Actual reflux

The recommended method to use to determine the actual theoretical stages at an actual reflux ratio is the Erbar/Maddox relationship. In the graph, N is the theoretical stages and R is the actual reflux ratio L/D, where L/D is the molar ratio of reflux to distillate. N, is the minimum theoretical stages and R, is the minimum reflux ratio. 

The actual reflux ratio that one pieks should be optimized from economics data. For a ballpark estimate use... 

After actual theoretical trays are determined (see Actual reflux and theoretical stages) one needs to estimate the actual physical number of trays required in the distillation column. This is usually done by dividing the actual theoretical trays by the overall average fractional tray efficiency. Then a few extra trays are normally added for offload conditions, such as a change in feed composition. 

N = Total equilibrium stages in the column including reboiler and partial condenser R = Actual reflux ratio... 

Example 8-9 Using Figure 8-24B to Solve Gilliland s Equation for Determining Minimum Theoretical Plates for Setting Actual Reflux (used by permission [122])... 

McCormick [97] presents a correlation for Gilliland s chart relating reflux, minimum reflux, number of stages, and minimum stages for multicomponent distillation. Selecting a multiplier for actual reflux over minimum reflux is important for any design. Depending on the com-... 

Assume actual reflux ratios of 1.2, 1.8, 2.25, 3.0 times the minimum and plot the effect on theoretical plates using Gilliland plot... 

Figure 8-28 presents the usual determination of optimum or near optimum theoretical trays at actual reflux based on performance. It is not necessarily the point of least cost for all operating costs, febrication costs or types of trays. A cost study should be made to determine the merits of moving to one side or other of the so-called optimum point From the Figure 8-28 ...

First choice actual reflux ratio, L/D = 1.33 Corresponding theoretical trays or stages, N =... 

The combined Fenske-Underwood-Gillilland method developed by Frank [100] is shown in Figure 8-47. This relates product purity, actual reflux ratio, and relative volatility (average) for the column to the number of equilibrium stages required. Note that this does not consider tray efficiency, as discussed elsewhere. It is perhaps more convenient for designing new columns than reworking existing columns, and should be used only on at acent-key systems. 

Eduljee [107] evaluated published data and corrected relationships for determining the number of actual trays versus actual reflux with reasonably good agreement ... 

R = Reflux ratio = External reflux ratio for a given separation, = L/D, L = liquid rectifying column R = Actual reflux ratio, O/D Rra = Minimum reflux ratio, O/D R = Pseudo minimum reflux ratio Rmin = Minimum external reflux ratio for a given separation... 

St = Theoretical trays/stages at actual reflux, L/D, including reboiler and total condenser Sopt = Optimum stripping factor (SR)i = Separation factor... 

Since the actual reflux is 8 per cent above the minimum,... 

The last step in the Hdist program is the calculation of actual trays with a given actual reflux input. Gilliland s [10] correlation of stages and reflux does this step. Gilliland s equation follows ... 

Actual number of theoretical stages calculated for input actual reflux ratio of 2.0 = 9.60 (add two more stages for condenser and reboiler, or 12 theoretical total)... 

Set the actual reflux ratio RR equal to 1.1 times the minimum and calculate the vapor boilup in the column ... 

Design procedure. The minimum refluxes computed for each section are compared with each other. The highest value is the minimum reflux for the column. From Eq. (2.35) the corresponding minimum liquid flow in the section is calculated. This flow can be multiplied hy a certain factor, commonly between 1.05 and 1.3 to give the optimum flow. Guidelines for selecting factors are given in Sec, 3.1,6, The liquid flow can now be resubstituted into Eq. (2.35) and the actual reflux ratio calculated. 

The intercept of each component balance line on the y axis can now be calculated from Eq. (2.40), using the actual reflux ratio, Each com-... 

Once the initial overhead composition has been determined in the trial-and -error process, both minimum and actual reflux ratios can be determined from the relationship. 

Selecting a reasonable actual reflux ratio (say, 1.3 x Rmin)i, tV... 

For water- or air-cooled columns, the actual reflux ratio l actuai is normally 1.1 to 1.3 times the minimum reflux ratio The optimal relationship between factual and f niin can be established by an economic analysis that compares the cost of energy (which rises with rising reflux) with number of column trays (which declines with rising reflux). For the present example, assume that... 

Table 13-6 shows subsequent calculations using the Underwood minimum reflux equations. The a and Xo values in Table 13-6 are those from the Fenske total reflux calculation. As noted earlier, the % values should be those at minimum reflux. This inconsistency may reduce the accuracy of the Underwood method but to be useful, a shortcut method must be fast, and it has not been shown that a more rigorous estimation of x values results in an overall improvement in accuracy. The calculated firnin is 0.9426. The actual reflux assumed is obtained from the specified maximum top vapor rate of 0.022 kg- mol/s [ 175 lb-(mol/h)] and the calculated D of 49.2 (from the Fenske equation). 

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