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Analytical Instrumentation
Analytical Techniques for Sulphur Determination
Mohammad Tajparast,Research and Development Specialist, C.I. Analytics Address 2085 Industrial Boulevard,Chambly, QC, Canada, J3L 4C5 Email :
mtajparast@cianalytics.ca Tel: (450) 658-4965
Stringent environmental regulations require sulphur reduction in petroleum products and fl ue gas to low levels. Furthermore, sulphur is toxic and corrosive to delicate catalysts and industrial units. Sulphur detection at low levels plays an important role in monitoring sulphur reduction in industries. There are a number of analytical techniques available in the market; each is associated with different pros and cons. Here, we focus on seven methods including gas chromatography, chemiluminescence, ultraviolet fl uorescence, diode array spectroscopy, X-ray fl uorescence, dry colorimetry, and tunable diode laser absorption spectroscopy.
The growing concern about air pollution has been generating a far higher need than ever for accurate analytical technology, to prevent further pollution of the atmosphere. People’s new understanding of the need for environmental protection, the severe regulations on sulphur content in petroleum products, and the poisonous and corrosive effects of sulphur to industrial catalysts and units call for an innovative analytical technique providing an extremely high degree of sensitivity and precision.
In industrial and monitoring applications the key factors on gas detectors are: high accuracy, excellent linearity, high sensitivity (low detection limit), good selectivity (free from interferences), versatility (multi compounds measurement), real-time and continuous measurement capabilities, easy to integrate and to operate (no sampling, user friendly, free of maintenance, no skilled operator). Moreover, operation in industrial units often means that the equipment has to be compact, fl exible, and so-called rugged (insensitive to harsh conditions such as shocks, noise, contaminants, and high temperatures).
Emission levels
It is impossible to clean the air, or in particular to reduce air pollution from industrial and transportation sectors, without getting sulphur out of fuels. Table 1 shows the regulated sulphur levels in diesel and gasoline in Canada, USA, and Europe.
Table 1. Regulations on sulphur levels in diesel and gasoline in Canada, USA, and Europe Region
Canada USA
Europe
Current sulphur level in diesel
< 15 ppm < 15 ppm < 15 ppm
Current sulphur level in gasoline
< 30 ppm < 30 ppm < 30 ppm
a:
http://gazette.gc.ca/rp-pr/p1/2013/2013-06-08/html/notice-avis-eng.html;b
Sulphur level in gasoline by 2017
< 10 ppma < 10 ppmb < 10 ppmc
:
http://yosemite.epa.gov/opa/
admpress.nsf/d0cf6618525a9efb85257359003fb69d/ce8984957ffefa6a85257c90004fe802!OpenDocument; c
http://www.greencarreports.com/news/1090661_sulphur-in-gasoline-to-fall-by-two-thirds-in-2017-to-cut-emissions
: Ultraviolet fl uorescence Analytical techniques
Various analytical techniques have been developed for the determination of sulphur in different industrial environments. In this article we focus on gas chromatography (GC), chemiluminescence detector (CLD), ultraviolet fl uorescence (UVF), diode array spectroscopy (DAS), X-ray fl uorescence (XRF), dry colorimetric detector (DCD) or Sensi-Tape technology, and tunable diode laser absorption spectroscopy (TDLAS).
The applicable ranges of these analytical techniques are summarised in Table 2. Table 2. Applicable ranges of the common analytical techniques Application GC-FPD/PFPDa
Total sulphur 0.04-500 ppm LDLb
: 40 ppb H2 S in natural gas H2 S in fuel gas H2 S in fl ue gas
SO2 gas
in fl ue
0.01-100 ppm LDL: 10 ppb
0.01-100 ppm LDL: 10 ppb
0.01-100 ppm LDL: 10 ppb
0.01-100 ppm LDL: 10 ppb
NA 0-5 ppm LDL: 100 ppb NA 0-5 ppm LDL: 100 ppb
5 ppb-1% LDL: 5ppb
5 ppb-1% LDL: 5ppb
0-5 ppm LDL: 100 ppb
0-250 ppm LDL: 100 ppb
0-500 ppm
NA 0-10 ppm NA 0-10 ppm NA
CLDc NAd
UVFe NA
DASf
The UVF method is an analytical technique utilising fl uorescence in the ultraviolet region. UVF spectrometry is a method for detecting low levels of H2
The light near 214 nm is also absorbed by SO2 in the fl ow cell and then re-emitted from SO2 S in hydrocarbons and fl ue gas. H2S highly
absorbs UV light in the UV range and is detected at a wavelength of preferably 228 nm to minimise interferences from other species such as hydrocarbons.
as fl uoresced
light near 330 nm. A photomultiplier tube can be used to measure the intensity of the fl uoresced light which may then be correlated with the total sulphur concentration (see the image above).
However, the UVF technique suffers from interferences such as Nitric oxide (NO). To prevent such interference from NO, an ozone (O3
XRFg DCDh
0-10 ppm 1 ppm-% 0.002-250 ppm LDL: 1 ppb
0-10 ppm NA
0.002-250 ppm LDL: 1 ppb
0.002-250 ppm LDL: 1 ppb
0.002-250 ppm LDL: 1 ppb
0.002-250 ppm LDL: 1 ppb
AUGUST / SEPTEMBER •
WWW.PETRO-ONLINE.COM TDLASi
0-20 ppm LDL: 500 ppb
0-20 ppm LDL: 500 ppb
0-20 ppm LDL: 500 ppb
-20 ppm LDL: 500 ppb NA
analyser. NO is easily oxidised into NO2 longer than that of the excited SO2
. Its sample cell set-up facilitates the technique to operate at high temperature (500
°C) and pressure (350 kPa – 15 MPa) and can be used to run at high levels of SO2 (0 – 10%) and H S (0 – 100%).
) injection technique has been adopted in the UVF sulphur by ozone whose radiation wavelength is
a: gas chromatography-(pulsed) fl ame photometric detector; b detector; d
: not available; e colorimetric detector; i : ultraviolet fl uorescence; f Gas chromatography
GC measures the species of interest in a compositional background. It fi rst separates the desired components from the bulk chemicals in a packed or capillary column and then directs the components to a detector. The commonly used detectors for sulphur analysis are (pulsed) fl ame photometric detector (FPD/ PFPD) and CLD. Packed columns are often chosen over capillary columns, since they allow greater injection volume.
Total sulphur in the gas and liquid phases may be determined measuring a number of individual sulphur species and adding up the total or fi rst converting all present sulphur species to SO2
and reporting the total sulphur as SO2 (or H2 S) concentration.
However, the GC technique is expensive, complex, and slow (i.e., total sulphur analysis takes 720 sec). It requires highly skilled analytical chemists to interpret the data.
Chemiluminescence detector
Chemiluminescence is a chemical process in which light is emitted as a by-product of a chemical reaction. This process can be used to measure total sulphur.
By mixing the fuel gas with oxygen and passing it through a pyrolyser furnace the sulphur species will react to produce SO2
ozone (O3 ) which has the effect of oxidising the SO portion of the effl uent into SO2
and sulphur monoxide (SO). Further, the effl uent of the pyrolyser is mixed with at an excited
electrical state. As this excited SO relaxes to its ground state it emits its extra energy in the form of photons. The intensity of the luminesced light, which is proportional to the SO concentration, can then be detected with a photomultiplier tube and related with the total sulphur concentration.
(or H2 S) : lowest detection limit; c : diode array spectroscopy; g : tunable diode laser absorption spectroscopy. : X-ray fl uorescence; h
: chemiluminescence : dry
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