CONDENSATE
P. T. 100 BULB
2.7 Development of LPVLE methods for higher pressures and temperatures
2.7.2 Designs for elevated temperatures
Experimental measurement of accurate pure component vapour pressures and binary VLE data at high temperatures has typically been the domain of high molecular weight hydrocarbons, most notably, n-alkanes (Chirico etal. 1989) and polycyclic aromatic hydrocarbons (Gupta et aI., 1991) Due to the inherent propensity of these compounds to undergo thermally-induced oligomerization or polymerization and decomposition reactions, the above task has often been met with failure. The high-pressure continuous flow dynamic (Joyce et al., 1999) and static methods (Morgan and Kobayashi, 1994) featured quite prominently amongst researchers investigating systems at high temperatures. There have been very few designs based on LPVLE methods that have been employed in high-temperature studies.
Equipment ofHaynes and Van Winkle (1954)
One of the early forays into the study of vapour-liquid equilibria of higher molecular weight aromatic +alkane mixtures was that of Haynes and Van Winkle, who employed the use of a Jones-Colburn-type still. Of particular interest to the research group was the overall study of VLE behaviour of systems containing high-boiling hydrocarbons at subatmospheric pressures.
Chapter 2. A Review of the Classification and Development of Vapour-Liquid Equilibrium Equipment
As discussed in the previous chapter, this type of apparatus is quite challenging to effectively control to allow for the acquisition of reliable VLE results. The authors designed the still with distinct thermally-controlled zones to ensure that in the operation of the still, the revapourized vapour condensate circulation was optimal. The research output from the group with regards to the vapour liquid equilibria of high-boiling hydrocarbons (alkanes, alkenes and polynuclear aromatics) was quite significant (Hirata et ai., 1975) and contributed greatly to the body of knowledge of the VLE characteristics of these types of systems.
Equipment ofMyers and Fenske (1955)
Myers and Fenske (1955) identified the importance of addressing the issue of the lack of vapour pressure data for high-boiling hydrocarbons to allow for optimal processing of the high-boiling distillation column bottoms. To this end, an apparatus based on the Othmer-type still was then constructed to allow for the acquisition of vapour pressures and vapour-liquid equilibrium data in the range of 0.1 mmHg to atmospheric pressure.
The apparatus of Myers and Fenske and its important features are shown in Figure 2.36. The principal construction material for the apparatus was chosen to be stainless steel (even though operation would be in the low-pressure region) to ease concerns over safe operation since high temperatures would be attained in the studies. A few considerations that influenced the design of the equipment related to the design of the reboiler and also ensuring even heating and proper mixing of the boiling liquid. With regards to the former, a reboiler with a large cross-sectional area was favoured to minimize the effect of the hydrostatic head of liquid in the reboiler in terms of its contribution to any superheating and "bumping" of the liquid mixture at low pressures. For the latter, a novel "combination heater and stirrer "was employed to allow for immersion heating and mechanical agitation in a single unit. Another consideration was that of the ease of disassembling of the apparatus with regards to cleaning (residues) and maintenance (change of gaskets). Consequently, the top of the reboiler was flanged, with a Teflon® gasket and brass compression fitting being used to provide the seal. A tempered glass window, shown in the upper half of Figure 2.36, allowed for visual observation of the contents of the interior of the still.
A variable-volume condensate cup was another notable feature of the equipment. The volume of the cup was varied through the use of an aluminium plug in the cup, with a Teflon® spring- loaded valve (manually operated with a rod) that can me raised or lowered to control the return flow rate of the condensate.
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Figure 2.36. Apparatus of Myers and Fenske (1955).
The equipment essentially shares the same flaws as in the original design of Othmer (1928), however, the incorporation of a combination heater-stirrer unit and the variable-volume distillate cup were novel features. If the apparatus were to be used for measuring binary or multicomponent VLE, and not just pure component vapour pressures, it would be shown to be seriously flawed. Equilibrium times of 4 to 5 minutes were reported by the authors, who used the experimental data obtained to construct vapour pressure charts for 26 different hydrocarbons including hexadecane, octadecane and eicosane.
Chapter 2. A Review of the Classification and Development of Yapour-Liquid Equilibrium Equipment
Equipment of Coon et al. (1989)
A comparative study was undertaken by Coon et al. with regards to activity coefficients obtained from solid-liquid equilibrium (SLE) and vapour-liquid equilibrium experiments. The compounds investigated were polynuclear aromatic hydrocarbons, which were combined with tetralin and decal in (solvents), to form combinations of binary mixtures. Isothermal results had also been obtained at temperatures of 1600C and 1800C for polynuclear aromatic hydrocarbon systems that included biphenyl, naphthalene, anthracene, carbazole and dibenzofuran.
The experimental apparatus for the determination of VLE was a commercially developed Stage- Mueller VLE still, constructed by Fischer Labor-und-verfahrenstechnik. The still featured a silvered vacuum jacket to prevent heat losses. The cooling system of the VLE still was that of a
"closed" one where excellent control over the temperature of the cooling fluid in the still was made possible so as to prevent any loss of volatile material through the condenser to the vacuum system and also any solidification of the high-boiling component in the condenser. An analysis of the results and the conclusions reached by the authors interestingly enough revealed that the vapour liquid equilibrium studies are "not the best method for obtaining activity coefficient data" for systems containing polynuclear aromatic compounds and related structures.
The experimental difficulties that were experienced by the researchers were cited as follows:
(a) Thesolidification ofthe less volatilecomponent in the condensers if the temperature was too low.
(b) Theloss of the more volatile component to the vacuum system if the condenser temperature was too high.
(c) Due to the considerable relative volatility of the systems studied, "temperature swings" in the equilibrium cell were experienced as a result of changes in the charge mixture composition returning to the heater.
(d) Thedilution ofthe sampleswas deemed as necessary to prevent solidification during the gas chromatographic analysis. This compromised the accuracy of the quantitative analysis of the liquid and vapour mole fractions as the detection limits of the detector are approached in the above.
In terms of the difficulties experienced, points (a) and (b) are valid in that when a system which exhibits high relative volatility i.e. when the boiling points of the two substances differ substantially or when one component is in great excess (for studies at or near the infinitely dilute region), control of the condenser fluid temperature is extremely difficult to satisfy both requirements in (a) and (b). The temperature swings experienced by the authors probably relate to flaws in the equipment design i.e. absence of stirring in the boiler chamber and an absence of heating in the return line. The results obtained by the authors with the VLE method was quite scattered and statistical treatment of the data revealed that the predictions of VLE using SLE derived parameters from UNIQUAC with the SLE experimental data was more useful than the reverse procedure.
The brief description of equipment designs and considerations in the measurement of high- temperature VLE dealt with above provides sufficient indication of the great challenges that researchers are faced in such a task. Since the high molecular weight hydrocarbons studied were frequently solids at room temperature, this necessitated special procedures for the initial charging of the mixtures and preventing any solidification in any part of the apparatus during a run. Of course with the greatest concern/s in the study being thermally-induced polymerizations and/or decompositions, an unenviable burden of monitoring the integrity of the components as a function of time (Joyceet aI., 1999) is necessitated for high-temperature investigations.