And how it differs from True Vapour Pressure and Kinetic Vapour Pressure
Correlation between True Vapour Pressure (TVP) and Reid Vapour Pressure (RVP) for similar products – in this case, petroleum distillates.
Reid Vapour Pressure – What is vapour pressure?
Every liquid is, to some extent, volatile. In other words, it gives off a vapour. For some liquids, it takes very little energy to convert the liquid to a vapour. Think of fuels. We are only too aware of just how volatile they are when filling up the car. And that vapour will inevitably exert a pressure.
Therefore, every fluid has a vapour pressure. Remembering back to school chemistry, we know that in gases, temperature, pressure and volume are all linked. Take a fluid like ethane. If the volume is kept constant, ethane will remain a liquid if enough pressure is applied to it, or alternatively, if it is kept cold enough.
To keep the fluid entirely liquid, a force needs to be applied to compress it. That force, in effect, is the true vapour pressure, which is usually abbreviated to TVP.
Reid Vapour Pressure – Is the TVP not enough?
The problem with the True Vapour Pressure is that it can be difficult to measure using process equipment. Measuring vapour pressure is complicated by the nature of fuels, due to the fact that they are complex mixes of hydrocarbons, some of which are more volatile than others. In essence, one can never be entirely sure what one is actually measuring.
Therefore, to overcome this, standard methods have been employed by the oil industry to measure vapour pressure under a standard degree of expansion (the ratio of vapour to liquid). The most common of these is the Reid Vapour Pressure, or RVP.
The advantage of the RVP over the TVP
To measure TVP, a high degree of pressure is required. In process equipment, this is not easy to do. RVP is measured using a 4:1 ratio of volume to liquid volume. The RVP is lower than the TVP, but the difference between the two exhibits a linear correlation, and so they can be calculated from each other (see graph above).
So, the main advantage of Reid Vapour Pressure is that it can be measured in-line by process equipment, or in a laboratory. However, it is a batch test and it differs from TVP.
Kinetic Vapour Pressure (KVP)
The KVP is a method by which a sample at constant temperature and pressure is subjected to a pressure drop. The reduction in pressure causes vaporisation of the sample and the measured ‘kinetic’ pressure of the vapour is then calibrated to RVP.
This is the method that the ATAC model 1354 Vapour Pressure Analyser utilises. The pressure drop is created by the passage of sample through an educator, where the sample velocity creates an area of low pressure in which a bubble forms. The fact that the sample generating the bubble is constantly refreshed means that the measure vapour pressure is very close to TVP.
Both the RVP & KVP can therefore be correlated to TVP but, in practice, the TVP is not usually required. It is generally taken that RVP and KVP are accepted as measurements of the vapour pressure in their own right.
The ATAC 1354 Vapour Pressure Analyser – A quality in-line physical property analyser of KVP which can be configured to read the RVP and the TVP.
Measuring Vapour Pressure – Conclusion
Obviously, there will be a variety of reasons for measuring vapour pressure, and a decision has to be made as to which of these measurements is most appropriate.
True Vapour Pressure (TVP) is a physical property of any sample, and it will be the same regardless of the method used. However, it can be difficult to measure. Fortunately, it can be calculated if the Reid Vapour Pressure or the Kinetic Vapour Pressure is known. Either of these measures are accepted as standard across the industry.
Kinetic Vapour Pressure (KVP) is close to the TVP as the method employed offers very little expansion and a continuously sample volume. The ATAC 1354 Vapour Pressure Analyser is designed to give an in-line measurement of KVP, and can be configured to read the Reid Vapour Pressure (RVP) and the True Vapour Pressure (TVP).
Contact Us
For more information about the ATAC 1354 Vapour Pressure Analyser, contact the experts at ATAC using sales@atacgroup.com or by phoning +44 (0)1380 818411. Alternatively, click below to visit the product page. We look forward to hearing from you.
Feature Photo courtesy of Sara Farshchi
