Packed Bed Scrubbers: Key Design Factors, Types and Configurations

Author: Anna Frank, Equipment Selection Expert

Packed bed scrubber media is utilized to remove pollutants from a gas stream by employing a liquid that interacts with the gas through layers of filling. They are applied to clean pollutants (e.g., acidic or alkaline contaminants, VOCs, dust).

The contaminated air stream is fed from the bottom (in countercurrent flow to the scrubbing solution) into the mass transfer section, passing through a packed bed.

The absorbent solution is introduced from the top, wetting the structured elements and forming a film on their surface. These components increase the contact area between the phases, ensuring efficient absorption of pollutants. As the air interacts with the solution, it gets purified and is released through the top of the column, while sludge is collected at the bottom.

Liquid Distribution
Uneven distribution of the absorbent solution can lead to "dry zones," where the cleaning process is ineffective. A well-designed liquid distribution system ensures uniform interaction with the air stream.
Gas Velocity
The flow rate must be controlled to avoid liquid droplet entrainment and excessive pressure losses, which would increase energy consumption. The velocity is typically selected to maximize phase interaction efficiency without causing mechanical issues.
Temperature and Pressure
Many absorption processes are temperature-dependent, making this a critical factor. For instance, gas solubility decreases at higher temperatures. Pressure affects the gas density and its interaction with the liquid. The optimal conditions are determined based on the physical and chemical properties of both the pollutants and the absorbent.

These parameters are essential for ensuring that the calculations for the packed bed scrubber design are both reliable and cost-effective.

1. Selection of Housing Material
The material of the packed bed wet scrubber design housing must be resistant to corrosion and chemical aggression. The choice is based on the chemical composition of the air stream, as well as the operating temperature and pressure. Materials like stainless steel, polymers, fiberglass, or specialized coatings may be used depending on these factors.

2. Sludge Removal System Design
A proper drainage configuration is essential to prevent sludge and contaminants from accumulating at the bottom of the tower. This reduces maintenance needs and keeps the equipment running efficiently.

3. Absorbent Selection
Choosing the correct absorbent is critical for the effective removal of pollutants. The choice depends on the type of contaminants, their concentration, and the specific chemical reactions that will occur during the scrubbing process.

4. Droplet Carryover Assessment
To prevent droplet carryover in the gas stream, it is important to configuration or install a mist eliminator. This device, located at the column outlet, captures droplets, preventing them from exiting with the cleaned air.

5. Control and Monitoring
Sensors and control systems should be incorporated to monitor key process parameters such as gas velocity, temperature, pressure, liquid levels, and efficiency. This ensures optimal operating conditions and allows for quick responses to deviations.

6. Energy Cost Evaluation
The design must account for energy consumption, particularly for the pumping of liquids and air through the equipment. Minimizing pressure drops and optimizing the performance of pumps, fans, and other equipment will help reduce energy costs.

7. Maintenance Requirements
Consider the ease of maintenance and replacement of key components. This helps minimize downtime and operational costs.

8. Emission Assessment and Regulatory Compliance
The equipment should be designed to meet environmental standards and emission regulations for pollutants released into the atmosphere. Process modeling and efficiency evaluations should be conducted to ensure compliance with these standards.

A packed bed scrubber calculation should be performed by a specialist, taking into account the input data and environmental requirements of the area. We are ready to perform the necessary calculations, so feel free to get in touch with us!

Packing refers to elements of various shapes and sizes with a developed surface dimension. They are categorized into random and structured types based on how they are arranged inside the equipment.

Random Bed Material is poured loosely into the mass transfer section without any specific orientation. The elements are distributed in a disorganized manner, creating irregular paths for the gas and liquid to travel through. It can be further divided into stationary (fixed) and floating types.

• Fixed Elements come in a variety of shapes, including ring-shaped, cylindrical, saddle-shaped, spherical, and others. The improvements in their configuration are typically achieved by increasing the number of bent units or adjusting the height-to-width ratio.

A fixed bed packed scrubber is suitable for a variety of applications, such as treating gaseous emissions from chemical processes, waste incineration, and more.

We have accumulated vast experience in the manufacturing and supply of fixed bed packed tower scrubbers. Here are some examples of our equipment:

• Floating or Fluidized Elements are available in shapes like spherical, cylindrical, spiral, ring-like, prismatic, or cubic. These components can be hollow, solid, or perforated, depending on the application and layout requirements.

Structured filling is arranged and oriented in such a way that a system of winding channels is formed, allowing phases to stream in a more controlled and efficient manner. These items are stacked in an ordered pattern to optimize phase interaction.

They can be classified into block and individually arranged types:

• Individually arranged regular components consist of items such as rings, prisms, angles, I-beams, and spheres that are positioned within the mass transfer section in a specific manner relative to each other. The components in adjacent vertical layers are arranged to avoid the formation of continuous channels. For ease of installation, individual items can be pre-assembled into containers that are then placed in the column housing. However, it has not been widely adopted in the industry due to the significantly increased labor intensity and installation costs.
• Block components refer to the type of structured media that is most commonly used in practice. It is installed in the column as blocks, which is how it gets its name. A block typically consists of a collection of perforated, corrugated, ribbed, or mesh sheets, each with various structural features and specific spatial orientations relative to one another.

Block-type has gained significant popularity in recent years due to its superior technical and economic performance compared to other types. They can be made from various materials, including metal, plastic, ceramics, porcelain, glass, graphite, and asbestos fibers.

A foam layer in a foam scrubber is formed through aeration, where gas is introduced into the liquid, creating bubbles. Mechanical agitation, the use of filling to increase the contact area, and the addition of foaming agents also contribute to the formation and stability of the foam.

Each type of packed bed scrubber packing materials is selected based on the specific requirements of the scrubbing process, with factors such as contact surface area, resistance to flow, and durability considered in the configuration.

The main geometric characteristics are specific surface dimension, free volume, and bulk density. Also important are:

• Low hydraulic resistance to the air flow.
• Good wettability (the ratio of the wetted surface area of the filling to its total surface area).
• Uniform stream distribution across the cross-section of the column.
• High frequency of surface renewal for the interaction between phases.
• Minimal droplet carryover with the gas.
• Sufficient mechanical strength and other considerations.

Sometimes, devices are enhanced with an additional purification stage — a fiber cassette. This unit is located after the filling layer.
Its main functions include:

• Droplet Capture: Acts as a mist eliminator, trapping liquid droplets that may be carried away with the cleaned stream.
• Increased Contact Surface: Provides additional surface space for contact, improving mass transfer efficiency.
• Filtration: Captures solid particles, aiding in the removal of fine particulate contaminants.

An example of such equipment is our BOREAS fiber bed scrubber. In addition to the cassette, it is equipped with a labyrinth mist eliminator and a high-efficiency spray nozzle system. The BOREAS configuration is horizontal, making it convenient for placement in small spaces and eliminating the need for special platforms for maintenance.

The parallel bed scrubber is characterized by the distribution of the contaminated stream across multiple parallel layers (reaction beds) of sorbent or absorbent. This arrangement increases the contact space, enhancing the efficiency of pollutant absorption while reducing pressure losses within the equipment. Additionally, a horizontal packed bed scrubber tends to have more compact dimensions, and its maintenance does not require specialized ladders or platforms, making it a more cost-effective option.

In contrast, the vertical packed bed scrubber features a vertical configuration where air flows from the bottom to the top through the elements, while the liquid descends from the top to the bottom. This configuration ensures effective interaction, compactness of the system, and high purification efficiency due to countercurrent flow and an increased contact area.