Determination of Unsaturates in Hydrocarbon Gases The Hydrogenation Method RICHARD F. ROBEY AND CHARLES E. MORRELL Esso Laboratories, Standard Oil Development Co., Elizabeth, N. J.
methods currently used t o determine the conof unsaturated compounds in gaseous hyM O Scentration T drocarbon mixtures were introduced years ago. The procedures have been modified occasionally to increase accuracy, but some methods give results which are inherently inaccurate and are retained either because of convenience or because errors have not been recognized or investigated. Recent improvements in older refining processes and the introduction of new processes, such as polymerization, alkylation, and the production of synthetic rubber raw materials, which involve olefinic compounds, have demanded higher accuracy in the analysis for these substances. A method for determining unsaturates in gaseous hydrocarbons by catalytic hydrogenation has been described by McMillan, Cole, and Ritchie (6). It is claimed to give results accurate within a few tenths of 1 per cent-and to be as rapid as others in general use a t present. The method is apparently satisfactory from the chemical standpoint: the hydrogenation is complete, free of side reactions, and sufficiently rapid for routine use, and the catalyst is easily saturated with gas and produces no interfering effects. However, the method, as originally described by McMillan, Cole and Ritchie, is based on the assumption that the lower hydrocarbons are ideal gases. Since these compounds are known to deviate considerably from the ideal gas l a m , the nature of the errors introduced by the assumption was studied. I n order to formulate a convenient procedure for correcting the analytical results for such deviations, it is necessary to consider the nature and sequence of the gas rolume measurements made during an analysis. These are:
I02
$
iIO, W
100
0
10
20
30
4C
50
60
70
80
MOLE PER CENT OF ISOBUTANE
FIQURE1. COMPR~SSIBILITY OF MIXTURESOF AND HYDROGEN
T.4BLE
Fraction
cz Ca C4
Cb a
I.
COMPRES8l[BILITIES OF Gas
Ideal gas Hydrogen Methane Ethylene Ethane Acetylene Propylene Propane Isobutane Isobutene 1-Butene n-Butane trans-&butene cis-2-butene Pentane
SOME GASES~
Compressibility at 760 Mm 00 c . 30' C.b 1.0000 1.0000 0 9994 (8 ... 1 0024 (81 1.0078 ( 2 ) 1 . ooii (6) 1.010 (1) 1 . 0 1 0 (9) 1.0204 ( 1 ) i . o i 5 i (6) 1.0207 (9) 1.0158(S) ... 1.0290 (S) ... 1.0288 (6) ... 1,0287 (6) ... 1.0328 ( 9 ) ... 1.0319 6) ... 1.0324 {6) ... 1.0418 (600 nim.)
:
...
compressibi,ity equals: PV extrapolated to 5ero pressure
pu at atmospheric pressure ' 6 Data of (6)have been obtained from the second virial coefficient, e, by
the approximation ' Compressibility = 11
+ (2.36 X
10-6 O)]
-1
I n general, however, the analytical procedure must be applied to hydrocarbon samples which are mixtures of individual hydrocarbons. The compressibility of a hydrocarbon mixture depends upon the compressibilities and relative amounts of the individual components. It is common practice, however, to distill such mixtures into fractions, each containing hydrocarbons of the same number of carbon atoms, prior to the determination of unsaturation. Strictly speaking, it is necessary to know the compositions of such fractions in order to calculate composite compressibilities from the values given in Table I. It has been found, however, that an average value may be chosen which expresses the compressibility of a given fraction with an accuracy sufficient for the purposes of the analysis. Thus, an average value of 1.031 has been chosen for the compressibility of butanebutylene fractions at 30" C. and 760-mm. pressure. I n general, the compressibility of a binary mixture of dissimilar gases such as hydrogen and a hydrocarbon does not vary linearly with the composition of the mixture. Leendertse and Scheffer (4) have discussed a method for approximating the deviation of binary gaseous mixtures from the ideal gas laws involving the compressibilities and the a and b 90 100 values of the van der Waals equation for the individual components. From their discussion, the following approximation of a binary ISOBUTANEhas been derived for the compressibility, gaseous mixture:
Volumes of hydrogen and hydrocarbon gas are measured se arately, the hydrogen being in excess. %he hydrogen and hydrocarbon sample are mixed and reacted over the catalyst, and the volume of the resulting gas, which consists of saturated hydrocarbons and excess hydrogen, is measured.
t ='
Hydrogen conforms to the ideal gas laws within the errors of the analytical measurements and hence requires no correction. However, corrections must be made for deviation of the hydrocarbon sample from ideal gas behavior and deviation of the mixture of saturated hydrocarbons and excess hydrogen resulting from the hydrogenation reaction. The extent of deviation of the measured volume of a pure gas from the ideal gas volume is conveniently expressed by the compressibility of the gas under the conditions of measurement. The compressibilities of hydrogen and a number of hydrocarbons a t 760-mm. pressure are presented in Table I.
November 15, 1942
ANALYTICAL EDITION
881
cylinders were used. Mixing was accomplished by several passes between the burets, all gas volume measurements
TABLE11. EXPERIMEXTAL RESULTS Expt. No.
Observed Volumes Hydrogen Isobutane Mixture M1. M1. M1.
Calculated Ideal Volumes CompressiIsobutanea Mixtureb bility e Isobutane M1. MI. M o l e yo
being made in One
1
Discussion The experimental results are summarized in Table 11. The compressibilities calculated from these data are plotted in Figure 1 together with the values calculated from Equation 1. The observed and calculated curves differ somewhat. The authors prefer t o express the unsaturate content of a given sample as the number of moles of hydrogen required to saturate 100 moles of the sample. For convenience, this value will be referred to as the mole per cent unsaturation, M . It is evidently defined by the following equation:
Q
5
b
buret'
Observed volume X 1.0290 = ideal volume of isobutane. Ideal volume of +butane volume of hydrogen = ideal volume of mixtuie. ideal volume of mixture = compressibility. Calculated by observed volume of mixture
+
M = 10O(V1C1
+ VP - V3Cd
( 3' VlC1 in which VI, Vz, and V 3are, respectively, the experimentally measured volumes of hydrocarbon sample, hydrogen, and hydrogen-saturated hydrocarbon mixture resulting from the hydrogenation reaction; and C, and Cs are the corresponding compressibilities. The degree of unsaturation as calculated by McMillan et al. without correcting for the nonideality of the gases will be called the volume per cent unsaturation, U:
U = IOO(V1
20
40
60
60
W
120
140
IM)
180
2W
VOLUME PER CENT UNSATURATION FOUND
FIGURE2. CORRECTION CHART FOR HYDROGEK4TION BUTANE-BUTYLENE-BUTADIENE FRACTION
OF
+ vz - V3)
(4)
VI
U is readily calculated from the analytical measurements. For this reason the authors have chosen to derive the corrections for gas nonideality in terms of an additive quantity, D, such that: M
Various volume ratios of hydrogen t o sample
=
U
+D
(5)
B y combining Equations 3,4, and 5 the following expression is derived:
where p1 and p z are the partial pressures of the two gases in the mixture, C1 and Cz are their respective compressibilities, and A is calculated by means of the following equation: = p,pz(RT)-2[2&*
- RT(b1
+ bdl
Accordingly, in Equation 6, D is determined (for a given value of C,) by the volume per cent unsaturation, U , of the
12)
I06
When p is expressed in atmospheres, R = 3.66 X mole volume-atmospheres degree-' mole-' For comparison with the calculated values the compressibilities of mixtures of hydrogen and isobutane were determined in the present work.
c
Experimental
L
The experimental RTork consisted of mixing accurately measured volumes of isobutane and hydrogen and measuring the volumes of the resulting mixtures. For these measurements the two 100-m1. burets of the hydrogenation apparatus (6)were employed, t h e catalyst tube being removed and replaced by a straight tube of known volume. The two joints TTere fitted glass-to-glass with rubber tubing. Although this technique is admittedly less precise than other methods described in the literature, it is sufficiently accurate to determine the compressibility-compohition curve within the experimental error of the analytical results to which the data are applied. Chemically pure isobutane and hydrogen from commercial
35
I04
d I03
m
g 102 0
0
IM
o
io
20
M
40
50
60
m
80
90
IW
MOLE PER CENT HYDROCARBON
FIOURE 3. CALCULATED COMPRESSIBILITY CURVESFOR MIXSATURATED HYDROCARBON GASESWITH HYDROQEN
TURES OF
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 14, No. 11
+O 8
TABLE111. ANALYBISOF PUREISOBUTYLENE AND BUTADIENE Volume of
tO4
Volume of Sample
volume Per Cent Unsaturation Found
Correction from Figure 2
Corrected Value
1.8 3.5 9.5 4.0
102.3 101.2 100.0 204.7
-2.1 -1.4 -0.2 (calcd.) -4.8
100.2 99.8 99.8 199.9
L!E!EEE
0 W
n
Gas
d: 0 0 W
Isobutylene
Z-04
Butadiene
z
0
y -00 L
0 -1 2
-1
8
0
20
40
bo
80
W
160
140
120
200
110
VOLUME PER CENT UNSATURATION FOUND
FIGURE 4. CORRECTION CHARTFOR HYDROGENATION OF ETHANE-ETHYLENE FRACTION OR ACETYLENE Various volume ratios of hydrogen t o sample
I
I
,
I
/
I
FIGURE 5. CORRECTION CHARTFOR HYDROGEKATION OF PROPANE-PROPYLENE-PROPADIENE FRACTIOK Various volume ratios of hydrogen to sample
v
sample, the ratio 2, ' VI of the volumes of hydrogen and hydrocarban sample taken initially in an analysis, and the compressibility, C3, of the hydrogen-saturated hydrocarbon mixture from the hydrogenation reaction. Values of C3 can be easily read from a compressibility-composition curve of the type given in Figure 1 if the composition in terms of the mole percentage, m, of saturated hydrocarbon in the final mixture is known. However, for purposes of reading values of C3 from the compressibility-composition curve, it is sufficiently accurate to calculate it thus: m =
100 v1 - 0.01 I'V?
v,+ v*
Samples of highly purified isobutylene and butadiene were analyzed. The isobutylene was prepared by dehydrating redistilled C. p. tert-butylalcohol, melting a t 24.1 o C., by means of c. P. anhydrous oxalic acid, condensing the resulting gas, and redistilling in a 60-em. (24-inch) vacuum-jacketed column fitted with Stedman-type packing. A middle fraction with a boiling range of 0.1 was taken for use. The butadiene was obtained from a commercial source and analyzed 99.4 per cent conjugated diolefins by absorption in molten maleic anhydride. The two purified gases mere hydrogenated in the presence of various volume ratios of hydrogen in the prescribed manner. The results are given in Table 111. All values are the average of two or more determinations agreeing within 0.2 per cent unsaturation or less. Since the corrected values are consistent and in good agreement with the probable purity of the gases, i t is felt that these data offer confirmation of the validity of the correction curves Corrections are also desirable for ethane-ethylene, propanepropylene, and pentane-pentene fractions. The pentanepentene fraction, although liquid at ordinary conditions, is easily analyzed in the gas phase after vaporization under reduced pressure and blending with a gas such as hydrogen. I n Figure 3 are shown the curves for the compressibilities of mixtures of ethane, propane, and n-pentane, respectively, with hydrogen under ordinary conditions, as calculated by the approximation Equation 1 mentioned above but adjusted by a factor equal to the ratio of the calculated and observed deviations from linearity as found in the isobutanehydrogen system (Figure 1). Although this method of deriving these curves is admittedly crude, the resulting shift of the ethane and propane curves is within the experimental error of the analytical data t o be corrected. The pentanehydrogen curve is verified by experimental work described below. The hypothetical value for the compressibility of pentane a t 30" and 760 mm. was obtained by extrapolating the data of Jessen and Lightfoot (3) from lower pressures. Using Equation 6 corrections for the hydrocarbons containing two and three carbon atoms have been derived and are presented in Figures 4 and 5, respectively. I n the case of the pentane-pentene fraction, the preliminary operations involving vaporization and blending with hydro-
t08 W
n w 00
Following the above procedure, it is possible to calculate v values of D for a range of values of U and 2. Figure 2 shows v1 results of such calculations for butane-butylene fractions. A value of 1.031 was assumed for C1 regardless of the value of U for the original hydrocarbon sample. It has also been assumed that the C3 values depend only on the per cent of butane in the final mixture, regardless of whether it is nbutane or isobutane. I n order to obtain some experimental confirmation of the corrections calculated in the above manner, a number of pure unsaturated hydrocarbons were analyzed by the hydrogenation method.
2 r 08
w
E- 6
s
-2 4 0
20
40
60
60
IW
120
140
VOLUME PER CENT UNSATURATION FOUND ON BASIS OF
I60
180
200
C5 C O N T E N T
CHARTFOR HYDROOESATION OF PENFIGURE6. CORRECTION TANE-PENTENE-PENTADIENE FRACTION I n 33.8 mole per cent blend with hydro en, using equal volumes of blend and additional %ydrogen
November 15, 1942
ANALYTICAL EDITION
TABLEIV. DETERMINATION OF UNSATURATIOK OF FIYECARBOX-ATOM HYDROCARBOXS AND MIXTURESTHEREOF BY HYDROGENATION Substance
%aoa
Found Corrected Synthesis M o l e Per Cent Cnsaturation b 202 0 199 6 200 0
Isoorene. nure 1.4217 Miiture ' 37.0% trimethylethylene 1.3871 28.0% piperylenes 1.4311 35.0% isoprene 1.4217 165 0 163 2 163 0 Xixture 46.4% n-pentane 1.3577 1 6 . 5 7 trimethylethylene 1.3869 37.1 piperylenes 1.4305 90.7 90.3 90.7 a Best literature valuea: isoprene, 1.4216; mixrd piperylmez. 1 4309: trimethylethylene, 1.3869: n-pentane, 1.3578 6 On basis of hydrocarbon content.
%
gen are carried out at constant volume, the pressure being measured. The liquid hydrocarbon is vaporized into a previously evacuated vessel until a pressure of 250 mm. is attained. Hydrogen gas is then admitted until the total pressure rises to 750 mm. The partial pressure of the hydrocarbon in the blend is well below the saturation pressure under the conditions of measurement. The blending operation is readily adapted to the ordinary Podbielniak distillation apparatus. It has been found convenient in this laboratory to prepare all blends of the pentane-pentene fractions a t these pressures and use a single correction factor (+0.5 mole per cent) for the composition of the blend. There are obviously many other ways in which the blending can be done, but the addi-
883
tive correction factor applies only to blends made up in the exact manner described. The corrections for the hydrogenation method when applied to a 33.8 mole per cent blend of pentane-amylene fraction in hydrogen have been calculated and the resulting corrections are plotted in Figure 6. These corrections are based on the use of equal volumes of additional hydrogen and blend in the analysis. To check the method, a series of pure five-carbon-atom hydrocarbons, and mixtures thereof, n-ere prepared, blended with hydrogen, and the unsaturation determined by hydrogenation. The results, presented in Table IV, offer very good confirmatory evidence for the validity of the method and the corrections.
-4cltnowledgmeiit The authors wish to thank H. K. Wiese for obtaining experimental data, IT. J. Troeller, Jr., and D. RI. Mason for some independent work on the corrections for Cd fraction, and the Standard Oil Development Company for permission to publish.
Literature Cited Batuecas, T., J. chim. phys., 31, 165-83 (1934). (2) International Critical Tables, Vol. 111, p. 3, New York, McGrawHill Book Co. (3) Jessen, F. W., and Lightfoot, J. H., IND.ENG.CHEM., 30, 312(1)
14 (1938).
(4)
Leendertse, J. J., and Scheffer, F. E. C., Rec. t
~ w chirn., . 59, 3-13
(1940).
(5)
McMillan, W. A., Cole, H. d.,and Ritchie, A. V., IND. ENQ. CHEM., ANAL.ED.,8, 105-7 (1938).
(6) Roper, E . E . , .J. Phuvs. C'hern.. 44. 83.5-37 f l R 1 0 ) .
Contact Sulfuric Acid Manufacture Evaluation of the Reich Test GERRIT DRAGT AND K. W. GREEXYAN (;rasselli Chemicals Dept., E. I. du Pont de Nemours & C o . , Inc., Cleveland, Ohio
T
HE Reich test for sulfur dioxide is commonly used in the determination of gas strength in sulfuric acid manufacture. I t s flexibility, ease of operation, and simplicity make it a desirable method for the routine testing of gas strengths in different parts of the gas purification or conversion system. Various modifications of the original test proposed by Reich (11) both as t o apparatus (2, 3, 4,7 ) and to the nature of the absorbing solution (6, IO), have been described, but so far as could be determined, no study of the accuracy and precision of which the test is capable has been reported. The present work evaluates the factors that influence the accuracy and precision of the test as used in brimstone-burning contact sulfuric acid plants. The test as described is applicable to chamber burner gases, but must be modified for use with chamber gases containing oxides of nitrogen.
Apparatus and iMaterials Keich test apparatus (Figure 1). Woulff bottle, 4-liter capacity, equipped with thermometer, inlet, and siphon tubes. In operation, this bottle is filled with water to the mark shown. Reich bottle, 350-ml. capacity, equipped with a 2-mm. bore glass tube with attached stopcock. Ga8 pressure bottle, 350-ml. capacity, equipped with T and vent tubes. Graduate, 125-ml. caparity, graduated in 0.5-ml. units. Oisat gas analysis apparatus, Bureau of Mines type. Sulfur dioxide gas mixture, 12 per cent sulfur dioxide, balance nitrogen. This gas mixture was stored in a regulation size cyl-
inder and kept above 21.11" C. (70" F.) to prevent liquefaction of sulfur dioxide. Barometer, mercurial. Chromic acid solution, 50 per cent aqueous. 0.1 N iodine solution, prepared from c. P. iodine crystals and potassium iodide (20 grams per liter). This solution was standardized against Bureau of Standards arsenic trioxide. Starch solution, 2 per cent.
Procedure After flushing the sampling line up to the tip of the inlet tube of the Reich bottle with the gas t o be tested, the stopcock was closed and the gas was analyzed according to the following procedure: Ten milliliters of 0.1 N iodine were added to the Reich bottle containing 175 ml. of distilled water, 5 ml. of starch solution, and sufficient iodine to give a light blue color. The Reich bottle was replaced in the Reich assembly, the clamp of the siphon tube removed (the siphon having previously been set), and the water from the Woulff bottle permitted to run to waste. When the flow of water had stopped, the graduate was placed under the siphon tip and the stopcock of the Reich bottle opened. The Reich bottle was shaken during the course of the absorption and the gas permitted to pass until the solution in the Reich bottle again assumed its original light blue color. As the end point vias approached, the stopcock was closed, and then momentarily opened to permit the passage of additional small volumes of gas by a rapid turn through a 180" angle. The clamp was again replaced on the siphon tube, after the system had reached its original pressure state as indicated by the cessation of water flon, and the volume of water was read to the nearest 0.5 mi. This volume of water represented the volume of gas analyzed with the
