Air Pollution Control Innovations

Venturi Scrubber Particle Size Distribution Test (PSD) Method

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Envitech is often asked to make recommendations for particulate removal on a wide range of industrial applications.  This might entail deciding if a Venturi scrubber can do the job or if a wet electrostatic precipitator (WESP) is required.  Envitech uses proprietary modeling that accurately predicts Venturi performance under various conditions of pressure drop and sub-cooling.  Because Venturi performance is highly dependent on particulate size for particles less than 1 micron diameter, an essential piece of data for making a performance guarantee is the particle size distribution (PSD) for the particles in the inlet gas.

There are several test methods for determining the particle size distribution, however the test method must determine the aerodynamic particle size to adequately precdict the Venturi performance.  The most common method for determining the aerodynamic particle size is to use a cascade impactor.  This can be a challenging test because the inlet gas stream may be at high temperature and contain a high particulate loading and moisture content.  It is important to select a testing company that has experience with this type of environment.  A company that has this experience is Air Source Technologies located in Kansas City, KS.  Envitech has worked with Air Source to test biomass gasification syngas and lime kiln flue gas.  Below are some references from the California Air Resource Board that describe acceptable test methods for this purpose.

• California Air Resource Board Note 95-6 - Particle size Test Method for Sampling High Temperature and High-Moisture Sources
• CARB Method 501 - Determination of Size of Distribution of Particulate Matter Emissions from Stationary Sources

A method that some companies use to to determine the PSD is to collect particles on a filter and analyze the dust with a particle size analyzer.  This test method will give the physical particle size.  However it can give an inaccurate estimate of the aerodynamic particle size.

The problems commonly associated with this method are outlined below:

• You may not get a representative sample. Different size particles end up in different layers on the filter. Agglomerates of particles can come apart. Individual particles can agglomerate and show up as one particle.
• This is a measure of the physical size and not the aerodynamic size. Venturi scrubbers primarily collect particles according to their aerodynamic size through inertial mechanisms. The aerodynamic size is a function of the density of the particle and the size of the particle. The greater the density, the greater the aerodynamic size. In some cases, particles may not be solid particles of inorganic material, but may be hollow glassy spheres, which will make them act more like soap bubbles rather than hard balls. Some combustion processes at extremely high temperatures can lead to hollow spheres.
• The size of the particle affects the aerodynamic size because, the smaller the particle, the easier it is to slip between gas molecules. As the particle size approaches the mean free path of the gas molecules this becomes a significant component of the aerodynamic size.
• To use data from a particle size analyzer, it is necessary to generate a log normal PSD curve of the physical size. This curve then needs to be corrected to provide an aerodynamic PSD curve. However, this requires making two assumptions: 1) particle density, and 2) particle shape (usually assumed to be a solid spherical shape). These assumptions may or not be valid and can lead to inaccurate Venturi performance predictions.

Before settling on a design path for particulate removal, Envitech recommends testing the process to generate good particle size distribution data.  This will help ensure meeting the performance requirements in the most economical way possible.

photo credit: azredheadedbrat