Ethylene is the core of the petrochemical industry. The ethylene cracking furnace is the core equipment of the ethylene production unit. Its main function is to process various raw materials such as natural gas, refinery gas, crude oil and naphtha into cracked gas and provide it to other ethylene units. Finally processed into ethylene, propylene and various by-products.
1. Ethylene cracking furnace structure
The cracking furnace generally consists of three parts: convection section, radiation section and quenching system.
A. The high-level heat energy required for the reaction is provided by burning fuel through the burner in the radiant section.
B. The purpose of the convection section is to recover the waste heat of high-temperature flue gas to gasify the raw materials, superheat them to the cross-temperature, and send them to the radiation section for thermal cracking; the excess heat is used to preheat the boiler feed water and superheat it by quenching High-pressure steam produced by boiler systems.
Each group of coils in the convection section of the cracking furnace is mainly composed of heat exchange furnace tubes (plain tubes or finned tubes) assembled and welded through return elbows. The end tube plates and the middle tube plates support the furnace tubes, and the inlets and outlets of some coil tubes are Collected together through collection boxes. Each group of coils is surrounded by the furnace wall to form a module.
C. The function of the quenching boiler system is to recover the energy of the high-temperature cracking gas leaving the radiant section to generate saturated ultra-high pressure steam. About 42% of the combustion heat provides reaction heat and temperature rise in the radiant section, about 51.5% is recovered in the convection section, about 1.5% is heat loss, and the rest is smoke exhaust loss.
2. Performance of CDQ refractory materials
The types of ethylene cracking furnaces can technically be divided into double radiation chambers, single radiation chambers and millisecond furnaces. From the furnace type, it can be divided into CBL cracking furnace, SRT cracking furnace, USC cracking furnace, KTI GK cracking furnace, millisecond cracking furnace and Pyrocrack cracking furnace.
2.1. CBL cracking furnace
The CBL furnace is a highly selective cracking furnace developed in my country in the 1990s by Beijing Research Institute of Chemical Industry, Sinopec Engineering and Construction Company, Lanzhou Chemical Machinery Research Institute and other units.
The convection section of the CBL cracking furnace is set on one side of the upper part of the radiation chamber, and a flue and an induced draft fan are set on the top of the convection section. The convection section is equipped with raw material, dilution steam, boiler feed water preheating, raw material superheating, dilution steam superheating, and high-pressure steam superheating sections. Injection of dilution steam: Type I and II are used for secondary steam injection, and type III is used for primary steam injection.
The main feature is to change the traditional primary mixing of dilute steam and hydrocarbons in the convection section to a new secondary mixing process. The ratio of primary steam to secondary steam should be controlled within an appropriate range. After adopting the new secondary mixing process, the temperature of materials entering the radiation section can be increased by more than 50°C.
In this way, when the cracking depth remains unchanged, the cracking temperature can be reduced by 5℃-6℃, the flue gas temperature in the radiant section can be reduced by 20℃-25℃, the maximum tube wall temperature can be reduced by 14℃-20℃, and the heat supply of the whole furnace can be reduced by about 10%.
The heat supply adopts a joint arrangement of side wall burners and bottom burners. The side wall burners are flameless burners and the bottom burners are oil and gas combined burners. The furnace has the characteristics of high cracking selectivity, flexible adjustment, and long operating cycle.
2.2. SRT cracking furnace
The SRT type cracking furnace is a short residence time furnace. It was developed by the American Lummus Company in 1963 and industrialized in 1965. After that, the furnace tube type and furnace structure were continuously improved, and SRT-Ⅰ was launched successively. ~ Type VI cracking furnace.
The continuous improvement of this furnace type is to further shorten the residence time, improve the cracking selectivity, increase the yield of ethylene, and have greater flexibility for different cracking raw materials. The SRT furnace is currently the most commonly used furnace in large ethylene plants in the world.
The convection section of the SRT cracking furnace is set on one side of the upper part of the radiation chamber, and the flue and induced draft fan are set on the top of the convection section. The convection section is equipped with preheating for feed, dilution steam and boiler feed water.
Starting from the SRT-VI furnace, the convection section is also equipped with high-pressure steam superheating, thus eliminating the high-pressure steam superheating furnace. In the process of preheating raw materials and diluting steam in the convection section, primary steam injection is generally used. When cracking heavy raw materials, secondary steam injection is also used.
Early SRT cracking furnaces mostly used side wall flameless burners to burn fuel gas. In order to meet the needs of the cracking furnace for burning oil, currently most use a combination of side wall burners and bottom burners. The maximum heat supply of the bottom burner can account for 70% of the total heat load. The thermal efficiency of the SRT-III furnace reaches 93.5%.
2.3. Folding USC cracking furnace
The USC cracking furnace (super-selective cracking furnace) of Stone-Webster (S.W) Company is a single-row double-radiation riser type cracking furnace, and the radiant coil is a W-shaped or U-shaped coil. Since the diameter of the furnace tube used is small, a single cracking furnace has a large number of coil groups (16-48 groups).
Every 2 or 4 groups of radiant coils are equipped with a USX-type (casing type) primary waste heat boiler, and the cracked gases from the outlets of multiple USX waste heat boilers are then aggregated and sent to a secondary waste heat boiler. Later, double-pass sleeved waste heat boilers (SLE) began to be used, and the two-stage waste heat boilers were merged into one stage.
The convection section of the USC cracking furnace is set on the upper side of the radiation chamber, and the flue and induced draft fan are set on the top of the convection section. The convection section is equipped with heat recovery sections such as raw material and dilution steam preheating, boiler feed water preheating and high-pressure steam superheating. Most USC cracking furnaces have one convection section corresponding to one radiation chamber, and there are also cases where two radiation chambers share one convection section.
When all the fuel in the device is gaseous fuel, the USC cracking furnace mostly uses side wall flameless burners; if the device needs to use part of the liquid fuel, a joint arrangement of side wall burners and bottom burners is adopted. The bottom burner can burn gas or oil, and its heat supply can account for 60%-70% of the total heat load.
Since the radiant coils of the USC cracking furnace are short tubes with small diameters and long furnace tubes, the processing capacity of a single tube is low, and each cracking furnace has a large number of coils. In order to ensure that the feed in the convection section can be evenly distributed to each radiant coil, a Venturi nozzle is installed at the inlet of the radiant coil.
USC cracking technology is named based on the selection of residence time, cracking temperature and hydrocarbon partial pressure conditions, so that the generated products contain less by-products such as ethane and have a higher ethylene yield. The short residence time and low hydrocarbon partial pressure make the cracking reaction highly selective.
2.4. Folding KTIGK cracking furnace
The early GK-I type cracking furnace was a double-row vertical tube cracking furnace, and the GK-II type cracking furnace developed in the 1970s was a mixed-row (double-row inlet section, single-row in the outlet section) branch reducing tube.
On this basis, GK-III type, GK-IV type and GK-V type cracking furnaces were developed successively. The GK-V type cracking furnace is a double-pass branch reducing tube. Due to the reduced tube path and shortened tube length, the residence time can be controlled within 0.2 seconds. GK type cracking furnace generally uses a first-level waste heat boiler.
The convection section is arranged on the upper side of the radiation chamber. In addition to preheating raw materials, dilution steam, and boiler feed water, the convection section also superheats high-pressure steam.
The GK cracking furnace adopts a joint arrangement of side wall burners and bottom burners. The bottom burner can burn oil or gas, and its maximum heat supply can account for 70% of the total heat load. The side wall burner is a gas-burning flameless burner.
Different furnace tube configurations are used for different cracking raw materials, allowing greater flexibility in raw materials. The new radiant section furnace tube has short residence time and high thermal efficiency.
2.5. Folding millisecond cracking furnace
Kellogg’s millisecond furnace is a vertical tube cracking furnace, and its radiant coil is a single-pass straight tube. The convection section is on the upper side of the radiant chamber. After the raw material and dilution steam are preheated to the cross-temperature in the convection section, they are sent from the bottom of the cracking furnace to the radiant tube through the cross-tube and pigtail tube. The material flows from bottom to upward and exits from the top of the radiant chamber. The radiant tubes enter the first waste heat boiler.
When cracking light hydrocarbons, a three-stage waste heat boiler is permanently installed; when cracking distillate oil, only a two-stage waste heat boiler is installed. The convection section also preheats the boiler feed water and superheats the high-pressure steam, with a thermal efficiency of 93%.
The millisecond furnace uses a large burner at the bottom, which can burn gas or oil. Since the tube diameter of the millisecond furnace is small and the number of tubes in a single furnace is large, in order to ensure uniform flow of the radiant tube, a pigtail tube is installed at the entrance of the radiant tube to control the flow distribution.
The millisecond furnace tube diameter is small and requires a large number of furnace tubes, resulting in a complex cracking furnace structure and relatively high investment. Because the cracking tube is one-pass, there are no elbows, the resistance drop is small, and the hydrocarbon partial pressure is low, so the ethylene yield is higher than other furnace types.
2.6. Folding Pyrocrack type
Linde developed the Pyrocrack cracking furnace in the 1960s. This type of cracking furnace usually has a double radiant section and single convection section structure. In order to adapt to different raw materials, the Pyrocrack cracking furnace uses three different furnace tube structures: Pyrocrack4-2, Pyrocrack2-2 and Pyrocrack1-1.
Among them, Pyrocrack1-1 type has high selectivity and short residence time. It has the smallest processing capacity of a single group of furnace tubes but high olefin production. The cracking furnaces designed by Linde Company after the 1990s mainly used Pyrocrack1-1 furnace tubes.
3. Energy-saving technology of ethylene cracking furnace
3.1. Improve thermal efficiency of cracking furnace
A. Reduce exhaust smoke temperature.
Under the premise that other conditions remain unchanged, the thermal efficiency of the cracking furnace is directly related to the exhaust gas temperature. Before 1975, the designed exhaust temperature of the cracking furnace was 190-240°C, and the corresponding thermal efficiency was 87%-90%. In the late 1970s, the exhaust temperature of the cracking furnace dropped to 120-140°C, and the corresponding thermal efficiency increased to 92%-93%.
B. Control excess air coefficient
Increasing the excess air can ensure complete combustion of the fuel, but at the same exhaust gas temperature, the exhaust heat loss increases and the thermal efficiency of the cracking furnace decreases accordingly. Therefore, on the premise of ensuring complete combustion of the fuel, reducing the excess air coefficient is also one of the measures to improve the thermal efficiency of the cracking furnace.
C. Strengthen thermal insulation.
At present, in addition to the silicon-aluminum series high-temperature refractory bricks composed of A12O3-SiO2-CaO, the furnace wall has also developed plastic refractory linings and ceramic fiber linings, which can reduce the heat loss of the furnace body by approximately 25%.
In addition, spraying a layer of ceramic lining on the inner surface of the furnace wall of the radiation chamber can further improve the radiation heat transfer and reduce the temperature of the furnace outer wall.
3.2. Improve cleavage selectivity
A. Use a new cracking furnace
In recent years, the residence time of new cracking furnaces has been shortened to about 0.2s, and millisecond cracking technology below 0.1s has emerged. The corresponding naphtha cracking temperature has increased to more than 840°C, and the millisecond furnace has reached 890°C; the cracking temperature of light diesel oil has increased to Above 820℃, the millisecond furnace reaches 870℃.
As the residence time is greatly shortened, the ethylene yield of millisecond furnace cracking products is greatly increased. For butane and distillates, the millisecond furnace cracking process can increase ethylene yield by 10%-15% compared with the cracking process with a residence time of 0.3-0.4s.
B. Choose high-quality cracking raw materials
Under the premise of the same process technology level, the ethylene yield mainly depends on the properties of the cracking raw materials. The comprehensive energy consumption of different cracking raw materials varies greatly. The choice of cracking raw materials determines the energy consumption level of ethylene production to a large extent.
C. Optimize process operating conditions
By optimizing the process operating conditions of the cracking furnace, not only the raw material consumption can be greatly reduced, but also the energy consumption of ethylene production can be significantly reduced. Different cracking raw materials correspond to different furnace types and have different optimal soil technology operating conditions.
3.3. Extend the operating cycle of the cracking furnace
A. Optimize raw material structure and process conditions
Generally, raw materials with high hydrogen content and low aromatic content have good cracking performance and are necessary conditions for the long-term operation of the cracking furnace. Hydrotreating raw materials with high unsaturated hydrocarbon content is an effective way to improve oil quality.
Low hydrocarbon partial pressure, short residence time and low cracking temperature are beneficial to extending the cracking furnace operation cycle. However, considering the olefin yield and steam consumption, the cracking depth and steam ratio control need to be optimized.
B. Use online burning
The online burning of the cracking furnace is to continue the burning of the waste heat boiler after the steam-air burning of the furnace tube is completed. Compared with traditional searing methods, online searing has obvious advantages. First, there is no heating and cooling process in the cracking furnace, which can extend the service life of the furnace tube and save the consumption of fuel and dilution steam during the heating and cooling process of the cracking furnace; second, due to online scorching, the cracking furnace has a short offline time, which can improve the operating rate. , and can increase the production of ethylene and ultra-high pressure steam.
C. Use coking inhibitors
Adding coking inhibitors to cracked raw materials or diluted steam can passivate the surface of the furnace tube and extend the coking period of the furnace tube.
According to reports, the CCA-500 coking inhibitor developed by Phillips can extend the furnace tube operating cycle by 2-8 times. After using Jiangyin Tianyuan Chemical’s coking inhibitor N-360 for two ethane furnaces in the 650,000 tons/year ethylene plant of Yangzi Petrochemical Co., Ltd., the operation cycle was extended from the original 45 days to more than 120 days.
D. Use new furnace tubes
Ceramic furnace tube technology generates a nanostructured spinel surface on the inner wall of the furnace tube to inhibit the formation of coke. This material can operate at higher cracking temperatures and is non-catalytic, so it does not form catalytic coke.
Stone & Webster Company conducted experimental tests on ceramic cracking furnace tubes. When ethane is used as the cracking raw material, the furnace tubes do not coke and the ethane conversion rate is high.
The French Petroleum Institute (IFP) and Canadian Nova Chemical Company developed high-temperature ceramic cracking furnace tubes. It is said that using this furnace tube, the conversion rate of ethane cracking is 90%, while the conversion rate of ordinary cracking furnace is only 65%-70%, and it can also effectively control the generation of cracking coke, shortening the operation cycle of the cracking furnace. Significantly extended.
3.4. Improve heat recovery of high-temperature cracking gas
When the cracking furnace all uses naphtha as the cracking raw material, the ultra-high-pressure steam generated by the convection section and the waste heat recovery boiler can roughly balance the power and heating steam required by the ethylene unit. Obviously, improving the convection section of the cracking furnace and heat recovery of high-temperature cracking gas will have a significant impact on reducing energy consumption in ethylene production.
