Deposit control agents

A negative attribute of orthophosphate is its tendency to precipitate with calcium hardness found ia natural waters. In recent years, deposit control agents that prevent this deposition have been developed. Owiag to its relatively low cost, orthophosphate is widely used as an iadustrial corrosion inhibitor. 

Pressure vessels and appurtenances should be constructed of stainless steel or other corrosion-resistant materials. Ideally, these steam generators should receive hot demineralized FW to minimize chemical treatment requirements. Alternatively, where a main boiler plant is installed, 100% steam condensate provides a good source of FW. In practice, it is very difficult to accurately control the correct amount of chemical feed. Chemicals are typically restricted to potable grade, deposit control agents such as polyacrylates, and other materials listed under the Code of Federal Regulations, CFR 21 173.310, or National Sanitary Foundation (NSF International) approval system. These boilers may be electrically heated or gas-fired. 

The tendency to form boiler waterside deposits is partly dependent on factors such as the solubility of the particular mineral species and the strength of physical adherence involved. As a general rule, the rate of deposition tends to increase with higher levels of BW dissolved solids. Also, the rate of deposition increases with increase in heat-flux density and with the inadequate dosage, inappropriate feeding, or otherwise usage of antisealants and other deposit control agents (DCAs). 

NOTE All-polymer programs employ various types of organic deposit control agents (DCA) such as phosphinocarboxylic acid (PCA) products, which tend to be high temperature-stable sludge dispersants, crystal modifiers, and hardness transporters. 

Polymaleic acid (PMA). The use of chemicals based on PMA and some derivatives has become standard practice for very brackish waters and seawater distillation processes around the world, where the TDS may reach 50,000 ppm TDS, or where total hardness levels exceed 500 to 1,000 ppm CaC03. Its use in RO systems is growing. However, PMA has limited dispersing properties and may need to be formulated with a dispersant chemical to provide satisfactory performance with some RO designs. It is claimed that PMA is also a successful silica deposit control agent and therefore may be incorporated into formulations where this is a problem. 

A phosphate-sludge conditioner blend may be employed because the deposit control agent or sludge conditioner limits and controls crystal formation (threshold and crystal distortion effects) and ensures particle fluidization (dispersion effect). 

The drawback is that the precipitation chemistry results in suspended solids that must be fluidized and removed from the boiler by BD, so a polymeric sludge conditioner (dispersant, deposit control agent) product and additional BD is required. The higher the FW hardness, the more BD is required because of the buildup of suspended solids, so there is a trade-off in terms of operating with lower quality FW and the resulting reduced efficiency. 

Deposit control agents (DCAs) or antiscalents to inhibit the deposition of CaC03 and other alkaline earth salts. 

Antifoulants or metal surface cleaners to reduce the risk of pitting corrosion and other forms of concentration cell corrosion initiated at the metal surface by shielding effects from inorganic deposits. These could also be called deposit control agents. 

Deposit analysis Deposit binding Deposit control agents... 

Table 3.3 Comparison of Treatment Cost Ratios for Magnetic Devices and Deposit Control Agent Chemical (Scale Inhibitor) at Typical 1997 Prices...

In addition, modem chemical polymer treatments tend to have wider applications than before, so, for example, many deposit control agents are also antifoulants as well as antiscalents, which only serves to confuse the issue. 

The development and ready availability in recent years of a wide range of modem polymeric deposit control agents (often with novel, multifunctional... 

While the standard, modem chemical treatment programs used today almost always contain a synergistic blend of polymers and other organic inhibitors, they often may not contain a specific calcium phosphate deposit control agent (as it would increase the cost of the program). This situation reinforces the need for vigilance. 

Although new and effective silica deposit control agents are available, which may permit silica levels of up to 250 to 300 ppm SiC>2 to be maintained in recirculating cooling water, it is unwise to allow silica to cycle up to these higher levels without a careful review of all operating parameters and water chemistries. 

Stabilizers to either inhibit or control the specific deposition of sulfates, phosphates, phosphonates, silicates, zinc, and other difficult salts in the cooling water. In reality, these are forms of deposit control agents (DCAs) and tend to act as sequestrants, or as dispersants and defloccu-lators for precipitated salts. 

SOME EARLY INHIBITORS, DEPOSIT CONTROL AGENTS, AND COOLING WATER PROGRAMS... 

Some important early cooling water inhibitors, deposit control agents, and cooling water programs leading up to the introduction of the modern deposit control agents (DCAs) are described in this section. 

Improved deposit control agents and phosphate/zinc stabilizers have permitted these programs to operate with high levels of calcium (up to 1200 ppm) and high pH (up to 9.0). 

As with many patented deposit control agents (DCAs), the patents on certain proprietary nonoxidizing biocides, or their applications, are also beginning to expire, and, as a result, secondary manufacturers are entering the market and helping to bring down the prices of the more expensive formulatory materials. 

Also, if this stream is close to the injection point of azoles, phospho-nates, or other deposit control agents (DCAs) into the cooling system, rapid oxidation of the inhibitor may occur. 

Any proposal for a future chemical inhibitor program needs to employ an alkaline zinc/stabilized phosphate base, similar to that currently used, as required by the customer. However, as noted previously, the inhibitor needs improved deposit control agent (DCA) performance. This is especially important, as a new program would not start with a properly cleaned cooling system (an on-line clean will be recommended, but is likely to have only limited effect). Thus the inhibitor would include better/more polymers. 

Deposition of scale does not occur instantaneously there is a time delay (scale induction time), which varies for each cooling system, based on its particular characteristics (including rate of recirculation, retention time, water chemistry, etc.). The induction time can be lengthened by the use of deposit control agents (DCAs). 

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