FILTRATION OF SUGAR SOLUTIONS Some Factors Determined by Laboratory Test Procedures A. B. CUMMINS AND L. E. WEYMOUTH Johns-Manville Research Laboratories, Manville, N. J. papers. Operation of this equipment has been made autoHE fundamental factors in a filtration are generally conmatic in so far as possible, and provision is made for the sidered t o be pressure, time, filter area, viscosity, and the accurate control of all variables influencing filtration. Clarity character of the solids to be filtered (usually expressed as measurements are made on filtrate cuts collected a t definite empirical constants which must be determined experimentime intervals. The clarity measuring apparatus is a spetally). For practical applications these factors become: cially constructed photometric Tyndallmeter of high sensitivity method of applying pressure, length of cycle, temperature, and accuracy, as required for the examination of highly density of the liquid, filter characteristics, cake formation, clarified liquid media. This apparatus, the early developtype and amount of filter aid, filterability, which includes the ment of which was reported nature and amount of solid by Cummins and Badollet ( I ) , material in the svstem. etc. The present discussion is in Laboratory and procedures emMa's described by Cummins and Miller (2). -4 brief dethe nature Of an introduction ployed in the experimental filtration of scription of the instrument and to a series of papers which will sugar are described* The fourits use is given below. consider each of the above factors experimentally (see unit bomb filter press assembly used for pages 398 and 403). It demany types of filtration work is described in Filtration Test Unit scribes the apparatus and gendetail, and the general filtration test proprocedures in cedures are outlined. The clarity appaThe filter unit illustrated in carrying out the experimental Figure 1 consists of four comwork and presents preliminary ralus used in quantitative measurement of plete bomb presses immersed data t o show some of the filtrate clarities and method of Use are in a common thermostatically general relations betxeen fildescribed. A series of sugar filtration tests controlled oil bath and conis presented to show some of the relations nected t o a common air prestration rate and the degree of clarification of sugar liquors. between filtration rate and clarity in sugar sure Figure shows The sugar refiner's primary the assembly during progress of interest in filtration is in the filtration using the equipment and procethe tests. Each of the bombs is . . clarification of solutions for dures described. provided with both mechanical subsequent decolorization so and air agitation and with a that he may obtain the desired separate reserve liquor tank. throughput of sugar solutions of the necessary quality a t the Pressure can be maintained either automatically by means lowest cost. While many related factors are involved, it is of a time-pressure regulator or manually by means of a handalmost universally recognized that the degree of clarification operated pressure-reducing valve. Each bomb is provided of the sugar liquor going to the char filters is of primary imwith a precoat chamber for applying a uniform filter aid preportance. Refinery practice in North America is largely coat under pressure a t the start of a run. Parts coming in based on the use of diatomaceous silica filter aids t o secure contact with the liquid being filtered are of brass, bronze, the necessary clarification. The manufacture and standcopper, or Monel construction. ardization of suitable filter aids for sugar refining has been Individual bombs (Figure 3) are of cast brass construction, pioneered by American interests, and for thirty years 5 inches in inside diameter and 11 inches deep, with diameter the Celite Company and the Johns-Manville Corporation narrowing t o 3 inches a t the bottom to reduce dead space have supplied filter aids for world-wide consumption from around the filter leaf. Air agitation is provided by a 3/ls-inchdiameter copper tube perforated in the bottom portion except the extensive deposit of high-quality diatomite near Lompoc, for the part directly under the filter leaf. T'olume of air Santa Barbara County, Calif. Progressive development in the improvement of filter aids and their more economic apemployed for agitation is observed by a flowmeter connected plication has been continuous for all of this period, a developto the air outlet. Mechanical agitation is by means of two three-bladed 1.25-inch-diameter bronze propellers on a shaft ment which has been made possible by the close cooperadriven through a bevel gear box inside of the bomb a t the top, tion of the sugar industry. I n the laboratory treatment of sugar liquors the two priand including a Monel universal joint on the shaft just above mary considerations are volume of filtrate and degree of the propellers. Propeller speed can be adjusted t o either 200, 300, or 450 r. p. m. by stepped pulleys on a drive, which is clarification, generally referred t o as rate of flow and clarity, respectively. Rate of flow without the measurement of from a common shaft a t the rear of the unit and is driven by a clarity is of little significance. Test methods for both flow 1/2-horsepower motor. Drive shafts to the bombs, each provided with a clutch, connect to spiral gear boxes on the rate and clarity have been developed, modified, and improved common shaft. Filter leaf generally used is of brass and is 1.5 in the Johns-Manville laboratories over a period of many inches in diameter. When no precoat is employed, a threaded years. This paper describes a test assembly which has proved applicable to many types of filtration work and was employed filter plate cap is used, or inch deep, depending on anticipated filter cake thickness. Filter leaf head of bronze for the series of studies to be described in this and subsequent
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INDUSTRIAL AND ENGINEERING CHEMISTRY AIR AGITATION CONTROL VALVES
\
COMPRESSED AIR FROM HAND REGULATOR OR FOXBORO PRESSURE STABILOG
RESERVE LIQUOR TANK
\
PRESSURE Y U G E
f
/ EATING COIL
393
-
U
BOMB +DRAIN
,&
RESERVE' LIQUOR PREHEATER
FIQITRE 1. FILTRATION TESTASSEMBLY construction is-held in position by four a/&wh steel nuts on steel studs. Removal of leaf from the bomb requires only removal of these four nuts. Tongue and groove construction a t the gasket connection between the filter leaf head and the bomb cover flange enables a tight joint to be easily made with the gasket. The bomb cover flange can be taken off by removing the eight S/8-inch steel nuts securing it. The 3.5inch-diameter opening in the cover flange, however, is sufficient to permit adjustments inside the bomb and cleaning and inspection of the interior without removing the cover flange. The maximum working capacity of the bomb is 0.687 gallon (2600 cc.), which allows a filtrate volume of 0.555 gallon (2100 cc.) * The precoat chamber, of 0.02-gallon (75-cc.) capacity, screws onto the filter leaf and replaces the regular filter leaf cap when operating with precoat. The precoat filter aid is deposited on the filter cloth from suspension in a suitable liquor in the precoat chamber by incoming main batch liquor at the start of the test. When the precoat filter aid has been completely deposited on the cloth, the precoat chamber is raised by pulling up the precoat rod through the stuffing box in the filter leaf head. The oil bath temperature is maintained by a steam coil in the bottom of the bath. The bath is agitated by a horizontal shaft with two propellers, driven by a 1/12-horsepower motor at one end of the bath. Temperature control is obtained with a Powers No. 14 regulator, which will accurately maintain any temperature within the limits 127" to 216OF. (53" to 102" C.). Automatic pressure control is obtained with a Foxboro 10inch, Model 10, time-pressure Stabilog, range 0-70 pounds pressure, with a 2-hour clock-driving control cam. Interchangeable cams of soft sheet aluminum can be cut for any desired pressure cycle, and a single cam can be cut for a gradual pressure increase during a 4-hour period or longer. Optional manual pressure control is provided by an Oxweld regulator. By closing a valve in the pressure manifold between bombs 2 and 3, i t is possible to operate bombs l and 2 with automatic pressure control while running bombs 3 and 4 simultaneously on a different pressure cycle with hand control. Each bomb filter is provided with a reserve tank of 0.687gallon (2600-cc.) capacity for supplying additional test liquor under pressure to the bomb when needed during a run. This tank is used when the volume of the test liquor initially filling the bomb is not sufficient for the duration of the run or when it is desired t o keep the test liquor hot for as short a time as possible before it is filtered. A gage glass on each
reserve tank permits observation of the amount of reserve tank liquor fed t o the bomb. Each reserve tank is provided with air agitation.
Filtration Test Procedure A suitable quantity of raw sugar or washed raw sugar is accurately weighed out in a pan, and the calculated amount d filtered water is measured or weighed out and added. Raw sugar tests have generally been run a t 60" Brix and washed raw commonly a t 65" Brix. I n runs with lime addition, somewhat less than the total amount of water required is initially added in order to allow for water later added as milk of lime suspension. The mixture is heated with agitation to approximately 158" F. (70" C.), the milk of lime suspension is added to bring the liquor to the desired pH, and water is added if necessary to adjust the liquor to the density desired. The liquor is then split up into as many separate batches as required by weighing into smaller pans. A previously weighed portion of filter aid is then added t o each and thoroughly mixed in, and the batches are heated t o the desired filtering temperature, most commonly 176" F. (SO" C.). The batches of liquor are transferred to the bombs by being poured through a 30-mesh screen to remove bag fibers, and agitation is started. While the sugar batches are being prepared, filter cloths are placed on each bomb filter leaf, and the precoaters are placed in position when they are to be used. The calculated amount of precoat filter aid is suspended in 0.02 gallon (75 cc.) of precoat liquor (clear liquor is generally used for this purpose), and the suspensions poured into the precoat chamber through a funnel. The filter leaves are then placed in position and filter leaf heads tightened in place. When precoaters are used, the precoat filter aid suspension is agitated just before the start of a test by raising and lowering the precoat rods. The plate a t the bottom of the rod and in the precoat chamber serves to mix the contents thoroughly. Pressure is then applied and the test started. An amount of filtrate corresponding t o the precoater volume is withdrawn before the start of the regular cycle. Tests are generally run on a definite pressure-increase schedule which is controlled either automatically or by hand. Filtrate volumes are read at definite time intervals, and filtrate fractions are removed at suitable times for clarity measurement. Ordinarily clarity fractions are collected during equal time intervals for the entire test although in a short 30minute test a single 15-30 minute filtrate fraction may suffice.
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clarified liquids are centrifuged first to remove all air bubbles. The cell is then placed in position on its mount in the apparatus and the primary light turned on. The superimposed fields of the Tyndall beam and the comparison beam in the photometer cube are viewed through t h e telescope. Photometric match of the two fields is obtained by movement of the calibrated neutral wedge, which varies the intensity of the comparison beam. The reading a t the point of photometric match is read from the vernier scale of the wedge. The position of the Tyndall beam in the liquid examined is fixed by settings of two mechanical stage movements which permit the test FIGURE 2. COMPLETEFILTRATION TESTASSEYBLYIN OPERATION cell to be moved in the direction of the incident light and also a t right angles to it. After centrifuging and cooling where necessary, the filtrate In this way one reading a t standard settings can be made, or a samples are ready for clarity measurement. series of readings a t different positions of the Tyndall beam can be quickly and accurately taken. A series of readings made of Clarity Measurement the incident beam and the emergent beam through different depths of liquids permits an extrapolated reading to be obThe amicroscopic turbidity of filtered liquids is due to the tained graphically which corresponds to a reading with zero presence of minute particles too small t o be visible by transdepth of liquid for both incident and emergent beam. By this mitted light. Such particles, however, have the capacity of method a value is obtained for the intensity of the Tyndall beam scattering light, which may be observed by examining the which may be considered to be independent of the color of the liquid a t an angle t o the path of a strong beam of light passing liquid. through the liquid. This is the well-known Tyndall pheScope of Tests nomenon. All colloidal systems possess this property of scattering light, and it has been recognized that the intensity of Raw sugar varies widely in color, purity, and filterability. This results in a considerable difference in filter aid and char the scattered light depends upon the concentration of the house costs as well as in the yield and quality of refined sugar. particles in the liquid as well as upon the size of the particles One of the best indices to the filterability or refinability of a and other properties of the liquid or dispersion medium. A sugar is the amount of filter aid required to obtain standard measurement of the intensity of the Tyndall beam, therefore, provides a means of estimating the relative amount of colrates of flow and a satisfactory clarity. A considerable amount of data has been published on the differences in raw loidal impurities in a liquid, or conversely its degree of sugars as shown by different rates of flow obtained with clarification or “clarity”. For well filtered sugar liquors the filter aids, but little information has been given on the clarity intensity of the Tyndall beam is relatively low, and a satisfactor. I n the work described here, clarity mas measured in factory apparatus must be capable of refined measurements all tests. Clarity values are expressed as foot-candles, the with light of such low intensity. The color of most sugar intensity of the Tyndall beam produced in the liquid under liquors also complicates the problem. With the clarity appathe test conditions. An optically void or absolutely clean ratus used, a sharply focused beam of high-intensity light is passed into the liquid under examination. The Tyndall water reads approximately zero. The purest sucrose solutions, however, give a measurable, low-intensity Tyndall beam thus produced in the liquid is compared photometrically beam with the Tyndallmeter described. The higher the footwith a light, the intensity of which can be varied and measured candle reading, the poorer the clarity or the higher the turquantitatively; a method is thus provided for expressing the bidity. Unfiltered raw sugars usually read more than 100 intensity of the Tyndall beam in the liquid in light units, or foot-candles. Filtered raw sugar liquors range from 5 to 30 in other units if desired. or more foot-candles. Filtered washed raw sugar ranges from The Tyndallmeter (Figure 4) consists essentially of a 2 to about 15 foot-candles, depending on the sugar, the type of primary light source, an incident beam system, the mounting filtration, filter aid used, and period of cycle when sample was and cell for the test liquid, the emergent Tyndall beam systaken. tem, the comparison beam system, the photometer unit, the Tests were carried out with several different raw sugars and telescopic observation system, appropriate light filters, an with one washed raw sugar. All filtrations were made with optical bench for mounting, and suitable housing for the the four-bomb filter assembly. Technique of operation was various parts. varied as described under the discussions of each series. The test liquid is placed in an observation cell which is conI n series I (Table I) the four raw sugars or mixtures of structed with plane optical glass sides cemented to form a them show a wide difference in flow rate for a given filter aid, cube of 0.65-inch (16.5-mm.) inside dimensions. Highly
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Raw sugar tests were at 60' Brix without lime addition; washed raw tests were at 65' Brix, The pH was 7.2. The clarities obtained for each of the five sugars tested with the slower filtering and finer particle size filter aids are superior to those for the faster flow rate or coarser particle size filter aids (Table I). The differences in clarities for two filter aids on different sugars, however, are not constant since the turbidities of some sugars are more readily removed than those from sugars of different characteristics. Thus sugar D shows relatively small clarity differences with different filter aids, whereas with sugar C the clarity differences are wide. The ratios between different filter aids are also quite different for both flow rate and clarity. Sugar D with 0.70 per cent mter aid, for instance, has a relatively good flow with FilterCel but an unusually low relative flow with the faster flow rate filter aids from Hyflo to Celite 535. With sugar C and 0.45 per cent filter aid, however, the flow with Filter-Cel is low, whereas the rate with Hflo and Celites 503 and 535 is rapid. These differences in the performances of filter aids are due to inherent differences in the colloid contents of the sugars.
U FILTRATE OUTLET
FILTER
HEAD
TABLEI. FILTRATIONS OF DIFFERENT TESTSUGARSWITH FILTERAIDSOF DIFFERENT CHARACTERISTICB
PRECOATER PERFORATED AIR INLET
FIGURE 3. INDIVIDUAL BOMBFILTER,SHOWINQ FILTER
LEAF, PRECOAT ATTACHMENT, AND MECHANICAL AND AIR AGITATORS
a n d varying amounts of filter aid are employed in order to bring the rates of flow somewhere near standard figures. Still more striking are the clarity differences observed. Both the flow rates and clarities are inherent characteristics of the individual sugars and are dependent upon the amounts and kinds of impurities in them. Five different filter aids were included in this study. They cover the range from the fineparticle-size Filter-Cel of extreme clarifying capacity to the coarse-particle-size Celite 535 which ordinarily does not provide satisfactory clarity for sugar liquors. Intermediateparticle-size filter aids cover the range of most interest to the sugar industry. Series I1 (Table 11) shows the effect of different percentages of filter aid as tested with three different raw sugars and four filter aids. Series I11 (Table 111) is a more detailed study of limed, washed, raw sugar filtration with two different percentages of three filter aids. I n this series the tests were for 4-hour periods, and the pressure cycles were made by gradual increases from the precoat pressure of 4 pounds up to the maximum of 50 pounds per square inch a t 3 hours. The procedure for series I11 corresponds to more or less conventional refinery practice, whereas the methods for series I and I1 are more commonly employed for filter aid testing. Series IV (Tables IV and V) includes tests made with two widely different raw sugars. Varying amounts of the different filter aids were employed. While this study is more applicable to filter aids than to refinery practice, the results indicate how a similar investigation on refinery liquors can be made useful for establishing a satisfactory filter practice for different sugars.
Discussion of Series I Bomb press tests were made a t 176' F. with pressure increase by steps to 40 pounds per square inch maximum.
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Standard Filter-Cel Super-Cel Sugar A 0 6% filter aid Filtraiioh rate5 Clarityb Su ar B 0 55% filter aid %iltrsLiohrate Clarity Su ar C 0 45% filter aid Biltr.aiioh rate Clarity Su ar D, 0.70% filter aid b r a t i o n rate Clarity Washed raw sugar, 0.20% filter, aid Filtration rate Clarity 0
b
Hyflo
Celite 503
Celite 535
5.1 10.8
10.0 12.4
32.7 24.2
56.5 40.2
95.5 61.8
6.3 11.6
12.6 13.5
35.5 19.7
64.2 31.9
102.2 40.2
4.1 10.0
8.8 14.3
42.6 31.6
79.6 44.1
114.7 49.4
6.1 8.6
11.3 9.4
22.5 11.3
35.9 14.8
49.8 29.8
7.8 3.0
15.1 3.4
37.1 4.9
56.0 7.6
104.0 8.9
In gal./sq. ft./hr. on a 0-30 minute period In ft.-oandles on a 15-30 minute period.
The results of the tests with 0.20 per cent filter aid with the limed washed raw sugar are of particular interest. Ratios for flow rates and clarities are as follows: Ratio Flow rate Clarity
Standard Filter-Ce1 Super-Cel 1.00 1.93 100 88
Hyflo 4.76 62
Celite 603 Celite 535 7.18 13.35 40 34
It should be noted that the clarities for the filtered washed raw sugar are of a different order of magnitude from those for the raw sugar liquors. Discussion of Series I1 The four-unit bomb press was employed with mechanical agitation, 60' Brix liquor was used, and the temperature was 176' F. Filter cycles were for 30 minutes with pressure increases by increments up to a maximum of 40 pounds per square inch. Flow rates are for the entire 30-minute cycle. Clarity measurements are on the 15-30 minute filtrate; this provides a better index of clarification than the filtrate for the entire cycle which would contain cloudy liquor from the start of the test. The three sugars all show different ratios in flow rates between Hyflo and Filter-Cel; the greatest variation was between sugar E with a ratio of 5.85 and sugar G with 3.47. The clarities of filtrates from both of these sugars are approximately the same with a given filter aid but differ widely
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with different filter aids. For sugar E the rates of flow with Hyflo and Celite 503 were considerably higher than with sugar G.
the 4-hour period Hyflo gave 46.1 gallons per square foot of clarified filtrate collected during the last 3"/4 hours of the test. This filtrate averaged 5.0 foot-candles in clarity, which is not equal t o the Standard Super-Cel clarity average. In the Hyflo run it was not until after 2l/2 hours that the clarity was TABLE 11. FILTRATIONS OF DIFFERENT SUGARS WITH DIFFERENT equal t o that of the 10-15 minute period for Standard SuperTYPESA N D PERCENTAGES OF FILTER AID Cel. With Celite 503, 60.2 gallons of clarified filtrate were --Sugar E--Sugar F--Sugar Gcollected during the last 3l/2 hours of the run. Clarity of yo Filter Aid 0.38 0.50 0.38 0.50 0.38 0.50 Celite 503 averaged 6.0 foot-candles for this period. 'CVhile Filter-Cel the final clarity obtained with Celite 503 in this run was relaFiltration rate= 4.7 6.1 5.7 6.7 4.2 5.4 Clarityb 7.4 7.8 7.8 7.7 7.2 7.5 tively good, the clarities for equal time intervals were defiStandard Super-Cel Filtration rate 8.9 11.3 9.9 12.4 8.0 9.7 nitely lower than for the corresponding Hyflo clarities. Claritv 8.5 9.0 9.0 9.0 8.4 8.8 Hyflo Considering the results with only 0.125 per cent filter aid Filtration rate 27.5 34.0 24.1 29.9 14.6 20.1 (Table 111), the rates of flow and total filtrates collected were Clarity 14.1 15.3 12.3 12.1 13.3 14.1 Celite 503 lower than for the larger amount of filter aid, but the same Filtration late ... 70.8 ... 62.1 22.5 31.7 Clarity ., . 27.8 ... 21.8 24.5 23.7 general conclusions as to clarity-flow rate relations are Flow rate ratio, apparent. I n these tests Standard Super-Cel gave 12.82 €Iyflo/Filter-Cel 5.9 5.6 4.2 4.5 3.5 3.7 gallons per square foot of filtrate and a clarity of 3.84 foot0-30 minute period. a I n gal /sq. ft./hr. on 6 I n ft.-candles on a 16-30 minute period. candles; Hyfio gave 22.90 gallons and 4.95 foot-candles, and Celite 503 gave 36.53 gallons and 6.75 foot-candles. For the last half hour of the 4-hour run Hyflo clarity (4.0 foot-candles) was about the same as that of Standard Super-Cel for the 25An important point to be brought out for this series, as for 30 minute period. With Celite 503 the final half-hour clarity series I, is that for a given sugar the clarity obtained with the was 4.3, about the same as that of Standard Super-Cel a t 20 faster flow rate filter aid is always less than that for the lower minutes. In these tests the smaller percentage of filter aid rate filter aid. affords a more severe and critical test for clarity than the 0.25 per cent filter aid. Discussion of Series 111 ~
I n these tests 65" Brix washed raw sugar solutions, limed to Discussion of Series IV DH 7.2. were filtered with the four-unit bomb filter assembly This series of tests was made with two raw sugars of widely Lnder Che following conditions: mechanical agitation, temdifferent characteristics. It was carried out in the bomb perature 176'F., precoat applied from pure sucrose a t 4 pounds per square inch pressure in an amount corresponding to 6 press for 60-minute periods with a gradual pressure increase from 5 to 50 pounds per square inch maximum a t 45 minutes. pounds per 100 square feet of filtering area. The pressure was increased from an initial 4 pounds a t a regular rate with A 60" Brix sugar was used a t 176' F. with no precoat and the Foxboro time-pressure regulator to a maximum of 50 no lime addition. Three different filter aids were employed pounds per square inch a t the end of 3 hours; it was mainwith percentages ranging from amounts too low for the most effectiveresults to amounts in excess of the economic optima. tained a t 50 pounds for the last hour. Filtrates were colFiltration rates are calculated for each 15-minute period. lected a t different time intervals for clarity tests. Standard Super-Cel, Hyflo, and Celite 503 mere employed in Clarity readings were made on filtrates collected during corresponding periods. amounts of 0.25 and 0.125 per cent, Results are given in The results are not submitted as directly applicable for reTable 111. Rates of flow as given for each time interval are the average fining practice, since the test conditions were selected for laboratory test purposes. The results, however, are believed for that period. The volumes of filtrates, however, are totals in gallons corresponding to each square foot of filter area for to indicate principles and t o be of value in pointing out some of the relations between variables such as type of sugar, kind the total time up to that of collecting the sample. Clarities and amount of filter aid, and rate of flow and degree of are for each interval for which flow rates were obtained. I n clarification obtained. I n particular, it is necessary to concalculating final flow rates, consideration was given to clarity; an attempt was made to count only the filtrate which could be sider the clarity of the liquors during all portions of the filtration cycles, and t o note the rates of flow and clarities obassumed to be of satisfactory brilliance as shown by the clarity tained during the latter part of the cycles (45-60 minute measurements. Thus for Standard Super-Cel all filtrates were considered clear. With Hyflo and Celite 503, hornperiods). ever, the liquors were not considered adequately clear until after 15 minutes for Hyflo and 30 minutes for Celite 503, a t which times the clarities were about 8.0 foot-candles. Relative flow rates may be calculated on the basis of total filtrates collected after reaching this arbitrary clarity value, even though it is not the equal of Standard SuperGel clarity. Standard Super-Cel gave a total of 19.27 gallons per square foot of filtrate for the 4-hour run. This filtrate had an average clarity of 3.93 foot-candles, with 4.83 for the first 5 minutes of the run FOR CLARITY MEASUREMENTS (WITHHOUSIXG REMOVED FIGURE 4. TYNDALLMETER TO SHOWOPTICALPARTS) and 3.34 for the final half hour. For
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Hyflo and Celite 503 are definitely inferior to that obtained with Standard SuperPressure a t End of -Standard Super-CelHyffo-Celite 503Cel, a still more striking Time Interval Flow Total Flow Total Flow Total rates filtrateb Clarityc rate filtrate Clarity rate filtrate Clarity factor brought out by the Min.' Lb./Sq. i n . 0.25 Per Cent Filter Aid tests is the low clarities 18.84 ... 16.75 found for the first 15-minute 10.54 ... 4.83 17.03 5.67 ... 5.6 0-5 17.80 ... 14.43 8.59 4.00 15.50 . . . 4.51 6.96 8.7 10-15 periods of the filtrations with 19.86 9.45 11.46 6.67 7.97 4.11 15.50 3.05 6.19 12.2 25-30 20.40 19.45 8.11 5.30 15.10 15.80 3.88 6.11 5.93 22.7 50-60 Hyflo and Celite 503. Here 21.32 30.11 7.31 4.89 22.68 3.79 13.75 9.05 5.89 32.0 60-90 4.89 13.12 29.25 19.02 39.62 5.81 clarities of 20.61 and 30.48 3.66 5.50 11.80 41.4 90-120 5.00 19.80 49.52 4.61 11.10 34.80 3.54 5.07 14.35 50.7 120-150 14.93 57.00 4.51 foot-candles, r e s p e c t i v e l y , 4.41 3.46 11.40 40.50 3.74 16.20 60.0 150-180 4.26 13.64 63.82 4.36 3.42 10.32 45.65 17.87 3.31 60.0 180-2 10 were obtained. ,Even with 4.06 11.65 69.65 4.31 8.90 50.10 3.34 2.84 19.27 60.0 210-240 17.43 ... 7.08 greatly increased amounts of 5.36 12.53 3.93 ... 4.82 ... 0-240 ... 46:io 5.00 ... ... 15-240 ... l7:ZO 60:ZO 6.01 filter aid (up to 0.8 per cent), ... ... ... 12.30 ... .. .. .. ... 30-240 go o d i n i t i a 1 clarifications 0.125 Per Cent Filter Aid could not be obtained with 28.35 13.30 0-5 5.6 6.70 ... 5.36 15.75 ... 10.21 the faster flow rate filter i:i7 2.30 ... ... ... ... 5-10 10:69 7:is 19:62 4.61 ii:Ss 5.41 ... 8.7 10-15 aids. In practice this means 8.89 16.26 10:36 i:+2 5.94 9.03 4.06 4.64 2.69 13.3 25-30 9.75 5.06 7.48 13.17 17.67 6.98 that prolonged periods of re3.88 3.87 4.75 22.7 50-60 6.59 11.40 20.34 13.00 4.94 6.54 3.88 3.48 6.49 32.0 60-90 5.81 circulation would be neces8.72 27.75 15.92 5.18 5.85 3.79 3.10 8.04 41.4 90-120 5.48 7.61 31.55 18.64 4.67 5.41 3.71 2.80 9.44 50.7 120-150 sary to secure satisfactory 7.22 35.15 5.18 21.15 4.46 5.07 3.58 2.67 10.77 60.0 150-180 4.72 6.10 38.20 4.26 4.30 23.32 clarification. Under some 3.50 2.24 11.90 60.0 180-210 5.03 40.70 4.31 4.02 3.74 25.20 3.42 1.85 12.82 60.0 210-240 10.18 7.15 conditions this is a fully 5.36 6.30 3.84 .. 3.21 ... 0-240 9.53 36:53 6.75 4.95 5.97 22:90 ... ... .. ... 10-240 warranted orocedure. Sugar I (Table V ) is both a Gal./aq. ft./hr. b Gal./sq. ft. C Ft.-candles. slow to filter and hard to clarify. Considerably higher amounts of filter aid are required with this sugar than with sugar H, since the total Tables IV and V show the applicability of the filtration colloid content and amount of suspended solids in I are higher apparatus and Tyndallmeter for the investigation of filter than in H. problems. Broad generalizations on filter aid filtrations With sugar I, 0.60 per cent filter aid gives rates of flow should not be made from these limited data. Most filtration which correspond roughly to those obtained with 0.20 per cent problems are more or less specific for certain prescribed condifilter aid with sugar H . Similarly, 1.0 per cent filter aid tions, and when this is the case carefully controlled laboratory with the difficult-filtering sugar was required to give roughly scale tests may frequently serve as the guide for satisthe same filter rate as 0.3 per cent with the easy-filtering sugar. factory and economical commercial practice. The main purWith sugar H, 0.13 per cent filter aid was inadequate to give pose of this paper is to describe apparatus and methods which suitable clarification, whereas with amounts from 0.2 to 0.8 have proved to be useful for this purpose, rather than to preper cent the final clarifications may be considered satisfactory. sent data which can be employed generally for all conditions. However, increased amounts of filter aid did yield satisfactory Sugar H was easy to clarify, and high flow rates with relaclarities more quickly. tively low percentages of filter aid were obtained. Thus 0.20 With sugar I, 0.4 per cent filter aid gave satisfactory final per cent Standard Super-Cel gave an average flow of 5.68 galclarities, but those for the earlier parts of the cycle were poor. lons per square foot per hour for a one-hour cycle and a final For all three of the filter aids 0.60 per cent gave the best final clarity of 7.48 foot-candles. The flow rate and clarity a t the clarities, but increased amounts gave appreciably better start of the cycle were not so satisfactory, however, and 0.30 clarities during the earlier portions of the cycles. When the or 0.25 per cent filter aid is indicated as a better test condition larger amounts of filter aid were used with this sugar, subfor either the comparison of this sugar with other sugars or stantially increased rates of flow were obtained, which refor comparative tests between different filter aids. sulted in the early formation of thicker filter cakes. Quicker With 0.3 per cent Standard Super-Cel, Hyflo, and Celite clarifications were the results of these conditions. As the 503, rates of flow corresponding to 8.41, 16.23, and 25.48 filtrations progressed, the filter cakes formed with the larger gallons, respectively, were obtained. Final clarities were amounts of filter aid were thicker and also more porous or 7.48, 10.09, and 12.57 foot-candles. While the clarities of FILTRATIONS OF WASHED RAWSUGAR WITH THREE FILTER AIDS TABLE111. LONG-CYCLE
--
OF EASY-FILTERING RAWSUGAR H WITH DIFFERENT TYPESAND PERCENTAGES OF FILTER AID TABLEIV. FILTRATION c
Yo Standard Super-Gel-
---.0.13
Time, Min.
0.13
0.20
0-16 15-80 30-45 45-60 0-60
1.20 2.67 5.93 3.78 3.57
4.04 9.11 10.66 14.45 16.68 8.60 10.66 11.45 13.25 13.76 6.19 7.91 9.28 11.44 12.30 3.87 5.76 7.22 9.46 10.15 5.68 8.41 9.65 12.15 13.22
0-15 15-30 30-45 45-60
0.30
0.40
0:60
0.80
0.40
0.60
Yo Celite 0.30
503 0.40
0.60
0.80
23 87 23:22 17.72 11.70 18.88
31.82 29.24 23.65 17.20 25.48
40 42 33:97 27.52 20.21 30.53
45.58 38.70 33.11 25.80 35.80
50 40 44:55 45.58 43.00 45.88
38.37 25.94 19.23 15.63
30.48 17.54 13.46 12.57
26.55 14.43 12.72 11.86
23.12 13.61 12.28 11.86
22.59 14.59 13.61 13.61
7
0.13
0.20
per Square Foot per Hour 22.80 25.80 30.10 33.71 17.63 20.47 24.47 27.00 14.10 17.03 21.84 24.08 10.40 13.07 17.89 19.61 16.23 19.09 23.65 26.01
15 91 17:54 15.48 9.46 14.59
Clarity, Foot-Candles 43.05 28.44 20.61 18.79 17.54 17.14 26.85 14.93 11.59 11.07 11.07 11.59 20.61 11.32 10.81 10.33 10.33 11.07 15.63 10.81 10.09 9.75 9.98 10.69
51.76 39.26 31.92 25.35
Gallons 15.57 13.77 9.97 6.80 11.52
0.30
.
0.80
Filtration Rate, 5.25 9.97 8.60 4.99 7.20
46.13 25.35 16.94 14.26 13.00 12.13 29.79 12.72 9.64 8.79 8.40 8.59 7.66 8.20 19.45 8.40 8.02 7.83 7.48 7.48 7.40 7.83 11.32 7.48
% HyAo 0.20
398
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 4
TABLEV. FILTRATION OF DIFFICULT-FILTERING RAWSUGAR I WITH DIFFEREKT TYPES AND PERCENTAGES OF FILTER AID Standard Super-Cel0.60 0.80 1.00
c
7 %
Time, hfin.
0.40
1.40
0.40
0.60
% Hyflo 0.80
1.00
1.40
0.40
0.60
21.24 18.49 15.48 11.87 16.77
21.93 20.21 18 06 15.65 18.96
10.15 18 92 12.90 8.17 12.53
28.44 17.95 15.63 15.28
26.55 18.37 16.37 16.37
63.68 33.42 22.08 15.45
% Celite 503 0.80
1.00
1.40
22.79 25.46 17 46 11.44 19.28
28.81 27 95 21.24 15.48 23.37
31.39 32.94 26 40 19.95 27.67
32.42 34 66 34.40 30.96 33.11
51.76 26.55 17.54 13.61
44.06 20.61 17.54 15.10
43.05 23.66 19.23 16.94
38.37 24.21 20.14 18.79
Filtration Rate, Gallons per Square Foot per Hour 15-30 30-45 45-60
0-60
0.43 2.32 6.71 4.30 3.44
6.28 8.00 6.02 4.30 6.15
6.97 8.86 8.43 5.50 7.14
10.05 9.63 8.43 6.71 8.71
10.75 10.49 9.29 7.74 9.57
0-15 15-30 30-45 45-60
54.20 31.19 18.37 11.59
25.35 13.30 11.32 10.54
22.59 13.30 11.32 11.07
20.61 13.30 11.59 11.59
18.37 13.30 12.13 11.86
0-15
6.88
8 43
6.54 4.56 6.60
14.79 15.48 11.61 8.60 11.97
16.60 16.51 13.33 9.98 14.10
Clarity, Foot-Candles 43.05 18.37 13.94 12.13
open, because of their higher ratios of diatomaceous particles to entrapped gelatinous impurities. The results were increased total filtrates but somewhat lower clarities than for some lower percentage of filter aid (0.6 per cent), at which point the filter cakes had maximum clarifying capacity a t the sacrifice of some rate of flow. This condition shows the advantage to be gained by carrying out for different sugars tests in which different filter aids are tried in varying percentages. An indication may be obtained for operating conditions such as (a) optimum balance between rate of flow and degree of clarification, (b) maximum throughput which can be secured
34.99 17.54 13.61 11.86
30.83 17.54 14.93 14.59
for a desired clarity, (c) best clarity obtainable for a given rate of flow, and ( d ) the most economic amount of filter aid to meet these conditions, etc.
Literature Cited (1) Cummina, A. B., and Badollet, M. S., IND. ENG.CHEM., ANAL. ED., 5, 328-32 (1933). ( 2 ) Cummins, A. B., and Miller, M. C., Am. Chem. SOC. Meeting, New York, 1935. PRBSENTED before the Division of Sugar Chemistry and Technology a t the CHEMICAL N SOCIBTY, Detroit, Mich. 100th Meeting of the A M ~ R I C A
Calcium Phosphate in the Filtration of Sugar Liquors A. B. CUMMINS Johns-Manville Research Laboratories, Manville, N. J.
HE use of calcium phosphate as a defecant in the refining of sugars is an old practice. I n cane sugar refining, calcium phosphate was employed generally for several decades in conjunction with Taylor bag filters of different designs and is still so used to a minor extent. The Williamson process of defecation is also based on the use of a flocculent calcium phosphate precipitate which is formed and handled under certain prescribed conditions. As a general procedure, the use of calcium phosphate with diatomaceous silica filter aids has not been practiced satisfactorily up to the present time for reasons given below. As employed previously, phosphoric acid (as orthophosphoric acid or one of the acid calcium phosphates of varying degrees of purity) is first added to the impure sugar solution. Calcium hydroxide is then added as milk of lime in amount sufficient to neutralize the phosphoric acid and precipitate it as tricalcium phosphate. This comes down as a voluminous flocculent precipitate which occludes, entraps, or otherwise serves t o remove some of the colloidal or finely divided impurities characteristic of unrefined sugars. The combined floc of calcium phosphate and impurities must then be separated from the defecated sugar solution by mechanical
T
methods, such as settling, flotation, or filtration. The calcium phosphate floc is removed with considerable difficulty since the precipitate has troublesome physical characteristics. It frequently passes somewhat readily through most filter media a t the start of filtration, t o give a turbid filtrate and incomplete removal of impurities. As filtration progresses, the rate of flow becomes slower and slower. When brilliant filtrates are obtained, the rate of filtration is generally unsatisfactory. Factors such as inversion losses, bacterial growths, etc. , frequently become troublesome. Thus, the calcium phosphate method for handling washed raw sugar in American refineries has been supplanted largely by other processes. The use of diatomaceous silica as a filter aid for washed raw sugar liquor has been usually carried out with lime alone as the defecating agent. Attempts to use lime, “phosphate paste”, and filter aids with the conventional filter equipment have not been considered satisfactory. The difficulty has generally been that the calcium phosphate floc plugs the pores and openings in the diatomaceous filter cake, with consequent low rate of flow. I n 1938 data were presented (1) t o show the possibilities of treating affination sirup with phosphoric acid and lime and
