Production Of Low Sulfur Fuel Oil

Abstract

Low sulfur fuel oil is prepared by distilling an asphalt-containing petroleum fraction to obtain a vacuum gas oil and vacuum residuum. The vacuum gas oil is passed downwardly with hydrogen through an upper bed of desulfurization catalyst; the residuum fraction is passed upwardly through one or more lower beds of desulfurization catalyst, and the desulfurized effluents are combined. It is found advantageous to pass the distilled fraction down through the desulfurization zone with less residence time and the residual fraction up through the zone to subject it to more back-mixing and turbulence and thereby effecting a longer residence time.

Description

This invention relates to the production of fuel oils of low sulfur content. More particularly, it is concerned with a method for the conversion of heavy petroleum hydrocarbon fractions such as atmospheric reduced crude into a fuel oil having a sulfur content of less than 1 percent. In one of its more specific embodiments it is concerned with a flexible process for the production of large volumes of low sulfur fuel oil when the demand is greater and reduced amounts of fuel oil when the demand is lesser.

In the refining of petroleum, the hydrodesulfurization of various fractions to reduce the sulfur content thereof is well known and many refineries have units designed particularly for this purpose. However, the demand for different petroleum products varies with the season. For example, in the summer months when pleasure travel is at its peak, there is a great demand for gasoline whereas in the winter months when the weather is not conducive to travel there is considerably less need for gasoline but there is a far greater demand for fuel oil, particularly for domestic heating purposes. The fluctuation in demand for industrial fuel oil is much less.

When a particular unit is installed in a refinery, it is most advisable for economical purposes to operate the unit at capacity. However, it is impractical to produce a product in excess of the demand therefor. It would be advantageous to produce various products in amounts corresponding to the demands changing with the seasons of the year. Accordingly, it is an object of this invention to provide a process whereby large volumes of low sulfur fuel oil can be produced when the demand is greater and smaller volumes are produced when the demand is lower. Similarly, the process is adapted to supply larger volumes of feedstock for catalytic cracking units when the demand for gasoline is greater.

According to our invention, there is provided a process for the production of fuel oil of reduced sulfur content which comprises subjecting an asphalt-containing petroleum fraction to distillation to produce a distillate fraction and a residual fraction, passing said distillate fraction in admixture with hydrogen downwardly through a first bed of desulfurization catalyst under desulfurization conditions, passing said residual fraction in admixture with hydrogen upwardly through a second bed of desulfurization catalyst under desulfurization conditions including a space velocity lower than that for the desulfurization of said distillate fraction and combining the desulfurized distillate and residual fractions.

Our process may be more readily understood by reference to the accompanying drawing which shows diagrammatically a flow scheme for the practice of one embodiment of the invention. For simplicity, various pieces of equipment such as pumps, heaters, compressors, coolers and the like have been omitted from the drawing.

The charge, preferably an atmospheric reduced crude, is introduced into the system through line 11 and charged to vacuum still 12 wherein it is separated into a vacuum gas oil removed through line 13 and a residual fraction removed through line 14. With valve 51 closed, the vacuum gas oil passes through open valve 50 in line 15 into desulfurization unit 16 where with hydrogen from line 17 it passes downwardly through a fixed bed of desulfurization catalyst in the upper section of desulfurization unit 16. The residual fraction from line 14 with hydrogen from line 18 is introduced into the bottom of desulfurization unit 16 where it passes upwardly through catalyst in the lower section of the reactor and the reactant streams merge and pass through open valve 53 in line 20 through which the mixed reactant stream passes to high pressure separator 22.

A hydrogen rich stream is removed from high pressure separator 22 through line 24 and is introduced into scrubber 26 where H.sub.2 S and NH.sub.3 are removed by contact with solvents such as water and diethanolamine. The purified hydrogen is recycled to the system through line 17 with make-up hydrogen being introduced into the system through line 30. The combined desulfurized stream is withdrawn from high pressure separator 22 through line 32 and sent to low pressure separator 33 from which a normally gaseous stream is removed through line 34, the liquid effluent being transferred through line 35 to still 36 from which a low molecular weight hydrocarbon stream is removed through line 37, a naphtha stream through line 38, a light gas oil stream through line 39 and a product fuel oil stream through line 40.

In a specific embodiment the lower catalyst bed in desulfurization unit 16 is separated into sections. To assist in temperature control, hydrogen for cooling purposes is introduced into the lower bed from line 18 through lines 21 and 23.

The above description presents the method for maximum fuel oil production. When it is desired to reduce the volume of residual fuel oil and increase the gasoline production, valve 50 in line 15 is closed and valve 51 in line 13 is opened and the vacuum gas oil recovered from vacuum still 12 is sent to catalytic cracking for conversion to gasoline. In the meantime valve 53 in line 20 connecting desulfurization unit 16 with high pressure separator 22 is closed and valve 54 in line 55 is opened thereby permitting residual fraction from vacuum still 12 to pass upwardly through the entire catalytic bed system of desulfurization unit 16 and then pass on through lines 55 and 20 to high pressure seperator 22. In this way only the residual fraction is processed into low sulfur fuel oil and the vacuum gas oil is converted to gasoline in a catalytic cracking unit, not shown. Valve 59 in line 19 may be opened to permit the introduction of additional cooling hydrogen.

The charge to our process may be any petroleum fraction containing asphaltic materials. However preferably the charge is an atmospheric reduced crude, that is, a fraction from which all materials volatile at atmospheric pressure and a temperature up to about 650.degree. F. have been removed.

Hydrogen used in the process of our invention need not be pure. Hydrogen of at least 60 percent purity is satisfactory with hydrogen having a purity of between about 70 and 95 percent being preferred. Hydrogen obtained as a by-product from the catalytic reforming of naphtha, by the partial oxidation of carbonaceous material followed by shift conversion and CO.sub.2 removal or electrolytic hydrogen are satisfactory for the purposes of this invention.

The desulfurization unit contains two catalyst beds, an upper and a lower bed. In the drawing the lower bed is depicted as having three sections but the invention is not limited thereto. In this specification and the appended claims, the lower bed through which the residual fraction passes is considered to be a single bed although as shown it may contain several sections.

The catalysts used in the upper and lower beds may be the same or different. In a preferred embodiment, catalysts having the same composition and physical characteristics are used in both beds. They are composed of a hydrogenating component supported on an inert base. Suitable hydrogenating components comprise Group VIII metals and compounds thereof such as oxides and sulfides used alone or in conjunction with a Group VI metal or compound thereof such as the oxide or sulfide. Particularly suitable catalysts are those containing nickel and molybdenum, nickel and tungsten, cobalt and molybdenum or nickel, cobalt and molybdenum. The Group VIII metal should be present in an amount between about 2 and 20 percent, preferably 3-10 percent and the group VI metal in an amount between about 5 and 40 percent, preferably 7-30 percent. The hydrogenating component preferably is supported on an inert refractory oxide such as silica, alumina, chromia, magnesia and mixtures thereof. The catalyst used in the lower bed should have a surface area of at least 250 m.sup.2 /g, a pore volume of 0.4-1.0 cc/g, an average pore diameter between 50 and 100 A. and at least 2 percent silica. Catalysts having these characteristics have been found to be superior both in activity and in life for the desulfurization of asphalt containing hydrocarbon fractions.

Broadly, the reaction conditions for the desulfurization of the different fractions are the same. Temperatures may range between about 400.degree. and 900.degree. F., pressures between 200 and 5,000 psig, space velocity in volumes of oil per volume of catalyst per hour between 0.2 and 5 with hydrogen rates of 1,000-20,000 SCFB (standard cubic feet per barrel of feed). For the desulfurization of the vacuum gas oil preferred conditions are temperatures of 600.degree.-800.degree. F., pressures of 500-2,000 psig, space velocities of 0.5-3.0 v/v/hr. and hydrogen rates of 3,000 to 10,000 SCFB. For the desulfurization of the residual fraction preferred conditions are temperatures of 650.degree.-850.degree.F., pressures of 500-2,500 psig, space velocities of 0.50 to 2.0 and hydrogen rates of 3,000 to 15,000 SCFB.

By operating the desulfurization unit in the manner of our invention, advantage is taken of the fact that it is better to desulfurize the vacuum gas oil downflow while it is in the vapor phase and the residual fraction upflow in the liquid phase. Since the gas oil has a lower sulfur content than the residual fraction, by contacting it in vapor phase with the catalyst there is less turbulence and less residence time and therefore not too much cracking takes place while effecting adequate desulfurization. On the other hand, by flowing the residual fraction upwardly through the catalyst bed, it is subjected to more back-mixing and turbulence thereby in effect having a longer residence time which is desirable as it has a higher sulfur content.

EXAMPLE

In this example, the charge, an atmospheric reduced Arabian crude, is distilled to yield a vacuum gas oil and a vacuum residuum. Characteristics of the charge and distillation products are tabulated below: --------------------------------------------------------------------------- TABLE 1

Charge Gas Oil Residuum __________________________________________________________________________ Yield, vol. % -- 55.0 45.0 Yield, wt. % -- 53.4 46.6 Gravity, .degree.API 15.4 19.8 10.4 Sulfur, wt. % 3.1 2.4 3.7 CCR*, wt. % 10.9 -- 23.4 Pour Point, .degree.F. 40 85 110 Distillation, vol. %. IBP-650.degree. F. 0.8 1.4 -- 650-1,000.degree. F. 54.5 98.6 9.7 1000.degree. F.+ 44.7 -- 90.3 __________________________________________________________________________

With 2,000 SCFB hydrogen (85 percent purity), the gas oil is passed downwardly through a bed of pelletized catalyst composed of 3.2 percent nickel and 18 percent molybdenum in the oxide form supported on alumina. The catalyst has a surface area of 178 m.sup.2 /g, a pore volume of 0.46 cc/g and an average pore diameter of 103.2 A. Reaction conditions are 700.degree.F., 1,500 psig exit hydrogen partial pressure and a space velocity of 1.5 v/v/hr.

The vacuum residuum with 5,000 SCFB hydrogen (85 percent purity) is passed upwardly through a bed of pelletized catalyst composed of 3.3 percent cobalt and 14.0 percent molybdenum in oxide form, 3.3 percent silica and the balance alumina. The one-sixteenth inch extruded catalyst has a surface area of 257 m.sup.2 /g, a pore volume of 0.56 cc/g, and an average pore diameter of 87.2 A. Reaction conditions are a temperature of 725.degree.F., an exit hydrogen partial pressure of 1,500 psig and a space velocity of 0.5 v/v/hr. A 97 wt. percent yield based on the original charge is obtained by combining the desulfurized gas oil with the desulfurized residuum. The product has the following composition. --------------------------------------------------------------------------- TABLE 2

Yield Wt. % __________________________________________________________________________ NH.sub.3 0.1 H.sub.2 S 2.3 C.sub.1 -C.sub.3 0.3 C.sub.4 -C.sub.5 0.6 C.sub.6 -400.degree. F. 0.1 400.degree. F. + 97.0 Sulfur 0.68 __________________________________________________________________________

In this way the entire charge of atmospheric reduced crude is converted to low sulfur fuel oil.

EXAMPLE II

In this example the gas oil obtained from the vacuum distillation of the same atmospheric reduced Arabian crude as Example I is sent to catalytic cracking for conversion to gasoline and the residual fraction is passed upwardly through all of the catalyst in the desulfurization unit. The reaction conditions are maintained the same as in Example I except that in this case the space velocity is now 0.33 v/v/hr. because of the larger volume of catalyst, and the temperature is 750.degree.F. The reduced space velocity compensates for the fact that no low sulfur gas oil is present to blend into the residual fuel oil product. The more severe conditions of higher temperature and lower space velocity result in a slightly greater conversion to products boiling below 400.degree.F. but the fuel oil product again has a sulfur content of less than 0.7 wt. percent.

Composition of the product appears in Table 3. --------------------------------------------------------------------------- TABLE 3

Yield Wt. % NH.sub.3 0.1 H.sub.2 S 3.3 C.sub.1 -C.sub.3 1.7 C.sub.4 -C.sub.5 0.7 C.sub.6 -400.degree. F. 3.0 400.degree. F. + 93.7 Sulfur 0.62 __________________________________________________________________________

Obviously, various other modifications of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

Claims

We claim:

1. A process for the production of fuel oil of reduced sulfur content which comprises subjecting an asphalt-containing petroleum fraction to distillation to produce a vacuum gas oil and a vacuum residuum, introducing said vacuum gas oil into a desulfurization zone containing a series of catalyst beds comprising an upper bed and at least one lower bed, each of said beds in said series comprising a Group VI-B metal or compound thereof and a Group VIII metal or compound thereof supported on an inert refractory inorganic oxide, said vacuum gas oil passing downwardly with hydrogen through said upper bed at a temperature between 400.degree. and 900.degree.F. a pressure between 200 and 5,000 psig, a space velocity between 0.2 and 5.0 v/v/hr. and a hydrogen rate between 1,000 and 20,000 SCFB, introducing said vacuum residuum into said desulfurization zone to flow upwardly with hydrogen through said at least one lower catalyst bed at a temperature between 400.degree. and 900.degree.F., a pressure between 200 and 5,000 psig, a space velocity between 0.2 and 5.0 v/v/hr. and a hydrogen rate between 1,000 and 20,000 SCFB. combining the effluents from said catalyst beds and recovering a fuel oil of reduced sulfur content from said combined stream.

2. The process of claim 1 in which the asphalt containing petroleum fraction is an atmospheric reduced crude.

3. The process of claim 1 in which the space velocity for the desulfurization of the gas 0:1 fraction is about three times that for the desulfurization of the residual fraction.

4. The process of claim 1 in which additional hydrogen at a temperature below the desulfurization temperature is introduced between catalyst beds when said vacuum residuum passes upwardly with hydrogen through more than one catalyst bed.

5. The process of claim 1 in which the catalysts are in the form of fixed beds of pellets.

6. The process of claim 1 in which the catalyst in said at least one lower catalyst bed has an alumina-silica support with a surface area of at least 250 m.sup.2 /g, a pore volume of at least 0.5 cc/g and an average pore diameter between 50 and 100 A.

7. The process of claim 1 in which the space velocity of the residual fraction through said at least one lower catalyst bed is lower than the space velocity of the distillate fraction through said upper bed.

8. The process of claim 1 in which the catalysts in both the upper and said at lease one lower catalyst bed have the same composition and physical characteristics.