September, 1929
I S D C S T R I A I , A S D EA1*GISEE&ISGC'HEMISTRY
819
Chemical Proportioning of Internal Feed Water Treatment' E. M. Partridge P A I G EA K D
JONES
CHEMICALC o , HAMMOSD, I ~ D
Operating results are given t o show that harmful scale formation m a y be prevented at low cost through the use of organic matter i n conjunction with the quantities of inorganic chemicals far below the quantities necessary to react chemically with the scale-forming constituents of the feed water. A means for conveniently applying the treatment i s described.
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HERE is probably no natural ;upply of water which cannot be benefited in some respects by treatment before use in steam boilers. Treatment by such agencies as lime and soda precipitation softeners or zeolite softeners has been difTerent.iated as external treatment in contrast to treatment by means of chemicals added to the water and expected to do their work in the boiler. There are connections between the two forms of treatment, however, which are not always clearly seen by those who in one sentence classify treatment either as external or internal. Lime and soda treatment, for instance, depends for its success, not entirely upon taking the incrusting salts out of the water treated, but in part upon adding an excess of treatment which will in the boiler prevent the unremoved incrustants from forming scale. Lime and soda softening is, therefore, partly internal in nature. Either lime-soda or zeolite softening may need to be followed by acid treatment or sodium phosphate to prevent boiler embrittlement, the action of such secondary treatment having to do with developments in the character of the water under boiler conditions and being essentially internal treatment. Again external softening is often followed by the use of antifoam. External treatment is thus related to internal treatment in that its use may make necessary a supplementary internal treatment or may for success be based on the internal effect of chemicals added as excess treatment in the external process. The most common interpretation of internal treatment is the use of chemical means to prevent hard boiler scale and it is with this phase of the subject that this paper will deal. There may be many variations of such treatment, but only one of two systems will be considered here. Addition of Excess of Alkaline Materials
The first system, which is not the one to be discussed except by contrast, is that in which alkaline chemicals are added to precipitate the calcium and magnesium salts together with an excess to insure proper reaction. Soda ash and sodium phosphate are the chemicals commonly used, although soda ash, being less expensive, has obtained the widest use. In its beat form this type of treatment is controlled by t,ests on the boiler water to insure the presence of the necessary excess chemicals. The use of these chemicals in amounts sufficient to secure this excess gives boilers free from hard scale, but increases the sodium salt content of the water by the conversion of the .sulfate hardness into sodium sulfate and by the excess alkali salt added to insure proper reaction. Unless a careful blow-dowi schedule is maintained, foaming results, and soda ash in particular has been known to be accompanied by pitting, due in part to the increased electrolyte in the boiler w a t e1'. Addition of Organic Matter
The second form of internal treatment does not depend upon complete neutralization of calcium and magnesium for 1
Receix-ed April 2 , 1929.
results. I t depends rather upon the addition of organic matter which will keep the carbonate hardness in solution or suspension long enough to allow it to mix with the sulfate scale, softening it and causing it to decompose under boiler conditions. I n previous papers (3) the writer has stated that scale-forming matter is first precipitated in the boiler water as suspended matter, and that the organic matter used in boiler compounds would determine whether or not this suspended matter would attach itself as boiler scale. Hall (2) has shown that adherent scale is formed by crystallization directly on the surface of the boiler metal, although suspended matter may be incorporated therein. This necessitates a modification of the theory of the effect of organic matter to be that its action on the suspended matter in the boiler water causes an increased incorporation of this suspended matter with the hard scale forming on the heating surfaces. This action should be carefully differentiated from that said by French to take place. To quote from French (1): Such organic reactions as occur usually produce bulky precipitates, relatively light for their weight, which, while continuing to circulate, seem to have coagulating and clarifying properties. In this way finely suspended matter which might otherwise be found later in scale is removed. The beneficial action which occurs is gained, not by keeping the products of the organic reaction out of the scale but rather, by causing them to be incorporated with the scale. The scale which would have had a dense crystalline structure is thus softened and caused to lose its cohesive property.
The average water used for boiler feed contains both carbonate and sulfate hardness. Upon entering the boiler the carbonate hardness is precipitated quickly as sludge or suspended matter, leaving the sulfate hardness in solution to form the hard scale appearing on the heating surfaces. It is a matter of common observation in railway operation that some waters containing carbonate hardness will lime up the injectors and other feed-water appurtenances, but that when antifoam or other tannin-containing compound is used this will stop. The average antifoam contains less than 80 per cent of liquid extract, and therefore not more than 40 per cent of solid extract. iidded to the water a t the rate of a pound per 6000 gallons there is less than 0.5 grain of tannin extract added, and yet this has been known to keep waters containing from 10 to 20 grains of carbonate hardness from forming an injector line deposit. This holding up of the carbonate apparently continues in the boiler water causing more calcium carbonate t o be mixed with the calcium sulfate scale than if organic matter were not added. This is regarded to be due to the exertion of a protective effect on the matter that would otherwise be quickly precipitated as sludge, keeping it in a sufficiently fine state of subdivision to allow easy incorporation with the adherent scale. It may be due in part to reactions between the organic matter and the carbonates to form compounds haying solubilities tending to cause their precipitation a t the heating surfaces. The following table shows the analysis of a raw water supply and two analyses of scale. The scale was found on the upper tubes of a locomotive just below the steam dome, the samples being taken before treatment was commenced and after treatment had been in use with water of the character indicated by the water analysis. It will be noted that the calcium carbonate in the scale has increased from 5.70 per cent without treatment to 19.20 per
INDUSTRIAL AND ENGTNEERING CHEMISTRY
820
cent with treatment. Since the alkali content of the compound used with this water constituted but 53 per cent of its make-up in terms of soda ash, its use a t the rate of a pound per 1500 gallons added only 2.67 grains of soda ash in amounts much less than that required to neutralize the 11.60 grains of magnesium sulfate present. It is a characteristic of such deposits, which have had their carbonate content built up by the addition of organic matter to the water, that a t the time the boiler is opened they are soft, being more of a mud than a scale. On standing they harden and become brittle, indicating that the calcium carbonate has been broken down to calcium hydrate in the deposit, and carbonates in the air after the boiler is opened. SCALE F R O M TOPRow TUBES
RAWWATER
L-ntreated Treated water water
Grains Silica Iron oxide a n d alumina Calcium carbonate Magnesium carbonate Magnesium sulfate Sodium sulfate Sodium chloride
per gal. 0.75 0.23 17.20 0.59 11.60 1 06 1 00
Per cent Silica 3.98 Iron,oxide a n d alumina 0.84 Magnesium hydrate 3.18 81.80 Calcium sulfate 5.70 Calcium carbonate Loss a n d undetermined 4.50
Per cent 3.64 2.12 4.12 58.58 19.20
12,34
It will be noticed from the preceding remarks that with this form of treatment absolutely clean boilers are not expected. The result is rather the prevention of extremely hard scale by the increased inclusion of floating particles, particularly calcium carbonate, with the crystalline scale forming on the heating surfaces, causing the formation of a mud a t that point rather than the dense scale which results without the treatment. It might seem from this that the actual volume of deposit would be increased, but the character of the deposit is such that it does not support continued growth except at places where the construction supports the formation of mud pockets and where the circulation of the water is insufficient to carry away the deposit. Much of the mud formed can be removed by blowing down and by boiler washing. This form of treatment has attained its widest use on the railroads. The following is a copy of a report by a railroad which has used this form of treatment for several years. Data o n Compound Treatment over 7 Months’ Period TREATMENT WATER INCRUSSCLFAIE Soda,ash Organic STATION PUMPED TANIS HARDNESS equiv. matter Gallons Grains per gal. Grains per gal. Grains Grains 2984,800 23.0 11.1 1.9 0.85 2205,000 25.0 8 0 2.5 0.90 3221,000 29.1 8.6 1.9 0.85 4 241.000 72.6 38.2 3.4 0.70 8 7 5 540;OOO 28.0 2.8 1.03 0.85 704,000 23.4 1.1 1.9 0.52 3917,000 21.3 3.5 1.4
;
SOLIDS REMOVED STATIONAT 60 PERCEST Lbs. 1 5878 2 4725 3 8050
4 6 7
1496 1295 1417 7155
GROSSSAVING A I
13 CEXIS
PER
COST OF
LE.
$764.14 604,25 1046.50 194.48 168.35 184.21 930.15
COMPOUND
1
$180.60 78.77 148.78 18.40 23.22 30.27 191.18
hrET
SAVING $583 54 525.48 897.72 176 08 145.13 153.94 748.97
T h e average flue mileage on nine engines in this district is 54,447 miles, which compares very favorably with t h a t on districts using lime-sodntreated water. There is no charge for operation and maintenance of feeders as there is none. Pounds of incrusting solids removed are figured a t only 60 per cent, which is very conservative. T h e boilers are practically free from scale, b u t some of t h a t is removed in washing and blowing down. Wash-out periods have not been increased or decreased, and cost is t h e same. A t no time has a locomotive been p u t in t h e shop from this district because of scale formation since compound has been used.
The figures for sulfate hardness and the amount of treatment have been added from the writer’s own records. It m-ill be noted that in all cases but one the soda-ash equivalent added by the compound is less than the sulfate hardness in the water. Yet the boilers are reported as practically free
Vol. 21, No. 9
from scale. The first station happens to be one a t which there was considerable trouble with clogging of injectors from deposits of carbonate hardness before the treatment was installed. After commencing treatment this trouble was eliminated. The switch engine using this particular water receives no other treatment and is reported as free from hard scale. The statement that no charge is shown for Operation and maintenance of compound feeders is explained when the form in which the treatment is put up and the device in which it is used is considered. The chemicals are pressed into dry one-pound balls, making weighing unnecess a r y a n d simplifying handling. Each day the regular p u m p i n g attendant a t the wayside station puts the required number of balls into a feeder (Figure l), which is set in as a bypass on the hard-water line to a storage tank. The balls are dissolved by part of the water flowing t h r o u g h t h e feeder and pass into the wayside tank with the water. T h e r e i s n o precipitate and no settling as a result of the addition of the compound, everything added going into the locomotive tender with the water when drawn. This method of handling has given rise to the name “ w a y s i d e tank method.” This form of treat- Figure 1-Wayside Treatment Tank ment has as its chief disadvantage its failure to remove completely or prevent any deposits in the boiler. Against this it has the advantages of low cost of installation and operation, great improvement in boiler conditions as regards scale, resulting in notable savings, and the elimination of certain troubles found with treatment of the type wherein an excess of chemicals is added. Since so little of the sulfate hardness is actually neutralized, thus forming sodium salts, there is no foaming or pitting through its use. These advantages are most apparent in hard water districts, but in these same districts a combination of external and wayside tank treatment is recommended, the first to remove as much of the solid-forming matter as possible without the formation of too many sodium salt5, and the second to keep down sodium salts and insure the presence of mud instead of hard scale, each treatment offsetting the disadvantages of the other and gaining their common advantages.
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Addition of Small Amounts of Inorganic Substances
In addition to adding organic matter to affect the proportion of carbonates included in the deposits on the heating surfaces, inorganic substances are used to obtain beneficial results. For instance, with some iron-bearing maters, the inclusion of sodium phosphate as part of the alkaline matter in the compound will tie this iron up and prevent the formation of carbonate crusts in cooler parts of the boiler which
September, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
would be cemented together by the iron. The iron being present in small amount as compared with the calcium and magnesium, the effect is achieved without sufficient alkaline matter being used to bear any definite relation to the hardness in the water. Again aluminum salts may be used with silica-containing waters. Some natural waters have a high silica content together with low hardness and sodium carbonate, and form hard silicate scales. The addition of a grain or so of aluminum sulfate per gallon will benefit, boiler conditions in such cases. However, the most widely used and successful feed-water treatments familiar to the writer which use smsll amounts of chemical as compared with that necessary for complete neutralization are mixtures of alkaline salts and organic matter, the latter usually tannin.
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The organic matter increases the cadbonate content of t h e scale deposits on the heating surfaces, softening them and rendering them less objectionable as insulating material between the boiler metal and the water. At the same time the structure of these deposits is weakened, keeping them from building up to any considerable thickness and in a condition to allow easy removal by a stream of washout water. Literature Cited (1) French, IUD E h G CHEM 15, 1239 (1923) (2) Hall, Carnegie Inst T e c h , Mzn M e t Inuesligalrons, 24, 17. (3) Partridge Pouer, 60, 56 (1924), J A m Wale7 W o r k s Assocn , 12, 288 (1924)
Zeolite-Deconcentrator Combination for Boiler Water PurificationL Elwood W. Scarritt ELGINSOFTENER CORPORATION, ELGIN,ILL.
The use of the zeolite water softener operating in combination with deconcentrating equipment eliminates the undesirable features of zeolite-softened water for boiler feed purposes, reduces operating cost, minimizes blowdown, and provides a clean boiler water which will produce clean steam. The high caustic concentration caused by zeolitesoftened water is replaced by insoluble matter permitting the normal sulfate content of the raw water to provide i n a majority of cases a n adequate sulfate-alkalinity ratio for the prevention of embrittlement. The combination system operates by softening but a portion of the water by the zeolite system and by-passing a large quantity of raw hard water. The reaction between the soft and hard water creates sludge, which is uniformly and continuously removed from the boiler by the deconcentrating system. The results obtained with a combination system covering a period of 200 days under boiler room conditions are shown with accompanying curves and tables.
H E use of zeolite-softened water for boiler feed purposes, with its attendant difficulties and dangers, initiated a series of inrestigations to determine the feasibility of utilizing the favorable properties of such water and the elimination of those properties which are unfavorable. The scope of the investigation has been limii,ed to boiler pressures under 250 pounds, but the waters selected for study have been of a mineral composition which would establish a high alkalinity. The equipment included a standard zeolite water softener installed in a typical boiler plant, with each boiler equipped with an external deconcentrating system. The work was divided into two parts, the initial phase consisting of the use of zeolite-softened water and the second phase consisting of ’the use of a mixture of zeolite-softened water with raw hard water for boiler make-up.
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Mechanism of Deconcentration
The utilization of deconcentrating equipment in combination with a zeolite softener permits the sodiuin carbonate and sodium hydroxide produced in the boiler from the use of the zeolite softener to soften ram hard water; and in so dcing Received April 2. 1929.
the use of sulfuric acid or acid salts is eliminated, the blovdown is reduced, and the absence of excess alkalinity permits the normal sulfate content of the water to establish a correct sulfate-alkalinity ratio for the prevention of embrittlement. The reaction between the soft mater and the hard water precipitates the scale-forming salts as sludge. I n the absence of deconcentrating equipment the sludge would accumulate within the boiler, introducing a problem difficult to control, and the method Tyould not be practicable. The use of deconcentrating equipment permits the circulation of a definite quantity of the boiler water t o a settling tank placed outside the boiler, where the sludge is collected, and the clarified water returns to the boiler. The two outstanding defects of zeolite-softened water for boiler feed purposes are the accumulation of soluble sodium salts and the concentration of sodium hydroxide within the boiler, The removal of these substances through greatly increased blow-down is wasteful, even though a portion of the heat is salvaged by means of a heat exchanger. Since the destruction of the sodium hydroxide with sulfuric acid is dangerous, often resulting in serious corrosion in spite of careful control, the feed water must be so conditioned that a minimum of soluble sodium salts will be introduced into the boiler and that sodium carbonate and sodium hydroxide will not exceed a predetermined low concentration. A great many natural water supplies are characterized by various proportions of carbonate and non-carbonate hardness. Certain waters, such as alkali water, are found containing sodium bicarbonate but free from non-carbonate hardness. As the other extreme, waters containing practically no calcium and magnesium carbonate are occasionally encountered, usually because of acid contamination. Both types are localized, and do not lend themselves to partial zeolite treatment. The great class of natural waters, which we may call ‘‘normal,” contains both carbonate and non-carbonate hardncsc: commonly designated as “temporary” and “permanent” hardness, respectively. When such a normal hard water is passed through a bed of zeolite mineral, the sodium bicarbonate in the softened water need not contribute t o an objectionable concentration of sodium carbonate and sodium hydroxide within the boiler. If some of the hard water is mixed with a definite quantity of zeolite-softened water, such that the sodium carbonate and sodium hydroxide of the softened water will be sufficient t o precipitate the non-car-
