Simple distillation columns design

Evolving the Design of Simple Distillation Columns to Improve Heat Integration 447... 

The procedure known as the list-processing method allows the designer to identify feasible sequences for separating ideal or slightly nonideal zeotropic mixtures by means of simple distillation columns. Table 3.13 presents a list of more specific heuristics for sequencing, supplementary to those in Table 3.7. The first one shows... 

For example, let us consider a simple distillation column in which we have specifications on both the distillate and bottoms products (v-ahK and x b lk We go through the design procedure to establish the number of trays and the reflux ratio required to make the separation for a given feed composition. This gives us a base case from which to start. Then we establish what disturbances will affect the system and over what ranges they will vary. The most common disturbance, and the one that most affects the column, is a change in feed composition. Next we propose a partial control structure. By partial we mean we must decide what variables will be held constant. We do not have to decide what manipulated variable is paired with what controlled variable. We must fix as many variables as there are degrees of freedom in the system of equations. 

Solvent-based separation through extractive distillation consists of two distillations. The first is an extraction column with two feed (Aspen Distill was used designing this column), while the second is a simple distillation column (the driving force concept was used for designing this column). The design was then verified by rigorous simulation using Aspen Plus . The residue curve map (see Fig. 3) was used... 

A second alternative, called indirect sequence , is depicted in the Figure 9.7-right. In the first split the pure high-boiler is recovered completely as bottoms, followed by the separation of the light and medium components. Note that the two alternatives make use of simple distillation columns, with one feed and two products. A suitable design can fiilfil any type of feed composition and purity specifications. 

Therefore, the use of some simple economic objective function usually serves the purpose of optimizing a distillation column design. We will use the TAC. As shown in Table 4.1, this measure incorporates both energy cost and the annual cost of capital. The units of TAC are /year. The units of capital investment are. The units of annual cost of capital are /year, and it is obtained by dividing the cost of capital by a suitable payback period. 

Industrial applications of the divided-wall (Petlyuk) column have expanded, so a new chapter has been added that covers both the design and the control of these more complex coupled columns. The use of dynamic simulations to quantitatively explore the safety issues of rapid transient responses to major process upsets and failures is discussed in a new chapter. A more stmctured approach for selecting an appropriate control structure is outlined to help sort through the overwhelmingly large number of alternative stmctures. A simple distillation column has five factorial (120) alternative structures that need to be trimmed down to a workable number, so that their steady-state and dynamic performances can be compared. 

The basic theory of batch distillation is given in Volume 2, Chapter 11 and in several other texts Hart (1997), Perry et al. (1997) and Walas (1990). In the simple theoretical analysis of batch distillation columns the liquid hold-up in the column is usually ignored. This hold-up can have a significant effect on the separating efficiency and should be taken into account when designing batch distillation columns. The practical design of batch distillation columns is covered by Hengstebeck (1976), Ellerbe (1997) and Hart (1997). 

The results received form the optimization using inherent safety as the objective function are somewhat different compared to those calculated with an economic objective function earlier (Hurme, 1996). With the inherent safety objective function the simple distillations were favoured more than with the economic function. Exceptions are cases where the extractive distillation could improve separation very dramatically. This is because in simple distillations only one column is required per split, but in extractive distillation two columns are needed, since the solvent has to be separated too. This causes larger fluid inventory since also the extraction solvent is highly flammable. The results of the calculation are well justified by common sense, since one of the principles of inherent safety is to use simpler designs and reduce inventories to enhance safety. 

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