US2347157A - Centrifugal purging and washing method for sugar mixtures and the like - Google Patents

Description

Patented Apr. 18, 1944 CENTRIFUGAL PURGING AND WASHING METHOD FOR SUGAR MIXTURES AND THE LIKE Eugene Roberts, Hastings, N. FL, and George E.

Stevens, Denver, 0010., assignors to The Western States Machine Company, Hamilton, Ohio, a corporation of Utah No Drawing. Application August 5, 1942,

Serial No. 453,726 4 13 Claims.

This invention relates to new and useful improvements in methods for purging and washing mixtures of crystals and syrup or mother liquor, and particularly to the centrifugal treatment of massecuites and magmas in the manufacture of sugar.

The principal objects .of our present invention are: To provide a new and improved method for the purging and washing of sugar massecuites and magmas and similar mixtures. To reduce the melting of sugar crystals in centrifugal washing operations. To reduce power requirements in certain centrifugal operations and render the same more economical than heretofore. To increase the efiiciency of centrifugal syrup separations. And generally, to alleviate various difficulties which are now experienced in the centrifuging of fine grained white sugar massecuites and the like.

Heretofore, the washing of sugar in centrifugals generally has been considered as subordinate to the purging of free syrup by centrifugal force, i. e., as a means of removing films of adherent" syrup remaining on the crystals after such purging. In accelerating a centrifugal charge of white massecuite or the like the syrup can be seen to move readily away from the inside face of the sugar wall in the early stage of acceleration, and it usually is assumed that the same action occurs throughout the sugar wall as the acceleration proceeds. Since acertain amount of wash water produces cleaner sugar and yields purer wash syrupif applied after as much as practicable of the mother liquor has been purged away and while the charge is spinning nearly at its full speed, the universal practice heretofore has been to apply wash water in that manner, after some 35 to 60 seconds or more of acceleration of each charge.

In upto-date practice on fine grain, high density, lower purity white massecuites, however, several conditions have been observed which seem inconsistent with premises inherent in the above. Even where the washing is done after accelerating the charge quickly to a centrifugal force as high as 1600 times gravity, there often is a. delay of 8 seconds or more between the start of washing and the time when wash syrup begins to be expelled from the centrifugal basket. Also, wash water requirements may be excessive; the wash syrup may not be as pure as the original massecuite; and to recover any wash syrup of such purity the syrup separator (curb or ring valve type) may have to be actuated as late as 10 to 15 seconds after the start of washing, with an attendant increase in green syrup purity. These conditions indicate that the wash water must do a great deal more than to remove thin films of adherent syrup from the sugar grains. They are explainable by the presence, when the washing begins, of accumulations of syrup and smear (exceedingly fine or false grains) in the outer zones of the sugar wall next to the basket screens, which clog the filtration spaces and have to be loosened and washed away before comparatively pure wash syrup can be recovered and before the sugar wall itself can become properly cleaned. Excessive wash water requirements are caused by this and also by the drying or cooling, or both. of syrup on sugar grains in the inner zones of the sugar wall during the purging period.

According to the present invention, we have discovered that the foregoing objects may be achieved and that the difficulties and conditions hereinabove mentioned may be largely overcome by carrying out the purging or elimination of the main body of syrup and smear from each charge, during the acceleration of the centrifugal, in conjunction with one or more regulated applications of hot aqueous liquid, such as to mobilize or dilute the syrup and smear that otherwise obstructs the filterability and the purging and washing of the sugar wall and to assist physically the purging of the syrup and smear by centrifugal force during the acceleration. Then, after a substantial time interval, a finish washing or cleaning operation may be performed with a clear hot liquid to finish the removal of impurities from the sugar grains while the charge is pervious and subject to a high centrifugal force suitable for the type of sugar being treated.

The present method therefore is one in which charges of the massecuite or magma are loaded and walled up in centrifugals, started on acceleration, and then washed with suitable hot wash liquid in a plurality of distinct operations or completed during the acceleration of the centrifugal and serves to mobilize and assist the purging of syrup, false grain or other filtration obstructions-without substantially washing or melting the recoverable sugar grain. In our preferred embodiments the first washing is carried out shortly after the charge has been walled up in the centrifugal basket, before it attains half its full centrifugal force, so that the hot wash liquid acts upon and mixes with the mass of syrup and smear in the charge and renders it readily purgable as a body. The first washing and any intermediate washing require only a small quantity of liquid to accomplish their function, depending upon the qualities of the massecuite, and by the time the centrifugal has reached its full speed, in the case of free-purging massecuites, the charge is rid of substantial obstructions of syrup and smear, and the sugar grains also have been kept hot and moist and in a readily washable condition. A final washing from chilling the syrup or mother liquor in the charge. In the treatment of high purity massecuites and various other mixtures, it may be preferable to carry out all of the washing operations with a hot watery medium as disclosed in our United States Patent No. 2,223,663, such as by releasing superheated water through a spray nozzle at a temperature between 100 and 150 C. suitable for the type of massecuite being treated. In the treatment of various mixtures, such as magmas or low grade or crystallizer massecuites, we may prefer to use a substantially saturated syrup or sugar solution for the first washing operation and any intermediate washing, to prevent any increase in green syrup or final molasses purity, and to finish the cleaning of the sugar with a hot watery medium, as aforesaid, if the process being carried out entails a final washing.

According-to one embodiment of this invention, a charge of sugar massecuite is loaded into a centrifugal in the usual manner, at a suitable operation then may be performed on a pervious sugar wall having good filtration qualities, and it needs only to complete the cleaning of the sugar crystals, so that the amount of liquid required is again small. For the best results, this final washing is started when the maximum purg'able'amount of syrup and smear has been spun off, which, depending upon the massecuite andthe rate of acceleration, usually is near the end of the acceleration period when the charge is'subject to a high'centrifugal force. At that time the wash liquidand resulting wash syrup flash quickly through the sugar wall with minimum damage to the finest and most delicate crystals.

This method should be practiced with centrifugals which at fullspeed impart centrifugal forces in excess of 800 times the weight of the charge, the top speedin any case being selected according to the type of massecuite or magma under treatment; and the temperature of the charges loaded into the 'centrifugals should be kept as nearly uniform as practicable by the use of suitable massecuite heat control means. The present method, however, overcomes the main problem which gave rise to the use of rapid centrifugal acceleration on white massecuites and the like, and makes it practical to use longer accelerations without sacrificing purging or washing results, at a great reduction in power requirements and other objectionable cost factors. In fact, although a plurality of washing operations is used for each charge, the total quantity of wash liquid required for proper results is very substantially reduced, with attendant reductions of grain melt, and the time involved for washing and drying the charge after acceleration also is reduced. Consequently, the acceleration stage may be prolonged, to reduce power requirements and other operating costs without lengthening the total cycle time or reducing capacity, and at the same time substantial improvements in purging and washing results may be obtained. A further important advantage is that the wash syrup produced from the final washing operation has an extraordinarily high purity, and all of it may be collected separately from the other centrifugal syrups to achieve a decided economy in sugar plant operations,

The wash liquid for the several washing operations may be any suitable aqueous liquid, such as water or a syrup or substantially saturated sugar solution, but the liquid should be applied to the sugar wall at a temperature above the pan temperature of the massecuite suificient to keep it loading speed, and when the charge has been walled up and started accelerating and the centrifugal curb cover has been closed (a cover should be used for best results), superheated water under pressure is released through the centrifugal spray nozzle for a few seconds, say 3 to 8 seconds, to complete a first washing operation before the centrifugal attains half its full centrifugal force. After a further substantial time interval during which the centrifugal continues accelerating and purges almost all of the syrup and smear from the charge, say 15 to seconds, depending on the acceleration rate and the type of massecuite, superheated water is again released through the spray nozzle until the desired degree of cleaning of the sugar wall has been accomplished. When a wash syrup separation is made, the syrup separator may be actuated at, or within a very few seconds after, the time of starting the final washing, depending upon the massecuite, and if done correctly a wash syrup of at least the massecuite purity will result. The drying period of the cycle follows the final washing operation in the usual manner, but it requires less time than heretofore.

According to another embodiment, the charge is loaded and started on acceleration, washed for a few seconds shortly after the loading, allowed to continue accelerating for several seconds without washing, then washed during another period of a few seconds, then allowed to continue accelerating without washing to substantially the full speed, and finally washed again for a few seconds to finish the cleaning of the sugar wall.

In still another practice, which may be preferable for low purity massecuites, the charge is loaded, started on acceleration and then washed with a hot, substantially saturated syrup for one or more suitable periods to be completed during the acceleration. After a further interval, when the green syrup purgable by centrifugal force has been spun off, a final washing is performed with a hot watery medium to finish the cleaning of the sugar wall. The duration of this interval depends upon the purging qualities of the massecuite, and in the case of crystallizer massecuites it may be a matter of minutes instead of seconds. 4 For efiicient and uniform operations according to this invention it is necessary to use automatic time controls for the centrifugals, such as comnism to begin and terminate the several washing operations at the desired predetermined intervals in each treating cycle. Various well-known types of electrical or mechanical timing devices may be used conveniently for this purpose, in various ways which will be apparent to those skilled in the art. Where superheated water or a mixture of steam and water is to be employed for washing a single supply system, such as heretofore provided for such work, may be used for all washing operations. Where one or more syrup washing operations are to be performed and to be followed by a hot water or steam-water washing, any suitable dual wash liquid supply system may be provided and operated under the control of the centrifugal timing mechanism. In all embodiments it is desirable to use movable covers on the centrifugal curbs, to reduce the cooling and drying effects of air currents during the accelerations and high speed operations of the machines.

Eacample I The following data illustrate treatments of the same white beet sugar massecuite, of extra fine and somewhat irregular grain size, in part (A) with multiple water washings according to the present invention and in part, for comparison, (B) according to the method regularly in use where the tests were made.

Massecuite: Brix, 91.3; purity, 92.7.

Centrifugals: 1550 R. P. M., Roberts belt-driven, with automatic timing controls. Acceleration period, to 1550 R. P. M., about 58".

Charge: 13 /2" deep, loaded dead=9.4 cu. ft., or 870#, containing (by calculation) 41'I# of sugar crystals.

Wash liquid: Water at 125 0., under 55# pressure.

M (A? 1 (B) Cycle components lump 6 Regular washing m th d method 6 0 Clutch in to start 1st Washing 19 8 Av. R. P. M. at start 1st washlng 550 l, cog Av. duration lst washing 3: 18 Av. interval 1st to 2nd washing Z Av. duration 2nd W&ShlI1g 10; Av. drying time 2.;

Av. total time to brake on 1'20 1'46" Average results Lbs. sugar from basket 375 330 Lab. color grade of sugar samples... 14.2 l5. 0 Lbs. sugar per cu. ft. of massecuitc 39. 4 34 6 Quarts Wash liquid used. a 11.7 l( Calculated loss of crystals ll] syrups. 42 81 These data show that the new practice gave a 52% reduction in the loss of crystallized sugar in syrups during centrifuging, increasing the crystal yield from about 79% to about 90%. Furthermore, the cycle time was shortened by about 26", which means a great increase in the producing capacity of the centrifugals.

Example II The following data illustrate differences in centrifugal syrup separations when using practices such as set forth in Example I. Here a number of pans of white beet sugar massecuite were treated successively according to each of the two practices (A) and (B). The massecuites treated therefore were not identical, but the average results shown nevertheless are significant in several respects:

Multiple R 5:2 washing method method No. of pans treated 1 26 Cu. ft. of white massccuite. 17,767 24, (S48 Average data Massecuitc:

Refractomcter brix Apparent purity Quarts of wash liquid per cyclcm. Lbs. of same Green syrup: Apparent purity Wash syrup: Apparent purity.

1 Start of 2nd washing. 9 5 after start of washing.

According to method (A), an average of about 3 quarts of wash liquid was used for the first washing operation. The average green syrup (mother liquor) purity, however, was not raised substantially by this washing (not more than ,/2 point in any observation), showing that the first washing served as an aid to purging without appreciably dissolving sugar.

By actuating the syrup separators in method (A) at the time of starting the second washing operation, a wash syrup was recovered having an apparent purity of 94.2-more than 1 point above the massecuite purity. In method (B), on the other hand, syrup separations made 5 seconds after the start of washing yielded wash syrup of only 92.4 apparent purity, which could not be recirculated to white massecuite boilings without lowering the massecuite purity.

The loss of crystals during centrifuging according to method (A) was over 40% less than in method (B), and the direct recovery of sugar from white massecuites was about 8% greater in (A) than in (B).

Example III Comparative tests were made, as follows, on refinery white massecuites, using temperaturecontrol equipment in the mixer, a superheatedwater Washing system, and a 48" Roberts geardriven centrifugal equipped with a ring valve syrup separator and automatic timing controls. The acceleration, from dead stop to a top speed of 1320 R. P. M., occupied '70 seconds.

1. Different charges of the same massecuite (a grade B white massecuite)l6.'l6 cu. ft. in each chargewere purged (A) by applying the washing medium for a period of 5 (6 /2 qts.) beginning 20" after the centrifugal clutch was thrown in to start acceleration, and (B) without any washing. The syrup purged from each charge was collected and weighed as it flowed from the centrifugal, then tested, with the fol,- lowing results:

Thus, by washing in the early stage of acceleration with about 13 lbs. of hot water, the green syrup collected in case (A) totalled 112 pounds more than in case (B) Where there was no washing, yet the purity of the green syrup was not appreciably increased.

2. Different charges of 3rd inboil white massecuite were treated (A) with multiple superheated-water washings and (B) according to the regular method in use where the tests were made, and the green and wash syrups produced were collected separately, weighed and tested:

Charges: 16.76 cu. ft. (1559 pounds).

Massecuite: Purity 94.3, Ref. Brix 91.9.

Cycle components (A) (B) Clutch in to "start 1st wash" seconds.. 20 45 R. P. M. at start 1st wash" 880 820 Duration lst wash seconds 17 Interval 1st to 2nd wash (10.... 50 R. P. M. at start 2nd wash 1, 320 Trip syrup separator Duration second wash seconds. 6 Drying time .do 15 15 Results Green syrup:-

Pounds 682 502 Apparent purity 89. 8 89. 2 Wash syrup:

Pounds 104 349 Apparent purity 94. 4 93.1 Washed sugar:

Lbs. per cycle 773 724 Lbs. per cu. ft. masse 46.1 43. 2

Per cent centrii. yield on total suga n masse 57. 2 53.6

Per cent moisture 2. 7 2. 6

Color (by light transmission)... 84 80. 5 Quarts wash water 14. 3 22.'l Lbs. masse. per qt. water 109 70. 6 Lbs. sugar per qt. water 54. 5 32.8

1 Start of 2nd wash. 1 7 after start of washing.

Method (A) yielded 49 pounds more of washed sugar per cycle than (B), from the same Volume of massecuite, yet the total amount of wash water was about less. The green syrup volume was greatly increased, without substantially increasing its purity, and the wash syrup purity was as high as the massecuite purity. Method (B), however, represents very good prior practice on the type of'massecuite involved.

Some of the more important features of the methods provided by this invention may be summarized as follows:

Upon applying a suitable amount of superheated water or other suitable hot wash liquid to the walled-up charge as soon as the syrup leaves the face of the sugar wall, while the force of the centrifugal is less than of its maximum force, this shot of liquid mobilizes and assist the purging of almost all of the syrup and smear from the crystals during continued acceleration, such that a sugar wall having excellent filtration and drying qualities is obtained at the full centrifugal speed.

The total quantity of wash liquid required for satisfactory purging and washing results is greatly reduced, with a corresponding reduction in the grain melt during centrifuging,

The eiiiciency of wash syrup separations is in While we have described preferred embodiments of'our invention in considerable detail and have set forth data to illustrate its practical use, it is to be understood that the invention may be utilizedin various ways without limitation to the details herein set forth and therefore should be ac-' corded a scope commensurate with its contribu-' tions to the art, which are intended to be defined by the appended claims.

We claim: l. The method of centrifuging mixtures of sugar crystals and syrup which comprises load-. ing a charge of the mixture into a sugar-cen-' trifugal and starting the acceleration thereof,

starting a washing of the charge with a, spray of hot aqueous liquid before the centrifugal attains half its full force and terminating said washing during the acceleration after applying,

a quantity of such liquid sufiicient to mobilize and assist the purging of the syrup and any falsemixtures of sugar crystals and syrup which com-' prises loading a charge of the mixture into a centrifugal, accelerating the centrifugal, and washing the charge with aqueous liquid in a plurality of distinct washing operations each separated from the other by a substantial time interval, the first washing operation being started shortly after the loading and continuing for only a few seconds and the last washing operation being completed after the purgable syrup has been spun from the sugar, at the full centrifugal speed.

4. The method of centrifuging and washing mixtures of sugar crystals and syrup which comprises loading a charge of the mixture into a centrifugal, accelerating the centrifugal, applying a portion of hot washing liquid to the charge shortly after the loading, waiting an interval, then applying a second portion of hot washing liquid before the completion of acceleration, waiting another interval andthen applying a third portion of hot washing liquid at the full centrifugal speed, to finish the washing.

5. The method of centrifuging and washing mixtures of sugar crystals and syrup which comprises loading a charge of the mixture intoa centrifugal and accelerating the centrifugal, applying a portion of hot washing liquid to the charge shortly after the loading to mobilize and assist the purging of syrup and false grain, waiting an interval, and applying another portion of hot washing liquid to the charge to finish the washing, at least part of said other portion being applied at the full centrifugal speed.

6. 'I'hemethod of centrifuging and washing mixtures of sugar crystals and syrup which comprises loading a charge of the mixture into a centrifugal, accelerating the centrifugal, washing the charge with liquid during a limited period shortly after the loading to enhance the purging of syrup and any false grain in the charge, and further washing the charge with liquid during at least one later limited period at a substantially higher centrifugal speed, the washing liquid being applied in each instance at a temperature at least as high as the temperature of the charge when loaded.

7. The method of centrifuging and washing mixtures of sugar crystals and syrup which comprises walling up a charge of the mixture in a sugar centrifugal and starting the centrifugal acceleration, then diluting and purging free syrup of the charge by subjecting the charge to a preliminary washin with heated aqueous liquid during an early period of the acceleration, and during at least one later period of the centrifuging, at a substantially higher centrifugal speed, washing the charge with heated water to finish the Washing.

8. The method of centrifuging and washing mixtures of sugar crystals and syrup which comprises walling up a charge of the mixture in an accelerating centrifugal, during acceleration diluting and purging syrup of the charge with a preliminary wash of hot substantially saturated sugar solution, and later applying a finishing wash of heated water with the centrifugal spinning at substantially full speed.

9. The method of centrifuging and washing mixtures of sugar crystals and syrup which comprises walling up a charge of the mixture in an accelerating centrifugal, for a limited period during the acceleration, beginning shortly after the loading, directing from a spray nozzle to the charge a mist-like spray of hot water and steam, and directing another such spray to the charge during a later limited period while the centrifugal is spinning at substantially full speed.

10. The method of centrifuging and washing mixtures of sugar crystals and syrup which cornprises walling up a charge of the mixture in a centrifugal and accelerating the centrifugal to a high speed sufficient to subject the charge to centrifugal force in excess of 800 times its weight, applying a portion of hot aqueous wash liquid to the walled-up charge before the centrifugal attains half its full force, waiting an interval, and applying another portion of hot aqueous Wash liquid to the charge to complete the washing when substantially the full centrifugal force has been attained.

11. The method of centrifuging and washing mixtures of'sugar crystals and syrup which comprises walling up a charge of the mixture in a centrifugal and accelerating the centrifugal to a high speed sufiicient to subject the charge to centrifugal force in excess of 800 times its weight, applying a highly atomized spray of hot water and steam to the walled-up charge for a few seconds before the centrifugal attains half its full force, waiting an interval of several seconds, then applying another highly atomized spray of hot water and steam for a few seconds, waiting another interval of several seconds, and then applying a third highly atomized spray of hot water and steam at the full centrifugal speed to finish the cleaning of the sugar wall.

12. The method of centrifugally separating mixtures of sugar crystals and syrup which comprises loading a charge of the mixture into a centrifugal and starting the acceleration of the charge in the centrifugal, releasing superheated water through a spray nozzle to the charge during a limited period of the acceleration beginning shortly after the loading, thereafter continuing the acceleration for a substantial period, and again releasing superheated water through the nozzle to the charge during a later limited period, while the charge is subject to centrifugal force exceeding 800 times the force of gravity, to effectuate the desired final purification of the sugar.

13. The method of centrifuging each of many successive centrifugal charges from a mass of sugar crystals and syrup which comprises walling up and starting the acceleration of the charge in a revolving sugar centrifugal basket, shortly thereafter releasing superheated water through a spray nozzle to the accelerating charge for a. predetermined period of about 3 to 8 seconds, then continuing the acceleration for a predetermined period of about 15 to seconds to finish purging the sugar, and then again releasing superheated water through the nozzle to the charge for a predetermined period to achieve the desired final degree of purification of the sugar wall.

EUGENE ROBERTS. GEORGE E. STEVENS.