Falling-Film High Vacuum Still LEROY U. SPENCE Rohm & Haav Company, Philadelphia, Pa. THIS dcvclopmeiit A pilot plant high vacuum falling-film still was deSimilar 1at)orator.v stills of high boiling esters veloped for distilling high boiling esters. The evaporating have been previously desurface was 12.75 inches outside diameter by 18 inches of dibasic3 acids for use as scribed hy .wveral earlirr plasticizers for polyvinyl high and was heated with Dowtherm vapor. The conworltrrs in this field ( 1 , ,?, denser was a concentric cylinder, 16 inches inside diamchloride rcsins, as ingredi5 ) . A 14-mm. outside dieter, cooled with water. Di-2-ethylhexyl sebacate was ents of sperial lubricants, ameter glass tube with a distilled at rates of 50 to 60 pounds per hour per square and for use in hydraulic condenser jacket of 35-mm. fluids, it \vas desirable to foot of area at pressures of 0.3 to 0.4 mm. Design of larger tubing was tried first. The stills is discussed, and an increase from the present size purify the products by disinner tube was wound with of 260 pounds per hour to 2000 pounds per hour seems 17 feet of Xirhrome wiiu? tillation. The presence of practical in a single unit. uniseacted ulcohols or of in a spiral to distribute the solvents used in the esterifoed over its surface, and it fication proress was harmwas heated hy rcfluying ful, giving a low flash point and a higher volatility than desirable. various liquids boiling in the range of 178" to 260" C. at atmosThe high boiling impurities were dark in color and gave high pheric pressure. Later a distilling tube consisting of a 1-inch standard iron pipe was used. A spiral groove was cut into the cloud points arid poor ekctrical properties. The esters of interest to this laboratory ranged from 340 to surface of the pipe over an 18-inch length to distribute the fred. 460 in molecular weight and had boiling points ranging from 230" These laboratory stills gave satisfac4oi-y products from the esters, and the pilot plant still w a s then constructed, to 280" C. at 10 mm. of mercury. Lower boiling csters, such as dibutyl phthalate and dibutyl sehacate had been successfully PILOT PLANT STILL distilled in batch vacuum stills at 5 to 10 mm. pressure and an Figure 1 is a photograph of the still and control board. Figure average holdup time of 8 to 12 hours, but the higher boiling esters 2 is a flow sheet of the unit, and Figure 3 is a cross-sertionttl and particularly esters of secondary alcohols arc decomposed drawing of the still itself. All equipment and piping in contart to a considerable extent under surh caonditions. with the feed and product were construrted of Type 347 stainlcss Use of a flash still, where the esters are heated rapidly in a steel, uith a few exceptions which will be pointed out later heat eschanger and the vapors are then flashed off in a rhamber Stainless steel was used so that the equipment would be useful containing entrainment separators is an improvement, b i n w the for a nuinher of other substances corrosive to iron. holding time may be rut down to 5 to 10 minutes. With this The crude ester to be distilled was stored in a 200-gallon alumitype still, esters up to a molecular weight of about 400 can be num feed tank (Figure 2), and was stripped under vacuum a i t h distillcd at 5-mm. pressure, but some decomposition occurs, steam before passing to the still. The preheater for the feed especially with esters of secondary alrohols, and entrainment is was a jacketed pipe heated with 190 pounds per square inch gage difficult to prevent. s t e m . The stripping column was 8 inches inside diameter Use of a centrifugal-type high vacuum still, tvhich \vas deand was packed with 13 feet of 0.5 X 0.5 inch stoneware Raschig veloped by IIickman ( 4 ) for the distillation of extremely heatrings. The ester leaving the stripping column was reheated in sensitive materials, was coniiidered. This still appeared to be a jacketed pipe by 190 pounds per square inch gage steam before unnecessarily complicated and espcnsive for the job to be done. reaching the still. A falling-film still appeared to offer the best possibilities. The still consisted of a 12.75-inch outside diameter cylindrical The rommerrially available falling-film stills were not particevaporating section, surrounded by a concentric water-rooled ularly attractive, because of the use of clectric heat, which made condenser section which was 16 inches inside diameter (Figure temperature rontrol at the distilling surface difficult. At low 3). The feed was introduced on the top of the evaporator section flow rates, bare spots on the surface would hecome overheated into a section of 3-inch pipe tacked to the top of the evaporator and cause charring. to distribute the flow evenly. The shell of the evaporator section Falling-film stills have been Lnown for a long time. Payne was extended 0.5 inch above the top head and was cut with 42 (6) described a falling-film still for the distribution of petroleum vertical slots */,e inch wide by */,inch deep to act as weirs to oils in a patent filed in 1929. -4nother falling-film still was dedistribute the flow over the surface of the evaporating cylinder. veloped by Zahn (9) in Germany in 1929. U'aterman and OsterThe shell of the evaporating surface was grooved to help maintain hog (8) and Carr and Jcnel (2)describe other early falling-film a uniform film over the surface. There are 48 grooves, ' / a inch stills. wide and '/,a inch deep, with B slope of 6 inches per turn. The Laboratory studies were made with vertical stills consisting of evaporating rylinder was heated internally by Dowtherm vapor, a heated vertical tube surrounded n-ith a condenser j:icket.
I"
1926
September 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
supplied by an electrically heated Dowtherm boiler (30 kw.) which was equipped with a pressure controller to maintain a constant Dowtherm temperature. The effective area for evaporation was 4.3 square feet, neglecting the area added by the grooves; the grooves are filled with liquid, and the area presented by the liquid film is considered in c:il(.ulating the evaporation rates. The distillate was condensed on the water-cooled outer shell and collected in a trough and removed continuously through a cooler by the distillate pump. The inside area of the condenser shell \vu9 6.3 square feet neglecting the dished head which waa not witor cooled. The condenser shell was cooled by spraying water over it from a circular sparger pipe in most of the runs and later 1)s providing an outside shell filled with water as shown Figure 1. Tho undistilled material was collected at the bottom of the still and removed through a similar cooler and pump. The d i 5 tillate and residue pumps were Nitralloy gear pumps (Northern Ordnance Company, Inc.), with a capaeity of 0.9 gallon per minute, and were immersed in a bath of distilled ester to prevent air Irakage through the stuffing boxes. The vacuum linc to the still was a &inch stainless pipe with a watcr jacket to condense any vapors escaping. The vacuum was iLpplied by a ICinney pump, Type VSD 8-8-11 with a displacaement of 43 cubic feet per minute at 330 r.p.m. This was protectod by a dry ice-cooled trap. A recording thermal conductivity gage (Televac, Precision Scimtific Company, Chicago, Ill.) was used to measure the pressure on the still, and Pirani gages were used for measurement of prcsaures at the inlet and outlet of the dry ice trap. An Alphatron gage wtir also used to check the other gages at intervals. OPERATION OF THE UNIT
The ester as supplied from the plant esterification process is essentially neutral and contains a small amount of unrencted alcohol. The terhnical grade ester was fed through a rotameter a t a rtltc of 250 to 530 pounds per hour through the preheatcr and
1927
introduced into the top of the stripping volumn :it a ternperaturc of about 175' C. The low boiling impurities arid dissolved sir were removed in the stripping tower by cbontacting with 20 to 25 pounds per hour of steam preheated to 165" C . The stripping colurnii wa.. operated under a pressure of 5 to 8 mm. of mercury. The stripped ester was fed through n 3Ginch seal leg and WILS preheated on the way to the still to a temperature close to the vaporization temperature at the vacuum at which distillntiori was to be carried out!. The still was operated at pressurcs in the range of 0.10 to 0.50 mm. its indicated by the Televav gage. The rate of distillation could be varied from 50 to 250 pountls per hour by adjusting the Dowtherm tempernture. Temperatures in the range of 180' to 280' C. could be obtained by regultrting the pressure on the Dowthcrm boiler. The distillate was collected in drums :ml the residue in a 30gallon stainless steel receiver (or in drums). The residue wm usually passed through the still agitin to rccovcr adttitioncil yicld of distilled product. Automatic controllers were provided for vacuum and tcmpcrature where important to the smooth opcration of the unit. The operation of the still was controlled by one skilled operator (nontechnical) per shift, and operation was generally continuous, with three shifts per day on a 5-day week. OPERATING VARIABLES
A large number of esters, such as dicupryl sobarate, dibetizyl sebacate, dicapryl phthalate, and di-2-ethylhexyl sebacate, have been distilled in this equipment. The boiling points of these esters aTe given in Table I along with some other esters on which the data are available. A rather thorough study of the operating variitbles in this still were made using di-2ethylhesyl sebacate. Pressure Drop. The difference between the reading on the Televac gage and the Pirani gage bcfore the dry ice trap is an indication of $hc pressure of organic vapors in the still. This pressure diffcrcnce varies with the Dowthcrm tcmpcrature :tnd th(*rate of distillation as shown in Table 11.
Figure 1. Pilot Plant Still
1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 42, No. 9
success. Boiling temperatures of the film were estimated from the vapor pressure curve a t presBoiling Point. C . . at *sure of sure observed in the still, and the temperature Ester iMol.Wt. 10 mm. 5 nun. 1 mm. 0.5 mm. 0 . 1 mm. difference was correlated with the rate of disDi-n-amyl sebaaate 342.28 228 217 180 170 142 tillation in Table 111. Dioapryl phthalate 390.29 250 231 197 184 154 Di - 2 ethylhexyl sebaHeat Transfer. Since the feed to the still was cate 426.38 266 249 216 203 175 below the vaporization temperature, part of the Dicapryl sebacate 426.38 265 250 216 202 174 Dibenzyl sebacete 283 264 236 222 195 still surface was used for preheating and the rest Di-tetradecyl pbthalate 400.24 (7) 558.47 278 263 235 223 198 for vaporizing the ester. Measurements were Di-tatradeoylsebacate made on the preheating heat transfer rate by (7) 594.56 300 283 256 243 218 feeding Over the at atmospheric presa Vapor pressure data were determined by D. R . Conlon of the Rolim & Ham Company Research Laboratories. sure and measuring the temperature rise. A t feed rates of 340 to 390 pounds per hour and a Dowtherm temperature of 220' C., over-all heat transfer rates of 30 to 40 l%t.u. per hour per At the maximum distillation rates studied the pressure drop square foot was obtained. was 0.35 to 0.40 mm. The reading of the Televac gage was Calculations of the heat transfer rate for distillation, after corprobably higher than the true pressure, as this gage was Calirecting for the preheating necessary, gave coefficients of 70 t,o 90 brrtted with air, and organic vapors diffusing into the gage would B.t.u. per hour per square foot per F. Possible errors in give high readings. A draft gage on the still at the maximum distillation rate showed a pressure drop of only 0.12 mm. compared with 0.39 mm. on the vacuum gages. Temperatures. Dowtherm vapor temperatures in the still TABLE 11* PRESSCTRE Ram (Feed rate constant at 370 pounds per hour in these runs) were varied in the range of 210' to 260" C. by regulation of the Mm. of Mercury pressure on the Dowtherm boiler, and the distillation rates were Pirani Pressure Distillation Dowtherm Televac observed. The ester fed into the still entered a t temperatures of Temp., C . reading gage drop Rate, Lb./Hour 180"to 150" C., dropping as the feed rate was increased, because 212 0.18 0.G3 0.15 140 0.19 170 22 1 0.22 0.03 of irisufficient heat transfer surface in the preheater a t the higher 234 0.265 0.03 0.235 215 rates. 243 0.310 0.03 0.28U 235 0.340 230 252 C.370 0.03 Attempts were made to measure the temperature of the ester on 261 0.40 0.03 0.370 250 the surface of the still and in the residue leaving the still without
TABLE I. BOILINGPOINTS OF ESTERSOF DIBASICA C I D S ~
-
O
LBJHR.
LOW
LEVEL CONTROL Figure 2.
Pilot Plant Distilliltion Equipment
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
September 1950
passed through the still again, distilling about SO%, and bringing the yield of distilled ester up to 97 to 98%- Further passes of residue permit recovery of essentially all of the di-2cthylhexyl sebacate content of thc feed.
TABLE111. TEMPERATVRE DIFFERENCE AND DISTILLATION RATE Feed Temperatures, e C. Boiling Point T ~ ~ Distillation ~ , Rate, Lb./Hour Feed Dowtherm O C. Mm. Difference, O C. Rate, Lb./Hour 295 370 370 370 370 420 460 490 630
181 171 168 164 167 163 158 155 159
258 212 234 252 261 256 258 256 259
190 183 191 190 198 191 193 194 196
0.25 0.18 0.265 0.37 0.40 0.28 0.30 0.31 0.37
68 29 43 58 63 65 65 62 63
TELEVAC
1929
260 140 215 230 250 235 220 240 225
DESIGN STUDIES FOR LARGER STILL
In order to simulate the effect of increasing the height of the still, runs werc made with the maximum feed rates. and the bottoms were collected and run through the still again at the rate at which the bottoms had been collected. This was designed t o show the results that would be obtained with a still with twice the height of vaporizing surface. The color of the distilled ester from the second paas was as good aa from the first pass, provided that sufficient bottoms remained to prevent dryness. It was ooncliided from these testa that a still could be built with twice the height of vaporizing surface and a distilling rate of 500 pounds per hour with a feed of 530 pounds per hour. Redesign of the feed inlet and weirs on top of the vaporizer would permit an increase of the feed rate to 700 pounds per hour without splashing. This would allow an increase in the height of the still to 50 inches and a distillation rate of 650 pounds per hour. Considerable increase in the diameter of the still seems practical, and this should permit going to 2OOO pounds per hour or more. An evaporator unit 50 inches high and 32 inches outside diameter should give a distillation rate of 2000 pounds per hour. Hugh Schwarz of National Research Corporation in Cambridge, Mass., cooperated in the h a 1 testing of this still, and a preliminary design of a unit to distill lo00 pounds per hour waa prepared, but has not yet been constructed. The operating cost of such a still would be about $10.00per hour or 1 cent per pound of distilled ester.
01STILLATE DISCUSSION
Figure 3.
.
Pilot Plant Falling-Film Still
measurement of the pressure and boiling point make these calculations subject to considerltblo inaccuracy. The heat transfer in this type evaporation would be expected to be much lower than is usually obtained with a boiling liquid, since there is no bubble formation from the film ut these low pressures. Approximately 24% of the heat supplied to the Dowtherm boiler was used in supplying latent heat of vaporization. Heat losses in the small still and Dowtherm system were high, and it is believed that in a large still, about 50% of the heat input to the vaporizer should be utilized in vaporization. Radiation of heat from the diefilling surface to the condenser surface in this type still R i responsible for the low heat efficiency. Feed Rate. The maximum feed rate that could be used without splashing crude feed onto the condensing surface waa 530 pounds per hour or 180 pounds per foot of circumference. Operating a t a Dowtherm temperature of 260" C. and a still pressure of 0.40 mm., the distilling rate remained about 240 pounds per hour as the feed rate was changed from 530 to 260 pounds per hour. The distilled ester remained practically water-white, 45 to 60 American Public Health Association (APHA) color, until the feed rate was reduced below 290 pounds per hour, when the color increased to 100 to 200 APHA. At the low feed rates, the bottoms takeoff was 5 to 10 pounds per hour, and much of the lower part of the still waa bare. It waa desirable to regulate the feed and distillation rat>esso that there were a t least 20 pounds of bottoms per hour. Yield of Distilled Ester. The yield of distilled ester will vary with the purity of the technical product to be distilled. On di-2-ethylhexyl sebacate, the dark colored impurities were less than 1% of the feed. On one pass, 86 to gO% of the feed waa distilled without sacrifice of color. The 10 to 15% resldue was
The falling-film still described is useful for the distillation of heat-sensitive materials boiling in the range of 180O to 280 O C,a t 0.6 mm. This corresponds to a range of 350 to 750 in molecular weight for esters of dibasic acids. Stills with a capacity up to 2OOO pounds per hour appear practical. The separation obtained is about one theoretical plate, which limits the separation to materials boiling at least Bo O C. apart, or a molecular weight difforence of about 200. ACKNOWLEDGMENT
F. W.Robinson carried out the original experimental work and aided in the design of the pilot plant unit. D. W. Kenny and J. H. Steen directed the testing of the pilot plant unit and cooperated with Hugh Schwarz of National Research Corporation on the design of a larger unit. LITERATURE CITED
(1) Burrown, G.,J . SOC.C h m . Ind (London),58T,61 (1939). (2) Cam,F. H., and Jewell, W., Brit. Patent 416,088(1934). (3) Detweiler, S.B., Jr., and Markley, I(.S., IND.ENQ.Cmu.,ANAL. ED.,12, 349 (1940). (4) Hickman, K. C. D., IND. ENQ.CEEX.,39, 880-94 (1947). (6) Jewell, W., Mead, T. H., and Phipps, J. W., J . SOC.C L m . Znd.
(L&),
ST, 69 (1939).
(6) Peyne, E. H. (to Standard Oil Company of Indiana), U. 8.
Patent 1,899,916(1933). (7) Perry, E. S. and Weber, W. H., J . Am. C h m . SOO.,71, 3726 (1949). (8) Waterman, H. I., and Osterhof, D., Rec. trap. chim., 52, 896 (1933). (9) Zahn, O.,Chm.-Z@., 53,807-8 (1929). REJO~VID March 23, 1950. Prasented before the Division of Industrial and Engineering Chemistry a t the 117th Meeting, AMmRlCAN CHaMICAL %L'ITT.'. Houston, Tex.
