Solvent selection for lube oil dewaxing Which of the following is an undesirable property for a solvent intended for dewaxing of lubricating oil?

Introduction / Context:
Dewaxing removes high-melting paraffins from lubricating oil to improve pour point and low-temperature flow. Solvent dewaxing relies on crystallizing wax from a diluted oil–solvent mixture at low temperature and filtering the crystals. Solvent choice critically affects wax crystallization and filterability.

Given Data / Assumptions:

• Typical dewaxing solvents: methyl ethyl ketone (MEK), toluene, methyl isobutyl ketone (MIBK), propane (for propane dewaxing).
• Process involves chilling to precipitate wax crystals for separation.
• We seek the property that is undesirable for effective wax removal.

Concept / Approach:
An effective dewaxing solvent should dissolve the oil well (to reduce viscosity and aid heat transfer) but should not dissolve wax at low temperature so that wax crystallizes out cleanly. Therefore, complete miscibility with oil is beneficial, while high wax solubility is detrimental because it suppresses wax crystallization and reduces removal efficiency.

Step-by-Step Solution:

Verification / Alternative check:
Phase diagrams and solvent selection literature for MEK–toluene blends show that wax solubility drops sharply at low temperature, enabling effective crystallization; processes are designed to exploit this behavior.

Why Other Options Are Wrong:

• Complete miscibility with oil: Desirable for viscosity reduction and uniform crystallization.
• Both (a) and (b) / Neither: Incorrect because only high wax solubility is undesirable.

Common Pitfalls:
Operating too warm or selecting a solvent blend that keeps wax in solution; both reduce filtration rates and leave residual wax, harming pour point.

Final Answer:
High solubility of wax in the solvent.