McCabe-Thiele diagram constant reflux

Figure 13.7. Features of McCabe-Thiele diagrams for constant molal overflow, (a) Operating line equations and construction and minimum reflux construction, (b) Orientations of -lines, with slope = ql q — 1), for various thermal conditions of the feed, (c) Minimum trays, total reflux, (d) Operating trays and reflux, (e) Minimum reflux determined by point of contact nearest xD.

Figure S.2 Batch distillation of benzene-toluene at constant reflux ratio, Example 5,3, ia-e) McCabe-Thiele diagram for progressively reducing still concentration to 0.13 mole fraction benzene.

Both modes usually are conducted with constant vaporization rate at an optimum value for the particular type of column construction. Figure 13.9 represents these modes on McCabe-Thiele diagrams. Small scale distillations often are controlled manually, but an automatic control scheme is shown in Figure 13.9(c). Constant overhead composition can be assured by control of temperature or directly of composition at the top of the column. Constant reflux is assured by flow control on that stream. Sometimes there is an advantage in operating at several different reflux rates at different times during the process, particularly with multicomponent mixtures as on Figure 13.10. 

FIGURE 12.20 Upper quadrant of a McCabe-Thiele diagram for batch distillation at constant reflux. Four theoretical stages shown. 

If a batch distillation is carried out at variable reflux ratio, it is possible u> maintain a constant purity of distillate and to do this until blocked by material balance limitations, A McCabe-Thiele diagram illustrating this method of operation is shown in Fig. 5.5-11. The value of xu is kept constant, and the slope of the operating line is varied in accordance with the overall material balance, The figure shows the conditions at time 0 and time t. Note that the three equilibrium stages include the stillpot. 

If a constant distillate composition is required, this can be achieved by increasing the reflux ratio as the system is depleted in the more volatile material. Calculations are again made with the McCabe-Thiele diagram as described by Bogart and illustrated by the following example. Other methods of operating batch columns are described by Ellerbe. ... 

The calculation of the minimum reflux and reboil ratios of nonpreferred separations is based on the fact that, in ideal mixtures, the states of constant reflux ratio = const, constitute a straight line in the triangular concentration space of Fig. 5.2-30. Their endpoints on the side lines of the triangle can easily be determined from the McCabe-Thiele diagram showing the equilibrium curves of the binary mixtures a-b and a-c. From a first estimation of the reflux ratio the operating line is drawn. Its points of intersection with the equilibrium lines dehver the endpoints... 

D6. We are separating 1,000 mol/h of a 40% benzene, 60% toluene feed in a distillation column with a total condenser and a partial reboiler. Feed is a saturated liquid. CMO is valid. A distillate that is 99.3% benzene and a bottoms that is 1% benzene are desired. Use the Fenske equation to find the number of stages required at total reflux, a McCabe-Thiele diagram to find (L/D), and the Gilliland correlation to estimate the number of stages required if L/D = 1.15(L/D)n. Estimate that the relative volatility is constant at a x = 2.4. Check your results with a McCabe-Thiele diagram... 

The batch distillation column can also be operated with variable reflux ratio to keep x constant. The operating Eq. 19-261 is still valid. Now the slope will vary, but the intersection with the y=x line will be constant at Xp,. The McCabe-Thiele diagram for this case is shown in Figure 9-7. This diagram relates Xs to Xp). Since Xp, is kept constant, the calculation procedure is somewhat different. 

Scheme J. This scheme directly adjusts the column material balance by manipulation of the distillate flow. The main advantage of this scheme is that it has the least interaction with the eneigy balance. In terms of a McCabe-Thiele diagram, this means that the slopes of the column operating lines can be held constant in spite of energy balance upsets. This independence ftom energy balance upsets is achieved by the scheme s ability to maintain a constant internal reflux even for variations in external reflux subcooling. When the temperature of the external reflux varies, the external reflux adjustment to maintain accumulator level offsets temporary internal reflux variations. If the accumulator level loop responds rapidly, the dis-tuibanoe will not propagate down the column, and the column s overall material balance remains undisturbed.

Figure 12.69 shows the original version of the Ryskamp scheme. This maintains constant a different definition of reflux ratio, i.e. R/ R + D). This is chosen because it is the slope of the top operating line on the McCabe-Thiele diagram - see Equation (12.18). The output of the level controller isR + D. From this is subtracted R to generate the SP for the distillate flow controller. The reflux ratio target is multiplied by the level controller ouQjut to generate the SP for the reflux flow controller. 

Figure 8.1.34. Batch distillation with reflux in a multiplate column, (a) Schematic of the device with total condensation, liquid reflwc and distillate product, (b) McCabe-Thiele diagram for constant reflux ratio with enriching section operating lines for three times, t = 0, t = tj, t = t2.

The minimum reflux occurs when the operating lines and the feed condition line intersect the equilibrium curve on a McCabe-Thiele diagram. Data for the equilibrium curve can be developed with the aid of Eqs. (6.22) and (6.16). Equilibrium constants are available in Table 6.2. Results are shown in Table 6.5 and plotted in Fig. 6.16. Since the feed to the column is saturated liquid, the slope of the feed condition line is infinite and the line extends vertically at Xp and intersects the equilibrium curve at j = 0.725. A line constructed from this intersection to = 0.95 on the x = y line represents the operating line in the rectifying section of the column under minimum reflux conditions. The slope of this line is given by... 

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