Control measures for coking in the fractionation system of heavy oil catalytic cracking units

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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 associated with 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 liquid flow 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 at the bottom of the column. 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 coke fragments from large oil and gas pipelines), the slurry at the bottom of the tower has the highest liquid phase temperature and the highest scale nucleation concentration of the entire tower. With the high temperature of the oil slurry at the bottom of the tower, the thermal polymerization reactions of the substances prone to coking 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 and elastic with high viscosity; 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 scale 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 coke can also be carried into 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 the 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 pipes: During various maintenance operations on Set II of the RFCCU, scaling in the slurry circulation pipes of the distillation 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 there, leading to scaling. The scaling in the slurry circulation pipes has the following characteristics: ① The degree of scaling is determined by the degree of coking of the scale-forming substances at the bottom of the distillation tower; the resulting scale layer is wavy in shape and soft in texture, making it easy to remove. Its coking macroscopic phenomenon is not obvious. ②When scaling is severe, it causes poor flow rate in the slurry pump, a drop in the pump outlet pressure, and a reduction in the slurry circulation volume. 　 1.3 Slurry heat exchanger (steam generator): The second set of RFCCCU slurry steam generators often gets 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 extent 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 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 bundles 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 layers of coke, 5–20 mm in thickness, 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 deposits 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 deposits, thus forming 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 substances 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 aggregation 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 quite hard in texture. 　 1.5 First and second stage trays and their tongue holes: In the II set of RFFCCU 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, while 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 from the superheating removal section 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 deposition fouling, while the waffle holes experience viscous deposition fouling. ②The degree of scaling in the tongue hole is controlled by the number of high-viscosity scale agglomerates in the oil and gas after washing, as well as by temperature. 　 2 Control Measures 2.1 Factors controlling 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 determined by the size and viscosity of those particles. Scaling in the tube bundles of the slurry heat exchangers results from 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 aggregate can be expressed by the magnitude of the increase in the average particle size Δd of the scale aggregate; the factors that influence Δd are shown in Figure 1. 　　As the increase in viscosity of the sludge 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 (1) Where: l1 —— the content of coking-prone substances (scale nuclei or scale deposits) in the reaction oil and gas, expressed in kg/t; Y —— the output volume of the oil slurry, in kg/h; F —— the total processing volume, in t/h; V1 —— the volume of the entire oil slurry circulation system, in m3; ρ —— 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 the changes in l1. ②Depending on the different raw materials and production methods, an appropriate degree of cracking should be controlled to prevent l1 from becoming too high. 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 300–350°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 of 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 equilibrium 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) in the tube bundle of the slurry steam generator: 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) (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 at a slightly lower level 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 mesh holes in the bottom filter from becoming clogged, it is possible to increase the diameter of these mesh holes 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 perforations The key control points for scaling and coking in these areas 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 holes 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 return oil slurry can remove excess heat from the reaction gas stream, thereby controlling the temperature of the first tray in the tower; it also helps to control the degree of scaling and coking in the desuperheating section of the distillation tower as well as in the trays above it. ③Based on the actual operating conditions of Unit II RFCCU, a lower limit value for the amount of slurry returning to the tower, FN, is given as follows: FN = max(7). In this formula, F1 represents the volume of fresh feed, in t/h; F2 represents the volume of reprocessed oil flowing back to the tower, in t/h; F3 represents the volume of reformated gasoline flowing back to the tower, in t/h; F4 represents the volume of terminator used, 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 reprocessed oil is determined by the saturation degree of the reaction gas 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 wicks. Excessive supersaturation results in the heat contained in the reaction gases and liquids not being effectively recovered at the bottom of the tower, which is detrimental to energy savings in the installation. Therefore, controlling the appropriate supersaturation of the de-superheated reaction gas and 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.