Electrostatic Precipitation

    What is Precipitation?

    In our business, the word Precipitation refers to the process of removing a substance from a medium.

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    What is Electrostatic Precipitation?

    Electrostatic precipitation utilizes one of the basic principles of electricity and electromagnetism: Magnetic Attraction. A charged particle will be attracted to and move toward a neutral or oppositely charged surface if enough attraction is present and the particle is not immobile.

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    What is an Industrial Electrostatic Precipitator?

    An Industrial Electrostatic Precipitator, often called an ESP, utilizes an electrode to produce a corona that ionizes and charges particles (dust, oil, fume, etc). Once charged, the particles naturally move toward surfaces of neutral or opposite charge. Those neutral or oppositely charged surfaces can be stationary, like collections plates or ductwork; or be dynamic, like other particles in the same gas stream. Once enough particles have been accumulated in an area, one of many removal methods can be utilized to remove the collected particles.

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    What is the history of electrostatic precipitation?

    The first recorded observation of the phenomena, that is the basis of industrial electrostatic precipitation, was in 1600 by William Gilbert, an English scientist. Gilbert wrote in his work De Magnete, that ….[electricks] entice smoke sent out by an extinguished light [flame]. The first large scale application of the electrostatic precipitation, for industrial applications, was performed in 1885 by Walker and Hutchings, who based their design upon observations and experiments of Sir Oliver Lodge, a British electrical physicist. The Walker and Hutchings precipitator had problems attributed to an inadequate power supply as well as an over abundance small highly resistive particles present in the flue gas stream.

    In 1906 the DuPont Corporation enlisted Frederick Gardner Cottrell to help them with the problem of separating arsenic from sulfuric acid. Cottrell's approach to the seperation resulted in a pure sulfuric acid mist. Cotrell then began to experiment with precipitation to condense the mist for the recovery of the sulfuric acid. He identified two major deficiencies in previous designs of precipitators: the power supply and the discharge electrode (used to generate corona). Utilizing the latest available power supply technology of the time (the synchronous mechanical rectifier and high voltage AC transformer) along with a pubescent (villous) discharge electrode. Cottrell filed his patent later that same year and received it in 1908.

    The basic design developed by Cottrell has not changed that much over the years. Examining the patent picture below, gases enter the chamber (A) via an inlet (B) and encounter the high voltage electrode (C 7 c) where the particles become charged and are attracted to (A). The high voltage from the power supply (I,J,K,L,M,N,O) enters the chamber through an insulating bushing (H,h) that is pressurized from the backside (G).

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    How Does ESP Work?

    In the simplest terms, a precipitator is a device that utilizes electrostatic forces for the collection of particulate. The particulate laden gases enter one area of the vessel and exit another area clean. Inside, high voltage electrodes impart a charge to the particles. Most modern precipitators utilize negative voltage electrodes though positive voltage electrodes do exist. These charged particles are then attracted to a collecting surface (grounded, neutral or oppositely charged). The particles build up on the collection surfaces over time and are then periodically removed, typically at set intervals.

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    What are the Different Types of ESP’s?

    The vast majority of Industrial Electrostatic Precipitators are of the dry type, though wet precipitators are gaining popularity. Dry Electrostatic Precipitators are often referred to as Dry ESP's or DESP's while the Wet Electrostatic Precipitators are referred to as Wet ESP's or WESP's. Regarding particulate removal, the surfaces in a Dry ESP are rapped or vibrated, and the tubes in a Wet ESP are flushed , causing the particles to separate from the collection surface. In a Dry ESP the particles are usually evacuated from the hopper by rotary screws or drag chain conveyors . In a Wet ESP the hoppers are designed to gravity drain or pump the flush water for designated treatment or disposal.

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    What can be collected through Electrostatic Precipitation?

    Precipitators can be used to collect many different kinds of particles with a wide range of physical properties. Some of the compounds that can be collected include:

    • Aluminum
    • Ammonium Sulfates
    • Arsenic
    • Asphalt
    • Bagasse Ash
    • Bentonite
    • Catalyst
    • Cement
    • Coal Ash
    • Coke
    • Copper
    • Diesel Smoke
    • Fluorspar
    • Fly Ash
    • Glass
    • Gold
    • Grease
    • Gypsum
    • Hydrochloric Acid
    • Hydrofluoric Acid
    • Incinerator Ash
    • Iron Oxide
    • Lead
    • Oil
    • Pigments
    • Plastic
    • Salt Cake
    • Smoke
    • Sulfuric Acid Mist
    • Tar
    • Wood Ash
    • Wood Acids
    • Wood Tar

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    What is Biofiltration?

    Biofiltration is a process to purify air and water biologically with the aid of micro-organisms, specifically bacteria.

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    What is Industrial Biofiltration?

    An Industrial Biofilter is a housing that contains and encourages the growth of vast numbers of bacteria through regulated temperature, humidity and pH for the destruction of volatile organic compounds (VOC's) and odor causing compounds (including NH3, CS2, H2S), in a way that can be quantified and measured, often for compliance purposes. Industrial biofilters utilize a group of aerobic organisms that are classified as chemotropic, meaning they derive the energy needed to live and degrade compounds from the reaction itself. As a point of fact organisms that derive their required energy from sunlight are classified as phototropic. Two sub-groups of the chemotrophs are the hetero-organotrophs and auto-lithotrophs. The sub group mainly utilized in biofilters are the hetero-organotrophs which have the ability to utilize the carbon in carbon compounds (i.e. methanol) as an energy source as well as the creation of cell components. As a point of fact auto-lithotrophs gain their energy through the degradation of non-organic compounds (NH3, H2S, S2O3, etc..).

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    What is the history of Industrial Biofiltration?

    One of the earliest known uses of biofiltration in the Industrial Era was in World War I (1914 to 1918). Stagnant air was sucked out of tunnels and trenches, blown through earth where it would permeate and re-enter the tunnels and trenches. The air would be "cleaned" through the actions of bacteria naturally present in the soil. In 1923 the concept of treating off gases from sewage treatment plants was developed. In 1953 R.D. Pomeroy received a patent (#2,793,096) for "De-Odoring of Gas Streams by the use of Microbiological Growths" that arose out of a successful commercial application in California.
    A high pressure blower (#12) is used to pull polluted gases (#11) through a pipe and inject it into a vertical header (#14) before it turns and becomes a horizontal header (#15) buried under a biologically active medium (#17) that is suspended above the header by a layer of permeable medium (#16) such as gravel or aggregate. The header is composed of perforated sections (#18) and non-perforated sections and couplings (#19). The basic design developed then is still used in many commercial applications today with the exception that the medium, or media, has changed. Since that patent, the major innovations in biofiltration have been in two areas: bacteria enhancement and media development.

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    How Does Industrial Biofilter Work?

    In the simplest terms, a biofilter is a device that utilizes natural biological oxidation for the destruction and / or removal of hydrocarbons (or CS2, H2S, NH3), that is to say biofiltration is the degradation of organic and inorganic substances by microorganisms. These microorganisms live in a biofilm coating that resides on the surface of a media composed of organic or inorganic (or a combination thereof) matter. The micro-organisms are stationary in regards to the system as a whole though they are mobile in their localized biofilm area. The process gases containing the contaminants to be treated flow through the media and as these gases flow by, molecules of contaminants pass very near to or directly contact the biofilm where they are absorbed into the biofilm. Noting that the biofilm is primarily composed of water one can see clearly that a compound's solubility in water will greatly impact ease of degradation because if the compound does not enter the biofilm then it cannot be decomposed by the micro-organisms in that biofilm. The biofilm also creates a fixed (more or less) inhabitable space that can only be utilized by a finite maximum number of organisms. The organisms will grow and expand until the available space (biofilm) is filled resulting in situation where no more effective growth can occur. This means that the effective amount of the summation of biomass (dead and alive) in the unit is relatively constant. An example of this is shown in the graph below.

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    What can be degraded in a Biofilter?

    Biofilters can be used to degrade many different kinds of compounds within a wide range of industries. Some of the compounds that can be degraded include:

    • Acetone
    • Aliphatic Hydrocarbons
    • Ammonia
    • Anthranilates
    • Aromatic Hydrocarbons
    • Butadiene
    • Carbon Disulfide
    • Esters
    • Ethanol
    • Ethers
    • Formaldehyde
    • Heptane
    • Hexane
    • Hydrogen Sulfide
    • Isopropanol
    • Isopropyl acetate
    • Ketones
    • Methyl Ethel Ketone (MEK)
    • Methanol
    • n-Propanol
    • N-propyl acetate
    • Pesticides
    • Phenol
    • Pinenes
    • Styrene
    • Terpenes
    • VM&P naphtha

    In general, more soluble compounds such as lower molecular weight alcohols, aldehydes and ketones are more easily treated in a biofilter than are aliphatic or aromatic hydrocarbons.

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    How are compounds degraded?

    The bacteria produce enzymes that act upon compounds to break them down into the elementary constitutes of water and carbon dioxide. Each enzyme has the capability of altering a reaction and in fact several enzymes will often work upon the same compound at the same time. The presence of many different species of bacteria means there is wide variety of enzymes available at any single given space and time. These enzymes may remove a water molecule, break a C-H bond or C-C bond or add a water molecule. The fewer the variety of compounds supplied the greater the specialization of the population as a whole. For example if a biofilter is fed nothing but methanol then the reproducing populations will be geared toward the degradation of methanol, that is to say they will produce enzymes that are especially successful at degrading methanol as well as more of those enzymes. A basic understanding of the degradation process can be obtained by the examination of the Michaelis-Menten kinetics model. where:

    • E is the enzyme being analyzed
    • L is the concentration of the compound to be degraded
    • EL is the developed complex (a complex is a group of two or more associated polypeptide chains)
    • P is the product that regenerates the original enzyme

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    On what processes are Biofilters used?

    Biofilters have been employed on various processes and pieces of process equipment such as:

    • Composting
    • Door & Window Manufacturing
    • Flavor & Fragrance Production
    • Food Processing
    • Frying Operations
    • Medium Density Fiberboard (MDF)
    • Paint Spray Booths
    • Particleboard Manufacturing
    • Pet Food Production
    • Petro-Chemical Plants
    • Pharmaceutical Production
    • Photo and Film Production
    • Printing
    • Pulp & Paper Manufacturing
    • Sewage Processing Plants
    • Tanneries
    • Textile Fabrication
    • Wastewater Treatment Plants (WWTP)

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    How is the size of a Biofilter determined?

    Each type of compound or gas has a different potential of being utilized by the bacteria as food and as such a semi-empirical model is generated from test reaction and diffusion data gathered in lab scale, pilot scale, and full scale applications. Once the model is constructed a biofilter's media volume can be determined by entering the concentration of each known compound in the gas stream and not just the concentrations of the compounds to be degraded because bacteria will eat the most easily digested molecules first. As a general rule the more insoluble a compound is the less biodegradable that compound is, or to put it another way the less soluble a compound is more media, and thus bacteria, it will take to degrade that compound.

    • Solubility of Various Compounds in Water
    • Compound mg/l @ 20°C
    • Acetone Miscible
    • Benzene 1,780
    • Butadiene 735
    • Ethanol Miscible
    • Formaldehyde Miscible
    • Heptane 3
    • Hexane 9.5
    • Isopropanol Miscible
    • Methyl Ethyl Ketone (MEK) 27,500
    • Phenol 8,300
    • Styrene 300

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    What factors can adversely affect Biofilters?

    Virtually any force that affects the biofilm can affect the operation of the biofilter. Factors that can affect the biofilm include: Moisture, temperature, residence time, particulate load, nutrient availability, pH and poisons.

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    How does moisture affect the Biofilter?

    The impact of moisture can be easily visualized, too little and the biofilm dries up and too much and overgrowth of organisms, desired and undesired, can occur. Each biofilter is different in terms of heat generation from the biological oxidation, heat capacity in terms of the mass and type of media, ambient conditions in terms of external heat loss or absorption, biofilter housing insulation coefficients, spacing of water irrigation nozzles and the irrigation nozzles themselves. In order to account for all of these variables industrial biofilters are irrigated with an over bed spray system (OBS) that is controlled by a microprocessor, usually a programmable logic controller (PLC), that allows the user to control water application by zone, by time of day and day of week. Some processes only operate eight hours a day, five days a week, so applying water on a 24/7 schedule would result in uncontrollable biological that would blind off the media resulting in excessive pressure drop. Increased pressure drop usually means either decreased flow or channeling (unbalanced flow through the media) resulting in operational issues with the plant.

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    How does temperature affect Biofiltration?

    Temperature has a direct impact biological activity of the bacteria, in other words how fast they can digest any given molecule, how fast they move and how fast they reproduce. Temperatures above 104°F start to encourage the growth of thermophilic microorganisms that do not have the ability to degrade or are less efficient at degrading the compounds. In recent years much money and time has been spent in developing and quantifying organisms that can efficiently degrade the compounds. The below graph is a typical example of the biological activity of mesophilic bacteria. It is important to note that increased biological activity does not necessarily mean increased appetite in terms of the rate of degradation or molecules per hour degraded by a particular micro-organism.

    In reality temperature effects in a biofilter are very difficult to predict because in addition to affecting biological activity, temperature can also affect the physicochemical properties such as solubility , diffusivity and Henry's law coefficient of the compounds in the gas stream. The chart below shows a biofilter's biological activity of bacteria versus temperature.

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    How does residence time affect Biofiltration?

    Like many other air pollution control technologies the longer any particular pollutant molecule (particulate matter, nitrous oxide, carbon monoxide, etc..) is within the treatment zone of the equipment the more time that equipment has to act upon that molecule. In a biofilter a longer residence time means an increased chance of any particular gaseous molecule will contact a biofilm, be absorbed into the water of that biofilm and subsequently captured and bio-degraded by an organism. Once captured the complexity of that particular compound will dictate the digestion time required to break it down into its simplest elemental constitutes (carbon dioxide, water, etc...).

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    How does particulate loading affect Biofiltration?

    Particulate matter comes in many different sizes and as such some can affect the biofilter while others do not. The two simplified vast extremes are the very small and the very large. Very small particles such as smoke, viruses and smog can pass through the biofilter without ever actually contacting a single water particle, media particle or biofilm. These particles are so small they follow the path of the flue gas and thus avoid any collisions. The very large particles such as sand, ash, sawdust and pollen can collect on the surface or fallout in the media. Over time these large particles can fill the void spaces that the air uses to travel through the media thus creating excessive pressure drop. A secondary affect can be that these large particles provide food for other organisms present in the media such as slimes and fungi with the result being the same; these organisms fill the voids resulting in excessive pressure drop. The gray area, those particles that are definitively not large and definitively not small, lies in the 1 to 10 micron range and their affect on the system can vary with each individual biofilter.

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    How does compound concentration affect Biofiltration?

    If, gas streams with high concentrations of a particular compound, the primary compound and low concentrations of a secondary compound then the resultant population of bacteria is geared toward the degradation of the primary compound and not the secondary compound, unless of course the secondary compound is part of the degradation path of the first compound. The graph below shows an example of how the inlet concentration has an effect on the elimination capacity of a biofilter.

    Coupling gas concentrations with degradation rates the below diagram can be used to visualize the relationship between the two. The easier a compound is to degrade the faster it is degraded and thus does not have a chance to move deeper into the biofilm.

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    How does solubility affect Biofiltration?

    One of the keys to successful biological oxidation is the degradation of the compounds. In order for this degradation to take place the compound must be where the bacteria and its enzymes can affect it, which is in the biofilm that is composed mainly of water. If a compound is not very water soluble then the molecules of the compound want to stay in the vapor phase and not dissolve in the water that makes up the biofilm. Coupling solubility, gas concentrations with degradation rates the diagram below can be used to visualize the relationship between the three. Note that from the previous graph the gas concentration is considerably higher than that in the biofilm.

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    How does increased pressure drop across the media affect plant operations?

    To understand how increased pressure drop in the system affects plant operations one must have a basic understanding of how to read a fan curve. In reference to the figure below CFM is the amount of airflow which is usually given in actual cubic feet per minute and SP is static pressure which is usually given in inches of water column. Point of fact there is 27.7 inches of water in 1 PSI. For a given system (fan, biofilter, bioscrubber, duct, etc..) there is a SYSTEM CURVE. This system curve is a plot of airflow (CFM) vs pressure drop (SP). Each fan will have static pressure curve (SP CURVE) developed by the manufacturer of the fan. Dropping a line straight down from the intersection of the SYSTEM CURVE and the SP Curve to the x-axis will yield the amount of airflow that the fan will move. The intersection of the drop line and the BHP CURVE will yield the brake horsepower (BHP) required by the motor to generate the air flow (shown on the second y-axis). Drawing a line left from the intersection of the SYSTEM CURVE and the SP CURVE will yield the static pressure at that intersection point.

    It is of vital necessity the process engineer or system designer take into account each potential pressure drop source (media, packing, transitions, support grids, direction changes, entrances and exits). Failure to accurately take these pressure drops into account can result in a system that has a desired operating point outside (above or to the right) of the SP Curve or in a situation where there is not enough horsepower available. Of these two the lack of horsepower is the easiest and most economical to fix though it may require pulling new cable and conduit but when you are also considering replacing 300hp motors and variable frequency drives (VFD's) the bill can add up to hundreds of thousands of dollars very quickly.

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    How can the physical or mechanical design adversely affect the biofilter?

    Mechanical design in a biofilter has ramifications from the inlet nozzle all the way through to the outlet nozzle. Improper inlet design can result in air flow imbalances across the width of the biofilter that can never be corrected short of full reconstruction of the inlet. Improper packing design can result in decreased efficiency removal in the bio-scrubber section as well as increased pressure drop. Improper demister design can allow water carryover onto the media surface that encourage the growth of unwanted micro-organisms. Improper selection of the media itself can result in high pressure drops and encourage the growth unwanted micro-organisms as well such as fungus. Improper outlet design can result in excessive pressure drop (ie in the 3" to 5" range).

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    How does the Process and Instrumentation Diagram for an Biofilter look?

    Various instrumentation, such as temperature, pressure, pH and humidity, are required on biofilter to monitor and predict the performance of the bacteria. A sample Process and Instrumentation Diagram (PID) is shown

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