Scientific Approach to Producing
Hydrocarbon Calibration Standards
Changes in ambient temperature may lead to phase changes with many hydrocarbon calibration mixtures. When a phase change occurs, the resulting composition may be different from the original composition. For a gaseous mixture, heavy components (low vapor pressure) may condense at low ambient temperatures. In liquid mixtures, vaporization of one or more lighter (high vapor pressure) components will occur as liquid is withdrawn from the cylinder, or if ambient temperature increases significantly above the temperature at which the mixture was made. Temperature and pressure changes may cause your single phase hydrocarbon mixtures to form two phases. Analytical or calibration work may be inaccurate if a cylinder is used in which phase changes have occurred. Air Liquide uses computer programs with the most accurate equations-of-state to predict the response of hydrocarbon mixtures to temperature or pressure changes. The benefit of these programs is two-fold:
- They allow Air Liquide to specify the pressure and temperature at which a
mixture can be produced and still maintain a single phase.
- They alert the customer to potential mixture composition problems due to
changes in ambient temperature.
With a database of hundreds of compounds and the ability to expand, Air Liquide can evaluate virtually any mixture. The types of information that can be supplied for a hydrocarbon mixture are as follows:
Gases
Maximum pressure or minimum temperature to avoid condensation of one or more components.
Liquids
Minimum pressure and maximum temperature to avoid vaporization of one or more components.
Specified Overall Composition
Fraction of mixture that is liquid or vapor and the composition of each phase.
Phase Envelope
Pressure vs. temperature diagrams will provide condensation and/or vaporization curves for a given mixture. These are available upon request.
Engineered to Your Specifications
To ensure mixture integrity and stability, Air Liquide technical representatives evaluate custom requests and recommend appropriate cylinders in terms of size and construction, ACULIFE™ cylinder treatment if required, and proper gas handling equipment depending upon the types and concentrations of components in the mixture. Computer programs, as described above, are available to help predict the phase behavior of mixtures. All products shown on this website are typical of those used in hydrocarbon applications but represent only a portion of our capabilities. For a more complete representation, contact us to discuss in further detail.
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Example of a Multi-Component
Hydrocarbon Mixture
Consider a hydrocarbon mixture of composition shown below (mole %).
0.2% Ethane
3% n-Butane
25% Propane
0.3% 1-Butene
68.2% Propylene
0.3% Isobutylene
3% Isobutane
The phase envelope diagram below shows how vaporization and condensation points vary with changes in pressure and temperature. At room temperature (72°F or 22°C), this mixture will start to condense at about 120 psig (8 bar) and will be mostly liquid at 135 psig (9 bar). This gas phase mixture would be supplied in a cylinder at less than 120 psig (8 bar).
A phase envelope diagram also indicates the temperature at which the gas phase mixture will start to condense. By examining this diagram, the user will see that storing the cylinder outdoors on a cold day may cause the heavier components to condense.
Phase envelopes are available upon request -contact us for more information.
