【Weekly Topic】On catalytic cracking of fractionated oil and catalytic cracking of residue?

This post was last edited by wyt1234 on 2011-1-26 at 15:56. Short reaction time and low conversion rate; it can reduce secondary reactions and increase the reprocessing ratio； The reaction time is long, which increases the conversion rate; however, an excessively long time leads to an overly high conversion rate, a decrease in the yields of gasoline and diesel, saturation of olefins in the liquefied gas, and a reduction in the yields of propylene and butylene. Generally, 2-4 seconds is used. The unit for processing residue oil adopts a shorter reaction time to reduce the coking rate. Reaction time cannot be adjusted arbitrarily in production; it is determined by the volume of the lift pipe (its diameter and length). In production, the reaction time varies – changes in the feed rate, as well as variations in the conversion rate caused by other factors, all lead to changes in the reaction time. The MIP process technology aims for a low space-time yield in the second reaction zone, with a long reaction time and prolonged contact time between the oil and the catalyst; this increases the degree of over-cracking and the chances of hydrogen transfer reactions, thereby reducing the olefin content in gasoline.

Overview of Catalytic Cracking Section 1, The status and role of catalytic cracking in the refining industry 1. The role of secondary processing of petroleum. Generally, after crude oil is often distilled under reduced pressure, 10 to 40% of light oil products such as gasoline, kerosene and diesel can be obtained, and the rest are heavy fractions and residual oil. Without secondary processing, they can only be used as lubricating oil raw materials or heavy fuel oil. However, the light oil products needed for the national economy and national defense The amount of oil is very large, and the development of internal combustion engines has put forward higher requirements for the quality of gasoline. Straight-run gasoline (lower octane number 40) is generally difficult to meet these requirements. The contradiction between the quantity and quality of light oil products that can be provided by simple processing of crude oil and the quantity and quality of light oil products required for production development has prompted the emergence and development of secondary processing. 2. The role of catalytic cracking in secondary processing. The process of catalytically cracking macromolecular hydrocarbons into small molecular hydrocarbons under the action of catalytic cracking agents and at a certain temperature and pressure is called catalytic cracking. The raw materials are cracked at 450-530°C, 1-3 atmospheric pressure and in contact with the catalyst to generate gas, gasoline, diesel, heavy oil, and coke. The secondary processing of petroleum includes thermal cracking, coking, hydrocracking and catalytic cracking, and the catalytic reforming process of gasoline. In the process of lightening heavy oil, the thermal cracking process is technically backward and has been eliminated. Hydrocracking has advanced technology, high product yield, good quality, and great flexibility, but it has complex equipment, high manufacturing costs, and large hydrogen consumption, which is subject to certain technical and economic restrictions. Catalytic cracking is the main means of lightening heavy oil. At the end of 2001, China's actual crude oil processing capacity was 280 Mt/a, and catalytic cracking processing capacity was about 100 Mt/a. Catalytic cracking accounted for 35.7% of crude oil processing capacity. At present, we* * 80% of gasoline comes from catalytic cracking. 2. Overview of the development of catalytic cracking technology. The process of the catalytic cracking unit. The catalytic cracking reaction is carried out on the surface of the catalyst. The decomposition reaction produces gaseous gasoline, diesel and other products with smaller molecules, leaving the catalyst and entering the product recovery system. The coke generated by the condensation reaction is deposited on the catalyst, causing Its activity gradually decreases. In order to continue the reaction, the coke on the surface of the catalyst must be burned off in time to restore its activity. This process is called "regeneration". It can be seen that it must include two processes. Catalytic cracking has a history of more than 60 years since it was industrialized in 1936. It has developed as follows: 1. Fixed-bed catalytic cracking The first fixed-bed catalytic cracking unit was put into operation in 1936. It is an intermittent operation, and several reaction vessels take turns to perform reaction and regeneration. This device has a complex equipment structure, small production capacity, large steel consumption, and troublesome operation, and has long been eliminated in industry. 2. Moving bed catalytic cracking In the early 1940s, moving beds appeared again. Moving bed catalytic cracking uses a catalyst with a diameter of about 3mm. The feed oil and catalyst enter the top of the reaction vessel at the same time. They contact each other, react while moving downward. When moved to At the lower part of the reaction vessel, a certain amount of coke has been deposited on the surface of the catalyst, so the oil gas is led out from the middle and lower parts of the reactor and the catalyst comes down from the bottom, and is then lifted to the top of the regenerator by the air riser, and then regenerated while the regeneration vessel moves downward. 3. Fluidized bed catalytic cracking Fluidized bed catalytic cracking adopts advanced fluidization technology. The catalyst used is a microsphere catalyst with a diameter of 20-100 μm. It forms a liquefied state with oil, gas or air in the reactor, and the fluidization between the two reactors is convenient. In 1942, the first type I fluidized catalytic cracking unit was put into operation. In 1952, ESSO Company's type IV fluidized catalytic cracking unit was put into operation. 4. Riser catalytic cracking In the early 1960s, molecular sieve catalysts came out. In order to give full play to the high activity characteristics of molecular sieve catalysts, riser reactors appeared. The original bed reaction was replaced by a plug flow reactor with high temperature and short contact time, thus overcoming the shortcomings of backmixing. 5. The development process of catalytic cracking technology in my country. In 1958, my country built a moving bed catalytic cracking unit. In 1965, a Type IV catalytic cracking unit (Fushun) was built. In 1974, a molecular sieve catalyst riser catalytic cracking unit was built. In 1983, the industrial production of fully refined atmospheric pressure residue was successfully tested. In 1998, the fully vacuum residue (Daqing) catalytic cracking unit (VRFCC) was successfully put into operation. Other process technologies: The MGD process that produces more liquefied gas and diesel, the MIP technology that produces more isoparaffins, the flexible double-effect FDFCC technology, the two-stage riser process, the DCC and ARGG processes that increase the production of propylene, the development process of catalyst technology, the natural clay catalyst, the amorphous silica-alumina catalyst, the molecular sieve catalyst, the multi-functional molecular sieve catalyst. The development of catalyst technology is the driving force for the development of catalytic cracking technology and processes. With the advancement of catalyst technology, the catalytic cracking process continues to develop. The early catalytic cracking of straight-run vacuum distillate was aimed at producing aviation kerosene, using light diesel fraction (200-350℃) as raw material, but was later cancelled. Vacuum side line 350-550℃ distillate (VGO) is the most conventional raw material. The VGO of paraffin-based crude oil is better, the VGO of naphthenic crude oil is worse, and the VGO of sulfur-containing crude oil needs to be hydrodesulfurized or moderately hydrogenated. ⑵ Delayed coking distillate oil The 320-500°C distillate oil (CGO) in the side line of the coking fractionation tower is also called coked wax oil. This raw material has high nitrogen content and aromatic hydrocarbon content, which is an unideal raw material. The usual refining ratio is 15-25%. (3 ) The extracted oil of lubricating oil solvent refining is obtained by extracting and refining furfural during the lubricating oil production process. It is mainly alkanes and condensed ring aromatic hydrocarbons. It is a poor catalytic cracking raw material. (4) Atmospheric pressure residual oil. Atmospheric pressure tower bottom residual oil (AR) with low sulfur, heavy metal content and residual carbon can be directly used as raw material, such as Daqing crude oil, Zhongyuan crude oil and other atmospheric residual oil. (5) Except for some crude oils, vacuum tower bottom residue (VR) is generally not used as a raw material alone, but is blended; the amount of blending depends on the properties of the vacuum residue. (6) Deasphalted oil adopts the solvent deasphalting process of propane or butane. Extracting 60% of the deasphalted oil (DAO) from the vacuum residue as raw material has become an integral part of the refinery processing flow. The heavy metal content of DAO is much lower than that of VR, which improves the properties of the raw material. (7) Combined process of raw material pretreatment, aromatics extraction - catalytic cracking combined process, dearomatization of catalytic cracking slurry. Residual hydrodesulfurization - catalytic cracking combined process, propane deasphalting - catalytic cracking combined process, asphalt residue treatment (ART) - catalytic cracking combined process 2, index to measure the properties of raw materials ⑴ Fraction composition Generally speaking, when the chemical composition type of the raw materials is similar, the heavier the fraction, the easier it is to crack, the more relaxed the conditions required, and the higher the coke yield. ⑵ Chemical composition The chemical composition is generally its family composition. The chemical composition of the raw material varies with the source of the raw material. ① Raw materials containing a lot of naphthenes are easy to crack, and the yields of liquefied gas and gasoline are relatively high. The octane number of gasoline is also high, so they are ideal raw materials. ② Raw materials containing a lot of alkanes are also easy to crack, but the gas yield is high and the gasoline yield is low. ③ Raw materials containing many aromatic hydrocarbons are difficult to crack, resulting in lower gasoline yield, high coke yield, and low liquefied gas yield⑶ The level of residual carbon contained in the residual carbon feed oil has a great influence on the operation of the device. It directly affects the amount of coke generated and the heat balance. The residual carbon content of the distillate oil raw material is generally not more than 0.4% (w). The residual carbon content of the atmospheric residual oil is high, about 4.0% (w) or more, which makes the reaction coke yield high. The device has excess heat. ⑷ Nitrogen-containing sulfur-containing compounds Nitrogen-containing compounds and basic nitrogen-containing compounds can seriously poison the catalyst, reduce activity, reduce light oil yield, increase coke yield, increase gasoline iodine value, and decrease stability. Sulfur has no obvious effect on the catalyst, but it increases the corrosion of equipment, increases the sulfur content of the product, decreases stability, and the carbon in the coke enters the flue gas, polluting the environment. ⑸ Heavy metals Heavy metals mainly refer to metals such as iron and vanadium. They exist in the form of metal organic compounds and are divided into two types: volatile and non-volatile. Volatile is equivalent to a compound with an average boiling point of 620°C, which can enter the vacuum distillate used as a raw material for catalytic cracking. Non-volatile heavy metal compounds are a liquid suspended in the residual oil. They need to be treated before they can be used as raw materials for catalytic cracking. We will discuss the impact of heavy metals on catalytic cracking in detail later. 2. Products and Product Characteristics The main product of catalytic cracking is light oil (gasoline, diesel), and liquefied gas, oil slurry and dry gas are obtained at the same time. The generated coke is used in the process. Under normal industrial conditions, the gas yield is about 10-20%. Gas products include: Product name Main component content, properties and uses Dry gas C1, C2, H2, H2S 10% Fuel gas or chemical liquefied gas C3, C4 90% 10%- 20% used for chemical gasoline C5-C11 30%-60% Research method octane number 80-90, good stability. Diesel C10-C20 0-40% contains more aromatics, low cetane number, poor stability, especially for catalytic cracking of residual oil. The oil slurry is mainly composed of condensed ring aromatic hydrocarbons, ranging from 5-7%, ranging from 5-10% of coke condensation products in chemical industry, mixed with residual oil, and burned in the high regeneration process. The catalytic cracking process is generally divided into four systems, namely, reaction regeneration system, fractionation system, absorption stabilization system, and energy recovery system. The new device also has an oil refining system. Here is a brief introduction to the reaction regeneration system and separation and utilization system. 1. Reaction - heating of raw materials in the regeneration system - entering the reactor - mixing with high-temperature catalyst - catalytic cracking reaction - separation of catalyst and products - catalyst stripping - entering the regenerator for charring - the charred and regenerated catalyst enters the reactor. 2, The reaction oil and gas from the reactor of the fractionation system enters the bottom of the fractionation tower, passes through the desuperheating section, uses the cooled oil slurry to cool the reaction oil and gas to a saturated state, and washes away the entrained catalyst dust, and then separates it in the fractionation tower into: top gasoline and rich gas, side line light diesel oil, heavy diesel oil and recycled oil, bottom oil slurry, light diesel oil and heavy diesel oil after stripping out of the device. 3. Absorption stabilization system The absorption stabilization system mainly consists of an absorption tower, a reabsorption tower, a desorption tower and a stabilization tower. The rich gas coming out of the oil and gas separator at the top of the fractionation tower contains gasoline components, and the crude gasoline also contains C3 and C4 components. The function of the absorption stabilization system is to use the method of absorption distillation to separate the rich gas and crude gasoline into dry gas, liquefied gas, and stable gasoline with qualified vapor pressure. Section 2 Catalytic cracking reaction of petroleum hydrocarbons FCC reaction of monomer hydrocarbons FCC reaction of petroleum fractions FCC reaction of residual oil Thermodynamic characteristics of hydrocarbon FCC reaction Kinetic rules of hydrocarbon FCC reaction 1. Catalytic cracking reaction of monomer hydrocarbons 1. Catalytic cracking reaction of monomer hydrocarbons (l) Alkanes are mainly decomposition reactions. Macromolecule alkanes are decomposed into a small molecule alkane and a small molecule alkene, or two molecules of alkenes plus two hydrogens. (2) Alkenes ① Decomposition reaction, one olefin molecule decomposes into two olefin molecules. ② Isomerization reaction, skeletal isomerism and double bond isomerization can occur to generate isomeric olefins. ③ Hydrogen transfer reaction, cycloalkane or naphthenic-olefin emits a hydrogen to saturate the olefin and gradually turn itself into a condensed ring aromatic hydrocarbon. It can also occur in two olefins. The hydrogen transfer reaction is affected by temperature and catalyst activity. ④ Aromatization reaction: cyclodehydrogenation of olefins to produce aromatic hydrocarbons. (3) The ring of cycloalkanes can be broken to form alkenes, which can then continue to react. Cycloalkanes can also be converted into aromatic hydrocarbons through hydrogen transfer reactions. Five-membered rings with side chains can also be isomerized into six-membered cycloalkanes first, and then further dehydrogenated to form aromatic hydrocarbons. (4) Aromatic hydrocarbons The aromatic nuclei of aromatic hydrocarbons are very stable under the conditions of re-catalytic cracking. Generally, ring scission does not occur, but the alkyl side chains on the core are easily broken to generate small molecular olefins. The reaction rate of polycyclic aromatic hydrocarbons is very low. Its main reaction is to condense into condensed ring aromatic hydrocarbons and finally generate coke to release hydrogen and saturate the alkenes. 2. Hydrocarbon catalytic cracking reaction mechanism. Carbon ion refers to the hydrocarbon ion formed by carbon lacking a pair of valence electrons. Its source: it is generated by obtaining a hydrogen ion from an alkene. The hydrogen ion originates from the surface of the catalyst. Illustrate the carbon ion theory through the catalytic cracking reaction of n-hexadecene (self-study). It is necessary to understand the key points of the reaction process explained by the carbon ion theory: fracture mode, β fracture. The stability of the carbon ion: tertiary position > secondary position > primary position. Use the carbon ion mechanism to explain the characteristics of catalytic cracking products. What are the main differences between catalytic cracking and thermal cracking 1. Catalytic cracking of alkanes. Thermal cracking carbon ion reaction. Free radical reaction 1. The reaction speed of isoparaffins is much faster than that of normal alkanes 2. There are many isomers in the product; 3. There are less olefins in the product. 4. The gas products are mainly C3 and C4. 1. i isomerization is faster than normal alkanes. 2. There are less isomers in the product. 3. There are more olefins in the product. 4. The gas products are mainly C1 and C2. 1. The reaction speed is close to that of alkanes. 2. The hydrogen transfer reaction is almost less than that of alkenes and diolefins. 2. Catalytic cracking of cycloalkanes, thermal cracking and carbon ion reaction, free radical reaction. 1. The reaction speed is similar to that of isoparaffins. 2. The hydrogen transfer is significant and a considerable amount of aromatics are generated. 1. The reaction speed is slower than that of normal alkanes; 2. The hydrogen transfer reaction is not significant 4. Catalytic cracking of aromatic hydrocarbons with alkyl side chains. Thermal cracking of carbon ion reactions. Free radical reactions. 1. The reaction speed is similar to that of alkenes. 2. Breaking at the connection between the alkyl side chain and the benzene ring (i.e. dealkylation) 1. The reaction speed is slower than that of alkanes. 2. When the alkyl side chain breaks, a short side chain of 1 to 2 carbons remains on the benzene ring. 2. Catalytic cracking reaction of petroleum fractions 1. Competitive adsorption between various types of hydrocarbons and blocking effect on the reaction. Hydrocarbon FCC is a gas-solid heterogeneous reaction. There is competitive adsorption and reaction blocking effect between various types of hydrocarbons. Gas-solid heterogeneous phase: reaction oil and gas (raw material) - gas phase, Cat particles (in the reactor) - solid reaction oil and gas: external diffusion → internal diffusion → adsorption on the Cat surface → reaction → desorption from the Cat surface → internal diffusion → external diffusion. There are seven steps. The order of the adsorption speed of various hydrocarbons: condensed ring aromatic hydrocarbons > condensed ring cycloalkanes > alkenes > monoalkyl chain monocyclic aromatic hydrocarbons > cycloalkanes > alkanes. In the same family of hydrocarbons: large molecules are faster than small molecules. The order of the speed of the cracking reaction of various hydrocarbons: olefins > macromolecular monoalkyl monoaromatic hydrocarbons > isoparaffins, alkylcycloalkanes > small molecule monoalkyl monocyclic aromatic hydrocarbons > normal alkanes > condensed ring aromatic hydrocarbons. Comparing the speed order of the two, we can find that there are big differences. The most prominent one is condensed ring aromatic hydrocarbons, which adsorb the fastest and react the slowest. 2. Parallel-sequential reactions of catalytic cracking (1) Parallel-sequential reactions Parallel reactions, a chemical reaction can proceed in several directions at the same time. In sequential reactions, the products of the first reaction can continue to react. Monomeric hydrocarbons can react in several directions at the same time during catalytic cracking, and the products of the first reaction can continue to react. The catalytic cracking reaction of petroleum fractions is also a parallel-sequential reaction. (2) The impact of parallel sequential reactions on product distribution As the reaction time prolongs, that is, as the reaction depth increases, the conversion rate continues to increase, and the yield of the final product gas and coke continues to increase. The gasoline yield increases for a period of time at first, but begins to decrease after passing a maximum point. This is because after reaching a maximum point, the rate of gasoline decomposing into gas is higher than the rate of generating gasoline. Diesel is also an intermediate product like gasoline, but its highest point occurs when the conversion rate is low. (3) Secondary reaction We call the reaction of the primary reaction product a secondary reaction. The secondary reactions of catalytic cracking are diverse, some of which are beneficial and some are disadvantageous. Favorable secondary reactions: olefin isomerization to produce high-octane components, hydrogen transfer of olefins and naphthenes to generate stable aromatics and alkanes, olefins are further cracked into dry gas, propylene and butene are saturated through hydrogen transfer, and olefins and high molecular aromatics are condensed into coke. We can control the secondary reaction through the reaction time. The riser reactor has such a function. 3. The reaction and the generation of catalytic carbon are different. The decreasing order of hydrocarbon coke generation rate is as follows: Bicyclic aromatic hydrocarbons > Monocyclic aromatic hydrocarbons > Alkenes > Cycloalkanes > Alkenes are mainly condensed ring aromatic hydrocarbons, which are the main source of coke. Alkenes: on the one hand, they act as hydrogen acceptors to generate saturates, and on the other hand, they can serve as hydrogen donors. They are strongly adsorbed on the surface of the catalyst, and finally generate coke from coke precursors. 4. In the catalytic cracking unit of recycled oil and recycled oil slurry, the single-pass conversion rate of raw materials is less than 100%. After the reactants are decomposed, the "unreacted raw materials" are converted into "unreacted raw materials". The "unreacted raw materials" mentioned here refer to the part of the reaction product whose boiling point range is roughly equivalent to that of the raw oil. It is called recycled oil or recycled oil slurry (circulating oil) in industry. In fact, recycled oil contains many reaction intermediates, so it contains a lot of condensed ring aromatic hydrocarbons, which is relatively difficult to crack. Three. Residual catalytic cracking reaction aromatics contain a lot of polycyclic aromatic hydrocarbons and condensed ring aromatic hydrocarbons. The characteristics of Chinese residual oil are: high colloid content (50%) and low asphaltene content. The boiling point of vacuum residual oil is very high, and it will not completely vaporize when it comes into contact with the catalyst at the FCC riser. Therefore, it is a gas-liquid-solid three-phase catalytic reaction. Molecular sieve catalysts with different pore sizes are used for the FCC reaction of residual oil. Fourth, The thermodynamic characteristics of hydrocarbon catalytic cracking reaction should be studied from two aspects: thermodynamics and kinetics for a chemical reaction process. Thermodynamics studies the direction of chemical reaction, chemical equilibrium and thermal effect, and kinetics studies the chemical reaction speed. 1. Chemical reaction direction and chemical equilibrium. Under the conditions of catalytic cracking reaction, the standard isobaric potential of hydrocarbon decomposition reaction is negative, and the equilibrium constant is very large, so it can be completely converted into small molecular alkanes and alkenes. Isomerization reactions, hydrogen transfer reactions, and aromatic condensation reactions are limited by chemical equilibrium. However, under the conditions of catalytic cracking, these reactions have not reached chemical equilibrium, so the reaction speed becomes the main factor in the depth of the reaction. Alkylation, aromatic hydrogenation, olefin superposition, etc., are extremely unlikely to occur. The main reaction of catalytic cracking is the decomposition reaction, and there is actually no chemical equilibrium limit, so people do not study its chemical equilibrium problem. 2. The heat of reaction is strong endothermic reaction - decomposition, dehydrogenation, cyclization reaction; weak exothermic reaction - isomerization, hydrogen transfer and condensation reaction. Generally speaking, catalytic cracking is a strong endothermic reaction. As the reaction depth increases, the reaction heat decreases. This is because as the reaction depth increases, hydrogen transfer and condensation reactions increase, so the reaction heat decreases. Calculation method of reaction heat: ①Based on fresh raw materials: 300-500KJ/kg fresh raw materials; (P330 Table 9-5) ② Based on the reaction product - the amount of (gasoline + gas) generated; (P330 Figure 9-5) ③ Based on the carbon (catalytic carbon) in the coke generated by the reaction. At 510°C, 9127KJ/kg catalytic carbon. Correction can be made at other temperatures (Textbook P331). Catalytic carbon = total carbon amount - Strippable carbon - additional carbon Additional carbon = fresh raw material FCC is a parallel sequence reaction, and the reaction rate also has an important impact on product distribution. 1. Several basic concepts (1) Conversion rate Conversion rate Conversion rate (single-pass conversion rate, total conversion rate) If the raw material oil is 100, then it is defined: Conversion rate (mass fraction) = (100-unconverted raw material)/100% or expressed by the following formula: Conversion rate = gas % + gasoline % + coke % (+ loss %) Single-pass conversion rate: refers to the conversion rate of the total feed (including fresh raw materials and recycled oil) passing through the reactor in one pass. Total conversion rate: refers to the conversion rate of fresh raw materials passing through the reactor in one pass. Space velocity and reaction time Catalyst storage capacity: In the fluidized bed catalytic cracking reaction unit, the catalyst continuously circulates between the reactor and the regenerator. However, at any time, a certain amount of catalyst is maintained inside each of the two reactors. The amount of catalyst regularly maintained in the two reactors is called the storage capacity. Catalyst circulation amount: the amount of catalyst entering the reactor per unit time, that is, the amount of catalyst leaving the reactor. Space velocity: refers to the ratio of the total feed amount entering the reactor per hour to the catalyst storage amount in the reactor. Unit: hour-1 (h-1). Mass space velocity = total feed amount (t/h)/storage amount (t) Volume space velocity = total feed amount (m3/h)/storage amount (m3) The reaction time, the reaction time "τ" of the homogeneous reaction can be expressed by the following formula: τ=VR/V The symbol in the formula: VR - the volume of the reactor, m3 V - the feed volume flow rate, m 3/h For heterogeneous reactions (FCC is heterogeneous gas-solid reaction): Use the reciprocal of the space velocity to relatively represent the reaction time, called pseudo reaction time "ω" ω=1/V0(h) For riser reactor: pseudo reaction time θ: θ= VR/VC VR - the volume of the riser reactor. V - the logarithmic average volume flow rate of oil and gas. 2. The basic factors that affect the catalytic cracking reaction rate. The hydrocarbon catalytic cracking reaction is a gas-solid heterogeneous reaction. The gas reaction process includes the following seven steps: ① Raw material molecules diffuse to the catalyst in the autonomous air flow; ② Raw material molecules close to the catalyst diffuse to the inner surface of the micropores; ③ Raw material molecules close to the catalytic surface are adsorbed by the catalyst; ④The adsorbed molecules undergo chemical reactions under the action of the catalyst; ⑤The generated product molecules are desorbed from the catalyst; ⑥The desorbed product molecules diffuse outward from the micropores; ⑦The product molecules diffuse from the outer surface of the catalyst into the main gas flow and then leave the reactor. Under general industrial conditions, catalytic cracking reactions usually manifest themselves as chemical reaction control. This section mainly discusses some of the main factors affecting the catalytic cracking reaction from the perspective of chemical reaction control. (1) Effect of Catalyst Activity on Reaction Speed ① Effect of Cat Activity on Reaction Speed: High Catalyst Activity - Increased Hydrogen Transfer Reaction Speed - Increased Saturation of Product - Improved Stability of Oil - Decreased Octane Number of Gasoline. High Catalyst Activity - Increased Isomerization Reaction Speed - Increased Gasoline Octane Number. High Catalyst Activity - Fast Reaction Speed - Shortened Reaction Time. - Increased Conversion Rate. - Increased Processing Volume. ② Increased Coke Deposition, Decreased Activity, Coke Deposition Amount C: C = αθb In the formula: a and b are constants, related to the properties of the raw materials. For fixed beds, θ is the length of the reaction cycle; for moving or fluidized beds, θ is related to the catalyst circulation amount and reactor storage capacity. θ = reactor storage capacity/catalyst circulation amount. The longer the catalyst is in the reactor, the more carbon will be deposited, and the greater the activity will decrease. ③ The influence of agent-oil ratio on reaction Agent-oil ratio (C/O) = catalyst circulation amount/total feed amount. A large agent-oil ratio will have less carbon deposits on the catalyst, and the activity of the catalyst will decrease less. A larger agent-oil ratio will have higher activity on the catalyst, and the reaction speed will be faster. However, if the agent-oil ratio is large, the agent-oil ratio will be larger than the coke. That is, more carbon that can be stripped will also affect the coke yield. (2) Effect of temperature on reaction speed Reaction temperature is the main parameter in production and the most sensitive parameter to product yield and quality. That is, the reaction temperature has the most direct relationship with reaction speed, product distribution and product rate. Analysis by the Arrhenius equation: K=Aexp(-E/RT) where: K—reaction rate constant; A—frequency factor; E—activation energy, KJ.mol-1; T—reaction temperature, K analysis: 1) When E is constant, T↑, k↑, conversion rate ↑; 2) When the temperature changes, the K changes of each reaction are different: FCC reaction E = 42~125 KJ.mol-1 kt = 1.1-1.2 Thermal cracking reaction E = 210~293 KJ.mlo-1 kt = 1.6~1.8 kt - temperature of reaction speed kt heat > ktFCC, so when the reaction temperature is raised, the proportion of the thermal cracking reaction increases. 3) The FCC reaction is still dominant, so gasoline and gas oil still occupy the dominant position, but slightly decreased. 4) The FCC reaction is a parallel sequence reaction, and temperature changes have different effects on the reaction rates of various hydrocarbons. Increasing the reaction temperature is beneficial to the gasoline production program. my country mainly focuses on gasoline and diesel production programs, and the temperature is lower, generally 470~520℃, which is 20~50℃ lower than abroad. Kt1-gasoline-Kt2-gas raw material Kt1-coke analysis: ① Effect on product distribution kt2 > ktl > kt3; When temperature T↑: gas ↑, gasoline ↓, coke ↓ ② Effect on product quality Decomposition reaction, kt of aromatization reaction > kt of hydrogen transfer reaction When temperature T↑: alkene, aromatic hydrocarbon content ↑, gasoline RON↑ (3) Effect of raw material properties on reaction rate ① When the family composition is similar, the higher the boiling point range, the easier it is to crack. This has little effect on molecular sieve catalysts. ② When the boiling point range is similar, raw materials containing many aromatic hydrocarbons are difficult to crack. ③ The residual coke in the raw material is high, and the coke yield is high. The regeneration coke load increases. ④ Heavy metals (Ni, V, etc.): Cause poisoning of the catalyst and worsen performance (activity, selectivity) ⑤ Non-hydrocarbons containing S, O, and N: cause catalyst poisoning, equipment corrosion, and product quality deterioration. (4) The effect of reaction pressure on reaction speed Strictly speaking, reaction pressure should be the effect of the reaction oil and gas partial pressure on the reaction speed. An increase in the oil and gas partial pressure means an increase in the concentration of reactants, and thus an increase in the reaction speed. As the reaction pressure increases, the coke generation speed increases, and the coke yield increases. The square of the reaction speed is proportional to the pressure, P↑, V↑. But the coking speed is ↑↑ (faster). Therefore, the reaction pressure is generally controlled to 1-3 atm, and w control means are generally not used. 3. Catalytic cracking reaction kinetic model (1) Tasks and significance of the reaction kinetic model 1) Optimize the engineering design plan to make the design of the reactor more reasonable; 2) Optimize the operating conditions of the FCC unit, and the purpose of production control optimization is more clear; 3), shorten the cycle of new process development and research, and enable targeted processing research. (2) Characteristics of FCC reaction kinetics research: ① Complexity of raw material composition ② Complexity of chemical reactions ③ Complexity affected by multiple operating conditions ④ Effect of catalyst activity, selectivity and deactivation ⑤ Influence of certain uncertain factors (3) Two typical methods of catalytic cracking reaction model: ① Empirical charts and empirical formula kinetic model method - correlation model: It is based on a certain kinetic equation, using various test data and production data to use mathematical regression and other methods to summarize correlation expressions for calculating various product yields and related properties ② Lumped kinetic model research method. Fluidized bed reactor - conversion rate X correlation expression is as follows: X = FP·Fsw·FT·FA·Fc·FF where: Catalyst-oil ratio, space velocity factor; FT - reaction temperature factor; FA - relative catalyst activity; Fc - carbon content factor of regenerated catalyst; FF - feed material factor. Lumped kinetic model research method The so-called "lumping" is to divide a complex reaction system into several lumped components according to the principle of similar kinetic properties, and treat them as a virtual multi-component system for kinetic processing. In 1959, RB Smith proposed a three-lumped kinetic model for catalytic reforming. In 1960, Weekman developed the three-lump dynamic model of FCC (P337). In the 1970s, a ten-lump model was proposed: Section 3 Catalytic Cracking Catalyst A catalyst is a substance that can change the reaction rate of chemical reactions that are likely to occur from a thermodynamic point of view under certain conditions. It can speed up some reactions and inhibit the progress of other reactions. But for those thermodynamically unavailable reactions Possible reactions have no effect. At the same time, for reversible reactions, it accelerates the forward and reverse reactions equally, that is, it does not change the equilibrium of the reaction. For example, under normal reaction conditions, judging from the thermodynamic point of view, hydrocarbons can undergo decomposition, isomerization, aromatization, and hydrogen transfer. There are many reactions such as superposition and alkylation, but the reaction speeds are different. In this way, the catalyst can be used to selectively accelerate the desired reactions in these reactions and suppress the undesirable reactions, thereby achieving the purpose of improving product quality and improving product distribution. This is not achieved by the thermal cracking process. For example, in order to increase the octane number of gasoline, try to make the catalyst selectively accelerate the isomerization reaction and inhibit the hydrogen transfer reaction and the further dehydrogenation of unsaturated hydrocarbons to coke. The role of the catalyst is to increase the reaction rate, and the fundamental reason is to change the chemical reaction process and reduce the activation energy of the molecule. Analysis by the Arrhenius equation: K=Aexp(-E/RT) where: K—reaction rate constant; A—frequency factor; E—activation energy, KJ.mol-1; T—reaction temperature. K analysis: 1) When E is constant, T↑, k↑, conversion rate ↑; 2) When the temperature changes, the K of each reaction changes differently: FCC reaction E = 42~125 KJ.mol-1 kt = 1.1-1.2 Thermal cracking reaction E = 210~293 KJ.mlo-1 kt = 1.6~1.8 Due to the different sizes of E, catalytic cracking is easier to proceed than thermal cracking. 1. Composition and structure of catalytic cracking catalyst 1, Types of Catalysts Industrial catalysts can be divided into three categories according to their development history: ① Natural soil, the main component is aluminum silicate (AlSiO3}. ② Amorphous synthetic aluminum silicate, φ20-100μrn, specific surface area 500-700m2/g. ③ Molecular sieve catalyst, molecular sieve. Also known as crystalline zeolite, it is an aluminosilicate with a regular crystal structure. 2. The composition and structure of the catalyst (1) The source of acidity of acidic sodium silicate (2) The relationship between catalyst acidity and activity ① The type of acid center is related to the catalytic effect. ② The catalytic activity is related to its acidity. When expressed by the value of acid strength H0, it is only relevant to the catalyst when the acid strength H0 is lower than +3.3. ③ The strength of the acidic center is different, and it has different activation effects on the reactants. ④The activity of the catalyst increases with the amount of acid. However, a quantitative relationship between the two has not yet been found. The range of acid strength H0 is H0 < -8.2, which is a strongly acidic center; -8.2 < H0 < -3.0, which is a medium acidic center; -3.0 The crystal collapse temperature of X-type molecular sieve REY-type zeolite molecular sieve catalyst is 870℃~880℃; The crystal collapse temperature of the ultra-stable Y-type (USY) molecular sieve catalyst is as high as 1010℃ ~ 1050 2. Selectivity: Indicates the catalyst's selective reaction ability to increase target products (gasoline, diesel) and reduce by-products (gas and coke). Highly active catalysts may not necessarily have good selectivity, so its selectivity must be considered when selecting a catalyst. FCC catalytic cracking reactions usually use "gasoline yield/coke yield" or "gasoline yield/conversion rate" to express the selectivity of the catalyst. Catalysts contaminated by heavy metals show a decrease in selectivity, generally expressed by the H2/CH4 ratio. 3. Different expression methods of density: True density: the ratio of the mass of the particles to the actual volume of the skeleton, 2~2.2 g/cm3 Particle density: the density of a single particle including the pore volume, 0.9~1.2 g/cm3 Packing density: the density of the catalyst including the pore volume and the void volume between the particles, 0.5~0.8 g/cm. The particle density of the catalyst has an important impact on the flow of the catalyst. 4. Screening composition, mechanical strength (1) In order to ensure a good sulfurization state, the sieving composition requires the catalyst to have an appropriate particle diameter distribution, that is, it has a certain sieving composition. The particle size distribution of the catalytic cracking catalyst is 20-100 μm. Generally, among fresh catalysts, about 50% have a particle size between 40-80 μm. %. Due to the mutual collision of catalysts in the device, some large particles will become small particles, so the sieving composition formula changes. (2) Mechanical strength In order to avoid crushing the catalyst during use to reduce loss and ensure good fluidization quality, the catalyst is required to have a certain mechanical strength. Our country uses "wear resistance index to evaluate the mechanical strength of the catalyst." 3. Types of industrial molecular sieve cracking catalysts 1, REY type molecular sieve catalyst 2, USY type molecular sieve catalyst 3, REHY molecular sieve catalyst 4, Principle 4 of catalyst selection, cracking catalyst additive 1, octane additive 2, metal passivator 3, CO combustion accelerator Section 4 Deactivation and regeneration of cracking catalyst 1. Deactivation of cracking catalyst Causes of catalyst deactivation: Hydrothermal deactivation: It is a slow process, related to the residence time. Reaction coke deactivation: It is related to the reaction coke rate, and a variety of coke models have been established for research. Poison poisoning deactivation: certain heavy metals (Ni, V, Na, etc.) 1. Hydrothermal deactivation (P345-346) Hydrothermal deactivation is when the surface structure of the cracking catalyst changes when the catalyst is at high temperature, especially in the presence of water vapor. The specific surface area is reduced, the pore volume is reduced, and the crystal structure of the molecular sieve is destroyed, resulting in a decrease in the activity selectivity of the catalyst. The crystal collapse temperature of REY type zeolite molecular sieve catalyst is 870℃~880℃; The crystal collapse temperature of the ultra-stable Y-type (USY) molecular sieve catalyst is as high as 1010°C ~ 1050°C. In gentle, medium-depth water treatment (816°C), there is little difference between the two. After exceeding 870°C, the rare earth Y-type crystals almost completely collapse, while the ultra-stable Y-type crystals still maintain good performance. In actual production, the temperature is strictly controlled: the deactivation problem at 730°C is more prominent. Generally, catalyst regeneration does not exceed 730°C. 2. Coking and deactivation Coking and deactivation: The reaction coke is deposited on the surface of the catalyst, covering the active center, reducing the catalyst activity and selectivity. The coke produced by industrial catalytic cracking can be considered to include four types of coke: ① Catalytic carbon: the coke generated during the reaction ② Additional carbon: the raw material neutralizes the coke precursor (condensed ring aromatic hydrocarbons) ③ Strippable coke: the heavy hydrocarbons remaining on the catalyst due to incomplete stripping. ④ Contaminated coke: Heavy metals deposit on the surface of the catalyst, promoting dehydrogenation and condensation reactions to produce coke. 3. Deactivation caused by poisons. In actual production, the poisons of catalysts are mainly certain metals (iron, nickel, vanadium, copper, other metals and sodium) and alkaline nitrides. (1) Heavy metal pollution. The order of heavy metal poisoning of the catalyst from weak to strong is: Lead