Vapor Pressure

What is Vapor Pressure?

Hydrocarbons are, for the most part, comprised of a mixture of different compounds. Under the right (or the wrong) conditions, these compounds can bubble out of the overall hydrocarbon mixture. The ambient pressure at which this process begins (at a given temperature) is referred to as the vapor pressure of that hydrocarbon. Within this broad definition are a number of subcategories that make certain assumptions about the conditions present. For example, the Reid Vapor Pressure (RVP) is the vapor pressure found by a very specific experimental method. Unless otherwise specified, assume that the term “vapor pressure” refers to the more general case.

Vapor Pressure in Condensate

Condensate, despite being an increasingly important part of the liquid hydrocarbon business, is also dangerous to store and transport. The fumes let off from condensate are both extremely flammable and toxic when inhaled. Because of these dangers, condensate must be stabilized and there are stringent requirements surrounding its storage and transport. The most common method used to ensure that these requirements are met is to measure the vapor pressure (RVP, TVP, and/or TPCRx) of the condensate.

Historically, it has been very difficult to measure the vapor pressure of condensate in real time. The most conventional method of doing so is that established by the ASTM. Unfortunately, this method operates at 100F. This relatively low temperate means that the paraffin present in your typical condensate sample will condense and begin to contaminate the analyzer. Without constant maintenance this paraffin build up will plug the sample lines, coat the measurement cells, and ultimately cause the analyzer to fail.

Vapor Pressure in Crude Oil

Relatively pure hydrocarbon liquids are generally quite predictable. If conditions are constant, they behave in a consistent manner. If conditions change, they react predictably. Crude oil on the other hand, being anything but pure, is also anything but predictable. The presence (and varying amount) of light ends in crude oil means that the vapor pressure can change wildly in response to variations in temperature and pressure. These changes in vapor pressure mean that it must be measured both regularly and accurately whenever crude oil is being transported or stored.

In the worst case, failing to properly monitor vapor pressure can have costly and potentially dangerous consequences. One example of this is pumping cavitation. This occurs when the vapor pressure of the crude oil exceeds the pressure of its surroundings. Bubbles form in the crude oil and eventually collapse. When these bubbles collapse, they will generate a shock wave that can damage nearby equipment.

NIRS and the Verax VPA

Near infrared spectroscopy (NIRS), combined with chemometrics, offers an alternative to the conventional vapor measurement processes. By passing near infrared light through the sample and measuring the absorption, it is possible, with the application of chemometrics, to determine the vapor pressure (and other aspects) of the process. More information on NIRS can be found here.

The core of the Verax VPA is of a pair of optical probes. It is these probes that project and measure the near infrared energy. These probes are capable of functioning at operational temperature and pressure and as such, can be mounted with direct access to the process. This positioning in turn eliminates the need for a sample collection system or a sample conditioning system.

In the case of condensate, this lack of conditioning means that the process is kept at a higher temperature and the paraffin in the process cannot condense. In practical terms, this means a decrease in operational costs and an increase in system up-time.

In the case of crude oil, the speed and accuracy of the analysis means that changes in vapor pressure can be detected quickly. Problems like pumping cavitation can then be prevented with prompt changes to temperature or pressure.

Click below to learn more about the Verax VPA and the benefits it can bring to your business.