US3436263A - Method of cleaning large storage tanks for petroleum products - Google Patents

Description

United States Patent 3,436,263 METHOD OF CLEANING LARGE STORAGE TANKS FOR PETROLEUM PRODUCTS Lynn A. Strenkert, Norwalk, Conn., and William H. Carls, Western Springs, lll., assignors to T he Perolin Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed May 13, 1965, Ser. No. 455,630

Int. Cl. 1308b 9/08, 3/10 US. Cl. 134-22 4 Claims ABSTRACT OF THE DISCLOSURE 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 upgrading. 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 oil products. 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 agglomerates.

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 residues. 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 residues.

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. Careful hand 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 previous methods. 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. Furthermore, 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 are 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.

The following examples, based on experimental cleaning operations conducted on tankers traveling from the New York area to Gulf ports, will serve to more clearly demonstrate the procedures involved in the new cleaning methods, but it is to be understood that these examples are given by way of illustration and not of limitation.

EXAMPLE I A tanker having 20 wing tanks of approximately 1,000 ton capacity and 10 center tanks of approximately 2,000 ton capacity had been carrying Bunker C oil and was now needed for higher grade cargo. Hot mechanical washing for extended periods of time, followed by hand hosing operations, failed to remove the heavy residue on the inner tank surfaces. Subsequent attempts to dissolve the residues by carrying cargoes of crude oil were unsuccessful. At this point, the captain agreed to conduct a test with applicants new cleaning method.

On a voyage between an east coast port and a Gulf port, the tanks were first washed and gas-freed by mechanical washing with hot (140160 F.) sea water for about 1 to 1% hours per tank, this operation being completed in about 2 days. After mechanical washing, examination of the tanks revealed the following:

Cargo Tanks Condition Floor Bulkheads, Frames, etc.

Center (No. 2 and N0. 10) Very dirty Cqlvlered with heavy deposits 23 Entirely covered with heavy oil film.

1e Center (No. 1, No. 4 and No. 6); Dirty Covered with deposits approx. 1" to Mostly cleanlarge areas covered with Port and Starboard (N 0. 4). 2" t k Remainder of tanks 55 gallon drum of cleaning agent within a period of about 1%. to 2 hours. With this type of 10 spray head assemblage, excellent results are obtained using an air pressure to the atomizing nozzle heads of 50 p.s.i. and a liquid pressure to the atomizers of 15 to 20 p.s.i.

In utilizing the spray assemblage in a tank which may be, for example, to feet deep, it is preferable to spray the cleaning material in three stagesfirst, at a point about 4 to 5 feet from the bottom of the tank; second, at approximately the mid-point; and finally, at a point about 6 feet from the top of the tank-discharging at each level about /3 of the total amount of cleaning material desired. Furthermore, the number of drops in a particular tank will depend upon the length and width of the tank. On a conventional tanker, for example, where the side or wing tanks are generally elongated lengthwise of the ship and normally equipped with 2 openings for mechanical washing apparatus, it is practical to make 2 drops with the atomizing units. In practice, this is done collectively, using 2 such units, in which event, a 55 gallon drum of cleaning material can be atomized within about to 60 minutes. In the larger center tanks on a conventional tanker, it is generally desirable to make 4 such drops. This can be done using 4 of the spray assemblage units simultaneously, or alternatively by using 2 such assemblages simultaneously, preferably dropping them at diagonally opposed positions in the tank and then switching them to the opposed diagonal positions. The larger capacity center tanks may, of course, require e Fairly good- Deposits of to 1 in some areas...

heavy oil film.

A liquid cleaning material consisting of 92 parts by volume of a mixed methylnaphthalene fraction having a boiling range of 400 to 550 F. and 8 parts by volume of a nonyl phenol ethylene oxide condensate containing 9 to 10 mols of ethylene oxide per mol of nonyl phenol supplied in 55 gallon drums, was then hand sprayed on inner tank surfaces using a 3-man crew per tankone spraying, one handling the hoses in the tank, and a safety man on deck tending the pump and equipment. The cleaning material was applied at the rate of 55 gallons per 1,000 ton tank, except in two instances (the dirty No. 4 wing tanks) where 82 gallons were used. In the 2,000 ton center tanks cleaning material was used in the amount of 137 gallons for the very dirty No. 2 and No. 10 tanks, gallons in the dirty No. l, 4 and 6 tanks, and 82 gallons in the other tanks. The tanks were ventilated with an air blower during the application of cleaning material, and the application required about 1%. hours per small tank and about 2 to 2 /2 hours per large tank.

After completing the hand spraying, a waiting period of 2 to 4 hours (or longer) was allowed to elapse, and the crew then again entered the tank to wash down the inner surfaces with cold sea water. The deposits were easily Washed from the tank surfaces, becoming emulsified in the water collecting at the bottom of the tank, which was pumped out and discharged into the ocean. The time required to rinse a tank varied from about 1% hours to about 4 hours depending on the tank size and the resi- 7 dence time of the chemical. When rinsing started within about 6 hours after application of chemical, it could generally be completed in about 1 /2 to 2 hours; but even when rinsing took place 2, 3, or 4 days after application of chemical, it could generally be completed in 3 to 4 hours.

The total elapsed time for cleaning the tanker after mechanical washing, i.e. from the start of chemical application to rinsing of the last tank, was 150 hours; and the consumption of the cleaning material being 53.5 gallons per 1,000 tons of tank capacity. The tanks were exceptionally clean and ready for the loading of gasoline or any other clean cargo.

In the cleaning and rinsing operations which were completed in a total of 150 hours elapsed time, the actual man'hours of labor in the six days were about 450 manhours. On four days three 3-man cleaning crews were working, and on the other days, two such crews were working. In reviewing the results of this test cleaning operation, the following conclusions were reached as to Ways of shortening the overall elapsed time and manhours of labor to accomplish the cleaning:

(a) By mechanical washing with water at about 110 F. instead of 14()l60 F. in the degassing operation, the time interval before tanks can be entered for applying chemical can be substantially reduced. Lower temperature also effects a substantial reduction in the steam requirements, and minimizes the reduction of the speed of the ship due to diverted steam.

(b) By using a four-man crew to apply chemical, particularly in the large center tanks, the application time and total man-hours involved can be reduced.

(c) By starting the rinsing of a tank after a chemical residence time of 2 to 6 hours, and preferably 2 to 4 hours, the rinsing time can be held to a minimum, effecting a saving of time and man-hours as much as 50% compared with the time required if the chemical residence time is 24 hours or more.

With these factors put in operation, it is estimated that the elapsed time for cleaning the ship (still working the conventional shifts) could be reduced to about 100 hours, and the total man-hours of labor could be reduced to about 300 man-hours.

EXAMPLE II A ship quite similar to that described in Example I was made available for testing a modification of the new cleaning method. This ship had 9 center tanks of approximately 1,900 tons cargo capacity each and 18 wing tanks of approximately 880 tons capacity each. Twenty-six tanks of 31,040 tons cargo capacity were to be cleaned, one center tank being clean. The prior cargo had been crude oil, and in a five-day trip from New York to Port Arthur, Tex., it was desired to prepare the tanks for loading of No. 2 oil.

All tanks were mechanically washed at 180 p.s.i. and 175 F., the operation being completed in approximately 28 hours with an expenditure of 157 man-hours of labor. After mechanical washing, the condition of the tanks was as follows:

In the wing tanks two atomizing units were operated simultaneously, at three levels-i.e., 4 feet from the bottom, mid-point, and 6 feet from the top of the tankspraying approximately 15 minutes in each position. In two, very dirty, wing tanks 82 gallons of cleaning material were sprayed as a wet fog, while all other wing tanks received gallons of chemical.

The center tanks received similar treatment except that to drops were made with the spray heads, first in one diagonal position, and then in the opposed diagonal position. In all center tanks, the amount of chemical used was 55 gallons per drop, or 110 gallons per tank.

The entire application of chemical was handled by one man, and involved 26 /2 man-hours of labor. The quantity of chemical applied amounted to 62 gallons per 1,000 tons of cargo capacity.

Rather than hand rinsing as in Example I, the rinsing was done with cold sea water fed through the Butterworth tank cleaning apparatus operated 30 minutes each at three levels in the tank. Three tanks were mechanically washed simultaneously following the order of chemical application, so that the residence time was at least two hours, but less than 6 hours. In this way, the rinsing was completed in 21 hours with two men handling the operation and shifting of hoses for the Butterworth apparatus (approximately 42 man-hours). In two wing tanks which had only one opening for inserting mechanical washing apparatus, slight hand rinsing was needed to complete the washing off of fore and aft ends which the jets of the Butterworth apparatus did not completely reach. All other tanks, however, were rinsed clean by the Butterworth apparatus.

The cleaning of all tanks was completed on the fourth day, nineteen hours before reaching port; and within one hour after reaching port, the tanks had been inspected and approved for the loading of a full cargo of No. 2 heating oil.

The total labor involved in this cleaning operation, after the initial mechanical washing, was approximately man-hours, only a small fraction of the labor required with the hand application and hand rinsing procedure of Example I. In addition, the overall time for chemical cleaning is substantially less in Example II than in Example I. Thus, the wet fog spray application of chemical and the mechanical cold water rinsing with the Butterworth apparatus is a distinctly preferable procedure.

The results obtained in the very dirty tanks of Example II were quite satisfactory, indicating that the procedure of Example II has general applicability. Even if extreme conditions were to require supplemental hand rinsing, or even supplemental application of chemical and rinsing of troublesome areas, the prolonged dispersing of chemical as a wet fog and the mechanical cold water rinse with the Butterworth apparatus is preferred as the first, and major, cleaning operation.

In both Examples I and II, the emulsion of chemical, removed deposits, and rinse water was discharged into the ocean without any indication of oil separation. The primary element of control would seem to be the application of enough chemical to emulsify all of the deposits Cargo Tanks Condition Floor Bulkheads, Frames, etc.

2 center, 6 wing Very dirty Covered with to 1" ol' 'i/oil film tops and sides,

thick oil.

to Q under beams.

5 center, 8 wing-.. Dirty Covered with M to of M to tops, sides and thic o1 1 center, 4 wing Fairly eleau Oil spots 34" to A thick..-

under beams. to M tops, sides and under beams.

Using the same cleaning material as described in Example I, this was applied by pressure atomizing as a wet fog using a multi-nozzle atomizing unit about 6 feet long, having at each end 4 angularly arranged and one axially arranged nozzles (the nozzles being Spraying Systems Company Spray Setup No. 29). The chemical was fed to the atomizing unit at a pressure l520 p.s.i. with air to the atomizing unit at 50 p.s.i.

use one or two of the cargo tanks as slop tanks to collect and store cleaning residue from all other tanks and to pump the residue to the storage tanks before emulsification was complete, i.e., with entrained lumps of sludge. Then after the ship is at sea, the stored residue can be discharged in two stages, first the emulsified portion, and then the additional emulsion formed by adding to residual sludge an appropriate amount of cleaning material and additional rinsing water. Such a storage use of one or two cargo tanks could, in some instances, be the most economical way of handling the disposal problem for in-port cleaning.

The initial mechanical washing to wash and gas free cargo tanks is normally desirable for two reasons. It removes a substantial amount of loosely adhering deposits on the inner tank surfaces, thus minimizing the amount of deposit to be removed chemically. Furthermore, the collected oil deposits are of value as a fuel supplement, or for further processing at a refinery. It should be understood, however, that if time were a factor (and realizing that the mechanical washing ties up much of a ships water circulating system for 1 /2 to 2 days) the initial mechanical washing can be eliminated, and cleaning material can be applied by prolonged dispersion as a wet fog directly to unwashed tanks. In such event, cleaning material must be used in greater amount, i.e. 70 to 80, or more gallons per 1000 tons of tank capacity. Chemical application time and rinsing time would be slightly greater than in Example II above when thus handling larger quantities of chemical and cleaning residues, but the overall saving in time and man-power by eliminating the initial mechanical washing could offset or justify the added cost of cleaning material required.

It is to be understood that generating a dispersion of cleaning material in the form of a wet fog in accordance with preferred adaptations of the present invention, has general applicability in the cleaning of large petroleum storage tanks, such as tanker cargo holds. Thus, application of any known liquid cleaning material by the novel prolonged wet fog generating technique will effect substantial savings in time and labor over conventional application methods, while at the same time, providing more complete and uniform coverage than is usually possible by manual application.

Various changes and modifications in the cleaning method herein disclosed may occur to those skilled in the art, and to the extent that such changes and modifications are embraced by the appended claims, they constitute part of the present invention.

We claim:

1. 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 optimum action of such cleaning material, and then washing with water to remove chemical and dislodged deposits from the 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 within the 10 to 40 micron range.

2. The improvement as defined in claim 1, wherein the cleaning material is discharged through a variably suspended atomizing unit, having a plurality of angularly disposed nozzles, said plurality of nozzles providing an atomizing capacity such that 55 gallons of cleaning material can be converted to a wet fog by said atomizing unit within a period of about 1 /2 to 2 hours.

3. The improvement as defined in any one of claims 1 and 2, wherein the cleaning material employed consists of to 94% by volume of a mixed methylnaphthalene fraction boiling within the range of about 400-500 F. having a flash point in excess of F., and 6 to 10% by volume of a liquid, water soluble, nonionic surfactant.

4. The improvement as defined in claim 3, wherein said surfactant is a C to C alkyl phenol ethylene oxide condensate containing about 9 to 10 mols of ethylene oxide per mol of alkyl phenol.

References Cited UNITED STATES PATENTS 2,023,496 12/ 1935 Todd 134-40 XR 2,748,084 5/1956 De Lew et a1 134-40 XR 2,911,371 11/1959 Weis 134-40 XR 2,937,112 5/1960 Boyer 134-40 2,952,571 9/1960 Freedman 134-22 3,030,238 4/1962. Cohn 13440 XR 3,078,189 2/ 1963 Baker 13440 XR 3,121,027 2/1964 Galanor 134-24 XR 3,281,269 10/ 1966 Watts 134-22 FOREIGN PATENTS 567,398 2/ 1945 Great Britain.

830,962 3/ 1960 Great Britain.

522,098 2/1956 Canada.

OTHER REFERENCES Butterworth; The Butterworth System, received in Patent Ofiice J an. 14, 1930*.

MORRIS O. WOLK, Primary Examiner.

J. T. ZATARGA, Assistant Examiner.

US. Cl. X.R. 134-24, 40