n . s.
MYERS
LOW PRESSUREDROP VACUUM COLUMN Distillation effectiveness can be bought with h a t and cooling water-instead
of with pressure drop
or some time our laboratory has been looking for a F good vacuum Titillation column, primarily to perform so called boiling-point distillations on hightrue
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boiling materials. The main requirement for such a column is that it have low pressure drop and yet maintain a reasonable efficiency. Unfortunately, most fractionating columns gain efficiency at the expense of pressure drop. In order to minimize pressure drop, vacuum columns that operate in the range below about 10 mm. of mercury are normally inefficient. The apparatus pictured at the left represents a section of a column which operates at low pressure drop, high - throughput.. and high efficiencv. It is the most convenient equipment we know of for obraining efficient, vacuum fractionation without excegnive pressure drop. Two important design considerations are: -a variable number of high capacity, low pressuredrop trays, or a short length of high efficiency packing is used in each section. -vapors are condensed as they leave each section, and are re-vaporized as thev enter the next section. This permits applying vacuum at the top ofeach section. In theory, as many sections as desired could be stacked on top of each other to produce any required number of trays, without increasing the total p m u r e drop above those across one section. This value could be minimized, if necessary, by decreasing the number of trays per section. In the extreme, all the trays could be removed, leaving only the reboiler in each.scction. With this arrangement the pressure drop would be only a haction of a millimeter.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Here, we are utilizing a method of buying trays with heat and cooling water instead of with pressure drop. For commercial columns, the idea may be attractive only if the product is valuable. But for small-sized equipment, utility costs are usually insignificant. For such cases, the multisection column described offers some intriguing possibilities. Appwatus
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trays under vacuum also show efficienciesover loo%, but not as high as those indicated in the current tests. At 10 mm. of mercury and 2.5 mm. of mercury, efficiencies appear to be slightly lower, but the reason might well be the use of relative voltility values that are higher than actual. In this range we were forced to extrapolate from the volatility data at 50 mm. of mercury. The uncertainty involved could easily account for the apparent decrease in efficiency. Pressure drop is nearly constant over most of the range of operation between the load point and the flood point. This is typical of the Perfo-Drip tray.
The column section in our prototype unit was made from 1-inch precision-bore tubing. We used three PerfoDrip trays in each section, spaced three inches apart. These are essentially sieve trays without downcomers, and have been described in INDUSTRIAL AND ENGINEERING CHEMISTRY,50, 1671 (1958). Trays were made from perforated stainless steel sheet with '/,&ch holes and 30% open area. The removable tray assembly rests on a glass packing support that is integral with the column. Thus the trays could be replaced with packing if desired. The trays are easily removed for cleaning. Our test column consists of three sections. The upper two of these sections are identical to the one shown opposite; the lower section is a conventional three tray PerfDrip column. At the bottom is the primary reboiler, heated with a heatingmantle; and at the top of the upper 1 I I section is a standard, liquid dividing head of the tilting0 400 800 IXK) 1600 funnel variety with an integral condenser. IHROUGHPUT, TOP OF COLUMN, ML. LlQUIO/HR. The figure opposite shows one section of the multisection column. Each section is, in effect, a reboiler, a column, and a condenser, although the condenser receives vapor from the section below it-not from the column in its own section. At the base of the section a 300-watt quartz heater, inserted through an O-ring seal, supplies the necessary heat. An overAow tube maintains a level in the reboiler just high enough to cover the beater. The loop in the tube forms a liquid seal thus preventing vapor bypass. Because of the high turbulence around the heater as well as in the main section of the reboiler, the operating holdup in the reboiler is quite low-probably just slightly greater than the holdup on an operating tray. I I I I 0 400 800 1203 It Vapor flow paths are shown on the diagram. Finally, THROUGHPUT, TOP Of COLUMN, ML. LlQUID/HR. a silvered vacuum jacket surrounds the entire assembly to minimize heat loss. Operating dola j o r sapmorion o j a &caru-tranr-dcculin mixture in the three section Pmfo-Drip column. Bacnlue o j thd m w h g tzchnique, throughput figures IMY ba mmwhat low. Since e&en&s Test Results at 10 and 2.5 nun. qf Hg depend on ax&a#olaLedwpa presme &to, tme &iemia moy bc h i g h &ti tho- shown The column was tested under vacuum with a decanetrans-decalin mixture. Results are shown in the two graphs. Throughputs were measured at the top of the column by setting the reflux divider for total take-off. This is an approximate technique, and gives results that are probably low. Some reflux was noted entering the top of the column even with the splitter in the total take-off position. Considering tray efliciencies, it should be remembered that the three reboiler trays are essentially perfect. At AUTHOR H. S. Myers is a Senior Research Engineer for 50 mm. of mercury the tray efficiencies are high, well C. F. Braun and Co., A l h b r a , Calq. The equipment over 100%. Our previously published data for these illustrated was fabricated by the James F. Scadon Co. VOL 5 5
NO. 5
MAY 1963
37
