WO2002081379A1

WO2002081379A1 - Process for treating waste water containing a nitrous organic components

Info

Publication number: WO2002081379A1
Application number: PCT/KR2002/000592
Authority: WO
Inventors: Joo-Hee Han; Min-Jeong Noh; Young-Jae Choi; Young-Ho Shin; Chang-Mo Chung; Gi-Woo Han
Original assignee: Hanwha Chemical Corporation
Priority date: 2001-04-04
Filing date: 2002-04-03
Publication date: 2002-10-17
Also published as: CN1501889A; KR100398799B1; KR20020078538A; CN1261363C

Abstract

Disclosed is a process for treating waste water containing nitrous organic components, comprising the steps of : feeding pressurized waste water stream of 130-250 atm to a hydrolysis reactor, in which the pressurized waste water stream is heated during feeding to the reactor by heat exchange with the hydrolyzed stream; hydrolyzing the nitrous components in the waste water stream by heating it to the liquid phase temp. range of 200-370°, which is below a critical point of water; transporting the hydrolyzed stream to a distillation column, in which the hydrolyzed stream is cooled during the transportation to the distillation column by heat exchange with the pressurized waste water stream to be fed into the hydrolysis reactor; and recovering ammonia as an overhead fraction, aqueous solution containing ammonia and carbon dioxide as a side fraction, and the remainder removed of ammonia and carbon dioxide as a bottom fraction, respectively, from the distillation column.

Description

PROCESS FOR TREATING WASTE WATER CONTAINING A NITROUS

ORGANIC COMPONENTS

TECHNICAL FIELD

The present invention relates to a process for treating waste water containing nitrous organic components. More specifically, this invention pertains to a process for treating waste water containing nitrous organic components, in which the waste water containing the nitrous organic components is hydrolyzed under conditions of temperature and pressure close to a critical point of water so that the nitrous organic components in the waste water are decomposed to ammonia and carbon dioxide, and then ammonia and carbon dioxide are recovered for using as raw materials.

Particularly, the present invention is a technology useful to treat a high concentration waste water containing refractory nitrous organic components, which is difficult to biologically treat, and which can be usefully applied to a near-critical water hydrolysis and ammonia recovery process, in which the organic components are hydrolyzed under conditions of temperature and pressure close to a critical point of water to ammonia and carbon dioxide, and then ammonia and carbon dioxide are recovered for using as raw materials. In addition, the process for treating waste water containing nitrous organic components according to the present invention enables ammonia and carbon dioxide to be used as raw materials in a process for synthesizing urea by recovering ammonia and carbon dioxide after the nitrous organic components are decomposed, and the treated water to be used as process water, thereby occurring of the waste water may be originally prevented.

PRIOR ART

Industrial waste water is a representative example of side products occurring with the advance of an industry. . Recently, a biological treatment for treating industrial waste water has been widely used, but it cannot be usefully applied to treat the waste water or liquid containing high concentration, refractory, or toxic organic components. In particular, most of nitrous organic components are refractory. Even though being decomposed, they are easily reduced to ammonia or oxidized to nitric acid, which causes additional problems in the ecosystem. The nitrous organic components dissolved in water may serve as a nutrient for microbes at a low concentration. On the other hand, when the concentration is high, aquatic life's existence is threatened because oxygen in water is exhausted owing to drastic increase of phytoplankton. In addition, the nutrient excessively contained in water causes secondary pollution such as green tide and red tide. Furthermore, ammonia or nitric ions dissolved in water have undesirable effects on the ecosystem due to toxicity thereof.

Nitrogen contained in waste water is classified into nitrate nitrogen, ammoniacal nitrogen, and organic nitrogen. Total content of these three kinds of nitrogen is called total nitrogen content, and the total nitrogen content of treated water to be discharged is restricted by laws. Conventionally, there are physical and chemical treatments for removing nitrogen. The physical treatments are exemplified by ion exchange and ammonia stripping methods. As for chemical treatments, a treatment for removing ammonia using a chlorination reaction, a biological treatment, and an incineration treatment have been known in the art. According to the ion exchange treatment, a selective ion exchange treatment is suggested, in which ammonia ions are removed by use of cation exchange resin, or nitric ions are removed by use of anion exchange resin. However, it is difficult to treat concentrated waste water which is produced during the regeneration of the ion exchange resin. Further, when organic components are contained in water at high concentrations, the removal of nitrogen and organic components is very difficult.

Therefore, it is not preferable for this treatment to be solely applied to waste water containing a large amount of organic nitrogen.

The ammonia stripping treatment removes gaseous ammonia by contacting the waste water with air while the waste water maintains pH of 10.5 to 11.5. However, this method is disadvantageous in that time for removing ammonia is too long, and the treatment cost is relatively high because ammonia is re-separated from moisture in gas phase. Furthermore, nitrous components other than ammonia and organics in waste water cannot be removed.

In the case of the treatment for removing ammonia by use of a chlorination reaction, waste water containing ammonium ions is added with chlorine to produce nitrogen gas and hydrochloric acid, thereby ammonia can be removed. However, this treatment is disadvantageous in that safety in work operations is threatened because excessive chlorine is used for removing ammonia, remaining chlorine in water should be again removed, and organic nitrogen is hard to remove.

The treatment for biologically removing organic components and nitrogen by use of nitrifying organisms and denitrifying organisms is useful to treat large quantities of sewage because of low operation costs. But, such treatment cannot be suitably applied to industrial waste water containing high concentrations of chemicals toxic to a microorganism, which requires a long treatment time, even though the treatment can be effectively applied to a domestic sewage in which toxic nitrous organic component is contained in small amounts. In other words, the treatment requires a long time because decomposition rate of pollutants by the microorganism is slow, and cannot be suitably applied to the industrial waste water, which is suddenly changed in organic components concentration according to chemical process conditions, because the microorganism does not sufficiently cope with sudden changes of the organic components concentration.

Most widely used to treat wastes containing high concentrations of nitrous organic components is the incineration treatment. But, this treatment has disadvantages in that nitrogen oxide (NO_x) is discharged during the incineration of wastes, and a lot of energy is consumed. Accordingly, an additional catalytic reactor should be equipped for removing NO_x, which incurs high treatment cost and makes the operation too complex. In addition, the incineration treatment causes secondary pollution and high energy consumption, and thus is being gradually substituted with a novel treatment in advanced countries.

Much effort has been made to avoid the above problems. For example, reference may be made to U.S. Pat. No. 4,341,640, which discloses a process for hydrolyzing urea in waste water discharged from the production of urea into ammonia and carbon dioxide and recovering them. According to the above patent, the urea contained in the waste water is decomposed into ammonia and carbon dioxide by hydrolysis at 120 to 250 ^°C, then the resulting ammonia and carbon dioxide are recovered through stripping, thus nitrous components are removed from the waste water. The process is advantageous in that both hydrolysis and stripping are conducted in a single column by integrally combining a hydrolysis reactor and a stripping column for stripping ammonia and carbon dioxide from the hydrolyzed waste water. In addition, U.S. Pat. Nos. 4,308,835 and 4,652,678 introduce a process for recovering ammonia and carbon dioxide from the urea-containing waste streams by hydrolyzing urea with increasing of temperature. Furthermore, U.S. Pat. Nos.

4,168,299 and 4,220,635 disclose a catalytic method for hydrolyzing urea, in which hydrolysis efficiency is improved with the use of a catalyst. However, the prior arts as aforementioned can hydrolyze urea, but cannot treat waste water containing refractory materials such as biuret, triuret, ammelide, ammeline, and melamine as well as urea, which are contained in waste water discharged from various processes such as a process for producing melamine.

As described above, the conventional treatment for decomposing nitrous organic components has problems in that decomposition time is too long (biological treatment), and secondary pollutants are produced and a lot of energy is consumed (incineration treatment). To avoid these problems, a process for hydrolyzing urea and recovering ammonia and carbon dioxide, which are raw materials used to synthesize urea, from the waste streams produced in the preparation of urea, has been developed. However, this process can hydrolyze nitrous organic components with low molecular weights such as urea, but cannot treat waste water containing refractory nitrous organic components such as the waste water produced from a process for producing melamine.

DISCLOSURE OF THE INVENTION

Therefore, it is an object of the present invention to provide a process for treating waste water containing nitrous organic components, in which the nitrous organic components are hydrolyzed into ammonia and carbon dioxide, then ammonia and carbon dioxide are separated from treated water and recovered, and the treated water is recycled, thereby environmental pollution is prevented.

Based on the present invention, the above object can be accomplished by a provision of a process for treating waste water containing nitrous organic components, comprising the steps of: a) feeding waste water stream to a hydrolysis reactor with the waste water stream pressurized to 130-250 atm, in which the pressurized waste water stream is heated during feeding to the hydrolysis reactor by heat exchange with the hydrolyzed stream from the subsequent step b); b) hydrolyzing the nitrous organic components in the waste water stream by heating the waste water stream to the liquid phase temperature range of 200-370 ^°C, which is below a critical point of water; c) transporting the hydrolyzed stream to a distillation column for recovering ammonia and carbon dioxide, in which the hydrolyzed stream is cooled during the transportation to the distillation column by heat exchange with the pressurized waste water stream to be fed into the hydrolysis reactor; and d) recovering ammonia as an overhead fraction, aqueous solution containing ammonia and carbon dioxide as a side fraction, and the remainder which is removed of ammonia and carbon dioxide as a bottom fraction, respectively, from the distillation column.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic flow diagram illustrating a process for treating waste water containing nitrous organic components according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION As described above, the conventional process can hardly treat waste water containing refractory nitrous organic components, for example, from a process for producing melamine. In addition, even though such nitrous organic components may be decomposed, the conventional process has problems such as discharge of secondary pollutants and excessive energy consumption.

The present invention is advantageous in that the nitrous organic components are decomposed with high efficiency; energy consumption is reduced; ammonia and carbon dioxide obtained from hydrolysis of nitrous organic components are recovered to re-use as raw materials; and the treated water is reused as process water. Typically, the nitrous organic components decomposable in this invention may be selected from the group consisting of urea, biuret, triuret, ammelide, ammeline, melamine, cyanuric acid, and mixtures thereof, which are commonly products, raw materials or side products, generated in the waste water discharged from production processes of urea or melamine. However, it must be noted that the components as mentioned above do not limit the scope of the present invention, but are only illustrative.

The present process for treating the waste water containing nitrous organic components is roughly divided into a hydrolysis step of the nitrous organic components and a recovery step of ammonia and carbon dioxide resulting from the hydrolysis treatment.

Referring to Fig. 1, a process for treating waste water containing nitrous organic components according to the present invention is schematically illustrated.

Waste water stream 10 containing the nitrous organic components is pressurized to about 130-250 atm, preferably the pressure close to critical pressure of water (about 218 atm) with a high pressure injection pump A and fed into¹ the heat exchanger B. When pH of the waste water stream is 7 or higher, a concentration of hydroxyl ions therein is sufficiently high to promote the subsequent hydrolysis reaction. Therefore, it is preferred that pH of the waste water stream is adjusted in the range of 7 or higher. In the heat exchanger B, the waste water stream 11 flowing from a pump A is heated by heat exchange with a hydrolyzed stream 14 having high temperature, which is discharged from a hydrolysis reactor D, while the hydrolyzed stream 14 is cooled. The waste water stream 12 heated in the heat exchanger B is re-heated by the heater C to a hydrolysis temperature, and then is fed into a bottom of the reactor D. To increase hydrolysis efficiency per unit volume of the reactor D, hydrolysis is conducted in a liquid phase. In this regard, the reactor D is operated under temperature and pressure of about 200-370 ^°C under 130-250 atm. For example, when the temperature in the reactor D is lower than 200 ^°C , a hydrolysis reaction rate is low, while when the temperature is higher than 370^°C , reactants are rapidly reduced in density because reactants are under supercritical state, and so the volume of the reactor D should be increased. Furthermore, when the pressure in the reactor D is lower than

130 atm, reactants are vaporized and gas and liquid coexist in the reactor D, and thus the average density of reactant is reduced. On the other hand, when the pressure is higher than 250 atm, energy efficiency is reduced. It is preferred that the hydrolysis is carried out under the condition close to a critical point of water (374 ^°C and 218 atm) since the nitrous organic components can be efficiently hydrolyzed in a short time.

The hydrolyzed stream 14, in which nitrous organic components are decomposed, is discharged in the liquid phase of aqueous solution containing ammonia and carbon dioxide from the reactor D through a discharge pipe positioned on the top of the reactor D. As described above, the hydrolyzed stream 14 is cooled by heat exchange with the waste water stream 11 in the heat exchanger B. Then, the stream

15 discharged from the heat exchanger B is further cooled to be near to an inlet temperature of a distillation column F for recovering ammonia and carbon dioxide by a cooler E followed by being fed to the distillation column F. In this regard, it is preferable that temperature of the stream 21 to be fed to the distillation column F is adjusted in the range of about 60- 100 ^°C .

At that time, pressure in the distillation column F is maintained in the range of about 18-21 atm, temperature at an upper part of the distillation column is about 50- 60 ^°C, temperature at a side discharging port of the distillation column is about 100- 150 ^°C , and temperature at a bottom of the distillation column is about 180-220 ^°C . Preferably, the distillation column F is operated in conjunction with a stripper and an absorber to recover ammonia and carbon dioxide from the hydrolyzed stream. In the distillation column F, low boiling point materials such as ammonia and carbon dioxide ascend to the top of the distillation column F.

Meanwhile, it is difficult to separate ammonia from a mixture of ammonia and carbon dioxide because a solution of ammonia and carbon dioxide in water forms an azeotropic mixture. For this reason, the distillation column is operated in such a way that upper part of the column is maintained at an excessive concentration of ammonia by additionally supplying ammonia thereto, whereby pure ammonia 22 and 23 can be recovered as an overhead fraction. A portion of the recovered ammonia may be utilized as a reflux of the distillation column, and the remaining ammonia can be obtained as product. A solution of ammonia and carbon dioxide in water, which forms an azeotropic mixture, is discharged through the side discharging port positioned at a middle part of the distillation column, thereby recovered as a side fraction. The side fraction may be sent to a process for synthesizing urea and utilized as a raw material. A bottom fraction 27 which is removed of ammonia and carbon dioxide is discharged through a pipe 26, and then may be recovered to re-use as process water. In the case of treating waste water generated during the production of melamine, the hydrolysis of nitrous organic components is controlled in such a way that the product melamine is slowly decomposed and thus left in the bottom fraction, thereby melamine can be reclaimed with high efficiency in a recovery process.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

EXAMPLE 1

Hydrolysis of a solution of urea in water

Urea of 1000 ppm was dissolved in an aqueous ammonia solution of 11 % and hydrolyzed. A hydrolysis reaction was conducted under 3700 psig with a temperature and retention time of a reactor being varied. After the hydrolysis reaction, final urea concentration indicating a degree of hydrolysis of urea was measured. Results are described in Table 1 , below.

TABLE 1

Conversion of urea according to reaction temperature and retention time

EXAMPLE 2

Hydrolysis of a solution of biuret in water

Biuret of 1000 ppm was dissolved in an aqueous ammonia solution of 11 % and hydrolyzed. A hydrolysis reaction was conducted under 3700 psig with a temperature and retention time of a reactor being varied. After the hydrolysis reaction, final biuret concentration indicating a degree of hydrolysis of biuret was measured. Results are described in Table 2, below. TABLE 2

Conversion of biuret according to reaction temperature and retention time

EXAMPLE 3

Hydrolysis of a solution of triuret in water

Triuret of 1000 ppm was dissolved in an aqueous ammonia solution of 11 % and hydrolyzed. A hydrolysis reaction was conducted under 3700 psig with a temperature and retention time of a reactor being varied. After the hydrolysis reaction, final triuret concentration indicating a degree of hydrolysis of triuret was measured. Results are described in Table 3, below.

TABLE 3

Conversion of triuret according to reaction temperature and retention time

EXAMPLE 4

Hydrolysis of a solution of an ammelide and ammeline in water

A mixture of ammelide and ammeline of 1000 ppm was dissolved in an ammonia aqueous solution of 11 % and hydrolyzed. A hydrolysis reaction was conducted under 3700 psig with a temperature and retention time of a reactor being varied. After the hydrolysis reaction, final ammelide and ammeline concentration indicating a degree of hydrolysis of ammelide and ammeline was measured. Results are described in Table 4, below.

TABLE 4 Conversion of ammelide and ammeline according to reaction temperature and retention time

EXAMPLE 5

Treatment of waste water discharged from a process for producing melamine

In a process for producing melamine, waste water was treated after melamine was recovered through a crystallization step. The waste water discharged from the process for producing melamine includes side products produced during synthesis of melamine, unreacted materials such as urea, biuret, ammelide, and ammeline, and melamine which is not crystallized nor recovered. Compositions of the waste water and the treated water after treatment of the waste water are described in Table 5, below. The hydrolysis reaction was conducted under 3400 psig for a retention time of 15 min.

TABLE 5

Composition of the waste water discharged from the process for producing melamine, and composition of the treated water

From the results shown in Table 5, it can be seen that organic nitrogen is removed at 99 % or higher efficiency, and hydrolysis efficiency is increased with increasing of temperature.

EXAMPLE 6

Treatment of waste water discharged from the process for producing melamine

Waste water discharged during production of melamine, which is different from that of Example 5 in terms of components concentration, was treated with varying of temperature, pressure, and retention time in a hydrolysis reactor as shown in

10 Fig. 1. Compositions of the waste water and the treated water after treatment of the waste water are described in Table 6, below.

TABLE 6

Composition of the waste water discharged from the process for producing

^•15 melamine, and composition of the treated water

'Time: Retention time

From the results shown in Table 6, it can be seen that hydrolysis efficiency is increased with increasing of temperature and retention time.

EXAMPLE 7

Recovery of ammonia and carbon dioxide from treated water

After the waste water discharged during production of melamine was treated, and ammonia and carbon dioxide were recovered from the waste water in the distillation column. The hydrolysis reactor was continuously conducted at 330 ^°C under 3200 psig for the average retention time of 12 min, then the hydrolyzed water was transported from the reactor to a distillation column. Ammonia was recovered through the distillation column with its top and bottom temperatures being 52 and 200 ^°C and its pressure being 276 psig. Flow rates and compositions of hydrolyzed stream 21, and overhead fraction 24, side fraction 25, and bottom fraction 27 of the distillation column were measured. Results are described in Table 7, below.

TABLE 7

Flow rates and compositions of hydrolyzed stream, and overhead fraction, side fraction and bottom fraction of the distillation column

COMPARATIVE EXAMPLE 1

After the waste water discharged during production of melamine was treated, and ammonia and carbon dioxide were recovered from the waste water through the distillation column. The treated water according to Example 7 was compared with the treated water according to the conventional process in terms of the composition of an inlet stream (A), overhead fraction (B), side fraction (C), and bottom fraction (D) of the distillation column, respectively. Results are described in Table 8, below.

TABLE 8

From the results shown in Table 8, it can be seen that nitrous organic components contained in the bottom fraction are removed at 99 % or higher efficiency and recovered in a form of ammonia and carbon dioxide in the case of the present invention. Further, the recovery of ammonia is increased from 2,762 to 3,111 kg/hr, i.e. by 12.6 %.

INDUSTRIAL APPLICABILITY

As described above, the present invention provides a process for treating waste water containing nitrous organic components. The nitrous organic components are hydrolyzed under conditions of temperature and pressure close to a critical point of water, and so organic components in the waste water are decomposed into ammonia and carbon dioxide, then ammonia and carbon dioxide are recovered for using as raw materials. The present invention is advantageous in that nitrous organic components discarded as waste materials can be recycled in the form of ammonia and carbon dioxide capable of being used as raw materials by hydrolyzing nitrous organic components contained in the waste water to ammonia and carbon dioxide at 99 % or higher efficiency, and the treated water which is removed of nitrous organic components is reutilized as process water. Furthermore, the present invention can be usefully applied to processes discharging nitrous organic components in large quantities such as a process for producing melamine or urea, thereby preventing waste water containing nitrous organic components from being released to the environment.

It is thus seen that the objects set forth above, as made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the construction set forth without departing from the spirit- and scope of the invention, it is intended that all matter contained in the accompanying drawing shall be inteφreted as illustrative, and not in a limiting sense.

Claims

1. A process for treating waste water containing nitrous organic components, comprising the steps of: a) feeding waste water stream to a hydrolysis reactor with the waste water stream pressurized to 130-250 atm, in which the pressurized waste water stream is heated during feeding to the hydrolysis reactor by heat exchange with the hydrolyzed stream from the subsequent step b); b) hydrolyzing the nitrous organic components in the waste water stream by heating the waste water stream to the liquid phase temperature range of 200-370 ^°C, which is below a critical point of water; c) transporting the hydrolyzed stream to a distillation column for recovering ammonia and carbon dioxide, in which the hydrolyzed stream is cooled during the transportation to the distillation column by heat exchange with the pressurized waste water stream to be fed into the hydrolysis reactor; and d) recovering ammonia as an overhead fraction, aqueous solution containing ammonia and carbon dioxide as a side fraction, and the remainder which is removed of ammonia and carbon dioxide as a bottom fraction, respectively, from the distillation column.

2. The process according to claim 1, wherein pH of the waste water stream fed into the hydrolysis reactor is 7 or higher.

3. The process according to claim 1, wherein the hydrolyzed stream is adjusted to a temperature of 60-100^°C, and then transported to the distillation column, said hydrolyzed stream being discharged from the hydrolysis reactor in the liquid phase of aqueous solution containing ammonia and carbon dioxide.

4. The process according to claim 1, wherein the nitrous organic components are selected from the group consisting of urea, biuret, triuret, ammelide, ammeline, melamine, cyanuric acid, and mixtures thereof.

5. The process according to claim 1, wherein the pressure of the distillation column is maintained in the range of 18-21 atm; and the temperature at the upper part, the side discharging port and the bottom of the distillation column is maintained in the range of 50-60 ^°C , 100- 150 ^°C and 180-220 ^°C , respectively.

6. The process according to claim 1, wherein the distillation column is operated in such a way that the upper part of the column is maintained at an excessive concentration of ammonia by additionally supplying ammonia thereto, whereby pure ammonia may be recovered as the overhead fraction

7. The process according to claim 1, further comprising supplying the bottom fraction as process water.