INDUSTRIAL AND ENGINEERING CHEMISTRY
1102
Vol. 20, No. 10
Zeolite Softening of Lime-Treated Water at Columbus, Ohio, Water Softening and Purification Works' Charles P. Hoover, Virgil L. Hansley,2 a n d Clyde Q. Sheely? WATER SOFTENING AND PURIFICATION W O R K S , COLCMBUS, OHIO
ECENT advances in water-softening by the lime-soda ash process and by the zeolite process have made it possible to use successfully a combination of the two in softening certain types of hard water. The purpose of this investigation was to determine whether or not it would be better and more economical for the City of Columbus, Ohio, to continue to soften its public water supply with lime and soda ash, as has been done for the past twenty years, or to use lime treatment in connection with zeolite as a substitute for the soda-ash treatment. If adopted, the entire supply of water will be treated with lime to remove the carbonate hardness, coagulated with alum, settled, carbonated, and filtered; then a sufficient quantity of i t will be softened to practically zero hardness by zeolite, so
R
7-
%FT
SAMPLE
II!
ZEOUTE.GPEEN SAND
t
-
-2
7
I
I BIRDSHOTGRAVEL MEDIUM GRAVEL b A P 5 E GRAVEL
INLET
t L
Figure 1-Diagram
The experimental plants are shown in Figures 1 and 2. One was a pressure-filter softener using greensand, and the other a gravity-filter softener using a gel-type zeolite, commercially known as Crystalite. Pressure Softener with Greensand The softener used for these tests was a type of domestic filter made by the Graver Corporation. (Figure 1) It was so devised that the softening might be made upward and the regeneration downward, or vice versa, by the simple manipulation of valves.
SOFT WATEROUTLET F~EEFMAKIdw
(1) Quantity of salt required t o remove 1000 grains of hardness (2) Efficiency of salt regeneration (3) Softening capacity of zeolite between regenerations-that is, number of grains of hardness removed per cubic foot (4) Per cent waste water used for regeneration and washing (5) Rate of softening a t beginning and end of softening period (6) Effect of mixing zeolite-softened water with lime-softened water
REGENERATIONWASTE
of Graver Household Pressure Softener
that by mixing the zero water with lime-softened water the resultant mixture will have the desired degree of hardness. The lime and zeolite process was given consideration at the Columbus plant about twelve years ago, but a t that time was given up because it was regarded as uneconomical. Original water-softening zeolites, although of comparatively high capacity, were low in rate of activity. They were slow in softening, and long periods of time were required for regeneration. They were also inefficient in salt consumption using from two to three times the amount of salt required with the modern rapid zeolites. The modern zeolites are greensand, found in extensive deposits in New Jersey and elsewhere, and the more recent gel types. Greensand must be thoroughly washed and screened to proper size before it is ready for use as a water softener, and is usually processed. The gel-type zeolites are synthetic and made by precipitation processes.
Data of Pressure Softener Depth of greensand in filter 34.9 inches Volume of greensand in filter 5 . 0 cubic feet 17.76inches Diameter of softener Area of greensand bed 1.72 square feet Rate of softening 7 . 5 gallons per minute Rate of regeneration 2.6 gallons per minute Discharge per square foot 4.38 gallons per minute Time of contact with sand 5 . 0 minutes
OPERATION-The raw water used for these experiments was lime soda-softened filtered water which ranged in hardness from 55 to 120 p. p. m., or from 3+ to 7.0 grains per gallon. The average hardness of the softened water was from 0.5 to 1.5 p. p. m. The flow was upward, and the rate was 4.38 gallons per square foot per minute. Regeneration was accomplished by placing a specified amount of 26 per cent brine in the %gallon regeneration chamber and then forcing it downward through the greensand at the rate of 1 I 2.6 gallons per minute. Regeneration was allowed to continue a t this rate for 18minutes, FREEBOARD at which time water was forced upward through the sand a t a
Scope of Investigation I n this investigation experiments were made using upflow through greensand in a pressure-type filter, and upflow in a gravity filter using Grystalite. Careful records were kept to determine : 1 Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 75th Meeting of the American Chemical Society, St. Louis, Mo., April 16 t o 19. 1928. Candidates for Ph.D. degree, Ohio State University, Columbus, Ohio.
Figure %Diagram
of Ex erirnental Crystalite Gravity Sottetener
October, 1928
INDUSTRIAL A N D ENGINEERING CHEMISTRY
rate of 1cubic foot per minute. When the backwash showed only 1 grain of hardness, the softening period was regarded as commencing. Results of operation are shown in Table I and Figure 2. The results show that: (1) There is no more grainage exchange from a n excess use of salt than from the smallest amount that produces effective softening. (2) Below the minimum effective amount of salt there is a grainage exchange proportional to the amount of salt used. (3) It is more economical to use too little salt than too much. (4) From 4.1 to 4.3 pounds of salt per 5 cubic feet of greensand was the most efficient salt rate. An average of the results using these amounts shows that 4.2 pounds of salt removed 12,452 grains of hardness, or 1000 grains hardness for each 0.337 pound of salt: and that each cubic foot of greensand removed approximately 2500 grains of hardness between regenerations. The waste in wash water amounted t o about 4.59 per cent.
Gravity Softener w i t h Crystalite
This type of softener was selected f o r t h i s test because it represented in reality a small section of a much larger water-works type of plant. (Figure3) The flow of a l l l i q u i d through the Crystalite bed i s upward. The GRAINS PER CU.FT. bed depth was such that CAPACITYOF GREEN SAND I N TERMS runs of at leitst 8 hours' OF CACO, REMOVAL PER C U . OF ~ SAND duration, including reFigure 3-Salt Efflciency a n d Softening generation, were obCapacity of Green Sand tained. Data of Gravity Filter Depth of Crystalite in filter 66 inches Volume of Crystalite in filter 11 cubic feet Diameter of softener 19 inches 2 square feet. Area of Crystalite bed 20 gallons per minute Rate of softening Rate of regeneration 6 gallons per minute Discharge per square foot 10 gallons per minute Depth of Crystalite on expansion 84 inches 5 minutes Time of contact with Crystalite
1103
into the softener was 3 gallons per minute per square foot of area. The ejected solution was tested for 5 per cent solution strength by means of a hydrometer. When the required number of inches were drawn from the brine tank, the ejector valve was closed and 6 gallons of water per minute were allowed to flow for 5 minutes. This is known as the slow wash or dilution operation. At the end of the 5-minute period of slow wash, the rate of wash was increased to 20 gallons per minute or 10 gallons per square foot of area; and this is known as the fast-wash operation, which finished the regeneration, usually taking 10 to 15 minutes. The time a t which the fast wash water shows 1grain of hardness is taken as the beginning of the softening period. The softener functioned for 8 t o 10 hours, depending on the influent hardness. Regeneration was repeated when the effluent water tested 1 grain hardness (soap method). Table I-Results RUN
1 2 3 4 5
; B
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
of Operation of Pressure Softener w i t h Greensand Av. HARDNESS SALTUSED SALTPER WATER OF HARDNESS PER 5 cu. FT. 1000 GRAINS SOFTENED INFLUENT REMOVEDGREENSAXD REMOVED Gallons P . p . m. Grains Pounds Pounds 1950 115 13,110 5.3 0.404 2700 82 12,960 5.3 0.410 3200 68 12,800 5.3 0.414 2708 82 12,960 4.9 0.377 2250 12,370 4.2 94 0.340 2565 12,800 4.54 0.358 85 2528 12,600 4.1 0,345 85 2175 11,950 4.1 0.346 94 2776 10,670 3.76 0.355 66 2295 10,430 3.42 0.328 78 2565 12,450 4.70 0.377 83 12,610 5.20 1800 0.419 120 12,760 5.7 3278 67 0.447 12,800 6.2 3945 55 0.484 12,600 3075 70 5.3 0 414 12,760 2715 5.3 80 0,420 2347 92 12.650 5.3 0.420 12,750 2407 5.3 91 0.416 12,750 2775 5.3 79 0.416 12,550 2512 6.5 85 0.518 3187 12,700 4.3 68 0 338 3052 12,800 72 4.3 0.336 2475 10,150 2.5 70 0.346 1665 8,830 3.0 91 0,339 1364 7,500 2.5 94 0.333 12,290 4.2 2145 98 0.341
Table 11-Operating Results w i t h Gravity Softener Crystalite Av. SALTUSED SALTPER HARDNESS HARDNESS HARD- PER 11 cu. 1000 REMOVED PER WATER OF NESS FT. CRYS- GRAINS cu. PT. RUN SOFTENED INFLUENT REMOVEDTALITE REMOVEDCRYSTALITS Gallons P . 9.m. Grains Pounds Pounds Grains 1 13,276 82 28.17 63,700 0.45 5791 2 13,510 80 62,422 0.44 28.17 5075 13,760 75 3 0.33 20.12 60,544 5504 80 4 0.35 12.070 20.12 56,729 5157 11,225 5 0.30 16.1 52,757 80 4796 0.31 16.1 51,900 10,380 85 4715 $ 0.24 12.0 49,500 85 9,900 4500 0.25 57,650 86 9,530 4332 12.0 9 35,154 92 6,510 3196 0.23 8.0 8,700 10 0.22 36,792 72 3345 8.0 ~~
A
OPaRATION-The raw water used for these experiments was lime soda-softened filtered water ranging in hardness from 72 to 92 p, p. m. or from 4.2 to 5.4 grains per gallon. The average hardness of the softened water was 0.5 to 1.5 p. p. m. The flow was upward and the rate was 10 gallons per square foot per minute. The method of handling the brine and salt was that practiced in industrial installations. (Figure 3) A bed of salt not less than 12 inches deep was kept evenly spread upon a layer of gavel. I n the lower part of the gravel layer a brine pick-up pipe system was inserted and connected through the side of the brine tank to a jet pump or ejector. The water level in the brine tank above the salt bed was kept at the top of the overflow, from which displacement measurements were taken. Saturated brine accumulated in the gravel layer (2.66 pounds to the gallon a t 60' F.)" For regeneration of the Crystalite, the specified inches of brine were drawn off from the bottom of the brine tank. Each inch d r a m off contained 8 pounds of salt. The saturated brine was ejected to the softener in such manner that the pressure water brought about a dilution to 5 per cent solution. The rate a t which the 5 per cent solution was ejected
Table 111-Wash
Water a n d Regeneration Water Used TIME TIME
TIME
EJECTOR EJECTOR RUN
WATER
WATER
SLOW WASE
SLOW WASH
FAST
WASH
FAST
WASH
WASTE
Gallons Minutes Gallons Minutes Gallons Minutes Per cent
9 10
69.4 84.4 62.4 72.4 69.0 59.0 55.5 75.5 37.0 37.0
7 8 6
5
50 55 45 40 45 40 40 40 40 55
5 5 5 5 5 5 5 5 5 5
200 180 270 250 245 245 235 230 220 230
8 8
10 6 12 11 10 11 10 10
2.4 2.5 2.7 3.0 3.2 3.4 3.3 3.6 4.3 2.7
Tables I1 and I11 show that: (1) From 16 to 20 pounds of salt were required for every 11 cubic feet of Crystalite-that is, each cubic foot of Crystalite removed 5000 grains of hardness between regenerations. An average of the results using these amounts show that 0.32 pound of salt is required for each 1000 grains of hardness removed. (2) Wash-water waste varied from 2.4 to 3.6 per cent.
Figure 4 shows the salt and waste water efficiency for Crystalite.
INDUSTRIAL AND ENGINEERIXG CHEMISTRY
1104
Mixing Zeolite-Softened Water with Lime-Treated Water The zeolite- and lime-softened waters showed the following analyses: ZEOLITELIMESOFTENED SOFTENED Total alkalinity Normal carbonates Bicarbonates Non-carbonate hardness Total hardness
P. p . m.
P.p . m.
52 12 40 52 0
55 0 55 96 151
-
The two when mixed together gave a mixture having an analysis corresponding to an average of the two. There is apparently no chemical reaction in the mixture. No apparent precipitate is formed either in cold or hot mixtures.
VOl. 20, No. 10
157 p. p. m., 45 parts alkalinity, and 112 parts non-carbonate hardness. The year 1927 was an unusual one for soft water. It is believed that 157 p. p. m. water is more representative of the expected average. Cost of soda-ash softening Cost of zeolite softening, basis pound aalt per 1000 grains
0.35
HARDNESS OF FINALWATER 70p.p.m. 80p.p.m.QOp.p.m. $128,369 $115,719 $103,070 95,592
85,900
77,366
No estimate has been made on cost of replacement of zeolite lost by disintegration. Capacity of plant required is indicated in Table VI. Table VI-Water Required to be Filtered through Zeolite under 1927 Operating Conditions a t Columbus Plant FINALHARDNESS FINALHARDNESS FINALHARDNESS 90 P . P. M. 80 P. P. M. 70 P. P. M. 1927 Av. Max. Min. Av. Max. Min. Av. Max. Min. M.g.d. M.g.d. Mg.d. M.g.d. M.g.d. M.g.d. M.g.d. M.g.d. M.g.d. Jan. 10.7 14.3 4.6 12.6 16.2 7.0 14.5 18.2 9 . 6 Feb. 9 . 8 11.9 7 . 4 11.1 1 3 . 5 9 . 9 1 4 . 1 1 5 . 1 1 2 . 1 Mar. 11.0 13.4 6 . 6 14.5 17.3 11.1 12.8 15.4 8 . 2 Apr. 10.4 13.9 8 . 5 1 4 . 1 1 7 . 3 11.8 1 2 . 3 15.4 10.0 9 . 3 13.4 6 . 2 11.5 15.6 8 . 1 13.5 18.0 9.7 May 10.8 17.3 6 . 2 5.7 13.0 0.6 8 . 1 15.0 3 . 2 June 1 4 . 5 1 8 . 2 10.7 12.8 15.0 8.6 10.0 1 2 . 0 6 . 7 July 12.6 16.5 1 0 . 9 15.1 19.2 1 2 . 8 10.6 14.0 8 . 2 Aug. 16.0 20.5 13.4 1 3 . 8 18.3 11.4 11.7 15.4 9 . 3 Sept. 17.1 2 0 . 0 14.9 15.0 17.5 12.9 1 3 . 0 1 5 . 1 11.1 Oct. 16.1 20.2 1 1 . 8 14.9 18.0 10.7 Nov. 13.2 16.3 9 . 6 14.6 18.7 8.9 12.4 15.5 6 . 2 Dec. 1 0 . 3 13.2 3 . 4 12.5 Av. 14.6 10.5 Max. 18.3 20.5 16.3 Min. 3.2 6.2 0.6
The tables show that: (1) By investing $200,000 the City of Columbus can save enough from operating expenses t o pay off the entire investment in twenty years; and in addition save, under 1927 operating conditions, from $7700 to $16,200 per year, depending on the hardness of the softened water desired. Under what is regarded as average operating conditions, the saving will amount t o from $25,700 to $32,800. (2) Under 1927 operating conditions sufficient equipment would have been provided to soften as much as 20 million gallons per day.
CAPACITYINGRAINS OF CACO, PER Cu.FT OF CRYSTALITE Figure 4-Salt
a n d Waste Water Efficiency for Crystalite
Tuble I V - O p e r a t i n g Costs for Softening Pretreated Lime-Softened Water b y Soda Ash a n d b y Zeolite (Figured from averages on summary sheet of daily operation of Columbus plant for the year 1927) SALT
FINAL BASIS0 . 3 5 YEARLY YEARLY COST HARDNESS WATER LBS. PER COST OFSALT THROUGH SODA 1000 GRAINS OF BASIS$ 6 . 5 0 WATER SOFTENED WATER PU?~P&DZEOLITE ASH HARDNESSSODAASH P E R T O N M.g. Tonsperday P. 8. m. M . g. d . $77,563 $36,161 7.43 15.2 90 28.1 10.5 90,771 38,840 16.4 12.5 8.70 80 28.1 104,269 49,822 9.99 21.0 70 28.1 14.6 Average hardness of lime-softened water 143 p. p. m., 45 parts being alkalinity and 98 parts being non-carbonate. Average hardness of water through zeolite 5.6 p. p. m. Plant cost to be $200,000. Cost of soda ash, $28.60 per ton. Cost of salt, $6.50 per ton. Table V-Recapitulation HARDNESS OF FINALWATER 7 0 p . p . m . 8 0 p . p . m . 90p.p.m. Bond charges, based on retiring bonds $15,400 $15,400 $15,400 in 20 years 36,161 38,840 49,822 Salt cost 10,672 8,968 12,483 Pumping 6,240 6,240 6,240 Personnel, 4 men at $130 per month 3,090 2,596 Wash water 3,609 500 500 Brine handling 500 Cost of zeolite softening, basis 0 . 3 5 pound salt per 1000 grains hardness 69,865 74,742 removed 88,054 90,771 77,563 Cost of soda-ash softening 104,269
The following is a similar summary when it was assumed that the average hardness of the lime-softened water was
Investigation Not Complete This investigation has not been completed. Tests have been made using two kinds of zeolite but not under identical conditions. The following differences in design and operation may have affected the results somewhat: (1) The natural zeolite bed was 34.9 inches deep. The synthetic zeolite bed was 66 inches deep. (2) The rate of flow through the natural zeolite was 4.38 gallons per minute per square foot. The rate of flow through the synthetic zeolite was 10.0 gallons per minute per square foot. (3) The direction of flow of brine and brine rinse water was downward with the natural and upward with the synthetic zeolite. (4) The general construction of salt-dissolving, brine-injecting, and water-distributing system was different for the two softeners.
The results so far indicate that greensand has a softening capacity of 2500 grains of hardness per cubic foot between regenerations when 0.35 pound of salt is used per 1000 grains of hardness removed; that Crystalite has a softening capacity of 5000 grains of hardness per cubic foot when 0.35 pound of salt is used for each 1000 grains of hardness removed; and that upward flow has certain advantages as applied to Columbus conditions of softening filtered water. These advantages are: (1) less repumping head or pressure loss; (2) constant pumping head or pressure loss thronghout the run, thus rate adjustment during the runs is simplified; and (3) deep beds and high rates of flow, which are possible by upflow, reducing the required plant area. No decision has been made as to whether greensand or gel-type zeolite or upflow or downflow will be used at Columbus. There are certain reported disadvantages to u p
INDUXTRIAL AND ENGINEERI,VG CHEMISTRY
October, 1928
flow softening which a small-scale, short-time experiment unfortunately cannot disclose. (1) Difficulty of distribution in large units to obtain uniformity of contact in upflow; (2) increased attrition and loss of zeolite with upflow as compared with downflow; (3) increased danger of gravel hills with upflow; and (4) upflow provides no filtration effect; in softening filtered water this would not seem to be important. Future runs will be made t o determine whether the zeolite can be made to show greater capacity between regenerations
1105
and whether consistent results during six to ten runs and r e generations for each set of constant conditions can be obtained. Salt-saving methods may also be investigated. Acknowledgment The authors wish to acknowledge valuable assistance and suggestions received from W. J. Hughes and A. S. Behrman, of the International Filter Company, and from S. B. Apple baum, of the Permutit Company.
A Study of the Synthesis of Methanol' Abstract Etienne Audibert and Andre Raineau S O C I ~ TNATIONALE $ DE RECHBRCHES SUR LE TRAITMENT DES COMBUSTIBLES, VILLERS-SA NT-PAUL(OXSE),FRANCE
three can be determined arbitrarily, leaving only the sixth one beyond direct control. Obviously the heat distribution within the catalyst mass depends both on the amount of heat developed by the exothermic reaction in question and upon the rate of dissipation of this heat. While the heat developed varies with the activity of the catalyst, the rate of heat removal depends only upon the arrangement of the catalyst chamber. The consequence is that two experiments, made with the same apparatus and under the same conditions with two catalysts of different activities, have different rates of heat transmission, and for that reason are not strictly comparable with each other. It is therefore necessary, if the results obtained are to have any meaning, to reduce to a minimum the effect of the variations in the (1-~)3 -27'000 + 3.5 log T + 2.9974-2 log P ( 2 ) rate of heat transfer between the catalyst mass and the surL o g X w = rounding medium for different experiments. I n order to where T represents the absolute temperature and P the secure operating conditions as nearly isothermal as possible, pressure in atmospheres, common logarithms being used. the following three precautions were resorted to, with the For another ratio of hydrogen to carbon monoxide in the result that the temperature difference between various points initial gas mixture, the conversion may be expressed as a in the catalyst bed was kept within 5" C. function of 2 by the law of mass action. For instance, when First, the gas mixtures employed contained carbon monoxthe initial gas mixture has the composition CO -t 5H2, the ide and hydrogen in the ratio 1:5, rather than in the theopercentage of carbon monoxide going to methanol, y, is de- reticalratio 1:2, thereby utilizing the diluent effect of the excess fined as a function of 2: by the equation hydrogen to keep the temperature down. -Second, the space velocity was regulated so as to give a low value for the heat (3) liberated per second per cubic centimeter of catalyst. Since The values of 2 and y as calculated from equations ( 2 ) and (3) a pressure of 150 atmospheres was consistently employed, are plotted as functions of temperature in Figure 1, showing the adopted space velocity of 5000 corresponded to a time of that only relatively low pressures are required to effect the contact of 28 seconds a t 250" C. and 23.5 seconds a t 350" C. alcohol synthesis as long as the temperature does not exceed Third, the apparatus was constructed in a form which insured the most rapid dissipation of heat from the catalyst 300" C. In the synthesis of methanol, as in any other ieaction be- bed. To accomplish this the catalyst was placed in a thintween gases in contact with a solid catalyst, the rate of prod- walled tube of small diameter provided with a large number uct formation is of utmost importance from the point of of longitudinal flanges. The catalyst tube, which had a view of industrial feasibility. This rate is best expressed as capacity of only 15 cc., was then placed inside a high-presthe amount of alcohol which can be obtained per unit time sure chamber of about 2000 cc. volume, maintained a t a gas per unit volume of catalyst, and is a function of the following pressure of 150 atmospheres. By this arrangement the variables: (1) the nature of the catalyst used, (2) the chem- catalyst tube was intimately in contact with a gas volume, the ical composition of the gas mixture a t the point of intro- specific heat of which was about 45 gram-calories, or more duction to the catalyst mass, (3) the temperature of the gas than ten times the amount of heat set free per second in the at that point, (4)the pressure of the gas, (5) the velocity catalyst zone. of the gas in the catalyst zone, and (6) the heat distribution Apparatus within the catalyst. For a given catalyst and gas comThe apparatus is illustrated diagrammatically in Figure 2. position the first two variables are fixed while the following The high-pressure chamber, E, capable of withstanding the 1 Received January 19, 1928. We are indebted to Per K. Frolich working pressure, is heated externally by an electric reand H. C. Hetherington for valuable assistance in translating and abstractsistance wire. The axial tube, T , is of small diameter with ing this article. The present paper is the first of a series presenting the results obtained in this study. large contact surface, and holds the catalyst in position by
T
H E synthesis of methanol consists in directly combining carbon monoxide with hydrogen a t high pressure in accordance with the equation CO 2H2 = CH30H 27,000 calories (1) This reaction liberates about 843 gram-calories per gram of alcohol produced and transforms into the liquid form 82.7 per cent of the heat content of the reacting gases, the remaining 17.3 per cent being dissipated in the form of heat. In a gas mixture having the theoretical composition of carbon monoxide and hydrogen in the ratio 1:2, the percentage, 2 , of the carbon monoxide converted into methanol a t equilibrium is defined approximately by Nernst's formula,
+
+
m
