Thermodynamic Losses and Their Decrease

The comparison of adiabatic and reversible distillation determines several sources of thermodynamic losses leading to increase of energy expenditures compared with separation work at the reversible process (Eq. [8.2]). 

A decrease in A i is always connected with an increase of necessary tray numbers and total capital costs. Therefore, it is justified only in the case of massive energy expenditures. 

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. 

Such separation takes place in flowsheets with prefracionator and in 

Thermodynamic losses caused by input into the column of unequiUbrium flows of reflux from condenser and of vapor from reboiler (A3). To exclude A3, it is necessary to replace condenser and reboiler by the input of liquid and vapor from the other columns (i.e., to turn from the flowsheet in Fig. 6.12d to the flowsheets in Fig. 6.12c,e,f). At such passage, parts of section trajectories Xd S and xb Sj, at which nonequilibrium of liquid and vapor flows being mixed at the trays is especially big, are excluded. It is very clearly seen in Fig. 8.5 (Petlyuk Platonov, 1965), which shows working and equilibrium lines for each of three components at the preferable split and mode of minimum reflux (ais = 5 0-23 = 2 Xfi = 0.1 Xfi = 0.6 xpi = 0.3 xbi = 0.0001 xz)3 = 0.0004 Lmin/F = 0,25). As is evident from the figure, the nonequilibrium at the end parts of the column xd S and xb Sj, if working with a condenser and a reboiler (the shaded regions correspond to them), exceeds many times the nonequilibrium in the middle part of the column at parts Sj x/ i and Xf. 

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