Decanter Centrifuge is mainly used in refinery wastewater treatment process to further separate solid and liquid from “three muds”. There are controllable and uncontrollable factors affecting the separation effect during operation. After system debugging and some countermeasures in the trial operation stage, the centrifuge can achieve the ideal working effect.
Decanter centrifuge process flow
The “three muds” generated in the front treatment process directly enter the thickening tank and use their own residence time to precipitate inside the thickening tank. By switching between thickening tanks, the concentration of the sludge continues to increase. The final initially concentrated and precipitated sludge is pumped into the thickening tank through a single screw pump for further concentrated precipitation, and then transported to the centrifuge through the centrifuge feed pump for centrifugal mud and water separation.
Composition and working principle
1. Unit composition：
The horizontal screw centrifuge is mainly composed of a drum, a screw, a differential system, a liquid level baffle, a drive system and a control system.
2. Working Principle：
Decanter Centrifuge uses the density difference between solid and liquid phases to accelerate the sedimentation speed of solid phase particles under the action of centrifuge to achieve solid-liquid separation. The specific separation process is that the sludge and flocculant solution are input into the mixing chamber of the drum through the inlet pipe, where they are mixed and flocculated. Due to the high-speed rotation and friction resistance of the rotor, the sludge is accelerated inside the rotor and forms a cylindrical liquid ring layer. Under the action of centrifugal force, solid particles with larger specific gravity settle to the inner wall of the drum to form a mud layer. The relative speed difference between the spiral and the drum is used to push the solid phase to the conical end of the drum. After pushing out the liquid surface, the sludge is dehydrated and dried, and pushed to the slag discharge port for discharge. The supernatant is discharged from the large end of the drum to achieve solid-liquid separation.
Problems and countermeasures in operation and debugging
During the trial operation of the centrifuge, the basic settings for the initial operation of the centrifuge were made according to the empirical parameters provided by the manufacturer. However, in actual operation, the operating conditions of the centrifuge cannot be fully met. For each parameter, through actual on-site debugging and rectification, the operating settings of the centrifuge in this process were revised. During the debugging process, the controllable and uncontrollable factors of the centrifuge itself and the supporting equipment were analyzed and compared, and adjustment experience was explored.
1. Factors affecting the centrifuge itself
a. Uncontrollable factors
(1) Drum diameter:
Increasing the drum diameter can increase the processing capacity of the centrifuge, but as the diameter increases, the maximum speed of the drum will decrease due to the material strength, thereby reducing the centrifugal force. For already formed equipment, this factor cannot be adjusted.
(2)Drum cone angle:
Under the action of centrifugal force, solid particles reach the inner wall of the drum and are transported forward by the screw propeller. At the same time, they also have a tendency to flow backward. The speed of the backward flow is related to the drum cone angle. For equipment that has already been formed, this factor cannot be adjusted.
(3)The ratio of drum diameter to length:
When the drum speed is constant, the larger the diameter-to-length ratio, the greater the processing capacity, the longer the relative sedimentation time of solid materials inside the centrifuge drum, the lower the moisture content of the solid materials being separated and processed, and the better the separation effect; conversely, the worse the separation effect. For the already formed equipment, this factor cannot be adjusted.
(4)Machine manufacturing materials:
The strength of the manufacturing material determines the maximum speed of the centrifuge, which directly affects the volume flow and corrosion level of the centrifuge. For already formed equipment, this factor cannot be adjusted.
b. Controllable factors
(1)Drum speed:
The drum speed affects the separation effect of the centrifuge and the processing volume per unit time. The higher the speed, the greater the centrifugal force, the faster the sedimentation rate of solid particles, the higher the separation factor of the centrifuge, and the better the separation effect. However, the speed is not proportional to the separation effect. When the critical speed is reached, the separation effect is not significantly improved by increasing the speed. On the contrary, too high a speed will destroy the sludge flocs, reduce the dehydration effect, increase the power consumption almost proportionally, greatly increase the operating costs, and increase the vibration of the equipment.
Countermeasures:
According to the actual desludging situation on site, the main motor frequency is adjusted through the on-site touch screen, thereby changing the rotation frequency of the motor to change the speed of the drum, so that the centrifuge reaches the operating state.
(2)Differential speed:
Differential speed refers to the difference between the drum speed and the spiral speed, that is, the relative speed between the two. The differential speed directly affects the slag discharge capacity, mud cake thickness and filtrate quality, and is one of the parameters that need to be adjusted according to the operating conditions during the operation of the horizontal screw centrifuge. When the amount of mud inlet is constant, increasing the differential speed is conducive to improving the slag discharge capacity, but the sediment dehydration time will be shortened, and the moisture content of the mud cake after dehydration will be large; reducing the differential speed will increase the sediment thickness, increase the sediment dehydration time, and reduce the moisture content of the mud cake after dehydration, but it will increase the load of the spiral pusher and reduce the processing capacity of the centrifuge.
Countermeasures:
During the operation, when the frequency of the main motor is constant, by changing the value of the differential speed on the touch screen, thereby changing the rotation speed of the auxiliary motor, the differential speed can be adjusted to control the separation of the treated sludge and supernatant.
(3)Liquid layer thickness:
The adjustment of the liquid layer thickness of the centrifuge is very important. It directly affects the separation effect and the vibration degree of the centrifuge, and also determines the residence time of the supernatant in the centrifuge. When the feed volume is constant, the greater the liquid layer thickness, the longer the liquid stays in the centrifuge, and the better the separation effect is achieved. However, if the liquid layer is too thick, part of the supernatant will overflow from the slag discharge port, affecting the dryness of the mud cake.
Countermeasures: The thickness of the supernatant can be controlled by adjusting the height of the supernatant baffle to make the centrifuge reach a good working state. However, during the operation, the adjustment of the adjustment plate needs to be opened when the equipment is shut down to adjust it. For this reason, after a period of trial operation, the above-mentioned variable factors are continuously adjusted to determine the adjustment plate.
(4)Knife gate valve opening and closing:
The knife gate valve automatically switches when the centrifuge reaches the set torque during operation, that is, when the torque is greater than the set torque during operation, the knife gate valve automatically opens to ensure that the dry mud automatically falls to the screw conveyor at the bottom of the centrifuge and is transported to the sludge field. When the operating torque of the centrifuge is less than the set torque, the knife gate valve automatically closes to ensure that the sludge with high water content is discharged together with the supernatant. However, in the actual operation process, due to the large differences in the changes of sludge and liquid medicine during the flocculation process, it cannot be completely guaranteed that dry sludge can only be produced when the centrifuge reaches a certain torque.
Countermeasures: Release the interlock between the torque value and the knife gate valve. During operation, the operator observes the mud discharge of the centrifuge through the centrifuge mud production observation hole on site, and manually operates the knife gate valve in the control cabinet.
2. External factors affecting centrifuge
(1)Dosing device operation adjustment:
The dosing device and the centrifuge are interlocked, and the continuity of the dosing device operation directly affects the continuous operation of the centrifuge. The dosing device is mainly composed of a dry powder storage tank, a water ejector, a dispensing tank, a pressurized water system, and a control system. During operation, the water ejector is blocked, the dispensing water pressure is unstable, and the medicine precipitation at the bottom of the dispensing tank blocks the medicine outlet pipeline, resulting in the dispensing system not being able to operate normally and the interlocking centrifuge stopping operation.
Countermeasures:
1. Reduce the amount of dry powder stored in the dry powder storage tank and increase the frequency of adding dry powder to the storage tank to avoid the water vapor combining with the dry powder in the storage tank and then hardening during the process of transporting the dry powder to the water ejector, thereby blocking the normal operation of the water ejector; 2. Add a pipeline booster pump to the fresh water pipeline of the drug preparation to keep the pipeline pressure at around 0.5MPa, so that the water and dry powder are fully mixed in the water ejector to better achieve the concentration of the prepared drug; 3. Regularly clean the deposited liquid at the bottom of the liquid storage tank to ensure the cleanliness of the bottom of the tank and avoid the deposited debris blocking the drug outlet pipeline.
(2)Sampling and analysis of centrifuge sludge:
The properties of the front sludge directly affect the sedimentation effect of the sludge in the thickening tank. The sludge has a high moisture content. Under the same sedimentation time, after entering the centrifuge, when the amount of sludge is constant, the moisture content of the sludge after centrifuge treatment will increase accordingly, and the solid content of the supernatant will also increase accordingly, showing a turbid state.
Countermeasures:
Sampling and analysis of the sedimented sludge inside the thickening tank should be carried out regularly and at fixed points. According to the moisture content of the sludge, the sludge sedimentation time in the thickening tank should be appropriately adjusted to ensure the process indicators of the centrifuge sludge.
Summarize
After the centrifuge has gone through the operation and debugging stage, we have fully mastered the adjustment methods for the control factors that affect the operation of the centrifuge. Under the same conditions, we can adjust the control factors in a timely and accurate manner to ensure that the centrifuge can run continuously under certain conditions and achieve the best working state.