Freeze out in natural gas systems: Utfrysning i naturgass-systemer
The motivation of the work is to increase the knowledge about thermodynamic modeling of freeze
outs in natural gas systems and a deeper understanding of the phase behavior of natural gas
mixtures, due to the problems experienced in cryogenic natural gas process-plants. Critical
components in natural gas mixtures introduce a risk of forming a solid coating and plugging the
process equipment. Hence, it is relevant to examine methane rich binary mixtures containing
components with high risk of freezing. Due to their high triple point temperatures, carbon dioxide,
benzene and cyclohexane are regarded as the most critical components.
The preferred thermodynamic method for modeling the solid fluid system is by describing the fluid
phases with a traditionally equation of state in combination with an expression for the solid phase
based on melting and triple point properties. This method is based on the assumption of a pure
component solid phase, which does not always represent the precipitated substances in natural gas
systems. However, it is the situation which represents the highest risk of crystallization at a given
This study was carried out by applying a simulation tool called NeqSim, where the equation of state
method is implemented based on the computational algorithms provided by Michelsen and
Mollerup. To investigate the reliability and accuracy of the equation of state method, it is used
experimental data from the literature as a foundation and further compared against two existing
simulation tools for freezing point predictions of natural gas mixtures, GPA and HYSYS.
The Soave-Redlich-Kwong equation was selected together with classical mixing rules and the use of
binary interaction parameters. The binary interaction parameters were discovered to be of crucial
importance to the accuracy of the predictions, both for the binary mixtures containing carbon
dioxide and the heavy hydrocarbons. The model showed promising results for carbon dioxide in
methane rich binary mixtures, after the binary interaction parameter had been optimized. However,
the interaction parameter dependency for solid-vapor equilibrium systems was discovered to be less
than in solid-liquid equilibrium systems. Predictions of freeze outs of heavy hydrocarbons were
discovered to be more challenging, due to numerical problems and a scarce experimental database.
One of the main issues concerning the development of reliable thermodynamic models for solid-fluid
systems is the lack of experimental data, which prevents extensive validation of the proposed
models. The experimental work related to the freeze out rig has mainly involved trouble-shooting
and experiments for determining the behavior of the rig. Hence, the experimental focuses were on
the gathering operational experience by running these experiments, and identify the main challenges
and the potential areas for improvement. Including a phase study of pure carbon dioxide where the
solid-liquid equilibrium and three-phase point was determined.
In order to investigate the binary mixtures of interest, where the composition and the component
exposed to freeze outs is known, the heat loss from the Dewar container and the temperature
difference between the air bath and the fluid has to be reduced. Hence, the solids will form in the
sapphire cell, where the freeze outs can be visually detected. Further, for studying multi-component
mixtures similar to real natural gas mixtures where the composition of the precipitating substance is
unknown; a solution for the sampling of the different phases has to be developed, including
extensively testing and validation.
Place, publisher, year, edition, pages
Institutt for energi- og prosessteknikk , 2009. , 161 p.
ntnudaim:4712, MTPROD produktutvikling og produksjon, Energi-, prosess- og strømningsteknikk
IdentifiersURN: urn:nbn:no:ntnu:diva-12889Local ID: ntnudaim:4712OAI: oai:DiVA.org:ntnu-12889DiVA: diva2:426928
Pettersen, Jostein, Professor IIFredheim, Arne OlavStang, JacobSolbraa, Even