Method for concentrating sodium hydroxide solution

Abstract

The present invention relates to a method for concentrating a sodium hydroxide solution in a step-by-step process for concentrating the sodium hydroxide solution using a concentration unit in which a plurality of evaporators from a first evaporator to an nth evaporator are connected in series, the method comprising: introducing the sodium hydroxide solution into the first evaporator and then evaporating same to obtain a first sodium hydroxide concentrate, and recovering steam evaporated from the sodium hydroxide solution; and introducing the first sodium hydroxide concentrate into a subsequent evaporator, and introducing the steam recovered from the first evaporator into a mechanical steam recompression device and compressing same, wherein the nth evaporator uses the compressed steam as a heat source.

Description

Method for concentrating sodium hydroxide solution

[0001] Cross-citation with related applications

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0131171 filed September 26, 2024, and all contents disclosed in the document of said Korean patent application are incorporated herein as part of this specification.

[0003] Technology field

[0004] The present disclosure relates to a method for concentrating an aqueous sodium hydroxide solution, and more specifically, to a method for utilizing steam generated when evaporating and concentrating the aqueous sodium hydroxide solution in the first evaporator as a heat source for the last evaporator in a stepwise concentration process of an aqueous sodium hydroxide solution comprising a concentration unit in which a plurality of evaporators are connected in series.

[0005] Sodium hydroxide (NaOH) is obtained by electrolyzing sodium chloride (NaCl) to obtain an aqueous sodium hydroxide solution with a concentration of about 30% by weight, and then evaporated and concentrated until the concentration of sodium hydroxide reaches about 50% by weight, and shipped as a product.

[0006] Specifically, a common method of evaporative concentration involves heating an aqueous solution of caustic soda (sodium hydroxide) in an evaporator to evaporate the water, and then separating the concentrated solution heated and concentrated in the evaporator from the evaporated water vapor. At this time, there is a need for the development of technology that can concentrate the sodium hydroxide solution more efficiently while recovering the latent heat of the steam generated in the evaporator to save energy and minimizing the amount of steam used during evaporator heating.

[0007] For example, there is a method of using a plate-type heat exchanger as an evaporator to concentrate an aqueous sodium hydroxide solution. In this case, while it is possible to miniaturize the device by improving the thermal efficiency of the heat transfer surface of the plate-type heat exchanger used in the evaporator, there are limitations to the energy saving effect.

[0008] The problem to be solved in this disclosure is to provide a method for concentrating an aqueous sodium hydroxide solution that can reduce the amount of steam and cooling water used in the sodium hydroxide evaporation concentration process and further simplify the process, in order to solve the problem mentioned in the background technology above.

[0009] However, the problems that this disclosure aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by a person skilled in the art from the description below.

[0010] According to one aspect of the present disclosure for solving the above problem, a method for concentrating an aqueous sodium hydroxide solution is provided, comprising a stepwise concentration process of the aqueous sodium hydroxide solution including a concentration unit in which a plurality of evaporators from a first evaporator to an nth evaporator are connected in series, the step of introducing the aqueous sodium hydroxide solution into the first evaporator and evaporating it to obtain a first sodium hydroxide concentrate and recovering steam evaporated from the aqueous sodium hydroxide solution; and introducing the first sodium hydroxide concentrate into a downstream evaporator and introducing the steam recovered from the first evaporator into a mechanical steam recompression device to compress it.

[0011] In addition, the nth evaporator may include using the compressed steam as a heat source.

[0012] According to the method for concentrating an aqueous sodium hydroxide solution of the present disclosure, the amount of steam and cooling water used in the concentration process can be reduced, and the process can be further simplified.

[0013] Specifically, high-quality high-pressure steam can be produced by compressing the steam evaporating from the sodium hydroxide aqueous solution fed into the front-end evaporator, and by using this as a heat source for the rear-end evaporator, the amount of steam supplied from the outside into the process can be reduced.

[0014] In addition, according to the present disclosure, an amount of high-pressure steam exceeding the heat source supply required in the evaporation process can be produced, resulting in excess steam, and since the excess steam can also be supplied externally, economic efficiency can be improved.

[0015] The effects obtainable from the present disclosure are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the present disclosure belongs from the description below.

[0016] FIG. 1 is a process flowchart for concentrating an aqueous sodium hydroxide solution according to one embodiment of the present disclosure.

[0017] Figure 2 is a process flowchart for concentrating an aqueous sodium hydroxide solution according to Comparative Example 1.

[0018] Figure 3 is a process flowchart for concentrating an aqueous sodium hydroxide solution according to Comparative Example 2.

[0019] Terms and words used in the description and claims of this disclosure shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of this disclosure, based on the principle that the inventor may appropriately define the concept of the terms to best describe his invention.

[0020] In relation to the description of the drawings, similar reference numerals may be used for similar or related components.

[0021] The singular form of the noun corresponding to the item may include one or more of the said item unless the relevant context clearly indicates otherwise.

[0022] In the present disclosure, each of the phrases such as “A or B”, “at least one of A and B”, “at least one of A or B”, “A, B or C”, “at least one of A, B and C”, and “at least one of A, B, or C” may include any one of the items listed together in the corresponding phrase, or all possible combinations thereof.

[0023] The term "and / or" includes a combination of multiple related described components or any of the multiple related described components.

[0024] Terms such as "first," "second," or "first" or "second" may be used simply to distinguish a component from another component and do not limit the components in other aspects (e.g., importance or order).

[0025] Additionally, terms such as 'front,' 'rear,' 'top,' 'bottom,' 'side,' 'left,' 'right,' 'top,' and 'bottom' used in this disclosure are defined based on the drawings, and the shape and location of each component are not limited by these terms.

[0026] Terms such as "include" or "have" are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in this disclosure, and do not preclude the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0027] When it is said that a component is "connected," "combined," "supported," or "in contact" with another component, this includes not only cases where the components are directly connected, combined, supported, or in contact, but also cases where they are indirectly connected, combined, supported, or in contact through a third component.

[0028] When it is said that a component is located "on" another component, this includes not only cases where one component is in contact with the other, but also cases where another component exists between the two components.

[0029] Additionally, terms such as "about," "substantially," etc., as used in this disclosure are used to mean at or near the numerical values ​​where inherent manufacturing and material tolerances are presented in the stated meanings, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content where precise or absolute numerical values ​​are mentioned to aid in understanding this disclosure.

[0030] As used in this disclosure, the term 'stream' may refer to the flow of fluid within a process, and may also refer to the fluid itself flowing within the piping. Specifically, the stream may simultaneously refer to the fluid itself flowing within the piping connecting each device and the flow of the fluid. Additionally, the fluid may include one or more components selected from gas, liquid, and solid.

[0031] Unless otherwise specified, the term "upper" as used in this disclosure refers to a point at a height of 0% to 20% downward from the top of the device, and specifically may refer to the top (top of the tower). Additionally, the term "lower" refers to a point at a height of 80% to 100% downward from the top of the device, and specifically may refer to the bottom (bottom of the tower).

[0032] Unless otherwise specified, “pressure” as used in this disclosure means absolute pressure measured relative to absolute vacuum.

[0033] The operating temperature of the evaporator mentioned in the present disclosure may include both the temperature of the stream fed into the evaporator and the temperature of the stream discharged from the evaporator.

[0034] The present disclosure will be described in more detail below.

[0035] According to the present disclosure, a method for concentrating an aqueous sodium hydroxide solution is provided, wherein in a stepwise concentration process of an aqueous sodium hydroxide solution comprising a concentration unit in which a plurality of evaporators are connected in series, the amount of steam used can be reduced by utilizing the steam generated when evaporating and concentrating the aqueous sodium hydroxide solution in the first-stage evaporator as a heat source for the last-stage evaporator.

[0036] According to one embodiment, a method for concentrating an aqueous sodium hydroxide solution according to the present disclosure comprises, in a stepwise concentration process of an aqueous sodium hydroxide solution comprising a concentration unit in which a plurality of evaporators from a first evaporator to an nth evaporator are connected in series, a step of introducing an aqueous sodium hydroxide solution into the first evaporator and evaporating it to obtain a first sodium hydroxide concentrate and recovering steam evaporated from the aqueous sodium hydroxide solution; and a step of introducing the first sodium hydroxide concentrate into a subsequent evaporator and introducing the steam recovered from the first evaporator into a mechanical vapor recompression (MVR) device to compress it.

[0037] According to one embodiment, the concentration unit is configured with a total of n evaporators connected in series, and the number of evaporators can be designed considering conditions such as the concentration of the sodium hydroxide concentrate and steam economy. For example, n may be 2 to 4, specifically 2 to 3, and more specifically 3. That is, the concentration unit may include 2 to 4 evaporators, specifically 2 to 3, and more specifically 3 evaporators.

[0038] The nth evaporator can use steam recovered from the first evaporator and compressed into an MVR as a heat source, and, if necessary, can additionally use steam supplied from the outside as a heat source. In addition, the steam recovered from the first evaporator and compressed into an MVR can be supplied to the outside as excess steam in addition to being used as a heat source for the nth evaporator.

[0039] For example, the steam evaporated from the sodium hydroxide aqueous solution in the first evaporator may have a temperature of 60°C to 70°C at a pressure of 0 bar to 0.1 bar. Additionally, the steam may be compressed through the MVR to a pressure of 5 bar to 10 bar and a temperature of 170°C to 180°C. By compressing the steam evaporated from the sodium hydroxide aqueous solution in the first evaporator to the above pressure range, the steam temperature may be raised to a level where it can be recycled as a heat source for the nth evaporator.

[0040] The number of stages of the above mechanical vapor recompression device (MVR) can be designed by taking into account conditions such as the heat source introduced into the nth evaporator, the temperature and pressure of sodium hydroxide, etc. For example, the MVR may be equipped with 3 to 10 stages, 5 to 10 stages, or 7 to 10 stages.

[0041] The first to n-1st evaporators may use steam evaporated from a sodium hydroxide concentrate fed into the downstream evaporator of each evaporator as a heat source for each evaporator. For example, when there are three evaporators, the first evaporator may use steam evaporated from a first sodium hydroxide concentrate fed into the second evaporator as a heat source, and the second evaporator may use steam evaporated from a second sodium hydroxide concentrate fed into the third evaporator as a heat source.

[0042] In addition, the n-1 sodium hydroxide concentrate obtained from the n-1 evaporator may be introduced into the n-1 evaporator, and the n-1 sodium hydroxide concentrate may be evaporated to obtain the n-1 sodium hydroxide concentrate. For example, when there are three evaporators, a sodium hydroxide aqueous solution may be introduced into the first evaporator to evaporate the sodium hydroxide aqueous solution to obtain the first sodium hydroxide concentrate; the first sodium hydroxide concentrate obtained from the first evaporator may be introduced into the second evaporator to evaporate the first sodium hydroxide concentrate to obtain the second sodium hydroxide concentrate; and the second sodium hydroxide concentrate obtained from the second evaporator may be introduced into the third evaporator to evaporate the second sodium hydroxide concentrate to obtain the third sodium hydroxide concentrate as a product.

[0043] At this time, the concentration from the sodium hydroxide aqueous solution to the nth sodium hydroxide concentrate may be increased stepwise. Specifically, the concentration of the sodium hydroxide aqueous solution introduced into the first evaporator may be 26% to 34% by weight, preferably 28% to 34% by weight, and more preferably 30% to 34% by weight. Sodium hydroxide (NaOH) is produced by electrolyzing an NaCl aqueous solution in an electrolytic cell, and the concentration of the NaOH aqueous solution obtained therefrom may be within the above range, and more specifically, the concentration of the NaOH aqueous solution may be changed according to the operating conditions of the electrolytic cell.

[0044] In addition, the concentration of the nth sodium hydroxide concentrate obtained through the nth evaporator may be 48% to 52% by weight, preferably 49% to 51% by weight. By concentrating the NaOH aqueous solution to satisfy the above concentration range, residual NaCl in the NaOH aqueous solution can be removed. Furthermore, by increasing the concentration of the NaOH concentrate to the above range, the viscosity increases, and consequently, the ion mobility decreases, so a NaOH concentrate with reduced corrosiveness can be obtained as a product.

[0045] For example, when the number of multiple evaporators is 3 (n=3), the concentration of the sodium hydroxide aqueous solution fed into the first evaporator is 26% to 34% by weight, the concentration of the first sodium hydroxide concentrate discharged from the first evaporator (i.e., fed into the second evaporator) is 34% to 40% by weight, the concentration of the second sodium hydroxide concentrate discharged from the second evaporator (i.e., fed into the third evaporator) is 40% to 44% by weight, and the concentration of the third sodium hydroxide concentrate discharged from the third evaporator is 48% to 52% by weight.

[0046] Meanwhile, the plurality of evaporators from the first evaporator to the nth evaporator may be operated such that the upstream evaporator is operated at a relatively lower pressure and temperature compared to the downstream evaporator. For example, when there are three evaporators, the first evaporator may be operated at a temperature of 50°C or higher and less than 80°C and a pressure of 0 bar (i.e., full vacuum) or higher and less than 0.1 bar, the second evaporator may be operated at a temperature of 80°C or higher and less than 120°C and a pressure of 0.1 bar or higher and less than 1 bar, and the third evaporator may be operated at a temperature of 120°C to 160°C and a pressure of 1 bar to 2 bar. In this way, by operating the downstream evaporator at a relatively higher pressure and temperature compared to the upstream evaporator, the sodium hydroxide concentrate, which has increased in concentration while passing through the upstream evaporator, can be further concentrated in the downstream evaporator. Additionally, the steam evaporated from the sodium hydroxide concentrate in the downstream evaporator may have a temperature and pressure at a level where it can be used as a heat source for the upstream evaporator.

[0047] According to one embodiment, in order to increase the thermal energy efficiency of the concentration process, the first sodium hydroxide concentrate to the n-1 sodium hydroxide concentrate discharged from each of the first to n-1 evaporators can be preheated by heat exchange with the n-th sodium hydroxide concentrate and / or the condensate of steam used as a heat source in the n-th evaporator. Since the operating temperature of the n-th evaporator, which is the last among the plurality of evaporators, is the highest, the first to n-1 sodium hydroxide concentrates, which are at a relatively lower temperature, can be preheated by using the n-th sodium hydroxide concentrate discharged from the n-th evaporator and / or the condensate of steam used as a heat source in the n-th evaporator, thereby allowing sensible heat to be recovered.

[0048] For example, the first sodium hydroxide concentrate to the n-1 sodium hydroxide concentrate discharged from each of the first to n-1 evaporators may be preheated by heat exchange with the n-th sodium hydroxide concentrate and then fed into the downstream evaporator of each evaporator.

[0049] As another example, the first sodium hydroxide concentrate to the n-1 sodium hydroxide concentrate discharged from each of the first to n-1 evaporators may be preheated by heat exchange with the condensate of the steam used as a heat source in the n-th evaporator, and then fed into the downstream evaporator of each evaporator.

[0050] As another example, the first sodium hydroxide concentrate to the n-1 sodium hydroxide concentrate discharged from each of the first to n-1 evaporators may each be partially branched and preheated by heat exchange with the n sodium hydroxide concentrate, and the remainder may be preheated by heat exchange with the condensate of the steam used as a heat source in the n evaporator, and then fed into the downstream evaporator of each evaporator.

[0051] Hereinafter, the present disclosure will be described in more detail with reference to the drawings to facilitate understanding.

[0052] FIG. 1 is a flowchart of a process for concentrating an aqueous sodium hydroxide solution according to one embodiment of the present disclosure, in which three evaporators (n=3) are provided in the concentration section.

[0053] Referring to FIG. 1, in a sodium hydroxide aqueous solution evaporation system comprising an evaporation section equipped with a first evaporator (1), a second evaporator (2), and a third evaporator (3), a sodium hydroxide aqueous solution (A) is supplied to the first evaporator (1). The sodium hydroxide aqueous solution is evaporated in the first evaporator (1) to obtain a first sodium hydroxide concentrate (A1), and steam (B1) evaporated from the sodium hydroxide aqueous solution is recovered. At this time, the operating temperature of the first evaporator (1) may be 50°C or higher and less than 80°C, and the operating pressure may be 0 bar or higher and less than 0.1 bar.

[0054] The steam (B1) evaporated from the above sodium hydroxide aqueous solution is fed into a mechanical steam recompression device (MVR) (30) and compressed into high-pressure steam (B1'). At this time, the pressure of the high-pressure steam (B1') can be adjusted to 5 bar to 10 bar, specifically 7 bar to 9 bar, more specifically 7.5 bar to 8.5 bar, and the temperature can be 170°C to 180°C. The high-pressure steam (B1') compressed by the MVR is supplied as a heat source to a third evaporator (3) located at the rear end, together with steam (C) supplied from the outside.

[0055] The first sodium hydroxide concentrate (A1) obtained from the first evaporator (1) is supplied to the second evaporator (2) through a pump (21). In the second evaporator (2), the first sodium hydroxide concentrate is evaporated to obtain a second sodium hydroxide concentrate (A2), and steam (B2) evaporated from the first sodium hydroxide concentrate can be recovered. At this time, the operating temperature of the second evaporator (2) may be 80°C or higher and less than 120°C, and the operating pressure may be 0.1 bar or higher and less than 1 bar.

[0056] The steam (B2) evaporated from the first sodium hydroxide concentrate can be supplied as a heat source to the first evaporator (1) and heat exchanged with the sodium hydroxide aqueous solution (A). At this time, the pressure of the steam (B2) can be 0 bar to 1 bar, specifically 0.1 bar to 0.5 bar, more specifically 0.3 bar, and the temperature can be 100°C to 110°C. The condensate (B2') after heat exchange with the sodium hydroxide aqueous solution can be collected in a vapor condensate drum.

[0057] The second sodium hydroxide concentrate (A2) obtained from the second evaporator (2) is supplied to the third evaporator (3) through a pump (22). In the third evaporator (3), the second sodium hydroxide concentrate is evaporated to obtain a third sodium hydroxide concentrate (A3) as a product of the desired concentration, and steam (B3) evaporated from the second sodium hydroxide concentrate can be recovered. At this time, the operating temperature of the third evaporator (3) may be 120°C to 160°C, and the operating pressure may be 1 bar to 2 bar.

[0058] The steam (B3) evaporated from the second sodium hydroxide concentrate can be supplied as a heat source to the second evaporator (2) and heat exchanged with the first sodium hydroxide concentrate (A1). At this time, the pressure of the steam (B3) can be 1 bar to 2 bar, specifically 1.3 bar to 1.8 bar, more specifically 1.6 bar to 1.7 bar, and the temperature can be 155°C to 165°C. The condensate (B3') after heat exchange with the first sodium hydroxide concentrate can be collected in a steam condensation drum.

[0059] Meanwhile, the high-pressure steam (B1') and external steam (C) supplied as a heat source to the third evaporator (3), which is the final evaporator, can be discharged as condensate (C'+B1") after heat exchange with the second sodium hydroxide concentrate (A2) in the third evaporator (3).

[0060] If necessary, the third sodium hydroxide concentrate (A3) and / or the condensate (C'+B1") may preheat the first sodium hydroxide concentrate (A1) and the second sodium hydroxide concentrate (A2), respectively, through heat exchangers (11, 12, 13, 14). Specifically, the third sodium hydroxide concentrate (A3) may be heat-exchanged in heat exchanger (13) with a portion of the second sodium hydroxide concentrate (A2) branched into a stream, and the condensate (C'+B1") may be heat-exchanged in heat exchanger (14) with the remainder of the second sodium hydroxide concentrate (A2) stream. Subsequently, the third sodium hydroxide concentrate (A3) that has passed through the heat exchanger (13) may be heat exchanged with a branched stream of the first sodium hydroxide concentrate (A1) in the heat exchanger (11), and the condensate (C'+B1") that has passed through the heat exchanger (14) may be heat exchanged with the remaining stream of the first sodium hydroxide concentrate (A1) in the heat exchanger (12).

[0061] At this time, a portion of the stream and the remaining stream of the first sodium hydroxide concentrate (A1), which are each preheated in the heat exchangers (11, 12), are combined again and fed into the second evaporator (2), and a portion of the stream and the remaining stream of the second sodium hydroxide concentrate (A2), which are each preheated in the heat exchangers (13, 14), are combined again and fed into the third evaporator (3).

[0062] Although the method for concentrating an aqueous sodium hydroxide solution according to the present disclosure has been described and illustrated in the drawings above, the description and drawings above describe and illustrate only the essential components for understanding the present disclosure. In addition to the processes and apparatus described and drawings above, processes and apparatus not separately described and illustrated may be appropriately applied and utilized to carry out the method for concentrating an aqueous sodium hydroxide solution according to the present disclosure.

[0063] The present disclosure will be explained in more detail below through examples. However, the following examples are intended to explain the present disclosure more specifically, and the scope of the present disclosure is not limited by the following examples.

[0064] [Example]

[0065] Example 1

[0066] As shown in FIG. 1, an aqueous NaOH solution (A) with a concentration of 32 wt% is supplied to a first evaporator (1) to obtain a first NaOH concentrate (A1) with a concentration of 37 wt%, and the first NaOH concentrate (A1) is introduced into a second evaporator (2) through a pump (21) to obtain a second NaOH concentrate (A2) with a concentration of 42 wt%, and the second NaOH concentrate (A2) is introduced into a third evaporator (3) through a pump (22) to obtain a third NaOH concentrate (A3) concentrated to a concentration of 50 wt%.

[0067] At this time, the operating temperature of the first evaporator (1) was controlled to 58°C to 62°C (i.e., the temperature of the NaOH aqueous solution introduced into the first evaporator is 58°C and the temperature of the first NaOH concentrate discharged from the first evaporator is 62°C), and the operating pressure was controlled to 0.07 bar; the operating temperature of the second evaporator (2) was controlled to 98°C to 106°C (i.e., the temperature of the first NaOH concentrate introduced into the second evaporator is 98°C and the temperature of the second NaOH concentrate discharged from the second evaporator is 106°C), and the operating pressure was controlled to 0.38 bar; and the operating temperature of the third evaporator (33) was controlled to 148°C to 162°C (i.e., the temperature of the second NaOH concentrate introduced into the third evaporator is 148°C and the temperature of the third NaOH concentrate discharged from the third evaporator is 162°C), and the operating pressure was controlled to 1.69 bar.

[0068] Additionally, the first evaporator (1) used steam (B2) at a temperature of 105°C and a pressure of 0.3 bar, evaporated from the first NaOH concentrate (A1) in the second evaporator (2), as a heat source, and the second evaporator (2) used steam (B2) at a temperature of 162°C and a pressure of 1.67 bar, evaporated from the second NaOH concentrate (A2) in the third evaporator (3), as a heat source, and the third evaporator (3) used steam (C) at a temperature of 172°C and a pressure of 8.16 bar supplied from the outside, and high-pressure steam (B1') compressed to a temperature of 171.6°C and a pressure of 8.16 bar through a 10-stage MVR (30) from steam (B1) at a temperature of 62.1°C and a pressure of 0.07 bar, evaporated from the NaOH aqueous solution (A) in the first evaporator (1), as a heat source.

[0069] In addition, the third NaOH concentrate (A3) discharged from the third evaporator (3) was obtained as a product after passing through heat exchangers (11, 13), and the condensate (C'+B1") of the heat source of the third evaporator (3) was discharged after passing through heat exchangers (12, 14), thereby preheating the first NaOH concentrate (A1) and the second NaOH concentrate (A2) before they were introduced into the second evaporator (2) and the third evaporator (3), respectively.

[0070] Meanwhile, the condensate (B2', B3') after being used as a heat source in the first evaporator (1) and the second evaporator (2) was collected in a vapor condensate drum.

[0071] Comparative Example 1

[0072] As shown in FIG. 2, an aqueous NaOH solution (A) with a concentration of 32 wt% was supplied to a first evaporator (1) to obtain a first NaOH concentrate (A1) with a concentration of 36.7 wt%, and the first NaOH concentrate (A1) was introduced into a second evaporator (2) through a pump (21) to obtain a second NaOH concentrate (A2) with a concentration of 41.8 wt%, and the second NaOH concentrate (A2) was introduced into a third evaporator (3) through a pump (22) to obtain a third NaOH concentrate (A3) concentrated to a concentration of 50 wt%.

[0073] Comparative Example 1 was operated in the same manner as Example 1, except that the steam (B1) evaporated from the NaOH aqueous solution (A) in the first evaporator (1) in the above Example 1 was not compressed and used as a heat source for the third evaporator. Specifically, the steam (B1) evaporated from the NaOH aqueous solution (A) in the above evaporator (1) was condensed using cooling water in the condenser (15) and then collected in a vapor condensate drum.

[0074] Comparative Example 2

[0075] As shown in FIG. 3, an aqueous NaOH solution (A) with a concentration of 32 wt% was supplied to a first evaporator (1) to obtain a first NaOH concentrate (A1) with a concentration of 36.2 wt%, and the first NaOH concentrate (A1) was introduced into a second evaporator (2) through a pump (21) to obtain a second NaOH concentrate (A2) with a concentration of 43 wt%, and the second NaOH concentrate (A2) was introduced into a third evaporator (3) through a pump (22) to obtain a third NaOH concentrate (A3) concentrated to a concentration of 50 wt%.

[0076] At this time, the operating temperature of the first evaporator (1) was adjusted to 58°C to 62°C and the operating pressure to 0.07 bar, the operating temperature of the second evaporator (2) was adjusted to 100°C to 108°C and the operating pressure to 0.38 bar, and the operating temperature of the third evaporator (33) was adjusted to 151°C to 162°C and the operating pressure to 1.69 bar.

[0077] The heat source of the first evaporator (1) used steam (B1) at a temperature of 62.2°C and a pressure of 0.07 bar, which was evaporated from the NaOH aqueous solution (A) in the first evaporator (1), was compressed into steam at a temperature of 188.6°C and a pressure of 0.30 bar through the first MVR (31).

[0078] The heat source of the second evaporator (2) used steam (B2) at a temperature of 105.3°C and a pressure of 0.38 bar, which was evaporated from the first NaOH concentrate (A1) in the second evaporator (2), and compressed into steam at a temperature of 180.1°C and a pressure of 2.51 bar through the second MVR (32).

[0079] The heat source of the third evaporator (3) used steam (C) at a temperature of 176°C and a pressure of 9.11 bar supplied from the outside, and steam (B3) at a temperature of 161.6°C and a pressure of 1.67 bar evaporated from the second NaOH concentrate (A2) in the third evaporator (3), which was compressed into steam at a temperature of 177.0°C and a pressure of 6.84 bar through the third MVR (33).

[0080] In addition, the third NaOH concentrate (A3) discharged from the third evaporator (3) was obtained as a product after passing through heat exchangers (11, 13), and the condensate (C'+B3') of the heat source of the third evaporator (3) was discharged after passing through heat exchangers (12, 14), thereby preheating the first NaOH concentrate (A1) and the second NaOH concentrate (A2) before they were introduced into the second evaporator (2) and the third evaporator (3), respectively.

[0081] Meanwhile, the condensate (B1', B2') after being used as a heat source in the first evaporator (1) and the second evaporator (2) was collected in a vapor condensate drum.

[0082] [Experimental Example]

[0083] In the above Example 1 and Comparative Examples 1 and 2, the amount of external steam (C) supplied to the third evaporator (3) was compared and is shown in Table 1 below.

[0084] Specifically, in Table 1 below, "steam usage rate" is calculated as a percentage of the external steam usage (y) in each example and comparative example based on the external steam usage (x) used in Example 1. This can be expressed as a formula as follows.

[0085] Steam Usage Rate (%) = [(yx) / x]*100

[0086] Steam Usage Rate Example 10% Comparative Example 1100% Comparative Example 211.1%

[0087] Referring to Table 1 above, it was confirmed that in the case of the embodiment of the present disclosure, the steam usage rate was lower compared to the comparative example, and thus the external steam usage was reduced.

[0088] More specifically, in the case of Example 1, where the high-pressure steam (B1') generated by compressing the evaporated steam (B1) generated in the first evaporator (1) with the MVR (30) is used as a heat source for the third evaporator (3), once the high-pressure steam (B1') begins to be generated, it is possible to supply a sufficient heat source to the third evaporator (3) using only the high-pressure steam (B1') without the input of additional external steam (C). Furthermore, in Example 1, the high-pressure steam (B1') was produced in excess of the heat source supply required by the third evaporator (3), and consequently, surplus steam was generated. This surplus steam can be supplied to other external processes that require a heat source supply, or it can be sold externally.

[0089] Meanwhile, in the case of Comparative Example 1, in which steam (B1) evaporated from the NaOH aqueous solution (A) in the first evaporator (1) was not utilized as a heat source, not only was the amount of external steam used significantly higher compared to Example 1, but additional cooling water was required to condense the evaporated steam (B1).

[0090] In addition, in the case of Comparative Example 2, evaporated steam (B1, B2, B3) was utilized as a heat source in each of the multiple evaporators (1, 2, 3), so the amount of external steam used was reduced compared to Comparative Example 1, but it was confirmed that the amount of external steam used was greater than that of Example 1. Furthermore, Comparative Example 2 has the problem that MVR is required for each of the multiple evaporators, which increases process equipment costs, installation site, and process complexity.

[0091] Although exemplary embodiments of the present disclosure have been described above, the present disclosure is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

[0092] [Explanation of the symbol]

[0093] 1, 2, 3: Evaporator

[0094] 11, 12, 13, 14: Heat exchanger

[0095] 15: Condenser

[0096] 21, 22: Pump

[0097] 30, 31, 32, 33: Mechanical Vapor Recompression Unit (MVR)

[0098] A: Sodium hydroxide solution

[0099] A1, A2, A3: Sodium hydroxide concentrate

[0100] B1, B2, B3: Evaporated steam

[0101] C: External steam

Claims

1. A stepwise concentration process of an aqueous sodium hydroxide solution comprising a concentration unit in which a plurality of evaporators from the first evaporator to the nth evaporator are connected in series, A step of introducing an aqueous sodium hydroxide solution into the first evaporator and evaporating it to obtain a first sodium hydroxide concentrate, and recovering steam evaporated from the aqueous sodium hydroxide solution; and The method includes the step of introducing the first sodium hydroxide concentrate into a downstream evaporator and introducing the steam recovered from the first evaporator into a mechanical steam recompression device to compress it. A method for concentrating an aqueous sodium hydroxide solution, wherein the above nth evaporator uses the above compressed steam as a heat source.

2. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, wherein the heat source of the above-mentioned nth evaporator further comprises steam supplied from the outside.

3. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, wherein the first to n-1 evaporators use steam evaporated from a sodium hydroxide concentrate introduced into the downstream evaporator of each evaporator as a heat source for each evaporator.

4. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, further comprising the step of introducing an n-1 sodium hydroxide concentrate obtained from an n-1 evaporator into the n-th evaporator, and evaporating the n-1 sodium hydroxide concentrate to obtain an n-th sodium hydroxide concentrate.

5. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, wherein n is 2 to 4.

6. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, comprising: the first sodium hydroxide concentrate to the n-1 sodium hydroxide concentrate discharged from each of the first to n-1 evaporators being preheated by heat exchange with the n-1 sodium hydroxide concentrate, and then being fed into the downstream evaporator of each evaporator.

7. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, comprising: the first sodium hydroxide concentrate to the n-1 sodium hydroxide concentrate discharged from each of the first to n-1 evaporators being preheated by heat exchange with the condensate of steam used as a heat source in the n-th evaporator, and then being fed into the downstream evaporator of each evaporator.

8. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, comprising: the first sodium hydroxide concentrate to the n-1 sodium hydroxide concentrate discharged from each of the first to n-1 evaporators being partially branched and preheated by heat exchange with the n-1 sodium hydroxide concentrate, and the remainder being preheated by heat exchange with the condensate of steam used as a heat source in the n-1 evaporator, and then being fed into the downstream evaporator of each evaporator.

9. In Paragraph 1, The concentration of the sodium hydroxide aqueous solution introduced into the first evaporator is 26% to 34% by weight, and A method for concentrating an aqueous sodium hydroxide solution, wherein the concentration of the above-mentioned nth sodium hydroxide concentrate is 48% to 52% by weight.

10. In Paragraph 1, The steam evaporated from the sodium hydroxide aqueous solution in the first evaporator has a temperature of 60°C to 70°C at a pressure of 0 bar to 0.1 bar, A method for concentrating an aqueous sodium hydroxide solution, wherein the steam is compressed through the mechanical steam recompression device and controlled to a pressure of 5 to 10 bar and a temperature of 170 to 180°C.

11. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, wherein the above mechanical vapor recompression device is equipped with 3 to 10 stages.

12. In Paragraph 1, A method for concentrating an aqueous sodium hydroxide solution, wherein the plurality of evaporators from the first evaporator to the nth evaporator are operated such that the upstream evaporator is operated at a relatively lower pressure and temperature compared to the downstream evaporator.

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