Application of activated carbon in petrochemical industry

Many years ago, in order to meet the needs of industry, people activated firewood or coal powder after burning to obtain black carbon with adsorption capacity and gave it a certain shape. This was the earliest activated carbon. Subsequently, a variety of activated carbons were developed, such as coconut shells, fruit shells, columnar, honeycomb, powdered, spherical and other types of activated car

 

Many years ago, in order to meet the needs of industry, people activated firewood or coal powder after burning to obtain black carbon with adsorption capacity and gave it a certain shape. This was the earliest activated carbon. Subsequently, a variety of activated carbons were developed, such as coconut shells, fruit shells, columnar, honeycomb, powdered, spherical and other types of activated carbon. Carbon black is an important factor in activated carbon. Early inventions found that powdered activated carbon can adsorb certain gases and remove turbidity and odor in sewage. It can be used as a clarifier and decolorizer in sugar production.

In the petroleum industry, factories will extract a considerable amount of light oil. In the synthetic tail gas, most of this light oil is adsorbed and recovered by powdered activated carbon and used in the processed gas light oil to increase the yield of light oil. Powdered activated carbon is often used in oil processing plants for recovery. Activated carbon is often used in oil and gas synthesis plants to remove sulfide gas. Excessive sulfide in the gas will lead to poisoning of the synthesis catalyst and loss of activity. Activated carbon can have a good effect in removing sulfide gas, so activated carbon plays an important role in the petroleum industry.

 

There are many methods for treating refinery wastewater, mainly gravity sedimentation, filtration, flotation, flocculation, biochemical, membrane separation, adsorption, etc. Here we introduce the application of activated carbon in refinery wastewater treatment. Activated carbon not only has strong adsorption capacity, but also has the function of strengthening biological treatment and coagulation, and is widely used in refinery wastewater treatment.

 

1. Refinery wastewater treatment process

The design treatment capacity of a refinery wastewater unit in a petrochemical plant is 700t/h. Under normal circumstances, the water flow of the unit is about 320t/h, and the sewage treatment rate is 100%. The upstream water is recovered through the grid, homogenization tank, and inclined plate oil separator (CPI) to recover the floating oil, and then the emulsified oil is removed by cavitation air flotation (CAF) and inclined plate pressurized air flotation (ADAF). After the first and second biochemical, membrane filtration facilities, sewage standard improvement and activated carbon adsorption, the pollutants in the sewage are removed to meet the reuse or discharge standards.

2. Characteristics and working mechanism of activated carbon

Activated carbon is an excellent adsorbent with a highly developed pore structure and stable physical and chemical properties. It is made by catalytic activation of carbon-rich organic materials, such as charcoal, fine coal, fruit shells, etc., at high temperature and a certain pressure. During the manufacturing process, its volatile organic matter is removed, and gaps are generated between the crystal lattices, forming a complex pore structure and a huge surface area. The strong adsorption capacity of activated carbon lies in its huge adsorption area.

 

The working mechanism of activated carbon mainly utilizes its strong adsorption capacity. Activated carbon is generally divided into two forms: powdered and granular. Powdered activated carbon has strong adsorption capacity, is easy to prepare, and has a low price, but it is very difficult to regenerate; granular activated carbon is more expensive than powdered activated carbon, but it can be regenerated and reused, and the working conditions during use are better, and the operation and management are convenient.

 

3. Application of activated carbon in refinery wastewater treatment

3.1 Powdered activated carbon enhanced activated sludge process Powdered activated carbon enhanced activated sludge process is a wastewater treatment method that combines powdered activated carbon (PAC) with biological treatment. The refinery wastewater unit adopts this process. When the secondary biochemical water is not good or the sludge performance is poor, powdered activated carbon is added to the secondary biochemical aeration tank, and it is added once every half a month on average. On August 7, 2017, the COD in the total incoming water was 2162mg/L, which greatly exceeded the design index of 1200mg/L, so powdered activated carbon was added to the secondary biochemical aeration tank. On August 7, the COD of the secondary biochemical effluent was 79.4mg/L, and the COD removal rate was 49%. After adding powdered activated carbon, the effluent COD on August 8 dropped to 44.9mg/L, and the COD removal rate increased to 69%, and the effluent water quality was significantly improved. After adding powdered activated carbon, the dissolved oxygen in the aeration tank increased from 1.89 mg/L to 2.13 mg/L, and the sludge concentration of the mixed liquor increased from 1377 mg/L to 3958 mg/L, and the sludge index decreased from 232 mL/g to 111 mL/g, indicating that the amount of sludge increased and the sludge settling performance was significantly improved. Practice has proved that the improvement of the system treatment effect by strengthening the activated sludge process with powdered activated carbon is not only due to the adsorption of activated carbon on difficult-to-degrade organic matter and toxic substances, but also due to the role of powdered activated carbon in increasing biomass, microbial metabolic activity and sludge settling performance in the system.

3.2 Powdered activated carbon enhanced flocculation and sedimentation process The refinery wastewater upgrading device adopts a high-efficiency sedimentation tank + aerated biological filter combined process. The high-efficiency sedimentation tank is composed of three units: coagulation, flocculation and sedimentation clarification, which mainly removes organic matter, colloids and suspended matter in sewage. Adding powdered activated carbon to the flocculation tank can enhance flocculation and sedimentation, reduce the content of colloidal substances in the water body, remove a considerable part of the organic matter in the water body after adding activated carbon, and reduce the surface viscosity. Because activated carbon has a large specific gravity and good adsorption performance, it is adsorbed on the floccules, increases the specific gravity of the floccules, removes a considerable part of the organic matter in the water, and has good coagulant performance. Therefore, adding powdered activated carbon to the flocculation tank not only has a good ability to remove organic pollution, but also has a good coagulant effect, which greatly improves the effluent quality. 3.3 Application of granular activated carbon in deep treatment of refinery wastewater   In order to realize the reuse of refinery wastewater after treatment, the refinery wastewater enters the activated carbon tower after passing through the sewage standard raising device. The refined wastewater is deeply treated by activated carbon adsorption, which can further remove trace COD, BOD, SS, high-concentration nutrients (nitrogen, phosphorus, etc.) and salts in the wastewater. In August 2017, the average removal rates of COD and ammonia nitrogen in the refined wastewater after treatment by the activated carbon tower were 36.1% and 33.7%, respectively. Practice has proved that after adsorption treatment by activated carbon tower, COD and ammonia nitrogen in sewage are further removed, and the effluent water quality meets the requirements for discharge and reuse.

4. Problems and countermeasures of activated carbon in the application of refinery wastewater treatment

In the application of powdered activated carbon in enhanced activated sludge process and enhanced flocculation sedimentation process, residual sludge is discharged, and the powdered activated carbon in this residual sludge still has great regeneration potential. At present, the discharged residual sludge is dehydrated and reduced in volume by a two-phase centrifuge, and then enters the sludge drying device and then enters the power station for incineration treatment. The powdered activated carbon in the residual sludge cannot be effectively utilized, resulting in great waste. If this part of the powdered activated carbon is regenerated and reused in the system, the operating cost can be greatly reduced.

In the application of granular activated carbon in the deep treatment of refinery wastewater, activated carbon has the problem of easy adsorption saturation and high regeneration cost. In daily operation, the amount of activated carbon can be reduced and the service life of activated carbon can be extended by controlling the water quality and water volume of the activated carbon tower inlet, and controlling the frequency and method of backwashing the activated carbon tower. Studies have shown that fixing microorganisms on activated carbon to form biological activated carbon can increase the adsorption capacity of activated carbon, extend the service life of activated carbon, and enhance the ability to degrade organic matter in water. Biological activated carbon technology can not only greatly extend the service life of activated carbon, but also remove some pollutants that cannot be removed when activated carbon and microorganisms act alone, thereby reducing the cost of using activated carbon.