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 VENTURI SCRUBBING FOR AIR POLLUTION CONTROL

Particulate Scrubbers are used to transfer solids, liquid droplets/mists from a carrier gas into a receiving liquid.  This transfer requires both the humidification of the gas by the host liquid and creation of target droplets into which the transference occurs.  This process requires energy to create the liquid droplets that will humidify and form the targets.

 A number of mechanisms are commercially available to produce these droplets.  Years of research has concluded that the finer size of the material to be collected, the finer will be the droplet needed to capture it.  Two means of droplet formation are used in most industrial applications.  The first involves liquid atomization by passing the receiving liquid into a number of types of nozzles where it is either pressured or collided with a pressured gas.  The second involves the use of the carrier gas itself to collide into a preformed curtain of the receiving liquid.  This collision will atomize the liquid and no pressurization or mixing gas is needed.

 The staff of D.R. Technology, with over thirty five years of hands on particulate collection experience, has concluded that the preformed atomization is the most cost effective method since it requires no compressors and maintenance intensive spray nozzles. The most effective design called a “Venturi Scrubber” utilizes a downflowing gas to achieve the needed atomization. If properly designed, this shape aerodynamically speeds a gas to a high velocity, maintains this velocity for a very short time (in a section called the “Throat”) and then slowly drops the gas speed down. This function must be done smoothly slowly and evenly.

Until a gas is completely humidified by the receiving liquid via evaporation very little material transference will take place. Also, the least amount of fresh receiving liquid required to do a collection is the best design concept. This means that if high volume to volume ratios of liquid to gas is needed, it may be necessary to recycle a portion of the liquid so it is used more than once, and only a small proportion need be added and removed. This results in collected material being carried along with the recycle liquid and exposed to the raw gas. As this liquid evaporates to humidify the gas, it concentrates the material already in it.

Design research has shown that if there is a transition between where the gas is dry and where it is humidified, deposition of solids occurs at this point. If this is a carrier vessel or duct, large solids buildup can occur at this wet/dry interface with potential clogging or damage even before material collection occurs. A gas downflow design can avoid this wet/dry line by indenting the gas into the contactor and introducing the carrier liquid above it in such a way that it completely covers the vessel walls and no unwetted dry spots occur.

The Venturi Scrubber design uses the upper (called “Converging”) section to both speed the gas and humidify it.  Recycled carrier liquid is introduced above the gas entry through open tangential pipes so that it spins down the convergent walls covering them completely.  This design results in rapidly humidified gas when the high velocity zone is reached.  This coverage can only take place in downflow configurations in order to create completely covered walls.

Once the liquid reaches the narrow “Throat” the shape of the converging section is such that the liquid flow turns horizontal before it drops further.  Designed properly, the liquid can jump horizontally 100 mm before falling.  This horizontal motion creates a water curtain in a properly shaped Throat that the gas must pass through before continuing downward.  The gas velocity can be accelerated to velocities as high as 140 Meters/Second.  The resulting collision will create liquid droplets as small as 5 micron (1 micron=1 millionth of a meter).

The gas/and formed droplets travel into the lower part of the Venturi called the “Diverging” or “Recovery” section.  It is here that the humidified particulate collides with the droplets, breaking their surface tension and being encapsulated by the larger droplet.  Typically, a collision velocity of 100 M/s can generate 10 micron droplets which can capture 99+% of particles 1 micron and larger and 90% of particles 0.5 to 1 micron.

Once the particles are transferred into the liquid droplets and grown, simpler inertial methods can be used to separate the resulting larger droplets from the gas and complete the transference.

What happens if the gas flow is greater than that value?   Do parallel wet venturis make sense?   Can the 100 mm horizontal shape be maintained while making the throat rectangular?   

The problem with parallel wet venturi scrubbers is expense, since numerous venturi’s would require split ducting, piping, etc.  The problem with a rectangular throat is that it will result in water curtain gaps at each end, and the downflowing gas will seek the path of least resistance so that a portion will pass through these dry ends unscrubbed. 

Fortunately there is a solution for this problem through use of a 200mm wide annulus. That is, the venturi will remain circular, but a tapered plug will be installed in the center to form the narrow gap. This design can maintain the gap with external diameters as big as 10M.  

Liquid is also introduced over the top of the plug to maintain the Wet Approach. Although a number of Venturi’s are commercially available, this type is the most aerodynamic and Maximizes particle collection at a given collision velocity at a fixed liquid to gas ratio. The tapered plug design can also be undermounted with a sealed support structure so it can be actuated from below to maintain a collision velocity when gas flow changes.

 D.R. Technology's years of experience in Venturi scrubbing of high pressure biogas and other synthesis gas has resulted in our having extensive performance data on scrubbers under these unusual conditions.  D.R. Technology also possesses proprietary performance data on prescrubber steam injection and upstream cooling to improve particulate collection. 

D.R. Technology venturi scrubbers capture particles down to submicron size while reducing opacity in applications as diverse as municipal incinerators, product dryers, coal and biomass fired boilers, crushers, grinders, reactors, metal furnaces and kilns and fluegas desulfurization.

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