During the storage of liquid phases such as oils, crude, refinery products and other petrochemical products, undesired sedimentation and thickening often occurs which will be discussed hereinafter in connection with the particular deposit-forming example of crude oil.
The liquid phase of crude oil is a mixture mainly consisting of hydrocarbons such as paraffins, aromatics and naphthenes, which are also accompanied by non-hydrocarbons or so-called impurities such as mud, water, dissolved salts, sulphur compounds, sand, etc. In certain circumstances prior to processing in refineries, the crude undergoes rough refining processes for the separation of impurities. It is then generally customary to store the crude oil to be processed and also the precleaned crude oil in large tanks. This involves holding times of varying lengths, which can be quite long in the case of stockpiling, and much shorter for operation-based storage locations.
Long holding times especially favor undesired deposit formation from crude in tanks. These deposits can be of different types, being e.g. favored by emulsions of water with hydrocarbon fractions, or can consist of segregates of heavy hydrocarbon fractions (hard waxes) or settled mud or salts. The result is a kind of oily mud which accumulates and is compressed on the bottom of tanks and leads to high costs and losses and which is referred to loosely as sludge.
Costs and losses are caused because the oil sludge reduces the capacity of the tanks and also binds crude oil or, in some cases, largely consists of thickened crude. Thus, apart from the costly space loss in the tank, storage leads to significant loss of raw material. In addition, the lost space cannot be recovered again if the sludge is stored in closed systems, i.e. tanks made available for this purpose, the undesirable alternative being a final storage which is prejudicial to the environment if the sludge is dumped into open pits or basins. In large tanks with a diameter up to 100 m, a height of 20 m and a corresponding capacity, sediment thicknesses of 1 to 2 m lead to a 5 to 10% capacity loss. In addition, the service and operation of the tanks is often made difficult because the oil sludge clogs the pumps, outflows from tanks have to be filtered, etc. Finally, down-times are linked with the removal of the oil sludge. If the oil sludge is not removed, it accumulates further and finally leads to the abandonment of sludged-up storage containers and the construction of new tanks. Apart from these storage costs the unprocessed oil sludge also represents a loss because, despite its impurities, it largely consists of utilizable hydrocarbons.
Several solutions are known for removing sediments from crude in tanks and two examples will now be given.
1) A first solution is proposed in U.S. Pat. No. 3,436,263 and French Patent 22 11 546, where cleaning substances are used for dissolving, or removing in bound form, the oil sludge. A disadvantage of this method is that, due to the cleaning substances which have been introduced, the liquified oil sludge is no longer usable because its composition has been changed by the additives and it must be disposed of in dumps or elsewhere. Such dumps are e.g. old tanks or so-called wasteland and constitute a serious pollution of the environment. Reprocessing of oil sludge is consequently not desirable using this method which, instead of combating the problem it only combats the effect. However, it is still possible to clean and reuse the tanks. The essential reason why the dissolved oil sludge cannot be processed is that the cleaning substances used represent impurities for processing in refineries, whose separation by standard cleaning methods involves great effort and is expensive and does not bear a positive relationship to the recovered crude.
2) Another solution is proposed in European Patent 160,805, wherein hydrokinetic energy is used in order to dissolve, or suspend back into the liquid phase by means of turbulence, sedimented residues in tanks. Thus, oil sludge dissolved by crude as the dissolving substance can be returned to the process and processed in the refinery following standard cleaning procedures. This method does not prevent the formation of oil sludge but instead merely eliminates it. For this purpose, planned turbulence or eddy flows are generated, whose successive remote action is able to dissolve the deposits in an effective manner, even outside the direct injection zone. However, this requires a considerable investment. Mechanically moving components within the tanks, such as e.g. rotary liquefying lances, which hydro-kinetically activate the oil sludge and dissolve it in crude represent considerable expense. Thus, although this method leads to a high oil sludge recovery level, it is expensive. Under extreme environmental conditions, e.g. in sand or ice desert regions, this is undesired.