Air Pollution Control Innovations

Acid Gas Dewpoint

Posted by Andy Olds on Mon, Aug 31, 2009 @ 09:00 AM

acid gas scrubberAcid gases can be found in the exhaust of a large number of combustion processes.  As a gas, the acid compounds usually are not particularly corrosive and are relatively easy to remove.  However, when the temperature of the gas drops below the acid gas dewpoint, an acid mist can form.  The acid mist can turn into a fine aerosol or it can condense on a cold surface.  Acid mist poses a number of design problems, due to the small size of the mist particles and the corrosivity of the liquid form of the acid.

Aerosol Formation

Aerosol formation occurs when the bulk temperature of the gas drops below the acid dewpoint of the gas.  Much like the formation of fog, the acid gas condenses into tiny liquid droplets.  The size of these droplets can vary widely depending on the acid, the amount of condensation nuclei present in the gas, and degree of supersaturation.

The most common problem that occurs with acid aerosol formation is the inability to capture the aerosol.  Many acid gas exhaust treatment systems utilize a packed bed scrubber to remove the acid.  Packed bed scrubbers are extremely efficient at removing acid in the gas, but unfortunately are ineffective at removing acid aerosol.

The solution to removing acidic aerosol mist is to use a high efficiency entrainment separator or Venturi scrubber, which can effectively capture particles to 1-micron or 0.5-micron, respectively.  If the acidic aerosol mist is primarily sub-micron in nature, a wet electrostatic precipitator also provides a useful solution.

Wall Condensation

Wall condensation occurs when a cold surface is in contact with a hot gas.  If the wall is cooler than the acid dewpoint, acid can condense onto the surface of the wall.  There are several dangers with wall condensation.

First, the material selection for an acid is dependent on its form.  Many acids are not corrosive as a gas, but are very corrosive as acids.  Engineers selecting materials under the assumption that the acid remains a gas often choose materials that are not compatible with the acid in its liquid form.

Second, when condensed, the acid is much more concentrated than it is in the bulk medium.  Instead of selecting a material for a gas containing 10 ppm of SO3 gas, the engineer now has to worry about a nearly pure sulfuric acid droplet.

Finally, the acid can condense in non-ideal locations, leading to pooling and further corrosion concerns.

The solution to preventing the effects of wall condensation is to insulate walls to prevent cold surfaces and select materials for the concentrated, liquid form of the acid in locations where wall condensation is unavoidable.

Acid Dew Point

All gases have a dew point that is dependent on the temperature, pressure, and concentration of the acid in the gas.  This article provides acid gas dewpoint equations for a number of acids.  Below are the formulae for a few of the more common acids in exhaust gases.

Tdp = Dewpoint Temperature, K

Pw = Partial Pressure of water, mmHg

Pa = Partial Pressure of acid, mmHg

Hydrochloric acid (HCl)

1000/Tdp = 3.7368 - 0.1591 * ln (Pw) - 0.0326 ln (Pa) + 0.00269 * ln (Pa) * ln (Pw)

Sulfur Dioxide (SO2)

1000/Tdp = 3.9526 - 0.1863 * ln (Pw) + 0.000867 ln (Pa) - 0.000913 * ln (Pa) * ln (Pw)

Sulfuric Acid (H2SO4)

1000/Tdp = 2.276 - 0.0294 * ln (Pw) - 0.0858 ln (Pa) + 0.0062 * ln (Pa) * ln (Pw)

 

Click on the button below to down load an Envitech packed bed absorber cut sheet for acid gas removal.

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Packed bed absorber cut sheet

Photo Credit: tinyfroglet

Topics: Venturi scrubbers, Scrubbers, wet electrostatic precipitators, Acid Gas

Instrumentation

Posted by Andy Olds on Thu, Aug 27, 2009 @ 02:25 PM

air pollution control instrumentationOne of the most difficult value engineering challenges I encounter is the selection of instrumentation.  Instrumentation within air pollution control systems may be subject to high salinity, high halide concentrations, pH swings, extreme temperatures, extreme ambient conditions, intrinsically safe environments, and abrasive particulate, just to name a few.  Selecting the proper instrumentation for each environment is a chore and can dramatically effect the cost.  It is a topic that both end-users and integrators should attempt to resolve before a system is built.

Extreme temperatures

Most know of the obvious differences between outside environments as opposed to inside environments.  Rain, snow, and sun all batter instruments outside; equipment inside rarely see any of the three.  Extreme heat though is not the exclusive domain of outside environments - I am currently working on a project where inside temperatures can reach 140F.  For that project, I have moved all of the temperature sensitive transmitters inside an air conditioned control cabinet to meet the temperature requirements of the analyzer.  When dealing with extreme temperatures, it is best to find ways to reduce the effect.  For extreme cold environments, use insulation or heat tracing.  For the extreme heat, look at moving sensitive electrical equipment into a temperature controlled environment.  Remember, plan for the extremes!

Be a copycat

A lot of the projects that I see are retrofits - the addition of new air pollution control equipment on existing processes or equipment.  In these cases, it is usually best to select instrumentation that is already working well within the plant.  First, copying instrumentation removes most doubts of instrument failures.  Second, operators have a familiarity with the instrumentation, and are more apt to use the instrumentation properly.  Finally, copying instrumentation reduces the number of spares that a plant needs to keep available, thus reducing the net cost of the instrument.  Whether you are buying the equipment for a customer, or buying equipment with prepackaged instrumentation, always find out what instrumentation is already working! 

Preventive Maintenance

Even after selecting the right equipment, there is still the need to properly maintain the equipment.  Acid gas scrubbers almost always require a pH probe for dosing basic reagents; those pH probes require regular calibration and often have lifetimes of six months.  A probe off by 1.5 pH units can cause a drop in acid gas removal efficiency from 99% to 75% - a significant problem if your stack tester is visiting! Conductivity sensors and hardness analyzer and ion selective electrodes also require regular maintenance.

For instruments that do require calibration, it is important to use bypasses to allow for calibration during operation.  A bypass loop with an instrument hold function allows an operator to carefully calibrate the instrument while the system continues to operate in a safe fashion.

Recommendations

Above all else, work with the end-user to identify instrumentation requirements.  A customer running a 24/7 operation has dramatically different requirements for instrumentation than one that is on a single shift operation or batch process.  Operators should be thought of as well.  Are the instruments accessible?  Can operators view indicators?

Remember, selecting the right instrumentation minimizes downtime; downtime that may cost you ten times the price of the instrument.

Topics: Venturi scrubbers, Scrubbers, wet electrostatic precipitators

Venturi Scrubber Particle Size Distribution Test (PSD) Method

Posted by Andy Bartocci on Mon, Aug 24, 2009 @ 06:54 AM

venturi scrubber particlesEnvitech 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.

cascade impactorThere are several test methods for determining the particle size distribution, however the test method must determine the aerodynamic particle size to adequately predict 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.  Below is a references from the California Air Resource Board (CARB) that describe acceptable test methods for this purpose.

 

  • 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.

To read more about Envitech's Venturi scrubbers, download the free case study on a Venturi scrubber treating the exhaust of a dryer.

Download Case Study

photo credit: azredheadedbrat

Topics: particulate control, Venturi scrubbers, wet electrostatic precipitators

Venturi Scrubber: Adjustable Throats

Posted by Andy Olds on Fri, Jul 24, 2009 @ 09:05 AM

Venturi scrubbers are used for the removal of fine particulate.  Gas is accelerated at a high speed through a Venturi throat.  Water is injected perpendicular to the gas flow.  The large water drops injected into the gas stream collide with the fine particulate through a process called impaction. 

The efficiency of this process is dependent primarily on the size and velocity of the particulate.  Superfine, sub-micron particulate are able to follow a stream line around the water drops and are not collected.  Micron-size and larger particulate are not able to slip around the water drops fast enough due to inertial effects.  The exact "cut" of the Venturi depends on the velocity; smaller particles are captured at higher gas velocities.  Venturi scrubbers are excellent particulate control devices for particulate at or above a micron in size.

venturi_scrubber_video.jpg

The performance of a Venturi though is dependent on maintaining the gas velocity at design conditions.  The above video details a Venturi scrubber with an adjustable throat.  In most industrial applications, the gas flow rate varies and with a fixed opening, the velocity of the gas through the Venturi would vary as well, impacting performance.  An adjustable throat offers one method for ensuring the Venturi works over a wide range of operating conditions.

The adjustable throat is a damper blade controlled by a positioner.  The blade is positioned to maintain a constant pressure drop across the Venturi.  The pressure drop across the Venturi is directly related to the gas velocity.  Essentially, the damper blade maintains the gas velocity in the Venturi even at much lower gas flow rates.  As stated above, gas velocity is critical to particulate removal in a Venturi.  An adjustable throat ensures that the gas velocity remains constant, so that particulate removal is unaffected by operation in a "real" environment.

Please click on the icon below to view a video of a variable throat Venturi.

Free Video

Topics: particulate control, Venturi scrubbers, videos