Sulphuric Acid Dew Point

1. Introduction

Calculating the Acid Dew Point (ADP) is essential in industries where acid components are present in the fuel. The acid dew point is the temperature at which acidic components in a gas mixture begin to condense into liquid form. This process can lead to corrosion of equipment and significant loss of efficiency in industrial processes.

To calculate the acid dew point, several correlations have been proposed. The key components include the composition of the gas mixture, the total pressure, and the partial pressures of individual acidic gases. Typically, sulfuric acid (H₂SO₄) is a major concern in industries dealing with flue gases from combustion processes.

In summary, calculating the acid dew point is a critical aspect of industrial safety and equipment maintenance. Understanding the composition of gas mixtures and employing appropriate methods to determine the acid dew point helps industries prevent corrosion and ensure the efficient and safe operation of their processes.

2. Sulphuric Acid Dew Point

The acid dew point is the stage at which gaseous sulfuric acid undergoes condensation, transitioning into a liquid state. This phenomenon can be expressed through the following formula:

Due to its high hygroscopic nature, sulfuric acid has a propensity to attract additional water, forming an aqueous solution:

3. Conversion of SO₂ to SO₃

The equilibrium curve for the reaction between sulphur dioxide (SO₂) and sulphur trioxide (SO₃) is typically represented graphically in the context of chemical thermodynamics. The reaction is as follows:

This reaction is an example of a reversible reaction, meaning it can proceed in both the forward and reverse directions. The equilibrium constant (K_eq) for this reaction is expressed as:

The equilibrium curve shows how the concentrations of SO₂ and SO₃ at equilibrium change with temperature and pressure.

The equilibrium curve for the SO₂-SO₃ reaction is influenced by Le Chatelier's principle, which states that if a system at equilibrium is subjected to a change in

temperature, pressure, or concentration of reactants/products, the system will adjust to counteract that change and restore a new equilibrium.

For the SO₂-SO₃ system:

Effect of Temperature:

The reaction is exothermic (releases heat). According to Le Chatelier's principle, an increase in temperature will shift the equilibrium position to the left, favoring the reverse reaction (SO₃ to SO₂).

Effect of Pressure:

Since the number of moles of gas does not change in the balanced equation, changes in pressure do not have a significant effect on the position of the equilibrium.

In summary, the equilibrium curve for the SO₂-SO₃ reaction would show how the concentrations of SO₂ and SO₃ at equilibrium change with temperature, and it would be influenced by the principles of chemical equilibrium. Graphically, you would observe shifts in the equilibrium position based on changes in temperature and pressure.

4. Conversion of SO₃ to H₂SO₄

The reaction between sulphur trioxide (SO₃) and water (H₂O) to form sulfuric acid (H₂SO₄) is an example of an acid-catalysed reaction. The balanced chemical equation for this reaction is:

This reaction is also reversible, and the equilibrium constant (K_eq) can be expressed as:

Now, let's discuss the equilibrium conditions and the factors that affect the equilibrium position:

Effect of Temperature:

This reaction is exothermic. According to Le Chatelier's principle, an increase in temperature will shift the equilibrium position to the left, favouring the reverse reaction (breakdown of H₂SO₄ into SO₃ and H₂O). On the other hand, a decrease in temperature will favour the forward reaction, leading to the formation of more H₂SO₄.

Effect of Pressure:

The reaction involves gases (SO₃) and a liquid (H₂O). Changes in pressure may not have a significant impact on the position of the equilibrium, especially if the reaction involves a liquid or a solid. However, high pressures may influence the solubility of gases in the liquid phase.

The equilibrium curve for the SO₃ to H₂SO₄ reaction would illustrate how the concentrations of SO₃, H₂O, and H₂SO₄ at equilibrium change with temperature. It would show how changes in temperature affect the equilibrium position, with higher temperatures favoring the reverse reaction and lower temperatures favoring the forward reaction. Graphically, the equilibrium curve might depict these shifts in equilibrium concentrations based on changes in temperature, as follows:

Knowing that the above favours H₂SO₄ at the lower temperatures and that more or less all SO₃ has been converted to H₂SO₄ below 200°C (400°F), technically the Acid Dew Point should be calculated using the amount of H₂SO₄, but as the conversion is 1mol to 1 mol, practically the amount of SO₃ can be used.

5. Formation of SO₂ to Condensation of H₂SO₄

Important is to understand where and how the SO₂, SO₃ and H₂SO₄ is formed and how the Acid Dew Point is influenced over the complete temperature range from fire box to stack.

This can be expressed in the following graph:

As the ADP is the temperature where the sulphuric acid starts condensing, it can be concluded from the above graph that when an heat exchanger condenses acid, the ADP at the outlet of the exchanger is lower than at the inlet. It is therefore beneficial to condense as much as possible when designing a condensing heat exchanger, because the flue gas leaving the exchanger will be less corrosive, which is beneficial for all downstream equipment.

6. Correlations to Calculate the Conversion of SO₂ to SO₃

Conversion of SO₂ to SO₃ can occur due to the following reactions:

• Chemical reactions in fuel beds or coal firing
• Reaction in the combustion chamber between SO2 and atomic oxygen
• Catalytic oxidation of SO2 by materials on the convection section surfaces (molecular oxygen)

The reaction in the combustion chamber is normally referred to as the first conversion. Where the reaction due to catalytic oxidation is referred to as the second conversion.

The whole process is shown in the following diagram:

To determine the amount of SO3 after the first conversion the equilibrium constant can be defined as follows:

The Kp constant can be obtained by the following equations:

Where T is the temperature of the gas in the firebox, where for fired heaters the temperature is lower compared to steam methane reformers. For Okker the temperature is set to 1000°C / 1273.15K

7. Correlations to Calculate the Acid Dew Point

Many correlations have been proposed over the years to calculate the sulphuric acid dew point. The following correlations are well known in the industry:

a) Ecoonex BV.

Where:
ADP = Acid Dew Point [°C]
ySO₃     = sulphur trioxide, volume fraction
yH₂O    = water vapour, volume fraction

P_gas        = total pressure of the flue gas, pascal abs

b) A.G. Okkes / Mobil Oil Company

c) R. Pierce

Where:

ADP      = Acid Dew Point [K]
pSO₃     = Partial pressure of sulphur trioxide , atm
pH₂O    = Partial pressure of water vapour, atm

d) Verhoff F.H. and Banchero J.T. method

Where:
ADP      = Acid Dew Point [K]
pSO₃     = Partial pressure of sulphur trioxide , mmHg
pH₂O    = Partial pressure of water vapour, mmHg

8. Example Calculation

O

O₂ = 1.7 Vol%

H₂O = 17 Vol%

SO₂ = 20 ppmvol

P_gas = -1000Pa g

T_firebox = 1000°C