JPH0722641B2 - Method for concentrating hygroscopic compound aqueous solution - Google Patents

Description

TECHNICAL FIELD The present invention relates to a vacuum concentration method for a solution consisting of water and a highly hygroscopic compound such as sulfuric acid.

[Conventional technology]

Generally, among strong acids such as sulfuric acid and phosphoric acid, strong alkalis such as caustic soda and potassium hydroxide, or chlorides and nitrates of various metals, a mixture of a highly hygroscopic compound and water (hereinafter referred to as an aqueous solution) cannot be dehydrated and concentrated. It is difficult and often requires a large amount of energy, and has a problem that it is highly corrosive.

These aqueous solutions can be concentrated by direct contact between the hot gas and the aqueous solution, such as spray tower or submerged combustion, by direct cooking, by multiple-effect can, by falling film method, or flash evaporation. There is a method of performing indirect heating mainly through a heat transfer surface, such as those described in (1), and these are appropriately used depending on the situation. At this time, as the water content decreases, the boiling point of the aqueous solution increases remarkably, and the concentration requires a high temperature. For this reason, the concentration operation is often performed under reduced pressure in order to lower the temperature in terms of materials such as corrosion and the problem of effective use of energy.

Now, the conventional method will be described by taking an example of concentration of sulfuric acid as follows.

(1) There are direct fire, Pauling type sulfuric acid concentration method, cascade type sulfuric acid concentration method, etc., but there is a material problem with the cast iron or silicon iron boiled container, and it cannot withstand long-term continuous operation. It is also said that the thermal efficiency is low.

(2) Well-known methods are those in which an acid solution is in direct contact with a high-temperature gas, those which are suitable for concentration to a relatively low concentration by in-liquid combustion, and those for high concentration which are Chemico drum type sulfuric acid concentration methods. These are operated near atmospheric pressure and have high heat utilization efficiency, but mist-like sulfuric acid mist is easily generated,
A large amount of equipment and cost are required for its removal.

(3) The vacuum method, which is different from the above (2), is indirect heating, and Simonson-Mantiu (Simonson-Mantiu)
s) It is a method known as the SM type sulfuric acid concentration method by the vacuum method or the initials. In this method, the main consists of the front and rear two-part condensing cans, except for the preliminary concentrating cans.
Under pressure of ~ 70mmHg, make 80% or more of sulfuric acid, 5 at the rear
85 ~ 93% concentrated sulfuric acid is obtained under the pressure of ~ 10mmHg.
In these cases, the temperature is maintained at a low temperature of 170 ° C. or lower. Therefore, decomposition of organic substances by hot sulfuric acid, decomposition of sulfuric acid itself, corrosion of equipment, etc. are greatly reduced. Then, the vacuum degree of each can is maintained by the steam ejector and the atmospheric leg condenser, and the steam booster is added to the high vacuum side.

Of these, the conventional sulfuric acid concentration method, which is a modification of the SM method, will be described with reference to the drawings.

In Fig. 2, dilute sulfuric acid as a raw material is passed through line 11 to a preconcentrator can.
Sent to F11. The can solution is circulated by the pump P11 and heated by the heat exchanger H11. The inside of the can is maintained in vacuum by steam radiators E12 and E13, condensers C11, C12 and C13, and atmospheric legs (not shown). The pressure is preferably low from the viewpoint of only evaporation, but it is usually around 60 Torr (vapor temperature of 41.5 ° C) because it is limited by the vapor pressure of water and the temperature of the cooling water due to the use of the atmospheric leg. Often suppressed. The evaporated water is discharged from the upper portion of the concentrating can F11 through the pipe portion 14, is suction-condensed by the action of each steam radiator and the condenser, and is discharged from the lower end of the atmosphere leg. On the other hand, the sulfuric acid aqueous solution that had been concentrated in the previous stage
Then, it is further concentrated by evaporation in a high-concentration canister F12 under high vacuum. Here too, the cans are heated and circulated by the pump P12 through the steam-heated heat exchanger H12. The vacuum of the highly concentrated can F12 is maintained at a high vacuum by which steam is hard to condense at normal temperature, for example, 1 to 10 Torr by the steam ejector E11 and the steam booster E14 using a large amount of steam. The steam booster E14 draws in a large amount of water vapor and compresses it to a pressure at which the atmospheric legs can be used, and thus requires a large amount of driving water vapor. The temperature here is determined by the concentration of the final product, the vapor partial pressure, the corrosiveness at high temperature, the melting point of some products, the presence or absence of crystal precipitation, etc.
When the solute is sulfuric acid, the temperature is about 170 ° C at maximum, usually 130 to 150 ° C, unless a high-grade material is used.

The vaporized water vapor is passed through line 15 and the steam radiator E1
The whole volume is sucked by 1, and steam booster E
The whole amount is sucked and compressed by 14, condensed in the condenser C11, mixed with the water from the pre-concentrator F11, and discharged from the atmospheric leg end. In each concentrator, some mist is generated along with water vapor and a loss of solute occurs, but this is rarely recovered because it is diluted with a large amount of condensed water in the condenser C11. Especially in the case of sulfuric acid, when the concentration is 85% or more, the vapor pressure of sulfuric acid becomes difficult to ignore, and the loss rapidly increases with increasing concentration. For example, 93% sulfuric acid reaches 23% by weight in the vaporized vapor.

Therefore, in order to perform the concentration differentially, it is also a common practice to partition the interior of the concentrator to multiple stages, but in any case, the solute that accompanies the vapor is often lost in the conventional method. The acid solution or the like diluted with the condensate to be diluted requires a detoxification treatment such as neutralization to prevent pollution.

[Problems to be solved by the invention]

In the above-mentioned concentration method by the vacuum method, since it is necessary to generate a high vacuum in which the atmospheric leg condenser cannot be used, the steam unit used by the steam radiator and booster is extremely large, and in the high concentration part of the evaporator. There is a problem in that effective mist recovery means for the generated mist or a solute portion entrained in water vapor due to evaporation is lacked and some post-treatment is required.

[Means for solving problems]

The present invention is intended to solve the above-mentioned problems, in which an aqueous solution of a hygroscopic compound is concentrated to a high concentration by using at least one preconcentrator and highly concentrated can. In addition, the water vapor generated by vacuum concentration from the highly concentrated can is contacted with an aqueous solution of the same compound having a water vapor partial pressure lower than the water vapor partial pressure to be absorbed, and an aqueous solution diluted by absorbing the water vapor is obtained. It is a method for concentrating an aqueous solution of a hygroscopic compound, which is characterized in that it is sent to a pre-concentrator and concentrated, and this concentrated solution is sent to a highly concentrated can to increase the concentration.

The present invention will be described in detail with reference to the drawings. FIG. 1 is an embodiment of the present invention. The raw material dilute sulfuric acid as an aqueous solution of the hygroscopic compound is passed through the conduit 1 to a highly concentrated can in the absorption cooler C4.
It absorbs the vapor generated in F2, dilutes and cools it to a predetermined concentration and temperature, and stores it in a storage tank T1 as necessary through a pipe line 1 '. This aqueous solution is sent by the pump P3 to the preconcentration canister F1 via the conduit 1 ″. The canister liquid is circulated by the pump P1 and steam-heated to a predetermined temperature by the heat exchanger H1. Inside are steam radiator E2 and E3, and condenser C
A vacuum is maintained by 1, C2, C3 and the atmospheric legs (not shown). Evaporated water is pipe 4 from the top of the preconcentration can F1.
Is discharged by the action of the steam radiator and the condenser described above, and is discharged from the lower end of the atmospheric leg. On the other hand, the sulfuric acid aqueous solution which has been subjected to the preconcentration passes through the pipe line 2 and is further evaporated and concentrated under a high vacuum in the next highly concentrated can F2. Here too, the cans are circulated and steam heated by the pump P2 through the heat exchanger H2. The vacuum of the highly concentrated can F2 is maintained at a high vacuum by the steam ejector E1 and the steam booster E4, but unlike the conventional method, the highly concentrated can F2
The steam generated from the water is sent to the absorption cooler C4 via the pipe 5, and at the same time is mixed and cooled by the raw material dilute sulfuric acid sent to the same place, and almost all of the steam passes through the pipes 1'and 1 "before the low vacuum. It is sent to the concentration canister F1, where water vapor is generated and discharged from the conduit 4. At this time, it is logical to use the conduit 1'as a so-called barometric clamp. The concentration of the raw material dilute sulfuric acid to be used needs to be such that the partial vapor pressure of the aqueous sulfuric acid solution after absorbing most of the steam generated in the concentrating can F2 is at least equal to or lower than the pressure in the highly concentrating can F2. The permissible concentration is governed by the temperature of the cooling water used for the absorption cooler, but it is 40-70 at around room temperature.
A concentration of% is sufficient. If the concentration of the raw material dilute sulfuric acid is not sufficient to secure this concentration, use a separate low-concentration concentrator to concentrate it before using it as the absorption cooling liquid, or if necessary, use the preconcentration can F1. It is also possible to increase the concentration by mixing part of the sulfuric acid concentrated in 1. or part of the product from the highly concentrated can F2 with the raw dilute sulfuric acid. The absorption cooler used here may be a type that performs absorption and cooling at the same time, but it is a combination of an absorption tower of a type that performs absorption and cooling in different places, and the absorption liquid is cooled and circulated by a separate heat exchanger. It may be a model of.

[Work]

In the case of concentrating an aqueous solution to a high concentration, in the conventional method, in order to maintain a high vacuum, a steam generator and two stages of steam boosters in series (E11 and E14 in Fig. 2) or a vacuum generator using at least one stage of a steam booster is used. In use, it was necessary to exclude from this system the vapors generated during the concentration, from the total volume. This is 10 when the vapor pressure of water is near room temperature.
~ There are several tens Torr, because the atmospheric leg condenser normally used cannot be economically used for high vacuum below about 30 Torr (sometimes a steam ejector is required after the atmospheric leg to remove a small amount of inert gas). For this reason, it was necessary to compress all the water vapor sucked by the steam booster and then compress it to raise the pressure (at least 30 Torr or more) at which the atmospheric leg condenser can be used.

The present invention drastically reduces the basic unit of steam used in this steam booster. That is, in the present invention, the steam generated in the high-concentration concentration canister F2 is not completely sucked by the steam ejector E1 and the steam booster E4, but simply kept to the minimum necessary to maintain a high vacuum, Almost all the remaining water vapor is absorbed and mixed with an aqueous solution having a required concentration under high vacuum to maintain the degree of vacuum. By doing so, the amount of steam used here can be saved more than 100 times (1/1
Can be less than or equal to 00).

Further, especially when the concentration increases, the solute that is sucked and discharged outside the system due to an increase in the vapor pressure of the solute itself or an increase in entrainment of the solute tends to increase. According to the conventional method shown in FIG. 2, it is clear that all the solute vapor generated in the highly concentrated can F2 or the entrainment of droplets containing the solute is removed by suction.
Most of it passes through the condenser C1 and becomes a dilute aqueous solution from the bottom of the atmosphere to be discharged to the outside of the system, so that its recovery is difficult. In the case of the present invention, most of the solute vapor or entrained liquid that passes through the pipe line 5 which is clear from FIG. 1 is captured by the absorption cooler C4, collected in the storage tank T1, and then recycled.
It is clear that the amount of loss can be ignored compared to the conventional method.

Note that the concentration of the aqueous solution sent to the highly concentrated can F2 is such that the vapor pressure at the evaporation temperature there is no choice but to use a steam booster, that is, it is difficult to condense using only the atmospheric leg condenser. It has no effect unless it is more than the concentration. This concentration is, for example, 85% by weight (hereinafter simply referred to as 85% by weight) at a temperature which is economically preferable in the case of sulfuric acid because of the corrosion resistance of the concentrator or the heating limit due to the usual inexpensive low pressure steam. Abbreviated, the same applies below). The vapor pressure of 85% sulfuric acid at 150 ° C is about 55 Torr, which corresponds to the vapor pressure of 40 ° C. In summer, the cooling water temperature may be close to 40 ° C, and it may be difficult to lower the pressure on the atmospheric leg below this temperature. Therefore, the sulfuric acid concentration when entering the highly concentrated can is 85
It turns out that there is a constraint of less than or equal to%.

The upper limit of the degree of enrichment in a highly concentrated can is theoretically 98%, which is the highest azeotropic composition, but the limit for economically concentrating is about 96% due to the vapor pressure and boiling point of sulfuric acid.

The concentration of the aqueous solution to be mixed with the steam from the highly concentrated can is the vapor pressure of the mixed aqueous solution after absorbing the generated steam from the concentrated can at the temperature near the water temperature of the absorption cooler is equal to the vapor pressure in the highly concentrated can. Or, the concentration needs to be lower than that. In the case of sulfuric acid, this is 40% or more, preferably 55%
Above 85% and below 95% sulfuric acid (170 ℃, PH ₂ O = 10.3mmH
g), for example, 60% sulfuric acid (40 ° C, PH ₂ O =
It is obvious that various choices such as 9.85 mmHg), 40% sulfuric acid (20 ° C., PH ₂ O = 9.95 mmHg) can be made.

The lower the temperature and the higher the concentration of the highly concentrated canister, the higher the concentration of the dilute solution required for absorption cooling and the lower the temperature. If the concentration of the raw material aqueous solution is too low for the above requirement, a preliminary concentration operation is naturally performed. Therefore, use an aqueous solution having an appropriate concentration as the absorption cooling liquid of the present invention, or an aqueous solution having a concentration in the middle. Part of the intermediate product of the preconcentration can, or if necessary, part of the product may be mixed with a dilute aqueous solution of the raw material to increase the concentration to the level required for absorption cooling before use. Is obvious, and the present invention covers such a case.

Although the present invention has been described above with respect to the case of concentrating an aqueous solution of sulfuric acid, it is clear that other than that, it is also applicable to hygroscopic inorganic compounds such as phosphoric acid and caustic, and the solute is not necessarily pure. It is not necessary to allow some mixing of organic and inorganic impurities, and the present invention includes all of them.

〔Example〕

FIG. 1 Table 1 shows the temperature, pressure, concentration, flow rate, etc. at the illustrated portions for the method of the present invention and FIG.

EFFECT OF THE INVENTION According to the present invention, the amount of steam used for heating and evaporating is less than that of conventional methods.
Although it slightly increases from 450kg / hr to 680kg / hr, 1190kg of steam for engineer including steam for booster
Since it significantly decreased from / hr to 20 kg / hr, the total amount of steam used was significantly reduced from 1640 kg / ton sulfuric acid to 700 kg / ton sulfuric acid. That is, the steam consumption rate can be greatly improved.

The water vapor discharged from the highly concentrated can has a sulfuric acid vapor partial pressure of 13% by weight with 92% sulfuric acid, and also contains a small amount of entrained water. Since all of them are diluted sulfuric acid aqueous solutions in the conventional method, it is difficult to recover and neutralize. However, in the present invention, most of these vapors are absorbed by the raw acid and are circulated and concentrated. There is almost no loss. Therefore, the load for detoxification is significantly reduced.

As described above, the present invention is extremely useful not only for energy saving but also for pollution prevention.

[Brief description of drawings]

FIG. 1 is a flow sheet for a sulfuric acid concentration according to an embodiment of the present invention, and FIG. 2 is a flow sheet for sulfuric acid concentration according to a conventional method. 1,1 ′, 1 ″, 2,3,4,5,6,11,12,13,14,15 …… Pipeline F1 …… Preconcentration can, F2 …… High concentration can H1, H2, H11, H12 …… Heat exchanger for solution heating E1, E2, E3, E11, E12, E13 …… Steam radiator E4, E14 ………… Steam booster C1, C2, C3, C11, C12, C13 …… Condenser C4… … Absorption cooler T1 …… storage tank, P1, P2, P12, P13 …… pump CW …… cooling water, STM …… steam

Claims (2)

[Claims] 1. An aqueous solution of a hygroscopic compound at least 1.
Water vapor generated by vacuum concentration from the highly concentrated can when condensing to high concentration by using individual pre-concentrators and highly concentrated cans has the same partial vapor pressure lower than that of the same compound. Moisture absorption characterized by contacting with an aqueous solution for absorption and absorbing the above-mentioned water vapor and sending the diluted aqueous solution to a pre-concentration can for concentration, and then sending this concentrated solution to a highly concentrated can for high concentration Method for Concentrating Aqueous Compound Aqueous Solution. 2. Water vapor generated from a highly concentrated can which is a vacuum concentrator for producing sulfuric acid whose hygroscopic compound is sulfuric acid and whose concentration concentration is 85% by weight or more, is 40% by weight or more and 85% by weight or more.
The method for concentrating an aqueous solution of a hygroscopic compound according to claim (1), wherein the method comprises contacting with an aqueous solution of sulfuric acid of not more than wt% to absorb the aqueous solution.