Finite reflux

Equation (5.3) can be written at minimum reflux and then at finite reflux, say, 1.1 times minimum reflux. The calculation is then repeated for all columns in the sequence. 

Using Figure 8-33 the separation from Xq, initial kettle volatile material to X3 as the distillate of more volatile overhead requires three theoretical plates/stages at total reflux. Using finite reflux R4, and four theoretical plates the same separation can be achieved with infinite theoretical plates and the minimum reflux ratio, Rmin- The values of reflux ratio, R, can be determined from the graph with the operating line equation as,... 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

Equation 11.1 can also be written at finite reflux, Figure 11.3. Defining RF to be the ratio R/Rmin (typically R/Rmin = 1.1) ... 

Figure 12.8 Mass balances for rectifying section at finite reflux conditions.

The distribution of nonkeys actually depends somewhat on the reflux ratio. For instance, in the case of Example 13.10, the distributions at minimum trays (total reflux) and minimum reflux are substantially different. Often it turns out, however, that the distributions predicted by Eq. (13.119) are close to those at finite reflux whenever R is near 1.2Rm, which is often near the economic value for the reflux ratio. Further discussion of this topic is by Hengstebeck (Distillation, 1961) and Stupin and Lockhart (1968) whose work is summarized by King (1980, p. 434). Knowledge of the complete distribution is needed for estimation of top and bottom temperatures and for determination of the minimum reflux by the method to be cited. 

Most efficiency data reported in the literature are obtained at total reflux, and there are no indirect VLE effects. For measurements at finite reflux ratios, the indirect effects below compound the direct effect of Fig. 14-42. Consider a case where apparent < OW and test data at a finite reflux are analyzed to calculate tray efficiency. Due to the volatility difference Rmin.apparent > hmin,tme- Since the test was conducted at a fixed reflux flow rate, (R/Rmia)appaieot < (R/RmiIJtme- A calculation based on the apparent R/Rmin will give more theoretical stages than a calculation based on the true R/Rmin. This means a higher apparent efficiency than the true value. 

The total reflux start-up period is ended when the unit reaches its steady state. Product is collected at some constant finite reflux ratio until the accumulated product composition reaches its desired purity. This type of operation is very common in practice and is known as constant reflux operation. Under this operation mode the column is operated using a fixed reflux ratio for the whole operation (cut), producing better than specification material at the beginning and below specification material at the end of the fraction (Barolo and Botteon, 1997 Greaves et al., 2001)... 

The above analysis suggests a third alternative (Figure 3.17) with only two columns. The first split could deliver high-purity acetone, while the second split would give chloroform with acetone as impurity. The representation predicts that chloroform purity would not exceed 98% for a reasonable amount of entrainer. Again, computer simulation gives a much better solution. The concentration profile for the first column shows clearly that the distillation border is crossed at finite reflux, and high purity can be obtained in the second split. 

Most efficiency data reported in the literature are obtained at total reflux. At total reflux, there are no indirect effects, and Fig. 7.6 shows the overall effect of VLE errors on column efficiency. For measurements at finite reflux ratios, the indirect effects below add to those in Fig. 7.6. 

Pinching is avoided. It has been recommended to pilot-test at total reflux (16). At finite reflux, pinching can convert small measurement errors into major errors in efficiency estimates (130). However, finite reflux testing is useful in supplementing a total reflux test and providing information on pinch-point location. 

L/V ratio. Most packed-column efficiency testing has been at total reflux, Some testB for both random (3,61,115) and structured packings (3,32,116) suggest that efficiencies at finite reflux are similar to those at total reflux. 

E.xample problems are included to highlight the need to estimate the entire set of products that can be reached for a given feed when using a particular type of separation unit. We show that readily computed distillation curves and pinch point cur es allow us to identify the entire reachable region for simple and e.xtractive distillation for ternary mixtures. This analysis proves that finite reflux often permits increased separation we can compute exactly how far we can cross so-called distillation boundaries. For extractive distillation, we illustrate how to find minimum. solvent rates, minimum reflux ratios, and, interestingly, ma.xinnim reflux ratios. 

There are two limits at which we can examine the behavior of a distillation column. The first is at total reflux (i.e., with an infinite reflux ratio, which is often called infinite reflux conditions). The other extreme is to operate at minimum reflux. In this section we shall limit our discussion to the total reflux case in later sections we shall look at operating columns at finite reflux (ratio) conditions. Intuitively, we tend to expect that a column will give its maximum separation when run at infinite reflux. While this is true for ideally behaving species, it does not have to be true when separating nonideally behaving species. Thus, we need to look carefully at running colunons all the way from minimum to total reflux conditions. 

Petlyuk, F. B. "Rectification of Zcotropic, Azeotropic, and Continuous Mixtures in Simple and Complex Infinite Columns with Finite Reflux." Theor. Found. Chem. Eng. (Engl. Tninsl.) 12, 671-678 (1978). 

Arbitrarily select a number of Oldershaw trays and set up the equipment to operate either at total or at finite reflux, as the situation demands. Oldershaws are generic, off-the-shelf units with varying numbers of trays and can be assembled rapidly. 

For the system to be studied, sun the column and establish the upper operating limit (flood condition). Take data at a selected approach to flood, using total reflux or finite reflux. At steady conditions, take overhead and bottom samples. 

Fewer equations are required to describe distillation columns at total reflux than are required to describe columns operating at finite reflux ratios. The condition that Ljt 4 J + x = 1 (in the limit as -> oo) eliminates the necessity for the determination of the total flow rates, and consequently the energy balance for each stage may be omitted from the set of equations to be solved. This type of total reflux appears to have been first proposed by Robinson and Gilliland.13... 

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