Water softener abstract
The monosodium and monopotassium citrates are improved iron-sequestering
agents in salt compositions for regenerating spent water softener
cation exchange resins. The dry compositions comprise a surfactant,
an alkali metal chloride selected from the group consisting of sodium
chloride and potassium chloride, and an iron sequestering agent
selected from the group consisting of monosodium citrate and monopotassium
citrate. The composition is preferably provided in the form of compacted
pellets or blocks. The method for regenerating the spent cation
exchange resin bed comprises the step of contacting the resin bed
with an aqueous brine solution containing the citrate and a surfactant
and having a pH of about 3.5 to 4.5.
Water softener claims
What is claimed is:
1. An aqueous composition for regenerating an iron-contaminated
cation exchange resin and removing about 90% or more of the iron,
said composition consisting essentially of:
(a) 400 ppm to 6000 ppm by weight of an iron sequestering agent
selected from the group consisting of monosodium citrate and monopotassium
citrate;
(b) an alkali metal chloride having a molecular weight of from
about 58 to about 75; and
(c) a surfactant.
2. The aqueous composition of claim 1 wherein said surfactant is
an alkali metal salt of an alkylated diphenyl oxide disulfonic acid.
3. The aqueous composition of claim 1 wherein said sequestering
agent is present at from about 1000 ppm to about 1800 ppm.
4. The aqueous composition of claim 1 wherein said sequestering
agent is monosodium citrate.
5. The aqueous composition of claim 1 wherein the amount of the
surfactant is from about 10 ppm to about 400 ppm.
6. An aqueous composition for regenerating an iron-contaminated
cation exchange resin and removing about 90% or more of the iron
contaminating said resin, said composition consisting essentially
of:
(a) 400 ppm to 6000 ppm by weight of an iron sequestering agent
selected from the group consisting of monosodium citrate and monopotassium
citrate;
(b) an alkali metal chloride having a molecular weight of from
about 58 to about 75; and
(c)10 ppm to 400 ppm surfactant;
wherein said composition has a pH of from about 3.8 to about 4.5.
7. The aqueous composition of claim 6 wherein the pH is from about
3.8 to about 4.0.
Water softener description
FIELD OF THE INVENTION
The present invention relates to an improved salt composition for
regenerating cation exchange resin beds in water softeners and inhibiting
fouling of the resin by the iron commonly found in water supplies.
BACKGROUND OF THE INVENTION
Water softening systems are used in households and by industry
to replace hardness cations such as calcium and magnesium with sodium
ions by passing an incoming water supply through a bed of cationic
exchange resin beads or particles. When the ion exchange resin bed
periodically becomes saturated with ions removed from the incoming
water and depleted of sodium ions, it is recharged by passing a
brine solution consisting essentially of sodium chloride through
the resin bed. This replenishes the bed with sodium ions and removes
the calcium, magnesium, or other ions previously removed from the
incoming water.
The iron problem is well known in the softening art, and attempts
have been made to remove iron in all its forms from water softening
resin beds. The exchange capacity of an ion exchange resin bed deteriorates
as the iron in the incoming water collects in the resin bed and
is not removed by the recharging process. Sooner or later, depending
on the level of maintenance of the resin bed and the characteristics
of the water supply being softened, the resin becomes "fouled",
meaning that the resin bed's capacity to soften water has diminished
so much that the resin must be specially treated to restore its
softening capacity or be replaced.
Iron can exist in the water supply as clear water iron, red water
iron, bacterial iron, colloidal iron, or tannate iron. Clear water
iron is iron in the divalent (ferrous), soluble state. Clear water
iron is not visible when the water is drawn, but when the water
is allowed to stand for a prolonged period the ferrous ions are
oxidized by air to become ferric or trivalent ions, which settle
as a precipitate of ferric hydroxide. The iron may also oxidize
after having been exchanged into a resin bed, which may prevent
it from being removed by regeneration. Red water iron is already
oxidized to the ferric state when it reaches the water softener.
Water containing red water iron is cloudy and orange when drawn.
This form of iron may be filtered by the resin bed or may be passed
and be present in the softened water. Bacterial iron is a third
troublesome form of iron, and is caused by iron crenothrix bacteria
which feed on the iron in the water supply. These bacteria thrive
in water softener resin beds supplied with ample iron, and the resulting
biomass clouds the water system, creates a bad taste and odor in
the softened water, and occasionally releases large, unsightly masses
of rust colored material. Colloidal iron is similar to red water
iron, but is composed of particles too small to settle. Colloidal
iron will normally pass directly through a water softener. Finally,
tannate iron, which is quite similar in appearance to colloidal
iron, is ferric iron complexed and held in solution by tannates
or other naturally occurring soil ingredients. This final form of
iron usually passes through a water softening resin bed. Iron present
in any of the previously discussed forms can foul the resin bed.
Oxidation of ferrous iron captured by the resin beads can crack
them, thereby physically degrading the resin bed as well.
Chelating compounds for sequestering iron, including citric acid,
are taught in U.S. Pat. No. 3454503. A method for regenerating
cation exchange resins fouled by iron by adding to the brine regeneration
medium any of a variety of organic acids, particularly citric acid,
is taught in U.S. Pat. No. 2769787. Citric acid is used commercially
in water softening salt compositions to remove iron from the system.
Compositions for regenerating resin beds, comprising a major proportion
of an alkali metal chloride, an alkali metal carbonate, and an alkali
metal carboxylate chelating agent such as sodium and potassium citrate
are disclosed in U.S. Pat. Nos. 4071446; 4083782; and 4116860.
These compositions have extremely high concentrations of sodium
citrate (5 to 15 per cent) and pH values of 7-9.5 and are not designed
for regenerating conventional sulfonate-based resins.
U.S. Pat. No. 4540715 teaches that regular use of a synergistic
combination of sodium citrate, as a sequestering agent, and a surfactant
identified as an alkylated diphenyl oxide disulfonate can protect
cation exchange beds against deterioration caused by the accumulation
of iron, other insolubles, oily and fatty deposits, and other impurities
found in various water supplies. Sodium citrate, the common name
for trisodium citrate dihydrate, imparts a pH of about 8 to a brine
containing it. Citrate ion, the active part of the sequestering
agent, constitutes only about 64% of its weight because of the high
sodium and water content.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a sequestering
agent that is more effective in iron removal from an ion exchange
resin than sodium citrate.
It is a related object of this invention to provide a novel sequestering
agent that operates at a pH as low as about 3.5.
It is another related object of this invention to provide a method
for regenerating an iron-contaminated cation exchange resin and
removing about 90% or more of the iron.
It is yet another related object of this invention to provide a
composition for use in said method.
These and other objects of this invention which will become apparent
from the following disclosure are achieved by a composition comprising
an alkali metal chloride having a molecular weight of from about
58 to about 75 and from about 400 to about 6000 parts, advantageously
from about 1000 to about 1800 parts, of monosodium citrate and/or
monopotassium citrate per million parts by weight of the composition
(hereinafter ppm). The alkali metal chloride is referred to hereinafter
as the water softener regeneration salt or, simply, regeneration
salt. The monosodium citrate and/or monopotassium citrate, hereinafter
referred to as the iron-sequestering agent, is suitably added to
the regeneration salt as a concentrated aqueous solution.
For the purposes of this disclosure, SC means trisodium citrate
dihydrate, MSC means monosodium citrate, and MPC means monopotassium
citrate. In the water softening art, a resin is exhausted when the
effluent hardness of the water is equal to the influent hardness.
In practice, however, regeneration must be initiated prior to total
exhaustion of the resin.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
A granular alkali metal chloride used conventionally as a compactor
feedstock and other components of the composition are mixed together
homogeneously and passed through conventional compacting equipment
to make pellets or blocks for use as water softening products in
this invention.
In one embodiment of the invention, the aqueous solution of the
sequestering agent is made by mixing highly concentrated solutions
of equimolar amounts of citric acid and sodium or potassium hydroxide,
or a mixture of said hydroxides. As a practical matter, the base
is added to the citric acid until the pH of the solution is about
3. The concentration may be as high as the solubility of each component
allows and it is highly desirable to add as little water to the
regeneration salt as possible. Because of the difficulties associated
with dissolving large quantities of solids in relatively small volumes
of water, the concentrations of the acid and the base may be as
much as about 50 % by weight. Thus, from about 900 to about 13000
ppm of the concentrated MSC or MPC solution is added to the regeneration
salt. The amount of said solution may be, for example, from about
2200 to about 3900 ppm of the regeneration salt, by weight.
The concentrated solution of the iron-sequestering agent may be
mixed with the regeneration salt at any convenient stage before
the salt is fed into the compacting apparatus, sprayed on the compacted
salt, or added to the salt as it is being conveyed to the compacting
apparatus by a screw conveyor. The pH of brine made from the thus
treated regeneration salt may be, for example, about 3.8 or from
about 3.5 to about 4.5.
The iron oxides and other particulate matter are often bound to
the resin beads by hydrophobic oily matter which defies removal
when the resin bed is backwashed or otherwise treated with aqueous
solutions. Surfactants have been recommended in the water softening
literature for restoring resin beds which are so fouled that regeneration
would be pointless without pretreatment. Thus, the composition of
this invention may include an amount of surfactant suited for its
routine use for the removal of iron while regenerating cation exchange
resin beds. Suitably, therefore, the composition of this invention
contains from about 10 to about 400 ppm of a surfactant, advantageously
from about 20 to about 300 ppm. Surfactants suitable for this invention
are anionic, safe for human consumption at low levels in drinking
water, low foaming at the normal level of use, soluble in saturated
brine, compatible with hard water, and stable at the high temperatures
encountered when compacting granulated salt to form products such
as pellets. Suitable surfactants for this invention have the following
formula: ##STR1##
wherein X is a hydrogen ion or an ion of an alkali metal having
an atomic weight of from about 23 to about 40 Y is an ion of an
alkali metal having an atomic weight of from about 23 to about 40
R.sup.1 is an aliphatic moiety having from 6 to 16 carbon atoms,
e.g., an n-decyl moiety, and R.sup.2 is selected from hydrogen and
R.sup.1. Said surfactants are commercially available under the trademark
DOWFAX from the Dow Chemical Company, Midland, Mich. DOWFAX 3B2
for example, is a mixture of the sodium salt of an alkylated diphenyl
oxide disulfonic acid wherein R.sup.1 in the above formula is n-decyl
and R.sup.2 is hydrogen and the sodium salt of another wherein both
R.sup.1 and R.sup.2 are n-decyl, and is available as a 45 % by weight
solution in water.
The method for regenerating the spent cation exchange resin bed
of a water softener comprises the step of contacting the resin bed
with an aqueous solution containing from about 50000 to about 150000
ppm of a regeneration salt having a molecular weight of from about
58 to about 75 from about 400 to about 6000 ppm of the iron-sequestering
agent, and, optionally, from about 20 to about 300 ppm of a surfactant.
The pH of the brine that contacts the iron-contaminated exchange
resin may be about 3.8 or from about 3.5 to about 4.5. The routine
employment of this method will avoid the necessity of taking more
extreme measures after the resin bed becomes fouled.
The invention does not exclude other ingredients which do not detract
appreciably from the utility of the compositions disclosed herein.
Thus, the following examples are provided solely for the purpose
of illustrating rather than limiting the invention.
EXAMPLE 1
Twenty-five (25) gram samples of a fouled cation exchange resin
having a certain iron content were weighed into each of 4 jars and
50 mls of a 10%.sub.wt sodium chloride solution in deionized water
with or without additives, as shown in Table I, was added to the
jars. The jars were capped and shaken for one hour and the solutions
were filtered through #42 Whatman paper. The filtrates were diluted
to a certain volume and analyzed for solubilized iron. The results
in Table I show that, at equal weights, monosodium citrate removed
68% more iron than the sodium citrate and 69% of the amount removed
by citric acid.
TABLE I Additive (0.35% of NaCl weight) mg iron removed None 85
Citric acid 610 MSC 420 SC 250
The following laboratory procedure was employed to measure the
effect of the improved salt composition on the exchange capacity
of iron-contaminated resins as compared to sodium chloride plus
sodium citrate and sodium chloride alone.
Water softener simulators, approximately 1/45 the size of a full
scale unit, were modeled after a residential water softener. These
simulators consisted of upright 1000 ml cylinders 167/8 inches (429
mm) tall, having an inside diameter of 23/4 inches (60 mm), plugged
at the top by a rubber stopper penetrated by two tubes or pipes.
A center tube extended to the bottom of the cylinder and was fitted
with a fine-mesh screen to prevent resin from washing out. The lower
end of a side tube was flush with the bottom of the stopper. Influent
water could be directed down the center tube for backwashing the
resin (upflow) or through the side tube to simulate the brine and
rinse, siphon, and softening cycles of a softener.
Approximately 200 ml of glass beads were placed at the bottom of
the cylinders before adding exactly 500 ml. of packed-down resin
beads collected from residential water softeners. The volume of
the beads was measured by placing a graduated cylinder containing
resin beads and water on a vibrating table until settling was complete,
then resin beads were added or removed to provide exactly 500 ml
and the stopper was tightly secured to close the system.
The resin was backwashed with 2150 ml. of untreated water at a
flow rate of 1080 ml/min to attain 50% expansion of the resin bed.
The backwash water was filtered and the filter pads were saved for
analysis. The water level was then siphoned down to an operating
level of about 3 inches above the resin before starting the brining
(regenerating) step.
The resin was regenerated with 481 g. of 10% sodium chloride solution
(6 pounds of NaCl per cubic foot of resin) (2.7 kg per 28 liters)
with no additives (Control) and with test additives. The brining
flow rate was 22 ml./min. After brining, the resin was rinsed slowly
with 660 ml. of deionized water at 33 ml./min. for 20 minutes, followed
by a fast rinse consisting of 840 ml. of deionized water at 84 ml./min
for 10 minutes. Effluents from the brine and rinse cycles were collected
in a 2 liter volumetric flask for subsequent analysis.
Well water, obtained by a submersible pump situated 160 feet below
the well head, was filtered through a 20 micron filter cartridge
to remove particulates and precipitated iron. The remaining iron,
in the form of clear water iron, amounted to 2 ppm. As little as
0.3 ppm iron is enough to cause iron-staining problems. The hardness,
expressed as calcium carbonate, was 375 ppm or 21.9 grains per gal.
On a weight basis, calcium represents about 62% of the hardness
while magnesium represents 38%. Approximately fifty (50) liters
of the well water was then passed through the resin at a flow rate
of 300 ml./min. Thus, approximately 100 mg of iron was exchanged
onto the resin in each run of the test procedure. In each run, the
regeneration, rinsing, and softening steps were repeated. An automatic
shut-off system is used to shut down the softening step when the
resin needs to be regenerated. A sodium ion electrode inserted in
the effluent water stream monitors the sodium ion concentration.
The electrode senses a slow drop in the sodium ion concentration
as it is replaced with hardness when breakthrough begins. In conjunction
with an oxidation/reduction potential (ORP) controller and a solenoid
valve, the hard water is shut off when the hardness reaches an arbitrary
value in the vicinity of 115 ppm (.about.7 grains/gal). The simulator
is then put into the backwash mode and water is passed up through
the bed to remove entrapped air. The final volume (or softening
capacity) of softened water was then recorded from a flow totalizer
connected to a flow meter. In addition, the capacity is corrected
by analyzing water hardness at shut-off and a factor applied in
an EXCEL spreadsheet to calculate the volume of water treated at
exactly 115.0 ppm hardness. The factor is derived from the slope
of the exhaustion curves for the resins variously treated in each
simulator.
The compositions of the regenerating salt solutions used as the
Control and as examples of this invention, and as a Comparative
Example (C. Ex.) are given in Table II. Test results are given in
Tables III-V. Table III shows the amount of iron removed per regeneration
cycle by the test formulations. Table V shows capacities and capacity
losses. Table VI shows analyses of composite softened water samples
taken during the 5-35 liter portion of a softening cycle. |