Selective Separation of Normal Paraffins from Slack Wax Using the Molecular Sieve Adsorption Technique

Abstract

The selective separation of heavy n‐paraffins (C₂₀–C₃₀) from refinery slack wax was performed by adsorption onto zeolite molecular sieves (5A). Sorbents were characterized by thermogravimetric analyses, N₂ adsorption, and mercury porosimetry. Three slack waxes: light, medium and heavy, were used, being analyzed by gas chromatography (GC) and simulated distillation. Several tests were carried out in a stirred batch reactor at high pressure and temperature for determining the system kinetics. Process variables were temperature (100–150°C), pressure (0.5, 1.5, and 2.5 MPa), and time (1–24 hr). Afterwards, adsorption isotherms in the range of 60–170°C were obtained for each wax by being fitted to a Langmuir type equation. Results showed that adsorption equilibrium is controlled by internal diffusion due to the large length of the normal paraffin chains, which have a diameter very close to the size of the zeolite channels. The amount of paraffin retained in the sieve is strongly influenced by the feedstock: lighter products adsorb more, also producing a higher purity n‐paraffin. The reverse trend is observed for the heavier feeds. However, adsorption of larger molecules (>n‐C₃₂) is strongly hindered. For temperatures above 170°C, cracking reactions become more and more important.

Keywords:

• n‐Paraffins
• Wax
• Zeolite
• Adsorption
• Molecular sieves

Acknowledgment

The authors are indebted to Repsol Petróleo Research Center in Cartagena (Spain) for allowing the use of their laboratories and equipment.