About the author: Wang Xin (1987—), female, doctor, lecturer. Research direction: Environmental pollution chemistry. E-mail: cherish8703@163.com
State Key Laboratory of Petroleum and Petrochemical Pollutants Control and Treatment, School of Chemical Engineering and Environment, China University of Petroleum (Beijing), Beijing 102249, China
Abstract: Taking the sewage treatment plant of a refinery and chemical enterprise as the research object, the composition characteristics and health risks of the exhaust gas outside the odor control facility were analyzed. The malodorous substances in the external exhaust gas are mainly ammonia, hydrogen sulfide, total non-methane hydrocarbons, aromatic hydrocarbons, and organic sulfur compounds; among them, ammonia, hydrogen sulfide, methyl mercaptan, methyl sulfide, dimethyl disulfide, carbon disulfide and The concentration of 4-ethyltoluene is higher than the odor threshold; the theoretical odor concentration of methyl mercaptan and hydrogen sulfide is the highest, and they are the key odor-causing substances; The contribution rate is the highest (77%); in the external exhaust gas, the non-carcinogenic health risk index (HI total ≈10 −4 ) and lifetime carcinogenic risk (LCR total ≈10 −8 ) of 20 malodorous pollutants are low.
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It is difficult for the sewage gathering and transportation systems and sewage treatment plants of refinery companies to be completely airtight. Pipelines, structures, and equipment all have volatile pollutants that escape from the water body and form waste gas emissions, becoming an important source of odor pollutants for refining and chemical enterprises. To this end, the sewage treatment plant odor pollution has become an important part of pollution prevention and control system in petrochemical enterprises [ 1 - 2 ] . There are two types of odorous gases emitted by sewage treatment plants: one is the high-concentration odor emitted from physical and chemical facilities, slop tanks, scum pools, etc. The composition is mainly non-methane total hydrocarbons, hydrogen sulfide, ammonia, and organic sulfur. The concentration of malodorous substances such as chemical substances is also relatively high; the second is the low-concentration odor emitted from biochemical facilities and sludge ponds (rooms). High-concentration odors are suitable for treatment by combustion methods (catalytic oxidation combustion, thermal storage oxidation combustion, etc.). Low-concentration odors are mostly treated with lye washing, adsorption and biological deodorization [ 3 ] .
There are strict requirements for source control and emission reduction of odor pollution at home and abroad [ 4 ] . Refining and chemical enterprises have fully implemented the "Petroleum Refining Industry Pollutant Discharge Standard" (GB 31570-2015), and the benzene (4 mg·m −3 ), toluene (15 mg·m −3 ) and toluene (15 mg·m −3 ) of the waste water treatment organic waste gas collection and treatment device have been fully implemented. ), xylene (20 mg·m −3 ) and non-methane total hydrocarbons (120 mg·m −3 ) have all put forward specific requirements and higher standards. Compared with the "Emission Standard of Odor Pollutants" (GB 14554-1993), the "Emission Standard of Odor Pollutants (Draft for Comment)" (2018 Edition) has made stricter regulations on the emission limits of 8 odor pollutants.
Previous studies pay more attention to the composition of characteristic odor of gas treatment facilities released, the main purpose is to provide a basis for odor removal process design [ 3 , 5 - 6 ] , while emissions odor control facilities, gas composition and health risk assessment, the Less research. This study took a sewage treatment plant of a northern refinery and chemical enterprise as the research object, and conducted a comprehensive analysis of the external exhaust gas composition characteristics of three sets of odor control facilities, determined the key odor-causing compounds, and evaluated the comprehensive odor pollutants. Health risks, and put forward corresponding pollution control strategies in a targeted manner, in order to provide a reference for the further improvement of the management and prevention and control of odor pollution in refining and chemical enterprises under the new standard.
1. Materials and methods
1.1. Sample collection
Take the odor control facility discharge gas of a sewage treatment plant in a refinery and chemical enterprise as the research object. As in FIG. 1 , the high concentration of the odor generated by using chemical and physical facilities "lye absorber Catalytic combustion" process for treatment, a high concentration of odor control flag (high-concentration odor treatment, HOT ) unit. The low-concentration waste gas generated by the biochemical facilities is treated by the "lye absorption" process, which is marked as low-concentration odor treatment unit I (low-concentration odor treatment, LOT-I). The low-concentration waste gas generated in the sludge dewatering room is treated by the "activated carbon adsorption" process, which is marked as low-concentration odor treatment unit II (LOT-II). The odor sampling point is located at the outlet of the exhaust cylinders of the 3 sets of odor control units, and the sampling is performed 3 times at an interval of 8 hours. The sampling method is in accordance with "Determination of Particulate Matter in Exhaust from Stationary Pollution Sources and Sampling Method of Gaseous Pollutants" (GB/T 16157-1996), using 1.5 L Tedlar air bag vacuum sampling.
1.2. Analysis indicators and methods
Index odor pollutants and detection methods, see Table 1 . These indicators include: 9 indicators of ammonia, trimethylamine, hydrogen sulfide, methyl mercaptan, methyl sulfide, dimethyl disulfide, carbon disulfide, styrene and odor concentration in the "Emission Standards for Odor Pollutants (Draft for Comment)" ; "Petroleum Refining Industry Pollutant Emission Standards" (GB 31570-2015) in the four indicators of benzene, toluene, xylene and non-methane total hydrocarbons; "Determination of volatile organic compounds in stationary pollution source waste gas solid phase adsorption-thermal desorption /Gas Chromatography Mass Spectrometry (HJ 734-2014) in 51 volatile compounds. The concentration of malodorous pollutants is the average of 3 sampling test results.
1.3. Analysis of Odor Contribution
Detection of a single component of the total proportion of substance and do not represent the contribution its malodour Theory odor concentration (theoretical odor concentration, thus introducing C OD ) to select for actuation the key odor substance [ . 7 - . 9 ] . C odis the ratio of the mass concentration of each malodorous pollutant component in the malodorous gas to the olfactory threshold. Calculate C od , i and the total C od of the pollution source based on the olfactory threshold and mass concentration of each odor pollutant component in the odor , and obtain its odor contribution Pby the proportion of each component C od , i in the total C od i , the calculation formula is shown in formulas (1)~(5).
Formula: C OD , I for the component C OD ; C I is the mass concentration of each component, mg · m -3 ; C OT olfactory thresholds for odor pollutants, mg · m -3 ; C OD the total of all components C OD , I ; P I odor contribution value for each pollutant component; C OD , n- is the region of the sources C OD sum; P n-odor contributions to the region of the sources.
1.4. Health risk assessment
Use the non-carcinogenic health risk index (HI) and lifetime cancer risk (LCR) to assess its non-carcinogenic risk and carcinogenic risk. Gas mainly affects people's health through respiration. Therefore, HI and LCR are calculated based on the US EPA environmental toxicants non-carcinogens' respiratory inhalation reference concentration (RfC) and carcinogen's respiratory inhalation unit risk (IUR) , The calculation formula is shown in formulas (6)~(7) [ 10 ] .
Where: C i is the mass concentration of a pollutant in the air, mg·m −3 ; ET is the daily exposure time, which is 8 h; EF is the continuous exposure frequency for one year, which is 250 d·a −1 ; ED is Exposure duration, 25 a; AT is the average life span of the population (LCR is 70 a, HI is 25 a); IUR is the carcinogenic risk per pollutant, m 3 ·mg −1 ; RfC is the non-carcinogenic reference concentration of the pollutant , mg · m -3 [ . 11 - 15 ] .
The health risk value of the mixed source is the sum of the hazard indexes of the various pollutants, and the synergy and antagonism between the substances are not considered in the evaluation process. For cancer risk, the LCR> 10 -4 , it indicates that there is a greater risk of potentially carcinogenic; the LCR ranging from 10 -6 to 10 -4 , it indicates the presence of a potential cancer risk; the LCR <10 -6 , it indicates that the cancer risk Within acceptable limits. For non-carcinogenic risks, when HI>1, it indicates that it will cause non-carcinogenic health risks to the human body; and when HI≤1, it can be regarded as not causing harm to the human body.
2. Results and discussion
2.1. Analysis of the composition characteristics of odor pollutants from external exhaust
A total of 42 odorous substances were detected in the exhaust gas of the 3 odor control units of the refinery sewage treatment plant, including ammonia, hydrogen sulfide, trimethylamine and 39 other odorous substances (2 alkane, 1 alkene, 11 aromatics). Hydrocarbons, 4 kinds of organic sulfur compounds, 13 kinds of oxygen-containing compounds and 8 kinds of halogenated hydrocarbons). Wherein, LOT-Ⅰ unit detected odorous substances most species, HOT minimum unit detects the type (see Table 2 ).
The main malodorous pollutants emitted by HOT units are ammonia, hydrogen sulfide, alkanes, aromatic hydrocarbons, methyl mercaptan, methyl sulfide, carbon disulfide and non-methane total hydrocarbons. Among them, non-methane total hydrocarbons have the highest content, which is close to the mass concentration limit specified in GB 31570-2015. The mass concentration of alkanes and aromatic hydrocarbons is also high, mainly because the odor sources of HOT units are oil traps, air flotation tanks and other physical and chemical facilities. The oily substances in these facilities produce a large amount of non-methane total hydrocarbons and aromatic hydrocarbons and other odor pollution. Things. Lu Xiurong et al. [ 16 ] conducted a screening of malodorous substances in a heavy oil refining sewage oil separation and air flotation treatment system and the results were consistent with this study. The main malodorous pollutants emitted by the LOT-I unit are ammonia, methyl mercaptan, hydrogen sulfide, carbon disulfide and non-methane total hydrocarbons. Among them, the content of methyl mercaptan is higher than that of HOT and LOT-Ⅱ units. This is mainly because the source of odor of LOT-I unit is biological treatment facilities, and there are more biological degradation products such as methyl mercaptan [ 17 ] . Between refinery wastewater treatment plant sludge dewatering is significant odor sources [ 18 is - 20 is ] . The main odor pollutants emitted by the LOT-II unit are ammonia, hydrogen sulfide, alkanes, carbon disulfide and non-methane total hydrocarbons. The content of ammonia and hydrogen sulfide is basically the same as that of the LOT-I unit, but the odor concentration is low.
The odor pollutants are divided into 8 types: ammonia, hydrogen sulfide, alkanes, alkenes, aromatic hydrocarbons, oxygenated compounds, halogenated hydrocarbons and organic sulfur compounds. The concentration of various types of odor emissions see FIG . Among them, the concentration of ammonia is the highest, accounting for 35.7%, 36.0% and 53.1% of the total odor concentration of HOT, LOT-I and LOT-II units, respectively. Alkanes and aromatic hydrocarbons are characteristic odor pollutants of refining and chemical enterprises [ 21 ] , and their concentration is still high after being treated by odor control facilities. The total emission concentration of the two accounts for 30.6%, 23.0% and 22.0% of the total mass concentration of odor discharged from HOT, LOT-I and LOT-II units, respectively. The mass concentration of organic sulfide discharged accounted for 12.3%, 28.3% and 6.7% of the total mass concentration of odor discharged from HOT, LOT-I and LOT-II units, respectively. HOT and LOT-I units mainly discharge organic sulfides from methyl mercaptan, and LOT-II units mainly contain carbon disulfide and methyl mercaptan. There are many types of oxygenated compounds and halogenated substances, but the emission mass concentration levels of the three odor units are low. The mass concentration of hydrogen sulfide is relatively low. The emission mass concentrations of the three odor units are 0.12 mg·m −3 , 0.08 mg·m −3 and 0.07 mg·m −3 respectively, accounting for HOT, LOT-I and LOT-II respectively. 7.0%, 3.1% and 4.1% of the total mass concentration of odor discharged from the unit. Olefins only contain 1,3-butadiene, and the concentration in the exhaust gas outside the three units is very low. Investigation of a conventional sewage treatment plant refining companies each processing process section consisting of odor found that the main type of contaminants including NMHC, organic sulfur-containing compounds, benzene, and other alkanes and cycloalkanes [ 6 , 21 is - 22 is ]. It can be seen that the main types of malodorous pollutants are basically unchanged before and after the treatment of HOT, LOT-I and LOT-II units, but the concentration of pollutants varies greatly.
2.2. Analysis of the contribution of malodorous pollutants
The chemical properties of China Research and Information Center for Environmental Health of Japan odor threshold of the malodorous substances [ 23 is - 24 ] , the odor control unit 3 in the exhaust gas detected by the odor threshold odor pollutants See Table 2 . Although the emission concentration of malodorous pollutants such as hydrogen sulfide, methyl mercaptan, methyl sulfide and dimethyl disulfide does not exceed the requirements of GB 14554-1993, its mass concentration is greater than its odor threshold. In addition, although 4-ethyltoluene is not included in the control standard, its mass concentration is greater than the odor threshold. These malodorous pollutants may make a greater contribution to the odor concentration of the exhaust gas outside the sewage plant of a refinery and chemical enterprise.
Odor control unit 32 has an outer exhaust malodorous contaminants olfactory clear threshold C OD and odor contributions P I , see Table 3 . The C od of methyl mercaptan in the exhaust outside the three odor control units are all the largest. Among them, the methyl mercaptan C od of the LOT-I unit is the largest, with a value exceeding 3 500. unit outside the exhaust gas P I are more than 0.7; hydrogen sulfide C OD exceeding 100, all odor pollutants P I second highest substance. Therefore, methyl mercaptan and hydrogen sulfide are the key odor-causing substances in the exhaust gas outside the refinery wastewater treatment plant. Further, dimethyl sulfide, dimethyl disulfide, and 4-ethyltoluene C OD of 1 to 10. The above malodorous pollutants contribute the most to the malodor of the wastewater treatment plant. The actual ammonia emitted has the highest mass concentration, but due to its high odor threshold, its malodor contribution is relatively small. Although the mass concentrations of aromatic hydrocarbons, alkanes, halogenated hydrocarbons and oxygenated compounds are also high, their odor contribution is small due to their high odor threshold. The main odor contributors of the waste treatment plant and the sewage treatment plant in this study have a certain degree of similarity [ 25 ] . However, the main odor contributors of different types of sewage treatment plants are not the same: the main odor contributors of municipal sewage treatment plants are hydrogen sulfide, ammonia and dimethyl sulfide [26 is - 27 ]; fermentative pharmaceutical companies sewage treatment plant is mainly contributed by hydrogen sulfide odor, butyl mercaptan and isopropyl mercaptan[ 28 ].
Each type of contaminants to the degree of contamination of the odor contribution see FIG . Among them, organic sulfur compounds have the highest contribution to odor pollution. The P i in the exhaust outside the three odor control units exceeds 70%, and the hydrogen sulfide P i in the exhaust outside the HOT and LOT-Ⅱ units exceeds 25%. Therefore, the key odor-causing pollutants in the refinery wastewater treatment plant are organic sulfur compounds and hydrogen sulfide. In addition, aromatic hydrocarbons, oxygenated compounds and ammonia are also the main odor-causing pollutants. Du Yafeng et al. [ 29 ] classified the odor pollutants of oil refinery wastewater treatment plants into four categories: benzene series, halogenated hydrocarbons, organic sulfur compounds and nitrogen-containing organic compounds. Among them, benzene series and organic sulfur compounds were the main odor-causing pollutants. The results of this study are basically the same. The main odor-causing pollutants of different types of sewage treatment plants are not the same. For example, the main odor-causing pollutants produced by domestic waste composting are oxygen-containing organic matter, followed by aromatic hydrocarbons [ 8 ] ; the main odor-causing pollutants in the automobile manufacturing industry are It is halogenated hydrocarbons and aromatic hydrocarbons, and 1,3-butadiene is its typical odor-causing substance [ 30 ] .
In Table 3 , the contribution value P n of the exhaust gas from the three odor control facilities to the total odor pollution in the refinery wastewater treatment plant areashows that the contribution value of the exhaust gas outside the LOT-I unit to the odor pollution accounts for 76.69%, which is the key cause Source of odor pollution. The P i ofmethyl mercaptan in the exhaust outside the LOT-I unitaccounts for 96.37%, so the methyl mercaptan discharged from the odor control unit of the biological treatment facility should be controlled.
2.3. Health risk assessment
A health risk assessment of some odor pollutants in the exhaust gas outside the odor control facility was carried out. In view of the incomplete information on IUR and RfC of chemical substances, this study only conducted health risk assessment on the 20 odor pollutants detected. The odor control unit 3 exhaust pipe height, displacement, gas temperature and other data, calculates the maximum concentration of each odor pollutants in the ground, and calculated based on each of its odor pollutants HI and the LCR (see Table 4 ). The non-carcinogenic and carcinogenic health risks of the exhaust from the three odor control units, from high to low, are LOT-I unit> LOT-II unit> HOT unit. The HI of the 20 malodorous pollutants is far less than 1.0. Among them, 1,3-butadiene is the malodorous pollutant with the highest non-carcinogenic risk, followed by benzene and perchloroethylene. The HI total of the odor pollutants in the exhaust outside the three odor control units is not much different, all in the order of 10 −4 , and the non-carcinogenic health risk is extremely low. The LCR of odor pollutants emitted by all units is far less than 10 −6 , 1,1,2,2-tetrachloroethane emitted by dibromomonochloromethane and LOT-I units are the odor pollutants with the highest carcinogenic risk, 1,3 -Butadiene and benzene are next. The total LCR of the odor pollutants in the exhaust outside the three odor control units is relatively small, and the total carcinogenic risk is on the order of 10-8 . It can be seen from the above that the sewage treatment plant of the refinery has effectively controlled the carcinogenic and non-carcinogenic health risks of the odor pollutants to the population through the treatment of odor before, and reduced the harm to the refinery employees and nearby residents [ 31 ] .
2.4. Pollution Control Strategy
After the odor pollution of the sewage treatment plant of the refinery and chemical enterprise has been treated, the external exhaust still contains a certain concentration of characteristic odor pollutants, mainly odor-causing substances methyl mercaptan, hydrogen sulfide, methyl sulfide, dimethyl disulfide, 4-ethyl Base toluene, and non-methane total hydrocarbons and ammonia with high pollutant concentration. In order to control odor pollution in the refinery area and protect the health of the population, odor pollution control facilities should be optimized and upgraded to improve the ability to control the above odor pollutants.
The main odor pollutants that HOT units need to control are non-methane total hydrocarbons, hydrogen sulfide, methyl mercaptan, methyl sulfide, dimethyl disulfide, and 4-ethyl toluene. The process adopted by the HOT unit is lye absorption + catalytic combustion, which can increase hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, etc. by optimizing the lye absorption process plan (such as increasing the concentration of lye, enhancing mass transfer, etc.) The absorption efficiency of sulfur-containing substances reduces the concentration of odor emission. In addition, the catalytic combustion process needs to be optimized to improve the removal rate of odorous substances such as non-methane total hydrocarbons and organic sulfur compounds [ 6 ] . When the catalytic combustion can not achieve good removal efficiency, applicability may be considered better, more energy efficient, higher purification efficiency of a regenerative oxidizer (regenerative thermal oxidizer, RTO) process [ 22 , 32 - 33 is ] , to reduce the The escape of malodorous pollutants is harmful to people's health. For the LOT-I unit, the main odor pollutants that need to be controlled are ammonia, methyl mercaptan, hydrogen sulfide, methyl sulfide, dimethyl disulfide, and 4-ethyl toluene. Among them, methyl mercaptan is the main malodorous gas substance in the LOT-I malodorous pollution control unit. The lye absorption process used in the LOT-I unit has limited removal efficiency for ammonia, methyl mercaptan, methyl sulfide, dimethyl disulfide and other malodorous substances, and it is not suitable for the removal of malodorous substances such as 4-ethyltoluene. Remove. Therefore, absorption in alkali of LOT-Ⅰ protection unit is adapted accordingly based on the operational performance of a process, such as adding activated carbon adsorption or catalytic combustion process, etc. [ 34 is - 35 ]. The concentration of odorous pollutants in the exhaust outside the LOT-Ⅱ unit is relatively low, but the emission of odorous substances such as ammonia, hydrogen sulfide and 4-ethyltoluene still needs to be controlled. The process used in the LOT-Ⅱ unit is activated carbon adsorption. In order to further reduce the concentration of ammonia and hydrogen sulfide emissions, the lye absorption process can be increased in a targeted manner, and the operating parameters of the activated carbon adsorption tower can also be optimized to improve the removal rate of 4-ethyltoluene, etc. [ 36 ] .
3. Conclusions and recommendations
1) A total of 42 kinds of odorous substances were detected in the external exhaust gas of the odor pollution control facility of the sewage treatment plant of a refinery and chemical enterprise. The main pollutants were ammonia, organic sulfur compounds, aromatic hydrocarbons and hydrogen sulfide; methyl mercaptan and hydrogen sulfide were It is a key odor-causing substance, and organic sulfur compounds have the highest contribution to the total concentration of odor; in the sewage treatment plant, the odor control unit of the sewage biochemical treatment facility has the highest contribution to the odor.
2) The non-carcinogenic and carcinogenic risks of malodorous pollutants in all external exhausts of the sewage treatment plant are extremely low. Among them, the non-carcinogenic health risk index and lifetime carcinogenic risk of malodorous pollutants in the exhaust of the odor control unit of the sewage biochemical treatment facility are the highest. .
3) It is recommended to optimize the "lye absorption + catalytic combustion" process of the physical and chemical treatment facility odor control unit, or adopt the "lye absorption + regenerative oxidation furnace" process; the biochemical treatment facility odor control unit adds "activated carbon adsorption" or "Catalytic combustion" process; the odor control unit in the sludge dehydration room adds the "lye absorption" process.
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