WO2025218930A1

WO2025218930A1 - A process for treating the waste water of a urea production plant

Info

Publication number: WO2025218930A1
Application number: PCT/EP2024/088689
Authority: WO
Inventors: Leonardo MARRONE; Alberto Benedetti
Original assignee: Casale SA
Current assignee: Casale SA
Priority date: 2024-04-18
Filing date: 2024-12-31
Publication date: 2025-10-23
Anticipated expiration: 2026-10-18

Abstract

A process for removing urea from waste water of a urea synthesis plant, comprising the step of contacting the waste water with a urease enzyme and subjecting the effluent of said contacting to steam stripping to remove ammonium salts.

Description

A process for treating the waste water of a urea production plant

DESCRIPTION

Field of application

The present invention relates to urea production.

Prior art

The industrial processes for urea production are based on the reaction of ammonia and carbon dioxide at high temperature and high pressure in a urea reactor; the reaction produces ammonium carbamate which decomposes into water and urea via a thermodynamically limited reaction. The reactor product is therefore a mixture of urea, ammonium carbamate, water, carbon dioxide and ammonia, the conversion after the urea reactor being approximately 60%. Said mixture is fed to one or more recovery sections at lower temperature and pressure where the urea is purified and the reactants are progressively recovered and recycled.

Most urea processes use the stripping process. In a urea stripping process, the effluent of the urea reactor is passed through a high-pressure stripper where ammonium carbamate is thermally decomposed into ammonia and carbon dioxide. A portion of the fresh CO2 can be used as stripping medium (CO2 stripping process).

In the stripping process, gaseous ammonia and carbon dioxide are removed from the urea solution, thus obtaining a purified solution and a vapour stream comprising predominantly ammonia and carbon dioxide. The purified solution is further processed at medium and/or low pressure; the vapour stream emerging from the stripper is condensed in a high-pressure carbamate condenser and recycled to the urea reactor. The high-pressure carbamate condenser usually receives also a carbamate-containing recycle solution from the recovery section(s). After recovery, an aqueous solution containing about 60-70% urea is obtained; if required, this solution can be concentrated by removing water in a suitable evaporation section to obtain a concentrated urea solution or a urea melt.

The urea processes are described in the literature, for example in Meessen, “Urea” Ullmann’s Encyclopaedia of industrial chemistry 2012, pages 669 to 677.

One issue of the urea production is the treatment of the waste water. The waste water may come from concentration of the urea solution and from other purge streams. The waste water of a urea plant typically comprises around 1 % urea, ammonium carbonate, ammonium bicarbonate, free ammonia and a small amount (about 50 ppm) of biuret. Therefore, the waste water cannot be discharged or recovered as such and must be processed to remove urea and the other contaminants. For example, removal of contaminants is typically required to use the waste water as boiler feed water for the production of steam.

In the current technique, urea is removed from waste water by a thermal hydrolysis process which must be performed at high temperature and pressure, typically around 210 °C and 20 bar. To maintain the above conditions, the hydrolyser is fed with medium pressure steam at around 23 bar. This valuable steam is generally not produced internally in the urea process and must be imported, thus being a relevant contribution to the overall consumption of the process.

Generally, 70 kg of medium-pressure steam are required for each m3 of waste water to be processed, which corresponds to around 50-55 kg of steam per metric ton of urea produced. The steam required by the waste water treatment (WWT) section may account for 10% of the import of steam for the whole urea process.

There is a strong demand to reduce the steam consumption of urea synthesis process, therefore the above drawback is significant and a solution is highly desired. Moreover, there is still a strong demand for processes performed at lower pressure and temperature than state of art.

Summary of the invention

The invention addresses the problem of how to downgrade the operating conditions at which the urea hydrolysis takes place and reduce the steam consumption of the waste water treatment in a urea production plant.

The problem is solved with a process according to claim 1 .

The novel and revolutionary approach of the invention is to remove urea from the waste water by contact with a urease enzyme. Urease enzymes are identified by EC number 3.5.1.5. A preferred enzyme is from Canavalia Ensiformis.

The notable advantage of the invention is that the process can be performed at low temperature and low pressure, such as atmospheric or near atmospheric pressure, without the need of medium-pressure steam. The invention goes against the conventional wisdom to perform the waste water treatment by hydrolysis at high pressure. A further advantage of operating at low pressure is the reduced cost of the equipment.

According to the invention, the waste water is subject to a steam stripping step after the contact with the enzyme. Said stripping step having the aim of removing carbonates, salts and free ammonia. According to other embodiments, the waste water may be subject to a stripping step also before the contact with the enzyme.

The process of contacting the waste water with the enzyme is preferably performed in a dedicated reactor (urease reactor).

Description of the invention

The invention concerns treating waste water effluent of a urea synthesis plant in a waste water treatment section wherein waste water is contacted with a urease enzyme before being subjected to steam stripping.

In a preferred embodiment of the invention, the waste water is contacted with the enzyme at a temperature not greater than 55 °C, preferably 30 to 40 °C and at a pressure preferably around atmospheric pressure. Particularly preferably, the relative pressure is not greater than 5.0 barg. The symbol barg denotes bar gauge. The above ranges of temperature and pressure provide a suitable compromise between the highest activity of the enzymatic process and a reasonable period of time before denaturation of the enzyme.

The step of contacting the waste water with the enzyme is performed, preferably, without the addition of chemicals other than the enzyme in the supplied form and external to the urea process. The term of chemicals external to the urea process refers to chemicals other than CO2, ammonia, water and urea. The treatment is performed in the presence of CO2, water, ammonia and urea naturally contained in the waste water of a urea process.

Preferably, the process of contacting the waste water with the enzyme is performed continuously in a urease reactor. Said urease reactor is preferably configured as a homogeneous tubular reactor (plug-flow reactor) where the enzyme is dissolved in a liquid phase.

According to other various embodiments, the enzyme may be fixed onto a solid support, thus the urease reactor may be configured as a column comprising internals or packing materials where the enzyme is fixed to increase the surface of contact between the enzyme and the waste water. According to another embodiment, the enzyme is a solid fluidized into a fluidized bed reactor.

An advantage of having the enzyme dispersed in the liquid phase is that a lower concentration of enzyme is needed for having the same urea concentration abatement, thus reducing the amount of enzyme to be replaced after denaturation. An advantage of operating with solid enzyme is the fact that it is not needed a processing step after said reactor to separate the enzyme from the effluent.

Before contacting the waste water with the enzyme, said waste water is preferably subject to any of the following processing steps, or a combination thereof:

- steam stripping to remove ammonium carbonates and ammonia that can reduce the effectiveness of the enzyme; preferably, this steam stripping step is performed in a top section of a desorber;

- processing to reduce pH to less than 9.0, preferably in the range 7.0 to 9.0, more preferably 7.5 to 8.0; pH reduction is preferably obtained by injecting CO2 that neutralizes NH3;

- processing to remove metal ions, preferably by passing the waste water over activated carbon filter.

Preferably, the processing step to reduce pH through injection of CO2 and/or the processing step to remove metal ions with an activated carbon filter is/are performed after the steam stripping performed in the top section of the desorber.

After one or more of the processing steps listed above, waste water is contacted with the enzyme. In the case of homogeneous biocatalysis, wherein the contacting between the enzyme and the waste water occurs with the enzyme dissolved within said waste water, said enzyme has a concentration at the inlet of the urease reactor that is preferably between 5.0 and 100 mg/L, more preferably 50 mg/L.

After contacting the waste water with the enzyme, said waste water has a content of urea preferably not greater than 10 ppm, more preferably between 1 and 10 ppm.

The urease reactor produces a stream of purified water wherein the enzyme is dissolved. Said stream is preferably treated in an ultrafiltration step to separate the enzyme from the purified water, producing an enzyme-rich stream that is recycled before the urease reactor, and a purified aqueous water.

According to a preferred embodiment, the ultrafiltration step provides a reduction of the concentration of enzyme not higher than 20 times, preferably not higher than 10 times, with respect to the concentration of the enzyme at the inlet of said ultrafiltration step. Said ultrafiltration step is preferably performed with a difference of pressure between upstream and downstream not higher than 20 bar, more preferably not higher than 10 bar.

Said ultrafiltration is preferably performed by membranes having a molecular weight cut-off not higher than 100 kDa.

The purified aqueous stream produced by the ultrafiltration has a concentration of enzyme preferably not higher than 1 ppm by weight. Preferably, the enzymerich liquid stream produced by the ultrafiltration step has a concentration of enzyme that ranges from 50 to 2000 mg/L, preferably 500 mg/L.

According to an interesting embodiment of the invention, the enzyme is replaced after a period not longer than 1 month, preferably from 1 day to 1 month (turnover time). The enzyme undergoes denaturation during its period of activity. Said denaturation leads to a reduction of the activity of the enzyme dissolved in the waste water. Preferably, the enzyme is replaced when said denaturation causes a reduction of activity not exceeding 90% of the initial activity of the enzyme.

Preferably, the concentration of the enzyme relative to the internal volume of said urease reactor is at least 140 U/mL, wherein U is the amount of enzyme which catalyses the production of 1 pmol [NH4+] per minute. In a preferred embodiment, the urease reactor has a volume such that a residence time of the waste water in said reactor is at least 10 min, preferably at least 20 min, where the residence time can be calculated as the ratio of the reactor volume to the volumetric flow rate of the waste water entering the reactor.

In an interesting embodiment of the invention, the purified waste water, after contact with the enzyme, is further processed to remove residual urea and/or to remove biuret. Removal of residual urea can be performed by contacting the purified waste water with a porous type strongly acidic cation exchange resin. A resin suitable for this purpose is AmberList™ 15WET available from Dow. Removal of biuret can be performed by contacting the water with a porous type strongly basic anion exchange resin. A suitable resin for this purpose is DIAION PA306S available from the company Resindion Sri. A resin for removal of biuret is also disclosed in US 3903158. Subsequent removal of biuret may be required in some embodiments because the urease enzyme does not reduce the content of biuret. Removal of residual urea may be required according to the quality specification of the water after treatment.

A preferred embodiment of the process of the invention includes treating the waste water in the first steam stripping to remove ammonium carbonates and ammonia, in a top section of a desorber. After said steam stripping, the waste water is cooled and sent to a urease reactor where it is contacted with the enzyme. In the case of homogenous biocatalysis, the waste water leaving said reactor is subject to an ultrafiltration step, and then to a second stripping step to remove ammonium salts in a bottom section of said desorber. The effluent of said bottom section of the desorber (purified waste water) may be treated to remove biuret and/or to remove residual urea, as above mentioned.

Description of figures

Fig. 1 discloses a preferred embodiment of the invention, and Fig. 2 discloses another embodiment of the invention, with the following main items.

1 desorber

2 top section of the desorber 1 3 bottom section of the desorber 1

4 process I process heat exchanger

5 process I cooling water heat exchanger

6 pump

7 carbon activated filter

8 urease column

9 CO2 admission valve

10 pH controller

11 feed pre-heater

12 biuret removal section

13 condenser.

20 plug-flow reactor

21 liquid-vapour separator

22 ultrafiltration section

According to a preferred embodiment of the invention shown in Fig. 1 , A waste water feed 14 is pre-heated in the feed preheater 11 and sent to the top section of the desorber 2 where it is steam stripped to remove ammonium carbonates and ammonia.

The effluent 15 is cooled through the heat exchangers 4, 5 and sent via the pump 6 to the urease reactor 20. Said reactor 20 is a tubular reactor configured substantially as a plug-flow reactor, said reactor operating at a temperature lower than 30 °C. Within the reactor the enzyme is dispersed and contacted with the waste water in the liquid phase, thus homogenous biocatalytic degradation of urea occurs. The reactor 20 has a volume suitable for providing 20 minutes of residence time to the waste water.

The effluent 23 of said reactor 20 is optionally treated in a liquid-vapour separator 21 to remove inert 24 before being introduced in an ultrafiltration section 22.

Said ultrafiltration section 22 provides separation of an enzyme-rich stream 26, containing the enzyme at a concentration around 10 times greater than the concentration at the inlet of the ultrafiltration, and a treated water stream 17 having a content of enzyme less than 0.1 ppm by weight. This separation is performed by means of membranes having a molecular weight cut-off less than 100 kDa.

The enzyme-rich stream 26 is joined with the waste water before the urease reactor 20, resulting in a stream 27 containing the enzyme at the desired concentration of about 50 mg/L for suitable performing the biocatalytic degradation of urea. Preferably, prior to introduction into the reactor 20, the water stream is added with CO2 for lowering the pH. The pH controller 10 detects the pH of the water at the inlet of the reactor 20 and controls the addition of CO2 via the admission valve 9.

The treated water 17 effluent from the ultrafiltration section 22 is heated in the process I process heat exchanger 4 with heat removed from the stream 15, and is sent to the bottom section of the desorber 3 where it is stripped with low-pressure steam to remove ammonium salts, resulting in a stripped effluent 18.

Said stripped effluent 18 is cooled by pre-heating the feed 14 in the pre-heater 11 and, before being withdrawn as purified water from the waste treatment section, it is optionally treated in a biuret removal section 12.

The vapours and steam from the bottom section of the desorber 3 are sent to the top desorber 2 where they act as stripping medium; the vapours 19 withdrawn from the top desorber 2 are condensed in the condenser 13 resulting in a carbamate solution which is partially recycled to the urea plant and partially returned to the top desorber 2.

Fig. 2 shows another embodiment of the invention wherein the effluent 15 of the top section of the desorber 2 is cooled through the heat exchangers 4, 5 and sent via the pump 6 to the urease column 8. According to this embodiment, the enzyme contained within the column 8 is solid and fixed onto a solid support, such as a resin; thus, heterogeneous degradation occurs. Another possibility is fluidizing said enzyme and performing the urea degradation reaction in a fluidized bed reactor.

Preferably, prior to introduction into the column 8, the water stream is added with CO2 for lowering the pH and passed over the carbon activated filter 7 to remove metal ions. The pH controller 10 detects the pH of the water at the inlet 16 of the column 8 and controls the addition of CO2 via the admission valve 9. In the column 8 the steam water inlet 16 is contacted with the urease enzyme for a residence time preferably greater than 30 minutes, to remove urea from said inlet 16.

Claims

1 . A process for removing urea from waste water of a urea synthesis plant, comprising the step of contacting the waste water with a urease enzyme, wherein the waste water, after contact with the enzyme, is subject to steam stripping to remove ammonium salts.

2. A process according to claim 1 , wherein the urease enzyme comes from Canavalia Ensiformis.

3. A process according to claim 1 or 2, wherein the waste water is contacted with the enzyme at a temperature not greater than 55 °C, preferably 30 to 40 °C, and/or wherein the waste water is contacted with the enzyme at a relative pressure up to 5.0 bar gauge.

4. A process according to any of claims 1 to 3 wherein the waste water is subject to steam stripping before contact with the enzyme, wherein said stripping removes ammonium carbonates and ammonia from the waste water.

5. A process according to any of the previous claims wherein the waste water, before contact with the enzyme, is processed to reduce the pH of the waste water to less than 9.0, preferably in the range 7.0 to 9.0 and more preferably 7.5 to 8.0.

6. A process according to claim 5, wherein the pH of the waste water is reduced by injection of CO2.

7. A process according to any of the previous claims wherein the waste water, after contact with the enzyme, contains no more than 10 ppm of urea, preferably between 1 and 10 ppm.

8. A process according to any of the previous claims wherein the waste water, after contact with the enzyme, is further processed to remove residual urea and/or to remove biuret.

9. A process according to any of the previous claims wherein the step of contacting the waste water is performed in a urease reactor, wherein the enzyme is dispersed in a liquid phase within said reactor, or wherein the enzyme is a solid fixed on a packing material or fluidized within said urease reactor.

10. A process according to claim 9 wherein the contacting between the waste water and the enzyme is performed in a liquid phase as homogeneous biocatalysis, and wherein the urease reactor is preferably configured as a plug-flow reactor.

11 . A process according to claim 10 wherein the residence time of the waste water in the plug-flow reactor is at least 10 min, preferably at least 20 min.

12. A process according to claim 10 or 11 wherein the enzyme at the inlet of the urease reactor has a concentration that ranges from 5.0 to 100 mg/L, preferably 50 mg/L.

13. A process according to any of claims 9 to 12 wherein the urease reactor produces a stream of purified water containing the enzyme, wherein said stream of purified water is treated in an ultrafiltration step to separate the enzyme, producing an enzyme-rich stream that is recycled before the urease reactor and a purified aqueous stream.

14. A process according to claim 12 wherein the enzyme-rich stream has a concentration of the enzyme not higher than 20 times, preferably not higher than 10 times, than the concentration of the enzyme of the purified water at the inlet of said ultrafiltration step.

15. A process according to claim 13 or 14 wherein the ultrafiltration step is performed with a difference of pressure between upstream and downstream said ultrafiltration not higher than 20 bar, preferably not higher than 10 bar.

16. A process according to any of claims 13 to 15 wherein the ultrafiltration step produces an enzyme-rich liquid stream having a concentration of enzyme that ranges from 50 to 2000 mg/L, preferably 500 mg/L.

17. A process according to any of claims 13 to 16 wherein the ultrafiltration step is performed with membranes having a molecular weight cut-off not higher than 100 kDa.

18. A process according to any of claims 13 to 17 wherein the purified aqueous stream effluent from the ultrafiltration step has a concentration of enzyme not higher than 1 ppm by weight.

19. A process according to any of the previous claims wherein the enzyme has a turnover time not higher than 1 month, preferably from 1 day to 1 month, after said turnover time the enzyme is replaced.

20. A process according to claim 19 wherein the enzyme is replaced when the denaturation of said enzyme results in a reduction of activity not exceeding 90% of the initial activity of the enzyme.

21. A process according to any of the previous claims wherein said step of contacting the waste water with the enzyme is performed in a urease reactor which is preferably a column, said reactor comprising internals or packing material where the enzyme is fixed.

22. A process according to any of claims 9 to 21 wherein: said waste water is subject to a first steam stripping to remove ammonium carbonates and ammonia, in a top section of a desorber; after said first steam stripping, the waste water is cooled and sent to said urease reactor where it is contacted with the urease enzyme; the waste water leaving said urease reactor is optionally subject to ultrafiltration and then subject to a second stripping step to remove ammonium salts in a bottom section of said desorber; the effluent of said bottom section of the desorber is optionally treated to remove biuret and/or to remove residual urea.

23. A process according to claim 22 wherein the waste water, after said first steam stripping and before entering the column, is added with CO2 to reduce pH of the waste water and/or is passed over an activated carbon filter to remove metal ions.