Gas Cleaning System - Venturi Scrubbers
June 22, 2003
Introduction
Venturi scrubbers are the most efficient fine particle collection devices to be
classed as wet scrubbers. They rely on the high kinetic energy generated
by accelerating the gas through a restriction to give good inertial collection
onto droplets distributed in the gas stream. This acceleration is achieved
at the expense of the gas side pressure drop and power consumption.
Classically a venturi is used for this purpose since this purpose since this
gives the maximum gas velocity for a given pressure drop and hence the maximum
theoretical collection efficiency.
Liquid is introduced at either the inlet of the venturi or at the venturi
throat. In both cases, the liquid velocity is low in comparison to the gas
velocity. The high relative velocity between gas and liquid ensures
good particle collection efficiencies even down to the submicron range. As
the droplets accelerate and attain the velocity of the gas, the collection
efficiency decreases due to the reduction inertia impactions between the droplet
and the particles to be collected. As the gas exits the venturi throat it
decelerates but the high inertia liquid droplets maintain their velocity.
Further inertia collection then occurs beyond the venturi throat.
The venturi is generally followed by an entrainment separator which removes the
dust laden droplets from the gas stream.
The collection mechanism in a venturi is predominantly inertia impaction.
For submicron particles, Brownian motion and possible electrostatic forces
contribute to the overall collection mechanism.
The simplest venturi is the fixed throat venturi. For this type of venturi
the gas cleaning efficiency will remain constant as long as the gas flow is
constant. When the gas flow varies and a constant gas cleaning efficiency
is desired, a variable throat venturi should be considered.
In general, the pressure drop across a venturi consists of two parts: a dry or
frictional loss and a wet portion. The dry portion are those losses that
occur when no liquid is present. The wet pressure losses are associated
with the formation of droplets and their acceleration. Five separate
components can be identified describing the total pressure drop across a
venturi:
- frictional pressure drop of the gas
- acceleration pressure drop of the gas
- acceleration pressure drop of the droplets
- acceleration pressure drop of the liquid film
flowing on the walls
- static pressure drop due to the difference in
height between the scrubber inlet and outlet.
Frictional Pressure Drop of the Gas
This is the loss due to the shear stress acting on the gas at the wall.
By analog with pipe flow it is proportional to the surface roughness and the
square of the gas velocity.
Acceleration Pressure Drop of the Gas
This results from the change in kinetic energy of the gas as it is accelerated
in the converging section of the venturi. The losses occur principally
in the diverging section as a result of flow separation. This loss can
be minimized by using shallow diffuser angles. Traditionally an angle of
less than 15º has been used in venturi designs.
Acceleration Pressure Drop of the Droplets
This pressure drop is a function of the venturi geometry, the means of liquid
introduction, throat velocity and liquid to gas ratio. The relative
velocity between the gas and the liquid causes a drag force which shatters and
accelerates the liquid droplets. If the liquid droplets are small and
the throat is long, the droplets may obtain the same velocity as the gas.
When the gas exits the throat it begins to decelerate. At some point the
gas velocity drops below the liquid droplet velocity resulting is a drag force
but this time in the opposite direction. The liquid droplets will
transfer energy back to the gas resulting in some pressure recovery.
Acceleration Pressure Drop of the Film
Some acceleration occurs in the converging section of the venturi but the bulk
of the pressure loss occurs at the start of the throat. At the point
where the liquid film is atomized into droplets the film component of the
pressure drop decreases and the pressure loss is taken up by the acceleration
of the droplets.
Gravitational Pressure Drop
This is the pressure rise resulting from the change in elevation of the gas
and liquid.
The pressure drop may vary within
a wide range depending on the application and cleaning efficiency required.
Venturis can be classified into three (3) groups depending on the pressure drop
across the unit. Low pressure drop venturis have a pressure drop less than
10" WC (254 mm WC). Medium pressure drop venturis have pressures
drops between 10" WC (254 mm WC) and 20" WC (508 mm WC). Venturis having
pressure drops higher than 20" WC (508 mm WC) are classified as high pressure
drop venturis.
Variable Throats
On certain applications the gas flow through the venturi can vary considerably.
When this occurs the cleaning efficiency will vary with the pressure drop
through the unit. To maintain a constant cleaning efficiency under varying
gas flow, a variable throat venturi is used.
Venturis with a rectangular cross section usually have a hinged wall or walls
which are adjusted to change the throat area. A series of gears and/or
levers and an actuator located outside of venturi moves and controls the
position of the hinged wall.
When the venturi has a circular cross section, adjustment of the throat area can
be achieved by having a fixed throat with an annular plug or disc located in the
middle which can be adjusted up and down to vary the size of the throat.
A slight variation of the
traditional venturi is the Radial Flow Scrubber (RFS) design offered by Lurgi.
The gas enters the throat area through a converging section similar to a
traditional venturi. A movable disc blocks the gas flow and forces the gas
to flow radially outwards to the inside wall of vessel. The operating
principle of the RFS is similar to a venturi where the annular space is the
equivalent to the venturi throat.
Corrosion and Wear in Venturis
The circulating liquid in a venturi will be a weak sulphuric acid solution
containing contaminants removed from the gas. These contaminants may
include heavy metals, dust, chlorides, fluorides, mercury compounds, etc.
The materials selected for the venturi must be resistant to these contaminants
from a corrosion and erosion point of view.
The parts of a venturi that are particularly vulnerable to corrosion and erosion
are the venturi inlet, throat, parts downstream of the throat and the outlet
elbow.
Venturi Inlet
Corrosion/erosion upstream of the throat can be prevented by injecting liquid
at the inlet to provide a liquid film on the surface. This can be
effectively done by injecting the liquid tangentially at the top of the
scrubber. The liquid feed nozzles may be shielded to prevent direct
contact with the gas.
Venturi Throat
The high gas velocity does not permit a stable liquid film to be formed on the
vessel surface so there is little protection for corrosion and erosion.
FRP has very good abrasion resistance and in all but extreme cases a liner
made from material resistant to corrosion and erosion can be installed in the
throat area.
Downstream of Throat
As with the throat itself, a specialty liner can be installed in these areas
to prevent corrosion and erosion of the vessel interior in the most extreme
cases.
Venturi Outlet Elbow
The outlet elbow is prone to erosion due to the high velocity gas and liquid
impinging on the outside radius of the elbow. A specialty liner can be
used in this area to minimize erosion in the most extreme cases.
Alternatively, the elbow can be constructed in a 'tee' arrangement. The
bottom of the 'tee' would contain a pocket of liquid which will protect the
bottom of the vessel.
Lurgi
Radial Flow Scrubber
Internal view of Radial Flow
Scrubber |
Typical Radial Flow Scrubber
Installation |
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