Through the analysis of the application status of the slurry filter in the catalytic cracking unit, it is considered that the feed temperature of the slurry should not be too high, otherwise the backwashing oil in the regeneration process will be passively used back for refining.
The design requirements of the slurry filter should be improved to lighten the oil used for backwashing.
The two currently used filter slag discharge and refining schemes are listed, and it is proposed that the filter slag should be directly sent to the riser, and the nozzle should be set up separately, and the feeding position should be higher than the raw oil nozzle.
The process scheme of the combined application of filtration method and sedimentation method for secondary separation of catalyst in filter slag discharge was preliminarily discussed.
Catalytic oil slurry is widely used in petroleum processing and petrochemical industry, but the oil slurry contains about 3000-7000ppm of catalyst particles, which restricts it from becoming a high value-added product. If the operation fluctuates, the catalyst content can even rise to 12,000 to 15,000 ppm. Backwash slurry filter has become the choice of many refineries, and its typical process is shown in Figure 1. The FCC oil slurry first enters the filter A, and the catalyst particle content in the filtrate is reduced to 50-100ppm. When the filter pressure drop or filtration time reaches a predetermined value, the slurry feed is switched to filter B, and filter A enters the regeneration process, and the filter residue is washed and discharged to the receiving tank by backwashing medium, and the whole process is controlled by the on-site PLC.
The key technology of slurry filters lies in filter elements and backwashing. Foreign companies such as MOTT and PALL, as well as domestic Bluslot Filter, have wedge wire filter elements of their own technology and are equipped with corresponding regeneration processes:
- MOTT company directly uses high-pressure pulse gas or liquid for explosive backwashing, and then uses filling oil to keep warm for backup.
- PALL company first soaks and softens the filter cake with replacement oil, then uses gas to backwash, and then uses filling oil to keep warm for later use.
- BLUSLOT first uses gas-assisted backwashing, then uses backwashing oil to soak and soften, and then gas-assisted backwashing again to achieve the purpose of cleaning.
CATALYTIC CRACKING UNIT AND SLURRY FILTER
The FCC unit is equipped with an oil slurry filter, mainly to fine-tune the process of the reaction and fractionation part:
Adjust the heat exchange process of the oil slurry system so that the oil slurry enters the filter at a suitable temperature.
Provide the backwash gas, replacement oil, filling oil, flushing oil, etc. required by the filter, and at the same time receive the filter slag.
Although the devices are different and the raw materials are different, the viscosity changes with temperature in the same trend. When the temperature is lower than 170℃, the viscosity of the oil slurry increases rapidly with the decrease of the temperature, the filter element is easy to be blocked, and the regeneration is frequent.
When the slurry temperature is higher than 345°C, coking may occur.
Auxiliary media that FCC units can provide for slurry filters include:
- Before desulfurization, dry gas, light diesel, heavy diesel, fractional distillation, oil refining, etc. (some FCC units do not have heavy diesel side lines).
- The dry gas pressure ranges from 0.9 to 1.0 MPa (G), which is the best choice for backwashing gas.
Table 1 lists the properties of light diesel, fractionated first-middle, heavy diesel, and back-refining oil by taking devices 1 and 2 as examples.
The smooth operation of the filter depends on maintaining a constant temperature in each link of filtration, regeneration, and backup. The feed temperature of the oil slurry basically determines which medium is used for the filter backwash regeneration.
|Parameter||Device 1||Device 2|
|Light diesel oil||Fractionation||Recycle oil refining||Light diesel oil||Fractionation||Heavy diesel||Recycle oil refining|
STATUS AND OPTIMIZATION
Application of partial backwash slurry filter. The feed temperature of the oil slurry is generally higher than 300℃. Low viscosity brings many benefits to filtration, but high-temperature feed also brings some disadvantages:
- The operating temperature is higher than the self-ignition point of the oil slurry. If the filter leaks, the danger is high.
- The filter has high requirements for thermal insulation and heat tracing. The filter designed according to the high temperature and low viscosity cannot withstand the influence of temperature drop.
- When the feed temperature of oil slurry exceeds 300°C, replacement oil, filler oil, etc. can only passively choose high temperature for oil refining.
MOTT’s recommended backwash media include N2, filtrate, LCO, HCO.
The backwash gases recommended by PALL are dry gas and N2, and the recommended replacement oils are LCO and HCO. LCO and HCO are similar to light diesel and heavy diesel in domestic FCC units.
It can be seen from Table 1 that light diesel oil is the first choice for filter backwashing, its temperature is 210 ~ 216 ℃, and the viscosity of oil slurry at this temperature is 1 ~ 2 cP.
The secondary selection is heavy diesel or fractional distillation one, heavy diesel is close to fractional distillation one, its temperature is about 277 ℃, and the viscosity of oil slurry at this temperature is 0.7 ~ 0.8 cP.
The final choice is oil refining, the temperature is above 330 °C, and the viscosity of the oil slurry at this temperature is 0.5 to 0.6 cP.
PALL company uses multi-layer metal wire sintered filter element, and the viscosity requirement is 0.8 ~ 4 cP, MOTT company uses metal powder sintered filter element, and the oil slurry temperature is required to be higher than 232 ℃.
After the slurry temperature rises to 200°C, the tendency of viscosity to decrease with the increase of temperature is not obvious. In order to pursue low viscosity, it is not advisable to increase the feed temperature to above 300°C.
Flux rate is defined as the flow rate of oil slurry through a unit filter area and is a key parameter of the filter. Advantages of using low flux rates:
- The regeneration frequency is reduced and the effective filtration time is prolonged.
- The linear velocity of the oil slurry entering the filter element is reduced, the formed filter cake is soft, and the backwash regeneration is more thorough.
- The amount of backwashing medium used in the regeneration process is reduced.
- Slurry filter energy consumption is reduced.
- The low flux rate brings many benefits, but also brings a substantial increase in investment, and the slurry filter design needs to be considered comprehensively.
FILTER SLAG TREATMENT
The filter slag is returned to the FCC unit for processing. There are two common methods:
- The slag is sent to the raw material oil tank by pressure.
- Set up a backwashing oil refining pump, and send the slag directly to the riser for refining.
At present, most domestic FCC units adopt the first method. The disadvantage is that the catalyst in the slag discharge pollutes the feedstock oil system. The feedstock oil pump, heat exchanger, pipeline, and feed nozzle are all subject to the abrasion of catalyst fine powder, especially the abrasion of the feedstock oil pump. It is more serious, which is unfavorable for the long-term operation of the device.
The second type is relatively good, but the periodic slag discharge of the filter will inevitably cause periodic fluctuations of reaction temperature and pressure, which will affect the operation of the riser.
A replacement oil or oil slurry can be introduced into the backwash receiving tank to dilute the catalyst concentration and set up a liquid control to realize continuous feeding of slag discharge. There are disadvantages in slag discharge when the feed position of the riser is close to the feed oil nozzle. In addition to catalyst fine powder, the slag discharge also contains polycyclic aromatic hydrocarbons, colloids, and asphaltenes. Asphaltene is difficult to gasify.
If the slag is preferentially contacted with the highly active catalyst, part of the catalyst activity will be affected, which is not good for the riser reaction.
Therefore, a separate nozzle should be set for slagging and refining, and the feeding position should be higher than the raw oil nozzle. The specific position should consider the type and layout of the FCC unit riser.
DISCUSSION ON SECONDARY SEPARATION OF SLAG DISCHARGE CATALYST
At present, the above two methods of the slag discharge of domestic oil slurry filters are adopted. Whether the slag discharge is sent to the raw material oil tank or directly sent to the riser, the catalyst in the slag discharge enters the settler cyclone.
The catalyst in the slurry was originally lost to the bottom of the fractionation column due to the insufficient ability of the cyclone to capture the catalyst particles, although some of the recovered catalysts entered the regeneration system.
However, there are still some catalysts carried by the oil and gas to the oil slurry again and enter the oil slurry filter to form a circulation process, which increases the load of the filter.
In order to break the cycle of catalyst fine powder, the catalyst in the filter slag should be separated for a second time, and light diesel oil should be selected as the oil for backwashing, which can greatly reduce the difficulty of secondary separation of the catalyst.
In addition to filtration, the separation of catalyst fine powder also includes electrostatic separation, centrifugation, and sedimentation.
The electrostatic separation method has a large investment and complicated equipment. After the FCC unit adopts an oil slurry filter, it is impossible to supplement it with an electrostatic separation method; the separation of oil slurry catalysts using a micro-cyclone has not yet been promoted in the industry.
The sedimentation method has simple equipment, low operating cost, and easy operation, while the separation of the catalyst in the slag discharge is not difficult, and the sedimentation method is an optional solution.
The combined application process of slurry filter and sedimentation method is shown in Figure 3. The slag is discharged into the sedimentation tank for sedimentation. The oil skimming in the tank is pumped to the fractionation tower or reaction system, and the catalyst is deposited at the bottom and periodically removed.
LIMITATIONS OF SLURRY FILTERS
The raw materials processed by the FCC unit include atmospheric residual oil, vacuum wax oil, vacuum residual oil, coking wax oil, hydrogenated residual oil, deasphalted oil, and their mixed raw materials.
There are very few FCC units using wax oil as raw material. Compared with foreign catalytic oil slurry, the quality of domestic oil slurry is generally poor, with a colloidal content of about 10% to 20% and an asphaltene content of about 3% to 4.5%.
Asphaltenes are an important factor affecting the operating life of slurry filters, so the actual operating conditions of some domestic backwash slurry filters are not ideal.
In addition, the properties of crude oil processed by domestic refineries fluctuate greatly periodically, which affects the properties of the feedstock oil of the FCC unit, and the properties of the catalytic oil slurry also change, which puts forward high requirements for the adaptability of the oil slurry filter.
In order to achieve continuous and stable operation of the slurry filter, in addition to optimizing the design of the filter body and the design of the matching process of the filter, attention should also be paid to the operation details, especially in the filter start-up process and the heat preservation operation after backwashing and regeneration.
The failure of the oil slurry filter is often caused by the misoperation or failure to change the operating parameters in time to adapt to the changes in the properties of the oil slurry.
- The feed temperature of the backwashing slurry filter is too high, and the regeneration process can only passively choose high temperature to return to the oil.
- Starting from the filter design, low flux rate is adopted to reduce the inlet temperature of oil slurry, and light oil can be selected as the regeneration medium to create conditions for production operation and subsequent optimization design.
- For the treatment plan of filter slag discharge, it is better to send the slag discharge directly to the riser for refining, and set up the slag discharge feed nozzle separately, and the feeding position should be higher than the raw material nozzle. The specific position considers the type and arrangement of the FCC unit riser pipe. .
- The process of secondary separation of catalyst in slag discharge by combined application of filtration method and sedimentation method is preliminarily discussed.The limitation of the application of the backwashing slurry filter is due to the poor nature of the domestic catalytic slurry. It needs to be optimized in all aspects of design conditions and operation details to realize the continuous and stable operation of the filter.