Natural gas analysis

Natural gas analysis has considerable economic importance. In fact, commercial contracts increasingly specify not just volume but the calorific or heating value as well. Today the calorific value of a natural gas calculated from its composition obtained by chromatography is recognized as valid. There is therefore a large research effort devoted to increasing the precision of this analysis. 

The economic value of natural gas is primarily determined by the thermal energy it contains, which is expressed in British thermal units (Btu) or calorific value (CV). Other important physical properties comprise the liquid content, the burning characteristics, the dew point and the compressibility. In order to enable the calculation of these properties from its composition, a natural gas analysis should contain a detailed determination of all of the individual components, even in the low-concentration range. 

Figure 14.8 shows a detailed schematic representation of a natural gas analysis System, which fully complies with GPA standardization (8). This set-up utilizes four packed columns in connection with a TCD and one capillary column in connection with an FID. The contents of both sample loops, which are connected in series, are used to perform two separate analyses, one on the capillary column and one on the packed columns. The resulting chromatograms are depicted in Figure 14.9. 

The popularity of GC as an analytical technique in many areas depends on the fact that all of the compounds of interest in an important sample can be detected. For instance, in petroleum and petrochemical labs, it is the rule that all of the compounds can be measured at very low levels with the flame ionization detector. In this case, the detector is "universal." In a natural gas analysis, however, the same detector would not have universal response. This is because several of the important constituents, such as N2 and CO, give little or no response on the FID. In this case a TCD is used, which is "universal" for this analysis. 

In September of 1983, a request was submitted by the Laboratoiy and Chemical Services Committee of the American Gas Association, which outlined the need for a cahbration gas for natural gas analysis by chromatography which was certified by a recognized ceitilying agency. This request was undertaken by the Gas Research Instimte. The result is a program that will allow cahbration mixture suppliers to prepare a gas that will have a certified analysis traceable to a mixture certified by the National Bureau of Standards. This program with the NBS will create a common source to which calibration mixtures can be traced. This common source of traceability will create a reference point that can be used to resolve aity dispute between parties on questions of compositional accuracy. 

There are several analytical procedures published these include American Society for Testing and Materials, Gas Processors Association, as well as those developed by various G.C. manufacturers and users. Mar r G.C. manufacturers have developed instruments specially designed for natural gas analysis. 

The basic method for natural gas analysis used is GPA Standard 2261. Components to be determined in a representative natural gas sample are physically separated by gas chromatography and compared to the known composition of a standardized reference gas which is determined under identical operating conditions. 

From the composition obtained in natural gas analysis using high accuracy gas chromatograplty discribed above, heating value of natural gas sample can be calculated from pure component gross heating value obtained from the physical constant of natural gas published by GPA Standard 2145. 

An accurate method would use available heat charts corrected for dissociation such as from reference 52, figure 9.7 and 13.4a, or figures 5.1 and 5.2 in this book, which give the following answers for a natural gas analysis of 90% methane (CH4), 5% ethane (CaHe), 1% propane (CsHg), and 4% nitrogen (N2), with 1800 F exit gas ... 

FIGURE 34. Natural gas analysis by GC-C-MS (a) m/z 44 trace (quantification), (b) relative to standard, (c) CPDB for each alkane (analyzed by Finnigan-Mat)... 

The aluminum oxide ALOT column is ideal for the separation of - C5 hydrocarbons. The column s upper temperature is 200 °C which allows elution of hydrocarbons up to Ciq- This is important, for instance, in natural gas analysis. Fig. 7-55 shows the analysis of natural gas on an aluminum oxide coated capillary. The benzene and toluene peaks (peak 8 and 10) are well separated from the other hydrocarbons. Very typical for the aluminum oxide ALOT column is the group-like elution of hydrocarbons of the same carbon number. All the (saturated) hydrocarbons seem to elute in quite a small window. In this analysis hydrocarbons elute up to Cg. In Fig. 7-56 a naphtha sample was analyzed which shows hydrocarbons up to 69. The same type of elution pattern is also observed for the Cg hydrocarbons. 

Figure 11. Natural gas analysis on an AFOj PLOT column...

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