Experimental Determination of Critical Points of Pure Components and Binary Mixtures Using a Flow Apparatus

Abstract

Equations of state can be successfully employed for the description of the phase equilibrium behavior of multicomponent mixtures using binary data alone. Especially in the vicinity of the critical point often only a limited number of phase equilibrium data or data of low quality are available. Critical points of mixtures provide additional information about the location and border of the two-phase region. The PSRK model (Predictive Soave-Redlich-Kwong) [1] combines the SRK equation of state with the group contribution model UNIFAC by the PSRK mixing rule. For the extension of the range of applicability and the improvement of the results of PSRK or other group contribution equations of state the systematic measurement of data required for the fitting of the group interaction parameters is desired. In addition to phase equilibrium data also critical data are of great importance. For this reason a new flow apparatus for the measurement of critical data of pure components and mixtures was developed, which can be operated at temperatures between 280 and 470 K and pressures up to 35 MPa.A common approach for the determination of critical points of mixtures in closed systems is the isothermal or isobaric method connected with an analysis of the gas and liquid phases for the determination of the two-phase region ending at the critical point. Another usual method is the visual detection of critical points where the temperature is varied until the phase border disappears while the temperature increases, or the meniscus reappears while the temperature decreases. The corresponding pressure automatically results from the phase equilibrium pressure. A prerequisite for the measurement of critical points is the exact adjustment of the critical density which requires a variable volume cell.A flow apparatus for the measurement of critical properties of pure components was introduced by Rosenthal and Teja [2]. The variation of density in this apparatus is enabled with the help of a regulation valve at the outlet of the optical cell. Because of the flow mode short retention times are achieved. That is why this technique also allows the determination of critical points of components which show a thermal decomposition. The critical data are determined at different flow rates. Extrapolation of these data to the retention time t = 0 allows the determination of the critical data of such unstable components.The apparatus developed in this study also works in a flow mode and is applicable for pure components as well as binary and multicomponent mixtures. An advantage of this approach is that phase equilibrium is reached before the substances enter the cell. Moreover, the content of the cell is pushed aside so that cleaning and controlled filling of the cell is not necessary. Unfortunately, the critical volume cannot be determined and quite large amounts of substances are needed. Principle of MeasurementA schematic diagram of the apparatus is shown in Fig. 1. Two thermostatted chromatography syringe pumps (Model 260D, ISO) in which the pure...

Prediction of the Binary Interaction Parameter of Carbon Dioxide/Alkanes Mixtures in the Pseudocritical Region

“…The results of the different predictive models are summarized in Table . The available experimental data of these mixtures in the open literature − are collected.…”

Application of Renormalization Corrections to SAFT-VR Mie

“…Predictions of the (a) p – T and (b) T – x relationship of the critical locus for propane + n -butane, + n -pentane, + n -hexane, + n -octane, and + n -decane obtained with SAFT-VR Mie (VRM, dotted blue line) and SAFT-VR Mie + RG (VRM + RG, solid black line). Experimental data from refs − .…”