Advantages and Disadvantages of Continuous Clarifiers for Refining J. M.BROWN AND W. A. BEMIS Revere Sugar Refinery, Charlestown, Mass.
C O N T I N U O U S clarification Differences in the various types of conbottom D. A with tank lengthwise having a sloping is a means of clarifying tinuous sugar clarifiers used in the sugar coils. defecated, washed sugar liquors bottom E, Same mchamber. D but with a falserefining industry are presented briefly. without filtration. The system of the advantages and disadvanSome F. Same as B but having a usuallymakes use of phosphoric tages encountered in actual operation of acid and lime defecation (6). ~ a ~ Y J ~ t ~ ~ r & ~ ,"r ~ e ~ ' ~ ~ ~ Essentially the process consists continuous clarifiers on phosphoric acid the steam coils in D, of impregnating the defecated and lime defecated sugar liquors are indihaving G. The five deep Jacobs troughs clarifier running (1) liquor with air and heating it in cated, including melting, treating, aeralen&hwise-in effect, five single flat vessels so that the air bubbles clarifiers (not included in Figtion, and char filtration. may expand, rise, carry with ui-e 1). them the flocculated material, All of these types have about the same dimensions: 12 and thus form a scum which is drawn off at the top by suitfeet long, 6 feet wide, and from approximately 2 to 4 feet able means. The clear liquor is drawn off a t exit ports located deep. Some types employ crosswise baffles and others operbelow the scum line. The filtrate goes to the char filters while the scums are treated to remove residual sugar and ate without baffles. The sloping bottom was adopted to prevent underrun of cold liquor which sometimes occurred in the finally discharged to the sewer. flat-bottom types. Some types have a single liquor outlet The entire continuous clarification system depends upon and others employ five individual outlets spaced a t equal disthe fact that, when phosphoric acid and lime are brought totances along the outlet end. All clarifiers have outlet ports gether in a sugar solution under the correct conditions, a voluminous flocculent or spongelikeprecipitate will form. The capable of adjustment to variable liquor heads. Numerous natural tendency is for the flocculated material to settle gradutypes of mechanisms are employed for this purpose. I n the earlier models scums were removed by flotation and scum ally to the bottom of any quiet sugar solution in which it is in wheels, but they have been replaced by the chain scraper or suspension. If, however, a means is devised to attach a bubscum drag in all the later types. ble of air to each floc or sponge and then heat the whole mixture, the bubble of air will expand, its lifting power will To achieve efficient performance and satisfactory separaincrease, the viscosity of the sugar liquor will decrease, and tion of impurities, i t is necessary that the upward movement instead of settling to the bottom, the floc will rise to the top. take place in a quiet, undisturbed medium; hence the clarifier During formation and in passing from bottom to top, these is constructed as a large flat vessel of such dimensions so that countless flocs will sweep through the liquor and collect most the rate of flow in any particular section of it is very low. In of the impurities in the sugar. Such physical materials as our clarifiers the average forward motion varies between 1.9 gums, waxes, dirt, and colloids, together with color bodies and and 2.8 inches per minute. Means are provided to prevent ash constituents, will be swept from the sugar solution and any further currents, eddies, or other undesirable stray moveappear a t the top surface as scums. Thus a rather complete ment of the aerated liquor. To date, a simple set of baffles separation of impurities may be obtained without resorting to has been found an excellent means of controlling these stray filtration. currents, but it is important that the baffles be located in The first clarifier of this sort was developed and patented in definite spots a t particular levels in order to secure satisfac1919 by Williamson (8); i t consisted of a flat tank with nutory results. merous crosswise steam tubes to heat the liquor passing The sugar liquor is treated by adding the predetermined through. Since then many types have been devised with amount of phosphoric acid (0.025 to 0.050 per cent Pz06on various modifications, but all are similar in principle. solids) and liming back to the desired alkalinity. The material is aerated (generally by nozzles or centrifugal pumps) Operation and passed into the clarifiers where i t is heated. During aeration the flocculated material is somewhat dispersed, and The various types have been as follows (Figure 1). upon reforming into larger particles, the air bubbles are enA. The original Williamson clarifier constructed with a flat closed. The expansion of the air bubbles and reduction of level bottom and crosswise heating tubes. the viscosity permits the flocculated material to rise rapidly B. Same as A with lengthwise heating tubes. to the top. The clear liquor is drawn off below the scum C . A level-bottom tank with a false bottom forming - a steam layer formed by the flocculated material. chamber without tubes. 419
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Performance Conditions The following conditions are essential for the successful operation of continuous clarifiers: The sugar liquor from the melter house must be of the highest quality consistent with good refinery practice (?'). The higher the quality of the sugar, the lower the treatment may be; and the higher the rate of flow which can be obtained in each clarifier, the fewer clarifiers will be needed to handle the melt. A constant supply of sugar liquor to the clarifiers is needed, as any interruption in continuous operation of these units is likely to destroy equilibrium and may result in an output of liquor which is below standard. A balance between steam and input must be reached before this equipment will deliver good liquor; whenever, for any reason, this balance is disturbed, a liquor output of subnormal quality may be expected until the unit balance is again established. Sugar liquor free from strings, trash, and other foreign material is an absolute requisite of this type of clarification. These impurities plug the nozzles and pumps, interrupt the flow of liquor to the units and so interfere with continuous operation. Sufficient steam a t reasonably constant pressure is required because the liquor must be heated toward the back section of the units. Heating well back in the unit lowers the viscosity and allows the air bubbles and phosphate floc to rise to the surface some time before the liquor reaches the output end of the equipment. Variations in steam pressure have a tendency to upset the clear liquor-scum boundary which may cause dirty liquor. Steady air and liquor pressure is necessary to secure proper aeration. Proper aeration is essential because the efficient separation of the floc and scums from the sugar liquor depend on the intimacy with which the air is mixed with the sugar liquor. Large air bubbles rise to the surface rapidly without carrying much material. Small air bubbles fail t o effect adequate aeration. A continuous melting of one kind of sugar is preferable to a mixture of sugars or frequent changes in the kind of sugar melted. Variations in types of sugars require that the treatment be altered with each change in sugar. When the type of sugar is changed frequently, dirty liquor may result due t o the inability of the operators t o keep informed or to anticipate these changes. With variable sugars the treatment is liable to be higher than necessary owing to the tendency always to have sufficient treatment. The longer the interval between changes, the more economical will be the operation and the more satisfactory the quality of the liquor output.
Advantages The following discussion of the advantages and disadvantages to be expected from the operation of continuous clarifiers is based on experimental and development data assembled by us, observation of results, and oqinions expressed by others and verified by us in experimental work. Continuous clarifiers should show the following advantages in operation over mechanical filtration operations: Savings in defecation costs are substantial. Operating data on approximately one fourth of our melt defecated by continuous clarifiers over a period of two years show that the material defecation cost is about half that for mechanical filtration (5). It appears that the labor costs will be the same. The savings secured by elimination of filter cloth and reduction in volume of sweet water, and benefits obtained from increased color and ash adsorption are obviously real and desirable although they have not yet been fully evaluated on a cost basis.
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The flow of liquor to the char filter storage tanks is oontinuous and steady. The steady rate of flow of liquor to these tanks results in steadier operation of the char filters, melter house, and blowups. In such an operation only a small amount of liquor need be held in the char filter supply tanks (2). This is a distinct advantage in saving of time in shutdown periods and in control of inversion which is ever present when relatively large liquor stocks are held in the refinery. Less sweet water is produced than from mechanical filters. A large amount of recirculation of impurities will be eliminated since the only sweet water handled is that from the scums. The calcium phosphate type of precipitation tends t o retain the impurities when they are on the alkaline side, whereas the diatomaceous earth cake tends to deliver some of the impurities back into the sweet water system when the mud is sweetened off in the filter and during the period in the settling tanks after sweetening off. Tests have shown that mechanical filters produce about three times as much sweet water as continuous clarifiers. The sweet water from mechanical filters is often poor in quality, containing more extracted nonsugar solids and some residual filter aid. Chemical treatment of the liquor results in substantial color and ash removal not secured in simple filtration. Phosphoric acid-lime defecation removes up to 40 per cent of the coloring material and up to 25 per cent of the ash constituents of the liquor, Since these impurities are enclosed in the flocculated material, they are removed permanently from the system. There is no filter cloth cost, except for scum filtration. The cost of cloth necessary in mechanical filtration is entirely eliminated. For scum handling, one filter equipped with wire leaves will suffice. The rate of flow is constant because there is no decrease in filtration efficiency as is the case when filter cycles are employed. Mechanical filter leaves must be redressed frequently, and the filtration efficiency of cloth covers drops off rapidly '(3). A cleaner station is obtained, with the result that less inversion and less labor are reauired. A filter station is of necessity a sloppy part of a refiiery operation, and any leaks or drips draining to the sweet water tanks necessitate reprocessing of a portion of the liquor. Clarifier liquor requires less char per unit than pressfiltered liquor. With the removal of a large percentage of the nonsugar impurities from the washed sugar liquor a t the clarification station, less bone char is required. The difference may be diverted for use on other materials, such as remelt and granulated sirups. The loss of hot water from the refinery is less. More clean hot water is required a t the melters instead of a t the mechanical filters. The same amount of water will be needed for the melt as is now used, but less of it will be sweet water. No mechanical filter sweet water is sent to the evaporators. The water evaporated in the last body of the evaporators is lost. The water saved is used a t the melters. This applies only t o a refinery requiring more hot water for melting than is supplied by sweet mater. Maintenance costs are negligible compared to those of mechanical filters. Maintenance costs of clarifier units consist chiefly of the repairs necessitated by accidental breaking of chains and the cleaning of strainers and base plates.
Disadvantages Scums must be desugared. This is a more exacting procedure than desugaring mechanical filter muds because the muds are discarded directly and must be low in sugar content. The filtration of scums is being successfully accomplished without the use of additional filter aid by handling the ma-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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CLEAR LIQUOR STEAM COILS
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Figure 1. Types of Continuous Clarifiers
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terial at 20-30" Brix and employing available sirup side mud as a spacer. Good flow rates and adequate cycles are obtained (4). Formerly considerable difficulty was encountered in the filtration of scums because the Iow-density settling methods then in vogue produced large volumes of sweet water with the accompanying danger of excessive inversion. Greater care and control are needed, and therefore labor must be specially trained and supervised. Because this is strictly a chemical treatment and requires careful control in operation to assure a satisfactory product, the operators must be trained thoroughly and understand the manner in which liquors are treated and handled in a clarifier. This does not constitute a serious drawback, but i t does emphasize the fact that control must be under the direction of the laboratory and the men must be laboratory-trained. A plugging effect may be obtained, and certainly will be, if dirty liquor is delivered by the clarifiers. Poor char filter sweetening-off performance also results. When a cargo of sugar is received which has a high fiber content, slowing of the rate of flow of the char filters may be experienced. The particles of fiber are relatively large, and the elevating power of the air bubbles is not sufficient to carry them upward to the scum layer. These pieces of fiber pass out with the clear liquor and form a mat on top of the char filter. Normally the amount of fiber is not sufficient to cause trouble of this nature. The same result will occur if dirty liquor containing floc passes onto the char filter. Sometimes certain types of raw sugar produce a liquor which does not floc immediately. Periodic cleaning of the base plates of the clarifier units is necessary. Certain raw sugars, condition of operhtions, or a component part of some raw sugar sometimes tends to form a scale on the base plates which acts as an insulator and reduces the heat transfer. This scale, consisting largely of calcium phosphate, adheres to the base plate but may be easily removed by covering the plate with hot water and adding a small amount of hydrochloric acid. Refinery startup and shutdown periods have to be rearranged. The startingup time has to be advanced to allow for filling the clarifiers. Several blowups will be needed to fill the clarifiers before they start to deliver to char filters. After they are filled they require about 30 minutes to heat to temperature; then, to ensure clean liquor, they should be rested a t this temperature for 30 minutes. Mechanical filters deliver to the liquor tanks within a few minutes after the material is delivered to the blowups from the melt house. At the end of the week most of
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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-
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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
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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
