Volatile organic compounds (VOCs) are seriously harmful to both humans and the environment. After being stimulated by VOCs, the human body will experience symptoms such as skin allergies, dizziness and headaches. Long-term exposure to VOCs will also increase the risk of cancer. As the country's attention to environmental protection continues to increase, petrochemical companies must also strictly control the emission of VOCs. In the operation of the company, there are different levels of VOCs in the exhaled air of the benzene storage tank, the leaked gas of the acid water stripping device, and the exhaust gas of the sewage treatment system. Once the emission exceeds the limit specified in the relevant standards, the petrochemical company will face fines, suspension of work for rectification and other penalties. Therefore, petrochemical companies must modify or build new VOCs treatment equipment and facilities in accordance with relevant standards to ensure that the VOCs content in the final exhaust gas is within the standard limit to achieve safe and clean production.
overview of VOCS Emissions of A certain enterprise
1.1.1
A petrochemical company has a crude oil processing capacity of 4.6 million t/a. In response to the "Comprehensive Treatment Plan for Volatile Organic Compounds in the Petrochemical Industry" issued by the Ministry of Environmental Protection, the company has renovated the equipment and facilities of the benzene tank area, acid water stripping unit and sewage treatment system that do not meet the national emission standards to reduce VOCs emissions and meet the relevant standards such as GB 31570-2015 "Petroleum Refining Industry Pollutant Emission Standards".
According to the investigation, the company's benzene filling area has not installed an oil and gas recovery device, and the VOCs emission is 18 t/a. The preliminary plan is to build a new benzene tank oil and gas recovery facility; the VOCs emission of the acid water stripping device exceeds the standard of 80 mg/cm3, and the preliminary plan is to transform the original equipment and add a VOCs gas catalytic combustion skid on the top of the acid water pipe; the VOCs tail gas of the sewage treatment system exceeds the standard of 80 mg/cm3, and the preliminary plan is to add a set of VOCs treatment facilities.
2 VOCs treatment plan for petrochemical enterprises
2.1 Treatment of VOCs in exhaled air from benzene storage tanks
The main components of exhaled air from benzene storage tanks are nitrogen and benzene vapor, and benzene series are important components of VOCs. The air intake flow rate of the tank is 6.5 m3/h,
the oil-gas ratio is 1.2, then the oil-gas recovery scale is 6.5×1.2=7.8 m3/h. A certain margin should be reserved when designing the oil-gas recovery device, so the final scale of the oil-gas recovery device is 8 m3/h.
2.1.1 Comparison of oil-gas recovery plans for benzene storage tanks
Under existing technical conditions, there are 3 oil-gas recovery plans for benzene storage tanks.
Scheme 1 is a combination of "flame arrester + breathing valve" as a pressure control scheme for the tank top exhaled gas collection system. The oil and gas discharged by the oil and gas manifold are transported to the oil and gas recovery facility. The recovered oil and gas are condensed and sent to the light oil system for storage in liquid form. The treated exhaust gas is discharged into the air [1].
Scheme 2 is a combination of "flame arrester + regulating valve", and the rest is the same as Scheme 1.
Scheme 3 is a combination of "flame arrester + liquid seal tank", and the rest is the same as Scheme 1.
Comparing the three schemes, the supporting equipment of Scheme 2 and Scheme 3 is relatively complex, while Scheme 1 has a simple structure and is easy to install and maintain. Therefore, it was finally decided to use Scheme 1 as the recovery scheme for exhaled gas from benzene storage tanks.
2.1.2 Oil and gas recovery process of benzene storage tank
The oil and gas recovery process of benzene storage tank based on Scheme 1 is as follows: When the oil and gas pressure generated by the benzene storage tank reaches the preset value, the oil and gas are discharged from the breathing valve (single call valve) on the top of the tank, and the oil and gas are transported to the oil and gas recovery device next to the benzene storage tank through the oil and gas manifold connected to the valve port. The tail gas is detected by the tail gas detection device. If it is qualified, it is directly discharged. If it is unqualified, it is re-input into the oil and gas recovery device for treatment. After condensation, the oil and gas changes from gas to liquid and enters the light oil device through the oil pipeline for storage.During the process, the acid water degassing tank and the acid water storage tank will leak VOCs. The leakage problem of the acid water degassing tank can be reduced or eliminated by replacing the valve type and strengthening the valve seal. However, there is no good treatment measure for the high-altitude emission of the top gas of the acid water storage tank after deodorization, and it must be treated.
2.2 VOCs treatment in sewage treatment plant
The designed treatment capacity of the company's sewage treatment plant is 500 m3/h, which can treat oil-containing, salt-containing, sulfur-containing and other sewage discharged from petrochemical production. The original sewage treatment plant will discharge waste gas when treating sewage due to old equipment. According to the on-site measurement, the composition of the waste gas is relatively complex, with the main components being hydrogen sulfide, as well as volatile organic compounds such as benzene, toluene, and xylene. For this reason, a new set of VOCs treatment equipment is required, and the designed VOCs treatment capacity is 35,000 m3/h.
2.2.1 Adsorption condensation pretreatment
Due to the high concentration of non-methane total hydrocarbons in the mixed waste gas discharged by the sewage treatment plant, the treatment effect of directly entering the biological treatment system is not ideal, so it is necessary to pretreat the waste gas with complex components. The optional options are "volume expansion absorption method" and "adsorption condensation method". In actual application, the former has problems such as the need to frequently add volume expansion agents and high operating costs, so adsorption condensation pretreatment was selected in this project. The principle is to use adsorbent to adsorb non-methane hydrocarbons in the exhaust gas, perform vacuum analysis after reaching saturation, and then lower the temperature to condense the VOCs gas into liquid, and then separate it from the gas. On the one hand, it can achieve the effect of purifying the exhaust gas, and on the other hand, it can also recycle and reuse VOCs. The advantages of adsorption condensation pretreatment are high purification efficiency (more than 85%), good stability, and low operating costs.
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