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by David E. Pierce, Diamond Alkali Co.
Temperature Cycles in Batch Distillation Effect of reflux holdup is little known but can beof great significance. Here's an interesting case history
I I I I I I I I I I I I I I I I I I I I I
WHEK
chemical engineers get to talking shop, their topics are not always "the remarkable things I have accomplished." In fact, they often tell about some of the problems that have made life so unhappy for them as to make any other \\a)- of life more attractive. I t is characteristic of the members of this profession, however, that they usually conclude their stories with analyses of where the real trouble occurred and how it could be avoided in the future. I t was in one of these discussions some months ago that the writer heard Phil .4rmstrong tell about the pilot plant vacuum distillation that had such a baffling rise and fall of temperature every 20 minutes. As designed, the 100-gallon still, with its column, condenser, piping, and controls. appeared normal in every way. Corrosion-resistant materials were used throughout and there ivas adequate instrumentation. Condensate from the vacuum condenser came down 40 feet to ground level, passed through a rotameter and was then divided, product discharging through a valve and reflux passing through a rotameter and back to the top of column about 30 feet above the iround. T h e general
arrangement, Ivith relative heights, is shown in Figure 1. The batch charged to the still \vas about 800 pounds of an organic
mixture containing four materials to be taken overhead and separated, leaving a still residue. Careful calculation had shown that the column as desiqned should effect the separation. T h e boilup rate was 120 pounds per hour and the reflux ratio was adjusted by means of the valve and rotameter arrangement. Initial operation was quite disappointing. The four fractions were not clean cuts but shomed overlapping of the tail end of one cut with the front end of the nest. Most annoying, too, was a temperature cycle that appeared during removal of the third fraction. As shown in Figure 2 , the top of column temperature rose from about 70" to more than 90", then dropped back to about 70". I t then rose steadily to about 93" but dropped back to about 77" before rising again to about 95". This procedure continued throughout the 3-hour removal of this cut, each cycle lasting about 20 minutes. Special attention \vas given to the third cut because its cyclical behavior might give a clue to the poor over-all performance. This particular cut was an intermediate consistinq of a mixture of two components, A being the low-boiler I
Figure
1.
Batch vacuum still
VOL. 48, NO. 9
SEPTEMBER 1956
51 A
m d EQUIPMENT AND
DESIGN
A Workbook feafure ~~
Figure 2. Overhead cycle during cut 3
temperature
and B the high-boiler. T h e relative volatility of A to B was high, 15.0 in fact. Figure 3 shows that the theoretical performance of the column should have been similar to curve 111, instead of what it actually was-like curve I. T h a t is, to get pure B coming overhead, distillation of about 22y0 of the charge should have been enough to remove A completely and make the product 100% B. Instead of this, 31y0 of the batch had to be removed before all of component A disappeared.
For comparison’s sake, curve I1 in Figure 3 shows what the Raleigh equation predicts for simple distillation of the mixture without any fractionation. According to this curve, pure B would appear only after about 44% of the batch had been distilled. T h e column, therefore, was doing some fractionation. but not what would be predicted. T h e capacity of the unit was low because an abnormally large intermediate cut had to be taken and returned to the next batch. Furthermore, the rise and fall of temperature was without rhyme or reason, something that couldn’t happen according to accepted principles. As would be expected, the difficulties with the column led to much discussion and to various hypotheses as to the cause of the peculiar behavior. Faulty instruments were blamed originally. However, they were checked carefully and found in good order. Reams of data kvere accumulated-both process and equipment performance-but the confusion continued. T h e logical explanation that finally came to the pilot plant engineers was excessive reflux holdup. They calculated that the volume of the 1-inch piping from the condenser down 40 feet to the ground level and back up 30 feet to the top of the column was enough to hold 40 pounds of distillate. This amount was about 5% of the total charge. Perhaps more to the point, this
amount was equivalent to 30y0 of one of the four components. I n short, for a batch distillation unit, there was a lot of reflux holdup. Frequent samples of overhead were now taken during the third cut with its cyclical temperature variations. Sure enough, there were corresponding variations in liquid composition, cycles of concentration of component B going from 67y0 down to 31y0 and back up again. The high relative volatility between A and B entered in a big way into the cycling. Evidently, while vapors rich in B were still flowing from the top of the column, condensing, and entering the reflux line, practically pure A in the return leg of the reflux loop was being fed back into the column. Since liquid containing only 10% ‘4 is in equilibrium with vapors containing 60y0 A , the effect of feeding rich reflux back into the column was to bring down the boiling temperature very rapidly into one of the valleys of the curves in Figure 2. Evidently, there was very little mixing in the reflux line itself, so that condensate of one composition rode down the pipe and pushed material of sharply different composition ahead of it back into the column. In discussing the solution of the problem, Phil Armstrong pointed out that it was just luck that the temperature cycling effect was amplified by the high relative volatility so that attention was called to the reflux holdup volume. Although the cyclic concentrations have been smoothed out in curve I of Figure 3, it is evident that there has been a serious adverse effect on the fractionation which, without any holdup, should have approximated curve
111.
MOL % DISTILLED
Figure 3.
52 A
Sharpness of separation curve
INDUSTRIAL AND ENGINEERING CHEMISTRY
Like many other problems, a clear understanding of the whole situation made the solution quite evident. Substantial reduction of the volume of the reflux piping was the answer. There has been considerable discussion in the literature about the effect of column holdup on batch distillation, but little attention has been given to the effect of reflux holdup. We are therefore grateful to Phillip E. Armstrong and to the Westvaco Chlor-Alkali Division of Food Machinery and Chemical Corp. for the information used in this column.
