Reboiler analogy

Another design option that can be considered if a column will not fit is use of an intermediate reboiler or condenser. An intermediate condenser is illustrated in Fig. 14.5. The shape of the box is now altered because the intermediate condenser changes the heat flow through the column. The particular design shown in Fig. 14.5 would require that at least part of the heat rejected from the intermediate condenser be passed to the process. An analogous approach can be used to evaluate the possibilities for use of intermediate reboilers. Flower and Jackson," Kayihan, and Dhole and Linnhofl have presented procedures for the location of intermediate reboilers and condensers. 

Thermodynamic efficiency is hurt by the large ATbetween the temperatures of melting and freezing. In an analogy to distillation, the high a comes at the expense of a big spread in reboiler and condenser temperature. Erom a theoretical standpoint, this penalty is smallest when freezing a high concentration (ca 90%) material. 

A column of four stages exists analogous to that shown in Figure El2.1 except that more than one feed can exist (the reboiler is stage 1 and the condenser is stage 4). Feed and product specifications are... 

As an alternative method, Qi et al. [14] proposed to use a reactive condenser (see Fig. 4.1) to predict possible top products of a countercurrent reactive distillation column. The feasibility analyses of the reactive condenser and the reactive reboiler are analogous to the flash-cascade approach. The latter authors used transformed... 

An analogous procedure is followed if the Murphree tray vapor efficiency is known. In this case, it is more convenient to start the graphical construction at the reboiler. 

When a finite value is specified for Lt and D is taken equal to F, the condition of total reboil (B = 0) in the stripping section is realized. The equations needed to apply the 2N Newton-Raphson method are formulated in a manner analogous to that demonstrated above for total reflux in the rectifying section. The condition that B = 0 is reflected by the equations for the Nth stage. The matrices of Eq. (10-38), the component-material balances, contain the following elements... 

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. 

BOTTOM PLATE AND REBOILER. The action at the bottom of the column is analogous to that at the top. Thus, Eq. (18.17), written for constant molal overflow, becomes, with L and later V used to denote flow rates in this section,... 

Whereas in ordinary countercurrent operation the richest possible extract product leaving the plant is at best only in equilibrium with the feed solution, the use of reflux at the extract end of the cascade can provide a product even richer, as in the rectifying section of a distillation column. Reflux is not needed at the raffinate end of the cascade since, unlike distillation, where heat must be carried in from the reboiler by a vapor reflux, in extraction the solvent (the analog of heat) can enter without a carrier stream. 

Reboiler oscillations can often be dampened by increasing flow resistance in the reboiler inlet lines or reducing flow resistance in the reboiler outlet lines. This shifts flow resistance from the outlet to the inlet lines. This is analogous to dampening U-tube oscillations by increasing friction in the bottom of the tube while easing the restriction at the open end of the tube. Design practices for reboiler inlet and outlet lines were discussed in Sec. 15.2. 

Wild (426) described a horizontal steam reboiler ceise history which is analogous to the problem above. A high liquid level in the reboiler shell (Fig. 15.10/) caused liquid heimmer. The remedy was to inject inerts when the level rose excessively. 

Analogously, at high-boiler separations the stripping section is operated with a surplus of reboil and, in turn, no pinch exists in this section. The rectifying section, however, is operated with minimum reflux characterized by a pinch immediately above the feed point. Again, pinch concentration differs from feed concentration. 

To remove traces of the absorbent always present in the overhead product of the absorber, some of the ethanol is recycled as reflux at the top of the absorber A-1. Analogously, a small part of the loaded absorbent is boiled up at the bottom of A-1 to remove most of the coabsorbed ethanol. However, reboil and reflux rates are rather small. Thus, the countercurrent flow within the column A-1 is dominated by the vapor feed (low boiling fraction) at the bottom and the liquid feed (high boiling fraction) at the top of the column what is characteristic of absorption columns. Thus, the process should better be called absorptive distillation instead of extractive distillation. 

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