Experimental And Thermodynamic Modeling Study On The Effect Of Pour Point Depressant On Wax Precipitation

When waxy oils are transported at low temperature,the plug of pipeline caused by wax precipitation has important influence on the transportation and production of crude oils.At present,many physical and chemical means have been used to solve this problem.Adding chemical pour point depressant(PPD) to the oils have been applied extensively because of its simple operation,low investment and non-after treatment.It is important to study the influence of pour point depressant(PPD) on wax precipitation.This research works on the effect of PPD on the thermodynamic properties of wax precipitation from the point of view of experimental and thermodynamic models.The amount and composition of wax precipitated from simulated oils with and without PPD at different temperatures were measured by high speed centrifuge and gas chromatography(GC),respectively.It can be concluded that the amounts of wax precipitated from oils treated with PPD in the experimental range are lower than that from untreated oils. However,the amounts of wax precipitated from oils treated and untreated with PPD are the same when the temperature drops to a certain temperature.At the same temperature,the wax precipitated from treated oils is richer in paraffins with higher melting point than that from untreated oils,so the wax formed from the treated oil mixture has a higher molecular weight than the wax formed from the untreated oil mixture.The thermograms of waxes with and without PPD were measured by differential scanning calorimetry(DSC).After the waxes were treated with additives,the DSC curves of waxes are all shifted to lower temperature.The experimental data show that the melting points and the solid-solid transition temperatures of paraffin mixtures are both decreased while they were being treated with PPD.However,the transition enthalpies have little changes especially the sum of solid-solid transition enthalpy and melting enthalpy.The lattice structure and morpholopy of wax crystals were studied by X-ray diffraction and the polarized light micsoscope,respectively.The X-ray diffraction results indicated that after adding PPD,the structure of wax is partly transformed from orthorhombic into hexagonal lattice.The orthorhombic lattice is an ordered phase at lower temperature and the hexagonal lattice is a disordered phase at higher temperature.The wax with PPD will prefer to crystallize in a less ordered state which is more akin to the liquid phase.This means that the wax precipitated from treated oils at a given temperature is corresponds to the wax precipitated from untreated oils at a higher temperature.The polarized light micsoscope results indicated that the function of PPD on the crystal morpholopy is increasing the size of paraffin particles,improving their symmetry,reducing their dispersivity,depressing their interaction and connection,so the characteristic temperature of non-Newton waxy oils is lowered.Based on the regular solution theory and polymer solution theory as well as the liquid-solid phase thermodynamic equilibrium,the regular solution model and the UNIQUAC local composition model were established in this work to predict the wax precipitated from oils with and without PPD,the prediction performance of these two models were also compared in this work.(1) Regular solution model:The activity coefficient based on a regular solution theory approach was used to account for the non-ideality of liquid and solid phases.(2) UNIQUAC local composition model:The fluid phase behavior was described by the modified LCVM,PR equation of state-G~E model and the solid phase non-ideality was represented by the UNIQUAC equation using the local composition concept.It is well known that the solid phases of all n-paraffins with more than nine carbon atoms in the chain consist of four distinct crystal structures,hexagonal(α_H),triclinic(β_T), monoclinic(β_M) and orthorhombic(β_O),respectively.When the temperature is slightly lower than the wax appearance temperature,the paraffin will precipitate with a disorderedα_H crystal structure,with the further decrease of temperature,and all of the solid paraffins become orderedβphase.The influence of solid-solid transition on the wax precipitation was considered in the correlation of standard fugacity ratio.From the point of view of the close relation between the crystal lattice structure and the thermal properties of paraffins,new correlations for fusion enthalpy,solid-solid transition enthalpy and solid-solid transition temperature of paraffins were established in this work based on the data from literatures.New correlations for the melting point and solid-solid transition temperature of treated paraffins were also established based on experimental results with differential scanning calorimeter (DSC),which show that these properties are both decreased after being treated with PPD.Based on the new correlations for fusion enthalpy,solid-solid transition enthalpy and solid-solid transition temperature of paraffins established in this work,a good calculation results as well as the reliability and precision of the models are demonstrated through a comparison calculation for three examples in the literature.The calculation results by these two models for the wax appearance temperature,the amount and composition of wax precipitated from oil with and without PPD at different temperatures have been compared with experimental observations.The predictions of the models agree well with the present experimental data.The calculation results indicated that the performance of wax precipitation models is strongly influenced by the thermodynamic properties of paraffins such as the fusion temperature and the solid-solid transition temperature etc.The results presented show that after adding PPD,the amount of wax precipitated from oils treated with PPD in the experimental range is lower than that from untreated oils,the wax precipitated from treated oils is richer in paraffins with higher melting point than that from untreated oils,which is consistent with the experimental phenomena of effect of pour point depressant on wax precipitaton.