Industrial water treatment

COOLING WATER TREATMENT IN INDUSTRY

Water is the most commonly used medium for removing heat. As a result large quantity of water is used for industrial cooling purposes. Water consumption will continue to increase with industrial growth and the cooling water systems designed to reuse water will also grow with time.
Cooling System Operation:
Cooling water is pumped from cooling water basin through the equipments requiring cooling. This water absorbs heat from system by conduction and circulates back to the cooling tower. The absorbed heat is released in the cooling tower by evaporation of a small portion of the recirculating water. Dissolved minerals do not evaporate and continue to remain and concentrate in the recirculating water. In addition to evaporative losses, water is lost as droplets known as ‘windage loss'. The extent of windage loss depends on the type of heat rejecting system employed and local operating conditions.
Any cooling water will have the following water associated problems.
1. Corrosion: Electrochemical reversion of base metal to its natural state. Metals getting oxidized in presence of moisture and oxygen through a galvanic cell established on the metal surface.
2. Scale: Dissolved metals in water lose their solubility and precipitate on the metal surface leading to tightly adherent non-conducting layer on the metal surface, which resists heat transfer and impedes equipment performance.
3. Fouling: Suspended and precipitated impurities in water along with airborne and waterborne physical matter, organic impurities etc, and settle along the flow channels leading to clogging of channels. Apart from restricticting the water flow, these can also provide active sites for corrosion and deposition to occur.
4. Microbiological: Waterborne microorganisms can secrete matter (slime) which can promote active sites for corrosion and deposition, some of the bacteria like sulfate reducers and nitrifiers can, through their process of metabolism, promotes an active corrosive environment in the system.
Prevention of these problems:
1. Corrosion control: Corrosion inhibitors are classified as anodic,cathodic or both depending upon the corrosion reaction each controls. Inhibitor molecule is absorbed on the metal surface forming a thin protective film either by itself or with metallic ions. Some inhibitors cause the metal to form its own protective film of metal oxide, thereby increasing its resistance. I the third type the inhibitor reacts with a potentially corrosive substance in the water.
Anodic inhibitors: Chromate/Orthophosphate/Phosphonate
Cathodic inhibitors: Zinc/Polyphosphate/Phosphonate
2. Deposit control: Deposit control is mainly pH and temperature dependent. At elevated pH and temperature most of the metal ions dissolved in water lose their solubility and tend to precipitate. Though the temperature encountered in common cooling water circuits are not critical, the pH is decided by the feed water characteristics and therefore assumes enormous importance. Extension of mineral solubilities at the operating pH and the temperature of the cooling water system, thereby preventing their deposition potential is the fundamental consideration. The introduction of dispersants based on polymers will help in extending the mineral solubility and also will help in avoiding sludging in the system through crystal modification and crystal distortion mechanism.
3. Fouling control: Synthetic dispersants will also help in effectively dispersing the physical matter and keep them in suspension so that they may be get rid of through blow down. The biological and organic foulants will be effectively dispersed by special products called Biodispersants.
4. Microbiological control: Use of micro biocides at fixed frequency will help in controlling the micro organic activity to a great extent. Chlorination is found to be the most effective and economic way of microbiological control, through chlorine has specific limitations with pH of the water.

................................