VALIDATION OF TRUE VAPOUR PRESSURE MEASUREMENT IN CRUDE OIL AND REFINED PRODUCTS
Testing the vapour pressure of crude oil and refi ned products is critical for meeting strict safety requirements and for complying with environmental regulations. Vapour pressure is a very important parameter for assessing a fuel’s performance, for maximising profi ts during blending operations, and for ensuring US Environmental Protection Agency (EPA) compliance at every custody transfer on its way to market.
This article explains the traditional, yet cumbersome, method of calculating true vapour pressure (TVP) and then describes the ability of new lab and online analysers to directly measure this parameter. Analyser technology has been developed that allows for the use of an inherently small sample size and short measuring interval. To perform this measurement, the analyser must use an expansion type measuring chamber. This will provide the various V/L ratios necessary for TVP extrapolation.
Measurement data showing the reliability of this method will be compared to calculations based on API Chapter 19 Section 2, as well as to an underlying model of the nature of vapour pressure in pure substances, refi ned products, and crude oils.
Background
Reid vapour pressure (RVP) and true vapour pressure (TVP) are two of the most closely monitored parameters of crude oil and fi nished gasoline. While the Reid method is performed at a fi xed temperature of 37.8°C, true vapour pressure is measured as a function of temperature. The true vapour pressure method is used to understand how the sample’s vapour pressure reacts to an increase or decrease in temperature. The API [3] calculation is the most recognised and practiced method today for reporting TVP. To use this method, two parameters need to be known: the RVP and the stock temperature of the sample. These values must be put into a complex mathematical equation to achieve the TVP. Separate equations are used for various vapour pressure ranges for both crude oil and fi nished gasoline.
Using expansion-type measuring chamber technology, it is possible to perform an analysis of TVP with a minimum sample. The pressure transducer permits the collection of vapour pressure readings at programmable expansion ratios, resulting in a TVP calculation within minutes. The section that follows shows the ability of the analyser to measure TVP through theoretical and real world data.
True Vapour Pressure (TVP)
According to International Safety Guide for Oil Tankers and Terminals (ISGOTT) [1] and the Internationale Maritime Organisation, the “True Vapour Pressure (TVP) or bubble point vapour pressure is the equilibrium vapour pressure of a mixture when the gas/liquid ratio is effectively zero. […] It is the highest vapour pressure which is possible at any specifi ed temperature.” According to this defi nition, the TVP is the vapour pressure at a
vapour-liquid ratio (V/L) = 0/1 at a specifi c temperature. Because of the V/L = 0/1, the True Vapour Pressure is a special condition of the Total Vapour Pressure.
Total Vapour Pressure
The term Total Vapour Pressure is commonly used to describe the sum of the vapour pressure of the liquid and the gases at a specifi c temperature and at a specifi c V/L ratio. Vapour pressure methods typically report a total vapour pressure at a V/L = 4/1.
It is important to note that dissolved air is one of the major contributors to vapour pressure for small V/L-ratios. The problem with dissolved air in the sample is that air cannot be separated from dissolved hydrocarbon gases like propane or butane. However, the problem of dissolved air and other gases, or the respective degassing process is not topic of this consideration, but is addressed in standardisation committee activity. Current ASTM work to review the ASTM D6377 crude oil method focuses on proper sample handling to prevent air saturation of the sample and loosing volatiles in the process of sampling.
It can be seen from the chart (Figure 1) that the total vapour pressure increases signifi cantly for smaller V/L ratios, especially if light gases (C1-C3) or air is present in the sample. At a V/L = 4/1, methods are virtually blind to the presence of gases in the sample. This is due to the fact that a big vapour space allows
outgassing of the sample during testing, with only a marginal increase in pressure. At a lower V/L ratio the presence of gases has a signifi cant effect on the measured total vapour pressure.
Figure 1 helps with understanding the problems faced by TVP measurement in testing high volatile crude oils. When the API calculation [3] was created, degassed (stabilised) samples that included only very little amounts of light gases were used to design the calculation. Because the calculation is based on that sample set, the API calculation will under predict the TVP for samples with high amounts of light gases and will tend to over predict the TVP for samples that do not contain C1-C3 gases at all.
Data in Figure 2 shows the increase in vapour pressure for V/L ratios down to zero for several samples, such as
• 2 different crude oil samples • 2 gasoline samples • 2 pure substances
Note that the crude oil samples have been stored at ambient pressure at zero centigrade. It can be assumed, that those samples are saturated with air, and some of the high volatiles may be lost
TVP Model
The TVP (True Vapour Pressure) model discussed will show how to use several Total vapour pressure measurements, tested at different V/L ratios, to extrapolate the TVP at a V/L =0/1.
The model uses the following assumptions: Assuming there is no decomposition of the samples, it is a good assumption that the vapour pressure of the liquid hydrocarbons are constant at a given temperature (Pliquid). It also can be assumed, that the vapour pressure of the substance is the sum of the contribution of the gas and the vapour pressure of the unchanged liquid. Assuming further, that the vapour pressure of the gas is well described by the ideal gas law (but including a volume of the dissolved gas in the liquid Va), the total pressure of the substance can be written as:
Fig 1. Infl uence of amount and types of dissolved gases in one hydrocarbon liquid as a function of vapour liquid ratio (V/L).
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