I
L.
H. DIMPFL, J. E.
GOODRICH, and
R. A.
STAYNER
California Research Corp., Richmond, Calif.
Evaluating Additives for Distillate Fuel Oil Storage Tests
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s
TORAGE instability of distillate fuels containing unstable cracked gas oils is a relatively recent problem. In the past demand for these products has been met by straight run distillates, which were sometimes chemically treated to improve their color stability or odor; but in general they were stable in storage. During World War I1 the demand for aviation gasoline and other cracked products was met by increases in refinery catalytic cracking capacity, requiring large volumes of straight run gas oils as charging stock. This resulted in a n increase of cracked gas oils in distillate fuels. Hill (4)has estimated that 70% of the distillate fuel oil on the market is cracked. Untreated cracked gas oils and their blends with straight run distillates deposit gums during storage at ambient temperature. These gums are higher in nitrogen and sulfur content than the oils from which they are deposited (8,9 ) . They often contain metals (3)and may be products of oxidation of the fuel, metallo-organic soaps, or condensation or polymerization products ( 5 ) . They clog filters, pump screens, and nozzles in fuel systems and in genergil cause diffi-
culties in service I n many fuel systems, water condensed from the air in the storage tank or introduced by accident aggravates the situation because these gums agglomerate at oil-water interfaces forming membranes of hydrated gum which readily clog screens and filters (7). The use of additives has economically solved the storage stability problem of untreated cracked gas oils. As a result, the demand for additives is great. According to Larson ( 6 ) ,approximately 12,000,000 pounds of distillate fuel additives were consumed during 1955. T o satisfy this demand, well over 60 additives for distillate fuel, claimed to be sludge inhibitors and/or dispersants, have been marketed. Some essentially eliminate sludge deposits, while others either have no effect or actually reduce storage stability. The situation is further complicated by the fact that an additive may perform quite effectively in one fuel but not in another. This poses a problem to the refiner or fuel oil distributor who desires an additive for his fuels. He needs a method of determining whether or not a
given additive will perform satisfactorily in his stocks, and if so, what the optimum dosages are. Also, a research laboratory wishing to develop an improved fuel may have to select the best compound from hundreds. T o do this, reliable laboratory test methods are necessary. Unfortunately, there are no industrywide standard test methods for storage stability of distillate fuels, either with or without additives. Two of the methods developed by California Research Corp. for this purpose are described here. In particular, this report shows how these methods may be used to evaluate the relative effectiveness of commercially available distillate fuel additives.
Filter Residue of Fuels Probably the most widely used test method for distillate fuels is determination of insoluble solids content. These insolubles may be rust or dirt or gums deposited from the oil. Many methods have been proposed for determining gravimetrically the filterable material i n distillate fuels. These VOL. 48, NO. 10
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Figures 1 and 2.
Response of gas oils, aged 24 weeks, to increases in storage temperature
differ in sample size, filter media, application of vacuum or pressure, determination of amount of deposit, and method of reporting results. Among the unique features of the filter residue method which we describe is the use of an asbestos pad filter and a correction for fuel-soluble polar compounds adsorbed by the asbestos. Asbestos is superior to many other filter media in that it is a n inert substance and forms a filter pad of relatively small pore size which can readily be reproduced. Procedure. A 500-ml. sample of gas oil is filtered with suction through a
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tared Gooch crucible containing a layer of asbestos. The deposit is rinsed with 500 ml. of petroleum ether, the crucible dried a t 190' F. for 2 hours, placed in a constant humidity vessel a t room temperature for 5 hours, and weighed to 0.1 mg. The filtered gas oil is refiltered through a second Gooch crucible which is rinsed, dried, and weighed as above. A correction is then made for the fuelsoluble material adsorbed by the asbestos filter pad. The corrected filter residue is calculated as follows:
INDUSTRIAL AND ENGINEERING CHEMISTRY
where F
= filter residue, p.p.m. by
weight
r.V, = increase in weight of first crucible, mg. W2 = increase in weight of second crucible, mg. A I = weight of asbestos pad, first crucible, g. Az = weight of asbestos pad, second crucible, g. 5' weight of fuel sample, g.
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The results are reported as parts per million of filter residue by weight (approximately equal to 12 times the mg. of deposit per 100 mL).
ADDITIVES For filter residues less than 90 p.p.m., the 95% confidence limits of the filter residue method are f 3 p.p.m. between different operators. Above 90 p.p.m. the reproducibility is somewhat poorer.
Accelerated Aging of Fuels Storage stability of distillate fuels and the effects of a given additive on this stability are most reliably determined by allowing the fuel to age a t a temperature approximating that a t which the fuel is stored in practice. Fuel-insoluble material deposited should then be measured as a function of time. As this requires a relatively long period, it is not suitable for routine testing or process control. Therefore, it is necessary to shorten the test period by accelerating the aging conditions, usually by raising the temperature at which the fuel samples are stored. Duval (2) has pointed out one difficulty-Le., that the different chemical reactions which form filter residues are not accelerated equally by increase in temperature. This is especially evident in distillate fuels of different crude sources, but it is also observed in fuels from the same crude processed differently in the refinery. T o illustrate this a number of gas oils were aged for 24 weeks at temperatures ranging from 70 to 140' F., and periodic determinations were made of the insoluble material which formed during storage. The stocks responded quite differently to increases in storage temperature as shown for two of the gas oils in Figures 1 and 2. Figure 3 is a semilog plot of the amount of filter residue formed during 4 weeks' storage as a function of the storage temperature. For gas oils, A, B, C, and D, straight lines can be drawn through the experimental points. The deterioration of these fuels can be expressed by log X t =
K
given set of conditions provided a method equally reliable for all base stocks. The test method should be as short as possible, yet the test should be made under conditions not too far removed from normal storage. The conditions selected were a storage time of 4 weeks at 140" F. These conditions have proved satisfactory for testing gas oils during the last 5 years. Because most distillate fuel systems contain some water, the test method provides for the determina-
IN FUELS
tion of the stabilities of distillate fuels under either wet or dry storage conditions.
Procedure. A 500-ml. sample of fuel is filtered through filter paper into a n unstoppered, I-quart glass bottle and stored at 140' F. for 4 weeks. For storage over water, 10 ml. of distilled water is added a t the beginning of the storage period. After storage the sample is filtered through a tared Gooch crucible.
+ K'T
where X,is the amount of filter residue formed during time t a t temperature T , and K and K' are constant characteristic of the fuel. These four gas oils, A through D, all contain catalytically cracked stocks from West Coast crudes, and the data suggest that they deteriorate by similar mechanisms. For gas oils such as E, rate of deterioration cannot be expressed as a simple function. I n such cases classical kinetic analyses are not possible. Gas oils E and F contain catalytically cracked stocks from a Rocky Mountain crude. Gas oil G is a West Coast straight run product. These factors were considered when the conditions for an accelerated aging test were selected. We realized that no
Figure 3,
Filter residue formed during storage VOL. 48, NO. 10
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Figure 4.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Full scale pumping test equipment
A D D I T I V E S IN FUELS as in the filter residue procedure. The filtrate is saved for a second filtration to determine the adsorption correction. The material adhering to the walls of the bottle is dissolved in 25 ml. of an 80/20 benzene-ethanol solvent, the gums are precipitated by the addition of 500 ml. of petroleum ether, and this suspension is filtered through the original Gooch crucible. The crucible is washed, dried, and weighed as in the filter residue test. The results are reported as parts per million of filter residue by weight.
Full Scale Pumping Test Although the accelerated aging test agrees in general with service results, the most desirable test to measure, or predict, the screen plugging tendency of a gas oil is one which actually involves passing the fuel through a screen. Many pumping tests have been proposed in the past, but these usually depend on some kind of accelerated aging procedure which speeds up the test but which leaves correlation with service open to question. The pumping test herein reported simulates a domestic furnace oil fuel system having an above ground storage tank. I t requires substantial volumes of fuel (100 gallons per test) and a relatively long time (several months) but yields results which are more reliable than any obtained from accelerated procedures.
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150
STORED DRY STORED OVER WATER
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100
150
250
200
ADDITIVE CONCENTRATION, PPM
6o
STORED DRY STORED OVER WATER
50 Procedure
n
The pumping test equipment, shown in Figure 4, consists of 12 identical fuel systems each composed of a 120-gallon domestic fuel storage tank, a Webster fuel unit having a 100-mesh screen and 3/8-inch copper suction and return lines 8 feet long. The suction line extends within 2 inches of the tank bottom. At the start of the test each tank is charged with 100 gallons of fuel plus 1 quart of distilled water to simulate actual wet storage conditions. The fuels are circulated by the pumps (at about 20 gallons per hour) for a 9-hour period each day. Significant differences in amount of screen deposit may be detected after only 2 weeks, but a longer time is required to build up substantial deposits. At the end of the test period the pump screens are removed and photographed, and the deposits are extracted and weighed. Uncompounded gas oils which form large amounts of filter residue during 4 weeks at 140' F. also coat the pump screens with deposit in the full scale pumping test. However, some additives are more effective in preventing gum deposition on screens in a circulat-
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ADDITIVE CONCENTRATION, Figures 5 and 6. Response of gas oils to additives. Figure 6. Gas oil J ,VOL 48, NO. 10
250
200
150
PPM
Figure 5.
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Gas oil H
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Figure 7.
Pump screens after 16 weeks' operation-Gas
oil
H
No additive
Additive 2 (250 p.p.rn.)
Additive 1 (250 p.p.rn,)
Screen deposit, 1 1 1 1 mg. Filter residue of fuel after 4 weeks at 140" F., 104 p.p.rn.
Screen deposit, 174 mg. Filter residue o f fuel after 4 weeks at 140" F., 4 0 p.p.m.
Screen deposit, 17 mg. Filter residue after 4 weeks at 140" F., 13 p.p.rn.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ADDITIVES IN FUELS ing system than in preventing the formation of fuel-insoluble gums. Thus screen deposit is often lower than would be predicted from the 140’ F. filter residue test when additives are present.
Evaluating Commercial Distillate Fuel Additives The distillate fuel additives on the market vary widely in their chemical composition. They may be divided into two general classifications-metallic and nonmetallic (7). The metallic additives include compounds such as barium or calcium sulfonates or phenates and the nonmetallic additives include organic amines and polymeric compounds. Most of the screening evaluations of the wide variety of additives were carried out using the 140’ F. aging test. The more promising additives were also evaluated in the full scale pumping test. Some of the results in the 4 weeks’ aging test at 140’ F. are shown in Figures 5 and 6. These figures show the effectiveness of two additives as a function of dosage in two base stocks, stored both dry and over water. They show that the effectiveness of an additive in preventing the formation of filterable gum depends to a large degree on the particular base stock used and whether the fuel is stored dry or over water. Gas oils H and J are 50/50 mixtures of catalytically cracked, light cycle oils and straight run oils. Gas oil H is from a West Coast crude, whereas gas oil J is from a Rocky Mountain crude. Using 25 p.p.m. of filter residue formed during 4 weeks a t 140” F. as a dividing line between stable and unstable gas oils, only additive 1 performed satisfactorily in gas oil H. I t would be expected to give satisfactory performance in this fuel a t a dosage of 50 p.p.m. (15 pounds per 1000 barrels of fuel). I n gas oil J additive 1 at 125 p.p.m. and additive 2 a t 25 p.p.m. would be expected to perform satisfactorily in a dry system. These results are presented as examples for two particular base stocks and, in view of the variation in additive response of fuels from different sources, would not be expected to hold true for all gas oils. I n the present state of knowledge, the only way of determining whether a given additive will perform adequately in a given fuel is to test that particular combination. Until the mechanism of gas oil deterioration is better understood, it will probably not be possible to “tailor make” a n additive to fit any given situation. Sometimes a t low additive dosages the amount of filter residue formed during accelerated aging is actually
greater than that of the uncompounded fuel, but a t higher additive dosages the fuel may be effectively stabilized. I t is possible that in a system where the concentration of gum particles is high relative to that of the additive, the latter may be precipitated from solution along with the fuel gum. At higher additive concentrations the oil solubilizing action of the additive peptizes the gum and prevents its precipitation from the fuel. Figure 7 shows the results of full scale pumping tests in which the use of additives brought about marked reductions in screen deposits. Although 1111 mg. of screen deposit was collected from the uncompounded fuel (gas oil H) during 16 weeks’ operation, this was reduced to 174 mg. by compounding with 250 p.p.m. of additive 2. Additive 1 reduced deposit to 17 mg. These results are consistent with the filter residue formed in samples of the fuels during 4 weeks’ aging at 140’ F.
Other Fuel Properties Affected by Additives This discussion concerns the effects of additives on the rate of formation of gums in distillate fuels during aging, because this is one of the most critical factors determining the service performance of a fuel. There are other fuel properties, however, which may be affected by additives. One of these properties is rust protection. Distillate fuels themselves are not corrosive to steel surfaces; but water, which is usually found in fuel systems, may cause these surfaces to corrode. An additive-containing fuel can protect those steel surfaces of the fuel system which contact both fuel and water from corrosion. At the bottom of storage tanks, however, where water settles, oil-solu ble additives may not be effective. Where tank bottom corrosion is a problem, water-soluble anodic-type inhibitors are effective. Because these additives are oil insoluble, they cannot be introduced into the fuel before it is delivered to the customer but must be added to the individual storage tanks. Color stability of distillate fuels i s not related to service performance; but fuel distributors prefer light-colored fuels, so color stability is an important marketing requirement. As is true of gum inhibition, additives vary in their effect on different stocks-Le., some additives improve the color stability of one fuel but have a deleterious effect on the color stability of another. We have found no direct relation in a given fuel between the relative effectiveness of a n additive as a gum inhibitor and as a color stabilizer.
Numerous types of additives other than gum inhibitors or dispersants are available for distillate fuels. These include such additives as combustion improvers and soot destroyers. In future years the trend will probably be toward the use of blends of various types of additives to impart to fuel oils all the properties necessary for optimum performance.
Summary Two test methods have been presented for evaluating the effectiveness of distillate fuel additives: one, a laboratory test involving the determination of the insoluble materials which form in a fuel during 4 weeks’ storage a t 140’ F. and the other a full scale pumping test carried out under conditions closely approaching those found in domestic fuel systems. Using these methods, it has been shown that additives may be remarkably effective in stabilizing distillate fuels which would otherwise deposit insoluble gums during storage and give unsatisfactory performance in service. These additives may be quite specific in their action and vary in their effectiveness. Optimum dosages depend on the fuel and whether or not free water is present in the fuel system.
Literature Cited (1) Chem. Week, 7 2 , No. 7, 65 (1953). (2) Duval, C. A., Petroleum Rejner 30, No. 2, 85 (1951). (3) Hershberger, A. B., Cowles, H. C . , Zieber, B., IND. END. CHEM. 35, 1104 (1943). (4) Hill, J. B., Advances in Chemistry Series, No. 5 , p. 249, AMERICAN CHEMICAL SOCIETY, 1951. ( 5 ) Korb, E. L., Scheule, H. J., Bender, R. O., Oil Gas J . 47, No. 49, 100 (1949). (6) Larson, C. M., Petroleum Engr. 2 7 , No. 3, C44 (1955). ( 7 ) Stayner, R. A., Dimpfl, L. H., Fuel Oil &3 Oil Heat 12, No. 12,71 (1953). (8) Thompson, R. B., Chenicek, J. A., Druge, L. W., Symon, T., IND. ENG.CHEM.43,935 (1951). (9) Thompson, R. B., Druge, L. W., Chenicek, J. A , , Zbid., 41, 2715 (1949). RECEIVED for review October 21, 1955 ACCEPTED May 4,1956 VOL. 48, NO. 10
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