I
SPERRY EHLERS Filtration Fabrics Division, American Machine and Metals, Inc., East Moline, 111.
Evaluation of Filter Media Performance Verify your selection of filter media for continuous vacuum filters by the techniques given here to get optimum operating results
X v i of continuous vacuum filters and their operation may be simulated TYPES
with a vacuum filter test leaf: 0
String discharge drum filter
0
Scraper discharge drum filter
0
Roll discharge drum filter
0
Media discharge drum filter
0
Precoat drum filter
0
Internal drum deflector discharge filter
0
Scraper discharge disk filter
0
Roll discharge disk filter
0
Scroll discharge horizontal filter
0
Tilting pan horizontal filter
A discussion of the various types of filtration equipment, theory, design, application, and media selection appeared in the July 1961 issue of I/EC. T h e current discussion will therefore be confined to the field of filter media and a practical method of evaluating filter media for optimum operating results on vacuum-type filtration equipment. Many filter equipment manufacturers make filter test leaves. The test leaf is usually one tenth of a square foot in effective filtering area and made from molded plastic with a ribbed drainage and media support section. Several manufacturers offer test leaves in other sizes for sound reasons. I t is not the purpose of this study to enter into the mathematics of filtration, and the onetenth square foot leaf simplifies the required calculations. The leaf is usually round but should be rectangular to permit string discharge action. Round leaves fitted with strings tend to cut through the filter cake as a series of knives rather than lift the cake as a support grid.
I n addition to the test leaf, other items are needed such as a vacuum gage (0 to 30 inches of Hg), vacuum receiver, slurry-container agitator, a source of vacuum, and enough hose to assemble the units and connect them to the vacuum supply. The test leaf may be connected by means of the vacuum hose directly to the single vacuum receiver which in turn is connected to an ample source of vacuum. The vacuum gage should be installed to provide vacuum readings, preferably after the vacuum receiver to prevent possible damage to the gage by corrosive slurries. An elaboration of this set u p would be to connect the hose from the filter leaf to a three-way cock, which in turn is connected to two vacuum receivers. One will be used as a filtrate receiver, and the other will be used as a wash receiver. A second vacuum gage should be installed to provide a means of determining the vacuum under which the entire system is operating. With this setup and the additional equipment required to heat the slurry, agitate the slurry, measure the volume of the slurry and the filtrate, weigh the cake, record temperatures, a small roll to simulate compression, a knife to cut precoats, a spatula to simulate a flapper, and a spray nozzle for washing, it is possible to duplicate the operating conditions of the above listed filters. T h e feed (slurry) is generally fed into the filter tank in which a drum or disks revolve or into revolving pans. As the filtrate is drawn through the filter medium, the solids deposit on the medium-covered drum, disks, or pan bottoms to form a cake. As the mechanism continues to revolve, the cake emerges and air is drawn into the filter, thus draining or air-drying the cake. As the cake revolves to the opposite side, a discharge mechanism removes it from the drum, disk, or pan.
The time during which the various drum section, disk sectors, or pans are immersed and under vacuum is called “filtering” or “form” time. Occasionally accessories are required, such as washing nozzles, compression rolls, and flappers for special treatment of the cake. These operations occur after the cake emerges from the feed. Following such operation is the period of time when air is pulled through the cake. This is called the “dewatering” or “drying” time. I n general, leaf tests do not give an accurate indication of progressive media blinding to filtrate flow or filter media wear. Leaf tests do, however, indicate filtrate clarity, production rates and residual cake moisture.
Test Procedure
Determine the per cent of solids by weight in the feed slurry. Select an appropriate filter media, and a t a vacuum of 15 inches of Hg make a n exploratory filtration, using a form time of 60 seconds. With vacuum on full, immerse leaf in clear water or filtrate and adjust air bleed to desired vacuum level. Shut off vacuum a n d empty the receiver. Reassemble, turn vacuum on and start form time by immersing leaf in agitated slurry. Remove after 60 seconds. During the form period, the vacuum will have a tendency to increase. Maintain the vacuum at 15 inches of H g by gradually increasing air bleed. Allow vacuum to remain on without further adjustment for a period approximately twice as long as the form timein this case 120 seconds. Measure depth of cake. Remove the cake by either string discharge or scraper knife or roll, in each case-observing the action of the cake removal by the VOL. 53, NO. 1 1
NOVEMBER 1961
8691
method you are using. Determine the weight of the wet cake and cake moisture, Determine weight of filtrate and ob3erve clarity. If the cake does not discharge factorily by any ofthe discharge methods, try longer form times and/or higher vacuums. If cake discharge is still unsatisfactory, another filter medium should be tried. If the cake discharges satis-
factorily, try shorter form times and both higher and lower vacuums. Some compressible cakes blind more quickly at high vacuums and can be handled better a t lower vacuums. Normally, the rate of cake build-up ,\,ill be proportional to the vacuum applied,
and the process slurry under consideration. Immerse the fabric-covered leaf in the agitated slurry for the predetermined form time. Record form time and vacuum. Remove leaf from slurry after the form portion of the cycle is completed and allow air to be pulled through the cake and leaf for rhe remainder of the cycle. Record dry time and vacuum.
the exploratory filtration, select several cycle times based on the type of filter equipment to be simulated
Types of Continuous Vacuum Filters and Their Operations Media Discharge Drum Filter. This method of media discharge involves removing the filter media from the drum over a discharge roll where the cake is either dropped or blown off from the media and a back-spray ma)- be used to wash the media. T h e media is then run back on to the drum just prior to submergence and forming operation. This is easily simulated on a test leaf by disconnecting the vacuum supply and gently blowing into the inverted leaf to discharge the cake. A back-wash can also be run into the leaf to simulate full scale back-washing. I n vieiv of the fact that successful media use on this type of filter depends more on mechanical belt tracking devices and the belt construction rather than the media construction, this filter opens u p to many applications a wider selection of possible medias. A good starting selection is the same fabric under String Discharge Drum Filter.
T h e cake should be lifted from the filter media on the test leaf by means of strings on either '2 or inch centers. The strings are placed in position over the filter media prior to the cake forming time. Some slurries contain slimes that blind the medium. This may be relieved to some extent by suppressing the initial cake formation until the drum is near the bottom of its rotation. thus favoring initial deposit of the courser material and subsequent deposit of the slimes on the cake rather than cloth. The chief advantage of the string discharge drum filter io low maintenance and very loiv filter medium wear. This allows the use of light weight. high thread count per inch fabrics. A good starting point in fabric selection for this type of filter is a fabric of the following construction details:
Nylon Multi-Filament Yarn
Roll Discharge Disk Filter. Again, simulating this unit is similar to the roll discharge drum filter. Again, the leaf should be held in a vertical position to more nearly simulate actual operating conditions. A good filter medium starting point is the same as Roll Discharge Drum Filter.
O u n c e w e i g h t p e r square yard
3.5
T h r e a d c o u n t p e r inch
212
Twist p e r
inch in (upper normal)
W e a v e pattern
Scraper Discharge Drum Filter. 'This is the most common of the vacuum drum filters. Maintenance and filter media wear are high on this type of filter. When considering scraper discharge, the optimum filtration rate is approximately the maximum drum speed or minimum cake that will discharge effectively. I n this instance, the cake that is formed is simply scraped from the fabric by means of a knife or spatula. Because of the high wear factor on the media, a good starting fabric for this type of filter is:
Spun-Staple Nylon Yarn O u n c e w e i g h t p e r square
yard T h r e a d c o u n t p e r inch Twist p e r inch W e a v e pattern Tensile strength p e r inch Air permeability
13.3 46
X
32
Normal
2/2 t w i l l 630 X 400 34. 0C
X
128
yarn
Tensile strength p e r inch Air permeability
10/10 1/3 satin 200 X 125
20,00
After a test with this fabric and evaluation of the results, i t should be possible to select the next fabric to try by referring to the fabric characteristics charts ( I ) . These charts will enable the worker to move in the direction of desired results by altering fabric construction drtails.
Tilting Pan Horizontal Filter. This is simulated in the same manner as the scroll discharge horizontal filter. However, the discharging mechanism involves simply turning the pan upside down and dumping the cake. Simulating this operation is the same as outlined under Media Discharge Drum Filter. T h e starting media would be the same as the Scroll Discharge Horizontal Filter.
Scraper Discharge Disk Filter,
Simulating this operation
is the same as simulating the scraDer discharge drum filtrr.
String Discharge Drum Filter, When simulating the string discharge drum filter, the limiting conditions are the thickness and the character of the cake required for discharge. Occasionally an excessive amount of solids may bleed through the cloth prior to the deposit of a restricting cake. I t then is ,desirable to use some flow restriction to reduce this initial velocity.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
except Table I1 should bue used foi determining the filtering cycle. Cake slippage is a problem with disk filters. however. for true indication of cake slippage, the entire operation should be carried out with the test leaf in the vertical position. If cake slippage is a problem that cannot be solved by proper media selection, a horizontal filter is indicated. (See Scraper Discharge Drum Filter for suggested starting fabric.)
FILTER M E D I A Discharge cake. At the end of cycle, immediately record Obtained and the amount Of the weight of cake obtained. Dry the cake in an oven and record the dry weight of the cake. The difference between the weight of the cake from the leaf and the oven dried cake will enable moisture content determination to be made.
Record CFM of the air pulled through the leaf during cycle time if possible. Determine per cent of solids in slurry when test series is completed. Calculate cake formation in pounds per hour per square foot of area. ~ i l trate rate of gallons per minute per square foot
