Distillation operating line

Condition of Feed (q Fine). The q line, which marks the transition from rectifying to stripping operating lines, is determined by mass and enthalpy balances around the feed plate. These balances are detailed in distillation texts (15). 

Minimum Reilux with Pinch Zone. There are some distillations where the minimum reflux does not occur at the intersection of the upper and lower operating lines and the q line. These cases arise when the equiUbrium is skewed from positive activity coefficients and when the operating line intersects the equiUbrium line in a 2one of constant composition, a pinch 2one, which is not at the line intersection. Figure 14 illustrates such a case. An example of such a pinch 2one in an ethanol—water column is available (37). 

Figure 14-6 illustrates the graphical method for a three-theoretical-plate system. Note that in gas absorption the operating line is above the equihbrium curve, whereas in distillation this does not happen. In gas stripping, the operating line will be below the equihbrium curve. 

In this equation, all of the terms except y +i and x but including x, are constant. Hence the relationship between and x is linear with a slope of L j(L + D) and a line representing the relationship on a graph of y vs jc must pass tlrrough y = jcd when x — jcd, since tire vapour and the liquid have the same composition in the product. This is called the rectifying operating line in a graphical representation of tire distillation process. 

After filling the receiver, reflux runs down the column at the same molar rate as the vapor back up (L = G). The operating line has a slope of 1.0. Then there are n plates/trays between composition Xp and xj (the mol fraction in distillate). As the distillation continues, the operating line moves closer to the 45° line of the diagram, and x and Xp (and Xj) become richer and leaner, respectively, until at the end xj becomes Xq and x becomes x. The required time is 02. 

Using Figure 8-33 the separation from Xq, initial kettle volatile material to X3 as the distillate of more volatile overhead requires three theoretical plates/stages at total reflux. Using finite reflux R4, and four theoretical plates the same separation can be achieved with infinite theoretical plates and the minimum reflux ratio, Rmin- The values of reflux ratio, R, can be determined from the graph with the operating line equation as,... 

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. 

For systems with one sidestream drawoff, either above or below the feed, Tsuo et al. [102] propose a method for recognizing that the minimum reflux ratio is greater for a column with sidestream drawoff. At the sidestream the operating line has an inflection. For multifeed distillation systems, the minimum reflux is determined by factoring together the separate effect of each feed [103]. 

Mols of component, i, at start of distillation Total mols of liquid in bottoms of still at time, Tj Total mols liquid (not including any steam) in bottom of still at start time Tq (batch charge) y intercept of operating line or constant at fixed pressure for Winn s relative volatility Mols of component, i, in bottoms No. components present, phase rule or no. components, or constant... 

Establish the location of the feed, bottoms and overhead compositions on the graph. Draw in the operating lines as for a distillation in a tray column. 

The slope of the operating line m will normally vary throughout a distillation so it will be necessary to calculate the HETP for each plate or a series of plates. 

Figure 14.7 illustrates what happens if the reflux ratio is fixed for a binary separation. Because the reflux ratio is fixed, the slope of the operating line is fixed, but its position varies through time. For a fixed number of stages, the distillate and the bottoms gradually become more concentrated in the heavier component. This means... 

The esterification by-product, water, is removed via a process column in a continuous steady-state mode of operation. The bottom product of the column, being mainly EG, flows back into the esterification reactor. The condensed top product consists mainly of water with small traces of EG. In cases where a reverse-osmosis unit is connected to the distillate flow line, the residual EG can be separated very efficiently from the water [124], The combination of a process column with reverse osmosis saves energy cost and capital investment. The total organic carbon (TOC) value of the permeate is sufficiently low to allow its discharge into a river or the sea without any environmental impact. 

The graphical construction of an extraction isotherm, an operating line, and the stepwise evaluation of the number of stages in this manner is known as a McCabe-Thiele diagram. Flistorically, it found great application in a variety of mass transfer operations, from gas adsorption through distillation to solvent extraction. Flowever, the advent of modern computational techniques has made it largely redundant, as it is often easier and certainly more accurate to calculate the cascade directly. 

Figure 13.9. Combined McCabe-Thiele and Merkel enthalpy-concentration diagrams for binary distillation with heat balances, (a) Showing key lines and location of representative points on the operating lines, (b) Completed construction showing determination of the number of trays by stepping off between the equilibrium and operating lines.

The constructions in Figure 2 represent a graphical calculation much like the McCabe-Thiele calculation in distillation. Here, the operating line of the upper con-... 

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