US 3850206 A
A layer of an aqueous foam is applied to the surface of crude oil during loading into vessels or tankage thereby preventing loss of hydrocarbon vapor and the resultant air pollution as the tank is filled.
Claims available in
Description (OCR text may contain errors)
United States Patent [1 1 Canevari et al.
FOAMED VAPOR BARRIER Inventors: Gerard P. Canevari, Cranford;
William M. Cooper, Jr., Basking Ridge, both of NJ.
Exxon Research and Engineering Company, Linden, NJ.
Filed: Dec. 6, 1972 Appl. No.: 312,580
[1.5. CI. 141/1, 21/60.5, 220/26 R Int. Cl B65b 3/04 Field of Search 21/605, 60.5 A; 61/5;
[ 1 Nov. 26, 1974  References Cited UNITED STATES PATENTS 2,907,627 10/1959 Cummings 220/26 R Primary ExaminerHouston 8 Bell, Jr. Assistant Examiner-Frederick R. Schmidt Attorney, Agent, or Firm-Harold N. Wells [5 7 ABSTRACT A layer of an aqueous foam is applied to the surface of crude oil during loading into vessels or tankage thereby preventing loss of hydrocarbon'vapor and the resultant air pollution as the tank is filled.
6 Claims, 4 Drawin glligures FOAMED VAPOR BARRIER BACKGROUND OF THE INVENTION Crude oil, although commonly thought to be heavy in nature, contains a wide spectrum of hydrocarbons ranging from those having very high boiling points to those which would exist as vapor if they were isolated. Owing to the presence of these latter mentioned volatile materials, a crude oil has a significant vapor pressure and generally will produce a substantial amount of vapor. When a tanker is filled the tanker compartment being filled initially contains air which has been drawn in when the compartment was last unloaded. Alternatively, the tank may have been purged by air or inert gases. Vapor is released when crude oil is pumped into an empty compartment. As the compartment is filled, this vapor is pushed out along with the air. Significant hydrocarbon losses, perhaps on the order of 0.02 percent occur during such filling operations. The economic loss is, of course, substantial when large amounts of crude oil are continually being loaded and unloaded. In addition, some air pollution may be created in the immediate vicinity owing to the discharge of these vapors. Although the vapors could be trapped and recovered, in general, it is not economically practical to provide recovery facilities for these vapors based solely on the limited value of the vapors recovered. In the past, such facilities have not been provided. With increased emphasis on reducing air pollution, provision may have to be made to prevent the discharge of vapors from loading operations. Also, under adverse atmospheric conditions, the concentration of such vapors may present a distinct safety hazard as well as an air pollution problem. Consequently, means are needed to recover those vapors which escape during loading operations. Such a method, owing to the relatively low value of the materials lost, must be as inexpensive as possible and yet at the same time perform satisfactorily.
Prior art methods which might be considered include the use of floating roof tanks such as are commonly used for highly volatile hydrocarbon fuels. In these tanks no vapor space is present, the tank roof merely floating up and down on the top of the surface of the liquid, thus minimizing the discharge of any vapors from the contents of the tank. Some vaporization and losses do occur, resulting from escape of vapors around the seal between the roof and tank shell and from the clingage to the tank walls. Floating roofs are expensive and ordinarily only employed where a serious safety hazard is involved, such as the formation of explosive mixtures within the tank.
Another possibility has been suggested by US. Pat. No. 3,146,060, i.e., the application of floating layers of microspheres to cover the surface of the oil. This technique has several disadvantages. The oil tends to wet the microspheres and escape through passages between the individual spheres. Further, the spheres are solids and thus difficult to remove from the tank and they may interfere with handling of the oil should they be entrained with the hydrocarbon materials. Although inherently cheaper than a floating roof, the actual cost of providing a floating roof of microspheres would be substantial. Another possibility is suggested by US. Pat. No. 3,639,258 which is the use ofa gelling material to create an integral roof by gelling the upper layer of the oil in a tank to provide a floating roof of the same material. This technique, while useful for some purposes, creates considerable difficulties in cleaning and maintaining tanks or tanker compartments.
The disadvantages of the prior art are principally those of high cost, difficulties in handling and disposing of added materials and inability to conform to irregular surfaces and obstructions inherently present, particularly in tanker compartments. A means of providing a simple inexpensive and easily disposed of covering for the surface of crude oil during loading would be most desirable. The novel technique disclosed herein satisfies all these requirements and provides a substantial advance in the art.
SUMMARY OF THE INVENTION A method of preventing discharge of hydrocarbon vapors during the loading of vessels or tankage comprises applying immediately prior to loading, a vapor barrier formed of aqueous foams which are stable for periods corresponding to that required for loading. Thereafter, the foams may be broken intentionally or permitted to gradually break down. They are disposed of with the off-loading of the oil since they contain only a very small quantity of water and extremely minute quantities of the foam forming agents.
For relatively cold crude oils a number of available foaming materials may be used to provide a suitable barrier which will prevent vapor discharge. Warmer crudes such as are frequently experienced will cause premature degradation of a foam suitable for cold crudes. Consequently, one feature of the present invention is the addition of a special foam stabilizer which maintains the foam intact and effective until the end of the loading period.
The foams disclosed more completely hereinafter are simple to apply, inexpensive and easily disposed of as part of the crude cargo. Essentially no hydrocarbon vapor is lost during the loading operation when applying this technique. Consequently, savings are made in retaining hydrocarbon materials otherwise lost and in avoiding the installation of expensive vapor recovery facilities. Further, no atmospheric pollution occurs.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows the schematic operation of the present invention during the oil-loading process.
FIG. 2 shows graphically the substantial improvement in hydrocarbon loss prevention which is obtained with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT The method by which the invention will be used in loading crude oil cargoes aboard tankers is illustrated in FIG. 1. Although a tanker compartment is illustated, and perhaps is the most useful mode of application of the invention, other uses will be immediately evident in view of this disclosure. For example, the method may be applied to shore tanks which receive the crude oil pumped out of the tanker compartments. The upper sketch illustrates an empty tanker compartment 10 prior to the admission of oil thereto. Foam 15 will be ordinarily applied through existing openings 16 used for the introduction of cleaning apparatus. A layer 17 of aproximately 3 to 12 inches, preferably about 6 inches of foam, is applied to the compartment. Although 3-12 inches of foam is sufficient to prevent significant vapor loss, more foam may be needed in order to assure complete coverage of the surface when the tanker compartments are filled non-uniformly owing to the internal structural members. Oil is introduced beneath the foam layer through the loading line 12 as shown in FIG. 1(b). As crude oil enters, the foam layer 17 rises and conforms to the irregularities present in all tanker compartments, sealing off effectively the surface of the crude which is being introduced. Hydrocarbon vapors are chemically dissimilar to the foam and do not readily pass through it. The vapor space is continually vented through outlet vent 14 as the liquid level risesv Since essentially no hydrocarbon reaches the vapor phase, none is vented during loading operation, requiring no recovery facilities and avoiding localized air pollution from hydrocarbon vapors.
FIG. 1(c) illustrates the tanker compartment when the loading process is being completed. If the loading process is continued any foam which passes out the vent 14 will be broken by contact with a foam breaking device located in or near the vent but, if not, the foam will naturally break over a relatively short period of time so that when the cargo is unloaded it will contain only very minor traces of water and the foam forming compound, which will enter the refining process and effectively disappear. A typical foam breaking device is a mesh screen comprising a hydrophobic material. As an alternative, injection of a defoaming agent, e.g. silicones may be employed.
FIG. 1 shows the results of experimental data taken when applying the invention to the loading process. The graph plots the amount of hydrocarbon in the vapor vented versus the fraction of the space which has been filled, illustrating the remarkable effectiveness of the foam in preventing hydrocarbon vapors from breaking through into the vented air. Without the foam, in the prior art, it can be seen that the amount of hydrocarbon reaches substantial levels once the tank is approximately 50 percent full repesenting significant losses and potentially serious air pollution in the immediate locale. When the foam is present there is no change in the hydrocarbon content of the vented oil illustrating the ability of water generated foams to suppress vaporization.
The foaming materials which can be used include any of a number of commercially available foam forming materials, for example, a 6 percent aqueous solution of Aerowater (a trademark of National Foam Co.) has been found to be effective. Others which may also be used, but which are not intended to limit the scope of the usable foaming materials, include fatty acid salts, e.g. sodium laurate, sodium myristate, and dodecyl sodium sulfate.
It has been discovered that, while many foam solutions may be satisfactorily applied to crudes handled at relatively low ambient temperatures, that these foams are not sufficiently stable in contact with naturally warm or heated crudes. In order to retain their integrity for the entire loading process, which may take as much as 20 hours, it has been discovered that the addition of small amounts of water thickeners may be added to the foam forming material before the foam is generated in order to strengthen the foam so that it will retain its effectiveness during the entire loading period. An example of such an application is in the use of 0.25 percent of a commercial water thickener (Kelzan produced by Kelco Co. of Clark, NJ.) to the 6 percent aqueous solution of Aerowater foam which has been mentioned above. Other water-soluble polymers could also be used to increase the viscosity of water, e.g. polyacrylamide, polyoxyethylene'and sulfonated polystyrene. A substantial improvement in durability was obtained by adding the water thickener, which apparently operates to effectively prevent drainage of water from the bubble films which comprise the foam structure. However, the amount of water thickener added must be carefully selected in order to improve the stability of foam. It has been found that an excessive amount of water thickener will result in an embrittlement of the foam and a loss of effectiveness since the foam in bulk may be broken. In contrast, most foams fail by drainage ofwater from the bubbles under the influence of gravity which causes the film to thin out until the bubbles rupture. Thus, an optimum amount of water thickener should be applied.
The scope of the invention is not to be limited by the description of the preferred embodiments given heretofore, but only by the scope of the claims which follow.
What is claimed is:
l. A method of suppressing vaporization of crude oil during the loading of substantially hydrocarbon-free containers comprising the steps of supplying a foam formed from an aqueous solution of a foam forming agent to said container sufficient to cover the bottom and having a depth adequate to serve as a barrier to bydrocarbon vapors formed from the crude oil during loading, loading said crude oil under said foam, and venting hydrocarbon-free vapor from above said foam during loading of said oil.
2. The method of claim 1 further comprising the step of adding a water thickener to said foam during its generation thereby to stabilize said foam.
3. A method of loading crude oil into tankers comprising:
a. supplying sufficient foam formed from an aqueous solution of a foam forming agent to each substantially hydrocarbon-free tanker compartment to cover the bottom thereof and having a depth adequate to serve as a barrier to hydrocarbon vapors formed from the crude oil during loading;
b. loading crude oil into each of said compartments under said foam; and
c. venting hydrocarbon-free vapor from the space above the crude oil and foam during the loading process.
4. The method of claim 3 further comprising the step of breaking a portion of the foam during the final stage of the loading by forcing said foam through a foam breaking means.
5. The method of claim 4 wherein said foam breaking means comprises a mesh screen of a hydrophobic material.
6. The method of claim 3 further comprising the step of adding a water thickener to said foam during its generation thereby to stabilize said foam.