Advances in
Separation, of Hydrocarbon and Related Compounds Four papers from the Symposium on Advances in Separation of Hydrocarbons and Related Compounds, presented before the Division of Petroleum Chemistry a t the 126th
CHEMICALSOCIETY,New York, meeting of the AMERICAN
N. Y.
Other papers in this sym-
posium are published in the February issue of lndusfrial and Engineering Chemisfry.
Separation and Analysis of Gases and Volatile liquids by Gas Chromatography H. W. PATTON, J. S. LEWIS, and W. 1. KAYE Research Laboratories, Tennessee Eastman Co., Division o f Eastman Kodek Co., Kingsport, Tenn.
possible to arrange conditions that produce sufficiently different rates of migration, the components are separated and can be detected or collected as they emerge from the column. The separation of gases and vapors by chromatographic methods has been reported by a number of workers. Hesse and Tschachotin ( f O ) , Cremer and coworkers (4-6), and Janak ( I S ) described techniques in which the sample materials are eluted from an adsorbent by a carrier gas. Instead of an adsorbent, Martin and James (11, 14), Phillips and coworkers (9, 12, Zj), Cropper and Heywood (7), and Ray (16 ) used columns packed with granules of a support covered with a high-boiling liquid. Development by displacement has been described by Claesson ( 3 ) and Phillips (9,IZ). Turner (18)described an apparatus in which hydrocarbon gases were separated on a charcoal column as they were moved through it by a heater. A later modification employing displacement by a vapor and other changes is available commercially ( 2 ) . Glueckauf and coworkers (8) obtained some isotopic enrichment when neon was passed through activated charcoal a t low temperatures. Wirth ( f g ) , Turkel’taub (17), and Barrer and Robins (1) have published the results of relevant investigations. Exploratory investigations of adsorption methods (elution and displacement from adsorbents) and the gas-liquid partition method (elution from liquid-covered granules) were made in this laboratory. Because of the early discovery that the method involving elution from adsorbents was readily adaptable t o the accurate determination of light hydrocarbons and some of the gases associated with them, the work was mainly concentrated on this technique. This paper is largely concerned with the separation and analysis of these gases and volatile liquids by elution from columns of charcoal] silica gel, and alumina
Gas chromatographic techniques were used to study the potentialities of selective adsorption and partition methods for the separation and analysis of gases and volatile liquids. Methods involving elution by hydrogen, nitrogen, or carbon dioxide from columns containing charcoal, silica gel, or alumina provided excellent separation of hydrocarbons and other nonpolar materials. Gas chromatographic elution is simple, accurate, and very effective for the identification and determination of the components of many gaseous and liquid mixtures, particularly of hydrocarbons.
I
S R E C E S T years the separation of materials by selectiye sorption, partition] and ion exchange processes has been developed into a very powerful and versatile tool for analysis and purification. Although many variations on the fundamental technique have been made and the scope of the method has been tremendously broadened, most of the work reported has involved samples of relatively nonvolatile materials in volatile liquid solvents. Some methods involving a gaseous phase have been used during the last decade, but this aspect of chromatography is still relatively new. The techniques and principles of gas chromatography are similar in many respects to those of the more common column procedures, except that the mobile phase is gaseous rather than liquid. In brief, the usual procedure is as follows: The material to be separated is adsorbed in a narrow band on one end of a column of adsorbent such as activated charcoal. .4n adsorbent or inert support coated with a high-boiling liquid is advantageous in some cases (gas-liquid partition). The sample is caused to move through the column by application of heat, passage of a carrier gas through the column (elution), passage of a carrier gas containing a strongly adsorbed vapor through the column (displacement)] or some combination of these methods. When it is
EXPERIMENTAL
Procedure. The general arrangement of the apparatus is shown by the block diagram in Figure 1. 170
V O L U M E 27, NO. 2, F E B R U A R Y 1 9 5 5 A stream of carrier gas p:tsses through the system continuously during a run. The flow rate is indicated by a flowmeter and controlled by a needle valve. Constant pressure is maintained a t the input to the flowmeter by allowing excess carrier gas to escape through a constant head of water. When the detector is a thermal conductivity cell, the carrier gas passes through its reference channel before entering the column. Gas samples are handled in a glass vessel provided with a bypass through which the carrier gas passes while the system is being prepared for a run. Liquid samples are injected onto the end of the column through a rubber serum cap with a hypodermic syringe. The carrier gas causes each sample component to move through the column a t a rate that depends on how strongly it is held by the adsorbent. In favorable cases, the mixture is separated and each component passes from the column into the sample channel of the detector at a different time. When it is desirable, the separated fractions are collected for investigation by infrared absorption or other means.
171 and dried in an oven a t 130" C. after being packed into the column, the carbon and silica gel adsorbents were dried further by passing the carrier gas through the column a t a temperature of about 150' to 200" C. for several hours. The adsorptive properties of alumina were markedly dependent on its water content. The material used seemed to give the most efficient and reproducible separations when it wap dried overnight by the carrier g:iq a t the temperature a t which it was to be used.
-b
CARRIER
Carrier Gases. The choice of carrier gas depends to a large extent on the materials to be analyzed. With a thermal conductivity detector, the sensitivity to a given substance can be changed tremendously by using different carrier gases. Of course, there is no response from that portion of a sample that is the same as the carrier. Nitrogen is a satisfactory carrier for many separations, but the response of carbon monoxide and the Cs and C, hydrocarbons in nitrogen is rather poor. A system using hydrogen as carrier is much more sensitive to hydrocarbons heavier than methane. The evplosion hazard and the possible hydrogenation of unsaturated materials are obvious disadvantages. Carbon dioxide hap been used with some success for the determination of oxygen, nitrogen, hydrogen, and the hydrocarbons through Cf. The stability of the thermal conductivity cells is excellent xith hydrogen, good with nitrogen, and poor with carbon dioxide. The flowmeter is of a conventional type that measures the pressure differential developed across a capillary. The efficiency of separation is not critically affected by moderate changes in the flow rate. Increasing the flow rate increases the instability of the thermal conductivity detectors, and decreasing the flow rate increases the time required for an analysis. A very satisfactory compromise for general use is 50 ml. per minute. The instability of the detectors is not objectionable with flow rates below 100 ml. per minutc. Reproduction of the flow rate to =t0.5% is necessary for quantitative work. Adsorbents. The adsorbents used in this study included activated charroal, silica gel, and alumina. Excellent separations were obtained on each of these materials. The choice of adsorbent depends largely on the composition of the mixture to be separated. Light hydrocarbons are readily separated on a column of silica gel or alumina a t temperatures of 25" to 100' C. The order of elution from columns of these two adsorbents was generally the same, but, for the materials used, the retention volume of a given substance was greater on silica gel than alumina. Different results were obtained with adsorbents from different sources, but it was possible to get approximately the same behavior on adsorbents of the same kind from different sources by adjusting the temperature of the column. The order of elution of saturated hydrocarbons from charcoal was the same as from alumina or silica gel. Unsaturated hydrocarbons preceded saturated ones from carbon columns, but the reverse was true with the other two adsorbents. The most active charcoals were capable of good separations of hydrogen, oxygen, and methane, whereas the aluminas and silica gels investigated were very much poorer for the separation of these materials. Elution of the liquid hydrocarbons from charcoal required either a very long time or a temperature of 250" C. or above. Compounds such as octane and toluene were separated on alumina or silica gel in less than an hour a t about 200' C. Separations seem to be somewhat better on finer adsorbents, but this factor is not critical and there is little to be gained in using particles smaller than 50 mesh. The most satisfactory size seems to be about 30 to 50 mesh. The adsorbents were sieved, washed with water to remove dust,
GAS
FLOW METER
4
NEEDLE'
BY -
I VALVE
PASS
Figure 1.
4DETECTOR
Schematic diagram of gas chromatography apparatus
The stability and reproducibility of any of the columns are improved by leaving them in operating condition continuously. If the carrier gas or column heater is turned off overnight, a warm-up period of 1 to 2 hours may be required to get the system in operation again. Unless the samples contain appreciable quantities of materials that are irreversibly adsorbed, the system can be used almost indefinitely without changing the adsorbent. For example, one silica gel column has been used for over 3 months for the routine determination of methane in a mixture of hydrocarbon gases without renewing the packing. Columns. Glass columns with various dimensions were used. With small amounts of sampleathe degree of separation seems to depend primarily on the length of the column. The amount of material that can be separated on a column of given length is roughly proportional to the cross-sectional area. The columns that were found most useful for general separations were about 0.5 cm. in diameter by 110 cm. long. All data included in this paper were obtained with columns having these dimensions, except where otherwise indicated. The work reported here was done u i t h columns a t or above room temperature. Accurate thermostating is not necessary, except for the most careful work. I t was found satisfactory in most cases to wrap the column closely with insulated Nichrome wire and then Rith asbestos tape. Electrical energy was supplied to the heater by means of a variable transformer. Continuously increasing the temperature of the column during a run makes possible a more efficient separation of materials having xidely different affinities for the adsorbent. However, the experimental difficulties attending reproduction of heating rate and niaintenance of constant flow rate eliminated the use of increasing temperature except for a few exploratory experiments. Sample Containers. Liquid samples were applied directly to the end of the column by injecting them through a rubber serum cap with a hypodermic syringe. The usual volume of liquid sample was 0.01 to 0.1 ml. Gases can be introduced in the same way for qualitative results, if the syringe plunger is lubricated with a nonvolatile liquid in which the sample is relatively insoluble. Glass sample containers constructed from two T-bore stopcocks were satisfactory for quantitative work with gases (see Figure 1). The sample was contained in one arm of the vessel, and the other arm was used as a bypass for the carrier gas until the system was ready for the sample. The volume of both arms
ANALYTICAL CHEMISTRY
172 was determined so that either could be used as the sample container. ,4sample volume of less than 15 ml. was generally used. Detectors. Two general types of detectors were used. I n the early stages of the work, the gases leaving the column were passed through a gas cell in an infrared spectrometer. This made qualitative and quantitative determination of the off gas possible a t any time. Although this system was useful during the exploratory studies, the obvious disadvantages are its slow response due to the large volume of the gas cell and the necessity for adjusting the spectrometer to a different characteristic nave length for each component.
2G
DETECT09
RESPONSE,
I C 0 - M L SAMPLE
SAMPLE
5
3
-
-
c
1
2
Figure 4.
CHANNEL
0
1 2 MINUTES
3
r
5
~
7
Elution of natural gas from alumina bj hydrogen
to provide a thermostat for the cell in locations where the varintion in the surrounding temperature did not exceed f5' C. For quantitative work, it was necessary t o control the current through the cell to within f1 ma. The thermal conductivity detectors were used satisfactorily with recording potentiometers having ranges from 0 to 6 mv. t o 0 to 30 mv. The stability of the background is believed to be sufficient to warrant use of a recorder having a range of 0 to 1 mv ,if the ultimate sensitivity to minor components is important. Thermal conductivity cells similar to those used in this study can now be obtained from the Gow-Mac Instrument Co.
SAMPLE CHANNEL
B
A
3
Figure 2. Thermal conductivity detector DETECTOR RESPONSE, MV
I
DISCUSSION AhD RESULTS
I
0
5
IO
,
,
15
e MINUTES
Figure 3.
Elution of a mixture of gases from charcoal by
The detectors used for most of the investigation were thermal conductivity cells. A cell similar to that described by Phillips (15) was found t o have a low output signal, a response time longer than desirable, and objectionable sensitivity t o ambient temperature changes. A commercially built cell (Gow-hlac Instrument Co., lladison, N. J.) was modified to pass the gases directly through two of the four filament chambers in order t o reduce the response time. The result was a very satisfactory detector. I t was considerably more sensitive to changes in flow rate than the unmodified cell, but the instability introduced by this factor was tolerable a t the rates used. These thermal conductivity cells were constructed so that the four filaments which were connected electrically t o form the arms of a Wheatstone bridge were contained within a small, cubical, brass block. Before entering the column, the carrier gas was passed through the unmodified reference channel, where it came in contact with the two reference filaments (Figure 2,a) The gas leaving the chromatographic column was passed through the modified sample channel (Figure 2,b). Thus all four filaments were exposed t o the carrier gas only, except when a sample component was being eluted. During elution of a component, two of the filaments (in opposite arms of the bridge) were exposed t o the mixture of carrier gas and the component leaving the column The resulting unbalanced voltage from the bridge was fed to a recording potentiometer which automatically plotted detector response against time. The response of the thermal conductivity cell was affected very little by changes in ambient temperature. It was not necessary
Separations. Gas chromatographic techniques involving elution from adsorbents seem t o be 20 most readily adaptable t o the separation of relatively nonpolar substances boiling below 150" C. at atmospheric pressure. Except for closely nitrogen similar isomers, hydrocarbons are usually sepnrated with ease. During elution, most polar materials produce tailing, which leads to poor separation. Displacement development or the gas-liquid partition method can sometimes be used to obviate this difficulty ( 3 , 1 2 ) . The results presented illustrate typical separations. Figure 3 shows a curve for the elution of 10 ml. of a synthetic mixture of hydrogen, oxygen, methane, carbon dioxide, acetylene, ethylene, and ethane from a column of charcoal. The carrier gas was nitrogen, and the column temperature was about 180" C. The separation of natural gas on alumina a t GO" C. with hydrogen as carrier ,
,
I
3
C 3 b
'ZH
03-ML SAMPLE
6
50-ML
SAMPLE
I
I rISO-C,H,, 0
~.
0
1
2
3
4
MINUTES
Figure 5.
Elution of commercial propane from alumina by hy-drogen
e
173
V O L U M E 27, NO. 2, F E B R U A R Y 1 9 5 5 is illustrated in Figure 4. This same column and conditions were used for the separation of commercial propane (Figure 5 ) , straight-run gasoline (Figures 6 and i ) ,and thermally cracked gasoline (Figures 8 and 9). The volume of the gasoline samples \vas 0.0'7 ml. Although there has not been sufficient invest,igation t o establish the identity of the component responsible for each of the peaks produced by the gasoline samples, the presence of some ceoinpounds was easily established from the curves. The straightr u n gasoline contained propane, isobutane, butane, a t least two i-oniers of pentane, and at least three isomers of hexane. In adclition to these, the sample of thermally cracked gasoline contained ethane, propylene, a t least, t v o isomers of butene, a t least tn-o isomers of pentene, and prohably some hesene?.
The logarithm of the corrected peak tinic is a linear function of the number of carbon atoms per molecule for saturated straightchain hydrocarbons, as indicated by Figure 11. James and Martin (11) and Ray (16) found similar relationships between the logarithm of the retention volume and the number of carbon atoms for the elution of homologous series of several types of compounds from gas-liquid partition columns. DETECTOR RESPONSE, MV.
15 r
lo
'II
DETECTOR RESPONSE. MV
---Lp0
I
3
2
I
HOURS
Figure 8. Elution of thermally cracked gasoline from alumina by hydrogen
0
3
2
I
I
HOURS
Figure 6. Elution of straight-run gasoline from alumina by hydrogen
11
''
The separation of a 2-nil. sample containing air, isobutane, butane, and cis- and trans-2-butene illustrates the efficiency of the method (Figure 10). The upper curve in the figure shows the separation obtained on a column of alumina 0.5 cm. in diameter by 110 cm. long a t 60" C., with hydrogen as the carrier gas The increased resolution obtained by using a similar column 220 em. long is demonstrated by the lower curve.
DETECTOP RESPONSE
I\
MV I
0
C
A B C
"
5
- AIR
I 0
- ISOBUTANE
nBUTENES PENTANES
I -
I
IO
5
25
20
15 MINUTES
- ETHANE - PROPANE
Quantitative Determinations. Quantitatively, the height of the elution peak or the area under it is approsimately proportional to the amount of the component that produced it. However, the relationship deviates from linearity for some gases, so that i t is generally necessary to establish its shape by several esperimental points. The peak height is influenced by the temperature of the column or any other factor that affects the activity of the adsorbent. It also depends somewhat on the volume of gawes in
.PENTANES
15 MINUTES
- AIR ETHANE -- PROPANE PROPYLENE
Figure 9. Elution of thermally cracked gasoline from alumina by hydrogen, M-ith expanded time scale
D -\SOBUTANE
10
AB C D E
GUTANE
10
20
25
Figure 7. Elution of straight-run gasoline from alumina by hydrogen, with expanded time scale
Qualitative Identification. -4s in other elution techniques, thc identification of a component can be based on its retention volume. As a constant flow rate was used, it was more convenient to measure the peak time-that is, the time elapsing between the introduction of the sample into the carrier stream and its appearance a t maximum concentration in the detector. The corrected peak time is the measured value minus the time that would be required for an unadsorbed gas to pass through the same system. To a first approximation, the peak time of e. given substance is independent of the amount of that component and of the presence of other materials. For a substance that tails during elution, the peak time decreases slightly as the quantity increases.
,
I
A -
t
DETECTOR
I
a -
c -
RESPOIvSE
'
110-CM
AIR I SOGUTA N E BUTANE
3 - TRANS- 2 -BUTEtUE CIS - 2 - B U T E N E
220- CM E
n i 0
15
-
-
-
~
0
15
30
45
60
MINUTES
Figure 10. Effect of column length on separation efficiency
ANALYTICAL CHEMISTRY
174
which the sample is contained. A given amount of a component produces a peak that decreases in height and increases in width as the total volume of sample increases. Therefore, if the peak height is employed for quantitative determinations, the vessels used for calibration must have the same volume as those containing the samples to be analyzed, or the results must be corrected for the change in peak height with sample size. This correction can be accomplished by making determinations with several volumes and extrapolating a plot of sample volume us. percentage to the volume used for calibration.
I 6
I
AREA UNDER PEAK, SO. CM.
./*
/*'
d
014 CH,,
010
ML (24O C 735 V,M I
Figure 12. Typical calibration curve
CORRECTED PEAK TIME, MIN I
IO
I
-
I
I
IO-
I
O i l 0
4: ' I
I
,
I
l--L-
1 2 3 4 5 CARBON ATOMS PER MOLECULE
6
Figure 11. Peak times for elution of n o r m a l paraffins f r o m a l u m i n a Since the area under the elution peak is practically independent of the sample size, it is generally a more satisfactory quantitative measurement than the peak height. A polar planimeter is useful for this purpose, but it is simpler to use the product of the peak height and its half width (the width a t one half the maximum height), as suggested by Cremer ( 5 ) . There is no experimental evidence to indicate that determinations based on the peak height and half width are any less accurate than those obtained from planimetered areas. The use of measured volumes of pure gases rather than mixtures greatly facilitates calibration. For this purpose, vessels similar to the sample containers but having much smaller volumes are used. Vessels having volumes that cover the desired concentration range are filled with the pure gas. The gas in each vessel is then put, through the column. A plot of gas volume us. response (area under the peak or peak height times half width) constitutes a calibration curve (Figure 12). A complete calibration usually requires less than an hour.
used as a simple, accurate, and inexpensive means of analyzing hydrocarbon gases. I t is estimated that hydrogen, methane, and ethane can be detected at a concentration of about 0.01%. The C3 and Ca hydrocarbons can be detected at a concentration of about 0.2%. Quantitative results are accurate to within about f1 or 2% of the amount found. In addition to the analyticalapplications based on the chromatographic elution curves, the method seems to have promise for the isolation of components for further study by other means. Concentration of minor components for identification by infrared absorption has already been accomplished. I t seems likely that the combination of gas chromatographic separation and mass spectrographic identification would be worth while. Other detectors based on ultraviolet absorption or a surface potential device might be used to enhance the sensitivity to some substances. LITERATURE C I T E D
Llarrer, R. M., and Robins, A. B., Trans. Faraday Soc., 49, 80715 (1953).
Burrell Corp., Pittsburgh, Pa., BUZZ.525. Claesson, s.,Arkiv Kem,i, Mineral. Geol., A23, No. 1 (1946). Cremer, E., and Muller, R., Mikrochemie ne?. Mikrochim. Acta, 36/37, 533-60 (1951).
Ckemer, E., and Muller, R., Z . Elektrochem., 55, 217-20 (1951). Cremer, E., and Prior, F., Ibid., 55, 66-70 (1951). Cropper, F. R., and Heywood, A., Sature, 172, 1101-2 (1953). Glueckauf, E., Barker, K. H., and Kitt, G. P., Discussions Faraday SOC.,NO. 7, 199-213 (1949).
Griffiths, J., James, D., and Phillips, C., Analyst, 77, 897-904 (1952).
Hesse, G., and Tschachotin, B., rafurwissenschaften, 30, 387-92 (1942).
James, A. T., and hIartin, .I. J. P., Analyst, 77, 915-32 (1952). James, D. H., and Phillips, C. 5. G., J . Chem. Soc., 1953,1600-10; 1954, 1066-70.
Janak, J., 'Chem. Listy, 47, 464-7, 817-27, 828-36, 837-41,
SUMMARY
1184-9, 1190-6, 1348-53, 1476-80 (1953).
Satisfactory gas chromatographic separations were obtained on charcoal, silica gel, and alumina columns using hydrogen, nitrogen, or carbon dioxide as carrier gases. A commercially built thermal conductivity cell was modified for faster response to produce an excellent detector. I n principle, the method is applicable to the qualitative and quantitative analysis of any mixture whose components can be separated by elution in the gaseous phase. It is already being
w
Martin, A. J. P., and James A. T., Biochern. J. (London), 50, 679-90 (1952).
Phillips, C. S. G., Discussions Faraday Soc., No. 7, 241-8 (1949). Ray, N. H., J . A p p l . Chem. (London), 4, 21-5, 82-5 (1954). Turkel'taub. N. M., Zhur. Anal. Khim., 5, 20Cb10 (1950). Turner, N. C., Oil Gas J . , 41, 48-52 (1943); Petroleum Refiner. 22, 140-4 (1943).
Wirth, H., Monntsh., 84, 15€-68, RECEIVED for review October 22, 1954.
W
741-50 (1953). Accepted December 27, 1954.
