Air Pollution Control Innovations

There are many applications that require packed bed absorbers for scrubbing SO₂. Some applications previously blogged about include refinery sulfur recovery unit tail gas treatment units (SRU/TGTU), geothermal power generation, and waste oil re-refining to name a few.  Another SO₂ scrubber example is gas cleaning equipment for a mineral processing application to recover gold and silver from mine sites with low grade ore.   Discovered and patented in the early 1900 by Charles Merrill and later refined by Thomas Crowe, the Merrill-Crowe process is a common separation technique for removing gold and silver from a cyanide solution using zinc dust.  Gold and silver precipitate (concentrate) is the product of the Merrill-Crowe process.  Furnaces are then used to recover ingots from smelting Merrill-Crowe precipitate.  Fluxes are mixed with the precipitate to bring impurities to the surface. The precious metals settles down and can then be easily removed.

Envitech is currently building an SO₂ scrubber to treat the exhaust gases from a melting furnace for a South American precious metals refinery mining project.  Furnace off-gases are in the 300^oF to 500^oF range and contain particulate and SO₂.  A bag-house removes particulate from the gas before passing downstream to the packed bed absorber for SO₂ removal.  Gas is first cooled to saturation in an evaporative quencher using re-circulated water. In the case of a pump failure, an emergency spray nozzle provides an independent source of water controlled by a thermocouple.  Water that has not evaporated flows from the quencher into the packed bed absorber sump. Gas from the quencher enters the bottom of the absorber and travels vertically upward through a packed bed.  Recirculated water is sprayed over the packed bed and mixes with the counter current gas.  A dilute solution of plant-supplied sodium hydroxide is metered into the scrubber recirculation line to neutralize acid gases and is controlled by pH of the absorber sump liquid.  The scrubbing water is collected in the sump and is re-circulated to the top of the packed bed and to the quencher. A blowdown stream is taken from the recirculation line to purge the system of reaction products. After the packed bed, the gas passes through an entrainment separator to remove water droplets entrained in the gas during scrubbing. The gas then exits the scrubber and is exhausted to atmosphere through an ID fan and stack.

The scrubber scope of supply includes a quencher, packed bed absorber, instruments, control system, recirculation pump, piping, valves, and fittings, ID fan, and metering pump. The unit will be pre-assembled to the fullest extent possible with pre-mounted instruments pre-wired to a control panel.  The assembly will be broken down as necessary for shipment and packaged for export to South America.  Bilingual submittals are provided for the O&M manuals and engineering submittals.  The scrubber will be delivered and started up in Q1 of 2019.

Click on the link below for a case study on the Merrill-Crowe Refining Furnace SO₂ Scrubber and a packed bed absorber cut sheet.

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.

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.

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.

In 2009, the US EPA revised the emission limits for the Hospital, Medical, and Infectious Waste Incinerator (HMIWI) MACT standard. You can follow the link to the blog piece published in May 2013 on the new standard. It dramatically reduced the emission limits for several pollutants including particulate (PM), lead (Pb), and dioxins and furans (D/F). Several existing medical waste incinerators in operation at the time were not capable of meeting the new limits, especially for lead (Pb) and/or dioxin and furans (D/F). Cost effective add-on controls were needed to bring existing system into compliance with the new rules and to allow them to continue to operate.

To meet this new challenge, Envitech designed a carbon bed adsorber and filter package to be installed downstream of existing wet scrubbers. The package is comprised of a new fan to overcome additional system pressure drop. Heat of compression from the fan and a re-heater duct heats the wet gas above the dew point to prevent condensation fouling in downstream filter and/or carbon bed adsorber. The system is delivered pre-assembled on a skid to reduce installation time and cost. A cartridge filter removes low concentrations of condensed Pb particulate. The carbon bed adsorber removes dioxins, furans and mercury (Hg). Envitech has upgraded four medical waste incinerators to meet the new MACT standards. All four are operational and compliant with the new standards.

In one case for Wyoming Medical Center (WMC), space was limited for add-on controls. The system had to be installed outdoors and capable of withstanding below freezing temperatures. The existing system did not meet the new limits for lead (Pb) and dioxins/furnace (D/F).The add-on controls included a cartridge filter and a carbon bed adsorber. The equipment was insulated and heat traced to maintain temperature above the dew point after re-heat. System features include:

•  Shop and skid mounted assembly for ease of installation.
• Insulation and heat tracing for outdoor operation in a cold climate.
• Silicon controlled rectifier (SCR) controller to control the heater duct.
• Compressed air pulse cleaning for automatic particulate removal from the cartridge filters.
• Pre-wired instrumentation to a control box located on the skid.
• Manways to facilitate maintenance access.

The system has been operational since 2014 and has been used on a routine basis during cold winter months.   The system comfortably passed a stack test in 2015. Compliance for lead (Pd) is 20 times below the limit and Dioxins/Furans (D/F) is 5 times below the limit. The re-heat and filter package has been used on several other medical waste incinerators and provide a cost effective solution for meeting stringent emission limits. 

Download a free case study to find out how Wymoming Medical Center met the new EPA HMIWI emission limits for their existing medical waste incinerator.

Download a free white paper from the 2010 Internationa Conference on Thermal Treatment Technologies and Hazardous Waste Combustors (IT3/HWC) on the 2009 HMIWI MACT standard for medicl waste incinerators.

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.

In October 2014, existing medical waste incinerators had to be compliant with the US EPA’s new Hospital, Medical, and Infectious Waste Incinerator (HMIWI) MACT standards. Nearly all of the systems that planned to continue incineratrion had to be upgraded with add-on controls to meet particulate (PM), lead (Pb), mercury (Hg), Cadmium (Cd), dioxins/furans (D/F), or a combination of the pollutants. Pb was the most common of those pollutants requiring additional capture.

Envitech upgraded scrubbers for three existing medical waste incinerators. In October 2015, I presented a paper at the International Conference of Thermal Treatment Technologies and Hazardous Waste Combustors (IT3/HWC) about using a wet electrostatic precipitator (WESP) on the National Institute of Health’s (NIH’s) Rocky Mountain Lab (RML) existing medical waste incinerator. Envitech also designed and built a fourth scrubber system which was permitted as a large (> 500 llb/hr of waste) new medical waste incinerator. The table below compares the previous 1997 standard for lead (Pb) for a large incinerator to the current (2009) standard for an existing incinerator and a new incinerator.

As shown, the current emission limit for an existing incinerator is just 3% of the limit for the 1997 standard. Add-on controls need to achieve 97% reduction in Pb for medical waste incinerators just meeting the previous limit. This is a significant reduction.

Lead (Pb) Emission Limits for Large Incinerators, mg/dscm

• 1997 standard                   1.2
• 2009 standard existing       0.036
• 2009 standard new            0.00069

Pb reduction for a new large medical waste incinerator is even more dramatic. The emission limit is a mere 0.06% of the 1997 standard. Compared to an existing system permitted to the new standard, a large new medical waste incinerator must emit 2 orders of magnitude less Pb.

Envitech’s scrubber for a permitted new medical waste incinerator recently passed the stack test and demonstrated compliance with Pb emission less than 0.00069 mg/dscm. We believe it’s the only systems in operation today that is compliant with the HMIWI MACT standard for a large, new medical waste incinerator.

It is interesting that despite the ultra low emission standards required by the HMIWI MACT standard, there is still significant public resistance to new permitted systems. It’s clear the public doesn’t understand the impact of these rules and how far technology has come to enable environmentally friendly and safe operation of these systems. The role of these captive systems (treating waste from the facility where it is generated) may become more important in emergency response plans of state and local governments. This was evident during the recent Ebola episode where large amounts of waste needed to be treated and disposed. Some would claim that treating the waste at the facility where it is generated poses less public risk than transporting the waste on public roads and highways to a centralized hazardous waste facility. More work needs to be done to educate the public on the capability of these advanced emission control technologies.

For more information on this topic, please read our paper at the IT3 conference.