(Originally printed in Chemical Engineering  September 1994)

Operators were able to recover the contents and regain use of a stagnant tank

If allowed to stand for any length of time in storage tanks, highly viscous hydrocarbon residue can solidify, making its removal so difficult that abandoning the tank is often the most cost-effective solution. Mohawk Lubricants, Ltd. (North Vancouver, B.C.), a re-refiner of used motor oil, faced such a problem after hydrocarbon residue was left in a 21- ft-tall, 10-ft-dia. storage tank for a four-year period.

The 8,000 gal of partially gelled residue that had collected in the 12,000-gal, flat-bottom tank was too viscous to pump, even after it was heated and agitated by direct steam injection. Dilution with a low-viscosity solvent would have made pumping easier. However, incorporating a low-viscosity solvent into the near-solid residue was a costly mixing job, which called for a powerful, high-torque agitator. The agitator was not only expensive, but it required a special mounting structure to be built on the tank.

Last March, Mohawk solved its problem by using the TNR 1000 [now the PTM-2000], a portable mixer from Pulsair Systems, Inc. (Bellevue, Wash.). Using pulses of compressed air, the mixer was able to fluidize the stagnant tank contents in just three hours. Once the residue was fluidized, a low-viscosity petroleum distillate was blended into it to make it pumpable.

Over the the course of several days, Mohawk used the mixer to blend an additional 3,600 gal of distillate and 1,060 gal of high-flash fuel into residue, to further reduce the viscosity, and to produce a mixture that was easily handled by transfer pumps.

The recovered mixture was eventually blended with other used motor oil in the feed lines to Mohawk's rerefining process. This turned an otherwise unusable waste material into a valuable product, and allowed the firm to place the tank back in service for storing incoming feedstock.

A Proper Pulse

The mixer used at Mohawk consists of 20-fl section of 1-in.-dia., steel pipe. A 12-in.-dia. accumulator plate made of carbon steel is threaded to the bottom of the pipe, and the pneumatic controller is attached to the top.

The entire apparatus was lowered into the tank until it came to rest near the bottom. Small "legs" held the plate 1/4 in. off the tank bottom, and the top of the assembly was attached to the top of the tank (Figure).

Using a patented process, the mixer sends pulses of compressed air through the pipe. As each pulse is released between the plate and the tank bottom, it forms a large bubble, which rises through the tank contents. This sequential stream of pulsed bubbles provides the circulation required to mix the tank contents. A controller on the compressed air inlet adjusts the pulse rate, as necessary. Mohawk operated the fluidizing mixer at a rate of roughly four pulses per second using 100-psi compressed air. This provided continuous agitation of the residue-distillate mixture.

The specific mixer design used at the Mohawk facility was an experimental device, which is now commercially available. Similar fluidizing devices have been used successfully for mixing paints, resins, slurries and asphalt emulsions, and for pH-balancing and temperature-equilibration operations. Both compressed air and inert gas can be used, and the mixer is effective in square and cylindrical tanks, as well as in rail cars.-- Pulsair Systems, Inc., Bellevue, Wash.

Edited by Suzanne Shelley