An alternative method for the estimation of critical temperatures of mixtures

Abstract

A knowledge of the critical temperature of mixtures is important, and because of the difficulty of measuring the critical properties of mixtures experimentally, it is desirable to have reliable methods for correlating and predicting these properties. A survey of the literature indicates that a variety of correlations have been advanced for predicting the critical temperature of mixtures. For example Spencer et al. (1973) reviewed and evaluated a number of proposed methods for estimating the critical temperatures of mixtures. In this note a new alternative method, based on the heat capacity fraction, is proposed to correlate experimentally determined critical temperatures.If the composition of a mixture is expressed as6i --the mixture's critical temperature can be estimated by wherey, is the mole fraction of i, C,, the molal heat capacity of i, T,, the critical temperature of i, and T , the mixture critical temperature. Here our attention is restricted to normal fluids; that is, to molecules that have zero (or small) dipole moments, no tendency to associate by hydrogen bonding or similar chemical forces, and that have sufficiently large mass to permit neglect of quantum corrections.In figure 1, the critical temperature of three binary systemsmethane-propane (Reamer et al. The behavior of the binary hydrocarbon mixtures that have been investigated raises the possibility of using this nearly linear property to estimate the critical temperature of ternary systems of hydrocarbons. To establish these values only the critical temperature and the molal heat capacity data for each component is required. The approach to the estimation of such temperatures is as follows.Since the critical temperature is not known, values must be assumed and the answer can be reached by a series of successive approximations. As a starting assumption the molal average pseudocritical temperature of the mixture (T, = &y,Tc,) can be taken as equal to the temperature sought. This allows us to calculate the heat capacity fraction, Eq. l , and the desired temperature, Eq. 2. The procedure is repeated until the new calculated value of T , checks with the last calculated value. The search procedure is straightforward and does not in general require a large number of iterations to find the critical temperature. To illustrate this method of calculation the following example is presented. Step A. From the data in Table 1 we calculate the molal average pseudocritical temperature for this mixture. The starting assumption is:

PVTx Properties of the R245fa/R1234ze(E) for Low-Temperature Organic Rankine Cycle

“…T c is the critical temperature. The critical temperature for the mixtures can be calculated by the eqs – according to the method proposed by Castillo and Li …”