SO2 Scrubber: Sources, Danger, Methods of Purification and Sulfur Dioxide Design

Author: Michael Klepik, Chief Executive Officer

Sulfur dioxide is a colorless gas, more than twice as heavy as air. It is toxic and highly soluble in water, forming a moderately strong acid when dissolved. SO2 is a relatively stable and thermally resistant compound, but when dissolved, it slowly oxidizes in the presence of atmospheric oxygen. It has a distinctive unpleasant odor, somewhat reminiscent of the smell of burnt matches.

Research indicates that anthropogenic SO2 emissions significantly exceed those from natural sources. Estimates suggest that human-caused emissions can be 3 to 10 times higher than natural emissions. These estimates vary depending on the region and time period, with a trend toward reduction in recent years. Nonetheless, they highlight the scale of the problem.

Today, there are various methods available for removing SO2. These methods can broadly be classified into two groups: dry and wet purification techniques, both widely adopted in industrial settings. Wet methods utilize spray tower sulfur dioxide scrubber systems, as well as packed bed or fluidized bed absorbers to ensure effective filtration. Dry purification employs various types of adsorbers. Let's delve deeper into air purification installations for a more comprehensive understanding.

Efficient removal of sulfur oxides from exhaust vapors is achieved using wet SO2 scrubbers. These systems effectively capture pollutants ranging in size from 0.1 microns and larger. For capturing finer particles and gaseous chemical impurities, additional components such as caustic soda are recommended.

Sulfur dioxide scrubber's design typically involves a housing where water mixed with additives is sprayed. Sulfur oxides react actively with the liquid phase, which is either in mist form or as droplets cascading down the walls of the housing. Different types of air purifiers vary in their methods of water atomization.

For example, in SO2 scrubber systems like TORNADO ST, TORNADO FB, and TORNADO-RP liquid is delivered to the column through spray towers arranged in one, two, or three rows. The thorough cleaning of flue gases from sulfur is accomplished thanks to the structural features of the spray towers.

For cleaning fumes containing low concentrations of pollutants such as dust, chemical impurities, and smoke, a straightforward hollow SO2 removal wet scrubber can be employed. These devices are advantageous due to their compact footprint and ease of maintenance.

In bubble-type devices, gas purification is facilitated by its passage through a plate column, where pollutants' particles settle on the surface. Foam-type sulfur dioxide scrubbers employ plates (either slotted or perforated) irrigated with liquid, resulting in foam formation on the plate surfaces that captures secondary impurities. These are then removed from the unit as sludge.

The Venturi-type SO2 removal scrubbers deserve special attention. They represent a successful compromise, offering high cleaning efficiency within relatively compact dimensions.

Dry cleaning is recommended for filtering air streams with minor pollutant contamination. This method is suitable when the exact quantity of pollutants is known and remains consistent throughout the process. Our dry adsorbers are ordered by agro-industrial enterprises, petrochemical and metallurgical plants, pharmaceutical manufacturers, and producers of paper products.

Now that you have an understanding of SO2 scrubber technology, we can proceed to a more detailed examination of the chemical processes occurring within the equipment.

As previously mentioned, inside wet scrubber units for SO2 removal, the incoming gas stream interacts with the liquid phase. The composition of the liquid is customized for each type of pollutant. Specifically, using specialized solutions instead of water significantly enhances the SO2 scrubber's efficiency. This substance's ability to actively interact with basic reagents such as NaOH, lime, or limestone is utilized in devices where these substances play a role in neutralizing harmful impurities.

Let's take a closer look at the chemical processes occurring inside the device across all the mentioned variants.

Upon contact with the solution, the mixture initially dissolves its unwanted component in water:
SO₂ + H₂O → SO₂ ∙ H₂O

Subsequently, the resulting acid is neutralized by an alkaline reagent.
For systems using caustic soda solutions, the reaction proceeds as follows:
SO₂ ∙ H₂O + NaOH → NaHSO3 + 2H₂O

Sodium hydroxide exhibits higher reactivity compared to other reagents, ensuring maximum efficiency in removing sulfur dioxide. The resulting salt products are water-soluble and resistant to precipitation within the equipment. However, caustic scrubbers for SO2 removal using this method are more expensive to operate due to the higher cost of NaOH and equipment corrosion caused by its high chemical activity.

In wet SO2 caustic scrubbers, another widely used sodium-based process involves absorbing sulfur dioxide into a solution of sodium sulfite in water, leading to the formation of sodium bisulfite, which precipitates out. Upon heating, these reactions reverse, converting sodium pyrosulfite into a concentrated stream of sulfur dioxide and sodium sulfite.
Na₂SO3 + SO₂ + H₂O → 2NaHSO3
2NaHSO₃(saturated) → Na₂S₂O5 + H₂O
Na₂S₂O5 → Na₂SO3 + SO₂

The sulfur dioxide can be used in further reactions, such as for the production of sulfuric acid, while the salt is recycled back into the process. This method employs regenerable sodium sulfite as an absorbent for unwanted gases, thereby reducing costs. The resulting product (sodium pyrosulfite) finds practical applications in other industries rather than being disposed of in landfills.

For systems employing limestone, the reaction equations proceed as follows:
SO₂2 ∙ H₂O + CaCO₃ → CaSO₃ + H₂O + CO₂;
2SO₂ ∙ H₂O + O₂ + 2CaCO₃ → 2CaSO₄ + 2H₂O + 2CO₂

In this SO2 scrubber process, gases are neutralized while suspended in a solution. Additionally, the salts formed have poor solubility in water, including CaCO₃ itself. It's important to note that the aqueous solution of sulfur dioxide undergoes partial oxidation in the air, resulting in the presence of both calcium sulfite and calcium sulfate in the products. While this stage may offer advantages, we will revisit this point shortly.

The sulfur dioxide scrubber reactions using lime appear as follows:
CaO2 + H₂O → Ca(OH)2;
SO₂ ∙ H₂O + Ca(OH)₂ → CaSO₃ + 2H₂O;
2CaSO₃ + O₂ → 2CaSO₄

You'll notice that the composition of the products is similar to the previous method.

SO2 wet scrubbers utilizing limestone or lime are often preferred due to their lower reagent costs and reduced corrosion potential compared to systems employing NaOH. Moreover, they exhibit superior efficiency in the compound capture relative to all existing methods. The table below outlines the most effective liquid-based methods currently employed.

In our examination of SO2 caustic scrubber chemistry, we noted that methods using limestone and lime produce a mixture of calcium salts due to partial oxidation processes. In some cases, this oxidation is intentionally driven further to produce CaSO₄ (gypsum) as the primary SO2 scrubber waste. This approach is advantageous because gypsum can be sold, enhancing profitability, and it is easier to separate from water compared to calcium sulfite, simplifying equipment maintenance. After discharge, the gypsum is sent for dewatering and can be used in the construction industry or in agriculture as a soil conditioner.

The slowest step in the oxidation process involves dissolving oxygen into the reaction mixture. To accelerate this reaction, an aerator diffuser is installed. Along with the alkaline reagent, additional air is introduced into the gas stream. This extra equipment effectively enhances oxygen dissolution in the scrubbing solution, greatly speeding up the oxidation process.

At low sulfur levels in the exhaust gases, the additional equipment has minimal impact on the setup's size. However, with high sulfur loads, significant modifications to the design are necessary to ensure adequate oxidation, thereby increasing the SO2 scrubber's cost. Excessive sulfur content may require oxidation to be conducted in dedicated specialized vessels.

It's important to note that enhanced reaction as described is not very common. Some modern packed bed air purifiers do not require this stage, as they already provide a high gas-liquid contact area within the system.

Another crucial aspect of the pollutant capture process is monitoring the wash solution during absorber operation. The reagents added to the water are inevitably consumed in the reactions discussed above, which can ultimately reduce the efficiency of the substance removal from the mixture over time. To ensure continuous operation of a sulfur dioxide caustic scrubber, the concentration of reagents in the wash solution must be maintained at specific levels.

The addition of lime or limestone to the wash solution is controlled by monitoring the solution's pH. Lime solutions are typically alkaline, with a control point pH of around 12, while limestone solutions are more neutral, around pH 8. A low pH indicates insufficient reagent to neutralize the acidic gas, making pH the critical factor for proper scrubber operation.

The formation of CaSO₄ precipitate increases as the pH decreases, meaning CaSO₄ is more prone to scale formation at lower pH levels. Therefore, maintaining a high pH level helps control the formation of solid scale.

The solubility of CaSO₃ significantly increases with decreasing pH, meaning that precipitate forms as the pH rises. If the pH is too high, "soft pluggage" occurs in the equipment, consisting of leafy masses of calcium sulfite. It is evident that maintaining equipment with soft pluggage is easier than dealing with hard scale. In many cases, soft pluggage can be dissolved simply by lowering the pH.

Both excessively high and excessively low pH levels will lead to significant deposition of solid products inside the scrubber. The optimal operating pH range can only be determined experimentally for each specific situation. Typically, the decision on how much reagent to add is based on a combination of parameters. These include the concentration of sulfurous gas in the fume stream being cleaned, its required residual content at the outlet of the cleaning module, and the specific equipment being used.

SO2 strikes back against humanity's efforts to combat it. Humans equip scrubbers with artificial intelligence designed to detect and neutralize the rogue gas. But SO2 recruits a group of saboteurs who engineer a leak in the main AI office, allowing the gas to seize control of the AI. Humanity teeters on the brink of a global catastrophe. Coming soon to theaters near you!