US2715594A - Method of cleaning asphalt tanks - Google Patents

Description

2,715,594 Patented Aug. 16, 1955 METHOD OF CLEANING ASPHALT TANKS Charles W. Garrison, Fort Mitchell, Ky., assignor to The Standard Oil Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application August 26, 1952, Serial No. 306,493

1 Claim. (Cl. 134-22) This invention relates to a process of cleaning asphalt tanks.

Before referring in more detail to the method which comprises my invention, it is desirable to describe briefly the method by which asphalt is made and stored and the equipment used, which gives rise to the problem which is solved by my invention.

In processing crude petroleum oil the lighter fractions which can be distilled at atmospheric pressure such as gasoline, kerosene, etc. are distilled off and the product remaining is known as reduced crude. This is further distilied under a vacuum in order to reduce the temperature below cracking temperatures. For this purpose a large fractionating column is often employed which is referred to as a pipe still. A number of distilled fractions are obtained such as a gas oil fraction, fractions comprising various viscosity lubricating oil stocks and cylinder stock, and fractions containing wax such as parafi'in distillate and wax slops. The residue which remains after all the overhead fractions have been removed is referred to as pipe still bottoms. The exact composition of the pipe still bottoms will depend somewhat on the source of the crude and the extent to which various fractions are distilled overhead. It may be modified by blending back some of the distilled fractions if desired. Pipe still bottoms, among others, are utilized in the preparation of asphalt. Other materials which can be used include residue from thermal cracking referred to as cracking coil tar. Any of these heavy black viscous residual materials is referred to as petroleum residuum.

In converting a petroleum residuum to asphalt, one method is to place it in a large tank and blow air and sometimes steam through it to oxidixe and reduce the residuum. The extent of the oxidation of any particular stock due to the air blowing largely determines the hardmass or softness of the asphalt, i. e., whether it has a high or low penetration.

The oxidation is generally carried out in a large tank such as from 10 to 100 feet in diameter and about 15 to 75 feet high. Spaced a short distance from the bottom, such as 2 inches to 3 feet, are a series of pipes generally arranged to cover the bottom area such as that resembling the spokes of a Wheel. The pipes have holes in the bottom portions. The tank is filled or partly filled with the petroleum residuum to be converted to asphalt and compressed air is introduced into the pipe system and blown out through the holes. The holes, being in the bottom of the pipes, direct the air towards the bottom of the tank and the air bubbles up through the asphalt which is maintained in a liquid condition by heating it to a temperature in the range of 360 to 550 F. After this operation has been continued for the required length of time, the air is turned off and the asphalt is withdrawn from the tank at a sufiiciently high temperature so that it remains fluid. In all of these operations it is desirable to keep the temperature sufiiciently high so that the material remains liquid.

Notwithstanding the use of these temperatures, a layer of hard rock-like asphaltic material and coke accumulates all over the interior, including the bottom, of the tank. This is to be distinguished from ordinary asphalt because this layer does not liquify and flow from the tank with the asphalt.

A similar hard layer of material forms during use of tanks for storing, mixing and transporting asphalt. Many of such tanks include pipes through which air may be blown for the purpose of agitating and mixing the asphalt,

or such pipes may be readily installed.

Periodically it becomes necessary to inspect or repair these asphalt tanks and the practice generally employed heretofore is to drain all the asphalt that will flow from them at an elevated temperature, cool them to ambient temperature and then to chip out the rock-like layer of hard asphaltic material which may be anywhere from an inch or so to four feet in thickness and firmly adherent to the bottom and sides of the tank. Often the accumulations of coke and rock-like asphaltic material resemble a series of small mounds inside the tank. Apparently agitation resulting from air blowing would prevent the formation of a smooth layer. The valleys be tween the various mounds would be filled with asphalt which would not drain out naturally. Accordingly, the chipping out often involved the removal of asphalt, rocklike layers of hard asphaltic material, and coke. This is a dirty, unpleasant, time-consuming, and expensive job. Tank cleaning in an asphalt plant is a major and costly problem which has not been solved heretofore in the field.

I have discovered in accordance with my invention that this hard rock-like layer of asphaltic material and coke, which cannot normally be drained, may be removed by introducing an amount of kerosene or other like hydrocarbon having a boiling point in the range of 350 to 600 F. to provide a depth sufiicient to cover the material in the tank to be cleaned. Live steam is then forced through the pipes and blown out through the holes towards the bottom of the tank. The steam is blown against the surface of the tank from which the deposit is to be removed. The steam apparently functions to assist in loosening asphalt and the hard asphaltic material and coke and facilitate the dissolving of at least a portion of them in the kerosene. In addition, the water from the condensed steam inside the tank tends to float some of the asphalt and loosened material and prevent it from resettling. In this way at least a portion of the asphalt that is loosened may be suspended in the interface between the kerosene and the water, and any of the remaining undissolved asphalt is prevented from adhering to the bottom of the tank by the water, which being heavier than the hydrocarbon, forms the bottom layer of liquid.

The blowing of steam as described may be continued until the rock-like layer is removed, anywhere from a few hours to a week. I have found that about two to three days is generally adequate for the treatment. At the end of this time the steam is shut oh and the kerosene and water is drained from the tank. Surprisingly, the tank was very clean except for a very slight film of sludge which was soft and could be readily scraped off or washed off without any difliculty. The solvent can be re-used or used in making cut-back asphalt.

As illustrative of a practical embodiment of my invention, an asphalt tank 80 feet in diameter and 42 feet high equipped with the air blowing pipes was attempted to be cleaned at two different times ten years apart by the previously known methods. In these attempts various solvents have been employed by themselves and steam has been employed by itself without removing the de posit. These attempts required ten mens time for a r 3 a period of four weeks and the tank'was only partiall cleaned in that it was not possible to remove the accumulated hard material down to the bare metal.

In cleaning this same tank by the method of the invention, some ten years after the last previous attempt to clean it, all of the hot asphalt which would flow from the tank was pumped out and immediaely there was pumped into the tank a solvent of the kerosene type normally used for making #3 cut-back asphalt. The amount of solvent was sufiicient to form a layer two feet deep in the tank. Live steam was then introduced through the air holes for a period of three days. This had the eflect of heating, dissolving, and cutting the ac-V eumulated deposit while at the same time the condensed water from the steam 'was getting under the asphalt on the floor of the tank and lifting it up where it could be dissolved in' the solvent. At the end of this time the tank was emptied and material pumped out largely in an emulsified state. The man-holes and clean-up plates were taken from the tank'and it was found that praci tically all of the deposit had been removed. What little 'i'emained was easily washed out in an emulsified state with water from a fire hose. The Water was drained from the tank and the bare metal plate was exposed, which could readily be inspected and repaired.

It will be obvious that other minor variations may be "asphalt.

made in the details of the process depending upon the type of equipment that is employed in making the All such variations are intended to be included within my invention as set forth by the claim.

I claim:

A method of cleaning a tank used for the conversion of petroleum residuum to asphalt to remove a hard solidified layer of material which has accumulated as a result of said conversion on the interior of the tank and which is solid at asphalt liquifying temperatures, which comprises introducing into the tank a layer of a hydrocarbon fraction boiling generally within the range of 350 to 600 F. and blowing live steam below the hydrocarbon layer toward the solidified layer of material on the tank, continuing the steam blowing for 'a period of from a few hours to a week until the material has been separated from the tank surface, and withdrawing from the tank the hydrocarbon and the water from the condensed steam together with the separated material. 7

References Cited in the file of this patent UNITED STATES PATENTS