Venturi scrubber removes pollutants by using high energy to atomize liquid into fine droplets, which trap pollutants effectively. Venturi scrubber is suitable for submicron particles since it create turbulence and intense contact between gas and liquid.
A venturi is designed with sudden contraction and expansion to increase the difference in fluid velocity. The differential velocity and turbulence created in the venturi throat creates thorough mixing of scrubbing liquid with pollutants at the throat and causes pollutants diffusion into the liquid.
Parts in Venturi Scrubber
- Liquid injection port
- Convergence section
- Venturi throat
- Divergence section
- Collection chamber and mist eliminator
- Liquid storage tank
- Instrumentation and control
Liquid injection port
The scrubbing liquid is injected directly at upstream before throat section with perpendicular direction to the exhaust gas stream. Spray nozzle will be used for injection to create fine droplet of liquid. These droplets are produced by specially designed spray nozzle to achieve maximum efficiency.
The gas is accelerated in a converging section and atomize the scrubbing liquid. This convergence section helps to achieve the throat velocity at venturi throat section.
It is a narrowed portion of venturi scrubber at which the velocity and turbulence will be very high. The exhaust gas breaks/shears the scrubbing liquid into enormous number of fine droplets. The formed droplets have capable of collecting particulates through inertial impaction. The throat section can be circular or rectangular depends on duct shape.
The exhaust gas with scrubbing liquid is passed through diverging section at which the velocity decreases suddenly. This sudden decrease in velocity causes more impaction and agglomeration.
Collection chamber and mist eliminator
A collection chamber is used to separate particulates entrained liquid from the gas. The collection chamber is simple round shaped tower with mist eliminator on the top of it. The exhaust gas after the venturi column is passed into the chamber tangentially. Part of the liquid drops into the bottom by gravity settling. The droplet remain in the gas will be removed by mist eliminator.
Liquid storage tank
A tank should be sized to provide continuous operation and minimize the frequency of change of scrubbing liquid. The liquid is recirculated through pipe using water pump. Due to the continuous operation, solid builds up and increases solid content in the scrubbing liquid as particulates is collected.
The solid concentration should be maintained below the limit up to which the spray efficiency does not get affected. In order to reduce the solid concentration, part of the liquid is blow down from the tank and fresh liquid is added. Also, the fresh liquid will be added to compensate water loss due to evaporation.
Instrumentation and control
Automated system can be incorporated into the system to avoid human intervention frequently to manage water level in the tank. This will be easy for operation and maintenance.
Liquid to gas (L/G) ratio
It is a volume of scrubbing liquid injected at venturi per volume of exhaust gas passed. Higher the L/G ration gives higher collection efficiency, also increases the pressure drop significantly. The optimum range of L/G ratio is between 7 and 10 gallong / 1000 cubic feet. Greater the (L/G) ratio, > 10 gallon/Cubic feet does not increase the efficiency but increase the pressure drop significantly.
Velocity and pressure drop
Relative velocity between liquid and gas increase the momentum and turbulence, thus causes fine particles to collect by impaction. We can increase the relative velocity by adjusting/reducing throat area. However, higher velocity causes higher pressure drops, thus lead to high energy consumption thereby increasing operating cost. Higher velocity occur at the centre point of venturi’s narrowest point.
The optimum velocity range: 150 to 500 fpm
Generally, increasing the pressure drop above 45 inch water column will not affect/increase the scrubbing efficiency significantly.
Particle size distribution