communication and measurement

TAIL GAS ANALYZERANALYSIS OF H2S AND SO2

The Tail Gas analyzer is part of the BAGGI BASE® InstrumentsSeries.

It is the result of combining the latest state-of-the-art-technologywith over 60 years of industry experience.

This real time process analyzer is ideal for the measurement ofthe percentage of H2S and SO2 within the outlet gas stream fromthe sulfur recovery unit process (Claus process unit). It usesultraviolet/visible spectrophotometry for achieving very accurateresults.

The fully automated instrument is accompanied with a samplingloop, tailored according to the Customer’s requirements.

An ATEX version is available for operation in potentially explosiveatmospheres.

The Claus process is the industry standard and so the mostsignificant gas desulfurizing process, recovering elemental sulfurfrom gaseous hydrogen sulfide.

The process is commonly referred to as a sulfur recovery unit(SRU) and is very widely used to produce sulfur from thehydrogen sulfide found in raw natural gas and from the by-productsour gases containing hydrogen sulfide derived from refiningpetroleum 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

The Claus reaction to convert H2S into elemental sulfur requiresthe presence of one mole of SO2 for each two moles of H2S:Claus gases (acid gas) with no further combustible contents apartfrom H2S are burned in lances surrounding a central muffle bythe following chemical reaction:

2 H2S + 3 O2 → 2 SO2 + 2 H2O

This is a strongly exothermic free-flame total oxidation ofhydrogen sulfide generating sulfur dioxide that reacts away insubsequent 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

Schematic flow diagram of the Claus Process:

Our scope is to supply a way to measure the presence of thevarious 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 savetime and money.

The Tail Gas analyzer of the BAGGI BASE® series provides

the required capabilities for the real time measurements of H2Sand SO2 in Tail gas stream.

The method of analysis is Ultra-Violet/Visible

spectrophotometry: faster, more rugged and less expensivethan Gas Chromatography.

The instrument provides high wavelength resolution and it canbe equipped with multiple cells and spectrometer module.

PRINCIPLE OF MEASUREMENT

The BASE® Series Instrument can Handle many of the well knowspectroscopy techniques.

Thanks to his modular design The BASE® Series Instrument isable to use a large area of the electromagnetic spectrum anddifferent 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

The measurement technique relies on the Beer-Lambert law. Thisone is a relationship that relates the absorption ofelectromagnetic waves energy to the properties of the materialthrough which the waves are travelling.

The process gas is introduced in a sample cell of a specificoptical path length.

The UV energy is transmitted into the cell via an optical fibercable, it passes through the sample cell and the residual energyis transmitted to the UV sensor by a second optical fiber.

The sensor is made by an array of photodiodes, each one ofthem tuned to a specific wavelength. Finally an embeddedcomputer collects the electrical signals from the diode array,analyzes the absorption spectrum and calculates theconcentration of the aromatic compounds.

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.

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 interms 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 signalI1= intensivity of outgoing signalL = length of the pathc= substance concentrationα = absorption coefficent of the

substance

The analyzer establishes the intensity of the signal transmitted

by the lamp of the spectrophotometer and measures theintensity 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.

The figures show some typical absorbance spectrum some

compounds (absorbance in the y-axis versus wave length inthe x-axis).

SO2 Spectrum at 1% concentration

H2S Spectrum at 1,5% concentration

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

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 inan embedded computer that is the heart of the system.

When required, an ATEX version is available. In this case thecomputer, 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 theplant instrument air system).

The computer is in charge of:

• Actuating the UV lamp of the spectrophotometer

• Acquiring the electrical signals from the CCD array (related to theintensity 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 thecalculated 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 aGraphical User Interface (GUI)

• Storing the status and the measurement archives into a data base(CSV format)

• Interfacing the human operator for system calibration andmaintenance transmitting remotely the information/alarms viaserial lines, Ethernet and WiFi;

The figure shows the computer’s display with the functional

keys, within the stainless steel pressurized ATEX certifiedenclosure or explosion proof ATEX enclosure:

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 eachwavelength composing the beam onto a specific diode of thearray.

The voltage emitted by the individual diodes is measured and thisinformation is acquired by the embedded computer through aserial line.

There are no moving parts.

The computer knows the amount of UV energy that has beentransmitted by the lamp and is able to draw the absorptionspectrum.

Finally it calculates the concentration of the components.

The spectrophotometer is controlled by the computer by meansof an internal USB/RS232 line and is housed in

the same enclosure.

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

Optical Cell

The optical cell, where the process sample is traversed by the UVbeam, is made under BAGGI design and different material can beprovided:

- 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 theoption of using two different cells connected in parallel. Thecomputer is able to select dynamically the cell more appropriate forthe actual value.

Some BAGGI design cell types are shown in the figures

belows. Cells are all modular in order to change the pathlenght:

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.

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.

BASE series instruments are disigned to be modular and the

can handle many measurement principles by changing theinternal sensor unit:

- UV/VIS - NIR SPECTROSCOPY

- GAS CHROMATOGRAPHY

- PHOTO-IONIZATION DETECTION

- ELECTROMAGNETIC ENERGY ABSORPTION

- PALLADIUM SENSORS

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

OTHER PRINCIPLES OF MEASUREMENT:

GAS CHROMATHOGRAPHY

In a GC analysis, a known volume of gaseous sample is injected into thehead of the column. As the carrier gas sweeps the sample moleculesthrough the column, this motion is inhibited by the adsorption of the samplemolecules either onto the column walls or onto packing materials in thecolumn. The rate at which the molecules progress along the columndepends on the strength of adsorption of each molecule.

Since each type of molecule has a different rate of progression, the variouscomponents of the sample mixture are separated as they progress alongthe column and reach the end of the column at different times (retentiontime); thus, the time at which each component reaches the outlet and theamount of that component can be determined.

Thank you for your attention