OWN AND TRAP FLASH TO 0O\LER5
STEAM SUPPLY
FEED PUMP STCR
PUMPS
SYSTEMAT THE PEORIA PLANT OF COMMERCIAL SOLVENTS CORPORATION FIGURE 1. WATERTREATMENT Threshold treatment system is shown a t left. Effluent cooling water from mash coolers is then softened and used for bailer feed, which is further conditioned with sodium sulfite a n d sodium metaphosphate.
THRESHOLD TREATMENT Elimination of Calcium Carbonate Deposits from
Industrial Waters OWEN RICE ARrD EVERETT P. PARTRIDGE Hall Laboratories, Inc., Pittsburgh, Pa.
W
HEN the system calcium oxide-carbon dioxide-water manifests itself in the chalk cliffs of Dover or the architecture of the Carlsbad Caverns, it arouses emotions in the observer which may be more lofty but are no more intense than those produced in an engineer by the unseen workings of this same system when it converts a 12-inch into an 8-inch pipe line or reduces the heat transfer in a heat exchanger to 40 per cent of its design value. The tendency for calcium carbonate to come out of solution in inconvenient places creates an operating problem wherever waters containing appreciable concentrations of calcium and bicarbonate or carbonate ions are used industrially. In the power plant, the distillery, the oil refinery, and throughout the process industries in general, calcium carbonate scales form on the water side of condensers or heat exchangers in just those regions where, for efficient operation, heat transfer should be least impeded. With regard to afterprecipitation, it is probably safe to say that nearly every pipe line following a cold process lime or lime-soda softener and many of the lines following hot-process softeners have accumulated calcium carbonate scale to an extent which has either increased pumping costs, seriously reduced capacity, or pyramided these undesirable effects. Incrustation of filter sand, distribution systems, and domestic water heaters, following a soften-
ing operation, is another variation of this problem which has contributed no little to the troubles of water-works operators. Altogether, calcium carbonate has been a consistent nuisance t o the engineer.
Traditional Methods of Preventing and Removing Scale The means employed to cope with calcium carbonate have varied with the local conditions and the degree to which the engineer in charge regarded scale as a necessary evil. In some industrial plants, tannin or compounds containing tannin as the chief constituent have been added to cooling water in the hope that the calcium carbonate precipitated would be peptized sufficiently to prevent or retard the accumulation of deposits. Some railroads have used tannin similarly in the attempt to prevent afterprecipitation in pipe lines carrying softened waters, and in injectors, boiler feed lines, and feed water heaters on the locomotives. The concentration of tannin necessary t o achieve satisfactory results is, however, high enough t o make the treatment rather expensive and to introduce a definite color into the water, which may be objectionable if the water is to be used in some types of industrial processing or for domestic consumption. 58
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
An alternative method of attack to colloidal treatment with tannin is partial neutralization. In the case of cooling water in industry, this usually takes the form of treatment with sulfuric acid. In the water works, recarbonation is employed. In both cases the usual price paid for freedom from scale deposition is corrosion accelerated by the lowered pH of the water. Instead of continuous or semicontinuous treatment with sulfuric acid to hinder scale formation, periodic removal of scale with hydrochloric acid is practiced in both process and power plants. In the choice between adequate time for dissolution of the scale and the necessity for returning equipment to service, the tendency is generally towards an incomplete job. Careful supervision is essential to ensure proper washing after the removal of the acid, and even where this is given and where inhibitors are used to limit the attack of the acid upon iron or steel surfaces, repeated cleaning must result in appreciable corrosion. Cooling water for the condensers of steam power plants has been zeolite-softened in a number of instances in the effort to obvia,tescale formation. The cost of this process, however, precludes its use except where a cooling tower is employed -in which there is little loss of water by windage in addition to the necessary evaporation loss. In such a system most of the dissolved salts present in the make-up inevitably concentrate in the circulating water. As a result, calcium carbonate scale forms iii time, although obviously much less rapidly than if unsoftened make-up had been used. The ultimate alternative to the various chemical methods of combating calcium carbonate scale has been t o let it form
Deposition of calcium carbonate from bicarbonate waters used as cooling media in condensers or heat exchangersis a widespread industrial problem, as is the afterprecipitationof calcium carbonate in filters and pipe lines following lime and lime-soda softeners. To obviate formation of these deposits, various methods have been employed in the past, involving use of tannin, partial neutralization with acid, or recarbonation, or deposits have been periodically removed by mechanical means or by dissolving them in acid. A new method, developed within the past two years, depends upon the fact that sodium hexametaphosphate, present in the w-ater to the extent of only a few parts per million, will prevent precipitation when a high-bicarbonate water is treated with alkali or heated to a temperature not exceeding the boiling point. This threshold treatment has been applied successfully to remove old deposits as well as to prevent new ones in railroad, industrial, and municipal water-softening plants and distribution systems, and in power-plant, distillery, and oil-refinery condensers. In once-through systems a continuous feed of from 5 to as little as 1 pound of sodium hexametaphosphate per million pounds of water is employed. Where cooling water is recirculated through a spray pond or cooling tower, an amount is introduced in the make-up sufficient to maintain a similar range of concentrationsin the circulating water. An advantage of the threshold treatment is the relatively high pH which may be maintained in order to decrease corrosion without causing precipitation of calcium carbonate.
59
until it so seriously interferes with operations that mechanical cleaning is undertaken. Such cleaning is a major item of expense. In some cases it has even been found less costly to install a new line than to remove the scale from an old one.
New Solution to the Problem Within the past two years a radically new and unusually effective method of preventing the deposition of calcium carbonate and of gradually removing old scale has been developed on the basis of the discovery by Rosenstein (6) that a few parts per million of glassy sodium metaphosphate, (NaPOJ., added t o a water high in bicarbonate and calcium would prevent precipitation of calcium carbonate even when 500 p. p. m. of ammonia were added. This discovery was made during the development of the use of liquid ammonia as a fertilizer to be applied in irrigation water. According to Rosenstein (0, where water was carried in closed lines, the addition of the ammonia caused extensive precipitation of calcium carbonate. In a search for some means of preventing this deposition, two types of substances were investigated in the laboratory. The first type comprised organic colloids, such as hay infusion, tannin, various types of glue, albumen, and caseinate. The second type included soluble substances which might be expected to have surface activity, such as citrates, urea, and the molecularly dehydrated phosphates. When the various substances were systematically tested, more or less positive results were obtained with all of them, but only in the case of the molecularly dehydrated phosphates were the results sufficiently startling t o induce further investigation. In the attempt to determine how much of these substances was required, the surprising effect of minute amounts was soon discovered. Immediate success attended field trials with sodium hexametaphosphate, which is now used regularly in concentrations of only a few parts per million with the ammonia treatment. Laboratory investigation (6)has since demonstrated that precipitation of calcium carbonate is inhibited consistently by the presence of 2 p. p. m. and in many cases by less than 1 p. p. m. of sodium hexametaphosphate, not only when ammonia is added to a water high in calcium and bicarbonate ions, but also when the carbonate-ion concentration of such a water is increased by treatment with sodium carbonate, sodium hydroxide, or lime, or by heating. The term “threshold treatment” which has been applied to this use of metaphosphate has a double significance, since the precipitation of calcium carbonate is evidently stopped on the threshold of crystallization, and a concentration of 1 p. p. m. is on the threshold of satisfactory measurement. The truly phenomenal effect of such vanishingly small concentrations of metaphosphate in not only preventing the formation of new scale but even in removing old deposits of calcium carbonate has been demonstrated in many applications, a few of which are described in the following sections.
Applications in Condenser Cooling Water Both figuratively and, in many cases, literally it may be said that rivers of water flow continuously through the condensers which are an essential part of distilleries, oil refineries, and steam power plants. In eachof these industries, threshold treatment has proved effective and economical. ONCE-THROUGH SYSTEM IN DISTILLERY.The Peoria, Ill., plant of the Commercial Solvents Corporation was the pioneer industrial plant in the use of the threshold treatment. The available water supply, drawn from wells, is high in calcium and bicarbonate ions, as indicated by the analyses in Table I. The double-pipe mash cooling system in this plant is designed to maintain certain final mash temperatures by automatic regulation of the flow of cooling water. With the system
INDUSTRIAL AND ENGINEERING CHEMISTRY
60
thoroughly clean, the effluent water will have a temperature of 170-175" F. (77-80" C.). As scale develops, increasing the resistance to heat flow, the water demand will progressively increase, and the temperature of the water leaving the system will drop. If much scale develops, it will be impossible to cool the mash to the desired point. Much of this water is used as feed in the boiler plant as well as for mixing fresh mash, an,d any unnecessary lowering of final water temperatures means an accompanying loss in the over-all plant heat economy. TABLEI. COMPOSITION OF WELLWATERS
a
Bicarbonate (HCOa) Sulfate (so4) Chloride (C1) Silica (SiOd Calcium (Ca) Magnesium (Mg) Suspended material Estimated.
( I n parts per million) A 475 173 36 15 135 40-50" :ABEONATE IN FILTERS
.
M7JNICIPAL W A T E l i SOFTENING P L a T . The operator of R rriuriicipel water plant using lime-soda softeners has always faced a dilemma, the two horns of which were afterprecipitstion and corrosion. By recarbonating the water following the softeners, he could prevent afterprecipitation more or less completely, but if he were successful in stopping all precipitation, he was likely to he plagued by "red water." The usual practice has been, accordingly, to recarbonate only
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INDUSTRIAL AND ENGINEERING CHEMISTRY
63
partially. As a result, the filter sand becomes incrusted with calcium carbonate (Figure 3) or even cemented into a solid mass (Figure 4).
were made to minimize the attack by the controlled feed of alkali to throw down a protective coating of calcium carbonate, it was found difficult to avoid both corrosion on the one hand and undesirably extensive scale formation on the other. Recent studies (1, 3) of the Langelier corrosion index have demonstrated how narrow are the limits within which TABLEIV. EFFECTOF THRESHOLD TREATMENT ON AFTERPREsuch a treatment must be maintained. CIPITATION AT WATER SOFTENING PL.4NTa Sodium silicate has been recommended particularly for .4v. Results, p. p. m., CaCOsHardness Pptn. on -Alkalinitycorrosion prevention; but when it is used in bicarbonate filtered filter sand Period Bettled Filtered waters, the increase in pH is likely to induce precipitation 103 33 1933 66 33 of calcium carbonate, which tends to take down with it the 122 34 1934 69 35 139 28 1935 66 38 silica. The net result is undesirable scaling close to the point 112 31 1936 73 42 108 30 1937 69 39 of addition and lack of protection beyond this zone. 7 -
1938, Jan. Feb. March April Mayb June July Aug.c 0
b 0
76 70 68 66 67 67 69 72
51 34 35 38 38 66 69 73
118 95 94 99 99 106 104 103
25 36 33 28 28 1 0
THRESHOLD TREATMENT
STARTED
I
-1
Data obtained from the Ohio State Board of Health. Threshold treatment with 2 p. p. m. was commenced at noon, May 25. Data for first 25 days.
The municipal water plant a t Delaware, Ohio, represents a typical case in which partial recarbonation has been employed to minimize afterprecipitation. Prior to May 25, 1938, when threshold treatment with 2 p. p. m. of sodium hexametaphosphate was initiated, the drop in total alkalinity through the filters varied between 28 and 36 p. p. m. of calcium carbonate. As indicated in Table IV by the monthly average of daily determinations of alkalinity on the settled and the filtered water, the afterprecipitation since the start of the threshold treatment has been practically zero; previously, in the course of a year approximately 33 tons of calcium carbonate precipitated on the filter sand. The change in the behavior of the water after the start of threshold treatment is indicated in Figure 5 by the values for total alkalinity of the water in the settling basin ahead of the filters, in the storage basin following the filters, and in the distribution lines in the city 3 miles from the plant. The difference in alkalinity between the settled and the filtered water prior to the start of treatment represents the normal extent of afterprecipitation. As the treated water began to accumulate in the storage basin, the difference decreased, disappearing on the third day. The water from the city tap showed a definite drop in alkalinity as compared to the filtered water for 3 days after the start of the metaphosphate feed, indicating that afterprecipitation had been occurring in the distribution lines as well as on the filter sand; but by the fourth day deposition of calcium carbonate had substantially ceased throughout the system (Figure 5). The filter effluent since the start of threshold treatment has been harder than before by the amount of calcium carbonate prevented from precipitating. If the incrustation and cementation of the filter sand seemed a negligible disadvantage compared to the advantage of a softer filter effluent, it would be advisable t o feed the metaphosphate following the filter instead of ahead of it, thus obviating further precipitation in the distribution system while gaining the softening effect of precipitation in the filters. The potability of water is in no way affected by threshold treatment with metaphosphate. In fact, much larger amounts of this substance than could possibly be ingested by the drinking of water treated in this manner are harmless.
Prevention of Corrosion The tendency of natural waters to corrode pipe has long been a concern of water-supply chemists. When attempts
SETTLED
FIGURE5. ELIMINATION OF AFTERPRECIPITACALCIUM CARBONATE IN MUNICIPAL WATERSYSTEM BY THRESHOLD TREATMENT
TION OF
After start of treatment, alkalinities of filtered and of city tap water approaoh and equal that of settled water, showing that afterprecipitation has ceased.
Threshold treatment now offers new possibilities in the prevention of corrosion. By the addition of 1 or 2 pounds of metaphosphate to a million pounds of bicarbonate water, precipitation of calcium carbonate will be prevented when alkali is added to raise the pH to 10 or even higher. Such an increase in pH will, in itself, minimize corrosion. Where controlled formation of a protective coating is t o be attempted in addition, there is the possibility of accomplishing this at the higher pH level by feeding even more minute amounts of metaphosphate, with the knowledge that deposition can be stopped and slow removal of scale can be started at any time by increasing the metaphosphate feed.
Acknowledgment Among the many persons who have contributed information for this paper, grateful acknowledgment is due particularly t o Bentley Brown of the Commercial Solvents Corporation, R. T. Armstrong of the New Mexico Power Company, and C. P. Hoover of the Water Purification Works of Columbus, Ohio.
Literature Cited (1) De Martini, F. E., J . Am. Water Works Assoc., 30, 85-111 (1938). (2) Hatch, G. B., and Rice, Owen, IND. ENG.CHEM.,31, 51 (1939). (3) Langelier, W. F., J . Am. Water Works Assoc., 28, 1500-21 (1936). (4) Rosenstein, L., personal communication. (5) Rosenstein, L., U. S. Patent Reissue 20,754 (June 7, 1938). RHICEIVED September 12, 1938. Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 96th Meeting of the Amerioan Chemioal Society, Milwaukee, Wis., September 5 to 9, 1938.
