Formaldehyde absorption system, absorption method, and method for designing absorption system

Abstract

[Problem] To provide: a formaldehyde absorption system which can be built at low cost yet demonstrates sufficient performance; an absorption method; and a method for designing an absorption system. [Solution] A first formaldehyde absorption system 10 collects a formaldehyde gas composition in the form of an aqueous formaldehyde solution by causing the formaldehyde gas composition to be absorbed by a liquid, the first formaldehyde absorption system 10 being mainly provided with one first absorption tower 100, a liquid supply unit 20 and a gas supply unit 30. The first absorption tower 100 is mainly provided with: a covered and bottomed cylindrical body part 110; five absorption sections 130a to 130e in which a formaldehyde gas composition is absorbed by a liquid; and three cooling sections 150a to 150c in which liquids in the absorption sections are cooled. [Selected drawing] FIG. 1

Description

Formaldehyde absorption system, absorption method, and method for designing an absorption system

[0001] The present invention relates to a system for absorbing formaldehyde, an absorption method, and a method for designing an absorption system.

[0002] A manufacturing apparatus is known that uses multiple absorption towers to react a reactant gas with water to produce an aqueous formaldehyde solution. In this manufacturing apparatus, the process of reacting the reactant gas with water is divided into a first absorption stage and a second absorption stage, with one absorption tower used in the first absorption stage and two to six absorption towers used in the second absorption stage. The total length of the towers in the first absorption stage is, for example, 2 to 8 m, and the total length of the towers in the second absorption stage is, for example, 4 to 6 m (Patent Document 1).

[0003] JP-A-51-56407 specification

[0004] However, increasing the number of towers or extending the total length of the towers increases the construction costs of the manufacturing equipment, while there is also the problem that the performance of the manufacturing equipment will be insufficient unless the number of towers and the total length are set to appropriate values.

[0005] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a formaldehyde absorption system, an absorption method, and a design method for an absorption system that can be constructed at low cost while still having sufficient performance.

[0006] The formaldehyde absorption system according to the first invention of the present application is an absorption system that absorbs a formaldehyde gas composition produced by a silver method with a liquid and recovers it as an aqueous formaldehyde solution, the formaldehyde gas composition containing at least 15.7% by weight to 27.0% by weight of formaldehyde, 17.3% by weight to 29.5% by weight of water, and 43.7% by weight to 65.0% by weight of an inert gas, the absorption system comprising one absorption tower or two absorption towers connected in series, the absorption tower having a cylindrical body with a lid and a bottom, and a formaldehyde gas recovery unit. The absorption system is equipped with an absorption section that absorbs the composition with a liquid and a cooling section that cools the liquid in the absorption section, the internal diameter of the absorption tower is 1,000 mm or more and 3,000 mm or less, the total length of all of the body sections included in the absorption system excluding the lid and bottom is 20,000 mm or more and 40,000 mm or less, the absorption system includes a total of two or more cooling sections, the liquid residence time per meter of length in the absorption section provided at the most downstream side of the absorption tower in the liquid flow is 0.8 minutes or more, and the cumulative residence time of the liquid in all of the absorption sections included in the absorption system is 150 minutes or more.

[0007] In the absorption section provided at the most downstream position of the absorption tower in the liquid flow, the liquid residence time per meter of length is preferably 1 minute or more and 2 minutes or less.

[0008] It is preferable that the cumulative residence time of the liquid in all the absorption sections included in the absorption system is 150 minutes or more and 2,000 minutes or less.

[0009] The absorption tower may further include a liquid supply unit that supplies liquid from above in the direction of gravity.

[0010] In the absorption section provided at the most downstream position of the absorption tower in the liquid flow, the liquid residence time per meter of length is preferably 0.8 minutes or more and 3 minutes or less.

[0011] It is preferable that the cumulative residence time of the liquid in all the absorption sections included in the absorption system is 150 minutes or more and 1,000 minutes or less.

[0012] The absorption tower may include an absorption tower provided on the upstream side and an absorption tower provided on the downstream side in the liquid flow, and the liquid discharged from the lower part of the absorption tower provided on the upstream side may flow into the absorption tower provided on the downstream side from the upper part of the absorption tower provided on the downstream side.

[0013] The absorption tower may further include a gas supply unit that supplies the formaldehyde gas composition from the lower part in the direction of gravity.

[0014] The absorber tower may include a plurality of absorption sections.

[0015] The formaldehyde absorption method according to the second invention of the present application is a method for absorbing a formaldehyde gas composition produced by a silver method with a liquid and recovering it as an aqueous formaldehyde solution, wherein the formaldehyde gas composition contains at least 15.7% by weight to 27.0% by weight of formaldehyde, 17.3% by weight to 29.5% by weight of water, and 43.7% by weight to 65.0% by weight of an inert gas, and the absorption system comprises one absorption tower or two absorption towers connected in series, and the absorption tower has a cylindrical body with a lid and a bottom, and a liquid absorber for absorbing the formaldehyde gas composition. The absorption system includes an absorption section that absorbs liquid by a body and a cooling section that cools the liquid in the absorption section, the internal diameter of the absorption tower is 1,000 mm or more and 3,000 mm or less, the total length of the body sections included in the absorption system is 20,000 mm or more and 40,000 mm or less, the absorption system includes a total of two or more cooling sections, the liquid residence time per meter of length is 0.8 minutes or more in the absorption section provided at the most downstream side of the absorption tower in the liquid flow, and a step of injecting water into the absorption tower so that the cumulative residence time of the liquid in all absorption sections included in the absorption system is 150 minutes or more.

[0016] In the absorption section provided at the most downstream position of the absorption tower in the liquid flow, the liquid residence time per meter of length is preferably 0.8 minutes or more and 3 minutes or less.

[0017] It is preferable that the cumulative residence time of the liquid in all the absorption sections included in the absorption system is 150 minutes or more and 2,000 minutes or less.

[0018] A design method according to a third invention of the present application is a design method for an absorption system that absorbs a formaldehyde gas composition produced by a silver process with a liquid and recovers it as an aqueous formaldehyde solution, the absorption system comprising two absorption towers connected in series, each of which comprises a cylindrical body with a top and a bottom, an absorption section that absorbs the formaldehyde gas composition with the liquid, and a cooling section that cools the liquid in the absorption section, and the formaldehyde gas composition is a mixture of 15.7% by weight or more and 27.0% by weight or less of formaldehyde and 17.3% by weight or more and 29.5% by weight or less of formaldehyde. and 43.7% by weight or more and 65.0% by weight or less of an inert gas, and this design method specifies that the internal diameter of the absorption tower is 1,000 mm or more and 3,000 mm or less, the total length of the body sections included in the absorption system is 20,000 mm or more and 40,000 mm or less, the absorption system includes a total of two or more cooling sections, the liquid residence time per meter of length in the absorption section provided at the most downstream side of the absorption tower in the liquid flow is 0.8 minutes or more, and the cumulative residence time of the liquid in all absorption sections included in the absorption system is 150 minutes or more.

[0019] The two absorption towers may include one existing absorption tower and one newly installed absorption tower, and the newly installed absorption tower may be connected in series to the existing absorption tower.

[0020] In the absorption section provided at the most downstream position of the absorption tower in the liquid flow, the liquid residence time per meter of length is preferably 0.8 minutes or more and 3 minutes or less.

[0021] It is preferable that the cumulative residence time of the liquid in all the absorption sections included in the absorption system is 150 minutes or more and 2,000 minutes or less.

[0022] The present invention provides a formaldehyde absorption system, an absorption method, and a method for designing an absorption system that can be constructed at low cost while still providing sufficient performance.

[0023] 1 is a schematic diagram of a first formaldehyde absorption system according to a first embodiment of the present invention; FIG. 2 is a schematic diagram of a second formaldehyde absorption system according to a second embodiment of the present invention; FIG. 3 is a table showing an example of the present invention; FIG. 4 is a table showing a comparative example; FIG. 5 is a table showing five conditions used in a numerical simulation and the simulation results thereof; FIG. 6 is a graph showing the relationship between liquid residence time per unit height and Liquid H2O / Feed F; FIG. 7 is a graph showing the relationship between cumulative residence time and cumulative formaldehyde conversion rate; and FIG. 8 is a graph showing the relationship between cumulative formaldehyde conversion rate and formaldehyde recovery rate.

[0024] A first formaldehyde absorption system 10, absorption method, and method for designing an absorption system according to a first embodiment of the present invention will now be described with reference to FIGS.

[0025] The first formaldehyde absorption system 10 is an absorption system that absorbs a formaldehyde gas composition produced by a silver method into a liquid and recovers it as an aqueous formaldehyde solution, and mainly includes a first absorption tower 100, a liquid supply unit 20, and a gas supply unit 30. In this embodiment, pure water is used as the liquid.

[0026] The first absorption tower 100 mainly comprises a cylindrical body 110 with a lid and a bottom, five absorption sections 130a to 130e for absorbing the formaldehyde gas composition into a liquid, and three cooling sections 150a to 150c for cooling the liquid in the absorption sections.

[0027] The body 110 is preferably made of stainless steel and comprises a right cylinder having top and bottom openings hermetically covered with a spherical crown-shaped lid and bottom. The length of the body 110 excluding the lid and bottom is the axial length of the right cylinder, in other words, the length of the linear portion of the right cylinder, and is 20,000 mm to 40,000 mm. The internal diameter of the first absorber 100, i.e., the internal diameter of the right cylindrical portion of the body 110, is 1,000 mm to 3,000 mm. A pipe 170 is connected to the spherical crown at the top of the body 110, and gas in the first absorber 100 flows out.

[0028] The absorption units 130a to 130e are stored in the body 110 in order from the top vertically. The absorption units 130a and 130d are so-called tray-type units, and each has a configuration in which trays with multiple holes are stacked in the direction of gravity, and liquid is injected from above the force of gravity and gas is injected from below the force of gravity, dissolving the gas in the liquid. More specifically, liquid is stored on the top surface of the tray, and gas that has passed through the multiple holes from below the tray passes through the liquid on the tray. At this time, the gas dissolves in the liquid.

[0029] The absorption units 130b and 130e are so-called filling types, and each has a configuration in which a large number of metal rings are randomly filled, and liquid is injected from above gravity and gas is injected from below gravity to dissolve the gas in the liquid. More specifically, liquid is sprayed onto the filled metal rings from above gravity, and the liquid drips between the metal rings. Gas rising from below gravity comes into contact with these droplets, and the gas dissolves in the liquid. The area filled with a large number of metal rings is called the filling unit.

[0030] The absorption unit 130c is of the aforementioned fill type. Liquid flowing in from above due to gravity passes through a large number of metal rings to dissolve gas.

[0031] The absorption units 130b, 130c, and 130e are each provided with a cooling unit 150a, 150b, or 150c that cools the liquid in the absorption units 130b, 130c, or 130e. Each of the cooling units 150a, 150b, or 150c mainly includes a heat exchanger 151a, 151b, or 151c and a pump 152a, 152b, or 152c. The pump 152a is connected to a pipe provided near the bottom of the absorption unit 130b and sends the liquid flowing out of the absorption unit 130b to the heat exchanger 151a. The heat exchanger 151a cools the liquid from the pump 152a and sends it upward under gravity into the absorption unit 130b. The sent-out fluid passes through the absorption unit 130b again. A pipe 153 is connected to the pipe between the heat exchanger 151a and the absorption unit 130b, and a diluted formaldehyde aqueous solution is injected into the absorption unit 130b through the pipe 153. The use of the diluted formaldehyde aqueous solution makes it possible to improve the amount of formaldehyde recovered in all of the steps of producing formaldehyde, including the step related to the first formaldehyde absorption system 10 and the steps before and after it. In this way, the liquid flowing out from the bottom of the absorption unit 130b is cooled through the pump 152a, the heat exchanger 151a, and the pipe, and is returned to the absorption unit 130b by gravity upward.

[0032] The pump 152b also sends liquid from near the bottom of the absorption unit 130c to the heat exchanger 151b, and the heat exchanger 151b also cools the liquid from the pump 152b and sends it upward due to gravity into the absorption unit 130c. In this way, the liquid flowing out from the bottom of the absorption unit 130c is cooled through the pump 152b, the heat exchanger 151b, and the piping, and is returned upward due to gravity into the absorption unit 130c.

[0033] The pump 152c also sends liquid from near the bottom of the absorption unit 130e to the heat exchanger 151c, and the heat exchanger 151c also cools the liquid from the pump 152c and sends it upward due to gravity into the absorption unit 130e. In this way, the liquid flowing out from the bottom of the absorption unit 130e is cooled through the pump 152c, the heat exchanger 151c, and the piping, and is returned upward due to gravity into the absorption unit 130e. A piping 160 is provided between the pump 152c and the heat exchanger 151c, through which the liquid in which formaldehyde is dissolved flows out. A portion of the liquid flowing out of the pump 152c flows out of the system 10 through the piping 160, and the remainder flows into the heat exchanger 151c.

[0034] The liquid supply unit 20 mainly includes a flow controller 21, a transmitter 22, a flow meter 23, a control valve 24, and a pipe 25, and supplies a liquid, for example, pure water, to the first absorption tower 100 from near the top thereof. The transmitter 22 transmits the value output by the flow meter 23 to the flow controller 21.

[0035] One end of the pipe 25 is connected to a storage tank (not shown), and the other end is connected to the vicinity of the top of the first absorption tower 100 via a flow meter 23 and a control valve 24. Pure water is placed in the storage tank, and the pure water is supplied to the vicinity of the top of the first absorption tower 100 via the pipe 25. The vicinity of the top is between the top cap and the top end of the absorption section 130a. That is, the liquid supply section 20 is connected from the storage tank to the first absorption tower 100 via the flow meter 23 and the control valve 24.

[0036] Flow controller 21 is connected to flow meter 23 and control valve 24, and opens and closes control valve 24 in accordance with the value of flow meter 23 while taking into consideration the flow rate of the liquid flowing through pipe 25a, thereby controlling the amount of liquid flowing into first absorption tower 100. Because a constant amount of liquid always flows through pipe 25a, the amount of liquid flowing into first absorption tower 100 is controlled by adjusting the flow rate of the liquid flowing through pipe 25a.

[0037] With regard to the inflow rates, the feed flow rates of the raw material gas and pure water are determined based on the formaldehyde concentration and production rate required in the final product. Because the feed flow rates of the raw material gas and pure water are determined by the liquid residence time, the liquid residence time is determined so that the feed flow rates of the raw material gas and pure water are the determined rates, and the internal diameter and longitudinal length of the absorption tower are determined so that the determined liquid residence time is achieved. In other words, because the feed flow rates of the raw material gas and pure water are limited by the internal diameter and longitudinal length of the absorption tower, the internal diameter and longitudinal length of the first absorption tower 100 must be determined based on the formaldehyde concentration and production rate required in the final product. Meanwhile, because the feed flow rates of the raw material gas and pure water are limited by the absorption section 130e provided at the most downstream position of the first absorption tower 100, the flow rates flowing through the absorption section 130e essentially become the feed flow rates of the raw material gas and pure water in the first absorption tower 100. Therefore, the size of the first absorption tower 100 is determined so that the liquid residence time per meter of length in the absorption section 130e, which is provided at the most downstream position of the first absorption tower 100, is a predetermined period. The length referred to here includes the distance between multiple trays in the tray type, and refers to the length of the packed metal rings in the packed type. The liquid residence time in each of the absorption sections 130a to 130e is the residence time of the liquid in that absorption section, and is calculated using the following formula. Note that the circulation flow rate in the following formula is the flow rate per unit time of the liquid circulating through each absorption section. (Liquid residence time in each absorption section) = (Liquid volume retained in each absorption section) / (Liquid outlet flow rate excluding circulation flow rate) The liquid residence time per meter of length is calculated using the following formula. (Liquid residence time per meter of length) = (Residence time in each absorption section) / (Length of each absorption section) From the above two equations, it can be understood that shortening the liquid residence time means that the amount of liquid retained in each tray in a tray type or in the packed section in a packed type is reduced. Reducing the amount of liquid retained also means that the internal diameter of the absorption tower can be reduced. Here, if the internal diameter of the absorption tower is reduced without changing the amount of formaldehyde gas composition supplied to the absorption tower, the linear gas velocity within the absorption tower increases. If the linear gas velocity exceeds a certain value, a phenomenon known as "flooding" occurs, making it impossible to operate the absorption tower.When the liquid residence time is less than 0.8 min / m, flooding occurs, making it impossible to operate the absorption tower. Therefore, the liquid residence time per meter of length is preferably 0.8 min to 3 min, and more preferably 1 min to 2 min.

[0038] Alternatively, the first absorber 100 is designed with respect to its internal diameter and longitudinal length such that the cumulative residence time of the liquid in all of the absorption sections 130a-130e included in the absorption system 10 is a predetermined period. The cumulative residence time is the sum of the residence times of the liquid in all of the absorption sections 130a-130e in the first absorber 100. The predetermined period is preferably 150 minutes or more and 2,000 minutes or less, more preferably 150 minutes or more and 1,500 minutes or less, and even more preferably 150 minutes or more and 1,000 minutes or less. The cumulative residence time that satisfies these periods and times is determined in advance by computational simulation or experiment, and the internal diameter and longitudinal length of the first absorber 100 are determined based on the predetermined cumulative residence time.

[0039] Formaldehyde exists as substances such as methylene glycol and hemiformal in the liquid of the absorption sections 130a to 130e due to various reactions. The proportion of formaldehyde converted into other substances relative to the amount of formaldehyde flowing into each absorption section 130a to 130e is called the formaldehyde conversion rate. The sum of the formaldehyde conversion rates in all absorption sections 130a to 130e in the first absorption tower 100 is called the cumulative formaldehyde conversion rate. The ratio of the amount of formaldehyde absorbed by the liquid in the first absorption tower 100 and recovered as a product to the total amount of formaldehyde supplied to the first absorption tower 100 is called the formaldehyde recovery rate.

[0040] The gas supply unit 30 is, for example, a formaldehyde reactor using a silver catalyst, and supplies a formaldehyde gas composition to the first absorption tower 100 from near its bottom. More specifically, the gas supply unit 30 generates a formaldehyde gas composition from a raw material gas containing a circulating gas such as methanol gas, water vapor, air, and an inert gas, and supplies the aldehyde gas composition as a gas from near the bottom of the first absorption tower 100. The formaldehyde gas composition contains at least 15.7 wt % to 27.0 wt % formaldehyde, 17.3 wt % to 29.5 wt % water, and 43.7 wt % to 65.0 wt % inert gas. The inert gas is, for example, nitrogen, but may also contain other inert gases.

[0041] Next, the function of the first formaldehyde absorption system 10 will be described.

[0042] First, the liquid supply unit 20 directly injects pure water from the pipe 25 into the first absorber 100. Then, the flow controller 21 controls the adjustment valve 24 so that the flow meter 23 indicates a predetermined inflow rate, thereby injecting an appropriate amount of liquid into the first absorber 100. The liquid supply unit 20 can also adjust the inflow rate as appropriate depending on the actual operating conditions. The inflow rate is the value described above. The pure water falls under gravity from the top of the first absorber 100, reaches the absorption unit 130a, and then passes through the absorption units 130b, 130c, 130d, and 130e in this order, before flowing from the bottom of the first absorber 100 into the pipe 160.

[0043] Gas supply unit 30 supplies the formaldehyde gas composition as a gas from near the bottom of first absorption tower 100. The formaldehyde gas composition rises due to gravity from the bottom of first absorption tower 100, passes through absorption units 130e, 130d, 130c, and 130b in this order, reaches absorption unit 130a, and then flows out from the top of first absorption tower 100 into pipe 170.

[0044] In the absorption section 130a, pure water drips from above under the force of gravity onto a plurality of trays and accumulates therein, and the formaldehyde gas composition passes through holes in the trays from below under the force of gravity, dissolving in the pure water.

[0045] The pure water containing the formaldehyde gas composition that has passed through absorption section 130a then reaches the next absorption section 130b. In the next absorption section 130b, the formaldehyde gas composition ascending from below comes into contact with the pure water sprayed onto the metal ring from above, and the formaldehyde gas composition dissolves in the pure water. This causes more of the formaldehyde gas composition to dissolve in the pure water than when it passed through absorption section 130a.

[0046] A portion of the pure water containing the formaldehyde gas composition that flows out of the absorbing section 130b flows into the cooling section 150a, where it is cooled, and then returned to the upward direction of the absorbing section 130b under the force of gravity. The formaldehyde gas can be liquefied by contacting the pure water containing the formaldehyde gas composition cooled by the cooling section 150a with the formaldehyde gas.

[0047] The pure water containing the formaldehyde gas composition that has passed through absorption section 130b reaches the next absorption section 130c. In absorption section 130c, the formaldehyde gas composition rising from below comes into contact with the pure water that has been poured onto the metal rings and trays from above due to gravity, and the formaldehyde gas composition dissolves in the pure water. This causes the formaldehyde gas composition to dissolve in the pure water in addition to when it passed through absorption section 130b.

[0048] A portion of the pure water containing the formaldehyde gas composition that flows out of the absorption section 130c flows into the cooling section 150b, where it is cooled, and then returned to the absorption section 130c by gravity upward. The formaldehyde gas can be liquefied by bringing the formaldehyde gas into contact with the pure water containing the formaldehyde gas composition that has been cooled by the cooling section 150b.

[0049] A portion of the pure water containing the formaldehyde gas composition that has flowed out of absorption section 130c reaches absorption section 130d. In absorption section 130d, the pure water drips onto a plurality of trays from above under the force of gravity and accumulates there, and the formaldehyde gas composition passes through holes in the trays from below under the force of gravity, dissolving into the pure water.

[0050] The pure water containing the formaldehyde gas composition that has passed through absorption section 130d reaches the next absorption section 130e. In the next absorption section 130e, the formaldehyde gas composition rising from below comes into contact with the pure water sprayed onto the metal ring from above, and the formaldehyde gas composition dissolves in the pure water. This causes the formaldehyde gas composition to dissolve in the pure water in addition to when it passed through absorption section 130d.

[0051] The pure water containing the formaldehyde gas composition that has flowed out of the absorption unit 130e flows into the cooling unit 150c and is cooled, after which a portion of the pure water is returned to the absorption unit 130e by gravity upward, and the remainder flows out of the system 10 through the pipe 160. At this time, the outflowing liquid is a 45 wt % aqueous formaldehyde solution. In this way, the first formaldehyde absorption system 10 absorbs the formaldehyde gas composition with the liquid and recovers it as an aqueous formaldehyde solution.

[0052] Next, a method for designing an absorption system that absorbs formaldehyde gas compositions with a liquid and recovers them as an aqueous formaldehyde solution will be described.

[0053] This design method resulted in an absorption tower having two cooling sections, an inner diameter of 1,500 mm, and a length of 24,200 mm excluding the lid and bottom of the body 110. In this case, as will be described in Example 2 below, 7,167 kg of an aqueous formaldehyde solution having a formaldehyde concentration of 45 wt % was obtained per hour.

[0054] According to the present embodiment, a formaldehyde absorption system, an absorption method, and a design method for an absorption system are obtained that can be constructed at low cost and yet have sufficient performance.

[0055] Next, a second formaldehyde absorption system 40, an absorption method, and a design method for an absorption system according to a second embodiment of the present invention will be described with reference to Fig. 2. Components similar to those in the first embodiment are designated by the same reference numerals, and descriptions thereof will be omitted.

[0056] The second formaldehyde absorption system 40 is an absorption system that absorbs a formaldehyde gas composition produced by the silver method into a liquid and recovers it as an aqueous formaldehyde solution, and mainly includes two absorption towers, a second absorption tower 400 and a third absorption tower 600, a liquid supply unit 20, and a gas supply unit 30. In this embodiment, pure water is used as the liquid.

[0057] The second absorption tower 400 mainly comprises a cylindrical body 410 with a lid and a bottom, two absorption sections 430d and 430e that absorb the formaldehyde gas composition using a liquid, and one cooling section 450 that cools the liquid in the absorption section 430e. The body 410 is a right cylinder with its top and bottom openings airtightly covered with spherical caps, and is preferably made of stainless steel. The length of the body 410 excluding the lid and bottom is the axial length of the right cylinder. The internal diameter of the second absorption tower 400, i.e., the internal diameter of the right cylindrical portion of the body 410, is 1,000 mm or more and 3,000 mm or less. A pipe 470 is connected to the spherical cap at the top of the body 410, and gas containing the formaldehyde gas composition in the second absorption tower 400 flows out. The pipe 470 is connected to the lower part of the third absorption tower 600 and supplies a gas containing a formaldehyde gas composition to the third absorption tower 600 from the lower part in the direction of gravity.

[0058] The absorbers 430d and 430e are stored in the trunk 410 in order from the top vertically. The absorber 430d has a configuration similar to that of the absorber 130a according to the first embodiment. The absorber 430e has a configuration substantially similar to that of the absorber 130e according to the first embodiment. Detailed description of the absorbers 430d and 430e will be omitted.

[0059] The absorption unit 430e is provided with a cooling unit 450 that cools the liquid in the absorption unit 430e. The cooling unit 450 mainly includes a pump 452. The pump 452 is connected to a pipe provided near the bottom of the second absorption tower 400 and sends the liquid flowing out of the absorption unit 430e to a heat exchanger 451a. The liquid flowing out from the bottom of the absorption unit 430e is cooled through the pump 452, the heat exchanger 451a, and the pipe, and returned to the upper side of the absorption unit 430e due to gravity. A pipe 460 is provided between the pump 452 and the upper side of the absorption unit 430e to allow the liquid in which formaldehyde is dissolved to flow out. A portion of the liquid flowing out of the pump 452 flows out of the system 40 through the pipe 460, and the remainder is returned to the upper side of the absorption unit 430e due to gravity. Note that the heat exchanger 451a may not be provided, and the liquid may not be cooled.

[0060] The third absorption tower 600 mainly comprises a cylindrical body 610 with a lid and a bottom, three absorption sections 630a, 630b, and 630c that absorb the formaldehyde gas composition using liquid, and two cooling sections 650a and 650b that cool the liquid in the absorption sections 630b and 630c, respectively.

[0061] The body 610 is a right cylinder with its top and bottom openings airtightly covered by a spherical cap, and is preferably made of stainless steel. The length of the body 610, excluding the lid and bottom, is the axial length of the right cylinder. The internal diameter of the third absorber 600, i.e., the internal diameter of the right cylindrical portion of the body 610, is 1,000 mm or more and 3,000 mm or less. A pipe 170 is connected to the spherical cap at the top of the body 610, and gas in the third absorber 600 flows out. The sum of the length of the body 610, excluding the lid and bottom, and the length of the body 410, excluding the lid and bottom, i.e., the sum of the axial length of the right cylinder of the body 610 and the axial length of the right cylinder of the body 410; more specifically, the total length of all the bodies 410, 610 included in the absorption system 40, excluding the lids and bottoms, is 20,000 mm or more and 40,000 mm or less.

[0062] The absorbent sections 630a, 630b, and 630c are stored in order from the top vertically inside the body section 610. The absorbent section 630a is substantially similar to the absorbent section 130a according to the first embodiment, the absorbent section 630b is substantially similar to the absorbent section 130b according to the first embodiment, and the absorbent section 630c is substantially similar to the absorbent section 130c according to the first embodiment.

[0063] The absorption sections 630b and 630c are each provided with a cooling section 650a or 650b for cooling the liquid in the absorption section.

[0064] The cooling section 650a mainly includes a heat exchanger 651a and a pump 652a. The pump 652a is connected to a pipe connected to the bottom of the absorption section 630b, and sends the liquid flowing out of the absorption section 630b to the heat exchanger 651a. The heat exchanger 651a cools the liquid from the pump 652a and sends it upward under gravity into the absorption section 630b. A pipe 653 is connected to the pipe between the heat exchanger 651a and the absorption section 630b, and a diluted formaldehyde aqueous solution is injected into the absorption section 630b via the pipe 653. The use of the diluted formaldehyde aqueous solution can improve the amount of formaldehyde recovered in all of the formaldehyde production processes, including the process related to the second formaldehyde absorption system 40 and the processes before and after it. The liquid flowing out from the bottom of the absorption section 630b is cooled through the pump 652a, the heat exchanger 651a, and the piping, and is returned to the upper part of the absorption section 630b under the force of gravity.

[0065] The cooling section 650b mainly includes a heat exchanger 651b and a pump 652b. The pump 652b is connected to a pipe connected to the bottom of the absorption section 630c, more specifically, to the spherical crown that forms the bottom of the third absorption tower 600, and sends the liquid that flows in from the absorption section 630c to the heat exchanger 651b. The heat exchanger 651b cools the liquid from the pump 652b and sends it upward due to gravity to the absorption section 630c. That is, the liquid that flows out from the bottom of the absorption section 630c is cooled through the pump 652b, the heat exchanger 651b, and the pipe, and is returned upward due to gravity to the absorption section 630c. A pipe 660 is provided between the pump 652b and the heat exchanger 651b, through which the liquid in which formaldehyde is dissolved flows out. A portion of the liquid flowing out of pump 652b is supplied through pipe 660 to the second absorber 400 by gravity, and the remainder flows into heat exchanger 651b. That is, of the two second and third absorber towers 400, 600, the liquid discharged from the lower portion, more specifically the bottom portion, of the third absorber tower 600, which is installed on the upstream side in the liquid flow, flows into the upper portion, more specifically near the top portion, of the second absorber tower 400, which is installed on the downstream side. This state means that the second absorber tower 400 and the third absorber tower 600 are connected in series.

[0066] The liquid supply unit 20 supplies liquid to the third absorption tower 600 from above in the direction of gravity. The configuration of the liquid supply unit 20 is substantially the same as that of the first embodiment.

[0067] The gas supply unit 30 supplies the formaldehyde gas composition as a gas from the lower portion of the second absorption tower 400 in the direction of gravity, in other words, from near the bottom of the second absorption tower 400. The formaldehyde gas composition is the same as in the first embodiment.

[0068] Next, the function of the second formaldehyde absorption system 40 will be described.

[0069] First, the flow controller 21 of the liquid supply unit 20 controls the flow meter 22 and the control valve 24 according to a predetermined inflow rate to inject an appropriate amount of liquid into the third absorber 600. The liquid supply unit 20 can also adjust the inflow rate as appropriate depending on the actual operating conditions. The inflow rate is the value described above. The pure water falls due to gravity from the top of the third absorber 600, reaches the absorption section 630a, passes through absorption sections 630b and 630c, and flows from the bottom of the third absorber 600 to the cooling section 650b.

[0070] The pure water then passes through a pipe 660 connected to the cooling section 650b and flows into the second absorption tower 400 from near the top of the second absorption tower 400. The pure water then falls under gravity from near the top of the second absorption tower 400, passes through absorption sections 430d and 430e, flows from the bottom of the second absorption tower 400 to the cooling section 450, and then flows out of the second formaldehyde absorption system 40 from the pipe 460.

[0071] The gas supply unit 30 supplies the formaldehyde gas composition as a gas from near the bottom of the second absorption tower 400. The formaldehyde gas composition rises due to gravity from near the bottom of the second absorption tower 400, passes through absorption units 430e and 430d, and flows out from the top of the second absorption tower 400 into pipe 470. The formaldehyde gas composition that has flowed out into pipe 470 is supplied to near the bottom of the third absorption tower 600. In the third absorption tower 600, the formaldehyde gas composition rises due to gravity from near the bottom, passes through absorption units 630c, 630b, and 630a, and flows out from the top of the third absorption tower 600 into pipe 170. The process of dissolving the formaldehyde gas composition in the absorbing units 630a, 630b, 630c, 430d, and 430e in pure water is the same as that of the absorbing units 130a, 130b, 130c, 130d, and 130e according to the first embodiment, and therefore will not be described here.

[0072] On the other hand, the pure water containing the formaldehyde gas composition that flows out of the absorption unit 630c flows into the cooling unit 650b and is cooled, after which a portion is returned to the absorption unit 630c under gravity, and the remainder is supplied from the pipe 660 to the second absorption tower 400. The pure water containing the formaldehyde gas composition that is supplied to the second absorption tower 400 passes through the absorption units 430d and 430e, and a portion flows into the cooling unit 450 and is cooled and is returned to the absorption unit 430e under gravity, and the remainder flows out from the pipe 460 to the outside of the second formaldehyde absorption system. At this time, the outflowing liquid is an aqueous formaldehyde solution having a concentration of 37 wt % or more and 55 wt % or less. In this way, the first formaldehyde absorption system 10 absorbs the formaldehyde gas composition with the liquid and recovers it as an aqueous formaldehyde solution.

[0073] Next, a method for designing an absorption system that absorbs the formaldehyde gas composition produced by the silver method with a liquid and recovers it as an aqueous formaldehyde solution will be described.

[0074] In this design method, a new absorption tower is added to an existing absorption tower with the aim of improving the performance of the absorption system. In the following description, it is assumed that the third absorption tower 600 is an existing absorption tower provided with two cooling sections 650 a, 650 b, has an internal diameter of 2,450 mm, and has a body section 610 with a length of 18,000 mm excluding the lid and bottom, and that the second absorption tower 400 is newly installed in this absorption tower.

[0075] The specifications for the second absorber 400 are such that the internal diameter is 1,000 mm or more and 3,000 mm or less, and the length of the body 410 is determined so that the total length of all the bodies 410, 610 included in the absorption system 40, excluding the lid and bottom, is 20,000 mm or more and 40,000 mm or less. Here, since the length of the body 610 of the existing absorber, excluding the lid and bottom, is 18,000 mm, the length of the body 410 of the second absorber, excluding the lid and bottom, is set to 8,200 mm, and the internal diameter is set to 1,800 mm.

[0076] The inflow rate is determined so that the liquid residence time per meter of length in the absorption section 430e is 0.8 minutes or more, and the cumulative residence time of the liquid in all of the absorption sections 630a, 630b, 630c, 430d, and 430e included in the second formaldehyde absorption system 40 is 150 minutes or more and 2,000 minutes or less. The inflow rate is determined in advance by calculation simulation or experiment so as to achieve these times.

[0077] The existing absorption tower produced 4,778 kg of an aqueous formaldehyde solution having a formaldehyde concentration of 45 wt % per hour, as will be described later in Comparative Example 2. The modified absorption system produced 7,167 kg of an aqueous formaldehyde solution having a formaldehyde concentration of 45 wt % per hour, as will be described later in Example 1. The addition of the new absorption tower to the existing absorption tower improved the performance of the absorption system.

[0078] Since the third absorption tower 600 is already provided with two cooling sections 650a and 650b, it is not necessary to provide a new cooling section, but a new cooling section 450 may be provided as in this embodiment.

[0079] By determining the specifications for the newly added absorber in this way, it is possible to improve the performance of the absorption system by utilizing the existing absorber without discarding it, and also to reduce waste because there is no need to discard the existing absorber.

[0080] Furthermore, since only one absorption tower and at least two cooling sections are required, the performance of the absorption system can be improved at low cost.

[0081] Furthermore, the total length of all of the body sections 410, 610 included in the absorption system 40, excluding the lid and bottom, only needs to be 20,000 mm or more and 40,000 mm or less, so the size of the newly installed second absorption tower 400 can be reduced, thereby improving the performance of the absorption system at low cost.

[0082] According to the present embodiment, a formaldehyde absorption system, an absorption method, and a design method for an absorption system are obtained that can be constructed at low cost and yet have sufficient performance.

[0083] Next, the components of the formaldehyde gas composition produced by the silver method will be described. The applicant conducted experiments using actual equipment for the first and second formaldehyde absorption systems 10 and 40 in the following cases 1 to 8. The longitudinal lengths of the absorption sections (T-1) 130a and 630a were 5,700 mm, the longitudinal lengths of the absorption sections (P-1) 130b and 630b were 1,500 mm, the longitudinal lengths of the absorption sections (P-2) 130c and 630c were 3,000 mm, the longitudinal lengths of the absorption sections (T-2) 130d and 430d were 1,200 mm, and the longitudinal lengths of the absorption sections (P-3) 130e and 430e were 3,000 mm.

[0084] [Case 1] In Case 1, a formaldehyde gas composition having a blending ratio of 15.7 wt % formaldehyde, 19.4 wt % water vapor, and 65.0 wt % inert gas was applied to the first absorption tower 100. The first absorption tower 100 had an inner diameter, i.e., an inner diameter of the right cylindrical portion of the body 110, of 2,450 mm, and a total length of the body 110 excluding the lid and bottom, of 18,000 mm. It was confirmed that the formaldehyde gas composition could be dissolved in pure water even when the formaldehyde concentration was as low as 15.7 wt % and the inert gas concentration was as high as 65.0 wt %.

[0085] [Case 2] In Case 2, a formaldehyde gas composition containing 25.4 wt % formaldehyde, 23.0 wt % water vapor, and 51.6 wt % inert gas was applied to the first absorption tower 100. The first absorption tower 100 had the same configuration as in Case 1, but the inflow rate of the formaldehyde gas composition was 125% of that in Case 1.

[0086] [Case 3] In Case 3, a formaldehyde gas composition containing 23.8 wt % formaldehyde, 25.9 wt % water vapor, and 50.3 wt % inert gas was applied to the first absorption tower 100. The first absorption tower 100 had the same configuration as in Case 1, but the inflow rate of the formaldehyde gas composition was 120% of that in Case 1.

[0087] [Case 4] In Case 4, a formaldehyde gas composition containing 25.0 wt % formaldehyde, 25.0 wt % water vapor, and 50.0 wt % inert gas was applied to the first absorption tower 100. The first absorption tower 100 had the same configuration as in Case 1.

[0088] [Case 5] In Case 5, a formaldehyde gas composition containing 21.8 wt % formaldehyde, 16.2 wt % water vapor, and 62.0 wt % inert gas was applied to the first absorption tower 100. The first absorption tower 100 had the same configuration as in Case 1.

[0089] [Case 6] In Case 6, the proportion of water vapor was increased by 20% compared to Case 3. A formaldehyde gas composition containing 22.7 wt. % formaldehyde, 29.5 wt. % pure water, and 47.9 wt. % inert gas was applied to the first absorption tower 100. Generally, in the formaldehyde production reaction in the gas supply unit 30, which is the first stage of the process of absorbing the formaldehyde gas composition, heat can cause the raw material methanol to explode, and the explosion range based on the proportion of each component in the reaction gas composition is known. Increasing the proportion of water vapor in the formaldehyde production reaction in the gas supply unit 30 eliminates this explosion range, enabling safe operation of the system. It was found that when the proportion of water vapor exceeds 29.5 wt. %, the formaldehyde concentration in the aqueous solution extracted from the absorption tower decreases, and the energy required in the post-treatment process performed outside the absorption tower increases, making this method unsuitable for practical use.

[0090] [Case 7] In Case 7, the proportion of water vapor was reduced to near the explosive range compared to Case 3, and a formaldehyde gas composition having a blending ratio of 26.6 wt % formaldehyde, 17.3 wt % water vapor, and 56.2 wt % inert gas was applied to the first absorption tower 100. The first absorption tower 100 had the same configuration as Case 1. Conditions under which the water vapor content was less than 17.3 wt % were not feasible because the process of producing a formaldehyde gas composition by a reaction in the preceding stage was dangerous as it was within the explosive range.

[0091] [Case 8] In Case 8, the proportion of recycled gas used in the gas supply unit 30 was 0%, as compared to Case 3. A formaldehyde gas composition containing 27.0 wt. % formaldehyde, 29.3 wt. % water vapor, and 43.7 wt. % inert gas was applied to the first absorption tower 100. The first absorption tower 100 had the same configuration as Case 1. The use of no recycled gas or a small amount of recycled gas in the gas supply unit 30 leads to an increase in the formaldehyde concentration in the formaldehyde gas composition, which is preferable from the viewpoint of producing a formaldehyde gas composition containing a high concentration of formaldehyde. The recycled gas in the gas supply unit 30 is also used for temperature control. However, without the use of recycled gas, it became difficult to control the temperature during the formaldehyde production reaction in the gas supply unit 30. It was confirmed that a formaldehyde gas composition could be obtained even when the formaldehyde concentration was as high as 27.0 wt % and the inert gas concentration was as low as 43.7 wt %.

[0092] Next, using Figures 3 and 4, the numerical parameters according to this embodiment will be described using the results of calculations by numerical simulation. First, Figure 3 will be described. The applicant performed numerical simulations for Examples 1 to 5 and Comparative Examples 1 to 5 for the first and second formaldehyde absorption systems 10 and 40. The longitudinal length of the absorption section (T-1) 130a, 630a was 5,700 mm, the longitudinal length of the absorption section (P-1) 130b, 630b was 1,500 mm, the longitudinal length of the absorption section (P-2) 130c, 630c was 3,000 mm, the longitudinal length of the absorption section (T-2) 130d, 430d was 1,200 mm, and the longitudinal length of the absorption section (P-3) 130e, 430e was 3,000 mm.

[0093] Example 1 A small second absorption tower 400 was added to an existing third absorption tower 600. The internal diameter of the existing third absorption tower 600, i.e., the internal diameter of the right cylindrical portion of the body 610, was 2,450 mm, and the total length of the body 610 excluding the lid and bottom was 18,000 mm. The internal diameter of the added second absorption tower 400, i.e., the internal diameter of the right cylindrical portion of the body 410, was 1,800 mm, and the total length of the body 410 excluding the lid and bottom was 8,200 mm. The second absorption tower 400 had one cooling section 450, and the third absorption tower 600 had two cooling sections 650a, 650b. That is, there were two absorption towers and three cooling sections. A formaldehyde gas composition containing 25.4 wt % formaldehyde, 23.0 wt % water, and 51.6 wt % inert gas was supplied to the second formaldehyde absorption system 40. As a result, 7,167 kg of an aqueous formaldehyde solution with a formaldehyde concentration of 45% was obtained per hour.

[0094] Example 2 The specifications of the existing first absorption tower 100 were modified. The internal diameter of the existing first absorption tower 100, i.e., the internal diameter of the right cylindrical portion of the body 110, was set to 1,500 mm, and the total length of the body 110 excluding the lid and bottom was set to 24,200 mm. The first absorption tower 100 was equipped with three cooling sections 150a to 150c. That is, the number of cooling towers was one and the number of cooling sections was three. A formaldehyde gas composition having the same composition and amount per hour as in Example 1 was supplied to the first formaldehyde absorption system 10 configured as above. As a result, 7,167 kg of an aqueous formaldehyde solution having a formaldehyde concentration of 45 wt % was obtained per hour.

[0095] Example 3 The amount of formaldehyde gas composition supplied to the existing first absorption tower 100 was 400% of that in Example 2. The internal diameter of the existing first absorption tower 100, i.e., the internal diameter of the right cylindrical portion of the body 110, was 2,450 mm, and the total length of the body 110 excluding the lid and bottom was 24,200 mm. That is, the number of cooling towers was one and the number of cooling sections was three. A formaldehyde gas composition having the same composition as in Example 1 but supplied at four times the hourly supply amount was supplied to the first formaldehyde absorption system 10. As a result, 19,112 kg of an aqueous formaldehyde solution having a formaldehyde concentration of 45 wt % was obtained per hour.

[0096] Example 4 An absorption tower having the same configuration as the existing third absorption tower 600 was added. The internal diameter of the existing third absorption tower 600, i.e., the internal diameter of the right cylindrical portion of the body 610, was 2,450 mm, and the total length of the body 610 excluding the lid and bottom was 18,000 mm. That is, the number of cooling towers was two and the number of cooling sections was four. A formaldehyde gas composition having the same composition as that of Example 1 but supplied in an amount four times higher per hour was supplied to the second formaldehyde absorption system 40. As a result, 19,112 kg of an aqueous formaldehyde solution having a formaldehyde concentration of 45 wt % was obtained per hour.

[0097] Comparative Example 1 One absorption tower having the following specifications was used. The internal diameter of the absorption tower, i.e., the internal diameter of the right cylindrical portion of the body, was 2,000 mm, and the total length of the body excluding the lid and bottom was 17,900 mm. The absorption tower was equipped with two cooling sections. The same formaldehyde gas composition as in Example 1 was supplied to this formaldehyde absorption system. As a result, 2,383 kg of formaldehyde aqueous solution with a formaldehyde concentration of 45 wt % was obtained per hour. In Comparative Example 1, the production amount per hour was low compared to the tower diameter, indicating insufficient capacity.

[0098] Comparative Example 2 One absorption tower having the following specifications was used. The internal diameter of the absorption tower, i.e., the internal diameter of the right cylindrical portion of the body, was 2,450 mm, and the total length of the body excluding the lid and bottom was 18,000 mm. The absorption tower was equipped with two cooling sections. The same formaldehyde gas composition as in Example 1 was supplied to this formaldehyde absorption system. As a result, 4,778 kg of formaldehyde aqueous solution with a formaldehyde concentration of 45 wt % was obtained per hour. In Comparative Example 2, the production amount per hour was low compared to the tower diameter, indicating insufficient capacity.

[0099] Comparative Example 3 One absorption tower having the following specifications was used. The internal diameter of the absorption tower, i.e., the internal diameter of the right cylindrical portion of the body, was 3,200 mm, and the total length of the body excluding the lid and bottom was 18,200 mm. The absorption tower was equipped with two cooling sections. The same formaldehyde gas composition as in Example 1 was supplied to this formaldehyde absorption system. As a result, 9,747 kg of formaldehyde aqueous solution with a formaldehyde concentration of 45 wt % was obtained per hour. In Comparative Example 3, the production amount per hour was low compared to the tower diameter, indicating insufficient capacity.

[0100] Comparative Example 4 The specifications of Comparative Example 2 were modified. The internal diameter of the absorption tower, i.e., the internal diameter of the right cylindrical portion of the body, remained unchanged at 2,450 mm, but the total length of the body excluding the lid and bottom was changed to 45,000 mm. The absorption tower was equipped with four cooling sections. The supply rate of the formaldehyde gas composition was 400% of that of Example 2. The supply rate of the formaldehyde gas composition was 400% of that of the absorption tower of Comparative Example 2. As a result, 19,112 kg of formaldehyde aqueous solution with a formaldehyde concentration of 45 wt% was obtained per hour. In Comparative Example 4, the absorption tower was too long, which increased the manufacturing costs of the absorption tower and the cost of building the factory, making it commercially unrealistic.

[0101] Comparative Example 5 Two small absorption towers were added to the absorption tower of Comparative Example 2. The internal diameter of the added absorption tower, i.e., the internal diameter of the right cylindrical portion of the body, was 1,800 mm, and the total length of the body excluding the lid and bottom was 8,200 mm. The added absorption tower had three cooling sections. That is, the number of cooling towers was three and the number of cooling sections was five. The same formaldehyde gas composition as in Example 1 was supplied to this formaldehyde absorption system. As a result, 19,112 kg of formaldehyde aqueous solution with a formaldehyde concentration of 45 wt % was obtained per hour. Since the number of absorption towers in Comparative Example 5 was three, it was not commercially practical due to the increased manufacturing costs of the absorption towers and the increased cost of building a factory due to the increased installation area.

[0102] In Comparative Example 2, it is possible to set the number of cooling sections to 16, and provide a cooling section for each tray in the tray type and for each filling section in the filling type, but this is not commercially realistic because the number of cooling sections would increase and costs would rise.

[0103] Next, the numerical parameters according to this embodiment will be described using the results of calculations performed by numerical simulations, with reference to Figures 5 to 8. First, Figure 5 will be described. The applicant performed numerical simulations for the first formaldehyde absorption system 10 under the following five conditions. The longitudinal length of the absorption section (T-1) 130a was 5,700 mm, the longitudinal length of the absorption section (P-1) 130b was 1,500 mm, the longitudinal length of the absorption section (P-2) 130c was 3,000 mm, the longitudinal length of the absorption section (T-2) 130d was 1,200 mm, and the longitudinal length of the absorption section (P-3) 130e was 3,000 mm.

[0104] Condition 1: 25.4% by weight of formaldehyde, H ２ O23.0% by weight, N ２ A 51.6 wt % formaldehyde gas composition was supplied to the first formaldehyde absorption system 10 at a flow rate of 35,680 kg / h. At this time, the flow rate of formaldehyde (Gas F) was 9072.6 kg / h, and H ２ Flow rate of O (Gas H ２ The flow rate of formaldehyde in the diluted aqueous formaldehyde solution supplied from the pipe 153 (DF) is 420.2 kg / h, and the flow rate of H ２ Flow rate of O (D H ２ The H O supplied from the liquid supply unit 20 is 1897.2 kg / h. ２ O flow rate (PH ２ At this time, Liquid H2O / Feed F (wt%) is 35.2 wt%, which can be calculated by the following formula: (Liquid H2O / Feed F (wt%)) = (H2O supplied from the outside to the absorption tower) ２ O flow rate) / (formaldehyde flow rate supplied from the outside to the absorption tower) where H ２ The O flow rate is (D H ２ O) and (PH ２The flow rate of formaldehyde supplied from the outside to the absorption tower is the sum of (Gas F) and (D F). In the simulation under condition 1, 46.2 wt% formaldehyde was obtained. In addition, at this time, the liquid residence time per unit height in the absorption section (P-1) 130b was 4.2 min / m, the liquid residence time per unit height in the absorption section (P-2) 130c was 3.9 min / m, the liquid residence time per unit height in the absorption section (T-2) 130d was 1.0 min / m, and the liquid residence time per unit height in the absorption section (P-3) 130e was 1.1 min / m.

[0105] Condition 2: 25.4% by weight of formaldehyde, H ２ O23.0% by weight, N ２ Another 51.6 wt % formaldehyde gas composition was supplied to the first formaldehyde absorption system 10 at a flow rate of 26,760 kg / h. At this time, the flow rate of formaldehyde (Gas F) was 6,804.4 kg / h, and H ２ Flow rate of O (Gas H ２ The flow rate of formaldehyde in the diluted aqueous formaldehyde solution supplied from the pipe 153 (DF) is 315.1 kg / h, and the flow rate of H ２ Flow rate of O (D H ２ The H supplied from the liquid supply unit 20 is 1422.5 kg / h. ２ O flow rate (PH ２ The flow rate (H2O / Feed F) was 700.0 kg / h. At this time, the Liquid H2O / Feed F (wt%) was 29.8 wt%. In a simulation under Condition 2, 47.4 wt% formaldehyde was obtained. Furthermore, at this time, the liquid residence time per unit height in the absorption section (P-1) 130b was 5.2 min / m, the liquid residence time per unit height in the absorption section (P-2) 130c was 6.2 min / m, the liquid residence time per unit height in the absorption section (T-2) 130d was 1.4 min / m, and the liquid residence time per unit height in the absorption section (P-3) 130e was 1.3 min / m.

[0106] In condition 3, formaldehyde was 26.2% by weight, H２ O20.6% by weight, N ２ A 53.2 wt. % formaldehyde gas composition was supplied to the first formaldehyde absorption system 10 at a flow rate of 25,956 kg / h. At this time, the flow rate of formaldehyde (Gas F) was 6,804.5 kg / h, and H ２ Flow rate of O (Gas H ２ The flow rate of formaldehyde in the diluted aqueous formaldehyde solution supplied from the pipe 153 (DF) is 315.1 kg / h, and the flow rate of H ２ Flow rate of O (D H ２ The H supplied from the liquid supply unit 20 is 1422.5 kg / h. ２ O flow rate (PH ２ The flow rate (H2O / Feed F) was 700.0 kg / h. At this time, the Liquid H2O / Feed F (wt%) was 29.8 wt%. In the simulation under Condition 2, 50.0 wt% formaldehyde was obtained. Furthermore, at this time, the liquid residence time per unit height in the absorption section (P-1) 130b was 5.0 min / m, the liquid residence time per unit height in the absorption section (P-2) 130c was 6.1 min / m, the liquid residence time per unit height in the absorption section (T-2) 130d was 1.5 min / m, and the liquid residence time per unit height in the absorption section (P-3) 130e was 1.3 min / m.

[0107] Condition 4: 25.4% by weight of formaldehyde, H ２ O23.0% by weight, N ２ Another 51.6 wt % formaldehyde gas composition was supplied to the first formaldehyde absorption system 10 at a flow rate of 13,380 kg / h. At this time, the flow rate of formaldehyde (Gas F) was 3,402.2 kg / h, and H ２ Flow rate of O (Gas H ２ The flow rate of formaldehyde in the diluted aqueous formaldehyde solution (DF) supplied from the pipe 153 is 157.6 kg / h, and the flow rate of H ２ Flow rate of O (D H ２ The H supplied from the liquid supply unit 20 is 711.7 kg / h.２ O flow rate (PH ２ The flow rate (H2O / Feed F) was 245.0 kg / h. At this time, the Liquid H2O / Feed F (wt%) was 26.9 wt%. In a simulation under Condition 4, 48.0 wt% formaldehyde was obtained. Furthermore, at this time, the liquid residence time per unit height in the absorption section (P-1) 130b was 7.3 min / m, the liquid residence time per unit height in the absorption section (P-2) 130c was 8.9 min / m, the liquid residence time per unit height in the absorption section (T-2) 130d was 3.5 min / m, and the liquid residence time per unit height in the absorption section (P-3) 130e was 1.5 min / m.

[0108] Condition 5: 26.2% by weight of formaldehyde, H ２ O20.6% by weight, N ２ Another 53.2 wt % formaldehyde gas composition was supplied to the first formaldehyde absorption system 10 at a flow rate of 12,978 kg / h. At this time, the flow rate of formaldehyde (Gas F) was 3,402.3 kg / h, and H ２ Flow rate of O (Gas H ２ The flow rate of formaldehyde in the diluted aqueous formaldehyde solution (DF) supplied from the pipe 153 is 157.6 kg / h, and the flow rate of H ２ Flow rate of O (D H ２ The H supplied from the liquid supply unit 20 is 711.7 kg / h. ２ O flow rate (PH ２ The flow rate (H2O / Feed F) was 245.0 kg / h. At this time, the Liquid H2O / Feed F (wt%) was 26.9 wt%. In a simulation under Condition 5, 50.7 wt% formaldehyde was obtained. Furthermore, at this time, the liquid residence time per unit height in the absorption section (P-1) 130b was 7.3 min / m, the liquid residence time per unit height in the absorption section (P-2) 130c was 9.2 min / m, the liquid residence time per unit height in the absorption section (T-2) 130d was 3.8 min / m, and the liquid residence time per unit height in the absorption section (P-3) 130e was 1.6 min / m.

[0109] Simulations based on the above conditions 1 to 5 were performed, and the graph shown in Figure 6 shows the relationship between the liquid residence time per unit height and the liquid H2O / feed F ratio. Referring to Figure 6, the absorption section (P-3) 130e at the bottom of the tower had the shortest liquid residence time per unit height, at 0.8 to 1 min / m. Generally, the absorption section at the bottom of the absorption tower has the highest gas flow rate and liquid flow rate compared to other absorption sections, and therefore is known to have the highest formaldehyde treatment rate, i.e., the amount of formaldehyde dissolved in pure water. Therefore, based on the required treatment volume for the system 10, the specifications for the absorption section at the bottom of the tower were determined, in this case, to satisfy a liquid residence time per unit height of 0.8 min / m. The inner diameter of the tower was calculated based on these specifications, resulting in a value of 1,000 mm to 3,000 mm. On the other hand, if the liquid residence time per unit height exceeds 3 min / m, it will be necessary to increase the inner diameter of the absorption tower and the total length of all of the body parts excluding the lid and the bottom, which will result in the formaldehyde absorption system becoming an excessively large facility, extending the amortization period of the equipment costs and worsening economic viability. Therefore, it is preferable that the liquid residence time per unit height is 3 min / m or less.

[0110] 7 is a graph showing the relationship between cumulative residence time and cumulative formaldehyde conversion rate obtained by simulations based on conditions 1 to 5. In all simulation results, it was found that when the cumulative residence time reached 150 minutes, the cumulative formaldehyde conversion rate reached 99%. In other words, it was found that a cumulative residence time of 150 minutes was necessary to obtain a cumulative formaldehyde conversion rate of 99% or more.

[0111] FIG. 8 is a graph showing the relationship between cumulative formaldehyde conversion and formaldehyde recovery rate obtained by simulation based on conditions 1 to 5. In a typical formaldehyde absorption system (industrial plant), a practical formaldehyde recovery rate is set to 99.5% or higher. Referring to the graph in FIG. 8, it can be seen that if the cumulative formaldehyde conversion rate is approximately 99% or higher, the formaldehyde recovery rate will be approximately 99.5% or higher. Furthermore, as shown in FIG. 7, the cumulative residence time at this time is 150 minutes. From the above, it can be seen that a cumulative residence time of 150 minutes or more is required to achieve a formaldehyde recovery rate of approximately 99.5% or higher. On the other hand, if the cumulative residence time exceeds 2,000 minutes, the formaldehyde absorption system will become excessively large, extending the amortization period of the equipment costs and reducing economic viability. Therefore, a liquid residence time per unit height of 2,000 minutes or less is preferable.

[0112] Furthermore, by limiting the number of absorption towers provided in the absorption system to one or two, the manufacturing costs and operating costs of the absorption system can be reduced.

[0113] Furthermore, since the total length of all the body parts included in the absorption system, excluding the lid and the bottom, only needs to be 20,000 mm or more and 40,000 mm or less, the size of the entire absorption system can be reduced, thereby improving the performance of the absorption system at low cost. Furthermore, by reducing the size of the entire absorption system, the cost of the factory building in which the absorption system is installed can be reduced.

[0114] In all the embodiments, the body portions of the first to third absorber towers 100, 400, and 600 have been described as being straight cylindrical, but they may also have a rectangular tube, an oblique cylinder, or a curved or bent shape.

[0115] It is not necessary to provide a cooling section for each absorption section, but two or more cooling sections may be provided in the first formaldehyde absorption system 10 and the second formaldehyde absorption system 40 .

[0116] In addition, in all the embodiments, the diluted formaldehyde aqueous solution is injected into the absorption section through the piping between the heat exchanger and the absorption section, but pure water may be injected instead of the diluted formaldehyde aqueous solution.

[0117] In any of the embodiments, the formaldehyde gas composition is produced by a silver method (silver catalyst method, excess methanol method) using silver as a catalyst.

[0118] DESCRIPTION OF SYMBOLS 10 First absorption system 20 Liquid supply section 21 Flow controller 23 Flow meter 24 Control valve 25 Pipe 25a Pipe 30 Gas supply section 40 Absorption system 100 First absorption tower 110 Body section 130a Absorption section (T-1) 130b Absorption section (P-1) 130c Absorption section (P-2) 130d Absorption section (T-2) 130e Absorption section (P-3) 150a Cooling section 150b Cooling section 150c Cooling section 151a Heat exchanger 151b Heat exchanger 151c Heat exchanger 152a Pump 152b Pump 152c Pump 153 Pipe 160 Pipe 170 Pipe 400 Second absorption tower 410 Body section 430d Absorption section (T-2) 430e Absorption section (P-3) 450 Cooling section 452 Pump 460 Piping 470 Piping 600 Third absorber 610 Body section 630a Absorption section (T-1) 630b Absorption section (P-1) 630c Absorption section (P-2) 650a Cooling section 650b Cooling section 651a Heat exchanger 651b Heat exchanger 652a Pump 652b Pump 653 Piping 660 Piping

Claims

1. An absorption system for recovering a formaldehyde gas composition produced by a silver process as an aqueous formaldehyde solution by absorbing the gas composition with a liquid, comprising: The formaldehyde gas composition contains at least 15.7 wt % or more and 27.0 wt % or less of formaldehyde, 17.3 wt % or more and 29.5 wt % or less of water, and 43.7 wt % or more and 65.0 wt % or less of an inert gas, the absorption system comprises one absorption tower or two absorption towers connected in series; the absorption tower comprises a cylindrical body portion having a lid and a bottom, an absorption section that absorbs the formaldehyde gas composition with a liquid, and a cooling section that cools the liquid in the absorption section; The internal diameter of the absorption tower is 1,000 mm or more and 3,000 mm or less, The total length of all the body parts included in the absorption system, excluding the lid and the bottom, is 20,000 mm or more and 40,000 mm or less, The absorption system includes two or more cooling units in total, the absorption section provided at the most downstream position of the absorption tower in the liquid flow has a liquid residence time per meter of length of 0.8 minutes or more; The cumulative residence time of the liquid in all of the absorption sections included in the absorption system is 150 minutes or more. Absorption system.

2. 2. The absorption system according to claim 1, wherein the absorption section provided at the most downstream position of the absorption tower in the liquid flow has a liquid residence time per meter of length of 1 minute or more and 2 minutes or less.

3. The absorption system according to claim 1, wherein the cumulative residence time of the liquid in all of the absorption sections included in the absorption system is 150 minutes or more and 2,000 minutes or less.

4. The absorption system according to claim 1 , further comprising a liquid supply unit that supplies liquid to the absorption tower from above in the direction of gravity.

5. 2. The absorption system according to claim 1, wherein the absorption section provided at the most downstream position of the absorption tower in the liquid flow has a liquid residence time per meter of length of 0.8 minutes or more and 3 minutes or less.

6. The absorption system according to claim 1, wherein the cumulative residence time of the liquid in all of the absorption sections included in the absorption system is 150 minutes or more and 1,000 minutes or less.

7. 4. The absorption system according to claim 1, wherein the absorption tower comprises an absorption tower provided on an upstream side and an absorption tower provided on a downstream side in the flow of the liquid, and the liquid discharged from a lower part of the absorption tower provided on the upstream side flows into the absorption tower provided on the downstream side from an upper part of the absorption tower provided on the downstream side.

8. The absorption system according to claim 1 , further comprising a gas supply unit that supplies the formaldehyde gas composition to the absorption tower from a lower portion in the direction of gravity.

9. The absorption system according to claim 1 , wherein the absorption tower comprises a plurality of the absorption sections.

10. An absorption method for recovering a formaldehyde gas composition produced by a silver method as an aqueous formaldehyde solution by absorbing the gas composition with a liquid, comprising: The formaldehyde gas composition contains at least 15.7 wt % or more and 27.0 wt % or less of formaldehyde, 17.3 wt % or more and 29.5 wt % or less of water, and 43.7 wt % or more and 65.0 wt % or less of an inert gas, the absorption system comprises one absorption tower or two absorption towers connected in series; the absorption tower comprises a cylindrical body portion having a lid and a bottom, an absorption section that absorbs the formaldehyde gas composition with a liquid, and a cooling section that cools the liquid in the absorption section; The internal diameter of the absorption tower is 1,000 mm or more and 3,000 mm or less, The total length of the body portion included in the absorption system is 20,000 mm or more and 40,000 mm or less, The absorption system includes two or more cooling units in total, a step of injecting water into the absorption tower so that the liquid residence time per meter of length in the absorption section provided at the most downstream side of the absorption tower in the liquid flow is 0.8 minutes or more, and the cumulative residence time of the liquid in all the absorption sections included in the absorption system is 150 minutes or more. Absorption method.

11. 11. The absorption method according to claim 10, wherein the liquid residence time per meter of length in the absorption section provided at the most downstream position of the absorption tower in the liquid flow is 0.8 minutes or more and 3 minutes or less.

12. The absorption method according to claim 10 or 11, wherein the cumulative residence time of the liquid in all of the absorption sections included in the absorption system is 150 minutes or more and 2,000 minutes or less.

13. A method for designing an absorption system for absorbing a formaldehyde gas composition produced by a silver process with a liquid and recovering it as an aqueous formaldehyde solution, the absorption system comprising two absorption towers connected in series, the absorption tower comprises a cylindrical body portion having a lid and a bottom, an absorption section that absorbs the formaldehyde gas composition with a liquid, and a cooling section that cools the liquid in the absorption section; The formaldehyde gas composition contains at least 15.7 wt % or more and 27.0 wt % or less of formaldehyde, 17.3 wt % or more and 29.5 wt % or less of water, and 43.7 wt % or more and 65.0 wt % or less of an inert gas, The design method includes: The inner diameter of the absorption tower is 1,000 mm or more and 3,000 mm or less, The total length of the trunk portion included in the absorption system is 20,000 mm or more and 40,000 mm or less, The absorption system includes two or more cooling units in total, In the absorption section provided at the most downstream of the absorption tower in the liquid flow, the liquid residence time per meter of length is set to 0.8 minutes or more, The cumulative residence time of the liquid in all the absorption sections included in the absorption system is set to 150 minutes or more. Design method.

14. 14. The design method according to claim 13, wherein the two absorption towers include one existing absorption tower and one newly installed absorption tower, and the newly installed absorption tower is connected in series to the one existing absorption tower.

15. 14. The design method according to claim 13, wherein the liquid residence time per meter of length in the absorption section provided at the most downstream position of the absorption tower in the liquid flow is 0.8 minutes or more and 3 minutes or less.

16. The design method according to claim 13 , wherein the cumulative residence time of the liquid in all of the absorption sections included in the absorption system is 150 minutes or more and 2,000 minutes or less.