December, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
1405
Mechanism of Formation of Aromatics from Lower Paraffins',' V. Schneider3and Per K. Frolich' RESEARCH LABORATORY OF APPLIED CHEMISTRY, DEPARTMENT OF CHEMICAL ENGINEERING, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
A n attempt is made to distinguish between the T HAS long been known Method of Identification primary and secondary reactions in the cracking of t h a t a r o m a t i c comT h e m e t h o d developed propane, by progressively varying the rate of flow and pounds can be produced here for identifying the indiextrapolating back to zero per cent cracking. By reby combined c r a c k i n g and vidual steps in the transforpeating this operation with the initial reaction prodpolymerization of t h e n o r mation of the lower aliphatic ucts, the secondary reactions are likewise analyzed. mally gaseous aliphatic hyinto aromatic hydrocarbons It is shown that higher hydrocarbons can be built drocarbons. H o w e v e r , in is based on the principle that up from the lower ones otherwise than by simple spite of the relatively high an infinitesimal a m o u n t of polymerization. That many seemingly dimolecular value of aromatic hydrocarc r a c k i n g should give only reactions are actually of the first order is confirmed, and bons as compared with the primary products, b e c a u s e the view that butylene is a probable intermediate in the cheap raw material available s e c o n d a r y reactions obviformation of butadiene from ethylene is disproved. in nearly unlimited quantities ously cannot take place until The following mechanism of cracking is suggested: in the form of natural and reafter the initial products have finery gases, the yields oba p p e a r e d . Although it is tained are ordinarily so low theoretically possible to dethat it is doubtful whether the c r e a s e t h e p e r c e n t a g e of process has as yet been opercracking until nothing but ated successfully on a comprimary reaction products are mercial scale. In view of the formed, excessive dilution of potential value of this transthe products with uncracked formation of aliphatic t o aromaterial makes the existing matic hydrocarbons, the presm e t h o d s of h y d r o c a r b o n ent research was undertaken in an attempt t o obtain a better insight into the little-known analysis break down long before this point is reached. This mechanism of the simple cracking and polymerization reac- difficulty is readily avoided, however, by varying the degree of cracking within the range where accurate analyses can be made, tions involved. and plotting the results in such a manner that extrapolation to Present State of Knowledge zero per cent cracking distinguishes the primary reaction Three different theories have been suggested in the past to products from the others. To determine the products of account for the formation of aromatics from paraffins. Ac- secondary reactions, a new series of experiments is made with cording to the early publications of Berthelot ( I ) , inter- the initial products as the feed stock, and, by repeating this mediately formed olefins are converted into acetylene, which procedure as many times as necessary, it is possible finally to on subsequent polymerization gives aromatics. Davidson identify each step in the reaction chain. The application of this method is illustrated by the dia( 2 ) and others (5, 6) conclude that the transformation of olefins t o aromatics proceeds by way of diolefins rather than grams, where the results are plotted as moles produced per acetylene. Finally, it has been proposed by Ipatiev (9) and 100 moles of feed cracked us. per cent of feed cracked. It will by Hurd and Spence (8) that the aromatic hydrocarbons are readily be seen that, when plotted in this way, the curves formed by dehydrogenation of cycloparaffins. The cyclo- representing products formed by secondary and subsequent paraffin structure may result from polymerization of olefins reactions must go through the origin. The method obvior free radicals, such as > CH,, or it may be produced by ously does not distinguish between products formed directly rearrangement of an olefin. (The latter mechanism refers to and those formed indirectly through an intermediate not stable olefins of higher molecular weight than those dealt with in enough to be detected in the cracked products. this paper.) Preparation of Materials Used Since the final products in any hydrocarbon cracking process come only in part from the initial reactions and largely The propane used was obtained by fractionating and refrom progressive polymerization, recracking, and repolymerization reactions, it follows that definite proof of any of the fractionating a commercial brand under pressure in a 10-foot foregoing theories depends on a reliable criterion for dis- (3-meter) column packed with small nails. The propane tinguishing the initial and secondary products. Past experi- thus prepared contained about 3.0 per cent propylene. After menters have concentrated mainly on simple identification removing the propylene by scrubbing with sulfuric acid, the of the constituents in the final products, irrespective of how propane analyzed 0.04 per cent ethane, 99.96 per cent propane, formed, and, hence, their data do not always throw the neces- and 0.00 per cent butane. The propylene was made by dehydrating isopropyl alcohol sary light on the underlying mechanism of reaction. by passing over heated kaolin. It contained a small but Presented before the Division of Petro1 Received March 17, 1931. varying amount of butylene. The method of removing the leum Chemistry at the 81st Meeting of the Amerisan Chemical Society, butylene was to fractionate twice in a Davis column ( 3 ) . Indianapolis, Ind., March 30 to April 3, 1931. * Part of thesis submitted by V. Schneider in partial fulfilment of the This was done only in the runs noted. requirements for the degree of doctor of science in chemical engineering at The ethylene was of quality suitable for anesthesia. Analythe Massachusetts Institute of Technology. sis without fractionation showed 0.2 per cent propylene, but * Present address, Vacuum Oil Co., Paulshoro, N. J. this was probably in error. 4 Present address, Standard Oil Development Co , Elizibeth, N. J.
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
1406
The butadiene was made by simultaneous cracking and polymerization of ethylene (see results given later). It was purified by brominating the butadiene fraction in carbon tetrachloride, filtering off the crystals, recrystallizing from .alcohol, and then regenerating the butadiene with zinc dust in hot alcohol by the method of Thiele (12). The melting point of the butadiene tetrabromide was 110-116° C. Titration of the regenerated butadiene with standard bromide-bromate mixture (4) showed two double bonds per molecule, its boiling point being -5.0" to -3.0" C. Experimental Procedure and Analytical Method
PROCEDURE-The
experiments were carried out as follows:
The hydrocarbon to be cracked (propane, propylene, ethylene, butadiene, and a butadiene-ethq lene mixture) was passed through scrubbers and drying tubes, a flowmeter, a silica preheater at 550' C., a 0.5 X 24 inch (1.3 X 61 cm.) silica reaction
Vol. 23, No. 12
accuracy in the analytical work seem worth noting. Close fractionation as a first step in the analysis cannot be overevaluated, and larger samples obviously increase the chances for a good fractionation. I n the present work about 25 liters of gas (under standard conditions) were cracked in each run. The gaseous products were not allowed to stand in gas bottles longer than necessary before analysis, as certain constituents-notably propylene-changed in concentration on standing over the salt solution used as the confining liquid. A titration method with standard bromide-bromate mixture was found to be the best method for determining butadiene, Toward the latter part of the work it became evident that better combustion data were needed, especially for the determination of ethane and butane in the propane runs. The following method was developed and used in the runs in which propane was cracked a t 650" C.: Six to ten explosions were made on each sample, and the average data calculated. Like combustions were then run on pure propane and averaged, and the known error here used to correct the data on the unknown sample. When combustion data are in error, it is found almost invariably that the ratio of decrease in volume on combustion to volume of carbon dioxide formed is too large. This may be due to traces of caustic in the manifold absorbing carbon dioxide, or it may be due to the formation of free carbon or carbon monoxide. Rather weak explosions with a large excess of oxygen were found to give the hest data. A separate caustic pipet was used to absorb the carbon dioxide, as it was found that an error was likely to result if the same caustic was used to remove bromine fumes. Discussion of Results (1) Figures 1 and 2, representing the results obtained with propane a t 725" C. and one atmosphere, show that a t zero per cent cracking, about 48 moles of both methane and ethylene are produced for each 100 moles of propane reacting. The reaction is obviously CaHs ----t C2H4 CH4. Likewise, it is seen that about 42 per cent of the propane reacting forms propylene and hydrogen. Ethane seems to be an initial product, but it is not clear what reaction produces it. The curves for butadiene and the higher hydrocarbons in Figure 2 both definitely pass through the origin, showing that these two are secondary products. The accuracy of the butylene and butane curves is doubtful, however, and does not justify any conclusions.
+
Figure I-Results
with Propane at 725O C. and I Atmosphere
tube where the gas was heated to the desired temperature, through a Cottrell tar precipitator, and finally through a condenser in carbon dioxide snow and acetone. The uncondensed gas passed out through a wet meter, a sample being bled off for analysis. The condensate was then fractionated in a Davis column, and each cut analyzed as outlined later. Propane was cracked at atmospheric pressure. All the others were cracked at a pressure of one-fifth atmosphere, except for a few runs made to determine the effect of concentration. The vacuum was maintained by recirculating water through three aspirator pumps in parallel, placed between the condenser trap and the wet meter. When operating under vacuum, a scrubbing liquid, such as pentane, hexane, or heptane, was used in the condenser trap. One important feature of these experiments is that provisions were made for measuring the true reaction temperature. The customary practice of measuring the temperature of a gas flowing inside an externally heated tube by inserting a thermocouple directly in the reaction space invariably gives readings which are too high, owing to radiation from the tube wall ( 7 ) . Since the magnitude of the error thus introduced varies with the rate of gas flow, it is impossible, when varyi?g the time of contact, to maintain constant temperature by means of an inserted couple. The true temperature of the gas was therefore measured by a thermocouple inserted in a small silica well extending near to the exit end of the reactor. To eliminate radiation errors as much as possible, the wall temperature at this point was made the same as the gas temperature by the use of an auxiliary heating coil and thermocouple. It is estimated that without these precautions the temperatures recorded would have been between 25" and 80" C. higher for the particular experimental conditions employed. ANALYSIS-A few things which helped to obtain greater
After a better method of obtaining combustion data had been developed, runs were made with propane at 650" C., as shown by the curves in Figure 3. It was hoped that better combustion data would indicate what primary reaction produced ethane and also show whether butane was an initial product. The results obtained suggest that the reaction 2C& +CzHe C4H10may take place, though the butane as determined is only about one-half that required by this reaction.
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1931
Since propylene is probably the most valuable of the main initial products from propane, the actual amounts obtainable from a given amount of propane is of interest. From Figure 3 it is seen that although about one-half of the propane reacting originally goes to propylene, secondary reactions materially decrease the yield of this olefin with progressive cracking. Instead of an initial concentration of 25 per cent propylene in the cracked gas, the maximum is only about 12 t o 15 per cent in practical operation with high conversions per pass. I
I
1
1
!
I
I
!
1407
C3Hs +'CH4 -I- C2Hz and C3Hs +C3H4 Hz
+
respectively. Acetylene was never satisfactorily determined; the one value given was by absorption in an alkaline potassium iodide-mercuric iodide solution (10). A later bromide-bromate titration on the gas not condensing a t -78' C. and one atmosphere pressure showed that acetylene was present to the extent of one-seventh of the ethylene, which is not quite twice that found by the absorption method. This was a t about 15 per cent cracking. Propadiene was determined quantitatively in only one run, and the value found may have been in error by 50 per cent. At any rate, these analyses demonstrate that the two hydrocarbons are formed, but the data indicate that neither exceeds 6 moles per 100 moles reacting. Therefore, most of the methane and part of the hydrogen found as initial products must come from other reactions than those producing acetylene and propadiene, unless i t be assumed that both these compounds are too unstable to build up in appreciable concentration in the product. The probable answer is that propylene, in addition to splitting out ethane and ethylene between two molecules (as mentioned previously) also reacts to form methane and hydrogen by the equations
2CsHe and 2GH8
+CaHs + CHI +CBHIOi- HZ
I n that case it follows t h a t a large part of the unsaturated hydrocarbons, assumed to be hexene, is 5- and 6-carbon diolefins. If the c u r e s were plotted with this correction, all, with the exception of the 5- and 6-carbon unsaturated curve, would be raised slightly. qWV 0
10
20
30 PER CENT
Figure 3-Results
OF
40 50 PROPMC REACTING
1 10
60
:
BO
w i t h Propane a t 650° C. a n d 1 Atmosphere
(2) From the course of the curves in Figures 1 and 3 it is apparent that the propylene formed by the initial cracking of propane is very susceptible to further reaction. To study the mechanism of these secondary reactions, experiments were made with propylene as the feed stock. Since the concentration of the olefin in the primary cracking products from propane is approximately 20 per cent (compare zero per cent cracking in Figure 3), the work with propylene was conducted a t a pressure of one-fifth atmosphere to duplicate as nearly as possible the conditions of the original experiments. The results, shown in Figure 4,bring out some interesting points: For each 100 moles of propylene reacting, 23 or 24 moles of both butylene and ethylene are formed as initial products. Hence, it must be concluded that about 48 per cent of the propylene reacts according t o the equation 2C3H6 +CzH4 -k C4Ha Likewise, it seems that about 10 per cent goes to form ethane and butadiene by the reaction 2CsH6 --f C2H6 C4H6
+
I
Figure 4-Results
I
I
1
I
I
I
w i t h Propylene a t 752' C. a n d Atmosphere
The higher olefins were, a t first, assumed to be hexene, which could have been produced by direct polymerization of two propylene molecules. The simplest reactions giving methane and hydrogen are
I
'0
/
10
1 20
Figure 5-Results
1 I I I 30 40 10 BO PER CENT PROPYLENE REACTING
I
70
T
(10
I
90
with Propylene a t 725O C. a n d 1 Atmosphere
(3) The results in Figure 5 , obtained with propylene a t atmospheric pressure, are not very satisfactory because of the lack of accurate data on butylene. They are given mainly to show that the relative importance of the initial products agrees approximately with those from cracking a t a pressure of one-fifth atmosphere. However, one other feature is worth noting: The slopes of the ethane curves show that, a t one atmosphere, quantities of ethane are produced by secondary reactions while, a t a pressure of one-fifth atmosphere, very little seems to come from the secondary reactions. Apparently the increased pressure has shifted the equilibrium sufficiently to make the hydrogenation of ethylene proceed more rapidly. (4) The data given in Figure 6 for ethylene cracked a t one-fifth atmosphere pressure show that there are formed for each 100 moles reacting about 41 moles of hydrogen, 36 moles of butadiene, 12 moles of propylene, and 3 moles of butylene. This indicates that about 72 per cent of the ethylene reacting forms butadiene and hydrogen as initial products: 2CzH4 c4H6 -k Hz The extra 5 moles of hydrogen may have come from a reaction producing acetylene, since a positive test for acetylene was always obtained. However, inaccuracies of acetylene analysis
Vol. 23, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
1408
when the concentration is low make a direct determination of little value. Within the accuracies of the data, ethane and methane are secondary products, though the curves are not drawn through the origin. That propylene is one of the initial products from ethylene, may, a t first, seem improbable. If the fractionations were poor, part of the propylene reported might be either butadiene or acetylene. However, a bromidebromate titration of the cut containing propylene showed that only a small fraction could have more than one double bond per molecule. There is no doubt therefore but that the propylene curve is a t least qualitatively correct.
that some benzene is formed by the interaction of two butadiene molecules and that some aromatics besides benzene are formed, That this is the case is indicated by a run made with pure butadiene. With about 68 per cent reacting, the main products were aromatics of which only about one-seventh was benzene. Another possible reaction using up butadiene is the formation of ethylene and acetylene. ( 7 ) I n addition to the reactions just given, there is undoubtedly some simple polymerization, but runs with ethylene and propylene indicated that polymerization of the olefins was a relatively unimportant reaction a t 725' C. and a pressure of one-fifth atmosphere. The analytical data from the experiments with the mixture of ethylene and butadiene show that there is very little polymerization to form olefins higher than those containing 6 carbon atoms, Summary and Conclusions
A summary of the more important experimental resuIts is given in Table I. T a b l e I-Composition
8 30
SUBSTANCE
PRES-
4 3
CsHs
''0°
CRACKEDSURE TEMP. INITIALPRODUCTS IN ORDEROR IMPORTANCE Aim. ' C. CaH8 ''0° 725 , a ~ ~ H 6CzHs, ], possibly CiHm and G H I
E
c",g]
20
B
0
5
15 20 PER CCNT RLACTING
IO
Figure 6-Results '/a
.
25
30
w i t h Ethylene a t 725OC. a n d Atmosphere
'j0
726
z?}, 22 1, L2;.2;/,
6 H * , and GHlo
5- and 6-carbon unsaturated hydrocarb o w CH4, Ha, CsHa , crHr, CiH4 GHs CaH4 1/5 725 Hz, CIHE,CaHs, &Ha, CaHa CdHe, 10% l / 5 726 Ha, CsH,, CaHa, 6-carbon unsaturates, assumed CaHa, C4Hs CZH4, 90% CiHsb l / 5 725 Aromatics, CZH4, Hn (CzHz not determined) a The braces indicate products approximately equal in quantity. b Only one run was made nith 100 per cent GHs. CaHs
10
0
of Initial Cracking Products
1/5
1
(5) The results discussed in the two previous sections bring out the interesting fact that by far more butadiene is formed from ethylene than from propylene. This observation, strengthened by results previously obtained by one of the writers (5),led to the choice of a mixture of butadiene and ethylene for the study of the third step in the reactions leading to benzene. The curves shown in Figures 7 and 8 for cracking of a 10 per cent butadiene-90 per cent ethylene mixture indicate the following reactions between butadiene and ethylene: CiHo and C4He
CzH4 CeHs + Hz +4-C2H4 -+-CeHe + 2H2 --f
It may seem probable that the first one of these is the primary reaction and that the benzene then results by further elimination of hydrogen from the triolefin, CcHe, followed by ring closure: CeHs = '&He
+ Hz
This undoubtedly happens to some extent, but the data suggest that both reactions proceed simultaneously and directly according to the first two equations. Figure 8 shows that some propylene is formed from the mixture of ethylene and butadiene. However, since the concentration of propylene is approximately the same when ethylene is cracked alone (Figure 6), there is no reason to assume that butadiene enters into its formation. The slopes of the curves indicate that secondary reactions are producing quantities of hydrogen and are consuming the benzene and the 6-carbon unsaturates. The data do not show what these reactions are, but they probably include such as
(11), these results permit certain definite conclusions in regard
(6) It should be noted that not all of the butadiene is accounted for in the last series of experiments. It is thought
to the mechanism of cracking the simpler hydrocarbons: 1-That cracking reactions are approximately first order and homogenous is confirmed, even in those cases where the initial products formed point toward a dimolecular reaction
+
CsHe $. C2H4 +CeHsCzH3 Hz and CeH6 f C4He +CldIs f 2Ha
PER CENT REACTING
Flgure 7-Results w i t h Butadiene-Ethylene Mixture at 725O C. a n d 111 Atmosphere
When interpreted in the light of the complete investigation
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1931
If anything, the order is even lower than first. Thus, while the initial products from both ethylene and propylene indicate that the main reactions are dimolecular, the amount of propylene reacting is actually increased threefold by lowering the concentration of the olefin from one t o one-eighth atmosphere by dilution with an inert gas. Also in cracking ethylene, where the main reaction is polymerization of two molecules with simultaneous elimination of hydrogen to form butadiene, it was found necessary to raise the temperature more than 25" C. in order to obtain the same percentage cracking a t atmospheric pressure as that obtained a t onefifth atmosphere, I n other words, instead of varying as the square of the pressure (as a simple dimolecular reaction should), the rate increased even less than the first power of the pressure. The absence of surface catalytic effects was shown by packing the cracking tube with broken quartz. As a matter of fact, the results indicate less cracking in a packed tube than in an open one. 2-It has been established that higher hydrocarbons can be built up from lower ones otherwise than by simple polymerization. Examination of the initial products from the cracking of ethylene and propylene has shown the importance, not generally recognized hitherto, of reactions of the type,
1409
was not sufficient to show any deviation from the data a t 650" C. 5-A study of the table of initial products indicates the steps by which most of the benzene is produced from propane a t the temperatures and pressures used. As a first step, only propylene and ethylene are formed in considerable quantities. These two, in turn, give a number of olefins, diolefins, and even a very small amount of acetylene, any of which might finally form aromatics. However, the main secondary products are ethylene and butylene (about 50 per cent) from propylene, and hydrogen and butadiene (about 70 per cent) from ethylene. From the slopes of the curves it appears probable that the butylene cracks rapidly to form mainly ethylene. The main road toward aromatics, therefore, seems to be through ethylene and butadiene, with the ethylene
2CsHs ----f CzH4 f C4Hs
Other reactions of this type which have been shown to take place are: 2CzH4 +CiHe f Ha 2C8H6 C2H6 f ClH8 CzHr f C ~ H -+ E C6Hs f Hz CzH4 f C4Hs +CeHe f 2Hz
And still others indicated but not definitely proved are:
+
2C3Hs +C?He C4HlO 2C3He ----f CeHio f Hz CHI 2C3H.s +CsHs
+
Reactions of this type may be explained satisfactorily on the assumption that free radicals exist. Thus, CZHI--f ( CHZ)
+ (CH2); (CHZ)f CZH4 +C ~ H B
In the absence of any definite proof, however, it is largely a matter of choice whether one prefers to explain the mechanism on the basis of free radicals or activated molecules. 3-That butylene is a probable intermediate in the formation of butadiene, as has recently been stated in t he literature (6), is disproved. -In the cracking of propylene, there is produced about five times as much butylene as butadiene; in the cracking of ethylene, butadiene is formed in quantities about fourteen times as large as butylene. This would be inconsistent if butadiene were formed by the intermediate production of butylene. Also the method of plotting the results, as explained in the foregoing, would clearly show butadiene RS a secondary product if it were formed through an intermediate as stable as butylene. 4-In the case of propane, changing the temperature from 725" t o 650" C. reverses the order of importance of the two main reactions. That is, a t the lower temperature, dehydrogenation to propylene predominates: C~HS = C3H6 f HZ
whereas a t the higher temperature there is more tendency for the C-C bond t o break with the formation of ethylene and methane: C3HS
+
C Z H ~ CH,
I n order to study the effect of a still lower temperature, a run was made a t 570' C. However, this one experiment
i
PER CENT REACTING
L
Figure 8-Result8 with Butadiene-Ethylene Mixture a t 725O C. and l/6 Atmosphere
coming both directly and indirectly from propane. Only a small amount of butadiene appears to be formed directly from propylene (5 moles per 100 moles of propylene reacting). Finally, it has been shown that butadiene and ethylene polymerize with liberation of hydrogen, the main products of this reaction being benzene and a 6-carbon unsaturate, assumed to be C6Hs. The data suggest that part of the benzene is formed directly while part of it originates by further dehydrogenation of the intermediately formed triolefin, CbHs. The complete reaction scheme in going from propane to benzene may therefore be written as follows:
It is to be noted that all these products are decidedly unstable and subject t o further reaction under the conditions a t which they are being formed. Although it is possible to obtain high yields of ethylene and propylene from propane by operating with a short time of contact, so as not to leave sufficient time for further reactions to occur, it becomes increasingly difficult to halt the reaction a t subsequent steps. Just as butadiene reacts with ethylene to form benzene, so benzene readily undergoes further reaction to yield alkylated derivatives, naphthalene, and eventually tarry compounds of high molecular weight. Acknowledgment
The writers wish to acknowledge the assistance rendered by P. R. Konz in obtaining part of the experimental data on propane cracking.
Vol. 23, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
1410
Literature Cited ( 1 ) Rerthelot, Comfit. rend., 62, 905 (1866); 63, 788,834 (1867). (2) Davidson, J. IND. END.CHEM.,10, 901 (1918). (3) Davis, I b i d . , Anal. Ed., 1, 61 (1929). ( 4 ) Davis, Crandall, and Higbee, Ibid., Anal. Ed., 3, 108 (1931). (5) Frolich, Simard, and White, IND. ENG.CHEM.,22, 240 (1930).
(6) Hague and Wheeler, J . Chem. SOC.,1923, 378. (7) Htrslam and Chappell, IND.END.CHEW.,17, 402 (1925). (8) Hurd and Spence, J . Am. Chem. Soc., 61,3561 (1929). (9) Ipatiev, Ber., 44,2978, 2987 (1911); 46, 1748 (1913). (IO) Lebeau and Damiens, Ann. chim., 8, 221 (1917). (11) Schneider, Doctor's Thesis, Mass. Inst. Tech., 1931. (12) Thiele, Ann., 308, 337 (1899).
Preparation of Potassium N.itrate from Solid Potassium Chloride and Nitrogen Peroxide' Colin W. Whittaker, Frank 0. Lundstrom, and Albert R. Merz FERTILIZER AND FIXEDNITROGEN INVESTIGATIONS, BUREAUOF CHEMISTRY AND SOILS,DEPARTMENT OF AORICULTURE, WASHINGTON, D. C.
The direct conversion of solid potassium chloride to solid potassium nitrate by the reaction of the chloride with gaseous nitrogen peroxide is suggested for the preparation of potassium nitrate. Thermodynamic calculations indicate that the free energy change is favorable a t ordi-
MONG the more promising salts for use in concentrated fertilizer mixtures is potassium nitrate. This salt contains two fertilizing elements, is much less hygroscopic than other fertilizer nitrates, and has recognized merits as a fertilizer. The present paper describes preliminary work on a method for its preparation from solid potassium chloride and nitrogen peroxide, the cheapest forms of potash and nitric nitrogen. Mehring, Ross, and Merz (17) have shown that, when nitrogen peroxide reacts with a saturated potassium chloride solution, hydrogen chloride is evolved, and a solution containing potassium nitrate and nitric acid is formed. When, however, in cyclic operation, the acid concentration reaches a certain value, nitrosyl chloride is formed and comes off with the hydrogen chloride. Now if nitrogen peroxide could be made to react with solid potassium chloride, there is presented the possibility of converting the solid chloride directly into the solid nitrate and of obtaining all the chlorine as nitrosyl chloride according to the reaction:
nary temperatures and that the reaction is exothermic. Experiments are described in which it was found that the reaction takes place a t ordinary temperatures in the presence of a small amount of water with good yield. Nitrosyl chloride is produced simultaneously.
A
2N02 (9)
+ KCI
(8)
= KNOs (a)
+ NOCl
There remains to be determined the free-energy change in bringing KNOJ in infinite dilution to the solid state or (since the activity of a solute in its saturated solution equals that of the solid solute) to saturation. The activity of potassium nitrate at the eutectic point (270.1 K.) is obtained from freezing-point data by means of the formula (16):
(g)
Thermodynamic calculations indicate a free-energy change favorable to the occurrence of the foregoing reaction at ordinary temperatures, and the experiments described later show that in the presence of a little moisture this reaction actually takes place. Thermodynamic Considerations
In the following calculations 4.182 joules have been taken
inwhichj = 1 where
Y
= number of ions formed
X = molal freezing-point lowering at infinite dilution by non-dissociating molecule ( = 1.858)
and
= freezing-point lowering rn = molality a = activity of solute
The value of the f i s t integral of Equation 8 up to m is given by
as equivalent to 1 calorie. To obtain the free-energy change of the reaction, KClG)
+ 2NO:k)
= KNOa(,)
+ NOCIk)
the free energy of formation of KCl(,) was first determined from the free energies of Na(.), K(,), KCl(.), and NaCI(,) by means of the free energy of formation of NaCl(.). Using the data presented in International Critical Tables (W), i t is found that AFftcl(.) = -97,361. The free energy of formation of KN03 in infinite dilution by the reaction between KCI(.) and NOz(,) is now easily determined. 1 Received
July 2, 1931.
-2 UX m
-jdlogrn
=
= 0.01
- 2.303a -(0.01)"
where a and p = 0.565 and 0.427, respectively (15).
and from m = 0.01 to m = 1.247 (the molality a t the eutectic point) by graphical integration. The value of the second integral is obtained by graphical integration also. By substitution of the value of a at saturation (m = 1.247), from Table I, in the equation A F = RT In a ( 1 3 )
it is found that Kf NOS- = KNOa(,); AFar0.1
+
-912
(94
