Fluid Properties and Thermodynamics

Fluid Types

Several types of fluid can be modelled by the Monitor computer program. Each type has different data requirements for physical properties. These data are often difficult to obtain but their accuracy will affect all calculated results.

Single Phase Hydrocarbon

This is the most common type of stream in a crude preheat simulation. American Petroleum Institute (API) procedures are used to calculate physical properties from gravity and UOPK factor. For liquid streams, you may supply kinematic viscosity data instead of UOPK factor.

Assay Stream

This type of stream is generally used in refinery simulation when a change of phase is expected. You must supply a D86, TBP, D1160 or D2887 assay curve and an average gravity. Other physical properties are calculated using the standard API procedures. The phase of the stream is determined from a flash calculation. Lightend components, which are part of the assay, may also be defined.

Defined Composition

This is the most general and flexible type of stream and is typically used for the hydrogen feed in HDS units. The stream is made up of components whose properties are stored in a library. You only need to specify the component names and the composition of the stream. The stream properties are calculated from individual component properties. The phase is determined from a flash calculation.


Internal steam table properties are used for all water and steam property calculations. Water may be vaporised and steam may condense. Saturated properties are always used for water. Steam properties are saturated or superheated as required.

Mixing Streams

Because of the different property calculation methods, the extent of mixing is not universal. Liquid hydrocarbon streams may mix with other liquid hydrocarbon streams or with water streams. Vapour hydrocarbon streams may be mixed with other vapour hydrocarbons. Assay, Defined Composition and Water/steam streams may all be mixed together.  

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