In a Double Contact Double Absorption (DCDA) sulfuric acid plant, thermal management is critical. The oxidation of sulfur to sulfur dioxide ($\text{SO}_2$) and the subsequent conversion to sulfur trioxide ($\text{SO}_3$) are strongly exothermic:
$$\text{S} + \text{O}_2 \longrightarrow \text{SO}_2 \quad (\Delta H = -297 \text{ kJ/mol})$$
$$2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3 \quad (\Delta H = -198 \text{ kJ/mol})$$
To prevent catalyst degradation above $600^\circ\text{C}$ and maintain equilibrium reaction rates at $400–450^\circ\text{C}$, heat exchangers, steam generators, and superheaters are arranged between each catalyst bed.
Equipment Arrangement Across Operational Stages
1. Furnace Exit: High-Pressure Waste Heat Boiler (WHB)
Hot gas exiting the sulfur burner at $1000–1100^\circ\text{C}$ passes directly through a Waste Heat Boiler (WHB).
- Function: Cools the $\text{SO}_2$-rich gas down to $420^\circ\text{C}$ before entering Converter Bed 1.
- Energy Recovery: Generates high-pressure saturated steam ($30–60 \text{ bar}$) from treated boiler feed water.
2. Converter Stage 1 to 3 Inter-Bed Heat Exchangers
As gas passes through each catalyst bed, reaction heat raises gas temperatures rapidly. Inter-stage heat exchangers cool the gas back down to the target $410–430^\circ\text{C}$ inlet temperature before entering the next bed.
Furnace (1000°C) ---> [ WHB ] ---> Bed 1 (420°C -> 600°C)
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[ Superheater ]
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Bed 2 (430°C -> 510°C)
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[ Cold Heat Exchanger ]
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Bed 3 (430°C -> 450°C)
- Bed 1 Exit ($\sim 600^\circ\text{C}$): Passes through a Steam Superheater. This cools the gas to $430^\circ\text{C}$ while superheating saturated steam from the WHB for power generation.
- Bed 2 Exit ($\sim 510^\circ\text{C}$): Passes through an Economizer or Gas-to-Gas Heat Exchanger to cool back to $430^\circ\text{C}$ before Bed 3.
3. Intermediate Absorption Loop (Cold / Hot Gas Exchangers)
Before entering the Intermediate Absorption Tower (IAT), gas temperature must drop from $\sim 450^\circ\text{C}$ to $\sim 200^\circ\text{C}$ to prevent acid evaporation. Conversely, gas leaving the IAT ($80^\circ\text{C}$) must be re-heated to $420^\circ\text{C}$ before entering Bed 4.
- Cold Gas Exchanger: Cold gas exiting the IAT ($80^\circ\text{C}$) absorbs heat from warm gas exiting Bed 3 ($\sim 450^\circ\text{C}$).
- Hot Gas Exchanger: The pre-heated gas is further heated by the hot gas exiting Bed 1 or Bed 2 to reach the $420^\circ\text{C}$ threshold required for Bed 4 activation.
4. Converter Stage 4 and Final Absorption
After exiting Bed 4, the gas ($\sim 440^\circ\text{C}$) must be cooled to $\sim 180–200^\circ\text{C}$ prior to entering the Final Absorption Tower (FAT).
- Final Economizer: Cools the gas exiting Bed 4 while preheating incoming boiler feed water sent to the WHB.
Thermal Control Summary
| System Component | Gas Inlet Temp | Gas Outlet Temp | Cooling / Heating Medium | Thermal Output |
| Waste Heat Boiler | $1000^\circ\text{C}$ | $420^\circ\text{C}$ | Boiler Feed Water | High-Pressure Saturated Steam |
| Steam Superheater | $600^\circ\text{C}$ | $430^\circ\text{C}$ | Saturated Steam | Superheated Steam ($400^\circ\text{C}+$) |
| Hot Gas Exchanger | $450^\circ\text{C}$ | $280^\circ\text{C}$ | Cold Gas from IAT ($180^\circ\text{C}$) | Heats IAT exit gas to $420^\circ\text{C}$ |
| Cold Gas Exchanger | $280^\circ\text{C}$ | $190^\circ\text{C}$ | Cold Gas from IAT ($80^\circ\text{C}$) | Heats IAT exit gas to $180^\circ\text{C}$ |
| Final Economizer | $440^\circ\text{C}$ | $180^\circ\text{C}$ | Water from Treatment Plant | Pre-heated Boiler Feed Water |