Distillation columns feed point location

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... 

Figure 1.11 illustrates the application of this mixing rule to a distillation column. Of course, a column separates instead of mixes, but the geometry is exactly the same. The two products D and B have compositions located at point xdi-Xd-2) and point xb -Xb ), respectively. The feed F has a composition located at point (zi-za) that lies on a straight line joining D and B. 

We wish to separate ethanol from water in a distillation column with a total condenser and a partial reboiler. We have 200 kmol/h of feed 1, which is 30 mol% ethanol and is saturated vapor. We also have 300 kmol/h of feed 2, which is 40 mol% ethanol. Feed 2 is a subcooled liquid. One mole of vapor must condense inside the column to heat up 4 moles of feed 2 to its boiling point. We desire a bottoms product that is 2 mol% ethanol and a distillate product that is 72 mol% ethanol. External reflux ratio is Lq/D = 1.0. The reflux is a saturated liquid. Column pressure is 101.3 kPa, and the column is well insulated. The feeds are to be input at their optimum feed locations. Find the optimum feed locations (reported as stages above the reboiler) and the total number of equilibrium stages required. 

Now we are ready to start drawing the flowsheet. We just click on the units we need and drag them from the palette to the main window. We click on the main window to leave the unit there. To cancel the selection of that particular unit, we click the right button. One important point is that we need to define the inputs and outputs of the process as feeds and products, respectively, using special unit operation block, the arrows. The rest of the units such as distillation columns, heat exchangers, and compressors have their inlets and outlets characterized by blue and red dots. We locate the units in the desired order to draw our flowsheet. If a particular operation is not defined within CHEMCAD, we can use either an Excel unit op or a Visual Basic unit op to build our own model. Both appear as units in the palette. 

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. 

Kirkbride equation An equation used for the design of distillation columns to determine the ratio of the number of trays above and below the feed point. It is therefore used to determine the location of the feed tray in the column ... 

We can locate various streams on this ternary diagram. For example, plotting a point with coordinates ZMeOH = 0.3 and Zjcs = 0.4 will indicate a feedstream to the column with these compositions. In the same way, the distillate and bottoms points can be located on the diagram. Because of the ternary mixing mle, the feed coordinates must lie on a straight line connecting the distillate and bottoms coordinate points. 

At this point, a question arises. Could we overcome the constraint of a distillation boundary The answer is not easy. In any case, the selection of a suitable entrainer is the key element. The number of columns involved in the separation sequence is not very important, because it can be either two or three. On the contrary, the recycle policy of the entrainer and of intermediate mixtures plays a major role. Sometimes the location of feeds, as for example the original mixture and the entrainer in the first column, may have also an effect on the overall feasibility. These issues will be developed in a lager extent in the next sections. 

Consider a column of the type shown in Fig. 8.9, but having feed streams Ft and F2 and no side stream. Assuming that the distillate and bottoms compositions as well as the reflux ratio on the top plate are specified, it is possible to locate the point P on Fig. 10.17. Next consider the two feed streams. If Fi and F2 are mixed, the resultant stream = F, + F2 can be located by material and enthalpy balance methods, or by line segment ratios, since F, /F2 = F2/F1. 

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. 

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