PAUL W .
LEPPLA
Good jilteraid practice is a combination of testing and experience.
A simple analogy to basic electricity makes the role ofjilteraids more clear
FILTERAIDS TO IMPROVE
A variety of particle shapes characierizes diatomite jlieraids. Although there are over 70,000 varieties of diatomites, only a few are suitable for jfilteratd use 40
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
FILTRATION
Obviously this will .readily paps through an 80-mesh wire screen having an opening 175 microns wide.
a filteraid can be used effectively, the W c B:cfore principles of filtration must be u n k m d .The
7 temperature, , a~.& ~ther controllable variables on a':particular operatien.m o t be minimized. But as 'technical ag filtration..fundamentals are, they can. be reduced to a~simplegraphic analogy without compkated mathematics or ca~cu~us. The d e played by a filteraid then becomes more clear, its use more qact.and less of a hit-or-miss propmi~m. ,:
effect of changes in
.
4oood-
. . .F
Must:
lwakuve U U d Fkkniar
m a Very P o t m Cake
saw
Porosity has nothing to do with the size of the pores. This should-not be confused with p..meability..w~d,,& the 'reciprocal of resistance. A cake having hieh resistance has low permeability. Porosity. meap 'centage of the cake that ConGsts of pores. made from diatomaceous earth or perlite have oddball shapes 'that prevent th& .'packing together d o d y . For that reason, there are rel+ively large spaces ~hvm the particks which'form a netyork of channels For flow. Good filtnaids form c&n. that are 85 to 90% pofes. This not only permits Mgh@itid flow ,but also provides room to trap out the filterable .&s and still main P . high percentage of.channelsQPWfor flow. -
I
tat
,
'
. . . Ham Low Surface Ar4a
,
, ' I n general, the slowest.@teraid that will the flow quiremeits should be used for' optimum results. This a a d w i l l . g i v e . t h e . b e s t darity.~ Optimuin results , q u i r e alno using the correct Too little wiU give poor clarity and uch can also impede flow by 'building up too thick a &e. 'sclcction of proper 'fltteraid at.the i ~c+ only be determined by test. Bomb, where the resdk'plot on a straight line close to 0.5 invariably mean good filtration, and the results can be extrapolated to longer with WEdence. If the line c u m .downwad, indicating ,@e ging, it may be caused by one of mnd -,fpclok. One that is sometimes overlooked, and which therefore warrants mention here,'is that low flow rate due to plugging can be caused by using too fast a filteraid. This pens if the liquid bdng filtend contains pmticles that &ate thrdugh the cake and at or near the septum, building up high flow realstance. No one can predict what grade of filteraid or what : d W . : i s optimnm. In the sasreh for higb,..@w, i t i k p c s i b l e to use too fast a.filteraid and'&@to use .',domueh. Detamining the op,timum gra& .end .the optimum dosage can only be achieved byte&.
Resistance to flow is created by the viscous shear of the liquid over the filteraid surface. Therefore, the. d e r the surface area, fhe higher the flow. Surface area; in tUrn.is d a t e d to particle size. A bed composed of veryfine particles, such as a p more s u & c % ' H ' t h a n a d tides. Surface area is .themain different filteraid grades. A t ' the?same cake d-9, Dicalite's Speedplus and: 4aQo::traYe the same PaFarity, or w h m e per cant pores, bufthe la-' h a sevrrat~~times:
( C d - a cn6x: pa@)
V O L 5 4 N b 5 MAY P$62
h
Fdtered-out solids are shown coming of /he k m j e of a rotaiy vacuum precoad j l t e r . Solids are largely pure carotene
ANALOGY REDUCES FILTRATION THEORY TO ITS BARE BONES
Ohm’s law is one of the most familiar relationships in science. Filtration can be compared with current flow and electromotive force. Development of the analogy is quite elementary, so much so that once examined closely, it will be retained. Ohm’s law says that for any given conductor, current is proportional to the voltage and to the cross-sectional area and is inversely proportional to the length of the conductor and its specific resistance, a characteristic of the conductor itself. The law is most often written :
I = (E
x
A > / ( r x L)
The simplest case of filtration is the flow of clean water through a fixed bed of powdered solid. Here the flow is proportional to the pressure and to the crosssectional area of the bed and inversely as the unit resistance of the particular powder we are dealing with and the thickness of the cake. It is written:
Note that it is analogous with Ohm’s law. A simple examination of the equation shows that you can predict the effect on filtration of all the common variables. iln actual filtration is only one step more complicated than the simple case of the fixed bed.
CHARACTERISTICS O F
Type and Composition Diatomaceoussilica
Perliticglassy silicate
Carbonaceous
Cellulosic
a
Packed Cake
Relative Retention Flow Density, on Coarse Rate Lb./Cu. Ft. Screens 1-15 16-22 Good
FI LTERAlDSa
Solubility at Room Temp. In alkali In acid Slight in dilute solutions
Typical Industries Using Chemical Petroleum Food, beverage Pharmaceutical
Manufacturers or Suppliers Great Lakes Carbon Johns-Manville Eagle-Picher
4-12
12-18
Fair
Slight in dilute solutions
Water Chemical Petroleum Food, beverage Pharmaceutical Metallurgical
Great Lakes Carbon Johns-Manville Tennessee Products & Chemical Silflo Corp.
8
16-18
Fair
Trace in strong solutions
Metallurgical Food, beverage Pharmaceutical
Great Lakes Carbon Barnebey-Cheney West Virginia Pulp & Paper
7-15
9-20
None in strong None in dilute alkali acids Excellent resistance
Water Food, beverage Metallurgical Petroleum Pharmaceutical
Brown Go. Great Lakes Carbon
Excellent
Within the physical characteristic ranges shown, filteraid grades are available in all types to meet the differing needs of the variety of filtrations met with in industry.
42
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
To increase flow
. . . Increase
pressure. This is too often neglected. Many plants can get the extra throughput they need by buying new pumps with higher discharge pressures. . . . Increase filter area. A greater need for filtered liquor may occur, and as the requirements go up, the filter operator is fighting flow rate to keep his plant running. When this condition occurs, an investigation should be made to determine the economics of installing new equipment. In most cases, under these conditions, an economic study of added filter area will show a fast payout in savings in filteraid, operating costs, labor, and maintenance. . . . Decrease D . This idea, to cut off deliberately a filter before the cycle is terminated by high pressure or by filling the cake space, is rarely used. . . .Decrease r. This is the heart of filteraid technology. I t is the most important factor in the equation and can be accurately determined for any particular filtration only by testing. Limited test results can be extrapolated by mathematics to arrive at conclusions which would take a long time by actual tests. Let’s examine a typical case. Take a sugar filtration at constant pressure, with 1% filteraid. In this particular case, the flow in the first minute is 100 ml. of filtrate. Assume we continue the cycle, measuring the volume delivered by the filter each minute. In the second minute the output is only 41 ml., the third minute only 32 ml. Now the obvious reason for the cut back is that at the start of each minute we have a thicker cake of deposited solids. The basic equation, similar to Ohm’s law, tells us that flow will be cut down as the bed depth increases. Look at the rest of the data: Time Flow Total flow
1 2 100 41
3 4 32 27
9
16 25 36 49
64
100 141 173 200 300 400 500 600 700 800
At the beginning of the cycle, with a thin cake, only one minute was needed for 100 ml. of output. Near the middle of the cycle, it took 11 minutes; at the end, 15 minutes were required. Plotted data look like this:
Time
Paul W. Leppla is Technical Director, Mining 63 Mineral Products Div., Great Lakes Carbon Cor$. H e has been active in the development ofjlteraids and has wrztten extensively on their use. AUTHOR
The basic problem is that we usually need to predict total flow over extended periods of time such as 8 hours rather than minutes. Obviously, if we have to test several different filteraids at several different dosages, many working days would be required to run the tests to completion. We badly need a means of calculating the flow over longer periods of time. Here is where the complicated mathematics comes in. T o extend the calculation we would use the basic formula, similar to Ohm’s law, and recalculate it for each successive minute, using the new cake thickness from the preceding minute. This is integral calculus. Fortunately the extrapolation can be made graphically and this involves the log-log plot given in the “Chemical Engineers Handbook” (John H. Perry, ed.). Replotting the data above on log-log paper results in a straight line that can be extended over longer time periods with confidence. There are a few limitations to this method which need not concern us here. There are also other things to be learned from the graph. The graph looks like this :
r
I Log Time Now look at the differential calculus equation for filtration rate, a modified Darcy equation : dv A X P - = d0 p X L X r This equation is almost identical to the one discussed before, except for the addition of a term p for the viscosity of the liquid being filtered. This gives us one more way of increasing flow by reducing viscosity. In filtering heavy sugar solutions, oils, or varnishes, using the highest possible temperature gives substantial increase in rate. By far the most important point to be learned by studying the filtration equation is that only moderate changes in rate can be accomplished through most of the terms. There are obvious limits to the percentage change that can be effected by increasing filter area, increasing pressure, or cutting down the thickness of the cake. By far the most important term is the r. By reducing the specific resistance of the filter cake through the addition of the correct amount of the right filteraid, tremendous improvements in flow rate can be accomplished. Very few people understand how powerful this tool is. For example, a filter cake from raw cane sugar has 10,000 times the resistance of a cake of a specific filteraid. The addition of the right filteraid in the correct amount can improve filtration many, many times more than any other variable in the formula. VOL. 5 4
NO. 5
MAY 1962
43
