The Claus process is the industry standard and so the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. The process is commonly referred to as a sulfur recovery unit (SRU) and is very widely used to produce sulfur from the hydrogen sulfide found in raw natural gas and from the by-product sour gases containing hydrogen sulfide derived from refining petroleum crude oil and other industrial facilities. There are many hundreds of Claus sulfur recovery units in operation worldwide. In fact, the vast majority of the 68,000,000 metric tons of sulfur produced worldwide in one year is by-product sulfur from petroleum refining and natural gas processing plants.

1. communication and measurement

2. TAIL GAS ANALYZER
ANALYSIS OF H2S AND SO2
The Tail Gas analyzer is part of the BAGGI BASE® Instruments
Series.
It is the result of combining the latest state-of-the-art-technology
with over 60 years of industry experience.
This real time process analyzer is ideal for the measurement of
the percentage of H2S and SO2 within the outlet gas stream from
the sulfur recovery unit process (Claus process unit). It uses
ultraviolet/visible spectrophotometry for achieving very accurate
results.
The fully automated instrument is accompanied with a sampling
loop, tailored according to the Customer’s requirements.
An ATEX version is available for operation in potentially explosive
atmospheres.

3. The Claus process is the industry standard and so the most
significant gas desulfurizing process, recovering elemental sulfur
from gaseous hydrogen sulfide.
The process is commonly referred to as a sulfur recovery unit
(SRU) and is very widely used to produce sulfur from the
hydrogen sulfide found in raw natural gas and from the by-product
sour gases containing hydrogen sulfide derived from refining
petroleum crude oil and other industrial facilities.
There are many hundreds of Claus sulfur recovery units in
operation worldwide.
In fact, the vast majority of the 68,000,000 metric tons of sulfur
produced worldwide in one year is by-product sulfur from
petroleum refining and natural gas processing plants.
INTRODUCTION

4. The Claus reaction to convert H2S into elemental sulfur requires
the presence of one mole of SO2 for each two moles of H2S:
Claus gases (acid gas) with no further combustible contents apart
from H2S are burned in lances surrounding a central muffle by
the following chemical reaction:
2 H2S + 3 O2 → 2 SO2 + 2 H2O
This is a strongly exothermic free-flame total oxidation of
hydrogen sulfide generating sulfur dioxide that reacts away in
subsequent reactions.
The most important one is the Claus reaction:
2 H2S + SO2 → 3 S + 2 H2O
The overall equation is:
10 H2S + 5 O2 → 2 H2S + SO2 + 7/2 S2 + 8 H2O

5. Schematic flow diagram of the Claus Process:

6. Our scope is to supply a way to measure the presence of the
various compounds (H2S and SO2) in the process gas.
An accurate analysis of the compounds composing the Tail
Gas is very important in order to have a real-time monitoring
and regulation of the production process.
With a well run and controlled process in plant you may save
time and money.

7. The Tail Gas analyzer of the BAGGI BASE® series provides
the required capabilities for the real time measurements of H2S
and SO2 in Tail gas stream.
The method of analysis is Ultra-Violet/Visible
spectrophotometry: faster, more rugged and less expensive
than Gas Chromatography.
The instrument provides high wavelength resolution and it can
be equipped with multiple cells and spectrometer module.

8. PRINCIPLE OF MEASUREMENT
The BASE® Series Instrument can Handle many of the well know
spectroscopy techniques.
Thanks to his modular design The BASE® Series Instrument is
able to use a large area of the electromagnetic spectrum and
different analytical techniques to measure many compounds.
This techniques can be summarised as:
- UV/VISIBLE Absorption and fluorescence
- NIR / SWIR / FTIR / FTNIR Absorption, Fourier Transform
- TLD Absorption

9. The measurement technique relies on the Beer-Lambert law. This
one is a relationship that relates the absorption of
electromagnetic waves energy to the properties of the material
through which the waves are travelling.
The process gas is introduced in a sample cell of a specific
optical path length.
The UV energy is transmitted into the cell via an optical fiber
cable, it passes through the sample cell and the residual energy
is transmitted to the UV sensor by a second optical fiber.
The sensor is made by an array of photodiodes, each one of
them tuned to a specific wavelength. Finally an embedded
computer collects the electrical signals from the diode array,
analyzes the absorption spectrum and calculates the
concentration of the aromatic compounds.

10. Beer-Lambert law
In essence, the law states that there is a logarithmic
dependence between the transmission of light (or UV waves)
through a substance and the concentration of the substance,
and also between the transmission and the length of material
that the light travels through.
The measurement is targeted at the wave length band where
the investigated material has maximum energy absorption.

11. The following relation holds:
I1/I0= 10- α L = 10- ε L c
where ε is the molar absorptivity of the absorber (e.g. Benzene).
The transmission of the signal through the sample is expressed in
terms of “absorbance”, which is defined as:
A = -log10(I1/I0)
This implies that the absorbance is linear with the concentration:
A = ε L c
I0= intensivity of incident signal
I1= intensivity of outgoing signal
L = length of the path
c= substance concentration
α = absorption coefficent of the
substance

12. The analyzer establishes the intensity of the signal transmitted
by the lamp of the spectrophotometer and measures the
intensity of the signal received by the photodiode array.
The signal is analyzed at wavelengths where the absorbance
of the measured substance is maximal.
Then the application software calculates the concentration
according to the measured values and the above formulas.
A multi-compounds analysis is possible, because each
compound has its unique absorbance spectrum.

13. The figures show some typical absorbance spectrum some
compounds (absorbance in the y-axis versus wave length in
the x-axis).
SO2 Spectrum at 1%
concentration
H2S Spectrum at 1,5%
concentration

14. ARCHITECTURE
The BASE® Series analyzer is composed by the following
main components:
• Embedded industrial computer
• Spectrophotometer
• Optical cells
• Fiber optics
• Interface modules
• Solenoid valves
• Power supply
• Sampling system
• Sample take-off probe

15. Embedded computer
The implementation of the H2S/SO2 analyzer follows the
general philosophy of the BASE Instruments Series.
The raw input data from the sensors (UV spectrophotometer)
are processed by algorithms provided by BAGGI, running in
an embedded computer that is the heart of the system.
When required, an ATEX version is available. In this case the
computer, together with the spectrophotometer and the power
converters, is within an enclosure provided with a protective
purge system and an optional Vortex cooler (connected to the
plant instrument air system).

16. The computer is in charge of:
• Actuating the UV lamp of the spectrophotometer
• Acquiring the electrical signals from the CCD array (related to the
intensity of the absorbed light)
• Calculating the concentration of the compounds
• Controlling the residual lifetime of the Xenon lamp
• Actuating the pumps and the valves
• Actuating the digital/analog conversion for outputting the
calculated values over 4…20 mA signals
• Interfacing the digital bus (e.g. Modbus)
• Actuating the output relays for handling the alarms
• Displaying the system status and the measurement data in a
Graphical User Interface (GUI)
• Storing the status and the measurement archives into a data base
(CSV format)
• Interfacing the human operator for system calibration and
maintenance transmitting remotely the information/alarms via
serial lines, Ethernet and WiFi;

17. The figure shows the computer’s display with the functional
keys, within the stainless steel pressurized ATEX certified
enclosure or explosion proof ATEX enclosure:

18. Spectrophotometer
The instrument is composed of an UV lamp and a diode array.
The UV beam, after passing through the measurement cell,
reaches a holographic grating disk. This one diverts each
wavelength composing the beam onto a specific diode of the
array.
The voltage emitted by the individual diodes is measured and this
information is acquired by the embedded computer through a
serial line.
There are no moving parts.
The computer knows the amount of UV energy that has been
transmitted by the lamp and is able to draw the absorption
spectrum.
Finally it calculates the concentration of the components.
The spectrophotometer is controlled by the computer by means
of an internal USB/RS232 line and is housed in
the same enclosure.

19. The UV/Visible band spectrophotometer schema is shown
below:
Xe Flashlamp
Lamp
Fiber optic cable
With SMA connection
Modular
Flow cell
Collimator and
window
Holographic
grating
Diode array
Sample fluid
inlet
Sample fluid
outlet

20. Optical Cell
The optical cell, where the process sample is traversed by the UV
beam, is made under BAGGI design and different material can be
provided:
- AISI 316L stainless steel.
- Hastelloy C276
- Monel
- Glass
- Other on request available
The length of the cell is a function of the range to be measured.
The smaller the concentration, the longer is the cell.
When the measured values can span a wide range, there is the
option of using two different cells connected in parallel. The
computer is able to select dynamically the cell more appropriate for
the actual value.

21. Some BAGGI design cell types are shown in the figures
belows. Cells are all modular in order to change the path
lenght:

22. Sampling system
The Baggi SensEvolution® products have been developed for
providing industrial analysis in many application fields.
The SensEvolution Sample® line comprises all the sampling products
developed for the SensEvolution® instruments and analysers, but also
special executions made under specific customers’ requirements.
For the Tail Gas analyzer an insulated and
heated sampling system is provided in
order to keep the temperature of the
sample gas about at 120°C to avoid
presence of solid sulfur inside tubing.
As per customer specification, heater
system can be composed of electrical or
steam heated pipes or by an electric atex
heater.

23. Sample take-off probe
A sample take-off probe
can be supplied. Probe
can be equipped with a
cooler and demister in
order to eliminate water
contents Filters,
pressure and
temperture gauges can
be directly mounted on
the process probe to
have a real-time
monitoring of the
sample point.

24. BASE series instruments are disigned to be modular and the
can handle many measurement principles by changing the
internal sensor unit:
- UV/VIS - NIR SPECTROSCOPY
- GAS CHROMATOGRAPHY
- PHOTO-IONIZATION DETECTION
- ELECTROMAGNETIC ENERGY ABSORPTION
- PALLADIUM SENSORS

25. OTHER PRINCIPLES OF MEASUREMENT:
THE UNIQUE INTERACTION OF HYDROGEN WITH PALLADIUM
• Both resistor and capacitor circuits for hydrogen measurement
capability from 15ppm to 100% v/v
• Palladium – Nickel alloy provides stable operation in pure hydrogen
at multiple atmospheres)
• On die temperature sensor and heater compensates for variations
in gas flow, gas composition and gas temperature.
• Unique semi-permeable coatings enable continuous operation in a
wide range of gas mixtures including harsh environments
Molecular hydrogen (H2) adsorbs
on palladium and dissociates into
atomic hydrogen (2H)
Atomic hydrogen is reversibly
absorbed into palladium
proportional to H2 partial pressure

26. OTHER PRINCIPLES OF MEASUREMENT:
GAS CHROMATHOGRAPHY
In a GC analysis, a known volume of gaseous sample is injected into the
head of the column. As the carrier gas sweeps the sample molecules
through the column, this motion is inhibited by the adsorption of the sample
molecules either onto the column walls or onto packing materials in the
column. The rate at which the molecules progress along the column
depends on the strength of adsorption of each molecule.
Since each type of molecule has a different rate of progression, the various
components of the sample mixture are separated as they progress along
the column and reach the end of the column at different times (retention
time); thus, the time at which each component reaches the outlet and the
amount of that component can be determined.

27. Thank you
for your attention