Distillation enriching operating line

Annual cost for distillation column. The McCabe-Thiele simplifying assumptions apply for this case, and the number of theoretical plates can be determined by the standard graphical method shown in Fig. 11-8. The slope of the enriching operating line is 1.5 /(1.5 + 1) = 0.6. From Fig. 11-8, the total number of theoretical stages required for the given separation is 12.1. 

The batch distillation of a binary is somewhat simplified, as L/V values can be assumed, and since there is only enrichment of the overhead involved, only one operating line is used per operating condition. Theoretical trays can be stepped off and xib values read to correspond. The plots involved are the same as previously described. 

Steam stripping is to be used to remove a solvent from contaminated soil. An enriching colunm will be used to recover the solvent from the stream. A vapor feed of 40 mol/hr with a composition of 20 mol% solvent and 80 mol% water enters an enriching column. The distillate stream is to have a flow rate of 5 mol/hr and a concentration of 90 mol% solvent. The internal reflux ratio is 0.875 and constant molar overflow (CMO) may be assumed. Graph the operating line to predict the number of equilibrium stages in this enriching column. 

In distillation, the liquid reflux returned to the top of the column plays, in a sense, very much the role of a solvent. In the course of its downward flow, it dissolves residues of the heavy component contained in the vapor phase and thereby contributes to its enrichment in the lighter, more volatile component. A reduction of reflux, or of the reflux ratio, may therefore be expected to result in an increase in the required number of stages in much the same way as happens in the case of the gas scrubber. Ultimately, when the operating lines simultaneously intersect the equilibrium curve, a pinch results and the number of stages goes to infinity. This is shown in Figure 7.19a. The reflux ratio at which this occurs can be read from the intercept Xpi/ R +1) and represents the minimum required to achieve the desired separation. 

Figure 8.1.20. Vapor-liquid equilibrium curve, operating lines for enriching section and stripping section and construction of ideal stages and q Une for a feed consisting of vapor and liquid in a multistage distillation column having ideal equilibrium stages the McCabe-Thiele method.

Figure 8.1.24. (a) Schematic for a partial condenser in a distillation column (feed and stripping section not shown), (b) McCabe-Thiele diagram for the partial condenser and the enriching section operating line. 

Figure 8.1.34. Batch distillation with reflux in a multiplate column, (a) Schematic of the device with total condensation, liquid reflwc and distillate product, (b) McCabe-Thiele diagram for constant reflux ratio with enriching section operating lines for three times, t = 0, t = tj, t = t2.

Distillation separates a feed solution by means of volatility differences. When the feed is at the top or the bottom of the column, the analysis parallels that for gas absorption. In the most common cases, the feed is in the middle of the column, and the feed is separated into a distillate enriched in the volatile component and a bottoms depleted in this component. In all cases, the analysis begins with mole balances, called operating lines and with free energy balances, called equilibrium lines. 

If, from a specified lube distillate we are required to make a given raffinate, R, and extract, E, and are given the reflux ratio for the enriching section of the column, operating points V and 0 for each section may be constructed on the diagram. A line constructed... 

The symbol used on a diagram for a plate column should indicate the type of tray used in the system bubble-cap, valve, or sieve. The first distillation column was invented in 1917. Today, a number of modifications allow modern process technicians to operate much more efficiently. The design, however, still includes the original still-on-top-of-a-still approach. The basic components of a plate distillation column are a feed line feed tray stripping section below the feed line enriching or rectifying section above the feed line overhead vapor outlet, side-stream outlet, and bottom outlet reboiler instrumentation for level, temperature, flow, pressure, and composition control outer shell and a top reflux line. 

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