Phosphates cycle

Phosphorus also occurs in all living things and the phosphate cycle, including the massive use of phosphatic fertilizers, is of great current interest.O 20) -pj.jg movement of phosphorus through the environment differs from that of the other non-metals essential to life (H, C, N, O and S) because it has no volatile compounds that can circulate via the atmosphere. Instead, it circulates via two rapid biological... 

Figure 12.2 Tite water-based phosphate cycle. ...

With high-hardness waters, the carbonate-cycle form of precipitation treatment is often preferred to the phosphate-cycle because it forms a less bulky and less dense sludge. The disadvantages of hard-water carbonate-cycle precipitation treatments include ... 

For higher pressures, or if the FW silica is too high, use a phosphate-cycle program. 

NOTE This chart assumes oxygen scavenger, polymer, and other necessary water chemistry controls are in place. If the FW is fully softened, there is no benefit in using this program. Employ a phosphate-cycle or alternative program. 

Phosphate-cycle programs, which first became available at the turn of the twentieth century and were researched and formalized in the 1920s, have gradually replaced the carbonate-cycle programs. Formulation developments include combined phosphate-carbonate cycle programs, which incorporate the best of both basic programs while minimizing the problems associated with carbonate breakdown. 

Related programs, such as phosphate-chelant and phosphate-polymer, are simply modifications of the basic phosphate-cycle, and they still rely on the precipitation of calcium (or other metal ions) as dispersible phosphate sludge. 

Some phosphate-cycle reactions are shown below, and, although for the sake of simplicity only calcium phosphate is shown as a precipitant, depending on the operational circumstances, the reaction produces either tricalcium phosphate, hydroxyapatite, or a combination of both salts. 

Typically, for FT boilers operating phosphate-cycles programs at pressures below 435 psig (30 bar) ... 

Where phosphate-cycle chemistry is employed in WT boilers, a different set of rules apply. However, for non-highly rated boilers, phosphate-cycle programs normally may be employed up to 1,200 psig (82 bar), and phosphate itself is generally satisfactory in industrial applications up to at least 1,885 psig (130 bar), or for any specific boiler application up to the phosphate hideout point. 

Tables 10.4 and 10.5 set out phosphate-cycle control limits. These control limits are for non-highly rated, FT and WT boilers and assume that modem polymeric sludge conditioners are employed (i.e., not starches, lignosulphonates, etc.).

Phosphate-cycle programs are a relatively low-cost, well-established technology that provides good, well-proven results in variable and... 

Table 10.4 Phosphate-Cycle Coagulation and Precipitation Program. Recommended BW Control Limits for Non-Highly-Rated FT Boilers Employing Hard, Partially Softened, or Fully Softened FW.

Two examples of typical phosphate-cycle formulations are shown here. 

Phosphate-Cycle Formulation for Simple Systems with Adequate FW Alkalinity Materials... 

Phosphate-Cycle Formulation for Systems with Moderate Alkalinity, Higher Hardness FW Materials Caustic soda 13.0%... 

Boiler water chelant reserves are minimal, so that any change in FW substrate contaminants demands additional chelant, unlike phosphate-cycle programs, which carry a much higher reserve and can manage occasional incursions without any change in product feed. 

Silica control requirements are the same as for phosphate-cycle programs. 

Lignins, as DCAs, are excellent for phosphate-cycle sludges in FT boilers where FW hardness may vary and also where iron oxides are present. 

Because of their high degree of hydrolytic stability and the extended abilities and effectiveness of these novel chemistries, wherever phosphate-cycle or chelant programs are employed, the circumstances generally permit all-polymer/all-organic programs to be used as technically and economically viable alternatives. 

For phosphate overlay programs, PAA is generally satisfactory, but where good phosphate control is required or for on-line cleaning of fouled boilers using phosphate-cycle, SS/MA or AA/AMPS should be used. These products also perform in the presence of iron. 

NOTE Phosphate-cycle programs may be employed at up to 1,200 psig. 

Where relatively expensive products such as terpolymers are employed in conventional programs, feed rates are not as high as, say, PAA, because of improved terpolymer performance. As an example, where a product such as Acumer 3100 is employed in phosphate-cycle programs, typical BW polymer requirement is 10 to 15 X COC active product, with a phosphate reserve of 30 to 60 ppm and perhaps 10 to 15 X COC. 

API AO alternative to phosphate-cycle program for commercial, HVAC and light industrial markets (15 to 150 psig)... 

Table 10.4 Phosphate Cycle FT Boiler Control Limits p428... 

Table 10.5 Phosphate Cycle WT Boiler Control Limits p429... 

Figure 10-8. Phosphate cycles and interchange of adenine nucleotides.

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