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Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
AD.11.07.03SOUR WATER ANALYSIS USING A DUAL COLUMN HEADSPACE SAMPLING
SYSTEMAndy Burgess, David Greaves, Daniel
Murphy, Sam Rolley
Applied Analytics, Inc.
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 2
Topics of Discussion
1)Sour Water
2)Absorbance Spectroscopy
3)Henry's Law
4)Headspace Sampling System
5)Testing
6)Conclusions
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 3
What is Sour Water?
• Simply put water that contains H2S
• Just about all waste water produced by oil refineries is sour water
• Ammonia
• Aromatics
• Mercaptans
• Hydrocarbons
• Particulates
• Phenols
• Sour water can also often contain the following:
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 4
Why Measure Sour Water?
• Ammonium Bisulfide is a salt formed by the reaction of H
2S and NH
3 in an aqueous solution
• Causes accelerated corrosion
• Can plug orifices & instrumentation
• Increases maintenance
• Ammonium Bisulfide:
• H2S and NH
3 must therefore be stripped from
sour water before it can be reused in the plant
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 5
How to Measure Sour Water
Utilizing UV absorbance spectroscopy
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 6
Absorbance Spectroscopy
Beer-Lambert (Beer’s) Law:
A A = absorbance
= molecular absorptivity
b = path length
c = concentration
When solved for concentration:
E
()*b is an experimentally determined constant
• a cbA )()(
• a b
Ac
)(
)(
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 7
Absorbance Spectroscopy
• Full Spectrum Analysis• Simultaneous
measurement of absorbance at several wavelengths
• Allows for multi-component analysis
• No moving parts; all solid state devices
• Very low maintenance
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
220 240 260 280 300 320 340
Wavelength (nm)
Ab
sorb
an
ce (
AU
)
0.250.50.7511.25
% H2S
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
220 240 260 280 300 320 340
Wavelength (nm)
Ab
sorb
an
ce (
AU
)
0.250.50.7511.25
% H2S
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
220 240 260 280 300 320 340Wavelength (nm)
Ab
sorb
an
ce A
U
0.250.50.7511.25
% SO2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
220 240 260 280 300 320 340Wavelength (nm)
Ab
sorb
an
ce A
U
0.250.50.7511.25
% SO2
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 8
Liquid Measurement
• Sour water does not lend itself well to spectroscopic analysis– Contains particulates which can block or scatter light
in the flow cell– Opacity can be very high which limits the transmission
of light through the sample – Contains compounds, such as phenol, which absorb
light in the UV and act as interferences.
• It would be better if a representative gas phase sample could be made from the process
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 9
Henry's Law
• Concentration = k x Partial Pressure
• The amount of a gas dissolved in a solution at a given temperature is directly proportional to the partial pressure of the gas above the solution
Phenol H2S NH3
Boiling Point oC
181.7 -60.28 -33.34
What effects partial pressure?
Liquid Temperature
Dissolved Gas Volatility
Dissolved Gas Boiling Point
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 10
Headspace Column
Process Liquid In
Process Liquid Out
Carrier Gas Out
Carrier Gas In
Process LiquidCarrier Gas
Heat Heat
Analyte Molecule
Mass Transfer
BU
LK G
AS
FLO
W
BU
LK LIQ
UID
FLO
W
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 11
Sample Conditioning
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 12
Sample Conditioning
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 13
Experiment• Calibration
– The analyzer was calibrated on H2S in N2 and NH
3 in
N2 standards
• Parameters– Liquid & Carrier flow - constant– Column temperature & pressure - constant
– Sample pH constant • Sample Preparation
– Samples were prepared in batches by adding a controlled amount of H
2S or NH
3 to clean water
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 14
Experiment
Procedure
Clean water run through column, parameters set to desired values
A known amount of NH3 in water is run through column for 1hr
The resulting concentration is logged by analyzer
Three more samples of NH3 in water are run through column
The resulting concentration of each logged by analyzer
Column is flushed with clean water, steps 2 & 3 are repeated with samples of H
2S in water
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 15
Results
Test Duration (hours) 1
Reading Stored (1/min) 60
Average Value (PPM) 999.99
Maximum Value (PPM) 1009.82
Minimum Value (PPM) 990.73
Standard Deviation (PPM) 4.09
Slope (PPM/reading) 0.02
Slope * 60 (PPM/hour) 1.10
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 16
Results
Test Duration (hours) 1
Reading Stored (1/min) 60
Average Value (PPM) 980.54
Maximum Value (PPM) 987.75
Minimum Value (PPM) 968.87
Standard Deviation (PPM) 6.54
Slope (PPM/reading) 0.36
Slope * 60 (PPM/hour) 2.35
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 17
Results
Liquid Concentration – Sample 5 (% of Cal Sample) 25Liquid Concentration – Sample 6 (% of Cal Sample) 50Liquid Concentration – Sample 7 (% of Cal Sample) 75Liquid Concentration – Sample 8 (% of Cal Sample) 100Gas Concentration – Sample 5 (PPM) 115.8Gas Concentration – Sample 6 (PPM) 184.15Gas Concentration – Sample 7 (PPM) 245.10Gas Concentration – Sample 8 (PPM) 314.06Linearity – R2 Value 0.9994
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 18
Results
Liquid Concentration – Sample 5 (% of Cal Sample) 43.5Liquid Concentration – Sample 6 (% of Cal Sample) 61.25Liquid Concentration – Sample 7 (% of Cal Sample) 91.25Liquid Concentration – Sample 8 (% of Cal Sample) 100Gas Concentration – Sample 5 (PPM) 1300Gas Concentration – Sample 6 (PPM) 2600Gas Concentration – Sample 7 (PPM) 3800Gas Concentration – Sample 8 (PPM) 4200Linearity – R2 Value 0.9996
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 19
Conclusions
• Measuring NH3 and H2S in liquid phase is complicated by interfering compounds
• As stated by Henry's Law, representative vapor phase sample can be made using a headspace column
• Spectroscopically measured concentration of vapor phase is very stable and linear with liquid phase concentration
• Column temperature can be optimized for particular compound of interest
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 20
Conclusions
• Interference from compounds with high boiling points mitigated
• Opaque or high particulate liquids can be measured spectroscopically
• Only a small portion of sampling system is exposed to the process liquid
• Headspace column is a scaled down version of process commonly used to strip H
2S and NH
3
• Technique can be used for other applications such as H
2S in crude oil
Set Your Sails For South Shore HarborThe 56th Annual Symposium of the Analysis DivisionLeague City, Texas, USA; 10-14 April 2011
Slide 21
Thank You
Questions?