(142e) Application of Response Surface Methodology to Optimize the Retrofit of the Side Column to DWC

the Side Column to DWC

Nguyen Van Duc Long^a and Moonyong Lee^†

^aSchool of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea

Email: allenthelong@yahoo.com

^†School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea

Email: mynlee@yu.ac.kr

Abstract

Distillation plays an important role in the chemical process industries and consumes the largest amount of energy with an estimated 3% of the world’s energy consumption [1]. Motivated by this large energy requirement for distillation, researchers have developed various column arrangements that can bring in savings in both energy and capital cost [2]. Any reduction of energy consumption will not only bring economical benefits but also environmental benefits in terms of reduced usage of fossil fuels and its associated emissions.

A side distillation column can be used to separate any nonazeotropic multicomponent mixture into three products [3]. Therefore, it can often substitute for a sequence of two columns. However, the purity of the side distillation is restricted by thermodynamics and by the nature of the distillation process. These columns are usually appropriate either as prefractionators (the side stream is fed to another column for further separation) or to generate recycle streams when there is no strict requirement of the recycle composition. They are also broadly used in the petroleum industry, where blends are produced rather than pure products.  

There is a significant amount of literature analyzing the relative advantages of dividing wall column (DWC) options offering a great chance at reduced energy consumption. These studies have shown that DWC systems are capable of achieving energy savings of up to 30% over conventional direct and indirect distillation sequences [4-8]. Besides, the investment and space saving can be gained because the number of reboilers, condensers and associated equipments such as pumps, their supports [2]. Therefore, DWC is attractive to many chemical and related industries in the current scenario of competition and environmental concerns, in order to reduce energy usage for distillation. One way is to make use of the existing equipments and operate the plant more efficiently, possibly with minor modifications and small investment. An investment with payback period of less than 3 years is favorably considered by the management.

Early in 1999 Krupp Uhde started the first revamp for a pyrolysis gasoline fractionation column [9]. The recovery of benzene is produced using the MORPHYLANE^® process [10]. Integrating an extractive distillation and a stripper column was performed. The total number of equipment items (heat exchanger, reflux drum, pumps etc.) can be reduced by 30%, resulting in lower engineering and hardware costs. In addition, 40% less plot area is required for the divided-wall column arrangement. The resulting energy savings for this process are remarkably high at 36%, reflecting impressively the advantage of the divided-wall column technology for such applications.

Recently, the potential of retrofit conventional 2-column systems and side columns for separating ternary mixtures into three products to DWCs was studied [2,7]. However, DWCs were optimized only one variable at a time, keeping constant the remaining ones. For each chosen variable value, the internal vapor and liquid flow to the prefractionator were varied to optimize energy consumption. The interactions between variables could not be identified and quantified by such a technique.

Retrofit a side distillation column to a DWC has potential for reducing energy.In this study, response surface methodology (RSM) is proposed for DWC structure design and optimization, which is easy and efficient to implement using Hysys and Minitab. A Box-Behnken design was employed under the response surface methodology to analyze how the variables interacted and optimize the system in terms of reboiler consumption and cost saving. Simulation run data were fitted to a second-order polynomial model and regression coefficients were obtained. This method requires a little computational effort while it allows control variables to be optimized simultaneously. It facilitates the evaluation of interaction effects between variables. The result from rigorous simulation also revealed that the DWC can save up to 25.16% of energy consumption, which showed good agreement with the predicted result by the RSM. Simple payback period can be calculated as additional capital costs divided by saving per year. In this case the value is 0.93 month due to high energy cost.  

  References

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