Gas Scrubbing – What is it?

The process generally known as “Gas Scrubbing” is the removal of a gaseous pollutant from a larger gas stream.

Traditionally, the term “scrubber” has referred to pollution control devices that use liquid to wash unwanted pollutants from a gas stream (“Wet Scrubbers”). In recent times this term has been expanded to also describe systems that inject a dry reagent or slurry into a dirty exhaust stream to remove acid gases (“Dry or Semi-Dry Scrubbers”).

In a broader sense, the term “gas scrubbing” can be expanded to include all devices that remove a gaseous pollutant from a larger gas stream.  These may include activated carbon systems, thermal oxidisers, catalyst systems and many more.

All gas scrubbing systems can be generally grouped into three major technology categories:

  • Absorption Systems – which seek to absorb the pollutant gas into a liquid.

  • Adsorption Systems – which seek to bind the pollutant gas to the surface of a solid.

  • Destruction/oxidation Systems – which seek to oxidise or convert the pollutant gas to a simpler compound in the gas phase

You can read further on each topic by clicking on the above topics or selecting them from the menu to the left of this page.

An Activated Carbon system removing H2S and odours from a sewage vent

An Activated Carbon system removing H2S and odours from a sewage vent

A Dynawave™ scrubber removing SO2 from the steel making process

A Dynawave™ scrubber removing SO2 from the steel making process

A DeNOx system using a catylst bed on a specialist materials plant.

A DeNOx system using a catylst bed on a specialist materials plant.

Absorption and Stripping

Absorption

Absorption

The basic process involved in “wet scrubbing” is the contacting of a polluted gas steam with a scrubbing liquid, with the intention of transferring a sufficient amount of the pollutants from the gas stream into the liquid stream to allow the cleaned gas to be discharged to atmosphere.

It’s the process in the liquid phase that further defines the type of Absorption technology:

  • Simple Water Scrubber
  • Chemical Scrubber
  • Bio Trickling Scrubber

Simple Water Scrubbing

Simple water scrubbers rely on the solubility of the pollutant gas.  Many of these scrubbers do not recycle the scubbing liquor, simply sending it to waste or recovery. An example of this would be the removal of methanol (CH3OH) from a gas stream using only water. Methanol is highly soluble in water, but evolves back out of the water if recycled.  These systems are identical to the chemical scrubbers described below except they only use water.  Click here to visit our Chemical Scubbing section.

QPAC_SPLASH_MR
Stripping

Bio-Trickling Scrubber

Also known as biotrickling filters, biological scrubbers or bioscrubbers, these use a modified form of packed tower technology – very much like the water and chemical scubbers above. A special packing is used that encourages and allows the growth of a highly active biological medium on the packing surface. The packing and biological medium are irrigated with water. The pollutants in the gas stream are absorbed into the water and aerobically degraded (“eaten”) by the biological medium. As it involves living organisms, this technology is applicable to most H2S and organic based compounds; that is odour control and VOC emissions from applications such as sewage treatment works, pump stations, rendering plants, food plants and similar.  

Stripping/Degassing

Also known as degassing or desorption, stripping is the opposite of absorption.  In this process we are seeking to remove gases that are dissolved in a liquid by contacting it with an air stream.  This style of system is used extensively to remove dissolved carbon dioxide from a water stream by contacting it with large volumes of air.  These systems are very similar to simple water scrubbers except we are trying to move the pollutant from the liquid to the air stream.  

Gas Scrubbing – Adsorption

Adsorption

Adsorption is defined as the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.  In air pollution control, adsorption refers to the adherence of pollutant gases to a solid surface.

Unlike other technologies, adsorption does not change or modify the nature of the pollutant gas – it merely captures it on the surface of a solid.  Because of this, the adsorbant material has a limit to the amount of a pollutant that it can adsorb on its surface.  Much like a sponge, it reaches a point where it can no longer adsorb which is known as saturation.  The amount that individual adsorbants can hold is heavily based on the chemistry of the adsorbant, active surface area and temperature.

So what do you do when an adsorbant becomes saturated?  In small applications it’s normal for the adsorbant to be disposed of in land fill and replaced with fresh adsorbant.  However in larger, more specialised applications, it’s possible to regenerate the adsorbant by heating it.  As mentioned above, adsorption is heavily dependant on temperature, with very low adsorption rates at high temperatures and higher adsorption rates at low temperatures.

Many materials can be used as adsorbants but the most commonly used substance is activated carbon due to its high specific surface area and good adhesion properties for organic compounds.  Another commonly used substance is alumina, which is used to remove HF in the aluminium smelting process.

Activated Carbon

Activated carbon is a highly porous, amorphous solid consisting of microcrystallites with a graphite lattice, usually prepared in small pellets or a powder. It is non-polar and cheap. One of its main drawbacks is that it is reacts with oxygen at moderate temperatures (over 300 °C).

Activated carbon can be manufactured from many carbonaceous materials, including coal (bituminous, subbituminous, and lignite), peat, wood, or nutshells (e.g., coconut). The manufacturing process consists of two phases, carbonization and activation. The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons from the raw material, as well as to drive off any gases generated. The process is completed by heating the material over 400 °C (750 °F) in an oxygen-free atmosphere that cannot support combustion. The carbonized particles are then “activated” by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. This agent burns off the pore blocking structures created during the carbonization phase and so, they develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time that they spend in this stage. Longer exposure times result in larger pore sizes.

Activated carbon is used for adsorption of organic substances in waste gas treatment. It is the most widely used adsorbent since most of its chemical (e.g. surface groups) and physical properties (e.g. pore size distribution and surface area) can be tuned according to what is needed. Its usefulness also derives from its large micropore (and sometimes mesopore) volume and the resulting high surface area.

At TAPC, we offer a range of quality activated carbons that can be engineered to suit your particular application.

AC
Portable system removing H2S and odours from a sewage pit

Portable system removing H2S and odours from a sewage pit

Activated Carbon system removing H2S and odours from a sewage vent

Activated Carbon system removing H2S and odours from a sewage vent

A DeNOx system using a catylst bed on a specialist materials plant.Benzene/p-Xylene recovery system in Malaysia

A DeNOx system using a catylst bed on a specialist materials plant.Benzene/p-Xylene recovery system in Malaysia

Gas Scrubbing  Destruction/ Oxidation

Destruction and oxidation technologies are all designed to chemically alter the pollutant gas.  Unlike chemical or biological scubbers, the changes to the pollutant gas all occur in the gas phase. For this reason, these technologies are generally used for substances that are not suited to wet scrubbing or capture with an adsorption system.

Some examples of this style of technology include:

  • Thermal oxidation
  • Catalytic Systems
  • Flares
  • Direct oxidation
  • Cold Plasma


Thermal Oxidation

Thermal Oxidizers are typically used to destroy Hazardous Air Pollutants (HAPs) and Volatile Organic Compounds (VOCs) from industrial air streams. These pollutants are generally hydrocarbon based and when destroyed via thermal combustion they are chemically changed to form CO2 and H2O.  Depending on the pollutants, small amounts of acid gases (HF, HCl, HBr, SOx etc.) may also be produced.  It is common to have a thermal oxidiser followed by a chemical scrubber as a “polishing” stage. 

The pollutant gases are oxidised at high temperatures typically 800-1100°C for a period of 1–2 seconds. Care must then be taken to rapidly cool the gas stream to avoid Dioxin/Furan formation.  As they require a combustable media (such as natural gas), flares can be costly to run.

Typical applications for thermal oxidisers include soil remediation plants and chemical plants.

Catalytic Oxidation

Catalytic oxidation occurs through a chemical reaction between the VOC hydrocarbon molecules and a catalyst bed that is internal to the oxidizer system. A catalyst is a substance that is used to accelerate the rate of a chemical reaction, allowing the reaction to occur in a lower temperature range to thermal oxidisers (275°C to 350°C).

Catalysts can be specifically design for certain compounds, such as Dioxins.  As the amount of heating required is small (or zero if the gas stream is already hot enough), catalytic systems have many running cost advantages over conventional Thermal Oxidisers.

Typical applications for catalytic oxidisers include oil refinieries and chemical plants.

Cat1
Cat3
Cat5
Cat2

Gas Scrubbing – Chemical Scrubbers

counterscr

Chemical scrubbers seek to absorb a pollutant gas into a liquid stream and then react the absorbed gas with chemical compounds in the scrubbing liquid with the aim of removing the dissolved gas from solution, which in turn allows further gas to be absorbed. An example of this would be the removal of sulfur dioxide (SO2) from a gas stream using a water based solution of caustic soda (NaOH). In this case the dissolved SO2 will react with the NaOH to form sodium sulfite and sodium bisulfite – hence removing the SO2 from solution and allowing more to be removed from the gas stream.  To create the best conditions for the pollutant gases to migrate to the scrubbing liquid, we use towers filled with complex geometry packing which provide a very high contact area between the gas and the liquid.


Vertical Packed Tower Scrubbers

   

These are the most common style of scubber that we build and range from small fume cupboard ventilation systems to large multi-stage scubbing systems. Some of the key features of these systems are:

  • Vertical tower configuration
  • Contains a bed of Q-Pac™ or Tellerette® packing material
  • High efficiency gas absorption (>99%)
  • Low pressure drop
  • Small footprint
  • Simple, low cost, trouble free design

Gas Scrubbing – Biological Scrubbers

Bio 1

Biological Scrubbers

Also known as trickling filters and biotrickling filters, biological scrubbers or bioscrubbers use a modified form of packed tower technology. A special packing is used that encourages and allows the growth of a highly active biological medium on the packing surface. The packing and biological medium are irrigated with water. The pollutants in the gas stream are absorbed into the water and aerobically degraded by the biological medium. The technology is applicable to most odour control and many VOC emissions control applications, such as sewage treatment works, pump stations, rendering plants, food plants and the like. 

The technology provides the following advantages:

  • High efficiency gas absorption, to over 99% on hydrogen sulphide if required
  • Minimized chemical consumption- very low operating costs compared with chemical scrubbers
  • Robust technology, able to handle widely varying inlet loads
  • The packing is inert and does not degrade over time
  • Chemical scrubbers can be readily converted to biological scrubbers to reduce operating costs

Gas Scrubbing – Particulate Scrubbers

venturiscr 1

Many of the gas scrubbing systems that we install are for the removal of gaseous pollutants.  However, there are many occasions where the gas stream includes particulates that need to be removed either before a gas scrubbing section, or as a stand alone process.  These scrubbers are generally referred to as “Wet Scrubbers”.


Venturi Scrubbers – Conventional    
  • Primarily for particle collection
  • Moderate efficiency for gas absorption
  • Gas and liquid pass through a narrow throat
  • Turbulence created provides gas-liquid contact
  • Particle collection down to 0.5 micron
  • Fixed and adjustable throats
  • High pressure drop/high energy use
  • Higher throat pressure drops increase fine particle collection
  • Can take a number of forms

Gas Scrubbing – Activated Carbon Adsorbers

AC

ACA systems clean industrial exhaust air of a wide range of pollutants, in particular volatile organic compounds (VOC’s) and hydrogen sulphide (H2S).

The VOC or H2S laden air is directed through one or more beds of carbon. The air can be pre-filtered, dehumidified or cooled if necessary to assist the adsorption. The carbon takes the form of small granules or extruded (8mm long) pieces. Very high rates of adsorption can occur, such that the exhaust air is essentially free of VOC’s or H2S. The carbon is replaced when its working capacity has been reached.

If there are high loadings of VOC it becomes uneconomic to replace the carbon after one use. In such cases regenerable systems are used. These systems have a duty and a standby bed. At any given time the duty bed is adsorbing VOC’s while the standby bed is being regenerated by steam stripping.

ACA systems have very wide application, ranging from bulk liquid hydrocarbon storage and handling facilities (tank farms) to sewage transport systems odour control.

Gas Scrubbing – Degassers

Degasser

The solubility of gas obeys Henry’s law, that is, the amount of a dissolved gas in a liquid is proportional to the amount gas above it. Therefore, placing a solution with a dissolved gas in contact with a gas stream with little or none of the dissolved gas forces it to migrate to the gas stream. This process is known as degassing and is the opposite of conventional gas scrubbing.

In industry, degassers are used extensively to remove dissolved gasses from a process liquid stream.  Typical applications inlcude RO permeate, ammonia removal and CO2 removal.


Vertical Packed Tower Degassers

   

Vertical counter-current towers are the most common style of degasser that we build. They are very similar to vertical chemical scrubbers but tend to run at significantly different air to liquid ratios.  Some of the key features of these systems are:

  • Vertical tower configuration
  • Contains a bed of Q-Pac™ or Tellerette® packing material
  • Low pressure drop
  • Small footprint
  • Simple, low cost, trouble free design

Gas Scrubbing – Mist Eliminators

kimre
Brink1
Brink2
Chevron
 
   
 

Kimre Mesh Pads

Kimre mesh pads offer high efficiency mist elimination. The pads employ a unique woven structure and a variety of filament sizes and weave densities. The woven material produced is in relatively thin layers (2.5 mm to 50 mm thick) with a pad being assembled from a number of layers which may be the same or may vary in weave and composition. As such a pad can be designed to achieve a given mist removal performance, liquid handling capability etc. This method of construction results in predictable, reliable performance and also allows for easy cleaning by pulling the individual layers apart and washing them. Kimre pads can also be used as mass transfer media in packed towers.

B-GON® Mist Eliminators operate at low pressure drops, giving higher throughput at lower energy usage. Kimre technology makes this possible because all B-GON® filaments are engineered to align perpendicular to your gas flow for maximum droplet removal efficiency.  This unique interlacing is in sharp contrast to the random orientation of competitive products of knitted mesh.  B-GON® mesh combines the best features of knitted mesh and plate type eliminators.  The ladder arrangement of the two sets of filaments causes a change in direction of vapor flow which enhances droplet removal by impaction, interception and centrifugal actions.  This also produces a cross-flow of captured liquid that flushes particulates from the media. Composite pads are a unique Kimre feature. Because particle size distribution and flow conditions are subject to unpredictable variations and in-plant data on them are not full or completely known, Kimre has developed composite pads for stagewise droplet removal. These uniquely designed B-GON® Mist Eliminators have several layers of differing coarseness (see picture above). Heavy loads of solid particulates or liquids can be stopped with coarse styles while less coarse styles eliminate small liquid droplets.  B-GON® composite pads are a robust design, less sensitive
to fluctuating operating conditions.

B-GON® is the registered trademark of Kimre, Inc.

Brink Mist Eliminators (BME’s)

Also known as candle filters and Brink mist eliminators, MME’s were developed by Monsanto to solve acid mist problems in sulphuric acid plants. They have since been developed to solve a range of mist problems, especially ultrafine acid and organic mists from sources such as chlorine plants, asphalt plants, sulphuric and nitric acid plants, vinyl curing operations and other applications producing condensed organic mists.

Chevron Mist Eliminator

Verantis engineers our chevron mist eliminators for the highest reliability and collection efficiency, with mist removal efficiencies often approaching 100%. Our mist eliminators rely on direct compaction combined with centrifugal force to remove liquid entrainment from gas streams. They guarantee high efficiency and low pressure drops in scrubbers, cooling towers, air washers, gas absorbers, and ventilation systems.
Our high performance chevron mist eliminators make sure your mist goes away:
  • Special chevron design allows for high velocity operation up to 5 m/s
  • Existing towers with mesh pads can be retrofitted to minimize fouling and pad replacement
  • Wide blade spacing provides excellent resistance against potential plugging in applications with high solids loading
  • Effectively removes liquid droplets down to 25 microns
  • Available in vinylester fiberglass reinforced plastic, polypropylene, carbon, and stainless steel

Gas Scrubbing – Technical Services

Photo 03

Service and Maintenance Work

TAPC have an expert crew of service technicians with many years of detailed experience in all types of gas scrubbing equipment.  Not only do we offer full servicing capabilities for our own equipment, but we also service our competitors equipment.  Behind our service technicians are a full team of project managers and engineers who can determine and resolve any issue that you may be having.

Currently TAPC services and maintains hundreds of different types of air pollution control equipment across Australia and SE Asia – many under long term maintenance contracts with the region’s largest customers.  If we’re not currently servicing your equipment, why not give us a call and give yourself peace of mind.

 

Engineering Studies

Many of our clients need assistance to evaluate their air emission applications, to identify solutions and to evaluate and troubleshoot existing ventilation and air pollution control systems.

TAPC have un-matched specialist expertise and knowledge to assist in these areas and have undertaken a variety of engineering studies over the past 40 years.

Gas Scrubbing – Spare Parts

DSC05294
tellerettesgroup
QPAC_SPLASH_MR

Tallerette Packing


Maximum

Dimensions O.D

Loop Height

Free Volume

Surface ft2/ft3

Average Packing Factor (ft)

Construction Material

Order Packing

No.1 Type R

1.81"

0.75"

87%

55


36

No.2 Type R

2.75"

1.00"

93%

38

18

No.3 Type R

3.75"

1.50"

92%

30

16

No.2 Type K

3.25"

1.25"

95%

28

11

No.3 Type K

4.125"

2.19"

96%

22

9

   Available in PE, PP, PVC, CPVC, Kynar, Noryl, Tefzel  = Available in PP only

At TAPC we offer a full range of spare parts for all types of air pollution control equipment, whether we manufatured the original equipment or not. In cases where we don’t hold these parts in stock, we can source the right parts for you in a timely and cost effective manner.

Tower Packing

TAPC supplies many types of tower packing from the traditional Tellerette tower packing, to more modern, highly efficient, low pressure drop Q-PAC™ tower packing from Lantec. The packing comes in a range of sizes and materials (polypropylene, polyethylene, PVC, CPVC, PVDF, Tefzel, Xydar) to suit your particular application. 

So whether you want to stick with your current packing, or you want improved performance, we have the packing for you

Lantec Q-PACTM


3.25" x 3.75"

Maximum Dimensions O.D.

2.1 lb/ft3

Weight in PP

11,000/ft3

No of Dripping Points

3.25" x 3.75"

Maximum Dimensions O.D.

30

Surface ft2/ft3

7

Average Packing Factor(ft)

96.3%

Free Volume

Fiberglass Fans

TAPC offer fiberglass fans for use in highly corrosive chemical environments. The range covers air flows up to 90,000 Am3/hr and static pressures up to 6 kPa. Both the impellers and the housings are solid fiberglass offering maximum corrosion resistance and structural integrity. The range covers the normal range of types, blade styles, accessories and arrangements as are available for metal fans.

Activated Carbon

Activated carbon is a highly porous, amorphous solid consisting of microcrystallites with a graphite lattice, usually prepared in small pellets or a powder. It is non-polar and cheap. One of its main drawbacks is that it is reacts with oxygen at moderate temperatures (over 300 °C).

Activated carbon can be manufactured from many carbonaceous materials, including coal (bituminous, subbituminous, and lignite), peat, wood, or nutshells (e.g., coconut). The manufacturing process consists of two phases, carbonization and activation. The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons from the raw material, as well as to drive off any gases generated. The process is completed by heating the material over 400 °C (750 °F) in an oxygen-free atmosphere that cannot support combustion. The carbonized particles are then “activated” by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. This agent burns off the pore blocking structures created during the carbonization phase and so, they develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time that they spend in this stage. Longer exposure times result in larger pore sizes.

Activated carbon is used for adsorption of organic substances in waste gas treatment. It is the most widely used adsorbent since most of its chemical (e.g. surface groups) and physical properties (e.g. pore size distribution and surface area) can be tuned according to what is needed. Its usefulness also derives from its large micropore (and sometimes mesopore) volume and the resulting high surface area.

At TAPC, we offer a range of quality activated carbons that can be engineered to suit your particular application.

AC

Contact us

+61 (0)2 4272 5233

sales@tapc.com.au