CN88101281A

CN88101281A - The hydrolysis of urea method of low concentration urea solution

Info

Publication number: CN88101281A
Application number: CN88101281.5A
Authority: CN
Inventors: 藤井英嗣
Original assignee: Toyo Engineering Corp
Current assignee: Toyo Engineering Corp
Priority date: 1987-03-13
Filing date: 1988-03-11
Publication date: 1988-09-28
Also published as: CN1015883B; JPH0691987B2; BR8801106A; IN168233B; JPS63224785A

Abstract

Adopt the first step and flow stripping and the method that the adverse current stripping in second step combines, will contain the whole hydrolysis eliminatings of urea in the low concentration urea solution of about 0.3～1.5% urea basically.

Description

Urea hydrolysis method of low-concentration urea aqueous solution

The invention relates to the treatment of low-concentration aqueous urea solutions, comprising subjecting an aqueous solution containing very small amounts of urea, ammonia and carbon dioxide to a heat treatment, hydrolyzing the urea and stripping it to gaseous ammonia and carbon dioxide, whereby the water obtained is substantially free of these components except water. More particularly, the invention relates to a process for separating the above-mentioned components contained in water, which are the main by-products of the urea production process, and which can be discharged outside the process.

The treatment of a low-concentration urea aqueous solution is important from the viewpoint of recovering useful substances, preventing pollution, achieving the absence of ammonium sulfate and ammonium nitrate by-products in the urea production and enlarging the production scale of plants. In principle, urea, which is present in very small amounts in a low-concentration urea aqueous solution, can be converted into ammonia and carbon dioxide by hydrolysis under pressure, and the ammonia and carbon dioxide can be evaporated and separated from the solution by distillation or stripping under a low pressure of less than a few atmospheres. Hitherto, there are four known hydrolysis techniques, as shown in FIGS. 2a to d, which are variously changed depending on the relationship between the flow direction of the injected steam and the flow direction of the solution to be treated. Fig. 2a and b are co-current flow diagrams, while fig. 2c and d are counter-current flow diagrams. In these processes, the urea content remaining in the waste water is about 50ppm when treated in the usual way.

However, in recent years, since the urea content in wastewater must be 10ppm or less strictly regulated, it is actually necessary to hydrolyze urea sufficiently. However, according to the above four methods, an extremely long residence time is required for hydrolysis, and the equipment cost is large, thus proving that the conventional method is basically not suitable for use.

The present inventors have found that the problem is attributable to the fact that ammonia and carbon dioxide generated by hydrolysis are not immediately completely evaporated and partially remain in the solution, and when a very small amount of urea is present in the solution, the hydrolysis is affected even if the amount of ammonia coexisting therewith is extremely small. The present inventors' results solve this problem and, according to the method provided by this results, the residual urea can be easily hydrolyzed to a content of 5ppm or less.

According to the present invention, when an aqueous solution containing a very small amount of urea, ammonia and carbon dioxide is subjected to a heat treatment to hydrolyze urea into ammonia and carbon dioxide, the hydrolysis of urea can be carried out in two steps, thereby obtaining water substantially free of these components. The first step is as follows: the aqueous solution is contacted under pressure with steam in cocurrent flow to hydrolyze the urea to ammonia and carbon dioxide until the residual amount of urea is 50 to 500ppm by weight, preferably 50 to 100ppm by weight, and the resulting ammonia and carbon dioxide are separated from the solution as a gaseous mixture. The second step is that: the aqueous solution still containing urea is brought into counter-current contact with steam at the same pressure as in the first step, further hydrolyzing the urea and separating the gaseous mixture of ammonia and carbon dioxide thus formed from the solution, thus obtaining water substantially free of urea.

FIG. 1 is a flow diagram illustrating the relationship between a urea hydrolyser and a stripper connected thereto for carrying out the present invention. FIG. 2 is a 4-flow diagram illustrating the relationship between a conventional urea hydrolyzer and a stripper connected thereto.

The condensate obtained by separation and condensation in the concentration step of the urea production process generally contains 0.3 to 1.5% by weight of urea, 0.5 to 5.0% by weight of ammonia, and 0.3 to 3.0% by weight of carbon dioxide. Introducing steam into a stripping tower and distilling to increase the pressure of the aqueous solution to 1-5 Kg/cm²Gauge pressure, and preheated to separate ammonia and carbon dioxide into a gas mixture. The pressure of the separated urea aqueous solution is increased to 10-30 Kg/cm²Gauge pressure, and then the hydrolysis treatment of the present invention.

The first and second steps of the process of the present invention may each be carried out in a separate column or vessel, but from the viewpoint of simplification of equipment and economy, the two-step treatment is preferably carried out in a single integrated apparatus. The closed container is divided into one chamber and two chambers, the container is a vertical cylinder, the solution can flow up and down, the steam flows upwards, and the container is divided into two chambers by a vertical flat plate or an inner cylinder into two annular chambers. There is no particular preference for the order in which the solution to be treated is first introduced into which chamber, based on the same pressure. To prevent breakthrough of the injected vapor, it is preferable to use multiple layers of perforated plates in both chambers. The pressurized urea aqueous solution is firstly introduced into the bottom of the first chamber, and simultaneously heated by steam which is in parallel flow with the pressurized urea aqueous solution, and the urea in the low-concentration urea aqueous solution is hydrolyzed at 180-230 ℃.

The ammonia and carbon dioxide produced by the hydrolysis of urea flow upward into the top of the chamber along with the solution vaporized by the injected steam. The gaseous mixture of ammonia and carbon dioxide that reaches the top is separated from the solution in a separation zone at the top. After the first step of treatment, the content of urea still remaining in the solution is reduced to 50-500 ppm. The reason why the ammonia and carbon dioxide formed have to be separated in this step and the amount of urea remaining as defined above is that a very efficient hydrolysis can be carried out in the second step. The trouble caused by the remaining ammonia should be attributed to the newly found reason as described above. If the amount of residual urea exceeds the upper limit, the ammonia formed in the second step is detrimental to the hydrolysis, whereas if the amount is below the lower limit, it causes an excessive amount in the first step. Therefore, the content within the above range is optimum.

The aqueous solution with the urea content reduced to the limited range is then fed to a second chamber where the urea remaining in the solution is hydrolysed. In the chamber, the aqueous solution flows downwards, while steam is injected in countercurrent. The remaining urea content is reduced to 1-5 ppm and the aqueous solution is discharged from the bottom of the chamber at 185-230 ℃. Most of the ammonia and carbon dioxide formed by the hydrolysis are carried by the injected steam to the top of the chamber and combined with the ammonia and carbon dioxide obtained from the first separation step and the combined gas mixture is discharged from the hydrolyzer together with the steam.

The discharged waste gas is decompressed and then returns to the stripping tower which is upstream in the process of the invention. Furthermore, the solution of the drainage is also returned, after any heat exchange and depressurization, to the same stripping column in which the stripping is completed in order to remove the remaining traces of ammonia and carbon dioxide.

After the low-concentration urea aqueous solution treated by the method passes through the stripper upstream of the process, the final wastewater of the urea synthesis process only contains 1-5 ppm (by weight) of urea, 1-5 ppm (by weight) of ammonia and 0-3 ppm (by weight) of carbon dioxide.

According to the method of the present invention, urea can be hydrolyzed to the residual amount of urea as described above very economically without enlarging the volume of the urea hydrolyzer.

FIG. 1 illustrates a preferred embodiment of the process of the present invention.

Examples

This example is described with reference to fig. 1.

The low-concentration urea waste liquid which is separated and condensed in the urea product concentration step of the urea synthesis process is sent into a pressure of 3Kg/cm through a conduit 1²The flow rates of urea, ammonia, carbon dioxide and water in the waste liquid were 18 kg/hr, 74 kg/hr, 55 kg/hr and 3.213 kg/hr, respectively, from the top of the stripping column 11U under a gauge pressure. In the stripping column, the solution is brought into counter-current contact with the ascending steam, which entrains ammonia and carbon dioxide so that at the top essentially all of the ammonia and carbon dioxide contained in the solution can be separated and the separated ammonia and carbon dioxide are discharged from stripping column 11U via exhaust line 4. On the other hand, the low-concentration urea aqueous solution flows into the liquid storage tank 11M, passes through the pipe 5, and flows to the pump 13, where the solution is pressurized to 16Kg/cm²The resulting pressurized solution is preheated in heat exchanger 14 and then passed through the bottom of hydrolyzer 12 into hydrolysis first chamber 12L.

The pressure is 20 through the steam pipe 17Kg/cm²The steam under gauge pressure was injected into the bottom of the first hydrolysis chamber 12L at a flow rate of 40 kg/h, and the ascending steam flow passed through the first hydrolysis chamber 12L constituted by a plurality of perforated plates while being in concurrent contact with the low-concentration urea solution reaching the top.

During this time, the urea content in the ascending liquid stream is hydrolyzed to 80ppm, while a portion of the ammonia and carbon dioxide produced by the hydrolysis is separated overhead in a gaseous state.

Then, the urea aqueous solution of low concentration, partially separated from ammonia and carbon dioxide, overflows into a second chamber 12R, constituted by a similar multi-layer orifice plate, and then flows through the chamber towards the bottom, while being in counter-current contact with the steam injected into the bottom chamber, at a flow rate of 40 Kg/h and a pressure of 20Kg/cm²And (4) gauge pressure.

During this time, the urea in the descending liquid stream was further hydrolyzed, reducing the bottom content to 1 ppm. On the other hand, the ammonia and carbon dioxide thus produced are carried into the top by the steam and flow out of the hydrolyzer together with the ammonia and carbon dioxide that have been separated in the one chamber 12L. The resulting gas mixture is reduced to the pressure of the stripper by means of a pressure reducing valve 15 and introduced into the bottom of the stripper 11U via a line 6 and finally discharged from the top of the stripper 11U via a line 4 together with the aforementioned ammonia and carbon dioxide. Since the solution leaving the bottom of the second chamber 12R via the line 7 still contains very little ammonia (40 ppm) and carbon dioxide (45 ppm), it is reduced in temperature by the heat exchanger 14, is reduced in pressure by the valve 16 and then enters the top of the stripper 11L consisting of a multilayer perforated plate. While the solution flows down through the stripping column 11L, a small amount of residual ammonia and carbon dioxide is evaporated by the steam sprayed into the bottom of the stripping column via the pipe 2 and finally discharged via the pipe 4. Water, practically free of urea, ammonia and carbon dioxide, can then be taken from the bottom of the stripper and discharged as sewage to be treated via the pipe 3.

In this series of steps, using a hydrolyzer of conventional first-order countercurrent or parallel flow type configuration, and requiring the same degree of urea hydrolysis as the residual amount of urea achieved by the process of the present invention, the volume of the hydrolyzer 12 required is 1.5 times that of the hydrolyzer used in the present invention.

Claims

1. By heat-treating an aqueous solution, urea is hydrolyzed into ammonia and carbon dioxide, thereby converting an aqueous solution containing very small amounts of urea, ammonia, and carbon dioxide into water substantially free of these components. The urea hydrolysis method comprises two steps. The first step is to directly contact the aqueous solution with steam under pressure in co-current flow to hydrolyze the urea to a residual urea content of 50 to 500 ppm (by weight), and to separate the ammonia and carbon dioxide produced by the hydrolysis into a gaseous mixture. The second step is to directly contact the aqueous solution still containing residual urea with steam in counter-current flow at the same pressure as in the first step to further hydrolyze the urea and separate the ammonia and carbon dioxide produced thereby into a gaseous mixture, thereby converting the aqueous solution into water substantially free of urea.

2. The urea hydrolysis method according to claim 1, wherein the first and second steps of urea hydrolysis are carried out in the same tower-type closed container.

3. The urea hydrolysis method according to claim 2, wherein the interior of the tower-type closed container is divided into two chambers by a partition wall composed of vertical plates.

4. The urea hydrolysis method according to claim 2, wherein the interior of the tower-type closed container is divided into two chambers by a partition wall composed of a vertical cylinder.

5. The urea hydrolysis method according to claim 2 or 3, wherein the top of the partition wall in the tower-type closed container forms an overflow weir structure.

6. The urea hydrolysis method according to claim 1, wherein the solution is in an upward flow in the first step and in a downward flow in the second step.

7. The urea hydrolysis method according to claim 2, wherein the tower-type closed container is equipped with a multi-layered perforated plate inside.

8. The urea hydrolysis method according to claim 1, wherein the first step and the second step are both carried out at a temperature of 180-230°C and a pressure of 10-30 kg/ ^cm² gauge.

9. The urea hydrolysis method according to claim 1, wherein the urea content of the low-concentration aqueous solution after the second treatment is 1 to 5 ppm.