Columns finite with infinite reflux

What parameters determine separation mode in the finite column with infinite reflux ... 

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. 

The second extreme we can imagine is to maintain finite flows for the feed and products but increase the internal flows for L and V to infinite values. This second case cannot really occur, as we would need a column with an infinite diameter. It is a limiting case. Both ways to think of infinite reflux are useful. In the latter case the column is still thought of as producing its products. 

This formulation of the Newton-Raphson method for columns with infinitely many stages is analogous to the 2N Newton-Raphson method for a column with a finite number of stages. First the procedure is developed for a conventional distillation column with infinitely many stages for which the condenser duty Qc (or the reflux ratio Lx/D) and the reboiler duty QR (or the boilup ratio VN/B) are specified and it is required to find the product distribution. Then the procedure is modified as required to find the minimum reflux ratio required to effect the specified separation of two key components. 

If one further assumes that the internal flows of the column are infinitely large, then drawing off finite amounts of overheads and bottoms products (as done in normal operation of a simple column, refer to Figure 1.1) will have a negligible effect on the internal flows. Of course, doing this then warrants the need for a feed stream to the column in order to obey mass balance. However, it can still be assumed that the internal flows are much larger than either the feed or products, and thus the same material balances put forward for total reflux operation apply, as shown in Equations 2.15 2.17. Thus, it can be stated that the path followed by the liquid in an infinite reflux column will follow a residue curve as well, with the temperatures in the column dictated by the profile. 

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. 

For azeotropic mixtures, the main difficulty of the solution of the task of synthesis consists not in the multiplicity of feasible sequences of columns and complexes but in the necessity for the determination of feasible splits in each potential column or in the complex. The questions of synthesis of separation flowsheets for azeotropic mixtures were investigated in a great number of works. But these works mainly concern three-component mixtures and splits at infinite reflux. In a small number of works, mixtures with a larger number of components are considered however, in these works, the discussion is limited to the identification of splits at infinite reflux and linear boundaries between distillation regions Reg° . Yet, it is important to identify all feasible splits, not only the spUts feasible in simple columns at infinite reflux and at linear boundaries between distillation regions. It is important, in particular, to identify the spUts feasible in simple columns at finite reflux and curvilinear boundaries between distillation regions and also the splits feasible only in three-section columns of extractive distillation. 

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). 

Minimum Number of Plates. The slope of the operating line above the feed is On/Fn, and as this slope approaches unity the number of theoretical plates becomes smaller. When On/Vn is equal to 1, Or/D is equal to infinity, and only an infinitesimal amount of product can be withdrawn from a finite column. Frequently it is assumed that total reflux corresponds to the addition of no feed or to the removal of no products. If such is the case, the tower is not meeting the design conditions. It is better to visualize a tower with an infinite cross section, which is separating the feed at a finite rate into the desired... 

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