K. C. D. Hickman and G. C. Mees
A
LTHOUGH no new kinds of Short-path high-vacuum stills have appeared during the past few years, existing types have been improved and enlarged to a point where they render relatively inexpensive, trouble-free service. The early commercial stills employed the falling-film principle, where the distilland was allowed to cascade down a heated circular pole contained within an evacuated tube. These were augmented in 1939 by centrifugal stills in which the oil was spun from the center over the surface of a heated disk about a foot in diameter, housed under a glass bell jar. In 1941 the falling-film stills were entirely replaced by 32-inch centrifugals having shallow cones rotating horizontally on a vertical axis. Experiments with larger sizes showed the desirability of using steeper conical evaporators rotating on vertical shafts. The distilland is admitted to the narrow end of the cone and climbs up the sides under centrifugal force. recently, and at once set interesting standards for high-vacuum evaporation.
They showed, for instance: (1 ) When using electrical heat, even without the advantage of countercurrent exchange or reflecting condensers, the power consumption was less than 0.1 kw. hr. per pound of distilland. (The consumption per pound of distillate varies too widely to be quoted exactly but is ordinarily within the range 0.2 to 0.5 kw. hr.) (2) Over-all operating costs, in
cluding power, labor, amortization and net factory overhead, are in the region of one cent per pound per pass of distilland. Of course, many chemical separations require multiple passes and feed-back of distillate which increase the cost of distillation.
There are residues and light ends of small value; in the case of the oil-soluble vitamins, there are chemical losses that must be debited against operating cost. (3) Capital cost in relation to volume handled compared favorably with conventional processes.
Indeed, a week’s throughput may represent a greater investment than the stills. A single 5-foot rotor can handle two tank cars of distilland in a week and evolve anywhere from a few hundred gallons of distillate in a stripping operation to 6000-8000 gallons in a complete distillation. The capacity of an unobstructed evaporator to transfer material at a saturation pressure as low as a micron (1/760,000 atmosphere) is quite large, a square yard evolving about 0.75 gram per second. The 5-foot rotor thus evaporates 30 pounds per hour at this pressure or, say, 300 pounds when the distillation is pushed up to 10 p. Evaporating
surfaces aggregating a quarter mile square and operating at a micron would transfer about 100,000,000,000 pounds a year, or about all the vegetable and marinç oil and a goodly proportion of petroleum residue output of the world.
The process itself has been used on a small scale in a few laboratories, notably at the U. S. Department of Agriculture, the University of Wisconsin, and various government regional labora
tories. Laboratory falling-film stills are known to be in operation in England, Russia, and India. With the exception of a plant operating at The British Drug Houses in England, the commercial development is believed to have been undertaken exclusively by Distillation Products, Inc. What follows is therefore necessarily a description of the work of that company.
Our publication policy has been to augment patents with somewhat copious technical publication. Six reviews, two of them lengthy, have appeared from our pen, and a valuable independent appraisal of the process has been issued by Theodore R. Olive.
Little can be said here that is not already available to the reader—
with one exception. Previous articles have stressed the difficulties and expense of molecular distillation; the present one definitely announces the stills’ abilities to compete in heavy industry.
Laboratory, falling-film, molecular stills are now being supplied under a reasonable agreement to academic and commercial re
search laboratories where they can be put to a variety of investi
gational uses. A now 14-inch centrifugal still has also been de
signed for laboratory purposes; and since this has a much higher capacity, its use is growing where larger volumes of material are required.
Since 1941 the 32-inch still units have been in practically con
tinuous operation on marine oils at Distillation Products, with minor modifications of design. These have now been brought to standardized patterns of single- and multiple-effect installations which operate reliably over long periods. Stills of this type have not been installed elsewhere but are now available to interested parties under license.
A battery of 5-foot stills, as mentioned, is in operation on a mis
cellaneous schedule of marine, vegetable, and mineral oils and heavy chemicals at a rate of 100 to 150 galjons of oil per hour, depending upon the type of product handled. A new vacuum system, devoid of mechanical pumps, has been devised which enables these large units to operate at pressures equal to or lower than the earlier models. The 5-foot stills are adaptable either to stripping a small percentage cut from the original material or to distilling practical!}' all of the input. A system of differential setting of the rotor and a multiple-zone condenser enables one to take off a number of fractions from a single plate.
28 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 1
G.
C. Meer is the son o f C. E. K . M ees, director o f research at the Eastman K o d a k Company.G. C. M ees was born in Croydon, England, in 1 9 1 0 , but was educated in the U nited States where he obtained his B.S. from the U niversity o f Rochester in 1 9 3 0 , and his M .B .A . from the Graduate School o f Business Adm inistration of H arvard U niversity in 1933. H e was comm ercial manager at Distillation Products, Inc., at Rochester from its inception in 1939 until last January when he was appointed vice president in charge of sales and purchasing.
In spite of the difficulties attendant on new developments during wartime, plans were completed for a new factory building which is now under construction. Besides being equipped with the usual clarifying, refining, and bleaching equipment, the factory will have batteries of molecular stills ranging from 3 inches to 10 feet in effective diameter. The 10-foot centrifugals have an anticipated capacity of one to two tank cars per day per rotor. All this equipment will be available for custom processing, and where indicated similar apparatus may be installed by license agreement elsewhere.
The molecular still has grown up in the service of the fat- soluble vitamin industry. While the early models could be used profitably for removing vitamin A esters from high-potency marine oils, present-day machines do not need the premium of
vitamin extraction to process oils profitably. Here are some of the uses to which the still has been put: removal of vitamins from natural oils, purification of plasticizers, reclamation of industrial residues, preparation of high-vacuum pump fluids, processing of heavy chemicals, separation of petroleum greases.
Molecular distillation seldom provides a complete manufac
turing process. It takes its place as a tool of industry to be applied at appropriate stages in more complex processes. It is not a substitute for alkali refining or solvent extraction or low-tempcra- ture crystallization; it is a unit process of unique character which may or may not perform a new and useful purpose, according to circumstances.
C o m m u n i c a t i o n No. 83 from the la boratories o f D istillation P rodu cts, Inc.
C om m ercial Size M o le c u la r S till
January, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 29
EVAPORATION
C O N T IN U E D F R O M P A G E 5
Several patents have been issued covering designs for the handling of salt and salt accumulations. Pendl (22) presents a method which involves the use of dilute liquor to remove in
crustations from the heating surface. This treatment has been common practice in the operation of forced-circulation evapo
rators and is called a "rinse” to differentiate it from the normal boil-out. Phillips (23) covers a method for the prevention of salt accumulations on evaporator side walls. He installs a cooling section around the body above the liquor level, and the resulting condensate keeps the walls washed clean. George (9) recommends a truncated cone baffle on the top tube sheet on an annular-downtake calandria. The top opening of the cone is approximately equal to the sum of cross-sectional areas of the tubes. Kermer (IS) describes a complete flow sheet for the con
centration of, and salt removal from, salt solutions with par
ticular emphasis on caustic cell liquor. Buch and Lilleike (2) suggest that foam in sugar evaporators can be controlled by the use of sulfonatcd phosphatides.
The problem of entrainment has become increasingly important as plant boiler pressures increase. It is common practice to use evaporator condensate for boiler make-up; during the war when many plants were far removed from a source of good water, many evaporators were counted on, not only for boiler feed water, but for water for process uses. This served to emphasize the need for entrainment prevention. The conductivity cell proved to be of considerable assistance in the handling of evaporator con
densate (11) and served to warn the operator of unsatisfactory performance. Arnold (1) discusses the cause and effect of en
trainment in the sugar industry, and describes the principles and operation of twelve entrainment separators. Unfortunately this Indian article is not easily secured so that it is not possible to summarize the material presented more completely. Kermer (14) describes an entrainment separator consisting of a special baffling arrangement.
The war has seriously curtailed the amount of work done on the theoretical aspects of evaporation problems. Staub (32) presents some data on the concentration of sodium chloride solutions in a long-tube vortical evaporator. Coefficients for both the boiling and nonboiling sections of the tubes are reported.
However, no indication is given of the relative lengths of the sections. The unit was run at constant steam pressure with varying feed rates and salt concentrations, and coefficients as high as 1500 B.t.u./(sq. ft.) (hr.) (° F.) are reported. Vapor velocities out of the tube are reported to be as high as 120 ft./
sec.
S&vavsk^ (27) discusses the coefficients obtained in the concen
tration of sugar in a pressure evaporator. He reports that the juice level is kept very low and the coefficient is relatively in
dependent of this level. The density of the juice has an ap
preciable effect on the coefficient. He further states that the use of high inlet steam velocity increases the coefficient.
Lukomskil (18) presents a summary of boiling coefficients.
He states that, in general, they can be expressed as U = C At'-' difference to the critical temperature difference (At/ At„u.)', further,
Larian (17) derives heat balance equations for multiple-effect evaporators. While these equations are simple and neglect some heat effects such as heat of solution, they are of interest in the teaching of such calculations. Ray and Carnahan (24) present a rather complete and thorough method for the calculation of heat balances for multiple-effect evaporator problems. This method depends on the r factor, which is the ratio of heat effects other than evaporation to the heat required for evaporation. It is shown that
Thus the material balance can also be ascertained. This method has much to recommend it, but the r factor has to be evaluated at each effect. Inasmuch as r is variable with temperature, the solution still involves trial-and-error calculations.
Egli and Lovell (7) present information on the growth and habit of crystals in an agitated evaporator. They conclude that the amount of mechanical agitation has no effect on crystal
(25) Reavell, J. A.t Ind. Chemist, 20, 119-30 (1944).
Group, Glasgow Sect., advance copy, March 26, 1943.
(32) Staub, S., Intern. Sugar J., 45, 40-3 (1943).
trifugal has been further proved during the past few years. The advantages of centrifugal classification are best realized when particle size cuts are made in the micron range, particularly finer than 20 microns. For example, a considerable tonnage of tita
nium dioxide and lithopone is classified in continuous centrifugals.
In the case of titanium pigment, the slurry is fed at ball mill con
sistency or with limited dilution, and the fine fraction which is 100% —5 microns is discharged at about 20% solids. The coarse fraction is returned, to the ball mill in the conventional manner.
An operation of this type permits direct filtration of the slurry comprising the fine fraction. Other materials such as clay, calcium carbonate, and other coating or filling materials are similarly classified.
The continuous centrifugal has been used in the same manner in the cement industry for closed-circuit grinding; it produces a thick slurry suitable for direct blending and calcination. In this instance the slurry is ground to 85% —200 mesh. In the cement industry it has also been used to classify cement slurry into two fractions, the cuts being made from 10 to 30 microns, and the ad
justment in performance is controlled by variable-speed motor drive. Approximately one million tons of solids have been proc
essed in two units, 54 inches in diameter and 70 inches long dur
ing thé past two years.
A still newer use for centrifugal classification is the desliming (rejection of 2-3 micron particles) of the slurry feed to froth flota
tion cells. Another application is in the separation of magnesium from water softening sludge where the sludge is burned for reuse in the system.
During the 1944 season on the Iron Range there was an inter
esting development in hydraulic classification. The mining companies were pressing to ship the maximum tonnage of ore to the blast furnaces. Washing (concentrating) plant tailings were examined for the purpose of determining how to recover additional tonnage of blast-furnace grade quickly and economi
cally. In one typical case the tailings from a washing plant at
—35 mesh contained about 22-23% iron and 63-65% silicon and represented about one third of the crude ore treated. Normal dewatering the concentrates produced by selected pockets of the Dorrco sizer. Briefly, this sizing device consists of a number of rectangular pockets, each fitted with a perforated bottom through which is introduced a controlled volume of hydraulic water for sizing. Feed is introduced into pocket No. 1, and all particles which will settle through the controlled water velocity are auto
matically discharged at the bottom. Particles carried over by the hydraulic water pass to succeeding pockets with diminishing upward velocities. The operation is controlled by pressure- regulating instruments set in accordance with the particle sizing desired.
In hydraulic sizing of iron ore and silica gangue, it has been established that a silica particle of a certain size and an iron ore particle 2- 2.8 meshes finer will overflow at substantially the same rate. The commercial significance of this fact is shown by the
duce a shipping grade of concentrate containing 61.45% iron and 8.92% silicon. The weight recovery for the washing plant as a whole was increased from 66.6 to 71%, representing ore which otherwise would not have been delivered to the blast furnace dur
ing the war.