The Role of Solvent in Dewaxing

As solvent is added to the waxy raffinate the oil is diluted and the viscosity of the oil solvent mixture decreases allowing filtration to take place more easily. The polarity of the oil-solvent mixture increases, decreasing the solubility of the wax and promoting the formation of more compact wax crystals. But as solvent is added the resulting filtrate becomes more dilute, loading up filtrate pumps and solvent recovery facilities. 

Solvent boiling point lower than the boiling point of the oil 

Ketone units typically use a dual solvent system consisting of MEK and either MIBK or Toluene. The MEK acts as an antisolvent to reject wax molecules from solution. This reduces refrigeration requirements but excessive MEK may cause oil phase separation. The second solvent keeps the oil in solution but also dissolves some wax. MIBK and toluene act as prosolvents. 

Solvent Wax Solubility g/100 ml Viscosity 0°C,cSt BP, C Latent Heat of Vaporization, cal/g Specific Heat, caVg-°C 

MEK/MIBK refrigeration requirements are lower than MEK/Toluene because the Pour-Filter spread is smaller due to the lower wax solubility. The Pour-Filter spread is the difference between the Dewaxed Oil pour point and the filtration temperature required to meet the Dewaxed Oil pour point specification. Wax has a higher solubility in Toluene than MIBK and MEK/Toluene systems will require a lower filtration temperature to achieve the same pour point. MEK/MIBK solvent mixture viscosity is lower than MEK/Toluene. Filtration rates are higher for MEK/MIBK. Toluene costs less than MIBK. 

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