Figure 219

Simple distillation column on all trays throughout each individual section. This constancy of molar flow rates is what we call "equimolal overflow." Primarily it assumes that the molar heats of vaporization of the components are about equal. In many systems this is a pretty good assumption.

A total condenser is used to produce liquid reflux and distillate product. The reboiler is a "partial reboiler" (vapor is boiled off a liquid pool). The composition of this liquid pool is the same as the bottom product composition. Thermosyphon, kettle, internal, and forced-circulation reboilers are all usually partial reboilers.

The parameter q will be used to describe the thermal condition of the feed. The ratio of the internal reflux flow rate LR to the distillate flow rate D is called the reflux ratio R.

Reflux ratio is widely used as an indication of the energy consumption.

Equations

A. Overall Balances

Mass and component balances can be written around the entire column system.

These relationships must be satisfied under steady-state conditions. Note that distillate and bottom product rates can be calculated from equations (2.23) and (2.24) if feed conditions and product compositions are specified.

B. Stripping Section

A light component balance around the wth tray in the stripping section yields (see Figure 2.20):

Rearranging:

This is called the operating-line equation. It has the form of a straight line: y = mx + b. The slope of the line is the ratio of liquid to vapor flow rates in the stripping section. This straight line can be plotted on an x-y diagram (see Figure 2.21).

The intersection of the operating line with the 45° line (where x = y) occurs at xB. This is easily proved by letting the point of intersection be x, = yt. Substituting into equation (2.27) gives:

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