CHFEN 5503
Memorandum
To: Dr. Terry Ring
From: Mr. Stan Holbrook
Date: October 13, 2005
Subject: Analysis of Natural Gas and an Unknown Using Gas Chromatography
On Sept. 22 & 27, 2005, a series of analyses were performed on the HP 5890 Series II
Gas Chromatograph. The resulting data was used in an attempt to determine the
concentrations of hydrocarbons in natural gas and unknown #1. These were found to
some level of accuracy, though there are reasons to believe that the calculation methods
used were flawed.
Theory
Chromatography is a procedure in which attractions between a mobile phase and a
stationary phase are exploited to perform a separation and can be used for large scale
separations or small scale analysis. Each compound is retained for a different amount of
time according to its attraction to the stationary phase. Therefore, a polar compound
interacting with a non-polar stationary phase will have a short retention time while a nonpolar compound will have a larger retention time.
Gas chromatography is a gas phase separation using the principles of chromatography.
The stationary phase is usually some type of polysiloxane with different substituent
groups added to achieve the desired polarity. Species are separated according to their
size and their adsorption on the stationary phase. Therefore, when a mixture of straightchained hydrocarbons is analyzed by gas chromatography the species with less carbons
elute first.
The detector used for these experiments was a Thermal Conductivity Detector (TCD).
This detector functions by measuring the thermal conductivity of the eluting gas
compared to that of a pure sample of carrier gas (He). As the conductivity changes, a
peak is given as a readout.
Results
GC readouts were obtained for natural gas, methane, ethane, propane, butane and an
unknown using the HP 5890 Series II Gas Chromatograph, U of U Inventory #188368.
Due to a problematic septum, there is an air impurity in each of the measurements,
showing up between 0.20 and 0.30 on the chromatographs. In natural gas (Figures 5-6),
it shows up as a little hump piggybacked on the methane peak. On the other
chromatographs (Figures 1-4,7), it shows up as the first peak. In addition to error
associated with air entering the system, some of the chromatographs show a value where
no species eluted. This is from drift, and those occurrences are marked accordingly.
Since the retention time of a species is the same regardless of the other species in the
mixture, the peaks on the natural gas chromatographs as well as that of the unknown can
be identified. Figures 1-4 are the chromatographs of pure hydrocarbons whose retention
times were used to label the peaks on the natural gas (Figures 5-6) and the unknown
(Figure 7).
Methane
Air
Figure 1. Gas chromatograph for a pure sample of methane
Ethane
Air
Figure 2. Gas chromatograph for a pure sample of ethane
Air
Propane
Figure 3. Gas chromatograph for a pure sample of propane
Butane
Air
Drift
Figure 4. Gas chromatograph for a pure sample of butane
Methane
Air impurity
and CO2
Ethane Drift
Propane
Figure 5. Gas chromatograph of a sample of natural gas
Butane
Hexane
Pentane
Methane
Air impurity
and CO2
Ethane Drift
Pentane
Hexane
Propane
Figure 6. Gas chromatograph of a second sample of natural gas
Air
Ethane
Propane
Butane
Drift
Figure 7. Gas chromatograph of a sample of unknown #1
By this method, the species in the unknown are determined. The concentrations of the
species are somewhat more complicated to determine. There are two simple ways that
would both seem to be able to determine the concentrations: comparing the areas under
the peaks (taking the total area under the peaks to be 100%), and comparing the values
for the area under the peak of known samples to those of the unknown.
According to the latter method, the volume percent of methane and ethane in natural gas
are 110% and 6.65%, respectively, while the combined percents of the three components
of the unknown equal 130%. Therefore, this method seems to cause physically
impossible results. This could be due to a number of reasons, one of which will be
treated here.
When the sample was taken, the syringe was filled from a pressurized vessel at room
temperature. Since the plunger did not push in or out, it is assumed that the gas was at
atmospheric pressure. Upon injection, the gas was forced into a heated chamber thereby
increasing the pressure in that chamber both by addition of material as well as the heating
and expansion of the entering gas. As the gas entered the column and eluted out, the
pressure in the system slowly decreased. Therefore, the partial pressure of the first
elutent is higher than that of the second (assuming similar amounts). As the pressure in
the column decreases, the concentration of any given compound seems to decrease. Also,
the pressure in the column at any given time in each analysis will vary.
As for the former case, the volume percents for the compounds in natural gas are fairly
accurate (Table 1). Comparing the values to those provided by Questar Gas, the error in
methane/N2/CO2 concentration is only 6.7%, much closer to reality than the results from
the latter method.
Table 1 Natural Gas Composition Data from Questar Gas and Experimental Data.
Vol%, GC,
Vol%, GC,
Component
Vol%, Questar Method 1
% error Method 2
% error
Methane/N2/CO2
94.29
110
16.7
88.0
6.6709
Ethane
4.376
6.65
52.0
7.8
78.245
Propane
0.902
2.75
205
2.2
144
Butane
0.317
0.03
90.5
0.5
57.7
Heavier HCs
0.115
1.5
1204
According to the more accurate method, the unknown was found to have the following
concentrations:
Table 2. Relative Concentrations of Species in Unknown 1
Species
Vol%
Ethane
22.0
Propane
74.6
Butane
33.4
The detector (TCD) used for the column, however, may cause problems with this second
method. The thermal conductivity varies according to the compound. Since a TCD only
measures thermal conductivity, it does not actually calculate the amount of substance
passing through, only the change in thermal conductivity. As the length of the chain
increases, the thermal conductivity decreases (Table A.1). That would mean that the
concentration of ethane is higher than that calculated since the deviation in conductivity
is smallest for ethane, giving a smaller peak than an equivalent amount of butane would.
One fault in both these methods is in inert impurities. Because the septum leaked,
unknown quantities of air were allowed to enter the system as previously mentioned.
Any inert species (CO2, N2, He, etc) in the natural gas and the unknown would have
been interpreted as entering air and neglected in these measurements. Therefore, the
volume percents shown in Table 2 are given as if there were no inert species in the
mixture, and their effects on the percents must be taken into account by some other
method (i.e. FTIR).
Gas chromatography proved to be a very efficient method of separation, though less
effective as a quick analysis tool. It is assumed that a standard method could be produced
in order to analyze the composition of mixtures, but for quick analysis such as this,
results are inconclusive.
Appendix A. Table and GC Output Data
Table A.1 Thermal Conductivity of Gases1
Species
k (mW/m*K)
Ethane
18.0
Propane
15.2
Butane
13.6
Helium
142.6
GC Output Data:
Sample Name = Methane 1
Instrument = 5890 II
Heading 1 =
Heading 2 =
Acquisition Port = DP#
Raw File Name = C:\CPData\GC Student Data\5890II TCD Data\Group Tito 9-22-05
B.0035.RAW Date Taken (end) = 9/22/2005 3:42:41 PM
Method File Name = C:\CPSpirit\Natural Gas Method 7-05\NatGasB.met Method
Version = 2
Calibration File Name =
Calibration Version = 0
Peak # Ret. Time
Name Amount
Amt % Area Area %
Type Width
1
0.36
0.00 N/A 453619
100.000
BB
0.05
Total Area = 453618.9
Total Height = 125325.9
Total Amount = 0
Sample Name = Ethane 1
Instrument = 5890 II
Heading 1 =
Heading 2 =
Acquisition Port = DP#
Raw File Name = C:\CPData\GC Student Data\5890II TCD Data\Group Tito 9-22-05
B.0036.RAW Date Taken (end) = 9/22/2005 3:49:09 PM
Method File Name = C:\CPSpirit\Natural Gas Method 7-05\NatGasB.met Method
Version = 2
Calibration File Name =
Calibration Version = 0
Peak # Ret. Time
Name
1
0.28
0.00
2
1.79
0.00
Total Area = 752257.4
1
Amount
Amt % Area Area %
Type Width
N/A 88660 11.786 BB
0.05
N/A 663597
88.214 BB
0.18
Total Height = 75625.82
Total Amount = 0
http://www.airliquide.com/en/business/products/gases/gasdata/index.asp
Sample Name = Propane 1
Instrument = 5890 II
Heading 1 =
Heading 2 =
Acquisition Port = DP#
Raw File Name = C:\CPData\GC Student Data\5890II TCD Data\Group Tito 9-22-05
B.0037.RAW Date Taken (end) = 9/22/2005 3:59:37 PM
Method File Name = C:\CPSpirit\Natural Gas Method 7-05\NatGasB.met Method
Version = 2
Calibration File Name =
Calibration Version = 0
Peak # Ret. Time
Name
1
0.27
0.00
2
5.98
0.00
Total Area = 751528.5
Amount
Amt % Area Area %
Type Width
N/A 296904
39.507 BB
0.05
N/A 454625
60.493 BB
0.28
Total Height = 116249.5
Total Amount = 0
Sample Name = Butane 1
Instrument = 5890 II
Heading 1 =
Heading 2 =
Acquisition Port = DP#
Raw File Name = C:\CPData\GC Student Data\5890II TCD Data\Group Tito 9-22-05
B.0039.RAW Date Taken (end) = 9/22/2005 4:26:15 PM
Method File Name = C:\CPSpirit\Natural Gas Method 7-05\NatGasB.met Method
Version = 2
Method Description = Natural gas example - channel B
Method Date =
7/20/2005 7:43:06 PM
Calibration File Name =
Calibration Version = 0
Internal/External = [No data] Calibration Date = [No data]
Run Time = 11.37925
Data Sampling Rate = 5.003257
Amount Injected = 0.05
Dilution Factor = 1
Sample Weight = 1 Int Std Amount = 1
Peak Width = 0.2
Operator =
Peak Threshold = 0
Manual Integration = not manually integrated
Peak # Ret. Time
Name
1
0.26
0.00
2
2.82
0.00
3
9.77
0.00
Total Area = 1355346
Amount
Amt % Area Area %
Type Width
N/A 12242 0.903 BV
0.06
N/A 297594
21.957 VB
5.84
N/A 1045510
77.140 BB
0.47
Total Height = 38646.04
Total Amount = 0
Sample Name = Nat. Gas 1
Instrument = 5890 II
Heading 1 =
Heading 2 =
Acquisition Port = DP#
Raw File Name = C:\CPData\GC Student Data\5890II TCD Data\Group Tito 9-22-05
B.0034.RAW Date Taken (end) = 9/22/2005 3:34:18 PM
Method File Name = C:\CPSpirit\Natural Gas Method 7-05\NatGasB.met Method
Version = 2
Calibration File Name =
Calibration Version = 0
Peak # Ret. Time
Name
1
0.36
0.00
2
1.83
0.00
3
2.35
0.00
4
6.38
0.00
5
9.06
0.00
6
10.09
0.00
7
11.31
0.00
Total Area = 746413.8
Amount
Amt % Area Area %
Type Width
N/A 499258
66.888 SBB 0.05
N/A 44127 5.912 TBV 0.10
N/A 179096
23.994 TVV 2.69
N/A 12507 1.676 TVB 0.21
N/A 2889 0.387 BB
0.30
N/A 2780 0.372 BB
0.31
N/A 5757 0.771 BB
0.55
Total Height = 122291.9
Total Amount = 0
Sample Name = Nat. Gas 2
Instrument = 5890 II
Heading 1 =
Heading 2 =
Acquisition Port = DP#
Raw File Name = C:\CPData\GC Student Data\5890II TCD Data\Group Tito 9-22-05
B.0040.RAW Date Taken (end) = 9/22/2005 4:41:02 PM
Method File Name = C:\CPSpirit\Natural Gas Method 7-05\NatGasB.met Method
Version = 2
Calibration File Name =
Calibration Version = 0
Peak # Ret. Time
Name
1
0.36
0.00
2
1.81
0.00
3
2.83
0.00
4
6.37
0.00
5
10.70
0.00
6
11.29
0.00
Total Area = 690227.9
Amount
Amt % Area Area %
Type Width
N/A 479598
69.484 SBB 0.05
N/A 36125 5.234 TBV 0.10
N/A 138817
20.112 TVV 3.18
N/A 30034 4.351 TVB 0.22
N/A 1575 0.228 BB
0.27
N/A 4078 0.591 BB
0.59
Total Height = 128193.1
Total Amount = 0
Sample Name = Unknown 1
Instrument = 5890 II
Heading 1 =
Heading 2 =
Acquisition Port = DP#
Raw File Name = C:\CPData\GC Student Data\5890II TCD Data\Group Tito 9-2705.0067.RAW
Date Taken (end) = 9/27/2005 3:21:01 PM
Method File Name = C:\CPSpirit\Natural Gas Method 7-05\NatGasB.met Method
Version = 2
Calibration File Name =
Calibration Version = 0
Peak # Ret. Time
Name
1
0.24
0.00
2
1.73
0.00
3
5.39
0.00
4
8.07
0.00
5
8.80
0.00
Total Area = 1348721
Amount
Amt % Area Area %
Type Width
N/A 243386
18.046 BB
0.04
N/A 146079
10.831 BV
0.11
N/A 339365
25.162 VV
0.17
N/A 270190
20.033 VV
1.36
N/A 349702
25.928 VB
0.23
Total Height = 153376.7
Total Amount = 0