Plate and frame filter press having modified flow pattern method

Method of filtering wherein the filtration rate of and/or consumption rate of filter aid for difficultly filterable slurries is reduced for a plate and frame filter press having alternate solid plates supporting filter means, e.g. a composite clothpaper media and hollow frames for accumulation of filter aid and adapted for compression between a fixed half end plate and a movable half end plate by provision within said press of a chamber immediately adjacent to said movable half end plate, said chamber provided with at least one filtrate input and the sole means for exit of the filtrate from said press.

BACKGROUND OF THE INVENTION 
The present invention concerns a plate and frame filter press. More 
particularly, the invention is concerned with a modification of a terminal 
frame utilized in such a press whereby the flow of filtrate through said 
press is altered in order to enhance the rate of filtration of difficultly 
filterable slurries. 
Plate and frame filter presses are widely used throughout the chemical 
industry. Typically a plate and frame filter press consists of alternate 
solid plates supporting a filter means such as a filtering fabric, for 
example, in the form of a composite cloth-paper media or the like and 
hollow frames in which there is a provision for the accumulation of filter 
aid to prevent blinding of the filter means thereby facilitating the 
filtration of the slurry. Such presses have been widely used throughout 
the chemical industry for many years for recovery of solids only or the 
clarification of a liquid only or the recovery of both solids and 
filtrate. No matter how complex the results of filtration may be, the 
principles of these plate and frame filter presses with their alternating 
flush plates and frames and the filtering means are basically alike. After 
the filter means are in place between the plates and frames and a closing 
device has pressed them tightly together, then the material to be filtered 
is pumped into the press at the slurry inlet. It passes through the feed 
channel to all of the chambers formed by adjacent plates and frames where 
the filter means stop suspended solids and allows only the clear liquid to 
go through to the discharge channel, leaving the solids in the chambers. 
Such a plate and frame filter press is described in U.S. Pat. No. 
1,722,497. In this patent, it is seen that the plates and frames are 
provided with side lugs so that they can be positioned on the side bars of 
a press frame. Each of the filter plates and/or filter means per se is 
constructed and arranged to be engaged and clamped between two of the 
filter frames in the usual manner. Usually these filter plates are 
provided with a ribbed or waffled surface to support the filtering fabric 
and provide channels for the filtered fluid to pass along the face of the 
plate to the discharge ports of the filter plate. In this frame and plate 
assembly, the slurry is fed into the press by means of a top feed to each 
of the filter assemblies and the filtrate is collected therefrom by means 
of a pipe secured to the lower part or bottom of the filter press. 
As noted above, the plate and frame filter press has remained basically the 
same through its evolution over the last one hundred years although the 
operating pressures have materially increased up to 250 psi and greater 
although normal operating pressures of 50-150 psi are used. The earlier 
metal plate and frames have been modified to take advantage of the newly 
developed engineering plastics which have further allowed for a wide 
diversity of structures with improved efficiency of filtration and reduced 
consumption of filter aid. In particular, the configuration of the plates 
and frames have been varied widely so as to provide not only for multiple 
feed ports but a washing capability whereby solubles can be dissolved or 
washed out of the filter cake while it still remains in the filter press 
chambers. Thus, there will be, in many instances, the presence of a wash 
channel separate from a feed channel which channels are provided through 
different corners of the plates and frames. 
In the filtering of slurries, it is necessary that the filter means receive 
a layer of filter aid to ensure that each frame receive a minimal coverage 
in order to prevent the filter means from blinding. It has been found that 
after termination of a typical run on a plate and frame filter press, the 
front frames are completely filled with filter cake (the mixture of filter 
aid and filtered solids) with the thickness of the cake decreasing toward 
the rear of the press until the cake is approximately one-half the 
thickness of the cake in the front frames. For this reason, large 
quantities of filter aid are admixed with the slurry prior to filtration 
in order to ensure that the last frames in the rear portion of the filter 
press receive sufficient filter aid to prevent blinding of the associated 
filter means. 
It is an object of this invention to provide a method to improve the 
filtration rate and/or filter aid consumption in the operation of a plate 
and frame filter press for the filtration of difficultly filterable 
slurries. 
SUMMARY OF THE INVENTION 
It has now been discovered that the foregoing comments and other 
disadvantages of the prior art plate and frame filter presses can be 
overcome by modification of the flow pattern of the filtrate through the 
press to provide that a filtrate discharge chamber is provided downstream 
of the rear filter plate immediately adjacent to the movable half end 
plate of the filter press. This has been accomplished by structure 
modification of the rear filter frame adjacent to the movable half end 
plate. The structure modification involves elimination of either two or 
three of the four holes in the corners of a conventional frame and not 
providing for, i.e. preventing, entrance of the slurry into the requisite 
filtrate discharge chamber. This modified frame is positioned between the 
movable half end plate and the adjacent rear filtrate plate. In one 
preferred form, the modified filter frame has one hole, however, almost as 
suitable is the frame without any hole, however, it is not readily 
fabricated from the conventional filter frame.

DETAILED DESCRIPTION OF THE INVENTION 
A conventional industrial plate and frame filter press is pictorially shown 
in FIG. 1. The plate and frame filter press 10 is an assembly of alternate 
solid plates 11, the faces (not visible in this figure) of which are 
waffled to allow drainage of the filtrate and the hollow filter frames 12 
in which the filter cake accumulates during filtration. A filter means 13, 
usually a layer of filter paper over a cloth fabric, covers both faces of 
each filter plate 11. The filter plates 11 and filter frames 12 are hung 
in a vertical position on a pair of parallel support rails 14 by means of 
support lugs 15. During the filtration process, the plates 11 and frames 
12 are pressed to a substantially liquid-type closure between two half end 
plates, one fixed half end plate 16 and one movable half end plate 17, by 
a capstan screw or some other closing device 18. Openings 19a, 19b, 19c 
and 19d in the corners of the fixed end plate 16 provide access to the 
passageways or pipes when the plates 11 and frames 12 are stacked together 
since the plates 11 and frames 12 are generally provided with similar 
holes. One of the openings 19a is used to introduce the unfiltered feed to 
the filter frames 12 where the solids in the feed are accumulated. Two of 
the openings 19b and 19c are normally used to remove the filtrate from the 
press 10 . The last opening 19d is normally used to flow water or solvent 
along the passageway backwash the filter cake at the end of a filter run 
by means of wash ports in the plates. Backwashing provides for removal of 
many of the solubles in the filter cake while it is still in the filter 
press chambers. This backwash process is accomplished by passing water or 
an appropriate solvent through the cake by means of ports in the plates to 
dissolve the solubles in the cake and wash out filtrate remaining in the 
cake. The wash liquor produced by the backwash process is removed via 
opening 19b with opening 19c valved off. 
Referring now to FIG. 2, a typical standard filter frame 12 is shown. The 
rectangular frame 12 consists of two horizontal sections 21 and two 
vertical sections 22 joined at their ends. At each of the inside corners 
arched members 23 extend from the horizontal and vertical sections to 
provide holes 24a, 24b, 24c and 24d. Hole 24a is aligned with the feed 
opening 19a thereby creating part of the feed passageway or pipe. 
Similarly holes 24b, 24c and 24d are aligned with half end plate openings 
19b, 19c and 19d respectively. At the inside corner arch forming the hole 
24a feed ports 25 connecting the open area of the frame 12 to passageway 
provided by hole 24a whereby the unfiltered feed enters the filtration 
chamber 26 and it is in this filtration chamber 26 that the filter aid 
accumulates. 
There is a support lug 15 on either side of the frame 12 enabling each 
frame to be properly aligned with the other alternating plates 11 and 
frames 12 of the press 10. 
A representation of a standard filter plate 11 is shown in FIG. 2. In the 
conventional press 10 and as well as in the press 10 as modified according 
to this invention, the filter plate 11 configuration is unchanged. The 
filter plate 11 consists of a solid metal plate 27 with a raised area 27' 
around its edges to provide a gasketing surface when placed against filter 
frame sections 21 and 22. The raised region 27' encloses a continuous 
imperforate plate 27 both surfaces of which are waffled to provide support 
for the filter cloth-paper layers while allowing drainage of the filtrate 
from the plate area 27 to the filtrate exit passageway by means of 
filtrate exit ports. On the raised area 27', at the corners of the plate 
11, there are holes 24a', 24b', 24c' and 24d' which match and are 
concentric with the corresponding holes 24a, 24b, 24c and 24d on the 
filter frame. One of the corners of the plate has the filtrate exit ports 
28 which run underneath the raised area 27' between the front and rear 
waffled plate centers 27 and the corner hole 24c'. This allows the 
filtrate to leave an individual plate 11 and flow out of the filter press 
10. The filter plate 11 also has support lugs 15 on either side. Both the 
front and the rear of the plate have the same details and configuration. 
The flow pattern of the filtrate and feed for the typical press 10 
utilizing the plates 11 and frames 12 of FIG. 2 is shown in FIG. 3. In 
this simplified representation of the flow in a conventional press which 
can have up to 100 plates and frames, the feed to be filtered is fed into 
hole 19a and initially through the filter frame 22 with its feed ports 25. 
The filtrate then passes through the filter means flows along the waffled 
plate 27 to the filtrate exit ports 28 through hole 24b' in the plate 11 
and from there back through the passageway in the frame 12 through the 
opening 24b to the exit hole 19b of the fixed half end plate 16. From FIG. 
3, it will be seen that each subsequent plate is reversed from the 
preceding plate thereby causing the filtrate from exit port 28 to flow 
through the frame hole 24c and finally through opening 19c. In this 
arrangement, it allows use of both exit openings 19b and 19c of the fixed 
half end plate 16. Finally, in this flow representation, it is shown that 
the filtrate slurry mixture passes into the last frame 12 through feed 
port 25 and then the resultant filtrate flows into exit port 28 of this 
movable half end plate 17 and back through the passageway to opening 19c 
in the fixed half end plate 16 where it can be collected. 
The consequence of this flow pattern which is embodied and illustrated in 
the conventional plate and frame filter press is that the pressure 
gradient across the filter means in the front of the press is much greater 
than the pressure gradient across the filter means in the rear of the 
press thereby creating a detrimental imbalance of fluid flow within the 
filter causing inefficiency of filtration. 
It was an object of this invention to improve the filtration rate of the 
feed through the filter press and/or reduce the consumption of filter aid 
required for filtration of the feed. This has been accomplished by 
modification of the last filter frame 12, i.e. that frame which is 
contiguous with the movable half end plate 17. These modified filter 
frames 12a and 12b are pictured in FIG. 4. Frame 12a has one hole 41a 
whereas frame 12b contains two holes 41a and 41d. In neither of these 
frames 12a or 12b are there any feed ports 25 as are normally present in 
conventional frames 12 in the arch 23. Aside from the above-noted 
differences frames 12a and 12b are substantially the same as frames 12 
being readily fabricated from frame 12 by removing the arches 23 from the 
appropriate corners. In practice, one need not have either hole 41a or 41d 
but is shown as such since frame 12a and 12b was modified from a 
conventional frame. The only requirement is that the feed does not flow 
directly into the rear chamber of the press. Thus one need only prevent 
feed flow from the last plate directly into said rear chamber as by any 
mechanical closure. By leaving arch 23 (no feed ports) with hole 41a in 
frame 12a or 12b, the movable half end plate 17 closes off the feed 
passageway and insures that no feed enters directly into the rear chamber 
space defined by the last frame 12a or 12b but rather must feed into the 
press through the feed ports 25 of the next to rear frame so that only the 
filtrate can enter into said rear chamber space. According to the 
invention, no filtration occurs in the rear or last filter frame. 
The consequence of modifying the rear filter frame according to either of 
the configurations of FIG. 4 results in the flow patterns of FIGS. 5 and 
6. The preferred modified frame 12a provides a flow pattern wherein the 
feed enters opening 19a and out into the first frame 12 through ports 25 
into the volume 26 defined by the frame sections 21 and 22 for filtration 
and the filtrate subsequently flows via the waffled portion of plate 11. 
The filtrate flows into the exit ports 28 of the first plate 11 and into 
the filtrate passageway 30c defined by holes 24c'. However, with this 
modification, the feed entering into the second frame volume via ports 25 
results in filtrate which can flow into either filtrate passageway 30c or 
30b toward the rear chamber volume defined by the modified last frame 12a 
since openings 19b and 19c are closed off along with closure of the wash 
ports of the plates. The cumulative filtrate which has flowed into said 
rear chamber volume defined by the modified last frame 12a and its 
contiguous rear plate and the movable half end plate 17 then flows back 
through the passageways defined by the alternating plates and frames as 
illustrated until it emerges from opening 19d of the fixed half end plate 
16 filtrate. Thus, the only filtrate exit of the press comes directly and 
solely from said rear chamber. 
The flow pattern provided by modified frame 12b is diagrammatically 
presented in FIG. 6. The feed again enters through the opening 19a in the 
fixed half end plate 16 but in this instance opening 19b is provided for 
removal of the product filtrate. As the feed flows in through opening 19a, 
it passes out through feed ports 25 into the filtration chamber 26 where 
the filtrate passes through the filter and filter aid and leaves via the 
exit ports 28. In this approach the usually alternated successive plates 
are horizontally rotated such that the plate exit ports open into 
passageway 30c which leads to the rear chamber volume. From said volume 
the filtrate flows along passageway 30b to opening 19b. 
In each of the modified embodiments according to the invention, it must be 
realized that all the fluid of the slurry passes or travels the same path 
length no matter which frame it passes through with all frames being 
exposed to approximately the same pressure differential. For this reason, 
all the frames will work at their full capacity for the entire run. 
The invention will be further understood by reference to the following 
example which illustrates one preferred form of the invention and compares 
this embodiment with a filter press using the standard configuration of 
alternating plates and frames. It is to be understood that this 
illustration is not to be interpreted as a specific limitation on this 
invention. 
EXAMPLE 1 
A difficultly filterable slurry composed of approximately 0.2 wt.% solid 
particles in an oil-based slurry was subjected to filtration in a standard 
36-inch Sperry plate and frame filter press manufactured by D. R. Sperry & 
Co. of North Aurora, Ill. and having 40 frames with alternating 
waffle-type plates using a filter paper-cotton cloth combination as the 
filter means. 1150 lbs. of diatomaceous earth filter aid was admixed 
thoroughly with about 100,000 lbs. of the slurry. The admixture was 
continuously fed into the press until a terminal pressure of 49 psig was 
developed after which filtration was terminated. Evaluation was made of 
the flow distribution in the filter press by means of an examination of 
the filter cake to determine the precoat and total cake thickness as a 
function of the frame number. This result was compared with a modified 
filter plate and filter press according to the invention by replacing the 
last plate with a modified plate of the configuration of that of 12b and 
horizontally reversing plates 1, 3, 5, 7, etc., (plates are numbered 
starting from the front full plate) along with closing openings 19c and 
19d. 
In the filter press employing the standard flow pattern, the total filter 
cake thickness generally decreased from the front to the 32nd frame. From 
the 32nd frame on, the filter papers were covered with a wet gelatinous 
sludge instead of the desired dry, firm filter cake. It was believed that 
this gelatinous sludge is a symptom of severe blinding so that it can be 
deduced that these frames receive little or no precoat. From this, it is 
possible that this poor distribution of precoat reduced the filter's 
capacity by up to 20%. In contrast, the precoat thickness in the filter 
employing the modified flow pattern resulting from the utilization of the 
modified filter frame 12b decreased rapidly from the front to the 16th 
frame. At frame 16 and beyond, the precoat thickness remained constant at 
1/8". This thickness is more than sufficient to prevent the blinding of 
the filter paper which will allow for reduction of the precoat treat rate. 
In the filter employing the modified flow pattern, the total filter cake 
thickness varied by plus or minus 1/16" around an average value of 5/16". 
This indicates that the flow is more or less evenly distributed to all the 
frames. The results of this comparison are shown in the following Table I. 
TABLE I 
______________________________________ 
CAKE THICKNESS AS A FUNCTION OF FRAME 
(in inches) 
Press 
of the Invention Conventional Press 
Frame # Precoat Total Precoat Total 
______________________________________ 
1 1/2 7/8 5/16 7/8 
4 5/16 1 1/4 1 
8 1/4 7/8 1/4 7/8 
12 3/16 15/16 3/16 7/8 
16 1/8 31/32 3/16 7/8 
20 -- -- 1/8 3/4 
24 1/8 7/8 1/16 3/4 
28 1/8 7/8 1/16 3/4 
32 1/8 15/16 .dwnarw. 
SLUDGE .dwnarw. 
36 1/8 31/32 .dwnarw. .dwnarw. 
40 1/8 31/32 .dwnarw. .dwnarw. 
______________________________________ 
Total # Bags 
Filter Aid 28 23 
Terminal Press. 
49 psig 50 
______________________________________ 
Frame numbers increased from 1 to 40 from the front to the rear of the 
filter. Filter cake samples were taken from the upper left side of each 
frame. 
The consequence of the modification of the filter plate in accordance with 
the invention and its utilization in a plate and frame filter press 
indicates that there will be a significantly reduced consumption of filter 
aid and that it is possible to achieve increased filtration rates and to 
increase the filter run length. 
It is to be understood that the examples present in the foregoing 
specification are merely illustrative of this invention and are not 
intended to limit it in any manner; nor is the invention to be limited by 
any theory regarding its operability. The scope of the invention is to be 
determined by the appended claims.