Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to the removal of FormAldehyde tJas
from air and other carrier gases.
Formaldehyde is used on a large scale in the chemical indus-
try and it is incorporated in many resin products which are extensively
used by the building industry and the public. Formerly it had been
considered harmless, however recently it was classified as a toxic
substance and a potencial carcinogen. A need has arisen for improved
protection of occupational personnel and members of the public,
exposed to airborne formaldehyde.
Commonly used methods for gaseous formaldehyde collection or
removal are based on its scrubbing with water or solutions. However
this process introduces moisture into the purified gas and hazardous
or undesirable secondary products, such as formic acid, may be formed
and released into the purified gas. ~his process also produces liquid
chemical waste, safe disposal of which is difficult and expensive.
Scrubbing systems are generally expensive and bulky, their maintenance
and operational cost are relatively high.
The physical adsorption process is not sufficiently effective for form-
aldehyde removal because several variable factors, such as temperature,
relative humidity, gas impurities and gas velocity, have significant
ef~ect on ~ormaldehyde adsorption equilibrium. Most adsorbents have low
capacj~y for formaldehyde adsorption under normal conditions and its
retention is not adequate because adsorption is a reversible process.
Positive change in the above parameters would result in significant
desorption of previously collected formaldehyde from the adsorbent.
A method has been patented (Canadian Patent No. 821.316) in which a
mixture of salts of sulphurous acid, a hydrophylic subs~ance and a hume-
ctant, formed with a large surface area, is used to remove formaldehyde
odor from air. No inFormation has been found in the specification on
the involved chemical reactions, on the efficiency and capacity of this
absorber for gaseous formaldehyde removal and on the volatility and
toxicity of reaction products. It can be expected that the absorbent
performance depends on the temperature and moisture of the air being
``~ purified and that under conditions of high humidity the hygroscopic
action of the collecting agents cause it to become water logged.
The method of recovering commercially usable formaldehyde from water-
methanol-formaldehyde vapours in Canadian Patent No. 6~3 062, is based
on scrubbing gaseous formaldehyde with an alkaline urea solution,
circulating through an absorbing tower at temperature of 25 to 100 C.
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Activated charcoal and silica gel were also used to collect reversibly
formaldehyde from air samples, to increase its concentration for analy-
tical purposes.
The disadvantages of the above methods may be eliminated by
absorbing formaldehyde gas with solid nonvolatile rnaterials, which form
stable~ nonvolatile reaction products. With properly selected absorbent
composition the adverse effect of the variability of operational parame-
ters can be minimized and the formation of harmfull products can be
eliminated.
The absorbers can be made as either active or passive systems:
In the active system gaseous formaldehyde is removed from the purified
gas, which is forced through the absorbent by a pressure difference
accross the bed. The absorption efficiency is proportional to the rate
of formaldehyde reaction with the absorbent components and to the resi
dence time of formaldehyde molecules in the absorber bed. With little
space requirements and low operational cost, the absorbers can purify
large volumes of gas from formaldehyde. Used absorbers can safely and
inexpensively be disposed, i. e. buried in ground or incinerated.
In the passive systems the formaldehyde absorbent is dispersed or
contained in a film which is applied on walls and other surfaces. No
gas mover is present in the passive system, the transport mechanism is
based on the diffusion of formaldehyde molecules and movement of the
carrier gas, e. g. air currents or turbulence in a room. This system
can be used effectively for gaseous formaldehyde removal from residen-
tial and working areas where large surfaces can be coated with the
absorbent film.
The following major aspects have to be considered in air
and gas purification applications:
- high absorption efficiency and capacity of the absorber for
gaseous formaldehyde under expected operational conditions
(temperature, pressure, gas velocity, etc.)
- low chemical affinity of absorbent components to the carrier
gas components and moisture,
- minimal volatility and toxicity of the absorbent components,
- minimal volatility and toxicity of the products of formaldehyde
, reaction with the absorbent components,
- low production, operational and maintenance costs.
Generally, high absorption efficiency is obtained by using absorbents
which, in solid form, have high chemical affinity to gaseous formalde-
hyde under normal conditions. Significant improvement in removalef~iciency and retention can be achieved by a process which combines
;; physical adsorption and chem;sorption. The physical adsorption extends
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the residence time and enables the chemical reactions to achieve equili-
brium. Catalysis can efficiently improve both the reaction rate and the
formaldehyde absorption ca~acity. Finaly, formaldehyde polymerization,
which also can be accelerated with physical adsorption and catalysis,
may further contribute to the process of formaldehyde fixation~
Many organic nitrogen compou~ds, contained in the following
groups, are known to react, mostly as aqueous solutions, with formalde-
hyde when reaction conditions are optimal:
Aliphatic and Aromatic ~nines, Amides, Diamides of Organic Acids, Imides,
Carbamates, Thioamides, Aminonitriles, Aminoacids and Prote;n;.
The suitability of the above compounds for gaseous formaldehyce removal
was preliminarily evaluated from published data. A brief summary follows:
~ormaidehyde readily reacts with primary amines and, at lower rates, with
secondary amines. Generally aromatic amines are less volatile than
aliphatic amines.
Formaldehyde reacts with solutions of some amides, e.g. urea, at room
temperature. Most of the reaction products are nonvolatile and more
stable than those produced from amines.
Cyanamide (melamine) has high capacity for formaldehyde absorption.
The reaction product is nonvolatile and stable, however the reaction
is slow under normal conditions.
The reactions of formaldehyde with diamides of organic acids and
imides are also relatively slow under normal conditions.
Carbamates and thiourea react with formaldehyde at room temperature,
however the products are not suffic;ently stable at sliyhtly elevated
temperatures.
Proteins have good chemical affinity for formaldehyde which readily
reacts with the ~ree amino groups~ forming methy1ene amine linkages, and
it may also involve the production of diformals. Chemically stable. non-
volatile products, resulting from these reactions, have increased molecu-
lar weight and improved resistance to alkalies and water at room tempera-
ture. The rate of formaldehyde reaction with most proteins increases
with increasing temperature and w;th the pH of the protein solutions.
Reactivity with formaldehyde varies for different proteins. The proteins
most commonly used in the production of formaldehyde modified products,
are suitable for gaseous formaldehyde removal, e. 9. gelatine, casein,
soyabean protein, vegetable proteins, keratin and glue.
Gelatine has good chemical affinity and good capacity for formaldehyde
absorption. Gaseous formaldehyde reacts with gelatine in dry formS with
a gel and most rapidly with solutions. However the reaction products
release formaldehyde ;n hot water and decompose in hydrochloric acid.
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The reactions of the above compounds with formaldehyde are ionic. Some
of them are sufficiently rapid in aqueous solutions however the reacti-
ons of solid compounds with folrmaldehyde ga's are typically much slower.
Because of their ionic character, the reactions can be catalysed with
both the hydroxyl and hydronium ions. However, for this application the
optimal pH ;s within the range of 7 - 14, because some reaction products
are not sufficiently stable in acidic conditions. Where alkaline pH is
undesirable, other known catalysts such as salts of sulphurous and halo-
gen acids can be used to accelerate the reactions of formaldehyde with
solid organic nitr~gen compounds. Alkali metal salts of ~he above acids
can also be applied as catalysts, additional to hydroxyl ions in the
m;xture of reactive components.
The reactions of formaldehyde gas with several organic nitrogen
compounds were e~perimentally investigated. The results confirmed that
the following compounds, in solid form, can react with formaldehyde gas
at room temperature:
amines (e.g. aniline, benzidine, aminopyrimidine, toluenediamine,
triethylenediamjne, diphenylamine, diaminodiphenylamine, etc~)
~ proteins (e.g. gelatine, soyabean protein, casein~ etc.)
- and also, at lower reaction rates, amides (e.g. ben~amide, melamine
urea, acetamide, etc.)
Their efficiencies for gaseous formaldehyde chemisorption were improved
with the assistance of one or more of physical adsorption, hydroxyl ions
and/or other catalysts. It was also found that the addition of small
quant;ty of triethylenediamine accelera~ed the reaction of formaldehyde
gas with several tested amines, amides and proteins more efficiently
than similarly alkaline pH from inorganic hydroxides. Tertiary amines,
which generally haYe alkaline character, apparently engage b~th the
hydroxyl ions and the tertiary amino group in their catalytic action,
therefore they qualify as convenient catalysts for this purpose.
For specific use requirements the above absorbent components
can be arranged in several combinations and physical configurations:
In active absorbe~ systems a large surface to volume ratio is
required to provide for the maximal contact of the absorbent with the
purified gas, sufficiently long reaction time and low pressure drop
across the absorber bed. The three principles, on which the active
absorber systems in this invention are based, are as follows:
- The absorbent, which is composed of one or more of amine(s3, amide(s),
protein(s), alkali(es) and/or other catalyst(s~, is incorporated
into an adsorbent which has the function of supportinq material and
the adsorption process also provides for longer residence time to
enable more efficient reaction of formaldehyde with the absorbent.
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- The absorbent consists of protein(s) or its mixture with one or more
of amine(s), amide(s), alkali(s) and/or other catalyst. The protein(s)
perform as formaldehyde absorbent and bind the reac~ive and catalysing
components to screens, fibres, sponge or other structure with large
surFace and low flow resistance.
- Protein(s), or their mixture with one or more of amine(s), amide(s),
alkali(s) and/or other catalyst, are selfstructured as fibres, screens,
open cell foam or sponge. This arrangement eliminates the need for
supporting materials.
- Wool, eventually impregnated with one or more of amine(s), amide(s),
additional protein(s), alkali~s) and/or other catalyst, is a typical
example of natural protein fibres which can be -Formed as a suitable
active system for formaldehyde removal. Artificial protein fibres,
produced by spinning protein solutions (casein, soyabean protein or
other protein materials) and partially hardened, can form an efficient
system for formaldehyde absorption. A mixture of natural and synthe-
tic protein fibres can also be used in some applications. Loose
fibres can be formed into a required filter shape or can be woven to
form a screen or fabric. The protein(s) can also be solidified as a
sponge or open cell foam for this purpose.
The active absorber systems are suitable for occupational respiratory
protection, atmosphere purification in worl(ing and living areas, formal-
dehyde absorption from process gases and for formaldehyde recovery.
In passive absorption systems the absorbent material is disper-
sed on surfaces. In the majority of practical applicatiorls it is applied
as a film to internal surfaces and walls of areas and systems from which
gaseous formaldehyde is to be removed.
The ~ollowing principles are applicable:
- Any of the active absorbent materials, dispersed on surfaces, can
perform as a passive system,
- The absorbent film can be formed of solid or gelatized protein(s)
which can contain one or more of amine(s), amide(s), alkali(s) and/or
other catalyst and fungicide.
- The film is formed of a binder, such as latex paint, the constituents
of which do not form any volatile or toxic reaction products with
formaldehyde and are inert to all reacti~e, pH conditioning and
catalysing components o~ the absorbent. Further, the binder is
inert to all carrier gas components.
The passive absorption systems are particularly suitable for formaldehyde
remoYal from operational and residential areas where the source is
releasing formaldehyde at relatively constant rate. Then the equilibrium
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airborne concentrations of formaldehyde are proportional to the rate of
its release and inversely proportional to the chemical affinity o~ the
absorbent to formaldehyde, the area coated with the absorbent film, air
temperature and the degree of air turbulence in the area.
Practical examples of two active absorbents and one passive
absorbent for gaseous formaldehyde follow:
Example 1 - Impregnated Adsorbent
An adsorbent, e.g. activated alumina, molecular sieve, charcoal, etc~
is impregnated with a solution containing one or more of amine(s),amide(s)
protein(s), alkali~s) and/or other catalyst. The loading of the impreg-
nating components is optimized for formaldehyde absorption capacity while
sufficient remaining surface activity of the adsorbent is provided.
An absorbent, identified as having a high performance, was made of acti-
Yated charcoal, impregnated with 5 % urea. This absorbent, filled in a
40 mm deep bed, removed , 90 % of gaseous formaldehyde from air within
the concentration range of 0.1 - 10 ppm, air flow of 300 m3-hr l-m 2
~face velocity of 5 m-min 1) and ~ 90 % relative humidity. The flow
resistance of this absorber was < 25 mm w.g. The efficiency of the
absorbent significantly increased under low relative humidity conditions.
Further improvement in its formaldehyde sorption efficiency was achieved
by adding a trace of triethylenediamine catalyst to the reactive
components mixture.
Both the formaldehyde removal efficiency and the flow capacity of the
absorption system can be improved by increasing the absorber depth, when
slightly higher flow resistance is acceptable.
All components of this absorbent, as well as the products of their reac-
tion with formaldehyde, are nonvolatile and nontoxic therefore the absor-
bent can safely be used for the purification of respiratory air.
This absorbent also can remove some other air contaminants, such as
acidic gases (S02, N0x, etc), however they somewhat reduce its efficiency
for~formaldehyde absorption.
Example 2 ~ Supported Absorbent
A suitable supporting fibre, screen or spongy material, e.g. cellulose,
cotton, plastic, metal, etc, is coated (by soaking, spraying, etc) w;th
a solution of protein(s) or its mixture with amine~s) and/or amide(s).
alkali(s) and/or other catalyst can be added to the viscous solution to
maximize the rate of dried components reaction with gaseous formaldehyde
and fungicide added to improve the long term stability of the absorbent.
An absorber of this type was made by soaking a cotton screen in hot
solution of gelatine and 2,4 Toluendiamine, containing a trace of Tri-
ethylenediamine as a catalyst. Its formaldehyde removal efficiency
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was higher than 90 % at room temperature, gelatine loading of 0.1 9
per 1 cm2 of the absorbent and an air flow 150 m3-hr~l-m-2.
The absorbent performance was not significantly influenced by humidity
conditions. The absorbent is reasonably stable when used, or stored
under normal conditions. No growth of bacteria or fungus occured in
nonstabilized absorbent which was exposed to air for six months at 25C
and 60 % relative humidity. For the use under high humidity conditions
its stability should be increased by adding a fungicide. The constitu-
ents as well as the products of their reaction with formaldehyde are
essentially nonvolatile, therefore this absorbent can be used in air
cleaning applications. The supported absorbent can be manufactured with
large surface to volume ratio which presents negligible pressure drop,
therefore a common low pressure air blower can provide sufficiently
high purification flow through a deep absorber of this type.
Example 3 - Absorber Film
An emulsion of a suitable coating material is homogenized with one or
more of amine(s), amide(s), protein(s), alkaline component and/or other
catalyst. In the emulsion the film forming components should permit
diffusion of formaldehyde to the absorbing constituents, in order to
provide for proper absorption capacity. Some paints already contain
amines as a stabiliser, however they were never intended for formalde-
hyde removal. Therefore they generally do not comply with the require-
ments, previously specified for air and gas purification.j
passive absorbent of this type was prepared by homogen;zing a common
commercial latex paint with 5 % of 2,aminopyrimidine and a trace of tri-
ethylenediamine. No visible change in the latex emulsion, no coagula-
tion, separation or o~her change occured during several months of storage.
The modified paint was also compatible with all tested pigments. The
appearance of the modified paint, after drying, remained the same as the
original paint and its abrasion resistance was not reduced. In a dynamic
efficiency test a foil of this absorbent removed more than 50 % of
airborne formaldehyde from passing air at 5 s. residence time. In an
exicator test the equilibrium concentration of airborne formaldehyde
from 0.05 % solution, was reduced by the factor of ~. The film of the
modified paint was exposed to 0.5 ppm of airborne formaldehyde for six
months period, after which no significant change in its efficiency was
found.
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A number of absorbents, similar to the above examples, were
prepared using several other amines, amides and proteins, catalysed
with alkali, tertiary amine and other above mentioned catalysts. Such
combinations were found with most of the tested compounds, which formed
a solid absorbent suitable for formaldehyde removal. The results of
their experimental evaluation substantiated the claims, that the inven-
tion is applicable for the use of one or more of amine(s), amide(s),
protein(s), or their mixture with akali(s) and/or other catalyst(s),
in the described physical configurations, as absorbents for gaseous
formaldehyde. The selection of optimal components and physical confi-
gurations will depend on specific performance requirements and operati-
onal conditions in individual practical applications.
The details in the above examples have been given only for
i11ustration. It is not intended to limit this invention by the
examples, description, or other details in the above disclosure.
