IL111694A - Method for producing feedstocks of high quality lube base oil from unconverted oil of fuel hydrocracker operating in recycle mode - Google Patents

Description

111694/2 nmxi niD^n mm miTT? ^un inur? ΊΤΠΤΙ ms"*? ntruj ΉΠ7ΓΤ3 131NI1 "7ΙΠ9Π D^T "703 IplpnTTQ 3Ό1Π N"?tD IDOJT] METHOD FOR PRODUCING FEEDSTOCKS OF HIGH QUALITY LUBE BASE OIL FROM UNCONVERTED OIL OF FUELS HYDROCRACKER OPERATING IN RECYCLE MODE YUKONG LIMITED C: 20611 METHOD FOR PRODUCING FEEDSTOCKS OF HIGH QUALITY LUBE BASE OIL FROM UNCONVERTED OIL OF FUELS HYDROCRACKER OPERATING IN RECYCLE MODE BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a method for producing feedstocks of high quality lube base oil from unconverted oil and, more particularly, to an improvement in efficiency along with a method for continuous production of high quality lube base oil from unconverted oil produced by a fuels hydrocracker in recycle mode.

Description of the Prior Art In general, a fuels hydrocracker is a process for converting vacuum gas oil (VGO) produced from a vacuum distillation unit (VI) into fuel grade hydrocarbons such as diesel (as shown in Figure 1). · The VGO feed contains a large amount of impurities such as sulfur, nitrogen, oxygen, metals and other materials not only harmful to the catalyst system but also undesirable in the products. Such impurities are removed in the hydrotreat ing reaction unit (Rl) and the resulting hydrotreated VGO undergoes hydrocracking in the main reactor (R2) to convert a major part of it into light hydrocarbons. The reactor effluents are first separated into hydrogen-rich gas and hydrocarbon liquid, the hydrogen rich gas is recycled back to above two reactors (Rl and R2 ) and the hydrocarbon liquid is fractionated into several different grades of petroleum products in a series of fract onators (Fs). Since it is essentially impossible to accomplish 100% conversion in the reaction, a portion of the feed not converted to diesel and lighter products ends up as the final fractionator bottom stream.

In fact, fuels hydrocrackers are normally designed such that the per-pass conversion (conversion achieved by a single passage through the hydrocracking reactor) is around 60%. The unconverted oil (UCO) is then either sent to storage as a semi-final product (this type of operation is called "once-through mode") or recycled back to the main reactor (R2) for further cracking to increase the overall conversion! this type of operation is called "recycle mode").

Being a mixture of highly saturated hydrocarbons, the UCO has many desirable characteristics such as high viscosity index, which is one of the most important properties for lube base oil. Table 1 shows typical properties of VGO and UCO for overall conversion of 85% and per-pass conversion of 60%.

Table 1 The Properties of the VGO and the UCO Propert ies VGO UCO API Gravity 22 3S Distillation* °C - IBP** / 5% 260/340 350/370 - 10% / 20% 372/396 385/398 - 30% / 40% 415/434 410/422 - 50% / 60% 445/460 435/446 - 70% / 80% 475/492 458/474 - 90% / 95% 516/538 496/515 - FBPm/ %recovery 547/98.5 536/99.0 Hydrogen, wt% 12.0 15.0 Nitrogen, wppm 800 4.0 Sulfur, wt% 3.0 0.0009 Aniline point "C 78 118 Pour Point 'C 33 38 Viscosity, est @ 40 'C 49.9 19.3 @ 60 "C 19.4 10.7 @ 100 *C 6.35 4.4 Viscosity Index 64 143 Saturation Degree of 31 98 Hydrocarbon, wt% * ASTM D-1160:, @ 760 mmHg " Initial Boiling Point "* Final Boiling Point From the economic standpoint, it is more advantageous to utilize the UCO for high quality lube base oil after further processing such as dewaxing and stabilization than use it as fuel oil blending stock or recycle it to the hydrocracking reactor. Some refineries are known to be producing lube base oil with very high viscosity index using the UCO generated from a fuels hydrocracker . For example, a refinery produces VHVI(Very High Viscosity Index) lube base oil at their lube base oil plant utilizing the UCO from their fuels hydrocracker with once-through mode. The hydrocracker plant is located far away from the lube base oil plant.

However, the above conventional method for manufacturing lube base oil from the UCO in that plant has several problems. The UCO generated from the fuels hydrocracker is fed to the lube base, oil plant. In that process, several existing units are being utilized including a vacuum distillation unit, a solvent extraction unit, a solvent dewaxing unit and so on in a "blocked mode" and becomes quite cumbersome with rather low operation efficiency.

For the above-mentioned plant, it becomes especially so, because the existing vacuum distillation unit was originally designed for processing atmospheric residue (AR). It is even necessary to blend the UCO with heavier stocks such as vacuum residue (VR) before feeding it to the existing vacuum distillation unit. For a better understanding of the background of the present invention, the description for a typical fuels hydrocracker in recycle mode is given below. And refer to the enclosed Figure 1.

Atmospheric residue (AR) is fed into the first vacuum distillation unit (VI) to produce a vacuum gas oil (VGO). The VGO is then hydrotreated in the first reactor (Rl) for the removal of impurities such as sulfur, nitrogen, oxygen and metals. The resulting treated VGO is then hydrocracked to yield a variety of hydrocarbon products in the second reactor (R2). These hydrocarbons are separated in a series of fractionators (Fs) to produce various light oil products and diesel oil.

However, not all of the cracked hydrocarbons are converted into diesel and lighter products. A substantial portion of the hydrocarbons remains unconverted. Most of such unconverted oil is sent back to the second reaction unit (R2) for further conversion. With high-endpoint vacuum gas oil feedstocks, however, heavy refractory hydrocarbons and condensed polynuclear aromatic compounds could gradually accumulate in the fuels hydrocracker ' s internal recycle oil stream. Excessive concentration of those compounds can cause rapid decline in catalyst performance and degradation in product selectivity. In order to avoid such operational instability, a small bleed stream of unconverted oil becomes necessary to purge those compounds from the system and to maintain a suitable level of reaction activity.. For that purpose, in general, the fuels hydrocracker in recycle mode recycles a small portion of the product fractionator bottoms back to the feed vacuum column (VI).

The purpose of such a recirculation scheme is to reject a portion of the refractory components and polynuclear ar;omatics to the vacuum residue. Such a scheme also minimizes the quantity of unconverted oil that must be purged from the product fractionator bottoms. The typical recirculation rate to the feed vacuum column is 15 to 25 liquid volume % of the total unconverted oil.

In addition, the unconverted oil from the fuels hydrocracker with high conversion has an average viscosity ranging from 4.0 to 4.5 est at 100*C, which is too low to make 150 Neutral lube base oil. The 150 Neutral lube base oil is one of the grades with high demand and has viscosities ranging from 5.5 to 6.0 est at lOO'C. Consequently, a considerable amount of the unconverted oil at most of the existing refineries as stated above is not being utilized for lube oil production, and wasted typically in the form of fuel oil.

SUMMARY OF THE INVENTION Therefore, the objectives of the present invention is to solve the above problems encountered in the prior arts and to provide a method for producing feedstocks of high quality lube base oil. The present invention will make it possible to use the desired portion of the unconverted oil efficiently during the operation of the fuels hydrocracker in recycle mode, thereby utilizing the facilities to the maximum.

And this invention is the first such approach to produce continuously feedstocks of high quality lube base oil with very high viscosity index and low volatility from the fuels hydrocracker in recycle mode.

In accordance with the first embodiment of the present invention(as shown in Figure 2A), the above objective can be accomplished by providing a method for producing feedstocks of high quality lube base oil, comprising the steps of distilling an atmospheric residue (AR) under vacuum in a first vacuum distillation unit (VI) to give a vacuum gas oil (VGO); hydrotreating the vacuum gas oil in a first reaction unit (Rl) to remove impurities therefrom; hydrocracking the treated vacuum gas oil in a second reaction unit(R2) to yield light hydrocarbons; applying a series of fractional distillations (Fs) to separate light oil products and an unconverted oil; feeding said unconverted oil to a second vacuum distillation unit (V2) to produce feedstocks of high quality lube base oil, having desired viscosities; and recycling the remaining portion of unconverted oil from the second vacuum distillation unit (V2) to the second reaction unit (R2).

In accordance with the second embodiment of the present invention(as shown in Figure 2B), the above objective can be also accomplished by providing a method for producing feedstocks of high quality lube base oil, comprising the steps of: distilling an atmospheric residue (AR) under vacuum in a first vacuum distillation unit (VI) to give a vacuum gas oil (VGO); hydrotreating the vacuum gas oil in a first reaction unit (Rl) to remove impurities therefrom; hydrocracking the treated vacuum gas oil in a second reaction unit (R2) to yield light hydrocarbons; applying a series of fractional distillations (Fs) to separate light oil products and an unconverted oil; feeding only a part of said unconverted oil to a second vacuum distillation unit (V2) to produce feedstocks of high quality lube base oil, having desired viscosities; and recycling the remaining portion of unconverted oil from the second vacuum distillation unit (V2) to the second reaction unit (R2), while recycling remainder of unconverted oil from said fractional distillations (Fs ) to said second reaction unit(R2).

BRIEF DESCRIPTION OF THE DRAWINGS Other objectives and aspects of the invention will become apparent from the following description of embodiments with reference to the following description of embodiments with reference to the accompanying drawings in which: Fig. 1 is a block diagram illustrating a conventional fuels hydrocracker in recycle mode; Fig. 2A is a block diagram illustrating a fuels hydrocracker and a method for producing feedstocks of high quality lube base oil according to the first embodiment of the present invention; and Fig. 2B is a block diagram illustrating a fuels hydrocracker and a method for producing feedstocks of high quality lube base oil according to the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, the preferred embodiments of the present invention will be, in detail, described with reference to the drawings above.

Fig. 2A illustrates a fuels hydrocracker and a method for producing feedstocks of high quality lube base oil according to the first embodiment of the present invention.

As illustrated in Fig. 2A, an atmospheric residue (AR) is fed into a first vacuum distillation unit (VI) to give a vacuum gas oil which is subsequently subjected to the treatment of hydrogenation in a first reaction unit (Rl).

The hydrogenating reaction proceeds , removing impurities, such as sulfur, nitrogen, oxygen and metals, from the VGO. The resulting treated vacuum gas oil enters a second reaction unit (R2) wherein the treated vacuum gas oil is hydrocracked to yield a variety of light hydrocarbons. These hydrocarbons are separated in a series of fractional distillation steps (Fs), to produce various light oil products including diesel oil .

In the meanwhile, a substantial quantity of feed hydrocarbons remains unconverted. All of this unconverted oil is sent to a second vacuum distillation unit (V2) wherein the UCO is distilled to produce feedstocks of high quality lube base oil in accordance with the first embodiment of the present invention. While the oils with desired viscosities are fractionated from the UCO in. the second vacuum distillation unit (V2) and subsequently subjected to dewaxing and stabilization so as to produce the lube base oil, the remaining part of the UCO is sent back to the second reaction unit (R2).

Fig. 2B illustrates a fuels hydrocracker and a method for producing feedstocks of high quality lube base oil according to the second embodiment of the present invention. As shown in this figure, a part of the UCO is taken to the second vacuum distillation unit (V2), whereas the other part is sent back to the second reaction unit (R2).

In accordance with the present invention, the additional vacuum distillation unit (V2) operating under vacuum is provided, wherein feedstocks of high quality lube base oil with appropriate viscosity grades can be produced. For example, 150 Neutral, a viscosity grade in high demand and 100 Neutral which has viscosities ranging from about 3.8 to about 4.2 est at 100°C can be produced as required.

It is preferable to operate the second vacuum distillation tower (V2) at temperature ranging from about 300 to about 380 *C and pressure ranging from about 20 to about 300 mmHg at the tower bottom, according to the present invention.

Turning now to Fig. 1 of prior art, the amount of the UCO that is recycled to the second reaction unit (R2) is approximately 60 to 70% of the VGO feed. Approximately 75 to 85% of the UCO (approximately 50 to 56.7% of the VGO) is sent back to the second reaction unit (R2) through line. 2, and approximately 15 to 25% of it (approximately 10 to 16.7% of the VGO) is sent back to the first vacuum distillation unit (VI) through line 1.

All or a part of the UCO proceed to the second vacuum distillation unit (V2) in accordance with the present invention, wherein it is fractionated to feedstocks of high quality lube base oil with desired viscosities. The lube base oil feedstock is approximately 15 to 25% of total UCO, which is equal to the amount sent back to the first vacuum distillation unit (VI) in the conventional process (Figure 1). The rest, which is approximately 75 to 85% of total UCO, is recycled to the second reaction unit (R2).

According to the present invention, the ratio of total UCO from the fractional distillation step (Fs) to the UCO recycled to the second reaction unit (R2) is preferably on the order of 1.05 to 2.0 : 1.

In accordance with the present invention, the ratio of the UCO proceeding to the second vacuum distillation unit (V2) to the UCO recycled to the second reaction unit (R2) from the second vacuum distillation unit (V2) is preferably on the order of 1.05 to .0 : 1.

As described above, it is unnecessary to send the UCO back to the first vacuum distillation unit (VI) in the present invention. This invention is the first approach to utilize the UCO for manufacturing high quality lube base oil with very high viscosity index and low volatility continuously from the fuels hydrocracker while recycling the unused portion of the UCO back to the hydrocracking reaction unit.

The preferred embodiment of the present invention will now be further described with reference to specific examples.

EXAMPLE 1 A vacuum gas oil with the properties shown in Table 1 was processed in a hydrotreating reaction unit (Rl) with a liquid hourly space velocity of 2.10 hr~ and treated with a catalyst, commercially available from Nippon Ketjen Company in Japan, model HC-K, at reactor average bed temperature of 386.1*C and reactor inlet pressure of 2,523 psig, using a hydrogen rate of 5,720 SCF/BBL of reactor feed.

Thereafter, the resulting vacuum gas oil along with the unconverted oil to be described later was processed in a hydrocracking reaction unit (R2) with a liquid hourly space velocity of 1.26 hr~ and treated with a catalyst, commercially available from UOP Incorporated in USA, model HC-22, at reactor average bed temperature of 393.8°C and reactor inlet pressure of 2,500 psig, using a hydrogen rate of 7,520 SCF/BBL of reactor feed.

Subsequently, all of the treated oil was subjected to a series of separations and fractional distillation steps (Fs) as shown in Fig. 2A, to obtain diesel and lighter products, and to give the 380°C + unconverted oil with the properties shown in the Table 1.

All of the unconverted oil was charged to a vacuum distillation unit (V2) wherein a tower top temperature, a tower bottom temperature, a tower top pressure and a tower bottom pressure are 80°C, 325*C, 75mmHg and 150mmHg, respectively and distilled, so as to give a light distillate(i) 33.0 LV%, an 100N distillate( ii ) 8.3 LV%, a middle distillate( iii ) 11.7 LV% and a tower bottom product(iv), 150N light distillate 47.0 LV%.

From the above distillates, the 100N and the 150N distillates amounting to 25% of the unconverted oil fed to the vacuum distillation unit (V2), i.e. 100N; 5% and 150N; 20%, were drawn out, and the rest was mixed and recycled to the hydrocracking reaction unit (R2).

The properties of the distillates are shown in the following Table 2A.

EXAMPLE 2 A vacuum gas oil with the properties shown in Table 1 was processed in a hydrotreating reaction unit (Rl) with a liquid hourly space velocity of 2.10 hr"1 and treated with a catalyst, commercially available from Nippon Ketjen Company in Japan, model HC-K, at reactor average bed temperature of 385.9 "C and reactor inlet pressure of 2,523 psig, using a hydrogen rate of 5,710 SCF/BBL of reactor feed.

Thereafter, the resulting vacuum gas oil along with unconverted oil to be described later was processed in a hydrocracking reaction unit (R2) with a liquid hourly space velocity of 1.25 hr" and treated with a catalyst, commercially available from UOP Incorporated in USA, model HC-22, at reactor average bed temperature of 384.1*C and reactor inlet pressure of 2,500 psig, using a hydrogen rate of 7,500 SCF/BBL of reactor feed.

Subsequently, the treated oil was subjected to a series of separations and fractional distillation steps (Fs) as shown in Fig. 2B, to obtain diesel and lighter products and to give the 380°C + unconverted oil with the properties ' shown in Table 1.

A half (50%) of the unconverted oil was recycled to the hydrocracking reaction unit (R2) and the other half (50%) was charged to a vacuum distillation unit (V2) wherein a tower top temperature, a tower bottom temperature, a tower top pressure and a tower bottom pressure are 80°C, 325"C, 75nunHg and 150mmHg, respectively and was distilled so as to give a light distillate(i) 32.9 LV%, an 100N distillate( ii ) 8.4 LV%, a middle distillate( iii ) 11.8 LV% and a tower bottom product, 150N distillate(iv) 46.9 LV%.

From the above distillates, the 100N. and the 150N distillates amounting to 50% of the unconverted oil fed to the vacuum distillation unit (V2), i.e. 100N : 10% and 150N : 40%, were drawn-out, and the rest was mixed and recycled to the hydrocracking unit (R2).

The properties of the distillates are shown in the following Table 2B. the 380°C + unconverted oil with the properties shown in Table 1.

A half (50%) of the unconverted oil was recycled to the hydrocracking reaction unit (R2) and the other half(50%) was charged to a vacuum distillation unit (V2) wherein a tower top temperature, a tower bottom temperature, a tower top pressure and a tower bottom pressure are 80°C, 325"C, 75mmHg and 150mmHg, respectively and was distilled so as to give a light distillate(i) 32.9 LV% , an 100N distillate ( ii ) 8.4 LV% , a middle distillate( iii ) 11.8 LV% and a tower bottom product, 150N distillate(iv) 46.9 LV%.

From the above distillates, the 100N and the 150N distillates amounting to 50% of the unconverted oil fed to the vacuum distillation unit (V2), i.e. 100N : 10% and 150N : 40%, were drawn-out, and the rest was mixed and recycled to the hydrocracking unit (R2).

The properties of the distillates are shown in the following Table 2B.

Table 2A The Properties of the Products from UCO Vacuum Distillation Unit(V2) (for Example 1) Light 100 N Middle 150 N Properties Distil. Distil . Distil . Distil.

API Gravity 38.8 38.6 38.4 37.8 Distillation*eC - IBP** / 5 LV% 278/289 377/405 341/408 424/437 - 10% / 30% 305/402 406/412 410/424 442/458 - 50% / 70% 405/414 421/431 434/447 471/493 - 90% / 95% 430/437 446/453 469/483 514/519 - FBP*** 462 482 520 523 Viscosity, est @ 60 °C 7.63 8.50 9.26 13.89 @ 100 'C 3.45 3.80 4.19 5.70 Viscosity Index 143 154 179 172 Flash Point(COC) 143 220 192 248 β r Noack Volatility, % 14.9 4.8 Average 347 387 403 456 Molecular Weight ' Watson K Value 12.73 12.88 12.93 13.04 Pour Point, °C 30.7 35.0 ASTM D-1160, @ 760 mmHg, eC, Distil. : Distillate Initial Boiling Point Final Boiling Point 16 Table 2A The Properties of the Products from UCO Vacuum Distillation Unit(V2) (for Example 1) Light 100 N Middle 150 N Properties Distil. Distil . Distil. Distil .

API Gravity 38.8 38.6 38.4 37.8 Distillation*°C - IBP" / 5 LV% 278/289 377/405 341/408 424/437 - 10% / 30% 305/402 406/412 410/424 442/458 - 50% / 70% 405/414 421/431 434/447 471/493 - 90% / 95% 430/437 446/453 469/483 514/519 - FBPtU 462 482 520 523 Viscosity, est @ 60 "C 7.63 8.50 9.26 13.89 @ 100 *C 3.45 3.80 4.19 5.70 Viscosity Index 143 154 179 172 Flash Point(COC) 143 220 192 248 ° r Noack Volatility, % 14.9 4.8 Average 347 387 403 456 Molecular Weight Watson Value 12.73 12.88 12.93 13.04 Pour Point, *C. 30.7 35.0 ASTM D-1160, @ 760 mmHg, "C, Distil. : Distillate Initial Boiling Point Final Boiling Point 16 Table 2Β The Properties of the Products from UCO Vacuum Distillation Unit(V2)(for Example 2) Light 100 N Middle 150 N Properties Distil. Distil . Distil. Distil .

API Gravity 38.9 38.6 38.3 37.8 Distillation**C - IBP** / 5 LV% 275/288 378/404 339/407 425/438 - 10% / 30% 306/402 406/413 411/424 442/457 - 50% / 70% 404/413 420/431 433/446 476/495 - 90% / 95% 431/437 444/453 467/483 516/521 - FBPm 463 484 518 525 Viscosity, est @ 60 *c 7.62 8.50 9.27 13.89 @ 100 *C 7 3.43 3.80 4.14 5.70 Viscosity Index 139 154 169 172 Flash Point(COC) 142 221 195 249 ° r Noack Volatility, % 15.Ό 5.0 Average 346 388 402 457 Molecular Weight Watson K Value 12.72 12.88 12.92 13.04 Pour Point, °C 30.9 36.1 * ASTM D-1160, @ 760 mmHg, *C, Distil. : Distillate " Initial Boiling Point *" Final Boiling Point As apparent from the above Examples and Tables, it is possible to produce feedstocks of high quality lube base oil 17 of 100N and 150N showing very high viscosity index and low volatility in accordance with the present invention.

In addition, withdrawing part of the UCO prevents the accumulation of heavy refractory hydrocarbons and condensed polynuclear aromatic compounds and frees capacity in the vacuum distillation unit (VI) and hydrotreating reaction unit (Rl), allowing treatment of the vacuum gas oil in the same amount as the withdrawn lube base oil feedstock. Therefore, it has been proved that the present invention could utilize the facilities very efficiently.

Although the preferred embodiment of the present invention has been disclosed for illustrative purpose, those skilled in the art will appreciate that various modifications, addition and substitutions are possible, without departing from the scope and spirit of the present invention as disclosed in the accompanying claims. r 18 - 19- 111694/2

Claims (10)

1. A method for producing feedstocks of high quality lube base oil utilizing the unconverted oil of a fuel hydrocracker, comprising the steps of: distilling an atmospheric residue under vacuum in a first vacuum distillation unit to provide a vacuum gas oil; hydrotreating the vacuum gas oil in a first reaction unit to remove impurities therefrom and provide a treated vacuum gas oil; hydrocracking the treated vacuum gas oil in a second reaction unit to yield light hydrocarbons; subjecting the light hydrocarbons to a series of fractional distillations to separate light oil products and an unconverted oil; feeding all of said unconverted oil to a second vacuum distillation unit to produce feedstocks of high quality lube base oil having desired viscosities and an unconverted oil remaining portion; and recycling the unconverted oil remaining portion from the second vacuum distillation umt to the second reaction unit.

2. A method according to claim 1, wherein the lube base oil feedstocks having a desired viscosity range are subjected to a further dewaxing and stabilization process, while recycling the unconverted oil remaining portion from the second vacuum distillation unit to the second reaction unit.

3. A method according to claim 1, wherein the second vacuum distillation unit is operated at a tower bottom temperature ranging from about 300° to about 380°C. under tower bottom pressures ranging from about 20 to about 300 mmHg.

4. A method according to claim 1, wherein the ratio of total unconverted oil from the fractional distillations to the unconverted oil remaining portion recycled to the second reaction unit is about 1.05: 1 to 2.0: 1.

5. A method according to claim 1, wherein the ratio of the unconverted oil sent to the second vacuum distillation unit to the unconverted oil remaining portion recycled to the second reaction unit from the second vacuum distillation unit is about 1.05: 1 to 4.0: 1. - 20 - 111694/2

6. A method for producing feedstocks of high quality lube base oil, comprising the steps of; distilling an atmospheric residue under vacuum in a first vacuum distillation unit to provide a vacuum gas oil; hydrotreating the vacuum gas oil in a first reaction unit to remove impurities and provide a treated vacuum gas oil; hydrocracking the treated vacuum gas oil in a second reaction unit to yield light hydrocarbons; subjecting the light hydrocarbons to a series of fractional distillations to separate light oil products and unconverted oil; splitting the unconverted oil into a first stream and a second stream; feeding the first stream of said unconverted oil to a second vacuum distillation unit to produce feedstocks of high quality lube base having desired viscosities and an unconverted oil remaining portion, while recycling the second stream of unconverted oil from said distillations to said second reaction unit; and recycling the unconverted oil remaining portion from the second vacuum distillation unit to the second reaction unit.

7. A method according to claim 6, wherein the lube base oil feedstocks having a desired viscosity range are subjected to a further dewaxing and stabilization process, while recycling the unconverted oil remaining portion from the second vacuum distillation unit to the second reaction unit.

8. A method according to claim 6, wherein the second vacuum distillation unit is operated at a tower bottom temperature ranging from about 300° to 380°C under tower bottom pressures ranging from about 20 to about 300 mmHg.

9. A method according to claim 6, wherein the ratio of total unconverted oil from the fractional distillations to the second stream of unconverted oil and unconverted oil remaining portion recycled to the second reaction unit is about 1.05:1 to 2.0:1. - 21 - 111694/2

10. A method according to claim 6, wherein the ratio of the unconverted oil sent to the second vacuum distillation unit to the unconverted oil remaining portion recycled to the second reaction unit from the second vacuum distillation unit is about 1.05:1 to 4.0: 1. For the Applicant, Sanford T. Colb & Co. C: 20611