Safe LPG Injection for Trace Sulphur and Permanent Gas Analysis

INTRODUCTION
LPG (Liquefied Petroleum Gas) has many industrial as well as domestic applications. It is used as ignition fuel for vehicles, as domestic fuel, as replacement for Chlorine and Fluor containing refrigerants, as chemical feedstock for e.g. polymers and for many other purposes.

Safe LPG sample handling is an important issue in every laboratory. Due to the high pressure nature of this sample type, special precautions have to be taken to guarantee high laboratory safety. This article describes the exercises followed on risk analysis for the Sample Securitiser from Global Analyser Solutions, using HAZOP study methodology, verifying PED (Pressure Equipment Directive) and ATEX Directives.  
Apart from safety, the accurate analysis of LPG is of great importance as well. Besides hydrocarbon composition, also trace amounts of other present species need to be known for the various usage of the LPG. This article focusses on the analysis of Sulphur compounds and permanent gases at ppb/ppm level, using customised analysers with (Pulsed) Flame Photometer Detector and Pulsed Discharge Detector.

INJECTION OF LPG
LPG contains primarily Propane and Butane and has a vapor pressure up to 8 bar at room temperature because of the components low boiling points (resp. -1 and -42 oC). In case of liquid LPG injection, pressures up to 20 bar are used to secure the liquid state of the sample. Applying such high pressures in the daily laboratory routine requires thorough risk analysis.

Figure 1
Sample Securitiser for safe LPG sample handling.  

RISK ANALYSIS  
Handling of LPG needs careful treatment. Both the high pressure cylinders and the flammability of LPG can create hazardous situations. For this reason compliance with adequate directives is required. At least the Pressure Equipment Directive (PED 97/23/EC) and the ATEX Directive (94/9/EC) are compulsory.

PED
The Sample Securitiser has been tested against PED 97/23/EC, and is categorised under Clause 3 sub 3, which is according this directive below specified pressure and volume thresholds. The directive states that specified equipment and assemblies accordingly must (1) be safe, (2) be designed and manufactured according sound engineering practice, and (3) bear specified markings. To make sure that our LPG injection instruments are compliant with the directive, harmonised standard EN13445 is used for product design, and all possible risks are taken into consideration. Several safety measurements like relief and check valves are used, and each instrument is extensively tested and delivered with a certificate of test compliance.  

 
Figure 2
High priority on safety
because of the involved
high pressure and flammability   
 
ATEX
Equipment and safety systems for use in potentially explosive atmospheres should be designed in such a way that ignition of the surrounding atmosphere is impossible. The Sample Securitiser will be used in non-hazard laboratories, but explosive substances might be released at certain moments. Therefore the unit does not have any ignition sources by design, however some subcomponents are utilised with ATEX approval. For this reason the Sample Securitiser is fully compliant with directive 94/9/EC.

Figure 3.
Compliance with ATEX directive.  

SAFETY CENTERED DESIGN: SCD
We have performed several dedicated risk assessments. Virtually all potentially hazardous circumstances have been taken into consideration in close contact with specialized safety consultancies. We also organised HAZOP assessments with representatives of expert laboratories on LPG analysis, and all possible situations and requirements were taken into account. Besides being compliant to the safely directives, also a user friendly device was designed in this way.
 
Figure 4
Sample Securitiser  

ANALYTICAL PERFORMANCE
There are two injection methods available for LPG analysis: injection as gas or as liquid. In case of gas injection, the sample is vaporised first, followed by injection though GSV (Gas Sampling Valve). This method works well for the analysis of permanent gases and hydrocarbons up to C4/C5.
When the sample contains higher boiling components as well, liquid injection is preferred to avoid loss of the heavier compounds. The liquid sample is injected through LSV into a heated Split injector for fast evaporation before entering the analysis column. For reliable quantitative results, complete filling of the sample loop is essential, and partial evaporation prior to injection must be avoided. This is achieved by applying a controlled pressure that is set above the sample pressure (10-20 bar). At this point the Sample Securitiser proves its importance by delivering high pressure resulting in good analytical performance under safe conditions.
LPG is analysed for several components, dependent on its usage. Measurement of hydrocarbon impurities is quite common, but especially when LPG is used as a feedstock in production which involve catalysts, analysis of trace impurities is of great importance. In this article the analysis of ppb/ppm Sulfur components and permanent gases is shown.

LOW LEVEL SULPHUR  
Figure 5 shows the analysis of various Sulphur species in liquid Propylene, according to ASTM D6228, D5303 or 5504. PFPD (Pulsed Flame Photometer Detector) is applied for ppb detection limit. Figure 6 shows the chromatogram of a calibration standard; figures 7 and 8 show excellent repeatability and detection limit. The linearity was tested from 100 ppb to 5 ppm, and r2 was >0.995 for all mentioned components.
The system has a highly inert sample path, which is a strict requirement for obtaining good peak shape and low detection limit. FPD (Figure 9) is also available at lower costs, when higher (about 5 times) LOD is sufficient.  

 Figure 5: Sulphur components in Propylene at ppb level using PFPD detection.

Figure 6: Calibration standard of Sulphur components in N2 (5 ppm each). Injection GSV; detection: PFPD.
Figure 7: Repeatability at 5 ppm concentration level. PFPD detection

Figure 8.
Caption: LOD Sulphur components (3*noise). PFPD detection
 

Figure 9: FPD Instant Connect Detector Module (Thermo Trace GC1300) is available when 100 ppb detection limit for Sulphur species is sufficient as an alternative to the more expensive PFPD.

LOW LEVEL SULPHUR using COMPACTGC4.0
Figure 11 shows the measurement of low level Sulphur in hydrocarbon matrices by CompactGC4.0, a fast 19” MicroGC, equipped with PFPD for this purpose (Figure 11). Two analytical channels with different columns are used for best possible separation. Both channels share the same PFPD.
Components like H2S, COS and Mercaptans are analysed at ppb level in this way. Additional channels are available for hydrocarbons and permanent gases with FID, TCD and PDD detectors. The instrument is calibrated using a gas standard, which can be diluted by the MK5 Calibrator module for multi-level calibration (Figure 12). This module can be equipped with permeation tubes with Sulphur components as well, avoiding the use of a calibration gas.    

 Figure 10: Analysis of H2S, COS and Mercaptans by CompactGC4.0 . 1 ppm level; PFPD detection

Figure 11: CompactGC4.0 19” GC equipped with PFPD

Figure 12: Calibrator module for generating/diluting calibration gases.

LOW LEVEL PERMANENT GASES
Figure 13 shows the chromatogram of low level permanent gases in LPG, according to ASTM D2504, using PDD (Pulsed Discharge Detector). A second channel for CO2 and other components according to ASTM D2505 is available as well.
After vaporising, the sample is injected by GSV. The LPG matrix is effectively back flushed, and therefore not seen in the chromatogram. The limits of detection are < 15 ppb for O2, N2 and CH4, and < 20 ppb for H2 and CO. A very low background level is mandatory in achieving such low LOD values, which is realised thanks to the use of diaphragm valves with internal purge option (Figure 14).
 

Figure 13: Analysis of permanent gases in LPG. Calibration standard at 10 ppm level. Detector: PDD; LOD<20ppb
Figure 14: Customised GC with auxiliary valve oven, diaphragm valves and PDD detection

SUMMARY
The Sample Securitiser from Global Analyser Solutions guarantees high laboratory safety and complies with Pressure Equipment Directive 97/23/EC and ATEX Directive 94/9/EC.
Besides high safety, accurate results are achieved as well. GAS offers dedicated solutions for low Sulphur species and permanent gases in LPG.
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