Practical Skills Competition Questions for Catalytic Cracking Unit Operators

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I. Process operations: 1. To reduce catalyst loss, what precautions should be taken when loading and unloading the catalyst during the start-up and shutdown of the plant? (7 points) Answer: When starting to add the catalyst, before the primary feed leg is blocked by the catalyst, most of the catalyst added is lost due to backflow through the primary feed leg. (1 point) At this time, large-scale feeding should be employed at the fastest possible speed, in order to reduce the velocity of the dilute phase and the concentration at the inlet of the cyclone separator; meanwhile, the main air flow rate should be reduced appropriately to shorten the time required to add the catalyst and block the feed leg. (2 points) For the catalyst installed before the sealing leg, it is best to use a balancing agent. (1 point) When stopping the operation for catalyst loading and unloading, ensure proper control of pressure and temperature, reduce the air flow, and carry out the catalyst transfer and unloading in sequence. (1 point) Increase the unloading speed while ensuring that the temperature does not rise too much; it is necessary to properly control the flow rates of the loosening air and conveying air to prevent them from entering the distillation system and causing accidents. (2 points) 2. What is a charcoal pile? What to do when a carbon pile forms? (7 points) Answer: When the ability to form coke exceeds the ability to burn it, the increase in the amount of coke on the catalyst becomes the main issue, leading to the formation of a coke pile. (3 points) When a carbon pile forms, it is common to reduce the reactor feed rate, (1 point) decrease the amount of reprocessed oil and reprocessed oil slurry, (1 point) and if necessary, cut off the feed to allow fluidized coking to occur. (1 point) Within the limits permitted by the regenerator material, the regeneration temperature and main air volume can be appropriately increased, while preventing secondary combustion. (1 point) 3. What is the purpose of the low-flow protection for reactor feed? (7 points) Answer: When the feed rate falls below a certain lower limit, it is not possible to achieve a low enough density in the lift pipe; (1 point) the normal pressure balance between the two units is disrupted; (1 point) the catalyst cannot circulate along the prescribed path; (1 point) moreover, catalyst backflow occurs, causing a large amount of oil and gas to enter the regenerator and leading to accidents. (1 point) In the case of low feed flow for self-protection, the following actions are automatically taken: the reactor feed is cut off and the feed is sent back to the distillation tower; emergency steam is introduced into the lift pipe to maintain the fluidization and circulation of the catalyst. (3 points) 4. Why are herringbone baffles used in distillation towers? (7 points) Answer: The feed to the catalytic cracking distillation column is superheated oil and gas with a temperature of over 450°C, (1 point) along with catalyst dust entrained in it. (1 point) At the lower part of the catalytic distillation column, there is an oil slurry heat exchange section; circulating oil slurry is drawn from the bottom of the column, cooled after heat exchange, and then returned to the column to come into counterflow contact with the rising oil and gas. (1 point) This process serves to rapidly cool the oil and gas in order to prevent coking. (2 points) It also helps to remove the catalyst dust entrained in the oil and gas. Due to the high temperatures and the presence of catalyst dust, triangular baffles are commonly used in this heat exchange section instead of trays. (2 points) 5. What are the similarities and differences between absorption and distillation? (6 points) Answer: The difference between absorption and distillation is that the former relies on the different solubilities of the various components in a mixture in a solvent to achieve separation ； (2 points) The latter achieves separation by taking advantage of the different volatilities of the various components in the mixture. (2 points) The common feature of absorption and distillation is that both are equilibrium problems involving gas and liquid phases. (2 points) 6. What should be noted when using cold reflux to control the temperature at the top of the distillation tower? (6 points) Answer: (1) Ensure proper water removal; enhance the water separation in the oil-gas separator. (2 points) (2) The amplitude of control should not be too large to prevent significant fluctuations in reaction pressure. (2 points) (3) Ensure that the dry point of the crude gasoline meets the required standards; adjust the temperature at the top of the tower based on the analysis results. (2 points) II. Accident identification and handling: 7. What are the causes, symptoms, and treatment methods for a catalyst being contaminated with oil? (7 points) Reason: (2 points) (1) The reaction temperature is too low. (0.5 points) (2) The amount of stripping steam is too low or the temperature of the stripping steam is too low. (0.5 points) (3) Changes in the stripping steam pressure affect the efficiency of catalyst stripping. (0.5 points) (4) The pressure in the settler is too high. (0.5 points) Phenomenon: (2 points) (1) The pressure increases again at stages one and two, and the bed temperature rises again at stage two. (1 point) (2) As the density difference between the two phases decreases, it even becomes negative. (1 point) Treatment method: (3 points) (1) Reduce the amount of oil slurry fed back for reprocessing, or the amount of oil used in such reprocessing; in severe cases, reduce the amounts of vacuum residue and vacuum wax oil as well. (1 point) (2) Slightly increase the amount of stripping steam, and use steam pressure control to maintain stability. (1 point) (3) Increase the reaction temperature. (1 point) The pressure in the settler is kept within the specified range. 8. What is the phenomenon of tower flooding in a distillation column? Reason? What’s the solution? (8 points) Answer: Phenomena: (2.5 points) 1) The liquid level in the distillation tower rises and the temperature in each section increases (0.5 points). 2) The instrument readings return to zero; the temperature in the upper section of the circulation reflux system increases, and the liquid level in D-201 rises (0.5 points). 3) The flow rate of the slurry pump and the re-refining oil pump is insufficient, resulting in overheating at the bottom of the tower and an increase in pressure (0.5 points). 4) The temperature in each section increases while the liquid level remains too low (0.5 points). 5) The pressure drop in the upper part of the tower increases (0.5 points). Causes: (3 points) 1) A sudden increase in the feed volume (0.5 points). 2) Interruption of the circulation reflux process (0.5 points). 3) Severe water contamination in the raw material (0.5 points). 4) Excessive amount of steam entering the tower or steam containing water (0.5 points). 5) High temperature at the bottom of the distillation tower and too low liquid level (0.5 points). 6) Salt formation on the trays at the top of the tower due to diesel extraction from the distillation tower (0.5 points). Solutions: (2.5 points) 1) First, activate the cold reflux at the top of the tower to control its temperature; then adjust the flow rates of various reflux streams to keep the temperatures in each section within acceptable ranges and the liquid levels under control. (0.5 points) 2). Quickly activate the cold reflux to control the top temperature of the tower, then identify the problem and take measures to address it (0.5 points). 3). Reduce the processing volume, maintain the reaction process, adjust the reflux flow at the top of the tower, and control both the top temperature and the liquid levels in various containers (0.5 points). 4). Adjust the heat extraction through the reflux flow at the bottom of the tower to control its temperature and liquid level (0.5 points). 5). Contact the personnel in charge of the reaction process to reduce the processing volume, apply cold reflux at the top of the tower, and compress the light components at the top into diesel for extraction; once the pressure drop returns to normal levels, resume feeding (0.5 points).

III. Equipment Use and Maintenance: 9. How is a hot oil pump preheated, and what precautions should be taken? (6 points) Answer: Slightly open the outlet valve of the preheating pump or the preheating line valve to allow hot oil to enter the pump from the outlet, flow through the pump body, and then be discharged through the inlet pipeline for preheating (2 points). It is not allowed for the pump to run in reverse during preheating, and measures must be taken to prevent the pump from running dry (2 points). At the same time, attention should be paid to the temperature rise; the pump should be rotated regularly, and the cooling water flow must be ensured to remain unobstructed (2 points).

10. Why must the inlet butterfly valve be opened to a certain angle when starting the main fan, rather than being fully open or closed? (7 points) Answer: If the inlet butterfly valve is fully closed when starting the fan, it is equivalent to starting the fan under no load, which results in a very high vacuum at the inlet (1 point). This can not only cause the fan to vibrate excessively but also pose a risk of the inlet pipe collapsing (2 points). Additionally, friction between the impeller and still air can cause the main fan to overheat. In the case of a motor-driven fan, there are significant changes in speed at the moment of startup (2 points). If the inlet butterfly valve is fully open, the load becomes too high, causing the motor current to increase excessively, which may lead to motor damage (2 points). Therefore, when starting the fan, the inlet butterfly valve should be opened to a certain angle.

11. What are the steps for switching between centrifugal pumps? (7 points) Answer: (1) First, check whether the standby pump is in good condition, whether the lubricant level is normal, whether it can be rotated easily, and whether the valve positions are correct; only then start the standby pump ； (3 points) (2) Once the standby pump is operating properly, slowly open the outlet valve, while closing the outlet valve of the pump that is currently in operation, trying to maintain a constant outlet flow rate ； (3 points) (3) After closing the outlet valve of the pump to be shut down, stop the pump following the prescribed sequence. (1 point) IV. Drawing and Calculation: 12. Please draw a flow diagram of the rich gas and dry gas streams in the facility, indicating the names of the equipment (5 points). 13. Given that the fresh feed rate is 150 T/h, diesel fuel is 35 T/h, reprocessed oil slurry is 20 T/h, the amount of reprocessed oil is 50 T/h, dry gas is 10 T/h, gasoline is 75 T/h, diesel fuel is again 35 T/h, and coke is 7.5 T/h, calculate: the single-pass conversion rate and the total conversion rate. (5 points) One-way conversion rate = (Gases + Gasoline + Coke) / (Fresh feedstock + Recycled oil + Slurry) = (10 + 75 + 7.5) / (150 + 20 + 50) × 100% = 42% (2.5 points) Total conversion rate = (Gases + Gasoline + Coke) / Fresh feedstock = (10 + 75 + 7.5) / 150 × 100% = 61.67% (2.5 points) V. Safety and quality knowledge: 14. For a catalytic cracking unit, what are the three key balances that need to be maintained for stable operation? And a brief explanation is provided for each. (5 points) ① Three types of balance: heat balance, pressure balance (or phase balance), and mass balance (1 point). ② Heat balance: The large amount of high-temperature thermal energy released during the coking and regeneration of the catalyst is just sufficient to meet the requirements of the cracking reaction, which takes place at lower temperatures. The catalyst that circulates between the reactor and the regenerator has an adequate quantity and heat capacity, and thus, in a sense, serves as a heat carrier for transferring heat. The heat carrier flows between the two devices, continuously absorbing heat from one end and transferring it to the other. (1 point) ③ Pressure equilibrium refers to the equilibrium of pressure between reaction systems. Pressure balance is crucial for maintaining catalyst circulation, as well as for ensuring the safe and stable operation of the plant. (1 point) In production operations, we maintain pressure balance among the three reactors by strictly controlling the differential pressure between them; this in turn ensures proper circulation of the catalyst. If the differential pressure between these reactors exceeds a certain limit, it can cause the catalyst to flow in the reverse direction. In severe cases, air may enter the reactor, or oil and gas may enter the regenerator, leading to serious accidents. (2 points) ④ Material balance: It mainly refers to the balance of various materials that enter and leave the system. For example: raw materials and products, one-way conversion rate and reprocessing ratio, green coke and burned coke, oxygen supply and demand, loss and replacement of catalysts, balance between gas production and the capacity of compressors, and so on. (1 point). 15. How should one handle a case of hydrogen sulfide poisoning? (5 points) Answer: It is necessary to report the situation to the relevant personnel immediately, (1 point) call the emergency number 120, (1 point) bring an air respirator with you before going to the scene to rescue people, (1 point) cut off the source of the poison and carry the poisoned person to a place with fresh air, (1 point) perform artificial respiration until the ambulance arrives. (1 point) VI. Others: 16. What are the characteristics of pollutant generation in catalytic cracking units? (5 points) Catalytic cracking is a production process in which, with the help of a catalyst and under certain temperature and pressure conditions, feed oil undergoes a series of chemical reactions to be cracked into light oil products. The feedstocks for catalytic cracking are heavy distillates and heavy oils (such as residue and alkaline residue), which contain large amounts of non-hydrocarbon elements such as sulfur, nitrogen, and oxygen. These non-hydrocarbon elements, along with heavy metal elements, are transferred to the products, catalysts, and process wastewater during the production process; some of them are released into the surrounding environment via flue gas and catalyst dust, causing pollution. High-power operating machinery and gas venting, among other things, cause noise pollution. (1 point) Its characteristics are as follows: (1) Pollutants of water. Mainly oil, sulfur, acid, COD, and cyanide. (1 point) (2) Pollutants in the atmosphere. They mainly include gases such as sulfur compounds, nitrogen compounds, CD, catalyst dust from recycled flue gas and furnace flue gas, as well as hydrocarbons resulting from leaks and uncontrolled emissions. (1 point) (3) Pollutants in waste residues. Mainly the alkaline residues emitted during the refining of gasoline, diesel, and liquefied petroleum gas ； Catalyst removal from the regenerator and catalyst dust recovered by the three-spinner separator in the regeneration flue gas. (1 point) (4) Noise pollution. Noise sources such as high-power main fans – air compressors, air-cooled fans, motors, and gas venting. Their sound levels all exceed 100 dB(A). (1 point)