Chapter 4:
From Phases to Method (Models) Selection
Abstract
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See the table of contentsList of examples
- 4-1: Calculation of the condensation enthalpy of the acetone + water mixture with NRTL at a given pressure (1 bar)
- 4-2: Distribution coefficients in an ideal mixture (propane + n-pentane)
- 4-3: Comparison of phase envelope predictions for the ethane + n-pentane mixture
- 4-4: Behaviour of a methane + n-decane mixture and its models
- 4-5: Behaviour of the benzene + n-hexane mixture and its models
- 4-6: Calculation of the eutectic of para- and ortho-xylene
- 4-7: Comparison of experimental values and different model with H2 + n-hexane mixture
- 4-8: Prediction of a heteroazeotrope with total liquid immiscibility
- 4-9: Formation of hydrates
- 4-10: Example of a vapour-liquid-liquid equilibrium of an acid gas system in the presence of water
- 4-11: VLE and LLE calculation of the methanol + n-hexane mixture
Example 4-7: Comparison of experimental values and different model with H2 + n-hexane mixture
Experimental values of VLE of hydrogen + n-hexane mixture have been published by Nichols et al. (1957)[1]. In figure 1, these experimental data at 377.6 K are compared with three models: Grayson-Streed, Peng-Robinson with BIP equal to zero and Peng-Robinson with BIP estimated by Moysan et al. [2]. The cubic EoS without BIP strongly underpredicts the bubble pressure, and is clearly not recommended for hydrogen-containing mixtures. This effect is adequately corrected up to a liquid containing 50 % hydrogen with the BIP evaluated with Moysan procedure. The Grayson-Streed model [3] is adequate for bubble pressure prediction up to 30 % of hydrogen for this mixture at this temperature. The dew line is adequately predicted by the cubic EoS with the Moysan BIP. Grayson-Streed is less accurate. The same behaviour have been obtained at different temperatures.
Figure 1: Comparison of experimental VLE and different models for a mixture of hydrogen + n-hexane at 377.59 K.
A comparison with additional models has been reported by Ferrando and Ungerer (2007 [4]). They have shown that the error in the hydrogen solubility prediction in the liquid phase remains close to 5 % for all paraffins, except for the Grayson-Streed model that overpredicts the hydrogen solubility by more than 20 % when the solvent is heavier than C16 (figure 2). All theses models predict the hydrogen concentration in the vapour phase with less than 2 % error.
Figure 2 : Relative error in the prediction of molar composition of hydrogen in the vapour phase in a binary mixture of H2 + n-paraffin [4].
References
[1] NICHOLS W.B., REAMER H.H., SAGE B.H., Volumetric and phase behavior in the hydrogen-n-hexane system, AIChE Journal, 1957, 3, n°2, p. 262-267. http://dx.doi.org/10.1002/aic.690030223
[2] MOYSAN J.M., PARADOWSKI H., VIDAL J., Correlation Defines Phase-Equilibria for H2, Ch4 and N2 Mixes, Hydrocarbon Processing, 1985, 64, n°7, p. 73-76.
[3] GRAYSON H.G., STREED C.W., Vapor-Liquid Equilibria for high temperature, high pressure hydrogen-hydrocarbon systems, Frankfurt, p. 233-245, 1963.
[4] FERRANDO N., UNGERER P., Hydrogen/hydrocarbon phase equilibrium modelling with a cubic equation of state and a Monte Carlo method, Fluid Phase Equilibria, 2007, 254, n°1-2, p. 211-223. http://dx.doi.org/10.1016/j.fluid.2007.03.016