Automatic and Continuous Filtration in Sugar Refining ARTHUR WRIGHT Arthur Wright and Associates, Upper Montclair, N. J.
Combining flocculation of liquor and use of inert filter aids permits automatic filtration of all refinery sirups. Flocculation was discarded when pressure filters superseded bag filters. The improved refining qualities of defecated liquors have revived use of precipitated calcium phosphate. High-pressure filtration “smears” the cake and blinds the filter cloth. With
cord medium, continuous filters can opererate at low pressure without clogging the filter members, and brilliantly clear filtrates and high flow rates are obtained. Continuous cconditioning and automatic filtration shorten the process time and reduce inversion losses. Bentonite augments flocculation at a wide range of pH. Activated carbons and finely divided decolorants are continuously filtered.
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N AMERICAN chemical engineering, continuous and automatic processing is always the goal. It is not so much that labor is then minimized as that Tabor’s productivity is increased. With automatic control of various factors, such as temperature, pH, chemical dosing, etc., optimum conditions are maintained so that maximum yields, efficiencies, and capacities are obtained. Losses are reduced and over-all economies greatly increased. Automatic and continuous filtration is assured for cane sugar refineries from the successful work on pilot-scale investigations. In 1913 at the Arbuckle Brothers’ refinery in Brooklp, N. Y . , a small hand-operated Sweetland filter press was installed to determine if the “self-dumping Sweetland clamshell” filter could be substituted for Taylor bag filters. This first modern filter was bought by Arbuckle Brothers for mud desugaring and also for liquor clarification. Before that unit was installed, however, the Federal Sugar Refining Company put in a pilot-scale Sweetland and earnestly went after liquor filtration. Success with the small unit resulted in the installation of a battery of five large machines. By the end of 1917 most of the refineries had bought or installed Sweetland filters, and in that year alone some $640,000 worth of Sweetland filters were sold to sugar companies. Several years prior to this, Henri Vallez, at the German American Company’s beet sugar plant, had developed the Vallez filter with rotatable leaves to overcome the tapering cakes formed in Sweetland filters in which agitation was insu5cient. Vallez claimed that with stationary nozzles and rotating leaves he had a better sluicing type of filter. Early in theJ920’s he‘made installations in sugar refineries. Coincident with better sluicing, Vallez advanced the use of fibrous filter aids in the form of paper pulp.
Filter Aid Principle I n the first work undertaken, basic principles were established for using insoluble filter aids for clarification and increased filtrate flow, and for precoating to help discharge deposited solids. Some thirty-two filter products were tried, including kieselguhr which was being mined in California and offered as a natural deposit just as good as European and African diatomaceous earths. Others among them were precipitated chalk, gypsum, pulverized bone char, sawdust, wood flour, stack dust, and paper pulp. Precoating at that time was done with clear water suspensions, and it was several years before filtrates were used to precoat or before overdoses of Celite, added to the unfiltered liquor, became standard precoating practice. Early in 1914 Filter-Cel teats were run at Arbuckle Brothers’ plant. The original idea was to reduce the amounts of phosphoric acid and lime in defecation to offset the cost of the earth. Tests were to be run using two thirds and one third of the normal defecation concentrations, and finally without any acid and lime. However, the order of the tests was reversed, and such excellent clarity, high rates of flow, and easy discharge of the deposited solids resulted that the other tests were never run and commercial operation for years never included defecation with chemicals. Filter-Cel was an improvement on kieselguhr, and intensive research developed the calcined Super-Cel which was superior to the natural product. Today Hyflo and competitive products permit selection, from a wide variety of filter aids, of that material best suited for the work in hand. The use of filter aids was first confined to that amount which balanced the costs of defecation, but when the various. 425
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The difficulties of filtration led to the ingenious frothing clarifiers. By entraining air in finely divided bubbles and raising the temperature so as to expand the bubbles and decrease the viscosity, the floc is carried to the top of the clarifier by the rising air and a clarified liquid is obtained. Improvements effected by Williamson and others account for the extended use of these machines. The advantages of the aerating clarifiers are reduced and largely offset by the difficulties in desugaring the scums. Where settling, followed by plateand-frame filtration, is practiced, the gravity of the sweet water drops to 13" Brix. The purity of this is rather low and indicates that nonsugars are carried back when they should be vented from the plant. Refrothing the scums in secondary flotation units still delivers Courtesy, Olzuer Unzted Fzlters, I n c . a low-density sweet water. Frothing units are not so successful on Open-TypeOliver Precoat Filter low-grade liquors. A well washed sugar from the affination of a good grade of raw is desired for best operation on these machines. Refineries making soft sugars or high-quality sirups are remarks of sugar were handled in the refineries, economical strained from adopting frothing clarifiers which cannot cope rates of flow demanded increases in the amounts used. Often with sirups or lower grade sugars. the cost of filter aid went far beyond the old cost of defecants. The problem of handling scums led to an investigation Re-use and recovery by burning out the organic impurities of the applicability of the cord filter medium, but the air in have resulted in economizing on the cost of new earth so that the solids direct from liquors of 62" Brix prevents filtration the general experience is satisfactory. even though large volumes of free fibrous filter aids are used. The air-binding effect is comparable t o efficiency reduction Improved Standards of centrifugal pumps or horizontal piping when they are airlocked, and a reduction of the gravity was found to be the Refining standards have gone up, and for many final sugars simplest answer to deaerating scums. At 48" Brix scums can it is necessary to filter the liquor after char decolorization. be handled and washed by displacement to less than 0.1 per Much of the murkiness is colloidal in character and should cent sucrose in cake, with a wash water consumption such that have been extracted at the clarification station. Insoluble the mixture of strong filtrate and wash filtrate results in a filter aids deliver filtrates without visible solids of suspension, gravity of 43" Brix. Purity is not harmed and the bagacillo but they do not adsorb colloids, they do not reduce ash, nor and beaten up waste newspaper, or other paper pulp, can be do they reduce color. These results were obtained in defecarecovered and re-used. tion with phosphoric acid and lime. The very foundation When this success was discussed with one of the leading of sugar refining is quality a t economy, and reprocessing is a proponents of defecation, he advised that automatic and constep in the wrong direction. Defecation is more than an tinuous filtration of all the liquors was more valuable than agglomeration of the suspended solids. Calcium phosphates are more insoluble than calcium sulfate, as witnessed by the use of phosphates as water softening agents for permanent hardness. The realization of this had led some refiners to return to defecation with acid and lime. I n handling blackstrap molasses to blend with dextrose of almost neutral flavor, it is necessary to refine molasses principally to reduce the lime content. It has been found that lime in excess of 0.03 per cent will curdle warmed milk, and the calcium ash of final molasses is well over 1.0 per cent. The initial step in this work is to add phosphoric acid when a voluminous floc of calcium phosphate is precipitated. Calcium phosphate is a gelatinous, weak-structured solid, and under pressure filtration the floc breaks down and smears the filter cloth. Low-pressure heads, as in the old bag filters, can cope with Discharge Side of Wright C o r d Filter and Drive End this weak structure.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
handling scums. This prompted the work at the National Sugar R e h i n g Company plants, tirst at Edgewater, N. J., and this past winter at Long Island City, N. Y. The initial attack was to limit the filtering pressure without lowering the density of the liquors as now processed in the house. If ailination washings could be handled, all liquors could be treated. We found that a t 4 inches of mercury vacuum we could get excellent capacity, and if defecation was correct. we obtained brilliant claritv. There seemed to be no gai; if the pressure went abov;? 8-inch vacuum. This low head is effective since there is no boiling a t this pressure and temperature (under 195' F.). At Long Island City the pilot filter was arranged to submerge the drum 60 per cent under the unfiltered liquor. The low filtering force suggested using barometric legs for the delivery of filtrate. For the tests we could not conveniently go through the floor, so we put centrifugal pumps on the cloudy and clear filtrate outlets. The foam that resulted from the direct use of these pumps on outlets was uncontrollable, and a lesson in handling low-grade liquors was cheaply but impressively learned. We then used a small vacuum pump and separate receivers, and drew out the filtrate without air mixing. We had no more foam headaches. Centrifugal pumps should not be used on low-grade liquors carrying entrained air. I n the scheme of operation we dosed the liquor in circulation about the drum with an excess of filter aid. The cake then dropped into a repulper to which unfiltered liquor was fed. Thus the filter aid was re-used to exhaustion. I n r e turning the cake with the filter aid, however, we were returning the calcium phosphate precipitate. Could we economize on the quantity of defecants since there is some adsorptive effect in the precipitate? Our work was not wholly conclusive on this, but it is reasonable to expect that the theme is correct.
Technique of Flocculation The second objective was to determine the best technique of flocculation. Flocculating affiation washings is very different from flocculating high-grade wash sugar liquors. It was
Ball-and-socket Bearing for Prevention of Shaft
Binding The filtrate outlets of this Louisville-Wright filter are the rubber hose connections.
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Single Set of Wright Enveloping -. Cords for W a s h i n g Cake
known that temperatures approximating 195' F. were necessary and that the optimum pH was around 6.9. But with colorimetric indicators this is not easy t o control. With some marks of sugar the range in pH a t which the floc is well formed is relatively wide but on others, especially some Hawaiian raws, the range is narrow. Flocs and not smears can be obtained, and for efficient work the break between flocs must show brilliant liquor. This difficulty in attaining the right flocculating technique might be overcome with pH control apparatus, but, not having this, we tried augmenting the calcium phosphate treatment with bentonite. This is typical of flocculating clays which will operate at any pH. Of itself it does a good job of flocculation but it does not reduce ash or color. Combining bentonite with acid and lime or with trisodium phosphate gave good results, and it was indicated that the total cost would be lowered rather than raised. The filter aid used was plant-recovered paper pulp. Absolute clarity was obtained, for it must be remembered that the pulp is not now required to hold back colloids but merely to arrest the large voluminous flocs. If the flocculation is lacking, the filtrates are not acceptable, but otherwise a free open filter aid is most satisfactory. The filter was equipped with a cloudy and clear filtrate port. The sharpness of break between murky initial filtrate and clear filtrate is a function of the volume of the drainage on the filter drum. With the cord filter medium this is a minimum, and hence some of our success in getting brilliant liquor can be attributed to the design and construction of cord filters. No attempt is made to desugar the cake on the clarifying filter. The unwashed cake is returned until its filter aid properties for clarification and economic flow rate are exhausted. The diverted cake is then pulped up in the sweet water from
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the washing arc of a low-submergence washing type filter. matic and continuous. This speeds up the operation of clariThe strong filtrate can go to the melter or evaporator as defication, and quicker work means less inversion loss. Sucrose sired. The density of strong wash filtrate approximates 30" liquors under heat tend to invert in proportion to the time of treatment. The entire process from melted liquor to material Brix. going to the char house can be measured in seconds, for it In bag filter practice the defecation called for an actual is only a few minutes before unfiltered liquor is brilliant filoverdose of lime and acid. The precipitated calcium phostrate with continuous machines. Batches are no longer phate was the fllter aid. The average dosage in green sirups was 0.24 per cent P,Oa on dissolved sucrose. With the connecessary. Cleanup a t shutdown is rapid, and evaporation of low-density sweet waters is reduced to a small fraction of tinuous filter one half this amount gave equal results in ash that now necessary. and color removal. With bentonite the acid and lime can be Labor in operating automatic filters consists largely in obrestricted to an amount just in excess of the chemical demand for ash removal. This is particularly true when the returned serving mechanical units. There are no liquors to control with valve manipulation, no time cycle for changing from cake helps with its adsorption. precoating to regular liquor, and none of the usual cyclic It was shown that on final sirups from char filters, defecahandling of valves that depends on the personal efficiency of tion, as above, resulted in increased purities with decidedly the operator for economical and quality operation. better tastes due to ash removal. Apparently the char gives The nonclogging feature of the cord filter medium provides up some of its adsorbed impurities, for there was evidence of a new tool for filter operators. At each revolution its filsulfides and other ash in proportions not contained in the trability is restored even with poorly conditioned liquors. liquor going on the char. When applied to existing filters, the conventional cloth of Liquors, filtered on continuous filters after defecation of the which carbonates or otherwise plugs up, the filter capacity unfiltered, are clear, both going on and coming off the char. and functioning become highly efficient. On one such appliHere is proof of the elimination of colloids by the precipitation cation a cloth that blinded after three revolutions of the drum of the defecants. went hour after hour without losing its initial porosity. Comparative char tests were run on defecated filtrates and It was thought that the cord filter medium could operate house product using filter aids only. That defecated showed only with coarse solids or fibrous products. A comparative 36 per cent less color going on the char, but from the final char test was run on the sluicings from Sweetland filters which are treatment there was no difference in final color. Some claim handled on a continuthis is proof that defeous string discharge cation removes only filter. The same liquor the color most easily fed to the plant filter adsorbed by the char was fed to the pilot and that its removal is cord filter. Vacuum of no economic value. pressure and rotative Others differ. They speed were maintained point t o the fact that equal for both units, those houses employSmooth braided cords ing defecation use less were used and with char per million pounds them a better clarity melted. Certainly of filtrate and equal small operators using capacity were obtained, vegetable carbons find The cords were changed defecation helpful in to provide a more open reducing the carbon demedium. Then the mand. Then, too, clarity was the same modern processes that and the cord filter had bleach sugar liquors some 30 per cent added use defecation to recapacity. At the time move close to half the of the test the sluicings color before applying were alkaline, 10.2 pH. the bleach. The free lime hydrate If no color is recarbonated readily and moved in the flocculathe woven cloth was tion, the removal of affected thereby, but the colloids and some the cords, being washed of the ash is beneficial each revolution, were to the char house. Reducing the work renot affected at all. The Oliver Filter with W r i g h t Cord Filter Medium cords are washed away quired of the char in mineral salts to be Filtering Whisky Stillage from the filter vat so that none of the washabsorbed and the coling liquor gets into the loidal solids t o be filter feed and none caught by the char of the slurry is diluted thereby. Since all porosity in the reduces the work in revivifying the char. These advantages cord medium is in the interstices between adjacent cords and are obtained even if no color is removed. Low-pressure no filtration takes place through the cords themselves, the filtration makes this possible, and the nonclogging cord washing of the cords means only a wash of their surface. A filter medium enables this work to be done automatically and fan spray is excellent. The washing medium need not be continuously. clean water, for unfiltered liquor works equally well when solWith continuous filtration it is easy to synchroni~ethe ids are present in small quantities. heating, dosing, and pH control so that they will be auto-
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Wright
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cord Filter (8-Foot Diameter X l8Foot Face) Sugar Muds
The cord filter medium requires no built-up drainage on the surface of the drum. One set of cords provides the grooves in which the second set of cords align. The space between the first cords and under the second set gives adequate drainage for the filtrate flow even when the rate is as high as one gallon per square foot per minute. At the point on the drum where the cake is conveyed from the drum down to the place where the returning cords again lie on the drum, the surface of the drum is exposed and there is nothing on it but the cross-division strips. There are no obstructions where solids can clog and putrefy. There is nothing to corrode, and the open accessibility allows the operator to keep the unit clean a t all times. Each set of cords is cleaned with each revolution, and drainage, as well as filtering members, is cleaned so that underclogging is impossible. I n sugar refinery work this is less important than in beet sugar filtrations, but it does make continuous filtration possible where undercloth clogging impedes maintenance of filtrate flow. Continuous filtration of liquors treated with vegetable carbons, finely divided bauxite, or other products is possible when the decolorant is used on the throw-away basis. An agglomeration of finely divided carbon by bentonite, or acid and lime defecation, makes the filtration of the fines quite simple. The technological advances made in the decolorization of sugars by activated carbons and other processes have resulted in the economical manufacture of acceptable white sugars from plantations and from small-capacity refineries. The large refineries on the mainland have great advantages in maintaining higher qualities, better packaging, and marketing conditions. It is necessary that such refineries adopt the most economical methods and equipment. In Puerto Rico it was a common sight to see bagasse going up from the mill and feeding the boilers, while the excess was carried to the storage bin. The steam so generated handled all the requirements for power and process steam for the raw house and, when operating on local products, the refinery and the distillery. When outside raws were refined or alcohol was distilled from molasses, fuel oil was required. I n addition to this fuel economy, the handling of the sugar going to the refinery was striking. A conveyor from the centrifugals delivered the sugar to a Weighometer, and the refinery was
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charged with that raw sugar without any bagging or storage. Affination sirup was returned to the raw house and credited with the sugar so returned. These economies exist, of course, only while the raw mill is operating. Refining or distilling after the crop requires fuel and other costs normal to refinery operation, but modern sugar refining competing with these obvious advantages of plantation must be economical in operation. The patented Sucro Blanc process has a great advantage over former bleaching methods of for Desugaring Raw Cane whitening sugars. It is held that some 50 per cent of the color is removed by the phosphate precipitation so that the bleach works on less than half of the original color. Present day practice can probably be further improved if continuous filtration is used, for then the liquors can be handled a t lower temperatures as the clear filtrate is several points lower than when the liquors rise to 208" F. When using adsorptive carbons to reduce excess chlorine from the final treatment, continuous filters can deliver brilliant filtrate without filter cloth or metal cloth worries. When brilliant clarity is desired, the Oliver precoat filter is recommended. Some day a bed will be made of a type that will admit ready flow of heavy sugar liquors, which have to be thinned to get rapid flows through conventional filter precoat beds.
Summary The advantages of continuous and automatic filtration of sugar refinery liquors include the following: 1. Continuous pretreatment and conditioning can be automatic in dosing, pH, and temperature control. 2. Continuous filtration coupled with continuous pretreatment speeds up transit through the filter station and reduces sucrose inversion losses. Operators become observers. 3. Cakes are washed free of sugar with a higher density of wash filtrate and a higher purity of recovered sucrose. 4. The lower pressure of filtration, employed in continuous filtration without vacuum pumps, permits use of flocculatin agents with the attendant advantages of colloid removal, and reduction of ash and color. 5. Open and fibrous filter aids are practical for brilliant filtrates, and maximum economy is obtainable in use of this filter aid. 6. The cord filter medium provides a nonclogging fabric with long life and minimum upkeep for additional economies by saving in ater cloths or metal cloths.
Thus we have seen the progress of refinery filtration from the bag filters with defecation, to pressure leaf filters with filter aids, and now continuous filters with a combination of defecation and filter aid. Coupling automatic preparation with automatic filtration makes the filter station really efficient and economical.
