Operating Reflux

As discussed briefly in Section 13.4, the operating reflux is an amount in excess of the minimum that ultimately should be established by an economic balance between operating and capital costs for the operation. In many cases, however, as stated there the assumptions R = l.2R , often is close to the optimum and is used without further study unless the installation is quite a large one. 

An improved relation that, however, requires more information is due to Akashah, Erbar, and Maddox [Chem. Eng. Commun. 3, 461 (1979)]. It is 

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McCabe-Thie/e Example. Assume a binary system E—H that has ideal vapor—Hquid equiHbria and a relative volatiHty of 2.0. The feed is 100 mol of = 0.6 the required distillate is x = 0.95, and the bottoms x = 0.05, with the compositions identified and the lighter component E. The feed is at the boiling point. To calculate the minimum reflux ratio, the minimum number of theoretical stages, the operating reflux ratio, and the number of theoretical stages, assume the operating reflux ratio is 1.5 times the minimum reflux ratio and there is no subcooling of the reflux stream, then ... 

Optimum Reflux Ratio The general effecl of the operating reflux ratio on fixed costs, operating costs, and the sum of these is shown in Fig. 13-39. In ordinary situations, the minimum on the total-cost cui ve wih geueraUy occur at an operating reflux ratio of from 1.1 to 1.5 times the minimum R = Lv + i/D value, with the lower value corresponding to a value of the relative volatility close to 1. 

FIG. 22-42 Four alternative modes of contimioiis-flow operation with a foam-fractionation column (1) The simple mode is illustrated hy the solid lines. (2) Enriching operation employs the dashed reflux line. (3) In stripping operation, the elevated dashed feed line to the foam replaces the solid feed line to the pool. (4) For combined operation, reflux and elevated feed to the foam are both employed. 

Using the same operating reflux (same fraction times the minimum) as was used in Example 8-10, calculate the theoretical plates required for feed of the following thermal conditions Use Figure 8-27. 

Note that these values for theoretical trays do contain corrections in overall efficiency, and hence are not the actual trays for the binary distillation column. Efficiencies generally run 50-60% for systems of this type which will yield a column of actual trays almost twice the theoretical at the operating reflux. 

Application may be (1) to determine a column diameter and number of plates or (2) to take an existing column and assume an operating reflux for the fixed trays and determine the time to separate a desired cut or product. 

This is not unusual to select an operating reflux ratio ten, or even fifty times such a low minimum. Selecting a higher reflux can reduce the number of trays required, and this becomes a balance of the reduction in trays versus operating and capital expense in handling the increased liquid both external to the column and internally. 

F = intermediate feed Theoretical Trays at Operating Reflux... 

The method of Gilliland [23] (Figure S-24) is also used for multicomponent mixtures to determine theoretical trays at a particular operating reflux ratio, or at various ratios. 

Figure 8-49. Brown and Martin operating reflux and stages correlated with minimum reflux and sttiges. Used and adapted by permission, Van Winkle, M Oil and Gas Jour. V. 182, Mar. 23 (1953).

Note than when (L/D) min is used as the starting basis, the Lr, Lg, Vr, Vg and their ratios will be for the minimum condition, and correspondingly so when the operating reflux is used. 

The minimum reflux ratio (L/D)min been determined to be 1.017. Using the Brown and Martin graph [9], evaluate the theoretical number of trays at an operating reflux of 1.5 times the minimum. The minimum number of stages was determined to be 22.1 including the reboiler. See Figure 8-49. 

From the theoretical trays at operating reflux the actual trays for installation are determined ... 

Operating Reflux and Theoretical Trays—Gilliland Plot Min trays = S j = 12.6... 

Example 8-25 Scheibel-Montross Minimum Reflux, 80 Minimum Number of Trays Total Reflux — Constant Volatility, 80 Chou and Yaws Method, 81 Example 8-26 Distillation with Two Sidestream Feeds, 82 Theoretical Trays at Operating Reflux, 83 Example 8-27 Operating Reflux Ratio, 84 Estimating Multicomponent Recoveries,... 

Formulas for the numbers of trays in the enriching and stripping sections at operating reflux also are due to Underwood (Trans. Inst. Chem. Eng. 10, 112-152, 1932). For above the feed, these groups of terms are defined ... 

Numbers of Theoretical Trays and of Transfer Units with Two Values of kL/kG for a Distillation Process An equimolal mixture at its boiling point is to be separated into 95 and 5% contents of the lighter component in the top and bottoms products. The relative volatility is a- = 2, the minimum reflux is 1.714, and the operating reflux is 50% greater. The two values of kjJkQ to be examined are —1 and... 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

Many industrial users of batch distillation (Chen, 1998 Greaves, 2003) find it difficult to implement the optimum reflux ratio profiles, obtained using rigorous mathematical methods, in their pilot plants. This is due to the fact that most models for batch distillation available in the literature treat the reflux ratio as a continuous variable (either constant or variable) while most pilot plants use an on-off type (switch between total reflux and total distillate operation) reflux ratio controller. In Greaves et al. 2001) a relationship between the continuous reflux ratio used in a model and the discrete reflux ratio used in the pilot plant is developed. This allows easy comparison between the model and the plant on a common basis. 

The operating reflux ratio is usually 1.5 to 10 times the minimum. By using the ethanol-water equilibrium curve for 101.3-kPa (1-atm) pressure shown in Fig. 13-123 out extending the line to a convenient point... 

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