JPH10120415A - Recovery of liquid ammonia from aqueous solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for recovering liquid ammonia from an aqueous solution, enabling to obtain the highly pure liquid ammonia by bringing an ammonia-containing aqueous solution into countercurrent contact with steam to release the ammonia. SOLUTION: This method for recovering liquid ammonia from an aqueous solution comprises supplying an ammonia-containing aqueous solution 1 subjected to preliminary treatments in a stirring tank 2 and a precipitation tank 5 into the upper stage of a distillation tower 11 operated at the approximately atmospheric pressure through a liquid-sending pump 8 and a liquid heat exchanger 9, supplying heating steam 15 generated in a reboiler 14 into the bottom part 13 to bring the steam into countercurrent contact with the ammonia aqueous solution, discharging the released ammonia from the overhead 12, guiding the ammonia into a primary partial condenser 20 to cool the ammonia with ordinary temperature cooling water by the use of an indirect cooling surface 21, returning the obtained primary partially cooled liquid 22 into the overhead 12, guiding the left primary non-condensed vapor 24 into a secondary partial cooling condenser 25 to cool the vapor at a temperature below the ordinary temperature, returning the secondary partially condensed liquid 27 into the overhead 12, sending the left secondary non-condensed vapor 28 into a vapor compressor 29 to compress the vapor, and further sending the compressed vapor into a whole condenser 31 to cool the compressed vapor with ordinary temperature cooling water to obtain the liquid ammonia.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a liquid ammonia (hereinafter referred to as "liquid ammonia") by compressing and liquefying concentrated ammonia vapor obtained by distilling free ammonia dissolved in water using a distillation column near normal pressure. ).

[0002]

2. Description of the Related Art Ammonia (NH ₃ ), molecular weight 17.0, boiling point −33.4 ° C./760 mmHg and water (H ₂ O), molecular weight 18.0, boiling point 10
It is well known to obtain a mixture of 0.0 ° C./760 mmHg by separating it into NH ₃ and H ₂ O of a predetermined purity, and the industrial purpose is achieved by distillation utilizing the difference in volatility between the two. It is customary.

[0003] distilled by NH ₃ and H ₂ O mixture NH ₃ and H easily planned by chemical engineering fractionation calculated using the vapor-liquid equilibrium relationship between the fractionating to ₂ O which such two components Can be set up.

FIG. 2 shows NH ₃ and H ₂ O under a normal pressure of 760 mmHg.
Is a gas-liquid equilibrium diagram of FIG.
₃ concentrations. The two curves XT and YT indicate the concentrations X and Y of the liquid phase and the gaseous phase in equilibrium with each temperature T.
For example, at a normal pressure of 760 mmHg and 40 ° C., the NH ₃ concentration in the liquid phase X1 = 24.4 mol%, while the gas phase concentration Y1 = 94.7 mol%
Are in equilibrium. FIG. 2 shows that pure H ₂ O has a boiling point of 100 ° C. under normal pressure, but needs to be cooled to −33.4 ° C. or lower in order to condense 100% of NH ₃ under normal pressure. Therefore, in order to distill the mixed solution of NH ₃ and H ₂ O under normal pressure and separate it into pure NH ₃ and H ₂ O, the bottom liquid at 100 ° C.
It is necessary to provide a means for heating as described above and for cooling and condensing NH ₃ emerging from the top to -33.4 ° C or lower.

It is a known means to carry out distillation under pressure in order to avoid such a low-temperature condensation means. For example, when pure NH ₃ and H ₂ O are taken out from the top and bottom of the column, respectively, if the cooling and condensing temperature of the top vapor is set at 20, 30, 40 ° C., the corresponding NH ₃ vapor pressure is 8.74, 11.9, 15.9
kg / cm ² pressurized system, the corresponding bottom temperature is 173.3, 18
The temperature rises to 6.7 and 200.1 ° C. FIG. 3 shows the vapor-liquid equilibrium relationship of a binary mixture of NH ₃ and H ₂ O under a total pressure of 16.0 kg / cm ² .

[0006]

The increase in the boiling point difference and the relative volatility between NH ₃ and H ₂ O indicates the ease of separation by distillation. However, equipment for cooling and heating means is required. Costs and utility costs may increase.

That is, if a plan is made to condense and obtain NH ₃ distilled from the top under normal pressure, the condensing temperature is −33.4
° C, and the temperature of the refrigerant to be supplied to the condenser requires a low temperature of -40 ° C or less. Therefore, the atmospheric distillation column itself can be installed relatively inexpensively, but requires expensive refrigeration equipment attached to the condenser and has to bear the operating cost.

On the other hand, if the refrigeration equipment is removed and cooling is performed using ordinary industrial water or circulating water from a cooling water tower, the cooling water temperature is generally expected to be 20 to 30 ° C., although it varies depending on the region and season. Whereas NH distilling from the top of the tower
Assuming that the temperature at which ₃ is condensed and liquefied is 30-40 ° C, the corresponding pressure will be 12-16 kg / cm ² . The bottom temperature at this time rises to 187 to 200 ° C. according to the above pressure, and when a heating source is required from steam, a high-pressure boiler exceeding 20 kg / cm ² is required. Therefore, canceling the refrigeration facility and using low-cost normal-temperature cooling water requires a high-pressure distillation column and a high-pressure boiler, and is subject to various laws and regulations, which may lead to an increase in the facility.

[0009]

As described above, as a means for separating an aqueous solution containing NH ₃ into a high-concentration solution and a bottom solution containing no free ammonia, not only equipment costs and operating costs but also safety regulations are required. There is no satisfactory process including the one. It is an object of the present invention to solve such an economic problem.

In order to achieve the above object, in the present invention, an atmospheric distillation column is used, and a stock solution is sent to an upper stage of the column, and NH ₃ mixed vapor distilled from the top of the column is connected in series with several stages of partial condensers. The condensing temperature was lowered successively using NH ₃ in the uncondensed vapor, and the concentration was continued until the concentration reached a predetermined concentration.After that, using a steam compressor, the pressure was raised to a predetermined pressure, and then all the condensers were cooled using normal temperature cooling water. The whole amount is condensed to obtain liquid cheapness.

Referring to FIG. 1, the outline of the solution will be described. An undiluted solution 1 of an aqueous solution of NH ₃ is fed after the pretreatment described below is performed in the stirring tank 2 and the precipitation tank 5 if necessary, or directly when the pretreatment is unnecessary. The liquid is fed as a supply liquid 10 through a liquid heat exchanger 9 by a pump 8 to a predetermined position in an upper stage of a distillation column 11 operated near normal pressure. The inside of 11 is composed of various packings or trays, and a gas-liquid contact portion equivalent to the number of theoretical plates necessary for performing predetermined separation between the tower top 12 and the tower bottom 13 is prepared. 13, a reboiler 14 is installed, and the bottom liquid is heated and evaporated by the heating steam 15. Steam 16 for direct injection without using 14 and 15
May be blown into the tower bottom 13 to generate the same amount of ascending steam as by 14 in the interior of 11. At this time, since the state of 13 is almost 100 ° C. at almost normal pressure,
Of course, 15 can be covered by low-pressure steam. Thus, gas-liquid contact between the descending liquid and the ascending vapor is performed at each point in the interior of the counter 11, and the light component NH
₃ is accompanied by the vapor, and the heavy component H ₂ O moves toward the top of the tower as a liquid and moves toward the bottom of the tower. The obtained bottom liquid 17 reaches a liquid at about 100 ° C. at normal pressure where substantially free NH ₃ has disappeared, and after performing heat exchange in the liquid heat exchanger 9 with the above-mentioned liquid feed, the discharged liquid 18 Become.

On the other hand, the overhead vapor 19 distilled off from 12 is the first vapor
The first indirect cooling surface 21 is guided to the next partial condenser 20 and is cooled by ordinary-temperature, low-cost cooling water such as industrial water or return water from a cooling water tower.
A part thereof is condensed to become the first condensed liquid 22, and the other passes as the first uncondensed vapor 24. At this time, the compositions of 22 and 24 are substantially in a gas-liquid equilibrium relationship, and are uniquely determined by the condensation temperature at that time. For example, under normal pressure 40
At ° C., the equilibrium concentration of 22,24 NH ₃ is X1 = 24.
4, Y1 = 94.7 mol%. 24 obtained is the second
In the next partial condenser 25, it is further cooled to a low temperature by low-temperature cold water, brine or the like obtained by a refrigerator or other means, and a part thereof is condensed on the second indirect cooling surface 26 to become a second condensate 27. The remainder leaves from 25 as second uncondensed vapor 28. The composition of 27 and 28 at this time is also determined by the condensation temperature as in the first order. For example, if the condensation temperature is set to 10 ° C., the corresponding NH ₃ concentrations of 27 and 28 are calculated to be X2 = 40.0 and Y2 = 99.3 mol%. The primary and secondary condensed liquids 22 and 27 obtained as described above are collectively sent to the reflux liquid 23 as 12.

The secondary uncondensed vapor (NH ₃ vapor) 28 having reached a predetermined concentration is then suctioned and pressurized by a vapor compressor 29, and the discharge-side vapor 30 is supplied to the entire condenser 31 by cooling water at normal temperature and low cost. The whole amount is liquefied to become a liquor 32, stored in the receiver 33, and then sent out of the system by a liquid sending line 34. The pressure of 30 is determined by the relationship with the temperature of the cooling water used at 31. If the temperature of the cooling water is 30 ° C. and the temperature of the condensate is 40 ° C., the pressure needs to be about 16 kg / cm ² .

[0014]

As described above, the present invention provides NH _{3 to} H ₂ O under normal pressure.
As shown in FIG. 2, the vapor-liquid equilibrium relationship of the gas phase concentration Y is remarkably higher than the liquid-phase concentration X in the entire concentration range as shown in FIG.
The first contrivance is to process using partial condensers in stages in series. Firstly, the first stage uses low-cost ordinary-temperature cooling water, and at a stroke, about 95 mol% of NH ₃ vapor is obtained by partial condensation. In the second stage, the remaining partial condensation is performed using cold water or brine. Subsequently, after obtaining NH ₃ vapor of a higher concentration, the liquid is obtained by using an ammonia liquefaction apparatus comprising a normal refrigeration ammonia compressor and a total condenser. NH exceeding 99 mol%
_{3 The} distillation column can be operated at normal pressure and open to the atmosphere to obtain steam. As utilities required for the distillation column, in addition to cooling water at room temperature, a small amount of relatively high-temperature cold water of about 0 to 10 ° C. for secondary condensation or brine and low-pressure steam for heating are sufficient.

The operation of the present invention for obtaining a liquid ammonia by obtaining a high-concentration NH ₃ vapor by means of the atmospheric distillation column constituting the present invention and the two-stage partial condenser having a different condensing temperature and then compressing and condensing this NH ₃ vapor. Was explained. However, in order to implement this, the mode is largely changed by each utility condition and applicable law. For example, in a cold region, cooling water around 0 ° C. can be easily obtained in winter.
Operation of the secondary condenser will not be necessary. Also, depending on the factory,
If a certain amount of groundwater around 15 ° C can be used, 3
Application such as dividing into stages is effective. In addition, strict regulations regarding refrigeration security and high-pressure handling exist for liquid safety equipment mainly for compressors of ammonia, but in the present invention, this is limited to a limited area mainly for compressors and condensers. Stops, stills and peripherals can be placed outside the limits of the pressure vessel, which helps reduce overall equipment costs.

Next, the effect of the presence of the third component in the raw aqueous ammonia solution on the practice of the present invention will be described. First, equations (1) and (2) show the equilibrium relationship (← →) between ammonia and water. NH ₃ (g) + H ₂ O (l) ← → NH ₄ OH (l) (1) NH ₄ OH (l) ← → NH ₄ ⁺ (l) + OH ^- (L) ... (2) In (1), NH ₃ (g) is in a gaseous state of NH ₃ , which is a liquid H ₂ O Combined with (l), NH in the liquid phase
₄ Equivalently present as OH (l) molecules,
In equation (2) this is further partially liquid phase for the ^{_{electrolyte, NH 4 + (l),}} OH - positive (l), which indicates that the presence dissociate negative ions. NH ₃ in the liquid phase in FIG.
The concentration X is determined by comparing the NH ₄ OH (1) in (1) with the NH ₄ OH in (2)
₄ ⁺ (l), while Y is NH ₃
This corresponds to (g).

The equilibrium of the equations (1) and (2) is simultaneously established between the gas and the liquid. According to this, at each point inside the distillation column, the temperature is shifted to the left side of the equation (1) by heating and raising the temperature. I do. As a result of multistage countercurrent contact at each point in the column, NH ₃ (g) is separated to the top and H ₂ O (l) is separated to the bottom,
Discharge from the top and bottom of each tower.

If a non-volatile acid such as sulfuric acid H ₂ SO ₄ is present as a third component in the liquid, NH ₄ OH (1) in (1) is used.
Undergoes a stoichiometric neutralization reaction as shown in formula (3) to produce ammonium sulfate (NH ₄ ) ₂ SO ₄ . 2NH ₄ OH (l) + H ₂ SO ₄ (l) = (NH ₄ ) ₂ SO ₄ (l) + 2H ₂ O (l) (3) Ammonium sulfate is converted into positive and negative ions in the liquid. It exists dissociated. _{(NH 4) 2 SO 4 (} l) ← → 2NH 4 + (l) + SO 4 - (l) ······· (4) Small amounts of H ₂ SO ₄ is present in a large amount of NH ₃ In the system, equations (1),
(2), (3), and (4) are simultaneously established.
At this time, as shown in the formula (3), 2 moles of NH ₄ OH in the liquid are quantitatively calculated based on 1 mole of H ₂ SO ₄ (NH ₄ ) ₂ SO ₄ (l)
Even when there is a change in composition due to operations such as heating and distillation, this always remains stably in the liquid, and the entire amount is discharged as the bottom liquid 17.

Therefore, non-volatile acid groups are present in the stock solution and NH
If _{the 3} is fixed as a salt, if free this NH ₃
If it is necessary to decompose and recover the salt, first add a non-volatile strong alkali such as caustic soda NaOH, calcium hydroxide Ca (OH) _2, etc. to a chemical equivalent or more to the stock solution to double-decompose the salt and remove the base component. fixed, it is necessary to convert the free NH ₃ to remove the NH _3. Equation (5) shows one example. (NH ₄ ) ₂ SO ₄ (l) + Ca (OH) ₂ (S) ← → 2NH ₄ OH (l) + CaSO ₄ (S) Equation (5) The Ca (OH) ₂ (S) and CaSO ₄ (S) in the parentheses indicate the state in which calcium hydroxide and calcium sulfate are suspended in the liquid as solid phases, respectively.

The stirring tank 2 and the precipitation tank 5 in FIG. 1 perform an operation of treating such a stock solution to convert fixed NH ₃ to free NH _3, and convert the above (NH ₄ ) ₂ SO ₄ (l) A slurry of Ca (OH) ₂ (S), which is the alkali 3 added in 2, is added to the stock solution 1 containing 2, and the metathesis reaction represented by the formula (5) proceeds toward the right side and is completed. The alkali adjusting solution 4 in which CaSO ₄ (S) is suspended is then separated under gravity in a settling tank 5 and separated into a settled sludge 6 and a supernatant 7. 6 discharged from the bottom is further disposed by known means such as filtration. Thus, in 7, all the fixed NH _{3 in the} stock solution 1 is changed to free NH ₃ and sent to the distillation column as it is for recovery.

[0021] Now in the stock solution, a part of the non-volatile acidic groups SO ₄ of NH ₃ as described above ^-, PO ₄ ^- in addition to which are stably fixed by such a volatile acid groups CN ^{- _{, HS -, HCO 3 -,}} HSO
₃ ^- it may have been loosely secured to create an equal and salt. For example, acidic ammonium sulfide (NH ₄ ) HS is in in-dissociation equilibrium in a liquid as shown in formula (6), but the components decomposed by heating and raising the temperature are directed to the right side of formula (7). . _{NH 4 HS (l) ← →} NH 4 + (l) + HS - (l) ············· (6) NH 4 HS (l) ← → NH 3 (g) + H 2 S (g) (7) Thus, both NH ₃ and H ₂ S, a volatile acid, escape into the gas phase. In such a case, in order to completely recover NH ₃ in the liquid by distillation with high purity, a non-volatile base sufficient to completely fix the acid group, such as NaOH, is charged into the H ₂ S gas phase. A pre-treatment is required to stop the migration of the compound and keep the solution as a non-volatile salt, Na ₂ S. Equation (8) illustrates this reaction. NH ₄ HS (l) + 2NaOH (l) → NH ₃ (g) + Na ₂ S (l) + 2H ₂ O (l) (8) This point is a volatile acid below H ₂ S, for example CO ₂ , SO ₂ , HCN
It is almost the same when any of the above is present, and in any case, it is impossible to remove the NH _{3 component} in the stock solution from the top of the column with a purity not containing a weak acid component.

When the volatile bases HCO ₃ ^- and CO ₃ ^- are present in the stock solution when the present invention is carried out, the ammonium salt has a complicated behavior inside and outside the distillation column. is required. When a large amount of NH ₃ present in comparison with the CO ₂ is in a liquid, wherein (9), NH ₃ in 2 equivalents to CO ₂ (10)
However, the decomposition proceeds toward the left side of (10) by heating, NH ₃ (g) escapes to the gas phase, and firstly, ammonium bicarbonate (NH ₄ ) HCO ₃ (l) remains. Further heating and decomposition progress,
Gas phase NH ₃ (g) and CO ₂ (g) move toward the left side of equation (9). CO ₂ (g) + NH ₃ (g) + H ₂ O (l) ← → (NH ₄ ) HCO ₃ (l) (9) (NH ₄ ) HCO ₃ (l) + NH ₃ (g) ← → (NH ₄ ) ₂ CO ₃ (l) ··························································································································································· In the vicinity, (NH ₄ ) ₂ CO ₃ and (NH ₄ ) HCO ₃ are decomposed to the left in equation (9) following equation (10) and the resulting NH ₃ (g), CO ₂ (g) moves to the gas phase and rises in the tower. On the other hand, the temperature in the tower is about 60 ° C at the top of the tower,
The rising CO ₂ (g) is partially captured by the descending liquid together with NH ₃ (g) and descends in the column, so that NH ₃ and CO _{2 in} the liquid and vapor finally distributed in the column are vaporized. The composition becomes considerably complicated, and the bottom liquid and the top gas follow it.

As described above, when CO ₂ is present as an ammonium salt in the raw NH ₃ liquid, a problem occurs due to ammonium carbonate vapor distilled off from the top in addition to the decrease in the purity of NH ₃ obtained from the top. There are cases. Solid ammonium carbonate evaporates with increasing temperature from around 50 ° C as its physical properties.
NH ₃ (g) and CO ₂ (g) are generated, but the mixed vapor generated in reverse is solid (NH ₄ ) CO ₃ (S) or (NH ₄ ) by cooling below 30 ° C.
It has the property of returning to HCO ₃ (S). Equation (11) shows such a sublimation phenomenon of ammonium carbonate. (NH ₄ ) CO ₃ (S), (NH ₄ ) HCO ₃ (S) ← → NH ₃ (g) + CO ₂ (g) + H ₂ O (11) The present invention When performing the above, NH ₃ (g) and CO ₂ (g) decomposed to the right side of the equation (11) and exiting the distillation column are cooled to a low temperature of 40 ° C. and 10 ° C. in the primary and secondary condensers. And is sucked into the vapor compressor. On the low pressure side of the steam compressor from the secondary condenser, the temperature of the steam is about 10 ° C., so NH ₃ (g) and CO ₂ (g) in the steam are reversed to the left side of equation (11). As a result, a solid ammonium carbonate salt is generated, and sublimates and precipitates as a solid between the outlet of the secondary condenser and the compressor blowing side as it is, so that the growth may be continued and the portion may be eventually closed. Therefore, in this case, the non-volatile alkali such as chemical equivalent or more NaOH shown in equation (12) to the stock solution was added CO ₂
It is necessary to carry out a pretreatment for neutralizing and fixing a basic component such as a non-volatile salt to form a non-volatile salt, and then supply the same to a distillation column and discharge the entire amount from the bottom of the column. (NH ₄ ) CO ₃ (l) + 2NaOH (l) = Na ₂ CO ₃ (l) + 2H ₂ O (l) + 2NH ₃ (g) (12) As an example of volatile ammonium salts, carbonates have been mentioned. However, when weak acid ammonium salts that cause the same amount of sublimation are present in the stock solution, pretreatment such as (12) is required for stable operation. is there.

[0024]

In the flow sheet of Embodiment 1 FIG 1, NH ₃ minutes 20.8
mol%, residual H ₂ O aqueous ammonia solution 100.0 kmol / h 4
The solution is sent to the liquid heat exchanger 9 by the solution sending pump 8 at 5.0 ° C. without performing the pretreatment, and the solution is heated to about 60 ° C. and then sent to the distillation column 11. The column body of the distillation column is composed of an ordered packing having a separation performance equivalent to 15 theoretical plates, and the reflux liquid is at the uppermost stage,
The stock solution is sent one stage below. About 16.3kmol /
h (280 kg / h), 2.0 kg / cm ² of saturated steam, 16 is blown. Heating of the bottom of the tower is performed only with 16 and reboiler 1
4 is not installed. Thereby, the feed liquid 10 is dissolved by the vapor rising from the bottom of the tower when the feed liquid 10 descends in the tower.
The theoretical number of columns in the column and the ratio between 10 and 16 are calculated so that NH ₃ is expelled and 0.1% mol at the bottom of the column. At this time, the bottom liquid 17 is subsequently led to the above-mentioned 9 and exchanges heat with 7 to become a discharged liquid 18. On the other hand, from the tower top 12
The overhead vapor 19 of about 60 ° C. distilled off including NH ₃ reaches the primary partial condenser 20, and is maintained at about 40 ° C. by adjusting the amount of cooling water supplied to 21 at about 30 ° C. Partial condensation is performed, and the liquid is separated into a primary condensed liquid 22 and a primary uncondensed vapor 24. The NH ₃ concentration of 24 corresponding to this condensation temperature is 94-9
Reaches 5% mol. 24 is a secondary partial condenser 25
In the same manner as in the first stage, the cooling water of 1 to 2 ° C. is sent to 26 to advance the partial condensation while controlling the flow rate while maintaining the condensation temperature of 10.0 ° C., and the second condensate 27 and the second Condensed steam 28
Divided into NH ₃ concentration of 28 obtained at this time is 99.0 to 99.
Reaches 3% mole. Each of the condensates 22 and 27 is returned to the top of the column as a reflux 23, while
Is sent to an ammonia vapor compressor 29 and is pressurized. The discharge-side vapor 30 obtained from the discharge side is cooled by room-temperature cooling water in the total condenser 31 and condensed in its entirety.

Table 1 shows the set specifications and operation results, and Table 2 shows an analysis of the energy load.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

The present invention is configured as described above and has the following effects.

From an NH ₃ aqueous solution of an arbitrary concentration, a concentration of 99.0 mo
Recover 1% or more of the liquid. At that time, the distillation column used is operated at about normal pressure, and each part is in the range of about 0 to 100 ° C., and is not subject to various regulations by the pressure vessel.

The top vapor is guided to a two-stage partial condenser under normal pressure in the same manner as the main body of the tower to obtain high-concentration NH ₃ vapor. In the first stage, partial condensation is performed using low-cost ordinary-temperature cooling water. After a large amount of water removal by the method,
By using a small amount of cold water at around ° C., high-concentration NH ₃ vapor having a residual moisture of 1% or less can be obtained. In the example of Table 2, the heat load of the secondary condenser is 以下 or less of the primary and １ ／ or less of the entire system. The equipment and utility costs for this are significantly lower than in a conventional one-stage partial condensation process.

[0031] During stock CO ₃ ^-, HCO ₃ ^- ammonium salt of a volatile weak acid groups are present, such as, decomposed into NH _3, CO _2, etc. within the distillation column, followed by a condenser, internal compressor or the like If there is a risk of sublimation precipitation as a solid of ammonium carbonate and clogging, a pretreatment of adding a non-volatile alkali such as NaOH to the stock solution in advance to fix the carbonate group in the solution as a non-volatile salt such as Na ₂ CO ₃ After that, a stable and safe operation is achieved by feeding the mixture to a distillation column and discharging the entire amount thereof as a bottom liquid.

After obtaining a predetermined concentration of NH ₃ vapor from the secondary partial condenser, the pressure is increased by a vapor compressor, and then a liquid ammonia is obtained by a total condenser using cooling water at normal temperature.
Only this part is a high-pressure system, but the main part of the well-known and widely used refrigeration system using ammonia as a refrigerant can be adopted as it is, and highly safe equipment that meets various regulations can be procured at low cost. Can be.

[Brief description of the drawings]

FIG. 1 is a flow sheet showing a flow of an entire process.

FIG. 2 is a diagram showing a two-component system vapor-liquid equilibrium relationship of ammonia water, with a total pressure of 760 mmHg.

FIG. 3 is the same as in FIG. 2, with a total pressure of 15.0 kg / cm ² .

[Explanation of symbols]

1 Stock solution 8 Feeding pump 2 Stirring tank 9 Liquid heat exchanger 3 Additive alkali 10 Feed solution 4 Alkali adjusting solution 11 Distillation tower 5 Settlement tank 12 Tower top 6 Settling sludge 13 Tower bottom 7 Supernatant 14 Reboiler 15 Heating steam 25 Secondary partial condenser 16 Injection steam 26 Secondary indirect cooling surface 17 Bottom liquid 27 Secondary condensate 18 Effluent 28 Secondary uncondensed vapor 19 Top vapor 29 Steam compressor 20 Primary part Condenser 30 Discharge side steam 21 Primary indirect cooling surface 31 Total condenser 22 Primary condensate liquid 32 Liquid safety (liquid ammonia) 23 Reflux liquid 33 Liquid safety receiver 24 Primary uncondensed vapor 34 Liquid safety liquid line

Claims (2)

[Claims] 1. An aqueous solution containing ammonia is sent to an upper stage of a distillation column 11 operated at about normal pressure, and is brought into countercurrent contact with steam rising from a column bottom 13 while descending in the column to dissolve in the liquid. To disperse the ammonia, and to the top 1
2, while the falling deammonified liquid is led to the bottom of the tower and discharged as bottom liquid 17, the top vapor 19 is led to the primary partial condenser 20 to use indirect cooling water at normal temperature. The first partial condensation is performed on the cooling surface, and the obtained first
The secondary condensate 22 is refluxed near the top of the tower, and the remaining primary uncondensed vapor 24 is led to a secondary partial condenser 25 to perform secondary partial condensation at a temperature lower than the primary condensation temperature, The second obtained
The secondary condensate 27 recirculates near the top of the column in the same manner as the primary, and the remaining secondary uncondensed vapor 28 is sent to a vapor compressor 29 to increase the pressure to a predetermined pressure, and then the total condensation using cooling water at normal temperature Container 31
To recover the liquid 32 mainly containing ammonia by condensing under pressure. 2. The method according to claim 1, wherein the ammonium carbonate or the like decomposes in an aqueous solution containing ammonia under atmospheric pressure from about 50 ° C. and emits into a gaseous phase, while it decomposes into a solid at 30 ° C. or lower. When the aqueous solution having the property of coagulation and sublimation was treated, a non-volatile alkali such as NaOH was added to convert the ammonium salt in the solution into free ammonium and a non-volatile salt, and a precipitate formed as necessary was removed. A method for recovering liquor, which is supplied to a distillation column later.