DynaWave Scrubber

DynaWave Reverse Jet Scrubber, a variation of the traditional wet scrubber device, is a form of air pollution control technology for particulate and acid gas removal, as well as hot gas quenching. This technology is engineered by MECS, Inc., a global leader in the design and construction of sulfuric acid plants and related high performance technologies for the phosphate fertilizer, oil refining, and metal smelting industries. The scrubbers are gas/liquid contactors utilizing patented Froth Scrubbing Technology. Liquid is injected counter current to the gas stream so that the momentums of the liquid and gas are balanced, which in turn creates a “Froth” zone, an area where the gas must pass through. This “Froth” zone is an area of extreme turbulence where mixing is so intense that it produces high rates of heat and mass transfer, as well as collecting small particles efficiently.

The Froth Technology used in DynaWave was invented by E.I. du Pont de Nemours and Company in the mid-1970’s. When DuPont was using the technology, they had installed over 140 froth scrubbers in over 40 applications. Originally the technology was treated as confidential and not published or made available outside of DuPont. In 1987, Monsanto Enviro-Chem Systems, Inc. purchased the technology from DuPont to market and supply Froth Scrubbing Technology in sulfuric, incineration and other industrial gas cleaning applications. In 2005, Monsanto Enviro-Chem (MECS, Inc.) management team acquired the company from Monsanto.

Reverse Jet scrubbers have several advantages over traditional gas/liquid contactors. The inherent design and operating simplicity, exemplified by the lack of atomizing spray nozzles, narrow passages or moving parts, results in high on-stream reliability and low maintenance costs. Overall system reliability and effectiveness are further enhanced by setting up a multiple nozzle system.

DynaWave Process Step by Step

* Dirty gas enters the top of a straight vertical duct called an inlet barrel
* The reagent chosen (Usually lime, limestone, caustic, or CKD) is added to the scrubber liquid
* Liquid/reagent is injected counter current upward into the down flowing gas stream
* The liquid collides with the gas coming down to create a froth zone, an area where quenching and SO₂ and particulate removal occur
* After the contaminates have been removed from the gas stream, the clean gas and liquid exit the inlet barrel and enter the gas/liquid disengagement vessel
* Liquid falls to the vessel sump where it is pumped back to the Reverse Jet nozzle, and injected into the gas stream to further capture acid gases and particulates
* The treated gas flows through a gas-liquid chevron separation device and exits the top of the scrubber

Froth Zone
The key to DynaWave scrubbers is the establishment of a froth zone, an area where quenching, as well as SO₂ and particulate removal occur. This happens when the reagent collides with the gas coming down. A proper balance of gas and liquid momentums and liquid to gas ratio is required in order to achieve the froth zone.

The froth zone scrubbing principle makes effective use of liquid phase and gas phase energy to generate interfacial area, and gas and mass transfer.

The liquid in the froth zone is not atomized; as a result, the amount of liquid entrainment in the exit gas stream is minimized and can be easily removed by conventional separators.

Reverse Jet Scrubber
The Reverse Jet is an annular orifice scrubber having one or more large-bore nozzles. These nozzles inject counter to the gas flow. The gas then collides with the liquid, forcing the liquid radically outward toward the wall. A standing wave is created at the point the liquid is reversed by the gas. Depending on relative gas/liquid power, the wave “floats” in the gas stream, moving up or down the barrel, not unlike a variable throat venturi but without moving parts. The wave or froth zone is added on the liquid side. Therefore, total scrubbing energy can be increased by raising nozzle pressure or increasing liquid flow. Gas side pressure drop is typically one-half that of a venturi scrubber for the same performance.

Reverse Jet scrubbers are used for quenching gases as hot as 2300°F while simultaneously removing approximately 90% of all particulate. Additional reverse jets, in series, can further improve removal efficiency.

In some cases, a jump jet is placed so as to inject liquid at a very high rate in the same direction as the gas. The froth zone develops where the gas velocity catches up to the liquid flow. Energy input is almost entirely in the liquid phase. Therefore, gas pressure drop is zero or can actually be negative. Due to the large liquid pumping rate needed for the type of jet, it is recommended only for less severe applications where gas pressure drop must be minimized.

Reverse Jet scrubbers can be retrofitted on existing scrubber vessels. This allows for increased collection efficiency while remaining cost effective.

Summary of Features and Benefits
*Virtually unpluggable: large open bore reverse jet nozzles; non-restrictive, open vessels
*Only wet scrubbing process that can use a variety of reagents: ; ; ; ; others specific to the process
*Accomplish multiple functions in one scrubber: Particulate removal; SO₂, H₂S, HCl and other acid gas absorption, hot gas quenching, in-situ oxidation
*High on-stream reliability: Simple operation with minimal control instrumentation; minimal use of high alloy materials reduces cost; small footprint; designed to handle high inlet temperatures above 2,200°F (1,200°C); versatility: can be integrated with other air pollution control equipment

Removing Particulate
One advantage to DynaWave Reverse Jet technology is that particulates are removed at the same time as SO₂ is. This is the reason that Reverse Jet technology is often used in situations where both particulates and acid gases are present.

Unlike acid gases that are absorbed into the scrubbing liquid by mass transfer, particulates must be captured by physical energy in the form of gas side pressure drop. The higher the pressure drop, the greater the solid removal is. Particulate extraction is also controlled by the particulate size. The smaller the particulate, the more difficult it is to remove. This is especially true for sub-micron particulates.

Absorption of SO₂ Using Caustic
The absorption of SO₂ with a caustic reagent is the simplest method of removing SO₂ from the FCC off gas. The acid-base reaction is fast and the equipment is minimal. Caustic is the most efficient reagent for the removal of SO₂ from a gas stream.

The reaction between SO₂ and caustic (NaOH) can be simplified as follows:

SO₂ + NaOH ---> NaHSO₃
SO₂ + 2NaOH ---> Na₂SO₃ + H₂O
When the oxidation step is included, the bisulfites and sulfites are then further oxidized to sulfates:

NaHSO₃ + 1/2O₂ + NaOH ---> Na₂SO₄ +H₂O
Na₂SO₃ +1/2O₂ ---> Na₂SO₄

The above acid-base reactions are relatively fast compared to the other alkali reagents commonly used, and all the products and reactants are soluble, and most easily handled from a pumping standpoint. The SO₂ is first absorbed into the liquid from the gas phase, and then reacted with the caustic per the above reactions.
The system pH of 6-7 is controlled by the addition of caustic into the sump of the DynaWave vessel. Air is injected into the sump liquid to oxidize the sodium bisulfite and sodium sulfite to sodium sulfate.

A blowdown from the scrubber is required to rid the system of reaction products and maintain the levels of dissolved salts in the scrubber liquor. The blowdown first passes through a rotary vacuum filter to remove catalyst particulates, then passes on to the disposal/treatment system.

Absorption of SO₂ with Limestone Slurry
Limestone, otherwise known as calcium carbonate, must first be dissolved into the liquid, before the acid-base reaction can take place. The dissolution of the calcium carbonate is often the limiting step in the overall conversion of SO₂ to calcium sulfate. After combining the intermediate steps, the net reaction is:

SO₂ + CaCO₃ ---> CaSO₃ + CO₂

Because of its crystal structure, cannot be filtered. To facilitate the disposal and handling of the by-products, the calcium sulfite must be further oxidized to calcium sulfate (CaSO₄) as follows:

CaSO₃ + 1/2O₂ ---> CaSO₄

The above reaction between SO₂ and calcium carbonate is relatively fast once the CaCO₃ is dissolved in the scrubbing liquid. To facilitate the dissolution of the CaCO₃, the scrubber liquid is controlled at a pH between 5 and 6.

The DynaWave Reverse Jet scrubber is well suited for slurry scrubbing because of its unique non-plugging design. As with the caustic example discussed above, the gas enters the inlet duct flowing downward. The limestone slurry is injected into the duct counter-current to the gas stream. Upon contact, a "froth zone" is developed between the gas and the slurry. SO₂ is absorbed into the liquid, where it reacts with the dissolved limestone, forming calcium bisulfite, calcium sulfite, and calcium sulfate. After disengaging the scrubbed gas, the slurry is then recycled through the circulation pump back to the Reverse Jet nozzle. The pH of the system is controlled with addition of fresh 20% wt limestone slurry. Air is injected into the sump of the vessel to complete the oxidation of the sulfites to sulfate. This oxidation is more difficult than the sodium sulfite oxidation and a longer contact time and residence time is required in the vessel. After the sulfite oxidation, a blowdown stream is withdrawn from the vessel and sent to a rotary vacuum filter. The liquid from the filter is then recycled back to the scrubber vessel as make-up water. The gypsum (calcium sulfate) can be filtered to a relatively dry 85 solid wt%, after which it can be disposed of as a non-leachable solid waste. In certain situations, this gypsum, along with captured catalyst fines, can be recycled to local cement facilities that add gypsum to their cement as an important additive.

Oxidation System of a Typical Scrubber vs. that of a DynaWave
A typical scrubber oxidation system is designed to oxidize only NaSO₃ resulting from the gas stream's captured SO₂. There may be other COD sources entering the system, i.e., organics in the makeup water that will not be oxidized by the scrubber. This must be considered when determining the scrubber's makeup water source and effluent COD requirements.

Reverse Jet wet scrubbers use a unique in-situ oxidation system. Air is injected directly into the scrubber vessel sump. This reduces the need for downstream treatment and lowers capital costs. A sparger-piping array in the scrubber vessel distributes air throughout the sump liquid creating fine bubbles. The vessel sump is then sized to maximize contact time with air and O₂ as bubbles migrate to the surface. The sump is also sized to provide adequate liquid retention time to allow ample time for oxidation.

List of Applications
DynaWave has been successfully installed in the following applications:
*Fluid catalytic cracking off gas
*Sulfur Recovery Unit off gas
*Coal-Fired Boilers/Flue-gas desulfurization
*Cement kilns
*Titanium dioxide
*Incineration
*Metal smelters & converters
*Sulfuric acid plants
*Phosphoric acid recovery
*Magnesium production
*Many others

FCC (Fluid catalytic cracking)
Introduction
Refiners are being required to drastically reduce sulfur dioxide (SO₂) and particulate emissions from fluid catalytic cracking. Wet gas scrubbing is one of the most accepted methods for FCC offgas treatment. Reverse Jet wet scrubbers were first used for FCC emissions control at the Navajo Refinery in Artesia, New Mexico, in 2003. Since then, Reverse Jet technology has been used in many refinery applications, including gas streams with extremely high SO₂ levels, such as sulfur recovery unit (SRU) tail gas. SO₂ levels in these gas streams can be as high as 50,000 ppm, especially during SRU bypass modes.

FCC off gases are characterized by:
*Emissions of: SO₂ (sulfur dioxide), SO₃ (sulfur trioxide), particulate (catalyst)
*Possible catalyst carryover
*Large range of gas flows
*Potential to plug or wear downstream equipment

Functions of DynaWave for FCC
*Absorbing the SO₂ with high removal efficiency of 99+%
*Quenching hot gases up to 1200 °C (2200 °F)
*Removing catalyst particulate

Advantages of DynaWave for FCC
*99+% removal efficiency of SO₂
*Low capital equipment cost
*Proven system with more than 300 installation references
*Small footprint
*Catalyst carryover has no effect on DynaWave performance
*Self compensation/high turndown
*Large bore Reverse Jet nozzle eliminates plugging
*Very few internal parts which equates to a very high on stream reliability
*Simplicity in operation with low operator attention required

Effluent Treatment
The extent to which a refinery must treat its effluent is site specific and ranges from none to total treatment and potable water discharge. In most cases, refineries must at least provide effluent that has been oxidized and contains no sulfite salts. This can be accomplished in the DynaWave by injecting air directly into the scrubber liquid and is known as in-situ oxidation. O₂ in the air reacts with sulfite, SO₃ to produce sulfate (SO₄). Reaction rates depend on several factors; but the most important ones are how much excess O₂ is present, the time O₂ is in contact with the liquid; and total retention time.

Example of Reverse Jet Technology in an FCC Application
Late in 2001, the Navajo Refinery, a 75,000 refinery that processes sour crude, in Artesia, New Mexico entered into a consent decree with the EPA, which meant that the wet gas scrubber had to be set up and running by Decemeber 31, 2003. The project was started immediately as there was less than two years to have the scrubber operational. An FCC turnaround was used to pre-install scrubber tie-ins even though a technology has not been chosen. When pilot tests were successfully completed for the reverse jet scrubber, Navajo quickly decided to move forward with full-scale installation

Construction began in autumn of 2002. Despite the aggressive schedule, construction was finished on time, and the scrubber was put into operation October 2003. The scrubber was shut down during December, contemporaneously with the planned outage to expand FCC capacity from 19,800 bpd to 25,000 bpd, refinery capacity from 60,000 bpd to 75,000 bpd, and bring a gasoil hydrotreater and associated equipment online. Since the refinery resumed operations in late December 2003, the scrubber has operated reliably, even through three power outages, without incident.

Because the reverse jet scrubber has a small footprint, no equipment relocation was necessary for the installation, with the exception of some underground duct banks and fire lines. The reverse jet scrubber itself is approximately 65 ft tall, with two froth zones, or gas cleaning stages, and in-situ oxidation in the scrubber sump. The scrubber's total footprint, including the stair tower and recirculation pumps is 30 ft X 30 ft.

The reverse jet scrubber contains minimal internals. Besides the nozzles, the only other internals are two chevrons in the gas/liquid disengagement vessel used to ensure scrubber liquor separation from the gas stream. The scrubber and piping were constructed of 316L stainless. The reverse jet nozzles are manufactured from abrasion resistant silicon carbide. All other scrubber internals are 316 stainless.

The scrubber requires only minimal instrumentation. A DP type bubbler transmitter provides level measurement and controls fresh water makeup. Two pH meters (one operating and one spare) measure circulating liquid pH and control adding NaOH to maintain a 6.5 to 7.5 pH in the scrubber sump. A density meter measures the scrubber sump specific gravity and modulates the system effluent, maintaining a specific gravity between 1.1 and 1.2.

Two filters were provided (one operating and one spare) for effluent filtration and captured catalyst fines removal. Upon startup, it was discovered that the catalyst fines particulate size was smaller than anticipated and a pre-coat system was added to the filters.

Since full scale system startup, outlet SO₂ emissions have consistently registered <1 ppmvd, corrected to 0% by the stack CEM. Testing conducted by Navajo has shown that the scrubber is outperforming design requirement on particulate removal. It is also easily meeting all environmental consent decree requirements and conditions.

Cement Kilns
Introduction
The major problem to address in cement kilns is SO₂ emissions control from the preheater exhaust (or directly from kiln in wet systems) using DynaWave technology. The raw materials used in this process may contain sulfides, FeS₂ (also called pyrites) which get roasted in the pre-heater. As a result, SO₂ is formed. The exhaust from the pre-heater contains this SO₂ as well as dust from the raw materials. If SO₂ is released into the atmosphere, it will react with atmospheric water vapor to form H₂SO₄ and produce acid rain. In many states and countries, governmental regulations limit the amount of SO₂ that can be released into the atmosphere. Even in the absence of regulations, SO₂ and dust can be objectionable due to proximity of residential communities.

Cement Kilns are characterized by:
*Regulated Emissions: SO₂, particulate matter, traces of metals
*Scrubber design will need to take into consideration possible corrosion, erosion, and plugging

Removal Efficiency for DynaWave
*90% to 97% for SO₂ and 80%-90% for dust particles for a single stage DynaWave
*97% to 99% for SO₂ and 90%-98% for dust particles for a two-stage DynaWave

Typical Reagents
*Lime, limestone, or CKD reagent is added to the scrubber liquid

What Are The Materials Of Construction?
* or flake glass lined carbon steel are the typical material used for the scrubber vessel; however alloy or stainless steel construction may be required for some areas

Advantages of DynaWave for Cement Kilns:
*Higher SO₂ removal efficiency
*Large bore nozzles and open design essentially eliminate plugging, even with sticky particles and slurries containing up to 30% solids. This is a plus when downtime for repair and maintenance is a concern
*Slurry design is based on experience and success in cement applications
*Smaller diameter and lower height compared to typical spray towers means less capital cost
*Can handle massive amounts of solids entering scrubber from baghouse or ESP failures and continue to operate without interruption
*Ability to utilize reagents such as limestone and CKD directly in the scrubber.
*Protection against corrosion, erosion, and plugging
*Since dissolved SO₂ forms corrosive weak sulfurous acid, the typical material of construction chosen for the vessel and piping is FRP
*Erosion is handled by use of special nickel hardened alloy for slurry pumps; piping design which reduces turbulence, and silicon carbide Reverse Jet nozzles
*Plugging is controlled by large bore Reverse Jet nozzles and piping design that discourages settling

Possible Synthetic Gypsum Production
A consequential benefit of using a lime/limestone scrubber to remove SO₂, is the capability of producing synthetic gypsum from the liquid effluent. Synthetic gypsum can be produced from the liquid effluent of the scrubbing process by forced oxidation and mixed into the final step of the cement making process.

To produce sulfate (gypsum), calcium sulfite must be oxidized. Normally, a small amount of calcium sulfite is naturally oxidized in the scrubber. Equipment and flow scheme can be designed so that forced oxidation occurs within the DynaWave scrubber (in-situ oxidation).

DynaWave Membrane WESP Combination
A cost effective control technology for reducing emissions of SO₂, SO₃, Mercury (Hg ) and fine particulate from flue gas.

Introduction to Process
After the dirty gas has been cleaned, the pre-cleaned saturated gas continues through the scrubber and flows upward into the Membrane Wet Electrostatic Precipitator. The liquid reverses direction and returns to the vessel sump for recycle back to the reverse jet nozzle.

Membrane WESP
The membrane WESP is a tubular type of configuration with its collecting surfaces being irrigated continuously. The fine particulate, SO₃, and are collected on the membrane and removed by a continuous stream of water. The membrane is continuously cleaned by a capillary washing action to maintain high efficiency. No downstream mist elimination devices are required.

Emissions Reductions
*SO₂, SO₃, Hg , soot, and fine particulate removal
*High SO₂ inlet reduced to regulatory levels
*PM2.5 compliant
*Very high SO₃ removal
*Minimal water consumption

Typical Reagents
*Lime/limestone

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