−301]. The objective function was developed into the minimum of the sum of the cost of modifications and the cost of utilities in one year, focusing on the changing heat transfer coefficients arising in the reused existing heat exchangers from stream splits. The HEN modifications included buying new heat exchangers, adding or reducing areas of the existing heat exchangers, moving the existing heat exchangers, and laying pipes. It was a mixed integer nonlinear program (MINLP) problem. A hybrid genetic algorithm was used to optimize the MINLP problem. In order to reduce the total cost of modifications, the heat exchangers and the corresponding matching structures between the hot and cold streams in the existing HENs were designed as elites introduced into the hybrid genetic algorithm. The Dittus−Boelter and Kern correlations were used to estimate the convective heat transfer coefficients of the corresponding streams. One example was given to show the effect of the novel retrofit strategy.

Synthesis of stream‐split heat exchanger networks using non‐structural model considering serial equipment in stream branches and submixing of substreams

“…This real‐world industrial problem is a distillation unit for crude oil. [ 41 ] 10SP is presented here to substantiate the economic value of incorporating submixes in splitting HENs. Seven out of 10 process streams in 10SP require cooling and three need heating to target temperatures.…”

“…[ 42 ] The data for streams is unvaried from those used in the works of Liu et al, [ 41,48 ] but cost correlations are different (see Table 3), whereby the fixed charge for heat transfer equipment is $8000/year. For more comparative data, this work has made use of an existing plant's HEN structure presented by Liu et al [ 41 ] and cost correlations used by Pavão et al [ 42 ] (which are also used herein), to compute the overall exchanger area, utility loads, and network TAC for the plant. These quantities are listed in Table 2 together with details for the solution reported by Pavão et al [ 42 ]…”

“…b Recalculated cost. [29] c Solution calculated from HEN structure presented by Liu et al [41] d Reworked cost. [47] economical concerning savings on operating expenses.…”

“…[42] The data for streams is unvaried from those used in the works of Liu et al, [41,48] but cost correlations are different (see Table 3), whereby the fixed charge for heat transfer equipment is $8000/year. For more comparative data, this work has made use of an existing plant's HEN structure presented by Liu et al [41] and cost correlations used by Pavão et al [42] (which are also used herein), to compute the overall exchanger area, utility loads, and network TAC for the plant. These quantities are listed in Table 2 together with details for the solution reported by Pavão et al [42] The HEN obtained in this study is displayed in Figure 8.…”