Scale control

One of the most effective methods for controlling crystallization fouling is by adding chemical inhibitors to a scaling water. Commonly used antisealants are derived from chemicals such as condensed polyphosphates, organophospates, and polyelectrolytes. 

Scale control in open recirculated systems is affected primarily by limiting the concentration of the scale-forming species and related parameters, especially by controlled acid additions to bring the water to a suitable pH range. Many of the commercial inhibitor formulations include scale-controlling additives (e.g., polyphosphates and chelating agents). 

---

In dmm boilers sodium hydroxide (caustic), sodium phosphate, or both may be added for pH and scale control. Sodium hydroxide is used more in Europe than in the United States, where sodium phosphate treatment is usually preferred. In boilers operating above 4 MPa (580 psia), caustic concentrations must be carefully controlled to prevent highly corrosive deposits from forming. In the lowest pressure boilers, phosphate treatment may be used to compensate for lower purity feedwater. As the boiler pressure increases, the allowable phosphate concentration decreases, and at 16.5 MPa (2400 psia) or above, equiUbrium phosphate treatment may be used. In this treatment, caustic is added to a low phosphate concentration in the boiler to maintain the proper pH (20). 

Magnesium can be exchanged ia a similar fashioa. To date, cost coasideratioas have preveated the use of ioa exchange for scale control. 

The option most promising today iavolves use of a new family of polymers, the so-called high temperature scale-control chemicals. These are compounds that, added ia 3—8 ppm, lead to lattice distortioa and the formation of a nonadhering scale. Belgard (CIBA-GEIGY) was the first compound... 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

The most commonly used scale inhibitors are low molecular weight acrylate polymers and organophosphoms compounds (phosphonates). Both classes of materials function as threshold inhibitors however, the polymeric materials are more effective dispersants. Selection of a scale control agent depends on the precipitating species and its degree of supersaturation. The most effective scale control programs use both a precipitation inhibitor and a dispersant. In some cases this can be achieved with a single component (eg, polymers used to inhibit calcium phosphate at near neutral pH). 

Of special interest in scale-control systems is the type in which the motion of the scale beam is sensed by a differential transformer or a group of load cells. The output of such devices is proportional to the displacement of the scale beam, which in turn is proportional to the amount of material in the weigh bucket. Many designs use loadsensing devices such as strain gauges or transducers. These eliminate the need for a scale-beam mechanism. The weigh vessel is mounted directly on the load-sensing devices. This provides many benefits in... 

Scale formation Controlled scale deposition by the Langelier approach or by the proper use of polyphosphates or silicates is a useful method of corrosion control, but uncontrolled scale deposition is a disadvantage as it will screen the metal surfaces from contact with the inhibitor, lead to loss of inhibitor by its incorporation into the scale and also reduce heat transfer in cooling systems. Apart from scale formation arising from constituents naturally present in waters, scaling can also occur by reaction of inhibitors with these constituents. Notable examples are the deposition of excess amounts of phosphates and silicates by reaction with calcium ions. The problem can be largely overcome by suitable pH control and also by the additional use of scale-controlling chemicals. 

Phosphonates, especially aminotri(methylenephosphonic acid) (ATMP) and l-hydroxyethylidene-l,l-diphosphonic acid (HEDP). Organophosphonates have the ability to control both scaling and corrosion by a combination of chelation and crystal nuclei growth supression (threshold) mechanisms they also provide scale control by crystal distortion. 

Treatment chemicals should preferably be fed to the FW tank to minimize sludge deposits in the coils. Hydroxide alkalinity in ppm (mg/1) CaC03 must be maintained at a sufficient concentration to keep silica soluble and avoid complex silicate deposits. These precautions are necessary because scale-control internal treatment chemicals usually are not employed to assist in the prevention of such deposits in coil-type steam generators. 

A. Rawlett, J. Chen, M. A Reed, J. M. Tom, Advances in Molecular Scale Electronics Synthesis and Testing of Molecular Scale Resonant Tunneling Diodes and Molecular Scale Controllers, Polym. Mater, Sci. Engin (Am Chem Soc, Div. Polym Mater) 1999, 81,140-141. 

High temperature reforming, 23 238 High temperature scale-control chemicals, 26 73... 

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

We will discuss, on the one hand, the spaghetti-like CNT electrodes on conductive surfaces or randomly dispersed in a polymer matrix and on the other hand, CNT arrays grown in situ on substrates with large-scale control of location and orientation. Finally, individual CNT electrodes will be briefly discussed. Figure 3.10 illustrates these different CNT electrode arrangements. 

Before entering the perraeator for salt removal, the feed is either acidified or modified with proprietary chemicals for scale control. While both approaches have their followers, despite its handicaps, acid dosing today appears predominant in the industry in fact some RO permeator manufacturers will only warranty their product for acid. Studies to be initiated shortly at our laboratory should hopefully quantitatively evaluate the relative merits of these approaches. It is hoped that at least equivalence if not superiority for proprietary scale control chemicals will be unequivocally established so that one can drop acid use and satisfy many who are justifiably reluctant to use acid, for reasons familiar to all. 

Scaling occurs when the concentration of dissolved materials, usually calcium- and magnesium-containing species, exceeds their solubility levels. The addition of scaling control chemicals allows a higher dissolved solids concentration to be achieved before scaling occurs. 

Medication should be considered an adjunct treatment for AN. Although many medications have been used in the treatment of AN, there are few large-scale, controlled studies demonstrating the efficacy of pharma-... 

Figure 5 shows the relationship of the several water cost components as a function of the TTD for a 50-stage sea water conversion plant with a blowdown concentration of twice that of sea water. Because the blowdown concentration was maintained constant, the cost of chemicals for scale control remained constant. 

Table I shows a detailed breakdown of the operating cost for this plant. The cost of steam represents about half of the water cost for the optimum plant. The capital charges for the evaporator plant, which includes amortization, interest on working capital, and real estate, represent about 30%. The remaining 15 to 20% is equally divided between the cost of chemicals for scale control and all the other costs. The converted water is estimated to cost approximately 42 cents per thousand gallons. This water cost represents a realistic figure for a large-capacity multistage flash evaporator that could be built today when the energy in the form of steam costs between 35 and 40 cents per million B.t.u.

Although large-scale controlled studies on chronic partial sleep deprivation and sleep extension are difficult to execute, we have been able to acquire a great deal of knowledge from the well-conducted investigations that have been carried out to date. Chronic partial sleep deprivation unquestionably leads to accumulation of impairment and progressively increased sleep tendency, the sum of which have been referred to as sleep debt. Once sleep debt has accrued, it can only be paid back by extra sleep. 

---