What are the common problems with using multiple thermo methods in a flowsheet?

This is a question commonly asked of Technical Support - so i figured we would post the answer here to help those that don't want to contact Technical Support. 

The use of multiple K-value sets within a flowsheet will not usually create serious discontinuities; however, the use of multiple enthalpy methods can cause serious discontinuities and erroneous results if not properly handled. 

For  example, both the SRK and Curl-Pitzer methods are based on deviations from ideal gas values.  However, they predict different enthalpy values for a mixture at any given temperature and pressure.  Thus an adiabatic flash using the SRK method predicts an incorrect temperature value for a feed stream with a Curl-Pitzer enthalpy.

Therefore, when it is desired to feed a process stream to a unit operation for which a different enthalpy method is to be used, it is recommended that the stream enthalpy first be reset.  This may be accomplished with an isothermal, dew point, or bubble point flash (as is appropriate) using the new enthalpy method.  This will reset the enthalpy to the new base and avoid the introduction of discontinuities.

For more information on multiple thermodynamic sets see Section 20.5 of the PRO/II Keyword Input Manual.

About PRO/II: PRO/II is the major product in the Process Engineering Suite (PES). This process simulation software optimizes plant performance by improving process design, operational analysis, and performing engineering studies. It is designed to perform rigorous heat and material balance calculations for a wide range of chemical processes. PRO/II allows users to evaluate the use of heat integration with case studies, rigorously evaluate process improvements with the help of a process flow diagram (PFD), or model crude distillation units. PRO/II offers a wide variety of thermodynamic models to virtually every industry and is incredibly cost effective, decreasing both capital and operating costs.

Technical Support: As always, feel free to reach out to your local technical support team for more information.