Distillation tower for improving yield of petroleum hydrocarbon distillate and feeding method thereof

Abstract

The present invention relates to a method for obtaining a fraction oil yield from petroleum hydrocarbons in a distillation column, wherein said distillation column comprises a fractionation stage, a vaporization section and a stripping stage from the top to the bottom of the distillation column. The method comprises preheating and sending a feedstock oil of petroleum hydrocarbons through a pressure-feeding system at a pressure of 100-1000 kPa higher than the vaporization section pressure of the distillation column, wherein said preheating is conducted in a heating furnace, wherein said heating furnace has an outlet pressure of 100-1000 kPa higher than the vaporization section absolute pressure, and an outlet temperature of 360-460 C.

Claims

1. A method for improving a fraction oil yield from petroleum hydrocarbons in a distillation column, wherein said distillation column comprises a fractionation stage, a vaporization section and a stripping stage from the top to the bottom of the distillation column, wherein said method comprises preheating a feedstock oil of petroleum hydrocarbons to be fractionated, sending the feedstock oil through a pressure-feeding system at a pressure of 100-1000 kPa higher than the vaporization section pressure of the distillation column into the vaporization section of the distillation column so that the feedstock oil is atomized and vaporized, in the vaporization section, and then distillation-separating the atomized and vaporized feedstock oil in the fractionation stage; wherein fraction oils are removed from column top and removed from one or more sidelines, and a non-vaporized heavy oil is removed from the column bottom, wherein the distillation column is an atmospheric distillation column having a column top absolute pressure of 110-180 kPa, a vaporization section absolute pressure of 130-200 kPa, and a vaporization section temperature of 330-390 C., wherein said preheating is conducted in a heating furnace, wherein said heating furnace has an outlet pressure of 100-1000 kPa higher than the vaporization section absolute pressure, and an outlet temperature of 360-460 C.

2. The method of claim 1, wherein the pressure-feeding system is at a pressure of 200-300 kPa higher than the vaporization section pressure of the distillation column.

3. The method of claim 1, wherein said heating furnace has an outlet pressure of 200-300 kPa higher than the vaporization section absolute pressure, and an outlet temperature of 380-430 C.

4. The method of claim 1, wherein in said distillation column, a liquid collection element is disposed below an inlet for injecting the feedstock oil and/or a foam removal element is disposed above the inlet for injecting the feedstock oil.

5. The method of claim 1, wherein said pressure-feeding system comprises a flow distribution system and one or more atomization devices; wherein said one or more atomization devices can be located in the vaporization section of the distillation column, or out of the distillation column, or both.

6. The method of claim 5, wherein said atomization device is one or more nozzles, and said one or more extend into the vaporization section of the distillation column and/or extend into an atomization vessel that is located out of the distillation column and fluid-communicated with the distillation column.

7. The method of claim 6, wherein said flow distribution system is located in the column and/or out of the atomization vessel and/or in the atomization vessel.

8. The method of claim 6, wherein the atomization vessel comprises an oil transfer line, a flash tank and a flash column.

9. The method of claim 5, wherein said atomization device comprises one or more nozzles that extend into an atomization vessel, said atomization vessel is located out of the distillation column and fluid-communicated with the distillation column, wherein a vapor-phase stream generated in the atomization vessel comes into the vaporization section of the distillation column, and a liquid-phase stream generated in the atomization vessel comes directly into the bottom of the distillation column and mixes with the bottom residual oil, or the vapor-phase stream and the liquid-phase stream generated in the atomization vessel comes into the vaporization section of the distillation column through a same pipeline.

10. The method of claim 5, wherein said atomization device comprises one or more nozzles extend into the vaporization section of the distillation column, wherein a feedstock oil of petroleum hydrocarbons to be fractionated is preheated, atomized and partially or totally vaporized through the pressure-feeding system at a pressure of 100-1000 kPa higher than the vaporization section pressure of the distillation column, and comes into the vaporization section; wherein the fraction oil is removed from the column top and removed from one or more of the sidelines, and a heavy oil is removed from the column bottom.

11. The method of claim 1, wherein the feedstock oil is atomized and vaporized completely in the vaporization section.

12. The method of claim 11, wherein the fraction oil is steadily and continuously removed from the column top and from one or more of the sidelines.

Description

ILLUSTRATIONS OF DRAWINGS

(1) FIG. 1 is a schematic flow chart for a conventional atmospheric distillation.

(2) FIG. 2 is a schematic flow chart for an atmospheric distillation according to the method of the present invention.

(3) FIG. 3 is a schematic flow chart for a conventional vacuum distillation.

(4) FIG. 4 is a schematic flow chart for a vacuum distillation according to the method of the present invention.

(5) FIG. 5 is a schematic flow chart, wherein the atomization vessel is an oil transfer line.

(6) FIG. 6 is a schematic flow chart, wherein the atomization vessel is a flash tank, and the vapor phase and the liquid phase are fed in admixture.

(7) FIG. 7 is a schematic flow chart, wherein the atomization vessel is a flash tank, and the vapor phase and the liquid phase are fed separately.

PREFERRED EMBODIMENTS OF THE INVENTION

(8) In the following context, the method for improving a fraction oil yield from petroleum hydrocarbons and the relevant apparatus of the present invention will be illustrated with reference to the accompanying drawings, but the scope of the present invention is not limited thereto.

(9) Referring below to FIG. 4 which is an example of vacuum distillation, an embodiment of the present invention is illustrated.

(10) FIG. 4 shows a vacuum distillation process according to the present invention. As shown in FIG. 4, the vacuum distillation column comprises a vaporization section 11, a washing stage 12 and a fractionation stage 13. A feedstock oil to be fractionated (atmospheric residue) is pumped by a feed pump 1 into a heating furnace 2 and preheated. The furnace outlet pressure of the heating furnace 2 is 100-1000 kPa, preferably 200-800 kPa, more preferably 200-600 kPa, most preferably 200-400 kPa or 200-300 kPa higher than the vaporization section pressure of the distillation column. The outlet temperature of the heating furnace is 360-460 C., preferably 380-430 C. The preheated feedstock oil is introduced into a lower part of the distillation column through a pressure-feeding system 3. Said pressure-feeding system comprises a flow distribution system 4 and atomization devices 5. The preheated feedstock oil is distributed by the flow distribution system 4 with a certain distribution ratio, atomized by the atomization device 5 into fine droplets, ejected into the vaporization section 11 of the vacuum distillation column, and vaporized rapidly. Because said fine droplets have large specific surface areas, the vaporizable fractions are thoroughly vaporized in a short time during the movement of fine droplets in the vaporization section. A foam removal element 9 is disposed above the atomization device 5, and a liquid collection element 10 is disposed below the atomization device 5. The fraction vaporized in the vaporization section 11 moves upwards, enters the washing stage 12 and the fractionation stage 13 of the vacuum distillation column, and is removed from the column top or sideline after fractionation to produce a fraction oil product. The structures of the washing stage 12 and the fractionation stage 13 are same as those of conventional vacuum distillation column. A heavy fraction, which is difficult to be vaporized, remains in the liquid state. Large liquid droplets are formed from the continuous agglomeration of fine droplets due to their collision with each other, collected under the action of the liquid collection element 10, fall down to the column bottom, and are removed as residual oil.

(11) Referring below to FIG. 5 which is an example of vacuum distillation, an embodiment of the present invention is illustrated, wherein the atomization vessel is an oil transfer line. A feedstock oil to be fractionated (e.g. atmospheric residue) is pumped by a feed pump 1 into a heating furnace 2 and preheated. The in-tube pressure of the heating furnace 2 is 100-1000 kPa, preferably 200-800 kPa, more preferably 200-600 kPa, most preferably 200-400 kPa or 200-300 kPa higher than the vaporization section pressure. The outlet temperature of the heating furnace is 360-460 C., preferably 380-430 C. The preheated feedstock oil is ejected into an oil transfer line 7 through a pressure-feeding system 3. The pressure and temperature in the oil transfer line 7 are 2.0-60.0 kPa (abs.) and 230-460 C. respectively. Fine droplets are thoroughly vaporized under a low oil-vapor partial pressure. The vaporized vapor stream is introduced into the vaporization section 8 of the vacuum distillation column 6. This embodiment can make fine droplets thoroughly vaporize, and therefore increase the distillation yield of the vacuum distillation column.

(12) Referring below to FIG. 6 which is an example of vacuum distillation, an embodiment of the present invention is illustrated, wherein the atomization vessel is a flash tank, and the difference from the embodiment with the oil transfer line as the atomization vessel as shown in FIG. 5 comprises the preheated feedstock oil is ejected into a flash tank 9 through a pressure-feeding system 3, wherein the pressure and the temperature in the flash tank 9 are 2.0-60.0 kPa (abs.) and 230-460 C. respectively. Since fine droplets have large specific surface areas, the fraction having a relative low boiling point is flashed and vaporized under a low oil-vapor partial pressure in the flash tank. The thoroughly vaporized vapor stream is introduced into the vaporization section 8 of the vacuum distillation column 6. This embodiment can make fine droplets thoroughly vaporize, and therefore increase the distillation yield of the vacuum distillation column.

(13) Referring below to FIG. 7 which is an example of vacuum distillation, an embodiment of the present invention is illustrated. This embodiment is similar with the embodiment with the flash tank as the atomization vessel as shown in FIG. 6 except that the fractions having a relative low boiling point are flashed and vaporized in the flash tank 9, the non-vaporized fractions in the fine droplets collide with each other, and re-aggregate into relative large liquid droplets, which fall into the flask tank bottom, as shown in FIG. 7. The vapor-phase stream in the flash tank is introduced into the vaporization section 8 of the vacuum distillation column 6 via a pipeline 10 from the tank top or the walls close to the tank top. The tank bottom liquid-phase stream is directly sent to the column bottom of the vacuum distillation column via a pipeline 11 and merges into the vacuum residue. This embodiment can better separate non-vaporized heavy fine droplets in the flash tank from the vapor stream, and therefore further reduce the entrainment in the vacuum distillation column.

Comparative Example 1

(14) Comparative Example 1 illustrates the effect of the fractionation of a mixed crude oil by an atmospheric distillation method of the prior art.

(15) The mixed crude oil to be fractionated has properties as shown in Table 1. FIG. 1 is a schematic flow chart for an atmospheric distillation method of the prior art. As shown in FIG. 1, the mixed crude oil was firstly heated by an atmospheric heating furnace 2 with an outlet temperature of the heating furnace of 368 C., was sent into an atmospheric distillation column 8 via an oil transfer line 7. Said atmospheric distillation column was a tray column having a diameter of 6.5 m, three sidelines and two middle section refluxes. The fractions such as straight run gasoline, kerosene and diesel were obtained. The operation conditions of the atmospheric distillation column and the product properties are shown in Table 2. The distillation yield of the atmospheric distillation column is 30.2%.

Example 1

(16) Example 1 illustrates the effect of the atmospheric distillation of crude oil according to the method of the present invention.

(17) FIG. 2 is a schematic flow chart for an atmospheric distillation according to the method of the present invention. As shown in FIG. 2, the used atmospheric distillation column 8 was same as that of Comparative Example 1. The feedstock oil to be fractionated was same as that of Comparative Example 1. The feedstock oil, after preheated by the atmospheric heating furnace 2, was ejected to the atmospheric distillation column 8 via a pressure-feeding system (comprising a flow distribution system 4 and atomization devices 5) at a pressure of about 500 kPa higher than the vaporization section pressure of the distillation column. The atmospheric distillation column was provided with the atomization device therein. Said atomization device was a rotary-flow type atomization nozzle. A rotary-flow core was disposed in the front of the nozzle. The top end of the rotary-flow core was equipped with a mono-pore plate. The rotary-flowed liquid, after ejecting via the pore, formed a cone-shaped liquid film. Due to relative high radial and angular velocities, the friction resulted from the velocity difference between the liquid film and the surrounding gases tore up the liquid film into fine droplets. Therefore, a good liquid phase atomization was accomplished. The operation conditions of the atmospheric distillation column and the product properties are shown in Table 2.

(18) TABLE-US-00001 TABLE 1 The properties of the mixed crude oil Items Value 20 C. density, kg/m3 871.4 True boiling point distillation: Temperature, C. Yield, wt % <90.0 2.4 140.0 6.5 180.0 11.2 250.0 19.5 300.0 27.4 350.0 34.5 400.0 44.0 450.0 54.8 500.0 65.0 >500.0 99.5 Loss 0.5

(19) TABLE-US-00002 TABLE 2 Operation conditions of the distillation column and the product properties Comp Items Ex 1 Ex 1 Column top residual pressure, kPa (abs.) 170.0 170.0 Pressure drop of the whole column, kPa 27.0 27.0 Vaporization section pressure, kPa (abs.) 197.0 197.0 Outlet pressure of the atmospheric heating 246.1 412.5 furnace, kPa (abs.) Outlet temperature of the atmospheric heating 368.0 372.0 furnace, C. Furnace tube surface temperature of the 568.0 550.2 atmospheric heating furnace, C. Vaporization section temperature, C. 365.5 364.8 Column top temperature, C. 118.1 119.5 Atmospheric distillation column, the first 193.1 193.8 side stream temperature, C. Atmospheric distillation column, the second 253.4 255.9 side stream temperature, C. Atmospheric distillation column, the third 304.0 308.5 side stream temperature, C. Column bottom temperature, C. 352.1 353.9 Product Product yield, wt % Atmospheric distillation column top product 5.0 5.2 Atmospheric distillation column, the first 7.0 7.2 side stream Atmospheric distillation column, the second 9.9 11.0 side stream Atmospheric distillation column, the third 8.3 9.8 side stream Atmospheric distillation column bottom product 69.8 66.8 Atmospheric distillation column distillation 30.2 33.2 yield

(20) It can be seen from Table 2 that when the method of the present invention is used in the atmospheric distillation, relative to the atmospheric distillation with a conventional feeding manner, the outlet pressure of the atmospheric heating furnace is increased by 166.4 kPa, and the outlet temperature of the atmospheric heating furnace is increased by 4.0 C. Under the substantially same vaporization section pressure and temperature, the distillation yield of the present distillation column reaches 33.2% and increases by 3% relative to the conventional feeding manner. The present method, if used in the atmospheric distillation column, can increase the distillation yield of the atmospheric distillation column.

Comparative Example 2

(21) Comparative Example 2 illustrates the effect of the vacuum fractionation of an atmospheric residue according to the prior art.

(22) The feedstock oil to be fractionated is an atmospheric residue and has properties as shown in Table 3. FIG. 3 is a schematic flow chart for a vacuum distillation according to the prior art. As shown in FIG. 3, the atmospheric residual oil was heated by a vacuum heating furnace 2 with an outlet pressure of the vacuum heating furnace of 30.0 kPa (abs.), a furnace tube surface temperature of the vacuum heating furnace of 593 C. and an outlet temperature of the vacuum heating furnace of 410 C. The preheated feedstock oil was sent into a vacuum distillation column 6 via an oil transfer line 7. The diameter for the furnace tube of the vacuum heating furnace increased from 152 mm to 273 mm continuously. The oil transfer line had a diameter of 2.0 m and a length of 33.0 m. The feedstock was subjected to a gas liquid separation by a feed distributor in the distillation column. The vacuum distillation column was a conventional full packed column with a diameter of 9.2 m and operated in a dry manner. Said vacuum distillation column comprised a vaporization section, a washing stage and a fractionation stage. The vaporization section temperature was 393.7 C. The washing stage was packed with ZUPAC2 packing (Tianjin Tianda Beiyang Chemical Equipment Co., Ltd.) of 1.5 meters. The fractionation stage was packed with two layers of ZUPAC 1 packing (Tianjin Tianda Beiyang Chemical Equipment Co., Ltd.). The vacuum distillation column comprised four outlets, named as Vacuum top, 1st Vacuum side stream, 2nd Vacuum side stream, and 3rd Vacuum side stream from top to bottom, and two middle section refluxes. A column top vacuum pumping system was operated in a three-level vacuum pumping manner. The operation conditions of the vacuum distillation column and the product properties are shown in Table 4. The distillation yield of the vacuum distillation column is 57.6%.

Example 2

(23) Example 2 illustrates the effect of the vacuum distillation according to the method of the present invention.

(24) FIG. 4 is a schematic flow chart for a vacuum distillation according to the method of the present invention. The feedstock oil to be fractionated was atmospheric residue which was same as that used in Comparative Example 2. The feedstock oil was heated by a vacuum heating furnace 2 with a furnace tube diameter of 152 mm. The preheated feedstock oil was sent into an oil transfer line, and ejected to the vacuum distillation column 6 via a pressure-feeding system (comprising a flow distribution system 4 and atomization device 5) at a pressure of about 300 kPa higher than the vaporization section pressure of the distillation column. The vacuum distillation column was provided with the atomization device therein as described in Example 1. The operation conditions of the vacuum distillation column and the product properties are shown in Table 4.

(25) TABLE-US-00003 TABLE 3 The properties of the atmospheric residue Items Value 70 C. density, kg/m.sup.3 909.1 D1160 distillation range data Yield, volume % Temperature, C. 5 344.0 10 368.6 30 438.6 50 505.3 60 548.4 65.3 571.8

(26) TABLE-US-00004 TABLE 4 Operation conditions of the vacuum distillation column and the product properties Comp Items Ex 2 Ex 2 Column top residual pressure, kPa (abs.) 2.6 2.6 Pressure drop of the whole column, kPa (abs.) 1.1 1.1 Vaporization section pressure, kPa (abs.) 3.7 3.7 Outlet pressure of the vacuum heating furnace, 30.0 279.0 kPa (abs.) Inlet pressure of the vacuum heating furnace, 470.0 470.0 kPa (abs.) Outlet pressure of the atmospheric column 1.05 1.05 bottom pump, MPa (abs.) Outlet temperature of the vacuum heating 410.0 428.0 furnace, C. Furnace tube surface temperature of the vacuum 593.0 560.0 heating furnace, C. Vaporization section temperature, C. 393.7 392.0 Column top temperature, C. 55.0 49.1 Vacuum distillation column, the first sideline 116.1 120.5 temperature, C. Vacuum distillation column, the second sideline 232.6 237.1 temperature, C. Vacuum distillation column, the third sideline 312.7 320.8 temperature, C. Column bottom temperature, C. 374.5 376.8 Product Product yield, wt % Non-condensable gas 0.3 0.2 Vacuum 1st sideline 5.2 5.8 Vacuum 2nd sideline 34.1 35.1 Vacuum 3rd sideline 18.0 19.1 Vacuum residue 42.4 39.8 Distillation yield, wt % 57.6 60.2 The properties of fraction oil Density(20 C.), kg/m.sup.3 905.3 912.4 Mixed wax oil residual carbon, %(w) 0.2 0.5 Mixed wax oil C7 insoluble(mg/kg) 60.0 120.0 Mixed wax oil heavy metal content(mg/kg) 0.2 0.5 Mixed wax oil distillation range ASTM D6352 Initial boiling point 282 282 50% 439 447 Final boiling point 540 565 Properties of residual oil Residual oil density(20 C.), kg/m.sup.3 977.3 985.8 Residual oil 100 C. kinematic viscosity, mm.sup.2/s 857.0 1189.0 Residual oil residual carbon, (mg/kg) 18 22 Residual oil <500 C. fraction content, % 4.3 1.3 Residual oil 500-550 C. fraction content, % 12.6 8.7 Residual oil 550-600 C. fraction content, % 18.0 14.1 Residual oil >600 C. fraction content, % 65.1 75.9

(27) It can be seen from Table 4 that when the method of the present invention is used in the vacuum distillation, relative to the vacuum distillation with a conventional feeding manner of Comparative Example 2, under the same vaporization section pressure and temperature, the distillation yield of the present vacuum distillation column reaches 60.2% and increases by 2.6% relative to the conventional feeding manner. The outlet temperature of the vacuum heating furnace increases by 18 C. The surface temperature of the furnace tube decreases by 33 C. The non-condensable gas content in the vacuum distillation column top product decreases from 0.3% to 0.2%. In the conventional feeding manner, the vacuum heating furnace has a furnace tube with a stepwise increased diameter and it is complex. According to the present invention, the diameters for the furnace tube and the oil transfer line are both 152 mm, which simplifies the structures of the furnace tube and the oil transfer line. In addition, in comparison with Comparative Example 2, the final boiling point for the vacuum wax oil increases by 25 C. All of density, viscosity, heavy metal contents and residual carbon content increase, but still meet the subsequent feedstock requirements by the downstream units. In the vacuum residue, the content for the fractions below 500 C. decreases from 4.3% to 1.3%, while the content for the fractions above 600 C. increases from 65.1% to 75.9%. The density, viscosity and residual carbon content of the residual oil increase remarkably.

Comparative Example 3

(28) Comparative Example 3 illustrates the effect of the vacuum distillation of an atmospheric residue according to the prior art.

(29) A mixed crude oil to be fractionated was introduced into an atmospheric distillation column, and fractionated into straight run gasoline, kerosene, and diesel fractions. The distillation yield for the atmospheric distillation column was 32 wt %. Similarly to FIG. 5 except that a pressure-feeding system 3 is not present, atmospheric residue from the atmospheric distillation column was sent to a heating furnace 2 of the vacuum distillation system via an oil pump 1, preheated, and then introduced into the vaporization section 8 of the vacuum distillation column via an oil transfer line 7. The outlet pressure of the vacuum heating furnace was 30.0 kPa (abs.). The furnace wall temperature was 561 C. The outlet temperature of the vacuum heating furnace was 386 C. The vacuum heating furnace had a furnace tube with a stepwise increased diameter. The vacuum distillation column was a high-efficient full packed column. The vaporization section temperature of the vacuum distillation column was 374 C. The properties of the mixed crude oil are shown in Table 5. The operation conditions of the vacuum distillation column and the product properties are shown in Table 6. The distillation yield of the vacuum distillation column is 29.8%.

Example 3

(30) Example 3 illustrates the effect of the vacuum distillation of crude oil according to the method of the present invention.

(31) The used atmospheric distillation column system and the mixed crude oil to be fractionated were same as those used in Comparative Example 3. The distillation yield of the atmospheric distillation column was 32 wt %. As shown in FIG. 5, the atmospheric residue from the atmospheric distillation column was sent to a heating furnace 2 of the vacuum distillation system via an oil pump 1, preheated, then ejected into then oil transfer line 7 via a nozzle 5, thoroughly vaporized in the oil transfer line, and then introduced into the vaporization section 8 of the vacuum distillation column via the oil transfer line. The inlet pressure of the oil transfer line was 14.0 kPa, and the inlet temperature was 386 C. The used nozzle is a centrifugal atomization nozzle. The furnace tube of the heating furnace had a constant diameter. The used oil transfer line and vacuum distillation column had the same structures as those of Comparative Example 3. The vaporization section temperature of the vacuum distillation column was 381 C.

(32) From the results of Table 6, it can be seen that by disposing a nozzle on the oil transfer line, under the same pressure of the vaporization section of the vacuum distillation column as that of Comparative Example 3, the outlet pressure of the vacuum heating furnace reaches 280.0 kPa, and the furnace wall temperature reaches 556 C., which is 5 C. lower than that of Comparative Example 3. The outlet temperature of the vacuum heating furnace reaches 418 C., which is 22 C. higher than that of Comparative Example 3. Fine droplets ejected into the oil transfer line are flashed and vaporized in the oil transfer line, and sent into the vaporization section of the vacuum distillation column, still remaining at the substantially same temperature as Comparative Example 3. Under the same vaporization section pressure as Comparative Example 3, the distillation yield after the feedstock passing through the vacuum distillation of Example 3 reaches 33.7 wt %, which is 3.9% higher than that of Comparative Example 3. The vacuum residue density and viscosity increase. In the vacuum residue, the mass content for the fractions below 500 C. decreases from 10% (Comparative Example 3) to 5.8%.

Example 4

(33) Example 4 illustrates the effect of the vacuum distillation of crude oil according to the method of the present invention.

(34) The used atmospheric distillation column system and the mixed crude oil to be fractionated were same as those used in Comparative Example 3. The distillation yield of the atmospheric distillation column was 32 wt %. As shown in FIG. 6, the used vacuum distillation column was same as that of Comparative Example 3, and the used vacuum heating furnace was same as that of Example 3. Exception is that a flash tank 9 was added after the vacuum heating furnace. The atmospheric residue was subjected to the flow distribution via the flow-distribution system 4, then ejected into the flash tank via a nozzle 5, thoroughly vaporized, and introduced into the vaporization section of the vacuum distillation column. The flash tank pressure was 6.1 kPa, and the temperature was 382 C. The other operation conditions and the product properties are shown in Table 6.

(35) It can been seen from Table 6 that by disposing the atomization nozzle and the flash tank at the outlet of the vacuum heating furnace in Example 4, the distillation yield of the vacuum fraction oil in the atmospheric residue reaches 34.5 wt %, which is 4.7% higher than that of Comparative Example 3.

Example 5

(36) Example 5 illustrates the effect of the vacuum distillation of crude oil according to the method of the present invention.

(37) The used atmospheric distillation column system and the mixed crude oil to be fractionated were same as those used in Comparative Example 3. The distillation yield of the atmospheric distillation column was 32 wt %. The used vacuum distillation column according to Example 5 was structurally same as that of Example 4. The used vacuum heating furnace was structurally same as that of Example 4. The used flash tank was same as that of Example 4. As shown in FIG. 7, the atmospheric residue was subjected to the flow distribution via the flow-distribution system 3, then ejected into the flash tank 9 via a nozzle 5, thoroughly vaporized, and introduced into the vacuum distillation column, wherein gas and liquid phases are in different pipelines. The flash tank pressure was 6.1 kPa, and the temperature was 382 C. The other operation conditions and the product properties are shown in Table 6.

(38) It can been seen from Table 6 that, in Example 5, by disposing an atomization-type pressure-feeding system and the flash tank at the outlet of the vacuum heating furnace, the distillation yield of the vacuum fraction oil in the atmospheric residue reaches 35.1 wt %, which is 5.3% higher than that of Comparative Example 3.

(39) TABLE-US-00005 TABLE 5 The properties of the mixed crude oil Items Value Density/(kg/m.sup.3) 798.6 Kinematic viscosity/(mm.sup.2/s) 65.7 True boiling point distillation Temperature/ C. Yield/(wt %) <90 C. 2.1 140 C. 6.9 180 C. 12.5 250 C. 19.8 300 C. 26.9 350 C. 35.2 370 C. 41.4 400 C. 44.6 450 C. 55.9 500 C. 67.4 >500 C. 99.2 Loss 0.8

(40) TABLE-US-00006 TABLE 6 Operation conditions of the vacuum distillation column and the product properties Comp. Items Ex 3 Ex 3 Ex 4 Ex 5 Main operation conditions Column top residual pressure, kPa 2.0 2.0 2.0 2.0 Pressure drop of the whole column, kPa 2.7 2.7 2.7 2.7 Vaporization section pressure, kPa 4.7 4.7 4.7 4.7 Outlet pressure of the heating furnace, kPa 30.0 280.0 300.0 300.0 Steam for nozzle atomization/Atmospheric 0.0058 0.0065 0.0065 column residual oil, m/m Outlet temperature of the vacuum heating 386 418 418 418 furnace, C. Furnace wall temperature of the vacuum heating 561 556 556 556 furnace, C. Vaporization section temperature of the vacuum 374 379 379 379 column, C. Column bottom Temperature, C. 369 374 373 373 Product yield, wt % Vacuum residue 29.2 25.3 24.5 23.9 Atmospheric distillation column distillation yield 32.0 32.0 32.0 32.0 Vacuum distillation column distillation yield 29.8 33.7 34.5 35.1 Residual oil properties Vacuum residue specific gravity, g/cm.sup.3 0.9773 0.9860 0.9871 0.9876 Vacuum residue <500 C. fraction content (%) 10 5.8 5.1 5.1 Vacuum residue 100 C. kinematic viscosity (mm.sup.2/s) 857 1391 1407 1431