Experimental pressure versus temperature isochoric - isoplethic curves for systems related to the supercritical hydrogenation of polybutadiene

Abstract

Linear low density polyethylene of low polidispersivity index could be obtained by hydrogenating low polidispersivity index polybutadienes (PBs). In conventional hydrogenation processes, the system always presents, at least, two fluid phases during the reaction progress. The presence of an interphase implies lower mass transfer rates. This could be overcome by using a supercritical fluid as reaction medium, so that the hydrogenation process occurs, in principle, under single fluid phase conditions. To hydrogenate the PB under supercritical conditions, the solvent (or solvent mixture) should be able to simultaneously dissolve the PB, the hydrogen and the reaction products. The high asymmetry of the reactive mixture, which is in part due to the large difference in size between the H2 and PB molecules, implies a high immiscibility level in the absence of the supercritical solvent or solvent mixture. In particular, the ‘PB + solvent mixture’ system should be homogeneous at the conditions of temperature, pressure and PB concentration range at which the PB hydrogenation is to be carried out. The purpose of this work is to experimentally find the conditions under which a number of ‘PB + solvent mixture’ systems, and related subsystems, are homogeneous. In a typical experiment, an in-house constant volume cell is loaded with a known amount of a known composition mixture. Next, the pressure is measured at set temperature, covering the temperature range of interest. Clearly, the measured and recorded pressure versus temperature curve is an isopleth (constant overall composition) and it is also an isochor (constant overall density). Slope changes in the curve imply changes in the phase condition, i.e., this experimental technique also provides saturation conditions. At high enough temperature, the system becomes homogeneous, and it remains so at higher temperatures. Pressure vs Temperature isochoric-isoplethic trajectories were measured in this work for a number of pure solvents, solvent mixtures, and systems made of PB and a solvent mixture. The experiments were performed at varying overall density and varying overall mixture composition.