Liquid purification or separation – Processes – Preventing – decreasing – or delaying precipitation,...
Reexamination Certificate
2000-12-28
2003-11-11
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Preventing, decreasing, or delaying precipitation,...
C210S701000, C252S180000
Reexamination Certificate
active
06645384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to the treatment of aqueous systems, and more specifically to inhibiting scale formation and other solid deposits in industrial heating and cooling systems.
2. Description of the Related Art
The water used in industrial systems such as in steam generating boilers, hot water heaters, heat exchangers, cooling towers, pipelines, gas scrubbing systems and related equipment accumulate various impurities derived from the water. These impurities generally include the alkaline earth cations, such as calcium, barium and magnesium, and some bicarbonates and carbonates, sulphates, phosphates, silicates and the like. The most common impurities in industrial water are the water-hardening metal ions including calcium, magnesium and the carbonate ions. In addition to precipitating as carbonates, calcium and magnesium, as well as the other metals such as iron or copper, react with the sulphates or phosphates to form the respective insoluble complex salts. These reaction products accumulate on the surfaces of the system forming scale and sludge, which substantially reduce the heat transfer efficiency by settling in the systems where they impede flow and insulate heat-transfer surfaces. Moreover, in addition to interfering with the fluid flow and heat transfer, corrosion of the metal surfaces is promoted, since the corrosion inhibitors added to the water are less able to reach the metal surfaces to provide effective protection against the corrosive components within the aqueous system. Further, scale deposits can harbor bacteria, the removal of which is expensive due to the delays inherent in the shutdown-treatment-restart sequence. Maintaining proper residual levels of water treatment chemical actives is critical to the success of high-performance water treatment programs. To provide optimum cost and performance, each active component in the water treatment program must be consistently maintained at residual levels sufficient to achieve treatment efficacy without relying on unnecessarily high levels of treatment chemicals.
Water conservation is another important consideration in the operation of industrial cooling and heating systems and will become more important in some regions as a result of moderate to severe water shortages caused by climate change, contamination and/or population growth. In the face of such shortages, the cost of both water consumption and discharge for industrial users will continue to increase. Additionally, both increased governmental regulation and a desire to reduce costs have motivated industrial users, particularly in industries that have traditionally utilized high levels of water consumption, to identify and apply methods that will enable them to reduce the incremental water consumption.
Improvements in water treatment technology, by allowing increased use of recycled water and permitting increased cycles of operation, have been significant factors in reducing industrial water consumption and discharge, ideally without requiring extensive process redesign or capital investment. However, both the use of recycled water and the use of higher operating cycles generally increase the potential for fouling and place correspondingly greater demands on the water treatment programs.
Early water treatment programs, particularly those for evaporative cooling systems, utilized acid to control the pH and thereby reduce the potential for scaling. More recently, however, the development and increasing use of acid-free organic cooling water treatment programs, also referred to as all-organic programs (AOP), has shifted the focus of high-cycle operation towards eliminating or controlling the deposition of calcium carbonate, calcium phosphate, and magnesium silicate scale. In addition to the scaling concerns, when using higher cycles of concentration in cooling systems, other makeup water components, including iron, ammonia, and the total dissolved solids (TDS), can place severe demands on the treatment program chemicals to control the resulting corrosivity and conductivity of the aqueous system. A treatment program that fails to address these additional concerns can result in galvanic corrosion, interfere with inhibitor film formation, and/or reduce the effectiveness of certain biocides.
The calcium carbonate deposition potential of a cooling water is frequently expressed as a scaling or saturation index. One such index is the Langelier saturation index (LSI) which provides an indication of calcium carbonate (CaCO
3
) stability in an aqueous system. The LSI is a function of the calcium hardness, alkalinity, conductivity and temperature of the aqueous system. A typical AOP can operate satisfactorily in aqueous systems having an LSI between 1.0 and 3.0, but few, if any, are able to function satisfactorily at LSI values above 3.0.
However, LSI calculations are based on bulk water concentrations of calcium and alkalinity and do not take into account other soluble species that may effect the activity of the calcium or carbonate ions. For this reason, the inventors prefer to use the calcite saturation index (CSI) for evaluating treatment program performance. The CSI defines the relative degree of saturation of calcium carbonate as a ratio of the ion activity product to the solubility product according to the formula:
CSI
=[Ca
2
+][CO
3
2
]/K
sp CaCO
3
Unlike the LSI method, the CSI calculation takes into account the effects of ion pairing and can be used to compare the scaling tendency of waters of with very different compositions. Commercially available software applications, such as Water Cycle™ from French Creek Software, Kimberton, Pa., permit rapid calculation of CSI and many other water parameters based on makeup water chemistry.
Most AOPs can achieve satisfactory results in aqueous systems having an operating CSI of between 100 to 200. Although there have been reports of an AOP that that can function satisfactorily in an aqueous system having an operating CSI of approximately 300, in practice such supersaturated waters generally present an unacceptable risk of total bulk water precipitation. Bulk water precipitation can be a catastrophic event in an industrial system resulting in, at a minimum, extensive fouling of the system, and, at worst, actual structural failure. Any practical treatment system must, therefore, provide a sufficient operating margin to avoid such an occurrence.
In addition to calcium carbonate scale, industrial systems must generally contend with silicates as well. Silicates may deposit on heat transfer surfaces as a scale of colloidal silica or as magnesium silicate and may, in some instances, become a limiting factor in a given aqueous system. In particular, the scaling potential of magnesium silicate increases for values of the system pH above 8, but does not typically become an significant concern until the system pH exceeds 8.5. Silica can co-precipitate with iron and magnesium hydroxides and the silicate may also precipitate with calcium salts. In systems with an alkaline pH, levels of silicate in general should be kept such that the product of the magnesium and silicon concentrations (in parts per million) is below 20,000 (i.e., Mg*Si<20,000). It is especially important to avoid the formation of silicates because, once formed, silicate deposits are particularly difficult to remove. To address this problem, various polymeric materials have been developed which show an ability to inhibit colloidal silica and magnesium silicate deposits. These materials are, however, relatively expensive and are thus usually restricted to specific applications where silicate is the dominant contaminate and the potential cost savings justify their use.
Increased phosphate levels in the available makeup water is also becoming an issue for many industrial applications that are attempting to maintain high-cycle operation. Phosphate may be present in surface waters as a result of agricultural run-off or industrial pollution. The concentration of phosphate in sur
Richardson John
Tribble Richard H.
Trulear Michael G.
Chemtreat, Inc.
Harness & Dickey & Pierce P.L.C.
Hruskoci Peter A.
LandOfFree
Method for inhibiting scale in high-cycle aqueous systems does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for inhibiting scale in high-cycle aqueous systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for inhibiting scale in high-cycle aqueous systems will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3129190