DIODE ARRAY PROCESS ANALYZER FOR SULFUR RECOVERY APPLICATIONS

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DIODE ARRAY PROCESS ANALYZER - FOR SULFUR
RECOVERY APPLICATIONS

• Aaron J. Rollo
• Applied Analytics, Inc.
• 4 Clock Tower Place,
• Maynard, Massachusetts 01754
• www.a-a-inc.com

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Objective

• To utilize a process analyzer based on UV
  absorbance techniques with diode array detection
  for a wide range of sulfur recovery applications.
• This paper reviews the utilization of
  spectroscopy in step with diode arrays, a
  time-tested, industry-accepted technology for
  sulfur recovery applications

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Instrumentation

• The analyzers consist of four major subunits
• A light source that generates electromagnetic
  radiation
• A sample area (flow cell or in-situ probe)
• A dispersion device that selects a particular
  wavelength from the broad band radiation of the
  source (concave holographic grating)
• A diode array detector to measure the intensity
  of radiation at each wavelength

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Spectrophotometer- Schematic Diagram
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Process Analyzer UV Diode-Array Spectrophotometer
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Application Variables

• When measuring the same analyte but at various
• concentrations, the following parameters can be
• adjusted
• Wavelength range
• Flow cell path length and
• Pressure

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1. Wavelength Range

• The measurement wavelength range is set per
  application. For H2S in the low PPM range the
  optimal wavelength range is 215-225nm, whereas
  for level H2S it is 230-240nm
• Optimization
• Light throughputs are lower in the low UV
• Absorbance signals are higher in the low UV
  (higher sensitivity)

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Absorbance Spectra

• Absorbance spectra of H2S PPM levels

Absorbance spectra of H2S PPM levels
- Different path lengths flow cells were used
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2. Flow Cell Path Length

• Optimization
• The longer the flow path the higher the
  absorbance signal, allowing for measurement of
  low level H2S
• Transmitting light in the low UV through a long
  flow cell is a challenge

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Flow Cells
1.

• 1. 600mm flow cell
• 2. 10mm flow cell
• 3-4. NeSSI type

2.
3.
4.
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High Pressure Flow Cell
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3. Pressure

• Samples at the same concentration, in gas
  application, have higher absorbance at higher
  pressures.

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Ideal Gas Law
P V n R T
Assuming All collisions between atoms or
molecules are perfectly elastic, there are no
intermolecular attractive forces. Molecules are
referred to as perfectly hard spheres which
collide but which otherwise do not interact with
each other.

• P pressure
• V volume
• N concentration
• T temperature
• R universal gas
• constant 8.3145 J/mol K

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Low UV Detection - Requirements

• High-resolution
• The resolution depends on the slit width
• The smaller the slit the higher the resolution
  (the light throughput is lower)1 nm resolution
• High signal-to-noise
• Low stray light.

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Resolution and H2S Spectra

• The importance of measuring the absorbance at
  high resolution is demonstrated

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Signal-to-noise Ratio (Low Levels Absorbance
Measurements)
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Interfering Components

• such as benzene can be corrected for in the
  calibration method

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Application The Monitoring of H2S 0-10 PPM /-
0.1 PPM in Sales/Sweet Gas.

• The spectroscopic approach allows for a direct
  measurement and does not require
• Separation
• Sample preparation
• Use of additives

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H2S (PPM ) Absorbance Spectra
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The Challenge Absorbance Signals are Very Low
for Sub-PPM Levels

• To enhance the signal
• Use a long flow cell path
• The challenge transmitting light in the low UV
  through a long flow cell
• Increase the pressure
• The challenge measuring at high pressures with a
  mechanical sealing flow cell that is still simple
  to maintain
• Measure absorbance in the low UV range
  (190-220nm)
• The challenge Obtaining high light throughputs
  with low stray light

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Application H2S 0-100

• Pre Sulfur recovery unit (SRU) analyzer.
• The same analyzer can be also used for the
  monitoring of 0-100 H2S.
• The signals coming from this analyzer are used to
  control the air input to the sulfur recovery
  (feed-forward control) and for evaluation of the
  process effectiveness. The methods wavelength
  range is higher, and a shorter flow cell is used.
  
• This application is relatively simple and
  straight forward.

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H2S Trends Graphs
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Application Tail Gas Air Demand

• The Claus sulfur recovery process is commonly
  utilized for the
• removal of sulfur compounds from fossil fuels.
  The process is
• optimized when the correct stoichiometric ratio
  of H2S to SO2
• (air demand) is achieved.
• The efficiency of the recovery process depends on
  accurate measurements of the H2S to SO2 ratio.
• To obtain accurate measurements, other stream
  components
• such as CS2 , COS and sulfur vapor have to be
  taken into account.

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Reactions

• The Claus sulfur recovery process
• Reaction furnace
• 3H2S 3/2 O2 SO2 H2O 2H2S
• 2. Catalytic converters
• 2H2S SO2 2H2O 3/xS(s)

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Components to Measure

• H2S 0-2
• SO2 0-1
• COS CS2 0-2000 PPM
• Sulfur vapor

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TLG UV Spectra
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Multi-component Analysis Method

• The analyzer measures a complete UV spectrum of
  the process at 1 nm resolution.
• Multi-component analysis is used for the
  quantification of the components by a
  mathematical de-convolution of the UV spectrum of
  the stream
• Standard gases can be introduced at any time to
  test the accuracy and reproducibility of the
  analyzer.

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Tail Gas Probe
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Tail Gas Analyzer Main Benefits

• No moving parts - diode array detection
• The measurements are instantaneous and continuous
  
• Flexibility in the selection of wavelength range
  to be used for the measurement.
• Calibration and / or verification of the analyzer
  can be done with cal gases. No need to rely on
  laboratory measurements, which are extremely
  difficult in this application due to the
  instability of the sample, or optical filters
  which do not represent the actual spectra of the
  measured components.
• Once the analyzer is calibrated there is no need
  to span or recalibrate only zero
• There is no drift associated with wavelength
  reproducibility since there are no moving parts
  associated with wavelength selection.
• Includes fiber optics allowing the flow cell to
  be installed further away from the electronics

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• In-situ detection cold finger demister probe
• Fast response
• No sample lines
• Steam, nitrogen and sample gases can be easily
  introduced into the probe
• Same analyzer can be used to measure an air
  demand ratios from 2 to 10
• Accurate concentration readings
• Long-life-lamp (5-10 years)

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Conclusions

• One analyzer for a variety of different sulfur
  recovery applications
• Technology
• Utilizing a high-resolution (1 nm)
  wide-dynamic-range (190-800) spectrophotometer
  allows for the measurement of different
  components, various concentration ranges and
  multi or single components.
• Benefits in using one analyzer for a variety of
  applications
• Availability of spare parts
• Training on one specific technology
• Ability to swap analyzers
• Service