Development of a multiphase flash in presence of hydrates: Experimental measurements and validation with the CPA equation of state

• A new multiphase flash calculation algorithm for complex hydrate forming systems is presented.
• The CPA-EOS is used for describing fluid phases and proves to be accurate in presence of hydrates.
• Vapor–Liquid–Hydrate equilibrium measurements performed for a natural gas are reported.
• The flash calculation results are validated with experimental results from 4 different examples.

A new robust flash algorithm was developed for equilibrium calculation in systems with clathrate hydrates. The algorithm is based on the minimization of the Gibbs free energy by solving the reformulated Richford–Rice equation. Special considerations have been taken for the algorithm to work with clathrate hydrates of different structures. In the current work, the cubic plus association equation of state (combined with the Peng–Robinson equation of state and a group contribution method to calculate binary interaction parameters between non-associating components) is used to calculate fugacity in fluid phases while the van der Waals and Platteeuw approach is used to calculate the fugacity of water in the hydrate phase.The developed algorithm has been applied to complex multicomponent example systems for which experimental data are available as well as new experimental data on both hydrate dissociation points and composition of vapour phase in equilibrium with hydrates inside the hydrate phase boundary for a natural gas. The new algorithm is fast and offers a simple routine for initial guess calculation and is capable of showing complex behaviours in hydrate forming systems including stability of more than one hydrate structure at equilibrium conditions and pseudo-retrograde behaviour in hydrate formation. For the examples used in this work, a good agreement between the experimental measurements and predictions is observed, demonstrating the reliability of the approach.