商品天然气烃露点测定方法及其溯源性

Determination of hydrocarbon dew point of commercial natural gas and its traceability

关键词:  反凝析现象  烃露点  冷镜式烃露点仪  潜在液烃含量  状态方程计算烃露点  溯源校准程序

Key words:  retrograde condensation phenomenon  hydrocarbon dew point  hydrocarbon dew point chilled mirror instrument  PHLC  calculation of dew point by state equation  traceable calibration procedure

摘要:

在输配系统特定工况下,高压输送的商品天然气有可能因降压而产生反凝析现象并在气相中形成重烃液滴。因此,应规定烃露点指标以保障生产系统的安全平稳运行。由于天然气组成极其复杂,其相态特性完全不同于其他单一组分和/或简单的多元混合物。因此,准确测定商品天然气的烃露点相当困难。扼要介绍了工业上常用的3种烃露点测定方法及其溯源途径。手动或自动式冷镜仪法均为物性测定方法,其测定结果不具备溯源性,无法评定测量结果的不确定度。PHLC法的技术特点为基于同一压力下冷镜法露点仪测得的烃露点值与被测气体中潜在烃含量的相关性而建立了冷镜法的溯源程序。目前,先进的自动冷镜仪测定烃露点的准确度已经达到±0.5 ℃。用状态方程计算烃露点的方法虽然有严密的化学热力学基础,但由于受取样代表性、色谱分析结果准确性及数据处理方式等诸多因素的影响,应用并不广泛。 

Abstract:

High pressure sales gas may appear to have retrograde phenomenon because of pressure reduce and condense liquid drops of heavy hydrocarbon in gas phase at the special operating conditions of gas transportation and distribution system. Therefore the dew point index should be stipulated to guarantee safe and smooth operation of the system. Because composition of natural gas is very complicated and its phase behavior is absolutely different from other gas mixtures of single or multiple components, measurement of hydrocarbon dew point for sales gas is extremely difficult. Three methods of hydrocarbon dew point measurement and relative traceable way are briefly introduced in this article. Methods using manual or automatic hydrocarbon dew point chilled mirror instrument are that of physical property determination. These methods are not provided with traceability and the uncertainty of results of determination can not be assessed. The technical characteristic of PHLC method is that of establishment traceable procedure for chilled mirror instrument based on substantial relationship between hydrocarbon dew point value and PHLC at same pressure. Presently the accuracy of an advanced chilled mirror instrument has already reached ±0.5 ℃. The method for calculation of hydrocarbon dew point by state equation is based on strict chemical thermodynamics and its application is not very popular because of influences of sampling representativeness, analytical accuracy and data processing.