JPS60262049A - Analysis of concentration of solution - Google Patents

Abstract

PURPOSE:To efficiently and continuously measure the concn. of a solution, by precipitating the crystalline substance in a specimen solution under cooling and detecting the change in the temp. within a measuring system caused by heat of precipitation while applying this temp. information to the memory system of crystalline substance and precipitation temp. CONSTITUTION:For example, gas containing ammonia, carbon dioxide steam and a minute amount of urea is introduced into an absorbing tower 2 under pressure of 17kg/cm<2>G at 125 deg.C through a line 1 and absorbed by the absorbing medium in said tower 2 while said absorbing medium is recirculated to a urea synthesis region 7 through a line 3, a pump 4, a strainer 5 and a line 3. A part of said medium is guided to the meandering tube 11 in a water bath 10 through a line 9 to be cooled at a cooling speed of about 5 deg.C/min and, when the temp. thereof reaches certain low temp. in a narrow path 14, a urea crystal begins to precipitate and the clogging of the narrow path 14 is judged at the point of time when the change in temp. T3 disappeared and the narrow path 14 is heated by a jacket 15 to melt the urea crystal while molten crystal is recovered to the line 1 through a line 16. Further, the concn. C of urea is preliminarily imparted to a measuring control system 12 as the function of water bath temp. T1, precipitation temp. T3 and pressure P to operate the concn. C.

Description

[Detailed description of the invention] 〔Technical field〕 The present invention relates to a method for analyzing solution concentration.

[Background technology]

Various chemical, electrical, or optical methods have been developed and put into practical use as methods for measuring the concentration of a solution. On the other hand, there are various chemical plants in which the concentration of solutions must be measured under high temperature and high pressure.

For example, in a urea production plant, there are many pipes that handle solutions that dissolve ammonia, carbon dioxide, and small amounts of urea, and sometimes the content of urea and/or ammonium carbamate increases and crystallizes and solidifies in the pipes. death,
It may interfere with continuous operation. In order to measure the concentration of a specific substance under such high temperature and high pressure conditions, a method is usually adopted in which a portion of the sample is extracted from the line and analyzed using conventional methods, but it is necessary to ensure the normal operation of the process. In order to do this, it is desirable to have an analysis system that is connected to the process without taking samples out of the process.

In view of the above-mentioned problems, the present inventor has conducted intensive research on a method for analyzing a solution containing and dissolving the four substances, and has completed the present invention.

[Disclosure of the invention]

That is, in the solution concentration analysis method of the present invention, a test solution containing and dissolving a crystalline substance is cooled.

The crystalline substance is crystallized, the temperature change in the measurement system due to the heat of crystallization at this time is detected, and this temperature information is sent to the storage system of the concentration of the crystalline substance and the crystallization temperature under a specific pressure to be measured. It is characterized by increasing the concentration of crystalline substances in the solution.

The solution targeted by the present invention is a solution containing and dissolving a crystalline substance. It goes without saying that even a gaseous mixture can be turned into a liquid by cooling and can be used as the test solution in the present invention. The test solution is sent to the crystallization process without being cooled. Crystallization is carried out, for example, by introducing the test solution into a bottleneck having one or several constrictions. l
The throttling at one or several locations produces a sufficient stirring effect within the bottleneck to prevent oversaturation. The number of apertures is determined as appropriate depending on the diameter and shape of the bottleneck. If cooling is continued at predetermined temperature intervals, the crystalline substance that is dissolved at a certain temperature will crystallize. At this time, a temperature change occurs due to heat of crystallization. This is taken as the crystallization temperature. The relationship between crystallization temperature and concentration at a certain pressure is measured in advance, and the concentration of the test solution is measured from the actually measured crystallization temperature.

The present invention will be specifically explained based on an example in which the present invention is applied to a urea production process.

As a method for producing urea, carbon dioxide and ammonia are reacted at a urea production temperature and pressure, and the resulting urea synthesis liquid is passed through a plurality of unreacted ammonium carbamate decomposition stages in which the pressure is sequentially reduced. Unreacted ammonia and mucarbamate are decomposed and separated as a mixed gas of ammonia, carbon dioxide and water vapor, and the resulting aqueous urea solution containing a small amount of ammonia and carbon dioxide is concentrated under reduced pressure to produce crystalline urea. A method is known as an established technique in which the separated gas mixture is sequentially absorbed into an absorption medium and recycled to the urea synthesis zone.

The absorption solution that sequentially absorbs ammonia and carbon dioxide obtained by decomposing unreacted ammonium carbamate contained in the urea synthesis solution usually contains 0 to 50% (by weight) ammonia and 0 to 50% (by weight) carbon dioxide. Besides, urea is 0
Contains ~100% (by weight). This absorption solution is circulated to the urea synthesis zone, and a portion of the absorption solution is taken out from this line to the concentration analysis system according to the present invention. The removed absorption solution is cooled at predetermined temperature intervals to avoid supersaturation of the crystalline material. This temperature information is transmitted to the measurement control system. Cooling is performed, for example, by making the line a spiral pipe and passing it through a water bath, and the water bath temperature is selected to be a temperature (stomach) at which crystalline substances do not precipitate during the cooling stage. The cooled absorption solution has a pressure (P)
and a line through which the temperature (T2) can be measured, leading to a bottleneck with one or several apertures. Pressure information (P)
shall be transmitted to the measurement control system. In this bottleneck, the absorption solution is sufficiently stirred by one or several constrictions, and when cooled to the crystallization temperature, part or all of the crystalline substance in the absorption solution is crystallized in the bottleneck. The end of the bottleneck is connected by a line to the inside of the system in which the separated mixed gas is sequentially absorbed into an absorption medium, and a thermometer is installed near the end of the bottleneck to measure the temperature (T3) in the line. The information is transmitted to the measurement control system.

Also, make sure that the bottleneck area can be heated with a jacket, etc. When some or all of the crystalline substance begins to crystallize within the bottleneck, the temperature T3 changes due to the heat of crystallization. At the same time, when all of the water crystallizes, the pressure (P) inside the bottleneck also changes. The measurement control system is given information in advance about the concentration of the crystalline substance in the absorption solution as a function of pressure, cooling temperature, and crystallization temperature, and the measured values T, , T3 . The crystalline substance concentration (C) in the absorption solution is determined by P. This concentration information is linked to the control system for the supply amount of the absorption medium in which the separated mixed gas is sequentially absorbed into the storage medium A, so that when the concentration exceeds a predetermined concentration, the concentration can be kept small. It is preferable to keep it. Of course, the supply amount of the absorption medium may be changed manually based on the concentration information.

After the measurement is completed, the crystalline substance crystallized in the bottleneck is heated and melted by a jacket, and the separated mixed gas is collected into a system that sequentially absorbs it into an absorption medium.

According to the present invention, it becomes possible to efficiently and continuously measure the concentration in a solution in which a crystalline substance is dissolved. If connected directly to a chemical plant process, the solution concentration can be analyzed while ensuring the normal operation of the process.

[Preferred Mode for Carrying Out the Invention] The present invention will be specifically explained below with reference to Examples.

Example Carbon dioxide and ammonia are reacted at the urea synthesis temperature and pressure, and the resulting urea synthesis solution is passed through a plurality of unreacted ammonium carbamate decomposition stages in which the pressure is sequentially reduced to remove unreacted components contained in the urea synthesis solution. Ammonium carbamate is decomposed and separated as a mixed gas of ammonia, carbon dioxide, and water vapor, and the resulting aqueous urea solution containing a small amount of ammonia and carbon dioxide is concentrated under reduced pressure to produce crystalline urea, which is then centrifuged. However, the separated mixed gas was sequentially absorbed into an absorption medium and circulated to the urea synthesis zone.

FIG. 1 shows a flow diagram in which a concentration analysis system as an example of the present invention is incorporated into the line circulating from the absorption tower to the urea synthesis zone during this process.

Pressure 17kg/cJG through line 1, temperature 125°
A mixed gas containing ammonia, carbon dioxide, water vapor, and a trace amount of urea in C was absorbed into an absorption medium in an absorption tower 2, and was circulated to a urea synthesis zone 7 via a line 3, a pump 4, a strainer 5, and a line 6. .

From line 6, a portion of the absorption solution was passed through line 9 with valve 7.8 to water bath 10. Inside the water bath 10,
The piping was a serpentine pipe 11. Water bath 10 is 5. C7ke. 61st grade is good. (This temperature information T was transmitted to the measurement control system 12.

The cooled absorption solution is directed through line 13 to bottleneck 14 . Pressure information P on the line 13 was transmitted to the measurement control system 12. The bottleneck 14 is made heatable by a heating jacket 15. The bottleneck 14 is connected to the line 16 via the line l
connected to. Temperature information T3 near bottleneck 14 of line 16
was transmitted to the measurement control system 12. When the temperature T3 became low and reached a certain temperature, urea crystals began to crystallize in the bottleneck 14, and as the crystallized, the temperature T3 rose due to the heat of crystallization, and when the bottleneck 14 was closed, the temperature T3 stopped changing. At that point, jacket 15 heated one passage 14 to dissolve the crystallized urea, which was recovered through line 16 to line 1. Information on the urea concentration C was given in advance as the number of patterns of the relationship between temperature T3 and time. By calculation in the measurement control system, the concentration of urea in this absorption solution is 20
% (weight), and the value exceeded the safe operating condition limit of 18%, so steam was immediately supplied to line 1 through a line (not shown) that supplied steam to line 1, and the value was reduced to 18%. (weight) It was diluted to the following.

Example 2 Formaldehyde, sodium hydroxide, and acetaldehyde are reacted to produce pentaerinistol, excess alkali is neutralized with formic acid, etc., and crude pentaerinistol is obtained by filtration separation, which is used as a high-purity product. For this purpose, it is poured into a re-dissolution tank, completely dissolved with a centrifugation-like mother liquor described later and pure water for concentration adjustment, and purified by passing through a layer of adsorbent such as activated carbon.

The treated liquid is concentrated and crystallized, and the crystals are separated using a centrifuge and dried to obtain the product "Pentaerinistol" crystals.

FIG. 3 shows a flowchart in which the concentration analysis system of the present invention is installed in the line for sending liquid from the redissolution tank to the adsorption tower during this process.

Crude pentol crystals are fed into redissolution tank 4 through line 1, dissolved by centrifuged mother liquor from line 2 and purified water from line 3, and pumped to pump 5. 6. Adsorption tower 7. The liquid was sent to a concentration system 9 through a line 8.

Line 6 also passed a portion of the lysate to water bath 13 through line 12 with valve 10.11. The piping inside the water bath 13 was a serpentine pipe 14. The water bath 13 was cooled at temperature intervals of approximately °C/min. This temperature information T was transmitted to the measurement control system 19.

The cooled absorption liquid is led through line 15 to bottleneck 16 . The pressure information P in the line 15 was transmitted to the measurement control system 19, the bottleneck 16 was made heatable by the heating jacket 17, and was circulated to the remelting tank 4 via the line 18.

Temperature information T3 near the bottleneck 16 of the line 18 was transmitted to the measurement control system 19.

When the temperature T3 decreases and reaches a certain temperature, crystals begin to crystallize in the bottleneck 16, and as the crystals crystallize, the temperature T3 decreases due to crystal heating.
3 rose, and when the insides of the bottlenecks 1 and 6 were blocked, there was no change in the temperature T3.At that point, the bottleneck 16 was heated from the 1□7e to dissolve the crystallized substance.

It was collected into the remelting tank 4 through the line 18. Temperature T3
The pattern of the relationship between time and time was similar to that shown in Figure 2. Information on the redissolved liquid concentration C as a function of the water bath temperature T1, crystallization temperature T8, and pressure P was given to the measurement controller 19 in advance. The concentration of pentaerinistol in this redissolved solution was determined to be 30% (by weight) by calculation in the measurement control system, and this value was approximately 31% below the operating limit.
Immediately, the amount of pure water in line 3 was reduced to approximately 31% (by weight).

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing Example 1 of the present invention. FIG. 2 is a graph showing a pattern of the relationship between temperature (T3) and time. FIG. 3 is a flow sheet showing Example 2 of the present invention. Patent applicant: Mitsui Toatsu Chemical Co., Ltd.

Claims (4)

[Claims] (1) The crystalline substance is crystallized while cooling the test solution that contains and dissolves the crystalline substance, and the temperature change in the measurement system due to the heat of crystallization at this time is detected, and this temperature information is used under a specific pressure. A method for analyzing the concentration of a solution, characterized in that the temperature of the crystalline substance in the test solution is determined by applying the concentration and crystallization temperature of the crystalline substance to a storage system. (2) The method according to claim 1, wherein the crystalline substance is crystallized by introducing the test solution into a bottleneck having one to several constrictions. (3) Claim 1 in which the crystalline substance is urea crystals
or the method described in Section 2. (4) The method according to claim 1 or 2, wherein the crystalline substance is a pentaerythritol crystal.