Method for reducing content of key impurity mononitrophenol in nitrobenzene preparation through benzene nitration

Abstract

The invention provides a method for reducing the content of mononitrophenol which is a key impurity in crude nitrobenzene. The method comprises the following steps: controlling mononitrophenol lt in crude nitrobenzene; 20 ppm; according to the method, the temperature of a nitration reactor is subjected to zoned control, and at least three temperature control areas are included; the temperature area comprises a temperature area A, a temperature area B and a temperature area C; according to the method, by adjusting the temperature and pressure of a sulfuric acid flash evaporator, the mass ratio of circulating sulfuric acid to feeding benzene and the excess rate of the feeding benzene, accurate control over reaction temperature zones is achieved, and mononitrophenol in crude nitrobenzene is controlled to be 1t; the content of mononitrophenol in prepared crude nitrobenzene can be reduced from the source within the range of 20 ppm, timely, rapid and effective regulation and control are achieved through the regulation and control strategy, and long-period safe and stable operation of a follow-up refining system is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of nitration of benzene to prepare nitrobenzene, in particular to a method for reducing the content of key impurity mononitrophenol in crude nitrobenzene. BACKGROUND

[0002] Nitrobenzene is an important basic chemical raw material, especially an important organic synthesis intermediate for preparing aniline and aniline derivative products diisocyanate and polyisocyanate.

[0003] The current commonly used method for industrial production is to use circulating sulfuric acid as a catalyst, benzene and nitric acid as raw materials to produce crude nitrobenzene by adiabatic nitration; the main feature of this process is to use a plunger flow reactor and a special design of a jet element to improve the conversion rate, and at the same time, the heat of the nitration reaction does not need to be cooled, and the circulating sulfuric acid is used as a reaction heat removal medium, and the heat is used to flash concentrate the circulating sulfuric acid, thereby reducing heat consumption. However, due to the existence of over-nitration and oxidation side reactions, by-products such as dinitrobenzene, mononitrophenol, and dinitrophenol are inevitably generated in the production process of nitrobenzene, among which the content of nitrophenol by-products is as high as 1600-3000 ppm, including 2-nitrophenol, 2.4-dinitrophenol, 2.6-dinitrophenol and 2.4.6-trinitrophenol. Nitrophenol substances contain benzene ring, nitro lipophilic group and phenolic hydrophilic group, and are a good surfactant. Due to the presence of only one -NO2 electron repelling group, compared with dinitrophenol and trinitrophenol, mononitrophenol has the largest acidity coefficient and is the most difficult to ionize at the same pH value. Through a large number of device tests and mechanism verification, the inventors found that mononitrophenol is the most difficult to remove in the ammonia washing and alkali washing and dephenolization process of the nitrobenzene refining system, and the emulsification phenomenon of the washing device is directly related to the content of mononitrophenol in the washing water, affecting the high load stable operation of the process. When the nitrophenol is not completely washed and the emulsification condition is not good, the nitrophenol enters the subsequent product rectification system, causing safety risks such as nitrophenol explosion.

[0004] For the current nitration of benzene to prepare nitrobenzene process, the generation amount of key impurity mononitrophenol is controlled by multiple factors, and its content fluctuates greatly, and there is no targeted and systematic monitoring and control means. At present, the running device monitors the amount of mononitrophenol by means of regular sampling analysis, which is complex and has poor timeliness, causing frequent washing and emulsification problems.

[0005] Currently, there are few relevant literatures reported for the control of mononitrophenol index in nitrobenzene. CN102209703B reported an adiabatic process for preparing mononitrobenzene, which uses a mixture of less than 3% nitric acid and 55-80% sulfuric acid, and the initial temperature is in the range of 60-96℃. By controlling the initial temperature and concentration of the mixed acid, the formation of nitrophenol and dinitrobenzene is effectively reduced, and the content of nitrophenol (polynitrophenol, mononitrophenol) is less than 1500 ppm, and the content of dinitrobenzene is less than 100 ppm. Based on the production practice verification, under the conditions described by Noram Company, the nitration reaction rate does not reach the expected value, and under the condition of excessive benzene, the nitric acid in the reactor outlet product still maintains a high level, which affects the thermal safety of the subsequent sulfuric acid concentration system. The content of mononitrophenol, which is a by-product of nitrophenol, is higher than the actual production, and the content is greater than 400 ppm, which is not conducive to the stable operation of the subsequent washing.

[0006] Therefore, it is necessary to establish a strategy for controlling the content of mononitrophenol in crude nitrobenzene in the production process, so as to realize timely, rapid and effective control, and realize long-period safe and stable operation of the subsequent refining system. SUMMARY

[0007] To solve the above technical problems, one of the purposes of the present application is to provide a method for preparing nitrobenzene by benzene nitration, which establishes a strategy for controlling the content of mononitrophenol in crude nitrobenzene in the production process, realizes timely, rapid and effective control, and realizes long-period safe and stable operation of the subsequent refining system.

[0008] In order to achieve the above-mentioned purposes of the application, the technical solutions provided by the present application are as follows:

[0009] A method for preparing nitrobenzene by benzene nitration, which controls the content of mononitrophenol in crude nitrobenzene to be less than 20 ppm; the method controls the temperature of the nitration reactor in zones, and at least contains three temperature control zones; the temperature zones include:

[0010] The reaction starting temperature T1 is the temperature of the recycled sulfuric acid after flashing, and the control temperature is 98-100℃, preferably 98.5-99.5℃;

[0011] Temperature zone A: T2-T1 temperature zone, main nitration reaction zone, T2 temperature monitoring point is set at 30%-40% length pipe section of the reactor, control temperature is 117-120℃, temperature zone A temperature rise control is less than 20.0℃, preferably 118-119℃;

[0012] Temperature zone B: T3-T2 temperature zone, secondary conversion zone of nitration reaction, T3 temperature monitoring point is set at 65%-75% length pipe section of the reactor, control T3 temperature is 127-130℃, control temperature zone B temperature rise is greater than 7.5℃, preferably 128-130℃;

[0013] Temperature zone C: T4-T3 temperature zone, residual raw material conversion and mononitrophenol main conversion area of nitration reaction, temperature point monitoring point is set at the reactor outlet, control T4 temperature 129-134℃, control temperature zone C temperature rise is greater than 1.8℃, preferably 130-132℃.

[0014] The inventors found that in the industrial production of nitrobenzene by benzene nitration, the generation mechanism of nitrophenol by-products can be represented as follows:

[0015] First step: oxidation reaction

[0016] The nitrobenzene is oxidized under the action of sulfate radical SO4* provided by the circulating sulfuric acid to generate mononitrophenol (mainly 2-nitrophenol);

[0017] Second step: dinitration reaction

[0018] The raw material nitric acid undergoes denitration under the action of SO4* to generate active agent nitrogen dioxide radical (NO*), and NO* reacts with mononitrophenol to generate dinitrophenol (2.4-dinitrophenol, 2.6-dinitrophenol);

[0019] Third step: trinitration reaction

[0020] The dinitrophenol is re-nitrated under the action of NO2* to generate trinitrophenol (2.4.6-trinitrophenol);

[0021] The first step of hydroxylation of nitrobenzene to generate mononitrophenol mainly occurs in the front section of the nitration reactor, and the concentration of SO4* is the key factor. Generally, the more the concentration of SO4* is, the higher the efficiency of oxidation reaction is, and the more mononitrophenol is generated. Temperature, sulfuric acid concentration, and acid-oil ratio (the ratio of circulating sulfuric acid and raw material benzene) are directly positively correlated with the amount of SO4* generated. Therefore, in the actual production process, if the above characteristics can be used, the generation of mononitrophenol in the front end can be reduced by controlling the above conditions.

[0022] In addition, since sulfuric acid is used as the catalyst, SO4* produced by the sulfuric acid can activate nitric acid to produce NO2* and nitration reaction to produce nitrobenzene, and the produced nitrobenzene is oxidized to produce mononitrophenol, which is inevitable under the condition of using sulfuric acid as the catalyst. Therefore, in order to reduce the content of mononitrophenol in the crude nitrobenzene, the second and third re-nitration reactions need to be strengthened to convert the mononitrophenol into polynitrophenol which has less influence. It is found that the process of converting mononitrophenol into polynitrophenol is mainly controlled by NO2*, that is, the higher the concentration of nitric acid in the middle and rear sections of the reactor, the more conducive to the production of polynitrophenol. The production process of nitrobenzene by benzene nitration is a benzene excess reaction, and the concentration of nitric acid in the middle and rear sections of the reactor can be adjusted by adjusting the benzene excess rate to control the amount of mononitrophenol converted into polynitrophenol.

[0023] According to the above reaction mechanism, it is found in the operation practice of the device that the content of mononitrophenol in the crude nitrobenzene can be continuously and stably controlled within 20 ppm by precisely controlling the temperature distribution in different regions of the plug flow reactor, and the long-term stable operation of the refining system can be realized.

[0024] In an embodiment of the present application, the method realizes precise control of the reaction temperature partition by adjusting the temperature, pressure, mass ratio of circulating sulfuric acid to feed benzene and mass excess rate of the feed benzene of the sulfuric acid flash evaporator; preferably, the temperature of the sulfuric acid flash evaporator is set to 98-100℃, preferably 98.5-99.5℃; preferably, the pressure of the sulfuric acid flash evaporator is 5-15kpa, preferably 7-9kpa; preferably, the mass ratio of the circulating sulfuric acid to the feed benzene is 19-23, preferably 20-21; preferably, the mass excess rate of the feed benzene is 5.5%-7%.

[0025] In an embodiment of the present application, the reactor of the method is a plug flow tubular reactor, which contains sieve plates or packing mixed elements inside; preferably, the number of mixed elements in the reactor is 8-18, preferably 12-15. The above-mentioned reactor, mixed elements and their number are well known in the art, and those skilled in the art can adjust them according to the needs, and the adjustment mode will not have a significant impact on the subsequent results.

[0026] Another object of the present application is to provide an application of the method for producing nitrobenzene by benzene nitration.

[0027] An application of the method for producing nitrobenzene by benzene nitration, the method is the above-mentioned method, and the method is used to realize precise control of the temperature of the continuous production nitration reactor to obtain a nitrobenzene product with a nitrophenol content of <1ppm.

[0028] Still another object of the present application is to provide a crude nitrobenzene product.

[0029] A crude nitrobenzene product, the product is prepared by the above method, in the crude nitrobenzene product, the mass concentration of mononitrophenol is less than 20 ppm, based on the total mass of the crude nitrobenzene.

[0030] Still another purpose of the present application is to provide a nitrobenzene product.

[0031] A nitrobenzene product, the product is prepared by the above method, the content of nitrophenol in the product nitrobenzene is <1 ppm. The nitrophenol is polynitrophenol and mononitrophenol.

[0032] Compared with the prior art, the beneficial effects of the technical scheme of the present application are that:

[0033] (1) In the continuous production process of nitrobenzene, the key impurity mononitrophenol in the crude nitrobenzene can be stably controlled within 20 ppm for a long time through reaction partition temperature monitoring, and the problems of high complexity and poor timeliness of sample testing are solved.

[0034] (2) The generation amount of the key impurity mononitrophenol is reduced from the reaction end, without the need to modify the equipment or introduce emulsion inhibitors or replace the washing equipment, without modification of the original process, and the process adjustment is simple.

[0035] (3) Through the control of mononitrophenol at the reaction end, the emulsification of ammonia washing and alkali washing caused by mononitrophenol no longer occurs, the explosion risk of nitrophenol precipitation in subsequent product rectification is greatly reduced, and high-load continuous operation of the device is realized.

[0036] (4) In addition, it is accidentally found in the implementation process of the present application that under the condition that the mononitrophenol is reduced to within 20 ppm, the nitrophenol substance in the crude product can be completely removed by two-step ammonia washing, and the subsequent alkali washing can be used as a protective washer, and the washing alkali consumption is reduced by 60%. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 It is a process flow diagram for the production method of benzene nitration to prepare nitrobenzene, in the diagram, the number explanations are as follows:

[0038] 1-mixer, 2-reactor, 3-phase separator, 4-flash evaporator, 5-flash heater, 6-sulfuric acid tank, 7-circulating sulfuric acid pump;

[0039] i-benzene stream, ii-nitric acid stream, iii-circulating sulfuric acid stream, iv-fresh sulfuric acid stream, v-sulfuric acid stream after reaction, vi-concentrated sulfuric acid stream, vii-crude nitrobenzene stream, viii-acid gas;

[0040] T1-reaction starting temperature, T2-temperature monitoring point of temperature zone A, T3-temperature monitoring point of temperature zone B; T4-temperature monitoring point of temperature zone C. DETAILED DESCRIPTION

[0041] The following further illustrates the specific embodiments of the method with examples, it should be noted that the examples are specific description of the preferred embodiments of the present application, and are not a limitation on the scope of protection of the present application.

[0042] <Raw material source>

[0043] Fresh sulfuric acid, commercially available, concentration ≥98wt%;

[0044] Fresh benzene, commercially available, purity ≥99.99wt%;

[0045] Raw material nitric acid, Wanhua Chemical nitric acid device, the concentration of nitric acid in the examples and comparative examples of the present application is 65wt%.

[0046] <Device>

[0047] In the following examples and comparative examples, the existing continuous process and continuous device for preparing nitrobenzene by adiabatic nitration of benzene are taken as examples to sample the flow on the production device, and the process for preparing nitrobenzene is as follows:

[0048] The nitration reaction system uses sulfuric acid as a catalyst, which removes the reaction heat by circulating in the system at a large flow rate through a sulfuric acid circulating pump. The raw material nitric acid aqueous solution is mixed with the circulating sulfuric acid at the inlet of the circulating sulfuric acid pump, and then fully mixed with the raw material benzene at the reactor inlet mixing element before entering the plug flow tubular reactor (12 blocks of packing mixing element). The benzene nitration reaction occurs under the catalysis of sulfuric acid to generate nitrobenzene. The obtained reaction liquid is separated into a sulfuric acid phase and a nitrobenzene organic phase in a phase separator. The separated sulfuric acid phase enters a flash evaporator for concentration, and the water generated in the nitration reaction is removed by flash evaporation under a low pressure environment, so that the circulating sulfuric acid is concentrated to recover 68-72%. The sulfuric acid after flash evaporation is stored in a sulfuric acid tank and recycled for participation in the nitration reaction.

[0049] In the continuous process and continuous device used in sampling in each example and comparative example, the device for preparing nitrobenzene by nitration reaction and its process flow as shown in Figure 1 are included.

[0050] <Testing method>

[0051] The component analysis method uses liquid chromatography (instrument: Agilent 1260), including:

[0052] 1) Standard curve establishment: mononitrophenol (2-nitrophenol, 4-nitrophenol) is weighed, diluted with methanol to prepare a standard, and diluted into samples of different concentrations. Liquid phase analysis is performed, and a standard curve is plotted on Excel, and an external standard equation is fitted.

[0053] 2) Weigh 1 g of the crude nitrobenzene to be tested (accurate to 0.001 g), add 0.1% sodium hydroxide solution 10 g and dichloromethane 9 g, after centrifugation, take 0.1 g of the aqueous phase, add 0.1% phosphoric acid solution to make it acidic, dilute with methanol to 2 g, then take 1 ml into a liquid phase vial for liquid phase analysis;

[0054] 3) According to the peak area of the substance, the content of mononitrophenol is calculated by the external standard equation.

[0055] The content of polynitrophenol is tested by similar characterization means.

[0056] Example 1

[0057] Under the pressure of 8.2 kpa of sulfuric acid flash evaporator, the circulating sulfuric acid temperature T1 is controlled to be 98.8℃ by adjusting the steam amount of the flash evaporator bottom heater, the mass ratio of circulating sulfuric acid to raw material benzene is 20.3, the benzene excess rate is 6.2%, the nitration reaction is carried out in the nitration reactor, the reaction section temperature monitoring point T2 is 118.6℃, the temperature zone A temperature rise is 19.8℃, the reaction section temperature monitoring point T3 is 128.2℃, the temperature zone B temperature rise is 9.6℃, the reaction section temperature monitoring point T4 is 130.6℃, the temperature zone B temperature rise is 2.4℃, the content of mononitrophenol in the crude nitrobenzene in this stage is <1 ppm (lower than the instrument detection limit), and the total amount of nitrophenol in the product nitrobenzene is <1 ppm (lower than the instrument detection limit).

[0058] Example 2

[0059] Under the pressure of 9.2 kpa of sulfuric acid flash evaporator, the circulating sulfuric acid temperature T1 is controlled to be 99.8℃ by adjusting the steam amount of the flash evaporator bottom heater, the mass ratio of circulating sulfuric acid to raw material benzene is controlled to be 20.1, the benzene excess rate is controlled to be 6.0%, the nitration reaction is carried out in the nitration reactor, the reaction section temperature monitoring point T2 is 119.6℃, the temperature zone A temperature rise is 19.8℃, the reaction section temperature monitoring point T3 is 130.0℃, the temperature zone B temperature rise is 10.4℃, the reaction section temperature monitoring point T4 is 132.6℃, the temperature zone B temperature rise is 2.6℃, the content of mononitrophenol in the crude nitrobenzene in this stage is 17 ppm, and the total amount of nitrophenol in the product nitrobenzene is <1 ppm (lower than the instrument detection limit).

[0060] Example 3

[0061] The temperature T1 of the circulating sulfuric acid was controlled to 99.0°C by adjusting the steam amount of the flash evaporator bottom heater, the mass ratio of the circulating sulfuric acid to the raw material benzene was controlled to 21.2, the benzene excess rate was controlled to 5.8%, and the nitration reaction was carried out in the nitration reactor. The temperature monitoring point T2 of the reaction section was 118.5°C, the temperature rise of temperature zone A was 19.5°C, the temperature monitoring point T3 of the reaction section was 129.1°C, the temperature rise of temperature zone B was 10.6°C, the temperature monitoring point T4 of the reaction section was 131.6°C, the temperature rise of temperature zone B was 2.5°C, the mononitrophenol content in the crude nitrobenzene was 3ppm, and the total amount of nitrophenol in the product nitrobenzene was <1ppm (lower than the instrument detection limit).

[0062] Comparative Example 1

[0063] Compared with Example 1, the difference was that the temperature T1 of the circulating sulfuric acid was controlled to 100.5°C by adjusting the steam amount of the flash evaporator bottom heater, and other conditions were the same as those in Example 1. The nitration reaction was carried out in the nitration reactor. The temperature monitoring point T2 of the reaction section was 122.9°C, the temperature rise of temperature zone A was 22.4°C, the temperature monitoring point T3 of the reaction section was 130.9°C, the temperature rise of temperature zone B was 8°C, the temperature monitoring point T4 of the reaction section was 131.3°C, the temperature rise of temperature zone B was 1.4°C, the mononitrophenol content in the crude nitrobenzene was maintained at 151ppm, and the nitrophenol content in the product nitrobenzene was 12ppm.

[0064] Comparative Example 2

[0065] Compared with Example 1, the difference was that the temperature monitoring point T2 was controlled to 122.9°C, the temperature rise of temperature zone A was >20°C, and other conditions were the same as those in Example 1. The temperature T1 of the nitration reaction was 98.8°C, the temperature monitoring point T2 of the reaction section was 122.2°C, the temperature rise of temperature zone A was 23.4°C, the temperature monitoring point T3 of the reaction section was 130.7°C, the temperature rise of temperature zone B was 8.5°C, the temperature monitoring point T4 of the reaction section was 131.6°C, the temperature rise of temperature zone B was 0.9°C, the mononitrophenol content in the crude nitrobenzene was 215ppm, and the nitrophenol content in the product nitrobenzene was 17ppm.

[0066] Comparative Example 3

[0067] Compared with Example 2, the difference lies in that the control temperature point T3 is 126.8℃, the temperature zone B temperature rise is <7.5℃, the mass ratio of the circulating sulfuric acid to the raw material benzene is increased to 27 by increasing the heat removal amount, and other conditions are consistent with those in Example 2. The circulating sulfuric acid temperature T1 is 99.8℃, the reaction section temperature monitoring point T2 is 119.6℃, the temperature zone A temperature rise is 19.8℃, the reaction section temperature monitoring point T3 is 126.8℃, the temperature zone B temperature rise is 7.2℃, the reaction section temperature monitoring point T4 is 128.6℃, the temperature zone C temperature rise is 1.8℃, the mononitrophenol content in the crude nitrobenzene in this stage is detected to be 179ppm, and the total amount of nitrophenol in the product nitrobenzene is 15ppm.

[0068] Comparative Example 4

[0069] Compared with Example 1, the difference lies in that the control temperature point T4 is 128.9℃, the temperature zone C temperature rise is <1.8℃, the benzene excess is controlled to be 8% to reduce the nitric acid content at the end of the reaction, the circulating sulfuric acid temperature T1 is 98.8℃, the reaction section temperature monitoring point T2 is 118.6℃, the temperature zone A temperature rise is 19.8℃, the reaction section temperature monitoring point T3 is 128.2℃, the temperature zone B temperature rise is 9.6℃, the reaction section temperature monitoring point T4 is 128.9℃, the temperature zone C temperature rise is 0.7℃, the mononitrophenol content in the crude nitrobenzene in this stage is detected to be 184ppm, and the total amount of nitrophenol in the product nitrobenzene is 15ppm.

[0070] Comparative Example 5

[0071] Compared with Example 3, the difference lies in that there is no reaction sub-zone monitoring and adjustment, the circulating sulfuric acid temperature T1 is controlled to be 99.0℃ by adjusting the steam amount of the flash evaporator bottom heater under the condition that the sulfuric acid flash evaporator pressure is 9.5kpa, the mass ratio of the circulating sulfuric acid to the raw material benzene is controlled to be 21.2, the benzene excess rate is controlled to be 5.8%, the nitration reaction is carried out in the nitration reactor, the reactor outlet temperature T4 is 129.7℃, the mononitrophenol content in the crude nitrobenzene in this stage is detected to be 36ppm, and the total amount of nitrophenol in the product is 3ppm.

[0072] The above only specifically describes the preferred embodiments of the present application, and does not limit the protection scope of the present application.

Claims

1. A process for the nitration of benzene to nitrobenzene, characterized in that, The method controls the mononitrophenol in the crude nitrobenzene to be less than 20 ppm; The method controls the temperature of the nitration reactor in zones, and at least includes three temperature control zones; The reaction starting temperature T1 is the temperature of the circulating sulfuric acid after flashing, and the control temperature is 98-100°C, preferably 98.5-99.5°C; Temperature zone A: T2-T1 temperature zone, the main nitration reaction zone, T2 temperature monitoring point is set at 30%-40% length of the reactor, the control temperature is 117-120°C, the temperature zone A temperature rise control is less than 20.0°C, preferably 118-119°C; Temperature zone B: T3-T2 temperature zone, the secondary conversion zone of the nitration reaction, T3 temperature monitoring point is set at 65%-75% length of the reactor, the control T3 temperature is 127-130°C, the control temperature zone B temperature rise is greater than 7.5°C, preferably 128-129°C; Temperature zone C: T4-T3 temperature zone, the main conversion zone of the residual raw material and mononitrophenol in the nitration reaction, the temperature monitoring point is set at the outlet of the reactor, the control T4 temperature is 129-134°C, the control temperature zone C temperature rise is greater than 1.8°C, preferably 130-132°C.

2. The method of claim 1, wherein, The method realizes precise control of the reaction temperature in zones by adjusting the temperature, pressure of the sulfuric acid flasher, the mass ratio of the circulating sulfuric acid to the feed benzene, and the excess rate of the feed benzene; Preferably, the temperature of the sulfuric acid flasher is set to 98-100°C, preferably 98.5-99.5°C; Preferably, the pressure of the sulfuric acid flasher is 5-15kpa, preferably 7-9kpa; Preferably, the mass ratio of the circulating sulfuric acid to the feed benzene is 19-23, preferably 20-21; Preferably, the mass excess rate of the feed benzene is 5.5%-7%.

3. The method of claim 1 or 2, wherein, The reactor of the method is a plug flow pipe reactor, which contains sieve plates or packing mixed elements inside; Preferably, the number of mixed elements in the reactor is 8-18, preferably 12-15.

4. The use of a method for producing nitrobenzene by nitration of benzene, the method being the method according to any one of claims 1-3, the method being used to realize precise temperature control of the nitration reactor for continuous production, so as to obtain a nitrobenzene product with a nitrophenol content of less than 1 ppm.

5. A crude nitrobenzene product, prepared by the process of any one of claims 1 to 3, characterized in that, In the crude nitrobenzene product, the mass concentration of mononitrophenol is less than 20 ppm, based on the total mass of the crude nitrobenzene.

6. A nitrobenzene product produced by the process of any one of claims 1-3, characterized by, The nitrophenol content in the product nitrobenzene is less than 1 ppm.

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