PREHEATING PROCESS MODULE INTEGRATED WITH COKE HANDLING SYSTEM FOR STEAM CRACKING OF HYDROCARBON FEEDSTOCK

Abstract

Steam cracking of naphtha is one of the major unit processes used in refineries for producing light olefins such as ethylene, propylene which is essentially a thermal cracking process wherein heat energy is supplied to crack the feed molecules. In recent times, the process has gained more importance due to emergence of requirement of increasing petrochemical production from crude oils. The furnace is the heart of the thermal cracking processes in which convection and radiation zone plays a role in providing heat required to crack the naphtha molecules. The conventional preheating of naphtha is done along with steam using heat load of the furnace which consumes huge amount of energy and in turn is expensive as well as results in significant CO.sub.2 emissions due to fuel burning. On the other hand, several refiners are exploring ways and means to find greener use of low value Fuel grade Petroleum coke, minimizing the carbon footprint. The present invention discloses a preheating process module integrated with coke handling system in which the overall CO.sub.2 emissions of Thermal steam cracking furnace can be reduced substantially by utilizing convection zone energy while making use of energy generation from petcoke coupled with carbon capture.

Claims

1. A preheating process module for preheating the hydrocarbon feedstock and routing the same into the hydrocarbon cracking furnace for cracking, wherein the preheating process module is integrated with coke handling system for supply of coke from coke yard required for combustion in the preheating module comprising steps of: a. feeding the coke lumps (1) after screening from coke storage yard to a hopper (2) and conveying to roller crusher (4) for crushing through conveyer belts (3) to obtain the crushed coke (5); b. feeding the crushed coke (5) after screening to second hopper (6) and conveying the coke of desired particle size (7) from hopper to coke storage vessel (9) through second conveyer belt (8); c. conveying the coke (11) from the coke storage vessel (9) to the heat source vessel (12), wherein coke powder (13) is combusted in presence of oxygen containing gas (14) to generate the heat which in turn heat up the heat carrier particles (15); d. transporting the heat carrier particles (15) from the heat source vessel to stripper (16), wherein oxygen and residual coke particulates is stripped off using steam (18) leaving as mixture from top (27) of vessel ensuring no oxygen reaches to heat sink vessel (20); e. transporting heat carrier particles (19) to the heat sink vessel (20) wherein hydrocarbon feedstock (21) supplied into the vessel is contacted with the heat carrier particles which in turn preheats the hydrocarbon feedstock; f. recycling back the heat carrier particles (22) to heat source vessel (12) for reheating; g. feeding preheated hydrocarbon vapors to cyclone separator (23) to remove any unwanted particles which may acts as a source of furnace fouling; h. contacting the flue gas (38) generated by combustion of coke with steam (25) in the upper section of heat source vessel using shell & tube type arrangement (26) resulting in the formation of dilution steam (17) at desired temperatures & pressure; i. cooling the exhaust gas mixture (28) in a cooler (29) and feeding to separator (30) to obtain CO.sub.2 rich gas stream (31) from top of separator; j. routing the dilution steam (17) from shell & tube type arrangement to the heat sink vessel (20) wherein it mixes with preheated hydrocarbon vapors to create mixed stream (35), while increasing the space velocity of mixture and reduces the partial pressure, thereby reducing the coke formation tendency in the vessel itself; k. feeding the mixed stream (35) to radiation section (36) of cracking furnace wherein thermal cracking of hydrocarbon occurs resulting in the formation of cracked gases (37), which are routed to further separation sections for recovery and recycle of unconverted gases back to the heat sink vessel (20).

2. The process as claimed in claim 1, wherein the hydrocarbon feedstock is selected from ethane, propane, C4 hydrocarbons, straight run naphtha, kerosene from atmospheric distillation unit, or paraffinic/olefinic naphtha, kerosene from secondary processing units of refinery or light oils produced from waste oils such as waste plastic pyrolysis oil, used lubricating oil, bio-oil and other waste oils and combination(s) thereof.

3. The process as claimed in claim 1, wherein the hydrocarbon feedstock preheating temperatures in the range of 550-650? C., preferably from 590 to 625? C.

4. The process as claimed in claim 1, wherein the location of injection of the mixed feedstock (35) in the hydrocarbon cracking furnace is made at a location in convection section or radiation section inlet which is selected based on the temperature of the mixed feedstock.

5. The process as claimed in claim 1, wherein coke used for combustion in the heat source vessel is selected from fuel grade coke, fluid coke, anode grade coke, bio char, coal or combination(s) thereof and the coke used for combustion in the heat source vessel is selected from Delayed Coker Units or pyrolysis units or coal plants

6. The process as claimed in claim 1, wherein the coke is subjected to size reduction and size separation to form coke particles of size in the range of 40 microns to 10 mm.

7. The process as claimed in claim 1, wherein the coke powder is injected into the heat source vessel by means of conveyer belt or pneumatic conveying system.

8. The process as claimed in claim 1, wherein combustion of coke for heat generation along with fluidization of heat carrier particles in heat source vessel is done either through pure oxygen or air or a combination thereof.

9. The process as claimed in claim 1, wherein for the combustion of coke, excess oxygen is supplied in comparison to stoichiometric oxygen requirement, in the range of 1 to 40 mol %.

10. The process as claimed in claim 1, wherein the operating temperatures of heat source vessel is in the range of 600-800? C., operating pressure is in the range of 0.5-3 bar.

11. The process as claimed in claim 1, wherein coke to hydrocarbon feedstock ratio is in the range of 0.02-1 (wt/wt).

12. The process as claimed in claim 1, wherein the operating temperatures of heat sink vessel is in the range of 400-700? C., operating pressure is in the range of 0.1-3 bar.

13. The process as claimed in claim 1, wherein the minimum fluidization velocity of heat carrier particles is in the range of 0.01-0.8 m/s, heat capacity is in the range of 300-1000 J/Kg-K.

14. The process as claimed in claim 1, wherein the heat carrier particles is selected from spent FCC catalyst, spent reformer catalyst, inert aluminosilicate particles, alumina balls, silicon carbide, fly ash, metallic oxides and combination(s) thereof with particle size distribution in the range of 40 microns to 3 mm.

15. The process as claimed in claim 1, wherein the heat carrier particles to hydrocarbon feedstock ratio is in the range of 1-10 (wt/wt), and hydrocarbon feedstock contact time with the heat carrier particles ranges from 0.5-30 sec.

16. The process as claimed in claim 1, wherein orientation of shell & tube heat exchanger is such that there is counter current flow between flue gases generated due to coke combustion and steam.

17. The process as claimed in claim 1, wherein temperature of steam (25) used for generation of dilution steam (17) is in the range of 110-300? C.

18. The process as claimed in claim 1, wherein temperature of dilution steam (17) is the range of 400-650? C.

19. The process as claimed in claim 1, wherein dilution steam (17) to hydrocarbon feedstock ratio is in the range of 0.1-1.5 and residence time of mixture in the heat sink vessel is in the range of 0.1-5 sec.

20. The process as claimed in claim 1, wherein the CO.sub.2 enriched exhaust stream (31) is sent for further purification, capture or utilization.

Description

BRIEF DESCRIPTIONS OF DRAWINGS

[0051] FIG. 1 illustrates process flow diagram of present invention

[0052] FIG. 2 illustrates the convection section of conventional furnace

[0053] FIG. 3 illustrates the convection section of present invention

DETAILED DESCRIPTION OF THE INVENTION

[0054] While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the scope of the invention as defined by the appended claims.

[0055] The following description is of exemplary embodiments only and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described without departing from the scope of the invention.

Feedstock

[0056] The hydrocarbon feedstock used in the process is selected from Ethane, Propane, C4 hydrocarbons, straight run naphtha from atmospheric distillation unit, other paraffinic/olefinic naphtha from secondary processing units of refinery such as Fluid Catalytic cracking, Hydrocracking, light oils produced from waste oils such as waste plastic pyrolysis oil, used lubricating oil, bio-oil and other waste oils and combination(s) thereof.

Coke Handling System

[0057] The present invention utilizes low value coke for combustion in heat source vessel of preheating module. This low value coke is transported to the heat source vessel via coke handling system which comprises of first coke screener to screen the coke lumps from coke yard, hopper for holding the screened coke, conveyer belt for carrying the screen coke to roller crusher, roller crusher for coke crushing, second coke screener for screening/particle size separation of crushed coke to obtain desired particle size coke particles of size in the range of 40 microns to 10 mm, hopper for storing the crushed and screened coke, conveyer belt for carrying the coke from hopper to coke storage vessel, which is finally injected to heat source vessel by means of conveyer belt or pneumatic conveying system.

Coke for Combustion

[0058] The present invention focused on utilizing the low value coke for generating the energy required for heating the heat carrier particles which in turn heat the hydrocarbon feedstock. Coke handling system is integrated with preheating module for supply of desired particle size coke in the range of 40 microns to 10 mm for combustion. This coke/coke powder is not produced internally within the system instead is taken from external source which can be either from delayed coker unit, or fluid coking unit or waste material (biomass, plastic, municipal solid waste etc.) pyrolysis plants, coal plant. The coke used for combustion in the process can be fuel grade coke, fluid coke, anode grade coke, bio char, coal or combination(s) thereof.

Heat Source Vessel Section

[0059] Heat source vessel is used for heating the heat carrier particles which in turn heats up the hydrocarbon feedstock. The vessel comprises of coke inlet from side wall and recycle heat carrier particles enters just above the coke inlet while oxygen containing gas enters from bottom of the vessel. Combustion reaction takes place between coke particles and oxygen containing gas, with excess oxygen is supplied in comparison to stoichiometric oxygen requirement, in the range of 1 to 40 mol %, which generates CO.sub.2 rich hot flue gases. The top section of vessel comprises of shell and tube heat exchanger orientated in a way to allow counter current flow steam for heat exchanging with flue gases generated due to combustion of coke. The operating temperatures of heat source vessel is in the range of 600-800? C., operating pressure is in the range of 0.5-3 bar, coke to feed ratio is the range of 0.02-1 (wt/wt). The temperature of dilution steam is the range of 400-650? C. and pressure in the range of 0.5-3 bar. The dilution steam coming out of heat source vessel is mixed with hydrocarbon feedstock in heat sink vessel such that dilution steam to hydrocarbon feedstock ratio is in the range of 0.1-1.5 and residence time of mixture in the heat sink vessel is in the range of 0.1-5 sec. The temperature of steam used for generation of dilution steam is in the range of 110-300? C., pressure in the range of 3-20 Kg/cm.sup.2. The steam used for generation of dilution steam can be LP or MP steam. Further, the additional steam generated in convection section can be utilized for power generation using turbines thereby obtaining low pressure & medium pressure steam which can be utilized back in this process itself. The exhaust mixture (28) containing CO.sub.2 rich gas and steam leave the top section of heat source vessel and is fed to separator where CO.sub.2 rich gas is obtained from top having CO.sub.2 in the range of 80-99% is obtained which can be further purified and utilized for carbon capture & utilization.

Heat Carrier Particles

[0060] Heat carrier particles is utilized for preheating the hydrocarbon feedstock wherein the same present in the heat source vessel is transported to heat sink vessel by fluidization using oxygen containing gas via stripper in between to ensure no oxygen reaches to heat sink vessel. The heat carrier particles comprise spent FCC catalyst, spent reformer catalyst, inert aluminosilicate particles, alumina balls, silicon carbide, fly ash, metallic oxides and combination(s) thereof with minimum fluidization velocity in the range of 0.01-0.8 m/s, heat capacity is in the range of 300-1000 J/Kg-K and particle size distribution in the range of 40 microns to 3 mm.

Heat Sink Vessel Section

[0061] The hydrocarbon feedstock stream supplied to the heat sink vessel is directly contacted with the heat carrier particles in the heat sink vessel, having contact time ranging from 0.5-30 sec and heat carrier particles to hydrocarbon feedstock ratio is the range of 1-10 (wt/wt), which in turn preheats the hydrocarbon feedstock upto desired temperatures. After contacting, heat carrier particles are recycled back to heat source section using pneumatic conveying through steam for reheating. The operating temperatures of heat sink vessel is in the range of 400-700? C., operating pressure is in the range of 0.1-3 bar.

Operating Conditions of the Cracking Section

[0062] The operating temperature of the radiation/cracking section is in the range of 750-950? C., pressure drop is in the range of 0.5-2 bar, residence time in the range of 0.1-1 sec, steam to feed ratio in the range of 0.3 to 1.

Description of Process and System Flow Scheme

[0063] In the process and system of present invention as depicted in FIG. 1, the coke lumps (1) after screening from coke storage yard is fed to a hopper (2) and conveying to roller crusher (4) for crushing through conveyer belts (3) to obtain the crushed coke (5). The crushed coke (5) after screening is fed to second hopper (6) and the coke of desired particle size (7) from hopper is conveyed to coke storage vessel (9) through second conveyer belt (8). The coke (11) from coke storage vessel is fed to the heat source vessel (12), using pneumatic conveying through oxygen containing gas (10), wherein coke (13) is combusted in presence of oxygen containing gas (14) to generate the heat which in turn heat up of the heat carrier particles (15). The heat carrier particles (15) present in the heat source vessel is transported with the help of oxygen containing gas (14) to stripper (16) wherein oxygen and residual coke particulates is stripped off using steam (18) leaving as mixture of oxygen containing gas & steam (27) from top of vessel ensuring no oxygen reaches to heat sink vessel (20) and from the stripper the heat carrier particles (19) is transported to the heat sink vessel (20) wherein hydrocarbon feedstock (21) supplied into the vessel (20) is contacted with the heat carrier particles which in turn preheats the hydrocarbon feedstock upto desired temperatures while recycling back the heat carrier particles (22) to heat source vessel (12) using pneumatic conveying through steam (24) for reheating. Preheated hydrocarbon vapors are then fed to cyclone separator (23) to remove any unwanted particles which may acts as a source of furnace fouling. The flue gas (38) generated by combustion of coke is then contacted with Steam (25) in the upper section of heat source vessel using shell & tube type arrangement (26) resulting in the formation of dilution steam (17) at desired temperatures and pressure leaving exhaust mixture of CO.sub.2 and steam (28). The mixture (28) is cooled in a cooler (29) and fed to separator (30) to obtain CO.sub.2 rich gas (31) from top of separator which can be utilized for carbon capture & utilization block (33) to produce value added products (34) and condensate (32) from bottoms. The dilution steam (17) from shell & tube type arrangement is then routed to the heat sink vessel (20) wherein it mixes with preheated hydrocarbon vapors to create mixed stream (35), while increasing the space velocity of mixture and reduces the partial pressure, thereby reducing the coke formation tendency in the vessel itself. The mixed stream (35) is finally fed to radiation section (36) of cracking furnace wherein thermal cracking of hydrocarbon occurs resulting in the formation of cracked gases (37), which are routed to further separation sections for recovery and recycle of unconverted gases back to the heat sink vessel (20).

[0064] In preferred embodiment, the hydrocarbon feedstock is selected from Ethane, Propane, C4 hydrocarbons, straight run naphtha, kerosene from atmospheric distillation unit, other paraffinic/olefinic naphtha, kerosene from secondary processing units of refinery such as Fluid Catalytic cracking, Hydrocracking, light oils produced from waste oils such as waste plastic pyrolysis oil, used lubricating oil, bio-oil and other waste oils and combination(s) thereof and requires preheating temperature up to 550-650? C., preferably from 590 to 625? C.

[0065] In another preferred embodiment, the location of injection of the mixed feedstock (35) in the hydrocarbon cracking furnace is made at a location in convection section or radiation section inlet which shall be selected based on the temperature of the mixed feedstock.

[0066] In another preferred embodiment, fraction of dilution steam (17) is injected in the heat sink vessel (20) and remaining fraction injection in hydrocarbon cracking furnace is made at a location in convection section or radiation section inlet which shall be selected based on the temperature of the mixed feedstock.

[0067] In another preferred embodiment, the coke used for combustion is not produced on heat carrier particles internally instead taken either from Delayed Coker Unit or fluid coking unit- or waste material (biomass, plastic, municipal solid waste etc.) pyrolysis unit or coal plant.

[0068] In yet another preferred embodiment, coke used in the process can be fuel grade coke, fluid coke, anode grade coke, bio char, coal or combination(s) thereof.

[0069] In yet another preferred embodiment, the combustion of coke for heat generation along with fluidization of heat carrier particles in heat source vessel can be done either through pure oxygen or air or a combination thereof. Further, excess oxygen is supplied in comparison to stoichiometric oxygen requirement, in the range of 1 to 40 mol %.

[0070] In yet another preferred embodiment, top section of heat source vessel comprises shell and tube heat exchanger which superheats the steam to generate dilution steam which is then mixed with vaporized hydrocarbon feedstock for reducing the residence time in heat sink vessel before routing to radiation section.

[0071] In yet another preferred embodiment, the steam used for generation of dilution steam can be Low Pressure (LP) or Medium Pressure (MP) steam.

Comparison of the Convection Section of Present Invention with Conventional Furnace

[0072] In conventional furnace as depicted in FIG. 2, the hydrocarbon feedstock (1) entered the convection section which comprises of upper preheater section (2) to obtain vaporized hydrocarbon feedstock (3). The boiler feed water (4) & dilution steam (6) enters the boiler water preheater & steam superheater section (5) to obtain superheated dilution steam (7) and very high-pressure steam (14). The vaporized hydrocarbon feedstock (3) is mixed with superheated dilution steam (7) to obtain resultant mixed stream (8) of hydrocarbon feedstock and dilution steam depending on the steam to hydrocarbon feedstock ratio. The resultant mixed stream (8) is then sent to lower preheater section (9) to obtain desired preheated stream (10) for routing to radiation section (12) which is maintained at cracking temperatures by combustion of fuel gas (11). The cracking of preheated stream (10) takes place in radiation section to obtain cracked gases (13). The upper preheater, boiler water preheater & steam superheater section and lower preheater section exchanges heat from hot flue gas (15) coming from radiation section to the convection section of the furnace. The hot flue gas after exchanging heat with above sections exits the furnace from top as stream (16).

[0073] In the present invention as depicted in FIG. 3, instead of feeding the hydrocarbon feedstock (20) and steam (22) to the convection section of the furnace for preheating using flue gases coming from radiation section, it is routed to a preheating process module (21) as described in FIG. 1 for preheating the hydrocarbon feedstock and steam at desired temperatures and routing the preheated mixture containing hydrocarbon feedstock and dilution steam (23) into the hydrocarbon cracking furnace (24) maintained at cracking temperatures by combustion of fuel gas (25) for conversion into cracked gases (26). The upper preheater, boiler water preheater & steam superheater section and lower preheater section of convection section of conventional furnace now can be combined as one preheating section (18) wherein boiler feed water (17) along with dilution steam (19) can be utilized for generating additional very high-pressure steam (27) by exchanging heat from hot flue gas stream (28) coming from radiation section to the convection section of the furnace. The hot flue gas after exchanging heat with above sections exits the furnace from top as stream (29).

[0074] It is to be note that the location of injection of the mixed feedstock (23) in the hydrocarbon cracking furnace is made at a location in convection section or radiation section inlet which shall be selected based on the temperature of the mixed feedstock. When we implement this system commercially, the additional very high-pressure steam can be generated from idle convection section or part of convection section using flue gases heat depending on the location of injection of the mixed feedstock in the hydrocarbon cracking furnace.

Example 1

[0075] As the underlying chemical engineering principles of the process could be estimated by mathematical calculations known in the art of chemical engineering, the process data given in the same has been generated using said approach. A single heater of steam cracker furnace was considered wherein Straight Run Naphtha (SRN) is taken with inlet temperature 60? C. & heated & dilution steam was heated at 200? C. upto 590? C. using heater. Taking the capacity of typical naphtha cracker unit as 0.5 MMTPA, the amount of heat required is close to 32 MMKcal/hr. Also, the quantity of coke required to maintain the temperature of heat source vessel at 700? C. & preheat the said capacity feed is close to 33.5 kTA (considering heat of combustion of coke ?32 KJ/gm coke). This way, we can say that by utilizing low value coke, we are saving almost 32 MMKcal/hr of energy required in the convection section which is further utilized in generating very high-pressure steam of 46 MT/hr which further can be utilized in generating electric power using steam turbines as shown in Table 1. The LP steam produced as result of that can be further utilized in plant operations and utility. The total amount of SCOPE-II CO.sub.2 which could have been produced due to generation of additional 46 MT/hr Very High-Pressure steam is close to 82 TMTPA per 0.5 MMTPA of hydrocarbon processed is getting reduced from the present process. The Coke utilized in generating the energy for preheating hydrocarbon feedstocks in turn results in reduction of equivalent SCOPE III CO.sub.2 of 107 TMTPA per 0.5 MMTPA of hydrocarbon processed, owing to carbon capture. Total CO.sub.2 emission reduction potential of present invention is close to 296 TMTPA CO.sub.2/0.5 MMTPA hydrocarbon processed compared to conventional steam cracker furnace as shown in Table 2.

Energy Balance of Convection Section of Furnace with/without Preheating of Naphtha Using Coke:

[0076] The calculations of heat requirements in convection section of conventional furnace and present invention are shown in Table 1 and CO.sub.2 emissions reduction is shown in Table 2 respectively.

TABLE-US-00001 TABLE 1 Heat requirements in convection section of conventional & present cracker furnace Conventional Cracker Scheme Capacity of Typical Naphtha Cracker, MMTPA 0.5 Steam to Hydrocarbon ratio 0.5 Amount of energy used in convection section, MW 37 Process of present invention Amount of heat carrier particles required, MT/hr 284 Heat of combustion of coke @ 700 C., KJ/g 32.3 Amount of Coke required, kTA 33.5 Heat energy saved, MW 37 Amount of energy required to produce 1 Kg 2.9 Saturated Steam, MJ/Kg Additional Quantity of Steam produced, MT/hr 46

TABLE-US-00002 TABLE 2 CO.sub.2 emissions reduction in present invention vs conventional furnace Conventional Scheme (Basis: 0.5 MMTPA capacity) CO.sub.2 emissions from FG burning in Steam Cracker furnace + DCU Petcoke burning Fuel Gas required in Steam cracker furnace (Assuming 9.59 100% CH4), MT/hr Amount of CO.sub.2 produced from Steam cracker furnace fuel 211 burning, TMTPA SCOPE-III CO.sub.2 emission by 33.5 TMPTA Petcoke produced 107 from Delayed Coker Unit, when subjected to burning in boilers (without CO2 capture) Total CO.sub.2 produced, TMTPA 318 Proposed Scheme (Basis: 0.5 MMTPA capacity) CO.sub.2 emissions from FG burning in furnace, TMTPA 211 CO.sub.2 emissions saved during generation of additional steam 82 in convection section of the Furnace of present process scheme, TMTPA (if done elsewhere without CO2 capture) Coke required for preheating naphtha, TMTPA 33.5 SCOPE III CO.sub.2 emission saved by preventing Petcoke burning 107 in boilers, TMTPA Total CO.sub.2 emissions in proposed scheme, TMTPA 22 Total CO.sub.2 reduction potential, TMTPA 296