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Spent lye
In the process of petrochemical production, NaOH solution is used to absorb H2S, alkali washing oil and cracking gas, which produces waste liquid containing a lot of pollutants
The waste lye is in
Petrochemical industry
In the production process, H is absorbed by NaOH solution
2
S, alkaline washing oil and cracked gas produced by the waste liquid containing a lot of pollutants. Due to contain
sulphide
Inorganic and organic sulfides such as mercaptan, so the waste lye has an unpleasant odor. Waste lye has a strong alkalinity. Without proper pretreatment, high-concentration waste lye will inhibit the growth and reproduction of microorganisms when it enters the sewage biochemical treatment system. In severe cases, a large number of microorganisms will die, thus affecting the normal operation of sewage treatment plants and the standard discharge of total wastewater.
- Chinese name
- Spent lye
- Foreign name
- exhausted lye
- industry
- Petrochemical industry
- temperament
- Usually has a foul odor
catalogue
Waste lye is absorbed by NaOH solution in the process of petrochemical production
2
S, alkaline washing oil and cracked gas produced by the waste liquid containing a lot of pollutants. Because of the inorganic and organic sulfides such as sulfide and mercaptan, the waste lye has an unpleasant odor. Waste lye has a strong alkalinity. Without proper pretreatment, high-concentration waste lye will inhibit the growth and reproduction of microorganisms when it enters the sewage biochemical treatment system. In severe cases, a large number of microorganisms will die, thus affecting the normal operation of sewage treatment plants and the standard discharge of total wastewater
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The main component of the waste liquid contaminant is the malodorous toxic sulfide (Na
2
S, mercaptan, thiophenol and thioether, etc.), phenols, naphthenic acid sodium salts, oils, heterocyclic aromatic hydrocarbons and reaction residues
Sodium hydroxide
Let's wait. The types and concentrations of pollutants vary greatly due to the types of processed crude oil and the different processing processes.
The treatment of waste lye has always been a core problem in the water pollution control of refineries and vinyl plants in China. With the increase of processing amount of high sulfur crude oil and the increasing scale of ethylene plant, the discharge of waste lye has also increased, and the treatment of waste lye has attracted the attention of researchers.
Foreign research and treatment technology status
Some developed countries such as Europe, America and Japan have applied wet air oxidation to treat waste lye for many years. Researchers in the United States have been working on the technique since the 1950s. The United States Resource Conservation and Reuse Act (RCRA) stipulates that the active sulfide in the waste lye is a class D003 harmful pollutant, so it is not allowed to neutralize and dilute the waste lye directly into the biological wastewater treatment facility, and requires the waste lye to carry out harmless pretreatment on the spot to convert the sulfide into relatively stable sulfur, insoluble metal sulfide or soluble sulfate.
Wet air oxidation is a series of oxidation and hydrolysis reactions under high temperature (150~320℃) and high pressure (2068~20684kPa), using oxygen in the air to oxidize sulfide in ethylene waste lye
Sodium thiosulfate
Or sulfate, thereby eliminating the foul odor in ethylene waste lye, while oxidizing part of the organic matter to
Carbon dioxide
To reduce the COD concentration in ethylene waste lye. The main processes are:
- 1.
Zimpro technology (high temperature, high pressure) developed by US Zimpro company;
- 2.
The NPC process developed by Nippon Petrochemical Company, which only oxidizes sulfide and does not oxidize hydrocarbon organic matter as the treatment target;
- 3.
LOPROX process (low temperature, low pressure) developed by Bayer, Germany;
- 4.
The supercritical wet oxidation process and NKT/RISO process developed by American MODAR company (temperature and pressure are slightly lower than Zimpro process).
Zimpro process is the first in various wet air oxidation processes, and its technology is the most mature and widely used. In Europe and the United States have been running hundreds of sets of WAO treatment equipment for waste lye, the use of Zimpro process accounted for the majority. However, the reaction process of Zimpro process requires high temperature and pressure, and the engineering cost is expensive, which hinders its further popularization.
The NPC process was developed specifically for waste lye treatment and is more economically reasonable, but out of water
Organic pollutant
Basically retained, further processing is required. The wet oxidation process using catalyst and pure oxygen can significantly reduce the temperature and pressure, which is convenient for popularization and application, and has become a research hotspot in recent years. However, air oxidation still fails to completely remove pollutants, and it is still necessary to use biological treatment of late sewage, so developed countries in Europe and the United States prefer comprehensive utilization of waste lye or wet oxidation pretreatment before entering the biochemical sewage system for treatment.
Current situation of domestic research and treatment technology
Domestic ethylene alkali washing waste liquid commonly used pretreatment and biochemical treatment combined with the method of treatment, that is, the neutralization method, oxidation method or biological method for pretreatment, and then sent to a comprehensive sewage treatment plant for biochemical treatment, in addition to comprehensive utilization method, biological oxidation method, etc., are introduced below
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.
Acid-base neutralization method
The pH of the waste lye is very high and cannot be discharged directly. It is necessary to add waste acid to adjust the pH to neutral and neutralize the released H
2
S, CO
2
It is an effective way to dispose of the waste lye by disposing of the gas separately after vaping it out.
>>> Acid neutralization method
This method is widely used in the treatment of waste lye in a batch of ethylene plants built in China in the 1980s. For example, the ethylene plant of Jinshan Petrochemical Complex and the ethylene plant of Yangzi Petrochemical Company use sulfuric acid-stripping incineration route to treat waste lye.
The method is to first remove the butter in the waste lye, and then acidize the ethylene waste lye with 98% concentrated sulfuric acid to pH=2~4, react in the neutralization tank, the sodium sulfide solution is converted into sodium sulfate solution, and sent to the sewage plant for biochemical post-treatment and discharge, and the H generated during the neutralization
2
S, CO
2
The gas is taken out of the air and sent to the torch for burning.
Hydrochloric acid neutralization and stripping technology was also used in the waste alkali treatment unit of the ethylene plant of petrochina Lanzhou Petrochemical Company.
Problems with acid neutralization:
- 1.
Equipment and pipeline corrosion, because the composition of waste lye often fluctuates, it is difficult to accurately control the amount of sulfuric acid added, and equipment and pipeline acid and alkali alternation, serious corrosion, resulting in greater safety risks;
- 2.
The butter in the waste lye must be thoroughly removed before acidification, otherwise the stripper is prone to coking and clogging.
- 3.
H 2 SO is generated after S combustion 2 Gas is still harmful gas treatment is not easy to cause secondary pollution.
>>>CO
2
Neutralization method
Yanshan Petrochemical Company uses CO produced by our ethylene glycol plant
2
The waste gas reacts with the waste ethylene lye to remove the Na in the waste lye
2
S, NaOH, etc. are converted into Na
2
CO
3
And NaHCO
3
, resulting in H
2
The S gas is then comprehensively treated to achieve the purpose of removing sulfide and neutralizing waste alkali.
The mass concentration of sulfide can be reduced to less than 40mg/L and the oil content can be reduced to undetectable after the treatment of ethylene waste lye. Na in lye treated by this method
2
CO
3
And NaHCO
3
The quality score can reach about 20%.
CO
2
The neutralization process is short, the equipment is simple, the material requirements of the equipment are low, and the process requires cheap CO nearby
2
Exhaust gas, and H generated by the process
2
S needs to be treated separately, and if it is not handled properly, it will cause secondary pollution.
Oxidation method
The process is mainly through the oxidation of various oxidants to convert the sulfide in the waste lye into harmless sulfate, thiosulfate,
sulfite
Let's wait.
There are in the country
Shanghai Secco Petrochemical Co., LTD
,
China Petroleum & Chemical Corporation Maoming Branch
,
China Petroleum and Chemical Corporation Guangzhou branch
,
China Petroleum Dushanzi Petrochemical Company
The ethylene plants of many companies adopt Siemens high temperature and high pressure wet oxidation process technology (from the US Zimpro process) to treat ethylene waste lye.
The disadvantages are:
- 1.
High one-time investment and operating costs;
- 2.
The treatment effect is not ideal, the COD of the treated sewage is still as high as 5g/L, which still cannot meet the requirements of the inlet water quality of the sewage treatment plant, and it needs to be diluted to increase the load of the sewage treatment plant.
- 3.
Pipeline is easy to block;
- 4.
High operating pressure, toxic gas (hydrogen sulfide), there are safety risks.
China Petroleum & Chemical Corporation Maoming Branch
The waste lye in the chemical division adopts wet oxidation +EM-BAF(engineered bacteria -
Biological aerated filter
Process treatment, put into production in 2009. After wet oxidation pretreatment, the discharge of ethylene waste lye is about 10t/h, the COD concentration is about 2500mg/L, the oil is less than 10mg/L, and contains a lot of inorganic salts (such as Na+, SO)
4
2 -
Plasma), salt content ≥8%, poor biodegradability, if discharged into the comprehensive sewage treatment plant will cause a large impact, so the WAO treated waste lye after dilution using EM-BAF process for biochemical treatment. After wet oxidation pretreatment +EM-BAF treatment, the waste lye can achieve the standard discharge.
According to the characteristics of ethylene and oil refining waste lye, CAI Hongmei of Fushun Petrochemical Research Institute developed a low temperature wet oxidation process. In the reaction temperature range of 150~180℃, the deodorization effect of waste lye is very good
2 -
The removal rate is greater than 99.7%, the removal rate of COD is generally 40%~70%, and the higher the temperature, S
2 -
Convert to SO
4
2 -
The larger the ratio of COD, the higher the removal rate of COD. This technology has been successfully applied in Shanghai Gaoqiao Refinery and Anqing Petrochemical Complex.
In short, the wet air oxidation method has higher requirements for equipment materials, control instruments and other aspects, the investment is also large, the operating cost is high, and the operation is difficult to run smoothly, so it has not been well promoted in China.
Yu Zhengzhe et al
Chemical precipitation method
Treatment of ethylene waste lye with photochemical method. The Na in the waste lye is removed by chemical precipitation first
2
S is regenerated into NaOH, so that the waste lye can be reused, and then the photochemical oxidation method is used to oxidize the organic matter in the waste lye, and the good effect is received
2 -
The removal rate of COD can reach more than 98%, and the total removal rate of COD can reach 87%. Although the effect of photooxidation is good, it is difficult to realize industrialization and only stays in the laboratory stage.
Comprehensive utilization of waste lye
The comprehensive utilization of ethylene alkali washing waste liquid should include the following processes:
- 1.
Removal of oily substances (including suspended solids);
- 2.
Sulfides (including Na 2 S and organic sulfur) and CO 3 2 - The use of;
- 3.
Utilization of surplus alkali.
NaOH and Na in ethylene waste lye
2
S is the active ingredient in the alkaline pulping cooking liquid, theoretically speaking, as long as the oil substances in the waste lye are removed, it can be used for pulping and paper making. For NaOH and Na
2
High levels of S, Na
2
CO
3
The content of waste lye is also high, such as the CO can be removed
3
2 -
Can be used for pulp and paper, while Na
2
CO
3
The waste lye with lower content can be directly used as paper making cooking liquor.
The key to whether the waste lye can be used for pulping and papermaking is whether the oil can be removed more thoroughly, but according to the process of treating the waste lye it is difficult to meet such requirements, the paper produced by the oil removal is not clean, and the implementation of this utilization method also depends on the nearby paper mill that can accept this lye
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Biological treatment
Biological treatment
It is through the metabolic action of microorganisms that pollutants such as organic matter in wastewater are
Microbial degradation
And the process of conversion into harmless substances. It is a widely used wastewater treatment technology, and economical. Domestic scholars have made some attempts to it.
Xie Wenyu and others passed on
Biological aerated filter
The aeration method of the process has been improved. The isolation biological aeration filter method has been used for pilot and industrial biological pretreatment of refinery alkali residue, and good experimental results have been obtained. Under suitable operating conditions, the average removal rate of CODCr from alkali residue of oil refining exceeds 85%, the removal rate of sulfide exceeds 99%, the removal rate of petroleum pollutants exceeds 85%, and the degradation rate of phenolic pollutants exceeds 88%.
Guo Guiyue et al. added a low content of waste lye to the acrylic fiber mixed wastewater, and the biological treatment system was slightly polluted
Biological activity
It is not affected, and better treatment effect is obtained. Huang Quanjing et al. used the internal cycle RBF biological oxidation technology to treat the waste alkali residue of the ethylene plant of Lanzhou Petrochemical Company. The test results show that the COD degradation rate of this technology can reach 93% and the sulfide removal rate can reach 99%.
To sum up, the ethylene waste lye treatment method with wet air oxidation technology is the most mature, and has achieved industrial application in petrochemical plants at home and abroad, but the technology needs to be carried out under high temperature and high pressure conditions, the reactor (high temperature, high pressure and corrosion resistance) requirements are high, so the equipment investment is large, and the operation cost is high, such as the waste lye composition is complex, but also easy to cause equipment abnormal operation.
- 1.
Chemical and photochemical oxidation methods have the problems of high cost, high cost and low utilization of light, which are mainly in the laboratory research stage.
- 2.
The comprehensive treatment method is an effective way to realize the comprehensive utilization of waste, but it is difficult to realize the problem of connecting the industrial chain.
- 3.
The biggest advantage of biological treatment of ethylene waste lye such as high organic matter, high sulfur and phenol containing wastewater is low cost, if the conditions are suitable, it is possible to recover the sulfur in the wastewater in the form of elemental sulfur, so as to eliminate the harm of sulfur and phenol pollutants in the waste lye.
According to public reports, the first set of full-biological oxidation technology ethylene waste alkali treatment device - Lanzhou Petrochemical company 128,000 tons of full-biological oxidation technology ethylene waste alkali treatment device officially introduced sludge and waste lye into the system on December 31, 2013, 10 days of trial operation, running well. The COD, pH and sulfide contents of the treated wastewater meet the design requirements
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