Methods and systems for controlling a continuous medium filtration system

A plurality of sensors sense a plurality of parameters including flow rate of a slurry being filtered sensed along a filtration medium and differential pressure across the medium which characterize the operation of a cross-flow filtration system having a moving, continuous filter medium. The plurality of sensors produce a plurality of signals representative of the plurality of parameters. A processor processes the plurality of signals to provide at least one control signal. The at least one control signal is applied to the filtration system to control at least one parameter thereof. As a result, a desired operating condition or a desired performance criterion is automatically maintained during operation of the filtration system.

RELATED INVENTIONS 
The present invention is related to the following invention which is 
assigned to the same assignee as the present invention: 
"Method and System for Monitoring and Controlling a Filtration Process" 
having Ser. No. 08/311,305 filed on Sep., 23, 1994, now U.S. Pat. No. 
5,492,632. 
The subject matter of the above-identified related invention is 
incorporated by reference hereby into the disclosure of this invention. 
FIELD OF THE INVENTION 
The present invention relates to continuous medium filtration systems which 
separate a solid material from a liquid material contained in a slurry. 
BACKGROUND OF THE INVENTION 
Many industrial processes result in the creation of liquid waste. Liquid 
waste may be in forms such as an unwanted process byproducts, used or 
contaminated solvents, and/or used or contaminated lubricants. Waste water 
is an example of liquid waste which is produced in various industrial 
processes. In many food canning processes, for example, a salt water 
byproduct is produced. Waste water is also a byproduct in paper production 
processes, and in bleaching and dying processes used by the textile 
industry in the manufacture of garments. Other applications in which waste 
water is produced include sewage processing and food processing. 
Typically, liquid waste is treated before subsequent disposal, recycling, 
or reuse thereof. One method of treatment entails diluting the liquid 
waste until a level of contaminants contained therein meets a 
predetermined standard. Thereafter, the diluted liquid waste is typically 
disposed into a nearby stream or lake. This solution is not 
environmentally sound since the contaminants introduced into the 
environment may be accumulative. 
U.S. Pat. Nos. 5,292,438, 5,256,288, and 5,259,952, issued to Lee and 
assigned to Cer-Wat, Inc., disclose methods and systems for separating a 
solid material and a liquid material contained in a slurry. A filtration 
system disclosed therein utilizes a continuous filtration medium, such as 
a traveling belt filter, on which a cake of the solid material forms 
within a separation chamber. This system may be utilized for treating 
liquid waste by filtering out the contaminants contained therein. Both the 
contaminants and the filtered liquid may then be reused or recycled. 
Further disclosed in U.S. Pat. No. 5,259,952 is an open-loop control system 
for controlling parameters of the filtration system. The open-loop control 
system commands the parameters to provide desirable steady-state 
separation conditions for a given slurry. However, the use of open-loop 
control results in a system which does not necessarily provide desirable 
transient separation conditions. Further, the open-loop control system is 
not capable of adapting the commands to changing conditions, such as a 
change in the concentration of solid material contained within the slurry.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Embodiments of the present invention advantageously utilize closed-loop 
control methods and systems for controlling the operation of a filtration 
system which employs a moving continuous filter medium. Consequently, the 
filtration system is robust to changing conditions which heretofore would 
have required intervention of an operator. As a result, the filtration 
system may be operated remotely. 
FIG. 1 is a block diagram of an embodiment of a system for automatically 
controlling a filtration system 10. The filtration system 10 receives a 
slurry 12, and separates a solid material 14 from a liquid material 16 
contained in the slurry 12. The solid material 14 is separated from the 
liquid material 16 by a continuous filter medium 20. The liquid material 
16 flows through the continuous filter medium 20, while the solid material 
14 adheres to a surface of the continuous filter medium 20 to form a 
cake-like deposit. 
The continuous filter medium 20 moves at a speed within the filtration 
system 10 such that the solid material 14 is transported out of the 
filtration system 10, and a clean surface of the continuous filter medium 
20 is provided for filtering. The solid material 14 is continuously 
removed from the continuous filter medium 20 so that the surface may be 
returned for further filtration. As a result, filtration is performed 
continuously within the filtration system 10. Typically, the continuous 
filter medium 20 includes a continuous loop belt (not specifically 
illustrated) containing a foranimous medium. 
The filtration system 10 has a plurality of parameters which characterize 
its operation. The parameters include the speed of the continuous filter 
medium 20, a flow rate of the slurry 12 over the continuous filter medium 
20, an amount of the solid material 14 deposited on the continuous filter 
medium 20, an amount of liquid material 16 extracted from the slurry 12, 
and a pressure drop across the continuous filter medium 20. 
A sensor 22 is utilized to sense a first parameter of the filtration system 
10 during operation thereof, and to generate a signal based upon the first 
parameter. A processor 24, operatively associated with the sensor 22, 
produces a control signal in dependence upon the signal generated by the 
sensor 22. The control signal is applied to the filtration system 10 for 
controlling a second parameter during operation of the filtration system 
10. 
The second parameter is controlled to maintain a desired operating 
condition or performance criterion, such as a desired filtration rate or a 
desired process efficiency. In a cross-flow filtration system, the second 
parameter may be controlled to automatically maintain a cross-flow 
condition within a predetermined section of the filtration system. In many 
applications, the second parameter differs from the first parameter; e.g. 
the speed of the continuous filter medium 20 may be controlled based upon 
the flow rate of the slurry 12. 
It is noted that the sensor 22 is representative of at least one sensor 
which senses at least one parameter of the filtration system 10 during 
operation thereof, and generates at least one signal based upon the at 
least one parameter. Further, the at least one sensor provides the at 
least one signal to the processor 24, which produces a control signal for 
controlling the filtration system 10. In preferred embodiments, the 
control signal is applied to the filtration system 10 for controlling the 
speed of the continuous filter medium 20. Here, it is preferred that the 
at least one parameter includes a parameter other than the speed of the 
continuous filter medium 20. In exemplary embodiments, a plurality of 
control signals are produced for controlling a plurality of parameters of 
the filtration system 10. 
FIG. 2 is a block diagram of an embodiment of a system for controlling a 
cross-flow filtration system 30. In a preferred embodiment, the cross-flow 
filtration system 30 is embodied by one of the methods and systems 
described in U.S. Pat. Nos. 5,292,438, 5,256,288, and 5,259,952, which are 
incorporated herein by reference. Although the subsequent discussion is 
directed to the cross-flow filtration system 30, it should be understood 
that the teaching may be applied to any filtration system which utilizes a 
continuous filter medium. 
The cross-flow filtration system 30 receives a slurry 32 and separates a 
solid material 34 from a liquid material 36 contained in the slurry 32. 
The cross-flow filtration system 30 defines a separation chamber 40 within 
which the slurry 32 is received and contained, and within which the solid 
material 34 is deposited on a continuous filter medium 42. The separation 
chamber 40 may be in the form of a box which sits above the continuous 
filter medium 42. A motor 44 drives the continuous filter medium 42 such 
that the continuous filter medium 42 moves at a speed relative to the 
separation chamber 40. 
The separation chamber 40 includes a cross-flow section 46 within which the 
slurry 32 flows over the continuous filter medium 42 at a flow velocity 
generally greater than the speed at which the continuous filter medium 42 
is moving. Having the flow velocity be sufficiently greater than the speed 
of the continuous filter medium 42 results in the creation of a cross-flow 
condition wherein the solid material 34 is prevented from settling onto 
the continuous filter medium 42. A compartment 50, located adjacent to the 
continuous filter medium 42, receives the liquid material 36 extracted 
through the continuous filter medium 42 within the cross-flow section 46 
of the separation chamber 40. 
The separation chamber 40 further includes a second section 52 wherein the 
flow velocity of the slurry 32 is not sufficient to create a cross-flow 
condition. As a result, the solid material 34 settles on the continuous 
filter medium 42 within the second section 52. A compartment 54, located 
adjacent to the continuous filter medium 42, receives the liquid material 
36 extracted through the continuous filter medium 42 within the second 
section 52 of the separation chamber 40. 
A sensor 56 senses the flow rate of the slurry 32 within the separation 
chamber 40. The flow rate of the slurry 32 is sensed along an axis 
generally parallel to an axis along which the continuous filter medium 42 
is moving relative to the separation chamber 40. Preferably, the flow rate 
of the slurry 32 is sensed at a region of the separation chamber 40 where 
the slurry 32 flows at a speed generally greater than the speed at which 
the continuous filter medium 42 is moving. Here, the sensor 56 may be 
located within the cross-flow section 46 to sense the flow rate of the 
slurry 32 therewithin. Alternatively, the sensor 56 may be located within 
the second section 52 to sense the flow rate of the slurry 32 therewithin. 
Various types of flow sensors may be utilized in the sensor 56. The sensor 
56 may include a mechanical flow sensor such as a propeller, turbine, or 
cup assembly located within the separation chamber 40. Alternatively, the 
sensor 56 may include a thermal flow sensor or an ultrasonic flow sensor. 
In a preferred embodiment, the sensor 56 includes an electromagnetic flow 
sensor having an electromagnet which generates a magnetic field transverse 
to the axis along which the flow rate of the slurry 32 is sensed, and a 
voltage sensor which senses a voltage induced in the slurry according to 
Faraday's law. The voltage provides a signal representative of the flow 
rate. 
Pressure sensors 60 and 62 are utilized to sense a pressure drop across the 
continuous filter medium 42. The pressure sensor 60 is positioned at a 
first location within the separation chamber 40. The pressure sensor 62 is 
positioned at a second location exterior to the separation chamber 40. As 
a result, a differential pressure, or pressure drop, may be sensed between 
the first location and the second location. 
The pressure sensor 60 is located either within the cross-flow section 46 
or within the second section 52 of the separation chamber 40 to sense a 
pressure exerted by the slurry 32 on the continuous filter medium 42. 
Preferably, the pressure sensor 62 is located either within the 
compartment 50 or within the compartment 54 to sense a reduced pressure 
beneath the continuous filter medium 42 produced by a vacuum 64 or a 
vacuum 65, respectively. 
A sensor 66 senses the speed of the continuous filter medium 42 during 
operation of the cross-flow filtration system 30. Preferably, the sensor 
66 senses the speed based upon an angular speed of either the motor 44 or 
a measuring roller (not specifically illustrated) mechanically coupled to 
the continuous filter medium 42. The sensor 66 generates a signal 
representative of the speed. 
In some cross-flow filtration systems, the separation chamber 40 defines an 
opening, or a gap, through which the cake-like deposit of the solid 
material 34 on the continuous filter medium 42 exits. The size of the 
opening determines the efficiency of these cross-flow filtration systems. 
If the opening is too large, a slurry 70 leaks out of the opening. The 
slurry 70 must be transported back to a slurry-receiving input of the 
cross-flow filtration system 30, which results in a reduction in process 
efficiency. In contrast, if the opening is too small, the solid material 
34 builds up within the separation chamber 40. As a result, the filtration 
rate of the system is reduced, requiring that the rate of slurry 32 
applied to the filtration system be reduced. 
In one embodiment of the present invention, a displacement sensor 72 senses 
an amount of the solid material 34 deposited on the continuous filter 
medium 42 during operation of the cross-flow filtration system 30. 
Preferably, the displacement sensor 72 senses a physical dimension, such 
as a thickness, of the solid material 34 deposited on the continuous 
filter medium 42. Further, it is preferred that the physical dimension of 
the solid material 34 is sensed exterior to the separation chamber 40 in 
proximity to the opening. In a preferred embodiment, the displacement 
sensor 72 includes an ultrasonic distancing sensor (not specifically 
illustrated) mounted above the continuous filter medium 42 in proximity to 
the opening. The ultrasonic distancing sensor is directed down toward the 
continuous filter medium 42 to sense the height of the solid material 34 
deposited thereon. 
In an alternative embodiment of the present invention, the displacement 
sensor 72 measures the gap or opening through which the cake-like deposit 
of the solid material 34 on the continuous filter medium 42 exits the 
separation chamber 40. In one embodiment, the displacement sensor is 
implemented using a linear variable displacement transformer coupled to 
the separation chamber 40, calibrated to yield a zero displacement reading 
when there is no gap or opening. Other displacement sensors can optionally 
be used including an ultrasonic or optical sensor or a simple mechanical 
gauge with an electronic interface. 
A sensor 74 senses an amount of the slurry 70 which leaks through the 
opening. Preferably, the sensor 74 includes a flow sensor (not 
specifically illustrated) which senses the amount of the slurry 70 
transported back to the slurry-receiving input, and produces a signal 
based thereupon. 
To monitor the filtration rate and process efficiency of the cross-flow 
filtration system 30, a sensor 76 is included for sensing an amount of the 
liquid material 36 extracted from the slurry 32. The sensor 76 may include 
a flow sensor which senses a rate of extraction of the liquid material 36. 
Alternatively, a mass sensor, volume sensor, or weight sensor may be 
employed to sense an absolute measure of the liquid material 36 extracted. 
The sensors 56, 60, 62, 66, 72, 74, and 76 sense a plurality of parameters 
of the cross-flow filtration system 30 during operation thereof, and 
generate a plurality of signals based upon the plurality of parameters. A 
processor 80, operatively associated with the sensors 56, 60, 62, 66, 70, 
74, and 76, produces at least one control signal based upon the plurality 
of signals. The at least one control signal is applied to the cross-flow 
filtration system 30 for controlling at least one parameter thereof. The 
at least one parameter may be controlled to maintain a cross-flow 
condition within a predetermined section of the separation chamber, 
regulate a physical dimension of the solid material 34 deposited on the 
continuous filter medium 42, maintain a desired filtration rate, and/or 
maintain a desired efficiency of the cross-flow filtration system 30. 
In a preferred embodiment, the at least one control signal includes a first 
control signal which is applied to an input of the motor 44. The first 
control signal is utilized to control the speed of the continuous filter 
medium 42 in order to maintain a cross-flow filtration condition within a 
predetermined section of the separation chamber 40. The predetermined 
section is typically within the cross-flow section 46 of the separation 
chamber 40. 
The at least one control signal may include a second control signal which 
is applied to an input of the vacuum 64 or the vacuum 65 to control the 
pressure drop across the continuous filter medium 42. The pressure drop 
may be controlled to modify the conditions for cross-flow within the 
cross-flow section 46, to prohibit a cross-flow condition within the 
second section 52, and/or to improve the process efficiency. 
The at least one control signal may include a third control signal which is 
applied to an input of a valve 82 to control the flow rate of the slurry 
32 entering the separation chamber 40. The flow rate of the slurry 32 may 
be controlled to maintain a cross-flow condition within the cross-flow 
section 46 of the separation chamber 40, to regulate an amount of slurry 
70 which leaks through the opening of the separation chamber 40, and/or to 
improve the process efficiency. 
In exemplary embodiments, the processor 80 produces a plurality of control 
signals used to control a plurality of parameters of the cross-flow 
filtration system 30 during operation thereof. 
FIG. 3 is a flow diagram of an embodiment of a method of automatically 
controlling a filtration system. As indicated by block 90, the method 
includes a step of sensing at least one parameter of the filtration system 
during operation thereof. The at least one parameter may include a flow 
rate of a slurry over a continuous filter medium, a speed at which the 
continuous filter medium is moving, a differential pressure between a 
first location and a second location separated by the continuous filter 
medium, an amount of the solid material deposited on the continuous filter 
medium, an amount of liquid material extracted from the slurry, a physical 
dimension of the solid material deposited on the continuous filter medium, 
and/or an amount of slurry which is transported back to a slurry-receiving 
input of the filtration system. Preferably, the at least one parameter 
includes a parameter other than a speed at which a continuous filter 
medium is moving within the filtration system. 
As indicated by block 92, the method includes a step of generating at least 
one signal based upon the at least one parameter. Preferably, each of the 
at least one signal is either an analog or a digital electrical signal 
representative of a corresponding parameter. 
The method further includes a step of controlling the speed of the 
continuous filter medium in dependence upon the at least one signal, as 
indicated by block 94. Preferably, the steps indicated by blocks 90, 92, 
and 94 are performed repeatedly during operation of the filtration system 
in order to automatically maintain a desired operating condition or a 
desired performance criterion, such as a desired filtration rate or 
efficiency. 
FIG. 4 is a flow diagram of an embodiment of a method of automatically 
controlling a cross-flow filtration system. As indicated by block 100, the 
method includes a step of sensing a first parameter of the cross-flow 
filtration system during operation thereof. The first parameter is either 
a flow rate of a slurry within a separation chamber, a speed at which a 
continuous filter medium is moving, a differential pressure between a 
first location within the separation chamber and a second location 
exterior to the separation chamber, an amount of the solid material 
deposited on the continuous filter medium, an amount of liquid material 
extracted from the slurry, a physical dimension of the solid material 
deposited on the continuous filter medium, or an amount of slurry which is 
transported back to a slurry-receiving input of the filtration system. 
As indicated by block 102, the method includes a step of generating a 
signal based upon the first parameter. Preferably, the signal is either an 
analog or a digital electrical signal representative of the first 
parameter. 
The method further includes a step of controlling a second parameter of the 
cross-flow filtration system in dependence upon the signal, as indicated 
by block 104. The step of controlling the second parameter typically 
includes steps of generating a control signal in dependence upon the 
signal, and applying the control signal to an input of the cross-flow 
filtration system. 
Preferably, the second parameter is either the speed of the continuous 
filter medium, the pressure at either the first location or the second 
location, or the flow rate of the slurry within the separation chamber. 
Hence, the step of controlling typically includes a step of applying the 
control signal to an input of a motor which drives the continuous filter 
medium, to an input of a vacuum, or to an input of a valve, respectively, 
within the cross-flow filtration system. Preferably, the second parameter 
which is controlled differs from the first parameter which is sensed. 
It is preferred that the steps indicated by blocks 100, 102, and 104 be 
performed repeatedly during operation of the cross-flow filtration system 
in order to automatically maintain a cross-flow condition within the 
separation chamber, and to automatically maintain a desired filtration 
rate. 
FIG. 5 is a flow diagram of another embodiment of a method of automatically 
controlling a filtration system. As indicated by block 110, the method 
includes a step of sensing the speed of a continuous filter medium. A step 
of generating a first signal representative of the speed of the continuous 
filter medium is performed as indicated by block 112. 
As indicated by block 114, the method includes a step of sensing the flow 
velocity of the slurry over the continuous filter medium. It is preferred 
that the flow velocity of the slurry be sensed along an axis generally 
parallel to an axis along which the continuous filter medium is moving. A 
step of generating a second signal representative of the flow velocity of 
the slurry is performed, as indicated by block 116. 
As indicated by block 118, the method optionally includes a step of sensing 
a pressure drop across the continuous filter medium in the cross-flow 
section of the separation chamber. A step of generating a third signal 
representative of the pressure drop is performed, as indicated by block 
120. 
As indicated by block 122, the method includes a step of processing the 
first signal, the second signal, and the third signal to produce a control 
signal. A step of applying the first control signal to an input of a motor 
which drives the continuous filter medium is performed as indicated by 
block 124. 
The control signal commands the motor to drive the continuous filter medium 
at a speed within a predetermined range proportionate to the flow velocity 
of the slurry. The predetermined range may be defined, for example, by an 
upper bound which ensures a cross-flow condition and a lower bound which 
ensures a sufficient filtration rate. If the step of sensing the pressure 
drop is performed, the predetermined range is determined in dependence 
upon the pressure drop. By performing the above-described steps, both the 
cross-flow condition and the filtration rate are maintained for the 
filtration system. 
FIG. 6 is a flow diagram of a further embodiment of a method of 
automatically controlling a cross-flow filtration system. The method 
includes a step of sensing a physical dimension of the solid material 
deposited on the continuous filter medium, as indicated by block 130. The 
physical dimension is sensed in proximity to the opening of the separation 
chamber through which the solid material exits. A step of generating a 
signal based upon the physical dimension is performed, as indicated by 
block 132. 
The method further includes a step of processing the signal to produce a 
control signal, as indicated by block 134. A step of applying the control 
signal to a valve at a slurry-receiving input is performed, as indicated 
by block 136. The above-described steps are employed to control the flow 
rate of the slurry in order to regulate the physical dimension of the 
solid material deposited on the continuous filter medium. In particular, 
the physical dimension is regulated in accordance with the size of the 
opening of the separation chamber so that the slurry does not flow out of 
the opening, and the solid material does not build up in the separation 
chamber. 
In one embodiment, the control signal commands the valve to reduce the flow 
rate of the slurry if the dimension is greater than or equal to a 
predetermined threshold. If the dimension is less than the predetermined 
threshold, the control signal commands the valve to increase the flow rate 
of the slurry. The predetermined threshold is based upon the size of the 
opening, and typically, is set approximately equal thereto. 
FIG. 7 is a flow diagram of a still further embodiment of a method of 
automatically controlling a cross-flow filtration system. The method 
includes a step of sensing an amount of slurry which flows through the 
opening in the separation chamber, as indicated by block 140. This is the 
slurry which must be transported back to a slurry-receiving input of the 
cross-flow filtration system. A step of generating a signal based upon the 
amount is performed, as indicated by block 142. 
The method further includes a step of processing the signal to produce a 
control signal, as indicated by block 144. A step of applying the control 
signal to a valve at a slurry-receiving input is performed, as indicated 
by block 146. The above-described steps are employed to control the flow 
rate of the slurry in order to regulate the amount of slurry which flows 
through the opening of the separation chamber. 
In one embodiment, the control signal commands the valve to reduce the flow 
rate of the slurry if the amount is less than or equal to a predetermined 
threshold. If the amount is greater than the predetermined threshold, the 
control signal commands the valve to increase the flow rate of the slurry. 
The predetermined threshold is based upon the size of the opening. 
Typically, the predetermined threshold is set approximately equal to, but 
slightly greater than, zero. 
The methods described herein and used in the various embodiments of the 
present invention are performed using the processor 24 or the processor 80 
as herein-described. The processors 24 and 80 can have a digital 
implementation using a microprocessor and a memory, wherein the 
microprocessor performs a series of programmed steps. Alternatively, the 
processors 24 and 80 can have an analog implementation using standard 
means for performing analog computations. The processors 24 and 80 may 
also be in the form of a custom integrated circuit, an 
application-specific integrated circuit (ASIC) or a programmable logic 
array. 
The term slurry has been used throughout this description and should be 
broadly defined to include any combination of a fluid and solid components 
including, but not limited to, a sludge or suspension. 
Thus, there has been described herein a concept, as well as several 
embodiments including a preferred embodiment of a method and a system for 
controlling a continuous medium filtration system. 
Because the various embodiments of methods and systems for controlling the 
continuous medium filtration system as herein-described form a control 
signal based upon sensed parameters, they provide a significant 
improvement in being able to adapt the operation of the filtration system 
to changing conditions. 
Additionally, the various embodiments of the present invention as 
herein-described sense two critical quantities, namely the speed of the 
continuous filter medium and the size of the opening of the separation 
chamber, so that operation of the filtration system can be quantified and 
optimized. 
It will be apparent to those skilled in the art that the disclosed 
invention may be modified in numerous ways and may assume many embodiments 
other than the preferred form specifically set out and described above. 
Accordingly, it is intended by the appended claims to cover all 
modifications of the invention which fall within the true spirit and scope 
of the invention.