JPH09201512A - Method for recovering sulfuric acid by exhaust gas desulfurization - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve desulfurization efficiency and to obtain high concentration sulfuric acid by a method in which a sulfur dioxide absorption part is converted into multiple stages to be combined in a series, absorbing liquid and exhaust gas are brought into contact with each other in a crossflow, the concentration of diluted sulfuric acid of the absorbing liquid is decreased from a stage to another in order of gas flow. SOLUTION: Absorption liquid is pumped up from each liquid receiver B-1, B-2, B-3 in the lower part of each absorption unit 1, 2, 3 to the upper part of each absorption unit 1, 2, 3 by pumps P-1, P-2, P-3 and flows down to the liquid receivers B-1, B-2, B-3 in the lower part respectively. Besides, exhaust gas containing sulfur dioxide is introduced from the exhaust gas inlet G-1 of the absorption unit 1 into an absorption apparatus and is brought into contact with the absorbing liquid in sequence in a crossflow in the absorption units 1, 2, 3, and the sulfur dioxide is absorbed by the absorbing liquid and discharged from a gas outlet. The absorbing liquid supplied to the absorption unit 3 is water, the absorbing liquid supplied to the absorption unit 2 is the absorbing liquid of the liquid receiver B-3, and the absorbing liquid supplied to the foremost absorption unit 1 is the absorbing liquid of the liquid receiver B-2.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for recovering sulfuric acid by desulfurizing exhaust gas containing sulfurous acid gas.

[0002]

2. Description of the Related Art Sulfurous acid gas contained in combustion exhaust gas of fuels such as heavy oil and coal has a great influence on the environment, and various technologies for removing this, that is, exhaust gas desulfurization technology, have been proposed in order to prevent pollution.

As one of these exhaust gas desulfurization techniques, there is a method of recovering sulfurous acid gas contained in the exhaust gas as sulfuric acid, which is roughly classified into the following three methods. (1) A method in which exhaust gas is brought into contact with diluted sulfuric acid to absorb sulfur dioxide gas and oxidized to form sulfuric acid.

SO ₂ + H ₂ O → H ₂ SO ₃ H ₂ SO ₃ + 1 / 2O ₂ → H ₂ SO ₄ (2) Wellman load method Exhaust gas is contacted with and reacted with an aqueous solution of sodium sulfite to obtain acidic sodium sulfite. It is concentrated, decomposed by heating and decomposed into sodium sulfite and sulfurous acid gas, and sodium sulfite is used again for contact and reaction with exhaust gas. To obtain sulfuric acid.

SO ₂ + Na ₂ SO ₃ + H ₂ O → 2NaHSO ₃ 2NaHSO ₃ → Na ₂ SO ₃ + SO ₂ ↑ + H ₂ O (3) Magnesium sulfite decomposition method Obtained, concentrated and roasted to decompose into magnesium oxide and sulfurous acid gas, magnesium oxide is dissolved in water and used again for contact and reaction with exhaust gas, and sulfurous acid gas is used as a raw material gas in a known sulfuric acid production process, Finally, the desired concentration of sulfuric acid is obtained.

SO ₂ + Mg (OH) ₂ → MgSO ₃ + H ₂ O MgSO ₃ → MgO + SO ₂ ↑ Each of the above methods has the following drawbacks. In the method (1), as the concentration of sulfuric acid becomes higher, it becomes difficult to absorb the sulfurous acid gas. Therefore, only sulfuric acid with a low concentration (for example, about 3%) can be obtained, and in order to obtain a high concentration of sulfuric acid, equipment for concentration and a huge heat source are required. The method (2) requires a large absorption tower and requires equipment and a large heat source for the concentration and thermal decomposition of NaHSO ₃ . The method (3) also requires equipment for concentrating and roasting MgSO ₃ and a large heat source.

[0007]

SUMMARY OF THE INVENTION The object of the present invention is to eliminate the drawbacks of the prior art. A method for improving the method (1) to obtain a high concentration of sulfuric acid with economical equipment. Is to provide.

[0008]

The present invention provides: Exhaust gas containing sulfurous acid gas is treated by a plurality of the following means. Means for collecting the absorbing liquid from the upper part in the form of spray and raindrops and collecting it in the lower liquid reservoir. 2. A means for introducing an exhaust gas containing sulfurous acid gas and bringing it into contact with a flowing absorbing solution in a cross flow to discharge it. A sulfite gas absorption unit having means for blowing air into the liquid reservoir for oxidizing the absorbing liquid, means for replenishing the absorbing liquid to the liquid reservoir, means for discharging the absorbing liquid from the liquid reservoir, and means for circulating the absorbing liquid to the upper part. , Is loaded into an absorption device that is continuously coupled in the direction of the flow of exhaust gas, and is contacted with the absorbing liquid flowing down in each unit to desulfurize, at this time, in the order in which the exhaust gas passes through multiple absorption units. Therefore, the units 1, 2. . . N is the last unit N
Is used for replenishing the absorbing liquid of the unit N-1, the absorbing liquid of the reservoir of the unit N is used for replenishing the absorbing liquid of the unit N-1, and the same is applied to the unit 1 for replenishing the absorbing liquid of the first unit. 1. A method for recovering sulfuric acid by exhaust gas desulfurization, characterized in that sulfuric acid is obtained as the discharge liquid of the absorption liquid of the unit 1 using the absorption liquid of the liquid reservoir of 2. The exhaust gas containing the sulfurous acid gas before being charged into the absorption device and the discharge liquid of the absorption liquid of the unit 1 are introduced into the column and brought into contact with each other, and the exhaust gas is cooled and supplied to the unit 1, Exhaust gas of the unit 1 is concentrated by utilizing the sensible heat of the introduced exhaust gas, and a cooling concentrating tower that is taken out as a concentrated sulfuric acid solution is added to the absorption device. It is a method of recovering sulfuric acid by desulfurization.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the method of the present invention is an improved method of the prior art method (1). The basis of the method of (1) is to absorb and remove sulfurous acid gas (hereinafter sometimes referred to as SO ₂ ) in the exhaust gas with diluted sulfuric acid, and to dissolve SO ₂ dissolved in diluted sulfuric acid.
To oxidize air to sulfuric acid.

Here, in order to obtain a good absorption effect as compared with mere physical absorption by dilute sulfuric acid, in the absorption tower, an oxidation reaction is caused by oxygen at the same time as the absorption of SO _{2 and} the absorbed SO is absorbed. _It is sufficient to change ₂ to sulfuric acid to bring it into a state of chemical absorption to increase the overall absorption rate.

Generally, the combustion exhaust gas contains oxygen due to the use of excess air at the time of combustion, and this oxygen is used for the oxidation reaction. However, in order to carry out the oxidation reaction more reliably, the lower part of each absorption unit is used in the present invention. A means for blowing air into the liquid reservoir was provided.

Further, it is known that the combustion exhaust gas contains an oxide of a metal contained in the fuel, which acts as an SO ₂ oxidation catalyst. If necessary, a known catalyst in the absorption liquid, For example, an iron catalyst can be added.

By blowing air into the absorbing liquid in the liquid reservoir, the SO ₂ is completely oxidized as described above and the residual SO ₂ is eliminated, and the absorbing liquid can contain a saturated amount of oxygen. SO _{2 in} absorption unit using liquid
Is efficiently absorbed. Next, factors that affect the removal efficiency of SO ₂ by the absorption tower will be examined. These are the following three.

(A) Concentration of diluted sulfuric acid which is SO ₂ absorbing liquid (b) SO ₂ concentration in exhaust gas at the inlet of absorption tower (c) Liquid gas ratio (L / G) The influence of these factors on the desulfurization rate has already been described. Well-known (eg "Gypsum and lime" No. 121, P
20-23. November 1972, Gypsum Lime Society).

As for (a), as shown in FIG. 4, the desulfurization rate decreases as the concentration of sulfuric acid increases. Regarding (b), as shown in FIG. 5, the desulfurization rate decreases as the SO ₂ concentration increases. (C) Regarding the liquid gas ratio, as a matter of course, as shown in FIG. 4 and FIG.
The larger the value, the higher the desulfurization rate. Summarizing this, (1) L / G is large, (2) the concentration of dilute sulfuric acid, which is the absorbing liquid, is low, and (3) operation with a low concentration of artificial SO ₂ results in high efficiency in SO ₂ removal. It is shown that. However, in reality, it is necessary to reduce L / G to reduce pump power, increase the concentration of dilute sulfuric acid to reduce the concentration cost of the diluted sulfuric acid to be recovered, and to operate efficiently even with exhaust gas with high SO ₂ concentration. Is.

Considering the above factors, a case where one conventional absorption tower (FIG. 8) is used and a case where the method of the present invention is used will be described in comparison. FIG. 8 is a schematic view of an example of an absorption tower used in the conventional method. The exhaust gas containing SO ₂ is introduced into the portion of the absorption tower 5 at the tower height H1, and the packing portion with the packing shown at the tower height H3 is located in the portion shown at the tower height H2. After contacting with the absorbing liquid dispersed through the nozzle and absorbing SO ₂ , it is discharged from the top of the column. The lower liquid reservoir B-O is provided with an air blowing port A-O so that the absorbing liquid can be replenished and discharged, and the absorbing liquid is supplied to the nozzle at the upper part of the tower by the pump P-O.

In such an absorption tower, from the relationship between the sulfuric acid concentration and the desulfurization rate shown in FIG. 4, the concentration of sulfuric acid used in the absorbing liquid is 1% (weight%, the same applies hereinafter) for the same L / G. Table 1 shows the absorption capacity at each concentration, where the absorption capacity of the absorption tower is 1.

[0018]

[Table 1] That is, when the sulfuric acid concentration is 8%, the absorption capacity is 1 / 13.3 lower than when the sulfuric acid concentration is 1%.

On the other hand, the method of the present invention is as follows. FIG. 1 shows that in the present invention, the number of units is 3 (N
= 3), the absorption device is schematically shown. In order to make it easier to understand, the liquid reservoir B-1 of each unit,
B-2 and B-3 are drawn away from the apparatus main body.

In the figure, the absorption liquid is represented by each absorption unit 1,
Pumps P-1, P-2, and P-3 pump up liquid from the lower reservoirs B-1, B-2, and B-3 of the pump 2, and flow down to the lower reservoirs, and SO ₂ The exhaust gas containing the is introduced into the absorption device from the exhaust gas inlet G-1 of the absorption unit 1 and sequentially contacted with the absorption liquid in the absorption units 1, 2 and 3 in a cross flow, SO ₂ is absorbed by the absorption liquid and is discharged from the gas outlet. Is discharged. As shown in the figure, the absorption liquid supplied to the absorption unit 3 is water, the absorption liquid supplied to the absorption unit 2 is the absorption liquid of the liquid reservoir B-1, and the absorption liquid supplied to the first absorption unit 1 is the liquid. It is the absorption liquid of reservoir B-2.

From the above-mentioned FIGS. 4 and 5, when the operation is performed so that the sulfuric acid concentration of the liquid reservoir B-1 becomes 8%, the sulfuric acid concentrations of the liquid reservoirs B-2 and B-3 are about 5% and 2.
8%, each unit capacity is 1 / 13.3, 1/3, 1/2 compared to 1% sulfuric acid operation, which is about 4 times the performance of the one-column system on average. Will have. In addition, the population S increases in the later stages of each unit.
Since the O ₂ concentration is low, the synergistic effect reduces the liquid volume to 1/5.
A small amount of liquid will suffice.

Further, in the example of the present invention shown in FIG. 1, the binding of each absorption unit is carried out via an eliminator. In addition, for the dispersion of the absorbing liquid, a simple sprinkler can be used such that the liquid is poured into a large tray as shown in FIG. 3 and the liquid is dripped from a small hole at the bottom. Further, in the method of the present invention, since the gas flow is made to cross the absorbing liquid flow, the pump head for sending the absorbing liquid to the upper part of the unit is substantially the same as that of the conventional example (FIG. 8) as shown in FIG.
Since it corresponds to 3, the pump power can be greatly reduced.

In addition to the method of degassing exhaust gas by using the above-mentioned absorption device to obtain dilute sulfuric acid, a method of cooling the exhaust gas charged into the absorption device and adding a step of concentrating the dilute sulfuric acid is also used. Included in the invention.

A schematic process diagram for explaining this method is shown in FIG. In FIG. 6, a part of the liquid feeding system of the absorbing liquid of the absorbing device and the like are omitted, and only a main part is shown. For example, combustion exhaust gas from a boiler is introduced into the cooling concentration tower 4 from the lower part of the tower and discharged from the top of the tower. During this period, the combustion exhaust gas is pumped from the absorber B-1 to the pump P-4 to form a liquid pool B-4 at the bottom of the tower.
Is further sent to the upper part of the tower by the pump P-5 from the liquid reservoir B-4, and is sprayed with the diluted sulfuric acid to be brought into gas-liquid contact and cooled, and the diluted sulfuric acid is concentrated.

In this case, the outlet exhaust gas has a large wet bulb temperature.
Operate using the degree of no effect as a guide. Inlet exhaust gas temperature
Degree, water content, diluted sulfuric acid concentration supplied from B-1, etc.
Of course, there is a limit to the concentration of sulfuric acid that can be obtained. 1
SO in the exhaust gas as an example _TwoConcentration of 1000ppm
(A), 1500ppm (B), 2000ppm (C)
And the temperature was 150 ° C. and the desulfurization rate in the absorber was 80%.
In this case, the supply sulfuric acid concentration of the cooling concentration tower and the sulfuric acid concentration after concentration
The relationship is as shown in FIG. 7, and the above (A), (B),
In the case of (C), 5%, 6.8% and 8.2% sulfur, respectively
30% sulfuric acid is obtained when the acid is charged into the cooling concentrating column.
In addition, in FIG. 6, the exhaust gas to be cooled is supplied from the lower part of the tower.
The diluted sulfuric acid introduced and concentrated is sprinkled from the top of the tower and directed.
They are in flow contact, but the contact of these gas and liquid is parallel flow or
Nothing like using a device that is done in a cross-flow
It doesn't matter.

[0026]

EXAMPLE An example of the recovery of sulfuric acid from exhaust gas corresponding to a heavy oil oil-fired 3-ton boiler is shown below. Using the series of devices schematically shown in FIG. 6, the inlet exhaust gas amount of the cooling concentration tower is 3000 Nm ³ / hr, the temperature is 150 ° C., the SO ₂ concentration is about 2000 ppm, L / G = 10 (therefore, the circulating fluid amount of each unit). Is 30 tons / hr) and the amount of makeup water is about 50 kg.
/ Hr, the data during steady operation are shown in Table 2.

[0027]

[Table 2] Table 2 ─────────────────────────────────── Gas temperature Cooling concentration tower outlet Absorption device outlet 57 ℃ 52 ℃ ─────────────────────────────────── SO ₂ concentration Unit 1 outlet Unit 2 outlet Unit 3 Outlet 1360ppm 690ppm 190ppm ─────────────────────────────────── SO ₂ removal rate Unit 1 Unit 2 Unit 3 32 % 49% 72% ─────────────────────────────────── Absorbing liquid B-1 B-2 B -1 Sulfuric acid concentration 8.1 wt% 5.5 wt% 2.8 wt% ──────────────────────────────── ──── Sulfuric acid recovery amount 30% Acid 71 kg / hr ─────────────────────────────────── As mentioned above, the desulfurization rate is about 90 in total. %, Sulfuric acid recovery amount is 1
A result of about 20 kg / hr was obtained as 00% sulfuric acid.

On the other hand, in the case of a one-tower type absorbing solution having a sulfuric acid concentration of 8.1%, the following results were obtained when the inlet gas conditions were the same.

[0029]

[Table 3] Table 3 L / G absorption liquid circulation amount Outlet SO ₂ SO ₂ removal rate (ton / hr) (ppm) (%) 30 90 90 900 55 55 50 150 640 68 100 100 300 320 320 84 150 450 180 180 91 Table 3 According to this, in the case of the one-column type, compared with the example of the present invention, in order to obtain the same desulfurization performance, the circulating liquid amount of the absorbing liquid is approximately 5
You will need twice.

[0030]

As described in detail above, according to the method of the present invention, efficient desulfurization and sulfuric acid having an increased concentration can be obtained by the multistage absorption tower in the desulfurization of exhaust gas. By using a cooling concentration tower together,
It has become possible to recover even higher concentrations of sulfuric acid.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating an example of the method of the present invention.

FIG. 2 is a schematic diagram showing an actual head of a pump for pumping an absorption liquid in the method of the present invention.

FIG. 3 is a schematic diagram showing an example of an absorbent spreading device used in the method of the present invention.

FIG. 4 is a graph showing the relationship between the concentration of sulfuric acid used in the absorbing liquid and the desulfurization rate.

FIG. 5 is a graph showing the relationship between the SO ₂ concentration at the inlet of the absorption tower and the desulfurization rate.

FIG. 6 is a schematic process diagram showing an example of the method of the present invention in which a cooling concentration tower is added to the absorption device.

FIG. 7 is a graph showing the relationship between the sulfuric acid concentration at the inlet and the sulfuric acid concentration at the outlet of the cooling concentration tower.

FIG. 8 is a schematic diagram showing an example of an absorption tower in the related art.

[Explanation of symbols]

 1, 2, 3 Absorption unit 4 Cooling concentration tower 5 Absorption tower A-0 to A-3 Air injection port B-0 to B-4 Liquid reservoir P-0 to P-5 Pump G-1 Exhaust gas inlet

Claims (2)

[Claims] 1. Exhaust gas containing sulfurous acid gas is treated by a plurality of the following means. Means for collecting the absorbing liquid from the upper part in the form of spray and raindrops and collecting it in the lower liquid reservoir. 2. A means for introducing an exhaust gas containing sulfurous acid gas and bringing it into contact with the flowing down absorbing solution in a cross flow to discharge it. A sulfite gas absorption unit having means for blowing air into the liquid reservoir for oxidizing the absorbing liquid, means for replenishing the absorbing liquid to the liquid reservoir, means for discharging the absorbing liquid from the liquid reservoir, and means for circulating the absorbing liquid to the upper part. , Is loaded into an absorption device that is continuously coupled in the direction of the flow of exhaust gas, and is contacted with the absorbing liquid flowing down in each unit to desulfurize, at this time, in the order in which the exhaust gas passes through multiple absorption units. Therefore, the units 1, 2 ,. . . Assuming N, the last unit N uses water for replenishing the absorbing liquid, the unit N-1 uses replenishing absorbing liquid for the unit N reservoir, and so on. A method for recovering sulfuric acid by exhaust gas desulfurization, characterized in that the absorbent of the liquid reservoir of unit 2 is used to replenish the absorbent of No. 1, and sulfuric acid is obtained as the discharge of the absorbent of unit 1. 2. The exhaust gas containing the sulfurous acid gas before being charged into the absorption device and the discharge liquid of the absorption liquid of the unit 1 are introduced into the column and brought into contact with each other, and the exhaust gas is cooled and then is supplied to the unit 1. Exhaust gas, characterized in that a cooling and concentrating tower that is supplied and concentrated by utilizing the sensible heat of the introduced exhaust gas as the discharged liquid of the unit 1 and is taken out as concentrated sulfuric acid liquid is added to the absorption device. A method for recovering sulfuric acid by desulfurization.