Evaluating Methods for Determining the Vapor Pressure of Heavy Refinery Liquids

Heavy refinery liquids such as fuel oil no. 6 and liquid asphalt are generally stored in heated tanks because at ambient temperatures they are too viscous to be pumpable. Studies have suggested that volatile organic compound (VOC) emissions from heated tanks may be underreported and may be contributing to the VOC inventory gap in some locations. Direct measurement of emissions from storage tanks is inexact and expensive, and reported emissions are based on equations that predict emissions based partly on thermodynamic principles and partly on empirically obtained values. The vapor pressure used in these equations has a significant influence on the resulting emissions estimate. However, in the case of heavy refinery liquids, the vapor pressure is very rarely measured. There is no agreed-upon method for measuring the vapor pressure of heavy refinery liquids, and there is no gold standard to compare to, so the accuracy of vapor pressure measurements cannot be quantified. Also, these materials have physical characteristics that present challenges during vapor pressure measurement. This presentation describes a project that was undertaken to identify the most reliable, convenient and cost-effective means of measuring the vapor pressure of heavy refinery liquids. None of the methods or mini-method instruments that were tested during the course of the project proved to be suitable, but much progress was made in understanding what needs to happen in order for the vapor pressure of these materials to be practically and reliably obtained.