28 Analytical Instrumentation
Accurate Determination and Quantification of Sulphur Content in Liquefied Petroleum Gas
Angela Seipel and Kristian Hoffman, Application Specialists, Thermo Fisher Scientific Mercers Row, CB5 8HY Cambridge, U.K Tel: +44 (0)1223347400 • E-mail:
analyze@thermofisher.com
Recently there has been increased focus on the need for accurate quantification of low-level sulphur content in liquefied petroleum gas (LPG) in the fuel cell industry, both to avoid sulphur related pipeline corrosion and to determine the levels of detection odorant that needs to be added to LPGs. Emerging total sulphur analysers are able to provide accurate determination of low-level sulphur content in LPG. Tis article will discuss the benefits of this new technology and describe a direct injection technique for the precise analysis of sulphur in line with ASTM D66671
requirements. Introduction
Monitoring of low-level volatile sulphur compounds in fuels such as natural gas and LPG presents significant measurement challenges within the petrochemical industry, namely due to the issues associated with the accuracy of these measurements. However, routine analysis in the industry is essential to ensure LPGs and the products of their combustion are not unduly corrosive or reactive to materials with which they come into contact. As a result, it is vital that petrochemical companies implement routine analysis methods to determine the precise sulphur content of LPG.
Due to the colorless and odorless nature of LPGs, the addition of odorants is necessary to aid leak detection. These odorants are normally the sulphur containing compounds mercaptans, which are added at an approximate concentration of ~10 mg/kg. Measurement of the total sulphur (TS) content of natural gas streams is therefore important, both to avoid sulphur related pipeline corrosion and to determine the precise quantity of odorant that must be added to LPGs.
LPG and other light hydrocarbons are also finding use as feedstocks for a variety of fuel cell technologies. Fuel cells and reformer catalysts are generally sulphur intolerant and the need for low-level sulphur measurements in the fuel cell industry is a growing trend to avoid the degradation of power generation performance. Furthermore, LPG is increasingly used as an automotive fuel and must therefore comply with legislation concerning the sulphur content of automotive fuels2
. A new analytical method
Traditionally, the introduction apparatus for combustion-based trace elemental analysis has depended on the boiling point of the sample. For light hydrocarbons, direct injection using a ceramic syringe or liquids module is commonly used, while for heavy hydrocarbons, boat inlet introduction is employed. As a result, two introduction modules are required by analysts to cover the range of boiling points analysed. This need has meant that analyses take five minutes for light hydrocarbons and twelve minutes for heavy hydrocarbons, significantly impacting on productivity.
The latest total sulphur analysers feature an innovative direct injection design providing the capacity to analyse both heavy and light hydrocarbons without changing introduction modules. As a result, it is now possible to automatically quantify sulphur in hydrocarbons from µg/kg level up to percent levels within three minutes. Compared to traditional techniques, this offers a significant reduction in the time spent from analysis to viewing results, which provides immediate cost savings and productivity increase.
The use of a direct injection, oxidative combustion technique can provide an effective solution for the measurement of ultra-low levels of total sulphur (TS) present in LPGs. To demonstrate the capabilities of the technique for sulphur analysis in LPG, an experiment was conducted using ultraviolet fluorescence, in accordance with American Society for Testing and Materials (ASTM) D6667. This method is applicable to the analysis of natural, processed and final product materials containing sulphur in the range of 1 to 100 mg/kg of sulphur.
Experimental A Thermo Scientific iPRO 5000TM
Table 1: System parameters used on the iPRO 5000 S analyser in the determination of sulphur per ASTM D6667.
The LPG was vaporised and mixed with argon carrier gas in the EGM-III gas/LPG module and injected directly into the iPRO 5000 S analyser, via the EGM-III adaptor. Complete oxidation of the sample occurred in the oxygen-rich environment of the high temperature furnace. Combustion products were dried using a permeable membrane drying tube contained within the instrument
and the total amount of sulphur was then determined by SO2 UV-fluorescence. Signal processing and analysis was handled automatically by the NSX Visual software.
Results Series Total Sulphur (TS) analyser fitted with a Thermo Scientific
EGM-III adaptor was used to perform the analyses, providing the capacity to determine low levels of sulphur in LPG by direct injection.
LPG was introduced into the iPRO 5000 S analyser using an EGM-III gas/LPG sample introduction module. The EGM-III gas/LPG module contained a ten-port rotary valve fitted with two sample loops (one for gas samples and one for LPG samples). The sample loops allow for the delivery of known quantities of gas or LPG into the iPRO 5000 S analyser, with the gas loop delivering 10 ml
per loop and the LPG loop delivering 100 µl per loop. The EGM-III gas/LPG module was used to transfer several loops of a thiophene doped butane LPG into the iPRO 5000 S analyser. The thiophene doped LPG contained 10.15 mg/kg of thiophene in butane. The total sulphur content,
Presented in table 2 are the results of injecting known quantities of sulphur-containing butane LPG into the iPRO 5000 S analyser. The data was used to generate a linear calibration line for the iPRO 5000 S analyser with the EGM-III gas/LPG module, see figure 1. Each loop delivered 3.8 mg/kg of sulphur into the analyser. The signal response of the sulphur detector exhibited excellent linearity with R2 = 0.998.
as a function of the number of loops, was then measured in triplicate and used to calibrate the instrument prior to determining the sulphur content of an LPG sample of unknown sulphur concentration.
The measurement process was controlled using the Thermo Scientific NSX Visual software which was supplied with pre-loaded ASTM-compliant methods. The analysis was performed in accordance with method ASTM D6667, which resulted in the system parameters shown in table 1.
Figure 1: First order calibration line for the iPRO 5000 S analyser and EGM-III gas/LPG module system. Each loop delivered 3.8 mg/kg of sulphur into the analyser. The signal response of the sulphur detector exhibited excellent linearity with R2
= 0.998. April / May 2012 •
www.petro-online.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
