CONDENSATE
2.6.2 Features of typical LPVLE Recirculating equipment
A diagrammatic representation of the key features of LPVLE recirculating equipment of the liquid phase and vapour condensate type is shown in Figure 2.4.
The liquid phase and vapour condensate recirculation method is indeed the most common and most successful LPVLE recirculating still design. The latter was consequently chosen as the most appropriate model for low-pressure recirculating equipment. With regards to the other types of methods, which are antiquated and no longer in use, further elaboration will be provided in the respective sections highlighting their development.
Due to some similarities between high-pressure and low-pressure VLE measurement methods, most of the considerations mentioned in Section 2.3 are also applicable here, however, the operational and functional design principles of LPVLE stills are fundamentally different to that of HPVLE methods and can be summarized as follows:
(a) Areboiler or boiling section is required to provide sufficient heat input to initiate and also perpetuate boiling ofthe mixture; usually under constant-pressure conditions.
Pressure Stabilization System
Temperature Probe
Equilibrium Chamber
Vacuum jacket
Coltrell Pump
t
~
...~
Condenser
Vapour Recirculation
Line
Reboiler
Stirrer
Mixer
Figure 2.4. Schematic of a LPVLE Recirculating apparatus.
Two types of heating duties can normally be distinguished i.e. internal and external heating.
External heating (shown as darkened regions around the reboiler) ensures that the influence of the external environment in the form of temperature gradients is negated together with maximizing energy efficiency.
The internal heating provides the bulk of the heating duty as it is positioned so as to be in intimate contact with the fluid (not shown in the diagram) and allows for the rapid, smooth and controlled boiling of the mixture. The appropriate use ofnichrome wire windings, heating tapes, cartridge heaters and various other types of heating elements (with provision for variable energy input) are employed to achieve the steady state boiling of the recirculated mixture.
Chapter 2. A Review of the Classification and Development of Yapour-Liquid Equilibrium Equipment
(b) A means ofagitationof the returning vapour condensate and liquid phase mixture serves to allow for intimate phase contacting to prevent concentration gradients and hence flashing of the lighter components upon re-entry into the reboiler. This is usually achieved with magnetically coupled internal stirrers and/or specially designed mixing sections in the return line, as shown in Figure 2.10. This is especially important for systems that exhibit a high relative volatility.
(c) The intermediate section between the reboiler and equilibrium chamber in the form of the Cottrell tube or pump(Cottrell, 1919), to allow for the transport of the superheated mixture via the "thermal-lift principle" to temperature probe well in the equilibrium chamber. The Cottrell tube has in the past been designed such that it enters at the side of the equilibrium chamber (as will be shown later in the many illustrations of the early designs). This design illustrated in the diagram shows the vacuum jacketed Cottrell tube entering through centre of the base of the equilibrium chamber, a design that has been favoured in recent designs (Raal and Muhlbauer, 1998). The Cottrell tube is a feature that is traditionally unique to ebulliometric (Swietolawski, 1945) and liquid phase and vapour condensate recirculation methods; however, modifications to the Othmer-type stills have featured this (Zudkevitch, 1992). Consequently, vapour recirculation and condensate recirculation equipment consist of a single chamber (as there is no Cottrell tube or separate chamber for equilibration to occur in), which is partitioned into the "liquid phase container" and the equilibrium chamber directly above it (Malanowski, 1982a).
(d) The design of theequilibrium chamberis crucial as proper phase contacting and subsequent disengagement occurs here and it has been subject to many modifications over the years. The equilibrium chamber can be packed or unpacked, where in the latter design, the Cottrell tube is considered as being sufficient to achieve phase equilibrium.It is imperative that the equilibrium chamber is maintained in a proper adiabatic state to ensure that erroneous phase compositions and temperature readings are not obtained.
Various strategies such as "vacuum jacketing", simple "draft boxes" or covering the chamber with insulating materials (shown as darkened regions in the diagram) are employed in this regard. The temperature probe is mounted in an appropriate position at the vapour-liquid disengagement interface in the equilibrium chamber to allow for the equilibrium temperature to be determined.
(e) Heat exchangers are very important auxiliary features in the equipment. For vapour phase recirculation method, no condensation of the vapour phase occurs; consequently no condensing facilities are required as the vapour phase is maintained (in a separate bath) at a temperature higher than that of the equilibrium cell.
For condensate recirculation stills, both phases enter the still as liquid phases. In revapourized condensate recirculation, the vapour phase condensate is revapourized prior to re-entry into the equilibrium chamber, whereas in the liquid condensate recirculation, the vapour phase condensate re-enters the equilibrium chamber in a liquid state. In some early designs, the use of liquid phase coolers (Brown, 1952) and sampling receiver coolers (Heertjies, 1960) has also been employed. The top of the condenser has been favoured in the majority of still designs for pressure connectionsi.e. to the atmosphere or to a suitable pressure regulating system.
(t) Sampling ports or traps serve to provide minimal holdup of the condensed phases to allow for sampling through static ports via septa or other appropriate sampling techniques (sampling valves, sample loops, etc.) where there is minimal disturbance of the still operation due to sampling. A drop counter, which leads into the vapour condensate trap, was a popular feature incorporated into the designs of early LPVLE stills, and still in use today, for providing an indication of the rate of condensation or flow of the vapour condensate and approach to the equilibrium condition.
2.6.3 Criteria for the proper design of LPVLE Recirculating equipment
The principles, upon which a properly designed recirculating still should be based, adapted from Malanowski (I982a), can be listed as follows below:
(a) The final design and form of the VLE still should beas simple as possible,eliminating many unnecessary design complications, especially with regards to the sampling of the vapour and liquid phases. Associated with design complications are additional costs; consequently a simple yet effective design is optimal.
(b) The requirement for the filling or charging of the still with the chemical components, for a single run, should be fulfilled by fairly small sample volumes. This is especially important for expensive or rare chemicals.
(c) Provisions for the monitoring of experimental variables (pressure and temperature) should allow for high accuracy. This has been discussed above in Section 2.3 (e).
(d) The time required for the attainment of a steady state should be short for the determination of single point. However, this factor, although related to the design of the equipment, is also highly dependent upon the nature of the system under study with regards to heat capacities, latent heat, relative volatilities,etc.
Chapter 2. A Review of the Classification and Development of Yapour-Liquid Equilibrium Equipment
(e) There should ideally be nopartial condensation of the vapour stream on the walls of the temperature sensor or in the equilibrium chamber prior to phase disengagement and sampling.
Any attempts to prevent the partial condensation of the vapour phase should also not result in overheating of the temperature sensor.
(t) Entrainment of liquid droplets in the exiting vapour streams must be prevented through proper design. This serious problem is also shared by single pass HPVLE dynamic methods (discussed in Appendix A).
(g) The recirculated streams of vapour (or its condensate) and liquid (at different temperatures and compositions) should be mixed thoroughly to ensure that flashing or differential vapourization of the more volatile components (secondary evaporation) does not occur when the mixture is boiled again. If this condition is not properly prevented, the phases will never reach a true steady state and this' is consequently a very important consideration in the design of any recirculating method.
This point proved to be a major undoing in the design of the equipment of Harris (2004).
Consequently, internal stirring or mixing sections must be incorporated into the equipment design for reliable phase equilibrium determinations.
(h) Theflow and compositions of the recirculated phases should remain constant once steady- state boiling has been achieved, to ensure that a steady state condition can be reached.
(i) There should be nopockets or voids in the equipment design, which would allow for dead volumes or the accumulation of material, out of the recirculation pathway.
(j) The provision for the withdrawal of samples without the disturbance of the equilibrium condition is also an important consideration for the design of the equipment to obtain representative samples. There should also be minimal holdup of the recirculating streams in the sample traps together with stirring to ensure homogeneity of the extracted samples.
2.6.4 Development of LPVLE Recirculation Methods
2.6.4.1 Vapour Recirculation Methods
The first experimenter to employ the use of this method was lnglis (1906) due to the apparent uncertainties in obtaining a steady-state with flow-type or single pass methods. lnglis (1906) investigated isothermal distillation of mixtures of nitrogen and oxygen and argon and oxygen.
The general principles of this method are shown in Figure 2.5. Although it was originally a low- pressure method, it was the basis for the HPVLE dynamic vapour recirculation method (see Appendix A), where the use of vapour recirculation pump is also employed, as in the original method where a vapour pump is used to recirculate the vapour phase through the sedentary liquid phase in the equilibrium chamber.
0 P
u~
Z
P
1.:1.31 Vs V
-
~..
E ..,""
T
* • -- . .. 2 2
-'-'. K,:.::: -.
.- - . . -
T,:,
constT2 ::
const ,.T1
,:r
2l~ 1
Figure 2.5. Schematic of a Vapour Recirculation apparatus: E, equilibrium chamber; K., liquid phase container; P, pressure sensor; T, temperature sensor; T.,Tz,constant-temperature baths; Vs, vapour path; Z., liquid phase(L) sampling valve;Zz,vapour phase sampling valve, Z3, valve for degassing.
The vapour recirculation method was the only low-pressure recirculating method that employed the use of a vapour pump to create the pressure differential required for the recirculation of the phase and is thus the only method affected by pressure drop, pump limitations, etc., as was summarized previously for HPVLE phase recirculation methods. Ithas largely been replaced by other low-pressure recirculating methods for LPVLE determinations and on the other hand it, has been modified extensively for HPVLE determinations, as mentioned above.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
2.6.4.2 Condensate Recirculation Methods
Vapour Condensate Recirculation
The first stills with vapour condensate recirculation were those of Yamaguchi (1913) and that of Sameshima (1918), which were modifications of the still of Carveth (1899). The most important modification implemented in the equipment of Sameshima (1918) was the condensate trap, with a 10 cm3 capacity that allowed for continuous circulation of the small quantity ofthe condensed vapour phase. This was an improvement on earlier methods, which required rather large liquid volumes. Included in this category is perhaps the most famous of all VLE stills (Malanowski,
1982a), although burdened with major shortcomings, in the form of the Othmer Still.
Equipment ofOthmer (1928)
Othmer attempted in 1928 to remedy the errors considered to be responsible for the principal inaccuracies of the vapour condensate recirculation method at that time. These were identified as partial fractionation of the evolving vapours on the cooler parts of the apparatus and errors resulting from a changing composition of the liquid phase in the boiler.
In his glass still design (shown in Figure 2.6), Othmer had intended to develop an apparatus which would be simple, compact, require small liquid volumes and provide accurate results given a short equilibration time. At the base of the still was a sample cock, for the drainage of samples of boiling liquid. At the top of the still, a thistle tube with a stop cock was blown to allow for the charging of the still. Above the liquid in the still, was a vapour tube that featured an elliptical hole near its base to allow for the passage of the generated vapour upwards, which was then "jacketed" with the vapours to allow for thermal equilibrium between the vapour and the inner walls of the vapour tube to be established; to prevent the partial condensation of the vapours inside the tube.
A thermometer, suitably supported (rubber, cork or Pyrex® stopper) was inserted into the vapour tube via the top. The vapours then moved up B, where the condensate and remaining vapour were passed through a condenser and a drop counter; the condensate then collected in a sample reservoir, from where the condensate overflow was recirculated through the overflow tube.
Figure 2.6. Apparatus of Othmer (1928): A, boiling chamber; B, vapour tube; C, vapour condensate receiver; D, thermometer; E, condenser; F, drop counter; G, liquid sampling valve; H, vapour condensate sample cock;I,heater.
The base of the condensate trap had a stopcock for withdrawal of vapour condensate samples.
The first amount of condensate was discarded due to uncertainty. A constant temperature reading, as detectable by the thermometer, was taken to be an indication of a steady state;
however, a further precaution taken by Othmer was to allow several circulations of the vapour condensate through the condensate the trap prior to any sampling. Equilibrium times were roughly 30 to 60 minutes.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
The many potential sources of errors in the design of the Othmer VLE apparatus and these can be summarised below as follows:
(a) The determination of the equilibrium temperature was inaccurate as the temperature of a phase transition is most accurately obtained at the interface of equilibrium mixture i.e. vapour- liquid interface of the phases and not in just one of either of the two phases (binary). If it were to be immersed in the boiling liquid phase, the effects of superheating the liquid phase would result in erroneous measurements and conversely, immersion in the vapour phase, which IS
subject to many heat losses in its path, would also result in incorrect measurements.
There were also uncertainties as to whether the vapour above the boiling liquid was truly in a state of equilibrium due to effects such as superheating the vapour phase, splash evaporation on the sides of a heated vessel, etc.
(b) The absence of any stirring mechanism in the boiling chamber resulted in the creation of concentration gradients when the relatively cold vapour condensate stream (richer in the more volatile stream) returned to the boiling vessel, where it was insufficiently mixed with the boiling liquid. This resulted in the flashing of the lower boiling cooler condensate stream, producing a vapour that differed from its true equilibrium composition as it contained an excess of the more volatile component.
The assumption that boiling itself (especially in the absence of an internal heater) was sufficient for the mixing of inhomogeneous streams is invalid, especially when the temperature difference of the two streams was large, as is always the case for a boiling liquid and its condensed vapour.
Consequently, in light of the above, the goal of Othmer to design an apparatus where the composition of the liquid in the boiling chamber would remain constant "by returning to the boiling liquid the same amount of each constituent as is being evolved in the vapour by means of the overflow", equilibrium had not been achieved. Othmer (1928), himself had stated in the journal article, that it was observed that the temperature had "decreased slightly" when the vapour condensate stream returned to the still.
(c) The problem ofpartial condensation ofthe vapour phase had not been avoided in the design of the apparatus as the possibility of partial condensation on the walls of the boiling vessel (with which the vapour was not in thermal equilibrium) still existed, together with the fact that the vapour jacket that these vapours formed around the vapour tube could not completely prevent the partial condensation of vapour phase in the vapour tube moving upwards towards the condenser.
This situation is indeed made quite worse for high-boiling systems (Hala, 1967) as heat loss and partial condensation becomes more significant as the temperature difference between the vapour and the wall temperature increases. The problem of partial condensation is extremely difficult to eliminate. If the upper part of the still were to be insulated or heated with nichrome wire to prevent partial condensation of the vapour, this would result in superheating the walls of the still and the possibility of producing non-equilibrium vapour due to the "splash evaporation" of the boiling liquid droplets thrown onto the superheated walls and this non-equilibrium total evaporation of liquid droplets (richer in the heavier component) on the vessel walls must be avoided.
(d) Thesize of the condensate trap was too large to allow for rapid circulation of the vapour condensate, which is crucial to prevent any unnecessary holdup of the recirculated phase. This ensures that a rapid approach to equilibrium is obtained where there is steady-state boiling of a constant-composition mixture, with minimal holdup in the recirculation lines and in the boiling chamber. In this regard, the principle of the design of the condensate trap of Sameshima (1918) with an 8 cm3capacity should have been employed into the design of Othmer's apparatus. An additional comment here is that the vapour condensate trap should also incorporate stirring of the condensed phase to ensure that there are no concentration gradients.
(e) There should ideally be internal and external heating of the boiler vessel to ensure that smooth steady boiling of the mixture occurs. In the design of Othmer, there was only external heating of the base of the still; which is very inefficient heat transfer to the still contents.
Internal heating (cartridge heaters, platinum wires, etc.) allows for more rapid heating and also provides nucleation sites.
(t) The last and possibly most blatant flaw in this design was that the sampling of the liquid phase from the boiling vessel did not allow for true determination of the equilibrium liquid composition. The liquid mixed with the some of the returning vapour condensate during the withdrawal ofthe liquid sample and as soon as boiling is terminated at the end of the run.
There was also partial condensation of any vapour present, further contributing to erroneous composition values for the liquid phase. Consequently, it was noted by researchers such as Ellis (1952) that the Othmer still boiling point curve lies inside those curves, where the VLE data sets have been acquired by stills where the liquid phase is sampled prior to mixing with the condensate.
In light of the above, there have been more than a hundred different kinds of modifications to this original flawed design ofOthmer (1928) to address the many drawbacks listed above.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
In one type of modification of the Othmer still that has incorporated the option of a Cottrell tube, this has allowed for the acquisition of data of better quality. It should be noted, however, that despite the hundreds of modifications to the original design, thermodynamic consistency tests indicate that the data obtained by these methods are indeed of poor quality (Malanowski, 1982a) and some authors (Raal and Muhlbauer, 1998) strongly advise against the use of this method for accurate determinations. The work of Zudkevitch (1992) can be referred to for a more current review of the flaws and mod ifications of Othmer sti lis.
Revapourized Condensate recirculation
In revapourized condensate recirculation, the vapour phase is initially condensed in a cooler and then transferred to a receiver or container, from where it is transferred to a heated section, where flash vapourization of the condensate is effected. The flashed condensate is then returned to the equilibrium chamber in the form of a vapour. The original idea for this type of circulation was conceived by Chilton (1935) and the first successful design of a still of this type was developed by lones et al. (1943).
With the development of the Othmer still (1928) and its many subsequent modifications, came a plethora of published data of poor quality. The statistical error analysis of the data indicated that the flashing of the improperly mixed and the cooler returning vapour condensate stream in the boiler was the principal flaw (Rala, 1967). Consequently, it was apparent that the mixing of the circulating vapour stream and the boiling liquid had to be improved upon to improve both the operation of the still and the quality of the data. Various strategies were employed to achieve this in the form of internal stirrers and heaters to eliminate concentration gradients and non- equilibrium vapourization of the condensate stream. The design of lonesetal. (1943) served to incorporate a mixing principle where the revapourized vapour condensate would be made to bubble through the boiling liquid phase until equilibrium was achieved.
Equipment ofJones et al. (1943)
The design of the apparatus of lonesetal. (1943) was motivated, in an analogous fashion to that of Othmer (1928), by the need to eliminate the inherent sources of error in preceding equipment designs. The two principle sources of error were identified in the study as being the production of a non-equilibrium vapour from the boiling liquid surface due to concentration gradients and the improper mixing of the returning vapour recirculation stream and the liquid in the still,