INDUSTRIAL AND ENGINEERING CHEMISTRY
910
The equipment required for the preparatiou of Insulin by the process described in this paper can be described briefiy. Throughout the process care is taken to avoid prolonged contact with undesirable metals. Hence the extraction of the glands is conducted in glazed earthenware vessels or glasslined tanks, and similar equipment is used for storage of the extract. The centrifuge used to remove the extracted glands from the mixture is of the basket type, a11 exposed metal parts being rubber-covered. An ordinary wooden plateand-frame filter press with cotton cloths is suitable for the filtration of the ammoniacal mixture prior to concentration in the stills. Glass-lined or stainless steel stills are used, 8 high vacuum being obtained by rotary pumps or steam ejeetors. Tubular condensers, which are cooled preferably by an
VOL. 32, NO. 7
ice machine, are satisfactory for the condensation of the distillate.
Literature Cited (1) Abel. J. J., Proc. Hall. dcad. Sci.. 12. 132 (1926). ( 2 ) Abel. .I. J., Geiling. E. hl. X., Roiiiller, C . A,, Bell. F. 0.. and Wiirtersteiner, O., 3 . Pharmeol., 31, 65 (1927). (3) Brucb, E., Arch. EzpU. Path. Pharrnakol., 173,439 (1933). (4) Emicir, Friedrich, and &id, Frits (Lr. by Schneidcr, Frank). "Minroehemieal Laboratory Manual". 2nd ed., p. 07, New York. John Wiley & Sons, 1952. (5) Quait. Bull. HeaUh Organization League Hotolions. Sgcoial No. 504 (Ne".,1030). (6) Scott, D. A., Biochern. J . , 28, 1592 (1934). (7) Scott, D.A, and Fisher. A. M., Ibid., 29, 1048 (1935). ( 8 ) Gtsllmann, B.,Ilid.. 185,77 (1937).
Chernistrv of Sulfur Dioxide Reduction J
KINETICS
ROBERT LEPSOE The Consolidated Mining and Smelting Company of Canada, Limited, Trail, British Columbia, Canada
tion of certain new details of procedure than on the chemistry of reduction. The present article is the second of a series dealing with the recovery of sulfur from waste gases. Substantially, the material presented is the result of studies conducted prior to 1935 in connection with the development of the Trail sulfur process. The previous paper dealt with the equilibria in sulfur dioxide reduction systems calculated from existing thermodynamical d a b (7). The present paper offers new experimental data on reaction rates, together witb interpretations applicable to the industrial process of sulfur dioxide reduction. The investigstions revealed here were pursued solely with a view to practical application. On the other hand, i t was evident that in the main ph$ses of sulfur dioxide reduction there was a fruitful field for fundamental scientific research.
Reduction of Sulfur Dioxide by Carbon Laboratory studies of the reduction with carbon were undertaken in the usual manner. Dry sulfur dioxide w89 passed thraunh a vertical extornallv heated auartz tube filled with e& eined~metallurgicdcoke (&he1 code oontaining 85 per -cent fixcd carbon, 1.5 per cent ash) screened between 4 and 8 mesh. The tube was evacuated and purged with nitrogen before each test. The reaction product8 %me collected in a gasometer after the first 4 liters had been run to wmte. The sulfur formed cou~,
~
.
~~
in the residual ms w& determincd by absomtion in acid~cua&s
I
N RECENT years the reduction of sulfur dioxide to elemental sulfur ha.s become a matter of couriderable industrial interest, especially in connection with the recovery of dioxide from smelter smoke, Little bas heen about the reduction of sulfur dioxide eroept in numerous patents in which more emphasis has been placed on the descrip
throuqh a' quartz tube containing platinum catalyst heated to 900" C. whereby carbon oxysulfide was reduced t o hydrogen a d fide which was subseouentlv absorbed in cndrnitirn sulfaie. The carbon oxvsu~fidacnn'tent \;.as eahulite2 from tile ea&&& SUI~~~
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
911
disulfide present under the conditions outlined. Carbon dioxide was determined by difference between the gas volume absorbed in alcoholic potash and the separate determinations for sulfur dioxide and carbon oxysulfide. The results are shown in Table I.
The terms in parentheses represent moles of carbon dioxide and sulfur dioxide, respectively. B~ selecting the values kl = 10 kz or (COJ = 1.1[(SOZ)O.’ - (SO2)]* we obtain figures for carbon dioxide, sulfur dioxide, carbon monoxide, and Sz which are in striking agreement with the observed figures. In Figure 1 TABLEI. REDUCTION OF SULFURDIOXIDEWITH CARBON these figures are plotted against the fractions of Time of -Calcd. Compn., Moles/MolTzmp., Contact, -Gas Compn., %SOz Treated Degree of undecomposed sulfur dioxide. C. hlin. Sot COz CO COS SOz COz CO Sz Reduction The constant kl may be defined best as the 850 6.5 57.0 38.4 1.3 2 . 0 0.590 0,395 0,034 0.205 0.41 “observed” rate of reaction constant for sulfur 0.91 850 2 . 0 2 1 . 5 0.090 0.720 0 . 3 6 0 0.455 22.0 7 . 5 59.0 900 3.8 5 0 . 5 4 6 . 0 Nil 1 12 .. 30 0.173 0 . 5 3 0 0.470 0.014 0.240 0.48 dioxide, and kz as the “apparent” reaction con0.83 0.760 0.148 0.413 900 9.4 16,s 6 9 . 7 1 . 5 950 3.2 36.9 53.6 0.7 6 . 2 0 . 3 9 0 0.570 0.074 0.305 0.61 stant for carbon dioxide. Actually i t is found, 0.91 2 . 7 1 9 . 7 0,089 0 , 7 8 0 0.257 0 . 4 5 5 950 6.5 7 . 7 61.7 when using carbon dioxide instead of sulfur 0.61 1.5 0 . 8 3 . 9 0.390 0 . 5 8 0 0.049 0.305 1000 37.6 56.8 1050 1 . 5 1 9 . 5 64.3 2 . 4 1 2 . 0 0 . 2 1 5 0.710 0.158 0.392 0.79 dioxide in the same furnace with the same 0.93 3 . 6 2 7 . 6 0.068 0.740 0.378 0.466 1150 2.6 5 . 6 61.2 1200 2.5 0 . 4 2 1 . 1 20.8 5 3 . 8 0.007 0.360 1.270 0.497 0.99 coke, that the reaction velocity of carbon dioxide is approximately one fifth instead of one tenth that of sulfur dioxide-i. e., The gas composition is given in per cent as found by analyksoZ (obsvd.) A 5kcoz A lOkco2 (apparent) sis. Since the amount of carbon oxysulfide that can be in equilibrium with carbon monoxide and sulfur vapor is very It is therefore evident that a third reaction must occur: small above 800” C. (7, Tables I1 and IV), most of the carbon oxysulfide found is obviously the result of the extremely fast so2 2co = 2coz 1/zs2 (5) reaction
+
co + 1 / 2 s 2
=
cos
which takes place during the cooling of the gas samples. Hence we are justified in assuming that substantially carbon monoxide rather than carbon oxysulfide was present in the original reaction products. From the gas analysis and stoichiometric relation the gas composition in moles is calculated. The oxygen in the reaction products is derived from the sulfur dioxide entering the furnace, so that by dividing the respective percentages of sulfur dioxide, carbon dioxide, and carbon monoxide in the exit gas by the sum of sulfur dioxide, carbon dioxide, and carbon monoxide, calculating carbon oxysulfide as carbon monoxide in each case, we obtain the composition in moles for 1 mole of sulfur dioxide treated. The number of moles of Szin the reaction products per mole of sulfur dioxide treated equals 1/2(1- moles sulfur dioxide in exit gas per mole of sulfur dioxide treated). The degree of reduction is (1 - moles of sulfur dioxide in exit gas).
+
This reaction also takes place a t the carbon surface; i. e., it is catalyzed by the ash and is therefore apparently of the first order, It has the effect of lowering the true ~ C O Zto kcoz
* If the initial gas, instead of being pure sulfur dioxide, is B mixture of sulfur dioxide and carbon dioxide or of sulfur dioxide and oxygen, the equation takes the following form,
where Xa and Yo denote the initial parts of eulfur dioxide and carbon dioxide (i. e., initial oxygen!, respectively.
Consecutive Reactions In none of these tests has equilibrium been attained (7, Tables V and VI). On the other hand, it is evident that sulfur dioxide cannot be reduced by means of carbon at any temperature without simultaneous reduction of carbon dioxide. I n other words, complete reduction t o carbon dioxide and sulfur cannot be attained. The figures of Table I show unmistakably the characteristics of consecutive reactions:
so2 + c = coz + + c = 2co
1/zs2
cot
(1) (2)
These reactions taking place a t the surface of carbon are all of the first order. Hence we write for the first reaction:
- -d(Soz) - d e
kl (SO,)
(3)
and for the second, -d(Coz) =
kl(SO2)
- kZ(CO2)
05
(4)
de Dividing Equation 4 by 3 and integrating, we obtain the following:
8
M O L E S Cop, CO,andS2 FIGURE1. REDUCTION OF SULFURDIOXIDEBY CARBON Numerals adjacent t o plotted points represent temperature in hundreds of degrees Centigrade.
912
INDUSTRIAL AND ENGINEERING CHEMISTRY
(apparent) and increasing ksoz to kso2 (obsvd.). By inserting the observed values in the equation
VOL. 32. NO. 7
diffuse through the stationary gas film around each carbon particle that the diffusion rate controls the speed of combustion.
and comparing with the true equilibrium constant (7, page 95), the apparent equilibrium constant K1 happens to be fifteen and five hundred times smaller than the true constant a t 1200" and 850" C., respectively. The existence of these consecutive reactions in commercial sulfur dioxide reduction furnaces has been confirmed by taking chilled gas samples from the coke bed a t various heights. The ratio k l / k z is not always equal to 10; it may vary from 8 to 10 but has never been found higher than 10.
Rate of Sulfur Dioxide Reduction by Carbon As observations of the time of contact in the above test data were not considered sufficiently accurate to serve for the determination of actual rates of reaction, a new series of tests was undertaken in which sulfur dioxide was passed through thP coke at such a high velocity that only a small fraction was reduced; the changes in volume were thereby eliminated. The coke was weighed before and after the tests, and the loss in weight was converted into terms of carbon dioxide producecli. e., sulfur dioxide reduced. Tests in which carbon dioxide replaced sulfur dioxide were included in this series. Two types of coke were used, each showing a temperature coefficient of approximately the same value, dR In k / d ( l / T ) = 46,000 - 51,000 kg.-cal regardless of whether the init>ialgas was sulfur dioxide 0 1 carbon dioxide. The rate of reduction of sulfur dioxide way five times as fast as that of carbon dioxide, and the one type of coke (Coleman) was four and a half times more reactive thaii the other (Michel). The temperature coefficient is in satiefactory agreement with that calculated from the carbon dioxide data of Clement, Adams, and Haskins ( I ) , but the reactivity of their coke was two thirds that of the Coleman coke.
'/r u 10'
FIGURE 2. REACTION CONSTANTS FOR SULFUR DIOXIDE REDUCTION, k802 (APPARENT), AND FOR PRODUCER GAS REACTION, koo2, BASEDON COLEMAN COKE,-4 8 MESH Dotted lines show rate as affected by diffusional resistance at
+
various velocities.
The results are plotted in Figure 2. The curve for thr carbon dioxide rates has been extended from 1100"to 1300"C.. and the sulfur dioxide curve has also been lengthened. There is, however, a definite limit to the extension of thr curves in this way. The limit is set by the rate of diffusion. It is known in combustion of carbon that all the oxygen is always consumed by the fuel within 3 to 4.5 inches from the grate, regardless of whether the rate of firing is 3 or 125 pound> of fuel per square foot of grate area per hour (4). In other words, the chemical rate of reaction, C O2 = CO?, is intrinsically so much greater than the rate a t which oxygen can
+
COKE BED-INCHES FIGURE 3. PRODUCER GASOPERATION
Evidently the same holds true for the producer gas reaction, C = 2C0, and for sulfur dioxide reduction a t high CO, temperatures ( 8 ) . Since the total rate of reaction depends on a chemical as well as a diffusional resistance, it follows that when the temperature rises, the former resistance becomes progressively smaller so that diffusion finally becomes the controlling factor. The temperature a t which this takes place must necessarily be much higher for carbon dioxide antl sulfur dioxide than for oxygen. From the plotted data of Hottel et al. (IO) the combustion rate of carbon with carbon dioxide a t 1230" C. is equal to that with oxygen a t 700" C., or oxygen reacts approximately a thousand times as fast as carbon dioxide. Assuming the diffusional resistance to be the same in both cases, we may extend the log ~ C O ,line until it intersects Hottel's oxygen diffusion curve (1700" K.), and hence infer that the producer gas reaction (carbon dioxide reduction) is controlled by diffusion rates above 1400" C. Applying the same rule to sulfur dioxide reduction, which is five times as fast as the reduction of carbon dioxide, it is found that the sulfur dioxide reduction is controlled by diffusion above 1200" c. The values plotted in Figure 2 denote the reaction rates for the producer gas reaction, kcoz, and for the sulfur dioxide reduction, k802 (obsvd.), with Coleman coke. The time, expressed in seconds, is based on the volume of -4 +8 mesh coke with 64 per cent voids. The dotted lines represent the envisaged diffusion-controlled rates a t various gas velocities. Since the process of diffusion for oxygen cannot without modification be evaluated to give the diffusion rate for COZ CO these diffusion rates are a t and more particularly for SOz Sz, present a matter of conjecture. The diffusion coefficient of the ambient atmosphere in the reactions COZ C = 2CO antl C O2 = COS may be similar, but it is probably quite different in the sulfur dioxide reduction. Fortunately, any
+
-
+
-
+
JULY. 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
913
REDUCTION PLANTUNDERGOISG ENLARGEMENT error in diffusion rate cannot be of great practical importance since the rates in the high-temperature range are so fast that only fractions of a second are involved.
Reaction Rates and Temperatures in the Coke Bed By means of the above data for reaction rates it is now possible to interpret the reduction of sulfur dioxide in the light of the producer gas operation. In several graphs the gas compositions and temperatures are shown in relation to the height of the coke bed. The objection may be raised that such curves, based solely on theoretical considerations and precluding the many variables involved in practical operation, can be rough approximations only. Actually, all the observations on practical operations that have been made so far-i. e., with gas mixtures comprising concentrated sulfur dioxide and oxygen or air-have been in fair agreement with the calculations. On the other hand, the picture would be modified somewhat in case a different type and size of coke were used, and the reactivity and diffusivity of the coke bed would thus be altered. A study was made of a standard water-jacketed gas producer 10 feet in diameter, operating a t a rate of 64 pounds of coke per square foot of grate area per hour and 5800 cubic feet per minute of air plus steam. The coke was sized between 3 / ~ and "8 inch. Temperature, gas composition, and heat loss (water-jacket steam) data were collected. These data do not differ much from what has been published elsewhere (6). In Figure 3 the temperatures and gas compositions are plotted, based on 100 moles of oxygen plus water vapori. e., 56 oxygen, 44 water vapor, 212 nitrogen, or 312 moles of initial gas a t 17.8 per cent oxygen. The oxygen curve is based on observation, except for the first 2 inches of the coke bed, for which it has been drawn arbitrarily. The water vapor and hydrogen are based on the observed gas analysis, but the carbon dioxide and carbon monoxide curves are calculated' according to the equation:
where CI, C2
=
a
=
k1
=
partial pressures or fractions of 1 atm. of COSbetween two stations in coke bed initial 0 2 concentration of 0.178 mean value of reaction constant kcOl between temperatures of two stations
The calculated points, as plotted on the carbon dioxide and carbon monoxide curves, checked remarkably well with the observed points. I n the high-temperature range the calculated rates are higher than observed but are lower in the lowtemperature range. For the whole coke bed the observed figures were ahead of the calculated figures by 0.2 second, equivalent to 3 inches of coke bed. Figure 3 also shows the theoretical temperatures which were calculated as follows: St'arting from ordinary temperature, the temperature TOa t which the reaction products leave the coke bed is given by the equation: C p ( g s s ) dT = Q ~ (reaction) Q ~ d 9 :
Since the gas and coke travel countercurrent to each other, the temperature a t which the reaction finishes in the coke bed is higher than TOas a result of heat exchange between any given station in the coke bed and the top level. The reaction temperature T I is thus given by the equation: 1 Use of calculated values for water vapor and hydrogen computed from Haslam's equation (3)for water gas resotions waa attempted. It w m found, however, t h a t the terms (I a n d 8, embodying the reaction constants. d o not remain consistent for t h e temperature range employed here; they give too high ratios of carbon dioxide t o water vapor a t the high temperatures and t o o low ratios a t the l o a temperatures.
INDUSTRIAL AND ENGINEERING CHEMISTRY
914 I500
6 0
VOL. 32, NO. 7
IN0
1400 1300
". I100 I200 %,.I100 I000 90 0
1400
1200
g,1100 1000
p
800
I -Rate 641bs.coke/hr., I9OOc.Em. at egg. SO,,IIj% 0:
100
Ib N
W R a k 64Ibs. coke/hr, 3900c.Em.a t 4970 SO2 517. air, preheated to 200 'C.
0 v) v)
W -I
0
x
\
40
IUb
IIb
\I
20
-Rate 641bs.coke/ h c , 187e 02 to base + s e c o n d a ~ SO2,equivalent to 1900 c.Cm a t 11% 02.
cos
INCHES OF COKE BED
.
I
I
IO
FIGURE4. SULFURDIOXIDEREDUCTION BY CARBON
30 40 INCHES OF COKE BED 20
I
50
FIGURE5. GRAPHICAL EXAMPLES OF SULFURDIOXIDE REDUCTION
Initial gas, 0.82 sulfur dioxide, 0.18 oxygen
C, (carbon) dT
where n, m
=
(7)
moles
C, = heat capacity Q = heat of reaction at 298" K.
TOis solved directly from Equation 6, and T I is found graphically by means of Equation 7. By repeating these calculations for a number of stations in the coke bed, the theoretical temperature curve is obtained. The difference between the theoretical and observed temperatures a t the top of the coke bed, 980" and 860" C., respectively, represents the heat loss, which on the basis of 312 moles initial gas is equivalent to 326,000 kg.-cal., corresponding to 11.3 per cent of Q298 (2,900,000 kg.-cal.) a t the final station, above which no further reaction takes place, the coke bed acting merely as a heat exchanger. Of the heat loss, 93 per cent was actually accounted for as water jacket steam. By deducting 326,000 kg.-cal. from Q for the various stations, a new set of temperatures was calculated. These new temperatures still do not agree with those observed in the hot zone area. The discrepancy is evidently due t o more heat radiated vertically from the hot zone than is lost by radiation to the water jacket, and to some heat lost in the ash. The exact calculation of the flow of heat in the coke column is obviously more complicated than is shown by the simple method of deducting the heat loss from the heat of reaction. The simplification is merely a convenient method for the approximate solution required here. In the reduction of sulfur dioxide the heat of the reaction, SO2 C = COz l/zSz, is 10,240 kg.-cal. This reaction does not proceed to completion without simultaneous production of carbon monoxide. The secondary reaction, COZ C = 2C0, requires 41,530 kg.-cal. In practice the two
+
+
+
reactions must cooperate to the extent that the exit gas carries a t least sufficient carbon monoxide to combine with the residual sulfur dioxide in a subsequent catalytic step. From Table I we see that this condition is attained a t or above a reduction degree of 0.90. The maximum combined heat, Q, per mole of sulfur dioxide entering reduction is thus only (0.90 x 10,240) - (0.13 X 41,530) = 3800 kg.-cal. This amount of heat is insufficient to support the reaction which, in order to attain the rate required for practical operations, must be carried out a t temperatures well above 1000° C. Extraneous heat, therefore, must be added, and the simplest method is t o introduce oxygen. As an illustration, a case will be considered in which a mixture of sulfur dioxide and oxygen containing 17.8 per cent oxygen and 82.2 per cent sulfur dioxide enters the same furnace as was used for producer gas, and coke is burned a t the rate of 64 pounds per square foot grate area per hour. Based on 312 moles of gas mixture, the reaction of oxygen and carbon produces 5,300,000 kg.-cal. and raises the theoretical maximum temperature to 1515" C. The sensible heat of the gas leaving the coke bed is 5,300,000 kg.-cal., the temperature is 800" C., and 98 per cent of the sulfur dioxide is reduced. The amount of carbon required is 380 moles, which is three times as much as was used for the same quantity of initial gas in the producer gas example. Based on the same coke rate (64 pounds per square foot per hour), however, the gas rate becomes 1900 cubic feet per minute, or one third of 5800 cubic feet per minute; and the heat loss, which otherwise would have been the same on account of the same peak temperature, becomes therefore three times as high (i. e., heat loss = 326,000 X 3 X 100/5,300,000 or 19 per cent). Besides the heat consumed for carbon dioxide reduction, the endothermic reaction, Sz = 2S, has been taken into account (7, p. 95) : H = 46,700 3.5 T - 0.0005 T 2
+
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
The dissociation of Sz aids in flattening the temperature curve. The heat absorbed a t the high temperatures is reclaimed a t the lower temperatures. The temperatures are plotted in Figure 4. Comparing the producer gas operation (Figure 3) and sulfur dioxide reduction (Figure 4), the former attains the maximum temperature a t the point where the oxygen is entirely consumed, whereas the temperature in the latter case continues to rise until 50 per cent of the sulfur dioxide is reduced and then falls as the endothermic carbon dioxide reduction becomes predominant. The producer gas operation shows a hot zone (oxidation zone) 4 to 6 inches in depth, whereas the sulfur dioxide reduction usually reveals a deeper hot zone. In the final, lower temperature range the exothermic reaction, CO 1/&3~= COS, tends to flatten the slope of the temperature curve in Figure 4. The rate of sulfur dioxide reduction is calculated using the equation:
+
8
(sec.)
=
C' 2.3 log C, +2 k" a __ 3
1
~
where C1', Cz' = partial ressures of SO2between two stations a = initial concentration (in this case 0.822) k" = mean value of reaction constant between temperatures of the two stations
80,
By substituting a = A / R and 1/T = X,whence dT = - (l/zz)dz, the integral is converted into the standard form
-Sed. X*
On account of the effect of diffusion, the above expression is only valid below 1200" C. The simplest method is, therefore, to use k curves derived from Figure 2, and the planimeter. In order to convert the time in seconds to inches of coke bed, the mean gas velocity is used. For example, if sulfur dioxide is reduced from 0.822 to 0.600 mole, the gas expands from 1 to 1.121 volumes; hence C1 = 0.822 and C2 = 0.535. The initial gas velocity which is 1900/78 X 60 = 0.41 foot per second (cubic feet per minute divided by the furnace crosssectional area multiplied by 60 seconds) is thus increased to 0.41 X 1.121 = 0.46 foot per second, and the mean velocity is 0.435 foot per second. One second is thus equivalent to 0.435 X 12 = 5.2 inches. In the temperature range 1400" to 1460" C. and at mean velocity 0.435 foot per second, k, = 2.3; hence 8 = 0.21 second, which corresponds to a distance in the coke bed of 1.1inches. A few more graphical examples are shown in Figure 5. From calculations it appears that a t least 11 per cent oxygen in the inlet gas to sulfur dioxide reduction is required to maintain the operation. Curves I a and Ib show that, with the heat zone moving to the top of the coke bed, this operation is unsatisfactory. The producer gas reaction and sulfur dioxide reduction are similar in that clinkering occurs if the temperature is too high, and much unburned coke remains in the ash if the temperature is too low. The producer gas operation is controlled by the rate of driving and by the amount of steam admitted. Similarly, sulfur reduction is controlled by the rate of driving and by provision for adequate oxygen supply. The unsatisfactory conditions with low oxygen concentration can be obviated by admitting a high concentration of oxygen to the base of the reduction furnace and adding secondary sulfur dioxide to the hot zone. Curves IIa and IIb represent a case in which the gas introduced a t the base contains 18 per cent oxygen, and two subsequent additions of sulfur dioxide
915
bring the total ratio of sulfur dioxide to oxygen t o the same ralue as in the previous case-Piz., 11.1per cent. Curves IIIa and IIIb illustrate the application of air instead of oxygen. The gas mixture to reduction consists of 49 per cent sulfur dioxide and 51 per cent air or, in moles, 82.2 sulfur dioxide, 17.8 oxygen, 68 nitrogen. The gas is preheated to 200" C. previous to entering the furnace (or oxygen may be added to effect an increase from 17.8 to 20 moles). If the gas is not preheated, the curves have unsatisfactory shapes similar to curve I. A fourth case has been calculated for dilute sulfur dioxide gas (roaster gas) containing 83 per cent nitrogen, 8 per cent sulfur dioxide, 9 per cent oxygen. The gas must be preheated to a t least 150" C ; in this case 5800 cubic feet per minute can be admitted to the 10-foot furnace, 35 pounds of coke are burned per square foot per hour, and 95 per cent of the sulfur dioxide is reduced in a 40-inch column of coke. The amount of carbon consumed depends on the quantity of oxygen used. With the minimum amounts of oxygen (11.1per cent) and secondary sulfur dioxide, the exit gas contains, in moles, 0.75 carbon dioxide, 0.04 sulfur dioxide, 0.42 carbon monoxide, 0.425 SZ. In order to have 0.20 mole of sulfur dioxide to react with 0.40 mole of carbon monoxide in the subsequent catalysis step, 0.16 mole of sulfur dioxide is added. A maximum of 0.525 mole of SZ may thus be produced Tith a minimum consumption of 1.14 mole of carbon. The carbon consumption is thus 0.41 ton per ton of sulfur. With air instead of oxygen, the carbon consumption is 0.44 ton, and with roaster gas, 0.64 ton if oxygen-free sulfur dioxide is used as the additional gas t o catalysis, but 0.8 ton if roaster gas is added. These are minimum figures. Actual carbon consumption would be higher, depending on the amount of carbon lost in the coke ash and the efficiency of catalysis. Coke is the major factor in the cost of sulfur dioxide reduction.
Reactivity of Coke In the foregoing, examples of two specific types of coke, exhibiting different reactivity a t temperatures below 1000" C., have been shown. It is generally known that metallurgical cokes may vary in reactivity characteristics. It has been shown that inorganic constituents, particularly iron in an easily reducible state, have a marked effect on reactivity (6). At elevated temperatures the gas diffusion rate controls the speed of reaction, and it may therefore be inferred that the ultimate rate of combustion will be the same for any type of carbon, provided the same amount of surface is exposed in each case (8). With domestic or other types of nonmetallurgical coke, high reactivity is due largely to the presence of hydrocarbons. Drakeley (2) reported that coke prepared at 450" C. was many times as reactive in the reduction of carbon dioxide to carbon monoxide as coke prepared a t 1000" to 1100" C. He found, however, that by continued use the reactivity of semicoke ultimately dropped to the same rate as that of metallurgical coke. In the reduction of sulfur dioxide with semicoke made from Michel coal and containing 9 per cent volatile hydrocarbons, it has been found that the reaction is fast a t 800" C., a relatively low temperature. However, a copious quantity of hydrogen sulfide is present in the reaction products. The reaction continues until 55 per cent of the semicoke is consumed, when the coke becomes "inactive"; i. e., the remaining coke has acquired the characteristics of metallurgical coke, and hydrogen sulfide no longer appears in the exit gas. With charcoal previously calcined a t 1200" C. the reaction commences a t 650" C. and continues at a fast rate a t 800" C. until 80 per cent has been consumed. With Michel coal the reaction commences a t 400" C. and passes through several stages
916
INDUSTRIAL AND ENGINEERING CHEMISTRY
of increasing temperature, a t the end of each of which the reaction (and the formation of hydrogen sulfide) slows down but again becomes fast when the next temperature interval is reached. At 670" to 740" C. the reaction is extremely fast, and about 80 per cent of the coal is consumed a t the end of this period. In the final temperature stage, 740" to 820" C., reaction started fast and continued until all the coal had been consumed.
VOL. 32, NO. 7
stable under conditions (calcination a t 1200" C.) in which the ordinary hydrocarbons are no longer capable of existence. There is a significant resemblance between these exceedingly reactive carbon-hydrogen compounds and the surface compounds in catalytic reactions. Coal as a reducing agent for sulfur dioxide would have the advantage of lower cost than coke and would require a much lower ignition temperature. It would perhaps seem preferable to operate concurrently (underfeed) to avoid the coking produced by the usual countercurrent (overfeed) method. The disadvantage of coal is the copious production of hydrogen sulfide which is much more difficult to catalyze completely than carbon monoxide or oxysulfide.
Reduction of Sulfur Dioxide by Carbon Monoxide and Oxysulfide
u
2o070
80 90 100% REACTION EFFl C I ENCY
FIGURE 6. REACTION EFFICIENCY OF VARIOUS CATALYSTS AT ONE MINUTECONTACT TIMEAND -4 8 MESH SIZE
+
Considering the reactivity of Michel coke as unity, the relative reactivities of semicoke, wood charcoal, and Michel bituminous coal were found to be 20, 80, and 500 to 1000, respectively. These values are probably indicative, in a general way, of similar products from other sources. By placing both coal and metallurgical coke in the quartz tube and passing sulfur dioxide through a t such a fast rate that the exit gas contained more than 90 per cent sulfur dioxide and thus assured equal opportunity for all parts of the charge to react with sulfur dioxide, and by increasing the temperature gradually to 800" C., it was found that more than 99 per cent of the coal and less than 1 per cent of the coke were consumed. The inferences from the above are that a t 800" C. or below: (a) Semicoke, having a nucleus of graphitic carbon surrounded by a hydrocarbon complex, is consumed only to the extent of the hydrocarbon complex, leaving the graphitic carbon practically intact; (b) a fraction of the coal, expressed by the formula CnHZn--6 is directly consumed, and another fraction of formula CnHZn--7 is formed which, in the next temperature interval, reacts with sulfur dioxide to be partly consumed and partly converted to CnH2,1--8.Concurrent consumption and degradation continues through the subsequent stages until the carbonaceous material is wholly consumed. According to the work of Riley and co-workers (9),so-called amorphous carbon may be defined as comprising a series of products containing varying amounts of graphitic carbon and hydrocarbon. They may be composed substantially of hydrocarbons (bituminous coal, charcoal, anthracite) or solely graphitic carbon (metallurgical coke). It may thus be inferred that in applying carbonaceous materials having more than one hundred times the reactivity of metallurgical coke (or correspondingly lower temperatures of reaction) , reduction is not by means of carbon but by unsaturated complex hydrocarbons which, in contrast to the simple compounds of the methane type, are highly reactive. I n the case of charcoal it was shown above that the carbon-hydrogen compound is
For laboratory tests the apparatus was the same as was used for reduction by carbon except that the quartz tube was filled with catalyst instead of coke. The mixture of dry gases, sulfur dioxide and carbon monoxide (the latter made from formic acid) or carbon oxysulfide (made from potassium thiocyanate), substantially in the proportion 2 to l by volume, was kept in a steel gasometer with mercury seal. Before each test the quartz tube was heated to the particular temperature of the test. The tube was flushed with 5 to 6 liters of nitrogen, evacuated, then flushed with 4 liters of sulfur dioxidecarbon monoxide (or sulfur dioxide-carbon oxysulfide) mixture a t a fixed rate, and finally with another 4 liters which were collected in a small gasometer and analyzed. Substantially all the sulfur formed during a test condensed in the lower part of the quartz tube. In order to express the completeness of the reaction, the term "reaction efficiency" has been introduced. This is obviously a better term than "sulfur conversion." Since the former term is synonymous with thermodynamic efficiency, i t indicates directly the efficiency of the catalyst, 100 per cent efficiency representing the equilibrium value, whereas "sulfur conversion'' is also dependent on the accuracy of mixing the initial gases in the proportion 2CO (or 2COS) to 1 SO2. The gas compositions and equilibria are shown in Tables 111 and IV of the first paper ( 7 ) . The following example is used to show the method of computing reaction efficiency; the figures relate to a test conducted a t 500" C.: Initial Gas. % ' Nil 63.3 Nil
COZ
Go
cos so2
Final Gas, % 90.0 1.2 0.1 6.8 1.9
35.4
Residual gas (presumably
1.3
N 2 1
According to Table IV of the first paper ( 7 ) , the gas a t equilibrium a t 500" C. contains 99.42 per cent carbon dioxide, 0.42 per cent carbon monoxide plus oxysulfide, and 0.2 per cent sulfur dioxide.2 The sulfur dioxide concentration in the final gas from this test is thus about thirty times in excess of the equilibrium concentration.
1 2/[SOn11 this
Since [CO] = -
excess should depress the carbon monoxide plus oxysulfide concentration to 0.1 per cent; hence, the final gas, if equilibrium had been attained, should contain 90 1.2 or 91.2 per cent carbon dioxide. Reaction efficiency is thus 90/91.2 = 0.99, or 99 per cent. The gas reaction
+
2CO(COS)
+ so2
=
2c01
+ %(;)Sz
Actually this gas composition refers t o a stoichiometrio proportion of = 2:1, whereas with a n initial gas of 2 C O : SOr the ratio becomes 4 : l . The concentration of Sz in the reaction products of 500' C. is. however, 80 small t h a t any change of the stoichiometric ratio haa practically no effect. 2
Con: Sz
INDUSTRIAL AND ENGINEERING CHEMISTRY
JULY, 1940
in the absence of a catalyst proceeds slowly even a t 800" C., but almost any kind of hot surface is capable of catalyzing the reaction a t this temperature. Run-of-mine pyrrhotite is efficient a t 700" C., and a t lower temperatures alumina, in various slightly hydrated and acid-soluble forms (boehmite), is a remarkably efficient catalyst. Lightly calcined Guiana bauxite and activated alumina are both satisfactory. Alumina may also be used on a carrier of porous cement or it may be sintered to crushed firebrick by means of a bonding agent such as sodium silicate. Although no detailed investigations regarding the catalyst mechanism have been made, in all probability surface compounds of sulfur dioxide and the catalyst are formed. The pronounced adsorption of sulfur dioxide is manifested by the way the catalyst tenaciously retains sulfur dioxide once it has been exposed to concentrated sulfur dioxide-gas mixtures. The efficiency of the alumina catalysts is partly lost if they are exposed to high temperatures for long periods. I n the range 300" to 600" C. i t appears that the reduction of sulfur dioxide by carbon monoxide or oxysulfide is a reaction of the first order. The temperature coefficient dR In Ic/d(l/T) has been found to vary between 14,000 kg.-cal. (bauxite) and 18,000 (pyrrhotite). The reduction with carbon oxysulfide is approximately four times as fast as with carbon monoxide. Figure 6 shows the characteristics of various catalysts sized to -4 $8 mesh, representing more than one hundred tests. The time of contact is based on the volume of the catalyst and on the volume of the initial gas a t normal temperature and pressure. Heats of reaction are as follows (7, page 95) :
917
750
PERCENT OF TOTAL HEAT REMOVCD 20 40 60 80 100 PERCENT OF TOTAL SULPHUR VAPOR AS 52.
DIOXIDE REDUCFIGURE 7. HEATIN SULFUR TION EXITGAS
It is calculated from thermodynamic data (7, page 95) based on an initial gas mixture, COz 0.42% Starting from 750' C. and cooling to 150' C., 49 per cent of the total heat reCO ' / Z S ~= COS; AH = 22,500 kg.-cd. moved is due to transformation of Sz to S g and SS,41 per cent is 2C0 SO2 = 2COz '/zSz; AH = 51,760 - 2.75T 0.0028T' sensible heat, and only 10 per cent is due to condensation to 2COs SOz = 2COa 3/2S2; AH = 6760 - 2.75T 0.0028T2 liquid sulfur. Theoretically these heats of reaction will raise the temperature Summary to 1830" and 230" C., respectively, for the carbon monoxide and carbon oxysulfide reactions. The reduction of sulfur dioxide by carbon is expressed Sulfur dioxide may be reduced by carbon monoxide directly satisfactorily by the consecutive reactions: if carbon monoxide is available as waste gas or is the cheapest fuel. Generally, however, the carbon monoxide and carbon SO2 c = coz '/zS2 (1) oxysulfide reactions come into use as a step subsequent to the coz c = 2 c o (2) reduction by carbon. The exit gas from the reduction furnace is passed on to catalyst chambers eventually, with addiThe rate of formation of carbon dioxide between 900" and tion of the appropriate quantity of fresh sulfur dioxide in order 1200" C., expressed as moles in the reaction products, is given COS) to 1:2. During the to bring the ratio of SO2:(CO by the formula, cooling of the gas from the reduction furnace, the conversion (COZ) = 1.11 [(SOZ)O*' - (SOJI of carbon monoxide to oxysulfide proceeds rapidly as a straight gas reaction. Assuming that catalysis takes place a t according to which the ratio of the apparent reaction con500" C. a t 97 per cent reaction efficiency, we may attain an stants for sulfur dioxide and carbon monoxide is 10 to 1. optimum conversion to elemental sulfur of 0.994 X 97, or 96 On the other hand, since the observed rate of sulfur dioxide per cent, the factor 0.994 denoting the conversion a t equireduction has been found to be only five times the rate of librium. carbon dioxide reduction in the producer gas reaction, I n order to maintain the efficiency of the alumina catalysts, the gas must be freed from dust; otherwise the surface of the CO2 C = 2CO (in the absence of SOJ catalyst becomes covered superficially with a coating of the it is inferred that a third reaction, much inferior "dust catalyst". The only positive method to achieve thorough purification prior to catalysis consists in so* 2 c o = 2cos '/A purifying the gas by electrostatic or other known means, after either converting all the sulfur produced in the reduction furalso takes place as a reaction of the first order a t the surface of nace to carbon oxysulfide or condensing all the sulfur. the coke with ash as a catalyst. Above 1200" C. the rate of sulfur dioxide reduction appears Condensation to be controlled by gas diffusion rates, substantially the same depth of fuel bed being required for the reduction of sulfur The final step in the process consists in the condensation of dioxide, regardless of gas velocity. the sulfur and the separation of mist which is accomplished Sulfur dioxide reduction and the producer gas operation by known means. The transformation and condensation show marked similarity. Both depend on oxygen (air) for curve for sulfur vapor is shown in Figure 7. It presents some heat. On the other hand, the extent of the hot zone in the unusual features due to the many changes: sulfur dioxide reduction process is greater than in the ordinary gas producer operating on coke. I n order to maintain high Sz(gas) -+ Se + Sa + S(1iquid)
+++
++
+
++
+
+
+
+
+
+
+
918
INDUSTRIAL AND ENGINEERING CHEMISTRY
reduction rates and smooth operation, the temperature of the hot zone should not be less than 1300’ C. The reduction of sulfur dioxide by means of carbon monoxide or oxysulfide is fast with any kind of catalyzing surface above 800’ C. At lower temperatures (250” to 500’ C.) alumina in the slightly hydrated and acid-soluble form is an efficient catalyst, and the reaction appears to be of the first order.
(5) Ibid., p. 553. ( 6 ) Jones, J. H., King, J. G., and Sinnatt, F. S., Dept. Sci. Ind. Research (Brit.), Fuel Res. Tech. Papers 22 and 25 (1929-30). (7) Lepsoe, Robert, IND.ENG.CHEM.,30,92-100 (1938). (8) Perrott, G . St. J., and Kinney, S. P.,Trans. Am. Inst. Mining E w s . , 69,585 (1923). (9) Riley, H.L.,Chemistry &Industry. 58, 391-8 (1939). (10) Tu, C. M., Davis, H., and Hottel, H. C., IND.ENG.CHEM.,26, 756, Fig. 12 (1934).
...
Acknowledgment The writer wishes to express his appreciation of the valuable assistance rendered by R. M. B. Roome, J. Melville, G. S. Ortner, and J. H. Salter in conducting the experimental work upon which this paper is based.
Literature Cited (1) Clement, Adams, and Haskins, U. S. Bur. Mines, BUZZ.7 (1911). (2) Drakeley, T.J., J. SOC.Chem. Ind., 50, 319-30T (1931). (3) Haslam, R. T.,Hitchcock, F. L., and Rudow, E. W., IND. ENG. CHEM.,15, 119 (1923). (4) Haalam, R. T., and Russell, R. P., “Fuels and Their Combustion”, 1st ed., p. 338, New York, McGraw-Hill Book Co., 1926.
VOL. 32, NO. 7
Correction In the previous paper on “Chemistry of Sulfur Dioxide Reactions”, which appeared in the January, 1938, issue of INDUSTRIAL AND ENGINEERING CHEMISTRY, some errors should be corrected : Page 95, table under “Monoatomic Sulfur” in the fourth column, the figure for /S/ should be 0.0085 instead of 0.085. Page 97, Table 111, the last figure under /CO/ should be 0.000005 instead of 0.055. Page 98, Table VI, the column headings “% CO” and “% COZ” should be transposed.
MULTICOMPONENT RECTIFICATION Optimum Feed-Plate Composition E. R. GILLILAND Massachusetts Institute of Technology, Cambridge, Mass.
I
The composition of the feed plate giving might be set to have a fixed conN DESIGNIKG rectification equipment for the continuseparation is centration of propane in the the fewest plates for a ous separation of a given bottoms and to recover a defi‘Onsidered for three different types Of feedmixture, the design engineer is nite fraction of the n-butane in action* equations are the feed with the bottoms; in often confronted with the problem of determining the plate on developed for these three cases, and they this case the key components serve as a method of estimating the optiwould be taken the same as bewhich the feed mixture should be fore, although the concentramum feed-plate composition. introduced into the rectifying column. In general, for given tion of the n-butane is not aboperating conditions the designer solutely fixed in the distillate, desires to introduce the feed to a plate having a composince the distribution of the intermediate component, isosition of the mixture to be separated, such that the total butane, would shift with the composition taken for the feed number of plates required for the separation will be a miniplate. I n general, little difficulty is found in selecting the mum; this will reduce the costs of the rectifying column. The two key components, although occasionally the problem of composition of the feed plate giving the fewest number of selection is somewhat involved but can usually be resolved plates will be termed the “optimum feed-plate Composition”, if the designer will analyze the fundamental purpose of the and a criterion of this composition will be of assistance to rectification in order to determine just what is fixed. the design engineer. I n performing the stepwise plate-to-plate calculations, The following derivations for the optimum feed-plate use is made of equilibrium data and of material balances composition will be on the basis of the two key components, expressed as the operating lines. Above the feed plate the where the “light key component” will be considered as the operating line1 is: most volatile component whose concentration is fixed in the bottoms, and the “heavy key component’’ will be considered as the least volatile component whose concentration is fixed in the overhead distillate. Thus, in the stabilization of a Below the feed the operating line is: gasoline containing saturated hydrocarbons ranging from methane UD to h e a w fractions. the design conditions mieht 1 Badger and MoCabe, “Elements of Chemical Engineering”. 2nd be set so as to have a’ definite concentratik of DroDane in ;he ed.. New York, McGraw-Hill Book Co., 1937; Robinson and Gilliland, “Elebottoms and of ,+butane in the distillate; in thi$ propane menta of Fractional Distillation”, 3rd ed., McGraw-Hill Book Co., 1939: be taken the light component and ” Walker, Lewia, MoAdams, and Gilliland. “Principles of Chemical Engineerthe heavy key component. Alternately, the conditions ing”, 3rd ed., McGraw-Hill Book CO.. 1937.
