US2793171A - Inhibitor sweetening of straight-run heating oils containing added olefins with a phenylene-diamine, alkali and oxygen - Google Patents

Description

United States iPatent l INHIBITOR SWEETENING F STRAIGHT-RUN HEATING OILS CNTAINING ADDED OLEFINS IvPI-IENYLENE-I)IAMINE, ALKALI AND James L. Keller, Brea, Calif., assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application April 22, 1954, Serial No. 424,921

4 Claims. (Cl. 196-29) This invention relates to 'the sweetening of straight-run heating oils, and other high-boiling, saturated fractions, such as may be derived by distillation of petroleum oils or shale oils. These distillates are distinguished from gasoline fractions by their higher boiling range. 'Ihe usual type of heating oils, i. e. stove oils, furnace oils, etc. ordinarily are composed of wide-cut distillates boiling within the range of about 350 to 750 F. Narrowboiling cuts within this broad range include for example diesel fuels, light and heavy gas oils, kerosene, and the like. The term straight-run distillates denotes herein those distillates obtained by simple distillation of crude oils, or reduced crudes, wherein essentially no cracking has taken place. The sweetening of cracked distillates for-ms no part of the present invention.

The term sweetening is well understood in the art as designating processes whereby mercaptans are either removed from hydrocarbon distillates, converted to hydrogen sulfide and hydrocarbon fragments, or converted by oxidation to more innocuous sulfur compounds which remain in the distillate. The present procedure is concerned with the latter category of sweetening procedures. It is believed that such processes operate by oxidizing the mercaptans to disuliides and other sulfur compounds which are relatively odorless. Many such oxidation procedures have been proposed in the past, mostly for the sweetening of gasolines.

The primary object of this invention is to provide sweetening procedures which are sufficiently economical to be applied to low-cost products such as heating oils. More specifically, an object is to provide sweetening methods which require a minimum of process equipment, and a minimum of handling. A further objective is to provide sweetening methods which do not require excessive lengths of time, thereby avoiding unduly lengthy holdup periods in the treating vessels. Another object is to provide sweetening procedures which do not appreciably impair the burning qualities of the fuel oil. Other objects will be apparent to those skilled in the art from the following description.

Essentially, the procedural steps involved in this inven- -tion comprise lirst admixing with the straight-run heating oil a small proportion, of a cracked distillate boiling within the heating oil range, a small proportion of a phenylene diamine type inhibitor, and then agitating the mixture in the presence of an aqueous caustic solution and air. The temperature of treatment should be maintained between about 85 and 140 F., under which conditions sour stove oils containing for example up to about 0.04% of mercaptan sulfur by weight may be rendered substantially sweet to the doctor test, and olfactorily, within a period of about 5 to 48 hours. At the end of the -treating period the mixture of oil and caustic is transferred to a settling tank where the aqueous caustic is allowed to settle and separate. The supernatant hydrocarbons are then drawn ofi and washed with water if necessary to remove traces of caustic, and' then sent to 2,793,171 Patented May 21, 1957 lCe storage. The phenylene diamine inhibitor remains ln the final product and imparts thereto the additional longrange benefit of retarding gum formation, which may result in lthe clogging of burner tips, valves and transfer lines.

It has long been known that certain petroleum fractions containing dissolved mercaptans could be sweetened by storing the same in the presence of air and the phenylene diamine type of gum inhibitors. This basic procedure is described in U. S. Patent No. 2,508,817. Ordinarily the preferred type of inhibitor is an N,Ndialkyl substituted para-phenylene diamine, such as the N,Ndi secondary butyl compound. The exact mechanism by which sweetening -is brought about under these conditions is not clearly understood, nor is there any -generally accepted theoretical explanation. It has recently been proposed (Rosenwald Petroleum Processing, September 1951, pages 969-973) that olefins may play an important role in this type of inhibitor or tank sweetening. However it is not possible to generalize, or to say with any certainty that this proposed reaction mechanism would extend to the sweetening of all types of hydrocarbons, regardless of their boiling range, and to all types of mercaptans regardless of their configuration and boiling range, and regardless of the configuration and boiling range of the olens concerned. It has been found in fact that certain types of oleiins appear to be much more active in their sweetening capacity than are other olens. Moreover, the secondary and tertiary mercaptans apparently are more diilicultly converted to disullides than arel the primary mercaptans.

Many other alternative oxidation procedures have been proposed for converting mercaptans to disuliides. However, all of the known methods possess one or more disadvantages which render them unsuitable for the economical treatment of low cost heating oils. For example, a seemingly simple procedure has been proposed (U. S. Patent No. 2,608,522) involving the treatment of heating oils with strong caustic solutions, ranging between 50 and 55 Baum, while agitating the mixture in the presence of air. The use of strong caustic solutions is highly disadvantageous because they tend to discolor the oil, and also show a marked tendency to become emulsiiied in Ithe oil, requiring -several stages of washing for complete removal. Moreover, there is considerable question as to whether this procedure will actually eiect sweetening, in the sense of actually extracting and/or oxidizing sutlicient mercaptan to give a sweet product. The strong caustic and air treatment is likely to produce peroxides or hydroperoxides, which interfere with the detection of mercaptans by the standard doctor test, or by the copper number test. Distillates treated by this process may appear sweet to the doctor test, but mercaptan analyses frequently show an actual mercaptan level above the minimum which is regarded as sweet by Ithe doctor test. To be doctor-sweet and odor-sweet, the distillates should contain less than about 0.0005% by weight of mercaptan sulfur, although this figure varies slightly for diiferent distillates. However, it is not always necessary that the oil be rendered doctor-sweet; the prime objective is to produce an odor-sweet oil, and heating oils are frequently observed to be odor-sweet at mercaptan sulfur levels as high as about 0.0025 by weight.

It has now been discovered that doctor-sweet and odorsweet straight-run heating oils may be obtained by simply adding to the oil about 5-25 pounds of the phenylene diamine inhibitor per 1000 barrels of oil, about 0.5% Ato 50% by volume ofrcracked distillates boiling within the same range, and then agitating the resulting solution for about 5-48 hours in the presence of air or oxygen and about 0.1% to 10% by volume of aqueous causticsolution, e, g, sodium hydroxide. The treatment maybe carried out in a vented storage tank to provide air by simple diffusion, or air may be continuously bled. 1n through a circulating pump which is used for agitatton. In sweetening such high-boiling stocks it has been found that low temperatures are relatively ineffective. Temperatures below about 75 F. require several days or weeks of storage for adequate sweetening. The cost of maintaining continuous agitation for such extended periods, and the large investment in storage equipment, are objectionable factors in producing cheap stove oils. It 1s therefore necessary to employ temperatures between about 85 and 140 F., and preferably between about 100 and 130 F. Higher temperatures tend to causediscoloration, and to promote the formation-of emulsions. The use of the cracked stock to provide olens has been found to be4 essential; an identicall treatment of the straight-run stock alone results in substantially no sweetening.

The cracked stocks employed herein must be within the boiling range of the heating oil, and may be either sweet or sour. lf sour oils are employed, they are sweetened along with thc straight-run oil. If low boiling stocks are employed, the flash point of the heating oil may be reduced to below the specified minimum. It is not essential that the cracked stock have the same boiling range as the heater oil, it must merely boil well within, i. e. above, the minimum boiling point of the heating oil. For this purpose cracked stocks boiling anywhere between about 350 and 800 F. may be employed. Such stocks include for example cycle oils from thermal cracking units, or catalytic cracking units, and the bottoms from such cracking operations. Such products'ordinarily will contain between about 10% and 30% by volume of olens, and will have a flash point of between about 140 and 220 F. The minimum stove oil specifications require a flash point of 115 F. In addition to thermal and catalytic cracking stocks, other olefin-containing, highboiling hydrocarbons may be employed, suchfor example as Coker-distillate fractions and the like. Preferably about 1% 105% by volume of such cracked stocks are employed. Higher proportions may sometimes be employed, but since these cracked oils usually promote gum and sludge formation, are undesirably dark in color, and may impair the burning quality of the stove oil, the lesse1 proportions, i. e. 5% or less, are preferred'for producing No. l stove oils.

The caustic solutions employed herein may comprise aqueous solutions of sodium hydroxide, potassium hydroxide, lithium hydroxide, etc. In order to avoid emulsion problems, color deterioratior, and undue peroxidation, it is highly desirable to use caustic solutions containing less than about 25% by weight of solid caustic. Referring to sodium hydroxide solutions, those containing between about 5% and 25% may be employed, corresponding to 7.4 to 31 B., respectively.

Reference is now made to the accompanying drawing which illustrates one particular method for carrying out the invention. The sour stove oil to be sweetened is brought in tt rough line 1 from the fractionation system, with or without any desired intervening pre-treatment. It is economically advantageous to bring the oil directly from the distillation system so as to avoid reheating the oil to the` desired sweetening temperature. However heating oils from other refinery sources may befemployed.

The high-boiling cracked distillate is brought in through line 2, and admixed with the sour stoveoil in line 3. The proportion of cracked distillate may be controlled by means of proportioning pump 4. The mixture of straight-run and cracked stock then ows further through liuc 3, and is admixcd thereinV with the inhibitor from line 5, and then with aqueous caustic fromline 6. The inhibitor may conveniently be-admitted ,in the form of a concentrated or dilute solution thereofEinhydrocarbons, alcohols, or other suitable solvent. The quantity of inhibitor and caustic may be controlled by means of proportioning pumps 7 andS respectively. Theforderof mixing the above ingredients is not critical, and hence any other sequence may be employed.

After admixing the desired proportions of heating oil, cracked distillate, inhibitor and caustic, the mixture is allowed to flow through open valve 10, line 11, bailleplate mixer 12, and line 13 into treating tank 15. Tank 1S may be of any suitable design, preferably heatinsulated, which will provide sutlicient capacity for the desired operation. In most cases it will be found that sufficient air may be provided by means of a vent 16, allowing for air diffusion and diurnal breathing in and out of the tank. Other types of treating tanks may provide inadequate aeration, in which case air may be introduced in a manner to be subsequently described.

After treating tank 15 has been lled with an initial charge, valve 10 is closed and valve 18 is opened to initiate the sweetening cycle. It will be understood that the aqueous caustic tends to settle out in treating tank 15, and means must hence be provided for continuously redispersing the aqueous caustic, as well as providing contact with air. These objectives may be accomplished in a variety of methods, only one of which is illustrated herein. In the illustrated procedure a small stream is continuously withdrawn from the bottom of tank 1S through line 19, passed through valve 18, line 17, circulating pump 20, line 21, air trap 22, line 23, and thence, through baffle plate mixer 12, back to tank 15. In the event that the tank 15 is not constructed to provide adequate aeration, air may be continuously admitted to line 17 through valved line 25. Passage of the mixture of air and oil through pump 20 and line 21 provides suicient agitation to saturate the liquids with air. The slurry of oil and air is then preferably admitted to an air trap 22 to separate undissolved air, which is continuously or intermittently exhausted through line 26. It is usually preferable to admit air in this manner, and to avoid the introduction of air directly into the large treating tank 15. The purpose of this procedure is to avoid frictional contact of air with oil in a large treating tank, which presents an extended liquid surface area. Such surfaces frequently accumulate large static electric charges, resulting in explosions. The small air trap 22 provides insuficient air-liquid interface area to accumulate any appreciable static charges. In another modication, air-trap 22 may be inserted in line 13, whereby more extensive air-liquid mixing is obtained in mixer 12.

After the sweetening cycle has been continued in this manner for a sutcient length of time, the circulating pump 20 is shut off, valve 18 is closed, and the contents of tank 15 are allowed to coalesce and settle for a period of e. g. 15 minutes to several hours. Valve 30 is then opened, permitting the lower layer of caustic to be withdrawn through line 31 and sent to storage vessel 32 for reuse with the next charge of stove oil. The sweetened stove oil is then withdrawn through line 33-and passed through a countercmrent water-washing tower 34 to remove traces of caustic. It has been found that the water washing step is not always necessary, especially where dilute-caustic solutions are employed. Frequently it is only necessary to pass the sweetened oil, after a suitable settling period, through a salt filter, which is packed with coarse granules of sodium chloride. Such a salt filter is illustrated herein at 35 for use in conjunction with the water-washed oil from tower 34. The salt filter 35 removes `the last traces of emulsied caustic and water haze from the treated oil, which is then passed through line 36 to ultimate storage 37.

The above described mode of operation is only one feasible procedure and it will be obvious that many other modifications could be employed. The followingl examples may serve to illustrate more specifically certain criticalfeatures of the invention, withouthowever limiting the same to the' non-critical features.

arsenal.

5. EXAMPLE I Sweetening experiments were carried outat 110- F. using a sour, straight-run stove oil obtained by distillation of a Montana crude oil. This stove oil was blended with various proportions of a cracked cycle oil from a thermal cracking unit. The cycle oil was also derived from a Montana crude. The principal characteristics of the stove oil and the cycle oil were as follows:

The resulting mixtures were then blended with various proportions of Tenamene No. 2 inhibitor (N,Ndisec butyl-para-phenylcne diamine) and agitated in the presence of air and 2% by volume of an aqeous 10% caustic soda solution. The results for various treating periods were as follows:

Table 1 Experiment No 1 2 3 4 Stove oil, percent by vol 100 100 98 05 Cycle oil, percent by vol-- 0 2 5 Inhibitor, 1b./M bbl 0 10 10 10 Analyses, initial:

Gravity, API 60 F 42. 8 Color, Saybolt +19 Mercaptan S, percent by w 0. 0207 Analyses after treatment:

Mercaptan S, percent by wt.-

after 8 11 0. 0201 0. 0174 0. 0075 0. 0043 after 24 hours- 0.0016 after 32 hours. 0. 0012 alter 2 days. 0.0010 after ys 0. 0006 Color, Saybolt after 5 days +12 +7 +9 Experiment No. 1 shows that substantially no sweetening takes place by treatment with caustic and air alone. Experiment 2 shows that adding inhibitor, but no cracked stock, gives only slightly improved results. In experiments 3 and 4, employing small proportions of cracked stock and inhibitor, the product is odor-sweet after 24 hours of treatment, and doctor-sweet after 4-5 days of treatment. By increasing the proportion of inhibitor to e. g. 1bs./ M bbl. of oil, and/ or by raising the temperature, and/or by increasing the proportion of cracked stock, the sweetening is further accelerated so that doctorsweet products are obtained after 24 hours of treatment. 1n all cases, the slight color degradation noted is not of critical significance, as is the case when strong caustic and/ or high temperatures are employed.

EXAMPLE Il Experiments similar to those of Example I were carried out at 80 F., using a stove oil and cycle oil derived from the same crudes, but exhibiting very slightly diierent properties. The results were as follows:

Table 2 Experiment No 5 6 Stove oil, percent by vol- 00 Cycle oil, percent by vol 5 10 Inhibitor, 1b./M bb1 20 30 Analyses, initial:

Gravity, API 60 F- 42. 0 41. 0 Color, Saybolt -I- +20 Mercaptan S, percent by wt 0.0224 0. 0218 Analyses after treatment:

Mercaptan S, percent by wt.-

after 2 days 0. 0190 0.0139 after 7 days 0. 0080 0. 0042 after 11 days. 0. 0077 0. 0037 after 17 days 0. 0040 0.0014 Color, Saybolt after 17 day -15 -10 The resulting products were odorand doctor-sour even after 17 days of treatment, proving that low temperature treatment is ineifective, even at high concentrations of inhibitor and cracked stock. The color degradation was also marked as a result of the extended treatment.

From the above it will be seen that the methods described herein provide highly economical means for producing odor-sweet and/or doctor-sweet heating oils. However, the foregoing disclosure should not be considered as limiting, since many variations may be made by those skilled in the art without departing from the scope or spirit of the following claims.

I claim:

1. A method for sweetening a sour, straight-run fuel oil distillate boiling substantially entirely above 350 F. without substantial color degradation thereof, which comprises admixing with said straight-run distillate a small proportion, between about 1% and 5% by volume, of `a cracked mineral oil traction boiling within the boiling range of said straight-run distillate and containing a substantial proportion of high-boiling olens, admixing therewith a minor proportion of an N,Ndialky1 p-phenylene diamine inhibitor, agitating the resulting mixture in the presence of `oxygen and in. intimate contact with between about 1% and 10% by volume of a caustic solution containing between about 2% and 25% by weight of solid caustic dissolved in water, continuing said agitation at a temperature between about and 130 F. for a period of time between about 5 and 48 hours, and then separating said aqueous caustic solution, thereby obtaining a sweetened, gum-stable product.

2. A process as dened in claim 1 wherein said caustic is sodium hydroxide.

3. A process for sweetening a sour, straight-run fuel oil boiling between about 350 and 750 F. without substantial color degradation thereof, which comprises admixing with said stove oil a small proportion, between about 1% and 5% by volume, of a cracked mineral oil fraction boiling within the range of said stove oil and containing a substantial proportion of high-boiling olefins, admixing therewith a small proportion of an N,N' dialkyl-p-phenylene diamine inhibitor, agitating the resulting mixture in the presence of air and in intimate contact with between about 1% and 10% by volume of an aqueous caustic solution containing between about 2% and 25% by weight of solid caustic dissolved in water, continuing said agitation at a temperature between about 100 and 130 F. for a period of time between about 5 and 48 hours, and then separating said caustic solution, thereby obtaining a sweetened, gum-stable stove oil.

4. A process as defined in claim 3 wherein said caustic is sodium hydroxide.

7 References Cited in the le of this patent UNITED STATES PATENTS 2,570,278 Ryder Oct. 9, 1951 2,616,833 Chenicek et al Nov. 4, 1952 5 2,694,034 Moriarty Nov. 9, 1954 8 OTHER REFERENCES Rosenwald, Petroleum Processing, vol. 6, (Sept. 1951), pages 969-973.

Claims (1)

1. A METHOD FOR SWWEETENING A SOUR, STRAIGHT-RUN FUEL OIL DISTILLATE BOILING SUBSTANTIALLY ENTIRELY ABOVE 350*F. WITHOUT SUBSTANTIAL COLOR DEGRADATION THEREOF, WHICH COMPRISES ADMIXING WITH SAID STRAIGHT-RUN DISTILLATE A SMALL PROPORTION, BETWEEN ABOUT 1% AND 5% BY VOLUME, OF A CRACKED MINERAL OIL FRACTION BOILING WITHIN THE BOILING RANGE OF SAID STRAIGHT-RUN DISTILLATE AND CONTAINING A SUBSTANTIAL PROPORTION OF HIGH-BOILING OLEFINS, ADMIXING THEREWITH A MINOR PROPORTION OF AN N,N''-DIALKYL P-PHENYLENE DIAMINE INHIBITOR, AGITATING THE RESULTING MIXTURE IN THE PRESENCE OF OXYGEN AND IN INTIMATE CONTACT WITH BETWEEN ABOUT 1% AND 10% BY VOLUME OF A CAUSTIC SOLUTION CONTAINING BETWEEN ABOUT 2% AND 25% BY WEIGHT OF SOLID CAUSTIC DISSOLVED IN WATER, CONTINUING SAID AGITATION AT A TEMPERATURE BETWEEN ABOUT 100* AND 130*F. FOR A PERIOD OF TIME BETWEEN ABOUT 5 AND 48 HOURS, AND THEN SEPARATING SAID AQUEOUS CAUSTIC SOLUTION, THEREBY OBTAINING A SWEENTENED, GUM-STABLE PRODUCT.

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