US3207627A

US3207627A - Centrifuge for the continuous recovery of sugar crystals

Info

Publication number: US3207627A
Application number: US239741A
Authority: US
Inventors: Dietzel Walter; Hillebrand Heinrich
Original assignee: BMA Braunschweigische Maschinenbauanstalt AG
Current assignee: BMA Braunschweigische Maschinenbauanstalt AG
Priority date: 1961-11-24
Filing date: 1962-11-23
Publication date: 1965-09-21
Anticipated expiration: 1982-09-21
Also published as: DE1181135B; GB971234A

Description

Sept. 21, 1965 w. DIETZEL ETAL 3,207,627

CENTRIFUGE FOR THE CONTINUOUS RECOVERY OF SUGAR CRYSTALS Filed Nov. 23, 1962 s Sheets-Sheet 1 Fig.1

INVENTORSZ Heinrich HilLebrand BY Walter Dietzel im kmi JV AGENT Sept. 21, 1965 W. DIETZEL ETAL CENTRIFUGE FOR THE CONTINUOUS RECOVERY OF SUGAR CRYSTALS Filed Nov. 25, 1962 5 Sheets-Sheet 2 INVENTORS;

Heinrich Hillebrand BY Walter DietzeL AGENT Sept. 21, 1965 w. DIETZEL ETAL GENTRIFUGE FOR THE CONTINUOUS RECOVERY OF SUGAR CRYSTALS Filed NOV. 23, 1962 3 Sheets-Sheet 3 INVENTORS: Heinrich HiLLebrand BY Walter Dieczel United States Patent 4 Claims. (Cl. 127-19 Our present invent-ion relates to continuously operable centrifuges and, more particularly, to centrifugal devices for the separation of solid material from liquids.

It is common in the process industries to employ a continuously operating centrifuge for the recovery of a solid centrifugate from a mixture thereof with a mother liquor in which the solid is soluble. Centrifuges of this type are conventionally employed in sugar-refining processes for the extraction of sugar crystals from a mother liquor. In this process the molasses is cooled to precipitate crystals of sugar, the resulting slurry or mixture being then fed continuously into the rotating basket of a high-speed centrifuge which centrifugally forces the liquid through the perforations in the basket into the surrounding molasses or liquor compartment while the crystals ascend the basket wall and pass into the sugar compartment. To this end the basket wall is usually of conical configuration with one or more centrifuge stages. The production efficiency of these centrifuges has generally been controlled by regulating the rate of flow of the mixture into the centrifuge basket. This arrangement has, however, the disadvantage that the centrifuge does not always operate under the most eifcient conditions since the mother liquor or syrup is highly viscous. It has been proposed, heretofore, to heat the basket at axially spaced locations to accelerate the passage of the syrup through the perforated wall. This method has also been found to be unsuitable, for the most part, since the zone method of heating results in the establishment of intervening unheated or cool regions which decrease the rate of fiow through the centrifuge.

Moreover, it should be noted that most earlier constructions made use of an open centrifuge housing whereby free air circulation was permitted into the basket as well as between the latter and the compartments. This also resulted in the formation of cool zones and/or the nonuniform heating of the basket and its layer of crystals.

It is, therefore, an object of the present invention to provide an improved method of operating a centrifuge for the separation of a solid from its mother liquor wherein the abovementioned disadvantages are obviated and an optimum separation rate can be achieved.

A more specific object of the invention is to provide an improved method of operating continuous sugar centrifuges which is capable of effecting a considerable increase in the operating efficiency thereof.

Another object of the invention is to provide a sugar centrifuge operable at a relatively high rate without the disadvantages inherent in earlier constructions.

These objects are attained, in accordance with the invention, by maintaining the basket of a centrifuge, which is rotatable about a substantially vertical axis and diverges conically upwardly to communicate with an outer compartment for collecting a solid centrifugate and is surrounded by an inner compartment for collecting the mother liquor in which the solid is increasingly soluble with increasing temperature, at a temperature below that at which substantial amounts of centrifugate dissolve in the mother liquor but sufficiently high to effect a relatively rapid passage of the mother liquor through the basket Patented Sept. 21, 1965 and the solid layer deposited thereon. It should be noted, in this connection, that the solid centrifugate may be in solution equilibrium with the mother liquor upon its admission to the basket, in which case the viscosity of the mother liquor will be well defined and characteristic of its inlet temperature. The basket of the centrifuge and advantageously, the layer deposited thereon is, according to the invention, raised to a temperature in excess of that of the admitted mixture so that further amounts of the centrifugate are capable of dissolving in the latter. Since, however, the viscosity of the liquor increases with temperature, the latter is so chosen that the liquor is capable of passing through the perforations in the drum or basket of the centrifuge at a much higher rate so that it is in contact with the centrifugate layer thereon for a short time and is thus not able to redissolve substantial amounts of the layer. Inasmuch as dissolution of the solid matter is a rate process and, therefore, time dependent, the reduced contact duration prevents any substantial dissolution of the solid deposited upon the centrifuge basket.

It is essential, for the purposes of the present invention, that the centrifugate should not pass through any cool zones as it travels axially along the wall of the centrifuge basket towards its upper end at which it is transferred to the centrifugate compartment so that undercooling is avoided. In conventional centrifuges the centrifugate (e.g. the mass of sugar crystals) encounters decreasing temperatures as it climbs the basket wall since it is introduced into the basket initially in a warm state and the circulation of air in the region of the upper end of the basket results in a cooling thereof at this end. It is, therefore, another feature of the invention to provide means for heating the basket at its upper end or, at least, to prevent its cooling in this region. We have found that, for optimum operation of a sugar centrifuge, best results are obtained when the basket is maintained, throughout its entire axial height, at a temperature between substantially 52 C. and C. and preferably between 65 C. and 75 C. The considerable amount of heat generated by frictional engagement between the rapidly rotating basket and the surrounding air has also been found to be sufficient to maintain the temperature of the basket within the proper range, especially when the mixture of sugar crystals and syrup is fed into the basket at a temperature just below the lower limit of this range, if bafile means are provided for limiting the circulation of air in the vicinity of the upper end of the basket and between the compartments. Thus We prefer to introduce the sugar slurry at a temperature of the order of 50 C. while maintaining the temperature in the molasses compartment between approximately 60 C. and C.

The limitation of the air flow sufiices, for the most part, to prevent the formation of cool zones axially along the basket. Since the frictional heat may be insufiicient to maintain the basket at the proper temperature when the slurry or mixture is introduced at a temperature substantially below that of the optimum range, We have found it advisable, in such cases, to provide additional thermal control means in the form of heating means capable of raising the basket to the optimum temperature without, however, formation of cool zones. It is necessary, under these circumstances, to avoid most prior-art methods which entail the heating of an open basket at axially spaced locations whereby cool zones can be formed intermediate the heated regions. We prefer to provide either dielectric heating, wherein an electrode is juxtaposed with the layer over substantially the entire height of the basket, or means employing a heating fluid. The latter may be admitted to the molasses or inner compartment and directed against the basket at its lower end so that eifective distribution of the heat upwardly is ensured. It will be noted, at this point, that the layer of sugar crystals traveling upwardly along the basket wall is relatively thin (e.g. having a thickness of only several millimeters) and is exceptionally temperature sensitive. Thus, it is relatively easy to heat this layer but also relatively easy to cool it. Chilling must be avoided, as previously mentioned, to prevent undercooling of the mother. liquor and consequent retention of solute. The importance of checking the circulation of air between the compartments and the interior of the basket will, therefore, be self-evident.

When heating means such as the dielectric or heatingfluid methods are employed, we may provide the centrifuge with temperature-responsive meanssensitive to the temperature of the basket and the layer for adjusting the heating means. Thus, when the heating means includes a stream of steam or hot air, the temperature-responsive means can include a flow-control mechanism for regulating the relative amounts of heating fluid and a cooling diluent (e.g. cold air) to be passed through the. fluid conduit. It will be noted that the temperature of the basket will be dependent upon the temperature within the molasses compartment so that the sensing means may be disposed within the latter.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is an axial cross-sectional view through a sugar centrifuge embodying the invention; and I FIGS. 2 and 3 are similar cross-sectional views of modified centrifuges.

The centrifuge shown in FIG. 1 comprises a generally cylindrical housing 1 to whose upper flange 1' a top plate 40 is bolted. The latter is provided with a pair of apertures 41 and 42 which releasably receive

covers

43 and 44 and afford access to the outer annular compartment within which the sugar crystals are collected. A partition 3 within the housing 1 forms, with the perforated basket 2 of the centrifuge, an inner annular compartment 4 wherein themolasses or mother liquor collects. An outlet 45 at the base of this compartment permits removal of the mother liquor 46. Housing 1 is mounted upon a base 9 whose vanes 47 extend radially upwardly in the direction of the axis of basket2 and carry a support plate 48 via I-beams 49. Support plate 48 is welded to a sleeve 50 provided with the usual bearings for a shaft 51 upon which the, basket 2 is swingably mounted but keyed for entrainment therewith. Shaft 51 is provided with a head 6 (e.g. of the ball type) to permit the basket 2 to swing about axes perpendicular to that of shaft 51, thereby preventing undue stresses from being applied to bearing sleeve 50. The latter is, however, provided with reinforcing ribs 52 for greater rigidity.

Themolasses compartment 4 is also provided withanother annular wall 53 which serves to deflect the liquor thrown outwardly by the basket in the downward direction. In addition, this wall 53 also limits circulation of air within the molasses compartment and .thus serves to retain heat within the region of the basket 2. The latter is provided with a hub 54 in the form of a sleeve whose upper end is articulated on the head 6 and whose lower end carries the perforated wall of the basket. The head 6 thus may be disposed substantially at the center of gravity of the basket for smooth rotation thereof.

The basket 2 comprises a perforated wall whose lower portion 55 is generally cylindrical and constitutes, with sleeve 54 and an annular flange 56, a well 57 for the slurry. The bottom portion 55 merges into a conically divergent wall portion 58 integral therewith and rigid with an outwardly extending flange 59 which also serves to join an outer perforated cage 60 to basket 2. Cage 60 reinforces the lower end of the basket and ensures an effective separation of the crystals from the mother liquor. The main wall portion 61 of basket 2 is provided, at its lower end, with a flange 62 bolted to flange 59 and conically diverges upwardly coaxially with respect to shaft 51. This main wall portion is provided with angularly equispaced reinforcing ribs 63, which extend along the outer surface thereof parallel to its generatrices, and axial: 1y spaced annular reinforcing rings 64 capable of withstanding the substantial centrifugal forces applied to the drum.

At the upper end of the main wall portion 61 of basket 2 we provide a heavier ring 65 which is bolted to an annular brace 66 whose ledge 67 is overlain by'and rigid with a guide plate 68 rotatable together with the basket. This guide plate extends radially beyond the molasses compartment 4 and co-operates with an annular baffle 69 to deposit sugar crystals in collecting compartment 5. Baffle 69 on partition 3 also prevents the flow of air between the molasses compartment 4 and the crystal-collecting compartment 5. A'further annular baflle 70 limits air circulation at the upper end of the basket 2.

The upper plate 40-of housing 1 carries a removable circular cover 12 which is seated upon an inner flange 71 of the plate and is provided with an inlet funnel 11 for directing a slurry or mixture of sugar crystals with mother liquor into the interior of the basket. This inlet 11 is aligned withan outlet nozzle 10 of a slurry conduit (which may be heated or insulated in the usual manner, to prevent blockage), whose valve 72 controls the rate of flow of the slurry into. the centrifuge, and with a further funnel 73 whose extension 74 guides the slurry within the interior 16 of basket 2 into its well 57. A cylindrical extension 15 at funnel 11 forms a baffle for preventing or checking the flow of air through this inlet. An axially extending annular baffle 14 depends from plate 40 and is in close-spaced relationship with the upper end of the conical wall portion 61 of basket 2 to reduce or prevent the flow of air between the interior 16 thereof and the compartments 4 and 5 while, nevertheless, permitting the thin layer of sugar crystals (e.g. with a thickness on the order of several millimeters) to pass into compartment 5. A bottom plate 13 forms with cover 12, baffle 14, guide plate 66 and partition 3 an enclosure for the basket 2 preventing exposure thereof to cooling effects.

The base or chassis 9 carries a motor 8 whose drive shaft 75 is provided with a V-belt pulley 76 which is connected by belts 7 to the driven pulley 77 of shaft 51.

A pump 78 maintains a continuous flow of lubricant from an oil pan 79 through the bearing sleeve 50 as indicated in dot-dash lines. A conduit 80 extends through plate 40 into the interior 16 of the basket to supply a heating or flushing fluid (e.g. air, steam or hot water), a washing agent .or an additive, as required. As a consequence of the provision of the aforementioned bafiles, the frictional heat generated upon rotation of the basket 2 is suflicient to maintain a temperature of the crystal layer S between substantially 52 C. and 80 C. and, preferably between 65 C, and 75 C., without the formation of any cool zones axially along the basket. Thus, the slurry 8 1, which is introduced at a temperature of, say, 50 C., is fed via funnels 11 and 73- into the well 57 whence the liquid is forced centri-fugally through the apertures in the basket while the solid material .(i.e. sugar crystals) climbs upwardlyv along the conical portions 58 and 61 of the basket t-o'pass over plate 68 into the sugar chamber. The molasses 46, however, collects within the compartment 4 from which it may be removed via outlet 45. The sugar crystals may be removed through openings 41. and 42 by suction.

In FIG. 2 .we show a similar arrangement wherein, however, an external source of thermal energy is employed .as an additional heating means. -In this case a conduit 17 directs a stream of heating fluid into the molasses compartment 4 and, advantageously, against the lower end of the basket 2 so that, upon rotation of the latter, the heat is distributed upwardly substantially uniformly. This conduit '17 is provided with a valve 22 for regulating the how of heating fluid into the molasses chamber, the latter being maintained at a temperature between substantially 60 C. and 110 C., preferably at the upper end of this range. An electrically controlled mixing valve 21 has one input connected to a steam or hot-air line 22a and another input fed with cold air from a line @211. [Lines 2 2a and 12% are also provide-d with manually adjustable control valves 82 and 83 for setting the proportions of hot and cold fluid to be passed through conduit :17. In cases where there is little thermal variation in the region of the basket 2, this proportion may be set permanently. Generally, however, a certain degree of temperature variation is unavoidable since the temperature of the incoming slurry may change from time to time. It is, consequently, advisable to provide means for compensatingly adjusting the heat supply to the basket in response to such thermal variations. To this end we provide a temperature-responsive sensing means $19 which is juxtaposed with the basket and, advantageously, may extend into the region between the latter and the partition :53 within molasses compartment 4. The electrical output of sensor '19 serves for the continuous adjustment of mixing valve 21 in order to increase the supply of heat upon a decrease in the temperature in the region of the basket and vice versa. It should be noted, however, that the sensor may also be provided in the interior 16 of the basket while it or another sensor may be disposed at the inlet 11 to detect the temperature of the incoming slurry.

FIG. 3 shows another method of heating the basket 2 and the layer S of crystals deposited thereon. In this case a dielectric heating electrode 18 is juxtaposed with the layer S and connected to a high-frequency alternatingcurrent source 84 whose other terminal is connected to the housing "1 which in turn is conduotively coupled with the metal basket 2, the latter thus forming a counterelectrode. Electrode '18 extends parallel to a generatrix of the basket which rotates at such high speeds that the heat produced between the electrodes as a consequence of the dielectric character of the sugar-crystal layer is substantially uniform throughout. Again, the intensity of heating may be controlled by a sensor 20 which is connected with the source 84 for adjusting the frequency and/or intensity of its output. 'In this system the temperature in the molasses compartment 4 may be maintained constant to effect a similar maintenance of the temperature of the basket and the layer 8.

The invention as described and illustrated is believed to admit of many modifications and variations within the ability of persons skilled in the art, all such modi fications and variations being deemed to be included in the spirit and scope of the appended claims.

We claim:

#1. A centrifuge for the continuous recovery of sugar crystals from a mixture thereof with a mother liquor, comprising a substantially closed housing having a substantially continuous cover; a centrifuge basket journaled in said housing for rotation about a substantially vertical axis, said basket being spacedly overlain by said cover and having a perforate wall diverging conically upwardly from a lower end to an upper end thereof; partition means in said housing forming with said perforate wall a first annular compartment surrounding said basket for collecting mother liquor passing therethrough and forming with said housing a second annular compartment outwardly of said first annular compartment communicating with the interior of said basket .at said upper end thereof for collecting said crystals; thermal control means in said housing for maintaining the temperature of the inner surface of said basket and a layer of crystals there on sufiiciently high to permit a rapid passage of said mother liquor through said basket; drive means for rotating said basket about a vertical axis; and baffle means within said housing for limiting the circulation of air therein between the interior of said basket and said com partment, said partition means and said housing means forming an enclosure for said basket substantially preventing the escape of ambient air in the region of said basket and the admission to this region of cooling currents of air from the exterior of the centrifuge, said baflle means including a ring depending from said cover and extending coaxially in closely spaced relationship with said upper end from above into the open upper end of said basket with small clearance from the inner surface thereof for permitting the flow of said crystals along said inner surface past said ring into said second compartment while restricting flow of air between said second compartment and the interior of said basket.

2. A centrifuge as defined in claim 1 wherein said thermal control means comprises heating means in said basket, and sensing means in said housing responsive to the temperature of said layer for controlling said heating means to maintain said temperature substantially constan-t.

3. A centrifuge as defined in claim 2 wherein said heating means comprises conduit means for introducing a heating fluid into the interior of said basket at a location intermediate said lower and upper ends thereof.

4. A centrifuge as defined in claim 2 wherein said heating means includes a dielectric heater juxtaposed with said layer.

References Cited by the Examiner UNITED STATES PATENTS 1,986,245 1/35 Berge 127-20 2,179,436 I l/39 Smith 127-56 2,688,405 9/54 Sharples 21-0''3 X 2,973,288 Q/6 1 Riedel 1-27-56 X FOREIGN PATENTS 1,221,231 1/-60 France.

OTHER REFERENCES Spencer-Meade: Cane Sugar Handbook, eighth edition, 1945, 'John Wiley and Sons, New York, pp. 233-236, 293, 294 and 353 relied on.

MORRIS O. WOLK, Primary Examiner.

Claims

1. A CENTRIFUGE FOR THE CONTINUOUS RECOVERY OF SUGAR CRYSTALS FROM A MIXTURE THEREOF WITH A MOTHER LIQUOR, COMPRISING A SUBSTANTIALLY CLOSED HOUSING HAVING A SUBSTANTIALLY CONTINUOUS COVER; A CENTRIFUGE BASKET JOURNALED IN SAID HOUSING FOR ROTATION ABOUT A SUBSTANTIALLY VERTICAL AXIS, SAID BASKET BEING SPACEDLY OVERLAIN BY SAID COVER AND HAVING A PERFORATE WALL DIVERGING CONICALLY UPWARDLY FROM A LOWER END TO AN UPPER END THEREOF; PARTITION MEANS IN SAID HOUSING FORMING WITH SAID PERFORATE WALL A FIRST ANNULAR COMPARTMENT SUURROUNDING SAID BASKET FOR COLLECTING MOTHER LIQUOR PASSING THERETHROUGH AND FORMING WITH SAID HOUSING A SECOND ANNULAR COMPARTMENT OUTWARDLY OF SAID FIRST ANNULAR COMPARTMENT COMMUNICATING WITH THE INTERIOR OF SAID BASKET AT SAID UPPER END THEREOF FOR COLLECTING SAID CRYSTALS; THERMAL CONTROL MEANS IN SAID HOUSING FOR MAINTAINING THE TEMPERATURE OF THE INNER SURFACE OF SAID BASKET AND A LAYER OF CRYSTALS THEREON SUFFICIENTLY HIGH TO PERMIT A RAPID PASSAGE OF SAID MOTHER LIQUOR THROUGH SAID BASKET; DRIVE MEANS FOR ROTATING SAID BASKET ABOUT A VERTICAL AXIS; AND BAFFLE MEANS WITHIN SAID HOUSING FOR LIMITING THE CIRCULATION OF AIR THEREIN BETWEEN THE INTERIOR OF SAID BASKET AND SAID COMPARTMENT, SAID PARTITION MEANS AND SAID HOUSING MEANS FORMING AN ENCLOSURE FOR SAID BASKET SUBSTANTIALLY PREVENTING THE ESCAPE OF AMBIENT AIR IN THE REGION OF SAID BASKET AND THE ADMISSION TO THIS REGION OF COOLING CURRENTS OF AIR FROM THE EXTERIOR OF THE CENTRIFUGE, SAID BAFFLE MEANS INCLUDING A RING DEPENDING FROM SAID COVER AND EXTENDING COAXIALLY IN CLOSELY SPACED RELATIONSHIP WITH SAID UPPER END FROM ABOVE INTO THE OPEN UPPER END OF SAID BASKET WITH SMALL CLEARANCE FROM THE INNER SURFACE THEREOF FOR PERMITTING THE FLOW OF SAID CRYSTALS ALONG SAID INNER SURFACE PAST SAID RING INTO SAID SECOND COMPARTMENT WHILE RESTRICTING FLOW OF AIR BETWEEN SAID SECOND COMPARTMENT AND THE INTERIOR OF SAID BASKET.