Report on Water Softening Plant

7/29/2019 Report on Water Softening Plant

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REPORT ON WATER SOFTENING PLANT

Submitted To:

Mr. Javed Iqbal Butt

Sr. Deputy Manager Utilities

Submitted By:

Gulfam Shahzad

Trainee Chemical Engineer

OCP #: TRN0702

August 2013

Olympia Chemicals Limited, WarchhaDist. Khushab


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Softening Plant

The raw water contains the ions that are to be removed by ionexchange. When these ions come in contact with theregenerated ion exchanger during its service, it displaces theions that was attached to the resin and takes their place on theresin. When hard water is passed through a bed of sodiumcation exchanger, the calcium and magnesium are taken upand held by the cation exchanger which simultaneously givesup an equivalent amount of sodium in exchange for them.

Ca} {(HCO3)2 + 2NaR Ca} R2 + Na2{(HCO3)2

Mg} {SO4 Mg} {SO4{Cl2 {Cl2

When the ability of a sodium cycle exchanger to producecompletelysoftened water is exhausted, the softener unit is taken out ofservice andregenerated with brine solution. This reaction may berepresented by the following:Ca} R + 2NaCl Na2R + Ca} Cl2

Mg} Mg}

Sodium Zeolite softener is used to remove hardness of calciumand magnesium from the water. Hardness removal lowersthe scale formation properties of water. The softener vesselcontains synthetic ion exchange resin. When water is passedthrough the resin bed, Sodium ion exchanged with thehardness (Ca and Mg ions). Sodium is very soluble and will notresult in scale formation in the plant vessel. Clean water with

lower hardness, alkalinity, silica and TDS leaves the softener.PH of the water is raised by adding Ca(OH)2 in catalyser. Limeis added to make calcium and magnesium compounds lesssoluble. Hardness (Ca and Mg) precipitate as CaCO3 andMg(OH)2 sludge. Some silica also precipitates out with Mg(OH)2sludge. The sludge is recirculated to the incoming water toincrease the reaction rate. The sludge level is controlled byblow down.

Main Equipment of Plant


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Catalyser Settler Coke Filter Sodium zeolite filter

Soft water storage tank MOL and Brine tank

Reactions taking place in softening plant

In Catalyser (Ca(OH)2 is added)

Mg(HCO3)2 + Ca(OH)2 Mg(OH)2 +Ca(HCO3)2

Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O

MgCl2 + Ca(OH)2 Mg(OH)2 + CaCl2

MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4

In Softener

Ca2+ + Na2Ze CaX + 2NA+

Mg2+ + Na2Ze MgX + 2Na

+

Regeneration (brining)

2NaCl + CaZe Na2X + CaCl2

2NaCl + MgZe Na2X + MgCl2

Final Product ParametersPH = 7 11 (limit), 8.3 9.0 (operating range)OH- = 50 ppm(limit) , 4 12 ppm (operating range)NaCl = 2000 ppm or 2%

Total Hardness = 100 ppm (limit), Nill (operating range)

Brine Solution Parameters


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PH = 11 12Concentration = 102 ttCa+ = 20 ppmMg+ = 10 ppm

MOL (Ca(OH)2) ParametersConcentration = 140 155PH = 11 - 12

Raw water ParametersPH = 7.8Conductivity = 208

TDS = 118 ppm

Resins CharacteristicsZeolite known as polystyrene resin, are most commonly usedresin now a days. Cost is reasonable, and it is easy to controlthe quality of the resin. They also have much higher ionexchange capacities than the natural material.

The ability of the resin to remove hardness from the water is

related to the volume of resin in the tank. Softeners shouldremove about 50,000 grains of hardness per cubic foot of resin.Resins hold hardness ions until they are regenerated with a saltbrine solution. The hardness ions are exchanged for sodiumions in the salt brine.

SoftnersThe interior is generally treated to protect the tank againstcorrosion from the salt. The units are normally of the downflow

type, and the size and volume of the units are dictated by thehardness of the water and the volume of treated water neededto be produced between each regeneration cycle. Resin issupported by an underdrain system that removes the treatedwater and distributes brine evenly during regeneration.Minimum depth of resin should be no less than 24 inches abovethe underdrain.

Softner regeneration

1) Backwash Cycle


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Backwashing, as the name implies, involves the passage ofwater upwardthrough the resin bed for the following reasons:1. Bed expansion releases any accumulation within the resin

bed and fluffs the bed to allow more efficient contact betweenbrine and resin during the brining step;2. Particle and resin fines removal prevents channeling, highpressure drop, and poor kinetics during the service step;3. Regrading or classification of the resin bed contributes to theuniform distribution of regenerant during the brining step.Once hardness breaks through, the softener must beregenerated. In down-flow units, the resin must first bebackwashed to loosen the resin (it becomes compacted by the

weight of the water), and to remove any other material that hasbeen filtered out of the water by the resin. The operator needsto apply enough backwash water to expand the resin bed byabout 50 percent. Distributors at the top of the unit provide foruniform water distribution and uniform wash-water collection.Under drains provide uniform distribution of the backwashwater on the bottom of the resin.

2) Regeneration (Brining)

In water softening, the primary factor determining capacity isthe regenerant level (pounds of sodium chloride per cubic footof cation exchange resin). Regenerant concentration (usually5% to 15% when introduced) and flow rate and kinetic loadingof the resin also influence capacity. Concentrated brine ispumped to the unit from the storage basin. Brine is dilutedthrough the injector to a solution containing about 10 percentsalt before it is passed through the resin. The time required forregeneration is about 20 to 35 minutes. The flow rate of brine

through the resin is measured in gallons per minute per cubicfoot of media. The brine needs to be in contact with the resinlong enough to allow for complete exchange of hardness ionson the resin with sodium ions in the brine. It is better to allowtoo much time than to not allow enough. If the resin is nottotally recharged, the next softening run will be short.

3) RinsingThe rinsing of bed removes remaining brine from the tank. The

total amount of rinse water needed is 20 to 35 gallons per cubicfoot of resin. The rinse is started at a slow rate (-2 gpm/square


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foot of surface area-) and continues until the chlorideconcentration of the effluent is quite low.

Salt StorageSalt is stored as a brine, ready to be used for regeneration ofthe resin. The amount of salt needed ranges from 0.25 to 0.45pounds for every 1,000 grains of hardness removed. The tankshould be coated with a salt-resistant material to preventcorrosion of the tank walls.Salt storage tanks should be covered to prevent contamination.A raised curb should be provided at each access hatch toprevent contamination by flood water or rain.

Filling a salt storage tank with water first and then adding saltis the preferred method for making brine. The brine is heavierthan water and settles to the bottom of the tank. The brine isusually pumped from the tank to the ion-exchange units. Whenmaking brine, water must be added through an air gap to avoidback siphonage of the brine to the water system.

Brine Feeding EquipmentIn water softening, the primary factor determining capacity is

the regenerant level (pounds of sodium chloride per cubic footof cationexchange resin). Regenerant concentration (usually 5% to 15%when introduced) and flow rate and kinetic loading of the resinalso influence capacity. Concentrated brine containsapproximately 25 percent salt. The brine should be diluted toabout 10 percent before added to the softener.

Silica removing

The residual silica can be predicted from the water analysis andthe dosage of adsorbent applied in the treatment process.Residuals range from 90% in cold process to as little as 5% inhot process, depending on the adsorbent addedor on the magnesium precipitated by lime softening.Silica is also removed in lime-softening processes. Themagnesium hydroxideprecipitated in the process is chiefly responsible for this. Theprocess is inefficient at ambient surface water temperatures in

a conventional lime softener with 1 h detention, but it becomesquite significant if the detention is increased to 4 h, the


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temperature increased to 120 to 14O0F (44 to 6O0C), or afavorable increase is made in both of these variables; it is veryeffective at typical hot-process softening temperatures inexcess of 22O0F (1040C). Contact time and density of the

adsorbent sludge are important factors.

Devices for BlendingA properly operated ion-exchange unit produces water withzero hardness, but with high corrosivity. Since a total hardnessof 85 to 100 mg/l is the most desirable, treated water from theion-exchange unit is generally blended with source water toraise hardness in the finished water. Blending is normallyaccomplished by metering both the effluent from the softener

and added raw water.

Resin BreakdownSynthetic resins normally last 15 to 20 years, but certainconditions can cause resin to breakdown earlier. Oxidation bychlorine is probably the most common cause of resinbreakdown. When chlorine is used to oxidize iron in the water,the chlorine should be removed before ion exchange.

Iron FoulingIron will significantly affect the ability of resins to removehardness ions. Ferrous iron can be oxidized during softeningand precipitate out as iron oxide on the resin, and no amount ofbrine will remove the iron fouling. If iron oxide is formed beforeion exchange unit, it can be filtered out by the resin andremoved during the backwashing of the unit. Normally if theiron concentration in the source water is high, iron removal is

provided ahead of the exchange unit to prevent fouling of theunit.

Suspended MaterialTurbidity, organic chemicals, and bacterial slimes resinsresulting in the loss of some of the resin exchange capacity.

The best solution is to remove of the suspended matter withcoagulation, sedimentation, and filtration before the softeningprocess.

Troubleshooting Softener Malfunctions


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Softener effluent may periodically indicate hardness leakage ordecreased exchange capacity. Normally, difficulties can beattributed to one of four causes, i.e. mechanical failures, resindegradation, improper operational controls, and changes in

feedwater chemistry. Mechanical failures take the form ofplugged or broken distribution laterals, improperly functioningmultiport valves, or clogged underdrain screens and supportmedia.

To determine whether the resin is performing poorly because ofchemicalor physical degradation, core resin samples should be checkedfor exchange capacity, physical stability, and foulants. Coresamples should be collected and analyzed annually.

Improper operational controls involve service flow rate;backwash flow rateand water temperature; regenerant concentration, dosage andflow rate; slow rinse flow rate and water volume; fast rinse flowrate and volume; and total hardness levels in the softenedwater and fast rinse effluent. To ensure that these controls areproperly set, each one should be checked periodically.Softeners are normally designed for an average feedcomposition. Key considerations are total hardness, metallic

compounds, suspended solids, and oxidizing agents. Apermanent change from the original characteristics may resultin low quality water, decreased capacity, and resin degradation.Operational and mechanical modifications can minimize theseproblems.

Resin DegradationThe various forms of softener resin degradation include osmoticshock, mechanical strain, thermal shock and iron, aluminum,calcium carbonate or magnesium hydroxide fouling.Osmotic shock is caused by excessive bead expansion duringthe servicecycle, and contraction during regeneration.Mechanical strain results in broken beads which lowers capacitydue to channeling and higher pressure drop. Thermal shock iscaused either by extreme temperature variations (i.e., when ahot service cycle is followed by a cold backwash) or bycontinuous operation above the maximum allowabletemperature for the resin in service.


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Iron foulants may originate from such sources as contaminatedwater supply, corroded water distribution piping, surface watersupply containing organically sequestered iron, orcontaminated regenerant. Most frequently, iron enters as the

insoluble ferric form, coating the surface of the beads, thusprohibiting efficient contact between water and resin. Washingwith a reducing agent such as sodium hydrosulfite will reducethe iron to the soluble ferrous form. Alternately, a hydrochloricacid solution may be used to dissolve the ferric form irondirectly.Aluminum, calcium carbonate, and magnesium hydroxidefouling are caused by carryover of coagulants or hardness fromthe pretreatment system. These precipitates coat the surfaces

of the resin beads thus reducing its capacity.

Report on Water Softening Plant

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