This invention relates to an apparatus for and method of transporting soluble solids in a tank. In particular, it relates to an agitator that is mounted on the top of a horizontal cylindrical railroad tank car and has a shaft that penetrates into the car at certain angles and rotates a propeller so as to cause fluid within the tank to move in a helical pattern around the tank.
Soluble solids, such as chromic acid, are shipped in lined steel rail cars. These solids are removed from the cars by dissolving all the solids in the car in a solvent, usually water, then removing the resulting solution from the car. Since most solids can not be dissolved in a reasonable time without at least a temporary increase of total system volume (volume of solvent+volume of solids+volume of solution), the cars can be filled with only that quantity of solids that will dissolve when the car is filled to the maximum allowable level with solvent, which is usually far less than the dry weight capacity of the car.
Many hours may be required to dissolve all of the contents of a railroad car, which can be about 6 to about 18 m long, about 2.4 to about 3.0 m in diameter, and can hold about 37,850 to about 94,625 L (about 10,000 to about 25,000 gallons). Agitators are used to reduce the time needed to dissolve the solids. These agitators are often mounted inside the car and direct a thrust either radially away from the agitator shaft or vertically downward. Because the length of a railroad car can be over three times its diameter, a single agitator can reach only a portion of the solids in the car and as many as 5 agitators are sometimes needed to dissolve all of the solids.
Agitation can also be accomplished by sparging air through the solvent/solids/solution mixture in the car. This procedure is also time-consuming, can result in airborne emissions, and is able to generate solutions approaching only 80 to 90% of saturation within a reasonable length of time.
In our invention, the agitator is mounted on top of the tank. Its shaft enters the tank at particular angles relative to the tank""s longitudinal and vertical axes and turns a propeller. We have discovered that if the shaft is within the angles specified and the flow generated by the propeller is within the proper direction, velocity, and volume, a single agitator will move the fluid within the tank in a highly structured helix or spiral flow pattern and the entire contents will be agitated and dissolved. Because the agitator of this invention is much more effective than prior agitators and air sparging, the time required to dissolve all of the contents of the car is reduced by as much as 2400%.
A major advantage of the agitator of this invention is that it enables a shipper to transport a much greater quantity of solids in the tank. While in the prior process the maximum amount of shipable solids was the amount that would dissolve when the tank containing solids was filled with solvent such that the total mixture of solids/solvent/solution reached the maximum volume that the tank could safely contain without overflowing, in our invention nearly the entire tank can be filled with solids. This is possible because, after initially filling the car with solvent, a brief agitation period, and removal of a portion of the solution, more solvent can be added continuously or in batches near the agitator and drainage point so that the resulting solution(s) can be removed without entraining undissolved solids. The volume of solids dissolved gradually expands from the space between the propeller and the drain until the entire contents of the tank have been dissolved and removed.