Sludge conditioners

In lower pressure boilers a variety of additional treatments may be appropriate, particularly if the steam is used in chemical process or other nonturbine appHcation. Chelants and sludge conditioners are employed to condition scale and enable the use of less pure feedwater. When the dmm pressure is less than 7 MPa (1015 psia), sodium sulfite may be added direcdy to the boiler water as an oxygen scavenger. It has minimal effect on the oxygen concentration in the system before the boiler. 

In addition to carbonates and/or phosphates, a variety of substances have also been used to act as sludge conditioners —e.g. starches, lignins, polyacrylates etc. Other substances, such as nitrites and sulphates are claimed to have some value in the suppression of caustic cracking (see Section 5.1). 

Hampered chemical treatments and interference with several types of organic sludge conditioners, such as tannins, lignins, car-boxymethyl cellulose, and others... 

These condensed tannins and their derivatives, all of high molecular weight, function as anionic polyelectrolyte sludge conditioners, tending to sequester hardness salts and hinder their precipitation as crystalline scales. In addition, when precipitation does occur, the condensed tannins coagulate the particles, resulting in a mobile sludge that can be easily blown down. 

Boiler compound formulations containing aluminate, silicate, and carbonate ingredients have commonly utilized various ratios of some or all of these constituents, together with caustic soda (for alkalinity control), sodium nitrate or sulfate (to prevent caustic embrittlement), sodium lig-nosulfonate (sludge conditioner), and other compounds. 

Sludge conditioners are required. Originally these were based on starches and lignins, but modem carbonate cycle treatments use carbonate-polymer programs, where the polymer (such as a phos-phinocarboxylic acid) provides a combination of threshold effect, crystal distortion, and sludge dispersion to minimize scaling and prevent sludge deposition. 

Sludge and other precipitated salts must be fluidized or mobilized (conditioned), so that they remains free-flowing and nonadherent, so sludge conditioners are essential. 

For more details on carbonate-cycle control limits, see Table 10.3. These control limits are for non-highly rated boilers and assume that modem polymeric sludge conditioners are employed (i.e., not starches, agar, etc., which tend to form colloidal suspensions and may be difficult to remove by BD). 

Hydroxyapatite produces a soft sludge that is not as sticky as tricalcium phosphate and can be more easily managed (dispersed) by a (polymeric) sludge conditioner. 

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. 

Lignin sulfonates (lignosulfonates), which are also sludge conditioners. 

All-polymer/all-organic programs are relatively thermally stable and multifunctional. Typically, they replace coagulation and precipitation programs, flocculation improvers, deflocculators, and sludge conditioners and also provide control over very specific and troublesome BW problems, such as silica scaling and iron transport. 

Many sludge conditioner and DCA chemistries are acceptable by various national governments for use as safe BW additives where steam may contact food (although under certain strict conditions and concentration levels). In the United States, for example, these conditions are contained in the Code of Federal Regulations (CFR) 21 173.310 Boiler water additives. 

Polyacrylamides are nonionic polymers, usually with much higher molecular weights (MW from 100,000 up to 12 or 15 M). They often are copolymerized with polyacrylates. Depending on the MW ratios employed, they may act as colloidal dispersants, sludge conditioners, or flocculants. Nonionics such as polyacrylamides (and isobutylenes) are particularly useful at dispersing uncharged particles. 

Tannin sludge conditioners are adversely affected by higher temperatures and, without the support of a supplemental PAA polymer, are limited to 350 psig. This probably is also true of lignosulfonates. 

Tannins are excellent pre-boiler and boiler section passivators and sludge conditioners but are nonvolatile, so that there is no specific postboiler oxygen scavenging ability. 

As an additional safeguard, it is common practice to use an appropriate polymeric sludge conditioner, which will maintain precipitates in suspension for removal with the BD. 

Aquatreat AR. [Alco] Acrylic acid derivs. dispersant, sludge conditioner, antisealant for water treatment... 

Maracell. [Borregaard LignoTech] Sodium lignosulfonate or oxylignins dispersant chelating agent sludge conditioner for boUer water treatment industrial cleaners stabilizer expand for lead acid batteries. 

Uses Emulsifier for acrylic, vinyl acetate, and SBR latexes, films textile dyeing assistant flocculant scale inhibitor sludge conditioner dispersant for cosmetics hair fixing agent for polyion complex antistat for paper, fibers, plastics artificial biomembranes photochemical metal plating brightener pharmaceuticals... 

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