Calcium temperature scale

Theoretically, controUed deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. However, low temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high temperature areas and interferes with heat transfer. Therefore, this approach is not used for industrial cooling systems. ControUed calcium carbonate deposition has been used successhiUy in some waterworks distribution systems where substantial temperature increases are not encountered. 

Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubiUties are exceeded in the bulk water. Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface. Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate. 

Consequently, a loss of free carbon dioxide in the water, because of either a rise in temperature (as occurs in a FW heater or boiler) or an increase in pH (all boilers operate at an alkaline pH) leads to a change of equilibrium and the resultant rapid and troublesome precipitation of insoluble calcium carbonate scale on heat transfer surfaces. The reaction is as shown here ... 

Calcium carbonate Aragonite CaC03 Low temperature scale. 

Scale Prevention. The scale normally formed on heat transfer surfaces of sea water evaporators consists of calcium carbonate, magnesium hydroxide, and/or calcium sulfate. The first two form as a result of the breakdown of bicarbonate in sea water, which is initially saturated with calcium carbonate. Calcium sulfate scale forms purely as a result of its inverted solubility curve. Sea water is not saturated with calcium sulfate and an economically reasonable amount of fresh water can be recovered from sea water without exceeding saturation with calcium sulfate. However, at the start of this investigation, the solubility of calcium sulfate in sea water was not accurately enough known to tell whether 30, 50, or 80% of the water content could be removed at various temperatures without encountering calcium sulfate scale. 

To increase the value of the demonstration plant, features have been incorporated to permit operation under other than demonstration conditions. It will be possible to operate the evaporator at first-effect temperatures up to 300° F., thus almost doubling plant output if calcium sulfate scale can be prevented, and to use the acid method of scale prevention in place of the sludge method. Provisions have been made for later installation of a vapor compressor, which would convert the plant to a combination multiple effect-thermocompression system. This would add about 15% to plant output and would permit performance evaluation of vapor compressors in sea water service. 

NOTE Calcium carbonate is in fact sparingly soluble (typically the solubility ofCaCOi is to the extent of 15 to 20 ppm depending on temperature and other factors). If a decrease in cooling water pH occurs and there is a resultant increase in CO2. in excess of the minimum necessary to establish equilibrium, this can, under certain conditions, resolublize some or all of the calcium carbonate scale. 

The temperature scale and the cell constant were calibrated using indium. Samples of approximately 4 mg were heated at 20 °C / min over the temperature range of 25-200 °C, under dry nitrogen purging (80 ml /min) in a pin-holed aluminum pan. The melting range of atorvastatin calcium was found to take place over the broad range of 158.4-178.03 °C. 

Low-pH cleaners are typically used to address calcium carbonate scale and iron oxide deposition. These cleaners are usually formulated using only acid, such as acetic, hydrochloric, or sulfamic. Figure 13.5 shows the effects of temperature and pH on the removal of calcium carbonate from a membrane.7 As the figure shows, lower pH, and higher temperatures are more effective at restoring permeate flow than higher pH and lower temperatures. 

For example, higher temperature and pH may be used to address biofilms and lower pH and higher temperature may be used to remove calcium carbonate scale (refer to Table 13.1). 

Compatibility tests should be conducted using reservoir brines at reservoir temperature. Formation of calcium sulfate scale is a serious problem because such a scale cannot be removed with hydrochloric or hydrofluoric acid, but can be removed, however, by first injecting sodium carbonate and then acidizing using hydrochloric acid. EDTA can be also used to remove calcium sulfate scale (37). 

Calcium sulfate scale can form when the solubility limit is exceeded in the sea water evaporator. This scale is formed only by the concentration of the brine as distillate is removed and by the decrease in solubility as the temperature of the solution is raised. 

Some saline waters have calcium sulfate scaling problems even more difficult than those of sea water. The partial ion exchange softening should allow operation at temperatures and pressures that up to now have not been possible. 

Calcium carbonate scale formation increases with increasing salt concentration and increasing temperature at a pH above 8.0. Below pH 8.0 the salt is quite water-soluble. It is frequently found on neutral sized fine paper machines located in hard water areas, where conditions for scale formation of this type are favourable, since the pH on such a machine is alkaline and the concentration of dissolved calcium and carbonate ions is high. On a paper machine it is not usually detrimental to paper production in itself unless it becomes extensive. However, it does provide an excellent surface for bacterial growth and deposition of organic material and for this reason should be prevented. 

No economical control method is available for calcium sulfate other than ensuring that the concentration-temperature process path is within the solubility confines of the various crystalline modification of calcium sulfate. The limiting top temperature at which seawater mey be evaporated without calcium sulfate scale deposition is of major design significance. 

In contrast with the two crystallisation studies discussced previously, for the single pipe flow experiment, the induction time was clearly observable and varied from 100 minutes to 400 minutes for the temperature range fiom 20 C to 40 C. The lower the temperature the longer the induction time. Thus the induction time of the calcium sulphate scale formation depends strongly on temperature. A correlation was formulated [15] as follows ... 

In this example, water is said to be saturated with calcium carbonate when it will neither dissolve nor precipitate calcium carbonate scale. This equilibrium condition is based upon an imdisturbed water at constant temperature which is allowed to remain imdisturbed for an infinite period of time. Water is said to be undersaturated if it can still dissolve calcium carbonate. Supersaturated water wiU precipitate calcium carbonate if allowed to rest. If water is undersaturated with respect to calcium carbonate, the SL value will be less than 1.0. When water is at equilibrium, SL will be 1.0 by definition. Water which is supersaturated with calcium carbonate will have a saturation level greater than 1.0. As the saturation level increases beyond 1.0, the driving force for calcium carbonate crystal formation or crystal growth increases. 

In the case of calcium carbonate scale, indices are typically calculated at the highest expected temperature and highest expected pH—the conditions under which calcium carbonate is least soluble. In the case of silica, the opposite conditions are used. Amorphous silica has its lowest solubility at the lowest temperature and lowest pH encountered. Indices calculated under these conditions would be acceptable in many cases. Unfortunately, cooling systems are not static. The foulants silica and tricalcium phosphate are used as examples to demonstrate the use of operating range profiles in developing an in-depth evaluation of scale potential and the impact of loss of control. 

Most calcium sulfate scales in oil-field work are gypsum, which has the formula CaS04.2H20. The solubility of gypsum is greatest at 43 °C. A temperature change can make either an increase or a decrease in solubility depending on its position on the curve. 

In this section, a novel and simple predictive tool is presented to estimate the formation of calcium carbonate scaling as a function of pH, temperature, ionic strength of the solution, calcium cation concentration, bicarbonate anion concentration, and carbon dioxide mole fraction when the water mixture is saturated with a gas containing COj, to evaluate the effect of solution conditions on the tendency and extent of the precipitation. The proposed method covers calcium cation concentrations, or bicarbonate anion concentrations up to 10 000 mg/L, temperatures up to 90 °C, total ionic strength up to 3.6, and pH values ranging between 5.5 and 8. 

Calcium hydride is prepared on a commercial scale by heating calcium metal to about 300°C in a high alloy steel, covered cmcible under 101 kPa (1 atm) of hydrogen gas. Hydrogen is rapidly absorbed at this temperature and the reaction is exothermic. 

Many other metal thiosulfates, eg, magnesium thiosulfate [10124-53-5] and its hexahydrate [13446-30-5] have been prepared on a laboratory scale, but with the exception of the calcium, barium [35112-53-9] and lead compounds, these are of Httle commercial or technical interest. Although thaHous [13453-46-8] silver, lead, and barium thiosulfates are only slightly soluble, other metal thiosulfates are usually soluble in water. The lead and silver salts are anhydrous the others usually form more than one hydrate. Aqueous solutions are stable at low temperatures and in the absence of air. The chemical properties are those of thiosulfates and the respective cation. 

The kinetics of the formation of the magnesium hydroxide and calcium carbonate are functions of the concentration of the bicarbonate ions, the temperature, and the rate of release of CO2 from the solution. At temperatures up to 82°C, CaCO predominates, but as the temperature exceeds 93°C, Mg(OH)2 becomes the principal scale. Thus, ia seawater, there is a coasiderable teadeacy for surfaces to scale with an iacrease ia temperature. 

Calcium Carbonate Protective Scale. The LangeHer saturation index (LSI) is a useful tool for predicting the tendency of a water to deposit or dissolve calcium carbonate. Work pubHshed in 1936 deals with the conditions at which a water is in equiHbrium with calcium carbonate. An equation developed by LangeHer makes it possible to predict the tendency of calcium carbonate either to precipitate or to dissolve under varying conditions. The equation expresses the relationship of pH, calcium, total alkalinity, dissolved soHds, and temperature as they relate to the solubiHty of calcium carbonate in waters with a pH of 6.5—9.5 ... 

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