Integrated Wax-Hydrate Formation in Real Oil Mixtures: What Factors Are Important from Thermodynamic Modelling?

The presence of unknown fractions in real mixtures, as opposed to synthetic mixtures, necessitates the application of a characterization scheme to provide accurate inputs for the thermodynamic models utilized for modelling wax and hydrate formation. Inappropriate evaluation of these inputs may result in poor predictions, even though accurate thermodynamic models are used to describe phases. On one hand, regardless of the type of the unknown fraction, it is essential for the purpose of wax precipitation modelling, to characterize it to identify the amount and distribution of the wax forming part in the mixture. On the other hand, accuracy of hydrate phase boundary and amount calculation in real oils, is indirectly dependent on characterization of the unknown fraction, meaning that if the fluid phases are described precisely the hydrate dissociation lines can be calculated with acceptable accuracy.
In this work, the modelling of formation of waxes and hydrates in real oils are discussed separately. It will be shown that the main complication for modelling combined wax-hydrate formation in real mixtures is with the wax modelling part and in particular the correct quantification of the distribution of normal paraffins in the unknown fraction. In the wax part, different schemes for characterization of real oils are explained and compared for evaluation of wax precipitation and melting temperatures, highlighting limitations and requirements of different models. It will be explained that amongst all of the existing methods for extending wax precipitation models to real systems, only those which (i) distinguish the wax forming (paraffinic) part from the non-paraffinic part for each unknown fraction and (ii) are capable of identifying the distribution of wax forming fractions in the heavy end, are reliable. Accordingly, the performance of the only two approaches which satisfy these criteria was evaluated. With a comparative approach, it is concluded that the method of Coutinho and Daridon [1] is the best fully-predictive extending scheme which gives the most accurate results with the least experimental information, specifically the distribution of n-alkanes in the mixture.
Our recently proposed UNIQUAC-CPA-vdWP integrated thermodynamic model for wax-hydrate formation [2] with the extension of Coutinho and Daridon method is finally applied for combined solids formation modelling in a real mixture for which experimental WDT data are available. Based on the results obtained it was concluded that hydrate formation could escalate the wax precipitation problem which agrees with the results achieved in our previous study for systems without unknown fractions.