THE JACOBS CONTINUOUS CLARIFIER For Phosphoric Acid and Lime Clarification of Melted Sugar Solutions Used for White Sugar Manufacture JOHN P. GREVEN P. 0. Box 684, Baton Rouge, La.
ent in the original solution; particular reference is made to the brown-greens caused by colloidal iron compounds (polyphenols) which yielded readily to phosphoric acid-lime defecation but were difficult to remove otherwise. Allowing sufficient time for precipitation, a brilliantly clear liquor was drawn off by decantation, but the precipitates offered a problem due to their gelatinous slimy condition and consequent slow filtration rate. Since pressure filtration was impractical for this problem, gravity filtration through Taylor bag filters was resorted to; this method, although an improvement over the settling process, was unsatisfactory because it required a large amount of labor and equipment, produced excessive amounts of sweet water, was unsanitary, encouraged inversion and bacterial growth, and resulted in high sucrose losses.
General aspects of P,O, and lime clarification are considered in its earlier applications, together with its subsequent replacement by pressure filtration by means of filter aids. The original Williamson clarifier and its objectionable features are reviewed, followed by a detailed consideration of the Jacobs developments in equipment and process incorporated in his clarifier. Processing comparisons are given between the cost of pressure filtration and P,O, clarification, with the use of either bone char or activated carbon as a final decolorizing agent. The fact is pointed out that a superior grade of so-called plantation-type white sugar can be made by melting raws, clarifying the melt, and recrystallizing for white sugar production, without the use of the other decolorizing agents.
T
HE use of phosphoric acid and lime in the production of white sugar from cane dates back to 1850, judging from the earliest patents on these processes. It was customary t o use an amount of phosphoric acid, in the form of diluted paste, of approximately 0.06 per cent on raws melted and also sufficient lime-milk, usually around 0.1 per cent on raws, to neutralize the acidity of the treated material and give a faint alkalinity, as determined by litmus indicators. Most of the organic acids present in the washed raw sugars were neutralized by a portion of the lime; the balance of the lime combined with the phosphoric acid t o form tricalcium phosphate. Subsequent heating, together with the chemical and physical reactions between the defecants used and the impurities in the sugar solutions, caused coagulation and the formation of a floc, heavier than the solution in which it was formed. Precipitation followed, and the greater part of the albumin, gums, pectins, colloidal impurities, bagacillo, etc., was enmeshed by the flocs and precipitated with the tricalcium phosphate. Incidental to this precipitation of suspended impurities, there was a definite reduction in color, due to the actual removal of from 20 to 35 per cent of the total color bodies pres-
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About 1915 nearly all the North American refiners discontinued the P2O6 clarification of the melt liquor and substituted pressure filtration of the lime-neutralized melt, using inert filter aids such as infusorial earth, paper pulp, etc., to increase filtration rates. Inert filter aids provide a protective layer of absorbent material on the filter surfaces and thus retard the fouling of filter screens or cloths, in addition to increasing the filtration areas. Their function is distinctly mechanical. A brilliantly clear filtrate can be obtained, but there is little or no color removal. Increase in purity and removal of colloidal impurities and ash are insignificant compared with the results of a chemical clarification with phosphoric acid and lime. While it is possible to employ small amounts of phosphoric acid in connection with pressure filtration, its use reduces the normal filtration rates. This loss must be compensated for by additional filter equipment, additional filter aid, or lowering the density of $he filtrate. After the adoption of pressure filtration, experimentation continued with various methods and equipment, in an attempt to solve the problem of realizing the advantages of the PZOsand lime clarification, without being hindered by its obstacles. Concurrent with this experimentation, the p H method of control was adopted by the sugar industry, making it possible to secure more efficient flocculation and establish an accurate control over the chemical phase of PzO6 clarification.
Williamson Clarifier Around 1919 G. B. Killiamson invented and patented a process and equipment for continuous clarification with PZO6 and lime, which made subsequent pressure filtration of the treated liquor unnecessary. It consisted of impregnating the flocculated liquid with air and then heating the aerated material to approximately 212" F. in Sat-bottomed, rectangular, 6 X 12 foot shallow tanks, by means of heating coils placed crosswise to the length, immediately above the bottoms of the tanks. The heat expanded the air bubbles, to which the flocculated but dispersed material was attached, and caused them to rise as a scum on top of the liquor surface. To obtain a directional flow and secure desired heat distribution, vertical baffle plates had to be used; they were adjustable to meet conditions. A slowly rotating roller, installed a t the scum level and a t the discharge end of the clarifier, was designed to constantly remove the top layer of the scum a t that point, while the clarified liquor was continuously drawn off through outlets located below the level of the scum blanket, a t the end of the tank opposite the liquor inlet; this provided the first reasonable means of securing the advantages of PzOsand lime clarification without the necessity of mechanical filtration. The Williamson equipment and process were adopted in a few of the continental North American refineries and tried out in some tropical plants, but during the life of the patent
May, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
few improvements were made on the original assembly and method to overcome the handicaps which developed in its operation. The principal disadvantages are as follows : delicacy of operating balance, sensitiveness to fluctuations in quality of product entering, long contact time, uncertain displacement, high operating temperatures, local overheating, appreciable inversion losses, and inefficient scum removal. Experimentation to overcome these difficulties was revived after the Williamson patent expired; as a result, modifications of his original assembly were developed, tried out, and put into operation. The best known of these modifications of the original Williamson clarifier are : different arrangement of heating coils (H. Edson design); modification in scum-removal rolls (J. S. Kent design) ; elimination of heating coils and substitution of heat through the bottom of the units, developed by G . B. Williamson and co-workers; improvements in aeration made by several experimentors.
Details of Jacobs Clarifier I n 1936 Harold J. Jacobs, in collaboration with J. J. Munson and C.F. Dahlberg, developed a new clarifier design and a new method of aeration which resulted in removing the handicaps of the Williamson equipment and of its subsequent modification. Their invention was patented in the United States and foreign countries in 1940 and has been made available to the sugar industry. They reduced the volume of treated liquor in their clarifier by an assembly of six separate troughs, each holding 100 gallons of liquor; they were parallel with one another but the trough bottoms were inclined sharply upward from inlet to outlet points, and with the tops formed a united, common surface. The inventors greatly increased the heating capacity and its efficient application by using the trough walls and bottoms for heat transmission; the available heating surface was thereby increased to approximately four times that of earlier types, and uniform temperature conditions were provided which are essential to proper operation. I n the Williamson clarifier and all of its subsequent modifications, this problem of proper heat distribution and uniform temperature conditions made it necessary to employ vertical baffles, placed crosswise to the liquor flow and at spacings and depths which had to be determined experimentally. The bottom of the scum blanket, resting on top of the clarifying liquor, functioned to serve as a seal of the baffle tops, and thus the path of the clarifying liquor was obstructed in a straight, horizontal direction. Instead the liquor was forced alternately up and down, following the baffles, and in this travel its temperature, flow speed, and flow direction were constantly subjected to fluctuations which were detrimental to efficient clarification. I n the Jacobs design all baffles are eliminated, with the result that a direct flow is obtained, parallel to the bottom, with positive displacement and greatly increased flow rate and stability of operations, The increased heating surface, its method of application, and the smaller volume and positive displacement of the liquor in process make it possible to operate at lower, safer, and more efficient temperatures. It was also found that the amounts of defecants necessary to produce the desired clarification can be reduced from quantities formerly necessary. Experimentation and research have developed the fact that the size and uniformity of the air bubbles introduced into the flocculated liquor is of great importance. A centrifugal aerator, of special design, was developed by Jacobs et al., which produces at low cost, the desired emulsification of air with the liquor carrying the flocculated material;
635
an extremely finely divided floc with very small uniform-size air bubbles is formed. As a consequence the floc rises to the surface immediately. The progressive and uniform increase in temperature as the liquor travels through the clarifier keeps forcing the floc to the surface, and a scum blanket of proper consistency is formed which acts as an insulator and retards heat losses from the liquor surface. The maximum temperature zone is a t the discharge end of the Jacobs clarifier where the liquor depth is least, and this temperature does not exceed 200" F. with normal liquors at 64" Brix (corrected). Refractory raws may require an optimum temperature of 205" F. The clarified liquor is unusually clear and does not show the appreciable increase in glucose ratio evident in prior types of clarifiers. With normal melt liquors, complete displacement takes place in 35 to 40 minutes at a flow rate of 900 to 1000 U.S. gallons per hour in the Jacobs clarifier as compared with an uncertain displacement, in 65 to 80 minutes, a t a flow rate of 600 to 700 U. S.,gallons per hour in the baffled types of clarifiers. For scum removal Jacobs and collaborators developed a rotating scraper drag, suspended above the entire length of the clarifier, and moved this drag at a speed which was concurrent with the speed of but countercurrent to the flow of the liquor. This provides continuous removal of the very top layer of the scum blanket, over its entire surface, without in any way disturbing the balance of the scum formed. The uniformity of the liquor flow through the six sections of the Jacobs clarifier is assured by a constant head on the centrifugal aerator feeding each clarifier and a fixed orifice in the pipe line supplying the aerators. Uniform height of liquor levels in the six sections is regulated by a novel leveling
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TANKS WITH AGITATORS HEATER IF NECESSARY TO M A I ~ T A I NCONSTANT TEMP. CLARIFIER SUPPLY TANK PUMP
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CLARIFIED LIQUOR TO CHAR, CARBON OR OTHER FINAL DECOLORIZING PROCESS OR DIRECT TO PANS FOR WHITE PLANTATION SUGAR
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device by which the angles of all six liquor outlet pipes can be changed simultaneously and at will, to suit conditions of flow rate and clarity as influenced by the characteristics of the liquor to be clarified. Exhaust steam, at pressures from 3 to 6 pounds per square inch, is used to heat the steam compartments. By having a constant steam pressure and supply, a constant temperature of the liquor entering the clarifiers, and the liquor levels under control, the entire process is so stabilized that one operator can handle twenty or more clarifiers.
Scum Clarification Jacobs and co-workers developed a scum clarifier which follows the general design of the liquor clarifier except that the scum drag travels in the same direction as the material in the clarifier and a t higher speed. Scums, removed from the liquor clarifiers, are diluted and reaerated before entering the scum clarifiers. Final scums are desugarized in any of the well-known types of pressure or vacuum filters without the need for filter aid. The sweet waters produced are of high purity, since there is no reabsorption of ash, etc., from the tricalcium scums, and they can be safely and advantageously used for melt dilution. There is no excess sweet water. Scums can also be diluted, settled, and decanted; the final mud is then filtered off by the usual methods and equipment. The great advantage of the scum clarifier is that it offers a continuous process, coordinated with production, which is very desirable when handling low-density material. One scum clarifier will handle the scum output of five liquor clarifiers. Economics of the Process When properly designed equipment is used, the clarification with P205and lime offers processing cost economies for any refining method, because a color removal of 30 t o 50 per cent and ash removal of 20 to 35 per cent can be obtained a t a cost for defecants of 5 to 6 cents per ton of sugar produced, based upon 0.035 per cent Pz06and 0.057 per cent lime on solids. Processing cost reductions are also due to elimination of process filtration equipment with its operating and mainte-
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nance costs, elimination of all filter aid, and increase in the decolorizing capacity of whatever subsequent process is used, whether bone char, activated carbon, or other material. Bone char plants have observed an increase of 10 to 20 per cent in the char house capacity. Activated carbon plants report a 50 per cent reduction in both carbon and filter aid used, with an increase of 10 to 20 per cent in the output capacity of the plant. I n both types of refining, the production of a greater percentage of water-white liquors accounts for color improvement in the sugars. It is well known that a sugar produced with P z O ~clarification has a brilliance and a sparkle which makes it decidedly superior in appearance. Due to almost complete colloid removal with PIOs and lime clarification, the afterproducts of the refinery, such as remelts, etc., are less viscous. Better exhaustion of final molasses helps to increase the sucrose recovery. The bacteria count of sugars produced with this process is consistently low, probably due to more nearly complete removal of coagulated impurities than can be obtained by other means, where coagulation is only gartial and mechanical filtration is used to remove the impurities. Cost comparisons are often misleading because operating conditions and costs vary greatly in different plants. The figures of Table I were obtained from a bone char process refinery, formerly using filter aid on a throwaway basis and not filtering the affination sirups, and from an activated carbon process refinery, using carbon on a throwaway basis after double filtering the melt liquor. In either case only the amounts and cost of the defecants used has been considered. Labor, power, char replacement, fuel for regeneration of char, etc., have not been included. TABLEI. COSTCOMPARISONS Material
Without PlOa Clarification Cost/ton of on melt melt
With PeOs Clarification C o W t o n of melt
% on melt
Bone Char Refinery Filter aid Lime
PZOS
Total defecant cost
0.30 0.04 None
$0.1290 0.0052 None
0.1342
None
0.057 0.035
None
50.0074 0.0490 0.0564
Activated Carbon Refiner>
T o t a l defecant cost
0.8807
Ash removal a t the beginning of the refining process is a valuable advantage, particularly in those methods of refining where ash removal in the process is negligible and accomplished principally by means of gradual ash concentration in the remelt system, and depends on the final molasses as the main channel of elimination. The value of this advantage is reflected in an increase in sucrose recovery, closely related to this ash removal in the beginning of the refining process. With the aid of PzOc and lime clarification it will be possible to produce a good grade of white sugar, as a direct output, in the raw sugar factory by following the normal practice of making A , B, and C raws, using the C sugar as a seed magma for the A and B , washing the A and B raws to high purity, melting these sugars, and purifying the liquor by PzOs and lime clarification.
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TABLB11. SUMMARY OF CLARIFIER TYPESAND PERFORMANCE Method of heating An le of bottom W i L or without baffles Heating surfacel sq. ft. Gross capac!ty, gal Soum caDacitv. nal. Liquor c'apachy;gal. Av. flow rate Gal. per hr, Gal. per min. Av. contact time. min.
Original Steam tubes at right anglea to flow of liquor
Williamson Clarifiers Modified A Modified B Steam tubes parallel to 1 steam chamber with flat bottom end top flow of liquor
Level With 100 1000 180 820
Level With 134 1000 180 820
Level With 72 1000 180 820
760
700
600
12.5 05
11.66
70
Three crystallizations of white sugar should be obtained, particularly if the original cane juice was sulfured lightly. This possibility should be very attractive for the smaller plants desiring to produce a white sugar far superior to the so-called plantation white sugars, without having to consider complete refining equipment. At present six North American refineries, with a combined melting capacity of 5000 tons daily, are using PlO6 and lime clarification as a pretreatment prior to bone char or activated carbon filtration with satisfactory results and considerable savings in operating costs. Thus far, continuous clarifica-
10.0 82
Modified C 1 steam chember with flat bottom and corrugated top Inclined With 210 990 180 810 700
11.60 69
Jacobs Clarifier
6 steam ohamberi im-
mereed in the liquor Inclined Without 400
760 160 600
1000 10.06 36
tion has been limited to sugar solutions made from washed sugars of high purity. A great deal of preliminary work and experimentation have been carried on to extend these limitations, and include products of lower purity. It is too early to report but we believe that prior limitations on continuous PzO6 and lime clarification will be extended to include the products of purity ranges considerably lower than has heretofore been found possible. PRES~XTB inDa group of papers on Filtration and Clarification before the Division of Sugar Chemistry and Technology at the 102nd Meeting of the AMERICAN CH~MICAL SOCIETY, Atlantic City, N. J.
SILVER PLATING OF OPTICAL GLASSWARE Triethanolamine as a Reducing Agent ROBERT D. BARNARD The Chicago Medical School, Chicago, Ill.
F
OR the uniform deposition of a layer of silver upon optical surfaces, the method of Brashear and the Rochelle salts process are most frequently used. The former employs sugar to reduce the silver ion to its metallic state, while in the latter the tartrates not only act as reducers but also probably serve to hold the alkaline silver complex in solution. Both of these methods have the disadvantage that the solutions are relatively unstable; there is also danger of silver fulminate forming from the action of ammonia, a possibility which, however remote, must always be taken into account. While investigating the utility of the ethanolamines as solvents for the alkaline copper complexes used in saccharimetry, we found that diethanolamine had no reducing action on cuprio hydroxide solution at 100' C. Triethanolamine did have a slight reducing action a t this temperature. This fact would place the reduction potential of triethanolamine at some point between that of alkaline tartrate and the reducing sugars. Its effect on silver ion bears out this conclusion. Since triethanolamine was found to be an excellent solvent for silver oxide as a preliminary to the reduction of the latter to metallic silver, the advantage of triethanolamine as a silver plating reagent was obvious. The cost of triethanolamine has been considerably reduced within recent years, chiefly because 'of the large demand for it as an emulsifying agent. It has therefore been adapted to a process of silvering glass which is felt to hold several advantages over those now commonly in use.
Preparation of Optical Surface and Reagents. The surface should be cleaned with hot chromate-sulfuric acid solution and thoroughly rinsed with distilled water. It is unnecessary to use the application of caustic soda so often
recommended. If a drop of distilled water will spread evenly over the entire surface, it may be considered free from grease or other organic matter. About a half liter of (1) a 10 per cent silver nitrate solution containing one or two drops of concentrated nitric acid and of (2) a 10 per cent solution of technical triethanolamine are conveniently made up. Both solutions will keep indefinitely.
Method of Plating. The surface to be mirrored is placed face upward in a clean Petri dish of sufficient diameter to accommodate it. To25 cc. of solution 1in a large test tube are added 10 cc. of solution 2; then with constant agitation further additions are made of 2 or 3 cc. a t a time, just to the point where the precipitate which forms on the first addition clears completely. The mixed solution is poured immediately over the object to be plated so as to cover it by a layer of at least 0.25 inch. The deposition of silver begins within a few seconds. For the half-reflecting surface required for interferometers, a layer of silver which transmits about as much light as it reflects is ideal. Such a layer has a distinct violet tinge and appears within 10 minutes a t room temperature. It is advisable to have several surfaces in preparation a t the same time and being plated with the same mixture of reagent but removed from the bath a t graduated intervals, and to select the one which has the proper depth of coating rather than attempt to plate a surface additionally which has been found to have been immersed for an insufficient time. For completely reflecting surfaces, the immersion may last for 24 hours; this particular bath is unique in that the deposition seems to be continuous for that length of time. The Petri dish gives good visual control of the extent of
