The next generation of hydrate prediction IV: A comparison of available hydrate prediction programs

The van der Waals and Platteeuw [Adv. Chem. Phys. 2 (1959) 1] hydrate equation of state, coupled with the classical thermodynamic equation for hydrates, has been used in the prediction of hydrate formation for over 40 years. The standard state used in these equations is a hypothetical empty hydrate lattice. In Part I of this series [Fluid Phase Equilib. 194/197 (2002) 371], we proposed an alternative derivation of these equations using a different standard state. The new hydrate equations were shown to be simpler to use. In Part II of this series [Fluid Phase Equilib. 211 (2003) 85], we proposed an aqueous phase model tailored specifically for the presence of mixed hydrate inhibitors such as salts and methanol in the aqueous phase. Part III [Fluid Phase Equilib. this volume] provided a prescription for the incorporation of the new hydrate and aqueous phase models into a multi-phase Gibbs energy minimization program (CSMGem), extending the work of Gupta and Bishnoi. V quantitatively compares the CSMGem program with four commercially available hydrate prediction programs: CSMHYD, DBRHydrate, Multiflash, and PVTsim. The work may be considered as a state-of-the-art accuracy overview of readily available hydrate prediction programs. Comparisons are given for two types of hydrate data: hydrate phase properties and hydrate formation temperatures and pressures. For the hydrate formation temperature and pressure comparisons, we show results for uninhibited and inhibited systems. Note that all data and predictions are at the incipient hydrate formation pressure (P) and temperature (T) due to lack of data for the interior of the phase diagrams. Such data await spectroscopic analysis for intrinsic variables and flash measurements for extrinsic properties, such as phase amounts, for comparison to flash calculation predictions.