W
ith this, the first 1 9 4 6 issue, we are b e g in n in g a new service fo r th e chem ical engineers— an in te rp re ta tio n o f progress in u n it o p e rations. W e hope to c o n tin u e this feature as a special lead section every January.A s w ill be seen from this in tro d u c to ry series, ou r plan is to have an e xp e rt w rite a concise review o f deve lo p m e n ts in a particu la r u n it o p e ra tio n . The articles cover all im p o rta n t th e o re tica l and practical advances scored d uring a previous p e rio d and re p resent a sp e cia list’s o p in io n o f th e significant happenings in a fie ld o f chem ical e n g i
neering.
The basic plan has been m o d ifie d s lig h tly in th is, the first survey, as most o f the reports cover m ore than a year, in th e m a jo rity o f cases g o in g back to the b e g in n in g o f the recent war. There was a g o o d reason fo r this tim e spread. D u rin g th e war m any o f o u r chem ical engineers, in th e service o r e x h a u stin g ly busy b u ild in g new plants, have been unable to keep abreast o f developm ents as th e y were re p o rte d . This s e ctio n , we h o p e , w ill save them tim e in b e c o m in g fa m ilia r w ith recent progress.
For the n ext re vie w , th e tim e p e rio d w ill be k e p t to one year. Since much new chem ical engin e e rin g k n o w le d g e , gained d u rin g secret war research, w ill be re
p o rte d in the n e xt few years, c ritic a l e va luation and c o lla tio n o f facts b y outstanding authorities w ill be useful and im p o rta n t to busy men. W e h o p e to have the same e x perts cover the same fie ld s, review after re vie w ! M o re u n it o p e ra tio n s w ill be added in tim e , and plans and extensions fo r th e n e xt volum e are already bein g considered.
For to o lo n g , p ith y , terse resumes have been missing from the literature o f th e chemical engineer and we h o p e , th e re fo re , that ou r new service w ill re c tify this oversight.
T H E E D IT O R S
S E D IM E N T A T IO N A N D H Y D R A U L IC C L A S S I
F IC A T IO N . . . R. P. K ite and A . J. Fischer 16 E V A P O R A T I O N
W . L. Badger and R. A . Lindsay
W arren L. M cCabe 1 8 C R Y S T A L L I Z A T I O N
FL U ID D Y N A M IC S W heat on W . Kraft 6
Fred D . D eV a n e y 20 F L O T A T IO N
H IG H T E M P E R A T U R E D IS T IL L A T IO N
T. J. W alsh D R Y IN G Samuel J. Friedm an 22
G eorge T. Skaperdas
H E A T T R A N S F E R C E N T R IF U G A T IO N James O . M aloney 24
M IX IN G J. H enry Rush ton 12 S O L V E N T E X T R A C T IO N M O L E C U L A R D IS T IL L A T IO N A B S O R P T IO N A N D H U M ID IF IC A T IO N
K . C. D. Hickm an and G. C. M ees 28 Harrison C. Carlson 1 4
w . L. Badger was born in M inneapolis in 1886 and attended the U niversity o f M innesota 1916, and has been director o f research and consulting engineer for the Swenson Evaporator Company since 1917. In 1936 he assumed the position o f manager o f the consulting en g i evaporation have been published recently. Deere and Brooks (5) trace the development and history of evaporators in the sugar industry. Waeser (S3) reviews recent progress in the heavy chemical industry; unfortunately this German article, which has several references, is unavailable at present.
Semple (31) covers multiple-effect evaporators but this English article is also difficult to secure. Wulfinghoff (36) presents a short discussion of evaporation. Rubush and Seavoy (26) report on the operation and performance of waste liquor evapo
rators in the paper industry.
During the past few years there have been no important changes in standard evaporator types. The war has acceler
ated the change to the outside-heating-clement, forccd-eircula- tion design for salting operations and to the long-tubc vertical design for nonsalting operations. The short-tube vertical evapo
rator with propeller agitation is losing favor as a salting evapo
rator, as a result of the higher coefficients and generally longer salting cycle for the submorged-heating-element, forced-circula- tion evaporator. 1
The war plants in the light metals industry served to emphasize this trend. In the aluminum plants, where evaporators were required to concentrate tremendous quantities of spent liquor (alumina mother liquor), long-tube vertical evaporators were used e x /^ iv o ly . This application is interesting in that a certain amount W silicate scale is encountered in this operation; never
theless, the long-tube vertical is very successful. In the mag
nesium industry, where magnesium chloride liquors required concentration, the horizontal-tube outside heating element forced-circulation evaporator was used.
Some minor trends have been noted in the design of standard types. In the long-tube vertical evaporator there is a tendency to dispense with the normal vapor head. Where this is done, the heating element discharges tangentially into a liquid-vapor separator. This makes the tubes more accessible, an important factor when scaling liquids are being evaporated. Further, it makes the construction considerably cheaper. The design of the forced-circulation evaporator is in a state of flux. The original design, which consisted of an internal vertical heating element, is being replaced by the outside heating element, which may be either horizontal or vertical. This newer design is in
tended to keep the liquid level high enough so that no vaporiza
tion occurs outside of the vapor head. The prototype of this machine is used at Trona, Calif., by The American Potash and Chemical Company. In these units four heaters are provided
for each body, and they can be cleaned individually without shutting down the machine. Another tendency in these ma
chines is to increase the tube size. Smaller tubes may be blocked by pieces of salt that have dropped off the side of the evaporator body. Larger tubes increase the pumping load at similar velocities, so there is a tendency toward lower velocities. In the calandria machine there is a tendency to increase the amount of artificial agitation; it is common knowledge that the liquor distribution is not uniform, and some work has been done to overcome this disadvantage (34).
Because of the scarcity of corrosion-resistant metals during the past few years, many substitute materials were tried. Brick lining and plastic coatings were used with some success, but proved to be both expensive and unreliable. The best substitute is Karbate for tubes and rubber for lining. Karbate tube sheets have been used, but they present a serious problem when field repairs are required. A successful application consists of a rub
ber-lined tube sheet and Karbate tubes. Rubush and Seavoy (26) discuss tube life in paper wastc-liquor evaporators. Un
fortunately they have been unable to publish their information but do state that there is no way to correlate the data. In to-evaporation efficiency comparable to a double-effect evapo
rator; inasmuch as it eliminated the need for steam-generating equipment, it offered considerable advantage during the war.
The principal shortcoming of the unit is its high entrainment.
Kokatnur and Jacobs (16) patented a method for concentrat
ing solutions, using kerosene to permit partial pressure evapora
tion of the water. The Dow Chemical Company developed a spray-type direct-fired evaporator (10, 2 8) similar in principle and operation to a spray dryer. The concentration of penicillin liquors presented a difficult problem of evaporation from heat- sensitive material; it was accomplished by very high vacuum and vaporization from the frozen solid.
There has been some work on the use of radiant heat in the evaporation of heat-sensitive liquids (25 ). This system is in
teresting in that, with the liquid opaque to the radiation and the
4 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
R.
A . Lindsay was born in Detroit, M ich ., in 1915. H e received his B.S. in 1 9 3 9 and his temperature above its boiling point at the pressure of the operation.
The development of evaporators for the manufacture of distilled water had great impetus during the war. Thousands of these units both stationary and portable, were built for the armed forces. The principle of all the designs is similar in that they use mechanical recompression with the compressor driven by an internal combustion engine. The make-up heat required is supplied by the waste heat of the engine. The economy of these units has increased tremendously; at the beginning of the de
velopment they required as much as 1 gallon of fuel for 10 gallons of distilled water; the present units require only a fraction of a gallon. Meier (21) discusses the application of thermocompression evaporators to milk, including the compres
sion of vapors from the evaporator for other uses.
The operation of evaporators has shown a marked trend toward higher temperatures. The principal reason has been higher fuel costs. More effects are being used; in some installations all the effects have been operated under pressure, and the vapor from the last effect has been used as process steam. Probably the highest temperature unit is a Dowtherm-heated caustic evapo
rator at the plant of Reichhold Chemicals, Inc., at Tuscaloosa, Ala. (35 ). This unit is designed to carry 50% caustic to approxi
mately 85%. A similar unit is now in progress of construction at the Pittsburgh, Calif., plant of The Dow Chemical Company.
Claasen (3) presents a study of the optimum number of effects and temperature range for a sugar evaporator. He arrives at the conclusion that a four-bodied triple-effect operated above atmospheric pressure is the best arrangement.
Rapid strides were made during the war in the control and instrumentation of evaporators. The control of the level in the unit has received considerable study. Clayton (4) discusses the control of the level in sugar evaporators and reaches the con
clusion that overflow is the best method. The use of floats and valves introduces the complications of electrical and air circuits plus the mechanical features of the valve. In this connection some manufacturers are producing valves in which the stuffing box is supplanted with a bellows. This eliminates the difficulty sometimes experienced with crystallization on the valve stem and subsequent failure of the packing. A level control of the float and valve type has been described (12 ). This unit consists of a float in a level tank outside the body which controls directly the position of the valve governing the air pressure on the motor- operated valve.
Much emphasis has been placed on the need for instruments m the control and operation of evaporators. Ziegler (37) reviews such control for the sugar industry and explains- the principles of the instruments required. Rubush and Seavoy (26, 30) explain the need for instruments in evaporators. They point out the particular value of the temperature difference at each
effect. These two articles indicate that without adequate in
struments it is not possible to determine which effects in a mul- tiple-effcct evaporator arc causing difficulty. Further, the proper boil-out cycle cannot be determined. Schiebl (29) de
scribes a method of controlling the effluent concentration by controlling the evaporation with a fixed feed composition.
Galainea y Quesada (8 ) describes a control for the specific gravity of sugar juices.. The rise in boiling point of salt solutions can be used for composition control. A differential thermocouple, with one connection in the liquid and the other measuring the saturated vapor temperature, can be used as the motivating force for the control. This control is suitable for solutions having a high boiling point rise but must be used with care. Where the characteristics of the system are such that during the salting cycle the pressure in the evaporator being controlled varies con
siderably, the boiling point rise-concentration relation will change and the instrument must be reset.
There has been considerable interest in the formation, pre
vention, and removal of scale from evaporator heating surfaces.
Scale is generally defined as deposition of a material which has an inverted solubility curve. Thus, maximum concentration occurs in the stagnant film at the heating surface. Scale differs from salt precipitation in that it is generally quite hard and smooth where salt is porous and rough. Rubush and Seavoy (26) report methods for the removal of scale from both the inside and outside of tubes in paper waste liquor evaporators. The out bothersome sodium-aluminum sulfate scale can be removed by circulating a 20% solution of sodium acid phosphate at 160° F. suggests that silicate scale can be removed with sodium fluoride and hydrochloric acid. Calcium sulfate scale can bo removed by using ammonium chloride with molasses as an inhibitor. For the removal of oil and carbon deposits on the steam side of evapo
rators, Dittmar (6) recommends that the surface be treated over
night with a solution of 1.5% permanganate and 2 % caustic at 212° F. The surface should then be rinsed and treated with a 1% ferrous sulfate solution containing 3 -5 % hydrochloric acid at 212° F. for 0.5 to 1 hour. Finally, it should be washed with hot water. This treatment is reported to cost about S0.01 per square foot of heating surface; Dittmar discusses the economics and indicates that considerable saving results in distilled water evaporators using exhaust steam. (Continued on page 30)
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 5
W h e a to n W . K ra ft was born in Spokane, W ash., in 1 9 0 3 . H e obtained his B.S. in chem
istry from the U niversity o f California in 1924 and his S.M . in chem ical engineering from Massachusetts Institute o f Technology in 1929., H e then became a process engineer with The Lummus Company, N ew York, and was prom oted to c h ie f process engineer in 1935.
Some o f the larger jo b s in which K raft has participated are butadiene plants for Neches Butane Products Com pany and Sincla ir Rubber, Inc., the styrene plant for Monsanto Chem i
cal Com pany, and aviation gasoline plants for several oil companies,- in the fie ld o f oil production em ploying solvent processing, the com plete lubricating oil refinery o f The British Am erican O il Com pany is outstanding. H e is a member o f the Am erican Chem ical S ociety, Am erican Institute o f Chem ical Engineers, and Am erican Petroleum Institute.