Finite reflux ratios

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)... 

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. 

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. 

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... 

The formulation of this method parallels the formulation presented in Chap. 4 for the 2N Newton-Raphson method for columns operating at finite reflux ratios. Again ideal solutions are assumed, and the condition that Lj/Vj+l = 1 (as Vj+l oo) eliminates the necessity for making energy balances. Suppose that the... 

In order to check the validity of the separation factor Rose and Biles [90] tested a column, arranged as in Fig. 88, with finite reflux ratios between 9.2 and 15.8. In the case of the mixture n-heptane-methylcyclohexane they found a good agreement... 

For finite reflux ratios, Schafer [148] evolved a method involving two nomograms and two diagrams. 

Of course, the above presentation of arithmetic methods is not exhaustive. Fohl [157] published numerical methods for ideal mixtures and batch as well as continuous operation at infinite and finite reflux ratios which make possible a rapid and relatively simple determination of the plate number. The contributions of Stage and Juilfs [71] should also be mentioned in which further accurate and approximate methods are summarized. The same applies to the book of Rose et al. [153]. Zuiderweg [158] reports a procedure which considers the operating hold-up (see chap. 4.10.5) and the magnitude of the transition fraction in batch distillation. 

On page 230 of Matz s book [73] detailed examples ot the calculation are given both for = oo and for a finite reflux ratio greater than the minimum value. 

F is the factor by which the number of separating stages, determined at v = oo, should be multiplied in order that the actual number of stages at a certain finite reflux ratio may be obtained ... 

Arrangement for testing a column with a finite reflux ratio... 

The plate equivalent is the minimum number of plates required at infinite reflux ratio to attain the same enrichment (xB- -Xg) as in a countercurrent distillation with a finite reflux ratio. All distillation conditions except the reflux ratio remain the same. Thus, in the McCabe-Thiele diagram the separating stages are drawn between the diagonal and the equilibrium curve v = oo). 

It can be seen from the table that the separation of low-boiling mixtures is relatively straightforward. Thus, it is usually sufficient to have columns with low efficiency. It has to be borne in mind, of course, that the data given refer to = oo. For finite reflux ratios the requirements have to be altered appropriately. 

Fig. 2.3.2-9 Balance lines and staircase construction, a) Minimum reflux ratio, infinite plate number, b) Finite reflux ratio, resp. finite plate number, c) Total reflux, minimum plate number.

But by analogy with extractive distillation, it can be expected that a second feed point would drastically widen the product region at a finite reflux ratio and thus also increase the conversion. Between the two feed points, the column profile is perpendicular to the distillation lines (Fig. 2.5). Since this effect is based on the finite nature of the reflux ratio employed, we can expect product purity and conversion to first increase with an increase in the reflux ratio and then slowly decrease again. The limiting value that is established for an infinite reflux ratio is determined by the azeotrope concentration in the methyl acetate/methanol system. 

Therefore, the convergence of the recycle loop is simple because the flow rate and composition of the recycle stream D2 are known exacfly. Remember, however, that the number of trays in each column and feed locations must be such that the specified stream compositions are achievable with finite reflux ratios. 

Figure 8-7 shows the characteristic pattern of distillation curves for ideal or close to ideal VLE with no azeotropes. All of the systems considered in Chapters 5. 6, and 7 follow this pattern. The y-axis (Xg = 0) represents the binary A-C separation. This starts at the reboiler (x = 0.01 is an arbitrary value) and requires only the reboiler plus 4 stages to reach a distillate value of x = 0.994. The x axis (x = 0) represents the binary B-C separation, which was started at the arbitrary value Xg = 0.01 in the reboiler. The maximum in B concentration should be familiar from the profiles shown in Chapter 5. Distillation curves at finite reflux ratios are similar but not identical to those at total reflux. Note that the entire space of the diagram can be reached by starting with concentrations near 100% C (the heavy boiler). 

Serafimov, L. A., Timofeev, V. S., Balashov, M. I. (1973a). Rectification of Multicomponent Mixtures. 3. Local Characteristics of the Trajectories Continuous Rectification Process at Finite Reflux Ratios. Acta Chimica Academiae Scien-tiarum Hungarical, 75,235-54. 

Theoretical Plates Required for Finite Reflux Ratios. The equation for the number of theoretical plates at total reflux is convenient for mixtures in which is relatively constant, and it would be useful to... 

Rectification without Liquid Holdup in Column Finite Reflux Ratio In this case, it is assumed that the distillation is carried out with a fractionating column, that the holjluii f liquid in the column is neg igi-... 

As a guide to the characteristics of multicomponent batch distillation, the case of (1) total reflux with no liquid holdup in the column and (2) finite reflux ratio with no liquid holdup by an approximate method, will be considered. 

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