Diatomaceous Silica Filter= Aid Clarification A. S. ELSENBAST AND D. C. MORRIS N.Y.
Johns-Manville Sales Corporation, New York,
sary to maintain his standard of sugar quality, and the refiner must know the efficiency of his char or carbon. The filter aid, or mixture of filter aids, which will give that clarity with the maximum flow rate is the proper choice around which the ideal filter station and its operation should be built. Because of the widely varying filtration characteristics of cane sugar liquors and, to a lesser extent, of beet sugar liquors, the refiner has little or no choice of the raw sugars he receives, nor has the direct manufacturer from cane and beet much control over his raw source. Hence the practical choice of a filter aid resolves itself to the one which gives satisfactory clarity with the difficultly clarified liquors, even a t a sacrifice of filter aid efficiency with the easily clarified liquors. The known particle size distribution of a filter aid plus the unknown particle size of suspended impurities in the sugar liquor are the two principal factors involved in choosing a filter aid to obtain desired clarity and flow rate. The following table shows the approximate particle size variations of several types of filter aids, and it is almost needless to call attention to the size distribution of the low-flow-rate, nonprocessed natural product and that of the highly processed one; this difference accounts for the fact that if a clear liquid is filtered with Celite 503 and with the less porous Standard Super-Cel, for example, the flow through the Celite 503 is much greater:
From time to time papers have been presented which dealt with various phases of clarification with diatomaceous silica filter aids, but for the most they have given a chemical or physical interpretation. Therefore, the mechanical application of such filter aids and the equipment in which they are used are discussed here for the benefit of the man in the factory.
D
IATOMACEOUS silica, then commonly known as kieselguhr, was first used in the sugar industry in this country about twenty-five years ago. Progress in its adoption was slow owing to lack of suitable filtration equipment except in beet sugar factories. A number of years went by before the economic and technical advantages of this new method of clarification were fully realized by refinery staffs. Increasing numbers of modern filtration equipment were installed, and added impetus was given with the advent of faster flowing filter aids in 1923. Within the next few years nearly all sugar refineries in this country and Canada had been converted to the use of diatomaceous silioa filter aids, as well as many refineries and other types of sugar factories in other countries. By 1930 nearly all beet sugar factories here were using the filter aids continuously, and now all beet liquors to the white sugar pans are being filtered in every factory.
Selection of Filter Aid The most important decision to be made in connection with a filter station is the choice of proper filter aid. The sugar man must first ascertain what degree of filtrate clarity is neces-
Particle Size, Microns
Filter-Cel (NaturalMilled)
>40 40-20
2.5 8.0 14.0
20-10
10-6 6-2 12
Typical Cotton Filter Cloth ( X 20) 412
19.0
37.5 19.0
Hyflo
Celite
503 (Flux-
Standard Super-Cel (CaloinedMilled)
Super-Cel (F1,ux-
Calcined, Special Prooessing)
4.5
6.0 15.5 33.5 22.0 21.5 1.5
12.0 25.5
10.0
20.0 24.5 33.5 7.5
CalcinedMilled)
29.0
19.5 13.5 0.5
60 x 60Monel Filter Cloth ( X 13)
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Celite 503, a Hiqh-Speed Filter Aid
(x-200)
The flow rate is therefore an inverse function of this change in percentage of the larger particles from one type to another. I n the presence of colloidal and suspended impurities such as are found in Hawaiian, Cuban, Natal, Fiji, Puerto Rican, and Australian sugars, however, the performance of each type of filter aid is radically altered, although each type will nearly always occupy the same relative position with respect to the others in the clarity-flow rate ratio. Since sugar liquors contain substances varying in size from colloidal coloring matter to gross fibers, the real filtration characteristics of a filter aid are changed by the amount of such substances entrapped, and the filter cake formed is a composite of filter aid and suspended trapped particles; in the final analysis it is the character of the filter cake itself that becomes so important. Since there are no apparent adsorption or absorption properties of diatomaceous silica filter aids in sugar liquors, the fine impurities can be removed only by screening through a tight cake. If there are no such fine substances and the impurities are uniformly large (as found in a few raw sugars), then a more open cake retains them. This usual wide range in particle size accounts for the fact that filtrate clarity varies inversely as flow rate (3). Since the flow rate is governed by the filter cake texture and the system of suspended matter is fixed a t the moment of filtration, the cake texture becomes controllable only through the kind and quantity of filter aid used. I n the lime and heat treatment process used in most refineries, a cake texture coarser than that obtainable with the Hyflo Super-Cel type, or equivalent mixture of aids, cannot be used without undue sacrifice to clarity; and with some types of sugars a less porous filter aid is necessary to provide the proper cake texture to maintain a predetermined clarity standard. Changing the physical character of the impurities by coagulation with phosphoric acid and lime (1) in the refinery and agglomeration a t higher densities in the beet sugar factory, for instance, will permit use of filter aids coarser than Hyflo Super-Cel to obtain a certain filtrate clarity. Without such treatments, under uniform conditions and with the help of an instrument which measures turbidity accurately and quantitatively ( 2 ) ]an inverse ratio of clarity to flow rate may always be expected. Laboratory data are helpful and experience is a guide, but actual plant tests are usually necessary to establish the type of filter aid that will best meet the over-all average conditions in a given plant filtering sugar liquors from a certain source. The insoluble impurities vary widely, depending on source of raw materials, climatic conditions, and prior extraction and processing methods. They vary in quantity] but of greater significance is their variation in physical nature with respect to nonrigid character and particle size distribution in a liquid. Considerable latitude in choice of filter aid is pos-
413
Standard Super-Cel, a Medium-SDeed Filter Aid ( X ZOO) sible where the impurities are more rigid in character and are uniformly larger in size; however, only a few raw sugars have been found where such conditions prevail under which rather fast-flow-rate filter aids could be used without a sacrifice of filtrate clarity. Since it is impracticable to stock a wide variety of filter aids to suit every condition which may arise, the sugar man must choose that filter aid or mixture of aids which produces the desired clarity when filtering the more difficult liquors.
Filter Station Equipment While following a general pattern, considerable variation in the filter stations exists. From the inception of filter aid filtration, the filters and accessories had to be fitted into already existing buildings and equipment, and in the case of the beet sugar factory the filters already there, antiquated in most cases, had to suffice. Later installations were better designed as the a r t of applying filter aids progressed] new and more efficient filter aids become available, and a higher estimate of the value of clarity was developed. Little change took place in the earlier installations to keep pace with development, however; today we may see wide differences between installations, many of which call for considerable resourcefulness in applying the proper filter aid efficiently in the light of present day knowledge. To describe the many types of arrangements is beyond the scope of this paper, so it will be confined to recommendations from the standpoint of the filtration engineer with many years of experience in applying filter aids in this and other industries.
Cane Sugar Factories and Refineries FILTERS. The leaf type of filter, with either stationary or rotating elements, is almost universally used in the refineries with a number of plate-and-frame installations in direct consumption factories. A choice as to type is based upon consideration of higher first costs, on the one hand, against lower investment but higher labor and cloth costs with lower production per unit of filter area, on the other. Regardless of type, serious consideration must be given to the drainage characteristics of the leaf or plate and the relationship of outlet sizes to the areas of the individual drainage surfaces. All too often do we see the production of fast-flowing liquids reduced because of these restrictions. The stationary leaf should have a grid of well defined, doubled, crimped 4-mesh wire or corrugated sheet metal where cloth cloth is used. T o keep the cloth from being pressed into the grid and restricting the flow, an 8- or 10-mesh screen should be superimposed over the grid. Clamping rivets t o
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INDUSTRIAL AND ENGINEERING CHEMISTRY
hold the cloth in place should be eliminated. Where metallic cloth is used, the superimposed screen is not necessary but the screen grid should be rolled, I n the rotary leaf filter a punched flat plate should not be employed as a separator between the cloth and grid. This flat surface with the screen pressed against it restricts flow and is a trap for solid impurities which wash out with difficulty or not a t all. I n plate-and-frame filters a 4- or 5-mesh screen should always be used to keep the cloth from being pressed into the drainage channels, and with this type of filter the outlet ports should be carefully examined to see that there is no restriction to flow a t that point. The large pressure liquor control valves generally in use should be by-passed by a much smaller line and valve, or an orificed plate installed in the flange after the valve, or the large valve replaced with a smaller one. At low pressures the shape of the opening in the large valve may become partially stopped by foreign material or, if the valve is worn, the disk may shift position and thus cause erratic pressure control. With a constant header line pressure of 70 pounds per square inch, a 1-inch orifice or equivalent on a filter of 1000 square feet, filtering washed sugar liquor, permits proper pressure control throughout the cycle. The restricted by-pass fitted with a lubricated core cock is preferable, for it leaves the large valve in place for emergency use (4). The leaf types of filters should be well baffled to prevent scouring of the precoat and cake. The rotary filters must be equipped with a vented gooseneck discharge, the loop of which is higher than the filter. The plate-and-frame 6lter should be bottom-corner inlet and top- and bottom-corner discharge, the former to be used while precoating. If the cake is to be sweetened off or blown in situ, the filter should have a washing inlet. RLTER CLOTH. Cotton drills or twills of about 10-ounce weight prove very satisfactory for the stationary leaf filters. Heavier twills of 16-ounce weight, in double thickness, are recommended for plate-and-frame filters. Ordinary duck is often used, but if it is tightly woven, the shrinkage may close the mesh so tightly that the cloth becomes very resistant. Cloth cloths are not recommended for rotary leaves unless spare rotors are available and labor is comparatively cheap. Metallic cloths of Monel or stainless steel receive favorable consideration, especially for use in higher purity liquors. The corrosive character of affination sirup, concentrated sweet water, and similar low-purity liquors makes the use of metallic cloths of questionable economic advantage. Twill weave cloths of 80 X 80 with a 0.0055 wire, 80 X 70 with 0.007 wire, 70 X 70 with 0.0065 wire, and 60 X 60 with 0.011 wire are very satisfactory. Coarser weaves can be used but only under exceptional conditions of quality of weaving, careful installation, and good filter operation. The cloths must be very taut when installed, and where considerable variation in temperature exists, the tendency to form wrinkles and subsequent splitting can be minimized by installing herringbone twill woven cloth. I n stationary leaf filters, plain Dutch weaves of 20 x 120 or 24 X 110 may be used, but the cloth should be installed so the shoot wires will be vertical. FILTER PUMPS,Double-suction, closed-impeller centrifugal pumps give best performance. They should be equipped with steel shafts and runners and have water seals on the glands in which water only is used. The lower speed of about 1700 is preferred over the higher speeds. The precoating pump should deliver not less than 20 gallons per hour per square foot of filter area at a pressure of 10 pounds per square inch at the filter. The pressure liquor pump must have ample capacity to maintain filters already operating st the then existing pressures when a new a t e r is started; a general rule is to assume that the entire volume
Vol. 34, No. 4
must be filtered a t a header line pressure of, say, 65 pounds, and then increase that estimated volume by 25 per cent to take care of surges. The resultant sacrifice in electrical and hydraulic efficiency is the cost of maintaining uniform pressure control which is necessary for optimum filter operation, FILTER AID ADDITION.Filter aid should be added uniformly in proportion to the liquor volume and, as well as can be predicted, to the character and quantity of the insoluble impurities in the liquor. Unless the addition can be automatically regulated to meet changes in volume, greater uniformity is realized in the individual batch blowup system than in the continuous blowup system. ’ Slightly higher efficiency is obtained by adding filter aid dry, provided agitation is sufficient to disperse the filter aid quickly and thoroughly in the liquor: but lacking proper agitation, better results will be realized by adding the filter aid in slurry form, BLOWUP TANKS.The precoating tank should be of proper size to precoat only one individual filter a t a time and should be located as closely as possible to the filter and preferably on the floor below to permit circulation through it during precoating. The return circulating line should discharge below the liquor level. The pressure liquor tanks may be of any convenient size, but the continuous blowup should be large so that the liquor volume can serve to even off any irregularities of filter aid feed, pH, and temperature. Location of these tanks above the filters to give a gravity filtering pressure of 10 to 15 pounds per square inch, followed by the pumped supply, is advantageous in eliminating malmanipulation of control valves and erratic pressure control when starting pressure liquor cycles. The blowup tanks should be cone-bottomed and equipped with closed steam coils, the area and the pressure of which are in such relation to the agitation that danger of localized overheating is eliminated. Agitation by mechanical stirrers rotating a t 15 to 20 r. p. m. with suitable baffles to reduce swirling are to be preferred over air agitation. The latter incorporates air into the liquor which, on reaching the filters, always causes disturbance in filtrate clarity and materially reduces filter capacity. This is particularly true with lower purity liquors or liquors which tend to foam.
Filter Operation LIQUORCONDITIONING. For years the standard method of conditioning liquors has been lime and heat defecation to a certain pH which may vary for different sugars and liquors; but the individual refinery has rather definite ideas as to what the pH value should be. The alkalinity, or acidity, of a liquor directly affects the filtration characteristics, especially where localized overliming occurs. Quick and thorough dispersion of lime, good agitation, low steam pressures, and a large heating area are recommended. During recent years the possibility of using phosphoric acid and lime with filter aids has been the subject of much investigation. FormerIy any attempt to use these chemicals resulted in failure due to plugging of the filter cake, sliming of cloths, and the difficulty of retaining the phosphate precipitate in the filter cake a t medium pressures. It has been found that under proper conditions of P20cdosage, pH, temperature, reaction time, and agitation, which vary somewhat with the sugar, a precipitate can be formed which is easily retained even with very porous filter aids. During the past year two refineries have adopted and are using this method in which dosages up to 0.01 per cent P20son sugar solids are used. Twenty-five to thirty per cent of the color is removed, and a filtrate turbidity about 40 per cent less than that formerly obtained with an untreated liquor through a much less porous filter aid is obtained. In both cases marked reductions in
April, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
415
A Vallez Fil.ter Installation in a Cane Sugar Refinery
bleaching requirements materialized, and the advantages seem to justify fully the increase in cost of filter aid and chemicals. Complete economic and technical data for all types of sugars are yet to be worked out. PRECOATINQ. As an aid in keeping elements clean and economizing on filter aid, once-filtered liquor should be used for precoating in all cases except when filtering washed sugar liquor; and it should be used in that case, too, when Gltering after phosphoric acid and lime or carbon treatment. The precoat volume should be about 25 per cent greater than that necessary to fill the filter, pump, and pipe lines, and the amount of Glter aid in that volume should be 8 to 10 pounds per 100 square feet of area in the filter. Where cloth cloths are used, considerable latitude is permissible in the precoat volume and in the relationship between it and the amount of filter aid. With metallic cloths, however, this relationship becomes an important matter. The mesh openings (many times larger than the average filter aid particle), the shape of the openings, the hard smooth surfaces of the wires, and the absence of fiber ends demand a certain minimum concentration to bridge the openings successfully. Large precoat volumes-conditions of wire mesh and weave and type of filter aid remaining the same-require excessive quantities of filter aid many times out of proportion to the filter area involved, to maintain this minimum concentration. With volume constant, a coarser mesh cloth requires a higher concentration of filter aid; this is true also with a finer filter aid as compared to a coarser, mesh being the same. Speed of precoating, or rate of flow through the filter, is an important factor too, and is closely interrelated to concentration, mesh, and type of filter aid. This interrelation can be worked out satisfactorily only in the plant; no two installations are alike with respect to these factors and the details adopted to coordinate them. The filter aid for precoating should be the same type as that used in the pressure liquor, but where unfiltered liquor is used for precoating or the filters are equipped with metallic cloths, a less porous filter aid may be necessary. It is applied
at pressures up to 5 pounds and at a speed short of washing or scouring the leaves near the inlet. Cloth-covered leaves coat quickly, and some latitude in pressure and speed can be tolerated. Metallic-cloth-covered leaves are more sensitive, and those in rotating leaf filters are particularly so. Turbulence adjacent to the inlets at high liquor flows, the friction caused by leaves rotating too fast, or an insufficiently large vent on the gooseneck outlet may cause slow precoating if not prevent it. Fast rotors should be precoated while stationary and turned over only a few times at the end of the precoating period to liquidate any turbid filtrate inside the leaf assembly; speed of precoating should be reduced to a flow obtainable at 1to 1.5pounds pressure, and the gooseneck discharge should be well vented to break the siphon effect. It is virtually impossible to precoat the conventional bottom-corner discharge, plate-end-frame filter evenly over the entire filtering surface. Installation of top-corner outlets on a bottom-inlet filter will permit an approach to the same degree of uniformity attainable on the leaf type filters. Where once-used washed sugar press cake is re-used in filtering lower grade liquors, it should be employed only in the pressure liquor. Only new filter aid should ever be used for precoating any filter. FILTERING CYCLE. Changing from precoating to filtering must be accomplished with a minimum pressure fluctuation. Where there is no restricted pressure liquor feed, the operator should tend the control valve closely to prevent pressure increase so that, when the precoat liquor in the filter has been displaced with unfiltered liquor, the pressure will be the same as when precoating. From that point the pressure increase should be slow for the first 25 per cent of the cycle, after which the increase can be more rapid to the maximum for the last 15 to 20 per cent of the cycle. Owing to the character of the suspended impurities, slow pressure rise and long filter cycles under otherwise comparable conditions produce the largest proportion of well-clarified filtrate at minimum filter aid expense. Filter aid dosage in the pressure liquor depends on the kind
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 4
of filter aid, the character and amount of impurities present, cycle with the help of 5 to 10 pounds of air to hold the cake in and the relation between the filter area and the amount of place, hot water a t blowup temperature or higher is admitted sugar solids to be put through it. The ratio of filter area to to the filter a t such a rate that there is no danger of cake melt is rather fixed in most plants and varies from plant to washing. Sweetening off should begin a t the point of maxiplant, depending on the melt capacity and quality of sugar mum filtering pressure and should be reduced sharply as the customarily received when the installation went in. Those resistance of the cake falls. Too high flow rates here scour with a high ratio of filter area to melt enjoy longer filter cycles the cakes and result in large volumes of dirty sweet water. and lower filter aid consumption than those with a low ratio As to plate-and-frame filters, efficient sweetening off in situ of filter area to melt. When handling poor filtering sugars, can be approached only when the cake space is filled and the the only way to maintain capacity in either is to (a) increase filter has a washing eye. Sweet water volume can be markthe filter aid dosage to which there is a limit from a filter edly reduced by fractionating into two parts a t about 2025" Brix and using the thin fraction as the initial charge in operation standpoint, (a) use a faster flowing filter aid with a 'resultant decrease in average filtrate clarity and perhaps an the next sweetened-off filter, followed by hot water. In some increase in filter aid consumption, and ( c ) "crowd" the presses instances the filter cake sluicings are fractionated, and the with short cycles and rapid pressure rises, which result in thin slurry is used to sweeten off. higher filter aid consumption due to an increase in the number If the sugar values are not recovered from the cake in situ, of precoatings and a reduction in average clarity. A typical the cake is sluiced out and the sluicings are put through example is a refinery with a filter area ratio of 3 square feet secondary filters. The pH must be controlled and the filper ton of sugar solids; when filtering a Philippine sugar, tration expedited as much as possible to minimize losses. 4 pounds of filter aid were consumed per ton of sugar solids, CLEANINQTHE FILTER.Possibly the most important and 6-8 hour cycles were obtained, while with a Cuban sugar phase of a filter cycle is the cleaning operation to put the filter aid Consumption was increased to 7 pounds and the cloths in a fit condition for the next cycle. All that has been cycles were only 2.5 hours. I n another refinery the filter aid said regarding filter aid consumption, filter aid feed, uniform consumption was increased from 6 to 19 pounds per ton of temperatures, pressure control, etc., come to naught if pracmelt when a cargo of 92 test raw sugar came in. tices prevail which permit the filter area to become plugged Temperature should be well controlled and be as high as and nonproductive in a short time to the extent of 30 to possible to enhance fluidity. If the flow through the filter 50 per cent of the total area. Depending on type of service, drops to a point where the B. t. u. input does not overcome cloth cloths should remain soft and porous for 1.5 to 3 months and be discarded then only because of weakness. radiation losses enough to prevent a material increase in visCareful adherence to a series of simple preventive measures cosity, the production falls off a t a rapidly accelerating rate. will accomplish much in maintaining high production throughThis is very marked when filtering the lower purity, higher viscosity liquids; their flow rates can be markedly improved out the life of a set of cloths. by installing a bleeder from the shell of the filter back to Considering the stationary leaf filter: the blowup 1. Grids should and circulating be thoroughly through i t sufficleaned when redressed. Any cient liquor so fouled wires or t h a t the temblinded areas reperature of the s t r i c t flow and liquor in the filprevent proper precoating, and a ter can be mainrapid spread of tained at a point blocked area ocabout 5 O F. lower curs. than the blowup 2. Cloths must be t h o r o u g h l y temperature. c l eaned after RECOVERY OF every cycle. Full SUGARVALUES reliance must not IN CAKE. Albe placed in the mechanical sluicthough the usual ing device, but it refinery practice should be augis to re-use the mented with washed sugar vigorous h a n d hosing once a day press cake in the where long cycles affination sirup obtain and once a pressure liquor, i t shift when cycles should be a t least are short. 3. Hand hosing partially sweetA Sweetland Filter Installation in a Cane Sugar Refinery should be done ened off to faciliwith a suitably t a t e cake recurved nozzle moval and t o which will reach Instead of a round jet the nozzle of the leaves. Wherever the filter the upper areas keep the leaves in a cleaner condition. cake goes, to discard or regeneration, i t must be completely s h ~ ~ l d ~ ~ ~a spray ~ and ~ not~ a jet, ~ sweetened off to the lowest economical point. Pressure on a sluicer should be 80-90 pounds, and the water temThe sweet water from the washed sugar filters goes to the perature at least 190" F. 5. Newly covered leaves should never be mixed with old ones. melter, and as there is seldom enough for melting, the tendency is toward carelessness in the sweetening-off step which, They take more than their share of the precoat, and the older if carried over to the affination sirup filters, for instance, clo:hs off, the may Cause tangible losses. I n the case of leaf filters, after the hand hose should be used entirely for removing the cake and the unfiltered "heel" has been removed a t the end of a filtering mechanical sluicer for rinsing only.
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April, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
7. Dirt must not be allowed to accumulate inside the cloth or screen. Cloth leaves should be eguipped with valves at the bottom, and metallic cloth assemblies can be cleaned with a highpressure jet of water directed at right angles to the leaf surface. 8. If filters are to be shut down for a time, they should be left open if the atmosphere is dry; otherwise they should be closed with a quart of formaldehyde inside to kill mold growth.
Considering the rotary leaf filter: 1. The filter should be emptied of all unfiltered liquor before cake removal is be un. Leaving the filter half full of liquor, with leaves rotating a n t strong air agitation, is bad practice. 2. Screens are best cleaned after sweetenin off, partially at least, in the usual manner but preferably with ?eaves stationary. Then the a t e r is filled rapidly to the shaft or above, and, with leaves rotating, vigorous air agitation is applied for three or four revolutions of the rotor. The slurry is quickly dumped and the rinsing sprays are turned on. In case the cake has not been partially sweetened off, two or more agitations with water and air are necessary to cleanse the screens thoroughly before rinsing.
Considering the plate and frame filter: The only way to clean this filter is to open it and scrape the cake out. Assuming the filter t o be dressed with two layers of clpth, the outer cloth should be removed after each cycle for laundering; the underneath cloth becomes the outer cloth and the new or freshly laundered cloth becomes the under cloth on redressing. In this way each cloth is laundered every second cycle. The cloths should be thoroughly washed to minimize danger of mold growth. During shutdownsor after long cycles, the laundered cloths should be hung up t o dry.
Re-use of Filter Aids
DIRECT RE-USE OF PRESSCAKE. A decided economy in filter aid consumption is effected by re-using the washed sugar press cakes in the pressure liquor of a lower purity sirup filtration. It should never be used, even in part, in precoating a filter. Only new filter aid should be employed in precoating, and in the case of sirup filters only once-filtered sirup should be used as a precoating vehicle. The washed sugar filter sluicings are sometimes settled in a thickener, the thickened sludge being measured to the sirup blowups. Such a sludge is subject to variation in filter aid content, and the preferred practice is now to filter the whole or thickened sluicings through or over a dewatering filter and then repug the cake to a uniform slurry. RECOVERY OF FINAL CAKE. Regeneration of filter aid appears to be justified in the larger refineries, but just where the line of demarcation exists apparently depends upon methods of estimating costs. There is considerable difference of opinion on the subject. Regardless of this, it is a definite fact that the regenerated earth has lower efficiency, both from the flow rate and clarity standpoints, than new filter aid; indeed, the use of regenerated earth may mount to as high as 50 to 60 pounds per ton of melt as contrasted to 10 to 15 pounds of new earth. The make-up of new earth to replace losses and discards varies from 5 t o 20 per cent of the circulating earth load and is usually introduced as precoating material or in filtering specialty liquors. The final sluicings are concentrated in settlers, and the subnatant mud is dewatered through or over a filter to 5060 per cent water, from which it goes to a rotary kiln or a hearth furnace, sometimes with prior briquetting. At times the organic content of the cake is sufficient t o maintain combustion after the furnace is once started. The burned material is subsequently milled and slurried for use. Beet Sugar Factories Technological advances in the beet sugar industry during the last ten to fifteen years have been noteworthy. Improved
417
economies and a higher quality finished product have resulted from a number of process changes; a keener appreciation of the importance of high clarification has been a contributing factor of major importance. Natural unprocessed filter aids were first used in this country some twenty-five years ago, chiefly as an emergency aid when spoiled beets were encountered, to help filter the juices. Little thought was given t o the quality of filtration, and until the advent of the processed filter aids only a few factories used filter aids continuously and with an evident appreciation of the value of high clarity. When the processed filter aids first became available, they were used to overcome production difficulties and high labor and cloth cost a t the filters, and it was not until several campaigns had passed that the quality of filtrate clarity obtainable began to be appreciated as a valuable processing feature. Higher density filtration came as a matter of course and permitted the use of filter aids of still faster flow rate until today 60 per cent of the factories are filtering melter liquor at 68" corrected Brix or higher, and all factories in this country and Canada are filtering liquors with filter aid t o the white pan. The necessity of filter-aid filtration as a means of controlling thermophiles and similar organisms has played an important role in promoting this development. While the vast majority of factories use the same type of flux-calcined filter aid, conditions exist in certain localities which demand the use of slower filter aids to retain the type of turbidity common to factory or beet conditions existing there. With the increasing practice of high-density filtration, many factories can safely use the higher flow rate aids. Quantities of filter-aid dosage vary widely, depending upon type of filters, kind of filter aid, beet purity, liquor purity, liquor density relation of filter area to daily slice, Steffens or non-Steffens house, percentage of foreign molasses being worked (if any), number of products being filtered, whether carbon is being removed in a filter aid filter, and degree of perfection of filtration technique. They may vary from 0.15 to 0.20 pound per ton of beets in a non-Steffens house filtering only the thick juice and high remelt from high-purity beets to as high as 1.25 pounds per ton of beets in a Steffens factory with limited filter area filtering thick juice and melted sugar, high wash, and high green, when beets are poor and a high ratio of molasses worked to beets sliced exists.
Filter Station Equipment
FILTERS. Most factories filter thick liquors through plateand-frame filters; there are many varieties, extending from those of recent manufacture to antique European importations installed in the early years of the industry in this country. All are of the bottom-corner open-discharge type which makes them unsuited for the high type of clarification and the assurance of thermophile removal demanded today. Where the conversion can be made, an increasing number have been changed to top-corner discharge with very satisfactory results by ensuring uniform cake deposition and a minimum circulation time to establish clarity. This change makes i t possible to form a precoat satisfactorily where occasion warrants, whereas formerly it was virtually impossible to establish a precoat over the entire filtering surface. The minimum plate-and-frame filter area should be considered as 0.75 square foot per ton of beets per 24 hours, filtering thick juice and melted sugar in a non-Steffens house. This is total area available, and about two thirds to three fourths of it would be in use at one time. I n Steffens houses and those filtering a standard liquor, the available area should be about 50 per cent larger. With such filters as the rotary leaf which can be quickly cleaned, the available area can be as low as 0.5 square foot per ton of beets per 24 hours.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The filter plate drainage surfaces should be well defined, and where there is any evidence of filtrate restriction, 4-mesh screen should be superimposed over the grid and outlet channels to prevent the cloth from being pressed into them. FILTER CLOTH. A double thickness of medium-weight, good-quality duck or twill cloth should be used, preferably preshrunk. Two layers ensure against contamination if one should break, the lighter weight cloths are more easily cleaned, a double cloth provides a better gasket, and less restriction of the drainage channels results. The upper cloth should always be removed for laundering at each press cleaning, and the underneath cloth be used as the upper on the next dressing. FILTER PUMPS.The same pump characteristics should apply here as in the refinery. The common fault in most cases is that the pumps are too small and it is impossible to maintain older presses at their current filtering pressures when a new filter is cut in. FILTER AID ADDITION.Because of the lower purity types of liquors in the beet industry, even greater care must be exercised to add filter aids uniformly; any means to that end is satisfactory. Many ingenious homemade feeders have been devised; most of them are slurry feeders for quick dispersion in melter or intermediate reservoir tanks where agitation is usually insufficient for dispersions of filter aid added dry. The slurry is often added a t the suction of the filter pump, but this practice is to be discouraged unless there is sufficient head on the pump suction so that air will not be drawn in through the slurry line. BLOWUPS OR FILTER FEEDTANKS.The blowup common to the refinery is virtually nonexistent in the beet sugar factory, though an increasing number are installing surge tanks between the melters and the filter pumps to maintain more uniform liquor supply. I n the short time of the usual campaign it is difficult to train operators to maintain a uniform output of thick juice from the evaporators and a uniform supply of remelt sugar and high wash, which, without a surge tank, make for erratic filter operation. This surge tank should be equipped with suitable gages which can be readily observed by the filter operator. If filtration is made direct from the melters, great care must be exercised that all sugar grain is melted. Unmelted sugar quickly stops a filter.
Filter Operation Except in a top discharge filter, precoating PRECOATINO. is not recommended and is not needed then except when the filter is used for refiltration or where carbon is being removed. The suspended impurities in the various liquors appear to be such that when the proper amount of filter aid is used to obtain best press performance, the filter cake is of such a texture that it parts from the cloth readily. Anticipating the starting time of a filter by three quarters to one hour and putting it on circulation for that period will permit the starting of a fairly uniform cake so that clarity will be well established by the time the filter is turned over to production. FILTERING CYCLE. The lower purity of beet liquors requires even stricter attention to controlled conditions than in the re-
Vol. 34, No. 4
finery. All of the following points are important in obtaining the optimum performance of the filter: 1. Complete elimination of air in the liquor. The production and life of a filter have been increased tenfold by this factor alone. 2. Uniform supply of liquor, with as uniform composition and density as possible. 3. Uniform and high temperature, not less than 195' F. at the filter. 4. Pressure increase as uniform as possible. Filtration is started at low pressures with the help of a restricted orifice if necessary. A filter should not be stopped and started after it is once started. In case of a slowdown, the pressures must be kept up on the older presses and the newest filter controlled to meet the flow demand; 5. A sufficient flow through the filter (not less than 5 gallons per hour per square foot at 195" F.) to maintain a good atering temperature in the filter. Heat is conserved by covering the filter with old cloths and keeping near-by windows closed. 6 . General cleanliness of filter, cloths, troughs, etc. Good filtration is useless, so far as sugar quality is concerned, if subsequent handling permits contamination. 7. Uniform filter aid addition in proportion t o liquor flow.
REFILTRATIOX.The refiltration of pan supply liquor is advantageous as a trap to remove any traces of contamination after the earlier filtration. A precoated filter of the top discharge type is best, and a precoat only is required. The filter cycle may run as long as a week, depending upon conditions. CAKEDISPOSAL. Little or no attempt is made to remove sugar values from the filter cake. Sometimes air or steam is used to partially dry an otherwise sloppy cake for easier handling. When the filtration is properly carried out, full firm cakes filling one-inch frames result which are easily removed. The cake is usually returned to one of the carbonation steps a t the head of the process or t o the Steffenshouse.
Literature Cited (1) Cummins, A. B., IKD. ENQ.CHEM.,34, 398 (1942). (2) Cummins, A. B., and Badollet, M. S., IWD. ENQ.CHEM., AN~L ED., 5 , 328 (1933). (3) Cummins, A. B., and Weymouth, L. E., IND.ENG.CHEM.,34, 392 (1942). (4) Elliott, R. D., U. S. Patent 1,978,254 (Oct. 23, 1934).
