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APPLIEDA N A L Y T I C S
TM
Application Summary
Analyte(s): hydrogen sulfide
Detector: OMA-300 Process Analyzer w/ Headspace Sample Conditioner
Process Stream: crude oil
Typical Measurement Range: 0-100 ppm
Introduction
Crude oil with low sulfur content (“sweet crude”) is coveted because it is more easily processed into usable gasoline. By contrast, sour crude contains a significant H2S concentration and requires more expensive processing. Online H2S analysis is required to determine how much processing a specific feed of crude oil will require and to differentiate different crudes by their commercial value.
The composition of oil presents significant challenges to direct optical analysis. These challenges include aromatic hydrocarbons and/or phenols which absorb heavily in the UV range and act as spectroscopic interferents, particulates which scatter light, and the opacity of heavier crudes (too dark to transmit a light signal and will not allow optical analysis).
The OMA system uses a ‘headspace’ sample conditioning system to circumvent the opacity of crude oil and continuously measure H2S concentration in the stream with proven reliability. The operating principle of the system is Henry’s Law: part of the liquid sample is evaporated into ‘headspace gas’— a vapor-phase sample that is totally representative of the liquid composition and can be analyzed very easily using UV spectroscopy.
[1] AN-009: Measuring Hydrogen Sulfide in Crude Oil
Measuring Hydrogen Sulfide in Crude OilApplied Analytics Application Note No. AN-009
Headspace Sampling & Henry’s Law
Henry’s Law states that the amount of a certain gas dissolved in a solution at a given temperature is directly proportional to the partial pressure of that gas above the solution. The major implication of this for crude oil analysis is that, given certain constant conditions, the chemical composition of the headspace gas correlates directly to the composition of the opaque liquid sample. The conditions which must remain constant (temperature, pressure, carrier gas flow rate, and liquid sample flow rate) are all held constant by the headspace system.
To create a representative vapor-phase sample from the liquid crude, the headspace system uses a temperature-controlled column (12” high by 2.5” diameter application dependent). The opaque liquid sample flows in from the top while carrier gas (typically nitrogen) flows in from the bottom. The carrier gas picks up the molecules that evaporate from the liquid sample and carries them out of the column and into the flow cell for optical analysis. The system is calibrated by correlating the H2S concentration in the flow cell (gaseous sample) to the H2S concentration of a standard liquid crude oil sample.
The held temperature determines the partial pressure mix of compounds in the headspace gas. This allows us to hold the temperature of the column at a point where we get an extremely useful vapor-phase sample that contains H2S but very low levels of interfering compounds like aromatics and phenols (due to the difference in Henry’s law constants between the analyte and the interfering compounds).
liquid sample(out)
carrier gas (out)
liquid sample(in)
carrier gas(in)
liquid sample
= Raschig rings for maximizing surface area within the column for interaction between liquid and gas. Common in distillation, these rings allow for much more efficient mass transfer.
carrier gas
[2]
Applied Analytics Application Note No. AN-009
Measuring Hydrogen Sulfide in Crude Oil
AN-009: Measuring Hydrogen Sulfide in Crude Oil
H2S Absorbance Curve
Any single photodiode measurement is vulnerable to noise, signal saturation, or unexpected interference. This susceptibility to error makes a lone photodiode data point an unreliable indicator of one chemical’s absorbance.
As accepted in the lab community for decades, the best way to neutralize this type of error is to use collateral data in the form of ‘confirmation wavelengths,’ i.e. many data points at many wavelengths instead of a single wavelength:
In the figures above, each diamond represents a single photodiode and data point. The nova II registers absorbance at each integer wavelength within the measurement wavelength region and produces an H2S absorbance curve. After being calibrated on a full spectrum of pure H2S, the OMA knows the absorbance-concentration correlation for each measurement wavelength; the system can average the modeled concentration value from each wavelength to completely eradicate the effect of noise at any single photodiode.
The OMA visualizes the H2S absorbance curve in this manner and knows the expected relation of each data point to the others in terms of the curve’s structure. This curve analysis enables the OMA to automatically detect erroneous results at specific wavelengths, such as when a single photodiode is saturated with light. The normal photometer, with a single data point, is completely incapable of internally verifying its measurement.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
210 220 230 240 250
abso
rban
ce (A
U)
wavelength (nm)
20 ppm H2S
15 ppm H2S
10 ppm H2S
5 ppm H2S
[3]
Applied Analytics Application Note No. AN-009
Measuring Hydrogen Sulfide in Crude Oil
AN-009: Measuring Hydrogen Sulfide in Crude Oil
Headspace Sample Conditioning System Technical Details
The standard headspace SCS design is detailed below. The design will vary to suit the needs of the measurement application.
B C D E F G
FLOW CELLOptical measurement interface; the carrier gas travels here from the headspace column. (600mm, 316L stainless steel)
HEADSPACE COLUMNRepresentative vapor-phase sample produced from opaque liquid sample.
TEMPERATURE CONTROLLER
COLUMN BAND HEATER (2X)
A
A. Air to valveB. Instrument air inC. Gas to flare/ventD. Sample returnE. Process inF. SpanG. Zero in (N2)
FLOW METER (4X)Controls flow of carrier gas and liquid sample.
ENCLOSURE48”H X 30”W X 10”D 304 stainless steel, NEMA 4X
PRESSURE GAUGE
[4]
Applied Analytics Application Note No. AN-009
Measuring Hydrogen Sulfide in Crude Oil
AN-009: Measuring Hydrogen Sulfide in Crude Oil
Application DataPerformance SpecificationsAccuracy Custom measurement ranges available; example ranges below. Accuracy specifications
represent headspace gas sample analysis validated with span gas.
H2S0-10 ppm: ±1 ppm0-100 ppm: ±1% full scale or 1 ppm*0-10,000 ppm: ±1% full scale
*Whichever is larger.
The specifications below represent performance of the OMA-300 Process Analyzer in a typical crude oil application.
For technical details about the OMA-300 Process Analyzer, see the data sheet:http://aai.solutions/documents/AA_DS001A_OMA300.pdf
All performance specifications are subject to the assumption that the sample conditioning system and unit installation are approved by Applied Analytics. For any other arrangement, please inquire directly with Sales.
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[5]
Applied Analytics Application Note No. AN-009
Measuring Hydrogen Sulfide in Crude Oil
AN-009: Measuring Hydrogen Sulfide in Crude Oil
is a registered trademark of Applied Analytics, Inc. | www.aai.solutions
© 2017 Applied Analytics, Inc. Products or references stated may be trademarks or registered trademarks of their respective owners. All rights reserved. We reserve the right to make technical changes or modify this document without prior notice. Regarding purchase orders, agreed-upon details shall prevail.
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Subject LocationOMA-300 H2S AnalyzerBrochure
http://aai.solutions/documents/OMAH2S.pdf
OMA-300 Process AnalyzerData sheet
http://aai.solutions/documents/AA_DS001A_OMA300.pdf
Advantage of Collateral DataTechnical Note
http://aai.solutions/documents/AA_TN-202_CollateralData.pdf
Multi-Component AnalysisTechnical Note
http://aai.solutions/documents/AA_TN-203_MultiComponentAnalysis.pdf
Further Reading
AN-009: Measuring Hydrogen Sulfide in Crude Oil[6]
Applied Analytics Application Note No. AN-009
Measuring Hydrogen Sulfide in Crude OilRevised 21 April 2017
