Particulate matter is an ongoing environmental concern. Known to contribute to health problems, from asthma to chronic obstructive pulmonary disease (COPD), many either implemented or plan on implementing more stringent controls to reduce public risk. Since 1987, the U.S. Environmental Protection Agency (EPA) has regulated PM10 (particulates greater than 10 microns in size), also known as coarse particulates. This was followed by additional controls on fine particles, known as PM2.5 (particulates greater than 2.5 microns in size). These PM2.5 regulations were first adopted in 1997 and have been revised twice since their inception. As of December 2012, the limit for PM2.5 emissions is 12 milligrams per cubic meter.
Currently, there exist two main ways to remove particulates: dry scrubbing and wet scrubbing. This article focuses on wet methods and compares and contrasts the benefits and limitations of each approach. Three main types of wet scrubbers available commercially are: Jet Venturis, high-energy Jet Venturis and wet electrostatic precipitators (WESPs).
The Jet Venturi has, for years, been used to scrub fumes such as acid gasses and odorous compounds like H2S and mercaptans. The operating principle of a Jet Venturi is relatively straightforward. A low-pressure gas is drawn into the jet by use of a high-pressure liquid. The Venturi effect and use of a converging/diverging throat section convert the velocity energy to pressure and impart a small increase in pressure (draft) across the unit as measured from the gas inlet to the two-phase outlet.
Because of the intimate contact between the two fluids and the high-velocity encounter in the throat section, particulates in excess of 10 microns can easily migrate from the gas phase to the liquid phase. The higher the liquid to gas ratio (L/G), the greater the rate of absorption of the particulate. For particle purposes, a minimum L/G ratio of 60 is recommended in which the ratio is calculated as follows:
L/G = gallons per minute liquid/1,000 actual cubic feet per minute gas
For particles from 10 microns down to 5 microns, the capture rate decreases in a modest linear fashion. The 5-micron particulate is captured at a rate of 0.5 percent less than the rate at which a 10-micron particulate would be captured. Below 5 microns, the capture rate drops off more sharply. Although a Jet operating with an L/G of 63 at a liquid pressure of 40 psig will capture 99 percent of the 5-micron particulates, only 90 percent of the 1-micron particulates will be captured.
Often the Jet Venturi discharges into a two-phase separator tank to allow the dirty liquid phase to be separated from the clean gas phase. Although the scrubbing liquid can often be recirculated, it is advisable to keep a continual purge to prevent the buildup of solids in the scrubbing fluid. Different manufacturers will vary their recommended purge rates; 10-to-2 weight percent is common. The Jet Venturi design can be useful for the control of PM10, but its use for control of PM2.5 is limited to cases in which an abundance of liquid is present to maximize capture rate.
High-energy Jet Venturi
The high-energy Jet Venturi was developed to handle submicron particulates. As such, it readily removes particles between 1 and 5 microns. The high-energy Venturi has a much different principle of operation than its Jet Venturi counterpart and uses less liquid per cubic foot of vapor. In the high-energy Venturi, water enters tangential to the gas flow, so the contaminated gas passes through a curtain of water. As the gas impinges on the fluid, a vortex is developed, and small particulates are forced into the liquid phase. Rather than creating a net pressure rise, the high-energy Jet induces a pressure drop across the Venturi throat. The greater the drop, the better the removal, but at the expense of power since a fan is often needed to pull the gas through the system unless the inlet vapor is under positive pressure. For most applications, the gas needs to be at a minimum of 20 inches water column to overcome the drop through the Venturi.
The high-energy Jet Venturi is always coupled with a suitably sized separator tank. The discharge of the Jet should enter this tank tangentially to impart a spin on the gas. Gas will flow up and out of the top, while the liquid and associated particulate matter will drop out of the bottom. To ensure minimal liquid entrainment, a mist
eliminator is placed at the top of the tank just prior to the gas outlet.
One of the advantages of the high-energy units is that they can be designed to have a variable throat to allow for maximum capture even under severe turndown conditions. Normally these variable throat designs are only required when turndown ratios are larger than 20 to 1. The throat diameter can be controlled manually or automatically. Often a butterfly valve is sandwiched between the converging and diverging cones of the Venturi to achieve the desired results.
Wet electrostatic precipitator
The wet electrostatic precipitator is also used for particulate control. This device is quite different from the Venturi design. As the name implies, this unit uses electrical energy to draw particles from a gas stream. The contaminated air flows between electrically charged plates. Much like rubbing a balloon across a wool sweater, a static charge is imparted on the particle as it passes through the magnetic field. The now negatively charged particles are attracted to the positive plate, resulting in their removal from the gas stream. Water is used to wash the particulates from the surfaces of the plates to keep them clean.
The main advantage of the WESP is its efficiency in collecting very small particles -— as low as 0.01 micron in size. The chief drawback is the operating cost, particularly the electrical requirement. The WESP requires large voltages to impart sufficient charge on the particles to facilitate their removal.
Know your particulates
No matter what technology is employed, WESP, Jet Venturi or high-energy Jet Venturi, reliable particulate distribution data is key to ensuring equipment is properly sized. Knowing whether the particulate is large, small or in-between helps one select the proper approach. For larger particles, a Jet Venturi should suffice. As one trends toward PM2.5 or smaller, the high-energy Venturi or WESP are the only options to adequately remove the particles. For very small particles (smaller than 0.5 micron) the WESP is the only option unless alternative methods, such as dry removal or filtration, are considered. If the particle is sticky or abrasive, that also affects the final equipment design.
In addition to particle size, particulate loading is an important factor. Finding the "sweet spot" will ensure maximum particulate capture. Too much loading will overload the equipment’s ability to capture particulates, while the chances that a particulate will pass through without being removed increases with too small a loading. Generally, loadings between 0.2 and 0.6 grain per cubic foot are common, where 1 grain=1/7,000 pound-mass. For very high loading or very low loading, collection efficiency drops off significantly.
In closing, particulates can be dealt with in many ways. Wet scrubbing in its different forms is a good option since it requires less maintenance when compared to filtration and is more effective for small particulates than comparable dry scrubbing techniques such as baghouses. Other than the periodic changing of spray nozzles, little can go wrong because there are no moving parts. When selecting a wet scrubber, particle size is the primary determinant of which technology should be used, while the particulate loading will govern the capture rate and the required utility consumption. Jet Venturis are best for coarse particulates (PM10), high-energy Venturis are used for PM2.5 and submicron particulates, and WESPs are used for ultra-fine particulates as small as 0.01 micron.
Gregory D. MacLeod, P.E., is engineering sales manager at CR Clean Air Group LLC. He has more than 16 years of experience, most of which was spent as a process engineer. He is familiar with a range of process equipment including heat exchangers, steam ejectors and scrubbers for pollution control. He holds professional engineering licenses in New York and New Jersey and holds a bachelor’s and a master’s degree in chemical engineering from WPI and Northeastern University, respectively. He also is an adjunct professor of Engineering at State University of New York, Orange.