REPORT ON WATER SOFTENING PLANT
Submitted To: Mr. Javed Iqbal Butt Sr. Deputy Manager Utilities
Submitted By: Gulfam Shahzad Trainee Chemical Chemical Engineer OCP #: TRN0702 August 2013
Olympia Chemicals Limited, Warchha Dist. Khushab
Softening Plant The raw water contains the ions that are to be removed by ion exchange. When these ions come in contact with the regenerated ion exchanger during its service, it displaces the ions that was attached to the resin and takes their place on the resin. When hard water is passed through a bed of sodium cation exchanger, the calcium and magnesium are taken up and held by the cation exchanger which simultaneously gives up 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 produce completely softened water is exhausted, the softener unit is taken out of service and regenerated with brine solution. This reaction may be represented by the following: Ca} R + 2NaCl → Na2R + Ca} Cl2 Mg} Mg}
Sodium Zeolite softener is used to remove hardness of calcium and magnesium from the water. Hardness removal lowers the scale formation properties of water. The softener vessel contains synthetic ion exchange resin. When water is passed through the resin bed, Sodium ion exchanged with the hardness (Ca and Mg ions). Sodium is very soluble and will not result 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. Lime is added to make calcium and magnesium compounds less soluble. Hardness (Ca and Mg) precipitate as CaCO3 and Mg(OH)2 sludge. Some silica also precipitates out with Mg(OH)2 sludge. The sludge is recirculated to the incoming water to increase the reaction rate. The sludge level is controlled by blow down.
Main Equipment of Plant
• • • • • •
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 Parameters PH = 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
PH = 11 – 12 Concentration = 102 tt Ca+ = ≤20 ppm Mg+ = ≤10 ppm
MOL (Ca(OH)2) Parameters Concentration = 140 – 155 PH = 11 - 12 Raw water Parameters PH = 7.8 Conductivity = 208 TDS = 118 ppm
Resins Characteristics Zeolite known as polystyrene resin, are most commonly used resin now a days. Cost is reasonable, and it is easy to control the quality of the resin. They also have much higher ion exchange 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 should remove about 50,000 grains of hardness per cubic foot of resin. Resins hold hardness ions until they are regenerated with a salt brine solution. The hardness ions are exchanged for sodium ions in the salt brine.
Softners The interior is generally treated to protect the tank against corrosion from the salt. The units are normally of the downflow type, and the size and volume of the units are dictated by the hardness of the water and the volume of treated water needed to be produced between each regeneration cycle. Resin is supported by an underdrain system that removes the treated water and distributes brine evenly during regeneration. Minimum depth of resin should be no less than 24 inches above the underdrain.
Softner regeneration 1)
Backwash Cycle
Backwashing, as the name implies, involves the passage of water upward through 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 between brine and resin during the brining step; 2. Particle and resin fines removal prevents channeling, high pressure drop, and poor kinetics during the service step; 3. Regrading or classification of the resin bed contributes to the uniform distribution of regenerant during the brining step. Once hardness breaks through, the softener must be regenerated. In down-flow units, the resin must first be backwashed to loosen the resin (it becomes compacted by the weight of the water), and to remove any other material that has been filtered out of the water by the resin. The operator needs to apply enough backwash water to expand the resin bed by about 50 percent. Distributors at the top of the unit provide for uniform water distribution and uniform wash-water collection. Under drains provide uniform distribution of the backwash water on the bottom of the resin.
2)
Regeneration (Brining)
In water softening, the primary factor determining capacity is the regenerant level (pounds of sodium chloride per cubic foot of cation exchange resin). Regenerant concentration (usually 5% to 15% when introduced) and flow rate and kinetic loading of the resin also influence capacity. Concentrated brine is pumped to the unit from the storage basin. Brine is diluted through the injector to a solution containing about 10 percent salt before it is passed through the resin. The time required for regeneration is about 20 to 35 minutes. The flow rate of brine through the resin is measured in gallons per minute per cubic foot of media. The brine needs to be in contact with the resin long enough to allow for complete exchange of hardness ions on the resin with sodium ions in the brine. It is better to allow too much time than to not allow enough. If the resin is not totally recharged, the next softening run will be short.
3)
Rinsing
The rinsing of bed removes remaining brine from the tank. The total amount of rinse water needed is 20 to 35 gallons per cubic foot of resin. The rinse is started at a slow rate (-2 gpm/square
foot of surface area-) and continues concentration of the effluent is quite low.
until
the
chloride
Salt Storage Salt is stored as a brine, ready to be used for regeneration of the resin. The amount of salt needed ranges from 0.25 to 0.45 pounds for every 1,000 grains of hardness removed. The tank should be coated with a salt-resistant material to prevent corrosion of the tank walls. Salt storage tanks should be covered to prevent contamination. A raised curb should be provided at each access hatch to prevent contamination by flood water or rain. Filling a salt storage tank with water first and then adding salt is the preferred method for making brine. The brine is heavier than water and settles to the bottom of the tank. The brine is usually pumped from the tank to the ion-exchange units. When making brine, water must be added through an air gap to avoid back siphonage of the brine to the water system.
Brine Feeding Equipment In water softening, the primary factor determining capacity is the regenerant level (pounds of sodium chloride per cubic foot of cation exchange resin). Regenerant concentration (usually 5% to 15% when introduced) and flow rate and kinetic loading of the resin also influence capacity. Concentrated brine contains approximately 25 percent salt. The brine should be diluted to about 10 percent before added to the softener.
Silica removing The residual silica can be predicted from the water analysis and the dosage of adsorbent applied in the treatment process. Residuals range from 90% in cold process to as little as 5% in hot process, depending on the adsorbent added or on the magnesium precipitated by lime softening. Silica is also removed in lime-softening processes. The magnesium hydroxide precipitated in the process is chiefly responsible for this. The process is inefficient at ambient surface water temperatures in a conventional lime softener with 1 h detention, but it becomes quite significant if the detention is increased to 4 h, the
temperature increased to 120° to 14O0F (44° to 6O0C), or a favorable increase is made in both of these variables; it is very effective at typical hot-process softening temperatures in excess of 22O0F (1040C). Contact time and density of the adsorbent sludge are important factors.
Devices for Blending A properly operated ion-exchange unit produces water with zero hardness, but with high corrosivity. Since a total hardness of 85 to 100 mg/l is the most desirable, treated water from the ion-exchange unit is generally blended with source water to raise hardness in the finished water. Blending is normally accomplished by metering both the effluent from the softener and added raw water.
Resin Breakdown Synthetic resins normally last 15 to 20 years, but certain conditions can cause resin to breakdown earlier. Oxidation by chlorine is probably the most common cause of resin breakdown. When chlorine is used to oxidize iron in the water, the chlorine should be removed before ion exchange.
Iron Fouling Iron will significantly affect the ability of resins to remove hardness ions. Ferrous iron can be oxidized during softening and precipitate out as iron oxide on the resin, and no amount of brine will remove the iron fouling. If iron oxide is formed before ion exchange unit, it can be filtered out by the resin and removed during the backwashing of the unit. Normally if the iron concentration in the source water is high, iron removal is provided ahead of the exchange unit to prevent fouling of the unit.
Suspended Material Turbidity, organic chemicals, and bacterial slimes resins resulting in the loss of some of the resin exchange capacity. The best solution is to remove of the suspended matter with coagulation, sedimentation, and filtration before the softening process.
Troubleshooting Softener Malfunctions
Softener effluent may periodically indicate hardness leakage or decreased exchange capacity. Normally, difficulties can be attributed to one of four causes, i.e. mechanical failures, resin degradation, improper operational controls, and changes in feedwater chemistry. Mechanical failures take the form of plugged or broken distribution laterals, improperly functioning multiport valves, or clogged underdrain screens and support media. To determine whether the resin is performing poorly because of chemical or physical degradation, core resin samples should be checked for exchange capacity, physical stability, and foulants. Core samples should be collected and analyzed annually. Improper operational controls involve service flow rate; backwash flow rate and water temperature; regenerant concentration, dosage and flow rate; slow rinse flow rate and water volume; fast rinse flow rate and volume; and total hardness levels in the softened water and fast rinse effluent. To ensure that these controls are properly set, each one should be checked periodically. Softeners are normally designed for an average feed composition. Key considerations are total hardness, metallic compounds, suspended solids, and oxidizing agents. A permanent change from the original characteristics may result in low quality water, decreased capacity, and resin degradation. Operational and mechanical modifications can minimize these problems.
Resin Degradation The various forms of softener resin degradation include osmotic shock, mechanical strain, thermal shock and iron, aluminum, calcium carbonate or magnesium hydroxide fouling. Osmotic shock is caused by excessive bead expansion during the service cycle, and contraction during regeneration. Mechanical strain results in broken beads which lowers capacity due to channeling and higher pressure drop. Thermal shock is caused either by extreme temperature variations (i.e., when a hot service cycle is followed by a cold backwash) or by continuous operation above the maximum allowable temperature for the resin in service.
Iron foulants may originate from such sources as contaminated water supply, corroded water distribution piping, surface water supply containing organically sequestered iron, or contaminated regenerant. Most frequently, iron enters as the insoluble ferric form, coating the surface of the beads, thus prohibiting efficient contact between water and resin. Washing with a reducing agent such as sodium hydrosulfite will reduce the iron to the soluble ferrous form. Alternately, a hydrochloric acid solution may be used to dissolve the ferric form iron directly. Aluminum, calcium carbonate, and magnesium hydroxide fouling are caused by carryover of coagulants or hardness from the pretreatment system. These precipitates coat the surfaces of the resin beads thus reducing its capacity.
