Dynamic self-cleaning filter for liquids

A continuous self-cleaning filter for liquids has an outer resonant tube concentrically mounted around an inner composite tube in spaced relationship providing an annular chamber between them. There is an inflow at one end of the annular chamber for the liquid and an outflow at the other end for residual concentrate. The inner composite tube has an outlet at one end for clear effluent and a closed opposite end. A relatively thin walled perforate liner enveloped in a filter blanket comprises the composite tube. For constantly cleaning the filter blanket a sonic sinusoidal wave inducing transducer is affixed to the exterior wall of the outer resonant tube at an antinodal point, thereby to excite the outer tube into a state of resonance, thereby to continuously transform the liquid to be filtered within the annular chamber into a state of intense vaporous cavitation energy thereby to implode the surfaces of the filter blanket material causing the contamination and build up residue to be continuously removed and returned to concentrate flow within the annular chamber.

New environmental standards limiting the volume and type of effluent 
discharged from a plant or process are having serious repercussions for 
the products finishing industry. As a result, and during approximately the 
last five years, separation technology has experienced increased 
commercial use in such industrial applications as, for example, 
electrocoating separation of oil emulsions, purification, product 
separation and recovery, concentration, chemical processing, and 
production of fresh water from sea water. In addition to minimizing 
pollution in a major way, separation technology thus employed does in turn 
produce considerable savings in plant operations by returning fresh water 
to the process system and providing a high percentage of product recovery 
approximately 90 percent in some cases. 
Although there are many variations in commercial equipment, this invention 
is specifically related to the membrane process of filtration. 
The membrane processes namely, reverse osmosis and ultrafiltration, offer 
the most significant advances yet achieved, in separation technology for 
liquid treatment. Membrane processes in combination with conventional 
processes make possible greater efficiency in the purification of liquid 
streams not considered heretofore practical to treat. 
The basic difference in reverse osmosis filtration and ultrafiltration is 
in the membrane itself. Although for both reverse osmosis and 
ultrafiltration semipermeable membrane materials are available 
commercially in many configurations, this invention relates to a new and 
novel tubular construction pertinent to the device whereby either reverse 
osmosis or ultrafiltration commercially membrane materials are wrapped 
around a stainless steel thin walled supporting tube which has openings 
throughout its length permitting passage of the filtrate flow. 
Reverse osmosis membranes have pores which are generally smaller than those 
used for ultrafiltration and are in the micron size thus permitting their 
use for processes which for example, filter salt out of ocean water. 
Reverse osmosis semipermeable membranes are available in several materials 
such as, for example, cellulose acetate and fine hollow fibers. 
Both reverse osmosis and ultrafiltration are low pressure processes which 
permit selective molecular separation of liquids. Reverse osmosis 
membranes are used for dissolved solids and ultrafiltration membranes are 
used for suspended solids. 
Ultrafiltration membranes generally consist of a porous tube or hollow 
fibers through which the waste stream passes when it is to be cleaned. 
Membranes with different pore sizes are available for separating the 
different sizes and concentrations of contaminants in various liquids. 
Ultrafiltration has certain advantages such as its ability to separate 
certain liquids from other liquids, such as oil from water solutions. 
The major problem that has confronted the industry involved in reverse 
osmosis and ultrafiltration has been the fouling factor of the membranes 
themselves. Membranes and porous tubes are prone to fouling by 
contamination since pores are generally of micron size and many of the 
liquids tend to impinge upon and penetrate the porous openings of the 
membrane. 
In conventional filtration processes filter cake builds up rapidly and the 
process must be frequently shut down for filter cleaning or replacement. 
Depending on the application, the process filter can operate only for days 
or a week generally before clean-outs are needed. Of the several methods 
commonly used to clean the filter elements during a downtime period, one 
is to use cleaning fluid or acid and to back flush and recirculate the 
fluid through the filter elements by utilizing special valving, piping, 
tanks, etc. This procedure is time consuming and expensive and usually 
only partially cleans the filter elements. 
The best and most common way of cleaning is to remove the filter elements 
for either cleaning or replacement. 
One of the common preventative methods to preclude buildup of filter cake 
is to increase the rate of flow, during operation, across the membranes 
fliter element, to continuously wash away a portion of the filter cake. 
This approach however reduces the filtering efficiency considerably and 
the filter elements must still be cleaned within days or weeks. 
Continuous fouling of the membranes reduces the operating efficiency, 
lowers the output capacity, adds appreciably to the plant operating costs 
and reduces plant output by repeated downtimes. 
It is therefore among the objects of invention to provide a new improved 
continuous self-cleaning filter. Another object of the invention is to 
provide a new and improved filter capable to operating at a rated capacity 
for long periods of time. Another object of the invention is to provide a 
new and improved filter which has low maintenance cost. Still another 
object is to provide a new and improved filter which will reduce the 
normally required downtime of the filter. Still further among the objects 
of the invention is to provide a filter which in terms of output capacity 
over long periods of time is low in cost and which has high capacity and 
low operating cost. Also included among the objects of the invention is to 
provide a new and improved filter which utilizes a cylindrical and 
longitudinal membrane shell design for maximum surface area and strength, 
which operates at low power and which is self-cleaning without addition of 
acid solutions or other cleaning agents.

In an embodiment of the invention chosen for the purpose of illustration 
there is shown a continuous self-cleaning filter device indicated 
generally by the reference character 10 shown set up and ready for 
operation. The device of FIG. 1 and consists, as shown in FIG. 2 for 
example, primarily of an elongated outer tube or shell 11 concentrically 
disposed with respect to an elongated cylindrical composite inner tube 12, 
the tubes being retained at the left end by headers 13 and 14 and at the 
right end by headers 15 and 16. On the header 13 is a foot piece 17 
resting on an isolation pad 18 which in turn is carried on a supporting 
foundation 19 by suitable plates 20. Similarly at the righthand end there 
is a foot piece 21 for the header 16 carried by the supporting foundation 
22 where it rests upon an isolation pad 23 and plates 24. 
The outer tube 11 is spaced from the composite tube 12 for the purpose of 
providing an elongated annular chamber. There is an inlet passageway 31 
adjacent one end of the chamber and an outlet passageway 32 adjacent the 
opposite end. An inflow pipe 33 adapted to supply the passageway at 31 may 
be separated from the passageway at 31 by an appropriate flexible nipple 
34. Similarly, an outlet pipe 35 may be connected to the outlet passageway 
32 by employment of a flexible nipple 36. 
The outer tube or shell 11 previously identified is of metallic material 
which in practice may be stainless steel or other metal suited to the 
particular filter process. The tube 11 must have a cylindrical shape and 
sufficient thickness to be self-supporting. It is especially important 
that it be of material capable of resonance throughout its entire 
circumference and length. For metal materials such as those made reference 
to, the resonance is capable of calculation and the tube can accordingly 
be designed for its job. The composite inner tube consists of an elongated 
cylindrical liner 37 which is relatively rigid and which additionally is 
provided with a multiplicity of relatively large perforations 38. In 
practice, the liner 37 can be relatively thinner than the outer tube 11, 
and concentric with respect to the outer tube. 
Surrounding the liner 37 is a blanket or filter pad 39 substantially 
thicker than the liner, the thickness depending upon the type of filtering 
to be accomplished and the particular composition of the blanket. 
On the exterior of the outer tube 11 there is affixed a sonic sinusoidal 
wave energy inducing generator, such, for example, as a transducer 40. It 
is of some moment that the transducer be mounted on the outer tube 11 at 
one of the wave length antinodal points 40', 41'. Nodal points are shown 
at 49 in FIGS. 5 and 6. In the embodiment of FIG. 1, only a single 
transducer is shown. It is contemplated, however, that there may readily 
be a plurality of such transducers each in turn being mounted at a wave 
length antinodal point as suggested by the broken line 41 of FIG. 1. 
Additionally, transducers may be mounted circumferentially when preferred 
at, for example, locations 42 and 43 of FIG. 6, these being also at 
circumferentially disposed wave length antinodal points 40", 40'". The 
number and distribution of such transducers is somewhat optional depending 
on results required and particular conditions existing in the 
self-cleaning filter device. The objective however in all instances is to 
set up a resonant condition in the outer tube 11 so that cavitation energy 
condition can be imparted to the liquid which flows through the annular 
chamber 30 and in particular such that cavitation energy can be imparted 
to the outer surface of the blanket 39, and the liquid which saturates it. 
For suitably sealing as well as mounting the outer and inner tubes in 
concentric spaced relationship there is shown at the left end as viewed in 
FIG. 2 a mounting seal 50 on which is an integral annular spacer boss 51 
and a central spacer boss 52. An annular flange 53 is clamped between the 
headers 13 and 14 by employment of bolts 54. In order to have the seal 50 
serve effectively the surfaces may be painted with a seal material or a 
gasket material (not shown) may be provided. 
At the righthand end as viewed in FIG. 2, a seal 55 is of slightly 
different construction in that it is provided only with an annular boss 56 
and an annular flange 57. For the righthand end there is provided a 
discharge passage 58 for the filtrate resulting from the process. 
As at the opposite end the flange 57 is clamped between the headers 15 and 
16 by appropriate bolts 59. Over the outer tube 11 is a soundproof pad 46 
held in place by a sheet metal jacket 47. 
In this form of the device there is provided a restricted orifice member 60 
in the outlet passageway 32. The restricted orifice member is a 
substantially conventional piece of hardware, details of which have been 
omitted but which in any event is present to restrict flow through the 
outlet passageway so that there is a pressure differential between 
pressure present in the liquid to be filtered which enters through the 
inlet passageway and the residual liquid which flows through the outlet 
passageway so that the pressure differential thus created urges a portion 
of the liquid through the filter blanket of the inner composite tube 12 
flowing from there through the perforations 38 into the interior of the 
liner where it becomes the purified filtrate ultimately passed through the 
discharge passage 58. In this way the annular passage 30 is kept full. 
As an alternative, there may be provided in place of the restricted orifice 
60 a pressure control valve 70 in a bypass line 71 subject to control by a 
pressure regulator 72 in the bypass line. A pressure gage 73 is employed 
to indicate the pressure present in the passageway 32. 
Although transducers have been shown by way of example for setting up a 
resonant condition in the attendant structure other sinusoidal wave energy 
inducing generators such for example as ultrasonic generators may be found 
usable on occasions. 
In a device 10 of the kind described there may for example be a low 
pressure liquid feed flow generally at approximately 100 PSI flowing 
through the inlet passageway 31 and through the wall of the resonant 
cylindrical outer tube or shell to the annular chamber 30 where it is to 
be filtered. The liquid feed flow passes horizontally through the chamber 
30 during which time it is filtered in part through the blanket 39 which 
is a tubular semipermeable filtration membrane or a tubular reserve 
osmosis membrane. The filtrate thus derived flows into the central chamber 
45, said filtered liquid flow being commonly known as a filtrate or 
filtrate flow. 
The continuous filtrate flow, at relatively low pressure flows through the 
discharge passage 58 in a purified and filtered condition. The filtrate 
thus may be discharged and either used commercially, or reclaimed and 
returned to the main process system. 
The remaining continuous feed flow passing through chamber 30 is generally 
known as a continuous concentrate flow and leaves the chamber 30 through 
the outlet pipe 35 whereby the concentrate is transported and reclaimed in 
some other process for further use or disposal as the case may be. The 
recovery of concentrate materials in an efficient filtrate system may 
reach a value of approximately 90 percent. The flow restricting orifice 
member 60 is used as a means to restrict and provide substantially a 
constant concentrate flow, providing of course that the inlet pressure is 
maintained constant composite. The semipermeable tube 12 may include a 
blanket of porous tubular cylindrical ceramic or metallic material or 
material of another construction consisting of cellulose acetate or 
semipermeable hollow fiber plastic membrane material that is commercially 
available, and which in turn is wrapped about and secured to the outer 
surface and throughout the length of a thin stainless steel or plastic 
liner 37. The semipermeable hollow fiber membrane material may be 
furnished either for reverse osmosis or for ultrafiltration as may be 
required. 
The longitudinal construction of the membrane tubular material with its 
supporting tube provides an exceptionally large semipermeable outer 
surface area which not only reduces the pressure drop through the tubular 
filter for a specific flow rate, but also tends to reduce the filter cake 
build up on the filter surface and provides a greater output capacity. 
Gradual fouling of the filter membrane by build up of filter cake occurs 
continuously in conventional process filters requiring constant and 
expensive periodical cleaning scheduling resulting in costly downtimes. 
It is therefore significant in this invention that a novel means of 
continuous self-cleaning of filter membranes or porous tubes be 
incorporated in the design of the filter structure to accomplish this end. 
The structure herein disclosed is based on certain principles of physics 
and fluid mechanics, and provides a means of generating a field of 
vaporous acoustical cavitation within a flowing concentrate fluid solution 
accommodated by a chamber 30 which is formed by the inner surface of a 
resonant outer tube 11 and the outer surface of a semipermeable membrane 
tubular filter or porous tube filter blanket 39. 
FIG. 1 shows the sinusoidal frequency generator 40 mounted at a preferred 
location on the resonant cylindrical outer tube 11 by means of a mounting 
pad 44 which is in turn brazed to the outer surface of outer tube 11. An 
acceptable frequency generator 40 is shown in patent application Ser. No. 
573,043, filed Apr. 30, 1975, and consists of a mass identified as an 
induction motor which is driven about its eccentric axis, its supporting 
motor shaft and its supporting line of center. The oscillating motor mass, 
in this case, is driven in a conical fashion about eccentric bearing at 
one end of its supporting frame by means of the shaft of induction motor 
generally at a speed of 3450 RPM. The induction motor housing is anchored 
at the opposite end of its supporting frame by means of a semi-rigid 
resilient mount. The other end of the motor mass consisting of a motor 
shaft is mounted on an eccentric end bearing which is secured to the 
supporting frame, through which the shaft turns, causing the motor mass to 
oscillate about its own axis. This in turn causes a sinusoidal force to be 
generated and transmitted to the cylindrical outer tube, the exciting 
frequency of which is the speed of the motor. The exciting sinusoidal 
force motion produces a sinusoidal longitudinal elastic wave motion and a 
circumferential elastic wave motion in the outer tube 11. 
The purpose of the longitudinal resonant outer tube 11 is to provide a 
simple structural means of generating and transmitting acoustical elastic 
wave energy approaching resonance throughout the extremities of the 
resonant outer tube 11. The cylindrical tube structure may be so designed 
that it can be excited into one of its chosen modes of natural or resonant 
frequencies as illustrated in FIGS. 5, 6, and 7. In this case the 
requirements are usually the 4th or 5th mode of resonant frequency of 
approximately 180 to 1,000 cycles per second. The design of such a 
resonant structure and the dynamics of structural response are well-known 
in the art. 
The structure of the invention with transducers rigidly attached to the 
resonant tube 11 is such that the entire tube serves as the common 
diaphragm of all the transducers. This means that the entire length and 
circumference of the tube is in resonance when in operation. As a 
consequence, sound wave energy is generated in a smooth sinusoidal pattern 
over the entire surface of the tube and is driven radially inwardly 
through the entire length and circumference of the annular chamber 30. 
This is in sharp contrast to indiscriminate vibration or random hammering 
which is devoid of a sinusoidal wave pattern, and incapable of creating a 
condition of resonance. 
The cavitation condition in the liquid, produced as a result of resonance, 
extends continuously throughout the liquid and also the filter pad as long 
as it is saturated with the liquid. Therefore, solids which would 
otherwise accumulate as filter pack are prevented from accumulating in the 
filter pad and are constantly being washed away. 
The longitudinal and circumferential elastic wave sinusoidal energy 
released at resonance and shown in FIGS. 5 and 6 is transmitted in 
perpendicular fashion from the inner surface of the outer resonant tube 11 
in the form of acoustical compressional sound waves through the flowing 
concentrate liquid in chamber 30 as shown in FIG. 2. The velocity of the 
transmitted compressional wave energy within the unpure liquid concentrate 
is estimated to be 5,500 feet per second. The shearing forces of the 
sinusoidal compressional wave energy traveling through the liquid 
concentrate causes an intense degree of kinetic energy reaction to take 
place within the fluid concentrate which in turn fractures and ruptures 
the fluid solution and transforms the energy released within the chamber 
30 into another known form of energy namely vaporous cavitation which is a 
commonly accepted term for such a condition. The cavitation thus developed 
can be produced at relatively low power. 
The phenomenon of cavitation and its physical sonic energy characteristics 
represents an energy source that is commercially applied to intense 
cleaning of materials within a liquid medium. In this invention the 
materials referred to are the semipermeable membranes or other porous 
filter tube materials that are in contact with the concentrate liquid 
medium which is in a state of cavitation. 
Cavitation energy serves to break down the molecular force or interface, 
commonly known as surface tension, that exists between the contamination 
or filter cake material and the filter membranes themselves. Once the 
molecular attraction of the contamination particles to the filter membrane 
material is broken, the surface contamination build up is loosened and 
washed away by the concentrate flow and the membrane filter surfaces are 
then further imploded and continuously cleaned thus preventing further 
build up of contamination on the filter membrane material. 
Other energy transformation that exist in a field of cavitation that also 
contributes to the continuous process of the filter material are the 
forces created by agitation and dispersion energy. The cleaning of the 
microscopic membrane filter material by cavitation is also aided by means 
of the great pressure differentials that are set up by the implosion of 
micron size occuring at a rate of 10.sup.-9 of a second in the microscopic 
pores of the membrane material and by the heat that is dissipated at the 
moment of implosion. The cavities or voids that occur during these 
implosions create a very intense vacuuming action on the surfaces of and 
within the membrane material and are instantaneously filled with new 
liquid that surrounds the surfaces and is driven by very high transitory 
pressure. 
The resulting pressures generated at the loci of these implosions have been 
measured up to 15,000 pounds per square inch. Furthermore, the heat 
dissipated at the moment of implosion has been determined to be in excess 
of 1000 degrees C. 
Direct mechanical agitation and dispersion created by the alternating 
sinusoidal compressional sound waves transmitted by the resonant outer 
tube 11 also assist in the cleaning action taking place at the filter 
membrane surfaces and to also disperse the removed contamination into the 
liquid concentrate of the annular chamber 30. 
One of the unique advantages of utilizing the principles of cavitation in 
this instance is that it can be generated anywhere that a compressional 
sound wave of sufficient intensity can penetrate, and cleaning will occur 
deep within the interstices of the membrane material which has complicated 
geometric configuration. Membrane surfaces are seemingly quite smooth to 
the naked eye, have microscopic pores, hollow tube configurations, cracks 
and grain boundaries. The specific action of cavitation and its resultant 
forces, penetrates these minute areas with very intense transitory energy 
and results in implosions within the membrane material at the microscopic 
level which can be equaled by no other known method in a cleaning process. 
Cleaning takes place in dynamic "in situ" fashion and does not require the 
addition of acid solvent solutions or other cleaning agents. 
Resilient elastomeric insolation pads 18 and 23 are shown which isolate 
substantially the energy from leaving the filter structure.