Comparison of Application of Multi-disc Screw Press Dehydrator and Horizontal Decanter Centrifuge in Sludge Treatment of Refinery

Due to the inferior nature of crude oil treatment in refineries in China, especially the refining of heavy oil and high acid crude oil, the quality of sewage is deteriorated. Meanwhile, the amount of scum and sludge produced in the process of sewage treatment is greatly increased. Sludge can be disposed in two ways: incineration or landfill as hazardous waste. In order to reduce the disposal cost, the moisture content of the dewatered sludge is an important index.

Compared with the centrifugal dewatering process which is widely used in refinery sludge dewatering, the screw press dehydrator has the advantages of energy saving, stable operation effect, low noise and so on. At the same time, it has obvious advantages in operating cost and operation management.

1. Analysis on the Characteristics of Refinery Sludge

1.1 Source of Refinery Sludge

The sludge of refinery sewage treatment plant is composed of oil sludge, scum and residual activated sludge, it is usually called “three sludge”. Oil sludge includes oil tank bottom sludge of oil separating tank bottom sludge. The oil tank bottom sludge is the mixture of oil, water, mud produced when the oil tank is heated and dehydrated. The oil separating tank bottom sludge is the sludge deposited at the bottom of tank after oil, solid and water three-phase separation. The scum comes from the flotation tank of oil-bearing sewage treatment in the refinery, and the remaining activated sludge comes from the secondary sedimentation tank of the biochemical treatment system of oil-bearing sewage.

1.2 Property of Refinery Sludge

The oil sludge is generally a stable suspended emulsion composed of oil, water and solid substances. Most of the sludge in the tank bottom is naturally settled oil particles, mud, slag and ash with good settling performance. The scum contains more waste oil, chemicals and bubbles. The residual activated sludge contains more microorganisms, bacteria and less oil. The volatile solid content accounts for about 80% of the total solid content.

In the process of refinery sewage treatment, the largest proportion of scum is the “three sludge”, followed by the residual activated sludge, and the proportion of oil sludge is only about 10%. In refinery sludge treatment, sludge and scum are usually mixed for treatment, and scum is the main component.

2. Comparison and Analysis of Sludge Dewatering Equipment in Sludge Treatment of Refinery

Due to the high moisture content of sludge, direct discharge will increase the number of subsequent sludge conveying equipment and the cost of sludge drying or incineration treatment. Therefore, it is necessary to dewater the sludge. At present, centrifugal dewatering technology is widely used in the sludge dewatering treatment of refineries. The application time of screw press dehydrator technology is relatively short, but because of its advantages in operating cost, it is gradually popularized.

2.1 Comparison of Working Principles

2.1.1 Working Principle of Screw Press Dehydrator

The screw press dehydrator is a new type of solid-liquid separation equipment, which uses the principle of screw extrusion and realizes the extrusion dehydration of sludge through the strong extrusion pressure caused by the change of screw diameter and screw pitch, as well as the tiny gap between the fixed ring and the moving ring.

The main body of the screw press dehydrator is a filtering device formed by the fixed rings and the moving rings overlapping each other and the screw axis penetrating through it. It integrates concentration and dehydration, with the front section as the concentration part and the back section as the dehydration part. Each moving ring is embedded in the middle of the two fixed rings, and the filter gap former between them is gradually smaller from the concentration part to the dehydration part. The screw pitch of the screw axis is the also gradually smaller from the concentration part to the dehydration part. The inner diameter of the fixed ring is the same as that of the screw axis, the inner diameter of the moving ring is smaller than that of the fixed ring, and there is a back pressure plate at the end of the dehydration part.

The main body of the screw press dehydrator is installed obliquely or horizontally, and the dehydration part is in the back section. The screw axis rotates to drive the moving rings to do circular motion, and the filter structure composed of the fixed rings and the moving rings is in a moving state, which is not easy to be blocked. The water flows out of the filter gap, creating a self-cleaning effect, and the sludge is continuously pushed forward by the screw axis. After gravity concentration and pressure extrusion, the sludge is transported to the dehydration part. With the gradual decrease of filter gap and screw pitch, the extrusion pressure increases gradually, and the sludge concentration increases continuously. The blocking effect of the back pressure plate produces great internal pressure, which extrudes a small amount of water contained in the sludge, and the sludge eventually turns into mud cake.

The dehydrator mainly relies on screw extrusion to dehydrate sludge. The difference of sludge water density and oil content will not affect the treatment efficiency of the equipment, so it can realize the dehydration of low concentration sludge. The rotation speed of the screw axis is very low (about 2r/min), so the dehydrator has these characteristics: low energy consumption, no noise and low operating cost. The dehydrator adopts a completely enclosed box with a reserved odorous gas treatment interface, which can effectively reduce the pollution of odorous gas (such as hydrogen sulfide) in sludge to the surrounding working environment.

The sludge with large particles and large hardness will cause great wear to the screw axis, the fixed rings and the moving rings. With the increase of the running time, the filter gap between the screw axis, fixed rings and moving rings will be larger, so the extrusion pressure will be weakened, then the sludge dehydration effect will be decreased.

Generally, when the filter gap becomes larger, it is effective to tighten the screw self-locking nut. If there is any wear, polish or replace the rings according to the severity. Therefore, the screw press dehydrator is not suitable for the dehydration of sludge dewatering containing large hardness particles, especially large amounts of sand and metal substances.

2.1.2 Working Principle of Horizontal Decanter Centrifuge

The horizontal decanter centrifuge is a kind of traditional sludge treatment equipment, using the centrifugal force generated by the high-speed rotation of the drum to separate the solid and liquid.

The centrifuge is composed of a drum and a screw conveyor with a hollow shaft. The sludge is fed into the drum by the hollow shaft, and immediately thrown into the drum chamber by the centrifugal force. Due to the different density of mud and water, solid-liquid separation is formed. Pushed by the screw conveyor, the mud is continuously transported to the cone end of the drum to be discharged. The water is discharged out of the drum by gravity.

The centrifuge realizes the solid-liquid separation mainly by the density difference of mud and water. The density difference of mud and water is the key to determine the dehydration effect. The density difference between oil sludge, scum, residual activated sludge and water is not large, meanwhile, the oil content of oil sludge and scum is very high. These factors increase the difficulty of treating oil sludge (mainly scum). In order to improve the effect of separation, it is necessary to inject polymer flocculant to form large flocs and increase the density difference of mud and water.

2.2 Comparison of Process Flow

Compared with the horizontal decanter centrifuge, the screw press dehydrator needs lower inlet pressure. Under normal working conditions, the sludge treated by the concentration tank can flow into the flocculation mixing tank by itself. But the horizontal decanter centrifuge needs a sludge transfer pump to transport the treated sludge.

Compared with the screw press dehydrator, when the horizontal decanter centrifuge is used to treated the sludge, the flocculant is needed to be added.

2.3 Comparison of Operating Cost

There is a big difference between the two kinds of equipments in chemical consumption and water consumption. The power consumption of the horizontal decanter centrifuge is about 10 times that of the screw press dehydrator. Compared with the horizontal decanter centrifuge, the operation, management and maintenance of the screw press dehydrator are simple. The sludge transfer pump can be saved and the operating cost can be reduced.

2.4 Comparison of Advantages and Disadvantages

Advantages of the screw press dehydrator:

(1) It is suitable for the dehydration of oily sludge (the main component is scum).

(2) It is suitable for the dehydration of residual activated sludge.

(3) It can realize the dehydration of low concentration sludge.

(4) Low rotation speed, power saving, and low noise.

Disadvantages of the screw press dehydrator: it is not suitable for sludge dehydration with large hardness particles, especially with large mounts of sand and metal substances.

Advantages of the horizontal decanter centrifuge: wide application range.

Disadvantages of the horizontal decanter centrifuge: high power consumption, high noise, violent vibration, high operating cost.

3. Conclusion

(1) Both the screw press dehydrator and the horizontal decanter centrifuge are suitable for the dewatering of oily sludge (mainly composed of scum) and residual activated sludge, but the former is not suitable for the dewatering of sludge with large hardness particles, especially with large mounts of sand and metal substances.

(2) The horizontal decanter centrifuge mainly relies on the centrifugal force generated by the high rotating speed of the drum to separate the mud and water. While the screw press dehydrator mainly relies on the extrusion of the screw to realize the sludge dehydration. The difference of working principle between them determines that: the operating cost of the screw press dehydrator is significantly lower than that of the horizontal decanter centrifuge.

(3) The screw press dehydrator can effectively reduce the pollution of odorous gas in sludge to the surrounding working environment.

(4) The treatment effect of them is basically the same. Compared with the horizontal decanter centrifuge, the SS concentration of the filtrated liquid of the screw press dehydrator is slightly lower and the effect is slightly better.

(5) After dehydration, the oil content of the oily sludge from is large, and the main component is emulsified oil. If it is directly discharged into the sewage treatment system, it may affect the whole system, so it needs to be treated with caution.