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INDUSTRIAL AND ENGINEERING CHEMISTRY
this time is recovered by running about 75 per cent of sugar liquor in the clarifier bodies to the clear liquor tanks. This procedure requires that the melt house start about 3 t o 4 hours ahead of its present schedule and finish the same amount of time ahead of the present shutdown schedule. There is danger of excess inversion unless proper precautions are taken. The clarifiers are operated a t an output temperature of 200” to 210” F. At this temperature sugar liquor will invert rapidly unless properly controlled. Experience has shown that even after long periods we get no inversion of this material if we keep the pH of the filtrate above 7.2. However, there is definite indication of inversion if we allow the p H to drop below this point. After heating for about 30 minutes the startup liquor may show slight inversion and a tendency to go over to the acid range. Material has been held in the clarifiers over the weekend without any appreciable inversion by carefully controlling the pH of the material. There seems to be less danger of inversion from the clarifier scums than is normally the case with light juices from a filter station. Cooling and straining equipment may be needed before char. With the entire melt going through clarifiers, i t may be necessary to have a cooling system t o reduce the temperature from 210’ to about 170” F. before the liquor goes t o the char. Tests now being conducted on a new type of sand filter indi-
Vol. 34, No. 4
cate that this mechanism may eliminate the need of a separate cooler by simultaneously straining and cooling the liquor before char filtration. Continuous clarifying equipment is more sensitive to changes in melt.
Conclusions Whether the advantages will offset the disadvantages in the operation of continuous clarifiers and whether the necessary changes in refinery equipment and procedure will justify this method of defecation is an individual problem in each plant which can be determined only by individual study and conditions.
Literature Cited (1) Jacobs, H. J., Dahlberg, C. F., and Munson, J. J., U. S.Patent 2,196,991(1940). (2) Lyle, Oliver, “Technology for Sugar Refinery Workers”, p. 306, London, Chapman and Hall, 1941. (3) Revere Sugar Refinery, Reports 5032 (1938)and 5450,5470,5513 (1939). (4) Ihid., 5774,5783 (1940)and 6171 (1941). (5)Zhid., 6195 (1941). (6) Spencer, G. L., and Meade, G. P., Handhobk for Cane-Sugar Manufacturers and Their Chemists, 7th ed., p. 152, New York, John Wiley & Sons, 1930. (7)Ihid., p. 159. (8) Williamson, George, U. 5. Patent 1,317,607(1919).
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Filtration of PhosphatelDefecated Affination Sirup H.I. KNOWLES Atlantic Sugar Refineries, Ltd., Saint John, New Brunswick, Canada
A
FFINATION sirup is a complex mixture containing most of the impurities of the original raw cane sugar. It contains in solution both sugars and salts; present in suspension are organic and inorganic matter. One may find in the sirup some of the soil in which the sugar cane was grown, finely divided particles of sugar cane, precipitated salts, waxes, and colloids in various degrees of dispersion. The characteristics of affination sirup change with the different kinds of raw sugar being refined, and these changes are reflected in the filtrability of the sirup. Much of the insoluble matter to be removed by clarification is extremely finely divided and of a character that readily clogs the filter medium. For these reasons it is customary to employ a finely divided filter aid and resort to single filtration under high pressure. On the other hand, one may employ defecation with phosphoric acid and milk of lime. The calcium phosphate precipitated adsorbs much of the colloidal matter and occludes other finely divided solids. This calcium phosphate complex, however, is a gelatinous mass which is easily compressed into an almost impervious film. It is therefore necessary to filter the phosphate-defecated sugar solutions a t low pressure. Formerly this was done with bag filters. The large amount of labor required in bag filtration plus the sugar losses, not to mention the messy operations involved, caused refiners to abandon bag filters and go in for simple pressure filtration. With the bag filters went
also the defecation with phosphoric acid and milk of lime. Since phosphate defecation has a considerable refining effect, the process is worth retaining, provided the objectionable features of bag filtration can be avoided. This has been accomplished by filtering the phosphate-defecated affination sirup a t low pressure using Vallez filters with paper pulp as the filter medium.
Description of Process The installation is designed for gravity feed only. The pressure at the filter is 5 to 7 pounds per square inch, depending on the level in the feed tank. Since the final thickness of cake on the filter leaves is less than 0.5 inch, it has been found practicable to equip the filters with forty-eight leaves in place of the conventional forty leaves. The nominal filtering area is 864 square feet per filter. The paper pulp is made from a blend of groundwood pulp and newsprint (overissue magazines and newspapers) by means of a paper cutter and a Claflin refiner or pulper. The cuttings from the paper cutter are carried in clarified sirup to the pulper through which the slurry is recirculated until the pulp produced is of the requisite “freeness”. This constitutes the make-up pulp. The remainder is reclaimed pulp obtained by passing used or dirty pulp, with its associated phosphate mud, through a washer which separates the pulp
I N D U S T R 1.A L A N D E N G I N E E R I N G C H E M I S T R Y
April, 1942
Heretofore phosphate-defecated affination sirup was filtered on bag filters. The filtration may be accomplished by means of Vallez filters using inexpensive paper pulp to precoat the filter leaves. The average operating filtration rate is approximately 0.4 gallon per square foot per
I Defecated Sirup
L
f I
423
hour for a cycle period of about 4 hours. The net filtrate output is 88 per cent of sirup filtered. Laboratory and refinery filtration tests indicate that, of the factors affecting the filtration rate, the most influential is the matter adsorbed and occluded by the phosphate precipitate.
+ Preooat Paper Cutter
Sweet Water I
4
lf
Air
Water
Pulp Washers
A. Dirty pulp Pump B. C . Preooat Filt. Sirup PumpPump
1
f
.
-
t
Flow Sheet of Filtration Process fibers from the phosphate mud. About 85 per cent of the pulp is recovered. The first operation carried out during a filtration cycle is precoating. This is accomplished by dispersing 125 pounds of -paper pulp (dry weight basis) in 2000 gallons of clarified sirup and filtering the mixture a t 150 gallons per minute. The precoat is followed by defecated sirup without interrupting the flow; and in due course the precoat left in the filter is displaced by defecated sirup. The first of the precoat that comes through contains a little paper pulp; this is diverted to the precoat preparation tank. Subsequent sirup is clear. Filtration of defecated sirup is continued 3 hours. Then the unfiltered sirup in the filter is forced back to the supply tank by compressed air admitted a t the top of the filter. A few gallons left in the filter are drained and returned to process. The filter cakes are desweetened in place with hot water in the usual manner. The water left in the filter a t the end of this operation contains approximately 30 pounds of sirup solids in solution. Therefore this sweet water is utilized in de-
sweetening the next filter by blowing it into the filter to be desweetened. Water is then admitted and agitated with air to dislodge the cakes and form a slurry in the customary manner. Finally the leaves are sprayed with water to remove the last traces of the pulp slurry. The time required to carry out the several operations of a filtration cycle is aa follows: Hrs.
Min.
The long filtration cycle permits of the operation of six Vallez filters by one man; in addition he prepares the precoat, except the new pulp, and operates the pulp washer or reclaimer.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 4
Operating Results The filtration data in Table I were taken from the records and indicate the results obtained during several years of operation. Any statement of rate of flow should be accompanied by an expression of the clarity of the filtrate. Unfortunately clarity cannot be defined in terms readily understood because of the absence of a standard method for determining clarity. However, it can be said that usually the clarity of the filtrate is equal to that obtained when a phosphate-defecated sirup is filtered through analytical filter paper.
TABLE I. FILTRATION DATA P20~used % on total sirup solids Temp. of hefecated sirup, E". (" C . )
0.2 1180-185 (82-85) 52-55 80-85 7.0-7.5O 1100-1500 6000-8000 1000-1500 500-1000 10-15
Brix of defecated sirup Purit of sirup p~ o%filtrate Gal. filtered per cycle Solids in filtrate, lh./cyole Gal. of sweet water Solids in sweet water, lh. Siruu solids lost in dirty pulp, lb. Average outturn of filter Gal./sq. ft./hr. japprox.) Filtrate, % of sirup filtered a
0.4 88
Made on solution of 2 parts sirup t o 1part of water.
TABLE IV. Brix
EFFECT OF VARYIXGBRIX OF SIRUP
------Filtration Gal./sq. ft./hr.
Rp.teSolids/sq. ft./hr.
Turbidity of Filtrate
6.05 5.65 5.85 4.81
0.07 0.08 0.06 0.06
Filtration rate in the tables is given in gallons per square foot per hour. Turbidity of filtrate is expressed as -log t a t 560 p wave length according to Balch's method'. Color of filtrate was determined by means of glycerol-caramel standards. Evidently there are several ways to increase the filtration rate. Since some enhance operation costs a t a subsequent stage of refining, the advantages must be considered in the light of costs. For example, decreasing the P205 may lower the color and colloid adsorption so that additional bone black filtration may be required. Evaporation costs increase with a reduction in the Brix of the sirup; and more pulp in the precoat means more sweet water. The p H and the rest period after defecation appear t o be operations that can be profitably controlled. A rest period of 30 minutes is indicated. Further study is required to form a conclusion concerning the optimum pH.
Factors Affecting Filtration Rate
TABLE V.
Since the quality of affination sirup may vary from hour to hour, it is not feasible to determine by factory observations the other variables that may affect the filtration rate. However, it seemed desirable to have an indication of the influence of the impurities associated with the phosphate complex in affination sirup. Therefore a phosphate-defecated and clarified washed sugar liquor was defecated and filtered in the same manner as affination sirup:
TABLE VI.
Filtration Rate
8.4 7.8 6.5
0.54 0.59 0.67
EFFECTOF AMOUNTOF PULPPRECOAT
Lh. Dry Pulp/lOO Sq. Ft. Filtering Area
59.30
INFLUENCE OF pH
p H of Filtrate
Filtration Rate
Turbidity of Filtrate 0.22
2.0 1.5
O:b8 0.18 0.10
The influence of controllable factors was studied in the laboratory on a specially constructed filter designed and operated to simulate factory filtration. The sirup used was a raw sugar factory blackstrap diluted with a granulated sugar solution t o 91.1 purity and 55.5" Brix. The sirup was defecated with 0.2 per cent P205 on solids (unless otherwise noted) and was filtered a t 180" F. (82" C.). Filtration period was 2 hours, and the flow rate was calculated accordingly. Some of the information revealed by the investigation is listed in Tables I1 to VI.
None of the factors so far discovered influence the rate of filtration as much as the adsorbed and occluded impurities; this is evident from the filtrability of the phosphate precipitated in a clarified or pure sugar solution. Apparently it is these impurities that determine the filtrability of affination sirup, no matter what method of clarification is used; and this is a reasonable explanation of the variable filtrability of affination sirup experienced in refinery operations. 1
OF VARYING P,06 IN DEFECATION TABLE11. EFFECT
P205,
% pn Solids
Filtration Rate
Turbidity of Filtrate
Color
0.05 0.10 0.20 0.30 0.40
0.75 0.64 0.59 0.59 0.58
0.75 0.42 0.07 0.12 0.06
380 270 230 220 200
BEFORE FILTERING TABLE 111. EFFECTOF RESTPERIOD
Min. of Standing
Filtration Rate
Turbidity of Filtrate 0.16 0.03 0.03 0.05 0.05
Balch, IND.ENG.CHFAI.,ANAL.ED.,3, 124 (1931).
