CN111944556A - Flexible preheating of boiler feed water and pyrolysis gas heat recovery method and ethylene cracking furnace heat exchange system - Google Patents
Flexible preheating of boiler feed water and pyrolysis gas heat recovery method and ethylene cracking furnace heat exchange system Download PDFInfo
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- CN111944556A CN111944556A CN201910399095.1A CN201910399095A CN111944556A CN 111944556 A CN111944556 A CN 111944556A CN 201910399095 A CN201910399095 A CN 201910399095A CN 111944556 A CN111944556 A CN 111944556A
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- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- C10G9/00—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils
- C10G9/14—Thermal non-catalytic cracking, in the absence of hydrogen, of hydrocarbon oils in pipes or coils with or without auxiliary means, e.g. digesters, soaking drums, expansion means
- C10G9/18—Apparatus
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- C07C4/00—Preparation of hydrocarbons from hydrocarbons containing a larger number of carbon atoms
- C07C4/02—Preparation of hydrocarbons from hydrocarbons containing a larger number of carbon atoms by cracking a single hydrocarbon or a mixture of individually defined hydrocarbons or a normally gaseous hydrocarbon fraction
- C07C4/04—Thermal processes
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- C07C5/00—Preparation of hydrocarbons from hydrocarbons containing the same number of carbon atoms
- C07C5/32—Preparation of hydrocarbons from hydrocarbons containing the same number of carbon atoms by dehydrogenation with formation of free hydrogen
- C07C5/327—Formation of non-aromatic carbon-to-carbon double bonds only
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Abstract
The invention belongs to the field of petrochemical industry, and relates to a flexible preheating of boiler feed water and pyrolysis gas heat recovery method and an ethylene cracking furnace heat exchange system. Boiler feed water from a battery compartment is preheated in a boiler feed water preheater of a convection section of the cracking furnace, the boiler feed water preheater comprises an upper boiler feed water preheater and a lower boiler feed water preheater which are independently arranged, and the boiler feed water from the battery compartment is divided into two paths which respectively enter the upper boiler feed water preheater and the lower boiler feed water preheater for preheating; pyrolysis gas at the outlet of the radiation section of the cracking furnace sequentially enters a first quenching heat exchanger group and a second quenching heat exchanger group for heat exchange. The flexibility process of the invention obviously improves the energy utilization rate and production flexibility of the device, realizes energy saving and consumption reduction in the production process, increases the yield of ultrahigh pressure steam, delays the occurrence of coking reaction and improves the overall economy of the ethylene cracking device.
Description
Technical Field
The invention belongs to the field of petrochemical industry, and particularly relates to a flexible preheating of boiler feed water and pyrolysis gas heat recovery method and an ethylene cracking furnace heat exchange system.
Background
The ethylene cracking furnace is a process device for generating ethylene and producing a byproduct of ultrahigh pressure steam by performing steam thermal cracking on petrochemical raw materials such as naphtha, diesel oil, hydrogenation tail oil, ethane, propane, LPG and the like, and is also the most energy-consuming part in the whole ethylene production device, and the energy consumption of the device accounts for 50-60% of the total energy consumption of ethylene production. Effectively improving the heat efficiency and the steam output of the cracking device, and is the key point for realizing energy conservation, consumption reduction and economic benefit improvement in the ethylene production process.
As shown in fig. 1, a general ethylene cracking furnace mainly comprises a cracking furnace body, a quenching heat exchanger and a high-pressure steam drum, wherein the cracking furnace body is divided into a convection section and a radiation section, and the radiation section is a core place of a cracking reaction. The cracking reaction process has high requirement on temperature, and the temperature of the flue gas in the radiation section can often reach over 1000 ℃. In order to fully utilize the heat of the high-temperature flue gas from the radiation section, a plurality of heat exchange sections such as a raw material preheater, a boiler feed water preheater, a dilution steam superheater, an upper mixing superheater, an ultrahigh pressure steam superheater and a lower mixing superheater are sequentially distributed in the convection section of the cracking furnace, and the heat recovery is realized through heat exchange. The boiler feed water preheater is a key module for heat recovery of the convection section and is also the key for steam drum steam quantity balance and energy balance.
Besides the boiler feed water in the convection section realizes the recycling of the heat of the flue gas, the boiler feed water also realizes the recycling of the heat in the quenching process of the pyrolysis gas. During normal cracking, the temperature of the cracking gas flowing out of the outlet of the radiation section of the cracking furnace is up to over 800 ℃, in order to inhibit the secondary reaction of the cracking gas and improve the olefin yield, the high-temperature cracking gas needs to be rapidly cooled in a quenching mode, and a quenching heat exchanger is a main place for recycling the heat of the cracking gas. At present, there are three quenching processes for cracking gas, which are a first-stage quenching heat exchange process (as shown in fig. 2), a second-stage quenching heat exchange process, and a third-stage quenching heat exchange process. For the first-stage quenching heat exchange process, high-temperature cracking gas at the outlet of a coil pipe at the radiation section of the cracking furnace enters a first-stage quenching heat exchanger, the cracking gas is cooled by high-pressure boiler feed water from a steam drum, the boiler feed water after heat exchange returns to the steam drum, and ultrahigh-pressure steam is a byproduct. For the traditional two-stage quenching heat exchange process, high-temperature pyrolysis gas at the outlet of the radiation coil of the cracking furnace sequentially passes through the first-stage quenching heat exchanger and the second-stage quenching heat exchanger to exchange heat with high-pressure boiler feed water from a steam drum to generate ultrahigh-pressure steam. For the traditional primary quenching heat exchange process and the secondary quenching heat exchange process, because the heat exchange medium is saturated boiler feed water from a steam drum, the saturation temperature of the boiler feed water is about 324 ℃ according to the pressure grade of steam, and the temperature of cracked gas cannot be further reduced to be lower through heat exchange. The third stage quenching heat exchanger in the three stage quenching heat exchange process usually uses raw material hydrocarbon or fresh boiler feed water as a heat exchange medium, so that the temperature of the cracked gas can be reduced to below 260 ℃, the heat of the cracked gas can be recovered to a greater extent, and the process is mainly used for cracking gas raw materials.
The inventor of the invention finds in research that in the existing three-stage quenching heat exchange technology, pyrolysis gas exchanges heat with raw material hydrocarbon or fresh boiler feed water in a third-stage quenching heat exchanger, the process improves the heat utilization rate to a certain extent, but in practical application, because the temperature of the raw material hydrocarbon or the fresh boiler feed water is lower, the pyrolysis gas is easy to condense and coke when entering the third-stage quenching heat exchanger, the heat exchange efficiency of the heat exchanger is seriously influenced, and thus the heat utilization of the three-stage quenching heat exchange process is not sufficient. And along with the continuous occurrence of condensation coking, the cracking gas outlet pipeline is further blocked, so that the heat exchange efficiency is further reduced, the pressure drop of the pipeline is greatly improved, the normal production is influenced, and the operation period of the device is shortened. Moreover, when boiler feed water is a heat exchange medium, the adjustment of the working condition enables the boiler feed water amount in the quenching process to have larger change, the traditional three-stage quenching flow is not flexible enough to adjust the boiler feed water amount, and the higher energy utilization rate is difficult to ensure all the time.
Therefore, how to optimize the boiler water supply scheme of the cracking furnace, optimize the quenching heat exchange flow of the cracking gas and improve the flexibility of the operation of the device is an important subject of the energy-saving research of the ethylene cracking furnace.
Disclosure of Invention
The invention aims to: the method solves the problems of high energy consumption and insufficient energy utilization in the existing cracking furnace heat exchange technology, solves the problems of poor operation flexibility, easy coking and short production period of the existing device, and provides a method for flexibly preheating boiler feed water and recovering heat of cracked gas and a corresponding ethylene cracking furnace heat exchange system. The invention can improve the flexibility of the device operation, is beneficial to the device to achieve higher energy recovery rate under different production conditions, and simultaneously reduces the occurrence of coking reaction of the device, so that the production device can safely, permanently and efficiently operate.
In order to achieve the aim, the invention provides a method for flexibly preheating boiler feed water and recovering heat of pyrolysis gas, wherein boiler feed water from a battery compartment is preheated in a boiler feed water preheater of a convection section of a pyrolysis furnace, the boiler feed water preheater comprises an upper boiler feed water preheater and a lower boiler feed water preheater which are independently arranged, and the boiler feed water from the battery compartment is divided into two paths which respectively enter the upper boiler feed water preheater and the lower boiler feed water preheater for preheating; pyrolysis gas at the outlet of the radiation section of the cracking furnace sequentially enters a first quenching heat exchanger group and a second quenching heat exchanger group for heat exchange; wherein, the shell pass of the first quenching heat exchanger group is communicated with the steam pocket through an ascending pipe and a descending pipe, and the pipe pass is respectively communicated with the outlet of the furnace pipe of the radiant section of the cracking furnace and the second quenching heat exchanger group; the shell pass of the second quenching heat exchanger group is respectively communicated with the outlet of the upper boiler feed water preheater and the steam drum, and the tube pass is communicated with the first quenching heat exchanger group;
boiler feed water preheated by the upper boiler feed water preheater completely enters a second quenching heat exchanger group to perform non-phase change heat exchange with pyrolysis gas so as to recover the heat of the pyrolysis gas; or part of the steam enters a second quenching heat exchanger group, and part of the steam enters a lower boiler feed water preheater and finally enters a steam drum.
According to the invention, the first quench exchanger bank may comprise one or two stages of quench exchangers, each stage of quench exchangers may comprise one or more. The second quenching heat exchanger group can be a second-stage quenching heat exchanger or a third-stage quenching heat exchanger according to the different stages of the first quenching heat exchanger group, and also can comprise one or more than one quenching heat exchanger.
According to a preferred embodiment of the present invention, as shown in fig. 5, the first quenching heat exchanger set comprises two stages of quenching heat exchangers connected in series, namely a first stage quenching heat exchanger and a second stage quenching heat exchanger; the second quench heat exchanger bank comprises a third stage quench heat exchanger; after coming out from a radiant section furnace tube of the cracking furnace, the cracking gas sequentially passes through a first-stage quenching heat exchanger, a second-stage quenching heat exchanger and a third-stage quenching heat exchanger; the pyrolysis gas exchanges heat with boiler feed water from a steam drum through thermosyphon in a first-stage quenching heat exchanger and a second-stage quenching heat exchanger to generate steam, and exchanges heat with the other path of boiler feed water preheated by a convection section in a third-stage quenching heat exchanger to realize heat recovery. In the present invention, this flow is simply referred to as flexible flow 1. The flexible scheme 1 is a preferred embodiment of the invention.
According to the present invention, in the above manner, preferably, the first stage quench heat exchanger of the first quench heat exchanger group is a vertical linear quench heat exchanger or a conventional quench heat exchanger; the other quenching heat exchangers (including the second-stage quenching heat exchanger of the first quenching heat exchanger group) are horizontal shell-and-tube quenching heat exchangers. Specifically, in the above preferred embodiment, the first-stage quench heat exchanger is a vertical linear quench heat exchanger; the second stage quenching heat exchanger and the third stage quenching heat exchanger are horizontal shell-and-tube type quenching heat exchangers.
Wherein, the linear quenching heat exchanger can be a double-pipe quenching heat exchanger, and also can be a U-shaped or inverted U-shaped quenching heat exchanger; the traditional quenching heat exchanger can be a double-sleeve type quenching heat exchanger and also can be a thin-tube plate type quenching heat exchanger.
In accordance with another preferred embodiment of the present invention, as shown in FIG. 6, the first quench exchanger bank comprises a first stage quench exchanger; the second quench heat exchanger bank comprises a second stage quench heat exchanger; after coming out from a radiant section furnace tube of the cracking furnace, the cracking gas sequentially passes through a first-stage quenching heat exchanger and a second-stage quenching heat exchanger; the pyrolysis gas exchanges heat with boiler feed water from a steam drum through thermosiphon in a first-stage quenching heat exchanger to generate steam, and exchanges heat with the other path of boiler feed water preheated by a convection section in a second-stage quenching heat exchanger to realize heat recovery. In the present invention, this flow is simply referred to as flexible flow 2.
According to the present invention, in the above-described manner, preferably, the first-stage quench heat exchanger is a vertical linear quench heat exchanger or a conventional quench heat exchanger; the second stage quenching heat exchanger is a horizontal shell-and-tube quenching heat exchanger. Wherein, the linear quenching heat exchanger can be a double-pipe quenching heat exchanger, and also can be a U-shaped or inverted U-shaped quenching heat exchanger; the traditional quenching heat exchanger can be a double-sleeve type quenching heat exchanger and also can be a thin-tube plate type quenching heat exchanger.
According to the present invention, the cracking feedstock of the cracking furnace may be various gaseous feedstocks such as ethane, propane, LPG, etc.
According to the invention, the boiler feed water preheater of the cracking furnace is divided into an upper boiler feed water preheater and a lower boiler feed water preheater, wherein the boiler feed water of the upper boiler feed water preheater is all low-temperature boiler feed water from a battery compartment, the boiler feed water of the lower boiler feed water preheater can be all low-temperature boiler feed water from the battery compartment, or part of the low-temperature boiler feed water from the battery compartment, and the other part of the boiler feed water is boiler feed water preheated by the upper boiler feed water preheater. In particular, the amount of the solvent to be used,
for the flexible flow 1, as shown in fig. 5, the boiler feed water from the battery compartment is divided into two paths to enter the upper boiler feed water preheater and the lower boiler feed water preheater of the cracking furnace, and meanwhile, a part of the boiler feed water from the upper boiler feed water preheater is sent to the third stage quenching heat exchanger, and the other part of the boiler feed water is merged with the fresh boiler feed water and enters the lower boiler feed water preheater to be heated continuously. Boiler feed water coming out of the third-stage quenching heat exchanger is directly sent to a high-pressure steam drum to be used for generating ultrahigh-pressure steam.
For the flexible flow 2, as shown in fig. 6, the boiler feed water from the battery compartment is divided into two paths to enter the upper boiler feed water preheater and the lower boiler feed water preheater of the cracking furnace, and meanwhile, a part of the boiler feed water from the upper boiler feed water preheater is sent to the second stage quenching heat exchanger, and the other part of the boiler feed water is merged with the fresh boiler feed water and enters the lower boiler feed water preheater to be heated continuously. Boiler feed water coming out of the second-stage quenching heat exchanger is directly sent to a high-pressure steam drum to be used for generating ultrahigh-pressure steam.
In the two technical processes, the boiler feed water going to the second quenching heat exchanger group is the boiler feed water preheated by the convection section. In the invention, the boiler feed water preheated by the lower boiler feed water preheater enters the steam pocket by adopting a conventional treatment mode. The boiler feed water preheated by the upper boiler feed water preheater can have two options, one is to enter the lower boiler feed water preheater for further preheating, and the other is to go to the second quenching heat exchanger group. According to an embodiment of the present invention, the boiler feed water to the second quench heat exchanger group is at least partially preheated by the upper boiler feed water preheater, and more preferably, the boiler feed water to the second quench heat exchanger group is preheated by the upper boiler feed water preheater, and the boiler feed water directly entering the drum is preheated by the lower boiler feed water preheater.
According to a more specific embodiment of the invention, the boiler feed water preheated by the upper boiler feed water preheater is divided into two streams, wherein one stream enters the second quenching heat exchanger group to exchange heat with the pyrolysis gas from the first quenching heat exchanger group, so that the temperature of the pyrolysis gas is further reduced, and meanwhile, the boiler feed water is further heated, so as to further recover the heat of the pyrolysis gas; the other branch is merged with the other path of boiler feed water from the battery compartment, and after merging, the merged stream continues to enter a boiler feed water preheater below the convection section for preheating, and finally enters a steam drum to exchange heat with pyrolysis gas through thermosiphon in a first quenching heat exchanger group.
In the present invention, further, the diversion of boiler feed water may be controlled by regulating valves arranged at the following positions: the boundary region comes from a boiler water supply main pipe, an upper boiler water supply preheater boiler water supply inlet pipe, a lower boiler water supply preheater boiler water supply inlet pipe, an upper boiler water supply preheater outlet main pipe, a second quenching heat exchanger group boiler water supply inlet pipe, and an upper boiler water supply preheater outlet branch pipe to a lower boiler water supply preheater. Under the guidance of the concept of the invention, a person skilled in the art can select a setting mode as required, the specific setting position of the regulating valve is not particularly limited, and the regulation and control of the amount of the boiler feed water entering the upper boiler feed water preheater and the lower boiler feed water preheater and the regulation and control of the amount of the boiler feed water entering the steam drum, the lower boiler feed water preheater and the second quenching heat exchanger group after the boiler feed water is preheated by the upper boiler feed water preheater can be realized. In particular, the amount of the solvent to be used,
in the flexible flow scheme 1, preferably, temperature regulating valves are arranged on the inlet of the upper boiler feed water preheater, the inlet of the lower boiler feed water preheater and the boiler feed water feed pipe of the third-stage quench heat exchanger. The boiler water supply amount of the upper boiler water supply preheater and the boiler water supply amount of the lower boiler water supply preheater are respectively controlled, and the total amount of the feed water of the cracking furnace boiler is determined by the sum of the two. When the operation days and the initial and final stages of the cracking furnace are different and the operation state and the temperature of the cracking gas are different, the water supply quantity of the boiler can be adjusted through a temperature adjusting valve on a boiler water supply feeding pipeline of the third-stage quenching heat exchanger, so that the maximization of heat recovery is realized.
In the flexible flow scheme 2, preferably, temperature regulating valves are arranged on the inlet of the upper boiler feed water preheater, the inlet of the lower boiler feed water preheater and the boiler feed water feed pipe of the second-stage quench heat exchanger. The boiler water supply amount of the upper boiler water supply preheater and the boiler water supply amount of the lower boiler water supply preheater are respectively controlled, and the total amount of the feed water of the cracking furnace boiler is determined by the sum of the two. When the operation days and the initial and final stages of the cracking furnace are different and the operation state and the temperature of the cracking gas are different, the water supply quantity of the boiler can be adjusted through a temperature adjusting valve on a boiler water supply feeding pipeline of the second-stage quenching heat exchanger, so that the maximization of heat recovery is realized.
According to the invention, the temperature of the pyrolysis gas can be reduced to 170-260 ℃ after the pyrolysis gas is subjected to heat exchange by the second quenching heat exchanger group. In particular, the amount of the solvent to be used,
in the flexible flow 1, after the pyrolysis gas is discharged from the third-stage quenching heat exchanger, the temperature is reduced to 170-260 ℃, and the pyrolysis gas can be directly sent to a downstream separation device without a quencher device.
In flexible scheme 2, the cracked gas is cooled to a temperature below 260 ℃, preferably 170-260 ℃ after passing through the second stage quench heat exchanger, and can be directly sent to a downstream separation device without passing through a quencher device.
The specific cooling temperature is determined according to the composition of the gas raw material, and particularly, for the ethane raw material, as the cracked product almost has no heavy components, the temperature of the cracked gas can be reduced to 170-180 ℃ after the cracked gas is subjected to heat exchange by the second quenching heat exchanger group. Other gaseous materials need to be determined on a case-by-case basis.
The second aspect of the invention provides an ethylene cracking furnace heat exchange system, which comprises a steam drum, a first quenching heat exchanger set and a second quenching heat exchanger set; the boiler feed water preheater of the ethylene cracking furnace comprises an upper boiler feed water preheater and a lower boiler feed water preheater which are independently arranged, wherein the lower boiler feed water preheater is communicated with the upper boiler feed water preheater; the shell side of the first quenching heat exchanger group is communicated with a steam pocket through an ascending pipe and a descending pipe, and the pipe side is respectively communicated with an outlet of a furnace pipe of a radiant section of the cracking furnace and the second quenching heat exchanger group; and the shell pass of the second quenching heat exchanger group is respectively communicated with the outlet of the upper boiler water supply preheating section and the steam drum, and the tube pass is communicated with the first quenching heat exchanger group.
According to the invention, the first quench exchanger group may comprise one-stage or two-stage quench exchangers; the second quench heat exchanger bank comprises a primary quench heat exchanger; preferably, the first stage quenching heat exchanger of the first quenching heat exchanger group is a vertical linear quenching heat exchanger or a traditional quenching heat exchanger; the other quench heat exchangers are horizontal shell-and-tube quench heat exchangers. By providing different numbers of quench heat exchangers, different flowschemes may be provided. The arrangement mode of the quenching heat exchanger group, the type selection of the quenching heat exchanger and the like are as described above, and are not described in detail herein.
According to the invention, further, a plurality of regulating valves are arranged on the pipeline of the heat exchange system of the ethylene cracking furnace to realize the diversion of boiler feed water, and the regulating valves are arranged at the following positions: the boundary region comes from a boiler water supply main pipe, an upper boiler water supply preheater boiler water supply inlet pipe, a lower boiler water supply preheater boiler water supply inlet pipe, an upper boiler water supply preheater outlet main pipe, a second quenching heat exchanger group boiler water supply inlet pipe, and an upper boiler water supply preheater outlet branch pipe to a lower boiler water supply preheater.
The technical scheme of the invention has the following beneficial effects:
(1) cracking furnace convection current section boiler feed water pre-heater divide into two independent pre-heaters in the flexibility flow, and the boiler feed water that comes by the boundary area divide into two the tunnel and get into above-mentioned two independent convection current section pre-heaters respectively and preheat, and the boiler feed water volume can be adjusted respectively, can carry out the distribution of flow according to the operating condition of difference for the device operation is more nimble.
(2) Boiler feed water to the second quenching heat exchanger group is boiler feed water preheated by the convection section, so that the wall temperature of the heat exchange tube is improved, and condensation of heavy components of cracked gas is effectively reduced, thereby slowing down coking, improving steam yield and further reducing energy consumption.
(3) The convection section boiler feed water preheater is divided into two sections to be independently arranged, compared with the traditional one-section arrangement, the heat transfer temperature difference between the flue gas and the boiler feed water is increased, the boiler feed water absorbs more heat under the same heat transfer area, the steam yield and the heat efficiency are both improved, and the heat transfer effect is better; under the condition of the same heat transfer quantity, the area of the heat exchange tube can be correspondingly reduced, and the equipment investment is saved. Meanwhile, the number of the heat exchange pipe flow paths can be reduced by independently arranging the two sections, and the design of the boiler water supply collecting pipe is simpler.
(4) One part of the preheated boiler feed water is directly sent into the steam drum, and the other part of the preheated boiler feed water is sent into a second quenching heat exchanger group to carry out further non-phase change heat exchange with the pyrolysis gas so as to recycle the heat of the pyrolysis gas and then enter the steam drum. The boiler feed water amount entering the second quenching heat exchanger group can be controlled and adjusted through the adjusting valve, and the flexibility is strong. When the number of days of operation of the cracking furnace is different in the early and late stages, the operation state and the temperature of the cracking gas are different, or the normal operation working condition and the decoking working condition are switched, the water supply quantity of the boiler can be adjusted through the adjusting valve, and the maximization of heat recovery is realized.
(5) Compare in traditional flow, the cooling of pyrolysis gas is whole to be accomplished by the rapid cooling heat exchanger in the flexibility flow, need not to set up oil quench cooler device, therefore the heat of pyrolysis gas is absorbed by boiler feed water as much as possible for produce superhigh pressure steam, thereby greatly increased superhigh pressure steam output, greatly reduced ethylene cracker's energy consumption.
(6) Compared with the prior art, the flexible process does not use hydrocarbon with lower temperature as a heat exchange medium, effectively reduces the occurrence of condensation coking in the quenching heat exchanger, not only improves the heat exchange efficiency of the device, but also is beneficial to prolonging the production period of the device and the safe, long-term and efficient operation of the production device.
Additional features and advantages of the invention will be set forth in the detailed description which follows.
Drawings
The above and other objects, features and advantages of the present invention will become more apparent by describing in more detail exemplary embodiments thereof with reference to the attached drawings, in which like reference numerals generally represent like parts throughout.
FIG. 1 is a schematic diagram of an ethylene cracking unit.
FIG. 2 is a schematic diagram of a primary quench heat exchange process of the prior art.
FIG. 3 is a schematic diagram of a two-stage quench heat exchange process of the prior art.
FIG. 4 is a schematic diagram of a three-stage quench heat exchange process of the prior art.
FIG. 5 is a schematic diagram of the three stage quench heat transfer flexibility process of the present invention.
FIG. 6 is a schematic diagram of the flexibility of the two-stage quench heat exchange process of the present invention.
Description of reference numerals:
1. a radiation section of the cracking furnace; 2. a convection section of the cracking furnace; a. an upper raw material preheater; b. boiler feed water preheater: (BFW-I: upper boiler feed water preheater, BFW-II: lower boiler feed water preheater); c. a lower raw material preheater; d. an upper mixer and a lower ultrahigh pressure steam preheater; e. a medium and ultrahigh pressure steam preheater; f. a lower ultrahigh pressure steam preheater; g. a dilution steam superheater; h. a down mixer; 3. a quench heat exchanger bank; 4. and (4) a steam drum.
Detailed Description
Preferred embodiments of the present invention will be described in more detail below. While the following describes preferred embodiments of the present invention, it should be understood that the present invention may be embodied in various forms and should not be limited by the embodiments set forth herein.
Example 1
This example 1 (flexibility scheme 1) utilizes an ethylene plant cracking furnace as shown in figure 5. The heat exchange system comprises a steam drum 4, a first quenching heat exchanger group 3 and a second quenching heat exchanger group 3; the first quenching heat exchanger group 3 comprises two stages of quenching heat exchangers in series connection, namely a first stage quenching heat exchanger and a second stage quenching heat exchanger; the second quenching heat exchanger group 3 comprises a first-stage quenching heat exchanger which is a third-stage quenching heat exchanger; the first stage quenching heat exchanger is a vertical linear quenching heat exchanger; the second stage quenching heat exchanger and the third stage quenching heat exchanger are horizontal shell-and-tube type quenching heat exchangers. The boiler feed water preheater b of the ethylene cracking furnace comprises an upper boiler feed water preheater BFW-I and a lower boiler feed water preheater BFW-II which are independently arranged and communicated with the upper boiler feed water preheater BFW-I; the upper boiler feed water preheater BFW-I and the lower boiler feed water preheater BFW-II are both directly communicated with a boiler feed water main pipe from a battery limits; the shell passes of the first-stage quenching heat exchanger and the second-stage quenching heat exchanger are both communicated with the steam drum, and the tube passes are respectively communicated with the outlet of a furnace tube of the radiant section of the cracking furnace and the third-stage quenching heat exchanger; and the shell pass of the third-stage quenching heat exchanger is respectively communicated with a BFW-I outlet of the upper boiler feed water preheater and the steam drum 4, and the tube pass is communicated with the second-stage quenching heat exchanger. And the inlet of the upper boiler feed water preheater, the inlet of the lower boiler feed water preheater and the boiler feed water feed pipe line of the third-stage quenching heat exchanger are provided with temperature regulating valves.
After coming out from a radiant section furnace tube of the cracking furnace, the pyrolysis gas sequentially passes through a first-stage quenching heat exchanger, a second-stage quenching heat exchanger and a third-stage quenching heat exchanger to exchange heat; the pyrolysis gas exchanges heat with boiler feed water from a steam drum through thermosyphon in the first-stage quenching heat exchanger and the second-stage quenching heat exchanger, and exchanges heat with a stream of boiler feed water preheated by the upper boiler feed water preheater in the third-stage quenching heat exchanger, so that heat recovery is realized.
Boiler feed water from a battery compartment is divided into two paths which respectively enter an upper boiler feed water preheater BFW-I and a lower boiler feed water preheater BFW-II of the cracking furnace for preheating, meanwhile, one part of the boiler feed water from the upper boiler feed water preheater BFW-I is sent to a third stage quenching heat exchanger, and the other part of the boiler feed water is converged with fresh boiler feed water and enters the lower boiler feed water preheater BFW-II for continuous heating. Boiler feed water coming out of the third-stage quenching heat exchanger is directly sent to a high-pressure steam drum to be used for generating ultrahigh-pressure steam.
Comparative example 1
An ethylene plant cracking furnace as shown in figure 3 was used.
After coming out from a radiant section furnace tube of the cracking furnace, the pyrolysis gas sequentially passes through a first-stage quenching heat exchanger, a second-stage quenching heat exchanger and an oil quencher; wherein, pyrolysis gas exchanges heat with drum boiler feed water in the first-stage quenching heat exchanger, second-stage quenching heat exchanger, and the cooling is realized through the direct mixing of quenching oil in the oil quencher.
In the flow, all boiler feed water is preheated by the boiler feed water preheater b and then enters the steam drum 4.
Comparative example 2
An ethylene plant cracking furnace as shown in figure 4 was used.
After coming out from a radiant section furnace tube of the cracking furnace, the cracking gas sequentially passes through a first-stage quenching heat exchanger, a second-stage quenching heat exchanger and a third-stage quenching heat exchanger; wherein, the pyrolysis gas exchanges heat with the boiler feed water of the steam drum in the first-stage quenching heat exchanger and the second-stage quenching heat exchanger, and exchanges heat with the raw material hydrocarbon in the third quenching heat exchanger.
In the flow, all boiler feed water is preheated by the boiler feed water preheater b and then enters the steam drum 4.
Example 2
Example 2 (flexibility scheme 2) an ethylene plant cracking furnace as shown in figure 6 was used. The difference from the embodiment 1 is that: only two-stage quenching heat exchangers are arranged in the process.
After coming out from a radiant section furnace tube of the cracking furnace, the cracking gas sequentially passes through a first-stage quenching heat exchanger and a second-stage quenching heat exchanger for heat exchange; the pyrolysis gas exchanges heat with boiler feed water from a steam drum through thermosiphon in the first quenching heat exchanger, and exchanges heat with a stream of boiler feed water preheated by the upper boiler feed water preheater in the second quenching heat exchanger, so that heat recovery is realized.
Boiler feed water from a battery compartment is divided into two paths which respectively enter an upper boiler feed water preheater BFW-I and a lower boiler feed water preheater BFW-II of the cracking furnace, meanwhile, one part of the boiler feed water from the upper boiler feed water preheater BFW-I is sent to a second-stage quenching heat exchanger, and the other part of the boiler feed water is converged with fresh boiler feed water and enters the lower boiler feed water preheater BFW-II to be continuously heated. Boiler feed water coming out of the second-stage quenching heat exchanger is directly sent to a high-pressure steam drum to be used for generating ultrahigh-pressure steam.
Table 1 below shows a comparison of the process parameters for each run under the same feed conditions:
TABLE 1
| Comparative example 1 | Comparative example 2 | Example 1 | Example 2 | |
| Phase of operation | Initial stage | Initial stage | Initial stage | Initial stage |
| Number of stages of quench heat exchanger | Three-stage | Three-stage | Three-stage | Second stage |
| Hydrocarbon feed volume ton/hr | 44.50 | 44.50 | 44.50 | 44.50 |
| Dilution ratio | 0.3 | 0.3 | 0.3 | 0.3 |
| Ultrahigh pressure steam yield ton/hr | 55.55 | 58.6 | 63.80 | 63.80 |
| Hours of annual operation | 8000 | 8000 | 8000 | 8000 |
| Exhaust gas temperature C | 104 | 108 | 103 | 103 |
| Thermal efficiency% | 94.5 | 94.3 | 94.6 | 94.6 |
| Energy consumption per ethylene kJ/kg (C)2H4) | 8948 | 7561 | 7351 | 7351 |
The flexible flow 1 with the three-stage quenching heat exchanger and the flexible flow 2 with the two-stage quenching heat exchanger have the same thermal efficiency and the same process parameters in the initial stage.
The steam production of the flexible scheme increased from 55.55 tons/hr to 63.8 tons/hr by a scheme modification over comparative example 1, indicating an increase in the energy utilization of the ethylene cracking furnace. Meanwhile, the unit ethylene energy consumption is reduced from 8948kJ/kg to 7351kJ/kg, which shows that the flexible flow has better energy-saving benefit.
Compared with the comparative example 2, the steam yield of the flexible flow path is increased from 58.6 tons/h to 63.8 tons/h through the flow path improvement, the thermal efficiency is improved from 94.3 percent to 94.6 percent, and the energy utilization rate of the ethylene cracking furnace is improved. Meanwhile, the unit ethylene energy consumption is reduced from 7561kJ/kg to 7351kJ/kg, which shows that the flexible process has better energy-saving benefit.
Therefore, the flexibility process of the invention obviously improves the energy utilization rate and production flexibility of the device, realizes energy conservation and consumption reduction in the production process, increases the yield of ultrahigh pressure steam, delays the occurrence of coking reaction and improves the overall economy of the ethylene cracking device.
Having described embodiments of the present invention, the foregoing description is intended to be exemplary, not exhaustive, and not limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments.
Claims (10)
1. A method for preheating boiler feed water flexibly and recovering heat of pyrolysis gas is characterized in that boiler feed water from a battery compartment is preheated by a boiler feed water preheater at a convection section of a pyrolysis furnace, the boiler feed water preheater comprises an upper boiler feed water preheater and a lower boiler feed water preheater which are independently arranged, and the boiler feed water from the battery compartment is divided into two paths which respectively enter the upper boiler feed water preheater and the lower boiler feed water preheater for preheating; pyrolysis gas at the outlet of the radiation section of the cracking furnace sequentially enters a first quenching heat exchanger group and a second quenching heat exchanger group for heat exchange; wherein, the shell pass of the first quenching heat exchanger group is communicated with the steam pocket through an ascending pipe and a descending pipe, and the pipe pass is respectively communicated with the outlet of the furnace pipe of the radiant section of the cracking furnace and the second quenching heat exchanger group; the shell pass of the second quenching heat exchanger group is respectively communicated with the outlet of the upper boiler feed water preheater and the steam drum, and the tube pass is communicated with the first quenching heat exchanger group;
boiler feed water preheated by the upper boiler feed water preheater completely enters a second quenching heat exchanger group to perform non-phase change heat exchange with pyrolysis gas so as to recover the heat of the pyrolysis gas; or part of the steam enters a second quenching heat exchanger group, and part of the steam enters a lower boiler feed water preheater and finally enters a steam drum.
2. The method for flexibly preheating boiler feed water and recovering heat of cracked gas as claimed in claim 1, wherein the boiler feed water of the upper boiler feed water preheater is all low-temperature boiler feed water from the battery limits, and the boiler feed water of the lower boiler feed water preheater is all or partially low-temperature boiler feed water from the battery limits.
3. The method for flexibly preheating boiler feed water and recovering heat of cracked gas as claimed in claim 1, wherein the first quenching heat exchanger group comprises two stages of quenching heat exchangers connected in series, namely a first stage quenching heat exchanger and a second stage quenching heat exchanger; the second quench heat exchanger bank comprises a third stage quench heat exchanger;
after coming out from a radiant section furnace tube of the cracking furnace, the cracking gas sequentially passes through a first-stage quenching heat exchanger, a second-stage quenching heat exchanger and a third-stage quenching heat exchanger; the pyrolysis gas exchanges heat with boiler feed water from a steam drum through thermosyphon in a first-stage quenching heat exchanger and a second-stage quenching heat exchanger to generate steam, and exchanges heat with the other path of boiler feed water preheated by a convection section in a third-stage quenching heat exchanger to realize heat recovery.
4. The method for flexible boiler feed water preheating and pyrolysis gas heat recovery according to claim 1, wherein the first quench heat exchanger bank comprises a first stage quench heat exchanger; the second quench heat exchanger bank comprises a second stage quench heat exchanger;
after coming out from a radiant section furnace tube of the cracking furnace, the cracking gas sequentially passes through a first-stage quenching heat exchanger and a second-stage quenching heat exchanger; the pyrolysis gas exchanges heat with boiler feed water from a steam drum through thermosiphon in a first-stage quenching heat exchanger to generate steam, and exchanges heat with the other path of boiler feed water preheated by a convection section in a second-stage quenching heat exchanger to realize heat recovery.
5. The method of claim 1, wherein the boiler feed water to the second quench exchanger group is preheated by an upper boiler feed water preheater, and the boiler feed water directly entering the drum is preheated by a lower boiler feed water preheater.
6. The method for flexibly preheating boiler feed water and recovering heat of pyrolysis gas as claimed in claim 5, wherein the boiler feed water preheated by the upper boiler feed water preheater is divided into two streams, one of which enters the second quenching heat exchanger group to exchange heat with the pyrolysis gas from the first quenching heat exchanger group, so as to further lower the temperature of the pyrolysis gas, and simultaneously, the boiler feed water is further heated, so as to further recover the heat of the pyrolysis gas; the other branch is merged with the other path of boiler feed water from the battery compartment, and after merging, the merged stream continues to enter a boiler feed water preheater below the convection section for preheating, and finally enters a steam drum to exchange heat with pyrolysis gas through thermosiphon in a first quenching heat exchanger group.
7. The method for flexible preheating of boiler feed water and heat recovery of cracked gas as claimed in claim 1, wherein the split flow of boiler feed water is controlled by regulating valves arranged at the following positions: the boundary region comes from a boiler water supply main pipe, an upper boiler water supply preheater boiler water supply inlet pipe, a lower boiler water supply preheater boiler water supply inlet pipe, an upper boiler water supply preheater outlet main pipe, a second quenching heat exchanger group boiler water supply inlet pipe, and an upper boiler water supply preheater outlet branch pipe to a lower boiler water supply preheater.
8. The method for flexibly preheating boiler feed water and recovering heat of cracked gas as claimed in any one of claims 1 to 7, wherein the temperature of the cracked gas is reduced to 170-260 ℃ after the heat of the cracked gas is recovered by the second quenching heat exchanger group.
9. The heat exchange system of the ethylene cracking furnace is characterized by comprising a steam drum, a first quenching heat exchanger set and a second quenching heat exchanger set; the boiler feed water preheater of the ethylene cracking furnace comprises an upper boiler feed water preheater and a lower boiler feed water preheater which are independently arranged, wherein the lower boiler feed water preheater is communicated with the upper boiler feed water preheater; the shell side of the first quenching heat exchanger group is communicated with a steam pocket through an ascending pipe and a descending pipe, and the pipe side is respectively communicated with an outlet of a furnace pipe of a radiant section of the cracking furnace and the second quenching heat exchanger group; and the shell pass of the second quenching heat exchanger group is respectively communicated with the outlet of the upper boiler water supply preheating section and the steam drum, and the tube pass is communicated with the first quenching heat exchanger group.
10. The heat exchange system of the ethylene cracking furnace of claim 9, wherein a plurality of regulating valves are arranged on the pipeline of the heat exchange system of the ethylene cracking furnace, and the regulating valves are arranged at the following positions: the boundary region comes from a boiler water supply main pipe, an upper boiler water supply preheater boiler water supply inlet pipe, a lower boiler water supply preheater boiler water supply inlet pipe, an upper boiler water supply preheater outlet main pipe, a second quenching heat exchanger group boiler water supply inlet pipe, and an upper boiler water supply preheater outlet branch pipe to a lower boiler water supply preheater.
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