Predicting Liquid Phase Heat Capacity of Ill-defined Hydrocarbons

  • Author / Creator
    Dadgostar, Nafiseh
  • There are currently no reliable methods for predicting liquid heat capacities of ill-defined hydrocarbons, such as bitumen and heavy crude oils and their partly processed fractions. Estimation techniques for liquid phase isobaric heat capacity of pure compounds include corresponding state and group contribution methods. These techniques require critical properties (Tc, Pc), molecular structure, or physical properties (Tb, ρ, sp gr) to be applied. Molecular structures and critical properties for many industrially relevant organic fluids remain speculative. In this work, a predictive correlation relating liquid specific heat capacity to absolute temperature and elemental composition was developed. It retained the quadratic form of the Lee-Kesler correlation, but the parameters were redefined as second order power series in a similarity variable based solely on elemental composition. This correlation yields significantly improved heat capacity estimates vis-à-vis prior practice. Since the correlation requires elemental analysis rather than molecular structure, liquid phase heat capacity of ill-defined hydrocarbon fluids, such as bitumen, pure compounds and polymers can be predicted with equal ease. Virtual Materials Group has implemented this correlation in their hydrocarbon refining/petrochemical/natural gas processing simulator (VMGSIM 6.5). In a further development, a simple correction term, which is a function of reduced temperature and molar mass, was added to the universal correlation to accommodate the critical region. This modification also addressed and resolved the variation of liquid phase isobaric heat capacity among isomers, with differing critical temperatures, at the same absolute temperature. With this modification, one can predict heat capacity of saturated organic liquids (CsatL) with 5% average relative deviation from experimental data up to the reduced temperature of 0.99. Constant pressure heat capacities for liquids are by default evaluated indirectly, in process simulators and in general purpose calculations, using ideal gas isobaric heat capacity values to which equation of state based departure functions are added. As ideal gas heat capacities are known or can be calculated from theory with small uncertainties, and typically comprise 75 % of liquid heat capacity values, the large relative deviations present in departure function calculations appear to be tolerated or ignored. Deviations between indirectly calculated and measured constant pressure liquid heat capacities can exceed 10 % but are typically smaller. Departure function evaluation is the principle source for deviations and uncertainty in these calculations. In this work, departure functions based on the Peng-Robinson and Soave-Redlich-Kwong equations of state are defined and then evaluated. The absolute and relative deviation between the calculated values and experimental data are compared with the Tyagi departure function correlation, and a difference calculation based on accurate and predictive universal correlations for liquid and ideal gas heat capacity. The poor quality of the predictions from typical cubic equations of state, both in magnitude and trend with absolute and relative temperature, are described. The Tyagi correlation is shown to be preferred if (Tc, Pc, ω, M) are available. The difference calculation based on correlations by Dadgostar and Shaw and Laštovka and Shaw is otherwise preferred. The potential use of departure function correlations for constraining the parameterization of equations of state, particularly for ill-defined hydrocarbons, is briefly discussed.

  • Subjects / Keywords
  • Graduation date
    Fall 2013
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.