Apparatus and method for sour gas analysis

A flame photometric detector for use with a gas chromatograph comprises a burner to produce a cool hydrogen flame and excite molecules of the gas sample to emit light characteristic thereof, a transparent chimney surrounding the burner and flame to enhance characteristic light emission, a selective light filtering means, and sensing means positioned so as to exclude the flame from its sensing area. The device and method of operation are characterized by a substantially broader range of detection and measurement than is possible with present commercially available equipment.

BACKGROUND OF THE INVENTION 
Technological developments have significantly increased the demand for more 
reliable and versatile detection and measuring equipment. The detection 
and measurement of sulfur and sulfur bearing constituents are a prime 
concern for both technological and environmental reasons. A suitable 
device should give a specific, clear response to, and a rapid recovery 
from, all the volatile sulfur compounds in the gas sample. The device 
should further minimize interference from other constituents such as 
hydrocarbons. Measurements from the low ppm, (parts per million) range to 
a high pph (parts per hundred) range should be well within the detector's 
capabilities. The device should possess a low noise level, operational 
stability and sulfur response repeatability. 
A special problem is presented by sour natural gas which may contain large 
amounts of hydrogen sulfide and comparatively smaller amounts of other 
sulfur bearing compounds such as carbonyl sulfide and lighter mercaptans. 
Generally, determination of the dilute components is frustrated because of 
the higher concentration contaminants, ultimately giving an unreliable 
analysis. 
One commercially available unit is a phosphorus and sulfur specific flame 
photometric detector for use with a gas chromatograph. The device 
comprises a burner, a mirror and optical filters for the sulfur and 
phosphorous components, ignition plugs and a photomultiplier tube for each 
of the optical filtering devices. The device is characterized by alignment 
of the photomultiplier optical axis and survey area with the uppermost 
portion of the burner flame. Such positioning increases the sensing 
element's sensitivity to hydrocarbon presence and interferes with a clear 
reading specific to the sulfur and/or phosphorous content. Furthermore, 
the device has a limited operational range of from approximately 5 ppb 
(parts per billion) to 5 ppm (parts per million). "Sulfur saturation" 
severely limits the unit's reliable operational range and makes the 
apparatus undesirable for sour gas analysis. 
SUMMARY OF THE INVENTION 
The present invention, to a large extent, overcomes the difficiencies of 
the prior art devices. The invention is directed toward a 
detection-measurement device and related method characterized by good 
versatility and a wide operating range to produce highly reliable and 
accurate measurements of the constitutent being analyzed. 
In a preferred embodiment, the device includes a burner having a relatively 
cool hydrogen flame which excites the molecules of a gas sample to produce 
characteristic light emissions. A transparent chimney surrounding the 
flame serves as a cooling chamber and concentrates the emissions in a 
preselected area. An optical filter allows only the characteristic wave 
lengths of the sulfur compounds to pass therethrough to a photo sensing 
device. The sensing device is so positioned as to exclude the burner flame 
from its optical sensing path, but include the area wherein the light 
emissions have been concentrated. Such positioning effectively eliminates 
flame noise and hydrocarbon interference from the sensing path of the 
photo sensing device. A recordable and measurable signal is generated and 
the same transmitted to a display or readout station. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the specification. For a better understanding of the invention, its 
operating advantages and specific objects obtained by its use, reference 
should be had to the accompanying drawings and the descriptive matter in 
which there is illustrated and described a preferred embodiment of the 
invention.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention generally relates to a method and apparatus for 
detection and measurement of sulfur and sulfur bearing compounds, and is 
more specifically directed at the detection and measurement of sulfur and 
sulfur bearing compounds in sour natural gas samples. 
Referring to FIG. 1, an inert carrier gas 10, such as nitrogen, entrains a 
sample of the gas to be analyzed in a sample injector 14. The carrier gas 
is piped to and from the injector 14 by lines 12 and 16 respectively. Line 
16 feeds into chromatograph column 18 wherein a physical separation of the 
sample's gaseous components, based on their respective partition 
coefficients, is carried out. The separation process is basically an 
adsorption process with the less mobile components of the gaseous sample 
(stationary phase) being physically bound to the column packing 20. In the 
present embodiment, the column is inert to sulfur compounds which are 
contained in the mobile phase. The latter phase, still entrained by the 
carrier gas, is not detained by the packing but flows from the column via 
line 22 and continues to the detection unit 24 wherein it mixes with 
hydrogen fuel stream 26. Combustion sustaining gas stream 28 is introduced 
into the detector and ultimately produces combustion products which are 
exhausted from the detector via line 30. Unit 24 produces a recordable and 
measurable signal which is transmitted to display 34 by output or lead 32. 
Referring to FIG. 2, flame detector 24 comprises a detector block 36 with 
an inspection plate 38. Block 36 is connected to a photomultiplier tube 40 
by light tube 42. Light tube 42 houses an optical filter (FIG. 4) and 
accessory devices such as secondary filters (not shown) and condensing 
lenses (not shown). The light tube may be operated with a slightly 
pressurized gas to clear the tube of scattering or absorbing substances 
and dissipate undesirable levels of heat. Lead 32 feeds the signal 
generated by the tube 40 to display 34 (FIG. 1). Hydrogen fuel is 
introduced into block 36 through port 44. A thermocouple (not shown), 
which allows continuous monitoring of detector block temperature, is 
inserted in port 46. Port 48 is for insertion of an electrically operated 
heater cartridge capable of producing an operating temperature of 
300.degree. C. (572.degree. F.). Port 50 (shown in phantom) is for 
introduction of the combustion sustaining gas into the block. 
FIG. 3 depicts block 36 in cutaway. Block 36 is formed with cavities 54, 
56, 58 and 60. Cavity 54 extends from port 52 and is used for introducing 
the gas sample from column 18 into burner or flame tip 62. The burner or 
flame tip is preferably made of a high nickel alloy or other material 
relatively unreactive with sulfur. Cavity 56 extends from port 44 and 
leads the hydrogen fuel to burner 62. A combustion sustaining gas, such as 
air or oxygen, enters block 36 through port 50 and flows through cavity 58 
to an annular gas chamber 66 which surrounds the burner. This gas then 
feeds into cavity 60 to support combustion of the hydrogen and gas sample. 
A transparent chimney 68 surrounds flame tip 64 and is held in position by 
support ring 70. The chimney is constructed of transparent material which 
is preferably quartz or a laboratory grade glass such as Pyrex. The 
chimney extends a preselected distance above the uppermost portion of the 
flame tip and is critical to substantially improved performance of the 
unit. 
Referring to FIG. 4, block 36 is formed with an enlarged bore 72 
transversing cavity 60. One end of the bore is used for access to the 
cavity and when not being so used is closed off by inspection plate 38 
(FIG. 2). The bore's opposite end is attached to the photomultiplier tube 
40 by the tightly sealed light tube 42. Light tube 42 houses selective 
optical filter 74. The light tube is critically positioned so that the 
line of sight of the sensing unit or its sensing area does not include any 
portion of the cool hydrogen flame. The optical filter is selective and 
allows only particular emissions to pass therethrough thus minimizing 
hydrocarbon interference. 
The analyzer, having received the carrier gas and mobile phase from the 
chromatograph column operates in the following manner. The hydrogen fuel 
and gas sample are mixed in the lower burner mixing area 65 and ignited by 
any conventional ignition arrangement. Combustion sustainig gas flows 
through the annular chamber 66 surrounding the burner and combusts with 
the hydrogen-sample gas mixture to produce a cool hydrogen flame. Such an 
arrangement improves operational stability. The flame temperature lies 
within the 300.degree. C.-800.degree. C. (572.degree. F.-1472.degree. F.) 
range rather than the 2300.degree. C. (4172.degree. F.) temperature of a 
common type of commercially available flame detection unit. The hydrogen 
to combustion gas (air) ratio and the hydrogen-carrier gas ratio are of 
the order of 3:1 and 5-8:1, respectively. In cases in which oxygen is used 
as the combustion gas, the hydrogen to oxygen ratio is about 15:1. The 
cumulative effect of these selected flow ratios significantly contributes 
to producing the cool flame and clean flow effect and compares favorably 
with the prior art fuel to combustion gas ratio of 2:3. The clean flow 
effect is the most significant factor contributing to the detector's rapid 
response to the sulfur bearing constituents. The relatively cool hydrogen 
flame significantly enhances formation of S.sub.2 molecules in a high 
energy state which produces the characteristic light emission at the 394nm 
wave length (1 nm = 0.000000001 meters) upon decay to a lower energy 
state. Evidence indicates that low energy S.sub.2 molecules absorb 394 nm 
light producing an undesirable quenching effect on the properly excited 
molecules. The high flow rate carrier gas, besides contributing to the 
cooling effect of the flame, sweeps spent S.sub.2 molecules (low level 
energy) from the detection device. 
Transparent chimney 68, which surrounds the uppermost section of the burner 
and the entire flame, functions as a contact type heat exchanger to cool 
the excited S.sub.2 molecules to a lower energy state where the 394 nm 
light emissions occur. The chimney further serves to concentrate maximum 
emission of the light into a specific area above the hydrogen flame in 
full view of the sensing unit scan area. This particular enfiguration is 
credited with giving a 400-500% more specific response to sulfur than 
prior art units. Detector dead volume, which represents areas of 
stagnation of the decaying molecules, is significantly reduced by the 
presence and operation of the chimney device. This reduction is credited 
with the definite and sharp signal peak produced by the inventive 
detector. Where polar and highly reactive species such as sulfur compounds 
are being analyzed, the chimney isolates such species from being adsorbed 
on or chemically reacting with surrounding metal surfaces. Interreaction 
of any type with metal housing can seriously impair the analyzer's 
reliability. Dimensional characteristics of the chimney are important for 
it to efficiently and effectively function in the multipurpose manner 
above described. For the common analytical instrument burner, the chimney 
should have an inner diameter of 0.75 centimeters (0.34 inches) and extend 
3 centimeters (1.35 inches) above the burner or flame tip. Variation from 
these dimensions leads to flame blow out and inefficient cooling of the 
over excited S.sub.2 molecules. 
The concentrated light emissions of the properly excited S.sub.2 molecules 
are selectively allowed to pass through filter 74 in light tube 42. Filter 
74 may be of a type which allows a number of varied wave lengths to pass, 
or it may be a narrow band pass filter specific for the 394 nm wave 
length. The latter type filter effectively filters out most other signals. 
The selectively filtered emissions pass through the light tube to the 
photomultiplier where, based on the intensity of the emissions, an 
electrical signal is generated and amplified or merely generated and 
transmitted to a recording or a display panel. 
Comparative testing of the present invention with prior art devices 
indicates superior performance over commercially available units. 
Referring to FIGS. 5 and 6, a calibration gas containing 0.5% of each of 
the following sulfur components was used to test and compare the present 
invention with a prior art device. 
______________________________________ 
REFERENCE 
COMPONENT NUMERAL 
______________________________________ 
Hydrogen Sulfide (H.sub.2 S) 
110 
Carbonyl Sulfide (COS) 112 
Methyl Mercaptan (CH.sub.3 SH) 
114 
Ethyl Mercaptan (CH.sub.5 SH) 
116 
Isopropyl Mercaptan (C.sub.3 H.sub.7 SH) 
118 
n-Propyl Mercaptan (C.sub.3 H.sub.7 SH) 
120 
t-Butyl Mercaptan (C.sub.4 H.sub.9 SH) 
122 
n-Butyl Mercaptan (C.sub.4 H.sub.9 SH) 
124 
i-Butyl Mercaptan (C.sub.4 H.sub.9 SH) 
126 
______________________________________ 
The respective detector scans clearly indicate that the present invention 
produced a more accurate and more specifically defined sample analysis 
than was produced by the prior art device. Furthermore, the present 
invention did not produce folding or tailing peaks which are indications 
of saturation and erratic unit performance. FIGS. 7 and 8 show the 
invention-detector's performance on gas samples containing 116 ppm and 
113,000 ppm of H.sub.2 S respectively. The gas samples also contained low 
concentrations of the other sulfur containing compounds listed and 
numbered as above. While attenuation adjustments were periodically needed 
with the higher concentration samples, reliably accurate results were 
obtained. The prior art device could not analyze the samples used for the 
data displayed in FIGS. 7 and 8. 
The present invention has also been found useful for analysis of sour 
natural gas samples and LPG fractions of petroleum. The device can detect 
a wide variety of sulfur bearing constituents in quantities from about 0.1 
ppm up to and in excess of 5000 ppm. Specifically, in addition to those 
listed above, the device can be used for determination of sulfur dioxide 
(SO.sub.2), carbon disulfide (CS.sub.2) dimethylsulfide (C.sub.2 H.sub.6 
S), ethylmethylsulfide (C.sub.3 H.sub.8 S), s-butyl, or i-butyl mercaptans 
(C.sub.4 H.sub.9 SH), and diethylsulfide (C.sub.4 H.sub.10 S). 
As used herein the term "light" includes not only visible light but also 
radiation having wavelengths longer and shorter than the visible spectrum. 
The terms and expressions which have been employed are used as terms of 
description and not of limitation, and there is no intention in the use of 
such terms and expressions of excluding any equivalents of the features 
shown and described or portions thereof, it being recognized that various 
modifications are possible in the scope of the invention.