INDUSTRIAL AND ENGINEERING CHEMISTRY
498
It is worthy of note that the temperature gradient of Figure 2 is nearly uniform above and below the feed plate. This is quite different from the corresponding plot of a binary mixture, where the temperature gradient is large near the feed plate and small toward both ends of the column. This difference is due to the fact that, in complex hydrocarbon mixtures of several components, the components of intermediate boiling point accumulate in the middle of the column in a way that effectively flattens out the temperature curve. It should be clear that the low-temperature portion of the curves is calculated down from the top of the column, and the high-temperature part up from the bottom. These two portions blend in tangentially where they meet. However, if the upper portions are continued down, they give a too low value of temperature and a too high value of the low-boiling constituents, because in calculating downward from the top it is impossible to allow for the high-boiling constituents. This portion of the curve is shown dotted in Figure 1. The only point regarding the concentration curve which merits special attention is the accumulation of butane in the middle part of the column. This is the component of boiling point intermediate between the bottom and top temperatures of the column, and, as already indicated, this accumulation in the middle of the column is characteristic. The slight rise in butane concentration on the first plate above the feed is more striking. While the general trend of butane concentration above the feed plate is downward, a rise of this sort may occur where the temperature is sufficiently high to give a value of P, sufficient to reverse the slope of the Hausbrand line. Dependable data as to plate efficiency in natural gasoline columns are exceedingly meager, but all the indications are that the efficiencies are high. Thus, a sample of the liquid on the plate in the upper part of a column gave upon analysis 15.7 mole per cent propane and 70 mole per cent
Vol. 24, NO.,^
butane. Using Raoult’s law, the vapor in equilibrium with this liquid should contain 47 per cent propane and 49.3 per cent butane. A sample of the vapor rising from this plate showed upon analysis 49.3 per cent propane and 53.8 per cent butane. In other words, the vapor rising from the plate was richer in butane and poorer in propane than it would have been had it left the plate in equilibrium with the liquid on it. This is another way of saying that the liquid on the plate had not completely dissolved the butane out of the vapor rising into the plate, down to the equilibrium value. However, the difference in composition is small. The vapor approached closely to equilibrium with the liquid, which means that the plate efficiency was reasonably high. It is very desirable that accurate determinations of actual plate efficiency and of the height of the equivalent theoretical plate for the different types of tower-filling used in the industry be made in order to serve as a suitable guide for designing and operating engineers. SUhlM.4RY
The Hausbrand equation for the calculation of plate-toplate concentration gradients in the rectification of binary mixtures can be applied directly to the calculation of gradients in the isopiestic rectification of mixtures however compIex, provided the composition of the feed and the point and sharpness of cut be known, and the components of the mixture follow Raoult’s law. These equations are, therefore, directly applicable in the design of rectification equipment for natural or refinery gasolines and, in general, for any mixture of hydrocarbons in which the concentrations of the individual components are known.
LITERATURECITED (1) McAdams, W.
H., and Morrell. J. C. I N D ENO
(’HEX..16. .
