Miscible and Immiscible Operations

MEK is an antisolvent for the wax and helps to reduce its solubility. If the MEK content is too high the Basestock may become insoluble and a phase separation will occur. MIBK or Toluene is added to help solubilize the oil. Both of these prosolvents have a higher affinity for wax molecules than MEK. The higher the concentration of prosolvent the more wax stays in solution, and ends up in the filtrate. This raises the pour point of the dewaxed oil and since the manufacturer must meet dewaxed oil pour point specification the manufacturer is forced to reduce the filtration temperature to remove more wax. The reduction in filtration temperature increases the viscosity of the slurry and filtration rates are slower and oil removal from the wax cake becomes more difficult. Thus the objective is to use the maximum amount of MEK without having a phase separation. A plot of the phase separation temperature or miscibility temperatures vs. solvent composition may be used to help set the optimum solvent composition. 

The ideal would be to operate as close to the miscibility curve as possible. However, this curve shifts depending on the Basestock. Wax molecules in heavier grades come out of solution earlier and this has the effect of shifting the miscibility curve to the left. Plants equipped with solvent splitters to separate the MEK from the MIBK or Toluene after it has been recovered in the DWO and Wax recovery sections may blend to the optimum solvent composition for each Basestock. Manufacturers without the capability to change solvent composition will set the plant solvent composition based on 

The shift from miscible to immiscible operation occurs over a narrow range of solvent composition. When the plant moves deep into immiscible operation and phase separation occurs, the oily phase, containing the desirable high VI molecules, will hang up in the filter cake and be very difficult to remove, increasing the oil in wax content of the wax and reducing dewaxed oil yield. Dewaxed oil properties may be adversely affected, and VI may decrease. Interestingly, in severely immiscible operation the dewaxed oil pour point may actually decrease as the waxy molecules remain in the wax cake. 

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