Deposit control

Additives for gasoline used in vehicles have been used for many years to improve the performance of the vehicle, reduce the emissions from the combustion of the fuel, and modify the physical and chemical properties of the fuel (4). 

One additive that has been used for many years is ethanol. However, the use of ethanol in a gasoline combusted in an internal combustion engine is well known to create harmful and imdesirable deposits on the fuel intake valves of the engine. 

In fact, ethanol fuel mixtures can reduce the formation of these intake valve deposits, but such a remediation typically shifts the problem to the combustion chamber, where unacceptable combustion chamber deposits are then formed. Therefore, an additive is needed which upon combustion will effect in the engine a significant reduction in the intake valve deposits and simultaneously effect a reduction of the combustion chamber deposits in the engine. 

It has been fotmd that the ethanol and the Mannich-base detergent work synergistically to control the formation of deposits, without 

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Clean boiler water surfaces reduce potential concentration sites for caustic. Deposit control treatment programs, such as those based on chelants and synthetic polymers, can help provide clean surfaces. 

If deposits are minimized, the areas where caustic can be concentrated is reduced. To minimize the iron deposition in 6.895-12.07 x 10 Pa boilers, specific polymers have been designed to disperse the iron and keep it in the bulk water. As with phosphate precipitation and chelant control programs, the use of these polymers with coordinated phosphate—pH treatment improves deposit control. 

The most effective way to prevent SCC in both stainless steel and brass systems is to keep the system clean and free of deposits. An effective deposit control treatment is imperative. A good corrosion inhibitor is also beneficial. Chromate and phosphate have each been used successfully to prevent the SCC of stainless steel in chloride solutions. 

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. 

The EPA summary (4) of Title IV states the basics of the acid deposition control amendments ... 

Title rV Acid Deposition Control - As we all know, acid rain occurs when sulfur dioxide and nitrogen oxide emissions are transformed in the atmosphere and return to the earth in rain, fog, or snow. Approximately 20 million tons of SOj are emitted annually in the United States, mostly from the burning of fossil fuels by electric... 

Deposit control is important because porous deposits, under the influence of heat flux, can induce the development of high concentrations of boiler water solutes far above their normally beneficial bulk values with correspondingly increased corrosion rates. This becomes an increasingly important feature with increase in boiler saturation temperature. In addition, deposits can cause overheating owing to loss of heat transfer. Finally, carryover of boiler water solutes, which can be either mechanical or chemical, can lead to consequential corrosion in the circuit, either on-load or off-load. Material so transported can result in corrosion reactions far from its point of origin, with costly penalties. It is therefore preferably dealt with by a policy of prevention rather than cure. 

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 The same polymer chemistries employed in the BW deposit control treatment market sector are also made available to other markets such as waste water, cooling water, potable water production from brackish or saline supplies, metal finishing, paint and coatings, electronics, pulp and paper, and more. 

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. 

Organic chelant compounds, such as the sodium salts of ethylenedi-aminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) are commonly used in BW deposit control treatments, often in combination with phosphates. 

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. 

Scale and deposit control. This is primarily control over the deposition of hardness salts and other scale-forming minerals. 

Hardness precipitation and deposit control, functioning at a stoichiometric level. Phosphate precipitation programs utilize this function. 

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. 

The most common chelants employed for boiler deposit control, solubilization, and cleaning duty include ... 

EDTA and NTA are essentially the only two chelants formulated into BW deposit control programs. The rationale for selection of one product over another is often a point of contention. Looking first at the relative chelation stability constants (chelate log values, Ks) ... 

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. 

Various phospono- and phosphinopolycarboxylic acids (PCAs) are available in the market. These polymers are similar to phosphonates and some actually are phosphonates. They tend to exhibit varying degrees of both deposit control and corrosion control properties. For BW applications, the acrylic acid/organic phosphate polymer (PCA type 16) is the only important phosphinopolycarboxylic and has a C-P-C bond (phosphonates have a C-P-O bond). 

Novel oxygen scavengers and polymer-based deposit control programs hardly figure in the operation of these large boilers because the treatment regimen is often simply hydrazine and ammonia. 

Boilers below 900 psig (6.21 MPa) with large furnaces, large steam release space, and internal chelant, polymer, and/or antifoam treatment may tolerate higher levels of FW impurities than those in the table and still achieve adequate deposition control and steam purity. Removal of these impurities by external pretreatment is always a... 

Some boilers may tolerate higher concentrations of FW impurities than those in the table and still achieve adequate deposition control. 

Here is a typical example of a stand-alone cleaner that temporarily supplants the existing deposit control product. The program still demands the use of an oxygen scavenger and condensate line treatment. 

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