A Rapid Approximate Method for the Estimation of Hydrocarbon Dew Points of Clean Pipeline Natural Gases

Abstract

Abstract A "theoretical" correlation of the effect of pressure on the hydrocarbon dewpoint temperature of clean pipeline gases is presented. The correlation wasdeveloped from calculated dew point data on thirteen gas analyses from ten gasfields in Alberta. The dew points were calculated by conventional trial anderror method and NGAA equilibrium ratio charts. The data are correlated bymeans of a wetness parameter based on the amounts of propane and heaviercomponents in the gas and equilibrium ratios for those components atarbitrarily selected conditions. The correlation allows the quick estimation of the hydrocarbon dew points ofclean pipeline gases without the usual trial and error procedure. For moreaccurate results the dew points calculated from this correlation may be used asthe first estimate in the conventional trial and error calculations. Thecalculated dew points are usually within +4°F of the results from conventionalcalculations over the normal range of pipeline operating pressure and, forpipeline natural gases free of contaminants, such as glycol or compressor oil, are within about the same limits when compared with measured dew points. The correlation does not apply to the prediction of dew point temperaturesfor pipeline gases containing even small quantities of alcohol, glycol orcompressor oil. Introduction Pipeline natural gases, which are composed mainly of a mixture of paraffinhydrocarbon gases, nitrogen, carbon dioxide and water vapor, normally exhibittwo kinds of dew points. These are hydrocarbon dew points and water dewpoints. The dew point is the temperature, at a certain pressure, at which the firstdrop of liquid is formed or is just about to form. In the case of water dewpoints, the liquid formed is water and in the case of hydrocarbon dew pointsthe liquid formed is a mixture of the hydrocarbons contained in the gas. If apipeline gas, as a result of processing, contains contaminants such as glycol, a third type of dew points may also be observed. The liquid formed in this casewill be a mixture of hydrocarbons and the contaminant. Because of the higherboiling point of the contaminant, the presence of this component in the gaswould raise the dew point temperature. Small amounts of nitrogen, carbondioxide and other non-hydrocarbon constituents may also be present in the dewformed, depending upon the concentration of these components.