Asphaltene Precipitation

 

-         All crude oils are stored near 0°C in sealed containers with an argon blanket.  These precautions are taken to minimize possible oxidation of the crude oil.

-         Remove crude oil can from refrigerator and place in paint can shaker for several three-minute cycles (Crude should be returned to room temperature before sampling).

-         Add 15-20 g of crude oil to a 1 L round bottom flask using 25 mL glass pipette and green pipetter.

-         Use large balance on bench top and cover the area with plastic-backed laboratory paper.

-         Add HPLC grade n-heptane to flask in a 40:1 by volume ratio to the crude (approximately 750 mL).  Depending on crude oil viscosity, shake flask by hand to completely disperse crude.

-         Place on shaker with a cork ring and set to ~4 (moderate).  Shake for 24 hours.

-         Asphaltenes are removed from solution by vacuum filtration through 15 cm diameter, 1.5 mm Whatman 934-AH glass microfiber filter paper.  The filter paper is supported on a ceramic Büchner funnel.  The funnel sits on a 1 L vacuum flask.

-         Pour the contents of the flask through the filter paper.  Rinse the round-bottomed flask with heptane to remove precipitates and transfer to filter.

-         Add ~250 mL heptane to the precipitated asphaltenes collected on the filter paper to completely remove remaining soluble material or until the effluent runs clear.  

-         Transfer the Büchner funnel to a new vacuum flask.  Dissolve the asphaltenes with methylene chloride (HPLC grade, Fisher) under a partial vacuum.  (Methylene chloride has a high vapor pressure and low boiling point causing it to evaporate quite rapidly and reduce the local temperature.  Atmospheric moisture would often condense and freeze on the filter paper hindering the flow of the organic phase.)

-         If you have more than one crude oil flask, use a new piece of filter paper.  Continue the process with the remaining flasks.

-         Refilter the soluble maltenes (deasphalted crude+heptane) to remove any residual asphaltenes.  Depending on the volume of maltenes generated they may need to be stored in clean 4 L screw-cap bottles.

-         Rotary evaporate the methylene chloride-asphaltene solution under partial vacuum at 50°C.  Transfer <300 mL of solution to a 1 L round bottom flask and place on the Rotavap.  Pull a relatively low vacuum and submerge only a small portion of the flask in the water bath.  This solution will very easily “bump” or boil unexpectedly and launch material into the condenser.  Watch the solution very carefully until stabilized and after a portion of the Methylene chloride has evaporated.  Once dry, add more solution and continue until finished. 

-         Add Methylene chloride back into the flask (enough to redissolve the asphaltenes and transfer to 1-2 glass jars with green PTFE lined caps.  Dry the slurry under an argon stream until most of the solvent has evaporated.  Place aluminum foil over the mouth of the jar and poke a few small holes.  Dry the asphaltenes in a nitrogen flushed vacuum oven at 50°C for 24 hours.  Excess solvent present in the jars will cause the asphaltenes to rise upwards and may spill out.