May, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
vessels for ease in cleaning and preventing contamination of succeeding batches. Figure 2 shows a self-supporting unit with counterbalance arrangement so that position of vessel can be changed if desired. Space has been gained in the vessel head by using a nozzle attached at an angle instead of the customary normal entry. The design details shown in Figures 1 and 2 can, of course, be combined in a n over-all design which would utilize structural steel cage support, swing joints, and flexible tubing so that the head may remain fixed while the vessel is lowered. The support of the vessel should be at the most comfortable height for the most difficult function involved, including the operations of loading, draining, cleaning, temperature measuring, sampling, and visual observation. Figure 3 shows a complete assembly for handling materials similar to paste, greases, etc. Provision for raising and lowering the mixing head is built into the equipment, and extreme flexibility is obtained by interchanging mixing elements. Speed is apparently vaned by changing flat belt pulleys. I n connection with handling paste, greases, eto., the choice of the mixing elements themselves is very critical. A piece of equipment, as illustrated, is extremely valuable to anyone experimenting in the paste range of materials. In fluid mixing, however, the type of impeller is not critical, the choice most frequently being made on a mechanical limitations basis. Therefore, in fluid mixing it is generally possible to equip the pilot plant with only a single type of impeller, provided this impeller type is capable of being scaled up to the largest anticipated commercial size, and provided the pilot plant is designed to develop the optimum conditions for the impeller through proper selection of size, variable speed, and other factors noted above. A single type or, at most, two types of impellers can be selected Q I ~the basis of final commercial size, physical characteristics of
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material to be handled, and over-all duty requirements of the agitator. Only when all of these factors are unknown at the time of designing the pilot plant is it necessary to make provision for trying out a wide variety of impellers. It is suggested as important that the complete pilot plant for any unit process include, wherever possible, actual equipment representing each unit operation essential in the unit process itself. If a pilot plant is to produce complete.results, both as to the chemical reactions involved ahd as to a study of the mechanical equipment and factors involved, greater attention must be placed on the mechanical equipment than appears to be common practice. For example, many times pumps are omitted from a pilot plant because the quantities handled are small enough to be transferred by buckets or by gravity. Consequently, no information is available for the engineers who design the commercial siae plant. It has also happened that mechanical methods for a unit operation are used in pilot plant work which are incapable of scaling up to commercial size. For example, a rocking tank producing agitation by shaking can hardly be scaled up beyond a relatively small batch size, owing to the massive equipment that would be involved. The authors do not suggest complete standardisation of that portion of the pilot plant involving agitation. They do believe, however, that the factors presented here are important i n unit process pilot plant design, and it is hoped that these factors will provide a general guide for those approaching such problems. LITERATURE CITED
(1) Cooper, C . M., Fernstrom, G . A., and Miller, S. A., IND.EN@.
CHEM.,36, 504-9 (1944).
(2) Gibson, A. H., “Hydraulics and Its Application”, 4th ed., 1930.
(3) Miller, F. D., and Rushton, J. H., IND.ENQ.CRBM.,36, 499 (1944).
OXIDATION of FERROUS SULFATE SOLUTIONS with OXYGEN Kenneth A. Kobe and William Dickey HE oxidation of ferrous sulfate solutions by atmoepheric oxygen has been studied considerably by analytical chemists, who have expressed surprise at the slowness of the reaction (6). They report a variety of conflicting results, probably arising from the differences in experimental methods. I n a physiGal-chemical study of the activation of the oxygen electrode, Lamb and Elder (6) studied a number of variables in this reaction and corrected many of the erroneous conclusions of earlier workers. I n general, they determined the hours required for a n oxidation of 1% of the ferrous ion a t 30” C. Their results will be compared with those obtained in this work.
UNIVERSITY
OF
TEXAS, AUSTIN, TEXAS
Industrial studies of this reaction were made by Reedy and Machin (8) who state that the initial concentration of ferrous sulfate makes little difference in the velocity of the reaction. They secured complete oxidation by circulating the solution five times over crushed pyrolusite. Posnjak ( 7 ) determined the hours required for oxidation of 0.1 and 0.5 N ferrous sulfate solutions at room temperature. Agde and Schimmel ( I ) bubbled air or oxygen through ferrous sulfate solutions at various temperatures a n d ’ pressures. As discussed by previous workers (7, 8),
The factors controlling the oxidation of the ferrous iron in a solution OF ferrous sulfate, ferric sulfate, and sulfuric acid have been studied, using both oxygen and air. The percentage oxidation of ferrous sulfate is shown here graphically on curves with respect to time, partial press!re of oxygen, temperature, acid concentration, and catalyst concentration. The oxidation OF ferrous sulFate in acid solution by oxygen is a very difficult reaction, and good conversions to ferric sulfate occur only at quite high temperatures and pressures over a considerable time period.
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the oxidatiori of ferrous sulfate is of importance in the "heap leaching" process in which cuprous and cupric sulfides in lowgrade ores are oxidized by ferric sulfate and the copper sulfate is leached away and recovered. The ferrous sulfate must be reoxidized to ferric sulfate in order to cycle the leaching solution (3,4). A totslly different application is in the hydration of acetylene to acetaldehyde. I n the low-acid process acetylene i3 bubbled through 6% sulfuric acid solution a t 70" C. The catalyst for this hydration is mercuric oxide which is dissolved in the acid. This catalyst gradually decreases in activity, being transformed partially to free mercury and partially to a heavy organic sludge. The mercury is recovered from both, reoxidized to the oxide, and reintroduced into the process. The life of the mercuric oxide catalyst can be ext'ended considerably by the addit'ion of mild oxidizing agents, such as chromates and ferric salts (5). I n another research project t,he ferric salt, as sulfate, a a s found to be preferable; but since it slowly reduced t o ferrous sulfate by the acetaldehyde formed in the reaction, the reoxidation of the spent solution became a n important phase of the hydration problem. The study of the initial phase of the oxidation (5) or the lengthy time reported ( 7 ) , both at, room temperatmure,made it desirable
Vol. 31, No. 5
to secure data concerning t,he speed of this oxidation at higher temperatures and pressures. OXIDATION PROCEDURE
The apparatus was a 4-inch-diameter steel autoclave, teated hydrostat>icallyto 1500 pounds per square inch. Through a packing gland in t'he cover came a quarter-inch shaft, t8helower portion of which was covered with a glass tube ending in glaas stirrer. The solution (300 ml.) being treated was placed in a lipless beaker within t,he bomb. The beaker was surrounded by slightly alkaline lvater. Thus, the solution being treated did not come in contact \$-it,hany metal. h mercury-in-glass thermonieter paused through a packing gland in the cover and indicated the temperature of the solution. The total internal pressure was indicated on a Bourdon spring type gage. The autoclave was immersed to the top in a water bath heated by gas burners. For temperatures over 100"C . a salt bath (10 parts potassium nitrate to 8.5 parts sodium nitrite, melting point 135" C.) was used. Oxygen or air was admitted from cylinders through a reducing valve and an inlet in the cover of the autoclave, through which the pressure was relieved a t the end of the experiment,. A synthetic solution of the same composition as the spent acetylene absorber liquor was used. I t was made from analytical grade chemicals to the composition 0.0909 M FeS04, 0.0328 41 Fe2(S04),, 0.887 M H2S04 (respectively, 13.8, 13.1, and 90.5 grams per liter). A t the end of the experiment a sample of the solution was quickly cooled, and t'he ferrous ion remaining was determined with standard permanganate. X duplicate sample was reduced, and total iron determined to ensure that no change in concentration had occurred, or the results were calculated to the basis of the original solution using total iron as t'he tie element. The experimental conditions, unless shoxri otherwise on the abscissa of the graphs, are for the composition of solution stated above: oxidized for 15 minutes a t 100" C.; rate of stirring, 300 r.p.m.; copper sulfate concentration, 0.01 molar. EXPERIMENTAL RESULTS
TIME
IN
MINUTES
Figure 1 . Effect of Time and Catalyst on Conversion at 100" C. and 275 Pounds per Square inch O x y g e n Pressure
CATALYST COSCENTIUTION.Previous work has shown copper sulfate to be the most satisfactory homogeneous catalyst (5, 7 ) . Figure 1 shows the percentage oxidation a t 100" C. and partial pressure of oxygen of 275 pounds per square inch using no catalyst and 0.01 and 0.02 molar copper sulfate. Another experi-
0
PARTIAL
PRESSURE
OF
Figure 2.
OXYGEN, psi.
50
100
PARTIAL
PRESSURE
I50 OF
Effect of Partial Pressure of O x y g e n on Conversion in 1 5 Minutes
0 =air] 0 =oxygen
250
200 OXYGEN,
psi.
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May, 1945
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80 2
P v)
5
5i
60
0
c
5
40
E
L
20
0 10
20 TIME
Figure 3.
30
40
50
60
I
TEMPERATURE,
IN MINUTES
Effect of Time on Conversion at 100' Various Pressures
C. and
Figure
4.
0
ment with 0.05 M copper sulfate showed results identical with 0.02 M solution. It appears that 0.01 M copper sulfate is t h e optimum concentration of catalyst, and this amount was used in all further experiments. TIME.Figure 1 also shows the effect of time on the conversion. All further experiments, unless otherwise indicated, were carried out for 15 minutes. Figure 3 shows conversions with respect to time a t 100' C. and various partial pressures of oxygen. PRESSURE. Figure '2 (left) shows the conversion at 100' C. in 15 minutes as a function of the partial pressure of oxygen above the solution. I n Figure 2 (right) both air and oxygen are used as sources of oxygen at temperatures of loo', 1 4 5 O , and 175' C. Figure 3 gives the effect of time at various pressures and 100" C. It is immaterial whether the oxygen is supplied as pure oxygen or as air. This result was obtained by Lamb and Elder (5),but Agde and Schimmel ( 1 ) found greater oxidation with air which they attributed to the greater surface of the air bubbles in their solution. TEMPERATURE. Figure 4 gives the conversion in 15 minutes as a function of the reaction temperature, using both oxygen and air at partial pressures of oxygen of 20, 100, and 275 pounds per square inch. ACIDCONCENTRATION. Previous work has shown the inhibiting effect of excess sulfuric acid. This has been checked, using the same concentration of ferrous and ferric sulfates b u t varying the molarity of the sulfuric acid. Figure 5 shows that the concentration used in this work has reached the minimum in the conversion curve. AGITATION AND DILIJTION. Preliminary work had shown that the method of bubbling employed by Agde and Schimmel (1) gave greatly decreased conversions when compared to agitation by stirring. Conversion in an unstirred solution dropped to 9% from 20% at the normal rate. Increased stirring rate did not increase the conversion. When the original solution was diluted to one half and one quarter the original concentration, the conversions remained unchanged. The effect Of concentration is reported differently by various workers; thus Reedy and Machin (&) state that the concentration of ferrous sulfate makes little difference in the velocity of reaction, Agde and Schimmel ( 1 ) report that the rate decreases, and Lamb and Elder (5) state that the initial velocity is proportional to the concentration. INHIBITORS. Addition of compounds present in the spent catalyst solution from hydration of acetylene to acetaldehyde gave decreased yields. Under conditions which gave PO% conversion with the synthetic solution, the addition of 10 grams
OC,
Effect of Temperature on Conversion in 1 5 Minutes at Various Pressures
-
air!
0= oxygen
$ 22
0
0.25
0.50
0.75 MOLARITY
Figure 5.
1.25
1.00
OF
1.50
1.75
H2S04
Effect of Sulfuric A c i d on Conversion in 1 5 Minutes at 100' C.
per liter of mercuric oxide dropped the conversion to 18%, and the addition of six drops of acetaldehyde to the latter solution dropped the conversionto 7%. When this latter solution containing acetaldehyde was evaporated to half its original volume to distill off acetaldehyde and rediluted to its original volume, the rose to 12yo.The addition of manganese sulfate as a catalyst for the oxidation of acetaldehyde to acetic acid did not improve the of ferrous sulfate. LITERATURE CITED
(1) Agde and Schimmel, 2. anorg. allgem. Chem., 225,29-32 (1935). (2) Chandler, U. S. Patent 2,039,950 (May 6, 1936). (3) Duden and Peters, d bid., 1,151,928 ( A ~ ~ . 31, 1915). (4) Elliott, Ibid.9 1,636,296 (July 19, 1927). Am* (5) Lamb and 137-63 (1931). (6) ~ ~ l"Treatise l ~ on ~ ~~~~~~~i~ , and Theoretical Chemistry", Vol. XIV, pp. 265-8, London, Longmans, Green and Co., 1935. (7) Posnjak3 Am* Inst* Mining Met+ Engrs.v NO.1615-D (1926). (8) Reedy and Machin, cHEM., 15, 1271-2 (1923).
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