Calorimetry in the near-critical and supercritical regions. Nitrous oxide + hydrocarbon mixtures

Abstract

Abstract:The high pressure calorimeters developed in the last 20 years are briefly reviewed. Most of them have been used to obtain excess enthalpy data (H E m ) in the critical and supercritical regions. Data are available for many mixtures formed by carbon dioxide and for a few involving other supercritical fluids such as ethane, methane, nitrous oxide, etc. Data for several N 2 O þ hydrocarbon mixtures were recently measured from 308.15 to 323.15 K and from 7.64 to 15.00 MPa. Excess enthalpies for N 2 O þ hydrocarbon mixtures were calculated using several cubic equations of state (EOS). Two noncubic EOS which combine the Carnahan-Starling hard-sphere term with the van der Waals and Redlich-Kwong attractive terms, were also used. The classical van der Waals mixing rules and those proposed by Wong and Sandler were used. The ability of the cubic EOS to correlate the excess enthalpies of the N 2 O þ hydrocarbon mixtures seems to be related to the proximity of conditions of temperature and pressure to the N 2 O critical point. CALORIMETRY IN THE CRITICAL REGIONThe increasing interest in supercritical fluids runs parallel with the increasing use of calorimetry in the near-critical and supercritical regions. The aim is to study the energetic interactions of molecules under these particular conditions. During the last two decades, high pressure calorimetry has shifted almost completely to supercritical fluid applications. The first experimental device to measure the endothermic excess enthalpies of simple mixtures such as CH 4 þ N 2 , CH 4 þ H 2 or CH 4 þ Ar was developed by Wormald et al. [1,2] in 1976. A new version is able to reach up to 700 K and 20 MPa, and to measure exothermic processes as well [3]. Simultaneously, Christensen et al. [4,5] were setting up a new highpressure flow calorimeter able to detect both exothermic and endothermic processes which was quickly applied to determine the excess enthalpies of CO 2 þ hydrocarbon mixtures in the critical region. From 1983 to his early dead, James J. Christensen published data of mixtures such as CO 2 þ ethane [6], þ pentane [7,8], þ hexane [9,10], þ cyclohexane [11][12][13][14], þ decane [15,16], þ toluene [17][18][19][20] and þ pyridine [21,22]. Figure 1 shows the experimental P,T conditions at which data are available for three of these systems. It may be seen that several types of mixtures-subcritical, supercritical, liquid-were studied. The data showed striking features: highly exothermic or endothermic enthalpies, large temperature and pressure effects, and two-phase linear sections in the H E m vs. composition diagrams. The same equipment has also been used to determine heats of reaction of electrolytes and amino acids. For instance, the heat of reaction of phosphate anions with hydrochloric acid as a function of temperature and pressure was used to determine the protonation constants of phosphate at high temperature [23], and the heat of reaction of amino acids with basic solutions as a function of temperature, pressure and concentration was used to determ...