“I’m saving energy; where are you? “Petrochemical 302: Measures to Improve the Yield of Light Oils through Delayed Coking 001”

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This post was last edited by Yitailong'er on 2016-4-23 at 16:39. Measures to improve liquid yield in delayed coking units. As the efficiency of refining units continues to decline, maximizing the liquid yield from these units has become a key factor for refineries aiming to increase their profitability. The increasing trend toward heavier and lower-quality feedstocks in coking units is the direct cause of a decrease in the liquid yield from these units, thereby affecting their economic viability. The high carbon residue content of the feedstocks leads to an increased amount of coke produced, which is the main reason for the decline in liquid yield. We conducted a comparative analysis of our delayed coking unit by comparing the production data from October 2014 with the recent actual production figures, in order to improve the liquid yield of the unit in future operations.

Project dates: 9.19–9.28, 10.8–10.11, 10.15–10.23; November 2014 (30% Dar oil was added after the 15th); March 2015.

Residue carbon content (%): 24.42, 22.89, 23.34, 24.97
Furnace exit temperature (°C): 494±2°C, 494±2°C, 498±2°C, 500±2°C, 500±2°C
Cycle ratio: 0.59, 0.62, 0.61, 0.59, 0.31
Coke volatiles (m/m %): 9.48, 10.56, 9.13, 10.37, 8.69
Distillation temperature of 95% diesel (%): Average values of 371.4, 376.2; both values are above 378; 374.7, 373.7
Coke tower pressure (MPa): 0.17, 0.17, 0.17, 0.155, 0.155
Residuum processing volume (t): 254, 758, 1872, 3123, 18740, 10
Percentage of processing volume relative to design capacity: 84.92%, 68.23%, 84.92%, 80.99%, 88.10%
Liquefied gas yield: 2.38%, 3.32%, 2.65%, 2.46%, 2.16%
Gasoline yield: 21.48%, 21.99%, 22.49%, 19.03%, 14.54%
Diesel yield: 28.33%, 30.43%, 30.37%, 31.97%, 29.62%
Total gasoline and diesel yield: 50.00%, 52.89%, 53.63%, 51.00%, 44.16%
Wax yield: 5.43%, 3.00%, 4.77%, 7.37%, 14.24%
Liquid yield: 57.62%, 58.74%, 60.28%, 60.83%, 60.57%
Coke yield: 33.37%, 33.32%, 32.56%, 33.49%, 34.34%

1. It is advisable to increase the furnace exit temperature, as well as to implement variable furnace exit temperature control. As can be seen from the table, the liquid yield during the period from 10.15 to 10.23 was 1.54% higher than that during the period from 10.8 to 10.11, after the furnace exit temperature was increased. Therefore, increasing the furnace outlet temperature can effectively increase the liquid yield. Variable-temperature operation involves lowering the temperature at the furnace outlet during the early stage of coke formation in the coke tower, in order to reduce coke hardness and prevent the formation of pellet coke; this temperature is then increased appropriately during the later stages of coke formation. An increase in the furnace outlet temperature leads to harder coke, making it difficult to remove cold coke, increasing the difficulty of coke removal, and exacerbating the tendency for coking in the furnace tubes. For example, the spontaneous combustion of coke in the early hours of November 6, 2014; abnormal swaying of the drill rods and abnormally high pressure in the high-pressure water pump during the coking process on April 6, 2015; and the surface temperature of tube TE1135 reaching 602°C on April 9, 2015 (the amount of steam injected into the tubes in my facility was higher than the designed value), whereas the designed maximum surface temperature for those tubes is 650°C. The temperature rise for TE1165 and TE1135 is significant: the surface temperature of the furnace tubes on March 1st and April 10th showed a temperature difference of 542°C for TE1165 and 578°C, resulting in a difference of 35°C, with an average daily increase of 0.85°C. For TE1135, the temperature difference was 533°C and 602°C, giving an average daily increase of 1.72°C. If operations continue in this manner, coking production can last another 27 days before reaching the upper limit for equipment operation. 2. It is advisable to reduce the circulation ratio slightly. As can be seen from the table, the residual carbon of the feedstock in March 2015 and from September 19 to 9 Month 28 is not very different; however, the liquid yield increased significantly after the recycle ratio was reduced. However, reducing the circulation ratio will deteriorate the properties of the feed to the heating furnace; the current raw materials are highly likely to cause oscillations in the coke tower and the production of pellet coke when operated at a low circulation ratio. The coke tower experiences shaking, and the production of pelletized coke presents various safety hazards such as cracks in the pipeline flange welds, leaks of high-temperature oil and gas, poor drainage, and issues related to the removal of coke. 3. Appropriate reduction of the pressure in the coke tower: As can be seen from the table above, reducing the pressure in the coke tower can also increase the liquid yield of the plant. However, if the pressure is too low, it will lead to an increase in the gas velocity within the tower, an elevation of the foam layer inside it, and easier transport of coke powder to the distillation tower, which can result in the formation of pelletized coke. At present, due to the large amount of coking material generated as raw material for coking, and with the processing capacity at 88% of the designed level, the height available inside the coke tower is only 11 meters (the safe height should be above 12 meters). If further pressure reduction is carried out, it will inevitably lead to increased carryover of coke dust. As evidenced by the cleaning of the bottom filter in the distillation tower on March 2, there was significant carryover of coke dust. On April 7, the bottom pump in two of the distillation columns of the coking unit experienced vacuum conditions repeatedly, and it was later found that the filters at the bottom of those distillation columns were blocked again. The interval between the first and second online cleanings was only about 35 days, and approximately 0.4 M3 of coking powder was removed in each cleaning. This means that nearly 10 kilograms of coking powder per day ended up at the bottom of the distillation column (excluding the amount carried away by various side streams), which undoubtedly poses a threat to the normal operation of the plant. To this end, while increasing the liquid yield, the workshop has developed a 22-hour coking plan; after it was approved by management, implementation began on the 11th. By adopting this measure, both the processing capacity of the plant has been increased and the amount of coke dust carried away has been reduced. However, the margin for adjusting the operating pressure of the current distillation column is already very small. 4. Optimization of the distillation temperatures at various side streams of the fractionation tower; the 95% distillation temperature of diesel was appropriately set at a higher level. Calibration conducted in September and October showed that an increase in this 95% distillation temperature led to a significant increase in the liquid yield of the plant. Based on the test data from March 2015, it can be seen that there is still room for improvement in the 95% distillation temperature of the diesel in my unit; the workshop will focus on addressing this issue in the next steps. 5. Reduce the volatility of coke. The volatiles in coke are mainly oils; reducing these volatiles increases the liquid yield of the plant. While ensuring the furnace outlet temperature is controlled as required, and in accordance with the plant’s overall steam balance requirements, the amount of feed steam should be increased as much as possible in order to vaporize the oils present in the coke and recover them through venting. The properties of cold coke water must also be carefully controlled; since the recycling system for cold coke water inevitably contains some oils, the frequency of overflow and oil removal from the workshop’s water tanks has been reduced from twice a month to once a week, thereby preventing any oils from entering the coke. 6. Recovery and utilization of the plant’s quench oil and sludge oil. Quench oil comes in two types: sludge oil produced by the plant itself and wax oil produced by the plant itself. After the quench oil is injected into the coke tower, pyrolysis reactions occur under high temperatures, resulting in the formation of some low-molecular-weight gas components; therefore, the temperature of the quench oil should be reduced as much as possible, and the amount of wax oil used should be minimized to avoid liquid losses. The workshop formulates plans based on these two scenarios regarding heavy paraffin oil: during normal discharge of heavy paraffin oil, a water cooler for heavy paraffin oil is put into use, while during its reprocessing, dirty oil is used as quenching oil to replace the water cooling system. The sources of coking sludge mainly include three aspects: sludge generated by the preheating of new towers ； The dirty oil generated by cooling, separation, and overflow of the old tower’s exhaust gas ； During cold coking, the dirty oil carried by the cold coking water during the drainage process is collected in the dirty oil tank after overflowing ； For these three types of contaminated oil, it is essential to carry out regular overflow collection, and to maximize the reprocessing of such oil in order to ensure economic benefits. 7. Increase the duration of the minor steam injection appropriately. In the next step, the workshop will extend this duration from 1 hour to 1.5 hours, without affecting the operation of the coke tower. By doing so, it is possible to transfer as many of the lighter components and heat present in the coke tower as possible to the distillation tower, thereby reducing losses of light oil and increasing the yield of liquids. 8. Can adding a residue fluidizer in collaboration with the relevant departments be considered? Fluidizers are generally added at the inlet of the heating furnace; according to available information, fluidizers developed specifically for a certain type of feedstock can indeed improve the liquid yield of the plant. If we plan to process 480# raw material on a long-term basis in the future, it is recommended to conduct an evaluation and analysis of the raw material, and develop appropriate fluidizers to improve the yield of liquid.