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

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1

220 240 260 280 300 320 340

Wavelength (nm)

Ab

sorb

an

ce (

AU

)

0.250.50.7511.25

% H2S

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220 240 260 280 300 320 340

Wavelength (nm)

Ab

sorb

an

ce (

AU

)

0.250.50.7511.25

% H2S

0

0.1

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220 240 260 280 300 320 340Wavelength (nm)

Ab

sorb

an

ce A

U

0.250.50.7511.25

% SO2

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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