The Benefits of Thermal Desorption Coupled with Gas Chromatography for the Analysis of Hydrocarbon Residues in Liquefied Petroleum Gas

Liquefied Petroleum Gas (LPG) is a hydrocarbon fuel produced from the refining of natural gas or the fractional distillation of crude oil. It is primarily a mixture of propane and butane that is used for a wide variety of field and industrial applications, including a fuel for motorised transport systems, a propellant for aerosols and as a gas for refrigeration purposes.

Once produced, LPG is transferred to pipelines, ocean tankers or terminal delivery systems for long-distance distribution. Once at a distribution centre, LPG is typically transferred to a bulk truck or rail car for short-haul transport to a retail plant. From there, it is distributed in cylinders or bulk trucks for delivery to the retail customer. Figure 1 represents a simplified schematic of the LPG distribution Chain [1].
The transportation and delivery of LPG can lead to potential sources of contamination, which can be harmful to engines, motorised systems or industrial processes. For example, if gasoline or diesel fuel has been used in the transportation tankers, it can result in contamination of those components in the LPG. When compressors are used to pump the LPG into pressurised tanks, the oil can contaminate the LPG. And finally, phthalates and similar plasticisers can end up in the LPG from the delivery hoses used to fill pressurised cylinders.
ASTM International (ASTM) D1835 ‘Standard Specification for Liquefied Petroleum (LP) Gases’ [2] designates ASTM Method D2158 ‘Standard Test Method for Residues in Liquefied Petroleum (LP) Gases’ [3], as the referee method for residue measurement. However, residue contaminants in LPG using this evaporation/gravimetric procedure does not achieve the detection limits required by industry. Besides being time consuming and labour intensive, the sensitivity of the method is not sufficient for many of the more challenging applications of LPG including fuel cells and micro turbines, which require keeping the contaminants below 20 ppm (µg/g) for the process to work efficiently. In addition, Method D2158 can also produce inaccurate results, because low boiling point compounds are lost during the evaporation stage. In addition, the method does not generate any information about the source of the contaminating residue, which is useful for troubleshooting purposes.
This study will therefore describe a new method using Automated Thermal Desorption (ATD) coupled with gas chromatography (GC), for the measurement of residue in LPG down to 5µg/g, as well as yielding the hydrocarbon range of the contaminants, to give an understanding about the source of contamination. This methodology has since become a new ASTM Method D7828, ‘Standard Test Method for Determination of Residue Composition in Liquefied Petroleum Gas (LPG) Using Automated Thermal Desorption/Gas Chromatography (ATD/GC)’ [4].

Standard Test Method for Residues in Liquefied Petroleum Gases

ASTM D1835 states that besides the four main constituents of, methane, ethane, propane and butane, the residue contaminants, particularly longer chain hydrocarbons C6-C40, should be kept to an absolute minimum, because they can lead to problematic deposits in liquid feed and vapour withdrawal systems utilised in end-use applications of LPG. These residues also have the potential to be carried over and can foul up regulating equipment, and over time, the ones that remain can accumulate, and could contaminate additional components.
ASTM Method D2158 involves taking a 100-mL sample of liquefied petroleum gas, which is evaporated at 38°C in a customised centrifuge tube, cooled with a condensing coil and cooling bath. The volume of residue remaining is weighed, measured and recorded. This test method has been used to verify heavy contaminants in propane and LPG products for many years. However, in addition to being time-consuming, labour-intensive, and often dangerous with harmful vapours escaping into the atmosphere, the test has precision limitations. Therefore, besides not being sensitive enough to protect some equipment from operational problems or increased maintenance, it also cannot identify the source of residue.
In fact, D2158 states that if the LPG is specifically being used for certain applications such as micro turbines, a new electricity generation technology being designed for stationary energy applications, or fuel cells, which are used to convert hydrogen/hydrocarbon gases into electricity using proton exchange membrane (PEM) technology, a more sensitive test is required. It has been estimated that to use LPG for these kinds of applications, a residue detection capability of < 20 µg/g is required in order to ensure the efficiency and trouble-free operation of the technology.

Thermal Desorption Coupled with Gas Chromatography

To meet the detection requirements of these innovative new technologies, it was decided to investigate the use of Thermal Desorption (TD) coupled with Gas Chromatography (GC) and flame ionisation detection (FID). The objectives of the study were to:

• Achieve acceptable recoveries of hydrocarbons from C6 to C40
• Not retain compounds lighter than C6 to minimise interferences
• Ensure the pressurised LPG enters the tube as a liquid
• Achieve a detection capability of less <10µg/g and a dynamic range of 3 orders of magnitude
• Prove accuracy through an LPG quality control sample
• Attain acceptable repeatability
• Offer the potential of identifying the individual residue component or hydrocarbon profile for troubleshooting purposes
• Enable sampling in the field, so the sorbent tubes can be sent to a laboratory for analysis, saving significant transportation costs associated with shipping pressurised cylinders
• Reduce costs associated with cleaning (labour and solvents) and purchasing cylinders
• Make it rugged enough to be a standardised ASTM method Thermal desorption is well-recognised as being an accurate and precise technique for the sampling and analysis of volatile and semi-volatile compounds by gas chromatography. It has become the industry standard for analysing soil gases, studying healthy building syndrome, fenceline monitoring, indoor/outdoor air analysis as well as addressing industrial hygiene concerns [5]. Sorbent tubes are small and light, making them easy to transport, and when applied to LPG samples at a remote site, it can result in reduced shipping costs compared to other sampling techniques. In addition, the tubes are easily cleaned during the desorption process, rendering them available for immediate re-sampling, which can be verified with a rapid GC analysis.

Summary of New Methodology

A sample of LPG is captured on a sample loop, which is maintained at a pressure above its bubble point as it is released directly onto the hydrocarbon selective absorbent tube material, thereby trapping the C6- C40 hydrocarbon residue. After the sorbent tube is sampled, it is brought or shipped to the laboratory for analysis by ATD/GC/FID. The tubes are placed on the autosampler and the operator starts the instrument, which initiates the process of moving the tube from the carousel into the primary desorption flow path. This process is shown schematically in Figure 2. The residue is desorbed from the sorbent tube using heat, inert gas flow and time. The effluent from the tube is focused onto a secondary (cold) trap. After residue recovery from the tube to the focusing trap is complete, the trap heats very rapidly to volatilise the components from the trap and the inert gas flow brings the effluent onto the analytical column of the gas chromatograph for separation and onto the FID for detection. This secondary desorption step is exemplified in Figure 3. This acquired (raw) data is stored in the data handling system for processing. The processing method, which contains the response factor (RF) and integration parameters from standards previously analysed, is applied to the sample, and the mass of residue in the sample is calculated.

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