Method and equipment for producing granular ammonium sulfate

Abstract

To manufacture granular ammonium sulfate with large grain diameter stably.SOLUTION: A manufacturing method of granular ammonium sulfate includes: reacting an ammonia-containing gas with a sulfuric acid aqueous solution to obtain an ammonium sulfate aqueous solution; introducing the ammonium sulfate aqueous solution to a crystal can; precipitating ammonium sulfate in the crystal can to obtain granular ammonium sulfate; and pulling a slurry containing the granular ammonium sulfate from a lower part of the crystal can. The manufacturing method of granular ammonium sulfate is characterized by that solid-liquid ratios at plural heights in the crystal can are measured and a pulling amount of the slurry is controlled based on the solid-liquid ratio.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing granular ammonium sulfate and production equipment therefor.

Background Art

[0002] As an industrial method for producing granular ammonium sulfate, a method using a crystallization tank is widely used. In this method, an aqueous ammonium sulfate solution is introduced into a crystallization tank, and granular ammonium sulfate is precipitated from the aqueous ammonium sulfate solution in the crystallization tank.

[0003] For example, in a steelworks, since ammonia is contained in the coke oven gas generated when coal is carbonized to produce coke, the coke oven gas is reacted with an aqueous sulfuric acid solution to remove ammonia. When ammonia is removed in this way, an aqueous ammonium sulfate solution is obtained, and thus granular ammonium sulfate is produced in a crystallization tank using the aqueous ammonium sulfate solution.

[0004] FIG. 1 is a schematic diagram showing a method for producing granular ammonium sulfate from the above-described coke oven gas.

[0005] First, the coke oven gas 1 generated in the coke production process and the aqueous sulfuric acid solution 2 are reacted in an absorption tower 3 to obtain an aqueous ammonium sulfate solution 4. The aqueous sulfuric acid solution 2 is prepared, for example, by reacting sulfuric acid 5 and water 6 in an intermediate tank 7 and sent to the absorption tower 3 by a pump 8. Inside the absorption tower 3, for example, the aqueous sulfuric acid solution 2 is sprayed from above to absorb ammonia in the aqueous sulfuric acid solution 2.

[0006] <0​​​​A vacuum device 13 is connected to the evaporator 10, and the ammonium sulfate aqueous solution 4 is concentrated under vacuum. That is, the degree of supersaturation of the ammonium sulfate aqueous solution 4 is increased by evaporating and removing water under reduced pressure. The saturated ammonium sulfate aqueous solution 4 is discharged to the bottom of the crystallizer 9 through a downpipe 14 connected to the bottom of the evaporator 10. As a result, the ammonium sulfate crystals already present inside the crystallizer 9 are stirred up, and at the same time, new ammonium sulfate precipitates on the surface of the existing crystals, causing crystal growth. Granular ammonium sulfate is obtained through this crystal growth.

[0008] During this process, the grown crystals settle to the bottom of the crystallization tank 9 due to their weight, while the relatively smaller crystals remain in the middle section of the crystallization tank 9. Thus, a classification effect is at work inside the crystallization tank 9, causing the sufficiently grown granular ammonium sulfate to accumulate at the bottom of the crystallization tank 9. The grown granular ammonium sulfate is then extracted from the bottom of the crystallization tank 9 using an extraction pump 15, while still in a slurry state mixed with the granular ammonium sulfate aqueous solution. The extracted granular ammonium sulfate is then dehydrated and dried to obtain the final granular ammonium sulfate.

[0009] The main use of granular ammonium sulfate obtained in this way is as an agricultural fertilizer, but from the viewpoint of ease of application, a larger particle size (for example, around 1.4 to 4.75 mm) is preferred. Therefore, there is a need for a method to stably produce granular ammonium sulfate with a large particle size. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 01-176214 [Patent Document 2] Japanese Patent Application Publication No. 05-213615 [Overview of the project] [Problems that the invention aims to solve]

[0011] However, conventional methods have made it difficult to stably produce granular ammonium sulfate with a large particle size. The reasons for this are explained below.

[0012] To control the particle size of the final granular ammonium sulfate, it is necessary to control the crystal growth occurring inside the crystallization chamber, and for that purpose, it is important to control the number of crystals present in the crystallization chamber.

[0013] Here, the number of crystals N in the crystallizer can is expressed by the following equation (1). N = N0 + N in -N out …(1) N0: Number of microcrystals generated N in : Number of crystals added as seed crystals N out : Number of crystals pulled out from the crystallizer

[0014] As can be seen from equation (1) above, the number of crystals N in the crystallization tank is determined by three factors, but the number of microcrystals N0 is extremely difficult to control. Therefore, the average particle size in the crystallization tank often decreased due to the unwanted generation of a large amount of microcrystals.

[0015] When the number of crystals in the crystallization chamber increases rapidly, it is common practice to increase the extraction rate to address this. However, this method does not allow for stable control of the particle size. The reason is as follows:

[0016] First, within the crystallization chamber, it generally takes about 3 to 10 hours for one crystal to grow and then extract it as granular ammonium sulfate. Because the crystals remain in the crystallization chamber for such a long time, even if the extraction rate is adjusted, it takes time for the effect to be reflected in the state of the crystallization chamber. During this time, crystals with small particle sizes continue to be produced. On the other hand, if the extraction rate is increased excessively in anticipation of the aforementioned delay in response, it becomes an overaction, which can actually lead to the generation of a large amount of microcrystals. This is because increasing the extraction rate too much reduces the amount of crystals in the crystallization chamber, and consequently, the total surface area of ​​the crystals also decreases. As a result, a large amount of ammonium sulfate that cannot precipitate on the crystal surface precipitates as microcrystals.

[0017] Such sudden fluctuations in the amount of microcrystals generated and over-actions caused large fluctuations in the number of crystals in the crystallization tank, making it difficult to stably produce granular ammonium sulfate of the desired size.

[0018] Therefore, Patent Document 1 proposes a method for predicting the number of crystals in a crystallization tank based on the average particle size of the ammonium sulfate product and the total volume of crystal particles measured from the liquid depth pressure inside the crystallization tank, and for controlling the amount of ammonium sulfate product extracted and the amount of seed crystals added.

[0019] This method uses the average particle size of the final product, granular ammonium sulfate, as control. However, there is a time lag between the extraction of granular ammonium sulfate from the crystallization tank and its eventual conversion to the final product. Furthermore, during the process of converting the extracted granular ammonium sulfate to the final product, the particle size may decrease due to factors such as crystal abrasion by the centrifuge during the drying process. Therefore, the particle size of the final product ammonium sulfate does not necessarily reflect the particle size of the granular ammonium sulfate in the crystallization tank. Moreover, since this method does not allow for the determination of the number of crystals generated, if a large amount of microcrystals are generated, the resulting granular ammonium sulfate will have a smaller particle size.

[0020] In Patent Document 2, a method is proposed in which the crystal concentration in the mother liquor withdrawn from the crystal elevated tank is measured, and when the crystal concentration becomes too high, the supply of the mother liquor from the crystal can to the crystal elevated tank is stopped, and instead, a recycled liquid is supplied to the crystal elevated tank. In this method, by keeping the concentration and liquid level in the crystal elevated tank constant, it is possible to suppress the mixing of impurities such as sulfuric acid liquid and tar into the ammonium sulfate crystals sent to the subsequent process. However, the above method focuses on the events occurring in the crystal elevated tank after the slurry is withdrawn from the crystal can, and does not focus on the crystal growth in the crystal can where crystal growth actually occurs. Therefore, with the above method, it is not possible to control the crystal growth proceeding inside the crystal can.

[0021] Thus, with the conventional method, it has been difficult to stably produce granular ammonium sulfate with a large particle size.

[0022] The present invention has been made to solve the above problems, and an object thereof is to stably produce granular ammonium sulfate with a large particle size.

Means for Solving the Problems

[0023] As a result of intensive studies, the present inventors have found that by measuring the solid-liquid ratio at a plurality of heights in the crystal can and controlling the amount of the slurry withdrawn based on the solid-liquid ratio, it is possible to stably produce granular ammonium sulfate with a large particle size.

[0024] The present invention has been completed based on the above findings, and the gist thereof is as follows.

[0025] 1. A method for producing granular ammonium sulfate, comprising reacting an ammonia-containing gas with an aqueous sulfuric acid solution to obtain an aqueous ammonium sulfate solution, introducing the aqueous ammonium sulfate solution into a crystal can, precipitating ammonium sulfate in the crystal can to obtain granular ammonium sulfate, and withdrawing a slurry containing the granular ammonium sulfate from below the crystal can, A method for producing granular ammonium sulfate, comprising measuring the solid-liquid ratio at multiple heights within the crystallization vessel and controlling the amount of slurry drawn based on the solid-liquid ratio.

[0026] 2. An absorption tower that reacts ammonia-containing gas with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution, A crystallizer for precipitating ammonium sulfate from the aforementioned ammonium sulfate aqueous solution to produce granular ammonium sulfate, An extraction means for drawing out the slurry containing the granular ammonium sulfate from the bottom of the crystallization container, A solid-liquid ratio measuring means for measuring the solid-liquid ratio at multiple heights within the crystallization vessel, A granular ammonium sulfate production apparatus comprising a withdrawal amount control means for controlling the withdrawal amount of the slurry based on the solid-liquid ratio. [Effects of the Invention]

[0027] According to the present invention, granular ammonium sulfate with a large particle size can be stably produced. Specifically, by measuring the solid-liquid ratio at multiple heights within the crystallization vessel, changes in the amount of microcrystals formed within the vessel can be quickly observed. Then, by controlling the amount of slurry drawn based on the obtained solid-liquid ratio, it becomes possible to quickly reflect the increase or decrease in the number of crystals in the crystallization vessel in operational adjustments. As a result, fluctuations in the number of crystals in the crystallization vessel can be suppressed, and ammonium sulfate crystals with a large particle size can be stably obtained. [Brief explanation of the drawing]

[0028] [Figure 1] Figure 1 is a schematic diagram illustrating a method for producing granular ammonium sulfate from coke oven gas. [Figure 2] Figure 2 is a schematic diagram showing the growth process of granular ammonium sulfate in a crystallization vessel. [Modes for carrying out the invention]

[0029] Next, a method for carrying out the present invention will be specifically described. Note that the following description illustrates preferred embodiments of the present invention, and the present invention is not limited in any way by the following description.

[0030] In a method for producing granular ammonium sulfate according to one embodiment of the present invention, an ammonia-containing gas is reacted with an aqueous sulfuric acid solution to form an aqueous ammonium sulfate solution, the aqueous ammonium sulfate solution is introduced into a crystallization vessel, ammonium sulfate is precipitated in the crystallization vessel to form granular ammonium sulfate, and a slurry containing the granular ammonium sulfate is withdrawn from the bottom of the crystallization vessel to produce granular ammonium sulfate.

[0031] Furthermore, a granular ammonium sulfate production apparatus in another embodiment of the present invention comprises an absorption tower that reacts an ammonia-containing gas with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution, a crystallization tank that precipitates ammonium sulfate from the aqueous ammonium sulfate solution to produce granular ammonium sulfate, an extraction means for extracting a slurry containing the granular ammonium sulfate from below the crystallization tank, a solid-liquid ratio measuring means for measuring the solid-liquid ratio at multiple heights within the crystallization tank, and an extraction amount control means for controlling the amount of slurry extracted based on the solid-liquid ratio.

[0032] In this invention, the same process as in the conventional method for producing granular ammonium sulfate can be employed, except for measuring the solid-liquid ratio and performing control based on the measured solid-liquid ratio, as will be described later. Therefore, unless otherwise specified, the manufacturing process and equipment shown in Figure 1 can be used in this invention as well. In the following description, Figure 1 will be referred to as needed.

[0033] [Ammonia-containing gas] The ammonia-containing gas is not particularly limited, and any gas containing ammonia can be used. For example, coke oven gas can be used as the ammonia-containing gas.

[0034] [Ammonium sulfate aqueous solution] The ammonia-containing gas is reacted with an aqueous sulfuric acid solution to obtain an aqueous ammonium sulfate solution. The method for carrying out the reaction is not particularly limited, but it is preferable to use an absorption tower 3 as shown in Figure 1. In the absorption tower 3, for example, an aqueous sulfuric acid solution 2 is sprayed from above, and ammonia-containing gas 1 is introduced from below, and the two are brought into countercurrent contact. As a result, the ammonia contained in the ammonia-containing gas is absorbed by the aqueous sulfuric acid solution, becoming an aqueous ammonium sulfate solution 4.

[0035] As the sulfuric acid aqueous solution 2, it is preferable to use dilute sulfuric acid obtained by mixing sulfuric acid 5 and water 6 in a separate tank (intermediate tank 7) to adjust the concentration, as shown in Figure 1.

[0036] [Crystallization Can] Next, the ammonium sulfate aqueous solution is introduced into a crystallization tank, and ammonium sulfate is precipitated within the crystallization tank to form granular ammonium sulfate. The crystallization tank is not particularly limited; any tank capable of precipitating ammonium sulfate from the ammonium sulfate aqueous solution inside can be used. Typically, as shown in Figure 1, a crystallization tank 9 equipped with an evaporator 10 on top can be used. In this case, the ammonium sulfate aqueous solution 4 is circulated between the crystallization tank 9 and the evaporator 10.

[0037] [Recruitment] The granular ammonium sulfate grown in the crystallization tank is ultimately withdrawn from the bottom of the crystallization tank. At this time, the granular ammonium sulfate is withdrawn in a slurry state along with a portion of the ammonium sulfate aqueous solution. Therefore, the granular ammonium sulfate production apparatus of the present invention is equipped with an withdrawal means for withdrawing the slurry containing the granular ammonium sulfate from the bottom of the crystallization tank. The withdrawal means is not particularly limited and any can be used, but typically, an withdrawal pump 15 can be used as shown in Figure 1.

[0038] The post-extraction treatment is not particularly limited, but generally, it is preferable to separate the solid and liquid using a centrifuge and then dry the solid, i.e., granular ammonium sulfate. After drying, the granular ammonium sulfate can optionally be separated according to particle size using a sieve or the like.

[0039] Here, we will explain the process by which granular ammonium sulfate grows inside the crystallization vessel and is eventually withdrawn.

[0040] Figure 2 is a schematic diagram showing the growth process of granular ammonium sulfate inside the crystallization chamber 9. First, the ammonium sulfate aqueous solution 4, which has been vacuum-concentrated in the evaporator chamber 10, is discharged to the vicinity of the bottom of the crystallization chamber 9 through the descending pipe 14 connected to the bottom of the evaporator chamber 10. As a result, new microcrystals are generated inside the crystallization chamber 9 and are swept upward (mainly to the middle region) along with the already existing ammonium sulfate crystals (Figure 2(a)).

[0041] Inside the crystallization chamber 9, crystals gradually grow, increasing in size. The larger crystals sink to the bottom of the chamber 9 due to their weight, while smaller crystals are stirred up and accumulate in the middle section. Thus, a classification effect is at work inside the crystallization chamber 9, causing the sufficiently grown granular ammonium sulfate to accumulate at the bottom of the chamber 9 (Figure 2(b)).

[0042] The grown granular ammonium sulfate is extracted from the bottom of the crystallization tank 9 using an extraction pump 15, while still in a slurry state mixed with the granular ammonium sulfate aqueous solution (Figure 2(c)). The extracted granular ammonium sulfate is then dehydrated and dried to obtain the final granular ammonium sulfate.

[0043] Furthermore, if the extraction rate is insufficient compared to the crystal growth rate, the large grains remaining in the crystallization can 9 will grow further, exceeding the desired particle size range (Figure 2(d)). Such excessively grown, coarse granular ammonium sulfate may be treated as a defective product.

[0044] Thus, a classification effect is at work inside the crystallization chamber, with microcrystals rising mainly to the middle section, while larger, crystallized particles sink to the lower section.

[0045] [Measurement of solid-liquid ratio] In the present invention, it is important to measure the solid-liquid ratio at multiple heights within the crystallization vessel in order to accurately understand the state of the crystals within the vessel as described above. By measuring the solid-liquid ratio, the amount of crystals at the measurement location can be quantitatively evaluated. Furthermore, by measuring the solid-liquid ratio at multiple heights, the distribution of crystals within the crystallization vessel can be determined. Therefore, the granular ammonium sulfate production equipment in one embodiment of the present invention is equipped with a solid-liquid ratio measuring means for measuring the solid-liquid ratio at multiple heights within the crystallization vessel.

[0046] Here, the "solid-liquid ratio" is the ratio of the weight of the solid to the total weight (the sum of the solid and liquid), that is, "weight of the solid / (weight of the solid + weight of the liquid)".

[0047] The method for measuring the solid-liquid ratio described above is not particularly limited and can be measured by any method.

[0048] In one embodiment of the present invention, a sample containing a mixture of solid and liquid is taken from inside the crystallization vessel, the weights of the solid and liquid contained in the sample are measured, and the weight of the solid is divided by the total weight (weight of solid + weight of liquid) to obtain the sample. The sample can be obtained, for example, by submerging a sampling container to a predetermined height (depth) inside the crystallization vessel, collecting the sample in the container, and then raising the sampling container. The sample may be collected manually or automatically by machine.

[0049] Furthermore, in other embodiments of the present invention, the solid-liquid ratio can also be measured indirectly. For example, a differential pressure gauge can be used to measure the differential pressure between the atmosphere and a predetermined height (depth) inside the crystallization vessel, and the solid-liquid ratio can be calculated from the differential pressure. In other words, the solid-liquid ratio measuring means may also include a differential pressure measuring means and a calculation means for calculating the solid-liquid ratio from the differential pressure measured by the differential pressure measuring means. The method for calculating the solid-liquid ratio from the differential pressure will be described below.

[0050] First, the differential pressure ΔP between a predetermined measurement position inside the crystallization vessel and the atmosphere is measured using a differential pressure measuring device. If Δh is the depth from the liquid surface at the measurement position, ρ is the specific gravity of the sulfuric acid slurry at the measurement position, and g is the acceleration due to gravity, then the differential pressure ΔP is expressed by the following equation (2). ΔP = ρΔh·g …(2)

[0051] By rearranging equation (2) above, the specific gravity ρ of the slurry can be expressed by the following equation (3). ρ = ΔP / (Δh·g) …(3)

[0052] Furthermore, if the solid-liquid ratio is V, the specific gravity of ammonium sulfate crystals is d, and the specific gravity of granular ammonium sulfate solution is D, the specific gravity ρ of the sulfuric acid slurry can be expressed as the weighted average of the specific gravity d of ammonium sulfate crystals and the specific gravity D of ammonium sulfate solution, as shown in equation (4) below. ρ = d·V + D(1-V) …(4)

[0053] From equations (3) and (4) above, equation (5) below can be derived. That is, the solid-liquid ratio V can be calculated from the differential pressure ΔP using equation (5) below. V = ΔP / {Δh g(dD)}- D / (dD) …(5)

[0054] Thus, since the method of determining the solid-liquid ratio from the differential pressure does not require sampling the solution in the crystallization tank, continuous monitoring of the solid-liquid ratio can be easily performed. Therefore, the granular ammonium sulfate production equipment in one embodiment of the present invention may be equipped with a differential pressure measuring means and a calculation means for calculating the solid-liquid ratio from the differential pressure measured by the differential pressure measuring means as solid-liquid ratio measuring means.

[0055] Furthermore, it is preferable to use a bubbler tube for measuring the differential pressure. A bubbler tube is a tube that is installed inside a crystallization vessel and bubbles gas from its tip. By using a bubbler tube, the differential pressure between the position of the tip of the bubbler tube and the atmosphere can be measured. Therefore, the granular ammonium sulfate production equipment in one embodiment of the present invention may be equipped with a bubbler tube as the differential pressure measuring means.

[0056] The number of locations where the solid-liquid ratio is measured is not particularly limited, as long as it is two or more, and can be any number. As will be described later, the desired effect can be obtained with measurement at two locations, but from the viewpoint of further enhancing the effect, it is preferable to measure at three or more locations. On the other hand, if measurements are taken at an excessively large number of heights, the effect will saturate, and the costs required for equipment and operation will increase. For this reason, it is preferable that the number of locations where the solid-liquid ratio is measured be eight or less, more preferably six or less, and even more preferably four or less.

[0057] For example, when calculating the solid-liquid ratio from the differential pressure measured by a bubbler tube, a bubbler tube should be installed at each measurement point. In this case, the bubbler tube should be installed so that its tip (gas outlet) is located at the differential pressure measurement point (i.e., the solid-liquid ratio measurement point).

[0058] The specific height at which the solid-liquid ratio is measured is not particularly limited and can be any height, but it is preferable to measure at least two locations, the lower and middle sections of the crystallizer, and more preferably three locations, the lower, middle, and upper sections. The reasons for this are explained below.

[0059] As described above, a classification effect is at work within the crystallization chamber, where grown granular ammonium sulfate accumulates in the lower section, while many microcrystals are stirred up and remain in the middle section. Therefore, by measuring the solid-liquid ratio in both the lower and middle sections, the state of the crystals within the crystallization chamber can be determined more accurately. Furthermore, although the upper section of the crystallization chamber is usually a region where no crystals exist, measuring the solid-liquid ratio in this upper section also allows for an even more accurate understanding of the state of the crystals within the crystallization chamber.

[0060] In one embodiment of the present invention, the lower section, middle section, and upper section are defined as follows. Top row: Regions where the solid-liquid ratio is less than 1% Middle section: Area where the solid-liquid ratio is 1% or more and less than 65%. Lower section: Areas where the solid-liquid ratio is 65% or higher.

[0061] When measuring the solid-liquid ratio at the above location, sampling and differential pressure measurements should be performed at a location where the solid-liquid ratio satisfies the above conditions under normal operating conditions.

[0062] In other embodiments of the present invention, the lower, middle, and upper stages may also be defined as follows, based on their relative height from the bottom surface of the crystallization vessel. Top row: Areas where the relative height is between 70% and 100%. Middle section: Areas where the relative height is 50% or more and less than 70%. Lower section: Areas with a relative height of 0% or more and less than 50%. Here, the relative height is the height from the bottom of the crystallization container, with the height from the bottom of the container to the liquid surface being defined as 100%.

[0063] In another embodiment of the present invention, the lower, middle, and upper sections can also be defined as follows, based on their relative volume from the bottom surface of the crystallization vessel. Top row: Regions where the relative volume is between 65% and 100%. Middle section: The region where the relative volume is 40% or more and less than 65%. Lower section: Regions with a relative volume of 0% or more and less than 40%. Here, the relative volume is the volume from the bottom of the crystallization container, where the volume from the bottom of the container to the liquid surface is considered to be 100%.

[0064] [Control of extraction amount] In the granular ammonium sulfate production method of the present invention, the solid-liquid ratio is measured at multiple heights within the crystallization vessel as described above, and the amount of slurry drawn is controlled based on the measured solid-liquid ratio. Furthermore, the granular ammonium sulfate production equipment of the present invention is equipped with a drawing amount control means for controlling the amount of slurry drawn based on the measured solid-liquid ratio. Below, specific examples of preferred methods for controlling the drawing amount will be described in several cases.

[0065] First, a preferred method for controlling the extraction amount will be explained using the example of measuring the solid-liquid ratio at three locations: upper, middle, and lower. In this embodiment, the extraction amount W is determined from the solid-liquid ratio based on equation (6) below. The extraction amount W can be expressed, for example, as the weight of solids extracted per unit time (t / h). For the weight of solids, for example, the dry weight of the solids (crystals) contained in the slurry may be used. W=α1×V1+α2×V2+α3×V3 …(6) Here, V1: Solid-liquid ratio in the upper section V2: Solid-liquid ratio in the middle section V3: Solid-liquid ratio in the lower section α1~α3: coefficients

[0066] The method for determining the coefficients α1, α2, and α3 is not particularly limited and can be adjusted as appropriate to obtain granular ammonium sulfate of the desired particle size. While the coefficients can be adjusted using the particle size of the final obtained granular ammonium sulfate as an indicator, they can also be adjusted using the lower solid-liquid ratio as an indicator. That is, among the solid-liquid ratios at the upper, middle, and lower positions, the lower solid-liquid ratio has the greatest influence on the particle size of granular ammonium sulfate. Therefore, the correlation between the particle size of granular ammonium sulfate and the lower solid-liquid ratio can be determined in advance, and based on this correlation, the lower solid-liquid ratio that yields the desired particle size can be set as the target solid-liquid ratio.

[0067] The correlation between the particle size of granular ammonium sulfate and the lower solid-liquid ratio can be determined, for example, by the following procedure. First, the granular ammonium sulfate production equipment is operated to produce granular ammonium sulfate. During this process, the lower solid-liquid ratio is measured. Meanwhile, the granular ammonium sulfate drawn from the crystallizer is subjected to solid-liquid separation, dried, and then its particle size is measured. For example, the median diameter in the mass-based particle size distribution can be used as the particle size. The particle size distribution can be measured using multiple sieves with different mesh sizes.

[0068] By performing the above measurements under multiple conditions, a correlation between the particle size of granular ammonium sulfate and the lower solid-liquid ratio can be obtained from the relationship between the measured lower solid-liquid ratio and the median diameter. In this case, it is preferable to approximate the median diameter of granular ammonium sulfate with a function of the lower solid-liquid ratio (for example, a linear function). Based on the correlation obtained in this way, the target lower solid-liquid ratio for obtaining granular ammonium sulfate of the desired particle size can be determined.

[0069] Once the target lower solid-liquid ratio is determined, the next step is to determine the coefficients α1, α2, and α3 using the target lower solid-liquid ratio as an indicator. The method is not particularly limited, but typically, a provisional coefficient is used to operate for a certain period and measure the lower solid-liquid ratio. If the measured lower solid-liquid ratio is lower than the target lower solid-liquid ratio, the coefficient is adjusted to reduce the extraction amount W. Conversely, if the measured lower solid-liquid ratio is higher than the target lower solid-liquid ratio, the coefficient is adjusted to increase the extraction amount W. By repeating this procedure, the coefficients at which the lower solid-liquid ratio stabilizes at a value close to the target lower solid-liquid ratio can be determined.

[0070] In this process, it is preferable to adjust each coefficient one by one in the order of α3, α2, and α1, rather than adjusting multiple coefficients simultaneously. This is because the lower solid-liquid ratio has the greatest impact on particle size, while the upper solid-liquid ratio has the least impact. For example, with α2 and α1 constant, adjust α3 so that the lower solid-liquid ratio is closest to the target lower solid-liquid ratio. Next, with α3 and α1 constant, adjust α2 so that the lower solid-liquid ratio is closest to the target lower solid-liquid ratio. Finally, with α3 and α2 constant, adjust α1 so that the lower solid-liquid ratio is closest to the target lower solid-liquid ratio. Note that if equation (7), described later, is used, the last step described above can be omitted.

[0071] When producing granular ammonium sulfate using the method of the present invention, the actual amount of extraction is adjusted so that the amount of extraction W is equal to the amount of extraction W calculated by equation (6) above. The amount of extraction can be adjusted, for example, by adjusting the opening degree of a control valve provided in the piping that extracts the slurry from the crystallizer.

[0072] There is no particular limit to the frequency of adjusting the amount of drawing; it can be done continuously or intermittently. However, there is generally a time lag of several tens of minutes to several hours between drawing the slurry from the crystallizer and drying the crystals to measure the amount of drawing, depending on the equipment. Therefore, adjusting the amount of drawing continuously or at a high frequency may result in overaction. For this reason, it is preferable to adjust the amount of drawing intermittently, with a time interval of 10 minutes or more, more preferably 20 minutes or more, and even more preferably 30 minutes or more. On the other hand, if the time interval is too long, it becomes difficult to control the particle size with sufficient precision. For this reason, the time interval is preferably 5 hours or less, more preferably 4 hours or less, even more preferably 3 hours or less, and most preferably 2 hours or less.

[0073] Furthermore, when measuring the solid-liquid ratio at two locations, the middle and lower sections, the extraction amount W can be determined using equation (7) below. W = α² × V² + α³ × V³ …(7)

[0074] As mentioned above, microcrystals rise to the middle stage but hardly reach the upper stage, so the solid-liquid ratio in the upper stage is usually less than 1%. Therefore, even when using three solid-liquid ratios, the influence of the upper stage's solid-liquid ratio on controlling the extraction amount is smaller than that of the middle and lower stages. For this reason, it is also possible to control the extraction amount using only the solid-liquid ratios of the lower and middle stages, as described above.

[0075] Conversely, when measuring the solid-liquid ratio in four or more stages, a term expressed as the product of the solid-liquid ratio Vn for the fourth stage and beyond and the coefficient αn should be added to equation (6) above.

[0076] Regarding the lower solid-liquid ratio V3, it may be used to control the extraction amount W by multiplying it by the coefficient α3 as shown in equations (6) and (7) above, but it is preferable to use a constant term β as shown in equation (8) below. W = α1 × V1 + α2 × V2 + β …(8) Here, β is a constant determined by the solid-liquid ratio V3 in the lower section.

[0077] For example, as illustrated below, the value of the constant term β is predetermined according to the range of the lower solid-liquid ratio V3. Then, the value of β used to calculate the extraction amount X is changed based on the measured lower solid-liquid ratio V3. Lower solid-liquid ratio V3 is 55% or less: β=1.5 Lower solid-liquid ratio V3 is over 55% and 60% or less: β=2.0 Lower solid-liquid ratio V3 is over 60%: β=2.5

[0078] As described above, when the extraction amount is determined using the constant term β, even if the lower solid-liquid ratio changes, it does not affect the extraction amount W as long as the value is within a certain range. Therefore, the frequency of changing the extraction amount W is reduced compared to when using equations (6) or (7) above. In the actual production of granular ammonium sulfate, after the slurry is extracted, the liquid is separated and dried, but if the extraction amount X changes frequently, it becomes necessary to adjust the drying conditions each time. Therefore, by treating the lower solid-liquid ratio V3, which has a large influence on the extraction amount W, as a constant term as described above, drying can be carried out smoothly. [Examples]

[0079] (Examples) To confirm the effects of the present invention, the extraction amount was controlled using equation (8) above, and the operation was carried out for two days. During the operation, the solid-liquid ratio was measured and the extraction amount was adjusted at 4-hour intervals. In equation (8), the coefficients were set to α1 = 0.15 and α2 = 30, respectively, and β was as exemplified earlier.

[0080] After drying the ammonium sulfate crystals obtained through the aforementioned operation, they were sieved using multiple meshes of different sizes. Of the sieved ammonium sulfate crystals, those with a particle size of 1.4 to 4.75 mm were deemed acceptable, and those of other particle sizes were rejected. The average final yield of large particles during the aforementioned operation period was 73%.

[0081] (Comparative example) For comparison, the extraction amount W was controlled based solely on the lower solid-liquid ratio V3, and the system was operated under the same conditions as in Example 1. That is, the extraction amount W was controlled using equation (9) above. W = β …(9)

[0082] As a result, the average final yield of large grains during the aforementioned operating period was 70%.

[0083] As described above, the method of the present invention achieved a high yield of 73% for large particles. This yield was 3% better than that obtained when control was made using only the lower solid-liquid ratio (comparative example). It should be noted that the production of granular ammonium sulfate from ammonia-containing gas, as in the present invention, is generally carried out on an extremely large scale, similar to the use of coke oven gas in steel mills. Therefore, even a 1% difference in yield can result in a difference of tens of thousands of tons in annual production, making the effects of the present invention extremely significant from an industrial perspective. [Explanation of symbols]

[0084] 1. Coke oven gas (ammonia-containing gas) 2 Sulfuric acid aqueous solution 3. Absorption Tower 4. Ammonium sulfate aqueous solution 5 Sulfuric acid 6 water 7 Intermediate tank 8 pumps 9 Crystallization Can 10 Evaporator 11 pumps 12 Heater 13 Vacuum equipment 14 Downcomer 15. Extraction pump

Claims

1. A method for producing granular ammonium sulfate, comprising: reacting an ammonia-containing gas with an aqueous sulfuric acid solution to form an aqueous ammonium sulfate solution; introducing the aqueous ammonium sulfate solution into a crystallization vessel; precipitating ammonium sulfate in the crystallization vessel to form granular ammonium sulfate; and withdrawing a slurry containing the granular ammonium sulfate from the bottom of the crystallization vessel, The differential pressure ΔP is measured at each of the multiple heights inside the crystallizer. From the differential pressure ΔP, the solid-liquid ratio V at each of the multiple heights is calculated using the following equation (1): A method for producing granular ammonium sulfate, wherein the amount of slurry drawn is controlled based on the solid-liquid ratio V. V = ΔP / {Δh・g(dD)}− D / (d−D) …(1) Here, Δh: Depth from the liquid surface at the measurement position of the differential pressure ΔP. ρ: Specific gravity of the slurry, g: gravitational acceleration, d: Specific gravity of ammonium sulfate crystals, D: This is the specific gravity of the ammonium sulfate solution.

2. An absorption tower that reacts ammonia-containing gas with an aqueous sulfuric acid solution to produce an aqueous ammonium sulfate solution, A crystallizer for precipitating ammonium sulfate from the aforementioned ammonium sulfate aqueous solution to produce granular ammonium sulfate, An extraction means for drawing out the slurry containing the granular ammonium sulfate from the bottom of the crystallization container, A differential pressure measuring means for measuring the differential pressure ΔP at each of the multiple heights inside the crystallizer, A calculation means that calculates the solid-liquid ratio V at each of the multiple heights using the following equation (1) from the differential pressure ΔP measured by the differential pressure measuring means, A granular ammonium sulfate production apparatus comprising: an extraction amount control means for controlling the extraction amount of the slurry based on the solid-liquid ratio V; and a granular ammonium sulfate production apparatus. V = ΔP / {Δh・g(dD)}− D / (d−D) …(1) Here, Δh: Depth from the liquid surface at the measurement position of the differential pressure ΔP. ρ: Specific gravity of the slurry, g: gravitational acceleration, d: Specific gravity of ammonium sulfate crystals, D: This is the specific gravity of the ammonium sulfate solution.

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