Dewaxing Solvent Recovery

Dewaxing (Figure 4.17) processes also produce heater stack gas (carbon monoxide, sulfur oxides, nitrogen oxides, and particulate matter) as well as hydrocarbon emission such as fugitive propane and fugitive solvents. Steam stripping wastewater (oil and solvents) and solvent recovery wastewater (oil and propane) are also produced. The fugitive solvent emissions may be toxic (toluene, methyl ethyl ketone, methyl isobutyl ketone). 

The dewaxed oil and wax phases discharged from the centrifuges are transferred through heat exchange equipment to individual solvent recovery units for continuous recovery of solvent. 

Solvent dewaxing a process for removing wax from oils by means of solvents usually by chilling a mixture of solvent and waxy oil, filtration or by centrifuging the wax which precipitates, and solvent recovery. 

A hyperfiltration process developed by Mobil Oil, now ExxonMobil, for this separation is illustrated in Figure 5.28(b). Polyimide membranes formed into spiral-wound modules are used to separate up to 50 % of the solvent from the dewaxed oil. The membranes have a flux of 10-20 gal/ft2 day at a pressure of 450-650 psi. The solvent filtrate bypasses the distillation step and is recycled directly to the incoming oil feed. The net result is a significant reduction in the refrigeration load required to cool the oil and in the size and energy consumption of the solvent recovery vacuum distillation section. 

Dry Waxy Solvent from wax recovery should be segregated from the Dry Clean solvent recovered from the Dewaxed Oil recovery section. Waxy solvent tends to have wax in it and carmot be chilled to the same temperature as that of the Clean solvent from the dewaxed oil recovery. It may be cooled and temperature blended with the Clean and used as filter wash. Flow restrictions may be set on flow to wax recovery to prevent wax carryover and downstream fouling. 

This dewaxing process is always installed in combination with Edeleanu solvent extraction because recovery of the solvents from the preceding step is not required. For example, if dewaxing follows the refining operation, the raffinate solution is taken directly from the extraction unit and the solvent composition is adjusted to that used for dewaxing by the addition of a benzene-rich solvent which is readily recovered from the dewaxed oil solution because of the wide difference in boiling points of the solvents. 

Recovery of solvent from the wax cake is accomplished in a manner similar to that from the filtrate, except that any water inadvertently entering the oil or solvent side of the dewaxing system quickly finds its way as ice into the wax cake. To remove this water, the wax cake is... 

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