JPS5824364B2 - Manufacturing method of sulfuric acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing sulfuric acid, and its purpose is to bring sulfur dioxide gas (S02 gas) produced by sulfur combustion into contact with catalyst layers in multiple stages to sequentially convert it into sulfur trioxide (S03). In a sulfuric acid production plant that adopts a method of converting and sequentially absorbing the converted SO3 into sulfuric acid in a multistage manner, the converter inlet gas temperature can be easily adjusted to the optimum temperature for conversion regardless of load fluctuations of the plant, Another object of the present invention is to propose a method for producing sulfuric acid that can prevent equipment and piping in the plant from being corroded at low or high temperatures by sulfuric acid mist.

Sulfuric acid is produced by burning sulfur under conditions of excess oxygen and using the resulting S02 gas as a raw material.

That is, SO2 gas is sent to a converter together with the oxygen in the air that was supplied in excess during the combustion, and is brought into contact with an oxidation catalyst here to be converted into SO3 gas, which is then absorbed into sulfuric acid to produce it.

The reaction from the sulfur combustion process to the sulfuric acid production process is expressed by the following reaction formula.

S+02→SO□10 heat of formation ・・・・・・・・・・・・
...(1) ■ SO2+-O□→SO3+ heat of formation...
... (2) S03 + H20 → H2SO4 + heat of formation ...
(3) Since the conversion reaction from S02 to SO3 shown in the reaction formula (2) is an exothermic reaction, the conversion equilibrium proceeds in inverse proportion to the temperature.

Therefore, in order to increase the conversion yield to 803, it is required to maintain the reaction heat generated in the converter at a preferable temperature at which SO3 production is effectively performed.

This also applies to the conversion yield of each stage in a converter employing a multistage contact method with a catalyst.

Therefore, even when the unconverted S02 gas after removing the S03 gas generated by the conversion in the absorption tower is subjected to the conversion reaction again, it is required to adjust the gas temperature to a preferable value so that the conversion to SO3 is effectively carried out. Ru.

A method for producing sulfuric acid (hereinafter referred to as a double contact/double absorption method) in which raw SO2 gas is brought into contact with a catalyst layer in two stages, and the S03 converted and produced at each stage is absorbed into sulfuric acid each time.

), but the conventional method for this method uses the heat generated by the conversion reaction to reheat the unconverted S02 gas as a means to adjust the S02-containing gas used for the conversion reaction to a preferable temperature, and the excess heat is used to reheat the unconverted S02 gas. The method used was to remove it for steam superheating or for heating feed water to waste heat boilers.

For this reason, in an actual plant, the temperature of the SO2-containing gas (to be cooled) and the gas after the conversion reaction are adjusted in response to load fluctuations in the system that introduces the S02-containing gas discharged from the first-stage absorption tower into the second-stage converter. Depending on the dynamics of the heat exchanger, the flow distribution may be varied by bypassing each of the gas streams, or the SO2 containing SO in gas
□The concentration was changed.

However, with this type of adjustment method, the optimum temperature cannot be obtained unless the three fluids are thermally balanced, and because the heat capacity within the plant is large, the response is slow and it takes time to maintain the desired temperature. As a result, the yield of SO3, sulfuric acid, etc. decreases, and unconverted SO2 is released into the atmosphere.
This led to an increase in gas levels.

In addition, the piping system becomes more complex, which increases the number of points susceptible to corrosion, which increases the number of failures.

Furthermore, the unconverted SO2 gas discharged from the first-stage absorption tower contains sulfuric acid mist, and in order to reheat such corrosive gas using the reaction heat of the gas after the conversion reaction from the second-stage converter, Usually, an AC type gas-gas heat exchanger is used to reduce the heat transfer area.

However, the temperature at the inlet of unconverted S02 gas in this heat exchanger may be below the condensation temperature of sulfuric acid, and in this case, corrosion due to sulfuric acid mist adhering to the heat transfer tubes or tube sheets may occur. be caused.

Conventional double contact/double absorption methods have problems as described in the paper, and improvements are desired.

The present invention has been made in view of the above-mentioned problems, and an embodiment thereof will be described below regarding a double contact/double absorption method.

In the drawing, 1 is a sulfur combustion furnace, and sulfur 3 is sprayed and supplied by a sulfur supply pump 2 through a main air blower.
Combustion is carried out with combustion air 6 sent by air 4 through a drying tower 5.

Combustion air 6 is sent so that the oxygen concentration is excessive relative to sulfur 3.

7 is a converter. Converter I has first to fourth catalyst layers 8A
8D, the first to third catalyst layers 8A to 8C can be used for the first stage conversion reaction, and the remaining fourth catalyst layer 8D can be used for the second stage conversion reaction.

For convenience, the portion having the first to third catalyst layers 8A to 8C will be referred to as the front stage converter 9, and the portion having the fourth catalyst layer 8D will be referred to as the rear stage converter 10.

Reference numeral 11 indicates a first absorption tower corresponding to the first stage converter 9, and 12 a second stage absorption tower corresponding to the second stage converter 10, into which sulfuric acid at a predetermined temperature and concentration is distributed and circulated.

S02 generated in the sulfur combustion furnace 1 becomes a high-temperature gas of 10 to 14 vol% and is subjected to a conversion reaction in the front converter 9 together with excess oxygen. The gas passes through the recovery boiler 13 and the heat exchanger 14 installed in parallel thereto, and the flow rate distribution from both is adjusted so that the inlet gas temperature of the pre-converter 9 becomes a temperature preferable for the conversion reaction.

In the heat exchanger 14, the unconverted S02 from the front absorption tower 11
The gas is heated to a temperature suitable for carrying out the conversion reaction in the second stage converter 10, but the first stage absorption tower 1
Since the unconverted SO□ gas from 1 contains sulfuric acid mist and is at a low temperature, the tube plate or heat transfer tube of the heat exchanger 14 is easily corroded.

Therefore, in order to avoid such adverse effects, the temperature of the gas at the high temperature side inlet of the heat exchanger 14 is maintained at a temperature sufficient to prevent low-temperature corrosion due to sulfuric acid mist, and this is of a parallel flow type.

The heat balance between the inlet gas to the pre-converter 9 and the high temperature side inlet gas of the heat exchanger 14 is balanced by the heat recovery boiler 13.

For this purpose, the heat recovery boiler 13 is equipped with a bypass pipe L1.
and a return pipe L2 for returning the outlet gas to the heat exchanger 14 side.

Water 17 from outside the system is fed to the heat recovery boiler 13 through the boiler water supply pipe L3 by the boiler feed pump 15, and this water 17 is heat exchanged with some high temperature gas from the sulfur combustion furnace 1 in the heat recovery boiler 13. Then, it becomes steam and is discharged to the outside through the steam pipe L4.

Steam super heater (steam superheater) 1 is installed in steam pipe L4.
8 is interposed.

On the other hand, the high-temperature raw material gas containing SO2 and surplus oxygen introduced into the pre-converter 9 at a preferable temperature comes into contact with the first catalyst layer 8A, and a portion of the SO□ gas is converted into 803 gas, increasing the temperature.

This mixed gas of S03 gas and unconverted S02 gas is sent to the second catalyst layer 8B via the steam superheater 18.

The steam superheater 18 lowers the temperature of the mixed gas so that the conversion reaction in the second catalyst layer 8B proceeds at a preferable temperature.

As a result, the unconverted SO2 gas in the mixed gas comes into contact with the second catalyst layer 8B and is further converted into SO3 gas and heated up.

Cold air that has passed through the drying tower 5 is blown into the passage between the second catalyst layer 8B and the third catalyst layer 8C by a cooling air blower 19, and the cold air is used as the high-temperature gas that has passed through the second catalyst layer 8B. The temperature of the high-temperature gas is lowered to the optimum temperature for the conversion reaction in the third catalyst layer 8C.

The SO3 gas and unconverted SO2 gas converted and produced by contact with the third catalyst layer 8C are then sent to the first-stage absorption tower 11.

Here, in order to achieve the most effective absorption, it is necessary to cool the high temperature SO3 gas to its preferred temperature.

Temperature adjustment for this purpose is performed by the boiler water supply pipe L from outside the system.
This is done by the first economizer 20 interposed in 3.

That is, in the first economizer 20, the gas is introduced into the first stage absorption tower 11 after the heat is recovered to a corrosion-proofing temperature higher than the dew point of the gas.

The S03 gas produced by conversion here is absorbed by sulfuric acid.

Note that when producing SO3 gas, SO3 gas is extracted from the gas before entering the first stage absorption tower 11.

The remaining gas from which the SO3 gas has been absorbed is in an extremely close equilibrium state with the circulating acid in the tower at the outlet of the first-stage absorption tower 11.

As mentioned above, the remaining low-temperature SO2 gas containing 802 discharged from the front absorption tower 11 is reheated by the heat generated by sulfur combustion in the heat exchanger 14, and the temperature becomes favorable for the conversion reaction in the fourth catalyst layer 8D. and introduced into the subsequent converter 10.

The inlet gas temperature of the second stage converter 10 is kept constant regardless of the inlet gas temperature to the first to third catalyst layers 8A to 8C in the first stage converter 9 over the entire operating range by adjusting the SO□ gas flow rate. can be held.

Therefore, the conversion reaction in the fourth catalyst layer 8D can be easily carried out under favorable temperature conditions.

After the conversion reaction in the fourth catalyst layer 8D is completed, the high-temperature gas produced by the conversion is heat-recovered by the second economizer 21 installed in the boiler water supply pipe L3, and sent to the latter stage absorption tower 12 and sent to the S
03 gas is absorbed.

In order to further improve the S02 conversion rate in the embodiment described above, cold air is blown into the exit gas of the fourth catalyst layer 8D, and after temperature adjustment, the remaining SO□ is further converted by the fifth catalyst layer 8E shown by the phantom line. Then the second economizer 2
1, and the SO3 gas is absorbed in the latter stage absorption tower 12.

Furthermore, instead of blowing in cold air, a heat exchanger such as a waste heat recovery boiler may be installed and used to adjust the temperature of the high-temperature gas entering the third catalyst layer 8C.

As detailed above, according to the present invention, it is possible to easily adjust the gas temperature preferable for the conversion reaction in all load ranges,
Further, the followability of temperature adjustment operations accompanying load fluctuations is improved.

Furthermore, since there is no need to install extra piping, the equipment can be simplified.

Moreover, since the high-temperature gas generated in the sulfur combustion furnace is introduced into the front converter through a heat recovery boiler and heat exchanger installed in parallel, the high-temperature gas is used for reheating in a high-temperature state to obtain a high temperature difference. Not only can the heat recovery boiler and the heat exchanger be downsized, but also the temperature of the heat transfer surface of the heat exchanger can be increased to prevent dew point corrosion on the low temperature gas side.

[Brief explanation of drawings]

The drawings are detailed illustrations of the invention. 1... Sulfur combustion furnace, 3... Sulfur, 7.
...Converter, 8A to 8E...1st to 5th
Catalyst layer, 9... Pre-stage converter, 10...
Post-stage converter, 11...Pre-stage absorption tower, 12...
...Latter stage absorption tower, 13... Heat recovery boiler, 1
4...Heat exchanger, 17...Water outside the system, 1
8...Steam super heater, 20, 21...
... Economizer, Ll ... Bypass pipe,
L2...Return pipe, L3...Boiler water supply pipe, L4...Steam pipe.

Claims (1)

[Claims] 1 A sulfur combustion furnace is one in which sulfur dioxide gas produced by sulfur combustion is used as a raw material, and this is brought into contact with a catalyst layer in multiple stages to be sequentially converted into sulfur trioxide, and the converted sulfur trioxide is sequentially absorbed into sulfuric acid in a multistage manner. The high temperature gas generated in is introduced into the front stage converter through a heat recovery boiler and a heat exchanger installed in parallel, and the high temperature gas after the conversion reaction released from this front stage converter is introduced into the front stage absorption tower through the first economizer, As a result, sulfuric acid is recovered, and the low-temperature gas discharged from the first-stage absorption tower is reheated in the heat exchanger and then introduced into the second-stage converter. The sulfuric acid is introduced into the latter absorption tower through an economizer, thereby further recovering sulfuric acid, while the cooling fluid from outside the system is passed through the first and second economizers and the heat recovery boiler to absorb heat. Method for producing sulfuric acid.