Mohawk Lubricants Ltd. returned a 12,000 gal storage tank to service by fluidizing and removing 8,000 gal of 4-year-old residue. A portable mixer, manufactured by Pulsair Systems Inc., Bellevue, Wash., was used to cut lighter oil into the partially gelled residue, thus enabling Mohawk to pump it from the tank.
While consulting with Pulsair regarding blending operations at its North Vancouver, B.C., used motor oil plant, Mohawk described the gelled residue. The company wished to place the residue storage tank back into service and either sell the removed residue or run it through the re-refining process.
The material involved is the distillation-tower bottoms from used oil processing--what Mohawk calls "tar residue." Mohawk had attempted to fluidize the residue at one time by applying steam, but the procedure did not produce a mixture that could be pumped out of the tank.
PROCEDURE
The 12,000 gal, flat-bottom cylindrical tank measures about 10 ft in diameter and 21 ft in height. The domed top has an 18-in. manway slightly off-center.
The tank is plumbed to fill and discharge via two pipes that penetrate the tank wall near the bottom. The tank contains no baffles, heating coils, or installed mixing apparatus.
A Pulsair prototype TNR 1000 mixer, a control unit, and a 12-in.-diameter accumulator plate were delivered to the site in March. Mohawk supplied compressed air to the mixing equipment and used lengths of 1-in. steel pipe as a probe.
The accumulator plate was threaded onto the bottom of the 1-in. pipe probe and the mixer was attached to the top of the probe. The air line was connected to the control unit and the system was started at a rate of about 60 pulses/min.
The probe was then lowered through the manway until the accumulator plate rested on the tank bottom. The probe was U-bolted to an angle iron, which was, in turn, bolted to the manway flange.
Because the mixer can accept 1-in. air hose and the only available supply was 3/8-in. hose, the pulse rate was increased to about 4 pulses/sec. At this speed, there was continuous agitation of the congealed residue, with an area of influence at the surface measuring about 5 ft in diameter around the probe.
Because Mohawk wanted to sell the residue, as little distillate as possible was used to dilute the product. Distillate 150, or No. 1 distillate, was selected, and 200 gal were added.
RESULTS
After mixing until the residue was visibly uniform, a sample was drawn and was determined to be too highly alkaline to sell. Because this meant that there was no reason to avoid further dilution, an additional 1,400 gal distillate was added.
Because the residue remained very viscous, 1,600 gal additional distillate was added. Mohawk then pumped in 1,900 gal residue from another tank, then added 400 gal distillate.
The mixture still was not very fluid, so 530 gal of high-flash fuel was added to the tank. This dilution increased the fluidity noticeably, and Mohawk decided to transfer into the mixture an additional 1,900 gal residue that had been causing pumping problems in another tank.
To eliminate any potential pumping problems, another 530 gal high-flash fuel was added. After this was mixed in, about 5,400 gal of the mixture was transferred to other tanks, leaving about 3,800 gal of diluted residue in the tank.
The mixer was then stopped, after a little more than 8 days of continuous use, and the tank was returned to service.
Pulsair concludes that the procedure could have been accelerated by adding the full amount of diluent sooner, or by adding heated diluent. Based on its calculations, the company believes these procedures could have returned the tank to service within 24 hr.
The cost of the operation is estimated to be the value of the mixer ($3,500) plus about 8 man-hr labor. The distillate is not included in the cost because it was returned to plant feed, and compressed air costs were not included because the refiner's compressor always runs at maximum output.
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