Wax precipitation in crude oils: DSC and model analysis
Advancing the chemical engineering fundamentals
Thermodynamics: Hydrocarbons & Petrochemicals (T2-1a)
Keywords: wax precipitation, crude oil, solid-liquid equilibrium, thermodynamic model
One of the well known problems in the flow assurance field is wax deposition in crude oils due to temperature decrease. Such deposition depends on crude oil nature, paraffin content and type, presence of additives, temperature conditions, etc. The prevention of precipitation or even full pipeline blockage requires a detailed characterization of the crude oil.
However, experimental determination and quantification of the precipitation process is quite complex and time consuming because of the increasing of viscosity of the system with temperature decrease, the small size of the wax crystals and the presence of absorbed and trapped crude oil in the solid phase.
Among the different procedures described in the literature to carry out such study, differential scanning calorimetry (DSC) technique has the advantage of its simplicity and fast response which make it appropriate to develop routine essays. In DSC analysis a crude oil sample follows a temperature profile and heat transfer is registered so that heat change due to phase transition can be quantified. Experimental difficulties are found in the application to crude oil samples: calibration at low temperature, broad and low-intensity signals, high sensitivity requirements, complex base line determination, integration of the thermograms not clearly defined, etc.
In this work, a procedure has been developed based on the calibration and the thermogram integration using values for pure n-paraffin. Such integration procedure yields both the wax appearance temperature (WAT) and the wax precipitation curve.
In order to check such procedure, several oil cuts were analyzed. Using petroleum fractions has the advantage of containing a much narrow n-paraffin distribution than a crude oil. Experimental characterization for the cloud point temperature, the n-paraffin distribution (determined by analysis of gas chromatography with mass detector, GCMS) and DSC was carried out.
A very simplified thermodynamic model has been checked to describe solid-liquid equilibrium in these systems. Solid phases were considered to be pure n-paraffin compounds and liquid phase was assumed to be ideal. Correlations for the melting temperature, melting enthalpy and heat capacities for liquid and solids versus the number of carbon atoms in the n-paraffin were taken from literature. Experimental n-paraffin composition was determined from GCMS analysis. Temperature profiles were simulated and the solid-liquid equilibrium conditions determined for each temperature value, thus obtaining the total amount in the solid phase against temperature.
WAT values determined by the analysis of DSC and calculated with the model compares favorably to the experimental cloud point values. The precipitation curve obtained from the integration of the DSC thermogram and that calculated by means of the model are in good agreement.
Presented Monday 17, 11:33 to 11:52, in session Thermodynamics: Hydrocarbons & Petrochemicals (T2-1a).
