I. Basic Concepts of Chemical Bubble Point and Dew Point

Bubble Point: The temperature at which a liquid mixture begins to boil and produce the first bubble under a certain pressure (bubble point temperature); or the pressure at which a liquid begins to boil at a certain temperature (bubble point pressure). The most volatile component in the liquid phase vaporizes first, forming a "vapor-rich phase".

Dew Point: The temperature at which a gas mixture begins to condense and form the first droplet under a certain pressure (dew point temperature); or the pressure at which a gas begins to condense at a certain temperature (dew point pressure). The least volatile component in the gas phase condenses first, forming a "liquid-rich phase".

Boiling Point: The temperature at which a pure component undergoes a phase change from liquid to vapor under a certain pressure. It only applies to pure substances and is a special case of bubble point and dew point.

Vapor-Liquid Equilibrium: The bubble point of the liquid phase is the dew point of the vapor phase. At this point, the composition of the gas and liquid phases reaches equilibrium. The smaller the difference between the bubble point and dew point, the closer the mixture is to a pure substance or azeotrope.

The bubble point and dew point are mainly affected by composition and pressure. When the pressure increases, both the bubble point and dew point rise, and the difference between them decreases.

II. The Special Nature of Sulfuric Acid Dew Point

The sulfuric acid dew point (also known as acid dew point), which is a key concern in sulfuric acid production, is significantly different from the common dew point. The sulfuric acid dew point is the temperature at which sulfuric acid vapor in the flue gas reaches saturation and begins to condense. It is the result of the combined effect of sulfuric acid vapor and water vapor. The content of SO3 is the most crucial factor; the higher the SO3 content, the higher the acid dew point. The more moisture there is, the stronger the condensation driving force, and the acid dew point also increases accordingly. Similarly, pressure has an impact on the sulfuric acid dew point, but the effect is relatively small. When the pressure rises, the acid dew point increases slightly.

A simple empirical calculation formula can be provided for the sulfuric acid dew point for use in daily production calculations:

Parameter explanations:

tADP: Acid dew point temperature of flue gas, unit: ℃

ϕH2O: Volume fraction of water vapor (%)

ϕSO3: Volume fraction of SO3 (%)

In sulfuric acid production processes, the sulfuric acid dew point significantly impacts three critical stages: drying, conversion, and absorption. Acid mist formation remains the "top challenge" in sulfuric acid manufacturing, closely correlated with dew point temperature. When gas temperatures drop below the sulfuric acid dew point, sulfur trioxide (SO3) combines with water vapor to form sulfuric acid vapor that condenses into acid mist (microscopic droplets measuring 1-10μm). These mist particles prove difficult for mist eliminators to capture, allowing them to enter subsequent equipment through gas streams. This leads to equipment corrosion and blockages, ultimately resulting in emissions through exhaust stacks that manifest as "white smoke" emissions.

Dew point corrosion is the most common form of corrosion in sulfuric acid plants, which can cause significant economic losses. The corrosion mechanism is as follows: SO3 and water vapor form a dilute sulfuric acid solution (with a concentration of 5-40%) on the metal surface, which is extremely corrosive. The resulting acid liquid penetrates into the metal interior, causing intergranular corrosion and stress corrosion cracking. The corrosion rate varies with temperature, and it is highest near the dew point (110-150℃). The equipment and locations prone to acid fog include:

Dryers, absorption towers, and their flue gas outlet pipelines; heat exchange equipment such as boilers, economizers, heat exchangers, and superheaters.

The control of sulfuric acid dew point is directly related to the product quality, environmental compliance and equipment safety of sulfuric acid production. Adopting reasonable control methods to control the generation of acid mist can effectively reduce the formation of acid mist. Here are a few suggestions to provide some references for industry colleagues:

1）Strictly control the moisture content of furnace gas to reduce dew point temperature at the source.

2）The gas temperature containing SO3 and moisture must be maintained 10-20°C above its sulfuric acid dew point temperature.

3）Select appropriate equipment materials, particularly for low-temperature areas, prioritizing sulfuric acid-resistant materials.

4） Regularly monitor gas composition and dew point temperature, and promptly adjust process parameters.

5） Periodically inspect equipment operation status, measure thickness, record variation patterns, and implement corrective measures.

In sulfuric acid production, understanding and controlling the dew point is one of the key technologies for achieving efficient, safe and environmentally friendly operation, which runs through the entire process from raw material processing to exhaust gas emission.