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Author:
Anna Frank, Equipment Selection Expert
Anna Frank, Equipment Selection Expert
A scrubber is considered one of the primary devices for reducing the concentration of harmful and hazardous substances in industrial exhaust gases. It allows the maximum permissible concentration of contaminants in the workflow to be brought within legally established standards.
In wet scrubber using the dynamic contact effect, the cleaning process occurs using nozzles that spray liquid under pressure into the internal volume of the housing.
In wet scrubber using the dynamic contact effect, the cleaning process occurs using nozzles that spray liquid under pressure into the internal volume of the housing.
Main Elements
The wet scrubber design consists of the following components: an inlet tube, a frame, a chamber where the inlet flow is cooled, a working chamber, a spray system, a separator, an outlet tube, and additional components (hydro-tanks, maintenance elements, chimneys, control units, etc.).
Nozzle system is located at the top of the housing. The supplied liquid completely covers the entire cross-section of the device, ensuring maximum contact with the contaminants. The dust-laden flow enters the bottom of the device and is cleaned by the liquid mist rising upward. The sludge, in the form of a pulp, flows into a special container and is then removed for disposal.
Nozzle system is located at the top of the housing. The supplied liquid completely covers the entire cross-section of the device, ensuring maximum contact with the contaminants. The dust-laden flow enters the bottom of the device and is cleaned by the liquid mist rising upward. The sludge, in the form of a pulp, flows into a special container and is then removed for disposal.
Device Schematic
Typical Design Configurations
The main types include:
- packed bed – with the body filled with "packing";
- tray tower – with horizontal plates;
- Venturi – with free flow of liquid and gas;
- spray tower – a hollow scrubber in which gas passes through spray nozzles.
Tray Tower (plate column) consist of a vertical column with several horizontal plates. Gas passes through the holes or valves of the plates, bubbling through the liquid and creating intense foam and bubbling. This effect ensures multiple phase contacts, increasing the efficiency of particle removal.
Venturi tube is a simple wet scrubber design and effective at removing coarse and medium-sized dust. The scrubber's operating principle is based on the aerodynamic force of a Venturi tube. It is an hourglass-shaped tube, composed of a diffuser, a throat, a converging section, and nozzles that supply the spraying fluid. Inside the hollow housing, water is sprayed from nozzles to create a highly dense liquid curtain in the path of the air masses. Air is supplied from below, and spraying occurs counter-currently or sideways, with the droplets subsequently falling freely against the airflow. Brief contact with water droplets is sufficient to capture medium and large particles.
The spray tower is suitable for processing all types of exhaust gases. The tower's simple design and ease of operation make it widely used for industrial gas purification. Gas enters the tower through the lower air inlet, fills the air inlet segment, and flows through the uniform flow section to the first-stage absorption section. On the filler surface, contaminants in the gas phase react with the liquid. The resulting reaction flows to the bottom of the tower with the absorption liquid. Incompletely absorbed gas continues to rise into the spray section. The absorption liquid is atomized at high velocity through a nozzle in the ejector segments, forming numerous fine droplets that continue to interact with the gas.
Step-By-Step Design Procedure
The wet scrubber system design requires careful analysis of numerous parameters, including gas flow characteristics, pollutant properties, cleaning efficiency requirements, and economic constraints. Modern design approaches are based on computer modeling and extensive practical experience. Key design parameters include: liquid-to-gas ratio, contact time, gas velocity and temperature, and droplet size.
How to build a wet scrubber? Development involves several stages.
In the first stage, the column diameter, taking into account the pipe's hydraulic resistance, as well as the throat dimensions, are determined using initial data.
How to build a wet scrubber? Development involves several stages.
In the first stage, the column diameter, taking into account the pipe's hydraulic resistance, as well as the throat dimensions, are determined using initial data.
Essential Data for Selecting a Unit
The second stage of calculations involves determining the minimum liquid flow rate, the diameter, type, and size of the packing, and the number of nozzles. This information allows us to calculate the hydraulic resistance and select an appropriate mist eliminator. Since many constructions don't have universally applicable characteristics, their design and production should be orientated on subsequent operational measurements.
Next, a thermal calculation is performed. If the scrubber will be cleaning highly heated gas, the calculations should determine the amount of heat diverted from the gas-air mixture. Based on these parameters, the optimal amount of water supplied is calculated. Standards for the amount of water supplied to the unit are taken into account. The calculations provide information about such wet scrubber design parameters as water flow rate, dimensions, hydraulic resistance, and purification quality.
Next, a thermal calculation is performed. If the scrubber will be cleaning highly heated gas, the calculations should determine the amount of heat diverted from the gas-air mixture. Based on these parameters, the optimal amount of water supplied is calculated. Standards for the amount of water supplied to the unit are taken into account. The calculations provide information about such wet scrubber design parameters as water flow rate, dimensions, hydraulic resistance, and purification quality.
Design Equations
The quality of gas purification is determined by the contact area between pollutant particles and water droplets. The diameter of the captured particles determined by the size of the drop. The smaller the drop, the less dust it can wet for subsequent removal from the gas. Dust concentration in the gas-air mixture is taken into account when calculating the unit dimensions and the number of shelves.
The wet scrubber design calculation involves several steps.
First, a heat balance wet scrubber design equations are developed. It requires knowing the parameters: the initial and final water temperatures, the dry gas consumption, the moisture content of the dry air, and the enthalpy of the gas at inlet.
The final equation for the change in heat energy is:
where G2 is the cooling water flow rate;
I2 is the enthalpy of the air at the scrubber outlet, measured in kJ/kg of dry air.
When calculating low-temperature gas-liquid scrubbers, Qp (heat loss to the environment) can be neglected.
The increase or decrease in water during heat and mass transfer is calculated using the formula:
Heat transfer is calculated using the formulas:
where Va is the active area of the apparatus;
∆t is the temperature difference;
F is the droplet surface area in the scrubber volume;
φ is the imperfection coefficient of the heat and mass transfer process;
α is the droplet surface heat transfer coefficient;
ku is the scrubber volume heat transfer coefficient.
The formula for calculating the droplet diameter in wet air scrubber design is as follows:
where ψ is a special coefficient that depends on the liquid used;
σg is the surface tension;
ωc2 is the liquid exit velocity from the nozzle;
ρg is the gas density.
The wet scrubber design calculation involves several steps.
First, a heat balance wet scrubber design equations are developed. It requires knowing the parameters: the initial and final water temperatures, the dry gas consumption, the moisture content of the dry air, and the enthalpy of the gas at inlet.
The final equation for the change in heat energy is:
where G2 is the cooling water flow rate;
I2 is the enthalpy of the air at the scrubber outlet, measured in kJ/kg of dry air.
When calculating low-temperature gas-liquid scrubbers, Qp (heat loss to the environment) can be neglected.
The increase or decrease in water during heat and mass transfer is calculated using the formula:
Heat transfer is calculated using the formulas:
where Va is the active area of the apparatus;
∆t is the temperature difference;
F is the droplet surface area in the scrubber volume;
φ is the imperfection coefficient of the heat and mass transfer process;
α is the droplet surface heat transfer coefficient;
ku is the scrubber volume heat transfer coefficient.
The formula for calculating the droplet diameter in wet air scrubber design is as follows:
where ψ is a special coefficient that depends on the liquid used;
σg is the surface tension;
ωc2 is the liquid exit velocity from the nozzle;
ρg is the gas density.
Factors Affecting Scrubber Size
The wet scrubber size is determined by the required capacity, which is determined during the wet scrubber design procedure, and the gas flow rate, which determine the required cross-section of the apparatus.
If the capacity of the gases being cleaned must vary during operation, this range must be taken into account when selecting a wet scrubber sizing.
If the capacity of the gases being cleaned must vary during operation, this range must be taken into account when selecting a wet scrubber sizing.
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We always perform precise calculations and offer expert assistance in selecting the optimal dust collection or gas cleaning systems, typically completing this process within 1 to 2 days
Head of Engineering,
Vladimir Nikulin
Vladimir Nikulin
CALCULATION AND SELECTION
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