Application of Slurry Filtration Technology in Heavy Oil Catalytic Cracking Unit

Catalytic cracking is one of the most important heavy oil lightning processes. In general, the amount of oil slurry thrown outside the catalytic cracking unit of an oil refinery accounts for about 5% of the raw material volume, and the annual catalytic cracking oil slurry produced in the country exceeds 5 Mt.

Catalytic oil slurry generally contains more than 2,000 μg/g of catalyst particles, whether it is used as fuel oil to leave the factory or as a raw material for cooking, or as a raw material for the production of carbon black and needle coke, or for aromatic extraction to produce rubber additives, etc. Further processing and utilization or the development of high value-added products all need to reduce the solid content in the oil slurry. 

Due to unfavorable factors such as high density, high viscosity, high freezing point, and solid catalyst particles contained in the catalytic oil slurry, many petrochemical companies have unsatisfactory results in purifying and separating catalytic oil slurry. 

For a long time, domestic and foreign petrochemical enterprises and scientific research institutions have paid great attention to the technology of purifying, separating, and catalyzing the slurry. 

Introduction of Heavy Oil Catalytic Cracking Unit and Slurry Filtration System 

Process Principle Of Slurry Filtration System 

The core element of the slurry filtration system is the wedge wire filter element

When the hot oil slurry passes through the filter element, the oil slurry liquid can pass through the filter element, and the clean oil slurry (filtrate) enters the downstream device, while the solid catalyst particles are trapped on the inner surface of the filter element. The filtered solid catalyst filter cake is backwashed with dry gas and discharged to the filter cake receiving tank. 
During the slag discharge process, the rapid pressure release and the reverse flow of the filtered liquid can quickly peel off the solids on the inner surface of the filter element, and make all the media inside the filter flow out of the filter through the slag discharge valve. 

After commissioning, the entire oil slurry filtration process is controlled by a programmable logic controller (PLC) program for preheating, pre-filming and filtering, backwashing, slagging, and parking. The control system controlled by the PLC program can realize automatic and manual control. There are two modes of operation, but since the oil slurry filtration system is controlled by 32 program-controlled valves, it generally only operates under automatic control and does not require manual intervention.

Process Flow Of Slurry Filtration System 

The oil slurry filtration system consists of 1 pre-filter and 2 fine filters, 1 filter cake receiving tank, 1 dry gas regulator tank, and supporting instrument valves. 

The external oil slurry of the catalytic device is cooled and the temperature is lowered to 280 ~ 300 ℃. After passing through the pre-filter, it is divided into two paths and enters the fine filter A and the fine filter B respectively. After being filtered, the clarified oil slurry is sent out of the device.  

Backwash dry gas enters from the top of the pre-filter, fine filter A, and fine filter B, and the filter cake is discharged from the bottom of the filter to the filter cake receiving tank. 

The filter cake in the filter cake receiving tank is sent to the catalytic cracking feedstock tank with dry air pressure along with the catalytic oil refining for recycling. The pre-filter, fine filter A, fine filter B, and the gas phase of the cake receiving tank is vented to the flare line through the vent valve. 
The re-refined oil used for replacement enters the slurry filtration system through backwashing, and enters the pre-filter, the fine filter A, the fine filter B, and the filter cake receiving tank respectively.  

The diesel oil process is the same as the oil refining process. When the device is shut down for a long time, the diesel oil replaces the oil refining to replace the oil slurry in the filtration system. The re-refined oil that replaces the oil slurry in the filtration system enters the fractionation tower through the re-refining return tower line. 

Another function of oil refining and diesel oil is to preheat the oil slurry filtration system to about 200 ℃ when restarting after the shutdown. 

Industrial Applications Of Slurry Filter

The slurry filtration system and the catalytic cracking unit were constructed synchronously and completed the purging, pressure test, and oil transportation processes. Each control valve, flow meter, level gauge, pressure gauge, and related control procedures need to be strictly tested. The catalytic device was put into use after it started running smoothly. The slurry filter has the following characteristics:  

  1. The oil slurry has a high viscosity at low temperature. In order to ensure that the oil slurry has a low flow resistance and is effectively separated from the catalyst particles in it, it must be filtered at a higher temperature. 
  2. High-temperature oil slurry is easy to ignite spontaneously, and the tightness of the filter is relatively high. 
  3. Program-controlled valves and operating procedures must be debugged. 

From the perspective of operation, the operation of the oil slurry filtration system is very convenient. In the automatic mode, the system can automatically complete all operations such as filling, pre-filming, filtering, backwashing, and slag removal. The PLC operating system is operated independently in the central control room, and there is a DCS screen for monitoring. 
During the shutdown process of the slurry filtration system, the system does not need any manual switching. The operator can directly perform the PLC “backwash shutdown” program, and the system will automatically complete the operation procedures such as replacement, backwashing, and slag removal. The slurry filtration system is an independent system, and its startup and shutdown will not affect the normal operation of the heavy oil catalytic cracking unit. 
The designed processing capacity of the oil slurry filtration system is 19.4 t/h. The processing capacity is relatively large, with an area of only about 10 m2. The filtered oil slurry is cooled to 70 ~ 90 ℃ and then sent out of the device. Since the pressure difference of the oil slurry filter is only about 0.002 MPa, the pressure drop is small and the operating pressure is high, so there is no need to increase the pressure of the pump for delivery, which reduces the investment cost. 
The oil slurry filtration system will cause a chain stop when the following conditions occur: The response time of the sub-programmable valve exceeds 15 s, the pressure difference of any one of the 3 filters exceeds 0. 35 MPa and the feed flow reaches an overload of 28 t/h. The inlet valves of fine filter A and fine filter B are closed at the same time. When the machine is stopped, the oil slurry will bypass the filter system. At this time, the bypass valve of the oil slurry filter is opened, and the valve for the oil slurry in and out of the filter is closed. Under fault conditions, the automatic interlocking shutdown of the slurry filter system has no effect on the operation of the catalytic cracking unit. 

Conclusion 

  • The oil slurry filtration system uses wedge wire filter element material as the filter component, which can realize continuous operation and automatically complete the processes of filling, pre-membrane, filtration, backwashing, and slag discharge, with a processing capacity of 19.40 t/h.The industrial operation and data filtering effect of the oil slurry filtration system, the ash content of the external oil slurry dropped from an average of 2070 μg/g to 120 μg/g, and the ash removal rate of the oil slurry reached 94.2%. 
  • After the oil slurry filtration system is put into use, there is no need to increase the delivery pump. Only the circulating oil slurry pump can meet the operation and delivery needs of the oil slurry filter, and the power and material consumption are very small. 
  • The slurry filter system can perform operations such as purging, pressure test, and oil transportation simultaneously with the catalytic cracking unit. At the same time, as an independent system, the startup and shutdown of the heavy oil catalytic cracking unit will not have any impact on the normal operation of the heavy oil catalytic cracking unit.