1.VDrSTRIAL A S D E.YGISEERISG CHEMISTRY
June, 1925
593
Influence of Reaction Rate on Operating Conditions in Contact Sulfuric Acid Manufacture’ By W. K. Lewis and E. D. Ries MASSACHVSETTS INSTITVIEO F TECHNOLOGY, CAMBRIDGE, MASS.
H E classic data as to the influence of operating vari- one cannot justify the excess oxygen solely on the ground of ables on reaction rate in the catalysis of SOz oxidation forcing the equilibrium itself to the 803 side of the equation. to 803 by means of platinum under plant conditions (It can readily be shown that, granting Bodenstein’s equation are those of Knietsch,2 who gave curves showing the con- for reaction rate, one will get maximum capacity with versions obtainable for different temperatures, rates of stoichiometric proportions of SO2 and 02.) The first outflow, and amounts of platinum. Later the problem was standing difference between the experimental conditions attacked experimentally by Bodlander3 and his associates of the two investigators lies in the fact that in Bodenstein’s and finally by Bodenstein. All investigators subsequent to experiments the catalyzing platinum was changing its acKnietsch employed a closed chamber in which the gases tivity throughout the course of the reaction owing to the were left in contact with the same platinum throughout change in composition of the gases with which it was the progress of the reaction. Furthermore, all employed in contact, whereas under Knietsch’s conditions. which simulated those of plant t e m p e r a t u r e s far below practice, each particle of those used in plant practice platinum was always in conand in consequence operated The data of Knietsch on conversion of SO2 to SO3 tact with gas of the same for very long periods of by platinum at commercial operating temperatures composition. Consequenttime. demonstrate that the catalytic activity of platinum ly, in Knietsch’s case there B o d e n s t ei n dismisses rises rapidly with the temperature to about 500’ C., was no tendency for it to Knietsch’s results with the then goes through a maximum at about 525’ C., change its catalytic activity observation that his experibeyond which temperature it falls off very greatly. due to change in amount of mental conditions fail to On the basis of these data it is shown that the temadsorbed gases of whatever provide adequate control of perature of the reaction gases should be lowered prosort. Furthermore, Bodent e m p e r a t u r e and time. gressively as conversion proceeds, necessitating the abstein’s experimental work Bodenstein’s purpose was to sorption not only of the heat of reaction but in addition was all conducted a t temstudy the mechanism of the some of the sensible heat of the gases. Two types of peratures far below those catalysis, and his results are converters working on this principle are suggested. commercially employed beof great value in throwing The optimum concentration of SO2 in the gases entercause his technic was inoperlight on the factors playing ing the converter when sulfur is used varies somewhat able a t the higher temperaa part not only in this rewith the operating conditions, but is normally between tures used by Knietsch. action but in the catalytic 9 and 10 per cent. Bod e n s t e i n brought the effect of solids upon gaseous gas and the reaction vessel reactions in general. He up to temperature and held concludes from his experimental results that the rate of oxidat’ion of SO2 is pro- them there during the coure 3f the reaction. As the gas portional to it,s partial pressure but is greatly retarded by could not possibly be brought to reaction temperature t’he presence of SO3. He expresses this retardation empiri- instantaneously, this method of procedure introduced a cally as inversely proportional to the square root of the SO3 certain error in measuring the time, an error which became concentration. He furthermore concludes that excess oxy- serious if the reaction time were too short, as it would be a t gen has no influence on the rate. Recent writers have prac- practical working temperatures. As will appear later, there t>icallyignored the work of Knietsch, perhaps because his is strong evidence that the reaction mechanism changes as the experimental technic was considered inferior to that of temperature increases, and in consequence one is not justified Bodenstein. 5 On the other hand, Bodenstein’s conclusions, in applying Bodenstein’s reaction rate equation to operating particularly with regard to the effect of oxygen, certainly do conditions in contact plants. Bodlander pointed out that a t high temperatures the renot apply to the conditions obtaining in ordinary commercial platinum convert’ers. Plant practice has absolutely justi- action rate is third order, changing its character, however, fied Knietsch’s conclusion that excess oxygen is necessary as the temperature is decreased. Apparently no one has for most successful operation, even when the catalysis is quantitatively analyzed Knietsch’s data from the reaction completed a t temperatures so low t,hat the conversion realized rate point of view. The authors have undertaken to do so is far from equilibrium-i. e., under conditions such that and to apply the conclusions to converter design. The reaction rate equation for the oxidation of SO2 to SOs is Presented before the Division of Industrial
T
~~~
1 Received March 9, 1925. and Engineering Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1925. 2 Ber., 34, 4093 (1901) 8 Bodlander and Koppen, 2. Eleklrochcm., 9, 559, 787 (1903). 4 Bodenstein and Fink, 2. p h y s . Chern., 60, 1 (1907). 6 A discussion of Knietsch’s data will be found in Rideal and Taylor’s “Catalysis in Theory and Practice,” 1919, p. 81, MacMillan C o . This book ignores the work of Bodenstein and Fink on the rate of this reaction. Taylor’s later hook Treatise on Physical Chemistry,” 1924, D. Van Nostrand Co., gives Bodenstein and Fink’s equation, but ignores Knietsch. The incompatibility of Bodenstein’s conclusions with the facts of plant practice has apparently never been recognized. ( ’
~
~
~
I n this equation the constants m and n depend upon the reaction mechanism and cannot be assumed from the reaction stoichiometry, but must be determined from a study of the actual data or reaction rate.6 Bodenstein called m = 1 a n d n = 0. a
et seg.
Taylor, “Treatise on Physical Chemistry,” Vol. 11, 1994, p. 952, D. Van Nostrand Company.
The simple reaction velocity equation is complicated by the presence of diluent gas-i. e., the nitrogen in the air. Although allowance can be.made for the presence of the nitrogen in a number of ways, the writers prefer to choose as a basis of computation 100 parts of the initial mixture subjected to catalysis. Under normal conditions this is SOsfree and consists of sulfur dioxide, oxygen, and nitrogen. Call the volume percentage of SO2 in the entering gases ZO, that of oxygen, a, and that of the nitrogen (plus any other inert gases), b. Let o represent time. Call the SO2 remaining unoxidized a t any particular time during the process of reaction, x. The corresponding oxygen is, obviously, u--1/2(~o-x). The total volume of all the gases, b ZO-'/Z including the SO3 produced, is therefore a (ZO--2); hence the partial pressure of each gas present is its amount divided by the total-i. e., that of SO2is
+ +
X
a
+b +
u
+b +
u
+ b + ~0-1/2
that of O2 . ,
XO-I/~
(xo-xj
u - 1 / 2 (xo-x) ~ 0 - 1 / 2 (xo-xj
and that of SOs
v#
where b = per cent of nitrogen in the gas. The time, e, is therefore proportional to bW/100B. Knietsch does not give the absolute quantities of either time or platinum but only relative values. However, it is entirely practicable to use his data on this basis. It is impossible to read Knietsch's curves with precision a t either the early or later stages of conversion. The range over which a curve can be read satisfactorily is broader the lower the temperature. One using the curves will find that above 450' C. the usable range is extremely narrow. At these higher temperatures the constants have been determined from never more than three points on any given isotherm. As already indicated, Knietsch's results are incompatible with a zero value of n. The data were tested on the assumption that both m and n are unity, but one finds large and progressive variations in the reaction rate constant a t constant temperature calculated on this assumption. On the other hand, when m is assumed equal to 2 and n equal to 1, the variation in the computed values of the reaction constants a t constant temperature is within the experimental error of Knietsch's data, as shown by the following table: Temperature, 400' C. Per cent conversion k 234,000 204,oon 180,000 181,000 21s,ooo km = 203,000
xn-x
(XO-X)
Consequently, one would expect the rate of oxidation of SO2 to be kxm l a - 1 1 2 ~ x o - x ) ] ~
dx
%
[a
+ b + x0-1/2
(x0-x)jmtn
unless further complicated by surface factors. However, at high temperatures SO3 dissociates to such an extent that the reverse reaction plays an important part. Unless toward the end of the reaction the mechanism changes completely, the reverse reaction must of necessity be such that the net oxidation rate becomes zero-i. e., the reverse reaction rate must equal
-K
k (XO-x)'"
[a
+b+
XO-I/Z
(XO-%)Im
where K is the equilibrium constant in the expression K =
(SOa)2 = ( S O Z )( ~0 2 )
At any time, therefore be dx
Vol. 17, No. 6
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
594
=f
(X)
=
5
= specific rate of forward reaction
k2
specific rate of reverse reaction
e, the net rate of oxidation of SO2to SO1 would kxrn[a-l/2
[a
(XO-X)
1"
+ 6 + ~ 0 - 1 / 2 (xo-x)]"+" k (xo-x)"' K [a + b + x o - I / Z
(XO-x)]"'
I n this expression the last term may be omitted at low temperatures and a t low percentage conversions. (At low temperatures, K is large: and at low percentage conversions (20-2) is small. The omission must be justified by trial in each case.) Although, if m and n are known, this equation is integrable by algebraic methods, it is simpler to handle it graphically; in other words, by plotting the reciprocal of f(z) against x, where f(x) is the right-hand side of Equation 1, and getting the area under the curve. The equilibrium constant K has been determined with particular care by Bodenstein and Pohl,' and since the computed values of the equilibrium mixture as determined from Knietsch's curve are obviously somewhat in error, Bodenstein's equilibrium values are used throughout this computation. As Knietsch clearly indicates, time of contact and amount of platinum are, in the case of steady flow, proportional. Call the mol9 of nitrogen per unit time B and the weight of platinum in the process, W . Then the total mols of gas is 7
Z. Elcklrochem., 11, 373 (1905).
Therefore, the equation may be written dx
f (4 =
k X 2 [a-1/2
[a
+b +
(XO-x)]
x0-1/2
-
K
(xo-x)la k (xo-x)' [U
+ b + ~0-1/2
(1) (XO-X)]~
It may be that the true reaction mechanism is different and that the character of results is due in a degree to compensation of errors, but unless these results are to be thrown out entirely, one must assume for the reaction under plant conditions a third order equation; and until there are available more reliable experimental data demonstrably applicable under those conditions, Knietsch's results must still be accepted as the basis for plant practice. Figure 1 shows the specific reaction rate constant, k, plotted against the temperature. It will be noticed that, whereas ordinarily specific reaction rate increases with the temperature, in this case the specific reaction rate a t first increases, goes through a definite maximum at about 525' C., and then decreases. Note-Reesea thoroughly appreciated the fact that even at the start of the catalysis i t was disadvantageous to have the temperature too high; but his data were not in such shape as to enable him to realize that the disadvantage was due to a decrease in specific reaction rate. MellorD states that the catalysis of this reaction with platinum-asbestos gives a maximum of the reaction rate itself a t about 400° C.; but this is true only at very high percentage conversions, and is not a t all the case for the average rate. It would seem that Mellor confuses the effect of reaction equilibrium with that of reaction rate
Furthermore, this decrease is so large that there can be no question but that it is outside any probable error in the data. The reason for this maximum is not clear and no experimental data are available to explain it. Bodenstein showed conclusively that a t low temperatures the platinum is kept inactive by an adsorbed layer of sulfur trioxide. The higher the temperature the less firmly would such adsorbed trioxide be held on the surface of the platinum, and therefore for a given partial pressure of SOs in the gas the larger would be the free surface of platinum available for catalysis. It is 8
J . SOL.Chcm. Xnd., 22, 351 (1903).
0
"Chemical Statics and Dynamics," 1914, p. 417. Longmans, Green
& Company.
INDUSTRIAL S N D ENGINEERING CHEMISTRY
June, 1925
not impossible that the catalysis of the oxidation requires the adsorption of SO2 on this SOa-free surface, and that, although at higher temperatures the free surface available increases, a temperature is finally reached a t which the adsorption of the dioxide decreases more rapidly than the rate of increase of trioxide-free area. Whatever the explanation, the presence of this maximum temperature for catalysis seems to be beyond question. Influence of Reaction Rate on Converter Operation
I n the design and operation of a converter the engineer has under his control three factors-namely, the composition of the entering gas, the temperature of operation of the converter, and the amount, character, and distribution of platinum employed for a given amount of gas. Because of the expense of this platinum it is desirable to use it in the most effective way possible-i. e., so as to be able to treat with it the maximum quantity of gas and secure the maximum possible conversion. The equations just developed are obviously those which enable one to determine the capacity and efficiency of the converter. One must keep clearly in mind the fact that the numerical value of the constant, k, depends on the character of the platinum-i. e., surface exposure, freedom from poisons, etc. Although in modern converter design the concentration of the platinum is sometimes varied from section to section, it is tentatively assumed that the activity is proportional to its mass, but it will be seen later that the character of the platinum in the last stages is what counts and that a different character in the early stages will not affect the conclusions reached below. Any method of preparation of the contact mass which increases the activity-i. e., k-will obviously be advantageous.
595
This quantity, zO/z, is the reciprocal of the ratio of residual to initial SO2. One minus the reciprocal of this quantity is the fractional conTersion. It is obviously desirable, other conditions being equal, to have this quantity as large as possible. For any given value of time of contact or, what is equivalent to the same thing, for any given value of platinum employed per unit volume of gas, this quantity is a maximum when a = 4A/10. Calling W the weight of platinum in the converter and V the mols of total gas entering the converter per unit time, the time of contact may be defined as e = W / V . Calling the production per unit time
P,
For any given value of x/-i. e., for any given per cent conversion-this production is obviously a maximum if zo = 8A/15. Since this is true for any value whatever of the per cent conversion realized, it must apply for all valuesi. e., under the conditions assumed above best results are obtained when the SO2 content of the gases entering the chamber is adjusted to the value zo = 8A/15. When sulfur is burned there is always a slight amount of trioxide formation in the burner and this trioxide is removed in the purifying system. Consequently, the sum of the percentage of SO2 plus that of oxygen in the entering gas is always a trifle less than that in air and is equal to approximately 20 per cent. Therefore, this value, A = 20, is assumed throughout. Hence for these particular conditions of operation the SO2 content of the gases sent to the converter should always be approximately 10.7 per cent, because this percentage will give a higher production capacity for the equipment than any other, whatever the conversion required.
Low-Temperature, Isothermal Conversionlo
Maximum Conversion Rate
If one operates a t temperatures below 475 O C. the equilibrium of the reaction lies so far to the right that during the early stages of the conversion the SOa term of Equation 1 is negligible. If one operates a t temperatures around 425" C. this term remains negligible even up to quite high conversions, and the lower the temperature of operation the higher the conversion to which one can go before this term begins to play an appreciable part. Under these conditions the re. action rate equation simplifies to
Inspection of Figure 1 shows that a t 525" C. the specific reaction rate is higher than a t any other temperature. Consequently, if one will introduce the purified gases into the converter a t this temperature, they will start to oxidize a t a 800,000
600,000 500,000 400,000
300,000
Although this equation can be integrated algebraically as it stands, the expression becomes so involved that it is better to make certain simplifying approximations. The denominator of the right-hand side changes but little during conversion under ordinary conditions and may for practical purposes be assumed constant. Furthermore, the amount of oxygen present may be given its average value, a-zso/4, without introducing serious error, where, as is normally the case, the per cent excess oxygen over that theoretically needed for oxidation of SO2 is large. The equation therefore simplifies to
+
where A = a integrates into
PO
=
initial oxygen
+ initial dioxide.
No - = 10
1
+ kxo ( A
- $)
8
As shown below, isothermal conversion should not be used.
100,a00 80,000
60,000 50,000
40,000
30,000
fJG. J
/o,ooo
This (4)
which simplifies to
zoo,ooo
300
350
400
NO 550 T&flf€RRTURL '6. 450
6W
650
higher rate than will be realized under any other conditions. On the other hand, as soon as the conversion has gone far enough to form appreciable quantities of SO3, the reverse reaction begins to play a part-i. e., the last term in Equation l reduces the effective reaction rate. Furthermore, this reduction becomes very serious a t such high temperatures because of the low value of the equilibrium constant. A
596
lSDVSTRI.4L AiYD EXGILVEERI,VG CHEMISTRY
study of the data will show that as oxidation proceeds and in consequence SO; concentration increases, the reaction rate, -dx/de, can be increased by lowering the temperature despite the fact that the specific reaction rate decreases. The reason for the increase is that the depressing effect of the SOa term in Equation 1 is lessened by the increase in the equilibrium constant with decrease in temperature to an extent which more than counterbalances the reduction in specific reaction rate. If, however, the temperature is depressed too far, the gain due to the change in the equilibrium constant is more than counterbalanced by the decrease in specific reactionrate-i. e., for any particular per cent conversion in a gas of a particular initial composition there exists an optimum temperature which will give a higher instantaneous reaction rate than any other. If one assumes 8 per cent SOzand 12 per cent 0 2 in the entering gas and computes for 98 per cent conversion the oxidation rate, -dx//de, for different temperatures, one obtains the curve of Figure 2. This curve possesses a maximum a t 416' C. The maximum for any other per cent conversion can be computed in the same manner. These maxima are shown in Figure 3 plotted against per cent conversion for this case of 8 per cent SO2 and 12 per cent 0 2 in the entering gas, and indicate the correct temperature gradient through the converter. This plot shows that the maximum activity is obtained from the platinum only when the temperature of the gases in the converter is reduced as conversion proceeds, starting a t 525' C., and falling to a lower temperature, the specific value of which depends on the per cent conversion. Similar curves drawn for gases of different initial composition show that the temperatures also depend on the composition, although to a much less degree than on conversion. Because of the heat of reaction the gases in a converter rise in temperature. To get the maximum efficiency of platinum they should fall in temperature. This can be accomplished only by absorption of heat as conversion proceeds, but to get the best results this heat abstraction must be controlled so as to keep the proper relation between temperature and conversion. The discussion of isothermal, low-temperature conversion indicated the desirability of an SO2concentration higher than that normally employed. However, it has just been shown that isothermal conversion is wrong in principle in that it does not get the maximum effect from the platinum. The question then arises-what per cent gas will give the best results in a converter designed to cool the gases to the optimum temperature a t each stage in the conversion? The preceding paragraphs have shown how to determine the optimum temperature of the gases a t each stage. It was pointed out that temperature is one of the variables under control of the engineer. It is now clear that it is not an independent variable, but one the value of which is fixed by the composition of the entering gas and the degree to which conversion has gone. Therefore, if one assumes the composition of the entering gas, one is in a position to compute the reaction rate a t each instant during the conversion for conditions of optimum temperature control. I n other words, z0 being known, for each per cent conversion construct the curves corresponding to Figure 2 and get the
Vol. 17, No. 6
corresponding maxima. The points on the curve of maxima show the temperature to which the gases should be cooled a t each stage in conversion and the rate of oxidation corresponding thereto. The total catalytic effect, is the integral corresponding to the area under the curve obtained by plotting the reciprocal of these maximum conversion rates against x. The accompanying table shows for an SO2 content of 8 per cent and 02 content of 12 per cent the following variables as functions of the per cent conversion: optimum temperat'ure, unconverted SOZ,f(x), -9 1 and
F,
Jh
f (4 so2
T
= 6 , 0 2 = 12
Per cent optimum conversion O C x f(XI 60 525 3 2 74 90 90 480 0 6 2 949 0 4 0 533 95 449 97 427 0 24 0 149 96 416 0 16 0 0493 Equation,f(r) = 4 64 (x-OO3)2.p4
1 f(x) 0 0 1 6
01335 336 67 70 20 25
Jj"
dx
0.0272 0.23 0.575 1.16
2 06
At first it would seem that the simplest way to integrate Equation 1 were to use the graphical method. However, this is not the case, since a study of f(z) shows that the values vary as much as 80,000 fold for a range of conversion from 0 to 99 per cent. Therefore, getting the value of this integral accurately by means of graphical integration involves dividing the curve into a large number of sections plotted on different scales and planimetering each separately. This introduces errors which may become serious, and to avoid the difficulty recourse was had to the following method: When the calculated values of f(x) as ordinates are plotted against the corresponding values of x as abscissas, on logarithmic paper, in every case the points are found to fall close to a straight line, though having a slight upward curvature. This curvature can be eliminated by subtracting a small definite value from each abscissa-i. e., an empirical equation of the form "f (xj = a (x-b)" 100 90
80 70
F
$B
60 50
Y
QO
0PTJflU.Y TEflPMiVURL
JP€ClRL CASE O f 30 20
IO 0 390 4W 410 420 430 440 450 460 470 480 490 500 SI0 SZO SXi EPlPERRWRE %.
can be written, where a and n are constants determined for each curve. The only restriction on the use of this method is that the equation ceases to hold when x becomes small compared with b i . e., beyond about 99 per cent conversion. As shown below, this equation is readily integrable algebraically and is accurate over the entire range employed. 1
---= f .r- b In -1
1
"
(xg- b ) - 1
a (%-I) e
(8)
June, 1925
I N D U S T R I A L A S D E S G I S E E R I S G CHEMI,STRI*
Figure 4 shows production plotted against initial SO, content for conversions of 90, 95, and 98 per cent. Two facts of importance will be noticed. The first is that cutting down conversion from 98 to 90 per cent increases the maximum capacity over tenfold. Hence by making an economic balance between sulfur loss and capacity on the basis of these curves, the correct conversion to be employed can be determined. The second is that there is a definite maximum production for any per cent conversion, determined by the initial SO, content. Inspection of the curves shows that these maxima lie between 9 and 10 per cent initial SO2 and that production falls off rapidly for other percentages in the entering gas. This falling off in production units is not so marked in the case of very high comersions, but the percentage decrease is greater than a t lower conversions. Points A and B show the calculated productions under isothermal conditions for 95 and 98 per cent conversion, respectively, a t 9 per cent initial SO2. Comparison of these points with those calculated for correct operating conditions shows that the production is from 15 to 40 per cent lower for the two cases shown, and since converter practice is often not so efficient as isothermal, the advantages of using a converter with the correct temperature gradient through it are obviously greater still in such cases. Design of a Controlled Temperature Converter From this discussion it is obvious that a very decided increase in efficiency may be obtained by controlling the temperature of the gases in the converter. To do this, the present type of converter must be modified so as to absorb heat during the process of conversion. With the large increase in efficiency, any reasonable cost of this added equipment will be readily amortized by the increase in production. Nofe-The sketches discussed in t h e next few paragraphs a r e purely diagrammatic and intended only t o make the thought clear. T h e y are not t o be considered in a n y sense a s working drawings.
Figure 5 is a strictly diagrammatic sketch of one type of converter employing this principle. The gases are fed t o the converter a t the desired initial SOz concentration through the heat exchanger 1, then through the cooling coils 4, to the bottom of the converter, being by-passed by valves 6 wherever necessary to maintain the correct temperature gradient. The gases up to this point are heated by the reaction of the gases already in the converter and reach the bottom a t about 400" C. From here the gases flow through the reaction chamber past the contact mass and the cooling coils and finally out past the yipes of the heat exchanger 1. The optimum temperatures for conversion in the various stages are obtained by heat balances on isolated sections of the converter, as in the following case. Assume the con-
597
centration of SO2 in the entering gas as 8 per cent. Call the temperatures below the contact masses C. and those above, B,, where n denotes the order in the direction of flow. To start reaction quickly, the gases should enter slightly below 400" C. It is obvious that we can follow the optimum temperature curve only in stepwise fashion, as it is impracticable to absorb the heat simultaneously with its evolution. It is also true that the larger the number of steps the closer one follows the curve. The number may be obtained by an economic balance between the profit gained from approaching the curve and the cost of the steps. In order to simplify calculation, assume enough contact mass on the first tray to yield 60 per cent conversion under the required production conditions. It is evident from Figure 3 that the temperature after reaction can rise to about 530" C. without losing efficiency on account of approaching equilibrium. A heat balance on this section can now be made, assuming that the contact mass a t each specific point in it remains practically constant in temperature, as would be the case when steady flow has been established. Since the heat of oxidation of SO2 to SOs a t constant pressure is 22,500 C. h. u. per pound mol (obviously numerically identical with the value in gram calories per gram mol) and is constant a t this figure, there is no change in heat capacity on reaction. Take as a basis 100 mols of entering gas, giving a total heat capacity of 760 C. h. u. The SO3 formed in the bottom tray is equal to 8 (0.6) or 4.8 mols. By a heat balance 14.8) (22,500) = i 6 0 1530-CI) Ci = 388' C.
e., th; temperature of the incoming gas should be 388" C. Considering the section in which the conversion rises from 60 to 90 per cent, the heat to be absorbed is the heat of reaction plus that necessary to cool the gases to 480" C.
-i.
+
:.
(530 - 480) (760) = C. h. u. = (8) (0.9 - 0 6) (22,500) 93 000 92,000 :. the temperature rise in the incoming gaqes = '-&= I
121' C., or when the converted gases are 90 per cent SO3 and a t 480' C., the inc o m i n g converted gases mustbe388-121 = 26i'OC.
60
l K - -i
Obviously, the method can be carried to the conversion required. This stepwise method of operation necessitates that the temperatures a t C should be somewhat below and those a t B somewhat above the temperatures corresponding to the curve of Figure 3, but the C temperatures ought to be less below than the B temperatures are above, since it is more important, especially a t low temperatures and high per cent conversions, to insure the rapidity of reaction than &Diagram of Controlled to maintain a given dis- FigureTemperature Converter from equilibrium* 1-Heat exchanger with excess surface In connection with the 2-Controlled temperature Converter 3-Contact mass control of this type of con- 4-coo1ing coils verter the following important point is to be noticed. If by controlling valves 6 we make the B and C temperatures as registered by thermocouples correspond to those calculated, the per cent conversions must be as assumed, as
~
=
~
~
598
I
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
otherwise no heat balance could be obtained. This gives an accurate and easy method of operation control. Figure 6 is a similar diagrammatic sketch of another type, which operates on a slightly different principle. The gases enter at the left near the top and go through the pipes of heat exchangers 1, progressing downward, and arrive a t the bottom of the converter a little below 400' C. From there they pass upward through the adiabatic converters 2, and heat exchangers 1, to the air dilution converter 3, and finally out through the top heat exchanger. In the converter 3 they are cooled 6and diluted by auxiliary air entering through ducts 7 , and controlled by valves 8 and 9. Pmfl The prelimiBLOU€R nary adiabatic converters a n d heat exchangers o p e r a t e in t h e same way as the trays and coils in the previous conv e r t e r . More than one adiabatic converter will usually be necessary toFigure 6-Diagrammatic S k e t c h of an Air Dilugether with ..the tion Converter 1-Heat exchangers 6-By-pass valves corresponding 2-Adiabatic converters 7-Auxiliary air lines 3-Air dilution converter 8-Auxiliary air valves heat exchangers 4-Hollow perforated grids 9-Main auxiliary air in order that the 5-Contact mass valve temDerature mav be kept down and that equilibrium be not to; closely a i proached. On account of the air dilution occurring later in the process, the initial concentration of SO2 should be about 8 per cent. At conversions up to 12 per cent, and of the 02, about 70 per cent this higher concentration of SO2 partially compensates for the loss due to the lowered reaction rate resulting from too rich a mixture; but above this point the concentration becomes important and auxiliary air is necessary. Heat balances may be made on the various sections in exactly the same way as in the previous converter. A L typical set of conditions is shown below:
6p-J -
Initial SO2 = 12 per cent; 0 2 = 8 per cent Gases entering at bottom, 393" C. Gases leaving first adiabatic converter, 550" C., 45 per cent converted Gases entering second adiabatic converter, 463' C. Gases leaving second converter (adiabatic), 550' C., 70 per cent converted Gases entering heat exchanger below air dilution converter, o.?-lo
r. L.
Gases leaving this heat exchanger, 307" C . Gases leaving second heat exchanger, 393" C.
Reese' gives experimental results obtained from a threesection converter where cooling was obtained by radiation. Although the data are not complete, it will be noticed that the temperatures employed by him in converter practice are approximately those shown in Figure 3. The officers and executive committee of the Division of Chemistry and Chemical Technology of the National Research Council for the year 1925-1926 are: chairman, William J. Hale; vicechairman, s. C. Lind; executive committee, William J. Hale, S. C. Lind, William McPherson, H. S. Miner, James F. Norris, C. L. Reese, and E. W. Washburn.
Vol. 17, No. 6
A Vacuum Door' By Robert F. Mehl and Donald P. Smith PRINCETON UNIVERSITY. PRINCETON, N. J.
D U R I N G an ipvestigation upon the preparation of very pure alloysZ it became necessary to devise a piece of apparatus for rapidly and safely admitting a gas into a high vacuum system. The presence of stopcocks in such a system is a source of continual annoyance because of leaks, and the piece of apparatus described herein has eliminated this trouble. The tube A connects the apparatus to the system. While the system is being evacuated, the mercury in B is drawn up through tubes C and D. Tube D is a drawn-steel tube of about 4 mm. inside diameter, and is contained within C, a stout tube of 12 mm. inside diameter. D is centered in C by means of bent wires conveniently placed. When the evacuation of the system is complete the mercury in C and D is held a t barometric height above the mercury in B. C and D are about 80 cm. long; when at its highest the mercury in these tubes stands a short distance below bulb G. The gas to be admitted into r the system is led through tube E , which is given great freedom of motion by means of two Vshaped bends placed a t right angles. The constricted end of E is placed within the enlarged opening of tube I), and the gas, the flow of which is controlled by means of a reducing valve, is caused to flow in a slow, steady stream. The small bubbles of gas enter D and rise slowly, increasing in size as they rise. F , the end of the steel tube D, is plugged, and just below the plug the tube is drilled with about a dozen 2-mm. holes distributed a t random. As the bubbles rise they force ahead sections of mercury, which finally reach F. At this point the mercury and the gas discharge horizontally into the large bulb G. The gas enters the evacuated system through A and the mercury falls into C, which by virtue of its large diameter permits the mercury quickly to adjust its height in accordance with the new messure existing in the system. When the apparatus is running correctly, the mercury-gas lift works smoothly and continuously. A spasmodic action may be corrected by an adjustment of the reducing valve. H is an auxiliary tube, concentric to C and D, under which E is placed until the bubbling is correctly adjusted or until the cleansing system, placed between the reducing valve and E , is operating satisfactorily. D is made of steel instead of glass so that i t will not break during the slight jarring produced by the discharge of mercury and gas a t F. This arrangement also takes care of any change in pressure that might occur in the system, the gas escaping through B in the case of an increase in pressure, or the mercury rising in C and D in the case of a decrease in pressure. 1 2
Received March 5, 1925. Mehl, Trans. A m . Electrochem. SOL,Preprint, October, 1924.
