Product Announcement from Bionomic Industries
Flux Force Condensation (FFC) scrubbing technology is used to capture fine (submicron) particulate at extremely high efficiency. It is a preferred method when using wet scrubbing to collect and control dioxins from combustion sources. As an added benefit, through the use of condensation, the water vapor plume commonly associated with a wet scrubbing system can be effectively eliminated. The overall collection efficiency of acid gases plus particulate is greater than "dry" systems since the acid gas removal efficiency is exceptionally high.
The technique uses the condensation of water vapor onto the particulate to coat them with a water film, thus making the particulate aerodynamically larger and easier to capture. This occurs in a device called a "Condenser/Absorber" or "C/A". Since water vapor is condensed, the downstream equipment (fan, stack, etc.) is physically smaller and often less expensive. The C/A circuit is also typically pH controlled to remove acid gases simultaneously.
Applications include hazardous and medical waste incinerators, boilers, calciners, kilns, and other sources that emit submicron particulate. It is most often applied to sources that provide saturation temperatures of over 145 deg. F but can be applied to lower temperature applications through the introduction of steam.
How it Works:
FFC technology mimics the particulate capture that occurs naturally in the atmosphere through the formation of raindrops. Raindrops primarily form onto dust particles in the air through a process called "nucleation". The atmospheric water vapor condenses around the particulate thus coating the particulate and making a heavier droplet that then falls to Earth. If it wasn't for this process, dust would remain in the atmosphere and air breathing Life forms would suffocate.
The nucleation and condensation effect was explored decades ago and was revealed during the Wilson Cloud Chamber experiments early in the 20th century. Vapors were observed to condense on particles which acted as nuclei.
Design engineers start the process by first saturating the hot gases with water or steam in a quencher then subcooling the gases in the C/A. In the C/A, the gases are placed into direct contact with cooled scrubbing liquid. This causes manmade "rain" to form in the C/A. Through differences in temperature and concentrations, a flux occurs pulling the dry particulate from the gas stream and encapsulating them with water. Now enlarged, the droplets are commonly removed by a venturi scrubber operating at a far lower pressure drop (and far lower gas volume) than if the gases were not cooled and condensed. In extreme applications, a wet electrostatic precipitator (WESP) is used after the venturi. Particulate outlet loadings of under 0.005 grs/dscf are achievable in many applications with over 99.9% removal of soluble acid gases.
FFC SCRUBBING INSTALLATION
The photograph is of a recent project. The vessel in the foreground is the condenser/absorber and the venturi scrubber and separator is located to the lower left of the photo. The stack is in the background.
On many installations, an adjustable throat venturi scrubber operating in the 35-45" w.c. range is used for particulate collection. Since the gas volume has been reduced (usually by about 50% versus a system without condensing), the size of the venturi scrubber and separator is reduced thus the cost is reduced. This is important since the internal components of the venturi are often made from expensive corrosion resistant alloys.
Common Materials of Construction:
Quencher: Refractory lined steel or corrosion resistant alloy (C-276, AL6XN, etc.).
C/A: Fiberglass reinforced plastic (FRP) or lined steel, thermoplastic media
Venturi: Fiberglass reinforced plastic (FRP)
Venturi Separator: Fiberglass reinforced plastic (FRP) with thermoplastic chevrons
Fan: Rubber lined steel with alloy wheel, all alloy, all Fiberglass reinforced plastic (FRP)
Stack: Fiberglass reinforced plastic (FRP)
Capacity Size Range:
Saturated gas temperatures of 145 deg F and greater at all gas volume ranges. The reason for the higher saturated gas temperatures is that only a certain amount of the water vapor condenses on the particulate using the particulate as condensation nuclei while the remainder auto-condenses and doesn't participate. If the saturation temperature is too low on certain applications, the saturation temperature can be raised by steam injection prior to the C/A.
Control Methods for FFC Scrubbing Systems
Though a "flux force condensation" (FFC) scrubbing system sounds like a complicated technology to control, in actual fact it can be efficiently operated as long as some important parameters are maintained.
These parameters are:
2. pressure drop
3. pH and
The amount of water vapor condensed is a critical element of the design. The C/A provides both the condensation function and an absorption function. Condensation is its most important role, however.
The outlet temperature from the C/A is a key operating parameter. Since the gases have already passed through a quencher and the packed bed of the C/A, they can assume to be fully saturated. Thus, by monitoring the outlet temperature of the C/A, and maintaining that temperature sufficiently low one can control its condensation performance.
Control involves monitoring the C/A outlet temperature using a thermocouple or RTD in the C/A outlet duct and using that signal to modulate the cooling water flow rate to the by-pass (see Figure 1, lower right) in the clean side of the plate and frame heat exchanger. Experience has shown that reducing the gas temperature to 100-110 deg. F. provides adequate and economical condensation without diminished returns. A control valve as shown is often used to modulate the clean water side by-pass flow rate.
The primary particulate removal is accomplished in the venturi scrubber. Its removal efficiency is a function of its pressure drop. Many permits require that this pressure drop be maintained above a certain minimum (the minimum drop typically verified by a stack test). If the venturi is equipped with an adjustable throat, some installations install a positioner on this throat and modulate the throat position based upon the signal from a differential pressure drop sensor that measures the pressure drop across the venturi throat. This pressure drop is often datalogged as part of the Permit requirements. On other systems, particularly those that are very draft sensitive (such as a hazardous waste incinerator), as Gas Reflux System can be used to recycle cleaned gases back to the scrubber inlet. The flow is modulated by an opposed blade damper and control logic circuit based upon a draft signal from the source. To prevent "hunting", the venturi throat is left in a fixed position. These type systems can control the draft to within 0.01" w.c..
If the system fan is VFD controlled, typically it is used for just general ventilation control and left at a speed away from any vibration causing harmonics. Sometimes a draft sensor on the source (oxidizer, kiln, calciner, etc.) controls the VFD based upon process draft requirements and the venturi is modulated separately. To prevent "hunting", the output to the throat positioner is dampened significantly (only adjusts every few minutes rather than continually).
The C/A also serves as an acid gas absorber. Therefore, the pH of its recirculation loop or sump post-reaction pH is controlled. This is performed in a conventional manner using a pH probe and controller.
To prevent the buildup of suspended and dissolved solids that could cause mechanical problems and reduce removal efficiency, a blowdown must be maintained. It is common to use a conductivity or density controller to adjust the blowdown from the venturi and C/A recycle circuits.
Most FFC systems "bleed forward" from the venturi stage to the quencher then "out". If a WESP is used, a bleed from the flush water circuit is sent to the venturi circuit from which point it is bled either to the quencher then "out" to sewer or water treatment or sent directly out.
This arrangement bleeds the reaction products and recovered particulate at the highest concentration (lowest volume) and highest temperature. The latter reduces the thermal load on the C/A thus improves the thermal efficiency. Various bleed schemes can be used depending upon the amount of particulate to be captured and its corrosive or erosive properties.
Magmeters are typically used to monitor the blowdown from stage to stage. Either manual or proportional control valves are used to meter the liquid flows.
The quencher blowdown line often is monitored using a magmeter and controlled proportionately based upon the process conditions to which the FFC system is attached. If the system is "base loaded" or producing a fairly constant emission rate, the blowdown can be a fixed amount. If the process varies, a conductivity, refractive index, or density meter can be used to adjust the blowdown. Volumetric control is the simplest and most reliable, however. The volumetric set points are usually established by grab sampling during commissioning to define the blowdown liquid parameters as required by the process. Process parameters (such as the beginning of a batch, feed weigh feeder settings, etc.) are then used as surrogates to set the blowdown rates.
Condensation scrubbing can be effectively applied to applications that emit both acid gases and submicron particulate and have or can be caused to have a saturation temperature of 145 deg. F or higher. Few air pollution control systems can match the overall pollutant collection efficiency, dry or wet, of this proven technique.
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