Robust equipment for the measurement of vapour-liquid equilibrium at high temperatures and high pressures.

Abstract

In this work VLE data was measured on three different pieces of equipment. Measurements were undertaken in the laboratory of Professor Gmehling in Oldenburg, Germany using two different static cells and in the Thermodynamics Research Unit (TRU), University of Natal, South Africa using a specially designed dynamic still. The three pieces of equipment used are as follows: i.) Static apparatus of Rarey and Gmehling (1993), ii.) Static apparatus of Kolbe and Gmehling (1985) as modified by Fischer and Wilken (2001), and, iii.) Dynamic apparatus ofHarris et al. (2003b). In total 370 data points were measured; fourteen sets of VLE data and eight vapour pressure data sets were measured. The work undertaken in Germany measured the systems hexane (1) + N-methylformarnide (2), benzene (1) + N-methylformamide (2), cWorobenzene (1) + N-methylformarnide (2) and acetonitrile (1) + N-methylformamide (2), at 363.15 K using the equipment of Rarey and Gmehling (1993). The systems CO2 (1) + Napthalene (2) at T = 372.45 K, 403.85 K and 430.65 K and CO2 (1) + Benzoic acid (2) at T= 403.28 K, 432.62 K and 458.37 K were measured on the equipment of Kolbe and GmeWing (1985) (as modified by Fischer and Wilken (2001)). Apart from the CO2 (1) + Napthalene (2) system at T = 372.45 K, all the above-mentioned data are new data. The equipment designed in the TRU was designed to operate between 300 and 700 K and between 1 kPa and 30 MPa. The equipment is of the dynamic recirculating VLE still type (DRVS) and is based on the principles of low-pressure stills. The still is constructed from uniquely machined Stainless-steel components and standard commercial Stainless-steel tubing and valves and is computer controlled to operate either isobarically or isothermally. Vapour pressures were measured on the new equipment for n-heptane, n-decane, n-dodecane, n-hexadecane, l-octadecene, 1-hexadecanol and d,l-menthol at low pressures and for acetone at high pressures. These vapour pressure measurements were used as test systems and ranged from 1.00 kPa to 1 000 kPa and from 308.33 K to 583.90 K. Cyclohexane (1) + ethanol (2) at 40 kPa and n-dodecane (1) + l-octadecene (2) at 26.66 kPa were measured as two isobaric VLE test systems. The VLE data measured for d,l-menthol (1) + l-isomenthol (2) at T= 448.15 K and n-dodecane (1) + l-octadecene (2) at P = 3.0 kPa represent new data measured on the equipment. All the VLE systems were modeled. Two data reduction methods were investigated: i.) the combined (r-rf) method, and, ii.) the direct method (H) method. Several different Gibbs excess models (Wilson, NRTL and UNIQUAC), equations of state (PengRobinson and virial) and mixing rules (Huron-Vidal, Wong-Sandler and Twu-Coon) were used in different combinations to find the best fit for the data. The Maher and Smith (1979) method was used to determine infinite dilution activity coefficients from the very smooth data of the N-methylformamide systems. Excess properties were determined for the CO2 (1) + Napthalene (2) and the CO2 (1) + Benzoic acid (2) systems. Although the equipment of Hams et al. (2003b) was able to measure data at high temperatures and elevated pressures, the precission of the data was not as good as was expected. Measuring the system temperature at elevated temperatures was especially problematic. The problem is attributed to the large mass of Stainless-steel used in the construction of the apparatus. To rectify this problem it is suggested that the equipment be modified to be lighter in weight and only capable of measuring VLE at moderate pressures (less than 3 MPa).