Design of a static micro-cell for phase equilibrium measurements : measurements and modelling = Conception d'une micro-cellule pour mesures d'é́́́quilibres de phases : mesures et mod́élisation.
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Date
2011
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Abstract
Vapour-Liquid Equilibrium (VLE), Liquid-Liquid Equilibrium (LLE) and Vapour-Liquid-Liquid
Equilibrium (VLLE) are of special interest in chemical engineering as these types of data form the
basis for the design and optimization of separation processes such as distillation and extraction,
which involve phase contacting. Of recent, chemical companies/industries have required
thermodynamic data (especially phase equilibrium data) for chemicals that are expensive or costly
to synthesize. Phase equilibrium data for such chemicals are scarce in the open literature since most
apparatus used for phase equilibrium measurements require large volumes (on average 120 cm3) of
chemicals. Therefore, new techniques and equipment have to be developed to measure phase
equilibrium for small volumes across reasonable temperature and pressure ranges.
This study covers the design of a new apparatus that enables reliable vapour pressure and equilibria
measurements for multiple liquid and vapour phases of small volumes (a maximum of 18 cm3).
These phase equilibria measurements include: VLE, LLE and VLLE. The operating temperature of
the apparatus ranges from 253 to 473 K and the operating pressure ranges from absolute vacuum to
1600 kPa. The sampling of the phases are accomplished using a single Rapid-OnLine-Sampler-
Injector (ROLSITM) that is capable of withdrawing as little as 1μl of sample from each phase. This
ensures that the equilibrium condition is not disturbed during the sampling and analysis process. As
an added advantage, a short equilibrium time is generally associated with a small volume apparatus.
This enables rapid measurement of multiple phase equilibria. A novel technique is used to achieve
sampling for each phase. The technique made use of a metallic rod (similar in dimension to the
capillary of the ROLSITM) in an arrangement to compensate for volume changes during sampling.
As part of this study, vapour pressure and phase equilibrium data were measured to test the
operation of the newly developed apparatus that include the following systems:
• VLE for 2-methoxy-2-methylpropane + ethyl acetate at 373.17 K
• LLE for methanol + heptane at 350 kPa
• LLE for hexane + acetonitrile at 350 kPa
• VLLE for hexane + acetonitrile at 348.20 K
New experimental vapour pressure and VLE data were also measured for systems of interest to
petrochemical companies. These measurements include:
• VLE for methanol + butan-2-one at 383.25, 398.14 and 413.20 K
ABSTRACT
• VLE for ethanol + butan-2-one at 383.26, 398.23 and 413.21 K
• VLE for ethanol + 2-methoxy-2-methylbutane at 398.25 and 413.19 K
• VLE for ethanol + 2-methylpent-2-ene at 383.20 K
These measurements were undertaken to understand the thermodynamic interactions of light
alcohols and carbonyls as part of a number of distillation systems in synthetic fuel refining
processes which are currently not well described. Two of these above mentioned systems include
expensive chemicals: 2-methoxy-2-methylbutane and 2-methylpent-2-ene.
The experimental vapour pressure data obtained were regressed using the extended Antoine and
Wagner equations. The experimental VLE data measured were regressed with thermodynamic
models using the direct and combined methods. For the direct method the Soave-Redlich-Kwong
and Peng-Robinson equations of state were used with the temperature dependent function (α) of
Mathias and Copeman (1983). For the combined method, the virial equation of state with the second
virial coefficient correlation of Tsonopoulos (1974) was used together with one of the following
liquid-phase activity coefficient model: TK-Wilson, NRTL and modified UNIQUAC.
Thermodynamic consistency testing was also performed for all the VLE experimental data
measured where almost all the systems measured showed good thermodynamic consistency for the
point test of Van Ness et al. (1973) and direct test of Van Ness (1995).
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2011.
Keywords
Vapour-liquid equilibrium., Liquid-liquid equilibrium., Phase rule and equilibrium., Chemical equilibrium., Theses--Chemical engineering.