Thesis (MScEng (Process Engineering))--Stellenbosch University, 2008. / Developments in the field of liquid detergents and cosmetics have increased the demand for
surfactants, processing aids and emollients. Alcohols are often used in liquid products where
they serve as solvents for the detergent ingredients, adjust the viscosity and prevent product
separation. Industrial scale oxygenation of the alkane to the alcohol is often incomplete, resulting
in a significant amount of residual alkane. Application of these alcohols often requires a low
residual alkane content and a post-production separation process is thus required.
Traditional separation methods such as distillation and crystallisation are not technically viable
as crossover melting and boiling points prevent the successful implementation of such processes
and it is envisaged to use supercritical extraction to separate a mixture of n-alkanes and 1-
alcohols.
The project scope revolves around a product stream consisting of detergent range alcohols and
corresponding n-alkanes that need to be separated. To model such a system, a typical detergent
range alkane – alcohol feed with an average of 13 carbon atoms was selected, although a large
number of the molecules have between 12 and 14 carbon atoms each.
Generally the presence of the hydroxyl group as well as an increase in the number of carbon
atoms decreases the solubility in supercritical solvents [17]-[19]. The most difficult separation
will therefore be that of the alcohol with the least number of carbon atoms, that is 1-dodecanol
(alcohol with 12 carbon atoms, CH3-(CH2)9-CH2-OH ) and the alkane with the most number of
carbon atoms, that is n-tetradecane (alkane with 14 carbon atoms, CH3-(CH2)12-CH3 ). To model
the system, it is assumed that the hydrocarbon backbone is linear and the alcohol is primary.
Therefore 1-dodecanol and n-tetradecane are used. If it is possible to separate 1-dodecanol and ntetradecane
with the use of supercritical fluids, it should be possible to separate an industrial
mixture.
The deliverables of this study are: a comparison of the high pressure solubility of n-tetradecane
and 1-dodecanol with a selection of possible solvents; a selection of potential solvents to be
tested on a pilot plant to confirm practical separation. From the literature and measured phase equilibria, all three solvents (carbon dioxide, ethane and
propane) have the ability to distinguish (based on a difference in the pressure required for
solubility) between 1-dodecanol and n-tetradecane. Both carbon dioxide and ethane have
favourable temperature considerations. Propane has superior solubility of n-tetradecane and 1-
dodecanol. Carbon dioxide demonstrates the highest selectivity.
Pilot plant experiments have shown that both carbon dioxide and ethane have the ability to
separate a 50-50% (mass percentage) mixture of 1-dodecanol and n-tetradecane. Both solvents
perform at their best at low temperatures. Carbon dioxide shows the best results at low
temperature and conditions of reflux. The highlight of pilot plant experiments with supercritical
carbon dioxide is achieving a selectivity of 16.4 with the mass percentage of n-tetradecane at 5%
and 82% for the bottoms and overheads product respectively. Very good separation is achieved
using supercritical ethane as solvent even without reflux. Attention is drawn to pilot plant
experiments where the selectivity is as high as 82 with the mass percentage of n-tetradecane in
the bottoms and overheads product at 1% and 82% respectively.
It is recommended to measure ternary phase equilibria for the system n-tetradecane, 1-dodecanol
and carbon dioxide/ethane to establish the interaction between n-tetradecane and 1-dodecanol.
The measured binary phase equilibrium data need to be expanded to include the vapour mass
fraction composition in the isothermal solubility data.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2036 |
| Date | 12 1900 |
| Creators | Bonthuys, Gert Johannes Kotze |
| Contributors | Knoetze, J. H., Burger, A. J., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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