R. P. Kite and A. J. Fischer
T
WO compensating factors are involved in preparing a review of recent, developments in almost any field at this time.
The helpful factor is the stimulus to new equipment de
velopment for war industries. The hindering factor results from development programs being deferred during the war.
Such projects will shortly bring new ideas into the market, but they involve products unproved at this time. Some liberties have been taken with the scope of the operations designated as sedi
mentation and hydraulic classification. Where the definition has been stretched, it was for the purpose of including proved devices which were interesting and at least bordered on the operations being discussed.
In the field of sedimentation the most significant advances during the past few years have been in mechanical “ conditioning”
or flocculating the feed prior to actual' sedimentation, and to improvements in settling tank inlet and outlet to increase settling efficiency. Other developments sufficiently new to warrant de
scription are combination type units where flocculation and sedi
mentation are carried out in the same container and/or where sedimentation is carried out in two or more compartments.
The various types of blanket-filtration units such as the Precipi
tator, Accelator, and Hydrotreator. may be regarded as falling into the former category.
Mechanical flocculation of solids-in-liquid suspensions involves a slow mixing whereby the solid particles are caused to agglomer
ate so that their settling rate is increased. It is obvious that once a rather fragile aggregate is formed, the suspension must be transferred into the sedimentation tank at relatively low veloc
ity so as not to destroy the delicate floe structure. Tests have shown that the critical velocity ranges between 0.6 and 1.2 feet per second for a wide variety of materials. A relatively short fall over an inlet weir is even more destructive to some floes than is an excessive velocity in the flocculating chamber.
The Clariflocculator was developed for the purpose of avoiding floe break-up as the feed is transferred from the flocculating to the settling zone. Here the ilocculator and settling compartments are concentric and circular in plan. Feed is introduced into the inner ilocculator compartment at the surface through a siphon feed pipe or overhead trough. It then passes down into the set
tling zone through a central opening in the floor of the floceula- tor tank. Clarified liquor overflows into a peripheral launder.
Mechanisms are provided in both the flocculator and the settling tanks, both mechanisms and flocculator tank being sup
ported on beams or a center pier. The flocculator mechanism con
sists of a rotating paddle made up of a series of V-shaped vertical blades extending up from two horizontal arms supported from a vertical drum. Intermeshing with these vertical blades are four sets-of downward-extending stationary V-blades supported from beams above the liquid surface. The purpose of this design is to afford maximum particle contact without causing excessive local velocity currents.
This unit was originally developed for use in the sewage treat
ment field. A number of units are now in use for water treatment, brine purification, and trade waste treatment. Flocculation periods generally employed are 30 to 40 minutes; settling tank overflow rates range from 600-1200 gallons per square foot per 24 hours (3.3 to 6.6 feet per hour), depending on the material being handled and the results desired.
The Multifeed clarifier was developed for the cane sugar industry. This unit is a multitray settling unit with a special flocculating compartment at the top. The design of the floccula
tor mechanism is similar to that used in the Clariflocculator.
The feed is transferred into the sedimentation compartments from the flocculation zone at relatively low velocity. The over-all benefits derived from this unit as well as the Clariflocculator are increased capacity per unit of floor space and lower overflow turbidities.
The various sludge-blanket type of clarification units were originally developed for water softening by the lime or lime-soda process. The common features of all units are the introduction of the feed into the mixing or coagulating zone where chemicals are added in the presence of large amounts of sludge, and upward
“ filtration” of the treated liquor through a sludge blanket. They differ in the arrangement of flocculating and settling zones and in mechanism design. In addition to water treatment, sludge- blanket type units are now in operation for clarification of white water from paper mills, production of magnesia from sea water, and brine purification! In water softening they are commonly operated at overflow rates up to 2700 gallons per square foot per 24 hours (15 feet per hour), and thus effect a considerable saving in ground area and installation cost. Their operation, however, is more delicate than the older conventional low-rate units.
16 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
A.
J. Fischer o f The Dorr Company, New York, was born in Philadelphia in 1 9 0 2 and been granted tw enty-eight U nited States patents involving flocculation, sedimentation, digestion, and filtration. H e is author o f thirty technical articles on sewage treatment.Rarely has it been necessary to design continuous sedimenta
tion devices for operation under pressure. In the petroleum industry it is conventional to use pressure separators, but these devices are almost exclusively used for liquid-liquid separations.
However, a problem was presented early in the aviation gasoline program which involved the removal of micron sizes of catalyst from a continuous flow of oil at 400-500° F. This was accom
plished by modifying the conventional multicompartment thick
ener and auxiliaries to make it suitable for oxygon-free pressure operation. Feed stock from the fractionating tower at the rate of 900-2500 barrels per day and containing 15-80 pounds catalyst per barrel was introduced through a closed, compartmented, feed box which distributed the flow equally to all thickener compart
ments. Clarified oil, usually containing from 0.2-1.0 pound catalyst per barrel was collected in a closed overflow box for transfer to the succeeding operation. Recovered catalyst in slurry form, containing from 125-400 pounds catalyst per barrel, was removed continuously and also transferred to the succeeding step. Pressures were equalized by suitable vents, interconnected with a related operation. The original work was done by the Standard Oil Development Company, and the idea was used widely in Fluid catalytic cracking plants.
A novel design of multicompartment sedimentation unit is the Trebler clarifier which has been used commercially in con
nection with milk waste treatment by means of trickling filters.
The unit consists of a standard clarifier separated into two com
partments by a vertical partition that extends from above liquid level to the mechanism rake arms. One compartment is used for primary settling and the other for secondary settling.
Thus, one tank and mechanism—for example, 40 feet in diameter
—will serve the same purpose as two 28-foot-diameter units, resulting in a lowering of plant installation costs. This type of unit is applicable only where a liquid is settled in two stages and where the initial settling step is followed by coagulation or oxida
tion ahead of final clarification. Short-circuiting of raw feed under the partial vertical partition may be prevented by hydrau- lically causing a backflow from the secondary to the primary compartment.
Many studies have been made in recent years on methods of improving feed distribution and overflow take-off in order to minimize short-circuiting. In most laboratory studies of feed and overflow arrangements, average detention time as measured and computed from dye and salt tests has been taken as the cri
terion of settling efficiency. Large scale tests have shown, how'- ever, that such is not the case, and that elaborate feed distribution and baffling means have little or no effect on actual solids removal.
The most exhaustive studies of the behavior of circular center- feed settling tanks were carried out on 126-foot-diameter units at the Sanitary District of Chicago. These tests showed that the only worth-while improvement was when the overflow launder was moved 12 feet in from the tank periphery. Similar tests run on cross-flow tanks, either rectangular or circular in plan,
have indicated that feed stilling wells and directional vanes are of some advantage, especially in wide tanks. As in the case of circular tanks, moving the overflow launders back from the dis
charge end of the tank has also been advantageous.
A departure from circular-tank conventional feed and over
flow arrangements is used in the Spiraflow clarifier. In this unit the feed is introduced peripherally near the tank bottom and over
flowed at the surface into a circular launder near the tank center.
Equal feed distribution is obtained by introducing the influent tangentially at a relatively high velocity into the circumferential feed well.
A radical departure from standard settling practice has been recently introduced in the sanitary and chemical fields where solid particles having a specific gravity close to 1.0 are caused to float at reduced pressure after pre-aeration. The flotation of the solids is caused by the attachment of minute air bubbles to the solids particles during aeration and their tendency to rise as the pressure is lowered. The floated solids are removed by a skim
ming device, and the clarified liquor is drawn off from a point below the liquid surface. The Vacuator as used in sewage and trade waste treatment, and the Pedersen and Adka Savealls as used in paper mill white-water recovery are specific examples of units based on this principle.
The use of the Vacuator lias recently been extended to the separation of high- from low-gravity solids in the sewage treat
ment field. In this application the settled sludge consisting of grit and organic matter is delivered to a classifier where the lighter organics are separated out and recycled to the aerator preceding the Vacuator. In this way a three-product separation giving relatively clean grit, scum, and clarified effluent is obtained.
In the metallurgical field it is difficult to define sharply those developments of the past year or so/which belong strictly to con
centration and those which belong to hydraulic classification.
There is considerable overlapping. Development and com
mercial work on a large scale have taken place on float-and-sink methods, using heavy-density slurry mediums. However, such developments are not considered as falling within the scope of this review.
A new device known as the selective-media concentrator was first placed under field development in 1944 on the Iron Range in Minnesota, and it may prove useful in other fields. Briefly, it consists of a rotating, ribbed construction cone surrounded by a stationary conical shell with the annular space between the cones available for holding a relatively high-density slurry of the material being treated. The high-density slurry is formed and held in place by the cone speed and a restricted lower opening at a point near the apex of the cone. The base of both cones is at the top, and feed in dilute slurry form is introduced into the annu
lar space. The finer and lower-dcnsity particles which cannot pass through the selective-media bed are rejected at the top about 270 ° from the point of feeding. The coarser high-density particles are discharged through the restricted (Continued on page 81)
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 17
W a rren L McCabe, of the Carnegie Institute o f Technology, Pittsburgh, was born in Bay City, M ich., 7899. H e obtained his B.S., M .S ., and Ph.D. degrees from the U niversity of M ichigan in 1922, 1923, and 1928, respectively. H e was an assistant and instructor in chem ical engineering at the Massachusetts Institute o f Technology from 1923 to 1925 and then became instructor, assistant professor, and associate professor in chem ical engineering at the U niversity o f M ichigan from 1925 to 1937. Since that date he has been professor and head o f the Chem ical Engineering Departm ent o f the Carnegie Institute o f Technology.
M cC abe is a member o f the Am erican Chem ical Society and the Am erican Institute of Chem ical Engineers. H is publications include the book "Elem ents of Chem ical Engi
neering" , w ithW . L. Badger, and research papers on unit operations and thermodynamics.
CRYSTALLIZATION
Warren L. McCabe
A
LTHOUGH progress during the last few years in the unit operation of crystallization has not been spectacular, some new methods and novel applications of old procedures have appeared. Interesting scientific treatments of the processes of nucléation and growth have also been presented.
The purpose of this article is to review briefly some of the ad
vances in the art and science of industrial crystallization. Spe
cifically the nucléation theories of Volmer, Stranski, Becker, and others, and the work of Van Hook on sucrose crystallization will be summarized, and recent developments in ammonium nitrate crystallization and in general crystallization practice will be described.
S c i e n t i f i c A d v a n c e s . A s usual, the literature of crystal
lization during the past few years has been extensive and scat
tered. Considerable work has been reported on various phases of sugar boiling and crystallization, theory of drop formation, effect of added substances on crystallization, various theories of growth and nucléation, and specialized problems of crystallizing Specific materials. Much of the current theoretical work is based on the work of Volmer (S), Stranski and collaborators (4).
Becker (7), and others. The ideas and approach of these in
vestigators have been applied effectively by Mold and collabo
rators (3) to metallurgical problems, and analogous results can be expected in the study of crystallization from solutions with the aid of the same basic concepts. Some of the ideas brought forward in these treatments are described in the follow
ing paragraphs.
All competent theories of crystallization divide the over-all process into two parts—nucléation and growth—and assign a rate to each. The nucléation rate is defined as the number of nuclei formed per unit time per unit, volume of reaction phase, and the rate of growth is expressed as the rate of linear transla
tion of a growing crystal face.
A process of crystallization would be completely described if the rates of growth and of nucléation were entirely known, and the objective of work in crystallization kinetics is to obtain quantitative knowledge of such rates.
Neither nucléation nor growth can occur unless the precipi
tated substance has a lower thermodynamic potential after precipitation than before. In a solution this means that nu
cléation and growth occur only in a supersaturated solution.
The over-all driving force is the negative value of the free energy difference:
A /-', = F — p
where F = molal free energy of crystallized material based on zero specific surface
m = molal chemical potential of the same component in the solution
The well-known Gibbs-Thomsen law shows that F increases with decreasing particle size and is very large for very small particles. It is convenient to divide the AF between a small particle and its mother phase into two parts:
AF = A Fi + A F-i (1)
where A/•’, = over-all free energy change, not including surface effects
AF2 = difference between free energy of a very large particle and the small particle under considera
tion
If the solution is supersaturated, AI<\ is negative, decreases with supersaturation, and is proportional to the bulk number of atoms in the crystal. It is, therefore, proportional to the cube of the linear size, but AF2 is positive and is proportional to the number of molecules in the surface of the crystal. It is, therefore, proportional to the square of the linear size. As the crys
tal size increases, the free energy change AF increases to a maxi
mum and then decreases for a definite crystal size, called the "criti
cal size” . The higher the supersaturation, the smaller is the critical size. A crystal smaller than the critical size will tend to dissolve, and one larger than the critical size can grow. The value of A F corresponding to the critical size represents an energy barrier that must be surmounted by a nucleus before it is stable. The energy required can come only from momentary and local fluctua
tions of both concentration and energy. The energy fluctua
tions are statistical in nature and arc of the usual kinetic type that give use to homogeneous reactions of all kinds. The con
centration fluctuation requires transport by molecular diffusion of the requisite number of molecules close enough to one another to form a nucleus large enough to exceed the critical size.
Becker (/) proposes the following equation for nuclcation rate:
N = ce-Q/kT e — A(T)/kT (2)
where N — nucleation rate, number per unit volume per unit time
Q = activation energy for diffusion
18 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
A{T) = work required to form surface of nucleus T = absolute temperature
c = a constant k = Boltzman constant
Several important qualitative deductions can be made from Equation 2. The work term A{T) increases markedly with de
crease in supersaturation and, is infinite at the saturation curve.
The term e~ Af7^ kT is, therefore, zero at saturation, and in
creases with increase in supersaturation. The term c~ Q/kT de
creases with decreasing temperature, since for a diffusion process Q is not independent of temperature; therefore, e- ®^7 decreases with increase in supersaturation. The Ar vs. T curve has a pro
nounced maximum that corresponds to a definite supersatura
tion. Also, the shape of the N vs. T curve is such that the value of N is very low for an appreciable supersaturation but increases rapidly when a definite suporsaturation is reached. The well known metastable region and the Miers supersaturation curve are explained by the N vs. T relation. Actually, the super
saturation curve represents a zone where N increases rapidly with T, rather than a sharp boundary. Also, if time enough is allowed, nucleation will eventually occur at any supersaturation.
The state of a crystal at the boundary is abnormal because of the unbalanced forces acting on the surface atoms. The surface molecules can possess a lower activation energy for dif
fusion. Also, the presence of the solid interface can affect the molecules in the mother liquor in such a manner as to affect the terms of Equation 2 and increase the nucleation rate N.
Such interface behavior may account for the inoculating effect obtained by seed crystals, or in some cases by foreign particles, in supersaturated solutions.
The growth process is also amenable to much the same kind of treatment as that for nucleation on the assumption that crystal growth is essentially a two-dimensional nucleation process.
Papers by Van Hook (7) on sucrose crystallization are of in
terest because his fundamental results are probably of general applicability in crystallization from solution. Van Hook shows that the rate of growth of sucrose crystals is proportional to the difference between the thermodynamic activity of the sucrose in the solution and the equilibrium activity at the temperature of the process. He shows that the rate may be in
creased, but is usually decreased, by the presence of electrolytes and other added substances. He also confirms previous beliefs that the rate-controlling step in the growth of sucrose crystals is a surface reaction and not diffusion of solute from the bulk of the solution to the surface.
creased, but is usually decreased, by the presence of electrolytes and other added substances. He also confirms previous beliefs that the rate-controlling step in the growth of sucrose crystals is a surface reaction and not diffusion of solute from the bulk of the solution to the surface.