Could that expert provide some information on the coking problem in the catalytic cracking fractionator slurry?

Control Measures for Coking in the Fractionation System of Heavy Oil Catalytic Cracking Units ZHOU Kang, Wuhan University of Technology (Wuhan, Hubei Province 430023) Abstract: This paper discusses the mechanisms and basic characteristics of scaling and coking in the fractionation system of the heavy oil catalytic cracking units at Wuhan Petrochemical Plant. It analyzes the control factors related to scaling and coking in different areas, conducts a quantitative analysis of these factors, identifies the quantitative relationships between them, and proposes specific control measures to address scaling and coking, thereby providing guidance for actual production. Keywords: catalytic cracking unit, fractionation tower, slurry, sediment formation, operating conditions, optimization 1 Characteristics of scaling and coking in different parts of the fractionation system 1.1 Bottom of the fractionation tower Since the operation of Unit II of the heavy oil catalytic cracking unit (RFCCU) at Wuhan Petrochemical Plant began, coking at the bottom of the fractionation tower has not affected safe production; however, the scale thickness there exceeds 800 mm, which has blocked the bottom filter. The nozzles of the stirring steam distribution ring and the slurry return distribution ring at the bottom of the tower are also clogged. As the slurry flows through the distillation system, and as the scale deposits grow, under the influence of gravity and viscosity, these deposits fall into the areas of slow flow in the liquid phase and settle on the walls of the vessels. The diameter of this distillation column is 3.8 m, and a stable liquid level is maintained at the bottom of the column; as a result, the oil slurry spends the longest time there. Furthermore, the linear velocity of the slurry flowing at the bottom of the tower is not uniform; there are slow-flow zones and dead zones at the bottom of the tower, as well as a stagnant boundary layer on the tower walls. These conditions create the necessary environment for the gravitational deposition of scale deposits. As a result of heat transfer and washing between the circulating slurry in the de-overshooting section and the reaction gas slurry (containing cracked coke from large oil and gas pipelines), the bottom slurry of the tower has the highest liquid phase temperature and the highest scale nucleation concentration among all sections of the tower. With the high temperature of the oil slurry at the bottom of the tower, the thermal polymerization reactions of the coking-prone substances on the scale deposits intensify. Additionally, when the level of the circulating oil slurry is high at the bottom of the tower, the residence time increases, which leads to more thermal condensation reactions. All of these lead to coking on the tube walls of the tower. The scaling and coking at the bottom of a distillation tower have the following characteristics: ① The structure of the scale layer at the bottom of the tower can be divided into three layers: the upper layer is a pile of scale particles, which is very soft and easy to remove; the middle layer is a transitional layer that is soft, elastic, and highly sticky; the lower layer is a coked layer that is hard and tough, making it extremely difficult to remove. ②Depending on the length of the production cycle, the temperature at the bottom of the tower, and the composition of the slurry, there are significant differences in the thickness and structure of the fouling layer at the bottom of the tower. When scaling and coking at the bottom of the tower are severe, a scale layer also forms on the tower walls. The height of the scale layer at the bottom of the tower exceeds that of the filter, resulting in a decrease in the oil slurry circulation rate. If the content of broken coke in the slurry is high, some very fine fragments of broken coke can also enter the slurry circulation system and clog the heat exchanger tubes. ③The bottom of the tower is the zone with significant thermal reactions. If the temperature of the liquid phase at the bottom of the tower is controlled too high, on one hand, substances prone to coking and scale nuclei will concentrate; on the other hand, the thermal polymerization reaction of these coking-prone substances on the scale deposits in the oil slurry at the bottom of the tower will intensify, increasing its viscosity. This facilitates scaling at the bottom of the tower, as well as in the oil slurry heat exchangers and oil slurry steam generators. ④The degree of scaling is controlled by the content of scale particles in the slurry and its viscosity. 　 1.2 Slurry circulation pipelines: During various maintenance operations on the second set of RFCCU, scaling in the slurry circulation pipelines of the fractionation system has been observed to some extent in every case. As the slurry flows through the circulation pipes, a laminar boundary layer exists at the bottom of the pipes. The deposits located at the very bottom of this laminar boundary layer settle at the pipe bottom due to gravity. Since the temperature at the pipe bottom is **lower than that of the flowing fluid, the viscosity of these deposits increases, leading to scaling. The scaling in the slurry circulation pipes exhibits the following characteristics: ① The degree of scaling is determined by the degree of coking of the scaling substances at the bottom of the distillation tower; the resulting scale layer is wave-shaped and soft in texture, making it easy to remove. Its coking macroscopic phenomenon is not obvious. ②When scaling is severe, it leads to poor flow rate in the slurry pump, a decrease in outlet pressure, and a reduction in the slurry circulation volume. 　 1.3 Slurry heat exchanger (steam generator): The two sets of RFCCU slurry steam generators often become clogged. During the emergency repair in April 1997, it was found that the thickness of the scale buildup in the tube boxes reached 500 mm, and one-third of the tube bundles were blocked. The steam generation capacity of the slurry steam generator has declined significantly. The scaling of heat exchangers has the following characteristics: ① The scaling process in the tube sheet and small floating heads occurs through gravitational deposition, and the degree of scaling is determined by the content of scale nuclei in the slurry as well as the flow rate. The scaling process in the tube bundle occurs through cooling-induced viscous deposition, and the degree of scaling is controlled by the presence of scale nuclei in the slurry, the amount of substances prone to coking, and the flow rate. ②The scaling on the tube box and small floating heads accelerates each other in conjunction with the scaling on the tube bundle: the scaling on the tube bundle reduces the flow velocity of fluid at the inlet of the tube box and small floating heads, increasing the degree of fluid stagnation there and thus leading to more severe scaling on those components. At the same time, as the scale layer thickens, on one hand, the inlet to the tube bundle near the scale layer is gradually blocked by it, and on the other hand, this increases the transport of scale particles toward that tube bundle, until the tube bundle becomes blocked. ③The scale in the tube bundles contains a high amount of substances prone to coking; although the scale is soft in texture, it is highly sticky and difficult to remove completely. The macroscopic phenomenon of coking in the fouling on the tube sheets and small floating heads is not obvious; the fouling is soft in texture and contains a high amount of organic matter. 　 1.4 Desuperheating Section: During various shutdown inspections of Unit II’s RFCCU, it was found that hard, compact carbon deposits ranging in thickness from 5 to 20 mm had accumulated on the chevron baffles and the tower walls in the desuperheating section of the distillation tower. When the gas-phase load increases significantly, the amount of slurry returning to the tower does not increase in a timely manner. This leads to a low liquid-phase spray density in the desuperheating section, as well as an unreasonable distribution of the gas-liquid phase loads, resulting in localized overheating. The reactivity of substances prone to coking and scale formations increases in these overheated areas, thereby triggering coking reactions. Scale deposits form at the cooling zones located on the chevron baffles or tower walls; these deposits have strong adhesive properties, allowing them to attach other substances prone to coking and scale formations, thus creating further scale layers. Due to the very high temperature of the liquid phase in the desuperheating section, which averages 320–370°C, once scaling occurs, the scale layer will undergo deep carbonization from the inside out, forming a char layer. Coking characteristics in the desuperheating section: ① The materials prone to form coke and scale exhibit rapid reaction properties. ②The scaling mechanism in the desuperheating section is the deposition of scale particles through excessive agglomeration and viscosity increase. The excessive condensation of coking-prone substances and scale aggregates causes the product to tend to form deposits on the walls; the degree of coking is determined by the temperature of local hot spots as well as the viscosity of these coking-prone substances and scale aggregates. ③In an environment free from superheating, the scale layer transforms into a char layer, on which a thin layer of scale adheres, being hard in texture. 　 1.5 First and second stage trays and their tongue holes: In the II set of RFCCU fractionation towers, dozens of tower flooding incidents occurred continuously from June to August 1999. The frequency of such incidents increased from 1–2 times every two weeks at first to 3–4 times per week later on; the duration of each incident rose from 1–2 hours to 5–8 hours. As a result, the fractionation system could not be controlled, the product quality was often substandard, and the safe operation of the facility was severely threatened. During the shutdown for inspection of the fractionation tower, it was found that, first, the first and second layers of the tongue-shaped trays were covered with a scale layer about 60–70 mm thick; the tongue holes in the first layer of trays were partially or completely blocked by this hard scale layer, with 90% of the holes being completely blocked, and in those that were partially blocked only the tip of the tongue portion remained unobstructed. The reason for the blockage of the orifice holes is that the oil and gas coming from the superheating removal section are not thoroughly washed; these oils and gases carry with them substances prone to coking, scale nuclei, and scale deposits from the slurry. These substances accumulate and settle due to gravity. Scaling occurs in the orifice hole area because it is a region with slow flow – under the influence of inertial forces and viscous forces, the substances prone to coking and scale deposits come into contact with the base of the orifice hole, forming scale deposits. In particular, high-viscosity macromolecular gums, asphaltenes, and their associated scale deposits further condense and carbonize in the high-temperature oil and gas environment, forming stable scale deposits that gradually accumulate over time. Fouling characteristics of the tray and its waffle holes: ① The tray suffers from gravity-driven deposition fouling, while the waffle holes experience viscosity-enhanced deposition fouling. ②The degree of fouling in the tongue holes is controlled by the number of high-viscosity fouling agglomerates in the oil and gas after washing, as well as by temperature. 　 2 Control Measures 2.1 Control factors for scaling and coking in the bottom of the tower and the slurry circulation system. Scaling in the slurry circulation pipes at the bottom of the tower and in the tube banks of the slurry heat exchangers is the result of the gravitational deposition of scale particles; the growth of these scale layers is controlled by the size and viscosity of those particles. Scaling in the tube bundles of the slurry heat exchangers is caused by the viscous deposition of scale particles and asphaltenes, and the growth of such scale layers depends on the viscosity, concentration, and flow rate of these particles and asphaltenes. Therefore, it is crucial to control the growth of these scale particles and the increase in their viscosity. The linear velocity of the tube bundle in the slurry heat exchanger is another key control parameter for tube bundle scaling. The degree of growth of the scale mass can be expressed by the magnitude of the increase in the average particle size of the scale mass, Δd; the factors that influence Δd are shown in Figure 1. 　　As the increase in viscosity of the scale mass, Δη increases, and Δd also increases. The control factors for Δη are shown in Figure 2. 　 2.1.1 Content of coking-prone substances, scale nuclei, and scale aggregates (L1) L1 is the average value of the content of coking-prone substances (scale nuclei or scale aggregates) in the bottom oil slurry at time t0 (kg/m3); L1′ is the average value of such content at time t0+Δt (kg/m3). Assuming the slurry system is homogeneous, the effect of the coking reaction at the bottom of the tower on the content of coking-prone substances (scale nuclei or scale aggregates) can be ignored. Its functional relationship is: L′1 = L1 + l1FΔtV1 – L1YΔtV1ρ. In equation (1): l1 represents the content of coking-prone substances (scale nuclei or scale deposits) in the reacted oil and gas, expressed in kg/t; Y denotes the output volume of the oil slurry, in kg/h; F is the total processing volume, in t/h; V1 is the volume of the entire oil slurry circulation system, in m3; ρ is the density of the oil slurry, in kg/m3. 　　 For Set II RFC CU, V1 can be expressed as: V1 = 27.20X/100 + 6.47 – 0.06 + 17.22 = 0.272X + 23.63. In equation (2), X represents the actual liquid level at the bottom of the distillation tower, in %. 　　l1 is related to the coking tendency of the feed oil, the degree of cracking, the coking content in large oil and gas pipelines, the metal oxide content, and the efficiency of the cyclone separator. The controls for L1 are as follows: ① To keep L1 in balance, when F or l1 or ρ increases, Y must also increase accordingly. It can be seen from this that Y is a control variable for L1. ②To prevent L1 from exceeding the limits, it is necessary to carry out proper monitoring, analysis, and control of L1. ③When an increase in F or l1 or ρ is detected, it is necessary to increase the amount of slurry discharged at the appropriate time and in an appropriate amount. The following requirements apply to the control of l1: ① Strengthen the monitoring of the properties of crude oil and slurry, in order to analyze changes in l1. ②Depending on the different raw materials and production methods, an appropriate cracking depth should be controlled to prevent l1 from becoming too large. ③During major overhauls, it is necessary to strengthen the coking removal and maintenance work on cyclone separators and large oil and gas pipelines. 　 2.1.2 Average residence time (τ1) of the slurry system: The residence time of the slurry in the distillation system refers to the time it takes for the slurry to enter the distillation tower and exit it, and can be expressed by the average residence time τ1 of the slurry system. For the fractionation system of Set II RFCCU: τ1=V1ρ/Y=(0.204X+17.72)/Y(h) (3) Where at temperatures between 300 and 350°C, ρ=750 kg/m3. Therefore, to control τ1, it is necessary to: ① Keep τ1 low when L1 is large, in order to maintain equilibrium in Δd. In fact, as can be seen from equations (1) and (3), increasing Y will reduce both τ1 and L1; thus, it is evident that Y is the control variable for Δd. ②For Set II RFCCU, τ1 is generally controlled at 4–6 hours. 2.1.3 Average residence time of the bottom slurry (τ2) The residence time of the bottom slurry refers to the time it takes for the slurry to enter the bottom of the tower and then leave it; this can be denoted by τ2. τ2 can be calculated by taking the ratio of the mass of the bottom slurry to the mass flow rate of the slurry in circulation. 　 　τ2=60V2ρ/Q=(11.9X+281)/Q (minutes) (4) Where the volume of the oil slurry at the bottom of the tower is: V2=27.20X/100+6.47-0.06=0.272X+6.41 (m3) (5) At 350°C, ρ=730 kg/m3; here, Q represents the total circulation rate of the oil slurry, in kg/h. Therefore, the control of τ2 is as follows: ① To maintain the balance of Δη, the value of τ2 is constrained by L1, T, and Δd. ②Q serves as a means to regulate both T and the temperature of the first tray, but it is also constrained by the design values of the slurry pump; therefore, the value of τ2 remains relatively stable, and it is generally not used as a means for regulating w. ③For Set II RFCCU, τ2 is generally controlled at 5–6 minutes. 　 2.1.4 Temperature (T) of the slurry at the bottom of the distillation tower: The bottom temperature refers to the average temperature of the slurry at the bottom of the distillation tower. By the time the slurry exits the outlet at the bottom of the tower, it has been well mixed, and the temperature here can represent the average temperature of the slurry at the bottom of the tower. The controls for T include: ① In the control function for Δη, production practice has shown that T and Δd are effective means for maintaining balance in Δη, with Δη being more sensitive to T. ②At 350°C, Δη experiences a sudden change; when T exceeds 350°C, coking intensifies. ③Set II of the RFCCU keeps T below 345°C over the long term. 　 2.1.5 Flow velocity (S) of the slurry steam generator tube bundle: In heat exchangers and pipes, if the flow velocity is too low, scale deposits can easily form due to gravity, leading to blockages in the slurry system. A recommended line speed is generally 1.5 to 2.4 m/s. 　 　S = Q′/3600ρA3 = Q′/185 (m/s) Equation (6). Where: A3 —— the cross-sectional area of the tube bundle in the first quadrant of the slurry steam generator’s tube box, taken as 0.0675 m2; Q′ —— the flow rate of the slurry passing through the slurry steam generator, in kg/h. 　 　At 310°C, ρ = 760 kg/m3. 　 　The controls for S include: ① The S value of the two sets of RFCCU is below the recommended value; only by reducing the value of Δη and keeping R slightly lower can scaling and coking be effectively controlled. ②When there is a surplus of heat available, the S value can be increased by blocking some of the tubes in the oil slurry steam generator’s tube bundle. 　 2.1.6 Other control factors ① The use of scale inhibitors can prevent the caking of scale deposits. ②Renovation of the slow-flow zone. To improve flow in the slow-flow zone, some plants have installed steam agitators at the bottom of the distillation tower, achieving certain results. To prevent the filter screens in the bottom filter from becoming clogged, it is possible to increase the diameter of these screen pores and thus expand their flow area. This approach not only enhances flow in the slow-flow zone but also extends the service life of the bottom filter. 　 2.2 Control factors for scaling and coking in the superheating removal section and the first and second tray levels The key control points for scaling and coking in the superheating removal section and the trough openings of the first and second tray levels are the formation and growth of scale deposits. The degree of coking in the desuperheating section is denoted by W1, and the factors affecting its formation are shown in Figure 3. 　　The coking degree of the trough openings on the first and second tray levels is denoted by W2; the factors affecting its formation are shown in Figure 4. The degree of fouling on the first and second tray levels is denoted by W3; the factors affecting its formation are shown in Figure 5. 　 2.2.1 Slurry Recirculation Rate (Q1) UOP recommends a minimum slurry recirculation rate of 120% of the total feed rate, or 37 m3/h per meter of tower diameter. The control of Q1 includes: ① As can be seen from the control diagrams of W1 and W2, in order to maintain the balance of W1 or W2, the value of Q1 is determined by L1 and l1. L1 and l1 cannot be used as control variables for W1 and W2; the control variable is Q1. ②The reflux oil slurry can remove the excess heat from the reaction gas stream, thereby controlling the temperature of the first tray in the column; it also helps to control the degree of scaling and coking in the superheating section of the distillation column as well as in the trays above it. ③Based on the actual operating conditions of the second set of RFCCU, a lower limit value for the amount of slurry returning to the reactor is given as FN: FN = max (7). In this equation, F1 represents the volume of fresh feed, in t/h; F2 represents the volume of reprocessed oil flowing back, in t/h; F3 represents the volume of reformated gasoline flowing back, in t/h; F4 represents the volume of terminator flowing back, in t/h; F5 represents the volume of water or another substance used as a terminator, in t/h. 2.2.2 Saturation degree and the amount of reprocessed oil returning to the tower (Q2): The amount of internal reflux below the extraction tray for the reprocessed oil is determined by the saturation degree of the reaction gases after overheating is removed. Too low a reflux flow rate in the first and second tray layers, too low a supersaturation level, or even a dry tray can accelerate coking in the first and second tray layers as well as their wick holes. Excessive supersaturation means that the heat contained in the reaction gases and liquids is not effectively recovered at the bottom of the tower, which is detrimental to energy savings in the plant. Therefore, controlling the appropriate supersaturation of the de-superheated reaction gas oil is important for the long-term operation of the fractionation tower and energy savings. 　 3 Conclusion In continuous production, it is very difficult, if not impossible, to completely eliminate scaling and coking in all parts of the distillation system. However, as long as people understand the patterns of scaling and coking in distillation systems, it is still possible to effectively control these phenomena, reduce their negative impact on normal operations, and ensure the safe and prolonged operation of the facility. Analysis of the causes of coking in distillation systems and countermeasures: In recent years, with the increase in imported crude oil and the rising proportion of residue used in blending, the feedstock for these processing units has become of lower quality and heavier in composition. As a result, the oil slurry stays in the bottom of the distillation tower for longer periods, has a low linear velocity in the heat exchange equipment, and contains high levels of solids and aromatics; hence, the problem of coking in the oil slurry system has become increasingly severe. Therefore, the causes affecting coking in the device’s fractionation system and corresponding countermeasures are discussed. (1) Bottom temperature of the distillation tower: The properties of the slurry change depending on the proportion of heavy oil blended in and the bottom temperature of the distillation tower. If the bottom temperature is too high, coking at the bottom of the tower can occur even with a short residence time; therefore, it is very important to select an appropriate bottom temperature. When putting the cleaned heat exchanger into use, maintain the circulation rate of the oil slurry, reduce the temperature at the bottom of the distillation tower from 355°C–360°C to 345°C–350°C, and continuously inject scale inhibitors to prevent insoluble substances in the oil slurry from sticking to the tube walls. While using the bottom stream oil slurry as quench oil to lower the bottom temperature of the tower, reprocessed oil can be directly injected at the bottom of the distillation tower if necessary. There are two purposes for injecting quench oil at the bottom of the distillation tower: one is to lower the temperature at the bottom of the tower, thereby reducing high-temperature polymerization and coking of the oil slurry ； Secondly, it can stir the oil slurry at the bottom of the tower, preventing the formation of flow dead zones and stopping catalyst particles from settling and clogging the filter for the oil slurry at the tower bottom. (2) Residence time of the slurry at the bottom of the distillation tower: The length of time the slurry stays at the bottom of the tower depends on the level of the liquid at that location and the volume of slurry circulating in the tower. When the slurry circulation rate is normal, it is appropriate to keep the residence time of the slurry at the bottom of the distillation tower between 5 minutes and 6 minutes. By adjusting the reaction depth and bottom temperature, the liquid level at the bottom of the distillation column is kept at around 50%, with 60% being an appropriate upper limit. By maintaining an appropriate liquid level, it is possible to avoid the pump of the slurry pump from running dry due to an excessively low liquid level, which in turn would cause an excess of heat in the distillation tower and disrupt the thermal balance of the entire distillation tower ； It can also prevent the liquid level from being too high, which would cause the slurry to coking due to prolonged exposure to high temperatures. Furthermore, the three-way valve at the slurry evaporator should direct as much flow as possible to the cold side, in order to maintain a high flow rate of the slurry within the heat exchanger tubes. At the same time, scale inhibitors should be continuously added to the slurry system, while keeping the flow rate of the slurry pump at its maximum level, so as to prevent the slurry from sticking to the inner walls of the heat exchanger tubes and avoid clogging of the slurry evaporator. This ensures a stable and high circulation rate for the slurry system, as well as a short residence time. (3) Relative density and solid content of the bottom oil slurry: Controlling the relative density of the oil slurry is also very important for reducing coking in the oil slurry system, as the density of the oil slurry indirectly reflects its composition as well as properties such as residue. Controlling the relative density of the slurry is mainly achieved by adjusting the amount of slurry discharged ； Next is adjusting the reaction depth; if necessary, the properties of the raw materials can be adjusted. The solid content in the slurry consists of catalyst particles and coke particles. It is also very necessary to analyze the solid content of the slurry in order to reduce coking in the slurry system. High solid concentrations cause severe wear on equipment, especially on high-speed components such as slurry pumps; extremely high concentrations can also lead to serious coking. The main approach adopted is to increase the amount of slurry discharged outside, thereby removing the large quantity of catalyst suspended at the bottom of the tower through the slurry pump ； To reduce the time it takes for the slurry pump to return to normal operation, a re-refining oil line is connected to the inlet of the slurry pump; a small amount of re-refined oil is pumped into the inlet of the slurry pump using the re-refining oil pump as a supplement, or the inlet pipeline of the slurry pump is opened so that the pump can return to normal operation in as short a time as possible ； An oil slurry pump with a high net positive suction head is used to ensure that it does not experience cavitation within the temperature range of 50°C to 390°C.

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Could anyone suggest ways to reduce the density of the slurry? The density of the slurry in my unit has remained around 1300 since the MIP upgrade; even after adjusting the amount of slurry sent back to the tower to 2.5–3.0%, the density did not decrease

Considering the large valves for oil and gas on small catalytic large-scale oil and gas pipelines as well as the coking phenomenon at the outlets, it seems that the causes of coking need to be analyzed more in depth