An Overview of Large and Small Scale Adsorption and by hwh10252

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                  An Overview of Large and Small Scale
                Adsorption and Catalytic Gas Processing
Technical article
  An Overview of Large and Small Scale
Adsorption and Catalytic Gas Processing

In terms of Molecular Products critical filtration refers to         Activated Carbon
the process of adsorbing various chemicals for air                   The process of activated carbon generation begins with the
treatment and purification by use of a catalyst.A catalyst is        selection of a raw carbon source.The element carbon occurs
defined as a chemical substance that increases the rate of           naturally in two different forms (or three if you count
a reaction without being consumed itself; after the reaction         Buckminster fullerene) viz. diamond and graphite.
it can potentially be recovered from the reaction mixture
chemically unchanged.The catalyst works by lowering the
activation energy required for a given reaction and thus
allows the reaction to proceed more quickly or at a lower
temperature. This is usually achieved via the shape of the
catalyst itself which acts as a ‘docking area’ for the various
reactants to come together and react more efficiently.
This is often referred to (in biology) as the ‘lock and
key method’.
                                                                     [The various forms of carbon. Buckminster fullerene (found
                                                                     in space on meteorites) consists of a closed ball like structure
                                                                     of 60 (or 70) carbon atoms in 5 and 6 membered rings (very
                                                                     stable). Diamond which is formed of regular tetdrahedral
                                                                     which can be cleaved in various planes. Graphite which forms
                                                                     sheet like structures which can be cleaved only along the
                                                                     planes of the sheets]



[In the above pictogram (left) there is only one reactant
(substrate) which if it ‘locks’ with the catalyst (enzyme) exactly
will react whereas if it is not quite the correct fit no reaction
will take place. More practically (right) there is a large           [The physical forms of Buckminster Fullerene, diamond and
molecular matrix (sticks) and within this is a zinc atom             graphite respectively]
(Zn) which acts as a catalyst which due to its position
(active site) and large charge attracts the other two                These sources are selected based on design specifications
reactants (ARG 127 and GLU shown as balls) to come                   since different raw sources will produce activated carbon
together in such a way that hydrogen bonds are formed                with different properties. Some of the more common raw
between the molecules (i.e. they have reacted).The chances           sources include wood, sawdust, lignite, peat, coal, coconut
of these molecules coming together without the presence              shells, and petroleum residues.The form used by Molecular
of the zinc are much more random. Note that once reacted             Products is from coconut shells and coal.
the zinc is unaffected.]

The Chemistry of Critical Filtration
Some of the critical filtration products require a base upon
which to act.This can be activated carbon (Chemsorb® 520,
620, 705, 1000-70, 1000-85, 1321, 1425, 1505, and 16103),
zeolitic (Ionex® - Type Ag 400 and Ionex® Type Ag 900)
Moleculite®, Hydrous Ferric Oxide (Ionex® -Type O-P) or
aluminium beads (Sofnosiv, Sofnofil and Sofnocat 514).




                                                                                                                                   1
Characteristics of importance in choosing carbon types           Activated carbon will remove organic chemicals from waste
include pore structure, particle size, total surface area and    or process streams in the liquid or vapor phase. Generally,
void space between particles. After selection of a source,       high molecular weight contaminants are more effectively
preparations for use are made. These preparations most           removed by activated carbon than are low molecular
often include dehydration, carbonisation, and activation.        weight contaminants. Activated carbon is a general purpose
Dehydration and carbonisation involve slow heating of the        adsorbent for organic vapours and gases. However, once
source in anaerobic (without oxygen) conditions. Chemicals       impregnated with other chemicals it becomes more specific
such as zinc chloride or phosphoric acid can be used to          in its adsorption and fortuitously is selective for a number of
enhance these processes. The stage of activation requires        hazardous gases and odours.
exposure to additional chemicals or other oxidising agents
such as a mixture of gases. All the MPI products use steam       MPI specializes in the impregnation of high activity coconut
activation. Depending upon the specifics of the process and      shell-based carbons. Coconut shell carbons have several
the source carbon, the newly activated carbon can be             advantages over carbons made from other materials.
classified according to density, hardness, and other             Coconut shell-based activated carbons have a high density,
characteristics. At the atomic scale, as can be seen in the      high purity, and are virtually dust-free, since they are harder
picture, this is directly related to the formation of pores      and more resistant to attrition. Also, the pore structure is
into which the agents to be removed can be adsorbed.             very uniform, with the majority of pores falling in the
Carbon is particularly good at adsorption since 1g can           micropore range (5-10 Angstroms), particularly effective for
provide an internal area of 1000 m2 (i.e. 50g has the same       air and water purification applications.
surface area as a full size football pitch!). As the activated
carbon is used the pores become full until the ‘breakthrough     The coconut shells are steam activated. They are first
point’ after which the carbon quickly becomes ‘spent’.           carbonised to create a char. High temperature kilns (up to
                                                                      o
However, the carbon can be regenerated/reactivated by            1000 C) are used to activate the char in a steam atmosphere
chemical treatment                                               i.e. are steam activated rather than chemical. The activation
                                                                 involves the reaction of the carbonised material with steam
                                                                 as follows:

                                                                           C + H2O ➞ CO + H2
                                                                           C + CO2 ➞ 2CO

[Carbonised and activated coconut shell]                         The activation process can be manipulated to produce
                                                                 desired characteristics in the product. Steam concentrations,
                                                                 temperature, and CO2 concentrations control pore
                                                                 development, which in turn affect pore size distributions
                                                                 and the level of activity.

                                                                 Un-impregnated or base carbon is a good adsorbent for
                                                                 organic vapours, but a poor adsorbent for low molecular
[The physical form of activated carbon – looks like coal         weight or polar gases such as chlorine, sulphur dioxide,
with woodworm!]                                                  formaldehyde, and ammonia. Impregnation is the process
                                                                 where activated carbon is treated with a chemical reagent
Adsorption is the process by which activated carbon              that reacts with these types of gases, binding them up on the
removes substances and is defined, as ‘the collection of a       carbon and thereby removing them from an airstream.
substance onto the surface of adsorbent solids’. It is a         This process, commonly referred to as chemisorption, may
removal process whereby certain particles are bound to an        involve neutralization or catalysis reactions.
adsorbent particle surface. Adsorption (or chemisorption)
is often confused with absorption (or physisorption), where      The impregnation process must be carefully controlled to
the substance being collected or removed actually                ensure correct loading levels and even distribution of reagent
penetrates into the other solid and alters its structure. The    on the carbon, without restricting access to the reaction sites
actual process used involves attractive electrostatic (van der   within the pores. Properties such as activity, moisture content,
Waals) forces which pull the particles out of solution/the       and particle size affect the performance of the adsorbent,
gas stream and onto the surface of the carbon. First the         and can be controlled to optimize filter efficiency and
substance will adsorb to the exterior of the carbon granules,    service life.
then move into the carbon pores and finally adsorb to
the interior walls of the carbon.

                                                                                                                               2
Due to their incredibly large surface area, activated and          As an example the most common zeolite is Zeolite A (also
impregnated activated carbons are popular for many diverse         called LTA or simply 4A) which is a major component in
applications. These are just a sample of the possible uses         Persil Automatic since it lets water and ‘dirt’ into the pores
for this versatile material:                                       but will not let the ‘dirt’ molecules out again. A unit cell
        • Air purification                                         (the smallest repeat unit) of LTA is actually 8 Sodalite cages
        • Catalyst support                                         joined together with 4 membered rings. Note that although
                                                                   LTA is symmetrical in every direction many zeolites have a
        • Decolourisation
                                                                   mixture of different channel sizes in different directions.
        • Deodourisation
        • Gold/metal recovery
        • Liquid purification
        • Emergency poison treatment
        • Solvent recovery
        • Sugar refining
        • Whisky manufacturing
                                                                   [Left: Sodalite which consists of 8 by 4 membered ring cages
Zeolites
                                                                   in 3D Centre and Right: Two representations of Zeolite A
From the Greek meaning ‘boiling stones’ which is a perfect         which consists of 8 Sodalite cages (called beta cages) joined
description since it was the Greeks who first noticed that         by 4 membered ring channels which give rise to an 8
these naturally occurring minerals would absorb water which        membered central ring (which forms a large alpha cage)]
could then be driven off with mild heating. Zeolites consist
of a framework of silicon/aluminium and oxygen in an               The way in which zeolites can absorb material is simple to
alternating fashion which form rings (commonly with 8, 12          understand and is dependent upon the Si/Al ratio. In a
and 16 oxygens which would be termed 8, 12 and 16                  neutral system there will be two oxygen atoms (valency -2)
membered rings respectively even though they actually              to every silicon atom (valency +4). It will be recalled that
consist of double that number of atoms the silicons and            aluminium only has a valency of +3 and hence the more
aluminiums are not included in the numbering).                     aluminium atoms there are in the framework the higher the
                                                                   negative charge on the rings. Hence the rings will trap any
                                                                   positively charged particles in gaseous of liquid phase
                                                                   (e.g. dirt). At higher temperatures (or pressures) these
                                                                   positively charged particles will gain enough kinetic energy
                                                                   to ‘escape’ the framework. Hence the zeolite can be
                                                                   recharged and acts as a catalyst.

                                                                   Some common uses include the filtering of radioactive
[Three representations of Faujasite. Left: the atomic
                                                                   cesium and strontium from nuclear waste, the removal of
structure showing the oxygen and silicon atoms alternating
                                                                   ammonia from sewage, as a scrubber to remove sulphur
around the rings Middle: pictogram showing the main
                                                                   dioxide (SO2) from coal-fired electric power station
central 12 membered ring plus surrounding 6 membered
                                                                   emissions, as a purifier for landfill gas for household utility use,
channels passing into smaller (Sodalite) cages Right:
                                                                   as a filter for mercury and other heavy metals from industrial
atomistic representation looking down the main channel]
                                                                   wastewater, as a remover of calcium (over sodium) from
                                                                   water in water-softening systems (it is no coincidence that
The rings are structurally very stable and will only be
                                              o                    large quantities of zeolites are found to be naturally
destroyed at temperatures in excess of 600 C.These rings in
                                                                   occurring close to Scottish whisky distilleries!), as an absorber
3-D form pores which can be used to ‘trap’ particles.There
                                                                   in cat litter trays and nappies, and as part of the cracking
are around 35 natural zeolites and many more that may be
                                                                   process (where long chain hydrocarbons are broken down
made synthetically – almost all end with – ‘ite’ i.e. mordenite,
                                                                   into smaller chains) in the petrochemical industry.
sodalite, faujasite (but not Moleculite® which is a hopcalite)
etc. Because of the various pore sizes the zeolites act as                                          [Zeolite ZSM 5, also called MFI,
(molecular) sieves and are often referred to by their pore                                          one of the most commercially
diameter e.g. 3A, 4A, 5A would represent zeolites that have                                         important zeolites used for
channels which are 3, 4 and 5 Angstroms (10-10m) in                                                 cracking crude oil to lower
diameter respectively.                                                                              molecular weight compounds
                                                                                                    such as paraffin]


                                                                                                                                     3
Critical Filtration Uses                                            Contaminants Versus MPL Product Activity
Critical filtration may be split into several categories i.e. gas
                                                                    Contaminant            Product             Notes
processing/safety and gas/liquid phase adsorption. In general
the applications fall into three types;To remove contaminants                                          ®
                                                                    Acidic gases           Chemsorb 1202       Used for escape gases.
from gases and non-aqueous liquids; to maintain breathable                                             ®
                                                                                           Chemsorb 16103
atmospheres in closed systems and; to control hazardous or                                             ®

unpleasant emissions from industrial processes.                     Alkaline Gases         Chemsorb 520
                                                                                                       ®
                                                                                           Chemsorb 620
                                                                                                       ®

Industrial Applications                                             Ammonia                Chemsorb 520        Where metal reagents
                                                                                                       ®
                                                                                           Chemsorb 620        cannot be used.
Acid Gas Absorption                                                                                    ®
                                                                                           Chemsorb 1425
VOC Destruction                                                     Arsine                             ®
                                                                                           Chemsorb 16103      Used for escape purposes
Acidic Gas Removal                                                                                     ®
                                                                    Carbon Monoxide        Moleculite
Oxidation of Organics                                                                      Sofnocat™


Nitrogen Removal                                                                                       ®
                                                                    Chlorine               Chemsorb 1202
Ozone Purification Systems                                                                             ®

Trapping of Gases from Electronic                                   Ethane                 Moleculite
                                                                                                       ®
Component Manufacturing                                             Ethylene Oxide         Moleculite
                                                                                                       ®
                                                                    Formaldehyde           Chemsorb 1505
Life Support Applications                                           General                            ®
                                                                                           Chemsorb 1000-70
                                                                                                       ®

Military Breathing Systems                                                                 Chemsorb 1000-85
                                                                                               ®
Carbon Monoxide Removal                                             Halogens               Ionex Type Ag 400
                                                                                               ®

Safe Haven Products                                                                        Ionex Type Ag 900
                                                                                               ®
Purification of Divers' Gases                                       Hydrogen/Oxygen        Ionex Type Ag 400
                                                                    Recombiner
                                                                                               ®
Liquid Phase Adsorption                                             Hydrogen               Ionex Type Ag 400   From vacuum spaces
                                                                                                       ®
                                                                                           Moleculite          in cryogenic systems
Wastewater Treatment                                                                       Sofnocat™


Drinking Water Treatment                                                                       ®
                                                                    Hydrogen Ortho to Para Ionex Type O-P      Liquefaction and storage
Groundwater Remediation                                                                                ®

Dechlorination                                                      Hydrogen Cyanide       Chemsorb 16103      Used for escape purposes
                                                                                                       ®

Landfill Leachate Treatment                                         Hydrogen Sulphide      Chemsorb 1202
                                                                                               ®
Chemical Decolourisation                                            Iodine                 Ionex Type Ag 900   Nuclear Waste Disposal
Solvent Decolourisation                                             Mercaptans             Sofnosiv™



Solvent Purification                                                Mercury                            ®
                                                                                           Chemsorb 1321
Pharmaceutical Purification                                         Methylamine            Chemsorb 620®


                                                                                                       ®
                                                                    Methyl Iodine          Chemsorb 705        Radioactive for nuclear
Vapour Phase Adsorption                                                                                ®
                                                                                           Chemsorb 1321       contamination
HVAC Air Purification                                               Organic Vapours                    ®
                                                                                           Chemsorb 1000-70
VOC Emission Control                                                (see also Volatile                 ®
                                                                                           Chemsorb 1000-85
                                                                                                       ®
Soil Vapour Treatment                                               Organic Compounds)     Moleculite
                                                                                                       ®
Tank Venting                                                                               Chemsorb
                                                                                                   ™
Hydrogen Sulfide Removal                                                                   Sofnocat
                                                                                                       ®
Landfill Bio Gas Treatment                                          Ozone                  Moleculite
Odour Control                                                       Phosphine                          ®
                                                                                           Chemsorb 16103      Used for escape purposes
Air Stripper Exhaust Purification                                   Sulphur                Sofnofil™
                                                                                                               Compounds of Sulphur
Mercury Removal                                                                            Sofnosiv™


                                                                                                       ®
                                                                    Sulphur Dioxide        Chemsorb 1202
                                                                                                       ®
                                                                    Volatile Organic       Chemsorb 1000-70
                                                                    Compounds (see also    Chemsorb 1000-85
                                                                                                       ®
                                                                    Organic Vapours)       Moleculite
                                                                                                       ®
                                                                                           Chemsorb
                                                                                                   ™
                                                                                           Sofnocat
                                                                                                   ™
                                                                    Water                  Sofnosiv

                                                                                                                                         4
MPL product versus contaminant and application

Trade Name          Type of Base          Application                         Phase   Size

Ionex Type Ag 400   Zeolite               Halogens                            Gas     7.4 & 2.2 Angstroms
                                          Hydrogen/Oxygen Recombiner
                                          Hydrogen from vacuum spaces
                                          in cryogenic systems
Ionex Type Ag 900   Zeolite (Mordenite)   Halogens                            Gas     4.0 Angstroms
                                          Iodine (Nuclear Waste Disposal)
Ionex Type O-P      Fe(OH)3               Converts Ortho to Para Hydrogen     Gas     N/A
                                          (Hydrogen Liquefaction & Storage)
Chemsorb 520        Activated Carbon      Alkaline Gas Removal                Gas     1000 m2 per g
                    (coconut shell)       Especially Ammonia
Chemsorb 620        Activated Carbon      Alkaline Gas Removal                Gas     1000 m2 per g
                    (coconut shell)       Especially Ammonia Methylamine
Main Product                              (Breathing Air Respirators)
Chemsorb 705        Activated Carbon      Radioactive Methyl Iodine           Gas     1000 m2 per g
                    (coconut shell)       (Nuclear Contamination)
Chemsorb 1000-70    Activated Carbon      General                             Gas     1200 m2 per g
                    (virgin coconut       Especially Organic Vapours
                    shell char)
Chemsorb 1000-85    Activated Carbon      General                             Gas     1250 - 1400 m2 per g
                    (virgin coconut       Especially Organic Vapours
                    shell char)
Chemsorb 1202       Activated Carbon      Acid Gases                          Gas     1000 m2 per g
                    (coconut shell)       Especially Chlorine
Main Product                              Hydrogen Sulphide
                                          Sulphur Dioxide
Chemsorb 1321       Activated Carbon      Mercury Vapour                      Gas     1000 m2 per g
                    (coconut shell)       Methyl Iodide (radioactive)
Chemsorb 1425       Activated Carbon      Ammonia (where metal reagents       Gas     1200 m2 per g
                    (coconut shell)       cannot be utilized)
Chemsorb 1505       Activated Carbon      Formaldehyde                        Gas     1000 m2 per g
                    (coconut shell)
Chemsorb 16103      Activated Carbon      Acid Gases                          Gas     1000 m2 per g
                    (coconut shell)       Especially Hydrogen Cyanide
                                          Arsine
                                          Phosphine
                                          (used for escape purposes)
Moleculite          Hopcalite             Removal of Carbon Monoxide          Gas
                                          Hydrogen
                                          VOC’s
                                          Ozone
                                          Ethylene Oxide
                                          Ethane
Sofnocarb           Activated Carbon      Removal of Organic Vapours          Gas
                                          & Gases
Sofnocat            Aluminium Beads       Removal of Carbon Monoxide          Gas     >250 m2 per g
                                          Hydrogen
                                          VOC’s
Sofnofil            Aluminium Beads       Removal of Sulphur                  Gas     >200 m2 per g
Sofnosiv            Aluminium Beads       Removal of Water
                                          Mercaptans
                                          Sulphur
                                          Compounds



                                                                                                             5
How to Pick the Right Filter                                       Challenge Concentration ([C]): The concentration of
Identification of Molecular Contamination                          contaminant gas in the air stream to be cleaned, often
                                                                   expressed as mg l-1, ppm, or even ppb.
The type of odour, gas, or vapour to be cleaned must be
identified to begin the filter design process. Bulk-fill filter    Residence Time (RT): The length of time that the
media such as activated carbon or zeolite are known as             contaminated gas is in contact with the adsorbent during
adsorbents, and are primarily used to remove molecular             flow through the filter bed. Residence time can be
contamination of a gas or air stream. Adsorbents are usually       calculated by dividing the bed depth by the linear velocity of
not used to remove solids or particulates - filter media           the air passing through the bed.
for particulates such as non-woven fibres are used to
mechanically filter airborne particles, like a sieve.              RT = Bed Depth/Linear Velocity

Selection of Adsorbent Type                                        Mass Transfer Zone (MTZ) or Critical Bed Depth:
Adsorbents use one of two mechanisms for                           The adsorbent bed depth required to adsorb the
capturing and removing gas-phase molecular contamination           contaminant gas molecules. The MTZ moves through the
- physisorption and chemisorption. Activated carbons               filter bed as contaminants are adsorbed, eventually
and other adsorbents remove contaminant gas                        reaching the filter outlet and causing breakthrough. The
molecules by physisorption, which concentrates the                 filter bed depth must be larger than the MTZ or
contaminant within the pores of the carbon while                   immediate breakthrough will occur. Additional filter depth
allowing clean air to pass through.                                results in longer filter life.

Low molecular weight or polar gases such as hydrogen               Breakthrough: The point at which the MTZ has reached
chlorine, sulphur dioxide, formaldehyde, and ammonia have          the filter outlet and contaminant gases can be detected in
little affinity for physical adsorption forces, and typically      the effluent. For testing filter life, a specific breakthrough
require chemisorption for removal. Activated carbons can           concentration is selected.
be impregnated (treated) with a chemical reagent that reacts
with these types of gases, converting them to solids or salts      Breakthrough Time (BT) or Filter Life: The time required
within the carbon and thereby removing them from an air            for the air passing through a filter to reach a specified
stream. This process typically involves neutralization or          breakthrough concentration when challenged with a
catalysis reactions, and adsorbent capacity is exhausted           contaminated air stream, expressed in time. Filter life cannot
when available impregnant chemicals are consumed or                be precisely defined without actually running laboratory
obstructed from the contaminant molecules. Ion-exchanged           bench tests with the exact adsorbent type used in the actual
molecular sieves or chemically-treated zeolites are also often     application. The test must also be run at the same conditions
used for chemisorption.                                            of humidity, flow, temperature, contaminant concentration,
                                                                   and other air contaminants that may be present.
Other adsorbent properties such as activity, pore size,
moisture content, and particle size affect the performance         Pressure Drop (dP) or Resistance to Flow: The differential
of the adsorbent, and are selected to optimize filter efficiency   pressure as measured upstream and downstream of the
and service life.                                                  filter, often expressed as inches or centimeters of water.
                                                                   Pressure drop across a filter bed depends on the size and
Physical Parameters                                                shape of the filter media particles, the depth of the filter bed,
                                                                   and the total flow through the filter. Often a maximum
Once an adsorbent is selected, the filter dimensions are
                                                                   allowable pressure drop is determined according to fan
determined based on existing system requirements or other
                                                                   specifications or other design restrictions.
design guidelines that depend on the final use of the filter.
Usually design restrictions already exist – for example, an
                                                                   Summary
existing filter tray design will be used, a minimum filter life
is required, or the maximum resistance to flow of the filter       For optimum filter bed dimension design, the filter media
is specified.                                                      should remain in contact with the contaminated air stream
                                                                   long enough to remove the contaminant to desired levels,
Important design parameters include the following:                 but should present a minimum resistance to air flow
                                                                   (pressure drop). Low pressure drop is desirable to minimize
Airflow rate (C): The total volume of contaminated air             the energy and expense of equipment used to move air
passing through the filter bed over a given time, often            through the filter bed, or in the case of a respirator cartridge,
expressed in liters or cubic feet per minute (l min-1 or CFM).     to ensure easy breathing through the filter cartridge.
                                                                   Low pressure drop is more easily achieved using larger

                                                                                                                                  6
cross-sectional area dimensions with minimal bed depth.             Sieve No. 4 6 8 10 12 14 16 18 20 30 40 50
However, bed depth must be large enough to prevent                  mm        4.75 3.35 2.36 2.00 1.70 1.40 1.18 1.00 0.850 0.600 0.425 0.300
premature breakthrough of contaminants through the filter,
so optimizing the dimensional design requires meeting               For example Chemsorb 1220 Moleculite® 814 has a
pressure drop criteria while maximizing residence time.             particle distribution of 0.85 - 1.70mm, and 1.40 - 2.36 mm
                                                                    respectively. The most common sizes used by MPI are 4x8,
When no filter dimensions are defined, as in a new filter           6x12, 8x16, 12x20 and 20x50.
design, the known process conditions are used to assess
filter bed dimensions based on residence time and pressure          Carbon Tetrachloride Activity
drop. As a general guideline, a minimum residence time of           Carbon tetrachloride activity is a means of determining
0.1 – 0.2 sec can be used, and a longer residence is                the degree of completion of the activation process, by
recommended to extend filter life.                                  measuring the ability of a carbon to adsorb an organic
                                                                    molecule (carbon tetrachloride).
The flow rate and the defined residence time can then be
used to assess different dimensional configurations for the         Carbon tetrachloride activity is determined by flowing
filter bed. Given a filter cross-sectional area and the flow rate   carbon tetrachloride-laden air through a dried sample of
of the air stream, the linear velocity of air through the filter    carbon of known weight, until there is no further increase in
can be calculated. The linear velocity can then be multiplied       the weight of the sample. Activity is then expressed as the
by the residence time to calculate the filter depth required        ratio (in percent) of the weight of carbon tetrachloride
to ensure the residence time. Filter depth can then be used         adsorbed to the weight of the original sample.
to predict the pressure drop through the filter, based on the
particle size of the adsorbent, using pressure drop curves.         Moisture Content
Usually, the smallest particle size range that meets the            Moisture content affects the ability of adsorbents to perform
allowable pressure drop is selected, since smaller particles        against certain gases and vapors, and is reported as a percent
have a higher capacity for the contaminants.                        by weight. For organic vapour adsorption on activated
                                                                    carbon, efficiency decreases as moisture content increases,
Physical Properties                                                 since some adsorption sites within the pores are filled with
The critical filtration products come in a variety of shapes,       water. For impregnated carbons, higher moistures generally
sizes and properties as follows:                                    result in an increase in efficiency, since the mechanisms of
                                                                    contaminant removal are chemical reactions that occur in
Mesh Sizes                                                          the reagent solutions contained within the pores.

                                                                    Apparent Density
                                                                    Apparent density is the dry density of a material, usually
                                                                    expressed in grams per millilitre or pounds per cubic foot.
                                                                    Density is essential for mass to volume conversions required
                                                                    for designing and filling equipment such as filter trays,
                                                                    cartridges, and adsorber vessels.
[Left: Granular and pelleted material, Middle - Moleculite 814,
Right - Activated carbon granules]                                  Hardness
                                                                    Hardness is useful for measuring the resistance of an
The known particle size distribution, or mesh, is important for     adsorbent to particle size degradation under service
achieving the proper contact of liquid or gas in a packed bed       conditions. Coconut shell carbons are among the hardest
of adsorbent, and affects the rate of adsorption and                carbons, with hardness numbers greater than 95%.
pressure drop across the bed. Smaller particles are more
efficient for adsorption, but also increase pressure drop, so
the choice of particle size must optimize adsorption efficiency
without exceeding acceptable pressure drop specifications.




                                                                                                                                           7
Glossary
Acidic Gases - Gases which have a pH value less than 7.           Methylamine - A toxic flammable gas, CH3NH2, produced by
This includes hydrochloric acid, hydrofluoric acid,               the decomposition of organic matter and synthesised for use
hydrobromic acid, chlorine, bromine, fluorine, sulphur            as a solvent and in the manufacture of many products, such
dioxide, hydrogen sulphide, hydrogen cyanide, chlorine            as dyes and insecticides.
dioxide and nitrogen oxides.
                                                                  Methyl Iodide - Methyl iodide is used as an intermediate in
Alkaline Gases - Gases which have a pH value more than 7.         the manufacture of some pharmaceuticals and pesticides,
This includes ammonia, phosphine and methylamine.                 in methylation processes, and in the field of microscopy.
                                                                  In humans, acute (short-term) exposure to methyl iodide by
Ammonia - A colorless, pungent gas, NH3, extensively used         inhalation may depress the central nervous system (CNS),
to manufacture fertilizers and a wide variety of                  irritate the lungs and skin, and affect the kidneys. Massive
nitrogen-containing organic and inorganic chemicals.              acute inhalation exposure to methyl iodide has led to
                                                                  pulmonary edema. Acute inhalation exposure of humans to
Angstrom - Unit of measurement for length suitable for            methyl iodide has resulted in nausea, vomiting, vertigo, ataxia,
atomic distances and wavelengths . Actually equal to 10-10m       slurred speech, drowsiness, skin blistering, and eye irritation.
but also defined in terms of the wavelength of the red line       Chronic (long-term) exposure of humans to methyl iodide
of cadmium. Symbol Å. For example a carbon-carbon                 by inhalation may affect the CNS and cause skin burns.
bond distance is 1.5Å.
                                                                  NOx - A generic term for the various nitrogen oxides
Arsine - Chemical formula AsH3, arsine is a highly toxic gas      (NO, NO2, NO3) produced during combustion. They are
with a garlic like odour. Arsine is so toxic that is has been     believed to aggravate asthmatic conditions, react with the
used as a chemical warfare agent. It is used as a dopant gas      oxygen in the air to produce ozone, which is also an irritant
during various chemical vapour deposition reactions.              and eventually form nitric acid when dissolved in water.
                                                                  When dissolved in atmospheric moisture the result can
Chemisorption - A process, related to adsorption, in which        be acid rain which can damage both trees and entire
atoms or molecules of reacting substances are held to the         forest ecosystems.
surface atoms of a catalyst by electrostatic forces having
about the same strength as chemical bonds. Chemisorption          In an internal combustion engine, a mixture of air and fuel
differs from adsorption chiefly in the strength of the            is burned. The combustion temperatures reach a high
bonding, which is much stronger in chemisorption.                 enough level to actually burn some of the nitrogen in the
                                                                  air, yielding various oxides of nitrogen. This is the main
Ethylene Oxide - A colorless, flammable, toxic, slightly sweet    component of smog.
smelling compound used especially in synthesis of other
chemicals (primarily ethylene glycol), and in sterilization       Ortho Hydrogen - Hydrogen has just 1 proton and
(of foodstuffs) and fumigation. Chemical formula, C2H4O.          therefore molecular hydrogen (H2) has two protons.
                                                                  Due to magnetic interactions the protons may either spin
Formaldehyde - A colorless gaseous compound, HCHO, the            so both electrons are aligned/parallel (ortho) or not
simplest aldehyde, used for manufacturing melamine and            aligned/anti-parallel (para). In this context para is from the
phenolic resins, fertilizers, dyes, and embalming fluids and in   Greek meaning against not parallel. Under standard
aqueous solution as a preservative and disinfectant.              conditions hydrogen is composed of about 25% of the para
                                                                  form and 75% of the ortho form (the so-called ‘normal’
Halogens - The group VII elements of the periodic table           form).The equilibrium ratio of these two forms depends on
i.e. fluorine, chlorine, bromine, iodine, and astatine            temperature, but since the ortho form has higher energy
                                                                  (is an excited state), it cannot be stable in its pure form.
HVAC - A system that provides heating, ventilating, and/or        In low temperatures (around boiling point), the equilibrium
cooling within or associated with a building.                     state is comprised almost entirely of the para form.

Mercaptans - Old name for any class of organic compounds          The conversion process between the forms is slow, and
in which the oxygen of an alcohol has been replaced by            if hydrogen is cooled down and condensed rapidly, it
sulphur and which have distinctive, often disagreeable,           contains large quantities of the ortho form. It is important in
odours (similar to garlic). Now called thiols. For example        preparation and storage of liquid hydrogen since the
CH3OH (methanol) becomes CH3SH (methanethiol or                   ortho-para conversion produces more heat than the heat of
methyl mercaptan)                                                 its evaporation and a lot of hydrogen can be lost by
                                                                  evaporation in this way during several days after liquefying.

                                                                                                                                8
Therefore, some catalysts of the ortho-para conversion               Phosphine - A colorless, spontaneously flammable poisonous
process are used during hydrogen cooling. The two forms              gas, PH3, having a fishy odour and used as a doping agent for
have also slightly different physical properties. For example,       solid-state components.
the melting and boiling points of para hydrogen are about 0.1
K lower than of the ‘normal’ form.                                   Physisorption - Physisorption is a process whereby a
                                                                     molecule adheres to a surface without the formation of a
Ozone - A form of oxygen, O3 (as opposed to O2).                     chemical bond, usually by van der Waals forces or
A powerful oxidizing agent that is considered a pollutant in         electrostatic attraction. The formation of a chemical bond
the lower troposphere (occurs as smog) but an essential              leads to chemisorption.
chemical in the stratosphere where it protects the earth
from high-energy ultraviolet radiation from the sun.                 VOC - Any organic compound which evaporates readily to
                                                                     the atmosphere. VOCs contribute significantly to
Para Hydrogen - Hydrogen has just 1 protons and                      photochemical smog production and certain health
therefore molecular hydrogen (H2) has two protons.                   problems. They produce noxious fumes and are found
Due to magnetic interactions the protons may either spin so          in many paints, caulks, stains, and adhesives.
both electrons are aligned/parallel (ortho) or not
aligned/anti-parallel (para). ). In this context para is from the    Summary
Greek meaning against not parallel. Under standard                   Molecular Products supplies a range of compounds for
conditions hydrogen is composed of about 25% of the para             the catalytic oxidation, absorption or absorption of gaseous
form and 75% of the ortho form (the so-called ‘normal’               contaminants.
form).The equilibrium ratio of these two forms depends on
temperature, but since the ortho form has higher energy              The range of products includes Moleculite®, Chemsorb®,
(is an excited state), it cannot be stable in its pure form.         Ionex®, Sofnocat™, Sofnofil™ and Sofnosiv™.
In low temperatures (around boiling point), the equilibrium
state is comprised almost entirely of the para form.                 Compounds removed include acid and alkaline gases,
                                                                     ammonia, arsine, carbon monoxide, chlorine, ethane,
The conversion process between the forms is slow, and if             ethylene oxide, formaldehyde, halogens, hydrogen
hydrogen is cooled down and condensed rapidly, it contains           compounds, iodide, mercaptans, mercury, NOx, VOCs,
large quantities of the ortho form. It is important in               ozone, phosphine, sulphur compounds and water.
preparation and storage of liquid hydrogen since the
ortho-para conversion produces more heat than the heat of            Key processes which require critical filtration products
its evaporation and a lot of hydrogen can be lost by                 include air purification, catalyst support, decolourisation,
evaporation in this way during several days after liquefying.        deodourisation, gold/metal recovery, liquid purification,
Therefore, some catalysts of the ortho-para conversion               emergency poison treatment, solvent recovery, sugar refining
process are used during hydrogen cooling. The two forms              and whisky manufacturing.
have also slightly different physical properties. For example,
the melting and boiling points of para hydrogen are about 0.1        When designing a filter system the contaminant, air flow
K lower than of the ‘normal’ form.                                   rate, challenge concentration, residence time, bed depth,
                                                                     breakthrough, and pressure drop all need to be considered.




Molecular Products Ltd                                                                                                                 9
Mill End,Thaxted, Essex       T +44 (0)1371 830676          E sales@molprod.com
CM6 2LT, United Kingdom       F +44 (0)1371 830998          W www.molecularproducts.com                         Version 2, 5/08/09 MCL, JS

								
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