CRYSTALLIZATION EQUIPMENT
Crystallization is controlled by many interrelated variables. To simplify design, think of a crystallizer as a number o f restraints, which can be imposed on a suspension of crystals to counteract adverse tendencies of the process hy W. C. SAEMAN, Metallurgical Laboratories, Olin Mathieson Chemical Corp., N e w Haven, Conn
C R Y S T A L L I Z A T I O N encompasses processes in which solid crystalline phases separate from liquids, vapors. or solids. I n the case of liquids the separation may be either from solution or melt. Here. consideration will be limited to crystallization from solutions. The object of the process is usually the recovery of the solute from the solvent, but not infrequently the situation is reversed. Crystallization is also closely related to precipitation. Often the line of demarcation between the two is indistinguishable, though the latter term is generally reserved for cases where solids are separated from solution in very fine or amorphous form. The scope of commercial crystallization operations covering the separation of solutes from solution is still quite broad and ranges from cases where solvent and solute are to be separated in any form whatever, by the cheapest or most expedient means available, to the highly specialized operations devised to grow monocrystalline structures for optical and piezo-electric applications. The particular operations of greatest interest to chemical engineers are intermediate between these two extremes and cover the separation, in substantial tonnages, of solutes under conditions where purity, size, shape, or structure of the solute is of significance in evaluating the performance of the operation. Design of crystallizers can be reduced to problems in :
0 Heat transfer 0 Mass transfer 0 Mixing and agitation 0 Size classification 0 Solids separations 0 Crystal shape or structure control
Variations in chemicals and raw materials Cost and performance requirements of the process \-ariatiom in plant environment Purity requirements for product Size requirements of product Special shape or structure requirements of product Design is also influenced by special features or styles emphasized by equipment manufacturers. This paper is intended to reflect the equipment user’s viewpoint instead of the equipment manufacturer’s viewpoint of crystallizer design. The above criteria are in approximate order of frequency in which they dominate the design of industrial crystallizers. Because of the wide variations in design due to these influences, any attempt to survey the principles of crystallizer design in brief form must be limited to certain general fundamentals with specific examples to cover more detailed aspects of design. Supplementary reading is also necessary to this type of coverage of crystallizer
Mode of operation Continuous Batchwise
Method of crystal suspension Hydraulic suspension Mechanical elevation Buoyancy Fixed support
Method of crystal size control Nucleation rate control Intermediate seed rate control
Method of growth inducement
Importance of these aspects depends on the following product and performance criteria :
612
INDUSTRIAL AND ENGINEERING CHEMISTRY
Evaporation Cooling Salting-out or chemical reaction
design. The prior work of Garrert and Rosenbaum (5)is highly recommended. The literature on crystallization equipment covers several overlapping systems of crystallizer classifications. A functional basis \vi11 serve most effectively to clarify the problem of classification. A breakdown of suggested functional aspects is given at left, below. These aspects are more or less independent, so that a more specific list of crystallizer classifications might include various combinations of these, numbering some 30 to 50 (2 X 4 X 2 X 3) separate cases. Also, there are 3 to 4 major sub-classifications of methods for maintaining hydraulic suspension, and as many or more ways of imposing other physical restraints on the size or shape of the crystal. It becomes apparent that any comprehensive classification of complex crystallizer systems will defeat its own purpose. L‘seful classifications are, therefore, generally limited to partial coverage of the variants in terms of the major functional coniponents which are required to attain specific product or performance characteristics. From this viewpoint, one may also differentiate between the specialty crystallizers for producing unique matariais and the general purpose equipment more widely used for tonnage production of a wider variety of materials. Some specialty crystallizers are eThose used to grow “hoppershaped”sa1t crystals e Scraped-surface pipe crystallizers @Those used to grow large monocrystalline structures for optical or piezo-electric purposes @Older or obsolete types having unique design features reflecting earlier concepts of the industrial crystallization processes I n the general purpose category are included most of the modern suspension-
type crystallizers which can be built in large sizes for high production rates. In suspension crystallizers, crystals are suspended hydraulically by agitation or circulation of mother liquor. Providing that the product is competitive with crystals produced by other methods, the suspension method will usually have the lowest production cost. Other older systems, often using mechanical levitation of crystals, are therefore gradually being displaced by these newer types. Objectives and Policies Optimum crystallizer design must balance the opposed objectives of product quality and cost to yield maximum possible profit. Sales, engineering, and operating management of the company must therefore define realistic objectives of the process. Some of the viewpoints to be reconciled are discussed in this issue (page 36 A). Acquisition of Know-How Since the behavior of each chrmical is unique, crystallizer design must be based on experimental data. But information and know-how cost money. A safety factor can sometimrs compensate
for lack of knowledge, especially where product specifications are broad. I n the case of a new product, there is no alternative to setting up laboratory procedures to develop basic design data. If one knows what to look for, simple studies with hot-plate and beakers can yield a great deal of information. Pilot plant studies are usually necessary to define the design problem. A pilot crystallizer should not be thought of as an actual model of any large crystallizer, but simply as an aggregation of physical restraints which may be imposed on a suspension of crystals, to counteract adverse tendencies of the process. The solution is introduced, and appropriate restraints are applied to give a set of conditions under which the desired shape, size, or form of product is produced. These restraints include Removing excess fines Varying attrition of the crystals Auxiliary seed preparation and control Minimizing crust formation Varying operating variables, such as temperature
Determining of the Relationships between Variables Design and production variables must be optimized within the restraints required, as determined above. At this point, one is interested particularly in quantitative evaluation of basic relations involved in the process. The relations of major significance in the determination of a design basis for suspension-type crystallizers are :
Solubility data of product a n d impurities-phase diagrams Heat content a n d heat of solution of dilute a n d saturated solutions Viscosity of saturated solution usvs, temperature Vapor pressure us, concentrationand temperature Rate of crystal growth QS. supersaturation a n d temperature Structural variations of crystal us. rate of growth Density of suspension us. crystal size Settling rates of crystals us, size Nucleation rate us. supersaturation is
Of these relations, a pilot crystallizer required only to determine the
LABORATORY TESTING A n example of exploratory testing o n a small scale intended primarily t o let a product talk for itself follows. A 2000-ml. beaker was equipped with a stirrer a n d set o n a hot-plate connected to a variable voltage transformer. T h e beaker was filled a b o u t with s a t u r a t e d solution i n a range 130° F. t o boiling, d e p e n d i n g o n the r a t e of crystallization a n d temperat u r e limitations of t h e process. Surface evaporation i n t o t h e atmosphere was used to control the r a t e of crystallization. Periodically, t h e agitation was stopped, coarser crystals were allowed to settle, a n d 10 to 2070 of the s u p e r n a t a n t solution with excess fine nuclei was dec a n t e d into a n o t h e r beaker a n d heated t o clarify t h e solution. T h e clear solution was t h e n r e t u r n e d to t h e agitated beaker. T h e h e a t r a t e t o the agitated beaker containing the suspension was reduced as required to compensate for excess h e a t r e t u r n e d with t h e clarified solution. A t higher temperatures, where surface evapo r a t i o n b e c a m e excessive, minor additions of w a t e r were also used t o assist in t h e clarification of decanted solution a n d i n limiting the r a t e of concentration d u e to evaporation. R u n s o n individual materials usually r e q u i r e d a b o u t 8 hours. I t is often claimed t h a t crystallizer operating stability improves as t h e scale of operation increases a n d t h a t the i n h e r e n t t e m p e r a t u r e instability of small scale d e m onstrations weakens t h e value of such demonstrations. T h e effect of such t e m p e r a t u r e instability is evidenced i n generation of excess nuclei i n t h e system. However, where excess nucleation is again brought u n d e r control by decantation a n d fines removal as cited above, t h e small scale operation becomes q u i t e controllable a n d has t h e further a d v a n t a g e of flexibility n o t possessed by crystallizer operations pursued o n a larger scale.
I n this way, it is easily s h o w n t h a t 0 Urea nucleates readily, is quite hard to manage, and tends to form such thin and hair-like crystals that classification by decantation is difficult Citric acid does not nucleate readily, yields crystals of large size rapidly, if seeded, and fudges if excessive supersaturations are maintained too long .Ammonium sulfate can be managed easily and 8-mesh crystals can be grown in less than 8 hours @ Ammonium alum forms jewel-like octa-hedral crystals and can be grown to large size easily
*
Aside from t h e behavior of t h e solution toward t h e formation a n d growth of nuclei, a n d the demonstration of crystal growth a s a preferred needle-like, plate-like, o r cubical trend, t h e beaker demonstration is also highly illuminating i n indicating t h e susceptibility of t h e solution toward crust formation o n t h e surface o r walls of t h e a p p a r a t u s . Although this low-cost, concrete introduction to t h e p r o d u c t to be crystallized m a y not solve t h e problem, it will, a t least, indicate t h e o r d e r of m a g n i t u d e of the difficulties to be surmounted a n d it will also serve to set m o r e practical targets for subsequent larger scale investigations of t h e problem. If carefully supervised, t h e beaker demonstration offers a potent method for achieving r a t h e r wide modifications in t h e size, shape, a n d s t r u c t u r e of crystals g r o w n d e p e n d i n g o n t h e intensity of agitation a n d t h e use of auxiliary seeding technique. T h e a d v a n t a g e of t h e beaker-scale procedure is t h e inherent flexibility of t h e smallness of operation, particularly i n m a k i n g exploratory investigations of t h e operation with solutions of varying compositions of chemical modifiers for inducing changes i n t h e nucleation o r structural aspects of crystal growth.
VOL. 53, NO. 8
AUGUST 1961
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rc=L,
AGITATOR
COOLING WATER COIL
4 VAPOR
---_ _ _ CRYSTALLIZER
MPELLER
F
1 PRODUCT Figure 1. left.
e Increased rate of growth increases production, but causes structural imperfections e Attrition decreases structural imperfections, but creates fines eExcess fines must be redissolved, lowering production Optimization therefore involves the use of attrition to attain the best balance between productivity and appearance. At this point, one has the basic data required to detail the basis of design for a larger plant. I t may appear that if all dominant variables and the major inter-relations of the variables are known, then scaleu p may be reduced to a sound basis. But scale-up itself is a dominant variable which affects the performance of the system. T h e other variables may be tested over the complete range of each variable, so that the effect of varying operating conditions can be inferred by interpolation. I n scale-up one is automatically excluded from testing a t the large end of the range, so that the effect of variation of scale has to be inferred by extrapolation, with an increased degree of uncertainty. The net result is that optimum operating conditions for large and small equipment may not coincide. Any flexibility
6 14
Design of a suspension c t'ystallizer must include these items
Where cooling is the driving force
nucleation rate us. supersaturation, and structural variations us. rate of growth. However, these are so variable or difficult to control or measure that the problem is best resolved by determining the degree of restraint required to achieve the desired product specifications. This can be expressed in terms of intensity of agitation, or the cross-sectional area of the fines trap (or seed rate dosage) in a pilot crystallizer. There is interaction between these two factors in this manner :
AG I TATOR DRIVE Right.
designed into the crystallizer a t a low cost to aid in optimizing the operation of large scale equipment may be of significant economic utility particularly where the production of newly developed crystalline products with tight product specifications is being undertaken. Precision of data is limited, as a crystallizer system seldom reaches equilibrium, either in the pilot plant or industrial operations. Comprehensive cross correlation analysis of operating data can yield the required cause-andeffect relationships, although the plant is not at steady-state. Elements of Suspension Crystallizer Design
General Considerations. A broad, qualitative review of considerations of significance in crystal production operations has been covered by Egli and Zerfoss ( 4 ) . Quite recently a more practical review along this line was also published by Caldwell (2). The most compact embodiment of the major elements which enter the design of a general purpose suspension crystallizer can be represented by the single body vacuum or coil cooled crystallizer with an internal impeller and draft tube or baffles for circulation of the suspension. Typical cross-sectional diagrams are shown in Figure 1. Equipment of this type may be operated either batchwise or continuously. This basic system may provide for agitation and growth of crystals by any of the three methods: evaporation, cooling, or salting out. Beyond this, any surther efforts toward control of the produce must be limited to variations in temperature, supersaturation, intensity of agitation, external seeding, or addition of chemical modifiers. Of these, supersaturation is usually the most influential.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Where vacuum evaporation is the force
The effect of supersaturation on the performance may be deduced from Figure 2 in which are shown typical, relative curves for crystal density, rate of growth, and rate of nucleation us. supersaturation. In practice, the supersaturation in suspension does not remain constant but varies cyclically with suspension turnover between definite minimum and maximum values. Duration and amplitude of the maximum values are of primary significance in correlating effects of supersaturation on nucleation and crystal structure. Crystal shape, structure, and quality vary with rate of growth. The presence of defects in crystals becomes evident by appearance well before any other effect, such as decrease of in density is noted. Appearance therefore is the simplest and best test of the structural quality of crystals. If both crystal size and crystal shape and structure are to be controlled strictly by control of supersaturation. there is a limiting supersaturation dictated either by the maximum tolerable rate of growth or rate of nucleation which cannot be exceeded. These maxima will not. in general, coincide as illustrated in Figure 2. Also supersaturation depends on the production rate per unit volume of suspension, the density of the suspension, the turnover rate of the suspension, and crystal size. However, as the density of suspension is pushed to higher values in an effort to increase production, while a t the same time lowering supersaturation, a new variable, attrition, starts influencing the situation. -4ttrition is also influenced by the intensity of agitation or the turn-over rate of the suspension. Increasing the turn-over rate decreases peak-to-average supersaturation in the suspension but also
CE/DESIGN MAXIMUM SUPERSATURATION FOR SOUND CRYSTAL STRUCTURE
CRYSTAL DENSITY
SUPERSATURATION LIMITED BY REQUIRED NUCLEATION RATE SUPERSATURATION
Figure 2. quality
Supersaturation is the maior variable controlling product size and
increases attrition. Thus, as either the density of suspension or the turn-over rate is increased, the conditions are enhanced for generating attritionspawned nuclei in suspension. Decreasing supersaturation and increasing attrition both favor the improvement in crystal structure, though tending to the growth of crystals of more spherical or rice-like shape. I t is, therefore, apparent that because of attrition, strong, dense crystals of small size can be grown more readily than those of large size. Increase in size of crystals increases attrition in the suspension. With crystal size determined by interdependent effects of supersaturation and attrition on nucleation, and with supersaturation and attrition in turn also dependent on crystal size, it becomes quite difficult to determine the quantitative relation among these variables. Nucleation and attrition are not easily measurable or controllable. The performance of the system under scale-up will, nevertheless, be governed by these relations. Because of this uncertainty in the design and performance relations of crystallizers the simple unrestrained suspension system described above is unsuited for optimizing performance of the crystallizer with respect to multiple antithetical operating variables which influence product criteria. The foregoing analysis also shows why facilities for the segregation and removal of excess fines can play a n influential part in improving crystallizer performance. The adverse influences of attrition in increasing the nucleation rate can be cancelled, while the beneficial effects of .attrition in improving structure and shape are retained. Crystal size plays an important part in the economic evaluation of crystallization processes. Larger crystals are, as a rule, more costly to produce. This increased cost may be off-set by reduced processing, handling, or storage costs in subsequent steps of the process or by increased sales value. Each situation needs be evaluated on its own merits. A further aspect of attrition concerns the recycling of mother liquor from the
centrifuge used to remove product crystals, to the suspension tank without intermediate clarification or dilution. I n this event, attrition fragments from the centrifuge may contribute to the seed supply in the crystallizer. Suspension Agitation There are two aspects to the choice of agitation; one dictated by internal crystal growth requirements and the other by environmental influences dictated by plant conditions or equipment manufacturers. I n view of their more or less arbitrary nature, the latter conditions can be dispensed with first. T h e choice of agitation from an environmental viewpoint has led to the adoption of internal gas lift or impellertype Pachuca circulation system; internal baffled and unbaffled impeller mixing systems, and external pump circulation systems. Here also are variations of circulation patterns such as the vertical vortex pattern created by a tangential return of circulating solution, horiiontal vortex patterns created by baffled mixing action, and conduit circulation characterized by fast-riseslow-fall, slow-rise-fast-fall, and balanced conditions of suspension movement.
Conduit systems are chosen over baffled mixing systems when it is difficult to maintain crystals in suspension. T h e draft tube method of agitation with internal impellers is favored for attaining maximum performance and flexibility at minimum cost because it is efficient, and consistent with rational principles of crystallizer design. Typical designs are illustrated in Figure 3. Airlift Pachuca operation is sometimes used for providing evaporative cooling and for holding easily maintained fine solids in suspension. An example is the Bayer crystallizers for aluminum hydrate. Some environmental conditions will justify the use of externally mounted pumps for maintaining the suspension, particularly where the solution must also be pumped through an external heat exchanger or evaporator. However, where such external pumping requirements are encountered, it must be recognized that crystallizer performance is dependent on the turn-over rate of the suspension. If the turn-over requirement and the external pumping rate are not of the same order of magnitude, the system should be designed with separate agitation facilities for the suspension, especially since power requirements for internal draft tube agitation can be reduced below that for externally mounted pump systems. Suspension agitation as viewed from crystal growth requirements has three functions : Circulation Mixing Turbulence Circulation is directed to thewefficient exposure of the solution to localized facilities for inducing supersaturation. Mixing function is directed to the rapid and efficient dispersal of the induced supersaturation among the crys-
SLOW- RISE FAST- F A L L
Figure 3. Conduit circulation systems are used when suspension of crystals is difficult. The fast-rise, slow-fall pattern requires least energy and yields a quiet and stable environment for fines trap operation. The slow-rise, fast-faII pattern, used in the Oslo-Krystal designs, can be used in cone bottom crystallizers, although crystals may settle in the cone. When the draft tube is enlarged to balance flow rates, relatively large impeller diameters are used to keep the bottom free of crystals VOL. 53, NO. 8
AUGUST 1961
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100
80 60 40 20
--- ---_ 10
20
30
40
50
60
TIME-SECONDS
Figure 4. Supersaturation does not remain constant, but varies cyclically with suspension turnover. Calculated desupersaturation rate for NH4N03. Basis: suspension density, 20 Ib./ft.3; crystal size, 1.O mm.
tals in suspension. Turbulence is directed to the degree of intensity of local agitation which determines the degree of local or general attrition among crystals in suspension or the degree of homogenization of local variations in supersaturation. I n viewing suspension circulation from crystal growth requirements, the situation is dominated by the desupersaturation time constant of the solution as illustrated in Figure 4. This requirement is determined by crystal chemistry, crystal size, and density of suspension. I t is independent of the scale of operation or the physical method of agitating the suspension provided the method of agitation is efficient-i.e., the solution is circulated through the growth-inducing zones in such a way that all of the solution is exposed equally to induced supersaturation in each cycle of suspension or solution turn-over. Conduit circulation is favored because of the greater ease with ivhich coarse crystals can be maintained in suspension and the improved circulation efficiency attainable with this method of operation. For cooling an evaporative type of crystal growth inducement, the generation of supersaturation is limited to surface areas immersed or in contact with the solution or suspension. Production and growth efficiency are enhanced by designing the suspension agitation system for effective mixing of the supersaturated solution with the remaining suspension. However, in the case of salting-out growth inducement a more extended three-dimensional dispersion of the salting agent within the suspension can be achieved so that the emphasis in agitation shifts more to efficient mixing without the need for circulation requirement noted above. A minor factor which will modify the shape of the desupersaturation curve in suspension is the dilution of suspension due to injection of fresh feed. T h e position chosen for feed introduction generally falls toward the lower end of the desupersaturation curve and causes a sudden depression of the curve depending on the strength and quality of fresh feed.
616
The question of isolation of the supersaturation inducing factors from the crystal growth factors has enjoyed a substantial degree of publicity in history of development of crystallizer design particularly for the Oslo-Krystal line of equipment. -4dvantages were claimed initially for this method of operation primarily because of the ease Lvith which excess fines could be segregated and removed on top of a classified suspension which was a n adjunct of this method of operation. There is no rational basis for suspecting beneficial influences from such a condition of operation, provided comparable conditions of operation with regard to turn-over rate, circulation efficiency: fines trapping facilities, etc., are otherwise maintained. As a matter of fact, the deliberate isolation of supersaturation inducing regions from the crystal growth regions ma>- lead, among other things, to severe limitations in productivitl- of the system, interference from crusts and obstructions in the conduits carrying supersaturated solutions, increased cost of the equipment, and increased difficulty in effective size or quality control of the product except a t relatively low production rates. These handicaps are overcome in the Krystal crystallizers by recycling suspension through the evaporator zone, thereby shifting over from a classified to a mixed suspension. Even after the desupersaturation time
Figure 5. Actual sup e rsa t u r a tion varies greatly with turnover rate, even at constant average supersaturation
INDUSTRIAL AND ENGINEERING CHEMISTRY
constant as a function of crystal size and density of suspension has been determined, one encounters further degrees of freedom in the choice of agitation parameters. O n the one hand, the turnover period may be relatively long compared to the desupersaturation time constant as illustrated in Figure 5. During intervals of low supersaturation, the suspension is idle leading to potential loss of productive capacity. Also, the supersaturation peaks are relatively high compared to the average. O n the other hand, if the turn-over period is unduly short, the peak-to-average supersaturation values in suspension may be merged a t the cost of increased agitation power and increased influences from attrition on the shape: size, and structure of crystals. Both rates may yield the same average supersaturation which in essence determines the productivity of the suspension. I n an otherwise unrestrained crystallizer, one would suspect that the optimum conditions for producing the largest and best crystals a t the maximum production rate would be attainable a t some intermediate turn-over rate where peak-supersaturations are reduced to a value below the critical value for spontaneously induced nucleation or inferior crystal structure and where attrition is not sufficiently severe to induce attrition-spawned nucleation. Aside from the turn-over rate, peak supersaturations are also dependent on the scale of operation, circulation efficiency-? and mixing action in the immediate vicinity of the growth inducing influences. As the scale of operation increases, peak supersaturation may also be increased due to a reduced surface to volume ratio of the suspension and reduced circulation efficiency past the evaporation surface. Evaporative cooling procedure is, however, highly favored from a purely technical vietvpoint in that flash evaporation a t the surface of the suspension occurs over a n appreciable range of submergence while the boiling action at the surface creates turbulence which is desirable to homogenize local peaks, in the degree of
CE/DESIGN supersaturation. This type of cooling is also relatively immune to fouling by crust formation. Such favorable conditions cannot be attained with fixed surface coolers for equivalent cooling rates. I n the latter case, high velocity flow past the cooling surface would tend to homogenize the degree of supersaturation, improve heat transfer, and minimize crusting or fouling of the surface, at the cost of extra attrition. Fixed surface cooling of the system may, nevertheless, be dictated by environmental factors which influence the cost of the operation. Removal of Excess Fines
I n a more restrained system with supplementary fines trapping facilities, the position chosen for the fines trap should be in the region of lowest supersaturation in the suspension. Depending on the direction of circulation this may be either the top or the bottom; however, for most stable operations, the bottom location with the fast-rise-slowfall pattern of circulation is favored. Since trapped fines should not be permitted to grow appreciably, the residual supersaturation at the trap should be allowed to decay to 5-20% of its initial value, and the suspension turnover rate may be computed accordingly. By minimizing the fines holding capacity of the trap, higher residual supersaturations can be tolerated, though a t a sacrifice in fines classification efficiency and at an increase in the required ratio of fines to product. Supersaturation Decay Constant
Supersaturation decay constant should remain constant with scale-up of the process. To this end, the turn-over period of the suspension should also be held constant under scale-up. There is a n absolute limit to the maximum useful size of suspension crystallizers with cooling or evaporative induced supersaturation, in that the average length of the suspension turn-over circuit increases as the size of the equipment increases. I n a medium size pilot crystallizer, the suspension circulation circuit may measure 10 feet, while in large industrial units the distance is extended to 100 feet or more. Where the desupersaturation time constant is relatively short (say 10 seconds), the energy requirements for pushing the suspension around this circuit in 10 to 30 seconds becomes significant aside from the fact that local attrition effects may also be aggravated. Maximum size must therefore be limited to dimensions compatible with the desupersaturation time constant to attain optimum performance.
Suspension Restraints Special features include : Varying seeding techniques beyond those dependent on spontaneous or attrition-spawned nuclei in suspension Using classification auxiliaries for controlling the crystal size distribution in suspension Using classification auxiliaries for controlling the crystal size distribution in the product Special arrangements for treating mother liquor outside of normal evaporative cooling or product removal requirements
Actual configuration of the suspension container will be determined by the number and type of such restraints included in the system. The degree to which suspension restraints are applicable is dependent on the settling rate of crystals and the viscosity of the mother liquor. Also, since larger crystals settle more freely than smaller crystals, auxiliary suspension restraints not only become more efficient and effective as the size of the crystal is increased but also become a necessary feature of the crystallizer in order to attain such larger sizes. As the crystal size and settling velocity decrease, the principles of suspension restraint may still be applied. I n the Bayer crystallizers for aluminum hydrate, large external classifiers are used to separate the suspension into product and seed portions in the 100- to 300-mesh size range in relatively viscous solution. However, more reliance must be placed on controlling the process in terms of rate-of-growth-effects and less reliance on the effects of attrition and excess nuclei removal as the rate of crystal classification slows down due to the smallness of the crystal, the absence of an adequate density difference between crystals and solution, or the presence of excessively viscous mother ljquor. Seeding Technique This restraint is used as a primary means for supersaturation control and crystal growth control. I n this case, the primary concern is with relatively small crystal sizes or materials with inadequate nucleation rates. The object is to provide sufficient crystal surface by artificial seeding so that the supersaturation corresponding to the required production rate can remain at a value for which the spontaneous nucleation rate is negligible compared to the seed requirement. I n the crystallization of borax from Trona, the solution can be processed in supersaturated conditions over extended periods of time without appreciable precipitation of nuclei and the crystallization of borax is finally induced by controlled seeding (3, 6 ) . Such conditions are also maintained in Bayer alumina plants. The outgoing
suspension is separated into fine and coarse fractions, the former being used to reseed the incoming supersaturated feed solution. I n another application, unclassified suspension of gypsum as encountered in the wet process phosphoric acid plants is recycled to seed the incoming feed with gypsum nuclei to limit the degree of supersaturation attained in the system. Size and purity control of refined sugar is controlled almost exclusively by carefully executed seeding techniques in batchwise sugar crystallizers ( 7 5 ) . I n other instances: fines separated from the product by dry screening may serve to reseed the suspension with sufficient force to suppress spontaneous nucleation. I n all these cases, it is important to vary slurry density and turnover rate of the suspension within the extreme limits of the process to seek, in so far as possible, the optimum conditions of minimum nucleation consistent with a given or required production rate. Data so obtained cannot always be submitted to simple correlation but must, at times, be submitted to more comprehensive rational analysis in order to account for effects noted. If material for artificial reseeding is unavailable, then a small auxiliary concentration or cooling circuit should be used, operating at a sufficiently high supersaturation to generate the required nucleation rate. A portion of the suspension could be subjected to such intensive mechanical agitation that the desired number of nuclei are formed by attrition. Fines Traps Where a sufficiently large crystal size is sought so that classification by elutriation or sedimentation proceeds readily, the logical location for classification auxiliaries is inside the crystallizer. This minimizes the length of pipes or conduits required to tie the facility into the suspension, eliminates the need for leak-proof construction, permits the use of lighter gage metal for construction by balancing forces due to weight or pressure of the solution, and eliminates interference from heat loss or chilling. The energy used to drive these classification devices can sometimes be drawn directly from the suspension without the need of auxiliary mechanical drives. Classification procedures on the small end of the crystal spectrum are far more important than classification at the large end. I t has been shown that under steady-state conditions, the distribution (on a weight basis) of crystals drawn from a suspension is dominated so strongly by the larger fraction of the product that the smaller fraction can usually be ignored provided adequate VOL. 53, NO, 8
AUGUST 1961
6 17
VAPOR
I VA IRIABLE erllrla
L. FROM FINES
1 ,I I r
-D
I s s o LV ER
DRAFT TUBE CLEAR SOLUTIQN
FINES SUBSISTENCE LEVEL /O$+
A G ITATOR
w Figure 6.
Vacuum crystallizer with internal self regulating fines trap
seed control methods are used for the system (7, 70, 72). From the same considerations, the effectiveness of suspension control a t the small end of the size spectrum is related directly to the solution processing rate through the fines classification equipment (72). Thus, to attain the maximum solution flow rate with the shortest and lightest solution conduits, the facility needs be located within the suspension. Such a facility in a suspension crystallizer designed for vacuum cooling is shown in Figure 6. Other configurations are also described in the patent literature (74) and technical publicatiom (9: 73). I n general, a trap having a crosssectional area u p to 807, of the suspension container cross-section can be installed with relatively minor modifications of the system and a t a moderate cost. Traps of this size are usually more than adequate for the effect desired. Where trap cross-sections of larger size are desired, more substantial modifications are entailed. Girth or height of the suspension container must be increased to accommodate the trap or, if a n external trap is used, large and heavy coupling conduits must be used to make the trap effective. Alternately, multiple deck traps may be installed internally by enlarging the height instead of the girth of the crystallizer. The data required for trap design are the free settling rate of the crystal and the density of suspension of the crystal as a function of size and superficial liquor velocity. Typical data of this sort for ammonium nitrate are shown in Figure 7 ( 8 ) . Since trap efficiency improves as the size of excess fines collected
61 8
decreases, the superficial liquor velocity must be held low to permit the smallest possible particles to settle out. However, the time required to segregate excess nuclei from the suspension in the trap decreases as the trap flow rate increases. Since fines continue to grow until they are trapped, the flow velocity through the trap must also be regulated with this factor in view. Segregation time requirements of fines in mixed suspension are given by : dC(A7,) - - RC(AT,) ___ d7
-
v
where C ( A T $ )= Concentration of nuclei in age 7% T
R V
increment ( A T & ) age increment denoted by index number i = elapsed time or age from origin of nuclei = trapflowrate = volume of suspension
=
This relation states that the rate of nuclei depletion in the suspension is proportional to the concentration of nuclei in the suspension and the relative flow rate of solution through the trap. The solution of this differential equation is c ( A 7 , ) = C(Aro)e--?R/V
where C(A7,) is the initial concentration on nuclei in interval (bo) and indicates that the concentration of nuclei AT,) in age bracket i has a n exponential decay pattern. Depending on the excess nuclei present, the factor must, therefore, be made sufficiently small to nullify the excess. Since the volume of suspension V is fixed, only the trap
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
flow rate R remains as a design variable for achieving the desired fines decay constant R/V. Thus, for a 100-fold excess of fines, R / V must be so chosen that e - T R / " = 1/100 = e-4,G for the time 7 during which the size will not increase beyond the maximum tolerable size for efficient trap operation. If the maximum tolerable size of fines in suspension is equivalent to an age of 46 minutes, then R must be adjusted to yield R / V = 10 minutes to permit a consistent simultaneous attainment of all conditions to achieve the anticipated performance of the system. The net effect of these considerations is that the trap needs to be made as large as possible provided exceptional cost factors are not otherwise encountered. When the required trap size exceeds 80% of the crystallizer cross-sectional, exceptional cost factors begin to enter the picture. One must then balance the extra trap cost against the increased production cost of the operation. A trap of deficient size or capacity results in a n increase in the ratio of fines to product which must be dissolved to meet specific size requirements of the product. Fines-to-product ratios less than 5y0 are usually tolerable without affecting production costs appreciably. The depth of the trap is a consideration quite distinct from the cross-sectional area. There are three distinct aspects. @Depth must be sufficient to satisfy the kinetic requirements for classification of the fines in the trap. Similar relations apply to any system of hydraulic elutriation. If one starts with a mixed suspension in a rising column of solution, how long does it take the large crystals from the top to segregate a t the bottom and vice-versa' This may be answered by relatively simple experimentation. .Trap must be deep enough so that turbulent fluctuations from the suspension which constantly stir about the lower layers of crystals in the trap a r e completely damped a t the upper layers. Actually, this consideration appears to be more significant in trap design than the kinetic classification in time requirement. I t is best determined by visual inspection of suspension movements i n the trap through sight glasses. One chooses a depth great enough so that the upper layer of fines are generally stable and quiescent. Six to 12 inches of depth in pilot crystallizers and 2 to 4 feet in plant crystallizers are typical ranges in which the depth requirement may fall. .Actual turbulence a t the trap entrance is also dependent on the mode of circulation used in the suspension. I n this respect, a fast-rise-slow-fall pattern creates less turbulence a t the trap entrance than other potential flow patterns which might br chosen.
CE/DESIGN Once excess fines are segregated in the trap, they must be disposed of, or the trap would be loaded to saturation and lose its effectiveness. The density of accumulated fines in the trap as a function of size and trap flow rate may be estimated from data of the type illustrated by Figure 7. Using this density, the external piping and fines dissolver are then designed to handle some maximum rate of fines solution-such as 10 to 20Qj, of the production rate-with the fines off-take located at the upper limit of fines in the trap. A number of sight glasses at this level will be required to confirm the fact that quiescent conditions are maintained a t this level and also for aiding in the adjustment of the trap flow rate to maintain the upper level at the fines off-take. Actually, the upper level reached by fines in the trap is also a function of fines size. As soon as all fines of a specific size and smaller have been withdrawn, the level of fines falls below the level of the off-take pipe and remains there. I n this manner, the fines concentration to the dissolver may vary from zero to maximum. T h e actual rate of removal of fines of the size for which the trap flow rate is adjusted proceeds only as fast as the fines of that size accumulate a t the upper level. All larger sizes of crystals are safe against removal provided quiescent conditions are maintained. This does not, however, prevent the fines dissolver from overloading with larger sizes of crystals very quickly if the lower layers of suspension in the trap are accidentally agitated. This is particularly true of a dead-ended fines dissolver from which solids cannot escape, regardless of size, without being taken baLk into solution. I n flow-through fines dissolvers, the holding time and dilution of the solution can be so arranged that only the fines
dissolve, and the coarser crystals which accidentally spill over are not completely redissolved but are returned to the suspension. The flow-through type of dissolver has considerable merit particularly in plant start-up because trap stability is often adversely influenced by light suspensions. During start-up on light suspensions a t low production rates, the accidental spillage rate into the dissolve? may easily equal the production rate so that little or no headway can be made toM. ard building up the suspension. Aside from the considerations of trap stability on trap depth noted above there is the further consideration of establishing a solution flow potential through the trap. This flow potential is established in a simple and effective, self-regulating manner by using the difference of apparent specific gravity of crystal suspension and its saturated solution to provide a flow potential through the trap. I n cases such as ammonium sulfate or ammonium nitrate with a specific gravity of 1.6 to 1.7 for crystals and 1.2 to 1.4 for saturated solution, the density of crystal suspensions may easily exceed the density of saturated solutions by 10% or more. I n such cases, over a n inch of hydraulic head is generated in the trap for each foot of trap height. I n plant crystallizers total trap heights of up to 6 feet are practical. Actually, this total head is not available for driving solution through the trap because part of the head is neutralized by the accumulation of suspension and fines in the trap. Net driving head kills to '/z or '/3 of the maximum. One needs, therefore, to insert in the upper portion of the trap variable flow orifices of such a size that the maximum tolerable trap flow rate is attained with '/z to '/j of the maximum potential head developed by the trap.
SUPERFICIAL SOLUTION VELOCITY
Figure 7.
FT/SEC.
Examples of data required for design of fines trap. left.
Settling velocity
These flow orifices should have throttle plates to permit variation of the flow rate from zero to maximum as indicated by visual inspection of the upper layer of fines in the trap and size analysis of crystals in suspension. This same source of flow differential may also be utilized in driving the solution from the trap to the fines dissolver and back into the suspensions at some higher level. Thus, by bringing the fines dissolver return back into the suspension some 15 feet above the fines off-take, a 15-inch head may be developed without auxiliary pumping. Such flow inducement will nct, however, be available until the suspension density has been built up to 108% of the density of saturated solution so that some start-up handicap may be occasioned, if auxiliary dissolver circula tion means are not provided.
Product Classifiers O n the large end of the crystal spectrum, suspension constraint can also be exercised by use of crystal classification auxiliaries. Several possible objectives are : Preferential removal of lumps and oversize crystals from suspension Preferential rejection of undersize crystals from product Increasing slurry density to product filter, separator, or centrifuge Again, for a mixed suspension, the access rate of the suspension a t the classifier inlet determines the effectiveness of the device and requires that the classifier and suspension be close-coupled to assure good access. Considerations of residual supersaturation are not particularly important so that the device may be located anywhere in the suspension consistent with minimum cost, maximum convenience in observation or operation, or other considerations. If sharp resolu-
LINEAR CRYSTAL SIZE-INCHES
NHkN03 crystals in saturated solution at 21-25°C. Right.
Density of suspension
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tion of crystal size is required, quantitative determinations on the kinetics of classification will need to be made to provide a basis for sizing the unit. I n other less stringent cases, a nominal holding time of 1 to 10 minutes in the classifier depending on the settling rate of the crystals will assure the preferential removal of oversize from the suspension and the preferential rejection of undersize from the product. A supply of clear saturated solution must be provided to assure proper functioning of the classifier. This may easily be diverted from the fines trap since the product classifier in general will be of smaller cross-section then the fines trap. By locating the product classifier in the upper regions of the suspension, the required head differential can be developed between the column of clear solution from the fines trap to the product classifier and the surrounding suspension to operate the product classifier without auxiliary external pumping. I n this case, the clear solution duct in the product classifier needs to be hooded a t the top to avoid the entry of crystals. The product classifier must be provided with dump gates on the bottom to avoid loading with a static bed of crystals when sufficient solution head to fluidize the bed is not available. Again, the density of suspension in the classifier can be varied by varying the clear solution flow rate in accordance with density of suspension data (Figure 7 ) . The clear solution rate to the product classifier must exceed the solution rate to the product removal system for effective classification action. The presence of product classifier places an increased duty on the fines trap, since the fines which would be removed in a mixed product are rejected, and must be removed separately if the distribution of large crystals is to be maintained constant. From the viewpoint of system dynamics the degree of stability of the system is actually decreased by operating with a product classifier particularly if such a classifier is used without a fines trap. This is: however, such a n unusual instability condition that it may safely be ignored. Although one suspension crystallizer is currently offered with a product classifier projecting axially from the bottom apex of the crystallizer cone ( 2 ) , the thought is offered that similar performance efficiency can also be had by locating this device elsewhere in the suspension, thereby eliminating the extra elevation and head room demanded by that type of construction. Furthermore. the apex of the cone appears to be the best location for the suspension agitator. Treating Mother liquor
Some processes require separating mother liquor from the suspension in
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substantial amounts because of special circumstances of the process of which the crystallizer may be a part. T o remove clear mother liquor from the suspension, one has the alternative of using mother liquor from the filter, centrifuge, or settling tank used to remove product crystals, if the volume requirements are adequate, or by the use of special internal or external classification apparatus. I n nonevaporative type of crystals growth inducement, the upper portion of the suspension may be left unagitated to permit crystal sedimentation and to permit withdrawal or overflow of clear mother liquor from the top of the suspension tanks (6). For evaporative growth inducement, the surface of the suspension is usually reserved for solvent evaporation. The fines trap will simultaneously serve the dual functions of fines segregation and mother liquor clarification in a sufficientiy large volume to meet most practical requirements. I n any case? the position chosen for removal of mother liquor should also fall rather Ioxv on the suspension desupersaturation curve.
Crystal Growth Inducement
This item is a major factor in the design of suspension crystallizers. T h e choice of equipment features for growth inducement is simple compared with the interacting influences on crystal size and quality. Evaporative growth inducement is indicated where the solubility of the solute is relatively constant or inverted with rising temperature such as NaCl or NazSOd (above 32.4' C.). Cooling or evaporative growth inducement may be used where the solubility increases very sharply with temperature, depending on the relative heat loads required for each method. Ammonia and potassium alums are typical salts responsive to cooling growrh inducement, particularly because temperature variations do not appear to influence crystal quality. Also, residual solute in the mother liquor can be reduced to negligible values by reducing the temperature of the solution sufficiently. Caution must be used; for cooling growth inducement if temperatures are encountered at which crystal form or crystal hydrate transitions occur, as such transitions may damage the structure. Ammonium nitrate is an example with a damaging rransition a t 32.1 C. I n the case of the cooling method, the equipment may operate either with evaporative cooling or with tubular or coil coolers. The latter methods are applicable where crusting or freezing of solute on the cooling surface can be easily avoided, otherwise the evaporative procedure is still preferred. I n special cases, the deposition of solute on the surfaces is avoided by using high
INDUSTRIAL A N D ENGINEERING CHEMISTRY
flow velocities with sufficient coarsc solids entrained to scour constantly the hear: transfer surfaces, or mechanical scrapers can be used to keep the surfaces clear. Costs of operating by these procedures will need to be evaluated against the cost of evaporative cooling where crusting is less of a problem to establish an objective basis for the choice of design. Salting-out is less frequently used as a means for crystal growth inducement but the method may present distinct advantages. I n the Solvay soda ash process, the reprocessing of spenr ammonium chloride solutions to recover ammonia constitutes a major step of the process. Some Japanese plants are using NaCl feed to salt out SHdC1 crystals for fertilizer use and restoring their process ammonia inventory with primary ammonia, simplifing the operation. I n other cases, the nature of the mother liquor may dictate the use of this technique, The process for purifying aluminum sulfate by crystallization (7, 7 7) utilizes alcohol as a salting-out agent for aluminum sulfate, This also thins the normally viscous mother liquor sufficiently to facilitate the steps of centrifuging and washing the crystals.
Process Scale-Up
Sound scale-up is predicted on knowledge of the dominant Crystallizer variables and the major interrelations among these variables. This basis pre-empts all other proposed methods of scale-up based on dimensional analysis (9), principles of geometric similitude, curvilinear extrapolation, or other empirical procedures which may be advocated. That is, scaleup itself is recognized as a dominant variable which may influence the performance of the system and which may have major interactions with other dominant variables. From this viewpoint, the final answer to the success of scaleu p therefore lies in experiment. From theoretical information established for crystal growth processes, supersaturation appears to top the list of important factors. This variable influences crystal growth processes to a major degree on a microscopic scale of space and time, and should be relatively insensitive to variations in macroscopic equipment geometry provided the microscopic environment is not altered too drastically. To this end, it is proposed above that the desupersaturation time constant and suspension turn-over period be held invariant on scale-up to assure as much as possible an invariant microscopic supersaturatLon environment for crystal growth. If these two parameters are fixed, the averaged peak supersaturation will also remain constant, but local variations about the average can increase as the scale of operation increases. Inter-
CE/DESIGN fering effects from this source can therefore best be eliminated by keeping the peak value well below the critical value for spontaneous nucleation. As the next factor which may influence the microscopic crystal growth environment, especially for crystals of larger size, one may consider attrition. Attrition is variable, ranging from negligible effects throughout most of the suspension to maximum effects in the vicinity of the agitator or other high velocity flow regions. I n this case, only the latter condition may be of significance. I n that event, scale-up would be properly executed, if the agitator for small and large scale equipment were designed for approximate equal intensity of local maximum attrition with relatively little regard for the lesser effects. One would not need to pursue this approach to scale-up far to observe that since relative importance of variables varies with different chemicals and with different objectives of the process, so the basis of scale-up will vary accordingly. With the inclusion in scale-up of the objectives of costs, performance, and quality, the best solution can be had with the maximum number of degrees of freedom of choice in design parameters of the process and equipment. Of these degrees of freedom, or major design parameters, some will dominate the physical properties of the product, some will dominate capital cost, some will dominate production cost, down the line until all objectives are satisfied. In some cases, conflict develops among major variables. In other cases, the dominant variables will not conflict and can be specified independently of one another. Interference among variables a t the secondary level of interaction is generally remedied by compensations in the noninteracting primary effects of dominant variables in each category. In following this philosophy of scale-up in a practical case, we would seek to satisfy the major requirements for crystal size and quality control by using the parameters of maximum supersaturation and turn-over rate. Nucleation conditions and seed requirements would be held under control by facilities for excess fines removal. Productivity requirements are met by specification of suspension volume and density. Minimization of capital cost would be dependent primarily on enlarging the scale of the operation sufficiently to reduce the number of parallel units to a minimum and to use height-diameter ratios to assure maximum volume with a minimum amount of metal. O n the other hand, height-to-diameter may also be chosen to minimize agitator driving power or to keep the size of the unit within clearance limits for rail transportation to avoid extra field fabricatioii
costs. If we now question the validity of the choice of height-diameter ratio exclusively on one or the other bases noted above, on the grounds that this ratio also affects the crystal growth environment at the secondary level of influence, the concept is advanced that the minor variations in growth due to major variations in equipment geometry can be compensated for by minor variations in turn-over rate and suspension density. Similar considerations apply to the other dominant crystallizer variables and to other cases of conflict in interactions below the level of primary cause-and-effect relations. Little more needs be said of this aspect of design other than to point-out that basis of scale-up advocating the principles of dimensional analysis and geometric similitude have been previously proposed by Bennett and Newman (9). The methods proposed are directed primarily to empirical concepts for evaluating crystallizer performance from a technical viewpoint. I n this respect, the methods proposed by Bennett and Newman are unduly restrictive by limiting the choice of parameters of crystallizer designs directed to the attainment of minimum capital and production costs of the operation. The degrees of freedom in crystallizer design normally available for allocation to the objectives of minimization of capital and production cost are frozen in dimensionless ratios and geometric similitude to remove them from the domain of independent choice in the design of crystallizers. With the reduced number of degrees of freedom, it may be inferred that fewer specifications are needed to design a crystallizer by the method of Bennett and Newman than is advocated above by the procedure utilizing the maximum of degrees of design freedom. From a performance viewpoint, the larger the number of degrees of freedom available to match the system against the problem, the better the potential for actually gaining optimum performance from the system. The alternate approach of restricting degrees of design freedom to promote standardization of equipment from the equipment vendors viewpoint may have occasional advantages, but this approach fails to present the design problem from a general or fundamental viewpoint and may limit the ultimate performance of the system substantially below optimum values. Crystallizer Operation and Control
The system should provide for: Start-up of the process Maintenance and operation Minimization of mishaps
Start-up Features. Special start-up considerations are usually directed to methods for building-up or introducing suspension. Further considerations may also be directed to auxiliaries which depend on a suspension of substantial density for their operation such as the gravity-induced flow of solution through a fines trap, fines dissolver, or product classifier. During the initial seeding operation from spontaneously induced nuclei, the fines dissolver is of value in correcting conditions of over-seeding in the early stages of suspension build-up. At this stage, the fines trap can be run wide open to keep it loaded with mixed or partly classified seed for disposal in the dissolver. If the fines dissolver circulation is normally dependent on gravity-induced flow, an auxiliary drive would be required during start-up. A product classifier is not needed during start-up. It matters little whether it works until a suspension has actually been built up. The most significant provisions for crystallizer start-up are usually concerned with the transfer of a previously grown starting suspension from a parallel crystallizer-in multiple installationsfrom an agitated holding tank, on even preparing it from dry storage. The productivity of a crystallizer varies as the weight of suspension contained. With this type of relationship, the rate of suspension growth follows an exponential pattern. In other words, the suspension will roughly double and redouble itself in a fixed period of time until the capacity of the equipment is reached. If we suppose that the initial suspension induced by spontaneous nucleation is 1% of the final suspension, then the weight of suspension will have to redouble itself some 6 or 7 times to reach the operating value of 1 0 0 ~ oand at this point the product size will be 6 to 7 times the seed size. If the doubling time is say 6 hours, then some 40 hours will be needed to get into production. If, on the other hand, provision is made to preload the crystallizer with suspension from some other source, then the unit can be brought into full production within say 6 hours of start-up. Under some conditions, solution and crystal can be stored in the suspension tank of the crystallizer, but to gain the objective of efficient circulation with a minimum of agitation energy in a crystallizer, the use of conduit or draft tube circulation systems is preferred. These systems will function only at the full solution level of the system. At lower levels the circulation stops and the suspension collapses. Unless a collapsed suspension is held at uniform, invariant temperature, it will progressively redissolve and recrystallize to form a solid cake which can then be removed only VOL. 53, NO. 8
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with considerable difficulty. Similar considerations apply to the wet storage of suspension outside the crystallizer but in this event, more energetic agitation systems can be used which operate a t variable solution levels to gain somewhat added flexibility in handling the suspension in wet form. Crystallizer Maintenance and Operation. Perhaps the most persistent nuisance which presents itself during crystallizer operations is the appearance of crusts on the walls of pipes and containers and lumps in suspension due to the occasional loosening of these crusts. Where the occurrence of crusts or lumps is persistent, the problem is kept in hand by desalting on a routine basis. During such a desalting procedure, the suspension may be drawn down as far as possible. Sufficient heat or solvent is then admitted to dilute the mother liquor for a sufficient period of time to redissolve all lumps and other accretions. Alternately, the mother liquor and suspension may be transferred to a storage tank and the crystallizer steamed: sprayed, or loaded with dilute solvent to clear all surfaces. Where frequent desalting appears to be unavoidable, the productive part of the cycle can be increased by minimizing peak supersaturations, by over-sizing the suspension container to reduce the average level of supersaturaticn, and by careful analysis of the suspension turnover requirements as dictated by the desupersaturation time constant. It may also be of value to equip the system with speciai trapping or indicating devices to detect over-size lumps which may stop up slurry-piping or other narrow clearances in the system. I n this manner. time between desalting periods can be increased. I t is also customary to provide suspension slurry piping with steam or water taps to wash out the pipes in the event of stoppage. Such connections should always be demountable or be vented to eliminate accidental dilution due to a leaky steam or water valve. At the same time, siqht glasses should be inserted in all critical lines and in critical areas of the suspension container to indicate flow stoppage, banked out suspension in the bottom or abnormal turbulence which is indicative of crusts or accretions on the walls. T h e degree of susceptibility to interference from crusts or lumps in suspension depends on the circulation pattern. Possibly the least susceptible design is a cone-bottomed tank with gravity movement of settled crystals into a central draft tube €or re-elevation to the surface. Dish-bottomed suspension tanks with a rising current in the peripheral regions of the tank are much more susceptible since smaller clearances are needed a t the bottom of the draft tube or return
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conduit in order to disperse the recycled solution with sufficient momentum to sweep u p settle crystals. Also, accretions on the bottom deflect the stream from the draft tube upward, permitting sheltered banks of crystals to build u p on the leeward side. I n one such case, in a dish-bottomed unit, it was not uncommon for 20 to 60y0 of the bottom of a 16-foot diameter suspension container to be covered with crusts or banked out crystal. This vacuum crystallizer operated near ambient temperature and was uninsulated. Deposits were located by heating the outer surface of the suspension tank with live steam. Where the interior was clean the outside temperature remained low. However, for crustcd areas, the metal remained hot for some time. Perhaps the most persistent source of crusting and lump formation is a t the liquid vapor interface on the vaporizer wall. Two approaches have been used. I n the one case, crusts are dislodged as fast as they form to keep them relatively small and sofr. These break down by attrition and impact from the suspension agitator into innocuous fines. Dislodging of crusts is sometimes achieved by periodic variations in solution level, by occasional sprays of water or steam, or by cooling the vaporizer walls immediately above the interface. This last method recondenses a small amount of vapor which then drains back into the solution, constantly washing the walls. Alternately, crusts can be prevented from forming by eliminating the variable wetted wall perimeter in the vaporizer by special solution manifolding. Minimization of Mishaps. The major mishaps are slurry line obstructions and power or utility failure. Slurry line obstructions will cause temporary curtailment of production but otherwise no great crisis is caused. The degree of nuisance depends largely on the accessibility of the lines for location of and clearing tlie obstruction. Operating supervision is generally quite prompt in changing in the system as dictated by experience to alleviate interference from this source. A mishap somewhat more in the crisis category is agitator power failure and a collapsed suspension. The agitator drive will need to be heavy enough to remobilize collapsed suspension if this mishap is not to be counted a major source of production curtailment. I n that event, no major consequence will result from power failure of limited duration provided evaporator or cooling adjustments are made immediately to prevent the precipitation of excessive amounts of fine nuclei owing to oversupersaturation in the stagnant solution. I n some cases, auxiliary ducting to convey clear solution from above the collapsed suspension to a point immediately preceding the
INDUSTRIAL AND ENGINEERING CHEMISTRY
agitator may aid in re-establishing circulation especially for coarse, dense, free-settling crystals. This modification can be installed after plant start-up if required. I n smaller crystallizers, remobilization of a collapsed suspension is not usually a problem. For larger crystallizers, the need for auxiliary suspension mobilization ducting may be greater. A mishap of limited occurrence confined to systems with barometric leg coupling between the evaporator and Crystallizer concerns the loss of vacuum. Unless sufficient free-board is maintained in the crystallizer to hold the evaporator contents, spillage byill occur. Of the various mishaps which befall a crystallizer, the most common is the weather, particularly during start-up. T h e severity of this source of interference is dependent on plant location and plant design considerations. However, if the effects of weather resulting in variable heat losses, chilling, quenching, drainage. etc., are carefully evaluated in pilot plant equipment, and the plant then is adequately protected, no great nuisance should result. I n extreme cases of sensitivity, the equipment may be housed i n a fully enclosed insulated building. Generally, outdoor erection with weatherproof insulation is quite common for the larger industrial crystallizer installations in mild climates.
literature Cited (1) Bransom, S. H., Dunning, \I7. J., Millard, B., Discussions Faraday Soc., No. 5, 83 (1949). (2) Caldwell, H. B., IND. ENG. CHEM. 53,115 (1961). (3) Chem. Eng. 65, 116 (August 1958). (4) Egli, P. H., Zerfoss, S.,Discussions FaradaySoc., No. 5, 61 (1949). (5) Garrett, D. E., Rosenbaum, G. P., Chem. Eng. 65,125 (August 1958). (6) Garrett, D. E., Chem. Eng. Progr. 54,65 (December 1958). (7) Gee, E. A., Cunningham, W. K., Heindl, R. A , , IND.ENG. CHEM.39, 1178 (1947). ( 8 ) Miller, P., Saeman, W. C., Chem. Eng. Progr. 43, 667 (1947), (9) Newman, H. N.? Bennett, R. C., Chem. Eng. Progr. 55, 65 (March 1959). (10) Robinson, J. N., Roberts, J. E., Can. J . Chem. Eng. (October 1957),p. 105. (11) Roller, P. S.,U. S. Patent 2,302,668 (Jan. 25, 1946). (12) Saeman, W. C., A.I.Ch.E. Journal 2, 107 (1936). (13) S;:man, W. C., "Separation Processes, Chap. 4, Reinhold, New York, 1961. (14) Saeman, W. C., U. S. Patents 2,737,451 (Mar. 6, 1956); 2,856,270 (Oct. 14, 1958); 2,883,273 (Apr. 21, 1959). (15) Shearon, W. H., Jr., Louviere, N. H., Laperouse, R. M., IND.ENG.CHEM. 43, 552 (1951).
RECEIVED for review March 7, 1961 ACCEPTED March 9> 1961 Division of Industrial and Engineering Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
