Bottoms product composition

An azeotrope limits the separation that can be obtained between components by simple distillation. For the system described by cui ve B, the maximum overhead-product concentration that could be obtained from a feed with X = 0.25 is the azeotropic composition. Similarly, a feed with X = 0.9 could produce a bottom-product composition no lower than the azeotrope. 

Answers are desired to the following two questions. First, what bottom-product composition Xb will the column produce under these specifications Second, what will be the top vapor rate Vv in this operation, and will it exceed the maximum vapor-rate capacity for this column, which is assumed to be 0.252 (kg mol)/s [2000 (lb mol)/h] at the top-tray conditions ... 

The problem as stated gives no bottom-product composition, so that whilst all flow-rates in the top of the column may be calculated, no information about the lower half can be derived. In the absence of these data, the feed rate cannot be determined, though the rate of distillate removal may be calculated as follows. 

Fig. 2 Top product, feed plate and bottoms product composition variations as a function of time.

Fig. 2 Column top and bottom product compositions approaching steady state from the rather arbitrary feed composition starting point.

Example 1.5. For a binary distillation column (see Fig. 1.6), load disturbance variables might include feed flow rate and feed composition. Reflux, steam, cooling water, distillate, and bottoms flow rates might be the manipulated variables. Controlled variables might be distillate product composition, bottoms product composition, column pressure, base liquid level, and reflux drum liquid level. The uncontrolled variables would include the compositions and temperatures on aU the trays. Note that one physical stream may be considered to contain many variables ... 

The two possible control configurations for a system with two inputs and two outputs are shown in Fig. 7.77. One example of this is illustrated in Fig. 7.73 where the overhead and bottoms product compositions of a distillation process are controlled using the reflux and steam-to-reboiler flowrates respectively as the manipulated variables. Theoretically, we could employ the reflux flowrate to control the bottoms product composition and the steam-to-reboiler flowrate to control the overhead product composition. It is possible to determine which configuration produces the least interaction by forming the system relative gain array A, where ... 

If the feed composition and the column pressure are constant, temperature can be used as an indirect measure of composition. When the bottom product composition is being controlled, the temperature sensor is located in the lower half of the column and when overhead composition is controlled, in the upper half of the column. The temperature sensor should be located on a tray that strongly reflects changes in composition (Figure 2.84). When two compounds of relatively close vapor pressures are to be separated, two temperatures or a temperature difference can be used instead of a single sensor. This configuration can also be used to eliminate the effects of column pressure variations. 

Here, xBf is the final bottom product composition and xB is a reboiler composition intermediate between xBf and xB0. 

Also 1.52 kmol of product C with an accumulated composition of 0.70 molefraction was obtained in the distillate tank. Figure 4.18 shows the accumulated distillate, feed tank and bottom product composition profiles for the operation and Figure 4.19 shows the holdup profiles in the distillate accumulator, feed tank and bottom product accumulator. 

Since the top and bottom product compositions are specified, the overall mass balance using Bt = 10.0 kmol and xBI = <0.6, 0.4> will give a total distillate product of 6.25 kmol and a total bottom product of 3.75 kmol. 

Here the feed rate is maximised while the reflux ratio is optimised. The bottom product composition imposes an additional constraint to the problem. The results are summarised in Table 11.8 which gives the maximum feed rate, minimum batch time, optimum reflux ratio, and total number of batches for each mixture and total yearly profit. 

Note that since both the top and bottom product compositions are fixed (and so are the total amounts of distillate and bottoms for each mixture) the profit shown in Table 11.8 represents the maximum achievable profit using the continuous column for equal time-sharing by the mixtures. Different profit figure is expected for unequal time-sharing of the column by the mixtures and for different start-up time. 

To develop the alternative process configurations needed to separate a given feed mixture into a set of specified products, we need to know just what distillate and bottoms product compositions we can reach when using a conventional distillation column. We shall start by examining this problem for ideally behaving mixtures. We shall then look at the much harder problem in which the mixtures do not behave ideally. 

The application of the superstructure for the separation of azeotropic mixtures requires some modifications. Separation is limited by azeotropes and the corresponding distillation boimdaries, which form distillation regions [7]. For a feasible separation, top and bottom product composition have to be in the same distillation region. Boundary crossing (where the feed and the two product compositions are located in different distillation regions) is possible in the presence of curved distillation boundaries, but is not considered in this work. 

Figure 14.24. Predicted versus experimental top and bottom product compositions in distillation of acetone-methanol-water system. Calculations by Taylor et al. (1987). Data from Kumar et al. (1984).

As an example of an ATV test, consider its application to a dynamic simulator of a C3 (propylene/propane) splitter. Figure 15.48 shows an ATV test and an open-loop test on the same time scale for the bottom product composition control loop. Note that the four cycles of the ATV test required 6 to 8 hr, while the open-loop test required in excess of 60 hr. The ATV results were used with TL settings, and the results for three different tuning factors are shown in Figure 15.49. 

A schematic of the stripping section of a distillation column with a ratio controller for the bottom products composition is shown in Figure 15.53. This application is similar to ratio control except that dynamic compensation is added to the measured column feed rate. If the steam flow to the reboiler were increased immediately for an increase in column feed rate, the corrective action would initially be an overcorrection. This results because, when a feed rate change occurs, it takes some time for the bottom product composition to respond. The purpose of the dynamic compensation (DC) element is to allow for the correct timing for the compensation for feed rate changes. 

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