Calcium sulfate scale deposition

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. 

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. 

Calcium Sulfate Deposition. The solubility of calcium sulfate is only slightly increased with increasing pH, and calcium sulfate scaling is related to the tendency of this material to form extensively stable supersaturated solutions. While chemical theory predicts that a given ionizable... 

Although the foregoing methods of scale control are particularly effective with the alkaline scales, there is evidence that calcium sulfate scale can also be controlled by codeposition with the alkaline scales. This is particularly true of the first two methods, and cases might well arise where small amounts of compounds such as sodium or magnesium bicarbonate might be added to waters containing high concentrations of calcium sulfate in order to control deposition of the latter. 

Although the foregoing scale removal techniques were developed originally for the alkaline scales, there is evidence that calcium sulfate scales can be similarly removed by deposition in admixture with alkaline scale components. 

Forced oxidation (in limestone systems) can increase oxidation to calcium sulfate to well over 95%. In fact, one supplier of forced oxidation systems guarantees 99.5-t% convosion to calcium sulfate and considers operation at 95% a result of process chemistry imbalances (Klingspor, 1993). The calcium sulfate so formed is precipitated in the absorber sump/reac-tion tank as gypsum, provided sufficient time and seed crystals are available. Hus reduces the amount of dissolved sulfite returned to the absorber, and minimizes the possibili of sulfite oxidation and sulfate scale deposition on equipment surfaces. 

Obtaining maximum performance from a seawater distillation unit requires minimising the detrimental effects of scale formation. The term scale describes deposits of calcium carbonate, magnesium hydroxide, or calcium sulfate that can form ia the brine heater and the heat-recovery condensers. The carbonates and the hydroxide are conventionally called alkaline scales, and the sulfate, nonalkaline scale. The presence of bicarbonate, carbonate, and hydroxide ions, the total concentration of which is referred to as the alkalinity of the seawater, leads to the alkaline scale formation. In seawater, the bicarbonate ions decompose to carbonate and hydroxide ions, giving most of the alkalinity. 

Seawater Distillation. The principal thermal processes used to recover drinking water from seawater include multistage flash distillation, multi-effect distillation, and vapor compression distillation. In these processes, seawater is heated, and the relatively pure distillate is collected. Scale deposits, usually calcium carbonate, magnesium hydroxide, or calcium sulfate, lessen efficiency of these units. Dispersants such as poly(maleic acid) (39,40) inhibit scale formation, or at least modify it to form an easily removed powder, thus maintaining cleaner, more efficient heat-transfer surfaces. 

Calcium carbonate (CaCO,) calcium sulfate or gypsum (CaSOJ and iron(II) carbonate (FeCO ) are the most common types of scales formed in drilling. If hydrogen sulfide is present, then there is a possibility of iron sulfide (FeS) scale depositing. 

Sulfates in surface MU water sources usually are present at lower concentrations (typically 20-60 ppm) but this level may rise to several hundred ppm in subsurface waters. The maximum solubility of calcium sulfate is dependent on temperature but is in the range of 1,800 to 2,000 ppm in cold water. This rate is significantly less in hot BW where boiler deposits occur, the sulfate scale normally is present as anhydrite (CaS04). Sulfate scales are hard and very difficult to remove, so treatment programs employed must be carefully controlled to avoid risks of scaling. 

The scale may consist of calcium carbonate, barium sulfate, gypsum, strontium sulfate, iron carbonate, iron oxides, iron sulfides, and magnesium salts [943]. There are monographs (e.g.. Corrosion and Scale Handbook [159]) and reviews [414] on scale depositions available in the literature. 

Scale deposits are converted to dispersed particles which can be circulated out of the wellbore. A chelating agent such as ethylenediamine tetraacetic acid can aid in dissolving calcium sulfate deposits. Hydrochloric acid following the basic treatment can also be used to dissolve calcium sulfate (167). 

Howden An early flue-gas desulfurization process using a lime or chalk slurry in wooden grid-packed towers. The calcium sulfate/sulfite waste product was intended for use in cement manufacture, but this was never commercialized. The key to the process was the use of a large excess of calcium sulfate in suspension in the scrubbing circuit, which minimized the deposition of scale on the equipment. The process was developed by Imperial Chemical Industries and James Howden Company in the 1930s and operated for several years at power stations at Fulham, London, and Tir John, South Wales, being finally abandoned during World War II. British Patents 420,539 433,039. 

Sulfur Emissicms Sulfur present in a fuel is released as SO2, a known contributor to acid rain deposition. By adding limestone or dolomite to a fluidized bed, much of this can be captured as calcium sulfate, a dry nonhazardous solid. As limestone usually contains over 40 percent calcium, compared to only 20 percent in dolomite, it is the preferred sorbent, resulting in lower transportation costs for the raw mineral and the resulting ash product. Moreover, the high magnesium content of the dolomite makes the ash unsuitable for some building applications and so reduces its potential for utilization. Whatever sorbent is selected, for economic reasons it is usually from a source local to the FBC plant. If more than one sorbent is available, plant trials are needed to determine the one most suitable, as results from laboratory-scale reactivity assessments are unreliable. 

In distillation the water closest to the heating surface is hottest and it is there that calcium sulfate is least soluble. Thus, calcium sulfate deposits, forming an adhering film that increases the thermal resistance and decreases the heat flux. The scale is continuously deposited until the tubes are cleaned or become plugged. For scale deposition the local concentration must be at least saturated in calcium sulfate. At 100° C. this occurs in concentrated sea water at a concentration 3.1 times that of ordinary sea water. A plant has been successfully operated continuously without calcium sulfate deposition by taking only part of the available water from the sea water, so that the liquid in the evaporator is never more than 1.8 times the concentration of sea water and the wall temperature is below about 250° F. ( ). This imposes technical and economic limitations on distillation plants. Similar considerations hold for plants distilling brackish water containing calcium sulfate. 

While the reverse solubility curve of calcium sulfate is often the main reason for scale deposition in fresh water boilers and in brackish water distillation, when the sea water is not chemically treated the cause is chemical rather than physical. Sea water contains bicarbonate ion. On heating, the bicarbonate ion reacts with water to form carbonate ion plus carbon dioxide, which tends to be evolved as a gas as shown in the equations... 

A typical deposit of calcium sulfate can be seen in Figure 2. On continuing the run the scale deposit increased, causing very high pressure drops, and in some cases the equipment became plugged with scale. 

PCA 16 is particularly effective for control of calcium sulfate deposition (and thus finds application, under different brand names and grades, as a calcium carbonate/sulfate scale inhibitor for RO systems treating brackish waters). This inhibitor is also useful for calcium phosphate control. It is stable against chlorine. 

Acid cleaning agents such as hydrochloric, phosphoric, or citric acids effectively remove common scaling compounds. With cellulose acetate membranes the pH of the solution should not go below 2.0 or else hydrolysis of the membrane will occur. Oxalic acid is particularly effective for removing iron deposits. Acids such as citric acid are not very effective with calcium, magnesium, or barium sulfate scale in this case a chelatant such as ethylene diamine tetraacetic acid (EDTA) may be used. 

During a study of the applicability of "spray" or "fog" evaporation to sea water desalination, it was found that this technique was particularly useful for scale deposition studies. Thus, test conditions are reproducible and heat transfer coefficients are very high, so that the effect of scale formation is readily apparent. Three novel methods for the control of scale deposits on the evaporating surfaces of a spray evaporator were explored. One involves the addition of small quantities of low molecular weight polyacrylic acid to the feed water, which prevents the formation of adherent scale. The methods are applicable under certain conditions to scales formed from sea water containing substantial amounts of calcium sulfate in addition to alkaline scale-forming substances. While spray evaporation appears to be of limited application in water desalination, the scale-control methods developed are probably applicable to other types of evaporator, particularly of the long-tube type. 

The study of scale deposition with and without vibration was limited to determination of the feasibility of scale prevention and removal by vibration of the heat transfer surface. Calcium sulfate solutions and sea water were used as scaling liquors calcium sulfate because of the difficulty of preventing its deposi-... 

The next approach to scale control was reduction of potential calcium carbonate by acidification of makeup water. Sulfuric acid is added to reduce alkalinity and convert most of the calcium bicarbonate to calcium sulfate. Calcium sulfate is soluble up to about 1,700 parts per million in cooling waters at ordinary temperatures, whereas calcium carbonate solubility is less than 30 ppm. So by slightly acidifying makeup water we greatly reduce tendency for calcium carbonate deposition. The calcium sulfate level in concentrated cooling water is controlled by bleed-off adjustment—manually or automatically. 

Earlier the standard industrial approach to prevention of calcium carbonate scaling by addition of sulfuric acid was described. Objectives were to reduce bicarbonate alkalinity, convert calcium carbonate to calcium sulfate, and regulate sulfate concentration by bleedoff. Corrosion inhibitors were added to protect system metals. A new approach to industrial cooling system treatment does not require addition of sulfuric acid. It involves application of phosphonate seques-trants, dispersants and special corrosion inhibitors, and provides deposit control equal to that obtainable when using sulfuric acid. Availability of phosphonate sequestrants makes possible combination scale control and corrosion inhibitors that can be used without the necessity of reducing cooling water alkalinity by acid feed. 

The steam-side condensing coefficient outside the tubes can be estimated using Eqs. (4.8-20H4 8-26). The resistance due to scale formation usually cannot be predicted. Increasing the velocity of the liquid in the tubes greatly decreases the rate of scale formation. This is one important advantage of forced-circulation evaporators. The scale can be salts, such as calcium sulfate and sodium sulfate, which decrease in solubility with an increase in temperature and hence tend to deposit on the hot tubes. 

Where sparingly soluble salts such es calcium carbonate, calcium sulfate, and calcium oxalate are present, they are essentially contaminants that can form scale deposits. The supersaturation potential is caused by both concentration and temperature effects. The inverse solubility characteristic acts to fevor deposition on the heat transfer surface. The contaminant may also be produced by a corrosion effect. 

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