Note: Descriptions are shown in the official language in which they were submitted.
~6~6
Continuous vacuum filters such as the rotary cylinder
type, vertical rotary disc type, horizontal rotary disc type and
horizontal band type are well known in the filtering art. All
of these filters can separate cake and filtrate continuously.
However, due to the open construction of these filters, their use
is undesirable when the filtrate is harmful or toxic, or a
dangerou~ substance having a low flash point~
On the other hand, it is also known in the case of
pressure filters that the filter cake cannot practically be
removed in a dry condition. However, if the ~iltrate is valuable,
it is desirable to separate the cake continuously as dry cake with -
a low iltrate content in order to improve the recovery rate of
~iltrate. ~
It is an object o~ the present invention to provide an ~;
improved continuous or semi-continuous filtering method which
avoid~ the above described disadvantages of the prior art filter~
ing methods.
.. :
It is another o~ject of thle present invention to provide
an improved continuous or semi-continuous filtering method which
~0 can treat a filtrate which is a readily inflammable or toxic -
substance and can carry out a filtering operation where attainment
of a viscosity suitable for filtration requires a high temperature,
using a closed type filter.
It is a further object o~ the present invention to
~r~ ,. ~ '` '.`
provide an improved continuous or semi-continuous filtering method
which can continuously extract cake in a dry state with a high
recovery of filtrate~
According to the present invention, a third party liquid
~k
. - . . . ~ : .: .
(hereinafter referred to as a T.P.L.) is introduced into a
closed type pr~ssure filter that i5 provided with means for
discharging cake continuously or semi-continuously from filtering
media or means positioned therein. As used herein, the kerm "T.P.L."
is used to designate a liquid separately introduced into the
continuous pressure filter as a third constituent in addition to
the solid or cake and the liquid or filtrate components of the
filter feed. The T.P.L. should have the following properties:
A. A specific gravity between those of the solids
and the liquid that compose the filter feed.
B. It does not react with, is not a solvent for, does
not emulsify and does not swell the filter feed (comprising cake
and filtrate~.
C. It should have low flammability, or be noninflammable.
D. It should be nontoxic and environmentally acceptable.
E. It should have low viscosity.
According to the method of the present invention two
layers are formed in the closed type pressure filter - one is a
filter feed layer that consists of solids and li~uid to be separated,
the other is a third party layer that consists of the T.P.L. and
cake discharged from the filter media.
A part of the T.P.L. layer containing the cake is with-
drawn from the pressure filter and fed to a continuous open type
ilter. The liquid contained in the pressure filter ~eed is isolated
at the pressure filter as a filtrate, and the solids in the pressure
filter feed are isolated at the succeeding open type filter as cake.
The T.P.L. is recovered at the open type filter as another filtrate.
Thus, by the use of the T.P.L. and a continuous open type filter,
. . - : - . . . . : ..................... . . :: . ......... : :
. : , - . . . . :
~346~
it is possible to perform a closed type pressure ilter operation
continuously or semi-continuously with a high rate of filtrate
recovery.
Further objects and advantages of the present invention
will become apparent from the following description and accompanying
drawings, in which: -
Fig. 1 is a flow sheet showing an embodiment of the
filtering method according to the invention employed where the
specific gravity of the solids forming the cake is greater than
that of the filtrate.
Fig. 2 is a view partly in section of a continuous
closed type pressure filter employed in the filtering method of
the invention when the speci~ic gravity of the solid forming the
cake is greater than that of the filtrate.
The system shown in Fig. 1 is an embodiment of the
present invention, applicable when the specific gravity of solids ~ -
of the filter feed is greater than that of the liquid, for example,
the following system:
Solid ~ - Coal powder
Liquid ---------~ - Toluene
Third party liquid ---~ - Water
~he filter feed con5isting of the solids and the liquid
is stored in a feed tank 1 and supplied to a closed type pressure
filter 5 through a suction pipe 2, a feed pump 3, a delivery pipe
4, a flow control valve 6 and a feed pipe 7. The feed pump 3
boosts the filter ~eed to a required pressure~ and the flow control
valve 6 controls the flow rate of the filter feed so as to correspond
to the performance of the closed type pressure filter 5. In the
presently described embodiment, there is no pump by-pass return
- 3 ~
flow through a by-pass pipe 9, because a by-pass valve 8 is closed
under normal filtration operation.
The mechanism and operation of the closed type pressure
filter 5 will be explained below.
Filtrate produced in the closed type pressure filter 5
is fed to a ~iltrate tank 12 through an outlet pipe 10, a valve
14 and an inlet pipe 11. If it is required to cool the filtrate,
a cooler 13 is installed between a valve 16 and the inlet pipe 11.
There is no by-pass flow through a valve 15, a back wash pump
delivery pipe 37, a back wash pump 35 and a back wash pump suction
pipe 36 because the valve 15 i9 closed and the back wash pump 35
is not operatecl under normal filtration operation.
The T.P.L. employed in the system is required to meet
the following conditions 1-5.
1. The speci~ic gravity is between those of the liquid
and the solids contained in the filter feed.
2. It does not react with, is not a solvent for, does
not amulsi~y and does ~ot swell the f:Llter feed.
3. It should have ~ow flammability or be noninflammable.
4. It should be nontoxic and environmentally acceptable.
5. It should have low viscosityO
ToP~L~ stored in a T.P~L. tank 16 is fed to the closed
type pressure filter 5 through a recycle suction pipe 17, a recycle
pump 18, a recycle pump delivery pipe 19, a flow control valve 20
and pipes 21 and 22. ~ -
- In the closed type pressure filter 5 herein described,
the T.P.L. layer consists of T.P.L. and sOlias or cake fxom the
filter media or means ~hereinafter this phase is referred to as
~ 4 - -
, .. , . .: . . . - . . ~ - . ~ - : :
- . ~ ,. . , ~ , - :
the slurry). The filter feed layer oc~upies the upper portion
and the T.P.L. layer occupies the lower portion of the filter 5,
and solids or cake discharged from the filter media transfer to
the T.P.L. layer due to the difference in specific gravity between
the layers.
Slurry formed in the bottom portion of the filter 5 is
withdraw~ by a draw off pipe 23, and the flow rate of the slurry
is controlled by a slurry level controller 24 and a control valve
25 to maintain the two layers. Withdrawn slurry is fed to a
continuous open type filter 27 through the pipe 23 and a pipe
26. If it is required to cool the slurry and heat the T.P.L., a
slurry/recycle T.P.L. heat exchanger 28, a slurry cooler 29 and a
recycle T.P.L. heater 30 may be installed as shown in Fig. 1.
The slurry is separated into T.P.L. and dry cake, which
i.s composed of the solids contained in the filter feed, by the
continuous open type filter 27.
In spite of the use of the open kype filter 27, operation
of the continuous open type filtration can be carried out safely
because harmful liquid of the ~ilter feed is not contained in the
sluxry and the T.P~L. is a safe liquid.
The T.P.L. fil~rate separated at the filter 27 is returned
to the T.P.L. tank 16 through a pipe 31 for recirculation and
reuse as recycled T.P.L. The separated cake is carried to a cake
~torage area 34 by a conveyor 33. Unrecovered T~P.L. is replaced
by a makeup of ~resh T.P.L~ from a makeup pipe 32.
I the discharge of cake accumulated on the filter media
can be performed continuously by means of, for example, scrapers
or by applying centrifugal foxce to the cake, a continuous filtering -
~
~ 0~ L6operation can be conducted and a back wash system is not required.
When using a closed type pressure filter that requires back washing,
however, the bacX wash sys~em shown in Fig. 1 has to be installed
and operated in the manner described below.
The feed pump 3 is stopped, and the flow control valve 6
and the valve 14 are closed. The by-pass valves 8 and 15 are
opened, and the back wash pump 35 is started. Filtrate for back
washing is drawn out o~ the filtrate tank 12 through the back wash
pump suction pipe 36 and hoo~ted up to a required pressure by the
back wash pump 35, and fed to the closed type pressure filter 5
through the back wash pump delivery pipe 37, the valve 15 and the
outlet pipe 10 thereby effecting back washing of the filter 5.
The cake accumulated on the filter media is cleaned out by back
wash liquid, and thus ilter performance can be restored. A
mixture of solids and liquid corresponding in volume to the volume
of the filtrate used for back wa~hing is returned to the feed tank
1 through the feed pipe 7, the by-pass pip8 9 and the valve s3 from
the closed type pressure filter 5. During the back washing operation
the T.P.L. recycle system is prefera~ly operated. After the back
- washing operation, the filtration system is switched to normal
filtration operation. Accordingly, when a closed type pressure
filter which requires back washing is used, the method of this
invention is performed in a semi-continuous manner.
Fig. 2 shows an axample of a closed type pressure filter
which may be employed in the embodiment of Fig. 1, where the specific
gravity of the solids of the cake is larger than that of the
filtrate.
Pressure tank 38 has a ~eed connection 39, a T~P~Lo feed
- 6 -
- . ....... . . .......... ~
- : - , .: . ........... . ;: , :
: ,, : ' '' ~. - ~ ' ~ '. ,' '' , ', ' ' ' '.- ; ' -: .
connection 40, a slurry draw-off connection 41, a filtrate draw-
off connection 42 and level gauge connections 43A and 43B. In
the pressure tank 38, the filtrate draw-off connection 42 is
connected to a gathering pipe 44 having a plurality of filter
leaves 45 at its inlets. On the filter leaves filter media 46
are provided. Solids in the filter feed cannot pass through the
filter media 46, and therefore accumulate on the filter media 46
and cohere to form layers of cake 47 thereon. A part of the cake
accumulated on each filter medium 46 i6 discharged by a scraper 49
mounted on a rotary shaft 48 and adjacently above each medium 46,
so that a predetermined filter performance can be maintained.
Most of discharged cake is in a coherecl state, therefore
di~charged cohered cake is able to transfer from the filter feed
]Layer to the T.P.L. layer due to gravity and procluces a slurry with
T~P~L. in the bottom portion 50 of pressure tank 38.
Operation need not be discontinued because of accumulation
of cake or solids in the pressure tank, because excess cake or
solids can be drawn off from the clo~ed type pressure filter
continuouslyO
If the specific gravity o the solids of the filter feed
is lighter than that of the filtrate, the T~P.L. layer is formed
in the upper part of the filter and the filter feed layer is formed
in the lower partO The space 50, therefore, has to be located in
the upper part of the pressure tank and filter leaves have to be
located in the bottom part of the pressure tank. Accordingly~
.
the arxangement of the i1ter feed connection and the T.P.L. feed
c~nnection, filtrate draw-off connection and slurry draw-off
connection has to be reversad from that of the embodiment described
abova wherein the specific gravity of the solids is greater than
that of the filtrat~.
As mentioned above, using a T.P.L. and a continuous
open type filter, a closed type pressure filter according to this
invention is able to provide dry cake and recovery of essentially
all of the liquid contained in the filter feed.
It is to be understood that the relative feed rates of
the ~ilter feed and the T.P.L. and the withdrawal rates of the
filtrate and the slurry are such as to maintain the two layers, or
phases, in the closed type pressure ~ilter at levels such that
the filtering of the filter feed is not interfered with nor is
the filter feed withdrawn through the slurry draw-off connection.
These rates are readily determined by those skilled in the art,
and may be controlled by means of a level sensing system controlling
the circulation rate of the third party liquid.
~ he invention is not to be :Limited to the examples
described above, but, instead, covers all changes and modifications
which do not constitute a departure from the true spirit and
scope of the invention.
- 8 -
