Column section distributed feed columns

The reader should be aware that the minimum reflux scenarios presented here are just one of three possible ways the minimum reflux limit can be obtained in distributed feed columns. The designs shown thus far all depicted minimum reflux when the vertex of the internal CS adjacent to the topmost rectifying section lies exactly on its profile, that is, a pinch occurs on the topmost rectifying CS. It is perfectly valid for the minimum reflux condition to be determined by the bottommost stripping profile, or indeed where the TTs of the internal CSs do not overlap one another. The latter case is shown in Figure 6.12 where the column reflux has been reduced and TTs cascade around one another, thereby limiting any further column reflux reduction. The general requirement for minimum reflux is however the same as for simple columns any reflux value below the minimum reflux value will lead to a discontinuous path of profiles, and minimum reflux is therefore the last reflux where a continuous path is still maintained. 

Henry s law constant for solute in feed liquid phase Henry s law constant for solute in solvent liquid phase equilibrium constant distribution coefficient molecular weight of feed without solute molecular weight of solvent without solute interfacial surface tension from Fig. 7.12, dyn/cm partial pressure of solute, atm raffinate density, column section 1, lb/ft3 entering solvent, lb... 

In the mode of minimum reflux, i min at sharp distillation without distributed components traj ectory of the top (bottom) section goes from the product point xd xb) to the trajectory tear-off point Sj Sj) into the boundary element, containing one additional component referring to product components, that is the closest one by phase equilibrium coefficient, then it goes from point S (Sj) to the point of trajectory tear-off S (S ) inside concentration simplex, then it goes from point to point X/ I (xf) in the feed cross-section of the column. Along with that, material balance should be valid in the feed cross-section. 

We have a considerable limitation of sharp extractive distillation process in the column with two feeds the process is feasible if the top product components number is equal to one or two. This Umitation arises because, in the boundary element formed by the components of the top product and the entrainer, there is only one point, namely, point iV+, that belongs to the trajectory bundle of the intermediate section. If Eq. (6.11) is valid, then the joining of the trajectories of the intermediate and top sections takes place as at direct split in two-section columns in the mode of minimum reflux. If Eq. (6.12) is valid then joining goes on as at split with one distributed component. 

In general, at intermediate sphts and splits with a distributed component, the calculation from one of the ends of the column for such splits encounters large difficulties. Determination of possible compositions in the feed cross-section of the column is of great importance for overcoming these difficulties. To estimate correctly the limits of change of component concentrations at the trays above and below feed cross-section, this limits have to be determined at sharp separation ([ / i] and [x/] ). 

State the feasible splits for the equimolar mixture of acetone(l), benzene(2), chlo-roform(3), and toluene(4). For each spht, determine the necessary tray numbers and liquid and vapor flow rates in the top and bottom sections of the column at optimal location of the feed tray and optimal distribution of the component and reflux excess coefficients and product purities as in item 1. 

Thermodynamic losses caused by mixing of flows of different composition in the feed cross-section of the colunm (A2). These losses always arise at separation of multicomponent mixture at any split without distributed components. The losses are absent only at the preferable split when the compositions of the liquid and vapor parts of feeding coincide (in the mode of minimum reflux) or are close (at the reflux bigger than minimum) to the composition of the liquid flow from the top section of the column and to the composition of vapor flow from the bottom section of the column, respectively. 

The entrance of a liqmd-flashing vapor mixture into the distillation column feed location requires a specially designed distribution tray to separate the vapors from the liquid, w hich must drop onto the packing bed for that section in a uniform pattern and rate. 

A tower comprised of rectifying (above the feed) and stripping (below the feed) sections is capable of making a more or less sharp separation between two products or pure components of the mixture, that is, between the light and heavy key components. The light key is the most volatile component whose concentration is to be controlled in the bottom product and the heavy key is the least volatile component whose concentration is to be controlled in the overhead product. Components of intermediate volatilities whose distribution between top and bottom products is not critical are called distributed keys. When more than two sharply separated products are needed, say n top and bottom products, the number of columns required will be n — 1. 

Almost all computer programs employed currently adopt the Thiele-Geddes basis that is, they evaluate the performance of a column with a specified feed, bottoms/overhead ratio, reflux ratio, and numbers of trays above and below the feed. Specific desired product distributions must be found by interpolation between an appropriate range of exploratory runs. The speed and even the possibility of convergence of an iterative process depends on the values of starting estimates of the variables to be established eventually. Accordingly, the best possible starting estimates should be made by methods such as those of Sections 13.7 and 13.8, or on the basis of experience. 

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