Separation thermally coupled distillation

Fig. 1. Three thermally coupled distillation schemes for the separation of ternary mixtures...

B.G. Rong and A. Kraslawski. Partially thermally coupled distillation systems for multicomponent separations. A.I.Ch.E. Journal, 49 1340-1347, 2003. 

Another class of separation problem attacked has been that of designing the most effective thermally coupled distillation column arrangement to separate a multicomponent mixture. Sargent and Gaxninibandara (1975) present a general column superstructure which they optimize. Imbedded in the superstructure are all the alternative thermally coupled and ordinary column sequences to be considered. The optimization eliminates those portions of the superstructure which are not economic leaving, hopefully, the optimal substructure. 

The thermally coupled distillation system shown below is to be used to separate a mixture of three components into three products. Determine for the system ... 

A mixture of three hydrocarbons is to be separated into three nearly pure products by thermally coupled distillation at 1 atm, as shown in Figure 9,30. 

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. 

The transition split divides direct-type sphts from indirect-type splits as discussed by Doherty and Malone (Conceptual Desisn of Distillation Systems, 2001, chaps. 4 andS) also see Fidkowski, Doherty, and Malone [AlChE J., 39,1301(1993)]. The upper line in Fig. 13-70 is the minimum vapor flow leaving the reboiler of the main column, which also corresponds to the minimum vapor flow for the entire system since all the vapor for the total wstem is generated by this reboiler. For P = 0 the minimum vapor flow for the entire thermally coupled system (i.e., main column) becomes equal to the minimum vapor flow for the side rectifier system (i.e., main column of the side-rectifier system see Fig. 13-65b or c) (Vsr) for P = 1 it is equal to the minimum vapor flow of the entire side stripper system (Vss) (which is the sum of the vapor flows from both the reboilers in this system see Fig. 13-66h or c). Coincidentally, the values of these two minimum vapor flows are always the same (Vsr), = (Vss)mm- For P = Pr the main column is pinched at both feed locations i.e., the minimum vapor flows for separations A/B and B/C are equal. 

FIGURE 4.2-1 Separation of fend stream into pure components A. B, and C through (a) ordinary distillation and (ft) a more complex arrangement dial includes thermal coupling and sidestream feed and removal. 

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). 

In this paper the Generalized Modular Framework (Papalexandri and Pistikopoulos, 1996) is used for the representation of the Petlyuk (Fully Thermally Coupled) column. The GMF Petlyuk representation, which avoids the use of common simplifying assumptions while keeping the problem size small, is validated for a ternary separation, by a direct comparison of its results to those obtained by a rigorous distillation model. 

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