Infinite reflux mode

As we will see in Chapter 3, the distillation region and subregion characterize those possible product compositions that can be produced from the given feedstock composition by distillation under one of the most important modes, in particular, under the infinite reflux mode. 

In Sections 1.3 to 1.5, the residue curve bundles, which characterize the direction of Uquid-vapor tie-lines in each point of the concentration space (i.e., the phase equilibrium field), were considered. As stated previously, such characteristics of the phase equilibrium field and structural elements related to it (bonds, distillation regions, and subregions) are the most important for one of the distillation modes, in particular, for the infinite reflux mode. 

When R = oo (infinite reflux mode), the number of theoretical trays is minimum, i.e., n = nmin (the steps between the equilibrium curve and the diagonal line are the largest ones). 

What are the distinctions in the representation of minimum, and more finite and infinite reflux modes in the diagram of McCabe and Thiele ... 

For azeotropic mixtures, not all the practically interesting splits are feasible at the infinite refiux. However, the sequencing should have the infinite reflux mode as its starting point because these splits are the easiest to realize at finite reflux. That is why we start systematic examination of distillation trajectories with the infinite reflux rate. It is also proved to be correct because the regularities of trajectories locations for this mode are the simplest. 

The analogy with the process of open evaporation favored the fact that this mode was investigated earlier than the others. Systematic examination of distillation at the infinite reflux was initially carried out in works (Zharov Serafimov, 1975 Balashov Serafimov, 1984). The analysis of infinite reflux mode in the infinite columns was made (Petlyuk, 1979 Petlyuk, Kievskii, Serafimov, 1977 Petlyuk Serafimov, 1983) that allowed general regularities of separation to be defined for the mixtures with any number of components and azeotropes. A number of important investigations was realized (Doherty, 1985 Doherty Caldarola, 1985 Laroche et al., 1992 Bekiaris et al., 1993 Safrit Westerberg, 1997 Rooks et al., 1998) and others. 

The reader should also be aware that infinite reflux columns are the smallest attainable columns in terms of column height however, they do require infinitely large internal flows and hence have operating expenses that are infinitely large to ensure continuous vaporization and condensation. This is the one extreme mode of operation, the other mode being minimum reflux which requires an infinitely high column to be built (and therefore have an initial capital investment that is infinitely large), but the smallest internal flows necessary for a desired separation. This is discussed in more detail in Chapter 4. 

When R = i min (minimum reflux mode), the number of stages is infinite (in the feed point, the step between stages becomes equal to zero - this is an area of constant concentrations or pinch). 

In the majority of cases, the product compositions under the infinite reflux coincide with the compositions of the product under a mode on the verge of the second and the third classes of fractionation. 

What is the arrangement of the distillation trajectory bundles under infinite and finite reflux modes dependent on ... 

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

Split 1 2,3 is feasible in two-section column at finite reflux, which is unfeasible according to the rule of connectedness (see Chapter 3) in the mode of infinite reflux. The feasibility of such separation was shown first by means of calculation in the work (Kondrat ev et al., 1977). 

Therefore, the conceptual calculation of infinite column with intermediate input and/or output of heat consists in two stages (1) calculation of minimum reflux mode for adiabatic column, and (2) determination of opt 7, opt opt 7, and opt < 5, ( pinch method ). 

Because in the mode of minimum reflux the intermediate section should be infinite, its trajectory should pass though one of its stationary points Sm or A+. Therefore, the following cases are feasible in minimum reflux mode (1) point A+ coincides with the composition at the tray above or below the cross-section of control feed (2) composition point at the trays of the intermediate section in the cross-section of control feed lies on the separatrix line, surface, or hypersurface of point Sm (i.e., in separatrix min-reflux region of intermediate section Reg , filled of trajectory bundle Sm — A+). In both cases, composition point at the tray of the top or bottom section, adjacent to the control feed, should lie in the separatrix min-reflux region of this section Re (5 - A+). 

We note that, for the top and bottom sections, the square filled up with the trajectory bundle is also maximum at (L/V)r = 1 and (E/V) = 1 (the mode of infinite reflux). 

The analysis of possible splits in the mode of infinite reflux with the purpose of synthesis of separation flowsheets was extended to multicomponent mixtures (Pet-lyuk, Kievskii, Serafimov, 1977a Petlyuk, Avetyan, Inyaeva, 1977 Petlyuk, 1979 Petlyuk, Kievskii, Serafimov, 1979 Baburina Platonov, 1990 Safrit Westerberg, 1997 Rooks et al., 1998 Sargent, 1998), (Doherty Malone, 2001). 

At some value of parameter (L/ trajectory bundles of sections Reg and Regi adjoin each other by their boundary elements - separatrix min-reflux regions Reg , s (5 => A+, shortly 5 - A+) and Re = (S N, shortly - N+), mostly remote from product points xd and xb, if one uses for determination of (L/ y) " the model in Fig. 5.29b, or if validity of condition (Eq. [5.18]) is achieved between some points of these boundary elements, if one uses the model in Fig. 5.29a. At this value of the parameter (L/F)J ", the distillation process becomes feasible in infinite column at set product compositions. Such distillation mode is called the mode of minimum reflux. It follows from the analysis of bundle dimensionality 5 - 1V+ and that, at separation without distributed... 

Figure 6.14 shows trajectories of the intermediate section for separation 1 1, 2 3 at different modes. Pseudoproduct points ( > — Dj+D) is located at side 1-2, and joining of the intermediate and bottom sections in the mode of minimum reflux goes on in the same way as for the simple column at indirect split. Trajectory of the intermediate section r tears off from side 1-2 in point Sn, and point of side product xd can coincide with point Sn (Fig. 6.14a) or lie at segment 1 - Sri (Fig. 6.14b). The first of these two modes is optimal because the best separation between top and side products (the mode of the best separation) is achieved at this mode. Zones of constant concentrations in the top and intermediate sections arise in point Sri = AC2- Therefore, in the mode of minimum reflux in the intermediate section, there are two zones of constant concentrations. At the reflux bigger than minimum, point 5 1 moves to vertex 2 and at i = oo this point reaches it (i.e., at i = 00, pure component 2 can be obtained in the infinite column as a side product). Therefore, for the colunuis with side withdrawals of the products, the mode of the best separation under minimum reflux corresponds to joining of sections in points 5 1 and of the trajectory bundle of the intermediate section (at sharp separation) or in its vicinity (at quasisharp separation). The trajectory of the column with a side product at minimum reflux at best separation may be described as follows ...

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