US2346517A - Method of crystallizing material - Google Patents

Description

-APll 11, 1944- J. H. THOMPSON Erm.

METHOD OF CRYSTALLIZING MATERIAL Filed Oct. 4, 1940 Patented Apr. l11, 1944 John Henry Thompson, Burlingame, and Edward Taber Winslow, Saratoga, Salif., assignors to J. D. & A. B. Spreckels Company, San Tram cisco, Calif., a corporation or California Application October 4, 1946i, Serial No.. 359,664

, l2 Claims.

Our invention relates to the crystallization of material from solution; and more particularly to the crystallization of sugar.

It is among the objects of our invention to provide a method for eiecting continuous crystalllzation.

Another object is to provide means for maintaining crystallization of the material at substantially its maximum rate. 1

Another object is to provide a method which gives positive control of the various factors affecting crystallization, whereby crystals may be grown in accordance with a predetermined schedule.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of our invention. It is to be understood that we do' not limit ourselves to this disclosure of species of our invention, as we may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing, the gure of the low rate of crystallization relative to inorganic salts.

Heretofore the sugar industry has largely depended upon using a vacuum pan in which evaporation of syrup and crystallization of the sugar occurs simultaneously. This is strictly a batch or discontinuous process; the capacity of the pan being the limiting factor, and its proper functioning depending upon the skill of the operator or pan boiler who by inspection and sampling determines when the strike has been made and the pan is to be emptied.

In attacking the problem of continuous crystallization of sucrose it became apparent to us that the conditions imposed Vby the vacuum panwere incompatible with either continuous operation or positive control, and that a new approach would have to be made. Thus, we appreciated the need for divorcing the concepts of evaporation per se and crystallization; that is, separating the step of evaporation (purely -for the purpose of concentrating the syrup) from the subsequent step of crystallization. By separating these steps, positive control of crystallization is possible.

We also recognized that when syrup is concentrated beyond saturation it first enters a. metastable state of supersaturation in which crystals grow but are not newly formed, and subsequently passes into a labile state in which new crystals are spontaneously formed. Since spontaneous formation of crystals is erratic, it became apparent to us that positivecontrol would require inoculation with preformed seed crystals of known characteristics. Such crystals can then be grown to desired size in a supersaturated solution maintained in the meta-stable state.

We also recognized that the rate of crystal growth depended upon the degree of supersaturation, purity of solution, surface area of growing crystals presented to the solution, and distribution of the crystals in the solution. With these factors under control, crystallization may be carried out at the maximum rate.

Our improved-method was developed with due regard to these factors, and provides a system for the continuous production of sugar under controlled conditions so that the crystals grow in accordance with a predetermined schedule.

In terms of broad inclusion, our method of crystallizing material from solution comprises maintaining a body of the solution under conditions favorable for crystal growth, continuously growing crystals in the body. and continuously discharging nished crystals from the body. The crystals are preferably grown from preformed seed crystals of predetermined number and size introduced into a lower zone in the body. and the growth of the crystals is preferably regulated by introducing controlled quantities of fresh solution of predetermined concentration and temperature into overlying zones of the body, and by maintaining the body under a predetermined absolute pressure. By this arrangement the soluvtion in the crystallizing body may be maintained Still speaking in broad terms, our method is illustrated by apparatus winch includes a crystallizer unitdesigned to retain the solution in different zones increasing in size from bottom to top of the unit. Separate means are provided for evaporatlng solution to a predetermined concentration and temperature and for introducing this fresh solution into the Qseveral zones in the crystallizer. Means are also provided for treating a portion of the concentrated solution to produce seed crystals and for introducingthe same into a lower zone. Means are further provided for evacuating the crystallizer to establish a predetermined temperature gradient in the body of solution; and a barometric duct is arranged to discharge nnished crystals from an upper zone.

In greater detail, and referring to the drawing, our preferred apparatus for carrying out the method of crystallzing sugar comprises a suitable evaporator 2 fed by a syrup line 3. In a vsugar refining plant this evaporating unit replaces the vacuum pan, it being understood that preceding stages of evaporation are carried out in the ordinary manner. The syrup is heated in evaporator 2, as by a steam c'oil 4, and is boiled under vacuum maintained through suction line 5. Evaporation is preferably controlled by suitable devices to continuously concentrate the syrup to some constant concentration, preferably not over the saturation pointy at a predeter.

mined temperature. Since unit 2 functions simply as an evaporator its operation may be made wholly automatic by a suitable motorized valve in line 3 responsive to boiling point rise in the evaporator to maintain a constant concentration. This type of evaporator regulation is a proved control under such conditions. f

From the evaporator the concentrated solution is recrculated through a closed circuit or duct 6 by a suitable pump l. A valve 8 is preferably provided for pressure control in the circuit. Since the syrup at this stage of the process is not concentrated above saturation it may be readily handled without danger of crystallization; and the recirculation system-provides a constant supply of hot, concentrated, -crystal-free syrup for the next stage of our process.

A crystallizer 9 is provided for holding a body of syrup. This unit preferably comprises a plurality of say three cylindrical sections having conical lower portions todivide the contained solution into a series of zones II, I2 and I3; the sections increasing in .diameter and size from bottom to top of the unit. While three zones are shown, it is understood that more may be provided. If desired the crystallizer unit may be in the form of a simple cone, but the sectional construction is preferred to provide more distinct zoning of the solution. l

Means are provided for introducing preformed seed crystals into lower zone I I of the crystallizer. For this purpose a duct I4 controlled by a valve or orice I 1 is connected with circuit 6 and a heat exchanger or cooler I8, preferably formed by a water jacket around the duct, is provided in this line to supersaturate the solution. The solution at this point is supersaturated to the labile state, say a Supersaturation above 1.6, wherecrystals may be shocked out of the solution by agitation. Duct Il is connected with a seeder unit comprising a housing having a cold water jacket 2 I, and containing a shaft 22 rotated by a pulley 23. A multiplicity of radial arms or spikes 24 function to violently agitate the solution flowing through the unit. This agitation serves to shock minute seed crystals out of the supersaturated solution. y

The number and size of the seed crystals shocked out depends upon the degree of supersaturation, rate of agitation and quantity of so-- lution being treated. These factors are controlled by the amount of cooling, speed of the agitator. and rate of solution flow, all for the purpose of supplying the crystallizer with a predetermined number of seed crystals of given size. The seed growth is further controlled by introducing controlled amounts of fresh concentrated, solution into end 26 of the seeder through line 21 having a valve or orifice 28. Means are also provided for destroying the Supersaturation of the solution surrounding the seed. For this purpose they end portion 26 of the seeder is unjacketed so that the temperature of the solution may rise. If desired a thin syrup may be added to the crystal bearing solution to assist in destroying supersaturation of the mother liquor. By thus destroying the Supersaturation the formation of seed is definitely stopped at a given point. The seed-bearing solution is discharged downwardly into lower zone II of the crystallizer through a duct 29.

The seed-bearing solution flowing into the crystallizer thus affords a continuous supply of seed crystals for inoculating the syrup in the lower zone of the crystallzing body. This treatment of a portion of the syrup to produce the seed crystals is an important feature of our invention because it not only provides a convenient and continuous supply of seed, but also provides a control for the size and number ofseed introduced into the crystallizer.

Means are also provided for continuously introducing fresh concentrated syrup from circuit 6 into the crystallizer. For this purpose a duct or line 30 controlled by a valve or orifice 3l leads from circuit 6 to the bottom of the crystallizer; and other lines 32 and 33 controlled by valves or orifices 34 and 35` lead from the circuit to the upper sections of the crystallizer. The latter lines have suitable heads 36 and 3l for uniformly distributing the fresh solution throughout zones I2 and I3; the heads preferably having a bottom plate with a slight clearance at the periphery to effect radial discharge.

Means are provided for maintaining the solution in the crystallizer in a predetermined state of supersaturation, preferably a Supersaturation of about 1.2 in the meta-stable range. This may be accomplished by cooling the concentrated solution by suitable means, but we prefer to effect it by evaporation. The hot solution is caused to boil at reduced pressure produced by evacuating the crystallizer through exhaust duct 38.

Supersaturation is also affected by the quantity. concentration and temperature of the incoming fresh solution, these factors also being under control at evaporator 2 and by the how regulating orices in the various ducts. After the flows and temperaturesbecome constant the supersaturation also becomes constant.

Means are further provided for agitating the solution in the crystallizer to disperse the crystals throughout the several zones and thus present a maximum crystal surface to the solution for opti-y mum crystal growth. This'could be accomplished by a mechanical stirrer, but in order to avoid moving parts, we prefer to utilize the agitation caused by boiling. This agitation, accompanying the formation of steam bubbles in the solution, operates to effectively distribute the growing crystals uniformly throughout the solution and n charging nished crystals from the crystallizer.

To this end a barometric duct 39 leads from the upper section of the crystallizer and extends downwardhr into a tank 4I which forms a liquid seal for the lower end of the duct. By this arrangement a vcontinuous flow of crystal bearing solution may be discharged from the upper zone without breaking the vacuum in the crystallizer. Solution in tank 4I overflows into a suitable mixer 42 and thence is discharged into a centrifugal 43 for purging the crystals from solution.. A series of centrifugals are preferably provided for taking care of the continuous flow from `the crystallizer.

Operation-In starting the apparatus the seed bearing solution is introduced into lower zone II through duct 29 together with fresh concentrated solution through duct 30. Barometric duct 33 is sealed by liquid or some mechanical means to permit initial evacuation of the unit, and vacuum is drawn in the crystallizer. Under reduced pressure the hot liquor boils to .supersaturate the 'solution and agitate the mass to disperse-the crystals. Under these controlled conditions the seed crystals grow in accordance with a predetermined schedule. At some point the rate of growth diminishes with respect to supersaturation, and is compensated for by the introduction of freshsolution in the upper zones through ducts 32 and 33. The crystals starting from seed are thus allowed to grow in the different zones of solution as the crystals flow upwardly through the body. Viewed in another way, the partially grown crystals in one zone provide nuclei for crystal growth in an overlying zone; in other words, one zone provides a fractional seeder for another zone. the upper portions of the crystallizer and discharges `through duct 39.

When temperatures and flows become constant the crystallizer runs steadily, taking in seedand fresh solution continuously and discharging finished crystals at a continuous rate through duct 39. In order to stop the apparatus, the seeder is stopped and solution flow is continued until all crystals in the unit are displaced.

During operation there are several important Finally the solution level reaches of the solution is desirable. The gentle boiling that takes place in our unit eil'ects uniform distribution of the crystals in any Agiven zone. Since the zones increase in size toward the top, the upper zones offer a greater volume of fluid in which the crystals may be dispersed. The agitation in the several zones is a function of the cross-sectional area, and is cumulative as the fluid height increases, thus insuring proper size distribution of the crystals.

Another feature loi our crystallizer is that the plurality of zones tends to produce nished crystals of substantially uniform size. A single section crystallizer may be considered to give a sizedistribution of from initial seed size to nal grain size, while an infinite number ofsections would produce crystals of identical size. Within practical limits however, a three section crystallizer provides sumcient uniformity in the grain.

Still another feature is that the final grain size may be varied by increasing the number of sections in the crystallizer. Furthermore, sugars of diiferent grain size may be produced all at the same time vby tapping different sections of the unit, the smaller grain sugars being drawn off from lower zones.

Example.-In apparatus. having a discharge capacity of 238 pounds per minute, the threesection crystallizer 9 is designed to hold about Y 10,008 pounds of material; 1533 pounds being retained in lower zone II, 5725 pounds in intermediate zone IZ, and 12,750 pounds in upper zone I3. In such a crystallizer the height of the discharge opening above the bottom of the unit is 'about seven feet, and discharge duct 33 has a length of about twenty feet. Syrup is continuously concentrated in evaporator 2 to about 83 Brix at 101 C., and pumped through circuit 6 at the rate of about 305 pounds per minute. This saturated solution is continuously introduced into the crystallizer through' ducts 30, 32 and 33 at the rate of 29 pounds per minute into zone II, 83 pounds per minute into zone l2, and 145 pounds per minute into zone I3.

The seeder is designed to discharge seed bearing solution into the lower zone of the crystallizer through duct 29 at the rate of about 3 temperature in the. crystallizing body along a predetermined gradient. The temperature decreases along a gradient in accordance with the laws of hydrostatic pressure, and the desired gradient is established by the degree of vacuum maintained in the unit. Since supersaturation is a function of concentration and temperature,

pounds per minute, of which 1.2 pounds comprises seed crystals having a size of about microns. To accomplish this the portion of the solution flowing through duct Il is cooled su'fciently to effect a supersaturation above say 1.6, and the seeder is operated at a speed suilicient to shock out seeds of a size and in the quantities above mentioned. y f

A vacuum of about 26 inches Hg is maintained in the crystallizer, causing thersolution to boil with a total evaporation rate of 22 pounds per minute; of which 2 pounds is evaporated from zone I I, 'pounds from zone I2, and 14 pounds from zone I3. When the iiows and temperatures become constant under these conditions the rate of flow between zones II and I2 is about 30 pounds per minute, carrying 14 pounds of sugar crystals having a Ysize of about 233 microns. At this point the m'other liquor has a concentration of about 81 Brix at 78 C. The rate of ow between zones I2 and I3 is about 107 pounds per minute, carrying 57 pounds of crystals having .a size of about 367 microns. At this point the mother liquor has a concentration of about 78 Brix at 68 C. The rate of discharge from zone I3 is about 238 pounds per minute. carrying 143 pounds of nished crystals having a size of about 500 microns. The mother liquor at this point has a concentration of about 75 Brix at a temperature of 59 C.

It will be observed that the temperature falls along a gradient, with a temperature drop of about C. per zone.l Under these conditions, and with concentrated solution flowing in at the rates indicated, the supersaturation is maintained constant at about 1.2 throughout the entire body.l

We claim:

1. The method of crystallizing material from solution, which comprises arranging the solution in a plurality of vertically disposed zones, introducing fresh solution of predetermined concentration and temperature into each of said zones, in sufficient quantity to maintain a substantially constant degree of supersaturation introducing seed crystals into a lower zone, maintaining a predetermined absolute pressure on the solution, and discharging finished crystals from an upper zone.

2. The method of crystallizing material from solution, which comprises passing crystals upwardly through a bodyof solution, and controlling concentration and temperature at different levels in the body of solution to maintain a substantially constant supersaturation throughout the solution by introducing regulated amounts of solution of controlled character.

3. The method of crystallizing material from solution, which comprises providing solution in vertically disposed zones increasing in size toward the top, introducing regulated quantities of fresh solution into said zones, introducing' seed crystals into a lower zone, and discharging nished crystals from an upper zone.

4. The method of crystallizing material from solution, which comprises providing solution in vertically disposed zones increasing in size toward the top, introducing fresh solution into said zones in quantities increasing with the size of the zone,

introducing seed crystals into a lower zone, and

discharging finished crystals from an upper zone.

5. The method of crystallizing material from.

solution, which comprises introducing regulated quantities of fresh solution of predetermined concentration into vertically disposed zones of the solution, introducing seed crystals into a lower zone, boiling the solution to supersaturate it and Adisperse the crystals, and discharging finished.

disposed zones of the solution, introducing seed crystals of predetermined size into a. lower zone, 'maintaining a predetermined absolute pressure on the solution, and discharging finished crystals from an upper zone.

7. The method of crystallizing material from a supersaturated body of solution which comprises introducing seed crystals into the lower portion of the body, agitating the body to effect dispersal of the crystals therein, continuously introducing regulated quantities of fresh solution into the body at different levels to feed the growth of the crystals, and removing finished crystals from the upper level of the body. Y

8. The method of crystallizing material from solution which comprises maintaining a body of the' solution in a plurality of vertically disposed zones increasingin size toward the top, introducing seed crystals into the lower zone, agitating the body to effect dispersal of the crystals therein, introducing regulated quantities of fresh solution into each zone to feed the growth of the crystals, and removing finished crystals from the upper level of the body.

9. The method of crystallizing material from a body of solution which comprises introducing seed crystals into the lower part of the body and causing them to move upwardly through the body to the top thereof, and introducing into the body at different levels fresh solution of predetermined volume, temperature and concentration.

10. The method of crystallizing material from a supersaturated body of solution which comprises introducing seed crystals into the lower part of the body and causing them to move through the body tothe top thereof, introducing fresh solution of predetermined concentration into the body at different levels in quantities such as to maintain substantially uniform supersaturation of the solution throughout the body, maintaining a predetermined absolute pressure upon the body, and discharging finished crystals from the top of the body.

1l. The method of crystallizing material from a supersaturated body of solution which comprises introducing seed crystals into the lower part of the body and causing them to move through the body to the top thereof, continuously introducing fresh solution of predetermined volume, temperature and concentration into the body at different levels for maintaining substantially uniform supersaturation, maintaining a predetermined absolute pressure upon the body. and continuously discharging finished crystals from the top of the body.

12. The method of crystallizing material from solution which comprises maintaining the solution in a plurality of vertically disposed zones increasing in size toward the top, continuously introducing into the lower part of each zone fresh solution of controlled character and amount, agitating the body of solution, and continuously discharging the finished crystals from the upper zone.

JOHN HENRY THOMPSON. EDWARD TABER WINSLOW.

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