Urea extraction crystallization

For the separation of n-paraffins from other hydrocarbon compounds, urea is added with an approximately 20-fold molar excess. The urea crystallizes in a hexagonal crystal structure with about 5.5 to 5.8 Å wide channels. In these channels the n-paraffins are included. If the concentration of n-paraffins in the mixture is too high, the mixture is diluted with a solvent.

In general, the reaction proceeds according to the scheme:

urea + adduct component -> urea adduct

The equilibrium of the reaction is dependent on the concentrations of the reactants, the solvent and the temperature.^[12] The necessary quantity of urea for the formation of inclusion compounds varies from about 1 to 0.8 mole of urea per methyl- and methylene group in a carbon chain.^[12] The urea is added as a supersaturated aqueous solution to compensate for losses due to adduct formation during the process. In order to avoid a too high concentrations of adducts in the dewaxed oil a solvent such as methyl isobutyl ketone or methylene chloride is used for dilution. The ratio of oil to water phase is about 1 to 0.5. The mixing of the oil and water phases occurs at slightly elevated temperatures of about 35 °C. In the course of the reaction the mixture is cooled to room temperature. Lower temperatures are advantageous for the formation of inclusion complexes.^[12]

The urea-paraffin-adduct can be filtered off and thereby separated from the iso-paraffins and other non-paraffinic components. By washing with a solvent a solid adduct residue is obtained. The washing of the clathrates with hot water at about 75 °C breaks up the clathrates and releases the paraffins. The obtained n-paraffins have a purity of about 99%. Losses of urea are small, the hot urea solution can be returned directly back into the process.

• Kenneth D. M. Harris: Fundamental and Applied Aspects of Urea and Thiourea Inclusion Compounds. In: Supramolecular Chemistry. 19, 2007, p. 47-72, doi:10.1080/10610270600977706.