Note: Descriptions are shown in the official language in which they were submitted.
SPECIFICATION
The present invention relates -~o an improved method
for repeatedly recovering and reusing liquid used to back-
wash a precoated filter.
Precoated filters are well known in the ar~, and
have a variety of uses in the liquid purification field.
Where ~he precoated filter is used for the removal of un-
dissolved solids, conventional filter aid materials may be
employed as the precoat, such as diatomaceous earth, re-
generated cellulose, expanded perlite, and the like. If it
is also desired to remove dissolved solids, the precoat
comprises finely divided ion exchange resin particles, pre-
~era'bly in the size range of 60 to 400 mesh. These ion ex-
change resin particles usually comprise a mixture of anion
and cation exchange resin particles, which exhibit the phe-
` nomenon of "clumping," reducing the pressure drop across -'
'' these particles, and increasing the efficiency of undissolved
solids removal. Because of this high efficiency in un- ~'
dissolved solids removal, finely divided ion exchange resins
are frequently employed for undissolved solids removal, even
where the removal of dissolved solids ls of little or no
importance. In addition, ion exchange resins are sometimes
used in combination with additional precoat layers of the
aforementioned conventional filter aid materials, or such
filter aid materials may be mixed with the ion exchange resins. ; '
~,~ One area of particular application for precoated
filters is in the production of electrical power with steam-
driven turbines. Such a system includes a steam generator
which produces steam to drive a turbine generator. The
steam is then condensed and returned to the steam generator
3~ for revaporization. The source of heat for the'steam
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generator may be either fossil fuel or nuclear energy.
It is essential that the water recycled through a
steam generator be of extreme purity, preferably having
dissolved and undissolved solids in the Low parts per billion
range, the particular purity requirement depending on the
type of system employed. In order to maintain these high
purity levels, a "condensate polishing" system is conven-
tionally employed to remove impurities from ~he condensate
stream. Precoated filters of the type described above form
a particularly suitable apparatus for performing this con-
densate polishing operation.
Precoated filters also have application in the -
product;on of electrical energy in connection with auxiliary
systems, for example for the treatment of fuel pool water
in nuclear reactors and in the disposal of radioactive i:
wastes.
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Precoated filters are periodically backwashed in
order to remove a precoat layer which has become exhausted
through the collection of dissolved and/or undissolved
impurities. Such removal is usually accomplished long
before absolute exhaustion of either the ion exchange or
particle absorption ability of the resins. The point at
which removal of the precoat is required depends upon whether
the undissolved solids capacity or ion exchange capacity
is exhausted first. If the undissolved solids capacity
is exhausted first (or if dissolved solids removal is rela-
tively unimportant), backwashing is initiated when a pre-
determined pressure drop is reached. If dissolved solids
capacity is exhausted first, backwashing is initiated upon
reaching a predetermined conductivity in the effluent.
Irrespective of the criterion for terminating the service
cycle, the point at which this is accomplished is referred
to herein as the "service cycle termination point." The
backwash is generally accomplished by the use of both liquid
and gas.
In order to avoid contamination, it is essential
that the backwash liquid be of a high purity level. Here-
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tofore, the backwash liquid has generally been demineralizedwater provided by external ion exchange sys~ems. The use
of such ion exchange systems is expensive, and it is there-
fore desirable to minimize the amount of water which must
be passed through such a system. When a nuclear power
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plant is involved, it is also important to minimize the
delivery of used backwash water to a drain, since such
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w~ter may contain radioactive impurities, and must there-
~ore be delivered to a suitable disposa:L -facili~y in or~er
to remove such impurities~ In nuclear systems, it is
therefore highly desirable to ret~in tl~:is b.lc~w~sh ~ tet
in the system, not only to minimize demand for additional
demineralized water, but also to minimize the need for
treatment of r~dioactive wastes.
In accordance with the present :invention, a method
is provided for recovering and reusing backwash liquid whic]
has been used to backwash a precoated filter, so tha~ the
need for demineralized makeup water and/or for radio~cti~e
waste disposal are both minimized. In carrying out the
method, the bulk of suspended precoat is removed fronl the
backwash e~fluent which has been used to remove the precoat
in a previous filter backwash cycle, and the e~fluent is
stored. A precoated filter is run to its service cycle
termination point, and the stored backwash ef~luent is ~-
passed through the precoated filter to remove impurities.
The treated backwash ef~luent is then used to backwash
the filter to remove the precoat. The bulk o~ suspended
solids are removed, and the liquid is again stored for
purification and use in a subsequent backwashing cycle
as previously described.
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More particularly, in one aspect the invention
comprehends a method ~or recovering and reusing backwash
li~uid contaminated with dissolved and undissolved im~
purities and which is used to backwash a filter at the
end of a service cycle, the filter being precoated with
ion exchan~e resin particles in the s~ze range of 60-400
mesh. The method includes separating a ma]or first portion
cf the u~dissolved impurities from a backwash liquid that
has been used to backwash a filter in a previous backwash
cycle so that a smaller second portion of the undissolved
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impurities remains with the liquid, placing the remaining
second portion of the undissolved impuri-ties in suspension
in the li~uid from which the major ~irs~ portion of the
undissolved impurities has been separated, delivering the
liquid containing the remaining second portion of the un-
dissolved impurities which has been placed in suspension :~
through the precoated filter at the end of a service
c~cle of the precoated filter to remove the dissolved
impurities and the remaining second portion of the un- :
dissolved impurities, backwashing the filter wi~h the
liquid from which dissolved and undissolved impurities
have been removed, and storing the liquid for use in a
later backwash operation.
Another aspect of the invention comprehends a
method for recovering and reusing backwash water contamin-
; ated with dissolved and undissolved impurities in a filter
system having first and second filters for alternate use
in a service cycle, the first and second filters having a
- precoat of ion exchange resin particles in the size range
2Q of 60-400 mesh. The method provides for backwashing the .;
first precoated filter by delivering water from a backwash
storage zone to the first precoated filter to remove the `~
precoat, delivering the backwash water from the first pre-
coated filter to a phase separation zone and separating ~ .
suspended solids from the water in the zone, delivering
the water from the phase separation zone to a backwash
receiving zone isolated from the phase separation zone,
delivering the water from the backwash receiving zone through
a second precoated filter after the end of the service cycle
for the second precoated filter whereby dissolved and un-
dissolved solids are removed from the water, and delivering
the water to the backwash storage zone. ~ -
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The invention, together with the objects
and advantages thereof, will be best unders-tood by
re~erence to the following detailed description, taken
in conjunction with the drawing, which is a diagrammatic
view of a system designed to carry out the present
invention.
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Referring to the drawing, an appara~us suitable for
carrying out the method of the present invention includes
first and second filter units, indicated respectively by
reference numerals 10 and :L2, which employ a precoat as
previously described. As those skilled in the art will
appreciate, the present invention is also adapted to use
with a system having only one filter unit, or a system hav-
ing more than two such units. Usuallyg a condensate polish-
ing system will employ at least two filter units, and some-
times more than two, so that at least one filter unit is onstream while another unit is being backwashed and coated
with a fresh layer of precoat. The specific embodiment of
; the invention described herein is in connection with a con-
densate polishing system employing two filter units and an
ion exchange resin precoat. It will be understood by those
skilled in the art that the invention is not limited to such
a system, but is applicable to other types of systems and
other precoat materials. The filter units 10, 12, are, for
example, of the type described in U. S. Patent No. 3,279,608,
although the exact design of the filter unit forms no part
of the present invention. ~ach of the filter units 10, 12
has a service inlet pipe 14 having a valve 16 and a service
outlet pipe 18 having a valve 20. During the service cycle,
condensate is delivered for polishing through the filter
units 10, 12 via the service inlet pipes 14 and service outlet
pipes 18. The service inlet pipes 14 each communicate with
an inlet pipe 22 having a valve 24, while the service outlet
pipes 18 communicate with an outlet pipe 26 having a valve 28.
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The system shown in the drawing also includes a
precoat tank 30 having a mechanical agitator 32 driven by
an electric motor 34. Any suitable system may be employed
' for agitating the precoat and maintaining it in suspension,
and the agitator 32 is shown merely by way of example. The
precoat tank 30 has an outlet pipe 36 having a pump 38, and
communicating with a transfer pipe 40. The precoat tank 3()
also,has an inlet pipe 42 having a valve 44, and communi~
cating with a transfer pipe 46.
In the lower portion of the drawing, there are
, illustrated a backwash storage tank 48, a backwash receiving
: tank S0, and a phase separator 52. The backwash storage
tank 48 has an upper inlet pipe 54 having a valve 56, and
communicating with the transfer pipe 46. The backwash
storage tank 48 also has a lower outlet pipe 58, having a
valve 60, which communicates with a vertical transfer pipe
, 62. The backwash storage tank 48 also has a makeup water :' :
inlet pipe 64 communicating with a suitable source of
I makeup water (not shown) in order to maintain an adequate
supply of backwash water.
, The backwash receiving tank 50 has a lower outlet
pipe 66, having a valve 68, communicating with the vertical
transfer pipe 62. The tank 50 also has an upper inlet pipe
70, having a valve 72, also communicating with the vertical ,~
transfer pipe 62. ~inally, a backwash conduit 74 is shown :,
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,~ communicating with the inlet pipes 22 of the filter units
10, 12, and also with~an upper portion of the backwash ' ,
receiving tank 50 vla a backwash inlet pipe 76 having a '
valvè 78. ~ : ~
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The vertical t~ransfer pipe 62 has a pump 80, which
is positioned to deliver liquid from either the outlet pipe
58 in the backwash storage tank 48 or the outlet pipe 66
in the backwash receiving tank 50. This vertical transfer
pipe 62 commllnicates with the trans-fer pipe 46, and has a
valve 82. The vertical transfer pipe 62 also has a back- -
wash delivery pipe 84 communicating therewith between -the
pump 80 and the valve 82. The backwash delivery pipe 84
communicates with both the inlet pipes 22 to the filter units
10~ 12, and has a valve 86 which can control flow to either
of these inlet pipes.
~ inally, the phase separator 52 provides a phase
separation zone, which is used to separate the bulk of the
ion exchange resin collected during a backwashing cycle from
the backwash liquid. To this end, the phase separator 52
has a lower outlet pipe 90 communicating with the phase
separator 52 at a point above the upper level of any ion
exchange resin collected in the phase separator 52. The
outlet pipe 90 has a valve 92, and communicates with a back-
wash transfer pipe 94 having a pump 96. The backwash trans-
fer pipe 94 is connected to a lower portion o~ the backwash ;
receiving tank 50, and has a valve 98 which is located be-
low the junction with the outlet pipe 90. The backwash trans-
fer pipe 9~ communicates with an interconnection pipe 100, . :~
; which interconnects upper portions of the backwash receiver ~.
; 50 and phase separator 52. The interconnection pipe 100
has a pair of valves 102, 104 located on either side of the ; .
juncture with the backwash transfer pipe 94 so that liquid :~:
may be delivered through the pipe 94 to either the backwash
receiver 50 or the phase separator 52.
Finally, the phase separator 52 has a lower outlet
pipe 106 having a valve 108, which may be used to deliver
exhausted ion exchange resin ~rom the lower portion of the
phase separator to a suitable disposal facility (not shown).
When the invention is used in connection with a nuclear
power plant, such a disposal facility must be appropriate
for disposal of radioactive wastes.
It must be emphasized that the particular apparatus
shown in the drawing is merely exemplary of an apparatus
that can be employed to carry out the method of the present
invention, and that this method could be carried out with
; significantly different apparatus. It will further be
appreciated by those skilled in the art that the location
of the various pieces of apparatus and pipes shown in the
drawing is purely arbitrary. In fact, in many instances,
the phase separator 52, for example, will be isolated and ~ :
located remotely from the backwash receiver 50, and, in
instances where this is not the case, it is even possible
to entirely dispense with the backwash receiver 50, since ~-
the phase separator 52 can also operate to provide a
suitable backwash receiving zone.
In describing the method of the present invention
in conjunction with the apparatus illustrated in the drawing,
it will be assumed that filter unit 10 has just been shut
down for a backwashing cycle, whi:Le filter unit 12 is in
normal operation. In a typical installation, at the begin-
ning of the service cycle, the ion exchange resin will pro-
duce an initial pressure drop of about 3-5 psi at a flow ``
rate of 4 gal./min./ft.2, and the service cycle will be
terminated when the pressure drop reaches about 25 psi at
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the same flow rate. Generally, the particle capacity of
the resin will be exhallsted before the ion exchange capacity,
and continuation of the run beyond a Z5 psi pressure drop
would cause the precoat particles to become embedded in the
filters, so that backwashing would be difficult. In in-
stances where the ion exchange capacity is exhausted before
the particle removal capacity, and when it is important to
remove dissolved materials from the stream, the service
cycle is terminated when a sharp increase in effluen~ con-
ductivity is noted.
At the termination of the service cycle, all valvesare closed with the exception of ~he inlet and outlet valves
16, 20 associated with the right-hand filter unit 12. At
this stage, the backwash receiver 50 contains backwash liquid -
that has been used ln the prior backwash cycle of -filter
unit 12, which has had the bulk of the solids removed in the
phase separator 52, in the manner hereinafter described.
In instances where the ion exchange capacity of the
precoat has been exhausted, it is sometimes desirable to
provide water that is demineralized for the backwash cycle.
To this end, a mixture of regenerated anion and cation ex-
change resins is added to the backwash liquid contained in
the backwash receiver 50 in an amount sufficient to de-
mineralize the liquid. Alternatively, an overlay ~recoat
of fresh resin may be placed on the filters in the filter
unit 10 a-fter termination o~ the service cycle and before
initiation of the backwash step. To this end, valves 28,
24, and 44 are opened and pump 38 is started to deliver
sufficient precoat from the precoat tank 30 to put a precoat
overlay on the exhausted resin in the filter unit 10 in an
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amount sufficient to demineralize the backwash liquid when
it is passed through the filter unit. The pump 38 is then
stopped, and valves 24, 28, and 44 are closed.
To initiate the backwashing step, the valves 68, 72
associated respectively with the backwash receiver outlet
pipe 66 and backwash receiver inlet pipe 70 are opened~ and
the pump 80 on the vertical transfer pipe 62 is started.
This causes backwash liquid to be recirculated through the
backwash receiver 50, so that any solids which were not
removed in the phase separator 52 are placed in suspension.
In addition, any fresh resin which has been added to de- :
mineralize the liquid is also suspended. While this step
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is being accomplished, a suitable hold pump (not shown)
may be utilized to continuously recirculate the liquid con-
tained in the filter unit 10, so that the precoat is main-
tained on the filters.
After the solids have been suspended in the back-
wash receiver 50, the valve 72 on the inlet pipe 70 is
closed, and the valves 24, 28, 86 and 56, located respectively
on the filter unit inlet pipe 22, the filter unit outlet
pipe 26, the backwash delivery pipe 84 and the backwash
storage tank inlet pipe 54 are opened. This operation per-
mits liquid to be pumped from the backwash receiving tank 50
through the filter unit 10 in the normal flow direction, so
that dissolved and undissolved solids are removed in the fil-
ter unit 10, and the liquid is then delivered to the backwash
storage tank 48 via the transfer pipe 46. If insufficient
liquid is available in the backwash storage tank 48, add-
itional makeup liquid may be added at this point via the
makeup water inlet pipe 64.
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In a condensate polishing system, the precoat will
normally reach its service cycle ter~ination point as a
result of absorption of suspended solids, so that the
pressure drop across the filter reaches its maximum? while
the resin retains ion exchange capacity. When this is the
case, the liquid from the backwash receiver 50 must be
pumped through the precoat at a flow rate below the normal
servlce cycle flow rate. For example, if the service cycle
flow rate is a typical 4 gal./min./ft.2, the liquid from the `~
backwash receiver 50 must be passed through the resin at a
lower flow rate, say 1 gal./min./ft.2, to avoid exceeding
the maximum pressure drop for the filter unit 10. Although ;
such a low flow rate is impractical during the service cycle,
it is adequate for treating the relatively small amount of
liquid needed for the backwashing step.
After the liquid from the backwash receiver 50 has
; been passed through the filter unit 10 and delivered to the
backwash storage tank 48, the unit is ready for the back-
washing cycle. In carrying out this cycle, backwash liquid,
20 which has been purified in the manner just described, is `
passed through the filter unit 10 in reverse-flow direction.
In order to accomplish this~ the valves 56, 68, 86 on the
backwash storage inlet pipe 54, the backwash receiving tank
outlet pipe 66 and the backwash delivery pipe 84, respect-
ively, are closed. At the same time, the valves 60, 82, and
78~ located respectively on the backwash storage tank out-
: let pipe 58, the vertical transfer pipe 62, and the backwash
inlet pipe 76 are opened. This operation permits liquid to
flow out of a lower portion of the backwash storage tank ;
through the pump 80 and vertical trans~er pipe 62 into the
transfer pipe 46, and then into the ou~let pipe 26 ~From
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the filter unit. Sînce ~his liquid is flowing into the
outlet pipe, it flows in reverse direction through the
filter, and removes the precoat. The backwash liquid and
suspended precoat then pass out of the filter unit 10 through
the outlet pipe 22, into the backwash conduit 7~, and then -
into the backwash receiver 50 through the backwash inlet
pipe 76. It will be understood by those skilled in the art
that this backwashing step may be accomplished in conjunction
with one or more air scour steps, which is well known in the
art, and which is described in aforementioned U. S. Patent
No. 3,373,104.
After completion of the backwashing step, all of the
backwash liquid and removed precoat will be in the backwash
receiver 50. The filter unit 10 is now ready for the appli-
cation o-f a new precoat. To this end, valves 60, 82, and 78
are closed, while the valve 44 on the precoat tank inlet
pipe 42 is opened and the pump 38 on the precoat tank outlet
pipe 36 is started. Liquid containing a fresh, suspended
precoat is thus delivered through the filter unit 10 in the
~`20 normal flow direction, placing a precoat on the surface of
the filter for use in the next service cyçle. As is well
known in the art, recirculation of this precoat suspension
through the filter is continued until a sufficient precoat
;,layer is applied. After application of the precoat, the
filter unit 10 is ready to be returned to its normal service
cycle, which can be accomplished by stopping the precoat
pump 38, closing the valve 44 on the precoat tank inlet
pipe 42, and by closlng the valves 24, 28 located on the
filter unit inlet and outlet pipes 22, 26 respectively.
30 The service cycle is then initiated by opening the valves ~
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16, 20 on the service inlet and outlet pipes 14, 18, re- ~
spectively. ~ .
While the precoating s-tep is being accomplished,
the backwash liquid and exhausted precoat in the backwash
receiver 50 may be transferred to the phase separator 52
~ by opening the valves 98, 104 on the backwash trans:fer pipe
-~ 94 and the interconnection pipe 100. The pump 96 is started,
and the liquid and suspended resin is pumped into the phase
separator 52~ where it is permitted to remain for a time
10 period sufficient for the bulk of the solids to settle out. . .
The open valves 98, 10~, are closed, and the valves 92, 102
on the phase separator outlet pipe 90 and the interconnection
pipe 100, respectively, are opened, so that the pump now
delivers backwash liquid from a lower portion of the phase
separator 52 back into the backwash receiver 50. The solids
collected in the lower portion of the phase separator 52
may be removed by opening the valve 108 in the lower outlet
pipe 106, and delivered to a suitable disposal facility.
It will be seen that, at this stage, the unlt is in the same ;
20 condition as it was at the beginning of the description of
the operation, except that the right-hand filter unit 12 ~ ;
is now ready to be taken o-ff stream for backwashing, while :-~
. the service cycle is handled by the left-hand fil~er unit
10. As will be appreciated by those skilled in the art, :
appropriate valving has been provided so that an identical
operation may be accomplished in the right-hand unit 12 ` .
as was previously described in connection with the left- :.
hand unit 10, so that such a description need not be .
undertaken.
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The following examples are intended to illustrate
- the present invention, and should not be construed as limi-
tative, the scope of the invention being determined by the
appended claims.
Example I
A system similar to that shown in the drawing was
used to treat a condensate stream in a nuclear power plant.
Each filter unit had replaceable tubular nylon-wound filter
elements of the type shown and described in U. S. Patent No.
3,279,608, having a total of 1500 ft.2 of filter surface
area. The filter cartridges were precoated with a mixture
of anion and cation exchange resin particles in the size
range of 60-400 mesh. The cation exchange resin was a
commercially available, strong-acid resin in the hydrogen
form, and the anion exchange resin was a commercially
available strong-base resin in the hydroxide form. The
resins were coated on the filter elements to a depth of
about 1/4 in.
The service cycle flow rate was maintained at 4
gal.jmin./ft.2 of filter area, and the initial pressure
drop was about 2-3 psi across the elements. The service
cycle was continued until the pressure drop reached 25 psi,
; while the effluent conductivity was less than 0.1 micromho.
The backwashing cycle was initiated by recirculating
15,000 gallons of used backwash water from a previous back-
washing cycle in a backwash receiving tank, so that any
'! particles were suspended. The backwash liquid was then
delivered through the filter unit at a flow rate of 1 gal./
min./ft.2, producing a pressure drop of about 7 psi. The
effluent was a clear stream of demineralizecL water, which
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was delivered to a backwash storage tank. lO() gallons of
demineralized water were added to the tank to produce a
total volume of about lS,100 gallons of backwash water.
The backwash liquid was then delivered through the filter
elements in reverse-flow direction at a flow rate of 1 to
1.5 gal./min./ft.2. Air was also passed through the filter
elements at a rate of 1 to 1.5 standard ft.3/min./ft.2 to
air-scour the filter elements and to aid in removing precoat.
This combination of backwash liquid and air scouring is well
known in the art, and is described in U. S. Patent No. 3,-
666,097, which is assigned to the assignee of this appli-
cation.
The backwash water and suspended precoat was de-
livered to a phase separator, where it was retained for one
- hour to permit the solids to settle, and was then trans-
ferred to a backwash receiving tank for use in a subsequent
backwashing cycle, as hereinbefore described. The filter
; elements are then coated with a fresh layer of precoat and
returned to the service cycle.
Example II
Example I was repeated, except that the service
cycle was terminated when the pressure drop across the
resin is only 20 psi. The cycle was terminated because
the effluent conductivity rose above 0.1 micromho, in- ;~
dicating that the ion exchange capacity was exhausted.
Because the precoat had exhausted its ion exchange
capacity, 5 pounds of resin precoat were added directly to
the backwash liquid in the backwash receiving tank to de-
mineralize the water. The backwash water was then passed
through the resin as in Example I. The filtered water was
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deli.vered to the backwas storage tank and was used to .
backwash the resin as in Example I. The filter was re~ .
coated and returned to service as in the preceding para-
graph.
Obviously, many modifications and variations of the
invention as hereinbefore set forth will occur to those :
skilled in the art, and it is intended to cover in the
appended claims all such modifications and variations as
fall within the true spirit and scope of the invent:ion.
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