JPH0759544B2 - Acrylonitrile purification and recovery method - Google Patents

Description

BACKGROUND OF THE INVENTION There are many methods for producing acrylonitrile.
This compound has become one of the most important and promising organic chemical intermediates available. It is a particularly desirable intermediate in the manufacture of a wide range of products (eg plastics, synthetic rubbers, synthetic fibers, soil additives, etc.). For many applications acrylonitrile must be of high purity and for this reason commercial production of acrylonitrile must meet stringent specifications.

"Each commercial method used to prepare acrylonitrile produces impurities and by-products from itself.
Each procedure has its own purification problems. "
(See US Pat. No. 3,459,639).

Different purification methods are required in that different methods of producing acrylonitrile result in the formation of different by-product impurities. Therefore, different methods may be required to remove this impurity and purify the acrylonitrile. Due to these constraints, none of the established acrylonitrile purification methods appear universally interchangeable in terms of their utility in purifying all acrylonitrile-containing compositions.

As shown in US Pat. No. 4,404,064, one very good and commercially practiced process for producing olefinically unsaturated nitriles is the catalytic reaction of ammonia with olefins. For example, acrylonitrile and methacrylonitrile can be produced by catalytic oxidation of propylene and isobutylene in the gas phase in the presence of ammonia, respectively. These methods produce a significant amount of impurities. The formation of acrylonitrile from ammonia and propylene results in the formation of significant amounts of acetonitrile, propionitrile, acetone, etc. It is necessary to remove the impurities of these byproducts to produce unsaturated nitriles suitable for polymerization with other products.

Borrel et al., U.S. Pat. Method) is disclosed. Separation of acrylonitrile from acetonitrile is carried out by extractive distillation at least at pH 5 (preferably 5-7), introducing an alkalizing agent into the distillation mixture.

Wu, U.S. Pat. No. 4,377,444, relates to the recovery and purification of olefinic nitriles, and more particularly to an improved process for recovering and purifying olefinic nitriles (eg, methacrylonitrile and acrylonitrile). Here, the olefinic nitrile is obtained from a mixture of the olefinic nitrile and the following substances.
Produced by ammoxidation of isobutylene and propylene: acetonitrile, hydrogen cyanide, propionitrile, butyronitrile, methacryloline, acrolein, acetone, acetaldehyde and the like.

Wu points out the following: When an olefin (eg, isobutylene or propylene) is subjected to a reaction with ammonia or molecular oxygen in the gas phase at elevated temperature in the presence of an ammoxidation catalyst, the corresponding olefinic nitrile (eg, methacrylonitrile) Nitriles and acrylonitrile) are by-products of the ammoxidation reaction (including acetonitrile, hydrogen cyanide, propionitrile, butyronitrile, methacrolein, acrolein, acetone, acetaldehyde), a mixture of the desired olefinic nitriles, And some of these by-products appearing in the eluate of the ammoxidation reactor, with varying amounts.

According to Wu's method, the products of the ammoxidation reaction are recovered by absorption in water in the first stage. During this stage, some heavy or high boiling organic compounds are formed by the polymerization, condensation, etc. of some of the light organic products. Therefore, this Wu method is an improved method for separating olefinic nitriles not only from by-products formed in the ammoxidation reaction but also from heavy organic compounds.

The method disclosed in US Pat. No. 3,051,630 to Hadley et al. Also relates to the purification of acrylonitrile. However,
This method is particularly applicable to the purification of acrylonitrile produced by the catalytic gas phase reaction of acrolein with ammonia and molecular oxygen. In such reactions, crude acrylonitrile is usually diluted in dilute aqueous solutions (which can also be mixed in varying amounts) by contacting the gaseous reaction product with water, preferably after neutralizing any unreacted ammonia. Containing acrolein and hydrogen cyanide)
Can be collected in the form of

Once a purified form of acrylonitrile is obtained, the acrylonitrile monomer is used to produce various products as shown above. The present invention relates to a method for purifying acrylonitrile from impurities when an excess of acrylonitrile is subjected to reaction with other reactants to produce a product. More specifically, excess acrylonitrile (which is an intermediate in its own right)
The product produced using
Methyl propane sulfonic acid, which is sold by Lubrizol Corporation under the trademark AMPS ^R. A stoichiometric excess of acrylonitrile is used during the production of AMPS ^R. Thus, even after the desired product is formed and separated, various amounts of acrylonitrile remain unreacted and are present along with other impurities. These impurities are often highly reactive with acrylonitrile and cause polymerization. This polymerization is more particularly an undesired copolymerization of acrylonitrile monomer units with monomeric units of impurities present. The unreacted acrylonitrile present together with impurities
In the synthesis of acrylamido-2-methylpropanesulfonic acid, the product obtained, if simply recycled, is
It does not have the desired purity and does not meet the desired specifications. Moreover, such impure acrylonitrile initiates unwanted polymerization. Moreover, all reaction products (eg, 2-acrylamido-2-methylpropanesulfonic acid) produced from such impure acrylonitrile must have the desired specifications (eg, no water-insoluble polymer particles, purity). Etc.) does not have. Accordingly, the present invention develops an improved method for purifying such unreacted acrylonitrile.

As pointed out above, in the production of acrylonitrile, the method of purifying acrylonitrile varies based on factors such as impurities present with acrylonitrile. The present invention differs from the methods discussed above in the following respects: The methods discussed above generally relate to purifying waste products from acrylonitrile products. In contrast, the method of the present invention uses unreacted waste streams generated during the production of a product (eg, 2-acrylamido-2-methylpropanesulfonic acid) produced using an excess amount of acrylonitrile. To purifying acrylonitrile of. In other words, the present invention is directed to a method of purifying unreacted acrylonitrile when the acrylonitrile is used as a reactant to produce other products. The prior art as discussed above is generally aimed at purifying acrylonitrile from the unreacted reactants used to make acrylonitrile. These unreacted reactants formed during the production of acrylonitrile are generally 2-acrylamido-2
Less reactive than the impurities present with unreacted acrylonitrile obtained from the production of methylpropanesulfonic acid. The higher the reactivity of these impurities, the greater the need to remove such impurities.

Different impurities are obtained when different reactants are used. Therefore, neither method is necessarily interchangeably useful in purifying acrylonitrile from all types of impurities.

SUMMARY OF THE INVENTION Excess acrylonitrile is combined with sulfuric acid and isobutene to synthesize 2-acrylamido-2-methylpropanesulfonic acid. The resulting reaction product contains 2-acrylamido-2-methylpropanesulfonic acid, along with significant amounts of unreacted acrylonitrile and other by-products. This 2-acrylamide-
2-Methylpropanesulfonic acid is a mixture of various residual acids,
It can be separated leaving acrylonitrile present along with acrylamide and other by-product impurities. This impurity is present in an amount of about 1-2% by weight, based on the weight of the composition.

The acid impurities include sulfuric acid, isobutylene monosulfonic acid, isobutylene disulfonic acid and small amounts of 2-acrylamido-2-methylpropanesulfonic acid, t-butylacrylamide and acrylamide. 1-2
If acrylonitrile containing% impurities (eg residual acid) is reused in the production of 2-acrylamido-2-methylpropanesulfonic acid, the resulting product (ie, 2-acrylamido-2-methyl) Propane sulfonic acid) has various undesirable properties. For example, the resulting product contains undesired polymers formed by the polymerization of acrylonitrile monomers and impurity monomers. Therefore, it is advisable to purify this acrylonitrile before reuse. The present invention is
It is aimed at such purification methods and the products obtained therefrom.

The amount of impurities (eg residual acid and acrylamide) contained in the acrylonitrile product purified according to the method of the present invention is negligible. Therefore, the purified acrylonitrile itself has a desired average molecular weight and excellent storage stability. Furthermore, this purified acrylonitrile can be utilized in the production of other substances. For example, this material is recycled for use in producing 2-acrylamido-2-methylpropane sulfonic acid, which also has high purity. This high purity is
This is particularly important because it is obtained by removing impurities that are particularly reactive with acrylonitrile. Such reactive impurities react with acrylonitrile to form a water-insoluble polymer. This polymer is 2-acrylamido-
The aqueous solution prepared by including 2-methylpropanesulfonic acid and water is clouded.

The main object of the present invention is to provide a method for purifying an impure acrylonitrile waste stream resulting from the synthesis of a compound produced by reacting the reaction components with an excess of acrylonitrile to obtain a product.

A feature of the invention is that the residual acid present with such unreacted acrylonitrile is neutralized by treatment with a base (eg lime or ammonia) and the salt formed is removed. Included.

Another object of the present invention is to provide an economical and efficient method for purifying acrylonitrile using vacuum and low temperature.

An important feature of the present invention is that such waste stream type impure acrylonitrile can be purified while avoiding polymerization of acrylonitrile during purification.

An advantage of the present invention is to provide a method for purifying acrylonitrile that is environmentally acceptable and approved by environmental law.

Another advantage of the present invention is to provide a method for purifying acrylonitrile which has a significantly reduced polymerization and the salts formed are removed early in the process, thus reducing the accumulation of precipitates in the equipment. is there.

An important feature of the present invention is that unreacted acrylonitrile used in producing 2-acrylamido-2-methylpropanesulfonic acid can be reused to produce a purified product such as: The purified product is provided with, for example, desired properties such as storage stability, and water-insoluble high molecular weight polymer, which is compatible with acrylonitrile and various reactive impurities. 2-acrylamido-2-methylpropanesulfonic acid, which is formed by the reaction) and is not present).

Yet another object of the present invention is to provide a process for producing purified acrylonitrile which itself has desired properties. The desired properties include, for example, the absence of water insoluble high molecular weight polymers, and improved and unexpectedly superior storage stability.

Another feature of the invention is to provide a purified acrylonitrile having the desired properties and capable of forming 2-acrylamido-2-methylpropanesulfonic acid, which can be a clear aqueous solution. Especially.

These and other objects, advantages and features of the invention will be apparent to those of ordinary skill in the art upon reading the disclosure of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a design diagram showing the outline of a purification system used in a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The chemical product 2-acrylamido-2-methylpropanesulfonic acid (trademark AM of Lubrizol Corporation.
Is produced as a product of PS ^R) are commercially used in a variety of different purposes. The 2-aculamide-2-methylpropanesulfonic acid product and other products can be produced by reacting excess acrylonitrile with sulfuric acid and isobutylene. Due to economic and environmental factors, it is desirable to reuse unreacted acrylonitrile to produce more 2-acrylamido-2-methylpropanesulfonic acid. However, unreacted acrylonitrile is present with other impurities such as acrylamide and residual acid; and, as such, is poorly storage stable and produces high quality 2-acrylamido-2-methylpropanesulfonic acid. Cannot be reused when manufactured. If not purified to the required extent, acrylonitrile reacts with the impurities present therein to form water-insoluble compounds. In order to produce a high quality 2-acrylamido-2-methylpropanesulfonic acid product, and to obtain effective use of acrylonitrile, the present inventors have found that unreacted impure acrylonitrile waste stream , Which occurs in the production of 2,2-acrylamido-2-methylpropanesulfonic acid).

The desired 2-acrylamido-2-methylpropanesulfonic acid product formed is not completely removed when an excess of acrylonitrile is used in the preparation of 2-acrylamido-2-methylpropanesulfonic acid Impurities such as sulfuric acid, isobutylene monosulphonic acid, isobutylene disulphonic acid, acrylamide, and small amounts of 2-acrylamido-2-methylpropanesulphonic acid can be removed leaving acrylonitrile present with 1-2% by weight. This acrylonitrile is the impure waste stream acrylonitrile mentioned above.

We have used acrylonitrile with strong bases (eg, lime (ie, calcium oxide), NaOH, NH ₃ , or Ca) to effectively purify unreacted acrylonitrile.
It was first found desirable to chemically treat the acrylonitrile by neutralizing it with (OH) ₂ ). When neutralizing with lime (ie calcium oxide), the amount of lime added depends on the acid neutralization number of the acrylonitrile to be treated. A certain effect can be obtained by using 0.5 to 1.5 equivalents of lime per impure acrylonitrile acid equivalent, but per acrylonitrile acid equivalent
It is generally undesirable to use less than 1.0 equivalent of lime. Furthermore, the inventor has found that about 1.1 equivalents of lime per acid equivalent of impure acrylonitrile are particularly preferred in the following respects: With less than 1.0 equivalents of lime all The acid is not completely neutralized and the use of lime above 1.1 equivalents is not economical.

When using lime for neutralization, water must be present. The solubility of water in acrylonitrile is about 3.2%
Nothing more. Therefore, water below 3.2% can be used. The inventors have found that the use of more than about 2.7% water makes the calcium salt viscous and difficult to handle. It has also been found that at least about 0.5% water should be present to provide an effective neutralization step. Therefore, the water is preferably about 1.0-2.5.
%, More preferably about 1.5% to about 2.0%.

Anhydrous ammonia can be used, but lime is not effective unless it is used in combination with a catalytic amount of water, as mentioned above. Unreacted acrylonitrile treated with base is neutralized to form a salt. For example, treatment with lime in the presence of catalytic amounts of water forms calcium salts. Some of the salts that form precipitate quickly and can be easily removed with unreacted lime. Any salts that do not precipitate immediately can be removed by settling or by subjecting the base treated material to mechanically assisted separation means such as centrifugation and / or filtration.

After applying one or more of the mechanical separation means mentioned above, most of the salts formed and unreacted lime are removed and the acrylonitrile is purified to some extent. However, at this point, acrylonitrile still contains about 1.5% or less impurities. This impurity must be removed by other physical means (ie various distillations).

Referring to the drawings, a base-treated acrylonitrile (which had salts removed by precipitation and / or various mechanical means) is fed to heat exchanger 2 via line 1. It Feed rates may vary depending on the particular equipment and implementation used. The present inventors have developed about 151 liters to 379 liters / minute (about 40 to 10 liters / minute).
A feed rate of 0 gal / min) is useful, about 265 liters /
We have found that minutes (about 70 gal / min) are preferred. The heat exchanger 2 is preferably a single pass shell and tubular heat exchanger. The inventors have found that it is particularly useful to use a 316 stainless steel pass shell and tubular heat exchanger that is approximately 750 square feet. The impure acrylonitrile in the heat exchanger 2 is heated to the outer shell 3 of the heat exchanger 2 by a heating fluid (which is preferably in the form of saturated steam, under reduced pressure of about 85
C. to about 102.degree. C. (maintained at a temperature in the range of about 185.degree. F. to about 215.degree. F.) is continuously circulated.
At 320 mmHg, a temperature below 78 ° C (172 ° F), preferably about 43 ° C to about 60 ° C (about 110 ° F to about 140 ° F),
More preferably maintained at about 120 ° F (49 ° C).

At atmospheric pressure, the temperature of the acrylonitrile must be kept below 78 ° C (172 ° F) to prevent polymerization. The inventors have found that it is desirable to maintain the temperature in the range of about 43 ° C to about 60 ° C (about 110 ° F to about 140 ° F) while maintaining the pressure under a reduced pressure of about 320 mmHg.

Steam temperature and pressure rise as the internal components of the heat exchanger collide. However, due to this temperature level, acrylonitrile does not undergo a significant amount of polymerization and the fouling of the components in contact with acrylonitrile is greatly reduced. This reduction in polymerization and deposition, which accumulates on the inner surface in contact with acrylonitrile, is a substantial advantage of the present invention. This heating fluid is flown around the shell 3 by the charging line 4. Here, the heating fluid circulates around the shell 3 and then via the discharge line 5 to the outlet. This keeps the acrylonitrile at a relatively constant temperature. Many different factors are interrelated and are determined by whether acrylonitrile reacts with impurities. As the temperature, pressure, amount of impurities, and degree of reactivity of impurities increase, the likelihood of formation of unwanted polymers and copolymers with the impurities themselves and / or acrylonitrile increases.

This acrylonitrile becomes a heated fluid substance containing both liquid and vapor substances, along with impurities. Some impurities have been removed from the vapor portion of this material. From the heat exchanger 2, the liquid / vapor material falls below the line 6 (here 78 ° C (172 ° F), preferably about 41 ° C).
To about 52 ° C (about 106 ° F to about 126 ° F), more preferably about 47 ° C (about 116 ° F), to the lower region of the steam column 7. Fried by. The tower 7 is composed of a cylinder wall portion. The inner part of it consists of several distillation trays 8,8 ',
It has a bottom opening 9 and a top opening 10, such as 8 ″, 8. This tower 7 can vary in size, shape, and number of trays. A tower, which has 6 trays, is 19 feet 8 inches high and is 6 feet in diameter, has been found to work particularly well, these trays being located about 2 feet inside. This tower 7 can be maintained under vacuum.
However, this column is maintained so that a pressure difference is obtained between the inside of the column and the outlet 11. That is, the internal pressure of the tower 7 is higher than the pressure at the outlet 11. As a result, pure vaporized gas (acrylonitrile) flows freely from the column 7 via the outlet 10 to the line 11. The present inventors have found that about 320 mmHg
The internal temperature of the tower is about 43 ℃ to about 60 ℃ (about 110 ℃ to about 1 ℃).
It has been found that this step works particularly well by maintaining a range of 40 ° F, more preferably at 49 ° C (120 ° F). This temperature and pressure range is preferred for many reasons. However, this temperature is about 78 ° C (about 17 ° C).
It should be noted that the melting point of acrylonitrile at atmospheric pressure is not higher than 2 ° F).

Of the liquid / vapor material added to column 7 via line 6, almost all of the liquid portion falls to the bottom of column 7 and is discharged via opening 9. Pure acrylonitrile has a low boiling point, rises to the top of the column and is removed from the opening 10 leading to line 11. The high-boiling-point acrylonitrile, which is present together with impurities, falls to the bottom of the column 7 and is removed from the opening 9 via the line 12. According to the first embodiment of the invention, almost all the material removed via line 12 is pumped into line 13 into heat exchanger 2 and
Therefore it is recycled. A small amount of bottom with many impurities
Removed from the bottom of tower 7.

According to the second embodiment of the invention, the material removed via line 12 is divided. Most of that material is line
It is recycled to the heat exchanger 2 via 13 (this line 13 can be directly connected to the heat exchanger 2 or fed back via the feed line 1). Then, a small portion of the material from line 12 is directed to thin film evaporator 15 via line 14. The second embodiment also includes removing impure bottoms via line 18 from the bottom of column 7 as well as from the bottom of thin film evaporator 15. The process of the present invention is economically advantageous in the following respects: It enables economical recovery of large amounts of acrylonitrile without evaporation of most of the liquid feed entering the heat exchanger 2. This process also greatly reduces the formation of polymeric material in the equipment by keeping the temperature low and reducing the portion of the feed to be vaporized.

The evaporator 15 has a temperature of less than 78 ° C (172 ° F) at about 320 mmHg, preferably about 49 ° C to about 63 ° C (about 120 ° F to about 145 ° C).
The temperature is maintained in the range of about 120 ° F, more preferably about 49 ° C (120 ° F). At atmospheric pressure, this temperature is about the melting point of the related substance (ie, the impurity acrylonitrile), which is about 78 ° C (172 ° F). The inventors have found that a 5.0 m ² (53.8 square feet) 316 stainless steel evaporator works particularly well. The evaporator is maintained at this temperature by continuously circulating the fluid along the outer surface with the fluid entering through the inlet line 16 and exiting through the outlet line 17. The heating fluid entering and exiting via lines 16 and 17 preferably does not enter the evaporator and is merely provided to control the temperature within the evaporator. The inside of the evaporator is maintained under reduced pressure. High boiling materials containing large amounts of impurities fall to the bottom of the evaporator and are removed via line 18 for further processing and / or removal. The partially purified low boiling acrylonitrile is withdrawn via line 19 connected to the upper opening 20 of the evaporator 15. The partially purified acrylonitrile transferred via line 19 is then returned to the distillation column 7 and enters this column 7 at point 215. This point 21 is preferably above the lower region of the tower where liquid / vapor material enters line 6 or the tower, but may be below or at the same height. Point 21
Is preferably below the column tray. Line 19
Most of the partially purified acrylonitrile product entering the reactor has a relatively low boiling point and is therefore withdrawn from the top of the distillation column and discharged via line 11. The purified substance withdrawn from the upper part of the distillation column 7 via the opening 10 and the line 11 is highly purified. The purified material has improved and unexpectedly superior properties (eg, molecular weight and storage stability). This substance is 2-acrylamido-2-
It can be utilized in producing methyl propane sulfonic acid, which also has the desired average molecular weight and has improved and unexpected storage stability.

The invention is disclosed and described herein in what is considered to be the most preferred embodiments. However, after reading this specification, those skilled in the art can make various modifications,
And it should be pointed out that such variations are considered to be included in the present invention.

Front Page Continuation (72) Inventor Hambrick, James L USA Ohio 44026 Chesterland, Wilson Mills Road 9431

Claims (17)

[Claims] 1. A method for purifying and recovering acrylonitrile present in a mixture containing a major amount of acrylonitrile and a small amount of acid and acrylamide, which method comprises the steps of: To remove the salt formed to obtain a treated mixture; the treated mixture in a heat exchanger at a temperature above the melting point of the treated mixture but at 78 ° C. Heating under vacuum to a temperature below (172 ° F.) and supplying heated fluid material; placing the fluid material in a distillation column maintained in vacuum;
And removing the purified low-boiling acrylonitrile from the column. 2. The method according to claim 1, wherein
Obtaining the treated mixture comprises treating the mixture with lime in the presence of a catalytic amount of water and removing the neutral calcium salt formed to obtain the treated mixture. And the distillation column is maintained at a temperature above the melting point of the treated mixture but below 78 ° C (172 ° F). 3. The method according to claim 1, wherein
The method wherein the mixture is a waste product produced by the production of 2-acrylamido-2-methylpropanesulfonic acid. 4. The method according to claim 3, wherein
A process wherein the production of 2-acrylamido-2-methylpropanesulfonic acid comprises the reaction of isobutene, sulfuric acid and excess acrylonitrile. 5. The method according to claim 1, wherein
The treated mixture is heated at 43 ° C to 60 ° C in the heat exchanger.
Heated to a temperature in the range of 110 ° F. to 140 ° F., and wherein the fluid material enters the distillation column in the lower region of the distillation column, the distillation column comprising a plurality of distillation trays, It has an upper opening when the purified acrylonitrile is withdrawn and a lower opening through which the fluid substance is withdrawn, while the purified acrylonitrile is removed from the upper opening, the fluid substance with a higher boiling point is opened at the lower opening. Pulled out of the department,
The method, which is returned to the heat exchanger. 6. The method according to claim 5, wherein
The method further comprising the steps of: placing a small portion of the high boiling fluid material withdrawn from the lower opening into a thin film evaporator;
The small portion of the fluid material enters the evaporator in the lower region of the evaporator, which is under vacuum at 49 ° C to 63 ° C (12 ° C).
A temperature in the range of 0 ° F to 145 ° F); removing high boiling impure fluid material from the lower opening of the evaporator; lower boiling portion from the upper opening of the evaporator Withdrawing the partially purified fluidic material; and returning the withdrawn partially purified fluidic material to an upper region of the column where the fluidic material enters the column from the heat exchanger. And, while the purified acrylonitrile is removed from the column and recovered, continues to be distilled in the column. 7. The method according to claim 1, wherein
The method wherein the total amount of the acid and acrylamide is less than 2% by weight of the total amount of the acrylonitrile, acid and acrylamide. 8. The method according to claim 1, wherein
The acid is sulfonic acid, isobutylene monosulfonic acid, isobutylene disulfonic acid, and 2-acrylamido-
The method is at least one selected from the group consisting of 2-methylpropanesulfonic acid. 9. A method according to claim 1, 5 or 6, wherein the base is lime in the presence of catalytic amount of water and is removed. Is a calcium salt. 10. A method according to any one of claims 1, 5 or 6, wherein the base is ammonia and the salt removed is an ammonium salt. 11. The method according to claim 10, wherein the ammonium salt is removed by centrifugation.
Method. 12. A method according to claim 10, wherein:
The method wherein the ammonium salt is removed by filtration. 13. The method according to claim 10, wherein:
The method wherein the ammonium salt is removed by precipitation. 14. Claims 1, 2, 5 or 6
A method according to any of paragraphs, wherein the salt is removed by mechanical means selected from the group consisting of centrifugation, filtration and sedimentation. 15. The method according to claim 5, wherein the base is lime present in water, and the lime is
The method, which is present in an amount of 0.5 equivalents to 1.5 equivalents per acid equivalent of the mixture. 16. The method according to claim 15, wherein the water is 1.0% by weight to 2.5% by weight of the mixture.
The method, which is present in an amount of% by weight. 17. The method according to claim 16, wherein the lime is present in an amount of 1.1 equivalents per equivalent of acid,
And the water is 1.5% by weight based on the weight of the mixture.
The method, which is present in an amount of 2.0% by weight.