Ifpa 2006 direct measurement of low levels of h2 s in

Direct Measurement of Low Levels of H2S in Moderate to Direct Measurement of Low Levels of H2S in Moderate to High Levels of Mercaptans in Refinery ApplicationsHigh Levels of Mercaptans in Refinery Applications

John A. Constantineau, Yael Barshad & Yoav Barshad John A. Constantineau, Yael Barshad & Yoav Barshad

Applied Analytics, Inc., Maynard, MA;Applied Analytics, Inc., Maynard, MA;

IFPAC 2006IFPAC 2006

Applied Analytics, Inc.


IntroductionIntroduction

Sulfur compounds, such as H2S, COS, CS2 and Sulfur compounds, such as H2S, COS, CS2 and RSH (mercaptans) in oil and natural gas cause RSH (mercaptans) in oil and natural gas cause environmental pollution.environmental pollution.

The process of removing these compounds form The process of removing these compounds form the oil or gas is called “Sweetening the oil or gas is called “Sweetening

Mercaptans with low molecular weight are Mercaptans with low molecular weight are particularly volatile toxic and corrosive, and are particularly volatile toxic and corrosive, and are frequently removed via caustic washing over a frequently removed via caustic washing over a catalyst bed. catalyst bed.



The catalyst needs to be regenerated every several The catalyst needs to be regenerated every several hours by heating and purging with a dry carrier gas hours by heating and purging with a dry carrier gas stream. stream.

TThe RSH and H2S in the carrier gas stream needs to be he RSH and H2S in the carrier gas stream needs to be

monitored.monitored. The application requires the simultaneous monitoring of The application requires the simultaneous monitoring of percent RSH and low PPM level H2S. percent RSH and low PPM level H2S.

Due to the large dynamic range this application is Due to the large dynamic range this application is traditionally done by a GC. traditionally done by a GC.



A UV process diode array analyzer was utilized to A UV process diode array analyzer was utilized to simultaneously monitor both components simultaneously monitor both components

The analyzer’s high resolution, high signal to noise ratio The analyzer’s high resolution, high signal to noise ratio and low stray light has made it feasible. and low stray light has made it feasible.

In addition optimization of the cell path length and the In addition optimization of the cell path length and the use of robust analysis procedures have insured a long use of robust analysis procedures have insured a long term low maintenance solution to the simultaneous term low maintenance solution to the simultaneous monitoring of ppm level H2S and percent mercaptans.monitoring of ppm level H2S and percent mercaptans.


LNG- Liquid Natural GasLNG- Liquid Natural Gas

LNG is the result of cooling natural gas LNG is the result of cooling natural gas to approximately 260 degrees below to approximately 260 degrees below zero. This cooling both liquefies and zero. This cooling both liquefies and condenses the natural gas so that it condenses the natural gas so that it occupies a space approximately 600 occupies a space approximately 600 times smaller than when in its gaseous times smaller than when in its gaseous state . state . Natural gas is composed primarily of Natural gas is composed primarily of methane. Small quantities of nitrogen, methane. Small quantities of nitrogen, oxygen, carbon dioxide, sulfur oxygen, carbon dioxide, sulfur compounds, and water may also be compounds, and water may also be found. found. The liquefaction process removes the The liquefaction process removes the oxygen, carbon dioxide, sulfur oxygen, carbon dioxide, sulfur compounds, and water. compounds, and water.


LNG ProcessingLNG Processing


ImpuritiesImpurities

Mercaptans - methyl mercaptan: CH3-SH and Mercaptans - methyl mercaptan: CH3-SH and ethyl mercaptan: C2H5-SH ethyl mercaptan: C2H5-SH

H2SH2S

COS & CS2COS & CS2

Mercaptans have the general formula R – S – H, Mercaptans have the general formula R – S – H, where R represents an aliphatic or cyclic radical.where R represents an aliphatic or cyclic radical.


H2S and RSH removalH2S and RSH removal

H2S and CO2 are impurities removed by an amine treating unit.

The RSH removal process is optimized by first eliminating the H2S which interferes with the mercaptan extraction efficiency and regeneration of the circulating caustic

A caustic regeneration system converts the extracted mercaptans to disulfide oil that is separated and removed, while the regenerated caustic is returned to the extraction section.

See reactions


The ReactionsThe ReactionsExtraction

RSH + NaOH RSNa + H2O

Oxidation and Caustic Regeneration over a catalyst

2 RSNa + ½ O2 + H2O 2 NaOH + R-SS-R

Reaction

2 RSH + ½ O2 R-SS-R + H2O

The resultant disulfides are separated and the caustic reused for extraction.


Catalyst RegenerationCatalyst Regeneration

Constitutes removal of the adsorbate from the Constitutes removal of the adsorbate from the molecular sieve bed by heating and purging with molecular sieve bed by heating and purging with a carrier gas .a carrier gas .Every several hours the system is switches Every several hours the system is switches between catalysts, where one is used for the between catalysts, where one is used for the oxidation and the other goes through a oxidation and the other goes through a regeneration.regeneration.For optimal results the concentration of RSH and For optimal results the concentration of RSH and H2S during the catalyst regeneration needs to H2S during the catalyst regeneration needs to be monitored.be monitored.


Fiber-optics-UV-VIS-Diode arrayFiber-optics-UV-VIS-Diode arrayschematicsschematics


Fiber optics – Fiber optics – Diode Array Diode Array Process Process AnalyzerAnalyzer


Flow cellFlow cell


DA to replace GC DA to replace GC

No separationNo separation

Instantaneous readingInstantaneous reading

Solid state – very low maintainsSolid state – very low maintains


Absorbance spectra of RSH at different flow paths

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

210 220 230 240 250 260 270 280

nm

AU

path A

2 x path A

5 x path A

10 x path A


Absorbance spectra of H2S at different path lengths

0

0.1

0.2

0.3

0.4

0.5

0.6

215 220 225 230 235 240 245nm

AU

path A

2 x path A

5 x path A

10 x path A


Absorbance spectra of H2S 100PPM & RSH 1% path length A

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

210 220 230 240 250 260 270 280

nm

AU

H2S

RSH

Total


Method optimization parametersMethod optimization parameters

Path length : Path length : – Increase path for higher signal for the low H2S levelsIncrease path for higher signal for the low H2S levels– Decrease path to allow for measurements in the low Decrease path to allow for measurements in the low

UV below 2 AU when full scale RSH are presentUV below 2 AU when full scale RSH are present

Wavelength Measurement rangeWavelength Measurement range– Low UV for higher H2S signalLow UV for higher H2S signal– Higher wavelength to allow for a high levels of RSHHigher wavelength to allow for a high levels of RSH

Application Application

H2S 0-20 PPMH2S 0-20 PPM

SO2 0-0.5%SO2 0-0.5%

ApplicationApplicationTail gas Tail gas

H2S 0-1%H2S 0-1%

SO2 0-0.5%SO2 0-0.5%


Absorbance spectra of H2S & SO2 in a tail gas application

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

220 230 240 250 260 270 280 290nm

AU

SO2

H2S


The Diode-Array Advantage:The Diode-Array Advantage:

Fast spectral acquisitionFast spectral acquisition

Simultaneous multi-wavelength measurementSimultaneous multi-wavelength measurement

Wavelength reproducibilityWavelength reproducibility

High sensitivityHigh sensitivity

Measurement StatisticsMeasurement Statistics

Very high reliability and ruggednessVery high reliability and ruggedness

Solid state no moving partsSolid state no moving parts

The diode-array spectrophotometer VS. conventional mechanical scanning The diode-array spectrophotometer VS. conventional mechanical scanning spectrophotometers, non dispersive device and GC:spectrophotometers, non dispersive device and GC:

Ifpa 2006 direct measurement of low levels of h2 s in

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