Abstract: The clogging of the furnace tube in the radiant section is one of the common faults in the cracking furnace of the ethylene plant. The clogging of the tube in the radiant section of the cracking furnace has occurred many times during the operation and heating process, which has brought serious problems to the safe operation of the cracking furnace and even the stable operation of the device. serious threat. This paper describes the main phenomenon and effective treatment measures of the tube blockage in the radiant section of the cracking furnace of an ethylene plant, and expounds the main reasons for the blockage of the tube in the radiant section, which provides a basis for avoiding similar phenomena in similar plants and their treatment.
The furnace tube at the end of the collecting tube in the spanning section is blocked
After the reformation of F0101-F0601 cracking furnaces, the high-temperature hydrocarbon/steam mixture on both sides leaves the convection section, collects respectively into a cross section manifold and then enters the radiant section furnace tube. Each radiant section furnace tube inlet is equipped with a critical flow venturi tube (also known as a Venturi tube) to ensure good flow distribution in normal operation. Each cracking furnace has 112 inlets (56 groups on each side of the furnace), corresponding to 112 groups of GK6 radiant section furnace tubes. After the transformation, the design operation period is 60 days, but when the actual operation is about 20 days, the outlet temperature of the waste heat boiler corresponding to the furnace tube at the end of the manifold at the end of the spanning section rises rapidly many times. On-site inspection found that the furnace at the radiant section at the end of the manifold The riser tube of the tube became red and bright (the corresponding down tube was still black), and there was a sign of blockage. Although measures such as greatly reducing the outlet temperature of the group of furnaces were taken, the furnace tube would still be blocked soon.
Cause Analysis:
After comparing the actual parameters with the design parameters, it is found that the actual spanning pressure is far lower than the design value. The reason was determined to be uneven distribution of materials, slow flow of materials in individual furnace tubes and TLE, excessive residence time, excessive cracking, and severe coking that caused blockage.
The cracking furnaces F0101-F0601 were replaced with Venturi tubes during capacity expansion and transformation, and the design spanning pressure was 280 kPa(g), which was never reached during actual operation. When cracking naphtha, the pressure at the manifold of the spanning section (hereinafter referred to as the spanning pressure) is generally around 220 kPa(g). When cracking heavy raw materials (hydrogenated tail oil) and (chemical light oil), the transverse pressure can generally only reach about 190 kPa(g), which is far lower than the design value. The pressure downstream of the Venturi is about 140 kPa(g) at the initial stage of oil injection, and after a period of operation, the pressure downstream of the Venturi rises to about 170 kPa(g), which is one of the coke cleaning indicators of the cracking furnace. The absolute pressure ratio of the Venturi outlet and inlet is close. Therefore, it is suspected that the hole diameter of the Venturi is too large after the transformation, and the pressure at the manifold of the spanning section is low, resulting in uneven material distribution at the Venturi. Take cracking furnace F0101 as an example. The end corresponding to the collecting tube of the crossing section on the A side is the TIA0111 group of furnace tubes, and the end corresponding to the crossing section of the B side is the TIA0118 group of furnace tubes. These two groups of furnace tubes are most likely to have less circulating materials and prolonged residence time. Excessive pyrolysis leads to severe coking and blockage. This situation mostly occurs in the middle stage of cracking furnace operation.
The feed is not clean and may contain a certain amount of impurities. Since the raw material has a climbing height of about 4m from the high-temperature convection section to the cross-section manifold, these impurities may accumulate in the horizontal pipe of the convection section. When the flow changes, is brought into the span manifold. However, the points of all downcomer inlets of the cracking furnace are not at the lowest part of the horizontally arranged spanning section manifold, but on both sides of the spanning section manifold. Therefore, these impurities are likely to be brought to the ends of the header tubes on both sides of the cracking furnace when the flow changes, and enter the corresponding TIA0111 group furnace tubes and TIA0118 group furnace tubes at the ends. As a result, the material flow rate in these two groups of individual furnace tubes is low, the residence time is prolonged, the coking is serious and then blocked, or the venturi throat is directly blocked by large particles of impurities (as shown in the figure below). This situation often occurs when the airflow changes greatly during the heating process of the cracking furnace or during the oil feeding process.
According to the calculation of the design department, the diameter of the throat of the Venturi pipe is too large at present and should be 18mm. In 2008, the Venturi tubes of F0101-F0601 were all replaced with new Venturi tubes with a throat diameter of 18mm (as shown in the figure below). Since then, the above-mentioned plugging phenomenon has been greatly reduced.
During the maintenance, it was found that the impurities at the end of the manifold and the Venturi of the spanning section are as shown in the figure below:
| The impurity components of the manifolds across the section | source of impurities | Responses |
| iron oxide | Iron oxides in metal pipes | Every three years, take advantage of the opportunity of furnace shutdown to cut open the end of the collecting pipe in the spanning section and clean it once |
| coke | There is a certain coking phenomenon in the convection section | Every three years, take advantage of the opportunity of furnace shutdown to cut open the end of the collecting pipe in the spanning section and clean it once |
| Aluminum oxide | One of the components of the arsenic removal catalyst | Go to furnace front feed line to add filter |
After taking the above countermeasures, F0101-F0601 did not have the above-mentioned similar plugging phenomenon.
The furnace tubes in the middle of the 14 sets of furnace tubes corresponding to each waste heat boiler are blocked
112 sets of GK6 radiant section furnace tubes enter 8 waste heat boilers, that is, each waste heat boiler corresponds to 14 sets of radiant section furnace tubes. Due to the variable diameter at the connection between the 14 groups of radiant section furnace tubes and the waste heat boiler, the seventh and eighth groups are located in the middle of the 14 sets of radiant section furnace tubes. It happened in the past operation that the pyrolysis furnace was in standby for a period of time. After heating up, it was found that the seventh and eighth groups of furnace tubes located in the middle of the 14 groups of radiant section were blocked. However, the inspection before cooling down did not find that the furnace tubes were blocked. .
Cause Analysis:
During the inspection, it was found that there were a lot of broken coke in the “U” bend at the bottom of the connection between the downcomer and the upcomer. At first, it was suspected that the pyrolysis furnace was not completely charred, and the furnace tube or the coke layer in the waste heat boiler fell off, resulting in blockage. However, if the furnace tubes are clogged due to the shedding of the coke layer, the randomness is relatively large, and it is the seventh and eighth groups of furnace tubes that will not only be blocked in the middle. The burning of the cracking furnace is carried out completely according to the plan, and the time and air volume of each step are guaranteed. Therefore, the possibility of incomplete burning of the cracking furnace that causes the furnace tube and waste heat boiler to fall off and cause blockage is basically ruled out. Judging from the disassembled waste heat boiler. There are a lot of broken coke in the upper tube sheet and outlet tube of the waste heat boiler, and a small amount of steam and condensate carry broken coke from the outlet tube and enter the waste heat boiler countercurrently. Because the seventh and eighth groups of furnace tubes in the middle of the 14 groups of radiant section furnace tubes are just in line with the waste heat boiler inlet pipe. Therefore, these broken cokes finally enter the seventh and eighth sets of furnace tubes in the middle of the 14 sets of radiant section furnace tubes of the waste heat boiler, and accumulate in the bottom “U” bend to cause blockage (as shown in the figure below). This kind of situation often occurs during the shutdown period, but it can only be found after the cracking furnace heats up.
Responses:
Improve the operation, close the coke cleaning valve in time to prevent coke steam after the cooling of the cracking furnace is completed, prevent steam and condensate from flowing back, and prevent broken coke along the way from being brought into the furnace tube system. At the same time, during the heating process, one hour before the dilution steam is introduced, the charred air is first introduced to disturb the furnace, and prevent the broken coke from agglomerating into lumps after the water in the furnace tube is evaporated, completely blocking the furnace tube.
Because two cracking furnaces share a decoking tank. When one cracking furnace is out of service and the other cracking furnace that shares a decoking tank with it is in a scorched state, the decoking valve of the out of service cracking furnace should be closed in advance. Prevent a large amount of steam and coke from entering the furnace tube system of the out-of-service cracking furnace and causing blockage.
When removing the head of the waste heat boiler, large vibrations should be avoided to prevent the coke from the upper tube plate of the waste heat boiler from falling into the furnace tube. After dismantling the waste heat boiler, seal the furnace outlet tube in time to prevent debris from entering the furnace tube.
Other conditions of cracking furnace tube blockage
In order to increase the operating flexibility of the device and the raw material adaptability of the cracking furnace. Added ethane and propane lines in the A-side radiation chambers of F0801~F0901, and added the working condition of mixed cracking of naphtha, ethane and propane. However, because there are no designed process parameters, the mixed feed amount, furnace outlet temperature, dilution ratio, and sulfur injection amount all rely on exploration. As a result, when F0801-F0901 were mixed with naphtha, ethane, and propane, the furnace tubes in the radiant section and the corresponding linear waste heat boiler tube bundles were blocked many times, which was difficult to deal with. At present, F0801～F0901 are rarely mixed with cracked naphtha, ethane, and propane. If there is a special need to mix cracked naphtha, ethane, and propane, the injection amount of dimethyl disulfide should be appropriately increased.
The device has also experienced a large area of pipe blockage due to dilution steam (DS) carrying water and entraining a large amount of impurities into the furnace pipe system.
Responses:
It was found that the A-side furnace tube of the F0801 cracking furnace DS with serious water was blocked in a large area. During the cutting process, it was found that a large amount of material blocked in the tube was sodium salt. It can be dissolved after soaking in water and recovered after treatment. But after the DS was passed again, the furnace tube was blocked again. It was analyzed that a large amount of sodium salt remained in the pipeline of the convection section, which caused the furnace tube to be blocked again. A temporary pipeline is prepared by pouring in the feed line, the boiler feed water is introduced into the furnace tube system, discharged at the discharge port of the decoking tank, and air disturbance is introduced at the pressure gauge downstream of the Venturi. The entire furnace tube system was fully and thoroughly washed with water to remove a large amount of deposited sodium salts. After the discharge port was sampled and analyzed for sodium ion concentration, it was restored, and the cracking furnace tube was not blocked again. This situation generally occurs at the cracking furnace at the end of the DS header.
Due to the serious deterioration of the quality of light diesel oil in the cracking furnace, the BMCI value rose from about 17 to about 21, and the average density rose from about 806kg/m3 to about 818kg/m3. This resulted in severe coking of the waste heat boiler and its outlet pipe, which almost caused all the furnace tubes in the radiant section corresponding to the waste heat boiler to be blocked.
Leakage at the inlet of the waste heat boiler caused blockage of the furnace tube in the radiant section. Due to the special location and angle of the leak point, the leaked high-pressure steam forms a fan to block the inlet of the waste heat boiler, resulting in poor circulation of cracked gas in the furnace tube of the corresponding radiant section, which is blocked by excessive cracking and coking.
Epilogue
(1) Monitor the surface temperature of the furnace tube in the radiant section of the cracking furnace every day, and if the deviation increases and is too high, it must be adjusted in time.
(2) Regularly check the combustion state of the burner to ensure that the combustion state of the burner is good and the distribution is even.
(3) Track the quality of raw materials every day, adjust the dilution ratio, furnace outlet temperature, etc. in time according to the quality of raw materials, and properly inject coking inhibitors into raw materials with low sulfur content.
(4) After installing the DS with water, it should be discharged at the end of the DS main pipe in front of the furnace in time to avoid bringing a large amount of Na salt accumulated in the tank for a long time into the cracking furnace tube and blocking the cracking furnace tube when the DS is filled with water.
(5) Operate carefully to avoid emergency shutdown accidents.
Prolonging the operation period of the cracking furnace is an eternal topic, and the blockage of the furnace tube in the radiant section is an important factor affecting the operation period of the cracking furnace. The clogging of the furnace tube in the radiant section of the cracking furnace of the ethylene plant is one of the common problems, and there are many factors causing the clogging.
