DESIGN SIZING CRITERIA
FILTRATION 18-87 At the start of the leaf test run, the hose between the test leaf and
filtrate receiver should be crimped by hand to bring the filtrate receiver to the operating vacuum level. The use of a valve at this point is not only less convenient but very frequently results in a hydraulic restriction. The net result, then, is a measurement of flow through the valve rather than the rate at which the filter cake is capable of form- ing. Hydraulic restriction is something which should always be kept in mind. If the filtrate runs at a high and full pipe flow rate into the fil- trate receiver, it is quite likely that there is some degree of hydraulic restriction, and larger tubing and piping should be considered. When very high air flow rates are obtained, the experimenter must be satis- fied that the rates being measured are limited by cake resistance and not by pressure drop through the equipment.
There will be many times when the quantity of sample is limited.
While it is best to use the 92.9 cm2(0.1 ft2) area leaf in order to mini- mize edge effects and improve accuracy, when the sample volume is limited it is much better to have several data points with a smaller leaf than only one or two using the larger leaf. Data from leaves as small as 23.2 cm2(0.025 ft2) are reasonably accurate and can be used to scale up to commercially sized units. However, it is usually prudent to employ a more conservative scale-up factor.
For top-feed applications, the most convenient assembly is that shown in Fig. 18-108. The depth of the dam must, of course, be suffi-
cient to contain the total quantity of feed slurry required for the test.
Since the test leaf is mounted on top of the vacuum receiver, it is nec- essary to provide a valve between the test leaf and the receiver so that the desired operating vacuum may be obtained in the receiver before the start of a test run. It is imperative, however, that there be no restriction in this valve. The preferred choice is a ball valve with the full bore of the drainage piping.
Test Program Figure 18-109 is a suggested data sheet which contains spaces for most of the information which should be taken during a leaf test program, together with space for certain calculated values. Additional data which may be required include variations in air flow rate through the cake during each dewatering period and chemi- cal and physical data for those tests involving cake washing.
It is difficult to plan a filtration leaf test program until one test has been run. In the case of a bottom-feed test, the first run is normally started with the intention of using a 30-s cake formation time. How- ever, if the filtrate rate is very high, it is usually wise to terminate the run at the end of 15 s. Should the filtrate rate be very low, the initial form period should be extended to at least 1 min. If cake washing is to be employed, it is useful to apply a quantity of wash water to measure its rate of passage through the cake. The results of this first run will give the experimenter an approximation of cake formation rate, cake washing rate, and the type of cake discharge that must be used. The
Company Address
FILTRATION LEAF TEST DATA SHEET – VACUUM AND PRESSURE
Filter Type Used Shim: No
Run No. Filter Module and/or Precoat Type Feed Temp., °F/°C As Prepared Back Calculated Form Wash Dry Form WashDewater Dry Temp.,°F/°C Temp.,°F/°C Dish No.Cake/Precoat Thickness, In. Dia. of Shared Area, In.Precoat PenetrationTo Crack or Gas Breakthrough After Form/WashAfter Cake Cracks
Yes
Leaf Size Ft.2
Mat’l as Received: Date Solids:
Analysis Analysis Liquid:
%
%
Test No.
Date Tested By
Location
Precoat Forming Liquid Temp. °F/°C
Vacuum = in. Hg.
Pressure = PSI.
(1)
ML. Clarity ML.
% Solids
in Food TIME, MIN. Air Flow Filtrate Wash Cake Weights
Tare GMS.
Wet
&
Tare GMS.
Dry
&
Tare GMS.
RUNS COMMENT COMMENT
REAGENT TREATMENT REMARKS: (1) Record Basis of Observation in Space Provided.
CAKE DISCHARGE
RUNS
FIG. 18-109 Sample data sheet.
rest of the leaf test program can then be planned accordingly.
In any leaf test program there is always a question as to what vac- uum level should be used. With very porous materials, a vacuum in the range of 0.1 to 0.3 bar (3 to 9 in Hg) should be used, and, except for thermal-drying applications using hot air, the vacuum level should be adjusted to give an air rate in the range of 450 to 900 m3/m2⋅h (30 to 40 cfm/ft2) measured at the vacuum.
For materials of moderate to low porosity, a good starting vacuum level is 0.6 to 0.7 bar (18 to 21 in Hg), as the capacity of most vacuum pumps starts to fall off rapidly at vacuum levels higher than 0.67 bar (20 in Hg). Unless there is a critical moisture content which requires the use of higher vacuums, or unless the deposited cake is so impervi- ous that the air rate is extremely low, process economics will favor operation at vacuums below this level. When test work is carried out at an elevation above sea level different than that of the plant, the ele- vation at the plant should be taken into account when determining the vacuum system capacity for high vacuum levels (>0.5 bar).
Generalized correlations are available for each of the operations which make up the full filter cycle. This means that simulated operat- ing conditions can be varied to obtain a maximum of information with- out requiring an excessive number of test runs. The minimum number of test runs required for a given feed will, of course, vary with the expertise of the experimenter and the number of operations per- formed during the filter cycle. If, for example, the operation involves only the dewatering of a slurry which forms a cake of relatively low to moderate porosity, frequently sufficient data can be obtained in as lit- tle as six runs. For more difficult tests, more runs are usually advis- able, and the novice certainly should make a larger number of runs as there is likely to be more data scatter.
Bottom-Feed Test Procedure The procedure for collecting data using bottom-feed leaf test techniques is as follows:
1. Fit the test leaf with a filter cloth expected to give reasonable results and seal the back of the leaf and side of the dam with silicone or other suitable material.
2. Hand-crimp the hose in back of the test leaf, and then turn on the vacuum pump and regulate the bypass valve on the pump to give the desired vacuum level in the receiver.
3. Agitate the slurry by hand or with a wide spatula to maintain a homogeneous suspension. Immerse the test leaf face downward to approximately one-half the depth of the slurry.
4. Simultaneously start the timer and release the crimped hose to begin cake formation. Maintain agitation during cake formation and move the leaf as may be required to ensure that solids do not settle out in any part of the container. It is not necessary to try to simulate the velocity with which the full-scale unit’s filtration surface passes through the slurry in the filter tank.
5. Remove the leaf from the slurry at the end of the cake-formation period and note the time. If the slurry is particularly thick and viscous, the leaf may be gently shaken to remove excess slurry and prevent the dam from scooping up extra material. Maintain the leaf in an upright position (cake surface on top) and elevated so that liquid within the drainage passages may pass to the receiver. Tilt and rotate the leaf to help the filtrate reach the drain outlet. Continue this dewatering period until:
a. the preselected time has elapsed, or b. the cake cracks.
6. If the cake is to be washed, apply a measured quantity of wash fluid and note the time required for free fluid to disappear from the surface of the cake. Pour the wash fluid onto a deflecting baffle, such as a bent spatula, to prevent the cake from being gouged. Washing must begin before cake cracking occurs. In particular, observe that there is no crack along the edge between the cake and the dam.
7. Continue with the various operations in the predetermined sequence.
8. During each of the operations record all pertinent information such as vacuum level, temperature, time required for the cake to crack, filtrate foaming characteristics, air flow rate during the drying periods, etc.
9. At the end of the run, measure and record the filtrate volume (and weight, if appropriate), cake thickness, final cake temperature (if appropriate), wet cake weight, and note the cake discharge character- istics (roll, sticks to media, etc.).
10. For runs involving cake dewatering only, it is usually conve- nient to dry the total cake sample, if the associated solution contains little or no dissolved solids.
11. When cake washing is involved, it is usually convenient to weigh the wet cake and then repulp it in a known quantity of distilled water or in water at the same pH as the filtrate, if precipitation of solute could occur in distilled water. The resultant slurry is then fil- tered using a clean dry filter and flask and a sample of the clear liquid analyzed for the reference constituent.
Should the mother liquor contain a significant quantity of dissolved solids, the filter cake should be thoroughly washed (after the sample for analysis has been taken) so that the final dry weight of the cake will represent suspended solids only. The quantity of reference con- stituent in the final washed cake can be readily calculated from the wet and dry cake weights and the known amount of distilled water used for repulping.
In cake-washing tests, it is important that the feed slurry liquid be analyzed for total dissolved solids and density as well as the reference constituent.
Top-Feed Test Procedure The sequence of operations with a top-feed leaf test is the same as in a bottom-feed test, except that the leaf is not immersed in the slurry. The best method for transferring the slurry to the top-feed leaf is, of course, a function of the charac- teristics of the slurry. If the particles in the slurry do not settle rapidly, the feed can usually be transferred to the leaf from a beaker. If, how- ever, the particles settle very rapidly, it is virtually impossible to pour the slurry out of a beaker satisfactorily. In this case, the best method is to make use of an Erlenmeyer flask, preferably one made of plastic.
The slurry is swirled in the flask until it is completely suspended and then abruptly inverted over the leaf. This technique will ensure that all of the solids are transferred to the leaf.
When the solids involved are coarse and fast settling, the vacuum should be applied an instant after the slurry reaches the surface of the filter medium.
Precoat Test Procedure Precoat filtration tests are run in exactly the same manner as bottom-feed tests except that the leaf must first be precoated with a bed of diatomaceous earth, perlite, or other shaveable inert solids. Some trial and error is involved in select- ing a grade of precoat material which will retain the filtered solids to be removed on the surface of the bed without any significant penetra- tion. During this selection process, relatively thin precoat beds of 1 to 2 cm are satisfactory. After a grade has been selected, bench-scale tests should be run using precoat beds of the same thickness as expected on the full-scale unit.
Where the resistance of the precoat bed is significant in comparison to the resistance of the deposited solids, the thickness of the precoat bed effectively controls the filtration rate. In some instances, the resis- tance of the deposited solids is very large with respect to even a thick precoat bed. In this case, variations in thickness through the life of the precoat bed have relatively little effect on filtration rate. This type of information readily becomes apparent when the filtration rate data are correlated.
The depth of cut involved in precoat filtration is a very important eco- nomic factor. There is some disagreement as to the method required to accurately predict the minimum permissible depth of cut. Some inves- tigators maintain that the depth of cut can be evaluated only in a quali- tative manner during bench-scale tests by judging whether the process solids remain on the surface of the precoat bed. This being so, they indi- cate that it is necessary to run a continuous pilot-plant test to determine the minimum permissible depth of cut. The use of a continuous pilot- plant filter is a very desirable approach and will provide accurate infor- mation under a variety of operating conditions.
However, it is not always possible to run a pilot-plant test in order to determine the depth of cut. A well-accepted alternative approach makes use of the more sophisticated test leaf illustrated in Fig. 18-110. This test leaf is designed so that the cake and precoat are extruded axially out the open end of the leaf. The top of the retaining wall on this end of the leaf is a machined surface which serves as a support for a sharp discharge knife. This approach permits variable and known depths of cut to be made so that the minimum depth of cut may be determined. Test units are available from Betts Advanced Metal, hompoc, Calif., (805) 735-5130.
FILTRATION 18-89
Lacking the above-described actual data, it is possible to estimate precoat consumption by using these values: nonpenetrating solids, 0.06-mm cut/drum revolution (0.0024 in); visible penetration, 0.15- to 0.20-mm cut/drum revolution (0.006 to 0.008 in); precoat bed density, 4.2 kg/m2⋅cm of bed depth (2.2 lb/ft2⋅in) for diatomaceous earth or 2.1 to 3.0 kg/m2⋅cm (1.1 to 1.6 lb/ft2⋅in) for perlite.
DATA CORRELATION
[Purchas (ed.), Solid/Liquid Separation Equipment Scale-Up,Uplands Press, Croydon, England, 1977.]
The correlations used are based partly on theoretical consideration and partly on empirical observations. The basic filtration data are cor- related by application of the classic cake-filtration equation, aided by various simplifying assumptions which are sufficiently valid for many (but not all) situations. Washing and drying correlations are of a more empirical nature but with strong experimental justification. If steam or thermal drying is being examined, additional correlations are required beyond those summarized below; for such applications, it is advisable to consult an equipment manufacturer or refer to published technical papers for guidance.
Dry Cake Weight vs. Thickness It is convenient to convert the test dry cake weight to the weight of dry cake per unit area per cycle (W), and plot these values as a function of cake thickness (Fig. 18-111).
Cake weight is measured quite accurately, while cake thickness mea- surements are subject to some variation. By plotting the data, varia- tions in thickness measurements are averaged. The data usually give a straight line passing through the origin. However, with compress- ible material, sometimes a slightly curved line best represents the data, since thinner cakes are usually compressed more than thicker cakes.
Dry Solids or Filtrate Rate Filtration rate, expressed either in terms of dry solids or filtrate volume, may be plotted as a function of time on log-log paper. However, it is more convenient to delay the rate calculation until the complete cycle of operations has been defined.
It is most useful to plot either dry cake weight (weight of dry solids/unit area/cycle) or filtrate volume (volume/unit area/cycle) as a function of time on log-log paper. These data should give straight-line plots for constant operating conditions in accordance with Eqs.
(18-55) and (18-56). The expected slope of the resultant rate/time plots is +0.50, as in Fig. 18-112. In practice, the vast majority of slopes range from +0.50 to +0.35. Slopes steeper than +0.5 indicate that there is some significant resistance other than that of the cake solids,
FIG. 18-110 Special test leaf for precoat filtration.
FIG. 18-111 Dry cake weight vs. cake thickness.
such as a hydraulic restriction in the equipment or an exceptionally tight filter cloth.
Data from precoat tests, however, generally produce filtrate curves with much steeper slopes. The precoat bed has a greater resistance than most filter fabrics, and the particles which are separated on a continuous precoat usually form a cake which has a relatively low resistance when compared to that of the precoat bed. Once the thick- ness of the deposited solids becomes significant, their resistance increases. Thus, at very short form times, the slope of the filtrate curve may be close to 1.0, but as form time increases, the slope of the curve will decrease and will approach +0.5 (Fig. 18-113).
There are some solids, however, which form a less permeable cake, even in very thin layers. With these solids, the resistance of the
deposited cake will be very high when compared to that of the precoat bed, and the slope of the filtrate curve will be +0.5 for all values of form time.
Effect of Time on Flocculated Slurries Flocculated slurries usually show significant decreases in filterability with time (Fig.
18-114). The rate of degradation may be established by running a series of repetitive leaf tests at frequent intervals on a flocculated slurry, starting as soon as practical after the addition of the flocculant.
If there is little change in the filtration rate, this factor need be given no more consideration. However, it is usually found that there is sig- nificant degradation.
When a flocculated feed is added to a filter tank, there is a definite time lag before this material reaches the surface of the filter medium.
FIG. 18-112 Dry cake weight vs. form time.
FIG. 18-113 Filtrate volume per cycle vs. form time.
FILTRATION 18-91
Since this lag time is not known at the time of testing, a lag time of 8 to 10 minutes should be allowed before starting the first leaf test on a flocculated slurry. Two, or perhaps three, tests can be run before the elapsed time exceeds the probable retention time in the full-scale fil- ter tank. With knowledge of the elapsed time after flocculation and data relating to the rate of degradation, the rates obtained on the leaf test runs can be adjusted to some constant lag time consistent with the anticipated full-scale design.
Cake Moisture Results on a wide variety of materials have shown that the following factor is very useful for correlating cake moisture content data:
Correlating factor =(m3/m2⋅h)(Pc/W)(ΘD/µ), (18-67) where m3/m2⋅h=air rate through filter cake measured at down-
stream pressure or vacuum
Pc=pressure drop across cake W=dry cake weight/unit area/cycle ΘD=dry time per cycle
µ=viscosity of liquid phase
For a more rigorous discussion of cake moisture correlation, the reader is referred to an earlier article by Nelson and Dahlstrom [Chem. Eng. Progress,53,7, 1957]. Figure 18-115 shows the general shape of the curve obtained when using the cake moisture correlating factor. The value of the correlating factor chosen for design should be somewhere past the knee of the curve. Values at and to the left of the knee are in an unstable range where a small change in operating con- ditions can result in a relatively large change in cake moisture content.
It is not always necessary to use all of the terms in the correlat- ing factor, and those conditions which are held constant throughout
FIG. 18-114 Degradation of flocculation with time.
FIG. 18-115 Cake moisture correlation.