Air Pollution Control Innovations

Venturi scrubbers are mechanical impaction devices that are used to control particulate on a wide range of applications. These include medical and hazardous waste incineration, potash drying, sewage sludge incineration and processing, coal drying, mineral wool manufacturing, and copper roasting to name a few.  In one of my last blog posts I discussed a Venturi collision scrubber used to treat exhaust fumes of a Banbury mixer for rubber compounding.  The Envitech collision scrubber is used in several Goodyear Tire manufacturing facilities around the world.   

A recent new application is a horizontal Venturi scrubber for an aggregate asphalt drum dyer.  The asphalt process is semi-stationary operating at fixed locations for several months at a time. The process is periodically broken down and moved to new locations for operation.  The horizontal arrangement makes the horizontal Venturi scrubber an ideal solution for mobile applications.  The equipment package is transportable on a single flatbed with only minor disassembly required for shipment and installation.  The equipment is mounted onto a skid with pre-assembled piping and wiring to facilitate equipment mobility. 

Particles are captured in the horizontal Venturi scrubber through a process of impaction by water droplets injected into the Venturi throat.  A fraction of particulate laden water becomes entrained in the gas which has a droplet size distribution.  The droplets are removed from the gas by a mist eliminator.   There are two pathways for particles to escape capture by the scrubber. The first is penetration.  Smaller particles lacking the mass for impaction penetrate through the scrubber.  The second is through carry over of particulate laden water droplets.  Droplet capture efficiency, or mist elimination, therefore impacts scrubber efficiency and overall plant emissions.

The horizontal Venturi scrubber uses a damper blade and actuator to maintain constant pressure drop across a wide range of gas flows.  A high efficiency chevron mist eliminator removes particulate laden droplets before the gas exits the scrubber.  A chevron mist eliminator removes greater than 99.9% of water droplets down to 27 microns with less than 0.25 inches W.C. of pressure drop.  By comparison, a cyclonic separator requires several inches W.C. of pressure drop to remove 98% of 27 microns droplets.   The advantages of a chevron mist eliminator are higher performance, larger turn down ratio, and lower pressure drop.

Envitech is currently building two new horizontal Venturi scrubbers sized for 45,000 acfm each for a peat drying operation. The compact design allows the scrubber to be easily installed inside a building. The high level of pre-assembly and lower profile reduces installation time and improves maintenance access for plant personnel.  The horizontal Venturi scrubber can be a cost effective alternative to many processes that use a typical vertical arrangement.

Click on the link below to download Venturi scrubber literature.

Packed bed absorbers are often used to treat gaseous emissions for reasonably large gas flow rates ranging from a few thousand cfm to greater than 70,000 cfm. Common emissions include SOx, HCl, HF, and NOx. The absorbers are often custom engineered for a specific plants and have been used for secondary lead smelters, geothermal power plants, waste oil-re-refiners, ceramic tile manufacturing, waste incinerators, and ethanol plants. The types of emission sources range from thermal oxidizers, regenerative thermal oxidizers (RTO’s), furnaces, kilns, direct fired heaters, incinerators, fermenters, vent tanks, and batch mixers.

Envitech developed a lab scrubber to provide an economic solution for smaller gas flow rate applications.  The lab scrubber is a packaged unit designed for high efficiency of water soluble contaminants and can handle up to 2,000 cfm of gas at a maximum temperature of 180°F. The system is engineered for reduced footprint at 4 ft x 4 ft and includes a pre-wired control panel and pre-piped service utility connections requiring minimal installation and maintenance costs. Scrubber units are configurable to different levels of automation and treatment applications.  

A recent lab scrubber is for a manufacturer of pharmaceutical products in Southern, California. The scrubber is designed to remove HCl from the exhaust of several small laboratory process vent streams. The vent streams includes low organic concentrations which are incompatible with many common plastic materials. Special resin was selected for the fiber reinforced plastic (FRP) vessel and ductwork.  Piping and valves were assembled using PVDF.  The scrubber is designed for a classified area with explosion proof motors and instruments and is provided as a turn-key installation.

Another example  is a process vent scrubber for a blending facility in South Carolina that produces crop protection products for agricultural markets.  The vent stream is 1,500 cfm and includes HCl and water soluble particulate greater than 3 micron in size. The Envitech lab scrubber was configured to include a low pressure drop Venturi for particulate control combined with a packed bed absorber for HCl control.  The system includes instruments, control system, recirculation pump, pre-assembled piping, valves, and fittings, interconnect duct, ID fan, and stack. 

A different use for a lab scrubber includes an ethylenediamine (EDA) scrubber installed in the South Eastern United States.  The storage of this precursor chemical requires extra handling than is typical with other common industrial chemicals.  With a relatively low exposure limit of 10 ppm, storage tanks must be properly engineered and scrubbed to remove excess vapors.  The Envitech lab scrubber is an ideal, low cost solution for this type of storage system.

Please click on the link below to download a brochure and case studies for the lab scrubber.

Sulfuric acid plants manufacture acid by burning sulfur with air in a smelter to form sulfur dioxide (SO₂), oxidizing sulfur dioxide catalytically into sulfur trioxide (SO₃), and absorbing sulfur trioxide into water to form sulfuric acid.  An unwanted coproduct of sulfur combustion is nitrogen oxide (NO).  In the catalyst beds, nitrogen oxide partially converts to nitrogen dioxide (NO₂) and the two react with sulfuric acid to form nitrosylsulfuric acid (NOHSO₄).  Candle filters located downstream of the catalyst beds collect most of the nitrosylsulfuric acid.

Candle filters require periodic maintenance. To maintain the equipment, operators wash the candle filters with water to remove the sulfuric acid contained within them before entering the candle filters for maintenance activities. Unfortunately, water reacts with built-up nitrosylsulfuric acid to reform NO_x. Since there is no diluting gases, NO_x concentrations leaving the candle filter during the wash may be very high, both dangerous to operators and potentially forming an opaque brown cloud.

Recently, Envitech developed an economical solution to this problem by designing a mobile NO_x scrubber. A mobile NO_x scrubber, rather than a fixed system, has several advantages in this application. First, a mobile system does not consume valuable footprint during normal operation. The area around the candle filters is often congested. A fixed system requires "shoe-horning" the equipment in a hazardous location, adding cost. A mobile solution provides flexibility to position the scrubber in optimal locations near the candle filter without permanently consuming valuable space. Temporary utility connections suffice for a mobile solution, further reducing installation costs. Second, a mobile scrubber can service multiple candle filters or other process vessels, even at different facilities. Finally, Envitech's mobile scrubber is multi-functional, designed to abate a number of hazardous chemicals in addition to nitrosylsulfuric acid, such as hydrochloric acid, sulfur dioxide, and other soluble gaseous emissions.

Envitech's mobile NO_x scrubber offers the ability to abate over 99% of the NO_x emissions during maintenance activities for flow rates of up to 1,000 acfm for a single reagent system and 15,000 acfm for a multi-reagent NO_x reduction system. Envitech's mobile NO_x scrubbers are fully automated, stand-alone scrubbers requiring only utility and gas connections, and configurable to handle a variety of pollutants. If you are interested in more information about Envitech's mobile scrubber, click on the link below to download a cut sheet for the scrubber.

Last March I gave a paper and presentation at the International Conference on Thermal Treatment Technologies and Hazardous Waste Combustors (IT3/HWC) in Houston, TX.  The paper discusses the challenges for meeting ultra-low emission limits for medical waste incinerators.  Wet scrubbers are used to control hazardous air pollutants (HAPs) on many hospital, medical, and infectious waste incinerators (HMIWI).   The Maximum Available Control Technology (MACT) standard for these incinerators was revised and became final in 2009.  The new standard has the lowest emission limits for incinerators today. The limits exceeded the capability of systems designed to the previous standard with respect to particulate matter (PM), lead (Pd), cadmium (Cd), mercury (Hg), and dioxin/furans (D/F).  By 2014 all existing medical waste incinerators were either shut down or upgraded to comply with the new standard.  Envitech successfully upgraded four existing medical waste incinerators.  A paper presented at the 2012 IT3/HWC conference describes one of these systems which was installed at the National Institute of Health (NIH), Rocky Mountain Labs.

The challenge moving forward will be new medical waste incinerators which have even more stringent, ultra-low emission limits.  Building a new incinerator requires critical decisions on control technologies and permitting.  The IT3/HWC paper reviews these issues for specific HAPs and discuss trade-offs between permitting a new medium size incinerator versus a large incinerator.   An example is provided of an air pollution control system meeting the emission requirements for a new large medical waste incinerator at the University of Texas Medical Branch (UTMB) in Galveston, TX.  Envitech is also building gas cleaning systems for two new medium size medical waste incinerators for a research facility which integrate NOx control using ozone injection.

Please click on the links below download the presentation and paper.

Ethylene amines are one of the major chemical building blocks.  Ethylene amines include ethylenediamine (EDA), diethylenetriamine (DETA), triethyltriamine (TETA) as well as other longer chained ethylene amines.  Amongst its many uses, ethylene amine is one of the two principal chemical ingredients in making ethylenediaminetetracetic acid (EDTA), the most common chelating agent in the world.  Diamines are also used in the manufacture of the ubiquitious textile Nylon. Due to its excellent properties, it is also used extensively in the asphalt, petrochemical, rubber, pesticide, and paper industries.

The storage of this precursor chemical requires extra handling than is typical with other common industrial chemicals.  Ethylene amines possess relatively low OSHA exposure limits of 10 ppm or less and have a vapor pressure akin to water; thus vent points in ethylene amine storage systems must be properly engineered to avoid hazardous conditions, and scrubbed to remove excess vapors.

Envitech manufacturers a packaged vent scrubber for the control of toxic compounds like ethylene amines. Envitech's vent scrubber is simple in design, constructed out of stainless steel or other corrosion resistant materials, and available in several sizes and customizations to meet individual storage needs.

The vent scrubber uses water to capture ethylene amine vapors from any process vent.  The scrubber is designed for batch or continuous use and can meet OSHA requirements for all classes of storage tanks.  The system only requires water, power, and a drain, and installation is easy and straightforward with single point connections for all utilities.  If you are interested in more information about Envitech's vent scrubber, click on the link below to download a cut sheet for the scrubber.

Venturi Scrubbers are used to control particulate on a wide range of applications including medical and hazardous waste incineration, pot ash mining, sewage sludge processing and incineration, coal drying, textile and mineral wool insulation manufacturing and copper roasting. A previous blog post in April discusses the mechanisms for particulate removal by a Venturi scrubber. One application for Venturi scrubbers is removal of fumes generated by a Banbury mixer. A Banbury mixer is an industrial mixer used in a wide range of applications including food, chemical, pharmaceutical, plastic, mineral, and rubber processing. Banbury mixers are used, for instance, to compound rubber material for manufacturing automobile tires. Uncontrolled fumes from the mixers can create a nuisance by settling around the facility. Envitech’s Venturi collision scrubber has been used to control these fumes. The figure on the right shows a typical Venturi collision scrubber for a 25,000 cfm mixer exhaust. The scrubber separates the exhaust into two streams internal to the scrubber. The streams are then directed to two opposing Venturi throats. Recirculated water injected into each throat is atomized into fine droplets as the gas is accelerated. Fume particles and droplets collide and are captured by the atomized water as the steams are recombined into a third Venturi throat. A diffusion section redistributes the gas to a horizontal chevron style mist eliminator to remove entrained water droplets. Water is collected and drained into a common sump and recirculated back to the Venturi throats. A blowdown stream purges the collected material.

The scrubber is designed for 24/7, semi-automatic operation and is skid mounted with redundant pumps, one operating and one spare. Instruments are pre-mounted into the piping assembly and pre-wired to a junction box. The systems are often provided with an ID fan which can be mounted on the roof of the building. Typical design conditions and performance are indicated in the table below.

*particles > 2.5 microns

Envitech Venturi collision scrubbers have been in operation at several tire manufacturing facilities since the early 80’s. Over 17 systems have been installed including several in recent years.

Click on the icon below to download a case study for Envitech Venturi Collision scrubbers learn how the scrubber solved the emission problems for Goodyear Tire & Rubber Company.

Venturi Scrubbers are used to control particulate on a wide range of applications including medical and hazardous waste incineration, pot ash mining, sewage sludge processing and incineration, coal drying, tire manufacturing, and copper roasting. One particular application is glass fiber manufacturing which can include both textile fiber and wool fiber insulation. Both types of fibers are manufactured by similar processes which use high-temperature to convert raw materials (predominantly borosilicates) into glass fibers. Emissions control is needed for both glass melting and fiber forming and finishing processes. A survey of stack test data from 10 manufacturing lines at different glass furnace operations demonstrate typical particulate emissions in the range of 0.0035 gr/dscf to 0.015 gr/dscf for volumetric gas flow rates ranging from 20,000 dscfm to 50,000 dscm. Most of these lines use a 10 in. W.C. pressure drop Venturi scrubbers. A few use wet electrostatic precipitators (WESP’s). Stack test data and particle size distribution (PSD) data indicate there can be significant differences in particle size distribution between different glass furnace manufacturing lines which account for the range in outlet concentrations. The figure on the right shows removal efficiency by particle size for a 10” pressure drop Venturi. It indicates that nearly all particles > 2 micron in size are removed by the Venturi. Performance drops off dramatically, however, for smaller particles. Mechanisms for particulate removal by a Venturi scrubber are discussed in more detail in an earlier blog post, dated April 14, 2016.

The image below shows a typical Venturi scrubber used for glass fiber manufacturing facility. The process exhaust gas passes through the Venturi scrubber throat for particulate removal. The Venturi has a variable throat damper that is pneumatically actuated for maintaining the Venturi scrubber pressure drop over a minimum and maximum gas flow rate.  The damper position is governed by proportional-integral-derivative control based on the differential pressure across the throat. 

After the Venturi scrubber throat the gas passes through a flooded elbow and enters a vertical entrainment separator through a tangential inlet. Large water droplets are removed by centrifugal forces by the spin induced by the tangential entry. After passing through internals to smooth the gas flow distribution, the gas passes through a vertical chevron style mist eliminator to remove remaining water droplets from the gas. A spray header provides a periodic wash to keep the chevrons clean from particulate and debris.   Liquid from the Venturi is collected in the entrainment separator sump and re-circulated to the Venturi throat. A blowdown stream is taken from the recirculation line to purge the collected particulate.

Venturi scrubbers have proven to be highly reliable on a wide range of applications, including several collecting fibrous material. Several considerations should be taken into account to design reliability into the system.  A well designed Venturi scrubber can operate continuously with just one or two shifts of maintenance per year. Although Venturi scrubbers are quite common on glass fiber manufacturing lines some sites have relied on wet electrostatic precipitators (WESP) to meet emission limits. This may be driven by a combination of the particle size distribution (PSD) of the process and site specific permit limits. In general, a WESP is used when there is a large fraction of submicron particulate that exceeds the capability of the Venturi scrubber to meet the permit limit. A WESP has higher capital cost, but will have lower operating cost from lower energy consumption.

Click on the icon below to view a video of a variable throat Venturi scrubber damper blade.

A common application for small scrubber systems is an emergency vent scrubber for laboratories. Envitech's lab scrubber is a packaged packed bed absorber designed for high efficiency removal of water soluble contaminants (e.g. HCl, HF, HBr, SO₂, NO₂, etc.) from the gas stream and can handle up to 500 acfm of gas at a maximum temperature of 180^oF.  The system is engineered for reduced footprint at 4 ft x 4 ft and includes a pre-wired control panel and pre-piped service utility connections requiring minimal installation and maintenance costs. Scrubber units are configurable to different levels of automation and treatment applications.  A typical application might be a facility with a gas cylinder filling stations laboratory hood vent.  The scrubber comes with a fan, pump, instrumentation, and control panel and is shop fabricated and assembled.

Click on the button below to download a free Envitech Lab Scrubber Brochure.

A Venturi scrubber is a common air pollution control device that is used to remove particulate. Because it is a wet scrubber, collected particulate is purged in a liquid discharge stream called the blowdown.

Venturi scrubbers are commonly used for industrial dryer applications (see photo). They have a relatively low inlet temperature or might have sticky particulate which prevent the use of a bag-house. Typical dryer applications include coal dryers, pot ash mining, CPVC plastics manufacturing, bio-solids sludge drying, or salt production. Venturi scrubbers are also used in insulation or glass manufacturing, magnesium mining, and hazardous and medical waste incineration.

It is important to saturate or pre-cool the gas before entering the Venturi throat to minimize evaporation. That is because during evaporation, water molecules leave the water droplet surfaces which push particles away from the droplets and reduce collection efficiency. It is also important to keep the inlet walls of the Venturi wetted to avoid fouling from wet-dry line interface.

In accordance with Bernoulli's equation, inlet gas accelerates at the converging section of the Venturi throat, increasing gas-liquid contact. As water is injected perpendicular to the gas flow in the throat, the accelerated gas particles are captured by water droplets upon collision. Three mechanisms account for collection in a Venturi which is summarized below. The adjacent graphic scrubber illustrates the three mechanisms.

• Diffusion – Particle is so small its path is erratic due to Brownian motion.
• Interception – Particle follows streamline around droplet, makes contact if within a particle radius.
• Impaction – Particle’s inertia cause it to leave stream line and impact the droplet.

Impaction is the dominant collection mechanism for large particles, greater than 15 microns. They can be collected with efficiency greater than 99%. Diffusion and interception are more prevalent for smaller particles. Collection efficiency is lower for these particles because their small size increases the probability they will flow around the water droplets and avoid collection.

The graph below illustrates the impact of particle size on collection efficiency. The vertical axis is collection efficiency and the horizontal axis is pressure drop. The curves represent different particle sizes ranging from 0.2 to 5 microns. It can be seen that collection efficiency increases for larger particles and higher pressure drops. The net result is overall collection efficiency is dependent on the aerodynamic particle size distribution (PSD).

After particle collection in the Venturi throat, the resulting droplets aggregate through the diverging section and are separated from the process gas by the mist eliminator (ME) in the entrainment separator (ES). The ability of the mist eliminator to remove water droplets from the gas stream can have a significant impact on the scrubber performance. Any water droplets that "escape" the ME will carry entrained particulate and increase the measured outlet emissions. A more detailed discussion on mist elimination can be found in the previous blog post on “Improving Entrainment Separator Design”.

To download a free case study on a Venturi scrubber used to remove particulate from a CPVC dryer.

Envitech has joined the paperless revolution by launching Interactive O&M manuals on electronic tablets (Apple iPads). Traditional paper based-manuals present several problems – they may be easily lost or damaged; large three-ring binders that are difficult to search and cumbersome to carry; and lastly, red-lined changes and as-builts often are not captured in the final revision.

Starting with 2015 projects, Envitech created web-based O&M manuals for its customers. Due to the positive feedback received to date, Envitech decided to phase out paper copies and fully implement interactive O&M manuals through the use of customized electronic tablets. Each tablet is equipped with 16GB of memory, Wi-Fi capabilities, video and audio calling, personalized e-mail account, video recording, camera, and text messaging. Our customers can access their online manuals and its features with the click of a button. Specific advantages of the Envitech interactive O&M manual are:

1. Portability - Tablets are thin, light and you can store more information than in 3-ring binders. Information is secure and readily accessible while you are out in the field. 
2. Updates - Online manuals allows us to make updates with ease. You have immediate access to the material rather than waiting for the manual to be re-printed and delivered. 
3. Troubleshooting - Our tablets allow you to contact our service team via text, e-mail or facetime.    

Portability

•  Compact tablet design eliminates the need for large storage capacity - 16GB of memory can store up to 4,096 photos, 3 hours of video, or +10,000 PDF files.
• Always available - Access your online manual 24-7 when and where you need it through the use of a tablet, phone or computer.
• Print select documents - Airprint is an Apple technology that allows you to print documents from your tablet.  
• Improved file management - Search and find files more efficiently.

 Figure 1. Click to navigate: simply click or swipe to access your files.

Always Up To Date

•  O&M Manual Updates - Red-lined changes and updates via online eliminates the need to re-print +500 page manuals.
• Offline capabilities - Access your manual without Wi-Fi connectivity.
• Revision management - Track changes made to your manual with online revision tables. 

Figure 2. Revision table

Interactive Troubleshooting

• Contact us - Each project has a unique username that allows you to reach our service team via text, e-mail, and facetime from your tablet.         
• Photo and video capturing - Send us photos/videos from your tablet to assess the situation.    
• Instruction videos - Access training videos and learn how to maintain/calibrate your equipment.   

Figure 3: Texting Support	Figure 4: Email Support	Figure 5: Facetime Support

To learn more about Envitech's service capabilities, please download our brochure.