Unique Automated LPG Residue Analysis at ppm/ppb Level by GC - Wilco Agterhuis & Ronald Verbeek

Analysis of LPG streams is very common on Refinery laboratories. The analysed components have a wide boiling point range, from methane to heavy oil fractions. Global Analyser Solutions offers a comprehensive range of instruments, using 3 different sample injection techniques, covering the complete boiling point range of all components of interest.

LPG samples are normally injected into the GC in gaseous state after vaporisation, using a gas sampling valve (GSV), or directly as a liquid, using a liquid sampling valve (LSV). The latter is the preferred method, avoiding discrimination of heavier components.

This technique delivers satisfactory quantitative results for volatile and medium volatile components, provided that the sample pressure is controlled. The required instrument configuration is discussed in this article. The automated analysis of high boiling components like DIPA (diisopropanolamine) and heavy oil fractions in LPG is a real challenge; many laboratories use the labor intensive and risky ISO 13757 method, which dictates evaporation of large amounts of LPG sample, and determination of the residue content by weighing. In this article, the automated and safe analysis of these demanding compounds at low ppm level is described.

Analyis of Volatile and Medium Volatile Components
The analysis of volatile and medium-volatile components in LPG streams is widely performed in many laboratories. The sample can be injected in gas or liquid state. In the first method, the sample is gasified using the
Vaporiser Facility, which is build around a heated pressure regulator, and subsequently the gas is injected on the column using a gas sampling valve.

When correctly applied, this method is satisfactory for volatile components up to C4/C5 hydrocarbons.

In case of medium volatile components, starting at C5/C6, liquid injection is preferred to avoid the loss of higher boiling components. The liquid sample is injected by LSV into the heated Split injector for fast evaporation before entering the analysis column. For reliable quantitative results, it is essential that (partial) evaporation of the sample in the sample loop is prevented by all means. This is achieved by raising and controlling the pressure at the sample cylinder and LSV, using pressures up to 20 bar. See figure 1 for the basic configuration. Figure 2 shows the utilised Pressure Facility.

Figure 3 shows chromatogram of the analysis of impurities in propylene with FID detection. In figure 4 PFPD (Pulsed Flame Photometer Detector) detection is applied for selective analysis of sulphur component at ppb level. Both channels can run simultaneously in one GC instrument with two LSVs and two columns, or using only one analysis channel by splitting the column effluent to both detectors. Figure 5 shows the optional
autosampler for LPG sample cylinders, with built-in Pressure Facility. A series of 10 cylinders is analysed by pushing only one button. Figure 6 shows the excellent repeatability.