409
JUNE 1947 the signs of Equations 6 and 7b must be reversed. I n some applications for asphalts and like materials, engineers are concerned with a so-called "pull-away" temperature, a t which the forces of contraction overcome the adhesion forces within the material. An interesting study of this action might be made on various materials with the method described for contraction. The device may also be used to study the change in expansion characteristics near the freezing points of materials showing sharp melting points and near the boiling points of liquids which may be solidified a t a temperature above -35" C. SUMMARY
The method described is similar in principle to the A.S.T.M. test for volumetric expansion of bituminous materials. Certain modifications in both apparatus and procedure result in the following advantages over the standard method : Results can be obtained in a shorter time because the system is easier to manipulate, and the sample preparation is simplified.
The method requires only ordinary laboratory glassware and equipment. Introduction of mercury under vacuum conditions ensures complete elimination of gaseous voids external to the sample. Contraction as well as expansion measurements can be made. The method avoids the health hazard of vaporized mercury. ACKNOWLEDGMENT
The writers wish to express their appreciation to C. F. Hill, manager, and to L. J. Berberich, section head, of the Insulation Department of Westinghouse Research Laboratories for their helpful suggestions in this work. LITERATURE CITED
(1) Abraham, H., "Asphalts and Allied Substances," Vol. 11, 5th ed., pp. 1033-50, New York, D. Van Nostrand Co., 1945. (2) Am. SOC.Testing Materials, A.S.T.M. Standards, 1'01. 111, p. 1126, Specification D 176-421,1942.
Laboratory Low-Temperature Fractional Distillation Optimum Charging Rates C. E. STARR, JR., J . S. kNDERSON, AND V. JZ. D.IVIDSOU Esso Laboratories Standard Oil C o m p a n y of .Yew Jersey, Louisiana Division, Baton Rouge, La. The analysis of mixtures of light hydrocarbon gases and gases such as hydrogen, nitrogen, oxygen, and carbon monoxide is generally conducted first by segregation of fractions by low-temperature distillation and second by analysis of the individual fractions. In such procedure the lightest components are taken overhead from the distillation column during charging of the samples. Published procedures for operation of low-temperature distillation equipment do not ordinarily include sufficiently detailed instructions for charging samples of widely varying compositions to ensure obtaining overhead fractions of maximum purity with minimum charging time.
T
H E analysis of mixtures of light hydrocarbon gases and gases such as hydrogen, nitrogen, oxygen, and carbon monoxide is generally conducted first by segregation of fractions by low-temperature distillation and second by analysis of the individual fractions. In such type of analytical procedure thr lightest components are taken overhead from the distillation column during the charging of the Pample. Published procedures (3, 4,5 ) for operation of lowtemperature distillation equipment do not ordinarily include sufficiently detailed instructions for charging samples of widely varying compositions to ensuw ohtaining overhead fractions of maximum purity x i t h minimum charging time. With the aim of improving such analyses a study a-as conducted to establish a more uniform procedure for handling samples of complex gas mixtures when conducting low-temperature fractional distillation, a necessary step in the segregation of compounds for specific chemical and physical tests. When such a gas mixture is charged to low-temperature distillation equipment, usually cooled by liquid nitrogen (boiling point - 195.8" C.), an overhead fraction is first segregated to contain, if present, all the hydrogen (boiling point, -252.7' C.), nitrogen, carbon monoxide (boiling point, -192" C.), and oxygen (boiling point,
A study has been made of charging rates for samples ordinarily encountered in petroleum refining plants. Overhead samples taken at different charging rates have been analyzed by mass spectrometer to determine the amount of contamination w-ith higher boiling constituents. The results obtained indicate that charging rates much higher than ordinarily employed may be used with equal accuracy; use of the higher charging rates effects considerable economy of time. Supplementary data obtained also show- that for special applications distillation rates may be increased ' severalfold over rates now generally employed.
- 183CC.),andaportio~ioftheniethane (boilingpoint, - 161.4"C.), ethylene (boiling point, - 103.7' C.), and ethane (boiling point, -89.0' (2.). During the charging period and while the gases listed above are being removed, the balance of the hydrocarbons are condensed into the still pot, and upon completion of the charge, are fractionally distilled. For accurate analysis of the overhead fraction it is desirable that i t be segregated in such a manner that hydrocarbons of three or more carbon atoms are excluded. Likewise for accurate determination of hydrocarbons of three or more carbon atoms in lean gases, it is important that the overhead fraction should not contain even small amounts of the heavier hydrocarbons, since such small amounts may represent appreciable portions of the total present. The overhead fraction is generally analyzed by chemical and combustion analyses on such equipment as the U. S. Steel (6) or Burrell ( 2 ) apparatus. I n this type of analysis ethylene is removed by chemical absorption and methane and ethane are determined by combustion. When methane is burned with oxygen the sample undergoes a contraction in volume equal to twice the volume of methane; the carbon dioxide produced is equal in volume to the methane. The water formed condenses and is of negligible volume. CH, 202 +COn 2H20 (1)
+
+
V O L U M E 19, NO. 6
410 The amount of methane present is equal to:
3s
m = -c + x 3 \\here
m
=
ml. of methane, c
=
x = ml. of contraction.
(2) ml. of carbon dioxide, and
When ethane is burned with oxygen, the sample undergoes a contraction equivalent to 2.5 times the volume of ethane present: the carbon dioxide produced is equal to twice the volume of ethane:
2CzHs
+ 702 +4coz + 6HzO
there are only the two determined values for computation.
For example, i f 1 ml. of propane is included in an overhead fraction containing 10 ml. of methane and 4 ml. of ethane, the usual combustion analysis would report values of 9 ml. of methane and 6 ml. of ethane, thus giving very appreciable errors. Traces of
rahydrocarbons in the mixture affect the results even more adversely.
This study has been made to measure the influence of charging rates and mixture composition on contamination of the overhead fractions by hydrocarbons of three or more carbon atoms.
(3) PROCEDURES
and the amount of ethane present is equal to:
where
e = ml. of ethane
Vhen methane and ethane are burned together the volunies present are calculated from the reactions shown in Equations 1 and 3 : e = - 4c - - 22 (5) 3 and m=c-Ze (6) Thus carbon dioxide production and gas contraction provide two determined values t o solve for the two unknowns (amounts of methane and ethane present). If, however, more than ttTo hydrocarbons are present during combustion the r e d t s are in error,
Table I.
Charging R a t e Nominal Actual hverage C c . / m i n . Cc./min. Mm./joo 120.0 62.25 100 112.0 58.20 100 116.0 60.00 100 159.5 82.80 150 160.1 83.30 150 108.0 208.0 200 104.00 200.0 200
Charging Rate Study of Low-Temperature Fractional Distillation (Sample A ) [Samples containing 5070 (total hydrocarbon basis) methane, ethylene, a n d ethane] AIS Analysis of Overhead Fractions Heavy Hydrocarbons in over- overHydrocarbons Distilled Over- Overhead on Total Total Sample Basis head head Fractions head Chart I1 CI C2 Ca Ch Csf No, Cz Ca C4 Cj+ Cz Ca C4 Csf I Travel Mole % M o l e 70 Mole % iWole I 0 . 0 8 0 . 2 0 0.47C 0.00 0 . 0 1 0 . 0 3 0 . 0 7 0.00 2283 14.8 a 34.68 1 1 . 2 2 1 . 2 1 4 . 2 4 . 0 I 0.09 0.13 0 . 1 2 0.07 0 0 1 0 0 2 0.02 0 . 0 1 16.1 2085 33.88 10.9 2 0 . 3 14.2 4.7 12.6 1885 I 0 . 0 5 0 . 0 3 0 . 2 2 0 . 0 4 0.01 0.00 0 . 0 3 0.00 37.8b 10.7 20.3 14.2 4 . 4 I 0 . 0 8 0.27 0 . 0 8 0.07 0.01 0.04 0.01 0 . 0 1 .14.6 2324 3 6 . 2 b 11.0 2 0 . 9 1 4 . 1 3 . 2 * 3 4 . 5 b 10.9 2 0 . 2 1 4 . 2 5 . 1 I 0 . 1 2 0 . 0 6 0 . 0 7 0 . 0 5 0.02 0 . 0 1 0 . 0 1 0 . 0 1 15.1 1818 15.2 I 0 . 0 0 0.00 0 . 1 6 0 . 1 0 0 . 0 0 0.00 0 . 0 2 0 . 0 2 2134 34.38 1 0 . 8 20.9 14.2 4 . 6 I 0 . 0 0 0 . 0 5 0 . 0 4 0 . 0 0 0.00 0.00 0 . 0 1 0 . 0 1 15.3 2262 a 35.18 1 0 . 6 2 0 . 5 1 4 . 0 4 . 5 Q
Q
Cylinder pressured t o 50% noncondensables I 5.6 I1 19.6 7 . 1 13.5 9 . 5 2.9 I 4.9 I1 22.1 7.3 14.3 10.1 3 . 2
106.7
55.3
2271
41.8
100
99.8
51.7.
1972
38.1
150
141.5
73.3
1788
43.5
U
23.56
7.3
13.4
9.4
150
142.8
74.0
2946
82.2
Li
34.36
7.2
13.6
9.6 3.1
200
166.9
86.5
1934
44,i
4.8
17.4
7.1
13.6
9.3
3.1
200
189.9
98.4
2128
42.2
3.7
20.6
7.3
13.6
9.4
3.2
200
209.4
108.5
2198
39.7
7.8
17.7
7.0
14.0
9.3
4.5
241.3
125.0
2261
39.2
7.2
13.6
9.2 3.4
1968
42.9
6.6
17 3
7.6
13.0
9.2
3.4
2024
41.2
5,2
20.2
7.6
13.8
9.4
2.6
7.2
13.7
9.4 2.0
100
250
127.3 250 245.7, Results using distillation r a t e recommended by Office of Rubber Reserved Mass spectrometer analysis of total sample
a 8
The equipment used in this study consisted of a Podbielniak Hyd-Robot distillation apparatus ( 5 ) , a Consolidated Engineering Corporation mass spectrometer (?), and conventional lahoratory equipment. It as necessary to standardize the charging, distilling, and sampling procedures employed and to adhere to these standards rigidly. I n general, the work involved the charging of samples into the distillation equipment a t various rates, the eampling of “overhead fractions” for analyses, and the analyses of these segregated fractions. (In this article the overhead fraction is considered to be that portion of the sample which is removed from the column while charging, plus that which is removed during distillation or flashing prior to the appearance of the methane plateau.)
(27.4)
67.7
2.9
I
0.00 0 . 0 3 0 . 0 2 S o t analyzed 0 . 0 5 0 . 0 9 0 . 1 1 0.02 0.02 0 . 0 3 0 . 0 8 0 . 1 2 0 . 0 2 0.00 0.11 0 . 1 2 0.10 0.02 0.04
I
0.00
0.17
0 . 1 2 0.00
0.00
0.05
0 . 0 1 0.00
I I1 I I1 I I1 I
0.08 0.11 0.05 0.04 0.02
0.03 0.06 0.06 0.14 0.00
0.00 0 . 0 4 0 . 0 2 0.00 0.01 0.02 0.03 0.00 0.03 0.01 Kot analyzed 0 . 0 3 0 01 0 00 S o t analyzed 0 . 0 4 0 01 0 04 0 . 0 4 0 00 0.00
0.01 0.00 0.04 0.01 0.00
0.01 0.00 0.02 0.00 0.01
0.04
0.01 0.00
0.03 0.01 0.13 0 01 0.03 0.01 0.02 0 03 0.03 0.04 0.00 0.02
0.11 0.00 0.02 0.00 0.07 0.00 0 01 0.00 0.10 0.00 0.10 0.00
I1
I I1
100
104.8
64.3
2212
71.8
100
105.1
54.5
2232
71.0
Cylinder pressured t o 757, noncondensablee I 3.3 6.1 I1 4.2 1 . 5 6 0 7.1 I 6.2 I1 3.3 7.3 4.3 1.5 6.4
150
164.8
80.2
2174
72.9
3.6
8.3
3.5
6.1
4.1
1.5
150
154.2
79.9
2187
72.6
4.6
7.6
3.2
6.0
4.3
1.7
250
243.6
126.2
2130
74.4
3.7
6.9
3.0
6.2
4.2
1.6
13.2.5 2148 7.5 73.9 3.7 250 255.7 Results using distillation rate recommended b y O 5 c e of 73.3 10.6 Rubber Reserved 2160 Mass spectrometer analysis 87.0 of total sample First analyses r u n , P a r t I 1 not segregated for analysis. CI plus overhead cut P a r t 11.
3.2
6.1
4.0
1.6
4.0
6.2
4.6
1.3
I
I1
I I1 I I1 I I1
0.06 0.03
0.00 0.12 0 . 0 9 0.07 0.03 0.12
0.06 0.03 0.03 0 06 0.07 0.05 0 04 0.04 0.11 0.01 0.07 0.03
0.02 0.08 0.04 0.06 0.02
0 . 1 6 0.10 0.05 0.06 0.07 0.03 0 04
0.06
0.02 0.08 0.09 0.11 0.02 0.03 0.14 0.12 0.13 0.08
0.02
0.02 0.10 0.11 0.00 0.03
0.05 0.00 0.02 0.00 0.05 0.00 0 03 0.00 0.08 0.00 0.05 0.00
0.01
0.00
0.01
0.03 0.04 0.01 0.00 0 . 0 1 0 . 0 0 0.05 0.04 0.01
0.00
0.00 0.00 0.00 0.01
0 . 0 4 0 . 0 2 0.00 0 . 0 0 0.00 0 . 0 0
3.1 .5.3 4.0 0.6 e Appears high, checked in duplicate a n d shown immediately below d Butadiene Laboratory Manual (9).
0.07 0.00
0.04 0.00 0.02 0.00 0.01 0.00 0.07 0.00 0.00 0.00
0.02 0.00
0.09 0.00 0.02 0.00 0.01 0.00
0.02 0.00 0.00 0.00
J U N E 1947
411 Table 11. Charging R a t e S t u d y of Low-Temperature Fractional Distillation (Sample B) [Samples containing 90% (total hydrocarbon basis) propane, butane, a n d pentane]
Charging R a t e Nominal Actual Cc./Min, Cc./min.
100
100.9
Average Mm./jog
Total Chart Travel
Overhead
I
Overhead I1
Mole %
52.3
2066
31.1
1.9
31s Analysis of Hydrocarbons Overhead Fractions Distilled Fractions Overhead Cz CI Ca CK+ KO. Cz Ca 0 Mole % M o l e 56
~
4 . 0 22.2 39.6 1.2
100
102,o
52.9
2088
30.8
2.0
4.3 22.0 39.6 1.3
250+
304.0
157.5
2069
30.6
2.6
3 . 8 22.5 39.2 1.3
250+
270.2
140.0
2167
31.2
1.3
4.0 23.0 39.4 1.1
1364
29.9
Results using distillation rate recommended b y Office of Rubber Reserve (3) Mass spectrometer analysis of total sample
2.9 32.9
Ca+
Heavy Hydrocarbons in Overhead o n Total Sample Basis Cz Cs C4 Csf Mole %
I I1 I I1
I I1
I I1
X o t analyzed
3.8 22.3 39.9 1.2 3.6 22.2 39.7 1.6
100
118.3
61.3
2028
150
166.1
86.1
2030
Cylinder pressured t o 70% noncondensables I 0.10 0.00 0.04 0.00 0.07 0.00 0.03 I1 68,O 1.4 9.9 17.8 0.9 0.00 0.10 0.32 0.03 0.00 0.00 0.00 2.0 I 0.08 0.04 0.08 0.04 0.05 0.02 0.06 68.5 I1 1.3 9.4 18.3 0.9 0.18 0.21 0.36 0.05 0.00 0.00 0.01 1.6 I 0.04 0.02 0.07 0.00 0.03 0.01 0.05 68.5 1.2 I1 2.0 9 , 6 18.0 0.7 N o t analyzed
150
160.1
77.8
1829
68.9
100
116.6
60.4
2063
2504-
271.4
140 6
2069
67.3
250+
331.1
171.6
1734
67.5
2063
67.5
Results using distillation rate recommended b y Office of Rubber Reserve ( 3 ) Mass spectrometer analysis of total sample
0.01 0.00 0.00 0.00 0.01 0.00 o.. n._ i
2.0
1.2
9.9 17.1 0.9
2.0
1.3 10.7 17.8 0.9
2.4
0.7 10.4 17.8 1.2
69.8
1.8
I I1 I
I1 I I1
0.00 0.00 0.03 0.00 0.00
0.02 0.08 0.07 0.01 0.01 0.06 0.05 0.01 0.00 0.16 0.16 0.01 0.00 0.00 0.00 0.00 0.07 0.33 0.09 0.02 0.05 0.22 0.06 0.01 Not analyzed
0.05 0.07 0.06 0.01 0.03 0.05 0.03 0.01 0.07 0.56 0.10 0.01 0.00 0.00 0.00 0.00
1.8 10.5 17.7 0.7 1.9 10.1 17.7 0.6
After the overhead fraction was removed, the distillation was continued as fast as possible without “sliding” from one boiling point to the next and the results, which indicate this method may be useful for certain applications, are given subsequently in detail Charging Procedure. The distillation column, receiving bottles, and auxiliary equipment n-ere completely evacuated before each analysis. I n order to reduce contamination due to previous distillations, every stopcock involved was removed, wiped clean, and regreased before each distillation. The same receivers were used for the overhead fraction only and heavier fractions were never allowed to enter these receivers. Before beginning this work, the receivers were evacuated and pressured to atmospheric pressure with air three times in order to remove any hydrocarbons which may have been left from previous distillations. Cylinder nitrogen was then added to the bottles and a sample was removed for analysis by mass spectrometer, This analysis indicated no more than a trace of contaminants. After completely evacuating the system, liquid nitrogen was introduced at the top of the column and allowed to flow downward between the column and vacuum jacket until the column was sufficiently cool t o allow a pool of liquid nitrogen to remain on top of the still. At this point the sample was introduced through the drying train int.0 the still until atmospheric pressure was reached without taking any of the sample overhead. The entering stopcock was then closed and the column was maintained a t atmospheric pressure for 5 minutes by opening the entering stopcock briefly at 1-minute intervals. This procedure allowed a heat transfer from the packing and inside of the still, in order to chill the system thoroughly before the bulk of the sample was charged. I t also probably allowed a very “thin” reflux to form. At the end of 5 minutes the entering stopcock JTas opened, and the overhead rate and cooling rate valves were adjusted. After the proper amount of sample had been charged, the entering was discontinued and the fraction taken overhead was segregated for analysis (overhead I). After charging was discontinued, the portion of the sample boiling (or flashing) below the methane boiling point was removed overhead in a separate receiver and analyzed separately (overhead 11). These two fractions were considered to be the “overhead fractions.” Sampling of Overhead Fractions. A 250-ml. sample container was thoroughly evacuated using a diffusion pump. The two stopcocks of this container were freshly greased before starting. The sample container was connected by rubber tubing to the receiving bottle through the manifold and sample outlet lines, and after these lines were evacuated, the gas sample was allowed to flow into the sample container. The Topler pump was not used for sampling, as it could be a source of contamination. The
stopcocks were closed, and the sample container was removed for analysis of the segregated fraction. Analysis of Segregated Fractions. The 250-ml. tubes containing the samples of overhead fractions were attached to the inlet system of the mass spectrometer by means of ground-glass joints. Before analysis of the sample by the mass spectrometer, a spectrum of the evacuated system was recorded to determine the contamination present in the apparatus itself. This contamination was small in every case but was subtracted from the sample analysis before completion. The sample was entered into the mass spectrometer in the conventional manner and the spectrum scanned. The recorded spectrum was analyzed and t,he data were calculated, employing a 12-equation electrical computer (1). Saturates and unsaturates were computed, but for convenience the results are reported as total CB,C4, and Cs. The mass spectrometer was selected for analysis of the “overhead fraction” because only a small sample (less than 1 ml. of gas) was required and because it was the best available method for accurately detecting the small quantities present in these samples. The saturates in these analyses may be determined on the average to within 0.05 to 0.1 mole %, the unsaturates to within 0.02 to 0.05 mole 70. I n the original samples employed, the ratios of unsaturates to saturates were approximat’ely 0.47 and 1.05. SAMPLES AIVD RESULTS
The samples employed in this study were taken from petroIeum refinery plant streams and represented the product gases from fluid catalyst cracking units (sample 9,Table I), and the off gases from gasoline stabilization (sample B, Table 11). The first sample contained considerable quantities of C1 and Czhydrocarbons, m-hile the second sample contained very little of the lighter hydrocarbons. The samples were caught in 25-gallon cylinders and were held at pressures belon- the devi point of the heavier hydrocarbons. ’ A complete set of analyses (6 to 8 runs) was made on the samples; these analyses consisted of duplicate distillations of samples charged a t various rates. At the end of each set of analyses the cylinders were repressured with refinery inert gas (goy0 nitrogen, 10% carbon dioxide), and another series of tests was made. Using the above method it was possible to vary the noncondensables while the ratios of the hydrocarbons to each other remained constant.
412
V O L U M E 19, NO. 6
After the sample was charged and the overhead fractions were +egregated, the remainder of the sample, consisting entirely of hydrocarbons, \yas distilled a t the fastest rate possible in order to obtain a product balance. I n some cases maximum voltage \vas placed on the heater and y a r m air blown upward inside the vacuum jacket. S o att’emptwas made to control the distillatioii rat,e other than hy the automatic valves, which were controlled only by column pressure fluctuations. The amounts of C2, C3, C1, and Ci and heavier hydrocarbons determined by distillation in the first series of analyses (sample a) checked so closely that mass spectrometer analyses were obtairied on all succeeding total samples to verify the distillation results. ;is the program progressed i t was decided that a further check of the analyses should he obtained. Therefore, the samples \yere analyzed by lox-temperature fractional distillation using the distillation rates recommended by t,he Office of Rubher Reserve (3). The complete results of overhead analyses by mass spectronieter and total sample analyses by distillation and mass spectrometer are shown in Tables I and 11. The nominal charging rates used were 100, 150, and 250 cc. per minute. Because of the pressure drop through the system 250 cc. per minute nominal rate was the maximum found practicable with this equipment employing no more than 2 pounds inlet pressure. The total amount of sample charged was the same for every sample within practical limits (approximately 2000 mm. of chart travel = 5500 ml. of gas). Interpretation of the data obtained in this study reveals the following:
1. Varying amounts of methane are taken overhead a t the different charging rat’es, but in no case does i t appear that too high concentrations are included in the overhead fractions to be accurately determined by the usual combustion methods. The total C1 and overhead fractions, calculated t o a total sample basis, were found to be essentially the same when determined a t each of the various charging rates employed. 2. The heavy hydrocarbons found in the overhead fractions hy maqs spectrometer analysis are shown in the right-hand col-
unins of Tables I and 11, calculated on the basis of per cent of the gas charged to the apparatus. While the heaiy hydrocarbons found would cause a positive error in the Cp percentage and a negative error in the CI percentage, the actual amounts of the heavy hydrocarbons determined to be present in any of the samples a t any of the charging rates employed are so low that the cxffect of this contamination of the C1 and Cp constituents is within the accuracy attainable by the Burrell or L-. S. Steel methods of :inalysis for C1and Cp hydrocarbons. 3. S o advantage is shown for s l o ~charging rates over the fwtest practical rate (250 cc. per minute). 4. Reproducibility of quantities of individual fractions is csxcellent a t the fast distillation rates used and is in good agreement v i th the slow distillation and mass spectrometer analyses of the samples. For samples in which the C,,, C p ,C3, C,, and.
