Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
Gas Cleaning System - Venturi Scrubbers
June 22, 2003
Corrosion and Wear in Venturis
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
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.
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.
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.
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.
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.
Internal view of Radial Flow Scrubber
Typical Radial Flow Scrubber Installation