Sludge Formation in Distillate Fuel Oils U
Storage of distillate fuel oils to meet the seasonal demand leads to the formation of small amounts of tarry sludge which interfere with operation of equipment and increase servicing requirements. The nature of this sludge has been studied and the mechanism of its formation is discussed. Methods for determining sludge content and sludge stability of furnace oils are described.
T
H E petroleum industry’s view of distillate fuel oil as a product of secondary importance is being revised as sales continue t o grow with the use of Diesel engines and increased domestic heating by oil. Table I shows that during the last nine years the average yield of distillate fuel and gas oil from petroleum in the United States has increased about 65 per cent while the average yield of gasoline has remained substantially constant. Furnace oils as conventionally produced from petroleum by distillation and cracking contain no appreciable amounts of sludge or insoluble materials. However, subsequent storage is often accompanied by development of insolubles which later may deposit in the strainers of consumer’s equipment. Sludge difficulties have been encountered in the strainers of high-speed Diesel engines even though using light fuel oil (6). One of the advantages of household heating by oil is its dependability and the lack of attention required. This could be improved further by decreasing the tendency of the oil to form sludge. The nature of the domestic-heating oil market, where consumption reaches a high peak in winter and falls to almost nothing in summer, makes storage of oil a necessity. Refinery yields of furnace oil are cut down in summer but some
R. J. HAWES AND F. M. MILLER Tide Water Associated Oil Company, Bayonne, N. J. must be produced and stored, or midwinter demand could not be met. There is also the storage in consumer’s tanks which usually are filled in summer to take advantage of lower prices. These unavoidable storage periods provide the opportunity for sludge formation in furnace oils which may have been entirely satisfactory when originally produced. I n studying sludge formation, No. 2 fuel oil (A. S. T. M. designation D396-39T) has been taken as a representative distillate fuel which is used for both domestic furnaces and for Diesel engines. TABLEI. AVERAGEY I E L D FROM PETROLEUM STATES (5) Year
I N THE UNITED
Per C e n t b y Volume Dist. fuel a n d gas oil
Gasoline
Analyses of Sludges The gummy, tarlike appearance of typical sludgss from distillate fuel oils suggested the application of solubility studies as an initial approach to analysis. Solubilities in naphtha and benzene were determined on numerous samples, and the insoluble material was burned to ash. Wide variations in solubilities of the different samples shown in Table I1 were due partly to differences in sample handling and partly, as will be shown later, to differences in the extent to which the sludge-forming reactions had progressed. TABLE 11. ANALYSESOF SLUDGES FROM CUSTOMER’S STRAINERS I N P E R C E N T BY W E I G H T Sample designation Sol. in n a p h t h a (adherent oil a n d resins) Insol. in n a p h t h a , sol. in benzene Insol. in n a p h t h a or benzene
Ash
FIGURE 1. BATHFOR POTENTIAL SLUDGE TEST
A 58 35 3.6 3.4
B 67 16 10.0 7.0
C 45
42.7
6.4
6.9
D 85 12.5 2.0
0.5
The analyses of Table 11,following the pattern of the Marcusson analysis of asphalts (d), indicate the presence of benzene-soluble asphaltenes and benzene-insoluble carbenes and carboids. However, asphaltenes, carbenes, and carboids together constitute the sludge, and in subsequent work have been grouped as naphtha insolubles with no further . subdivisions. Oils which had formed naphtha-insoluble sludges were found to contain naphtha-soluble resins. These resins were determined by adsorption from the naphtha-soluble portions
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
October, 1941
on 300-mesh fuller’s earth which was added incrementally with agitation until no further color change resulted. The oil was then filtered off, the clay was washed with naphtha, and the resins were extracted from the clay with a mixture of half benzene and half ethyl alcohol, The solvent was evaporated off to a constant weight of resin. The ultimate analyses of typical naphtha-soluble resin and naphtha-insoluble residue are shown in Table 111. The similarity of these components to asphalt constituents is substantiated by their carbon-hydrogen ratios which correspond closely with the values of 7.9 and 10.2 given by Thurston and Knowles (4)for asphalt resins and asphaltenes from Mexican asphalt. TABLE111.
ULTIMATn
ANALYSES OF
Component Carbon % by wt. HydroLen % by wt. Ash, % b; wt. Sulfur 7 0 b wt Oxygen, % g y wt. (b.y difference) Carbon-hydrogen ratio
1319
the rate of oxygen absorption and the distribution of the oxygen among the various products of the reactions. From their results they postulated a reaction mechanism involving the successive steps of oxidation, dehydration, and decarboxylation followed by polymerization. The general reactions occurring may apparently be described as oxidation-condensation type. Hillman and Barnett ( 1 ) showed that the molecular configuration changes from “alkyl aromatics of one or two rings to increasingly more condensed polyring aromatics with fewer and shorter side chains” as the lowmolecular-weight resins progress to asphaltenes, carbenes, and carboids.
SLUDGE COMPONENTS
Naphtha-Insol. Sludge
Naphtha-Sol. Resin
60.2 6.3 19.5
81.2 10.3 1.0 7.5 7.9
t:tl 9.6
Mechanism of Sludge Formation The ultimate analyses of resin and sludge (Table 111) show considerable oxygen, and this indicates that oxidation must be involved in their formation. Their likeness to asphalt resins and asphaltenes suggests that their behavior and reactions should be similar to those of naturally occurring asphalt components. Thurston and Knowles conducted oxidation experiments with various asphalt constituents, measuring AT2OBd / %
FIGURE2. SLUDGE FORMATION vs. TIMSO UNDER CONDITIONS OF POTENTIAL SLUDGE TEST A . Cracked gas oil B . Blend of 40 per cent cracked gas oil and 60 per cent straight-run gas oil
The compositions of the resins and sludges from consumers’ strainers indicate that a similar progressive oxidationcondensation is involved in their formation and accumulation. The mechanism appears to be that essentially oxygen-free hydrocarbons oxidize and condense t o resinous material, TABLE IV. ANALYBES OF LABORATORY RESINAND SLUDGES PREPARED FROM CRACKED GAS OIL
Carbon % by wt. Hydrogh % by wt. Oxugkn.%‘b Sulfur % {Y wtwt. (by difference) Ash 7 by wt. Carbon-hydrogenratio
f
NaphthaSol. Resin
NaphthaInsol. Sludge A
NaphthaInsol. Sludge B
81.0
79.1
71.6
8.5 0 7.7
12.5
10.5
8.4
Trace 9.4
r
6.8
i1$:; 7.1 10.6
3
which further reacts in similar manner to form the more insoluble sludge. This hypothesis applies also to resins and sludges produced by accelerated oxidation of distillate fuel oils in the laboratory by procedures described later. Analyses of such materials made from cracked gas oil are shown in Table IV.
Determination of Sludge Content DISTILLATION UNITAT TIDEWATER’SBAYONNE REFINDRY
I n the usual examination of distillate fuels no test is included which indicates the presence of sludge in low concentra-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1320
tions. The test for water and sediment by means of centrifuge (A. S. T. M. designation D-96-35) cannot measure sludge in concentrations of less than 0.01 per cent. This value is of the order of ten times that which may cause strainers to plug. The method also uses benzene as a solvent so that material insoluble in the oil may be dissolved by the solvent and escape detection. Sludge content was determined in this work by a simple filtration procedure. A relatively large volume (1.5 liters) of oil is filtered through a Gooch crucible prepared with asbestos and Celite analytical filter aid; the residue is mashed with A. S. T. M. precipitation naphtha and dried to constant weight a t 320" F. (160" C.). The naphtha-insoluble sludge is reported in parts per million by weight. The naphtha wash is necessary to remove adherent oil and to maintain a standard basis for all determinations. The naphtha-soluble resins removed by this wash are not considered as sludge. Values obtained by this procedure are referred to as preformed sludge. Small concentrations of sludge are usually found in distillate fuel oils even though freshly distilled and blended. This is due to mechanical entrainment, contamination with pipe scale, atmospheric dust, etc., as well as to contamination during sampling and sample handling. Complete elimination of such Contamination has been found difficult and unnecessary, since troublesome amounts of sludge are at a distinctly higher level.
Oil Stability Tests Sludge content after storage consists of the original insolubles plus sludge formed by oxidation-condensation of some constituents of the oil, as shown from the foregoing analyses. Considerable work has been done to develop accelerated storage tests for studying the mechanism of sludge formation and the relative stabilities of refinery stocks and t o assist the study of the prevention of sludge formation. The first stability tests were carried out in a manner similar to that conventionally used to determine the gum stability or induction period of gasoline (100 ml. under 7 kg. per sq. cm. oxygen pressure a t 100" C., A. S. T. M. designation D525-39T). With cracked distillate fuel oil no pressure drop indicative of the end of the induction period was found within any reasonable time. This test also has the disadvantage that the conventional equipment does not accommodate enough charge to permit determination of the final sludge content of the oil after the bomb test.
Vol. 33, No."lO
in a boiling water bath for 24 hours and then determining its content of sludge. A 2-liter Pyrex beaker (approximately 125 mm. in diameter and 190 mm. high, such as Eimer & Amend catalog No. 17,614) containing the oil under test is suspended by its lip in boiling water as shown in Figure 1, with the bottom of the beaker 3 inches (76 mm.) below the water level. A bath temperature of 212" F. (100" C.), a t normal barometer, is maintained by open steam admitted through a perforated pipe in the bottom of the bath, regulated t o give moderately vigorous agitation of the water. The oil rapidly reaches and maintains a temperature of 208 =t1' F. (97.8 * 0.8" C.). The beaker is left uncovered to allow free circulation of air to the oil surface. After 24 hours the beaker is removed and cooled t o 100" F., and the sludge content of the sample is determined as in the "preformed" sludge test above. This sludge, as parts per million based on the weight of original sample, is reported as potential sludge. Reproducibility of both preformed and potential sludge tests is shown in Table VI for which one oil was tested on several different days. The reproducibility of * 10 per cent probably is susceptible t o improvement by further studies of the causes of error but has been found adequate for most purposes.
T.4BLE
VI. REPRODUCIBILITY O F SLUDGE TESTSO X
USSTABLE
CR4CKED G 4 S O I L Preformed Sludge, P. P. M. 16.5 16.3 16.5 20.7 18.0 18.0
Potential Sludge, P. P. M. 214 209 236 212 234 232
Interpretation of the potential sludge data requires further knowledge of the sludge-forming reaction. This knowledge is advanced by curve A , Figure 2, which shows sludge formation as a function of time under conditions of the stability test for a cracked gas oil which is somewhat more stable than that used for the data of Table VI. Curve B of Figure 2 shows the rate of sludge formation for a typical blended oil. It would be ideal to have curves of this type for each oil tested, but the much shorter met'hod of determining the stability after 24 hours has been found adequate for most purposes.
Literature Cited TABLE v.
SLUDGE FIED
FORMED FROM REFINERY STREAMS (MODIINDIANA OXIDATIONTEST)
Kerosene Straight-run gas oil Cracked gas oil
yo by Weight Naphtha-insol. Naphtha-sol. sludge resin 0.075 0.50 0.490 0.60 1,121 8.13
The next stability method tried employed a temperature of 341" F. (171.7" C.) in equipment similar to that used for Indiana oxidation tests ( 3 ) on lubricating oils. Results obtained in a 96-hour test with air injected during the first 16 hours only are shown in Table V. The values obtained were not easily reproducible and were so high that comparisons with actual storage at atmospheric conditions were questionable. A more useful sludge stability test which is less severe and gives values of the same order of magnitude as found in storage consists of holding a larger volume of oil (1500 ml.)
(1) Hillman, E.
S.,and Barnett, B.. Refiner Natural
Gasoline M f r . , 18,533 (1939). (2) Nlarcueson, J., Burchaytz, H., and Wilke, P., "Die natiirlichen und kiinstlichen Asphalte", p. 92,Leipsig, Verlag von Wilhelm Engelman, 1931. (3) Rogers, T. H.,and Shoemakei, B. H., IND.EKG.CHEW,ANAL. ED., 6, 419 (1934). (4) Thurston, R. R.,and Knowles, E. C., 1x11. ENG.CHEM..28, 88 (1936). ( 6 ) U. S.Bur. of Mines, Market Repoits. (6) Wilson, G. O., Proc. A m . PetroZeumI%st., 111, 20,33 (1939).
