The slurry system has coked up – do you use diesel as a make-up stream?
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As the title suggests: The slurry system has coked up. Do you use diesel as a make-up stream? What are the requirements for reaching the top line, such as the amount of diesel and the time it takes to reach the top line? Is it safe to use diesel as a top-up? Could it lead to some unforeseen consequences?
Introduction to the analysis and treatment of coking at the bottom of the distillation tower under accident conditions. Abstract: This article describes the cause of coking at the bottom of the distillation tower and the procedures taken to address it, during a failure of the slurry pump in a heavy oil catalytic unit and the subsequent shutdown of the slurry system. Practice has shown that due to the heavy nature of the feedstock in heavy oil catalytic units, the high density of the slurry, and its high aromatic content, the operation of the distillation tower can be maintained during a short-term shutdown of the slurry system. However, if the shutdown lasts more than 12 hours, an emergency shutdown is necessary. Otherwise, it will cause significant coking at the bottom of the distillation column. Keywords: Heavy oil catalytic unit; Fractionator; Slurry pump; Coking; Treatment 1. Unit overview: A certain heavy oil catalytic unit is designed with a processing capacity of 1.5 Mt/a, using 60% desulfurized residue and 40% mixed wax oil in its feed. Since October 2001, the plant has been blending regular slag and deslagging agent, with the remainder being mixed wax oil; this corresponds to a 60% blending ratio of deslagging agent, and the processing capacity has reached 4800 T/D. The yield of gasoline + light diesel is over 69.5%, with the amount of oil slurry lost at around 2.3%. The slurry density ranges from 1000 kg/m3 to 1060 kg/m3, and its solid content is 2.0 g/L. The properties of typical raw materials and slurry are shown in Table 1. Table 1: Typical data on the properties of mixed feedstocks and oil slurry
| Parameter | Mixed Feedstock | Oil Slurry | Mixed Paraffinic Oil |
|-----------|-----------------|------------|----------------------|
| Density (20°C)/kg•m-3 | 901.9 | 1016.3 | 873.3 |
| Enneberg viscosity/° | At 60°C: 6.04; At 80°C: 3.31 | At 60°C: 8.89; At 80°C: 3.80 | At 60°C: 2.00; At 80°C: 1.55 |
| Residue content/m% | 5.26 | 11.08 | 0.09 |
| Boiling range/°C | 238.0; 392.0 at 10%; 463.0 at 30%; —— at 50%; 500.0 at 70% | 186.0; 420.0 at 10%; 465.0 at 30%; 437.0 at 50%; 530.0 at 70% | 188.0; 358.0 at 10%; 389.0 at 30%; —— at 50%; 490.0 at 70% |
| Distillate yield at 350°C, % | 4.5 | 2.8 | 9.0 |
| Distillate yield at 500°C, % | 41.0 | 50.0 | 93.0 |
| Composition by type, % | Saturated hydrocarbons: 59.59; Aromatic hydrocarbons: 23.52; Resins + asphaltenes: 11.81 | Saturated hydrocarbons: 33.52; Aromatic hydrocarbons: 51.13; Resins + asphaltenes: 10.79 | Saturated hydrocarbons: 75.65; Aromatic hydrocarbons: 14.83; Resins + asphaltenes: 5.19 |
| Solid content, g•L-1 | —— | 2.0 | —— |
2. Introduction to the distillation system: The distillation tower has 32 trays arranged from bottom to top; in June 2002, trays 20 through 32 were all replaced with ADV-type floating valve trays. Levels 1 to 19 are the originally designed tongue-shaped trays. The lower part of the distillation tower is equipped with 10 layers of herringbone baffles. A DN80 stirring steam coil is installed at the bottom of the tower and is used in daily production. The oil slurry extraction port at the bottom of the tower is of a squirrel-cage structure. The diameter of the fractionation tower is 4800 mm/5800 mm, with the diameter reduction occurring between the 20th and 21st trays. The top of the fractionation tower and its side lines correspond to the overhead vapor system, top circulation system, light diesel extraction system, absorbent oil system, first intermediate system, re-refining oil system (second intermediate system and re-refining oil reflux system to the tower), and slurry system, respectively. To prevent coking in the slurry distillation system, a slurry scale inhibitor system is installed, and during normal operation, the scale inhibitor is added uniformly as per the design. Through previous inspections, it was found that there was no coking in the bottom of the fractionation tower, the slurry pipelines, or the heat exchangers. The slurry system at the bottom of the distillation column of this unit is equipped with two slurry pumps, one in operation and one as a backup. After being drawn from the bottom of the distillation tower, the slurry is pressurized by a slurry pump; it first exchanges heat with fresh feedstock to heat the feedstock to 200°C, and then enters a slurry steam generator to produce 3.5 Mpa saturated steam. After heat exchange, it is divided into three streams. The slurry is sent back for reprocessing into the riser ； Along the way, passing by the water cooler’s external discharge device ； Finally, it passes along the slurry upward and downward return lines to enter the distillation tower. Under normal conditions, the control flow rate at the upper return tower is 140 T/H to 200 T/H, entering the split plate of the distillation tower ； The flow rate at the bottom of the reboiler is 270 T/H to 300 T/H, entering the bottom of the fractionation tower. The liquid level of the oil slurry at the bottom of the tower is generally maintained between 40% and 50%. The flow diagram of the distillation system is shown in Figure 1. 3. Analysis of the causes of coking in the distillation tower: At 6:10 on April 28, 2003, a fire broke out in the power distribution panel of the slurry pumps, resulting in the failure of those two pumps to operate and thus the shutdown of the slurry system. Two hours later, the distribution panel was repaired and the slurry pump was started urgently; however, both slurry pumps failed to operate properly, and the slurry system stopped circulating. During this period, the slurry pump was urgently repaired. At 2:00 on April 29, one of the slurry pumps was repaired, but when it was started, it could not pump any fluid. At around 9:00 on April 29, feed supply was interrupted due to coking in the oil slurry lines at the bottom of the distillation tower and at the pump inlet. 3.1 Changes in the operation of the reverse and regeneration systems during the shutdown of the slurry system: After the slurry system is shut down, this is done to reduce the complexity of operating the distillation tower. The plant’s processing capacity was reduced from 200 T/H to 150 T/H, and the oil slurry reprocessing was stopped. The reprocessing capacity of the re-refinery has been reduced from 65 T/H to 55 T/H. The reaction temperature was reduced from 502°C to 498°C. The slurry system was shut down, resulting in the raw material preheating temperature dropping to 130°C. To improve the atomization effect of the raw material, the proportion of atomization steam used in the nozzles was increased from around 5% to around 8%. 3.2 Changes in the operation of the fractionation system during the shutdown of the slurry pump: After the slurry system is shut down, the heat and vapor-liquid load on the fractionation tower need to be readjusted. The top circulation rate was increased from 300 T/H to 400 T/H; heat extraction from Stage 1 and Stage 2 was enhanced to control the operation in the middle and lower sections of the distillation column. The reflux rate from Stage 1 was raised from 200 T/H to 350 T/H, while the internal reflux rate in Stage 2 was increased by 30 T/H, in order to wash the catalyst powder carried above the herringbone trays by the oil-gas stream. Through the above adjustments, the extraction temperature of the re-refined oil is kept below 270°C. The bottom temperature of the tower is controlled by adding feed oil (the properties of the feed are shown in Table 1). With the slurry pump shut down, the bottom of the fractionation tower is not equipped with external discharge lines, so the oil slurry at the bottom of the tower cannot be discharged for replacement. Through the adjustments mentioned above, from 6:00 to 18:00 on April 28, the operating temperatures of all trays above the second tray of the distillation column up to the top of the column (including the side streams) were essentially the same as normal. However, the temperature at the bottom of the tower and the temperatures above and below the chevron baffles exceeded the normal operating temperatures; since the slurry at the bottom of the tower could not be discharged externally, it could not be replaced, making it impossible to control the temperature there. After 18:00 on April 28, the operating conditions at the lower part of the distillation tower began to deteriorate significantly, and a tendency for coking at the bottom of the tower gradually emerged. 3.2.1 During normal production, the bottom temperature of the distillation tower is controlled at 345°C to 350°C. After the slurry system was shut down, the bottom temperature of the tower gradually rose to 440°C. After 18:00 on April 28, the temperature at the bottom of the tower began to drop. In order to remove the oil slurry from that area, around 20:00, the connection between the reprocessing line and the slurry pump was opened, and the reprocessing pump was used to draw out the oil slurry from the bottom of the tower. This pump operated for about 10 minutes before it became unable to continue functioning, forcing the activation of another reprocessing pump to maintain circulation in the reprocessing system. By 2:00 on April 29 (20 hours after the shutdown of the slurry system), one of the slurry pumps had been repaired, and the temperature at the bottom of the tower had dropped below 300°C. The slurry pump was started urgently; it failed to draw in any fluid, indicating that the pipelines were blocked by coke deposits. At this time, the liquid level at the bottom of the tower gradually increased, and by 4:00 on April 29, it had reached over 100%. 3.2.2 During the entire period that the slurry pump is out of service, the quality control department shall be arranged to analyze the solid content in the reprocessed oil every two hours. To replace the reprocessed oil, reduce the solid content, and prevent coking and blockages in the pipelines and heat exchangers of the reprocessed oil and the second system, a reprocessed oil circulation line is activated at the nozzle for the reprocessed oil, along with an emergency bypass and a feedstock circulation line, to continuously discharge the reprocessed oil outside the plant. By the time the unit was shut down, the solid content in the re-refined oil did not exceed 6 g/L. From 6:00 on April 28, when the slurry system was shut down, until 4:00 on April 29, the quality of the products from all outlets of the distillation tower was satisfactory, with no outlet meeting standards that were not met. The yields at each distillation outlet are basically normal. After 6:00 on April 29, due to an obvious trend of coking at the bottom of the distillation tower, the plant decided to shut down for emergency repairs. .3 Analysis of coking at the bottom of the distillation column: The slurry system was shut down, and the feed to the unit was stopped for 27 hours. During the shutdown of the slurry system, the heat at the bottom of the distillation tower cannot be removed, resulting in a continuous rise in the temperature at that location. To reduce the bottom temperature of the distillation column, dilute the oil slurry with a high solid content at the bottom of the column, prevent rapid coking of the oil slurry there, buy time for the repair of the oil slurry pump, and intermittently supply raw material to the bottom of the column. Over the 12-hour period from 6:00 on April 28th to 18:00, intermittent oil replenishment to the bottom of the distillation tower did prove effective. Each time the liquid level at the bottom of the tower was raised to around 60%, it would gradually drop to 20%, and this level could be maintained for about 3 hours. During this period, the temperature of the liquid phase at the bottom of the distillation column increased gradually from around 340°C to around 440°C. After that, oil was added to the bottom of the column again; after each addition, the temperature at the bottom of the column rose slowly while the liquid level decreased slowly. As can be seen from Table 1, the distillation yield of the oil slurry at 500°C is only 50%; the distillation temperature for 30% of the content is 465°C, and the aromatic content reaches 51% ； The distillation yield of the mixed crude oil at 500°C is 41%, and the distillation temperature at 30% yield is 463°C. Since the heat at the bottom of the distillation tower could not be removed continuously, the temperature continued to rise to 440°C, reaching the conditions required for the coking process – in which hydrogen-poor heavy fractions undergo deep thermal cracking and condensation reactions – and coking began to occur. According to normal production data, the amount of slurry generated accounts for about 6% of the raw material volume (with no reprocessing of the slurry). Additionally, considering the heavy components that do not distill at the bottom of the tower due to the raw oil added there, the amount of heavy oil remaining at the bottom of the distillation tower is around 6 T/H. In a short period of time, although the solid content in the liquid phase at the bottom of the distillation tower continued to increase, the amount of coking was not significant. At this point, by resuming the circulation of the slurry system and utilizing the DN400 pipeline for the slurry at the tower bottom, it is possible to supply large amounts of oil to the distillation tower through the operation of the slurry pump, thereby allowing for continuous discharge of large quantities of slurry – thus ensuring that feed to the plant can continue without interruption. After 18:00 on April 28, as the solid content at the bottom of the tower increased, the density of the slurry rose further, more heavy fractions accumulated, and the coking process became increasingly severe. After each refueling, the liquid level at the bottom of the tower basically stops dropping and instead rises slowly; the temperature of the liquid phase at the bottom of the tower decreases gradually, while a coking layer accumulates slowly at the bottom of the tower, causing the coked surface to rise progressively. The coking layer temperature decreases slowly under the effect of anti-coking steam; the additional feed oil cannot penetrate into the coking layer, and the liquid level does not drop. As shown in Table 2, at the end of the unit shutdown, the fractionation tower is washed with water to cool it down. The coke lumps removed from the bottom of the fractionation tower contained no large lumps; in terms of their appearance, they were all small lumps with distinct edges, containing a large amount of catalyst powder. Their interior showed a clear layered structure, along with elliptical-shaped voids. Coke quality is extremely similar to that of coked coke. Table 2: Comparison of Parameter Values Between Normal Operation of the Typical Fractionating Tower and Shutdown Condition of the Slurry System
Parameter | Normal Operation | Slurry Shutdown Condition
---|---|---
Processing Capacity/T•H-1 | 200 | 150 | 150
Reaction Temperature/°C | 502 | 498 | 498
Settler Pressure/Mpa | 0.212 | 0.212 | 0.2142
Top Pressure of Fractionating Tower/Mpa | 0.171 | 0.136 | 0.1347
Top Temperature of Fractionating Tower/°C | 108.4 | 108.6 | 108.6
Top Circulation Volume/T•H-1 | 220 | 400 | 400
Temperature of Top Circulation Extract/°C | 124.7 | 123.7 | 124.0
Cooling Return Flow Volume/T•H-1 | 30 | 30.0 | 30.0
Absorbed Oil Flow Volume/T•H-1 | 28 | 28.0 | 28.0
Temperature of Light Diesel Extract/°C | 190 | 196.3 | 195.6
Control Temperature for Stage 1/°C | 232 | 232.0 | 233
Circulation Volume for Stage 1/T•H-1 | 200 | 350 | 350
Extract Temperature for Stage 1/°C | 266 | 265 | 266
Extract Temperature for Stage 2/°C | 338 | 339 | 339
Circulation Volume for Stage 2/T•H-1 | 100 | 100 | 100
Temperature of Stage 2 Return to Tower/°C | 200 | 204 | 250
Internal Reflux Flow Volume/T•H-1 | 30 | 60 | 60
Re-refining Oil Re-refining Volume/T•H-1 | 65 | 55 | 55
Temperature on the Wedge Tray/°C | 357 | 360–440 | 410–440
Temperature Below the Wedge Tray/°C | 406 | 400–460 | 430–460
Circulation Volume of Slurry Returning to Tower from Above/T•H-1 | 160
Circulation Volume of Slurry Returning to Tower from Below/T•H-1 | 300
Temperature of Slurry Returning to Tower/°C | 270
Liquid Phase Temperature at Tower Bottom/°C | 347 | 350–440 | 180–440
Steam Volume for Tower Bottom Stirring/kg•H-1 | 200 | 500 | 500
Tower Bottom Liquid Level/% | 40–50 | 10–50 | 50–100
4. Treatment of Coking in the Tower Bottom and Slurry Pipelines
After the plant was shut down urgently at 9:00 on April 29, the slurry, re-refining oil, and feedstock systems were cleaned out. The anti-reaction and distillation systems have been completely shut down. The problems that need to be solved are: ①. How to remove the oil remaining in the bottom of the distillation tower and in the inlet pipeline of the slurry pump. ②. How to clean the coke from the distillation column bottom to the slurry pump inlet pipeline. ③. How to clean the coke from the bottom of the distillation tower to the inlet pipeline of the re-refining pump. First, connect a temporary DN80 line from the DN80 vent line at the bottom of the fractionation tower to the inlet of the reprocessing pump; then connect a DN80 line from the outlet of the reprocessing pump to the line leading out of the slurry unit. The distillation column bottom slurry was pumped out using a re-refining pump, and by repeatedly introducing 1.0 Mpa steam for backwashing, the liquid level in the distillation column was emptied over a period of nearly 12 hours. The inlet pipeline of the slurry pump is purged repeatedly with steam and the oil at the pump inlet is drained to remove as much oil as possible from the pipeline. The slurry system was shut down for a total of 27 hours, which likely caused catalyst powder and heavy oil to accumulate on the middle and lower trays as well as on the cross-trays in the distillation tower. To remove the existing catalyst powder and heavy oil, as well as to replace the oil stored at the bottom of the distillation tower and cool the coking layer there; this is done in preparation for removing the blind flanges from the oil and gas pipelines leading into the distillation tower and for cleaning the coking from the manholes in the tower. We wash the fractionation tower with hot water at 100°C. Using a cold return pipeline, fresh water is injected into the distillation tower from the 32nd tray. At the same time, the stripping steam for the light diesel stripper and the stirring steam at the bottom of the distillation tower are turned on, thereby heating the fresh water injected into the distillation tower to around 100°C. The distillation tower is then washed with this hot water until clear water appears in the outlet at the bottom of the tower. After taking the above measures, the manhole at the bottom of the distillation tower was opened, and it was found that the area from the bottom of the distillation tower to the inlet for the reaction oil and gas inside the tower was completely filled with sludge and coke pieces mixed with a large amount of catalyst powder, forming a layer of coke with an axial height of 4 meters. However, there were no coking deposits in the oil slurry returning to the tray coils or in the bottom stirring steam coils. Subsequently, when the inlet and outlet valves of the slurry pump were opened, it was found that in the 80-meter-long DN400 pipeline running from the inlet of this slurry pump to the bottom of the distillation tower, the 30 meters of pipeline near the slurry pump were completely blocked by coke deposits; approximately half of the remaining 50 meters of pipeline near the bottom of the distillation tower also had coke deposits. This explains the reason why the slurry pump started up in an emergency in a vacuum condition with no flow. The coking at the bottom of the distillation tower is relatively easy to remove; the difficulty lies in how to remove the coking in the 80-meter-long DN400 slurry pipeline, especially in the section near the slurry pump. After careful analysis, it was found that the accumulation of coke in the 30 meters of pipeline near the slurry pump was quite dense, while in the 50 meters of pipeline extending from the bottom of the fractionation tower, the coke accumulation was less dense. Given the short time available for repairs, we decided to use a method of replacing sections of pipe in the 30 meters of pipeline near the slurry pump in order to remove the coke, whereas for the remaining 50 meters of pipeline, we used water flushing. After the coking at the bottom of the distillation tower is completed, seal the manhole at the bottom and fill the distillation tower with fresh water. When fresh water is filled into the chevron baffle, the large discharge valve at the bottom of the distillation tower is quickly opened, and the high-pressure, fully filled pipeline of fresh water is used to carry away the coke deposits in the 50-meter section of pipeline near the bottom of the distillation tower. In the actual coking removal process, this method has achieved very good results. The inlet pipelines of the two slurry pumps were each flushed with water once to completely remove any coke deposits from within the pipelines. **It reduced the time required for emergency repairs and saved funds needed for pipe replacement. 5. Conclusion According to available data, similar slurry pump failures have also occurred in the wax oil catalytic units of some domestic refining companies, resulting in the cessation of circulation in the slurry system. However, since the raw material for wax oil catalysis is relatively light, more than 70% of the wax oil added to the bottom of the distillation tower can evaporate from there, with only about 30% of the heavier fractions remaining at the bottom. Moreover, the amount of wax oil catalytic slurry produced is less than 2%, and its aromatic content is low; as a result, the amount of coking that occurs in the heavier fractions at the bottom of the distillation tower is minimal, and no clogs are formed. Therefore, in cases where the slurry system stops circulating, most of these units have been able to continue operating for over 20 hours by adding more wax oil to the bottom of the distillation tower, without any need to shut down the feed supply urgently. From this incident involving the termination of circulation in the slurry system of the heavy oil catalytic unit, as well as the subsequent forced shutdown due to the interruption of feed, we have drawn the following lessons: ①. In cases where circulation in the slurry system of a heavy oil catalytic unit is terminated, by reducing the processing volume and adjusting the operation of the distillation tower, it is possible to ensure that the quality of the products at the outlet meets requirements and to maintain low-load operation for a certain period of time. . ②In heavy oil catalytic units, due to the heavy nature of the feedstock and the large amount of slurry generated, once the slurry system stops circulating, it is possible, in a short period of time when the tendency for coking is not yet significant, to replenish light feedstock (wax oil) from the bottom of the distillation tower. At the same time, the catalyst powder located below the two lower trays in the reprocessing column can be washed with recycled oil. This allows the unit to continue operating without shutting down the feed supply; once the slurry system resumes normal circulation, an ample amount of oil can be added from the bottom of the distillation tower, along with the removal of large quantities of slurry, thereby restoring normal production. ③When the slurry system of the heavy oil catalytic unit is shut down and coking at the bottom of the fractionation tower becomes severe, it is necessary to cut off the feed.
| Parameter | Mixed Feedstock | Oil Slurry | Mixed Paraffinic Oil |
|-----------|-----------------|------------|----------------------|
| Density (20°C)/kg•m-3 | 901.9 | 1016.3 | 873.3 |
| Enneberg viscosity/° | At 60°C: 6.04; At 80°C: 3.31 | At 60°C: 8.89; At 80°C: 3.80 | At 60°C: 2.00; At 80°C: 1.55 |
| Residue content/m% | 5.26 | 11.08 | 0.09 |
| Boiling range/°C | 238.0; 392.0 at 10%; 463.0 at 30%; —— at 50%; 500.0 at 70% | 186.0; 420.0 at 10%; 465.0 at 30%; 437.0 at 50%; 530.0 at 70% | 188.0; 358.0 at 10%; 389.0 at 30%; —— at 50%; 490.0 at 70% |
| Distillate yield at 350°C, % | 4.5 | 2.8 | 9.0 |
| Distillate yield at 500°C, % | 41.0 | 50.0 | 93.0 |
| Composition by type, % | Saturated hydrocarbons: 59.59; Aromatic hydrocarbons: 23.52; Resins + asphaltenes: 11.81 | Saturated hydrocarbons: 33.52; Aromatic hydrocarbons: 51.13; Resins + asphaltenes: 10.79 | Saturated hydrocarbons: 75.65; Aromatic hydrocarbons: 14.83; Resins + asphaltenes: 5.19 |
| Solid content, g•L-1 | —— | 2.0 | —— |
2. Introduction to the distillation system: The distillation tower has 32 trays arranged from bottom to top; in June 2002, trays 20 through 32 were all replaced with ADV-type floating valve trays. Levels 1 to 19 are the originally designed tongue-shaped trays. The lower part of the distillation tower is equipped with 10 layers of herringbone baffles. A DN80 stirring steam coil is installed at the bottom of the tower and is used in daily production. The oil slurry extraction port at the bottom of the tower is of a squirrel-cage structure. The diameter of the fractionation tower is 4800 mm/5800 mm, with the diameter reduction occurring between the 20th and 21st trays. The top of the fractionation tower and its side lines correspond to the overhead vapor system, top circulation system, light diesel extraction system, absorbent oil system, first intermediate system, re-refining oil system (second intermediate system and re-refining oil reflux system to the tower), and slurry system, respectively. To prevent coking in the slurry distillation system, a slurry scale inhibitor system is installed, and during normal operation, the scale inhibitor is added uniformly as per the design. Through previous inspections, it was found that there was no coking in the bottom of the fractionation tower, the slurry pipelines, or the heat exchangers. The slurry system at the bottom of the distillation column of this unit is equipped with two slurry pumps, one in operation and one as a backup. After being drawn from the bottom of the distillation tower, the slurry is pressurized by a slurry pump; it first exchanges heat with fresh feedstock to heat the feedstock to 200°C, and then enters a slurry steam generator to produce 3.5 Mpa saturated steam. After heat exchange, it is divided into three streams. The slurry is sent back for reprocessing into the riser ； Along the way, passing by the water cooler’s external discharge device ； Finally, it passes along the slurry upward and downward return lines to enter the distillation tower. Under normal conditions, the control flow rate at the upper return tower is 140 T/H to 200 T/H, entering the split plate of the distillation tower ； The flow rate at the bottom of the reboiler is 270 T/H to 300 T/H, entering the bottom of the fractionation tower. The liquid level of the oil slurry at the bottom of the tower is generally maintained between 40% and 50%. The flow diagram of the distillation system is shown in Figure 1. 3. Analysis of the causes of coking in the distillation tower: At 6:10 on April 28, 2003, a fire broke out in the power distribution panel of the slurry pumps, resulting in the failure of those two pumps to operate and thus the shutdown of the slurry system. Two hours later, the distribution panel was repaired and the slurry pump was started urgently; however, both slurry pumps failed to operate properly, and the slurry system stopped circulating. During this period, the slurry pump was urgently repaired. At 2:00 on April 29, one of the slurry pumps was repaired, but when it was started, it could not pump any fluid. At around 9:00 on April 29, feed supply was interrupted due to coking in the oil slurry lines at the bottom of the distillation tower and at the pump inlet. 3.1 Changes in the operation of the reverse and regeneration systems during the shutdown of the slurry system: After the slurry system is shut down, this is done to reduce the complexity of operating the distillation tower. The plant’s processing capacity was reduced from 200 T/H to 150 T/H, and the oil slurry reprocessing was stopped. The reprocessing capacity of the re-refinery has been reduced from 65 T/H to 55 T/H. The reaction temperature was reduced from 502°C to 498°C. The slurry system was shut down, resulting in the raw material preheating temperature dropping to 130°C. To improve the atomization effect of the raw material, the proportion of atomization steam used in the nozzles was increased from around 5% to around 8%. 3.2 Changes in the operation of the fractionation system during the shutdown of the slurry pump: After the slurry system is shut down, the heat and vapor-liquid load on the fractionation tower need to be readjusted. The top circulation rate was increased from 300 T/H to 400 T/H; heat extraction from Stage 1 and Stage 2 was enhanced to control the operation in the middle and lower sections of the distillation column. The reflux rate from Stage 1 was raised from 200 T/H to 350 T/H, while the internal reflux rate in Stage 2 was increased by 30 T/H, in order to wash the catalyst powder carried above the herringbone trays by the oil-gas stream. Through the above adjustments, the extraction temperature of the re-refined oil is kept below 270°C. The bottom temperature of the tower is controlled by adding feed oil (the properties of the feed are shown in Table 1). With the slurry pump shut down, the bottom of the fractionation tower is not equipped with external discharge lines, so the oil slurry at the bottom of the tower cannot be discharged for replacement. Through the adjustments mentioned above, from 6:00 to 18:00 on April 28, the operating temperatures of all trays above the second tray of the distillation column up to the top of the column (including the side streams) were essentially the same as normal. However, the temperature at the bottom of the tower and the temperatures above and below the chevron baffles exceeded the normal operating temperatures; since the slurry at the bottom of the tower could not be discharged externally, it could not be replaced, making it impossible to control the temperature there. After 18:00 on April 28, the operating conditions at the lower part of the distillation tower began to deteriorate significantly, and a tendency for coking at the bottom of the tower gradually emerged. 3.2.1 During normal production, the bottom temperature of the distillation tower is controlled at 345°C to 350°C. After the slurry system was shut down, the bottom temperature of the tower gradually rose to 440°C. After 18:00 on April 28, the temperature at the bottom of the tower began to drop. In order to remove the oil slurry from that area, around 20:00, the connection between the reprocessing line and the slurry pump was opened, and the reprocessing pump was used to draw out the oil slurry from the bottom of the tower. This pump operated for about 10 minutes before it became unable to continue functioning, forcing the activation of another reprocessing pump to maintain circulation in the reprocessing system. By 2:00 on April 29 (20 hours after the shutdown of the slurry system), one of the slurry pumps had been repaired, and the temperature at the bottom of the tower had dropped below 300°C. The slurry pump was started urgently; it failed to draw in any fluid, indicating that the pipelines were blocked by coke deposits. At this time, the liquid level at the bottom of the tower gradually increased, and by 4:00 on April 29, it had reached over 100%. 3.2.2 During the entire period that the slurry pump is out of service, the quality control department shall be arranged to analyze the solid content in the reprocessed oil every two hours. To replace the reprocessed oil, reduce the solid content, and prevent coking and blockages in the pipelines and heat exchangers of the reprocessed oil and the second system, a reprocessed oil circulation line is activated at the nozzle for the reprocessed oil, along with an emergency bypass and a feedstock circulation line, to continuously discharge the reprocessed oil outside the plant. By the time the unit was shut down, the solid content in the re-refined oil did not exceed 6 g/L. From 6:00 on April 28, when the slurry system was shut down, until 4:00 on April 29, the quality of the products from all outlets of the distillation tower was satisfactory, with no outlet meeting standards that were not met. The yields at each distillation outlet are basically normal. After 6:00 on April 29, due to an obvious trend of coking at the bottom of the distillation tower, the plant decided to shut down for emergency repairs. .3 Analysis of coking at the bottom of the distillation column: The slurry system was shut down, and the feed to the unit was stopped for 27 hours. During the shutdown of the slurry system, the heat at the bottom of the distillation tower cannot be removed, resulting in a continuous rise in the temperature at that location. To reduce the bottom temperature of the distillation column, dilute the oil slurry with a high solid content at the bottom of the column, prevent rapid coking of the oil slurry there, buy time for the repair of the oil slurry pump, and intermittently supply raw material to the bottom of the column. Over the 12-hour period from 6:00 on April 28th to 18:00, intermittent oil replenishment to the bottom of the distillation tower did prove effective. Each time the liquid level at the bottom of the tower was raised to around 60%, it would gradually drop to 20%, and this level could be maintained for about 3 hours. During this period, the temperature of the liquid phase at the bottom of the distillation column increased gradually from around 340°C to around 440°C. After that, oil was added to the bottom of the column again; after each addition, the temperature at the bottom of the column rose slowly while the liquid level decreased slowly. As can be seen from Table 1, the distillation yield of the oil slurry at 500°C is only 50%; the distillation temperature for 30% of the content is 465°C, and the aromatic content reaches 51% ； The distillation yield of the mixed crude oil at 500°C is 41%, and the distillation temperature at 30% yield is 463°C. Since the heat at the bottom of the distillation tower could not be removed continuously, the temperature continued to rise to 440°C, reaching the conditions required for the coking process – in which hydrogen-poor heavy fractions undergo deep thermal cracking and condensation reactions – and coking began to occur. According to normal production data, the amount of slurry generated accounts for about 6% of the raw material volume (with no reprocessing of the slurry). Additionally, considering the heavy components that do not distill at the bottom of the tower due to the raw oil added there, the amount of heavy oil remaining at the bottom of the distillation tower is around 6 T/H. In a short period of time, although the solid content in the liquid phase at the bottom of the distillation tower continued to increase, the amount of coking was not significant. At this point, by resuming the circulation of the slurry system and utilizing the DN400 pipeline for the slurry at the tower bottom, it is possible to supply large amounts of oil to the distillation tower through the operation of the slurry pump, thereby allowing for continuous discharge of large quantities of slurry – thus ensuring that feed to the plant can continue without interruption. After 18:00 on April 28, as the solid content at the bottom of the tower increased, the density of the slurry rose further, more heavy fractions accumulated, and the coking process became increasingly severe. After each refueling, the liquid level at the bottom of the tower basically stops dropping and instead rises slowly; the temperature of the liquid phase at the bottom of the tower decreases gradually, while a coking layer accumulates slowly at the bottom of the tower, causing the coked surface to rise progressively. The coking layer temperature decreases slowly under the effect of anti-coking steam; the additional feed oil cannot penetrate into the coking layer, and the liquid level does not drop. As shown in Table 2, at the end of the unit shutdown, the fractionation tower is washed with water to cool it down. The coke lumps removed from the bottom of the fractionation tower contained no large lumps; in terms of their appearance, they were all small lumps with distinct edges, containing a large amount of catalyst powder. Their interior showed a clear layered structure, along with elliptical-shaped voids. Coke quality is extremely similar to that of coked coke. Table 2: Comparison of Parameter Values Between Normal Operation of the Typical Fractionating Tower and Shutdown Condition of the Slurry System
Parameter | Normal Operation | Slurry Shutdown Condition
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Processing Capacity/T•H-1 | 200 | 150 | 150
Reaction Temperature/°C | 502 | 498 | 498
Settler Pressure/Mpa | 0.212 | 0.212 | 0.2142
Top Pressure of Fractionating Tower/Mpa | 0.171 | 0.136 | 0.1347
Top Temperature of Fractionating Tower/°C | 108.4 | 108.6 | 108.6
Top Circulation Volume/T•H-1 | 220 | 400 | 400
Temperature of Top Circulation Extract/°C | 124.7 | 123.7 | 124.0
Cooling Return Flow Volume/T•H-1 | 30 | 30.0 | 30.0
Absorbed Oil Flow Volume/T•H-1 | 28 | 28.0 | 28.0
Temperature of Light Diesel Extract/°C | 190 | 196.3 | 195.6
Control Temperature for Stage 1/°C | 232 | 232.0 | 233
Circulation Volume for Stage 1/T•H-1 | 200 | 350 | 350
Extract Temperature for Stage 1/°C | 266 | 265 | 266
Extract Temperature for Stage 2/°C | 338 | 339 | 339
Circulation Volume for Stage 2/T•H-1 | 100 | 100 | 100
Temperature of Stage 2 Return to Tower/°C | 200 | 204 | 250
Internal Reflux Flow Volume/T•H-1 | 30 | 60 | 60
Re-refining Oil Re-refining Volume/T•H-1 | 65 | 55 | 55
Temperature on the Wedge Tray/°C | 357 | 360–440 | 410–440
Temperature Below the Wedge Tray/°C | 406 | 400–460 | 430–460
Circulation Volume of Slurry Returning to Tower from Above/T•H-1 | 160
Circulation Volume of Slurry Returning to Tower from Below/T•H-1 | 300
Temperature of Slurry Returning to Tower/°C | 270
Liquid Phase Temperature at Tower Bottom/°C | 347 | 350–440 | 180–440
Steam Volume for Tower Bottom Stirring/kg•H-1 | 200 | 500 | 500
Tower Bottom Liquid Level/% | 40–50 | 10–50 | 50–100
4. Treatment of Coking in the Tower Bottom and Slurry Pipelines
After the plant was shut down urgently at 9:00 on April 29, the slurry, re-refining oil, and feedstock systems were cleaned out. The anti-reaction and distillation systems have been completely shut down. The problems that need to be solved are: ①. How to remove the oil remaining in the bottom of the distillation tower and in the inlet pipeline of the slurry pump. ②. How to clean the coke from the distillation column bottom to the slurry pump inlet pipeline. ③. How to clean the coke from the bottom of the distillation tower to the inlet pipeline of the re-refining pump. First, connect a temporary DN80 line from the DN80 vent line at the bottom of the fractionation tower to the inlet of the reprocessing pump; then connect a DN80 line from the outlet of the reprocessing pump to the line leading out of the slurry unit. The distillation column bottom slurry was pumped out using a re-refining pump, and by repeatedly introducing 1.0 Mpa steam for backwashing, the liquid level in the distillation column was emptied over a period of nearly 12 hours. The inlet pipeline of the slurry pump is purged repeatedly with steam and the oil at the pump inlet is drained to remove as much oil as possible from the pipeline. The slurry system was shut down for a total of 27 hours, which likely caused catalyst powder and heavy oil to accumulate on the middle and lower trays as well as on the cross-trays in the distillation tower. To remove the existing catalyst powder and heavy oil, as well as to replace the oil stored at the bottom of the distillation tower and cool the coking layer there; this is done in preparation for removing the blind flanges from the oil and gas pipelines leading into the distillation tower and for cleaning the coking from the manholes in the tower. We wash the fractionation tower with hot water at 100°C. Using a cold return pipeline, fresh water is injected into the distillation tower from the 32nd tray. At the same time, the stripping steam for the light diesel stripper and the stirring steam at the bottom of the distillation tower are turned on, thereby heating the fresh water injected into the distillation tower to around 100°C. The distillation tower is then washed with this hot water until clear water appears in the outlet at the bottom of the tower. After taking the above measures, the manhole at the bottom of the distillation tower was opened, and it was found that the area from the bottom of the distillation tower to the inlet for the reaction oil and gas inside the tower was completely filled with sludge and coke pieces mixed with a large amount of catalyst powder, forming a layer of coke with an axial height of 4 meters. However, there were no coking deposits in the oil slurry returning to the tray coils or in the bottom stirring steam coils. Subsequently, when the inlet and outlet valves of the slurry pump were opened, it was found that in the 80-meter-long DN400 pipeline running from the inlet of this slurry pump to the bottom of the distillation tower, the 30 meters of pipeline near the slurry pump were completely blocked by coke deposits; approximately half of the remaining 50 meters of pipeline near the bottom of the distillation tower also had coke deposits. This explains the reason why the slurry pump started up in an emergency in a vacuum condition with no flow. The coking at the bottom of the distillation tower is relatively easy to remove; the difficulty lies in how to remove the coking in the 80-meter-long DN400 slurry pipeline, especially in the section near the slurry pump. After careful analysis, it was found that the accumulation of coke in the 30 meters of pipeline near the slurry pump was quite dense, while in the 50 meters of pipeline extending from the bottom of the fractionation tower, the coke accumulation was less dense. Given the short time available for repairs, we decided to use a method of replacing sections of pipe in the 30 meters of pipeline near the slurry pump in order to remove the coke, whereas for the remaining 50 meters of pipeline, we used water flushing. After the coking at the bottom of the distillation tower is completed, seal the manhole at the bottom and fill the distillation tower with fresh water. When fresh water is filled into the chevron baffle, the large discharge valve at the bottom of the distillation tower is quickly opened, and the high-pressure, fully filled pipeline of fresh water is used to carry away the coke deposits in the 50-meter section of pipeline near the bottom of the distillation tower. In the actual coking removal process, this method has achieved very good results. The inlet pipelines of the two slurry pumps were each flushed with water once to completely remove any coke deposits from within the pipelines. **It reduced the time required for emergency repairs and saved funds needed for pipe replacement. 5. Conclusion According to available data, similar slurry pump failures have also occurred in the wax oil catalytic units of some domestic refining companies, resulting in the cessation of circulation in the slurry system. However, since the raw material for wax oil catalysis is relatively light, more than 70% of the wax oil added to the bottom of the distillation tower can evaporate from there, with only about 30% of the heavier fractions remaining at the bottom. Moreover, the amount of wax oil catalytic slurry produced is less than 2%, and its aromatic content is low; as a result, the amount of coking that occurs in the heavier fractions at the bottom of the distillation tower is minimal, and no clogs are formed. Therefore, in cases where the slurry system stops circulating, most of these units have been able to continue operating for over 20 hours by adding more wax oil to the bottom of the distillation tower, without any need to shut down the feed supply urgently. From this incident involving the termination of circulation in the slurry system of the heavy oil catalytic unit, as well as the subsequent forced shutdown due to the interruption of feed, we have drawn the following lessons: ①. In cases where circulation in the slurry system of a heavy oil catalytic unit is terminated, by reducing the processing volume and adjusting the operation of the distillation tower, it is possible to ensure that the quality of the products at the outlet meets requirements and to maintain low-load operation for a certain period of time. . ②In heavy oil catalytic units, due to the heavy nature of the feedstock and the large amount of slurry generated, once the slurry system stops circulating, it is possible, in a short period of time when the tendency for coking is not yet significant, to replenish light feedstock (wax oil) from the bottom of the distillation tower. At the same time, the catalyst powder located below the two lower trays in the reprocessing column can be washed with recycled oil. This allows the unit to continue operating without shutting down the feed supply; once the slurry system resumes normal circulation, an ample amount of oil can be added from the bottom of the distillation tower, along with the removal of large quantities of slurry, thereby restoring normal production. ③When the slurry system of the heavy oil catalytic unit is shut down and coking at the bottom of the fractionation tower becomes severe, it is necessary to cut off the feed.
In our facility, coking due to internal leakage in the backup slurry heat exchanger has occurred; diesel was used for cleaning, but the entire slurry system was not treated with diesel