SS2100 - Flare Gas Analyzer Solution

Application Note

H2S in Refinery Flare Gas
Refinery flares present a special challenge in measuring H2S due the variability of the background stream composition. See Table 1 for a typical composition of refinery fuel gas. Nearly all analytical technologies are sensitive to rapid changes in background composition to some extent. Fortunately, the regulations recognize this fact and have an exception for upsets and leaks through relief valves.

Specifically, 40 CFR Part 60, Subpart J, paragraph 60.104 (a) (1) states:

“The combustion in a flare of process upset gases or fuel gas that is released to the flare as a result of relief valve leakage or other emergency malfunctions is exempt from this paragraph.”

So an important part of specifying and designing an analyzer for H2S in Flare Gas is to define the “normal” gas. To prevent in the ingress of atmospheric air into the flare header, a purge or sweep gas is used. This may be refinery fuel gas, natural gas or some other light gas that is inert and non-corrosive. The design basis of the analyzer should be this purge gas composition.

Online H2S Analyzers
Many refinery sites have installed on-line gas chromatography (GC) systems for this measurement. In addition to their initial cost, GC’s have high consumable costs for carrier gas, columns and other parts. Additionally, they require frequent maintenance, especially those utilizing a flame photometric detector (FPD). Lead-Acetate tape systems can be used, which are expensive, use consumables, and require constant maintenance. Additionally, the spent tape is considered hazardous waste and is expensive to dispose.

SpectraSensors’ Solution
Only SpectraSensors employs Differential TDL spectroscopy to get accurate measurements in the changing background gas composition of flare gases. The sample is scrubbed using a proprietary Copper nanoparticle absorbent that removes only H2S, and the background spectrum is acquired. Then the spectrum of the raw, un-scrubbed sample is acquired and the background spectrum is subtracted, leaving only the laser light absorbance due to H2S. The same background spectrum can be used until the background gas changes which is detected automatically by the smart controller logic looking at the background spectrum. Since the scrubber is only used intermittently, its lifetime is typically a minimum of 18 months. Software calculates the loading of H2S on the scrubber and gives a warning when it is time to replace the scrubber, and an indicating scrubber downstream warns if the scrubber fails prematurely. The scrubber is easily replaced in a few minutes; the spent scrubber contains no hazardous materials and can be exchanged at a reduced price for a new scrubber.

The laser and detector are isolated from the sample and protected against corrosive gases. The analyzer is extremely fast (typically 16 sec/measurement) and the results are highly reliable. Requiring no consumable gases or lead acetate tapes, the cost of ownership is extremely low.

http://www.gpoaccess.gov/cfr/index.html

Component	Minimum (Mole%)	Normal (Mole%)	Maximum (Mole%)
Hydrogen Sulfide (H2S)	0	150 ppm	300 ppm
Hydrogen	25	40	65
Nitrogen	0	4	20
Oxygen	0.1	1	5
CO	0	0.5	5
CO2	0	1	5
Methane	15	30	55
Ethane	5	8	15
Ethylene	1	6	15
Propane	1	5	15
Propylene	1	2	5
i-Butane	0	1	5
n-Butane	0	1	3
C5+	0	1	5

 

The background stream composition must be specified for proper calibration and measurement performance. Specify the Normal composition, along with the minimum and maximum expected values for each component, especially H₂S, the measured component. Other stream compositions may be allowable with approval from SpectraSensors. 

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Data Sheet

Performance
Target Components	H2S in Flare Gas
Typical Measurement Ranges	0 – 10 through 0 – 300 ppm*
Typical Precision	±0.5 ppm or 2% of Full Scale
Measurement Response Time	1 to ~60 sec
Principle of Measurement	Differential Tunable Diode Laser Absorption Spectroscopy (H2S Scrubber Included)
Environmental Temperature Range	-20º to 50ºC (-4º to 122ºF) -10º to 60ºC (14º to 140ºF) optional
Sample Inlet Pressure	70 kPag (10 psig) typical 210 kPag (30 psig) maximum
Cell Temperature	Maintain at 50ºC +/- 0.2ºC
Maximum Cell Pressure	70 kPag (10 PSIg)
Sample Flow Rate	3-4 L/min (6.4 to 8.5 scfh)*
Validation	Certified blend of H2S in Nitrogen balance
Electrical
Power	100-240 VAC, 50-60 Hz standard
Max Current	Controller: 1 A @ 120 VAC
Controller to Cell Cable Length	1m standard (3m, 5m & 10m available optionally)
Communication	Current Loop Output 4-20 mA Isolated, 1200 ohms @ 24 VDC max load Serial: ASCII Text RS232C standard Modbus RS232C
Digital Outputs	4 ea. 12 VDC for valve operations Scrubber (if required), Process/Val, Val 1, Val 2 5 SPDT (Form C) Dry Contact Alarms Common Fault, Val 1 Active, Val 2 Active, Val Fail, one user-assignable to any alarm
LCD Display	Concentration, Cell Pressure and Temperature, Diagnostic Data
Physical
Controller Enclosure	NEMA 4X - Stainless Steel standard
Controller Dimensions	343mm H x 305mm W x 165mm H (13.5" H x 12" W x 6-7/16" D)*
Weight Approximately	13.1 Kg (28.6 lbs)*
Sample Cell Dimensions	28m Herriot Cell 559 mm H × 127 mm W (22” H × 5” W)
Sample Cell Construction	316L Series Polished Stainless Steel standard
Number of Sample Cells	1 (single channel SS2100) or 2 (dual channel SS2100)
Area Classification	CSA Certified for Class I, Division 2, Groups ABCD Temp Code T3C  ll 2G Ex d llB+H2 T5; Tamb : -20 ÷ +60 °C
Dimensions with Sample System	1678 mm H x 613 mm W x 427 mm D (66” H x 24-1/8” W x 16-13/16” D)
Weight with Sample System (approx.)	68 kg (150 lbs.)
* Application specific; consult factory

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