Feed-point location distillation

Some distillation columns must handle two or more feed streams simultaneously. Furthermore, alternative feed nozzles are often provided to allow the actual feed-point locations to be altered. By optimizing the feed-point locations, energy consumption in the reboiler can often be minimized. 

Since it is desired to separate pure A and B from the mixture, two crystallizers are needed (Fig. 11.4(a)). To achieve the separation objective, one must visit compartments A and B. By inspection of the phase diagram, it is found that the feed (located in compartment AB2) can be brought to compartment A by adding S (Fig. 11.4(b)). This implies that A should be separated first, and B recovered next However, it is not possible to move from compartment A to B, because the two compartments are not adjacent to each other. One option to deal with this problem is to crystallize AS, which is an adduct that can be easily separated using distillation. Instead of A, the adduct AS is recovered in the first crystallizer (Fig. 11.4(c)). Solvent removal is then used to cross from compartment AS to B, followed by crystallization of B. The final mother liquor (point 4) is recycled (Fig. 11.4(d)). To complete the flowsheet, two units are added a dissolver to introduce solvent S to the feed, and a distillation column to recover A from the adduct (Fig. 11.4(e)). S is recycled back to the dissolver. Cooling-type crystallizers are used for both Cl and C2, since the boiling point of... 

Identify mislocated feed. For binary distillation the feed point should be where the q-line intersects the equilibrium curve. For multicomponent distillation this may or may not be the case. So for multicomponent distillation feed location, key ratio plots and d/b plots are preferred and discussed next. 

In die figure, (he upper and lower operating lines are shown inrerescdng at the feed point with each other and with die q line. The location of die lines depends on dieir slopes and on die compositions of the distillate (overhead) and residue (bottoms) products, x0 and xa. These compositions usually are specified on die hasis of process needs for pority and recovery. The slope of die upper operating litre is deteimined by the reflux ratio to be used. This nnio is... 

Another illustration may further clarify the procedures. Figure 10.18 shows a column having one side stream S along with the usual feed, bottoms, and distillate. Again, it is assumed that point P has been located. 

As one can see in Fig. 4.2, the trajectory of each section at sharp reversible distillation consists of two parts the part, located inside the (n -1) component boundary element C i of concentration simplex, lying between the product point Xd or xb and the tear-off point of the trajectory from this boundary element x[, and the part located inside concentration simplex C , lying between the tear-off point of the trajectory and the feed point xp. Only the second part should be located inside a region of reversible distillation Reg y orRegJ g, and product point Xd or xb can lie outside this region. 

We previously examined the process of reversible distillation for a given feed point. Below we examine trajectories of reversible distillation sections for given product points located at any -component boundary elements Q of the concentration simplex (xd e C or xg e Q). If / < (n - 1), then in the general case such trajectories should consist of two parts the part located in the same -component boundary element where the product point lies and the part located at some (k+ l)-component boundary element adjacent to it. Along with that, the product point should belong to the possible product composition region Reg or Reg for the examined ( )-component boundary element, and the boundaries of this region can be defined with the help of Eqs. (4.19) and (4.20). 

An example of the steps outlined above is a two distillation column system designed to separate desired product B from component A (Fig. 4). The first is an extractive column (Cl) where the A/B mixture is fed at a location below the feed point for component S. The feed rate of S to Cl is to be maintained at a fixed ratio to the feed rate of A/B. Component A goes overhead from Cl as the vapor product from a partial condenser. The bottoms product from Cl is a B/S mixture, which is then fed to the second column (C2). C2 separates component B as the vapor overhead product from a partial condenser. The bottoms from C2, component S, recycles as feed back to Cl. 

Improved response speed can be combined with accurate composition control by means of a cascade loop. It requires a temperature controller whose sensing element is located somewhere between the end of the tower and the feed tray, manipulating distillate flow. The set point of the temperature controller is then positioned in cascade by a composition controller sensing product quality. 

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