Vapor-liquid equilibrium apparatus

Figure 1. Schematic Diagram of the Dual-Sampling Vapor-Liquid Equilibrium Apparatus. Components are labeled as follows 6PG, Gas pressure generator GRP, Gas recirculation punqp HZ, Heated zone LPG, Liquid pressure generator LRP, Liquid recirculation pump P, Pressure gauge RV, High pressure sanqpling valve SPV, Sapphire pressure vessel VG, Vacuum gauge. Hatchmarks on the lines in the saxtqpling section of the apparatus indicate where heaters have been wrapped on them. The path of vapor saxtqples is indicated by light arrows and the dark arrow shows the path of liquid sanples.

Figure 1. Schematic flow apparatus used for NH3-H20 (and electrolyte) vapor-liquid equilibrium measurements...

Apparatus. All vapor-liquid equilibrium measurements were made by using a modified Othmer still provided with an external electric heater. Total volume of the still was about 500 cm3, of which about 300 cm3 was occupied by liquid. The liquid loaded in the condensate receiver was about 7 cm3. Details of the still are described in a previous paper (5). 

Vapor-liquid equilibrium data obtained for the 2-propanol-water binary system at 75 °C agreed well with the values calculated from the total pressure data used in the numerical method of Mixon et al. (9). Thus, the apparatus used in this work gives consistent data. 

Bobbo, S., Stryjek, R., Elvassore, N., Bertucco, A. (1998) A recirculation apparatus for vapor-liquid equilibrium measurements of refrigerants. Binary mixtures ofR600a, R134a andR236fa. Fluid Phase Equilibria 150-151, 343-352. 

The schematic diagram of the high-pressure vapor-liquid equilibrium circulation-type apparatus is shown in Fig.l. The main piece of the equipment is a high-pressure phase equilibrium cell of approximately 100 cm3. The apparatus includes a compressed-air actuated piston-pump that allows to circulate one or both phases to bring the vapor and liquid in close contact with each other. This pump, the cell and all the related valves were placed in a constant-temperature water bath to have and to keep uniformely the desired temperature. 

Donald F. Othmer while at Eastman Kodak during the 1920 s experimented using salts to concentrate acetic acid (14). He also developed an industrial process for distilling acetone from its azeotrope with methanol by passing a concentrated calcium chloride brine down the rectification column (15). Pure acetone was condensed overhead, and acetone-free methanol was recovered in a separate still from the brine which was then recycled. The improved Othmer recirculation still (16) has been the apparatus generally favored by investigators who have studied the effects of salts on vapor-liquid equilibrium. 

Rarey, J. R. Gmehling, 1. Computer-operated differential static apparatus for the measurement of vapor—liquid equilibrium data. Fluid Phase Equilib. 1993, 83, 279-287. 

While we have shown various examples of vapor-liquid equilibrium data, we have not discussed the methods by which such data are obtained. There are two general methods, referred to as the dynamic method and the static method. In the dynamic method, the vapor-liquid mixture is boiling, and samples of both the vapor and liquid can be withdrawn and their compositions determined by gas chromatographic or other methods. As the pressure and temperature are also measured, a data point consists consists of a- P-T-x-y point. One apparatus for such measurements is shown in Fig. 

A static cel apparatus could also be used to measure high-pre.ssure vapor-liquid equilibrium. Schematically, the equipment would look similar to that in Fig. 10.2-9, except that the glass flasks would be replaced with metal vessels. 

Figure 1-17 shows an cperimental apparatus introduced by Fischer [1.61] to find vapor liquid equilibrium data. 

The separation of a mixture of volatile liquids by means of fractional distillation is possible when the composition of the vapor coming from the liquid mixture is different from that of the liquid. The separation is the easier the greater the difference between the composition of the vapor and that of the liquid, but separation may be practicable even when the difference is small. The relation between the vapor and liquid compositions must be known in order to compute fractional distillation relationships. Usually this is obtained from information concerning the composition of the vapor which is in equilibrium with the liquid. On this account a knowledge of vapor-liquid equilibrium compositions is usually essential for the quantitative design of fractional distillation apparatus. In most cases the study is made on the basis of the composition of the vapor in equilibrium with the liquid. However, this is not a fundamental requirement and any method that would allow the production of a vapor of a different composition than that of the condensed phase, whether equilibrium or not, could be used for separation. However, most of the equipment employed depends on the use of a vaporization type of operation, and the equilibrium vapor is a good criterion of the possibilities of obtaining a separation. 

Fig. 1-1. Circulation apparatus for vapor-liquid equilibrium measurements.

A6.5.1 A suitable apparatus is shown in Fig. A6.1. For the freezing point of water, a Dewar flask filled with crushed ice and water can be substituted. For the boiling point of water, use an equilibrium still or ebulliometer, a tensimeter or other apparatus for measuring vapor-liquid equilibrium. 

In Section 13.5.4 we discussed that use of the so-called static apparatus in vapor-liquid equilibrium measurements provide only the temperature, pressure, and liquid phase composition values of the equilibrium system. Let us now examine how the vapor phase composition is determine from such measurements, by considering the case of a binary system. 

The data output of the automatic apparatus is in the form of a pressure-volume record on a strip chart. The chart advance and chart pen drive operate in such a manner that the relative pressure scale reads horizontally and the volume scale reads vertically. The horizontal pen position records the difference in pressure between the vapor in equilibrium with the sample and that in a liquid nitrogen thermometer in the sample bath. The chart drive mechanism moves the chart 0.1 inch for each valve cycle involved in adding, or removing, a unit quantity of nitrogen gas. 

Potentially interesting donors were first screened on the basis of the saturation pressure of the adduct. (The term saturation pressure refers to the total pressure of vapor in equilibrium with a sample of molecular addition compound. The vapor may consist of free acid, base, undissociated complex, or a combination of all of these constituents.) Measured quantities of BF3 and the donor were equilibrated at a given temperature in the apparatus shown in Reference 14. Manometric observations (corrected for the free volume of the equipment) were made over that part of the liquid range which lay between room temperature and the freezing point of the complex. Estimates of the heat of association of the complexes were obtained from the temperature dependence of the saturation... 

Now consider the case depicted in figure 3.20c, an isotherm at the UCEP temperature (see figure 3.19). At the UCEP pressure there is a vapor-liquid critical point in the presence of solid. This requires the solid-liquid equilibrium curve to intersect the liquid-gas envelope precisely at the binary liquid-gas critical point and, hence, exhibit a negative horizontal inflection, i.e., (dPldx)T = 0. Notice that the vapor-liquid envelope has not shrunk to a point, as it did at the naphthalene-ethylene UCEP. The solid curve shown in figure 3.20d is the solubility isotherm obtained if a flow-through apparatus is used and only the solubility in the SCF phase is determined. This solid curve has the characteristics of the 55°C biphenyl-carbon dioxide isotherm shown in figure 3.17. So the 55°C isotherm represents liquid biphenyl solubilities at pressures below 475 bar and solid biphenyl solubilities at pressures above 475 bar. 

For the design of RD processes, besides information on the reaction, information on phase equUibria is of prime importance, especially on vapor-liquid equilibria and in some cases also on liquid-liquid equilibria (see above). The systematic investigation of phase equUibria for the design of RD processes will generally involve also studies of reactive systems (see examples above). Studies of phase equUibria in reactive systems generally pose no problem if the reaction is either very fast or very slow as compared with the time constant of the phase equilibrium experiment (high or low Damkohler number Da). In the first case, the solution will always be in chemical equUibrium, in the second case, no reaction will take place. The definition of the time constant of the phase equilibrium experiment win depend on the type of apparatus used. If the RD process is catalyzed and the catalyst does not substantially influence the phase equilibrium, the phase equilibrium experiments can often be performed without catalyst and again no or only little conversion will take place. 

This brief survey shows that there are many options for measuring phase equilibria in reacting systems, which allow to carry out such studies for a wide range of systems and conditions. The main limitation for experimental investigations of reactive vapor-liquid equilibria is related to the velocity of the reaction itself if phase equilibrium measurements of solutions are needed, which are not in chemical equilibrium, the reaction must be considerably slower than the characteristic time constant of the phase equilibrium experiment. Apparatus are available, where that time constant is distinctly below one minute. For systems with reactions too fast to be studied in such apparatuses, it should in many cases be possible to treat the reaction as an equilibrium reaction, so that the information on the phase equilibrium in mixtures, which are not chemically equilibrated is not needed. 

The basic equation for the vapor-liquid phase equilibrium forms the major design criteria for apparatus that separate liquid mixtures by distillation or selective absorption. 

Flash vaporization, or equilibrium distillation as it is sometimes called, is a single-stage operation wherein a liquid mixture is partially vaporized, the vapor allowed to come to equilibrium with the residual liquid, and the resulting vapor and liquid phases are separated and removed from the apparatus. It may be batchwise or continuous. 

In the pressure drop technique, a known number of moles of gas (determined by accurate measurements of temperature and pressure in a precalibrated volume) is secured in one section of the apparatus. The IL is metered into another section, whose volume is also known accurately. Then a valve is opened and the gas is allowed to expand into the entire apparatus, dissolving in the IL. Measurement of the full pressure drop when equilibrium is reached allows the number of moles of gas to be determined in the vapor phase and, subsequently, the number of moles in the liquid phase to be determined by difference. 

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