INDUSTRIAL AND ENGINEERING CHEMISTRY BIBLIOGRAPHY Brooks, H. B., and Spinks, A. W., Bur. Standards J . Research, 9. 781-98 (1932). Bruun, J. H:,IND. ENQ.CHEM.,Anal. Ed., 1, 212-13 (1929). Ibid., 2, 187-8 (1930). Bruun, J. H., J. Chem. Education. 8, 1930-44 (1931). Bruun, J. H., and Hicks-Bruun, M. M., Bur. Standards J . Research, 5, 9 3 3 4 2 (1930). Ibid., 6, 869-79 (1931). Ibid., 7, 607-15 (1931). Ibid., 8, 583-9 (1932). Ibid., 9, 53-9 (1932). Ibid., 9, 269-70 (1932). Ibid., 10,465 (1933). Bruun, J. H., Leslie, R. T., and Schicktanz, S. T., Ibid., 6, 363-7 (1931). Bruun and Schicktanz, Ibid., 7, 851-82 (1931). Hicks-Bruun, M. M., Ibid., 5, 575-83 (1930). Hicks-Bruun and Bruun, Ibid., 7, 799-809 (1931)
(16) (17) (18) (19) (20) (21) (22) (23) (24j (25)
(26) (27) (28) (29) (30)
Vol. 25.
KO.8
Ibid., 8, 525-40 (1932). Leslie, R. T., Ibid., 8, 591-9 (1932). Ibid., 10, 609-19 (1933). Leslie, R. T., and Schicktana, S. T., Ibid., 6, 377-86 (1931). Liddel, Urner, and Kasper, Charles, Ibid., to be published (1933). Mair, B. J., Ibid., 9, 457-72 (1932). Rossini, F. D., Ibid., 6, 37-49 (1931). Ibid.. 7. 329-30 (1931). Washburn, E. W., I b k , 4, 221-46 (1930). Ibid., 5, 867-90 (1930). Washburn, E. W., IWD. ENQ.CHEM.,22, 985-8 (1930). Washburn, E. W., Bruun, J. H., and Hicks, M. M.. Bur. Standards J . Research, 2, 467-88 (1929). White, J. D., and Rose, F. W., Jr., Ibid., 7, 907-11 (1931). Ibid., 9, 711-19 (1932). Ibid., 10, 6 3 9 4 7 (1933).
March 7, 1933. Publication approved by the Acting Director, RECEIVED National Bureau of Standards. Thia work is part of Project No. 6 of the American Petroleum Institute.
Densities of Hydrocarbon Mixtures E. W. THIELE AND W. B. KAY, Standard Oil Company (Indiana), Whiting, I n d .
T
H E densities of petroleum liquids a n d vapors, under given conditions of temperature and p r e s s u r e , are constantly required in engin e e r i n g c a l c u l a t i o n s in the petroleum industry. Thus in the calculations of the pressure drop in pipe lines, in the design of pipe stills a n d of c r a c k i n g equipment, a n d in determining bubble tower d i a m e t e r s , knowledge of densities is essential. This p a p e r is a r e v i e w of o u r p r e s e n t knowledge of t h e subject, i n a n attempt t o indicate the best m e t h o d s of computation, a n d t h e p o i n t s where more data are needed. HYDROCARBON VAPORSAT Low
PRESSURES
b o i l i n g p o i n t of t h e cut in question. Wilson and Bahlke (15) give a compilation of such boiling points, For a given crude it will of course be possible to determine once for all the relation between any suitable property and the molecular weight. This has been done for Midcontinent crude by but, FitzSimons and Bahlke (8), so far as the writers are aware, no similarly extensive data are a v a i l a b l e f o r o t h e r crudes. Density is usually the most suitable property for light fractions, and viscosity for the h e a v i e r ones. If sufficient information were a v a i l a b l e , it would perhaps be p o s s i b l e to c o r r e l a t e all crudes and cracked stocks by m e a n s of two properties-for e x a m p l e , mean boiling point and gravity, or viscosity a t two temperatures. For mixtures of the lightest hydrocarbons, for which a Podbielniak analysis is available, the molecular weight can be determined by simple calculation.
Available methods f o r computing the densities of hydrocarbon liquids and vapors at a n y given temperature and pressure are reviewed. For vapors at low pressures, where the question is one of molecular weight, mean boiling point affords a rough correlation, but actual molecular weight determinations o n the particular type of stock are necessary for accurate correlation with a n y given property. For vapors at high pressures, the curves of Cope, Lewis, and Weber are good, but they require a knowledge of the critical pressure, and this is not readily calculable. For liquids, a new set of curves is presented, based mainly on the work of Jessup. These curves require a knowledge of the density at 60" F. and the viscosity. Some preliminary data on the change of volume of petroleum oils on mixing show that contractions of at least 0.25 per cent m a y occur.
At low pressure the gas laws are undoubtedlv s a t isf a c t o r v for engineering"purposes, the pressures at which they cease to be satisfactory being probably somewhat lower near the saturation line. The question of density then reduces to one of molecular weight. It is quite practicable to determine directly the molecular weight of any given cut, the cryoscopic method being probably the most satisfactory. Gullick (9) has worked out a suitable procedure for hydrocarbon oils; by his methods an accuracy of 1 to 1.5 per cent is possible. In general, however, it will be desirable to determine the molecular weight from other properties of the oil. While the data for such correlation are scanty, it is certain that no single ordinarily determined property can be related unambiguously to molecular weight without reference to the source of the oil. If reliance is to be placed on one property, however, that which comes nearest to being a function of molecular weight is the mean boiling point (or for simplicity, the A. S. T. M. 50 per cent point). In a rough and ready way, where no better information is available, it is suggested that the molecular weight be taken as that of the paraffin hydrocarbon having a boiling point equal to the mean
HYDROCARBON VAPORSAT HIGH PRESSURES Where the gas laws fail, apparently the only reliable guide, apart from direct experiment, is the law of corresponding states. The application of this law requires, however, a knowledge of the critical temperature, pressure, and volume of the mixture in question. For hydrocarbon mixtures the only determinations are the two made by Bahlke and Kay (1) on a gasoline and a naphtha, and the quite exceptional case of acetylene and ethane (11). The former workers have made a correlation on the basis of the law of corresponding states for the only hydrocarbons (except methane and ethylene) for which extensive data in the superheated vapor region are available. These are N-pentane (IS), isopentane (18), and N-hexane (17). The agreement is quite satisfactory.
August, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
Unfortunately, of the three critical constants, only the critical temperature can be readily determined from other data a t the present time; it is also the only one for which there exists any considerable body of published data. The formula of Eaton and Porter ( 7 ) , which relates the critical temperature to the boiling point and density, agrees well with the results of unpublished determinations made in this laboratory on thirty-one samples. While no data are available for light mixtures where the Podbielniak analysis is known, some unpublished data on mixtures of light and heavy cuts indicate that the principle of Pawlewski (12)-that critical temperatures are additive on a weight basis-is substantially correct. Published data on critical pressures of llydrocarbon mixtures with the exceptions noted are practically nonexistent. It is almost certain, however, that the matter is more complicated than in the case of the temperatures, and that the width of the cut will be an important factor. The question of the critical volume is in very much the same condition as that of the critical pressure. However, Cope, Lewis, and Weber ( 5 ) have given a method, based in effect on a reduced equation of state, which does not require a knowledge of this property, the molecular weight being substituted. I n another paper ( 1l a ) presented a t this symposium their method has been revised. In the development of their method the P-T-T data on pure hydrocarbons were correlated by means of an equation of state of the form: pv
where 2 = q = f = e =
As a test of the applicability of the revised method to complex petroleum hydrocarbon mixtures, calculations of the deviations from the perfect gas law have been made for naphtha and gasoline and the values compared with the values found experimentally. The results of this comparison are shown in Table I. TABLEI. DEVIATION OF VAPORSOF GASOLIXE AND NAPHTHA FROM PERFECT GASLAW (Comparison of experimental values w i t h those calculated by method of Cope, Lewis, and Weber ) p--TR PR Obsvd Calrd. Deviation ~~
Superheated vapors
RT Pc
v
correction term which is a function of T / T 0and 2 some function of T / T , base of natural logarithms (The remaining terms have their usual significance)
For evaluation of the terms f and + below the critical point, the data reported in the literature on the saturated vapors of a considerable number of hydrocarbons were used. Above
0.874 0.938 0.976 1.004 1.02 1.00
0.184 0.368 0.554 0.737 0.906 1.000
0.946 0.797
0.88 0.80
-6 0
0.685
0.504 0.372 0.261
0.73 0.67 0.57 0.30
+++345313
0.918 0.157 0.196
0.908 0.906
0.92 0.89
+1 +2
0.994 0.170 0.214 0.395 0.635
0.907 0.915 0.914 0.735
0.93 0.91 0.84 0.72
+2 -1 -7 -3
1.007
0.764 0.709 0.645 0.575 0.490 0.408 0.321
0.76 0.725 0.635 0.58 0.51 0.33
0 +3 +4 10 18 1-25 +3
1.034 0.448 0,922 0.997
0.553 0,490 0.431
0 64 0.57 0.46
16 +-i+7 16
1.057
0.922 0,997
0.613 0.567 0.522
0.67 0.63 0.54
10 10 4
1.084 0.848 0.922 0.977
0.674
0.595
0.71 0.68 0.63
6 5
0.~54 0.627 0.701 0.775 0.848 0.922 0.997
0.848
I
~r - - - -- 05t) l6ff
014
nE
05
I
1
m
&
02
“[I
MODULUS I/DZ LOG IOOOS
k’lGURE
1. JESSUP’SDATAUSING MODULUS(10)
SQUARE OF
the critical point only the data on ethylene were used. Using this equation and knowing the critical pressure and temperature of the substance dealt with, it is possible to calculate the deviation from the perfect gas law-hence the volume of unit quantity, V , a t a temperature, T,and pressure, P.
0.636
0.68
t6
++
6
SAPHTH.AC
Saturated vapors
Superheated vapors
0,231 0.461 0.692 1.000
0.813 0.69 0.586 0.265
0.60
0 . 9 1 3 0 184 0.230 0.400
0 862 0.862 0.750
0.90 0.87 0.75
+4 +I 0
0.989 0,205 0.256 0.461 0.731
0.8115 0.883 0.796 0.631
0.91 0.90 0.80 0.64
+a
t 2 0 +2
1.002 0.876 0.968
0.524 0.408
0.56 0.45
++810
1.047
0.894 0.895 0.828 0.713
0.93 0.91 0.82 0.66
-5 +1 -1 -7
0.840 0.913 0.96 1.00
0.219 0.274 0.507 0.873
a p = PV/RT. b P c = 542 l b / s q in ; T C = 586‘ C Pc = 434 lb./sq 1x1 ; Tc = 591’ F.
OD
%
GISOLIYE~
Saturated vapors
e-z/+ 6
RT
895
0.815 0.71 0.30
0 -3 -2 10
+
F
Table I shows that the calculated deviations are in fair agreement with the experimental values except in the critical region. The lack of agreement in this region is probably due chiefly to inherent differences between pure substances and mixtures, as appears to be indicated by the larger deviations in the case of gasoline (a wide-boiling fraction) as compared with the narrower boiling naphtha fraction. The test points are mostly chosen very close to the critical, so that the table as it stands might give a false impression a t first glance. More recently Brown, Souders, and Smith (3) have correlated the P-V-T data on pure hydrocarbons for the superheated region in a manner similar to that employed by Cope, Lewis, and Weber and have included the published data on S-pentane and isopentane which were apparently overlooked by the latter. The results of the calculation on naphtha and gasoline, using the curves for pentane, however, are in good agreement with those calculated using the curves of Cope, Lewis, and Weber which were based on data on ethylene.
INDUSTRIAL AND ENGIKEERING CHEMISTRY
896
Unfortunately, the lack of information on critical pressures prevents the full utilization of this work for mixtures, though for close cuts it has much value. More work on critical pressures is desirable, and also, if possible, more directly determined P-V-T values for typical petroleum products. I n principle, this type of work presents no unusual difficulties below cracking temperatures. It is possible that data on densities can best be extended into the cracking region by
I
3
a 1
MODULUS l/D2 LOG 1000 S
FIGURE2. CORRELATION OF EXPANSIONS OF LIGHTERHYDROCARBONS ON BASISOF SQUARE OF JESSUP'SMODULUS
means of throttling experiments, which can be made to yield such data by the use of thermodynamic relationships (14)'
DENSITY OF LIQUID
Vol. 2.5, KO.8
curvature a t the higher moduli. At 167" F. the curves are straight for a wide range of values, and afford a reasonable means for extrapolation (Figure 2). An attempt was made to use the law of corresponding states to extend Jessup's data to higher temperatures, using isopentane for the comparison, and fitting the known data as well as possible. I n some cases the assumed critical temperature for the petroleum oil which gave best agreement with the data for isopentane could be compared with that calculated by other means. The result was that for gas oils (modulus about 1) the assumed critical temperature was much too low, though it was reasonably correct in the case of the naphthas (modulus about 2). The curves which were developed as most reasonably representing the kno\m facts are shown in Figure 3. They show the density as a fraction of the density a t 60" F. and one atmosphere for gage pressures of 0, 400, and 800 pounds per square inch and for values of the square of Jessup's modulus (l/d2 log 1000s) up to 2.4. This covers everything from the heaviest asphalts to a rather light gasoline. In drawing these curves, the following points were taken as correct: For temperatures up to 572" F. and values of the modulus up to 1.2, Jessup's data were taken. For the lowest temperature up to 170" F. the extrapolation of Figure 2 was used. For the curves of modulus 0.2, 0.4, 2.0, and 2.2 the extrapolation of the data based on the law of corresponding states was adopted. I n the other regions the points were interpolated in what seemed a reasonable way, having regard for the probable values of the critical temperatures. S o attempt has been made to carry the results very close to the probable critical region. An alignment chart (Figure 4) is also presented by means of which the modulus can be quickly found.
The literature on the expansion of liquid petroleum products a t low temperatures and pressures is quite extensive, and the results, as embodied in the publications of the Bureau of Standards ( 2 , 4 ;6), seem entirely adequate for any ordinary engineering purpose. For very accurate work, where the tables are considered to be not sufficiently accurate, the actual determination of the density a t two temperatures is probably the best solution. I n making use of experimental data, it should be recollected that the densities are much more nearly linear with temperature than the volumes. Our knowledge of the density of petroleum oils in the liquid state a t high temperatures and pressures has been much increased by the work of Jessup ( I O ) . He examined thirteen oils a t pressures up to about 50 atmospheres and a t temperatures to 572" F. He gives a correlation of his data on the basis of the modulus:
l/ddkZ-iiBB There d S
= =
density of oil at 60" F. kinematic viscosity at 100" F.
Oils of the same modulus behave in nearly the same way in the liquid state with changes of temperature and pressure. Jessup has also compared his results, so far as possible, with those of Zeitfuchs (19) and of Bearce and Peffer ( 2 ) ,and finds good agreement. I n addition to this, Bahlke and Kay (1) give some data on the liquid volumes of naphtha and gasoline, and Young ( I ? ) has provided extensive data on the volume of liquid isopentane. I n correlating Jessup's data, it was found that the square of his modulus was more suitable for extrapolation and interpolation, since it made the volume-modulus relation a: a given temperature a straight line (Figure I). However, extrapolation to higher values of the modulus was not found satisfactory even a t 302" F., since there was considerable
FIGURE3. VoLi:>ms
OF
LIQUIDHYDROCARBONS
I N D U S T R I h L A \-D
August, 1933
E KG I N E E R I N G CH E M ISTR Y
897
,
the opposite direction from that for octane. From the data now available, it appears likely that the mean boiling point would be a better means of correlating the expansions of light distillates than is Jessup's modulus, but the mateiial is too scanty to attempt anything a t present for high temperatures and pressure&. For low tempeiatures and pressures Ci agoe and Hill ( h ) have found the mean boiling point to be much more satibfactory than the density. As mill be seen from the discussion, the curves presented herewith cannot be regarded as definitive. They represent a tentative arrangement and extrapolation of existing data, and are offered as the best available a t present. Here as elsewhere more experimental work is needed. CHAXGES IS T r OF ~ ~I I X I S ~G ~ Khile it is geneially realized that s m e slight change in volume occurs whenever two dissimilar liquids ale mixed, it has been generally assumed that in the case of petroleum oils the effect was negligible, and that additivity of volume could be assumed. So far as the authors are aware, there is no experimental evidence for or against this in the liteiature, although Cragoe and Hill ( 0 ) have pointed out that an expansion of 0.36 per cent may occur when Iniving benzene and gasoline. Ylonen ( I O ) reports an expansion of 0.5 per cent on mixing benzene and kerosene. FIGURE 4. CHART
FOR
DETERMISING y.ILUE
OF R I O D C L L S
Since all available data weie on straight-run stocks, it was thought desirable to obtain results on cracked stocks. Accordingly, values were obtained for certain samples a t atmospheric pressure by J. J. Alexander of this laboratory. A few unpublished values on the volume increase of oleum spirits were also available, obtained by one of the authors in the apparatus described previously (1). Comparisons of the values predicted from the charts and those experimentally found are given in Table 11. TABLE 11. COMPARISON O F DATA O F CHARTS A S D EXPERIMENTS s.4blPLE
IV~ODULCSPRESJURE TEMP
Vso/Vt Otmvd. Charts
I
gage
Cracked heavy naphtha
Distillate fuel nil (from pressure distillate)
Pressure still cycle stock
Pressure sti!! re=iduum
1.663
1.235
0.848
Atrn.
.\tm.
Atm.
168.3 206.3 258 313
0.940 0.921 0.888 0.850
0,938 0.916 0.884 0.851
167 210.3 271.2 299.6 350
0.946 0.926 0.896
0.856
0.947 0.924 0.893 0.878 0.861
0 936 0.912 0.896 0.869
0.932 0.906 0,890 0.865
182.1
0.952 0 938 0 017
0.050 0.938 0.914
210 422 472 537 593
0.918 0.786 0,747
0.914 0,779 0.743
167,6 210.3
267 300 355 0.564
htm.
21?.6 271.3 362.7
Oieum spirits (straightrun)
1.635
Gasoline (Bahlke and Kaya)
2.24
Atm. 27 54 107 322
0.880
:E;
176 362 0.784 0.785 248 422 0.726 0.727 420 422 0.732 0.739 a Bahlke and Kay's results on naphtha are very closely repreeented b y the charts.
The charts were also tested bv the available data for uure hydrocarbons. The data for decane and doclecane, &ch are not very extensive, are well represented by the charts but the data for octane are not, and the data for benzene, toluene, m-xylene, and cyclohexane are quite different from the values predicted from the charts, but the deviation is in
J 9
PERCENT
wn cf WTHA
2
Y
FIGURE5 , CHANGE13 VOLUME ON MIXINGLIGHTA N D HEAVYNAPHTHA Some measurements of volume change on mixing together different distillates have been made a t this laboratory. Since this work has not yet been completed, no general conclusions can be given except to say that the change is measurable and in some cases might be of practical importance. A few preliminary results given here will serve t o indicate the magnitude of this change. The difference in volume of the final mixture and the sum of the volumes of constituents before mixing was determined for a mixture which approximated crude oil. The following table gives a list of the constituents and the proportions in which they were mixed: P.ARTS BY
DENSITY (82.4'A TF.) 28" C.
VOL.
Light naphtha Heavy naphtha Gas oil Parsffin distillate Residue Density of mixture (calcd ) Density of mixture (exptl )
21.4
7.8
29.5 20.8 18.8
0.71903 0.77261 0.82411 0.87300 0.94176
0,83063
0 83253
The weight and density of each cut before mixing and the density of the resulting mixture were carefully determined, the density being determined by means of a pycnometer
~
~
.
898
INDUSTRIAL AND ENGINEERING CHEMISTRY
with the usual correction for the buoyancy of air. From the weights of the distillates and their densities, the density of the resulting mixture was calculated. As shown in the table the shrinkage in volume on mixing amounted to 0.23 per cent. A study was also made of the effectof mixing two distillates in various proportions. Figure 5 shows some results on mixing a light and heavy naphtha, which are typical of other binary systems of distillates that have been studied. The inspection data on the distillates that were mixed are as follows: GRAVITY
Naphtha 1 Naphtha 2
(60" F.) 9 A. P.I . 72.7 44.6
9.R . T. M.DIsrx.
Initial
50%
Max.
140 348
156 398
202 470
ACKNOWLEDGMENT The authors wish to acknowledge the assistance of the late
W. L. MacKusick, who made many of the preliminary calculations in connection with the densities of liquids. (Refer
Vol. 23, No. 8
LITERATURE CITED Bahlke and Kay, IA-D.ENG.CHEM.,24, 291 (1932). Bearce and Peffer,Bur. Standards, Tech. Paper 77 (1916). Brown, Souders, and Smith, IA-D.ENG.CHEM.,24, 513 (1932). Bureau of Standards, Carc. 154 (1924). (5) Cope, Lewis, and Weber, IND.ENG.CHEJI.,23, 887 (1931). (6) Cragoe and Hill, Bur. Standards J . Research, 7, 1133 (1931). (7) Eaton and Porter, IND.EKG.CHEM.,24, 819 (1932). (8) FiteSimons and Bahlke, Oil Gas J . , 28, 164 (Dee. 7, 1929). (9) Gullick, J . Inst. Petroleum Tech., 17,541 (1931). (10) Jessup, B u r . Standards J. Research, 5, 985 (1930). (11) Kuenen, 2.physik. Chem., 24, 667 (1897). (1la)Lewis and Luke, IND.ENG.CHEM.,25, 725 (1933). (12) Pawlewski, Ber., 15, 460 (1882). (13) Rose-Innes and Young, Phil. Mag., 47,353 (1899). (14) Wien-Harms Handbuch der Experimentalphysik, Band VIII, Teil 1, p. 519, Akad. Verlagsgesellschaft, Leipzig, 1929. (15) Wilson and Bahlke, IND.E m . CHEM.,16, 115 (1924). (16) Ylonen, E . W., Soc. Sci. Fennica Commentationes PhysicoMath., 1, No. 7, 18 (1922). (17) Young, Sydney, Trans. Chem. SOC.,67, 1071 (3896). (18) Young, Sydney, 2. physik. Chem., 29, 193 (1899). (19) Zeitfuchs, ISD. ENG.CHEM.,17, 1280 (1925). (1) (2) (3) (4)
RECEIVED April
15, 1933
t o July issue, pp. 723-735, for additional symposium papers.)
Formation of Floc by Ferric Coagulants EDWARD BARTOW, State University of Iowa, Iowa City, Iowa, A. P. BLACK IND WALTER E. SAXSBURY, University of Florida, Gainesville, Fla.
T
H E fact has long been recognized that ferric compounds Rice, and Bartow (3) on aluminum sulfate. Aluminum and may be employed in the formation of floc for the re- iron salts are known to differ in their action when used as moval of color or turbidity from natural waters, but coagulants in water treatment. It is the purpose of this paper it is only in recent years that they have become available in to determine and explain these differences. adequate quantity and a t a price yhich has made it possible FERROUS SULFATE AS COAGUL.4NT to compete with alum. Ferrous sulfate, or copperas, has been much employed as Ferrous sulfate in connection with lime was first used by W. B. Bull of Quincy, Ill. According to Gwinn (IO), the a coagulant since it was introduced in 1898 a t Quincy, 111. experiments were begun in July, 1898, with lime and ferrous Since the coagulation takes place after oxidation to ferric sulfate, the latter made by burning sulfur and passing the salt by the oxygen of the air, we are really dealing with a ferric coagulant. Ferrous sulfate is unsuitable for t i e treatgas into water and over scrap iron ment of certain types of waters. F e r r i c c h l o r i d e , made by Soft colored waters or soft turbid treating scrap iron with gaseous Ferrous sulfate, ferric chloride, chlorinated waters are best coagulated on chlorine obtained directly from copperas, and ferric sulfate are used as coagulants the acid side of the n e u t r a l a c h l o r i n e cell, w a s used in water purification. Natural waters contain point, p H below 7. With colored e x p e r i m e n t a l l y i n 1910 i n sulfate, chloride, sodium, calcium, etc., ions. waters, the color appears to beChicago and Toledo by Bull (4). come fixed or set by the addition Recently, ferric chloride in conThese ions affect the formation of floc with ferric of alkali. The use of ferrous c e n t r a t ed solution shipped in salts at diflerent p H values: O n the acid side sulfate is therefore limited to rubber-lined tank cars has been sulfate ion has a much greater effect than chloride those waters where alkalinity available. ion; a n increase f r o m 25 to 250 p . p . m. sulfate will not interfere with color reChlorinated copperas made ion causes little change; between p H 6.5 and 8.5 moval. It has been shown bv by adding chlorine to a solution Miller (16) and later by Cornog there is a zone in which ferric floc forms slowly of ferrous sulfate was used by and Hershberger (6) that, when M o h l m a n (17) a t Chicago i n or not at all; in and beyond this zone, sodium alkali is added to a dilute soluthe coagulation of sewage sludge. and calcium ions are most effecthe. tion of ferrous sulfate, a small It has since b e e n a d a p t e d t o The assumption of a change in the sign of the fraction of one equivalent inwater purification. colloidal ferricfloc f r o m positive, where it is more creases the p~ to about 7.5, It Ferric sulfate in dry form has remains practically constant a t affected by sulfate or chloride ions, to negative will b e e n r e c e n t l y placed on the this figure until 0.9 of an equivamarket. With the possibility explain the zone of no floc formation and the lent Of has been added; of increased use of these ferric more effective action of the sodium and calcium then it again r a p i d l y rises to. coagulants the writers have unions beyond this zone. about pH 11.0. Further addition dertaken a study of them simiResidual iron in solution is roughly proporof akali up to the theoretical lar to those made by B a r t o w tional to the time required f o r the floc to f o r m . two equivalents produces little and Peterson (1) and Black,
