Chemical Engineering October 2011 - 53

Gregg-Taylor Associates
THREE FILTRATION TECHNIQUES
Media
only
When you have:
* Crystalline, easily filtered solids
* Very low solids concentration
* Particle size about the same as
media opening
Use:
* Bags, cartridge, plate or leaf filters
Figure 10. Shown here are a typical
bag filter housing, a selection of synthetic
cloth and felt bags, and baskets
that hold the bags in the housing
leave the outlet valve partially open
and let the air pressure in the filter
build up. Once the pressure reaches
30 to 40 psig, quickly open the outlet
valve and partially close it again when
the air pressure drops to 10 psig. Repeat
this procedure for about 10 to 15
minutes in order to dry the filter cake
by blowing out as much moisture as
possible. Once this cake drying step
is concluded, relieve the pressure in
the filter, open the drain valve and the
air vent, close the air blow down valve
and open the filter tank bottom in the
vertical tank or the tank closure in the
horizontal tank.
Either retract the tank or pull out
the leaf bundle exposing the filter leaf
bundle with the filter cake and activate
the air vibrator to shake the cake
off the filter leaves. The discharged filter
cake will then fall into a tote bin
below the filter to contain the cake.
The filter cake discharge is completed.
If the filter has an internal washing
feature, it is used at this time to wash
the filter internals of any residual filter
cake or solids.
Running an air vibrator in the filter
is always recommended when filling
the filter in the beginning of the
precoat step to help loosen any precoat
material left on the filter media and
thus ensure that the filter leaves are
clean for a new cycle.
Filter cake options
Disposal or further handling of the
filter cake varies depending on the
application. In some applications
where the products filtered are very
valuable, washing of the filter cake is
prescribed to recover as much of the
mother liquor as possible to reduce
residual product in the filter cake for
further processing. This application
applies to the plate-type batch filters
as well as the Nutsche filters and even
Media
+
precoat
When you have:
* No slimy solids
* Up to 5% solids
* Fine retention needs
Use cake filtration:
* Plate or leaf filter
Media
+
precoat
+
body feed
When you have:
* Slimy, difficult to filter solids
* Up to 5% solids
* Fine retention needs
Use cake filtration:
* Plate or leaf filter
Figure 11. To perform a liquid filtration process, there are three different techniques
to consider when designing the filter station. The technique used will depend
on the nature of the solids being filtered
to the pressure leaf filters. The washing
in place of the filter cake with a
solvent is commonly done at the end
of the cycle. The solvent or wash liquid
is introduced and pumped through
the filter cake or pushed through the
filter cake with air or gas to displace
the mother liquor out of the cake. This
procedure is done taking into consideration
the pressure drop or resistance
to flow at the end of the filter
cycle. Generally, as the mother liquor
is displaced, the pressure drop is reduced.
This displacement procedure
normally should take about 10 minutes
or slightly more depending on the
initial pressure drop and resistance to
flow through the filter cake. Laboratory
testing is helpful in defining this
washing step.
Disposing of the filter cake in some
applications can pose problems, depending
on the toxicity of the resulting
filter cake. In the case of the plate filters,
the filter cake is on top of the filter
paper. As the filter is cleaned, the filter
paper is removed from the filter plate,
folded up and disposed into a waste receptacle
or sent to be incinerated.
In the pressure leaf filters with a wet
cake discharge the resulting slurried
cake is sent to either a waste sump or
transported off site for ultimate disposal.
The cake from filters with dry
cake discharge is typically held in the
waste tote for transportation off site
or sent for further processing either at
the plant or another location that may
recover certain wanted products from
the processed, spent filter-cake waste.
Sometimes the non-toxic spent cake is
used for landfill or soil conditioning or
even animal feed.
TroubleshooTing
Some of the more common situations or
problems that affect the performance
of a filter are now covered to assist you
in the troubleshooting process.
Bleedthrough
This situation is identified by different
names like cloudy filtrate or particle
migration. Sometimes these situations
are caused by tears or pinholes
in the filter media, or poorly cleaned
filter elements with residual cake
from the previous cycle that prevents
good seals. Channeling due to erosion
of the filter cake by the incoming
liquid not being properly baffled
to prevent disturbing the cake is
another cause of the problem. If possible,
the filter needs inspection once
it is precoated or prior to the cleaning
step to determine if the cause of the
channeling or bleedthrough is due to
pinholes or tears. You can readily see
the holes in the cake or the disturbed
flow path across the cake where this
short circuit has occurred to cause the
bleedthrough. Poorly cleaned filter elements
leave particles inside the filter
element structure that eventually are
released by the flow once the differential
pressure equalizes.
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Chemical Engineering October 2011

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