HVOS model

By filtration mechanism. Although the mechanism for separation and accumulation of solids is not clearly understood, hvo models are generally considered and are the basis for the apphcation of theoiy to the filh ation process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within tne pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration. 

The correlation of data for the methane and pentane binary system is shown in Figure 4.2.1. In this case the van Laar excess Gibbs free-energy model has been used in the HVO model the two model parameters were fitted to VLE data on the 277 K isotherm, and the vapor-liquid equilibria at higher temperatures were predicted with the same temperature-independent parameters. The results are very good in this case and similar to those obtained with the IPVDW and 2PVDW models. 

To demonstrate the differences between the WS and the HVO models, the results of VLE predictions for the 2-propanol and water binary system at 353 K with the parameters obtained from the DECHEMA tables at 303 K are shown in Eigure 4.3.9 in which the solid line is the prediction with the WS mixing rule and the dashed line describes the results of the HVO model. The significant advantage of the WS model over the HVO model in predictions is clearly visible in this figure. 

Experience has shown that the choice of activity coefficient model to be coupled with the EOS has some effect on the predictive performance of both the WS and HVO models. For example, as the complexity of the mixture increases, the NRTL and the UNK UAC models, which have a temperature dependence of their parameters, usually give better results over a range of temperatures than the simple van Laar model. 

Equations of State The MHV , MHV2, LCVM, and HVOS Models... 

This is a hybrid model and lacks a firm theoretical basis because it combines of the HVO model, which is evaluated at infinite pressure, and G of the Michelsen model, which is evaluated at zero pressure. Nevertheless the authors have shown that this model can be used to obtain successful correlation and prediction of the VLE of various nonideal systems. As we will discuss in Section 5.1, part of this success is due to a cancellation of errors. This model is also included in the computer programs on the disk accompanying this monograph and is tested below in this section for VLE correlations and predictions. 

In this model, eqns. (4.4.12 and 3.3.8) are used to obtain the EOS parameters a and b. This model is referred to as Huron-Vidal as modified by Orbey-Sandler (HVOS) model in this monograph and is also included in the programs supplied on the accompanying disk. It is an approximate model but is in agreement with the spirit of the van der Waals hard core concept, and it is algebraically very similar to several of the commonly used zero-pressure models mentioned in this section. Yet it does not... 

Group Pair HVOS Model LCVM Model... 

Figure 6.2.2. Excess enthalpy for the benzene and cyclohexane system at 293 K (dots) and at 393 K (triangles), Tlie lines denote correlations at 293 K and predictions at 393 K using various models. The solid line reflects predictions using the 2PVDW model, the dotted line represents the predictions using the van Laar activity coefficient mode , the short dashed lines signify predictions using the HVOS model, and the long dashed line denotes predictions made with the WS model. Data are from the DECHEMA Chemistry Series (Gmehling and Onken 1977, Vol. 3, Pt. 2, p. 992).

This results in the selection of the HVOS model for the mixing rule.)... 

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