Determination of composition of natural gas and its gross heating value (GHV) at the custody transfer is important for the seller and buyer to calculate the energy content transferred in million BTU (MMBTU). The objective is to develop right analytical technique and equipment to ensure collecting representative sample at high pressure and accurate determination of its composition and gross heating value.
The low pressure gases at different pressure are compressed and dehydrated to remove moisture. The liquid condensate is dehydrated in a stripper and its pressure is boosted to 150 bar and mixed with dry gas. A special flow and temperature controlled vaporizer is used and spot samples are collected in high pressure cylinders for gas and piston cylinders for liquid. GC with thermal conductivity detector (TCD) and flame ionization detector (FID) with automated gas conditioning unit are used for determination of gas composition. TCD detects the permanent gases and hydrocarbons from C1 to C5 and the FID detects C6 and break down compounds up to C14 including heavier aromatics. Their concentration is derived based on their response factors and bridge calculation using i-pentane in FID. As the gas is sour containing high H2S, special safety precautions are taken with appropriate personal protection.
The gas composition analysis was performed as per GPA 2286 using a gas chromatograph. The low pressure gas contained 35 - 45 mol % of methane while the high pressure gas had 80 – 85 mol % methane. Ethane, propane, butanes and hydrocarbons up to C14 were present. H2S and CO2 were present up to 6.0 mol%. The GHV was calculated from the gas composition and the energy content of the gas was derived from the flow rate and GHV. The laboratory data was compared with online analyzer and corrective actions were taken in case of any abnormality. The designing a special vaporizer and a customized gas chromatograph helped us to determine the export gas composition and its energy content accurately. The trend and variation of gas quality are correlated with feed gas supply and helped for adjusting process conditions to achieve operational excellence.
The measurement of gas composition and its energy is important for the seller and buyer. Any difference in the gas quality will have direct cost implications in the sale and it is important to standardize the sampling and analysis techniques to be best accuracy possible.