Remote Condenser Falling-Film High Vacuum Still F. C. BENNER AND A. DINARDO National Research Corporation, Cambridge, Mass.
A pilot falling-film type distillation unit, for performance in the high vacuum range of pressures, is described. The principal feature of this design is high evaporation rate per unit of evaporator area. The advantages of compact equipment size and high production rates lead to minimum operating costs. Use of the unit in distillation of plasticizer-type esters of which di-2ethylhexyl sebacate is a typical example is described also. Operating data for this liquid are reported.
H
IGH vacuum distillation is a potential tool for the processing
of high boiling chemicals which are heat sensitive. Despite t,he fact that many liquids cannot be distilled except a t pressures in the high vacuum range, high vacuum distillation is not yet used extensively. The major published applications ( I ) have been for the processing of extremely heat-sensitive products, which must be distilled a t as low a temperature as possible with
resultant low production rates based on distillate produced ppr unit area of evaporator surface. Compounds vary widely in their thermal stability. Many chemicals may be too sensitive to heat to withstand the temperatures and contact times cncountered, even under reduced pressure distillation in the range of 20 to 50 mm. of mercury, but nevertheless may not decompose seriously at temperatures corresponding to vapor pressures in the range 0.1 to 1.0mm. of mercury. The distillation unic described in this paper is useful for processing liquids of this type. For distilland pressures of this order of magnitude, high rates of evaporation per unit area can be achieved. The still described here has been operated to study distillation characteristics of liquids and to obtain data for design of plant scale distillation equipment. The data presented relate to a particular class of compounds, but it will be apparent that the scope of application is not necessarily limited. DESCRIPTION OF APPARATUS
Figure 1. Photograph of Still Assembly
The still is of the falling-film, wetted-wall type. Figure 1 is a photograph of the complete assembly; Figure 2 is a diagrammatic representation showing the principal components; and Figure 3 is a cross-sectional drawing of the still itself. In addition to the evaporator and condenser, which are constructed as a single unit, there are two storage tanks, a liquid circulating pump, feed line preheater, overhead feed tank, piping and valves suitable for directing flow of bottoms or distillate as described, and the vacuum pumping system. Stainless steel has been used in the construction wherever possible for those parts of the equipment coming in contact with the product. Still. The evaporator section (Figure 3) is an 8-inch diameter stainless steel tube, jacketed externally and heated by Dowtherm vapor. Liquid is fed to the evaporator by means of a notched overflow weir built into the inner circumference of the top flange. Two feed lines situated 180" apart aid in uniform distribution of liquid to the weir. The overflow weir is leveled by three leveling screws to obtain uniform flow of liquid to the wall. Maintenance of a uniform film on the wall is aided by spiral grooves machined into the surface of the evaporator. The general contour of these grooves is showri in Figure 3. A t low feed rates much of the liquid flow takes place entirely within the grooves, but a t higher feed rates the high points of the grooves are completely covered. For purposes of calculating evaporation rates, superficial area has been used rather than total area. The evaporator wall is 8 inches long and Dowtherm-jacketed over a length of 5.75 inches; evaporation area is 0.916 square foot. Bottoms running off the evaporator wall are collected in an alembic and drained off for storage or recycling. The use of Dowtherm vapor for heating the evaporator has proved satisfa(.tory and vastly better than electrical heaters, which have been found t o cause serious charring on parts of the wall due to uneven heating even after only a few hours of operation. Charring has been negligible in the present Dowtherm-heated system. The condenser is situated below the evaporator and is an %inch inside diameter stainless steel tube, water-jacketed over a length of 9 inches. Placement of the condenser below the evaporator
1930
September 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
w
1931
TABLE I. DISTILLATION OF DI-ZETHYLHEXYL Liquid
4 \.. Dowtherrn B o t t o m Temp., O F. 373 328 410 346 442 359 374 469 376 485
__Temp., ' F. Feed 320 332 336 340 334
Val. 42, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
1932
Atm
Feed Rate, Lb./Hr./
F.' 49 71 95 112 130
Ft. 75.5 119.5 107.5 102,5 107.5
Bottoms Distillate QI Rate, Lb./ Rate, Lb./ B.t.;./ Hr./Ft. Hr./Bq. Ft. Hr. 70 11.8 676 111.5 17.3 1880 97 23.2 2780 86 36.5 3820 90 38.5 5050
permits location of the feed weir s t the head of the still and out of the path of the vapor flow, thus eliminating any possibility of entrainment of feed. Splashing at the weir must, of course, be avoided. Vapor condensed on the wall of the condenser is collected and drained off at the bottom of the section.
Qz B.t.u:/ Hr. 1112 1630 2185 3440 3630
SEBACATE
+
___ Qi Qz, A B.t.u./Hr./ Sq. Ft. 1950 3830 5420 7930 9480
u B t,u./ Hr.i! , F t . ( %.) 40 54 57 71 73
Pressures, Microns Hg AlphaDistron tilland
