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.
82 1
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 in a majority of cases an 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.
T
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-
I S D V S T R I A L A X D E-VGISEERISG CHEJlIIsTRY
822 HYPOTHETICAL COMBISATIOKS
T a b l e I-Mineral Analyses of W a t e r s R a w WELLWATER ZEOLITE-SOFTENED WATER MAKE-UPWATER P. p . m. Gr. p e r g a l . P. p . m. G r . p e r gal. P.p . m. Gr. p e r gal.
VOl. 21, No. 9 FILTERED BOILERWATER
P.p. m.
Gr. p e r gal.
INCRUSTANTS
Calcium carbonate Magnesium carbonate Magnesium sulfate Iron and aluminum oxide Silica
I Sodium Sodium Sodium Sodium
carbonate sulfate chloride hydroxide
205.0 96.3 54.4 6.0
8.4
12.00 5.63 3.18 0.35 0 49
3.4 4.2
0.20 0.25
...
...
6.0 8.4
0.35 0.49
164.8' 108.2 .,.
9.64 6.33 ,..
7.6 3 . 3 Mg(OH)?
0.44 0 . 1 9 Mg(0H)L
...
...
6.0 8.4
0.35 0.49
2 2 9.1
0.13 0.53
27.5 84 1 16 1
1.61 4.91 0.94
121.4 2465,O 692.0
7.09 144.00 40.40 2.73
HOS-INCRUSTASTS
...
...
19.5 16.1
1.14 0.94
330.0 84.1 16.1
19.30 4.91 0.94
...
...
...
46.7
SUMMARY
Total dissolved solids Total hardness as CaCOa Total alkalinity as CaCOa Half-bound COa as CaCOs
405.7 365.0 320.0 320.0
23.73 21.35 18.70 18.70
452.2 8.5 320.0 320.0
26.44 0.50 18.70 18.70
bonate hardness as calcium carbonate and magnesium hydroxide, the scale-forming impurities of the hard water will be changed to sludge and the excess alkalinity will be destroyed. The precipitated sludge introduced into the boiler by the reaction between the soft and hard water is controlled by the deconcentrator system. Operation of Deconcentrator Deconcentration, defined as the separation of the insoluble matter from the boiler water without the waste of blow-down, is accomplished by installing a settling tank outside the boiler. (Figure 1) The equivalent of 25 to 50 per cent of the hourly evaporation is circulated to the tank, where the suspended matter deposits and the clarified water returns to the boiler. Circulation is accomplished by means of the thermo-siphon principle without the use of pumps, ejectors, or other mechanical means.
415 1 294.0 320.0 320.0
24.27 17 18 18.70 18.70
3347.3
Under severe conditions, where the insoluble matter is present in large quantities, it is possible to circulate the equivalent of 150 per cent of the hourly evaporation, separate the suspended matter from the boiler water, and return the clarified water to the boiler. Sedimentation is adequate to separate a large portion of
...
...
the suspended matter. Attempts have been made in the past to use a filter bed for clarification of the boiler water, but experience has shown that more effective results will be secured by sedimentation. The continuous and uniform circulation of a definite quantity of boiler water maintains the boiler in what is known as equilibrium, with a low coefficient of insoluble concentration, permitting operation a t high overloads without the production of wet steam. Survey of Deconcentrator Operation The benefits to be realized from the operation of a deconcentrator in combination with a zeolite softener are disclosed in the following sur\-ey. The investigation covered a period of 200 days, under actual boiler room conditions, using a horizontal return tubular boiler with a heating area of 1500 square feet, operated a t 125 pounds pressure and 150 per cent of rating. Return condensate amounted to 60 per cent of the total evaporation, with make-up water 40 per cent. The well water available contained a total hardness of 365 p. p. m., composed of 320 p. p. m. carbonate hardness, and 45 p. p. m. non-carbonate hardness, as indicated by the analysis in Table I. Other deconcentration data are given in Table 11. T a b l e 11-Deconcentration Type boiler Pressure Rating Returns Make-uD Boiler make-up: Hard water Zeolite-softened water Boiler feed water: Hard water Zeolite-softened water Returns Boiler water: Allowable soluble concentration Allowable insoluble concentration Coefficient of soluble concentrafion, C, Coefficient of insoluble concentra,tlon, C, Blow-down per cent of evaporatlon Circulation'through deconcentrator Deconcentrator settling efficiency Suspended solids removed per hour Potential insoluble matter entering boiler Sludge blown out per hour, 5 per cent solids
F i g u r e 1-Deconcentrator S y s t e m for W a t e r - T u b e Boiler w i t h E x t e r n a l P r e c i p i t a t i n g Tank
195.51 0.77 10.78
13.2 184.6
Data
H. R . T. 125 Ibs. 150% 60 %
3500 p. p. m. 280 p. p. m. 68.5 2.42 1.46% 48 6% 85 0% 0 . 8 6 Ib. 0 86 Ib. 1 7 . 2 lbs.
USE OF ZEOLITE-SOFTENED WATER horn-The initial phase of the investigation covered 14 days, using zeolitesoftened water alone for make-up. The mineral composition of the zeolite-softened water is shown in Table I. An initial alkalinity of 186 p. p. m. increased to 2050 p. p. m. in 6 days. Heavy blow-down to reduce the concentration of soluble salts lowered the alkalinity to 1617 p. p. m., but by the end of the thirteenth day the alkalinity had again increased to 2545 p. p. m. The peak alkalinity represented over 17 pounds of sodium hydroxide and sodium carbonate per 1000 gallons of boiler water. The sodium hydroxide alone reached a concentration of 754 p. p. m., equal to 5 pounds of sodium hydroxide per 1000 gallons of boiler water.
September, 1929
I S D CSTRIA L A X D EiYGIh’EERISG CHEXISTRY
823
USE OF ~ I I X T UOF R EZEOLITE-SOFTEXED WATERAND RAW immediately became greater than 1:l and remained far WELL WATER-AS the initial step in the second phase the above the danger zone during the remainder of the 200-day zeolite softener was shut off and raw well water was sup- period. By keeping the alkalinity a t a lo^ value and allowing the sulfates in the natural water to concentrate within the boiler, a safe sulfate ratio may 3 be maintained in the great majority of cases without the addition of sulfuric acid, acid salts, or sodium sulfate. a
8
Economy i n Operating Costs
1000
500
In addition t o the control of the concentrations which accumulated within the boiler, the zeolite DAYS I N O P E R A T I O N deconcentrator combination was found to effect a Figure 2-Results of 200-Day Survey Showing Total Alkalinity and Total Sodium Hydroxide for Zeolite-Softened Water Alone a n d with Deconcentrator l’ery considerable Saving in Operating Costs. During the time the zeolite-softened water alone plied for make-up. l~igure2 indicates how rapidly the reac- was used for make-up, the boiler required 360 gallons per hour, tion of the sodium carbonate in the boiler miter with the 24 hours per day, and 300 days per year. The total annual non-carbonate hardness in the raw feed water reduced the requirements of soft water amounted to 2,600,000 gallons, carbonate alkalinity, the sodium hydroxide in the boiler requiring 27,500 pounds of salt for regeneration of the mater combining with the magnesium of the raw water t o softener. The cost of the salts mas $10.00 per ton, resulting in an annual salt cost of 813i.50. reduce the hydroxide alkalinity. Ctilizing the zeolite deconcentrator combination, only i 2 Na2C03 CaS04 --f Na2S04 CaC03 gallons of zeolite-softened water \yere required per hour, makMgCO3 ---t Na2C03f RSg(0H)z ing a total of 520,000 gallons per year. The annual cost for 2NaOH regeneration of the softener totaled S27.50, a saving of 80 In boiler water I n hard water Sludge to deconcentrator per cent. Following continued operation in this manner, the total alkaThe blow-down necessary t o maintain a non-foaming conlinity of the boiler water by the seventeenth day was reduced dition when operated with zeolite-softened water alone was to 215 p. p. m. Since most of the excess alkali in the boiler 6.9 per cent of the total evaporation, while with the zeolite dewater had been exhausted, the zeolite softener was again concentrator combination a blow-down of only 1.46 per placed in service. For each 1000 gallons of hard water sup- cent was required, a saving of 78.8 per cent of the blowplied to the heater 250 gallons of zeolite-softened water were down. added. This ratio provided sufficient sodium ‘OD carbonate to precipitate the hardness of the raw water and maintain an alkalinity of approximately 200 to 250 p. p. in. For the remainder of the f 200-day period the predetermined ratio of soft and hard water was maintained. The hardness of the boiler water did not exceed 15 p. p. m., the boilers did not accumulate scale, and foam- 3 4 0 J ing was not experienced. It is to be noted that, although the make-up water amounted to 40 per cent of the evapora- z tion, the blow-down was less than 1.5 per cent, 51 the circulation through the deconcentrator was k 10 nearly 50 per cent, and the sludge removed { 8 amounted to 17.2 pounds per hour and carried K 6 2o 4o ~o 8o 120 140 IBo IBo 2oo 5 per cent solids. D A Y S IN OPERATION
+ +
+
3
E
Importance of Proper Sulfate Ratio Embrittlement of boiler d a t e mav be definite17 prevented by the maintenance of a proper ratio of sodiurn sulfate to total alkalinity, as shown by the work of Parr and Straub ( 1 ) . -4lthough other possible inhibitants, such as tannates, phosphates, and other salts, are being investigated, the work is still in the experimental stage and should not be relied upon at present for protection from embrittlement. As shown in Figure 3, the ratio of sodium sulfate to total alkalinity was very low during the first 14 days of the present survey, owing to the high alkalinity of the boiler water when zeolite-softened water alone was used for boiler make-up. According to the A. S. 11. E. recommendation, the sulfate ratio must be a t least 1:l for pressures up to 150 pounds. The low values exhibited here placed the boiler water definitely in the embrittlement danger zone. On the fifteenth day, however, when t,he deconcentrator was p1acc.d in service and raw water was fed to the boiler, the sulfate ratio
Figure 3-Sulfate-Alkalinity
Ratio
Conclusion The survey establishes a new method for the conditioning and control of boiler water. The utilization of the alkalinity for a profitable purpose eliminates one of the serious and unfavorable features of the zeolite system. The reduction in salt consumption, reduction in blow-down with its saving in fuel, and the maintenance of a correct sulfate-alkalinity ratio without the use of sulfuric acid, acid salts, or sodium sulfate makes the method effective in reducing operating expense. The maintenance of clean boiler water provides the initial step in the production of clean steam, and the simplicity of operation makes the method practicable for the boiler plant. Literature Cited (1) Parr and Straub, IND ENG CHEM, 19, 620 (1927) (2) Universityof Illinois, Eng Expt. Sta., Bull 177 (1928).
