Method for determining and using the fill amounts of material for pressing in solid/liquid separation with a filter press

A determination of the amounts of fill of material (7) to be pressed in the solid-liquid separation by means of a filter piston press with a pressure element (6) for several successive pressing operations is made with the aid of a consideration in the yield/output diagram. Under a presupposition regarding the position of characteristic curves connecting various operating points in this diagram and by the interposition of an imaginary operating point it is possible to determine the changes in yield and output for each pressing operation and therefore the amounts of refill to be used in such a way that a maximal product of yield and output results for the solid-liquid separation operations when predetermining free process values. The method provides an automatic adaptation of the fill time to the compressibility of the materials. By means of this it is made possible to feed in material (7) of very different compressibility automatically and without having to predetermine reference values in such a way that an optimal behavior is achieved in respect to the yield and the juice extraction behavior of a filter press.

FIELD OF THE INVENTION 
The invention relates to a method for the determination and use of the 
amounts of fill with material to be pressed in connection with the 
solid/liquid separation by means of a filter press, comprising a pressing 
chamber in which liquid is pressed out of the material to be filtered by 
the operation of a pressing element charged with a pressure force during 
several successive stroke actions, wherein a fill amount is placed into 
the pressing chamber during each stroke operation in the course of one 
filling phase of the separating process. 
BACKGROUND OF THE INVENTION 
In discontinuous filter presses of this type the liquid portion of the 
material to be pressed is let off to the outside via filters by means of 
the action of a compressing pressure. In the process the compressing 
pressure is applied directly to the material to be pressed via a rigid 
pressure plate or pneumatically or hydraulically via a flexible diaphragm. 
The question arises at the start of feeding-in the material to be pressed 
as to what amounts must be fed into the pressing chamber so that there is 
a press pad sufficient for a first pressing. It should be noted in this 
connection that in the extended position of the pressure plate or the 
diaphragm the ratio between the effective filter surface and the 
instantaneous pressing chamber volume is greater than with the pressing 
element retracted. 
The question arises in connection with the subsequent further fill process 
as to what amounts should be refilled per stroke of the pressing element 
so that an advantageous juice extraction behavior is achieved. Different 
problems arise with organic and inorganic materials regarding the material 
to be pressed. In connection with organic materials it is typical that the 
processability in the press (compressibility) greatly changes from batch 
to batch. Accordingly, a known continuous adaptation of the process 
parameters by hand assumes great experience and continued monitoring of 
the press during the filling operation by the operators. 
Known attempts to automate required adaptations of the process parameters 
have remained without results. A usable understanding of the processes 
during the pressing operation by means of a model has not been successful 
so far. 
Filling of the presses in particular makes very high demands on the 
operator. For example, the following set points must be predetermined in 
connection with a horizontal filter press for fruit material: 
Total amount of the fill. This is greatly dependent on the compressibility 
of the material to be pressed. Hard to compress materials permit only 
small amounts of fill, while easily compressible materials permit large 
amounts of fill. 
Amount of pre-fill. The same requirements apply here as with the total 
amount of fill. Too small or to large amounts of pre-fill have a very 
negative effect on the yield/output behavior. 
Amount of fill per piston stroke. Following the end of pre-filling, a 
defined amount of material to be pressed is added per piston stroke in 
known pressing operations. These batch-wise filling pulses take place 
until the predetermined total amount of fill has been achieved as the sum. 
The appropriate selection of this amount of fill as a process value also 
very heavily depends on the compressibility of the material. 
The result as a whole is that the pressing results are very different, 
depending on the ability and experience of the operator since, because of 
the required guesses, manual predeterminations of the process parameters 
very seldom result in an optimal yield/output behavior of the pressing 
operations. 
SUMMARY OF THE INVENTION 
It is therefore the object of the invention to resolve the cited problems 
by means of an optimized method for determining and applying the fill 
amounts of material to be pressed in a filter press. 
In accordance with the invention, this object is attained by means of the 
following steps: 1) a fill and pressing operation is performed while 
measuring the output and yield, which leads to a first operating point 
with known output and yield in the yield/output diagram; 2) in at least 
one subsequent second fill and pressing operation, which leads to a second 
operating point in the yield/output diagram, at least one process value is 
determined for the second operating point, and subsequently, by means of 
employing relationships regarding the changes in output and yield of the 
solid/liquid separation in the course of the fill and pressing operations 
by the interposition of an imaginary operating point, a fill amount is 
determined and used which is required so that a maximum product from yield 
and output results during the separating process, wherein the transition 
from the first operating point to the imaginary operating point takes 
place by means of a purely fill operation and the transition from the 
imaginary operating point to the second operating point takes place by 
means of a purely pressing operation and wherein it is presupposed that 
the straight-line connections of those operating points which differ by 
means of a purely pressing operation intersect in the yield/output diagram 
at a common operating point with maximum yield and vanishing output, or 
are parallel with each other. 
Advantageous embodiments of the method ensue from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 schematically shows a horizontal filter piston press of a known 
type. It includes a press jacket 11. A pressure piston 6 fastened to a 
piston rod 14 is located inside the press jacket 11. The piston rod 14 is 
movably seated in a hydraulic cylinder and performs the pressing 
operations via the pressure piston 6. The material 7 to be pressed is fed 
by means of a pump 8 into the press jacket 11 through a closable feed 
opening and traverses a multitude of drain elements, not shown. 
In the course of the pressing operation the drain elements conduct the 
liquid phase of the material 7 to be pressed through the action of the 
pressure piston 6 into a drain line 10 outward. The material to be pressed 
can be fruit and therefore the liquid phase fruit juice. 
In the normal case the known course of the pressing operation is as 
follows: 
Filling operation: 
The pressure piston 6 is retracted and at the same time the material 7 to 
be pressed is fed in through the opening. 
Pressing operation: 
The entire press unit shown in FIG. 1 is rotated around the center axis, 
The pressure piston 6 is advanced under pressure, 
The juice is separated from the material to be pressed by pressure, 
The compacting pressure is turned off. 
Loosening operation: 
The pressure piston 6 is retracted while the entire press unit shown in 
FIG. 1 is rotated, in the course of which the remaining material to be 
pressed is loosened and torn open. 
Further pressing operation: 
The process steps of pressing and loosening are repeated several times per 
batch of material to be pressed to constitute the pressing, until a 
desired final pressed-out state has been achieved. 
Emptying operation: 
The remainder of the pressing is emptied out by opening the press jacket 
11. 
The course of the process in connection with a filter piston press will be 
described in more detail, making reference to FIG. 1. Besides the already 
described plan of the filter piston press, associated graphic 
representations are shown there which show the piston strokes between the 
positions HM and HS and the fill function F over the time t. As 
represented by means of the time diagrams next to the press jacket 11, at 
the start the material 7 to be pressed is continuously fed by means of the 
pump 8 into the pressing chamber through an opening. In the course of this 
the pressure piston 6 is moved, starting at a position HM and, after 
reaching the position HS, is immediately retracted into its initial 
position HM. This process is repeated several times. A bar identified by F 
illustrates the continuous operation "pre- filling" taking place 
simultaneously. 
The "pre-filling" operation is terminated as soon as the pressure piston 6 
no longer reaches the position HS in the course of its advance. After 
that, in a following step, filling is performed only in discontinuous 
phases, which respectively start with the retraction of the pressure 
piston 6. In the course of this it is assured by means of a fill control 
that the pressure piston 6 always reaches the same end position, located 
in front of HS, during each piston operation. 
In a further step, in the course of the advancing filling of the pressing 
chamber, the pressure piston 6 reaches positions which are continuously 
farther away from HS. In the process the fill control sees to it that the 
yield or the output of the pressing operation remains constant during each 
stroke and press operation. If, in the process, the pressure piston 6 
reaches the position HE in its advance, it is then returned to the 
constant end position of the pressure piston 6 in the succeeding step, 
until the desired total amount of the material to be pressed has been fed 
in and further pressure strokes only take place without filling operations 
F. 
In a representation similar to that of FIG. 1, wherein the same reference 
symbols identify the same functions, FIG. 2 shows fill and pressing 
operations which are separated from each other. Prior to the start of 
"prefilling" which can be seen at the bar F, the pressure piston 6 
advances to an end position HS. The pressure piston 6 is not locked during 
subsequent pre-filling, it is pushed back into a position HM by the 
pressure of the pumped-in material without performing pressure strokes. 
Following the termination of pre-filling, "pre-pressing" takes place by 
means of several strokes without filling operations, which is then again 
followed by further filling without pressure strokes as soon as the 
pressure piston 6 moves past a stroke position HN. Finally, the further 
pressure strokes only take place without any filling operations F. 
The differently controlled pressing operations described so far as examples 
can be represented in a yield/output diagram suitable for basic 
considerations, as shown in FIG. 3. In this case the following conventions 
apply 
Output L=(supplied amount of material to be pressed)/(work time used) 
and 
Yield A=(amount of juice generated)/(amount of material to be pressed 
used). 
An operating point, identified by 1 in FIG. 3, corresponds to an 
instantaneous operational state of the press, such as occurs immediately 
following the end of a stroke operation within a series of individual 
pressings of the type described in connection with FIGS. 1 and 2. The 
pressure piston 6 is still in the pressing position at the operating point 
1, but the working compression pressure has already been released. The 
previous stroke operation occurred at the operating point 1'. Thus, the 
operating points 1', 1 only differ by this one stroke movement. If at the 
operating point 1 a defined fill amount of material to be pressed is fed 
in, the operating point 1 changes to an operating point 3', wherein the 
output L is increased and the yield A is decreased. Thus, the operating 
points 1, 3' only differ by this fill operation. 
Since stroke operations and fill operations take place combined in actual 
use, as described in connection with FIG. 1, the transitions 1', 1 and 1, 
3', as well as the operating point 3', are imaginary Likewise, a stroke 
operation 3', 4' following 3', wherein the yield A is increased because of 
the amount of juice generated and the output L decreases because of the 
work time used. It is now assumed that the intersections A01 and A04 of 
the extended straight-line connections of the operating points 1', 1 and 
3', 4' coincide with the A-axis, corresponding to an output zero. In 
accordance with the invention this makes it possible to predetermine a 
process value for the operating point 4' and to determine the amount of 
fill then required in such a way that the result is a maximum product of 
yield and output. 
Although the fill amounts determined in this way lead to optimal results, 
in actual use the result is an operating point 4 of lesser yield, 
deviating from the operating point 4'. To determine the next following 
pressure stroke operation, the actually reached point 4 is combined with 
the previously determined imaginary point 3', corresponding to the pair 1, 
1' of the previous stroke operation. 
FIG. 4 shows, as a summarizing supplement to FIG. 3, a straight course of 
the press characteristic curve in the course of several purely pressing 
operations. Because of a lesser total amount of fill, the material to be 
pressed is in a state a) here. In comparison with this, another 
straight-line course applies to a state b) with a larger total amount of 
fill in the end state. Under idealized conditions, the extended courses a) 
and b) pass through a common intersection A0 with the yield axis, 
corresponding to an output value zero. In actual use this intersecting 
point A0 can change its position in the course of processing a batch of 
material to be pressed. 
FIG. 5 in comparison shows the output-yield Combinations which can be 
achieved with various controls of the pressing operations. Starting with a 
pre-filling operation R1 and with constant output L and increasing yield 
A, the pressing operation R2 shows a control with the goal of constant 
output with an approximately sufficient refilling input. A pressing 
operation R3 without refill follows this. The course b represents a 
pressing operation with insufficient refilling. The course a finally shows 
three parts which result in sequence with a constant end position of the 
pressure element for each pressure stroke, with a constant yield and 
finally after termination of the filling. 
FIG. 6 shows the course of an individual pressing operation in the 
yield/output diagram, in which the output is kept constant between the 
operating points 1 at the beginning and 4 at the end. An improvement in 
the product of output and yield can be seen. 
FIG. 7 shows the course of an individual pressing operation in the 
yield/output diagram, in which the amount of material to be pressed fed in 
between the operating points 1 at the beginning and 4 at the end is 
determined in such a way that the yield is kept constant. With a changed 
compressibility of the material it is also possible that a point 4 with 
increased output results to the right of point 1. 
FIG. 8 shows the course of a pressing operation in the yield/output 
diagram, in which no refilling is performed during a pre-pressing 
operation following the pre-filling R1 and comprising several piston 
strokes. This operation was described in connection with FIG. 2. A 
pressure-free refill operation follows the pre-pressing, which leads from 
the point. 4 to the point 3'. Several pressing operations without 
refilling then again follow during the transition from point 3' to point 
4'. The work time used for the pressure-free refilling operation is 
represented by a transition to a virtual operating point 1'. 
FIG. 9 shows how the effect of a supplied refill amount can be detected in 
the yield/output diagram by means of a theoretical consideration. 
Similarly as already described in connection with FIG. 3, the operating 
point 1 corresponds to the instantaneous operational state directly at the 
end of an individual previous pressure stroke. The pressure piston 6 (FIG. 
1) is still in the pressing position HS, but the compression pressure has 
already been released. The pressing residue is thinned by the refill 
amount and the yield is reduced. At a point 2, reached virtually without 
using up any time by purely filling, the yield will be reduced while the 
output remains the same. 
If G1 identifies the amount of material to be pressed fed in up to point 1, 
G2 is the amount fed in up to point 2, and A1 and A2 identify the yields 
at points 1 and 2, 
EQU A2=A1 (G1/G2) (1) 
At a virtually reached point 3 the output will rise, while the yield 
remains the same. If L1 and L2 identify the outputs at points 1 and 2, 
EQU L3=L1 (G2/G1) (2) 
Since the output is calculated from the amount of material to be pressed up 
to now and the time expired at this time, the output increases in the 
course of a feed of material to be pressed. Similarly to the way described 
in connection with FIG. 3, the virtually reached point 3 constitutes the 
starting point for the theoretical determination of the subsequent 
pressing step leading to point 4. The required consumption of work time 
.DELTA.t for this pressing step is predetermined by the pressing 
installation. Since it is furthermore presupposed by the invention that 
the straight-line extensions of the characteristic curves for the pressure 
stroke operations leading to point 1 and to point 4, for an output zero 
lead to the same point A0 on the yield axis, the process values L4 and A4 
for the point 4 can be determined by connecting the points 3 and A0. 
If G4=G3 again identifies the amounts supplied up to point 4 and .DELTA.t 
the press time leading to point 4, 
EQU L4=L3 (G3/(G3+L3*.DELTA.t)) (3) 
and 
EQU A4=A0-((L4/L3)*(A0-A3)) (4) 
EQU L4=L3 ((A0-A4)/(A0-A3)) (5) 
From the presuppositions made and from the equations (1) to (5) it is 
therefore possible to determine in accordance with the invention the 
amounts of fill per stroke operation to be used as the differences 
.DELTA.G of the amounts G4-G3 or G1, supplied up to the points 4 or 1 in 
accordance with 
EQU .DELTA.G=G4-G1. 
The following table shows the initial values A1, L1, the end values A4, L4, 
the refill amounts delta G determined in accordance with the invention and 
the actual strokes achieved for eight sequentially following 
fill--pressure stroke operations of a filter-piston press as a part of 
greater sequences of such operations for processing a total amount of 
materials to be pressed of 10,000 kg at a predetermined approximately 
constant pressure time of two minutes per pressure stroke operation and a 
path (stroke) of the pressure element constant for all pressure stroke 
operations of 500 mm as the predetermined process value: 
TABLE 
______________________________________ 
A1 A4 
weight- L1 .DELTA.G 
Stroke weight- 
L4 
n % t/h kg mm % t/h 
______________________________________ 
1 53.56 17.63 330 499.9 55.65 16.57 
2 55.65 16.57 220 493.4 57.13 15.78 
3 57.13 15.78 240 498.7 58.63 15.09 
4 58.63 15.09 210 497.5 59.85 14.51 
5 59.85 14.51 210 500.3 61.28 13.90 
6 61.28 13.90 190 494.4 62.16 13.50 
7 62.16 13.50 220 502.5 63.51 13.01 
8 63.51 13.01 180 494.3 65.19 12.42 
______________________________________ 
Similarly to FIGS. 3 and 9, FIG. 10 shows the operating points in a 
yield/output diagram which result if, for a second operating point 4, 
reached by means of a single fill and stroke operation from the operating 
point 1, the condition is predetermined, that the associated values of 
yield A4 and output L4 of this operating point 4 define a point 4 on the 
connecting straight line between the first operating point 1 and an 
operating point AF on the yield axis, which corresponds to a fixed, 
maximal theoretical yield value for the respective material to be pressed. 
The determination of such a condition is practical particularly in the case 
where a material to be pressed, which has a mediocre compressibility, is 
to be processed. For a material of this kind a determination of a constant 
path (stroke) in the manner of the example shown in the above table would 
produce a lesser pressing result.