Distillation Sequencing Using Thermal Coupling

The final restriction of simple columns stated earlier was that they should have a reboiler and a condenser. It is possible to use material flows to provide some of the necessary heat transfer by direct contact. This transfer of heat via direct contact is known as thermal coupling. 

Both the side-rectifier and side-stripper arrangements have been shown to reduce the energy consumption compared to simple two-column arrangements3,7. This results from reduced mixing losses in the first (main) column. As with the first column of the simple sequence, a peak in composition occurs with the middle product. But now, advantage of the peak is taken by transferring material to the side-rectifier or side-stripper. 

The side-rectifier and side-stripper arrangements have some important degrees of freedom for optimization. In these arrangements, there are four column sections. For the side-rectifier, the degrees of freedom to be optimized are  

All of these variables must be optimized simultaneously to obtain the best design. Some of the variables are continuous and some are discrete (the number of stages in each column section). Such optimizations are far from straightforward if carried out using detailed simulation. It is therefore convenient to carry out some optimization using shortcut methods before proceeding to detailed simulation where the optimization can be fine-tuned. 

Partition columns offer a number of advantages over conventional arrangements  

Similarly, Fig. 5.15a shows a thermally coupled indirect sequence. The condenser of the first column is replaced by a thermal coupling. The four column sections are again marked as 1, 2, 3, and 4 in Fig. 5.15a. In Fig. 5.156, the four column sections are arranged to form a side-stripper arrangement.  

Various studies have compared the thermally coupled arrangement in Fig. 5.166 with a conventional arrangement using simple columns on a stand-alone basis. These studies show that the thermally coupled arrangement in Fig. 5.166 typically requires 30 percent less energy than a conventional arrangement using simple columns. The fully thermally coupled column in Fig. 5.166 also 

Although side-stripper arrangements are common in the petroleum industry, designers have been reluctant to use the fully thermally coupled arrangements in practical applications until recently. 

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Long, N.V.D. and Lee, M. (2014) Review of retrofitting distillation columns using thermally coupled distillation sequences and dividing wall columns to improve energy efficiency. Journal of Chemical Engineering of Japan, 47, 87-108. 

Caballero, J.A. and Grossmann, I.E. (2014) Optimal synthesis of thermally coupled distillation sequences using a novel MILP approach. Computers Chemical Engineering, 61,118-135. 

Dynamic Study of Thermally Coupled Distillation Sequences Using Proportional - Integral Controllers... 

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

Due to the tremendous costs associated to distillative separations, many alternate schemes to the simple column shown above have been proposed over the past several years both to improve on some of its inherent costs. Traditionally, when purifying a multicomponent mixture, an entire series of distillation columns are used in series, and the way in which these columns are sequenced may make a tremendous difference in the eventual process costs. However, due to the large energy requirements of even the most optimal sequence, more complex column arrangements have been proposed and subsequently utilized. These arrangements include thermally coupled columns such as side rectifiers and strippers, the fully thermally coupled columns (often referred to as the Petlyuk and Kaibel columns). 

A comparative study of the energy requirements and control properties of three thermally coupled distillation schemes and two conventional distillation sequences for the separation of ternary mixtures is presented. The responses to set point changes under closed loop operation with proportional-integral (PI) controllers were obtained. Three composition control loops were used, and for each separation scheme, the parameters of the PI controllers were optimized using the integral of the absolute error criterion. The effects of feed composition and of the ease of separability index were considered. The results indicate that there exist cases in which integrated systems may exhibit better control properties than sequences based on conventional distillation columns. 

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