Computer Driven Static Apparatus 1

Computer-Operated Differential Static Apparatus for the Measurement of Vapor-Liquid Equilibrium Data

Starting in 1988 a computer - operated static apparatus was developed. It can be used for the mesurement of binary and ternary VLE data up to 3.5 bar, activity coefficients at infinite dilution, gas solubilities, pure component vapor pressure data and isothermal compressibilities of liquids.
After a few recent modifications the apparatus is capable of very precise measurements in the small homogeneous region of systems with large miscibility gaps (1-2 mol % mutual solubility).
A scheme of the apparatus is shown in the following figure together with a detailed drawing of the equilibrium cell and the pressure sensor:




The individual steps of a measurement are shown in the next figure:


The piston injector has to be filled manually with carefully purified and degassed components. All other operations are automatically performed by the computer. The directly obtained data are feed composition zi pressure P and temperature T of the system. From this information isothermal P (x) - data can be obtained.

Typical results are shown in the figure below. It can be seen, that the apparatus can also be used for precise measurements in the very dilute range. The apparatus is in steady use for the measurement of data for the development of group contribution methods as well as for industrial applications.

Fig.1 Typical result

A very difficult measurement was performed in the system 2-propanol - tert. butanol, which show nearly identical vapor pressures in the normal temperature range and show azeotropic behavior. Variation of pressure as function of composition is less than 0.1 % of the available pressure range. Three isotherms were determined (40, 54.9 and 70oC). The xP- and xy-behavior of the system is shown in the following plot at 54.9°C. It can be seen, that the equilibrium curve can only hardly be distinguished from the diagonal line (x=y) in the xy-diagram:

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Fig.2 xP- and xy-behavior of the system 2-propanol(1) - tert. butanol(2)

From VLE-data at different temperatures the heat of mixing can be calculated. Due to limited precision of most VLE data, this procedure usually leads to unsatisfactory results. In case of the results obtained with the computer-operated static apparatus, the prediction of excess enthalpies derived from VLE does not differ significantly from the direct correlation of hE-data by Gmehling and Krentscher as shown in the figure below.

Fig.3 hE-data from the system 2-propanol(1) - tert. butanol(2) by Gmehling and Krentscher

To determine the random error in composition, measurements were performed in a system, where the pressure changes very strongly but linearly with composition in the very dilute region. A rather large random deviation in pressure is observed, which is due to the error in composition.
The following three plots show the xP - behavior, the error in pressure with respect to a linear behavior and the probable error in the amount of the second component. As a stepping motor was used for the movement of the piston of the injection pump, the amount is given in motor steps (12800 motor steps = 1*10 -3 dm 3). To our astonishment the error is usually below one motor step, which is the mechanical resolution of the injection pump.

Fig.4 xP - behavior, the error in pressure with respect to a linear behavior and the probable error in the amount of the second component

Due to the high precision and the mostly automatic operation of the apparatus, this device is very useful for the routine measurement of VLE-data and activity coefficients at infinite dilution in industry. The typical time required for an isotherm is approx. 2 - 3 days but requires only little human attention. A commercial version of the apparatus was developed at Mitsubishi Chemical Company in Kurashiki, Japan. A picture of this apparatus is shown below.


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