Experimental Measurements and Thermodynamic Modeling of Paraffinic Wax Formation in Undercooled Solutions

Abstract

The crystallization of paraffinic materials from hydrocarbon mixtures subject to a cold environment is one of the problems faced by the petroleum industry. It can be found during oil production and in the transport and even in the use of refined materials such as diesels and fuels. A better understanding of the crystallization process and the capacity to model the wax formation would help to overcome this problem. This work is focused on the study of the two-phase region by developing a new experimental technique for measuring the composition of the phases in equilibrium and the amount of paraffins precipitating from a hydrocarbon mixture. The crystallization of several model systems, constituted by a mixture of normal alkanes between n-C19H40 and n-C28H58 dissolved in ethylbenzene in mass concentrations up to 25% weight, was promoted in an optical cell. The composition of the phases in equilibrium at several different temperatures below the cloud point was determined by chromatography. The amount of waxes formed and the cloud points were obtained by differential scanning calorimetry (DSC). The new technique is shown to be simple and accurate. Some particularities of paraffin crystallization were studied as well as the influence of aromatic compounds on the wax formation. A thermodynamic model was used to describe the measured data. It is based on the Flory-free-volume model for the liquid phase and on the predictive local composition model, a predictive form of the Wilson equation, for the nonideality of the solid solution. The model is shown to provide a good description of the SLE data for the studied systems.

The Use of Differential Scanning Calorimetry in Studies of Wax Deposition: Measuring the Solid Formation and Binary Solid-Liquid Equilibrium Phase Diagrams

“…For the binary, the solid fractions measured by calorimetry are compared with those calculated from the phase diagram presented by Ghogomu et al [6]. For the other mixture, the solid fractions are compared with data determined by crystallisation [3]. There is, in both cases, a very good agreement between the data obtained by the two different procedures giving a good indication about the capacities of the proposed method to measure the solid fractions.…”

“…Due to the gelling of mixtures with paraffin deposition, it is impossible to separate the phases to evaluate the amount of solids. Alternative techniques have been proposed [2,3] but they are very complex requiring the establishment of the equilibrium at each temperature of interest and the measurement of the composition of the phases present. DSC can be an adequate alternative to this process allowing a much simpler and faster determination of the solid formation by analysis of the plot of enthalpy variation with temperature as described below.…”

Liquid-Solid Phase Equilibria of Paraffinic Systems by DSC Measurements

“…The DSC technique allows also the determination of the wax precipitation or solubility curve (amount of precipitated wax at different temperatures) as it has been reported [18,28,[32][33]. It is carried out by assuming that the amount or fraction of precipitated wax in the total wax content is proportional to the percent of accumulated heat released in the total heat released (Crystallization enthalpy), thus the amount of precipitated wax at different temperatures can be determined by dividing the accumulated heat released by the heat of crystallization.…”

“…Some experimental techniques reported in literature such as Microscopy and X-ray diffraction are powerful methods to determine the crystal structures but give limited insight into the crystallization process [16], while others methods used to get the liquid-solid equilibrium of paraffins have been used [17][18], but they are very complex due to they require the establishment of the equilibrium at each temperature of interest and the measurement of the composition of the phases present. Finally, visual methods have been also reported to measure solubility and phase behavior of paraffin waxes [1,19]; however, these methods cannot be applied to test dark samples (e.g.…”

Fundamental Study of Wax Deposition Under Real Flow Conditions

“…It is usually obtained through either filtration or centrifugations methods (Han et al, 2010;Coutinho and Ruffier-Méray, 1997;Coutinho et al, 2000). However, in both methods, trapped liquid prevents the complete separation of solid phase and increases the uncertainties of the experimental results.…”