WO2019097123A1

WO2019097123A1 - A process for producing aqueous acrylamide solution, aqueous acrylamide solution and use thereof

Info

Publication number: WO2019097123A1
Application number: PCT/FI2018/050845
Authority: WO
Inventors: Ville HAKKARAINEN; Marko Laakkonen; Jaakko SIMELL
Original assignee: Kemira Oyj
Priority date: 2017-11-20
Filing date: 2018-11-20
Publication date: 2019-05-23

Abstract

The present invention relates to a process for producing aqueous acrylamide solution by hydrating acrylonitrile in an aqueous solution in the presence of a biocatalyst. The present invention further relates to aqueous acrylamide solution and use thereof. Aqueous solution of acrylamide in high concentrations is produced with new acrylonitrile feed and process temperature profiles.The temperature and the acrylonitrile feed rate are maintained high at the beginning of the reaction to achieve fast reaction rate and short synthesis time.

Description

A PROCESS FOR PRODUCING AQUEOUS ACRYLAMIDE SOLUTION, AQUEOUS ACRYLAMIDE SOLUTION AND USE THEREOF

Field of the invention The present invention relates to a process for producing aqueous acrylamide solution by hydrating acrylonitrile in an aqueous solution in the presence of a biocatalyst. The present invention further relates to aqueous acrylamide solution and use thereof. Background art

Acrylamide (AMD) has been on market since mid-1950s and acrylamide market has grown steadily since that time. Acrylamide is used primarily in the production of polyacrylamide, which is used in many applications fields of water treatment, crude oil recovery, papermaking industry and mining processes. Acrylamide is produced from acrylonitrile (AN) by hydrolysis reaction in the presence of a catalyst.

Conventionally, production of acrylamide was based on chemical catalyst process, sulfuric acid-catalyzed hydration reaction and copper-catalyzed hydration which slowly replaced the sulfuric acid process. Enzyme catalyzed acrylamide production processes were developed in 1980s. Advantages in contrast to the conventional process are low reaction temperature, atmospheric operating pressure, complete conversion, low by-product selectivity and easy downstream processing.

There are two types of nitrile-degradation metabolic pathways existing in micro- organisms; nitrilase and nitrile hydratase (NHase) pathways. Three different enzymes are involved in these reaction; nitrilase, (NHase) and amidase.

Nitrilase catalyzes the hydrolysis reaction of nitriles directly into corresponding carboxylic acid and ammonia products. In NHase pathway, NHase and amidase react in series. NHase hydrolyzes nitriles into the corresponding amide product at first. In the presence of amidase, amides may be further converted into corresponding acid and ammonia products. Methods for producing acrylamide from acrylonitrile in the presence of a biocatalyst, e.g. nitrile hydratase are described in numerous patent publications.

For example, EP 2749637 B1 relates to a bacterial strain belonging to the genus Rhodococcus which is a producer of a nitrile hydratase. The document also relates to a method for producing acrylamide by hydration of acrylonitrile using a biomass of the bacterial strain.

Document US 2013/059349 A1 relates to a method for producing acrylamide from acrylonitrile by a microbially-derived biocatalyst having nitrile hydratase.

Even though there has been developed numerous methods for producing acryla- mide from acrylonitrile in the presence of a biocatalyst, there is still a need for a novel more simple and efficient process for producing acrylamide in high concen- trations.

Summary of invention

An object of the present invention is to provide a process for producing acrylamide from acrylonitrile in the presence of a biocatalyst.

A further object of the present invention is to provide a process for producing an aqueous solution of acrylamide, from acrylonitrile in the presence of a biocatalyst, in high concentrations.

Another object of the present invention is to provide a process for producing acrylamide from acrylonitrile in the presence of a biocatalyst, wherein consumption of the biocatalyst is reduced.

Yet, another object of the present invention is to provide a process for producing acrylamide from acrylonitrile in the presence of a biocatalyst, wherein acrylonitrile accumulation to reactor is avoided.

A further object of the present invention is to provide a more cost-efficient process for producing acrylamide from acrylonitrile in the presence of a biocatalyst.

The inventors have surprisingly found that aqueous solution of acrylamide in high concentrations (at least 50 wt%) can be produced with novel acrylonitrile feed and process temperature profiles. Cooling of the reactor is typically needed to keep the reaction mixture in a reaction temperature. The temperature and the acrylonitrile feed rate are kept high at the beginning of the reaction to achieve fast reaction rate and short synthesis time. Then the reaction mixture is started to cool down when acrylamide concentration approx. 25 wt% to 38 wt% is reached since biocatalyst deactivation is notably smaller in lower temperatures. Acrylonitrile feed rate is kept small during the last hours of the process to avoid acrylonitrile accumulation to reaction mixture.

As the temperature is kept low at end of the process of the present invention less acrylic acid is formed in the process. Activation energy of reaction forming acrylic acid is higher than activation energy of the main reaction (formation of acrylamide). The amount of acrylic acid in the aqueous acrylamide solution of the present invention is at most 300 ppm, preferably at most 200 ppm, more preferably at most 100 ppm. Low amount of acrylic acid in the aqueous acrylamide solution is advan- tageous when cationic polymers are prepared from the acrylamide solution.

Over 50 % savings of biocatalyst consumption can be achieved by using the process of the present invention compared to former methods. Additionally, low residual acrylonitrile quantity in the final acrylamide solution is achieved with the process of the present invention

The present invention provides a process for producing aqueous acrylamide solution as depicted by claim 1 .

The present invention provides an aqueous acrylamide solution obtainable or obtained by a process of the present invention.

The present invention provides aqueous acrylamide solution as depicted by claim 16.

The present invention provides use of aqueous acrylamide solution as depicted by claim 22.

Brief description of the figures

Figure 1 shows temperature profiles as function of time of the process of the present invention and a comparative process.

Figure 2 shows temperature profiles as function of acrylamide concentration of the process of the present invention and a comparative process. Figure 3 shows acrylonitrile mass fraction as a function of time (5h feed and 1 h maturation time) of the process of the present invention and a comparative process.

Figure 4 shows acrylamide mass fraction as a function of time (5h feed and 1 h maturation time) of the process of the present invention and a comparative process.

Detailed description

According to the first aspect of the present invention there is provided a process for producing aqueous acrylamide solution.

More particularly, there is provided a process for producing aqueous acrylamide solution comprising providing a slurry comprising water and a biocatalyst having Nitrile hydratase activity; feeding acrylonitrile into a reactor comprising said slurry to provide a reaction mixture comprising acrylamide, acrylonitrile and biocatalyst; and maintaining the temperature of the reaction mixture at a temperature range of 15 °C to 25 °C; cooling down of the reaction mixture (e.g. after the reaction mixture has been maintained at a temperature of 15 °C to 25 °C) is started when acrylamide concentration reaches at least 27 wt%, preferably reaches 27 wt% to 38 wt%; cooling of the reaction mixture is continued so that when the acrylamide concentration reaches 37 wt% to 55 wt% temperature of the reaction mixture is within range of 10 °C to 18 °C, or 10 °C to 21 °C; optionally the reaction mixture is matured at the temperature of 10 °C to 18 °C, or 10 °C to 21 °C, (e.g. after the reaction mixture has been cooled until the acrylamide concentration reaches 37 wt% to 55 wt% and the temperature of the reaction mixture is within the range of 10 °C to 18 °C, or 10 °C to 21 °C, and/or after the feeding of acrylonitrile into the reactor has ended) preferably matured until final concentration of the acrylonitrile is at most 1000 ppm, at most 100 ppm, at most 90 ppm, preferably at most 75 ppm, more preferably at most 50 ppm, and most preferably at most 10 ppm.. The slurry may be produced by any known method in the art, such as mixing water and the biocatalyst in a receptacle or a reactor. Preferably the slurry is homogenous. Strongly agglomerated slurry is less active than homogenous slurry. The biocatalyst is more active in homogenous slurry.

The biocatalyst may be fresh i.e. straight from fermentation; stored such as stored as frozen (frozen as wet); or dry before the production of the slurry.

After the fermentation the biocatalyst slurry may often typically be washed, or otherwise or in addition to suitably treated before entering the slurry or before storage, e.g. by freezing.

The biocatalyst may be any biocatalyst having Nitrile hydratase (NHase) activity known in the art.

In accordance with any one of the embodiments of the present invention, the bio- catalyst capable of converting acrylonitrile to acrylamide may be a microorganism which encodes the enzyme nitrile hydratase (NHase) or any part of said microorgan- ism. With this regard, it is not relevant for the present invention whether the microorganism is naturally encoding nitrile hydratase, or whether it has been genetically modified to encode said enzyme, or whether a microorganism naturally encoding nitrile hydratase has been modified such as to be able to produce more and/or enhanced nitrile hydratase. In a very specific embodiment the biocatalyst is a nitrile hydratase (NHase).

In context with the present invention, microorganisms encoding nitrile hydratase (e.g. naturally encoding or genetically modified to encode nitrile hydratase) or any part of said microorganism, which can be used as biocatalyst in any one of the embodiments described herein, comprise species belonging to a genus selected from the group consisting of Rhodococcus, Aspergillus, Acidovorax, Agrobacterium, Bacillus, Bradyrhizobium, Burkholderia, Escherichia, Geobacillus, Klebsiella, Mesorhizobium, Moraxella, Pantoea, Pseudomonas, Rhizobium, Rhodopseudomonas, Serratia, Amycolatopsis, Arthrobacter, Brevibacterium, Corynebacterium, Microbacterium, Micrococcus, Nocardia, Pseudonocardia, Trichoderma, Myrothecium, Aureobasidium, Candida, Cryptococcus, Debaryomyces, Geotrichum, Hanseniaspora, Kluyveromyces, Pichia, Rhodotorula, Comomonas, and Pyrococcus. In preferred embodiments of the invention the biocatalyst is selected from bacteria of the genus Rhodococcus, Pseudomonas, Escherichia, and Geobacillus. Typically, the biocatalyst is selected from the group consisting of Rhodococcus, Aspergillus, Acidovorax, Agrobacterium, Bacillus, Bradyrhizobium, Burkholderia, Escherichia, Geobacillus, Klebsiella, Mesorhizobium, Moraxella, Pantoea, Pseu domonas, Rhizobium, Rhodopseudomonas, Serratia, Amycolatopsis, Arthrobacter, Brevibacterium, Corynebacterium, Microbacterium, Micrococcus, Nocardia, Pseu- donocardia, Trichoderma, Myrothecium, Aureobasidium, Candida, Cryptococcus, Debaryomyces, Geotrichum, Hanseniaspora, Kluyveromyces, Pichia, Rhodotorula, Comomonas, and Pyrococcus, or any part of said microorganism.

In some embodiments the biocatalyst is selected from the group consisting of Rhodococcus, e.g. Rhodococcus pyridinovorans or Rhodococcus rhodochrous or Rhodococcus aetherivorans, Pseudomonas, Escherichia and Geobacillus, or any part of said microorganism.

In preferred embodiments the biocatalyst is Rhodococcus aetherivorans or Rhodo coccus rhodochrous.

In one embodiment amount of the biocatalyst is 0.1 kg dry cells/m³ to 5 kg dry cells/m³ of reaction mixture.

In another embodiment the amount of the biocatalyst is from 0.1 g dry cells/kg 100% AMD to 3 g dry cells/kg 100% AMD, based on the final AMD amount, preferably from 0.2 g dry cells/kg 100 % AMD to 3 g dry cells/kg 100 % AMD, preferably 0.2 g dry cells/kg 100 % AMD to 2.5 g dry cells/kg 100% AMD. In another embodiment the amount of the biocatalyst is from 0.5 g dry cells/kg 100 % AMD to 2 g dry cells/kg 100 % AMD or preferably 1 .1 g dry cells/kg 100 % AMD to 1 .5 g dry cells/kg 100% AMD.

Yet in another embodiment the amount of the biocatalyst is from 0.5 g dry cells/kg 50% AMD to 1 g dry cells/kg 50% AMD, based on the final AMD amount, preferably from 1 .6 g dry cells/kg 50 % AMD to 1 .8 g dry cells/kg 50% AMD.

Yet in another embodiment the amount of the biocatalyst is 0.1 kg dry cells/m³ to 1 .5 kg dry cells/m³ of reaction mixture at the end of the maturation of the reaction mixture. In another embodiment the amount of the biocatalyst is 0.1 kg dry cells/m³ to 1 .0 kg dry cells/m³ of reaction mixture.

During the process more of the catalyst may be added, for example, if acrylonitrile starts to accumulate to the reactor. The feeding of acrylonitrile into a reactor comprising said slurry provides a reaction mixture comprising acrylamide, acrylonitrile and biocatalyst.

The temperature of the reaction mixture is maintained at 15 to 25 °C. In one embodi- ment the temperature is maintained at 19 to 25°C, preferably at 20 to 22°C and most preferably at 22°C. In an embodiment the temperature is maintained in the desired range by measuring the temperature of the reaction mixture and either cooling the mixture or heating the mixture so that the temperature stays in the desired range. The cooling and/or heating of the reaction mixture may be conducted with known methods in the art.

As the feeding of the acrylonitrile is started it reacts to acrylamide in aqueous solu- tion in the presence of the biocatalyst having NHase activity. Thus, a reaction mix- ture comprising acrylamide, acrylonitrile and biocatalyst is formed.

The feeding of the acrylonitrile may be continued throughout the process, preferably is continued throughout the whole process but not during the maturation.

Feed rate of the acrylonitrile may vary during the process.

During the maturation substantially no, preferably no acrylonitrile is fed to the reactor. During maturation acrylonitrile monomers still present in the reaction mixture reacts to acrylamide. The reaction mixture is maturated until desired features are reached.

The feeding may be continuous or intermittently. The feed rate of the acrylonitrile depends on the reaction rate of the acrylonitrile to acrylamide and biocatalyst deactivation.

In one embodiment feeding of the acrylonitrile is continued throughout the process. The feeding may be continuous or intermittently.

Course of the reaction is preferably monitored by on-line measuring at least one of the acrylonitrile and the acrylamide concentrations in the reactor, and preferably temperature of the reaction mixture. The monitoring and measuring may be per- formed with any suitable means and methods in the art.

In one embodiment the acrylonitrile feed rate is adjusted during the process to avoid acrylonitrile accumulation into the reaction mixture. The acrylonitrile is fed during the process with such a rate at which the acrylonitrile converts to acrylamide. Preferably the acrylonitrile amount in the reaction mixture is maintained as less than 2 wt%, preferably less than 1 wt%, more preferably less than 0.5 wt% relative to the total amount of reaction mixture.

In the present invention the temperature and the feed rate of the acrylonitrile are high at the beginning of the reaction to achieve fast reaction rate and short synthesis time. The reactor is started to cool down after, for example, 60 minutes since deactivation is notably smaller in lower temperature. Feed rate is small during the last hours to avoid acrylonitrile accumulation to reactor.

During the maturing of the reaction mixture substantially no acrylonitrile, preferably no acrylonitrile is fed to the reactor.

The inventors found that the biocatalyst starts to deactivate in around 25 wt% to 38 wt% acrylamide solution. Biocatalyst deactivation caused by acrylamide is strongly dependent on temperature and cooling of the reaction mixture reduces biocatalyst deactivation notably.

In one embodiment the cooling of said reaction mixture is started when acrylamide concentration reaches 28 wt% to 30 wt%.

The cooling of the reaction mixture can be performed by any suitable method and means known in the art, such as by cooling the reactor.

When the cooling is started the temperature may be same, higher or lower than in the beginning of the process.

Cooling of the reaction mixture is continued so that when the acrylamide concen- tration reaches 37 wt% to 55 wt% the temperature of the reaction mixture is within range of 10 °C to 18 °C, or 10 °C to 21 °C. In other words, the time period of the cooling to the temperature of 10 °C to18 °C, or 10 °C to 21 °C is the time period when the acrylamide concentration of at least 27 wt% (preferably 27 wt% to 38 wt%) increases to acrylamide concentration 37 wt% to 55 wt% (preferably 40 wt% to 50 wt%).

In one embodiment the cooling of the reaction mixture is continued so that the temperature is within range of 10 °C to 16 °C, preferably 13 °C to 16 °C and most preferably 15 °C. In one embodiment the cooling is started e.g. after the reaction mixture has been maintained at 15 °C to 25 °C.

In one embodiment the reaction mixture is cooled by at least 10 °C, preferably at least 5 °C, most preferably at least 4 °C.

The cooling may be conducted linearly or stepwise, preferably linearly.

In one embodiment the reaction mixture is maturated at temperature of 10 °C to 18 °C, or 10 °C to 21 °C. During the maturation substantially no, preferably no acrylonitrile is fed to the reactor. During the maturation unreacted acrylonitrile in the reactor reacts to acrylamide. Preferably the maturation is continued until final concentration of the acrylonitrile in the reaction mixture is at most 1000 ppm, at most 100 ppm, at most 90 ppm, preferably at most 75 ppm, more preferably at most 50 ppm, and most preferably at most 10 ppm, or 0 ppm.

In one embodiment the maturation is started e.g. after the reaction mixture has been cooled until the acrylamide concentration reaches 37 wt% to 55 wt% and the temperature of the reaction mixture is within the range of 10 °C to 18 °C, or 10 °C to 21 °C, and/or after the feeding of acrylonitrile into the reactor has ended.

The produced aqueous acrylamide solution may be centrifuged to separate acrylamide and unreacted biocatalyst.

The reactor may be any suitable reactor, such as a semi-batch reactor, a continuous reactor, continuous reactors in series or stirred tank reactors in series, preferably a semi-batch reactor.

The reaction is conducted at ambient pressure, preferably at 1 bar.

In one embodiment of the process of the present invention the temperature of 18 °C to 25 °C is maintained for 30 min to 90 min, such as 45 min to 60 min; the cooling of the reaction mixture to the temperature of 10 °C to 18 °C is performed during a period of time of 45 min to 120min, such as 60 min to 120 min.

In another embodiment of the process of the present invention 38 % to 48 % of total amount of acrylonitrile fed to the reactor is fed during 0 min to 60min from the beginning of the process; 22 % to 30 % of total amount of acrylonitrile is fed during 60 min to 120 min of the process; 12% to 18 % of total amount of acrylonitrile is fed during 120 min to 180 min of the process; and 8 % to 12 % of total amount of acrylonitrile is fed during 180 min to 240 min of the process. To reach balance of 100 % of fed acrylonitrile, the rest acrylonitrile is fed during the process, but not when maturing the reaction mixture.

In a second aspect of the present invention there is provided aqueous acrylamide solution. More particularly there is provided aqueous acrylamide solution charac- terized in that

- concentration of the acrylamide solution is from 35 wt% to 55 wt%., preferably 50.5 % to 55 %,

- concentration of residual acrylonitrile in the solution is equal or less than 1000 ppm, measured by HPLC.

- turbidity of the solution is equal or less than 20 measured from filtrated 0.45 pm acrylamide sample.

The turbidity value is measured in following way: 0.7 ml of HCI (0.1 N) and 7 ml of acetone and 2.3 ml of filtered (0.45 pm) acrylamide sample is placed in a cuvette, and is mixed. Turbidity 450 nm is then measured.

In one embodiment the turbidity of the solution is equal or less than 15.

In one embodiment of the present invention the aqueous acrylamide solution is obtained or obtainable by the process of the present invention.

The amount of residual acrylonitrile is measured by HPLC method, likewise concentrations of acrylamide and acrylic acid.

In one method a 1 100 Series HPLC device (Agilent Technologies, USA) equipped with a Kinetex® 5 pm C18 100 A LC Column (250 ^c 4.6 mm) are used to determine the concentrations of acrylonitrile, acrylamide and acrylic acid. The components are detected by UV absorbance with a 1260 Infinity Diode Array Detector (Agilent Technologies, USA) at 200, 225 and 260 nm depending on each component and its linearity. The flow rate is 1 ml/min and 3.8 ^c 10 ² M H₃P0₄ is used as the eluent. The samples for HPLC are prepared by accurately measuring about 0.2 g of reaction mixture and diluting it into 100 ml of Type 1 MilliQ water. MilliQ water is pretreated with UV radiation for one hour to decompose impurities. A 1961 ppm internal acrylamide standard is run 3 times prior to measurements to ensure that the device is calibrated correctly. Acrylamide 0.1 pi program is used in Agilent software. In one embodiment colour of the aqueous acrylamide solution is equal or less than 20 measured with spectrophotometer PtCo (455nm) from filtrated 0.45pm acrylamide sample. When the colour of the acrylamide solution is at most 20, polyacrylamide produced from the acrylamide solution has a bright colour.

In another embodiment concentration of the acrylamide solution is from 34 wt% to 55 wt%, preferably from 38 wt% to 40 wt%.

Yet in another embodiment the concentration of the acrylamide solution is from 38 wt% to 55 wt%.

In still another embodiment, the concentration of the acrylamide solution is from 50.5 to 55 wt%.

In one embodiment the concentration of residual acrylonitrile in the solution is at most 1000 ppm, at most 100 ppm, at most 90 ppm, preferably at most 75 ppm, more preferably at most 50 ppm, and most preferably at most 10 ppm. In one embodiment the concentration of residual acrylonitrile is 0 ppm.

In a preferred embodiment the aqueous acrylamide solution is produced with the method of the present invention.

In a third aspect of the present invention there is provided use of the aqueous acrylamide solution. More particularly there is provided use of acrylamide solution produced with the process of the present invention or the acrylamide solution of the present invention in manufacturing of polyacrylamide.

Hereafter, the present invention is described in more detail and specifically with reference to the examples, which are not intended to limit the present invention.

Examples

Biocatalvst preparation

All kinetic experiments were started by making a biocatalyst slurry. The slurry was prepared day before the synthesis. At first, biocatalyst ( Rhodococcus aetherivorans, VKM Ac-2610D, having ability to produce nitrile hydratase) was weighed and added to a slurry preparation vessel accurately. After that, 200 g of distilled water was weighed accurately. About 50 % of water was added to the vessel first. This mixture was mixed about 15 min. Then the rest of distilled water was added to the vessel and the slurry was mixed until it was totally homogenous. The slurry was then put to refrigerator to avoid possible thermal deactivation.

Acrylamide synthesis

The reactor setup was checked and cooling unit was turned on. After that, empty reactor was weighed and desired amount of biocatalyst slurry, distilled water and possible AMD solution were weighed accurately to the reactor. Then reactor lids, AN feed pipes, reflux condensers and agitators were installed. Agitators and reactor cooling were turned on instantly to avoid biocatalyst settling and possible thermal deactivation. Speed of the agitator was set to 400 rpm. Once reactor temperature reached the target, the experiment was started by turning on AN feed pump. In batch experiments, AN was charged to the reactor at first and the experiment started when biocatalyst slurry was added to the reactor. Amount of biocatalyst was 1 .35 g dry cat/kg 100% AMD.

Samples were taken by using a long Pasteur-pipette. Each 2 ml sample was filtered through 0.45 pm porosity syringe to separate the biocatalyst and to stop the reaction in samples. Filtered samples were stored in refrigerator to wait HPLC analysis.

Determination of the concentrations of acrylonitrile (AN), acrylamide (AMD) and acrylic acid (AA) by HPLC Analysis

Samples were analyzed by using Agilent 1 100 series HPLC equipment. 200 to 300 mg of sample weighted accurately to 100 ml measuring bottle. Then the sample was diluted into 100 ml of Milli-Q water which was prepared with ultrasound for 60 minutes. Diluted samples and AMD standard sample were pipetted to vials and vials placed to HPLC. HPLC device injected 1 pi of the sample. Computer, which was linked with the HPLC, calculated mass fractions of AMD, AN and AA based on the area of the peak.

With the process of the present invention 50 wt% acrylamide (AMD) solution can be manufactured for example in a semi-batch reactor. The process of the present invention includes new acrylonitrile (AN) feed profile and temperature profile.

In table 1 . are shown new developed AN feed profiles according to the process of the present invention and comparative AN feed profiles (Comp.).

Tablel . AN feed profiles

240“ 300 3 2_.3 % 53 % 7%% 63

¾0 - fO % %ø% $3%

Table 1 and Figures 1 to 4 show the AN feed profiles and temperature profiles used in the process of the present invention (New) and comparative process (Comp.)

Figure 2 shows temperature profiles as function of acrylamide concentration of the process of the present invention and a comparative process.

Higher amount of AN can be fed in the process of the present invention within the first 60 minutes since the reactor temperature is higher and reaction occur faster. In the method of the present invention the reactor is started to cool down after 60 minutes, at that point AMD-concentration of the reactor is about 30 wt%.

Biocatalyst starts to deactivate around in 30 wt% AMD-solution. Biocatalyst deactivation caused by AMD is strongly dependent on temperature and cooling of the reactor reducing biocatalyst deactivation notably. AMD also inhibits the main reaction. AMD formation in presence of high AMD- concentration is substantially smaller than initial reaction rate. Lower reactor temperature is also reducing reaction rate. To avoid AN accumulation to reactor, AN feed rate should be small during the last hours. Since the reaction rate is higher at the beginning of the synthesis, it is possible to reduce feed rate at the end of synthesis without increasing total synthesis time.

Figure 3 shows acrylonitrile mass fraction as a function of time (5 h feed and 1 h maturation time) of the process of the present invention and a comparative process.

As it can be seen from Figure 3 that AN-residual was 2.9 wt% with the comparative process (comp.), whereas AN residual was 0 ppm with the process of the present invention. AMD-product specification is AN <1000 ppm. Figure 4 shows acrylamide mass fraction as a function of time (5h feed and 1 h maturation time) of the process of the present invention and a comparative process.

As it can be seen from Figure 4, the AMD concentration reaches 50 wt% with the process of the present invention, and comparative process only approx. 47 wt%. To reach acrylamide concentration of 50 wt% with the comparative process more catalyst must be used. In other words, with the process of the present invention less biocatalyst is needed. Thus, the process of the present invention is more economic.

Claims

1. A process for producing aqueous acrylamide solution comprising providing a slurry comprising water and a biocatalyst having Nitrile hydratase activity; feeding acrylonitrile into a reactor comprising said slurry to provide a reaction mixture comprising acrylamide, acrylonitrile and biocatalyst; and keeping temperature of the reaction mixture at a temperature range of 15 °C to 25 °C; cooling down of the reaction mixture is started when acrylamide concentration reaches at least 27 wt%, preferably reaches 27 wt% to 38 wt%; cooling of the reaction mixture is continued so that when the acrylamide concentration reaches 37 wt% to 55 wt% the temperature of the reaction mixture is within range of 10 °C to 21 °C, or 10 °C to 18 °C; optionally the reaction mixture is matured at the temperature of 10 °C to 21 °C, or 10 °C to 18 °C, preferably so that the final concentration of acrylonitrile is at most 1000 ppm.

2. The process according to claim 1 , wherein the final concentration of acrylonitrile is at most 100 ppm, at most 90 ppm, preferably at most 75 ppm, more preferably at most 50 ppm, and most preferably at most 10 ppm.

3. The process according to claim 1 or 2, wherein course of the reaction is moni- tored by on-line measuring at least one of the acrylonitrile and the acrylamide concentrations in the reactor, and preferably temperature of the reaction mixture.

4. The process according to any one of claims 1 to 3, wherein the acrylonitrile feed rate is adjusted during the process for avoiding acrylonitrile accumulation into the reactor.

5. The process according to any one of claims 1 to 4, wherein the cooling of said reaction mixture is started when acrylamide concentration reaches 28 wt% to 30 wt%.

6. The process according to any one of claims 1 to 5, the cooling is continued until acrylamide concentration reaches 40 wt% to 50 wt%.

7. The process according to any one of claims 1 to 6, wherein amount of the biocatalyst is 0.1 to 5 kg dry cells/m³ of reaction mixture.

8. The process according to any one of claims 1 to 7, wherein the temperature is maintained at 19 °C to 25 °C, preferably 20 °C to 22 °C and most preferably 22 °C.

9. The process according to any one of claims 1 to 8, wherein the temperature is maintained for 30 min to 120 min, preferably 30 min to 90 min.

10. The process according to any one of claims 1 to 9, wherein cooling of the reaction mixture is continued so that the temperature is within range of 10 °C to 16°C, preferably 13 °C to 16 °C and most preferably 15 °C.

1 1 . The process according to any one of claims 1 to 10, wherein the reactor is a semi-batch reactor, a continuous reactor, continuous reactors in series or stirred tank reactors in series.

12. The process according to any one of claims 1 to 1 1 , wherein 38 % to 48 % of total amount of acrylonitrile fed to the reactor is fed during 0 min to 60 min from the beginning of the process.

13. The process according to any one of claims 1 to 12, wherein the biocatalyst is selected from the group consisting of Rhodococcus, Aspergillus, Acidovorax, Agrobacterium, Bacillus, Bradyrhizobium, Burkholderia, Escherichia, Geobacil lus, Klebsiella, Mesorhizobium, Moraxella, Pantoea, Pseudomonas, Rhizobium, Rhodopseudomonas, Serratia, Amycolatopsis, Arthrobacter, Brevibacterium, Corynebacterium, Microbacterium, Micrococcus, Nocardia, Pseudonocardia, Trichoderma, Myrothecium, Aureobasidium, Candida, Cryptococcus, Debaryomyces, Geotrichum, Hanseniaspora, Kluyveromyces, Pichia, Rhodo- torula, Comomonas, and Pyrococcus, preferably the biocatalyst is selected from the group consisting of Rhodococcus, Pseudomonas, Escherichia and Geobacillus.

14. The process according to claim 13, wherein the biocatalyst is Rhodococcus rhodochrous or Rhodococcus aetherivorans.

15. Aqueous acrylamide solution obtainable by a process according to any one of claims 1 to 14.

16. Aqueous acrylamide solution characterised in that

- concentration of the acrylamide solution is from 35 wt% to 55 wt%, - concentration of residual acrylonitrile in the solution is equal or less than 1000 ppm, measured by HPLC

17. The aqueous acrylamide solution according to claim 16, wherein colour of the solution is equal or less than 20 measured with spectrophotometer PtCo (455 nm) from filtrated 0.45 pm acrylamide sample.

18. The aqueous acrylamide solution according to claim 16 or 17, wherein the concentration of acrylamide solution is from 34 wt% to 55 wt%, preferably from 38 wt% to 40 wt%.

19. The aqueous acrylamide solution according to any one of claims 16 to 18, wherein the concentration of acrylamide is from 38 wt% to 55 wt%.

20. The aqueous acrylamide solution according to any one of claims 16 to 19, wherein concentration of the residual acrylonitrile in the solution is equal or less than 100 ppm, equal or less than 90 ppm, preferably more preferably equal or less than 50 ppm, still more preferably equal or less than 10 ppm, most preferably 0 ppm.

21. The aqueous acrylamide solution according to any one of claims 16 to 20, wherein the turbidity of the solution is equal or less than 15.

22. The aqueous acrylamide solution according to any one of claims 16 to 21 wherein the aqueous acrylamide solution is obtainable by the process of any one of claims 1 to 14.

23. Use of the aqueous acrylamide solution according to any one of claims 16 to 22 and/or aqueous acrylamide solution produced by the process according to any one of claims 1 to 14 in manufacturing of polyacrylamide.