Research progress on coking mechanism and protective measures of ethylene cracking furnace tube
Introduced the three coking mechanisms of ethylene cracking furnace tube catalytic coking, free radical coking and gas phase coking, as well as the development of coking suppression technology at home and abroad in recent years, and how to improve the cracking raw materials and furnace tube materials, and optimize the cracking process and structure. The use of coking inhibitors and surface coating technologies are introduced in detail. Finally, relevant suggestions are put forward on how to reduce the coking situation and improve the service life of the furnace tube. The comprehensive use of a variety of measures to inhibit coking can effectively reduce the amount of coking, such as the combination of coating technology and metallurgical technology, coating technology and optimization of cracking raw materials combine etc. This has certain reference value for the on-site production operation of ethylene cracking furnace tubes.
Ethylene cracking furnace is the key equipment for ethylene production. Among them, the cracking furnace tube is the core component of ethylene cracking furnace. At present, most enterprises at home and abroad adopt the technology of thermal cracking hydrocarbons in tubular cracking furnace to produce ethylene. In the process of ethylene production by pyrolysis, the active carbon atoms will be precipitated from the cracking feed gas in the furnace tube of ethylene cracking furnace. The activated carbon atoms diffuse and gather to the inner wall of the furnace tube driven by carbon potential, and finally form a coking layer on the surface of the furnace tube. Coking will increase the heat transfer resistance of cracking furnace tube wall, reduce the heat transfer coefficient, lead to the temperature rise of furnace tube outer wall, and local overheating phenomenon, which will shorten the service life of cracking furnace tube. The thickening of the coking layer makes the inner diameter of the furnace tube gradually smaller, which increases the pressure drop of the fluid in the tube, reduces the hydrocarbon treatment capacity, reduces the olefin yield and increases the energy consumption of the production process. When the coking is serious, the furnace tube will be blocked, which will force the ethylene production unit to shut down , which will bring huge economic losses and safety risks to the petrochemical industry.
Research Progress on coking mechanism of furnace tube
The existence of coke layer will cause the microstructure change and performance weakening of furnace tube material . However, due to the secondary reactions such as condensation and polymerization in the process of cracking hydrocarbons to produce ethylene, coking will inevitably occur . Exploring the mechanism of furnace tube coking plays an important role in promoting the development of the technology of restraining tube coking. In the past decades, researchers at home and abroad have done a lot of research work on the coking mechanism of furnace tubes. It is generally accepted that the coking mechanism proposed in reference .
Metal catalytic coking
HK or HP series alloys are mainly used as furnace tube materials for ethylene cracking furnace, and these two alloys are Fe Cr Ni Alloys, which mainly contain transition elements Fe, Cr and Ni of the first kind.
Li chusen et al.  have proved through experiments that Ni and Fe are catalysts for coking of furnace tubes. The experimental results also provide evidence for the catalytic coking of Fe, Ni and other elements. As shown in Fig. 1, in the early stage of coking, the Cr2O3 protective film on the inner wall of the furnace tube cracks under the action of complex stress, resulting in the bare metal Fe and Ni in the furnace tube. The activated carbon atoms generated from the pyrolysis raw material are adsorbed on the inner wall surface under the driving of carbon potential and contact with the catalytic metals Fe and Ni. The two main transition elements Fe and Ni can form unstable transition carbides with carbon, In the high temperature environment, the transition carbide is unstable, and decomposes into carbon and metal particles, completing the diffusion of carbon into the particles . When the stress increases to a certain value, the catalytic particles will be jacked up from the surface of the furnace tube, and the metal particles lifted up will be exposed to the carbon atmosphere, which provides conditions for repeating the above catalytic process. The above process makes carbon atoms continuously gather at the bottom of metal particles. When the subsequent diffused carbon atoms continue to gather at the bottom of the metal particles, longitudinal filamentous Coke will be formed . Song ruokang et al.  used the cr35ni45 radiant section furnace tube which had been used for 2.5 years from the ethylene cracking unit of Petrochemical Company as the experimental material, and observed by electron microscope that the coke body close to the inner wall of the furnace tube is many filamentous coke, which confirms the conclusion that catalytic coking produces filamentous coke.
Fig.1 Principle diagram of metal catalytic coke
In the late stage of catalytic coking, hydrocarbon free radicals and molecules in the cracking gas gather around the carbon lattice with structural defects to form transverse filamentous coke. Moreover, with the continuous accumulation of carbon deposited in the vapor phase, the active carbon atoms pass through the metal surface and gather around the carbon fiber, so that the transverse filamentous coke is connected to form a carbon layer on the metal surface. At this time, the catalytic particles such as Fe or Ni at the top of the filamentous coke are completely covered by the carbon layer, and the catalytic coking is finished . Li chusen et al.  found that Fe and Ni exist in the inner surface of coke, but not in the outer surface after scanning coke with energy dispersive X-ray surface, which also demonstrates the evolution process of catalytic coking mentioned above.
Free radical coking
As shown in Fig. 2, the free radicals take the filamentous coke and carbon black particles formed by metal catalytic coking and gas-phase coking as the coking parent, and react with the particles formed by the aggregation of small molecular substances with relative molecular weight less than 100, such as acetylene and ethylene, to generate molecules with acyclic long chain structure. This molecular structure is easy to crack in high temperature environment to generate molecules with other structures, After a series of reactions, the ring structure, namely polycyclic aromatic hydrocarbons, is formed. The polycyclic aromatic hydrocarbons are further dehydrogenated and condensed to produce new coke and new free radicals [9,11]. Due to the instability of acyclic long chain structure molecules, this kind of dehydrogenation reaction is very rapid and can generate more free radicals continuously. Moreover, with the accumulation of coke, the surface area and temperature of coke surface will increase, and the polycondensation reaction will be intensified, which further increases the amount of free radicals on the surface of coke. These free radicals, as “active substances”, become a part of the gas phase components, accumulate with the free radical phase generated by the pyrolysis of raw materials, react with the particles coalesced with the above small molecular substances to form coke. In this way, the coking matrix increases rapidly and coke particles are formed.
Fig.2 Principle diagram of free radical coke
Gas phase coking
Gas phase coking is a coking method that generates coke in the main gas stream, independent of the alloy material of the furnace tube itself, only related to the cracking raw material itself . In reference , the process of gas-phase coking was studied in detail by means of electron microscope and micrograph. It was considered that aromatics produced by cyclization of olefin polymerization was the most important intermediate of gas-phase coking. During the pyrolysis of heavy feedstock, these aromatics polymerize to form spherical, brown film or black flake coke. As shown in Fig. 3, aromatics produced by olefin reaction or aromatics contained in raw materials are condensed and dehydrogenated in the gas phase to form condensed tar of polycyclic aromatic hydrocarbons. After further cyclization and condensation reaction, tar drops are generated. After the formation of oil droplets, they impact on the inner wall of furnace tube, and most of the oil droplets will be adsorbed on the inner wall of furnace tube and finally enter the coking layer, The rest of the oil droplets rebound back to the gas phase. Because the tar drops in the gas phase are not completely dehydrogenated, the possibility of hydrogen capture reaction of free radicals in the gas phase is increased, so the volume of coking layer is further increased and semi tar drops and carbon black particles are formed through dehydrogenation reaction. However, limited by the temperature range of hydrocarbon cracking, less coke is generated by gas-phase coking under the working condition of less than 700 ℃ .
Fig.3 Principle diagram of gas phase coking
Protection measures for coking
The ethylene industry causes huge losses due to coking every year. Although the coke accumulated in the tube can be removed temporarily by coke cleaning technology, the outermost layer of the internal carburized layer is likely to be transformed into an oxide layer mainly composed of Fe and Cr when the production is started after complete coke cleaning. The Fe existing on the inner surface of the furnace tube will further cause coking, destroy the oxide film, expose the substrate of the furnace tube material, change the material composition and properties, and reduce the coking resistance [6,13]. Therefore, researchers have developed many measures to inhibit coking based on the research of coking mechanism, which mainly include the following aspects.
Optimization of cracking feedstock and process conditions
Optimization of cracking feedstock
It is an effective measure to control coking by adjusting cracking feedstock. Aromatics are the main materials causing coking in raw materials, and the coking rate is proportional to its content. The coking of furnace tubes varies with raw materials and aromatic content. Literature  shows that when light diesel oil containing 15% aromatics is used as cracking raw material, the industrial operation period reaches 60 days, while when Kuwait vacuum diesel with 46% aromatics is used as cracking raw material, the industrial operation cycle is only 7-20 days. With the increase of feed oil weight or olefin content, the amount of coking is also increasing. Therefore, light and high-quality cracking raw materials have obvious effect on reducing coking amount, which can not only prolong the operation cycle of the equipment, improve energy utilization and reduce energy consumption. It can also increase the yield of triene (ethylene, propylene and butadiene) . In reference , it is proposed that the process technologies such as raw material hydrogenation and aromatics extraction can reduce aromatics content and saturate olefins. By increasing the hydrogen content of raw materials, polycyclic aromatic hydrocarbons (PAHs) can be converted into cycloalkanes, thus improving the quality of oil products.
Optimization of process conditions
After the feedstock is determined, coking can be restrained by changing the reaction conditions. Low hydrocarbon partial pressure, short residence time and low reaction temperature are favorable to improve coking. However, the olefin yield needs to be guaranteed by a certain cracking depth, so it is considered to reduce hydrocarbon partial pressure to improve furnace tube coking . In addition, another measure to improve coking mentioned in reference  is to increase the diluted steam flow rate to 160% of the design value before feeding, form a steam film in the furnace tube, and control the residence reaction time of raw materials in the tube. This process can reduce the coking amount in the feeding process. However, in actual production, the optimal cracking conditions should be determined by combining on-line analysis with sampling analysis, considering olefin yield and cracking furnace operation cycle.
New material and new structure furnace tube
Improving tube material and optimizing tube structure can effectively inhibit coking, especially catalytic coking, and effectively improve the heat transfer efficiency of furnace tube.
New material furnace tube
Among foreign research institutions, Oak Ridge National Laboratory  of the United States developed a new material by CO extruding and co casting aluminides into the furnace tube materials during the manufacturing process of furnace tubes. The surface of the furnace tubes has a 3.2 mm thick aluminide coating. The aluminide protective layer can effectively inhibit the development of coking. The ceramic impregnation tube developed by S & W company and the ceramic radiation cracking furnace developed by IFP gazde France company in France can achieve the designed conversion rate under high temperature conditions without catalytic coke formation, and can effectively control the generation of high temperature pyrolysis coke . The alloy MA956 furnace tube developed by Incoloy company  not only has high creep resistance, but also has outstanding anti coking effect. Compared with cr25ni35 alloy, which is widely used in cracking furnace tube, the coking rate of furnace tube is reduced by 50%, and no subsequent carburizing phenomenon occurs. There are few research results on new furnace tube materials in China, and most of them are in the experimental stage. Among them, the new anti coking composite furnace tube produced by the Institute of metals, Chinese Academy of Sciences, is more successful. By coating the materials that do not produce catalytic coking and no carbonization reaction on the surface of the materials, the anti coking ability is 3 times higher than that of the ordinary furnace tubes currently used .
New structure furnace tube
In terms of improving the furnace tube structure, many research institutions at home and abroad have developed a variety of new furnace tubes based on the idea of destroying the boundary laminar flow layer and reducing the boundary layer temperature to slow down coking, such as Mert furnace tube (mixed unit radiation furnace tube) developed by Kubota, inner spiral finned tube used by kelgo company, and inner spiral plum blossom tube adopted by Lummus company, etc., which can effectively inhibit coking . The Institute of metals, Chinese Academy of Sciences and other units  have successfully developed cracking furnace tubes with twisted plates. The twisted plate structure will rotate the high-speed material in the tube at 180 ℃, which will destroy the laminar flow layer of the material boundary. Moreover, the temperature of the inner wall of the furnace tube decreases, which slows down the coking and prolongs the operation period of the cracking furnace up to 10 ~ 20 days.
Addition of coking inhibitor
At present, it is feasible to add the inhibitor to the coking tube of ethylene furnace. Adding coking inhibitor to the cracking furnace can ensure that the furnace tube in the radiation section is at a lower temperature, effectively reduce the fuel consumption and coke amount, and improve the service performance and service life of the cracking furnace. The main types of coking inhibitors for industrial application are: organometallic compounds, rare earth compounds, phosphorus compounds, boron compounds, sulfur compounds, alkali metals and alkali metal salts, phosphorus compounds, etc.
The representative enterprise of sulfur and phosphorus compounds is Nalco chemical company . The company’s inhibitors range from monoester and diester of phosphoric acid or phosphite to thiophosphoric acid or phosphite monoester and diphenylphosphine oxide, and then to triphenylphosphine oxide and triphenylphosphine oxide. Philips oil company  is a representative company for the production of metal compound coking inhibitors. The main components are: Ge and Sn, sb; P and Sn, Sb, etc. The coking inhibitors developed by Bates research company  are mainly boron compounds, rare earth elements and their compounds. These non phosphorus series inhibitors are suitable for the operating temperature higher than 760 ℃. However, boron compounds can effectively inhibit coking when the temperature is lower than 1050 ℃.
For example, the coke yield of Shanghai Petrochemical Co., Ltd. can be improved by 50.0% due to the co-development of several strong coke cracking agents from East China University of science and technology to Shanghai Petrochemical Co., Ltd. Li Changming et al.  and Luan Xiaojian et al.  of the Research Institute of Lanhua company studied the coking law of naphtha cracking and the coking inhibition effect of DMDs. In the range of 0 ~ 200 μ g / g, the higher the concentration of inhibitor, the less coking. Moreover, adding 200 μ g / g DMDs into the pyrolysis feedstock has the best inhibition effect on coking, which can reduce the coking rate by more than 20%, and prolong the coke cleaning cycle by more than 20%. A new type of high-efficiency coking inhibitor for ethylene cracking furnace independently developed by Beijing Institute of science and technology  has obtained the invention patent right granted by the State Intellectual Property Office. The introduction of non-toxic and highly effective components (such as phenyl mercaptan compounds, metal passivators, etc.) has completely changed the shortcomings of traditional dimethyl sulfur inhibitors, such as pollution, corrosiveness and large amount.
Preparation of coating on furnace tube surface
By preparing specific inert anti coking coating on the inner surface of radiation section furnace tube , the catalytic activity of Fe, Ni and other particles on the surface of furnace tube can be reduced, the adhesion of coking precursor can be reduced, and coking can be effectively inhibited. There are a lot of uneven carbon deposition and filamentous coke on the surface of uncoated samples . The surface of the sample with Al Si Cr coating is granular coke, the coke layer is even and flat, and the diameter is very small. The coating preparation methods include: high temperature sintering , atmosphere treatment , vapor deposition , solid powder embedding infiltration [29, 32], etc.
In China, in reference , a layer of Al Si Cr coating with thickness of about 130 μ m was prepared on the surface of cr25ni35nb alloy by solid powder embedding infiltration method, and the coking inhibition rate was 72.5%. In reference , the anti coking glass coating was prepared on the inner surface of HP40 alloy furnace tube by high temperature sintering method with SiO2, Bao, Cao and Al2O3 as raw materials. The anti coking performance of furnace tube was increased by more than three times than that of ordinary furnace tube. In reference , 35cr45ni alloy was oxidized in a low oxygen partial pressure atmosphere of H2-H2O, and oxide films with Cr2O3 and mncr2o4 as the main components were formed on the surface of the alloy. At 800 ℃, 900 ℃ and 1000 ℃, the coking inhibition rates reached 81%, 93% and 56%, respectively. In reference , researchers prepared SiO2 or silicide coating on the inner surface of alloy furnace tube by chemical vapor deposition method, which has good coking resistance. The solid powder embedding infiltration used in reference  refers to the process of heating, constant temperature and cooling after acid pickling of furnace tube to make the metal elements in the penetration agent penetrate into the inner wall of the furnace tube to form an anti coking alloy layer, so as to enhance the anti coking performance of the furnace tube.
In the research of coating preparation abroad, several successful representatives are: the composite coating technology of SK company of Korea  not only improves the bonding performance of coating and furnace tube, but also limits the diffusion of metal elements and carbon atoms, and effectively reduces the amount of coke produced by thermal cracking. Novartis chemical company  prepared nano spinel surface on the inner surface of furnace tube to inhibit coke formation. This technology can prolong the cleaning period of furnace tube by 10 times, up to 516 days. After 3 years of operation, its service activity still remains at 50%. The advanced double diffusion system developed by alon company  can prepare double layer structure of Cr Si bottom layer and Al Si outer layer, Cr Si bottom layer protects furnace tube, and Al Si outer layer inhibits coking. This technology can double the cleaning cycle, prolong the life of the furnace tube to 10 years, and increase the heat temperature in the furnace tube from 1000 ℃ to 1200 ℃.
The ultimate purpose of the research on coking of ethylene cracking furnace tubes is to restrain coking and improve the operation cycle and service life of the equipment. This requires that in the actual production, according to different stages and different situations, effective measures should be taken flexibly to slow down coking. A variety of protective measures can be taken to meet the production protection requirements, such as combining coating technology with metallurgical technology, combining coating technology with optimized raw materials, etc., which can effectively reduce the coking amount. At the same time, the equipment is in different stages, and the protective measures should be selected differently. In the stage of equipment design, coating, new material or new structure can be used. In the operation stage, coking inhibitor should be added as the main means. The comprehensive and flexible application of these technologies will certainly achieve ideal results in prolonging the operation cycle of the equipment, increasing ethylene yield and reducing energy consumption.
The authors have declared that no competing interests exist.
Author: Sun Xiaoru, Shen Limin
Source: China Seamless Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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