Measuring the amount of crystallinity in solutions using DSC

Abstract

The amount of solid that forms in a solution at temperatures below the cloud point, is an important quantity in the design of crystallisation processes. A new method using differential scanning calorimetry (DSC) to measure the fraction of crystallised solution, as,o,utio,,, is proposed. A very good agreement is obtained between the data measured by the new calorimetric method and using alternative techniques. A simple graphic version of the calculation procedure is also presented. It is shown to produce quick and fair estimates for the solid fraction by a simple analysis of a plot of enthalpy variation with temperature.La quantite de solides qui se forme dans une solution a des temperatures au-dessous du point de trouble est un facteur important dans la conception des procedes de cristallisation. On propose une nouvelle methode utilisant la calorimetrie differentielle a balayage (DSC) afin de mesurer la fraction de solution cristallisee, Un tres bon accord est obtenu entre les donnees mesurees par la nouvelle methode calorimetrique et d'autres techniques utilisees. On version graphique simple de la procedure de calcul est egalement presentee. On montre qu'elle peut produire des estimations rapides et fiabies pour la fraction de solides par simple analyse d'un trace de variation d'enthaipie avec la temperature.Keywords: solid-liquid equilibrium, calorimetry, DSC, crystallisation.he knowledge of the amount of solid precipitating from T a solution is required in designing and operating plants where either crystallisation is promoted for purification purposes or when solid formation is to be prevented, as for instance with the wax deposition problem. For simple systems when the phase diagram is known the evaluation of the hction of precipitated material can be readily obtained by means of a simple calculation. However this is not the case when dealing with complex mixtures for which neither experimental phase diagrams nor calculation procedures exist to estimate the amount of precipitate being formed at a given temperature. For those systems this quantity can only be assessed experimentally.To evaluate the quantity of precipitated forming at a given temperature a crystallisation of the solution problem can be performed measuring directly or indirectly the fraction of solid material (Van Winkle et a1.,1987; Coutinho and Ruffier-Meray, 1997), but this is time consuming and may be a difficult technique if the phases do not separate easily. An alternative method would be the use of the Differential Scanning Calorimetry (DSC). This is a well known technique but its use for measuring solid fractions is not well understood and seldom used. The procedure presented in some DSC manuals for these measurements is poorly accurate and, to a large extent, empirical. In this work a new methodology for such measurements, along with its thermodynamical background, will be presented. The assumptions and simplifications used in the derivation of the equations are thoroughly stated and from them the limits of applicability of the techni...

Low-Pressure Modeling of Wax Formation in Crude Oils

“…This is altogether a more well-established and reliable technique to measure low-temperature phase behavior of oils . Using the approach proposed before for DSC data analysis, , it was possible to compare the two techniques and assess the quality of the NMR data. A comparison for oil 3 is presented in Figure , and similar results can be obtained for all the other oils.…”

Measurements and modelling of wax formation in diesel fuels

“…This is but an approximation, albeit the best available at the moment, and some incertitude is introduced in the calculations by its adoption. Using this base line the fraction of solids dissolved is obtained using an approach that proved adequate for complex synthetic mixtures and is described elsewhere [11].…”

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

“…The fraction of solute melted, α L , is thus simply given by the ratio of the distance between the enthalpy curve and the line ϕ measured at temperatures T and T c as shown in Figure 1. This gives a simple and fast method to estimate the amount of solid paraffins at a given temperature once a calorimetric measurement as been performed [4].…”