Multiple bed filtering apparatus and process

A filter regeneration method comprising mixing precipitate deposited on a bed of filtering particles throughout the bed to thereby reduce the pressure drop across the filter generally occurring during use. Regeneration is conveniently accomplished by fluidizing the precipitate and filtering particles into another portion of the filter. Two types of apparatus are described in which the regeneration method may be used.

BACKGROUND OF THE INVENTION 
This invention relates to a filtering method and an apparatus to filter 
solid impurities from fluids. 
One of the most common types of filters is the bed filter. Such a filter 
may be used in gravity flow or pressurized systems. In a gravity flow bed 
filter, a fluid (usually a liquid) is fed to the upper surface of the 
filter media and the flow of the liquid through the bed is promoted by 
gravity. As the liquid flows through the filter media, suspended solids 
are trapped and precluded from passing through the media, thus filtering 
and purifying the liquid. In a pressure system, the fluid (either liquid 
or gas) is forced to pass through the filter because of the pressure 
exerted on the fluid. In a pressure system, the fluid may flow in any 
direction; upward, downward or in a generally horizontal direction. 
Consequently, the pressurized system is not limited to the downward flow 
as is the gravity filter. Otherwise, a pressure filter works in a manner 
almost identical to that of the gravity filter. The fluid flows through a 
filter media, where the suspended solids are trapped, while the fluid 
passes through the media. Pressure filters may be built more compactly to 
accommodate a given flow rate of fluid. Another difference is the fact 
that both gases and liquids may be filtered in a pressure filter, while 
only liquids may be filtered in a gravity filter. 
With both types of filters, as fluids are being filtered, a layer of the 
filtered material (precipitate) accumulates on the filter media. This 
precipitate continues to build up, causing an increasingly greater 
pressure drop across the filter. After a period of time, the pressure drop 
becomes unacceptably high. Generally, the high pressure-drop problem is 
solved by employing a process known as "backwashing", i.e., the flow of 
the fluid through the filter is reversed. Such a reversal causes a large 
portion of the precipitate to be removed from the surface of the filter 
media, thus reducing the pressure drop across the filter. If the 
backwashed material is again run through the filter, the precipitate 
immediately builds up on the filter surface again and the pressure-drop 
across the filter rapidly increases to about the original unacceptable 
level. To avoid this rapid pressure increase, the backwashed material is 
usually removed from the filtering system. 
SUMMARY OF THE INVENTION 
The present inventive method of regenerating a particulate bed filter with 
particulate impurities therein comprises mixing the filtering particles 
with the precipitate particles to form an admixture. The mixture is then 
formed into a bed wherein the precipitate particles are distributed among 
the filtering particles. 
This regeneration method reduces the pressure drop across the filter as 
compared to the pressure drop before the regeneration. Two types of 
devices are described which may be used as a filter in which the 
regeneration method may be used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a filtering process employing a bed of filtering material through which 
a fluid passes, a layer of precipitate forms on the surface of the 
filtering material as the fluid is filtered. This precipitate layer causes 
the pressure drop across the filter to increase. A method has been 
developed to regenerate the filter by distributing the precipitate 
throughout the filtering material, thus reducing the pressure drop across 
the filter. A preferred method includes fluidizing the filtering material 
and the precipitate, and allowing the mixture to settle in a manner such 
that the precipitate is distributed substantially throughout the filtering 
material. 
FIGS. 1, 2 and 3 illustrate an apparatus which may be used as a filter and 
be regenerated by the described method. A filter 101 with elongated hollow 
first and second sections 103 and 105, respectively, connected at a second 
end 106 by a passageway 107 adapted to pass a fluid, filtering particles 
and solid precipitate between the first and second sections. At a first 
end 108 of the elongated hollow sections 103 and 105, there are fluid 
outlets 109 and 111 and valves or other flow regulating devices (not 
shown) to control the flow through the outlets. The sections 103 and 105 
have fluid inlets 113 and 115, respectively, and a valve or other flow 
regulating device to control the flow through the inlets (not shown). The 
fluid inlets 113 and 115 are located in sections 103 and 105 between the 
respective fluid outlets 109 or 111 and the passageway 107. The volume of 
the elongated hollow sections 103 and 105 between the respective fluid 
inlets 113 or 115 and the fluid outlets 109 or 111 are at least partially 
filled with filtering particles forming stationary filtering beds 117 and 
119. The material comprising the stationary filtering beds 117 and 119 are 
of a size such that the beds are capable of filtering particulate 
impurities contained in a fluid to be filtered. Optionally, each of the 
stationary filtering beds may be held in place by restraining means 121 
and 123. The restraining means may be a device such as a wire screen or 
other suitable restraining devices. 
In operation, a first movable bed of filtering particles 125 is placed in 
one of the sections 103 or 105. For illustration, the first movable bed 
125 is so positioned in the second section 105 to occupy at least a 
portion of the volume of the second section between the fluid inlet 115 
and the passageway 107. The particles of the bed 125 should be of a size 
so that the bed can remove particulate impurities from a fluid as it flows 
through the bed of particles. The particles in the movable bed 125 may be 
of the same or different composition and size of the particles comprising 
the stationary beds 117 and 119. 
A fluid which contains solid, particulate impurities is flowed into the 
filter 101. For the sake of illustration of the invention, the fluid is 
flowed through inlet 113 into the first section 103, through the 
passageway 107 and into the second section 105. The fluid is flowed 
through the first movable bed of filtering particles 125, through the 
stationary bed 119 and then out of the filter through the fluid outlet 
109. The flow control devices on the fluid outlet of the first section and 
the fluid inlet 115 of the second section are closed during the filtering 
operation when the fluid passes from the section 102 through the beds 123 
and 125. 
As the fluid is flowed through the filter 101, at least a portion of the 
particulate impurities contained in the fluid are removed by the movable 
bed 125. As the particulate impurities are removed, they form a 
precipitate cake 127 at the surface of the first movable bed 125. This 
precipitate cake 127 is composed of particles generally smaller in 
physical size than the particles comprising the first movable bed of 
filtering particles 125. As more fluid is filtered, the thickness of the 
precipitate cake 127 increases. As the precipitate cake 127 increases in 
thickness, the fluid flow through the bed 125 decreases and the pressure 
drop across the filter increases. 
The pressure drop across the filter 101 may be decreased by regenerating 
the filter. FIG. 2 shows the filter 201 being regenerated. Flow control 
devices on the fluid outlet 209 of the second section 205 (not shown) and 
on the fluid inlet 213 of the first section 203 (not shown) are closed 
during the regeneration while the fluid is flowed into the second section 
205 of the filter 201 through fluid inlet 215. The fluid should be flowed 
through the second section 205 at a rate sufficient to fluidize the 
material comprising the first movable bed of filtering material 225 and 
the particles comprising the precipitate cake and form a 
fluid-precipitate/filtering-particle admixture 229. The admixture 229 is 
flowed through the passageway 207 and into the first section 203. The 
admixture is flowed through first section 203 to and through stationary 
bed 217 and out of the filter through the fluid outlet 211. As the 
admixture flows through the first stationary bed 217, the particulate 
material in the admixture is filtered from the fluid, forming a second 
movable bed of filtering particles 231. The second movable bed of 
filtering material 231 is a mixture of the particles from the first 
movable bed of filtering particles (FIG. 1, 125) and the particles from 
the precipitate cake (FIG. 1, 127). The two types of particles are 
intermixed. This distribution of the two types of particles after 
regeneration, as opposed to the layer formation of the two types of 
particles before regeneration, results in the pressure drop across the 
filter being reduced. 
FIG. 3 shows that the flow of the fluid may be continued and that 
particulate impurities will be removed from the fluid forming a 
precipitate cake 333. When further regeneration of the filter is desired, 
the regeneration process described above may be repeated by directing the 
fluid flow in a direction which causes the second movable bed of particles 
331 and the precipitate cake 333 to be moved back into the second section 
305 to form a third movable bed of filtering particles, wherein the 
filtering particles and the precipitate particles are admixed. The 
regeneration procedures may be repeated as many times as desired. 
FIGS. 4, 5 and 6 illustrate another embodiment of an apparatus which may be 
used as a filter in which the present regeneration method may be used. 
FIG. 4 shows a filter 400 with one section 402. At a first end 403 of the 
section 402, there is placed a first stationary bed of filtering particles 
404 and at a second end 405 of the section 402 is placed a second 
stationary bed of filtering particles 406. Each stationary bed is held in 
place by restraining means 408 and 409, such as a wire screen or a similar 
restraining-type apparatus. There is a first fluid inlet and a first fluid 
outlet, 410 and 412, respectively, at the first end 403 of the section 402 
and a second fluid inlet 414 and a second fluid outlet 418 at the second 
end 405. Each fluid inlet and each fluid outlet has a flow regulating 
means (not shown) by which the flow may be controlled or stopped. 
A first movable bed of filtering particles 422 is inside section 402. The 
bed 422 occupies at least a portion, but not all, of the volume of the 
section 402 located between the first stationary bed 404 and the second 
stationary bed 406. In operation of the filter, the location of the 
movable bed of filtering particles 422 depends upon the direction of the 
flow of a fluid containing particulate impurities. For illustrative 
purposes, the fluid is flowed into the filter 400 through the first fluid 
inlet 410 and then through the section 402 to the surface of the first 
movable bed of filtering particles 422. The fluid is flowed through the 
first movable bed 422, through stationary bed 406 and then out of the 
filter through fluid outlet 418. As the fluid passes through bed 422, 
particulate impurities contained in the fluid are at least partially 
removed and form a precipitate cake 424 at the surface of the bed 422. 
When regeneration is desired, the flow direction of the fluid is changed, 
as shown in FIG. 5, so that the fluid enters the filter through the second 
fluid inlet 514, passes through the movable bed of filtering particles 
522, through the section 502, through the first stationary bed of 
filtering particles 504 and exits the filter 502 through the first fluid 
outlet 512. The fluid is flowed at a rate sufficient to fluidize at least 
a portion of and preferably substantially all of the bed of filtering 
particles 522 and the particles comprising the precipitate cake forming an 
admixture 503. The direction of the fluid flow and its velocity forces the 
filtering particles and the precipitate cake particles toward the first 
end 505 of the section 502. 
As the flow of fluid is continued, the admixture forms a second bed of 
movable particles 602, as shown in FIG. 6. In the second bed of particles 
602, the precipitate particles are distributed throughout at least a 
portion of the second movable bed of particles 602. This distribution 
results in the pressure drop through the filter being reduced as compared 
to the pressure drop through the filter prior to regeneration. 
There are other devices which may be used to practice the invention and the 
two examples shown do not limit the type which may be used. Rather, they 
merely serve to illustrate the fact that the pressure drop across a bed 
filter may be decreased by regenerating the filter using a method which 
results in a distribution of the precipitated particles, throughout at 
least a portion of the particles comprising a bed of filtering particles. 
Likewise, any method used to distribute the precipitate throughout the 
filtering bed is contemplated by the present invention. 
Optionally, the fluid inlet and the fluid outlet may connect to the section 
through one conduit. In such a design, the fluid should be flowed through 
a stationary bed of filtering particles on its way into a filter or as the 
fluid exits a filter. 
Any fluid which is in a liquid state or a gaseous state and has particulate 
impurities to be removed and which is compatible, i.e., nonreactive with 
the filtering particles and with the exposed surface of the filter, may be 
successfully filtered. 
Any particulated material which may be fluidized and is nonreactive with 
the fluid being filtered and with the filter body itself may comprise the 
stationary beds of filtering particles and may comprise the movable bed of 
filtering particles. For example, sand and various ion exchange resins are 
particularly well suited as filtering particles. 
The following examples illustrate the invention: 
EXAMPLE 1 
Two clear plastic tubes with an inner diameter (I.D.) of 2 inches were 
connected at one end by a piece of 1/2 inch I.D. tube as shown in FIG. 1. 
About 4 inches of 20-35 mesh sand was placed in the bottom of each column 
forming a stationary bed of filtering particles. This was covered with 12 
inches of 50-100 mesh ion exchange resin which acts as the movable bed of 
filtering particles. River water with an average solids content of 25-150 
parts per million (ppm) was flowed through the filter. The test was run in 
accordance with the description given concerning FIGS. 1-3. The test ran 
for 16 days, during which time the filter was regenerated approximately 
every 30 minutes by fluidizing the movable bed of filtering particles and 
the precipitate particles and flowing the particulate mixture into the 
opposite section, and forming a new movable bed of filtering particles 
wherein the fluidized precipitate particles were distributed throughout at 
least a portion of the new bed. Regeneration reduced the pressure drop in 
each case. The solids content of the river water was reduced to an average 
of 2 ppm. The average flow rate of the river water was 20 gallons per 
minute per square foot (gpm/ft.sup.2) of resin area (0.43 gallons per 
minute). 
EXAMPLE 2 
The filter and procedures described in Example 1 were used to filter a 
saturated NaCl aqueous brine solution having a solids content of 350 ppm. 
The flow rate of the brine was 7 gpm/ft.sup.2. The flow from the filter 
contained less than 2 ppm solids. The filter was regenerated about every 
30 minutes throughout the run. After each regeneration, the pressure drop 
was reduced.