A SEQUENTIAL CRACKING PROCESS

Abstract

A sequential cracking process for the thermal cracking of a hydrocarbon feedstock in a cascade of cracking units wherein said hydrocarbon feedstock is heated in a furnace to a predetermined maximum temperature and thermally cracked in the cascade of cracking, such that the formation of coke is reduced.

Claims

1. A sequential cracking process for the thermal cracking of a hydrocarbon feedstock in a cascade of cracking units wherein said hydrocarbon feedstock is heated in a furnace to a predetermined maximum temperature and thermally cracked in said cascade of cracking units, the process comprising the following steps: a. heating said hydrocarbon feedstock in said furnace to a cracking temperature T1; b. introducing the heated hydrocarbon feedstock into a first cracking unit operating at a temperature T1; c. passing the product stream from said first cracking unit to a first fractionation; d. recovering as separate streams from said first fractionation a light fraction boiling below 370 degrees C. and a heavy fraction boiling above 370 degrees C.; e. introducing said heavy fraction from said first fractionation into a second cracking unit operating at a temperature T2; f. passing the product stream from said second cracking unit to a second fractionation; g. recovering as separate streams from said second fractionation a light fraction boiling below 370 degrees C. and a heavy fraction boiling above 370 degrees C. and h. introducing said heavy fraction from said second fractionation into a third conversion unit operating at a temperature T3, wherein temperature T1 is not equal to temperature T2, and temperature T2 is not equal to temperature T3, wherein the temperature in the first cracking unit T1, the second cracking unit T2 and the third conversion unit T3 is in the sequence of T1<T2<T3.

2. The process according to claim 1, wherein said heavy fraction from said first fractionation is heated before introducing into said second cracking unit.

3. The process according to claim 1, wherein said heavy fraction from said second fractionation is heated before introducing into said third conversion unit

4. The process according to claim 1, wherein the temperature ranges for T1, T2, and T3 are 250 degrees C. to 430 degrees C., 390 degrees C. to 460 degrees C., and 300 degrees C. to 530 degrees C., respectively, wherein T3 is in the range from 440 degrees C. to 530 degrees C. if said third conversion unit is of the type coker drum, wherein T3 is in the range from 300 degrees C. to 530 degrees C. if said third conversion unit is of the type hydrocracking unit.

5. The process according to claim 1, wherein the conditions prevailing in said first fractionation correspond to the conditions prevailing in said second fractionation.

6. The process according to claim 1, wherein the residence time of the feedstock in said third conversion unit is longer than the residence time in any one of said first and second cracking unit.

7. The process according to claim 1, wherein said third conversion unit is a slurry hydrocracker.

8. The process according to claim 1, wherein said hydrocarbon feedstock comprises hydrocarbons originating from a crude oil distillation unit (CDU) and/or vacuum distillation unit (VDU).

9. The process according to claim 1, wherein the feed to at least one of said first cracking unit, said second cracking unit and said third conversion unit is mixed with a solvent before introducing the mixture of feed and solvent into the respective unit, wherein said solvent comprises a total concentration of aromatics plus resins in a range of 60-95 wt. %, based on the total weight of the solvent.

10. The process according to claim 9, wherein said combined mixture of the feed and solvent prior to entering to at least one of said first cracking unit, said second cracking unit and said third conversion unit has a S value, measured as per ASTMD7157-12, of greater than 1.

11. The process according to claim 9, wherein said solvent is a vacuum gas oil cut boiling in the range of 350 degrees C. to 550 degrees C. from petroleum crude oil distillation, and said solvent is chosen from the group including crude oil atmospheric tower bottoms, crude oil vacuum tower bottoms, steam cracker cracked distillate and mixed plastic pyrolysis oil, or a combination thereof.

12. The use of a cascade of cracking units for the thermal cracking of a hydrocarbon feedstock wherein the thermal cracking conditions from first to subsequent cracking unit(s) increase from least severe to most severe.

13. The use according to claim 12 for the reduction of the formation of coke.

Description

[0039] The invention will be described in further detail below and in conjunction with the attached drawing.

[0040] FIG. 1 is a schematic illustration of an embodiment of the process of the invention.

[0041] Referring now to the process and the apparatus 101 schematically depicted in the sole figure, there is shown a crude oil distillation unit (CDU)/vacuum distillation unit (VDU) 1 from which the bottom stream 2 is sent to a heater 15. In a preferred embodiment a solvent stream 25 is mixed with stream 2 before entering first cracking unit 3. The stream 20 thus heated is sent to a first cracking unit 3 operating at a temperature T1. The effluent 4 from the first cracking unit 3 is sent to a cooler 16 and its effluent 21 is sent to a first fractionation 5. The first fractionation separates the effluent 4 from first cracking unit 3 into a light fraction 6 boiling below 370 C. and a heavy fraction 7 boiling above 370 C. The separation here should be carried out such that fouling is minimized, for example with minimum internals and specifically designed entry and outlet ports. Heavy fraction 7 is sent to a heater 17 and its effluent 22 is sent to a second cracking unit 8 operating at a temperature T2. In a preferred embodiment a solvent stream 26 is mixed with stream 7 before entering second cracking unit 8. The effluent 9 from the second cracking unit 8 is sent to a cooler 18 and its effluent 23 is sent to a second fractionation 10, which fractionation 10 provides a light fraction 11 boiling below 370 C. and a heavy fraction 24 boiling above 370 C. Heavy fraction 24 from second fractionation 10 is sent to a heater 19 and its effluent 12 is sent to a coking unit 14, e.g. a coker drum, operating at a temperature T3. In a preferred embodiment a solvent stream 27 is mixed with effluent 12 before entering coking unit 14. Coking unit 14 provides an effluent 13. The step of cooling in cooler units 16, 18 can be carried out by flashing and/or heat exchanging, or a combination thereof.

[0042] The temperature ranges for T1, T2 and T3 are (250-430) C., (390-450) C. and (440-530) C., respectively.

[0043] It should be appreciated that the example below is merely exemplary, and is not intended to be limiting.

EXAMPLE

[0044] A combined feed is prepared by mixing a 10% by weight Arab heavy vacuum residue (SARA analysis 5.4/10.4/60.3/23.9) with 90% by weight Jinzhou vacuum residue (17.2/29.6/51.3/1.9) upstream of first cracking unit and heated to a temperature of 410 deg C. The effluent from first cracking unit reactor is separated into gas stream and a liquid stream boiling above 370deg C. by fractionation. The liquid boiling above 370 deg C. is heated in a 2nd heater to 450 deg C. and fed second cracking unit R-2. The effluent from R-2 is separated again into a gas stream and a liquid boiling above 370 deg C. This liquid boiling above 370deg C. is fed to a coker drum operating at 450-530 degC. Alternatively this liquid boiling above 370 deg C. from 2nd fractionation is fed to a hydrocracker, preferably a slurry hydrocracker, operating at 300-530 deg C. The combined feed to first cracking unit reactor and the streams exiting first cracking unit reactor and second cracking unit reactor or their heaters are evaluated as per ASTM D7157-12 to ensure asphaltenes are stable and dissolved. Reactor or heater temperatures are adjusted to ensure these stable solutions.