Measuring and testing – Gas analysis – Gas chromatography
Reexamination Certificate
2001-04-24
2003-06-10
Kwok, Helen (Department: 2856)
Measuring and testing
Gas analysis
Gas chromatography
Reexamination Certificate
active
06575015
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of process chromatography is concerned with analyzing gas samples flowing through a process pipeline. A sample from a gas pipeline may be taken by use of a sample probe or other sampling device, which then provides the sample to a gas chromatograph. The gas chromatograph separates the sample into its individual components, using a variety of detectors to analyze the concentration of the resulting component bands in the sample. In the oil and gas industry the knowledge of what fluid is being transported by the pipeline is useful for a variety of purposes, such as source identification and custody transfer.
FIG. 1
shows a known gas chromatograph system (not to scale). Gas flows through a process pipeline
110
, a sample of which is taken by a sample probe
120
prior to being introduced to gas chromatograph (GC)
100
. The gas sample may be filtered and heat traced generally along tubing
130
before flowing into gas chromatograph
100
. Heating may be required for gases that may condense into a part gas, part liquid flow at cooler temperatures. After being analyzed by the gas chromatograph, the gas sample is either returned into the process pipeline
110
, or vented to the atmosphere. As used herein, the term gas chromatograph is being used in its broad sense, to include what is traditionally known both as the sample handling system and as the carrier pre-heat system.
Referring to
FIG. 2
, gas chromatograph
100
includes valve assembly
210
connected to multiple columns
220
and detectors
230
, in this case, thermal conductivity detectors (TCD's). A gas sample generally follows path
240
through valve assembly
210
, columns
220
and TCD's
230
. The valve assembly allows the selection of columns
220
which contain a liquid phase, or porous polymer, or other material. Two types of columns are: 1) packed columns, filled with a liquid coated solid support or porous polymer; and 2) capillary columns, coated with a liquid or porous polymer. In either case, these materials act to separate the gas sample into multiple fractions, each fraction that is to be analyzed being sequentially directed to the TCDs
230
. For example, a gas sample may contain various molecular weight hydrocarbon components such as ethane, methane, and heavier molecules. Ideally, each of these components would be analyzed individually. The resulting analysis could be normalized to minimize the effects of varying sample size from one injection to the next. In general, column
220
separates the gas sample so that more volatile components would elute from the column first, followed by less volatile components (although the use of valve switching may cause the components not to elute at the detector in that order).
Referring to
FIGS. 3A and 3B
, the operation of a sample valve is shown. Valve
300
includes a plurality of valve ports, labeled
1
-
6
. Incoming line
310
provides a gas sample to valve
300
. Exhaust line
320
expels the gas sample from the valve
300
. Solid lines
330
show open passages between ports, whereas dotted lines
340
indicate blocked passages between the ports.
A solenoid (not shown) places valve
300
into either an ON position, as shown in
FIG. 3A
, or an OFF position, as shown in FIG.
3
B. When a valve is in the ON position, sample gas flows from incoming line
310
, through port
1
to port
6
, through line
315
and finally through port
3
to port
2
and out exhaust line
320
. When the valve is in the OFF position, sample gas flows from incoming line
310
, through port
1
to port
2
and out through exhaust line
320
. At the same time, carrier gas flows through port
5
to port
6
into line
315
where it displaces the sample gas. The carrier gas then flows from port
3
to port
4
and injects the sample onto the column. Of course, the designation of OFF versus ON is somewhat arbitrary and the opposite nomenclature could also be used.
FIG. 3C
illustrates how a pair of valves may operate either alone or in combination with additional valves (not shown). A first valve
300
includes an array of six valve ports. A second valve
350
also includes an array of six valve ports. Associated tubing
310
,
315
,
320
,
325
and
390
, and columns
360
and
370
are also shown as well as dual TCD's
380
.
Incoming line
310
is attached to a sample transport line (not shown). When first valve
300
is in an OFF position, gas sample flows from incoming line
310
to port
1
to port
2
of the valve
300
and out exhaust line
320
. When valve
300
is in an ON position, however, gas sample flows from port
1
to port
6
and then through sample loop
315
. That gas then flows from port
3
to port
2
of valve
300
and is expelled out exhaust line
320
. At this time, the sample loop
315
is filled with a gas sample. This means that, if valve
300
is turned OFF at this time, a gas sample is trapped within the sample loop
315
.
Turning now to valve
350
, when it is in an OFF configuration, carrier gas flows from carrier gas input line
390
through port
2
of valve
350
, to port
1
and then through carrier tubing
325
. At this time, valve
300
is also in an OFF configuration, so that the carrier gas in tubing
325
is forced through port
5
to port
6
and through gas sample tubing
315
. Consequently, this action forces the gas sample down column
360
via ports
3
and
4
. The gas sample can then additionally be forced through column
370
and into the dual TCD
380
via ports
4
and
3
. Thus, the valves may be connected in series to form “channels.” Each channel feeds into a corresponding thermistor pair (a measurement thermistor and a reference thermistor), which measures the amount of a component in the process sample. Alternatively, downstream analyzer valves can be arranged in the system to select a desired column or detector. The graph on which the data are presented has a series of peaks corresponding to the detected components (such as ethane, methane, etc.), and is generally referred to as a chromatogram.
FIG. 7
shows an example of a chromatogram. As various molecules elute from the columns
460
based upon their volatility, they are measured by a concentration-dependent detector such as a thermal conductivity detector (TCD), a flame photometric detector (FPD), a photoionization detector (PID), a helium ionization detector (HID), or an electrolytic detector. The measured values appear on the chromatogram as a series of peaks. The peak maximum corresponds to the absolute retention time (i.e. time elapsed from injection of sample) for each component in the gas chromatograph system, with the area under each peak being related to the concentration of that component in the sample. To operate the system most efficiently, the valve switching directs the samples from column to column at predetermined times. The columns are sized to provide adequate time between critical components (i.e. for valve switches).
FIG. 4
illustrates a simplified gas chromatograph
400
as is broadly known in the art. Sample valve
410
connects to sample-in line
420
, sample out line
430
, carrier-in line
440
and column line
450
. Sample-in line
420
connects to sample shut-off valve
470
upstream of the sample valve
410
. Immediately upstream of sample shut off, sample in line
420
connects to a sample pre-heat coil. Further upstream, sample-in line
420
connects to, e.g., a process pipeline (not shown). Downstream of the sample valve
410
, column line
450
connects to column
460
. Column
460
, in turn, connects downstream to the remainder of the gas chromatograph, including TCD
480
, with measurement line
481
and reference line
482
.
During operation, a sample of fluid is delivered from a process pipeline or similar source through sample-in line
420
. Once the sample is inside the sample valve
410
, sample shut off valve
470
is actuated, closing off sample valve
410
from the upstream sample source. At this time, the sample in the sample valve
410
is allowed to equilibrate with atmospheric pr
Lechner-Fish Teresa
Runyan Robert Jeffrey
Conley & Rose, P.C.
Daniel Industries Inc.
Kwok Helen
Politzer Jay L
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