Thermal Properties Critical Temperatures

When a fat or oil is heated, thermal instability may cause decomposition, and depending on the temperature reached, subsequent combustion of volatile gaseous decomposition products (Mehlenbacher, 1960). The thermal stability of fats and oils is thus essentially a chemical characteristic. However, stability is characterized by measuring certain critical temperatures, the smoke, flash and fire points, at which certain heat-induced changes become apparent. It is appropriate, therefore, to include here methods for measuring these critical points. 

Measurement of smoke, flash and fire points is carried out subjectively by observing the surface of an oil sample while the sample is being heated. (The critical temperatures are higher than the upper limit of the melting point range of a fat.) The smoke point is the temperature at which the sample begins to give off a continuous stream of bluish smoke, observable 

Standard methods for the determination of the critical temperatures are published by the AOCS (AOCS Official Methods Cc 9a 48 (smoke, flash and fire points by an open cup method), and Cc 9b-55 and Cc 9c 95 (flash point by open cup methods), Firestone, 1998) and by the British Standards Institution (BS 684 Section 1.8 1976 (smoke point), BSI, 1976b and BS 684-1.17 1998/ISO 15267 1998 (flashpoint by a closed cup method), BSI, 1998b). 

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Critical Temperature The critical temperature of a compound is the temperature above which a hquid phase cannot be formed, no matter what the pressure on the system. The critical temperature is important in determining the phase boundaries of any compound and is a required input parameter for most phase equilibrium thermal property or volumetric property calculations using analytic equations of state or the theorem of corresponding states. Critical temperatures are predicted by various empirical methods according to the type of compound or mixture being considered. 

Principles and Characteristics Water is an interesting alternative for an extraction fluid because of its unique properties and nontoxic characteristics. Two states of water have so far been used in the continuous extraction mode, namely subcritical (at 100 °C < T < 374 °C and sufficient pressure to maintain water in the liquid state) and supercritical (T>374°C, p>218 bar). Unfortunately, supercritical water is highly corrosive, and the high temperatures required may lead to thermal degradation of less stable organic compounds. However, water is also an excellent medium for extraction below its critical temperature [412], Subcritical water exhibits lower corrosive effects. 

Homogeneous Liquids. The physical properties important in determining the suitability of a liquid for propellant application are the freezing point, vapor pressure, density, and viscosity. To a lesser extent, other physical properties are important such as the critical temperature and pressure, thermal conductivity, ability to dissolve nitrogen or helium (since gas pressurization is frequently used to expel propellants) and electrical conductivity. Also required are certain thermodynamic properties such as the heat of formation and the heat capacity of the material. The heat of formation is required for performing theoretical calculations on the candidate, and the heat capacity is desired for calculations related to regenerative cooling needs. 

Table 1 gives the components present in the crude DDSO and their properties critical pressure (Pc), critical temperature (Tc), critical volume (Vc) and acentric factor (co). These properties were obtained from hypothetical components (a tool of the commercial simulator HYSYS) that are created through the UNIFAC group contribution. The developed DISMOL simulator requires these properties (mean free path enthalpy of vaporization mass diffusivity vapor pressure liquid density heat capacity thermal conductivity viscosity and equipment, process, and system characteristics that are simulation inputs) in calculating other properties of the system, such as evaporation rate, temperature and concentration profiles, residence time, stream compositions, and flow rates (output from the simulation). Furthermore, film thickness and liquid velocity profile on the evaporator are also calculated. 

At a fundamental level, the process of spontaneous ignition depends strongly on the thermal properties (p, k, C) and the reaction constants, and weakly, on the viscosity ( a) and permeability (K). The final parameter is the eigenvalue of the problem corresponding to the ignition temperature, Tc. The critical ambient temperature, 7 0, and the critical volume Vc are truly not physical parameters controlling spontaneous ignition, but the result of the mathematical analysis. 

Besides the critical issue of containment and sealing, the choice of the materials for the membrane and other membrane reactor components affects the permeability and permselectivity, operable temperature, pressure and chemical environments and reaction performance. Important material parameters include the particular chemical phase, thickness, thermal properties and surface contamination of the membrane, membrane/support microstructure, and sealing of the end surfaces of the membrane elements and of the joining areas between elements and module components. The conventional permeability versus permselectivity dilema associated with membranes needs to be addressed before inorganic membrane reactors are used in bulk processing. 

In 1822, Cagniard de la Tour showed the existence of a critical temperature for each individual substance above which such a substance can only occur as a fluid and not as either a liquid or a gas. This critical point is reached as one moves upward along the gas-liquid coexistence curve, where both temperature and pressure increase. The original liquid becomes less dense through thermal expansion and the gas becomes more dense as the pressure rises. At the critical point, the densities of the two phases are equal, the distinction between the gas and liquid vanishes and the coexistence curve comes to an end at the critical point, where the substance is described as a fluid. Supercritical fluids exhibit key features such as compressibility, homogeneity and a continuous change from gas-like to liquid-like properties. 

For biomaterials that are thermally unstable and decompose before reaching the critical temperature, several estimation techniques are available. We have used the Lydersen group contributions method ( ). Other techniques available for predicting critical properties have been reviewed and evaluated by Spencer and Daubert ( ) and Brunner and Hederer Qfi). It is also possible to determine the EOS parameters from readily measurable data such as vapor pressure, and liquid molar volume instead of critical properties (11). We used the Lydersen method to get pure component parameters because the vapor compositions we obtained were in closer agreement with experiment than those we got from pure component parameters derived by Brunner s method. The critical properties we used for the systems we studied are summarized in Table II. 

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