3.2 Calculation of water Volume for Low-temperature heat recovery

1) The total water volume required by the low-temperature heat recovery system

In the previous text, a scenario was set where the first conversion rate was 95%, the total conversion rate was 99.8%, and all the sulfur trioxide in the first conversion was absorbed through low-temperature heat recovery, generating sulfuric acid with a concentration of 99.6%.

The mass of 100% concentration sulfuric acid generated by low-temperature heat recovery absorption =100/98/99.8%*95%*98=95.19t/h

The required water addition =95.19/98*18+95.19/99.6%-95.19=17.87t/h

2) Distribution of water addition volume

Secondary acid carries water To ensure the complete absorption of flue gas in the high-temperature absorption tower, a two-stage absorption system is set up in the high-temperature absorption tower. The secondary acid should not only guarantee the absorption efficiency but also not corrode the equipment. Generally, the acid concentration is controlled at around 98.5%. Additionally, the flue gas temperature at the outlet of the high-temperature absorption tower needs to be controlled. If the temperature is too high, it will burn out the demister; if it is too low, the temperature of the fourth stage of conversion cannot be controlled. The amount of secondary acid also needs to be controlled. For a facility with an annual capacity of 800,000 tons, the temperature is generally controlled at around 114 cubic meters per hour and 60℃.

Calculate the amount of water brought in by secondary acid as 114*1.7968-114*1.7968*98.5%/99.6%=2.26t/h

Diluter water addition: Similarly, the acid concentration in the low-temperature heat recovery tower and the liquid level in the circulating acid tank are in a dynamic balance. Under the condition that the acid valve of the drying string diluter is not open, the only sources of water are secondary acid and diluter water addition.

The water addition volume of the diluter =17.87-2.26=15.61t/h

The acid produced by the low-temperature heat recovery system consists of secondary cross-acid and acid produced by absorption

Low temperature heat recovery capacity acid = 114 * 1.7968 * 98.5% / 99.6% + 95.19/99.6% = 298 t/h

If the extreme weather mentioned earlier is 38℃ and the humidity is 80%, to ensure that the dry acid concentration is 98%, 218.5t/h of diluted acid needs to be added for drying

Calculate the water volume brought in by the drying string diluter as 218.5-218.5*98%/99.6%=3.51t/h

In this case, the amount of water added to the diluter =17.87-2.26-3.51=12.1t/h

3.3 Calculation of secondary suction water addition volume

The secondary suction circulation acid tank undertakes multiple processes such as drying and acid transfer, low-temperature heat recovery for acid production, and secondary suction absorption for acid production. These can be calculated separately, but this method is relatively complex. We can consider the entire system as a whole, and the calculation will be simpler.

The total water addition of the device =100/98*18+100/98.5%-100=19.89t/h

The secondary suction water addition volume = the total water addition volume of the device - the water volume brought in by air - the water addition volume of the diluter =19.89-3-15.61=1.28t/h

Through calculation, it can be seen that under normal working conditions, a small amount of water is still added in the second suction to maintain the stability of the acid concentration. But let's look at it under the above-mentioned extreme weather conditions.

Calculate the total sum of the water added to the diluter and the water carried by the air as 12.1+8.38=20.48t/h. At this point, the water added at both locations has exceeded the total water added to the device. Not only does the second suction not need to add water, but it also fails to retain the acid concentration. If the original dry acid concentration and the corresponding amount of cross-acid are maintained

A simple calculation shows that the secondary acid concentration =100/(20.48-100/98*18+100)=97.94%(ignoring the change in secondary acid concentration during low-temperature heat recovery).

At this point, the second inhalation without adding water can only maintain around 98%. At this point, the acid concentration enters the high-temperature absorption tower in the form of secondary acid, which will corrode the equipment and shorten its service life. Therefore, we must also take measures to increase its acid concentration.

3.4 Adjustment Calculation of Acid balance

Under the premise that the amount of acid in the drying string of the low-temperature heat recovery diluter cannot be increased, we can only reduce the concentration of the dried acid, allow more water to enter the low-temperature heat recovery diluter, and reduce the amount of water added for dilution to maintain the stability of the entire secondary absorption acid concentration.

First, determine that the maximum water addition of the low-temperature heat recovery diluter under the above extreme conditions = total system water addition - air water carrying capacity =19.89-8.38=11.48t/h

The water volume of the drying string diluter = the water volume required for low-temperature heat recovery - the water volume brought in by the secondary acid - the maximum water addition capacity of the diluter =17.87-2.26-11.48=4.13t/h

If the maximum acid transmission volume of the above-mentioned diluter is maintained, calculate the minimum concentration of the dried acid concentration

Acid concentration = (218.5-4.13) *99.6%/218.5=97.58%

Through calculation, it can be known that the acid concentration of the drying system can be controlled below 97.58%, and the acid concentration of the entire system can be maintained by keeping the above-mentioned acid flow in the diluter. However, increasing the acid flow in the diluter also increases the acid production of the low-temperature heat recovery system. This method will reduce the thermal energy utilization rate of the low-temperature heat recovery and does not utilize the overall thermal energy recovery of the device. Therefore, the acid flow in the diluter of the drying system needs to be reduced. At this point, it is necessary to reduce the concentration of the dry acid to maintain the constant flow of water.

If the acid flow is reduced to below 115t/h, calculate the minimum acid concentration under extreme conditions for drying

Acid concentration = (115-4.13) *99.6%/115=96%

Drying an acid concentration of over 96% can meet the requirements of the device and also ensure a high steam production rate of the low-temperature heat recovery system under extreme conditions.

Acid concentration is the key to the operation of the device and also the guarantee of the service life of the equipment. Currently, it is controlled by a single variable of the acid concentration meter. If the instrument malfunctions, the acid concentration will fluctuate greatly. In severe cases, it may cause environmental protection accidents and damage the equipment. The introduction of multi-variable control for water balance can achieve predictive effects even under various factors such as instrument distortion, changeable weather, and load fluctuations, ensuring the long-term stable operation of the device.

In the previous water balance calculation formula (I), the temperature data used to calculate the working condition gas volume was incorrect. However, to maintain the uniformity of the calculation, the data from the previous text is still applied in this paper.