Separation of 1-dodecanol and n-tetradecane through supercritical extraction.

Bonthuys, Gert Johannes Kotze (2008-12)

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.

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