Method of cleaning large storage tanks for petroleum products
In a process for cleaning petroleum deposits from storage tanks, including tanker cargo holds which may be of 1,000 to 2,000 ton and higher cargo
capacity, involving the steps of applying a liquid chemical cleaning material to inner tank surfaces, allowing a residence time for opti mum action of
such cleaning material, and then washing with water to remove chemical and dislodged deposits from tank surfaces;
the improvement for providing uniform application of cleaning material to all inner tank surfaces including inaccessible surfaces of structural and
support members, that comprises generating a dispersion of said cleaning material within said tank in the form of a wet fog having a particle size
predominantly in the to 40 micron range.
As long as crude oil and oil products have been stored and transported, there has been a nagging problem of how to clean out storage tanks and
bunkers to permit the carrying of other products without objectionable contamination.
As long as the same cargo such as crude oil or residual fuel
oil is to be carried in a particular hold or storage tank, the need for cleaning may not arise except for the conventional washing with water to free
the space of gas fumes. Substantial layers of deposit on the tank walls formed by carrying a particular cargo such as crude oils will not particularly
contaminate additional cargoes of crude oil.
On the other hand, even small amounts of such deposits will contaminate subsequent cargoes of higher grade, such for example, as distillate oil,
kerosene, gasoline, or the like.
In fact, one method of upgrading a tank or tanker in the past has been to shift to progressively higher grade cargoes contaminating each with a
small amount of residue left by the prior cargo. Such a process, however, may require many months to achieve the desired upgrading.
Furthermore, it is often difiicult to follow this procedure because of unavailability of the particular type cargoes needed in various stages of the
Even so, it has sometimes been the practice to travel substantial distances without cargo in order to carry cargoes of the grade for
which the tank is suitable or of the grade needed in the upgrading process.
Various methods have been used in the past to effect rapid cleaning of tanks, but all have been extremely cOStly in materials employed and/ or in
time and hours of manual labor involved.
It has sometimes been the practice, for example, to use light oils or kerosene type fractions to wash the
bulk of sludge and oil deposits from tank surfaces, but this may require the use of the lighter fraction in a proportion of about 10 to percent of the
total volume of a tank, and obviously becomes prohibitive unless the resulting mixture can be returned to the refinery and reprocessed.
Another procedure has been to have men enter the tanks and manually scrape deposit from the inner tank surfaces, expensive, due to the man
hours involved in cleaning each but this is unpleasant, tedious, and hazardous, as well as tank.
Furthermore, after scraping away the bulk of the
deposits, it is still necessary to remove the residue with 3,436,263 Patented Apr. 1, 1969 lighter fractions, such as kerosene, or with suitable
chemicals if a really clean tank is desired.
The problems involved are large-scale problems when it is realized that individual tanks to be cleaned may have a capacity of 2,000 tons or more of
A typical cargo tank might measure, for example, 30 feet by 30 feet and have a height of 40 feet.
Furthermore, while large surfaces
may be relatively smooth and accessible, there are inevitably inwardly extending reinforcing members, frequently with overhanging portions which
provide inaccessible surfaces which are extremely difiicult to clean.
In conventional cleaning operations, extensive use is made of Butterworth (trademark) tank cleaning apparatus or similar mechanical tank washing
apparatus for hot and cold water washing of tanks.
Such apparatus involves a number of movable high-pressure water jets which in an extended
period of operation at different levels in a tank, can contact most surfaces to physically erode loose deposits and work them to the bottom.
From the tank bottom, the water and dislodged oil deposits are pumped to a slop tank from which, after Stratification, the water layer can be
pumped back to the sea.
The oil residues, however, are extremely hydrophobic, and when attempting to clean a tank containing, for example,
residual fuel oil, it is not uncommon even after extensive mechanical washing with hot water to find deposits remaining which are A to inch, and
sometimes as much as one inch in thickness.
Furthermore, inaccessible areas which cannot be reached by such apparatus may retain still heavier deposits. Thus, even after extensive
mechanical washing, there may be several barrels of sludge or oil deposits adhering to the walls and bottom of a single tank. On a ship which may
have 25 to 30 or more cargo tanks, therefore, a thorough cleaning of all tanks may involve the removal of one hundred barrels or more of deposits
from the tanks.
It has now been discovered in accordance with the present invention that it is possible by applying certain chemicals to troublesome oil and sludge
deposits, under conditions which will assure a substantially uniform contact of controlled duration with such deposits, to so soften and react upon
the deposits that they can not only be dislodged by streams of water, as by hosing or mechanical washing, but are at the same time converted to
an oil-in-water emulsion which can be easily removed from the tank being cleaned. Furthermore, the resulting emulsion is of such a nature, that
either directly, or with a minimum of further treatment, it can be discharged into the ocean without any objectionable separation of oil
The chemical to be employed in the new method is a type previously used for the cleaning of machinery, the cleaning up of oil spillages on floors
and the like, and similar marine and industrial housekeeping in areas which tend to become contaminated with petroleum products and sludge.
It comprises a mixture of a highly aromatic petroleum fraction, and a completely miscible emulsifying agent which is capable of producing
oil-in-water emulsions. Particularly effective as emulsifying agents are the liquid nonionic surfactants.
Preferably, the aromatic fraction should be
a mixed methyl-naphthalene type fraction (including mono-, diand poly-methyl-naphthalene) boiling within the range 350'700 F., and having
a flash point above 150 F.
The composition should contain about to 94% by volume of aromatic fraction and 6 to 20% by volume of emulsifying agent.
Within this range
the preferred amount of emulsifying agent will depend upon the nature of the cleaning operation and the intended manner of disposal of collected
Thus, while compositions containing to 94% by volume of aromatic fraction, and 6 to 10% by volume of emulsifying agent are generally
preferred, somewhat greater proportions of emulsifying agent may sometimes be desirable to provide complete emulsification of the petroleum
The emulsifying agents for use in the cleaning compositions can be selected from a wide variety of commercially available surfactants which form
oil-in-Water emulsions and are miscible with the aromatic petroleum fractions.
Suitable nonionic surfactants include those in which hydrophobic
moieties are condensed with a lower alkylene oxide to form hydrophilic oxyalkylene chains.
Hydrophobic moieties can be provided by fatty
alcohols, alkyl phenols, such as nonyl phenol, octyl phenol, and the like, by polypropylene glycols, and other relatively high molecular weight
compounds having free hydroxyl groups.
As is well known in the surfactant art, condensing such an alcoholic compound with a lower
alkylene oxide, such as ethylene oxide, has a solubilizing effect which increases with the length of the ethylene oxide chain; but also has a viscosity
increasing effect as the molecular weight of the resulting condensates is increased, with the higher molecular weight condensates becoming
pastes, or even solids.
For use in the present compositions, those surfactants should be selected which are liquid and highly water soluble,
as well as being miscible with the petroleum fractions.
A preferred aromatic fraction for use in the cleaning composition is a mixed methylnaphthalene fraction having a boiling range of about
400 to 550 F.; and with this methylnaphthalene fraction preferred surfactants include octyl phenol and nonyl phenol ethylene oxide condensates
containing about 9 to 10 mols ethylene oxide per mol of alkyl phenol.
Such a composition is a transparent liquid of very low viscosity, and
generally light amber in color, and has a flash point in excess of 175 F.
In carrying out the new method, the chemical, or cleaning composition above described, is applied as uniformly as possible to all oil and sludge
coated surfaces in an enclosure at the rate of about 50 to 80 gallons per thousand tons of tank capacity.
Within this range, selection of the
optimum amount will depend generally on the nature and thickness of deposits to be removed, and the amount of additional surface Within the
tank by reason of beams, bays, and other braces protruding from the walls.
The dosage of chemical can also be expressed as about to 9 gallons per
thousand square feet of surface to be cleaned. In practice, however, it is more practical to relate the dosage to the tank capacity, since this figure
will be readily available to operating personnel.
The application of chemical to contaminated tank surfaces can be accomplished by an operating team of 2 or 3 men entering the tank with hoses
and spraying all surfaces as uniformly as possible.
This method of application permits complete wetting and coating of deposits in inaccessible
placesbehind beams and bulkheads, and the like but is subject to variation in the care exerted by the workmen in reaching all areas.
spraying of chemical on inner contaminated surfaces of a tank having a capacity of 800 to 1,000 tons can be accomplished within a period of about
one hour to one and one-half hours.
Following the application of chemical to the contaminated tank surfaces, the tank is allowed to stand to permit a 2 to 6 hour, and preferably
a 2 to 4 hour, residence time of the chemical on the tank deposits.
During this period the chemical has a chance to thoroughly penetrate
the deposit reacting upon and softening even the caked sludge formed in repeated carrying of cargoes, such as residual fuel oil.
At the end of the 2 to 6 hour residence period, washing the treated tank with either heated or unheated sea water dislodges virtually all of
the softened deposits, leaving the walls substantially free of any petroleum residue.
The washing can be done with hand hoses, requiring
a workcrew of 2 to 3 men, or can be done with mechanical washing equipment, the operation in either instance requiring about 1 hours for
a 1,000 ton tank.
This removal of the deposits progresses in three ways.
The jets of water from the hoses or mechanical washing apparatus may physically loosen large chunks of softened deposit, permitting them
to fall to the bottom of the tank.
Adhering films and deposits are eroded and emulsified in the moving films of water.
At the same time,
under the action of the emulsifying agent and the agitation of water as it is being pumped from the bottom of the tank, the semi-solid deposits
which may fall to the bottom of the tank are broken up and dispersed in the water.
When washing is done by mechanical washing, it is preferable,
after the mechanical washing to have a man enter the tank with a hand hose to rinse inaccessible areas which may not have been reached by
the mechanical washing apparatus, and to aid in the removal of semi-solid residue which may not have been completely dispersed before the water emulsion is pumped from the tank.
In this final clean-up operation there is also occasionally solid residue, such as rust flakes which have been dislodged from the walls, and which should be collected and removed in providing a thoroughly clean tank.
The residence time of 2 to 6 hours, and preferably 2 to 4 hours, should be adhered to for optimum results with the new method,
as the subsequent rinsing or washing operation tends to be prolonged with a longer residence period.
Tanks have been found to rinse clean,
however, even when washing is done two or three days after the cleaning material has been applied.
The procedure as above described permits tanks to be cleaned much more thoroughly and in a small fraction of the time required by
Furthermore, it requires substantially less manpower than previous methods, although the step of hand spraying chemical
on the inner tank surfaces remains a rather unpleasant operation.
Most important, however, is the fact that the new cleaning method makes
possible for the first time in an economically practical way the upgrading of a tanker from crude or residual oil cargo to clear cargoes (gasoline, etc.) in a single brief cleaning operation.
A prefered adaptation of the new tank cleaning method which further reduces cleaning time and costs while eliminating this unpleasant operation
of hand application of cleaning material, and the chances of human error in the hand application, involves the generation within the tank of
a dispersion of cleaning material in the form of a wet fog.
In spraying terminology, the term wet fog is used to designate a particle size of about
10 to 40 microns.
This compares, for example, to a dry fog having a particle size of about 2 to 5 microns, and a misty rain having a particle size of
about 50 to microns.
For the purpose of the present invention, the particle size of the sprayed material can be within the range of about 5 to
50 microns, provided that the bulk of the material is in the wet fog range of 10 to 40 microns.
To better visualize the significance of particle size,
attention is directed to the following tabulation:
Within a confined area such as an oil storage tank, it will be apparent from the slow rate of particle fall, and the distance which particles can travel
in a mildly agitated system (such as a 3 mph wind) that dispersed particles of cleaning material will have ample opportunity to impinge and settle
upon all inner tank surfaces, including surfaces behind beams and bulkheads which are normally considered inaccessible.
in the confines of a tank in which the total exposed surface area is only a small fraction of an acre, the number of particles per square inch
impinging upon the surfaces will be many times the numbers indicated in the above tabulation.
Furthermore, when it is realized that the cleaning
material is applied at the rate of about 50 to 80 gallons per thousand tons of tank capacity, it will be apparent that the spraying of a wet fog will
distribute the cleaning material as a continuous film of appreciable thickness on all inner tank surfaces.
It should be noted in this connection,
that the cleaning material penetrates rapidly into oil and sludge deposits, thereby minimizing accumulation at the surface of the deposits and
runoff of the impinging cleaning material.
The generation of a wet fog of the type suitable for use in the present method can be accomplished with commercially available spray equipment,
preferably involving the use of pneumatic atomizing nozzles providing a wide and dense spray pattern, to which cleaning agent and atomizing air
feed at separately and properly controlled pressures to give the desired particle size distribution.
Suitable nozzles can be obtained from various
suppliers, but particularly good results have been obtained wtih Spray Setup No. 29, produced by Spraying Systems Company of Bellwood,
Ill., and utilizing their fluid nozzle No. 60100 and air nozzle No. 1406-52-70, which provides a wide angle round spray pattern.
It is preferable to
use a number of these nozzles collectively both to multiply the directions in which the spray is initially aimed, and to provide the capacity desired
for discharge of cleaning material at a practical rate.
A preferred apparatus has been an elongated unit with a cluster of nozzles at each end which
may be passed through the openings for mechanical washing apparatus, standard on most oil tankers, and the like, and which can then be
suspended at different heights within a tank from its midpoint.
A unit approximately 6 feet long with a total of nozzles, 4 angularly disposed,
and 1 axially disposed at each end of the unit have been found to provide excellent spray distribution, and a practical feed rate at the desired
operating pressures, permitting discharge of a 1% to 2 times as much cleaning material as previously indicated.
Following the wet fog spray application of cleaning material, the rinsing or washing operation is carried out in the manner previously described
after a residence time of the cleaning material of about 2 to 6 hours, and preferably 2 to 4 hours.
Whether the washing is done by means of hand
hoses, or by mechanical washing apparatus, the cleaning results are excellent, generally surpassing to some extent, the results obtained when the
cleaning material is applied by hand.
This is understandable, since the wet fog application of cleaning material provides a uniformity of contact
with contaminated surfaces which is difiicult to achieve with hand application.