Dewatering of "three sludge" (oil sludge, scum and activated sludge) generated in refinery wastewater treatment has always been a difficult problem. The dehydration of oil sludge and scum is particularly prominent. In the past, vacuum drum filter was used for dewatering. Due to its low efficiency, high energy consumption, high labor intensity and other shortcomings, it cannot be used normally, which leads to the impassability of refinery sewage treatment, so a large amount of sludge has to be transported out for treatment.
in recent years, Zhejiang Zhengda Environmental Protection Equipment Co., Ltd. kept learning and researching on this issue, and finally independently developed the horizontal spiral sedimentation through unremitting efforts Centrifuge 。 Tarim Oilfield started to adopt horizontal type Spiral sedimentation centrifuge (hereinafter referred to as centrifuge) has been used for "three sludge" dewatering in refinery and sludge dewatering in PTA unit, and achieved good results.
Feeding properties and conditioning According to the working properties of the centrifuge, there must be a density difference between solids and liquids to achieve separation. This is easy to achieve for TA sludge generated in PTA sewage treatment, and the settling performance of TA residue is very good. However, some work needs to be done for the "three sludge" of refinery sewage, because its solid particles are extremely small, the density difference is very small, and the suspended particles of sludge are charged, its colloidal properties are stable and difficult to separate, the thickened scum and activated sludge almost do not settle, and its water content is up to 98%. On the basis of consulting a large amount of data and research, it is believed that the sludge needs to be conditioned first to destroy the colloidal stability of its solid medium, so as to make it agglomerate, so as to achieve the separation of sludge and water and good agglomeration of the agglomerate. Therefore, more than 30 kinds of domestic reagents were tested in the laboratory, and 6 kinds of reagents were screened out for industrial test, and good results were achieved‘
1. Selection of centrifuge
1.1 Differential ratio
The settling speed of solids in different media is different, and the degree of condensation is different, so different differential ratio should be selected. When other parameters remain unchanged, the higher the differential speed, the faster the solids are pushed out, and the greater the amount. At the same time, the liquid content increases, and the shaft power generally decreases, but the separation effect will decrease, that is, the solid content in the filtered liquid will increase. The solid reason is that the differential speed is increased, which actually plays a stirring role in the drum. The selection of proper differential ratio is the first factor to be considered when selecting a model.
1.2 Length diameter ratio
The length diameter ratio is the ratio of the axial length of the centrifuge drum to its diameter. The working area of the centrifuge is divided into feeding, settling, drying, unloading and other areas. The length of the settling zone is the main parameter that determines the separation effect. The longer the settlement area, the greater the length diameter ratio, the better the separation effect, but the greater the shaft power. Different length diameter ratio shall be selected according to different medium properties (mainly settlement performance), and shall be considered in combination with differential ratio.
1.3 Power
Selecting larger power can improve the processing capacity, especially when the settling performance of the medium is very good. Users have some difficulties in selecting models. When facing new media or media without successful use experience, relevant tests shall be carried out.
2. Determination of operating parameters
2.1 Adjustment of overflow port diameter
There are four diameters of overflow port for general centrifuge. The larger the diameter of the overflow port, the longer the drying zone, the lower the water content of the filter cake, but the higher the solid content of the filtered liquid, so it is necessary to adjust the appropriate diameter of the overflow port. For the "three sludge" that is not easy to settle, it should be smaller, while for the media that is easy to settle, such as TA residue, it should be larger.
2.2 Speed
According to the working principle of the centrifuge, the higher the rotating speed, the better the separation effect, the drier the filter cake, the clearer the filtered liquid, and the greater the shaft power. This is an important parameter in actual operation, but it does not mean that the higher the rotating speed, the better the separation effect. Generally speaking, for the medium that is easy to settle, the higher the rotating speed is, the greater the shaft power is, which is likely to cause the machinery or solid materials to be pushed out too late, resulting in material blockage. Therefore, appropriate rotating speed shall be selected according to factors such as power of centrifuge, feeding, dehydration effect, etc. For the "three sludge" that is not easy to settle, when the rotating speed reaches a certain value, the separation effect will decline, the sludge production will decrease, and the solid content in the filtered liquid will increase. When the rotating speed is reduced, the sludge production fork will increase. This is because the materials slip in the drum and cannot be pushed out. At the same time, with the increase of rotating speed, the absolute differential speed increases and the stirring effect of the spiral drum increases.
3. Operation effect
3.1 PTA sludge dewatering
For PTA sewage, whether it can reach the discharge standard depends on whether it can remove TA residue before entering the biochemical device. This link depends on acid precipitation and TA residue dehydration. The oA) has been running since May 2000, and its effect is satisfactory. After acid precipitation and centrifuge dehydration, the removal rate of TA reached 80%~90%. The solid content of centrifuge feed is about 20%, the liquid and solid content after filtering can reach below 0.1%, and the water content of TA residue is 25%~40%, which is in powder form. The filtered liquid is clear and transparent.
3.2 Refinery "three sludge" dehydration
After a series of adjustments, the treatment of "three sludge" in the refinery has also achieved satisfactory results. Especially for flotation dross, vacuum drum filter and belt filter press can not be used normally, while centrifuge has good effect. The water content of scum and oil sludge after concentration is about 98%, and then dehydrated by centrifuge. The filtered liquid water content is 99.9%. The filter cake water content is 35% - 85%, and the dosing concentration is 250~300 mg/L.
4. Summary
Domestic centrifuges have been more and more widely used in sludge dewatering in refineries and oilfields due to their excellent dewatering effect, stable mechanical performance, simple operation, low price, less land occupation, continuity and closure of operation, etc
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