GERG-2008方程计算天然气压缩因子及烃露点

Calculation of natural gas compressibility factor and hydrocarbon dew point by the GERG-2008 equation

GERG-2008方程是国际标准ISO 20765-2推荐的天然气热物性参数计算方法,全面评估GERG-2008方程对不同组成、不同相态天然气的压缩因子、密度及天然气烃露点的计算准确性,对于合理选用天然气物性参数计算方法、提升天然气物性计算准确性及质量控制水平都具有重要的意义。为此,在收集大量实验数据的基础上,评估了用GERG-2008和AGA8-92DC方程计算气态天然气密度(压缩因子)的准确性,以及用GERG-2008方程计算高含硫天然气压缩因子、LNG密度及天然气烃露点的准确性。研究结果表明:①对标准规定的组成较简单的管输天然气,GERG-2008与AGA8-92DC方程的密度计算准确度相当,在管输温度、压力范围内,准确度均在±0.10%以内;②对重组分含量相对较高的天然气,当临界凝析温度接近计算温度时,在管输天然气压力下,上述两个方程的密度计算准确性均变差,GERG-2008、AGA8-92DC方程的相对平均绝对值偏差分别约为0.30%和0.50%,前者表现略优;③GERG-2008用于C_(5)^(+)摩尔分数低于0.20%的LNG的密度计算时,与测试值相对平均绝对值偏差小于0.10%;④GERG-2008可用于不含或含微量C_(6)^(+)同分异构体天然气的烃露点计算,其准确度优于PR或SRK类状态方程。结论认为,GERG-2008方程具有准确、全面的天然气物性计算能力,可在天然气工业多个环节得以广泛应用,建议天然气行业相关组织加快制订相关国家标准。

The GERG-2008 equation is recommended in international standard ISO 20765-2 to calculate the thermodynamic properties of natural gas.Comprehensively evaluating the accuracy of GERG-2008 equation in calculating the compressibility factor,density and dew point of natural gas with different compositions and phase states is of great significance to select the calculation method of natural gas thermodynamic properties appropriately and to improve the natural gas quality control level.After collecting abundant experimental data,this paper evaluates and compares the calculation accuracy of the GERG-2008 and AGA8-92DC equations on the density(compression factor)of gaseous natural gas,and evaluates the calculation accuracy of GERG-2008 on the compression factor of high-sulfur natural gas,on the density of LNG and on the hydrocarbon dew points of natural gas.The following research results can be obtained.Firstly,as for the pipeline natural gas with simple compositions as stipulated in the standard,the accuracy of GERG-2008 equation is similar to the AGA8-92DC equation on the density calculation,which is within±0.10%under pipeline transmission temperature and pressure.For natural gas with high content of heavy compositions,when the critical condensation temperature is close to the calculation temperature,both equations'density calculation accuracy get worse under the pipeline natural gas pressure and their relative average absolute deviation is 0.30%and 0.50%,respectively.The GERG-2008 equation is slightly superior to the AGA8-92DC equation.Secondly,when the GERG-2008 equation is used to calculate the density of LNG with C_(5)^(+)mole fraction lower than 0.20%,its relative average absolute deviation from the measurement results is lower than 0.10%.Thirdly,the GERG-2008 equation can be used to calculate the hydrocarbon dew point of natural gas without or with trace C_(6)^(+)isomers.Its calculation accuracy is higher than that by PR or SRK equations of state.In conclusion,the GERG-2008 equation is capable of calculating the physical property of natural gas accurately and comprehensively and can be widely applied in natural gas industry.Therefore,it is recommended that the relevant organizations of natural gas industry speed up the formulation of relevant national standards.