Fig. 1-3. Dynamic flow method.
Thus, in the determination of the vapor-liquid equilibria for systems such as ammonia and water, ammoniacal solutions are placed in the vessels, and a gas such as nitrogen is bubbled into the first of these and the resulting nitrogen-ammonia-water vapor mixture is passed through the succeeding vessels obtaining a closer approach to equilibrium. Equilibrium obtained in such a manner is not the true vapor-liquid equilibria for the water vapor-ammonia system. It closely approaches true equilibrium for the binary system under a total pressure equal to the partial pressure of the ammonia and the water vapor in the gaseous mixture. Even this is not exact. The carrier gas has some solubility in the liquid phase, and the partial pressure of these added constituents modifies the energy relations of the liquid and vapor phases. Usually for low-pressure operation these errors are not large in magnitude, but as the pressure becomes higher the errors are serious and the method can give erroneous results if the true vapor-liquid equilibria for mixtures without the carrier gas are desired.
Dew and Boiling-point Method. In essence this technique consists in introducing a mixture of known composition into an evacuated equilibrium container of variable volume (Refs. 6, 7, 9, 15, 17, 18, 20, 28). The system is maintained at a constant temperature, and by varying the volume the pressure is observed at which condensation commences and is completed. The dew- and bubble-point curves of pressure vs. temperature for a number of these prepared samples are obtained and, by cross-plotting, conditions of phase equilibrium may be found by locating points at which saturated liquid and saturated vapor of different compositions exist at the same temperature and pressure.
The pressures are determined in two ways. One involves the measuring and plotting of the PV isotherm, the dew point and bubble point being indicated by the discontinuities in the curve at the beginning and the end of condensation. The other employs a glass or quartz equilibrium cell, and the conditions are determined visually.
The advantages of this method are that it allows the critical conditions to be determined, gives data on specific volumes of mixtures at high pressures, and requires no analysis of the phases since the total composition of the mixture is accurately determined gravimetrically upon charging.
There are disadvantages, however. For certain conditions the dew and bubble points are not sharply defined; hence they require measurements to be made with highly refined precision instruments. The simpler units using mercury as a variable volume confining fluid cannot be used below the freezing point of mercury. In addition, the materials used must be very pure and free in particular from traces of fixed gases, for in the critical region the saturation pressure is quite sensitive to small amounts of fixed gases. Further, a large amount of experimental work must be done in order to define completely and accurately the phase equilibria over all ranges of liquid and vapor composition. The major limitation, however, is the fact that the method can be used only on binary systems. As the phase rule dictates that more complex systems are not a unique function of pressure and temperature, dew and bubble points alone cannot define the composition of two phases in equilibrium.
Dynamic Distillation Method. The four previous methods involved repeated contact of the vapor with the liquid and thus afforded the time necessary for the attainment of equilibrium. The dynamic distillation method (Refs. 2, 5, 11, 19, 24, 26, 34, 39) involves a different procedure (see Fig. 1-4). In this system a distilling vessel is connected to a condenser and a receiver.
In the simplest case, a small sample of distillate is taken, and the compositions of this sample and the liquid in the still are determined. During such a distillation the composition of the distillate and the liquid in the still changes, and the samples represent average values.
To reduce this composition variation the quantity of liquid in the still is made large in comparison to the quantity of distillate. Frequently successive samples of condensate are obtained, and these are analyzed and the composition plotted vs. the quantity of liquid that has been distilled. An extrapolation of this curve back to zero quantity of liquid removed is taken as the composition of the vapor in equilibrium with the original liquid.
The method involves a new assumption, namely, that the vapor obtained by boiling a liquid is in equilibrium with the liquid. There has been no adequate proof of this assumption, and theoretical considerations would tend to indicate that equilibrium should not be obtained. The few experimental data that are available would indicate that the difference in the composition between the vapor obtained in this manner and the true equilibrium is not great in most cases, but in a few systems significant differences have been found.
After the vapor leaves the liquid, any condensation in the upper part of the distilling vessel will change the composition of the vapor and therefore introduce errors. Such condensation is usually reduced or eliminated by having the upper part of the system jacketed and at a higher temperature than the condensation temperature of the vapor. However, this higher temperature can introduce errors; for example, in such a boiling system there is a certain amount of spray and splashing. The spray that lands on the heated walls will tend to vaporize totally and give a vapor of the composition of the liquid rather than of the equilibrium composition.
The pressure involved in such a system is of course essentially that prevailing in the receiver, and this method can be used either for normal pressures, high pressures, or vacuum. The exact temperature of the operation is usually not known because the liquid is generally superheated. The vapor and the liquid therefore are not in thermal equilibrium, and it is doubtful whether they are in true composition equilibrium. The apparatus has been extensively used because of its simplicity, and the results are of sufficient accuracy to be of real value in distillation calculations.
In order to obtain a closer approach to equilibrium, various complicating arrangements have been used; for example, Rosanoff modified the system to obtain a second contact of the vapor with the liquid.
Continuous Distillation Methods. Continuous distillation methods involve distilling a liquid, condensing the vapor sample, and recycling the condensate back to the still. A schematic drawing of such an equilibrium still is given in Fig. 1-5. This system was developed by Yamaguchi (Ref. 38) and Sameshima (Ref. 29) and has been modified and improved by a number of other investigators (Refs. 1, 8, 22, 30, 31, 32, 33). This method has been widely used and has the great advantage that it is simple, and the unit can be placed in operation and allowed to come to a steady state without any great amount of attention. The same precautions relative to entrainment, condensation and total vaporization of splashed liquid must be observed in the still as was indicated for the dynamic distillation method. The condensate collects until the level is high enough to flow over the trap and back to the still. At the end of the distillation, this condensate is removed and analyzed to determine the composition of the vapor, and a sample is removed from the still to determine the still composition.
This method suffers from the same difficulties as the dynamic distillation method in that it is open to the question of whether the vapor formed by boiling a liquid is in equilibrium with the liquid. It is also difficult to obtain the true liquid temperature because of the superheating effects. The pressure is maintained by the pressure in the exit tube, and in normal pressure determinations this is usually open to the atmosphere. This theoretically offers the possibility of errors in that it allows Oxygen and nitrogen to dissolve in the condensate sample, which is then recycled back to the still. At low pressures the solubility of such gases is usually small and the error is slight, but in high-pressure operations the use of this gas system can lead to serious errors.
The gas pressuring system, however, is extremely desirable in that it regulates the condenser cooling capacity so that it exactly balances heat input to the still. At high pressures the errors become so serious that this benefit must be foregone. Figure 1-6 indicates a type of apparatus in which the heat input and removal are adjusted so that the pressure remains constant without the necessity of a sealing gas.
Another source of error in the system is possible because the condensate returned to the still is of a different composition from the liquid in the still and in general is of lower boiling point. If this vaporizes before it is completely mixed with all of the liquid in the still, this vapor composition will not be an equilibrium vapor.
Fig. 1-5. Continuous distillation equilibrium still.
Although the apparatus appears to be of the recirculation type and it might be supposed that the successive contacts would tend to give a closer approach to equilibrium, this is not the case. If the vapor evolved from the liquid is not an equilibrium vapor, this type of recycle
Fig. 1-6. Continuous distillation still for high-pressure operation.
Fig. 1-6. Continuous distillation still for high-pressure operation.
system does not give a closer approach with repeated recycling since new vapor is formed and the recycled material is not brought to equilibrium by successive contacts. The recycling does give a steady-state condition, but the approach to equilibrium is only that obtained by boiling the liquid.
In order to eliminate some of the sources of error in continuous distillation systems, various modifications have been made. The most important of these (Refs. 1, 8) would appear to be one in which the condensate stream is revaporized before it is returned to the still; i.e., the heat is added to revaporize the condensate stream instead of forming a new vapor in the still. Such an apparatus is shown in Fig. 1-7. In this case, the result is equivalent to recycling the vapor, and the operation tends to be equivalent to the usual recycle system. It is more difficult to operate than the conventional continuous distillation system. The condensate must be completely vaporized. If any liquid is allowed to return to the still, the purpose of the system is defeated and the rate of distillation decreases; i.e., less vapor leaves the still. If the vapor returned to the still is greatly superheated, it will cause additional evaporation in the still and the operation will speed up. By proper adjustment a satisfactory balance can be obtained. It is believed that this apparatus is a definite improvement over the regular continuous distillation system, and comparative data on the same system taken with this and the usual continuous distillation system show definite differences of the type that would be anticipated.
Both the continuous distillation system and the modifications of it suffer from the difficulty that the vapor must be totally condensable under the operating conditions^ This is usually not a serious difficulty,
It is also necessary that the condensate be a homogeneous mixture. , Thus, if the condensate separates into two layers, the operation is not satisfactory. The other vapor-liquid equilibrium methods are suitable for multilayer systems either in the still or in the vapor sample.
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