| It is known that in critical state the thermal physical properties of water vary considerably at slight temperature and pressure/density alterations. For example, thermal capacity or thermal conductivity of water in sub- or supercritical state displays anomalous behavior, which appears in a form of extremum functions of temperature or pressure. Anomalous fluid properties must affect the course of the process in the reactor. Therefore, calculation of alteration of the fluid properties in supercritical state is the necessary condition for the modeling of the reactor and apparatuses.
The mixture thermal capacity p c and molar enthalpy H, which display strong fluctuations near critical point, is determined in the above model by the additivity rules. To calculate thermal capacity and enthalpy of individual compounds we used the methods of non-ideal thermodynamics. The physical properties of mixture were calculated using the partial molar values of individual components.
Non-ideal character of individual components was accounted by thermodynamic functions
connecting free Gibbs energy, enthalpy, entropy and fugacity:
Figure 32. Heat capacity of water depending on temperature. P =27. MPa. Solid line – the
calculation by equation (8), dots with points – the tabular data [11].
Figure 33. Partial molar heat capacity of individual components and heat capacity of mixture vs. temperature. P=27 MPa. 1-H2O; 2-CH3OH; 3-O2; 4 – CO; 5 – mixture.
Here, ( ) y , P , T Py f i i i F = , ( ) y , P , T i F - fugacity and fugacity coefficient of i-th component of the mixture.
In the above equations y stands for the vector of molar composition; subscript “0” indicates respective thermodynamic values for ideal gas, calculated at P = 0.1 MPa at process temperature T. These values were obtained from tabular data by tenth order polynomial approximation on T. The fugacity coefficients were calculated by the RKS equation of state. Heat efficiency of chemical reaction in SCF calculated by (9) differs from the heat efficiency in ideal approach, Fig. 34.
Figure 34. Heat efficiency of equilibrium chemical conversion of model mixture vs.
temperature. P = 27.0 MPa: 1 – Calculation in ideal-gas approach, 2 – Calculation in nonideality
of mixture.
Calculations showed that the feed temperature at the reactor entry part increased considerably due to exothermal reactions of methanol oxidation and hydrogen peroxide decomposition, whereas thermal capacity decreased down, because feed parameters progressively moved away from critical values.
Both methanol and hydrogen peroxide convert substantially at this entry part; the CO concentration proceeds through the maximum. The oxidation of acetic acid is a slow reaction; its concentration decreases almost linearly along the reactor length, Fig. 35. While the feed concentration of acetic acid is low, the increase of adiabatic temperature in reactor due to oxidation of acetic acid is insignificant (~15K). Thus, the role of methanol as the fuel is to provide sufficient temperature for the acetic acid oxidation at relatively short initial part of the reactor. Variation of fed methanol and organic wastes concentration allows controlling the temperature in the reactor.
Figure 35. Variation of temperature (curve 1) and thermal capacity (curve 2) along the heatexchanger.
Figure 36. Variation of concentrations of methanol (curve 1), carbon monoxide (curve 2) and acetic acid (curve 3) along the reactor.
The performed calculations of simplified SCWO process prove that the supercritical fluid properties affect essentially the chemical and thermal physical processes in the reactor. The proposed mathematical and thermodynamic models of individual process units can form basis for the calculation of a whole class of chemical processes in supercritical state. Currently, primary data for the designing of a pilot plant for SCWO of nitro-compounds are calculated based on the proposed models. The pilot-plant is planned to be multi-functional, and suitable for oxidation and decomposition of diverse organic compounds in SCW. |