U.S. patent application number 12/724840 was filed with the patent office on 2010-07-08 for system for scrubbing alkyl halides from gases.
This patent application is currently assigned to Value Recovery, Inc.. Invention is credited to Roman Bielski, Peter J. Joyce.
Application Number | 20100172807 12/724840 |
Document ID | / |
Family ID | 36206391 |
Filed Date | 2010-07-08 |
United States Patent
Application |
20100172807 |
Kind Code |
A1 |
Joyce; Peter J. ; et
al. |
July 8, 2010 |
SYSTEM FOR SCRUBBING ALKYL HALIDES FROM GASES
Abstract
Alkyl halides can be effectively scrubbed from gas streams
containing them by passing the gas streams through dispersers
having a very small pore size into an aqueous scrubbing liquid
containing a nucleophile such as thiosulfate. Optionally, the
addition of a water-soluble organic compound to the scrubbing
liquid may increase the rate of destruction of the alkyl halide.
Alkyl halides such as methyl bromide can be effectively scrubbed
from enclosed spaces containing the halide, for example in
enclosures used for fumigation.
Inventors: |
Joyce; Peter J.; (Mullica
Hill, NJ) ; Bielski; Roman; (Coopersburg,
PA) |
Correspondence
Address: |
RATNERPRESTIA
P.O. BOX 980
VALLEY FORGE
PA
19482
US
|
Assignee: |
Value Recovery, Inc.
Bridgeport
NJ
|
Family ID: |
36206391 |
Appl. No.: |
12/724840 |
Filed: |
March 16, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10972875 |
Oct 25, 2004 |
7678353 |
|
|
12724840 |
|
|
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Current U.S.
Class: |
422/168 |
Current CPC
Class: |
B01D 2257/2062 20130101;
B01D 53/70 20130101; B01J 19/24 20130101; B01J 4/001 20130101; B01D
2251/21 20130101 |
Class at
Publication: |
422/168 |
International
Class: |
B01D 53/34 20060101
B01D053/34 |
Claims
1. An apparatus for removing an alkyl halide from a gas volume, the
apparatus comprising: a reactor assembly comprising a reaction
vessel containing a single continuous liquid phase comprising water
and a nucleophile, and one or more gas dispersers having therein a
plurality of holes, the holes being between 1 and 200 .mu.m across
at a widest part thereof, said one or more dispersers immersed in
the liquid phase.
2. The apparatus of claim 1, wherein a majority of the holes are
between 1 and 200 .mu.m across at a widest part thereof.
3. The apparatus of claim 1, wherein a distance from an upper end
of the one or more dispersers to an upper surface of the liquid
phase is between 6 and 120 inches.
4. The apparatus of claim 1, wherein a distance from an upper end
of the one or more dispersers to an upper surface of the liquid
phase is between 12 and 36 inches.
5. The apparatus of claim 1, wherein the holes are between 10 and
50 .mu.m across at a widest part thereof.
6. The apparatus of claim 1, wherein the nucleophile comprises
thiosulfate.
7. The apparatus of claim 1, wherein the nucleophile is selected
from the group consisting of aliphatic thiols, aromatic thiols,
salts of aliphatic thiols, salts of aromatic thiols, aliphatic
disulfides, aromatic disulfides, aliphatic polysulfides, aromatic
polysulfides, sulfide anion, bisulfide anion, thiosulfate anion,
sulfite anion, bisulfite anion, and thiocyanate anion.
8. The apparatus of claim 1, wherein the single continuous aqueous
phase is essentially free of suspended undissolved material.
9. The apparatus of claim 1, wherein the aqueous phase further
comprises between 1 and 99 wt % of an organic compound dissolved
therein.
10. The apparatus of claim 1, wherein the aqueous phase further
comprises between 1 and 25 wt % of an organic compound dissolved
therein.
11. The apparatus of claim 9, wherein the organic compound does not
boil below 125.degree. C.
12. The apparatus of claim 9, wherein the organic compound is a
compound according to the formula H--(OCH.sub.2CHR).sub.n--OH,
wherein n is an integer from 1 to 20 and R is H or CH.sub.3.
13. The apparatus of claim 12, wherein R is H.
14. The apparatus of claim 9, wherein the organic compound is
tetraethylene glycol.
15. The apparatus of claim 9, wherein the organic compound is
dimethyl formamide, N-methylpyrrolidone, or dimethyl sulfoxide.
16. The apparatus of claim 1, comprising a plurality of reactor
assemblies connected in series.
17. The apparatus of claim 1, comprising a plurality of reactor
assemblies connected in parallel.
18. The apparatus of claim 1, wherein a distance from an upper end
of the one or more dispersers to an upper surface of the liquid
phase is between 12 and 36 inches, the holes are between 10 and 50
.mu.m across, the nucleophile comprises thiosulfate, and the
organic compound is a compound according to the formula
H--(OCH.sub.2CHR).sub.n--OH, wherein n is an integer from 1 to 20
and R is H or CH.sub.3
19-36. (canceled)
Description
FIELD OF THE INVENTION
[0001] This invention relates to destruction of pollutants in a
gas. More particularly, this invention relates to removal and
destruction of alkyl halides from a gas.
BACKGROUND OF THE INVENTION
[0002] Many alkyl halides possess a degree of toxicity, sometimes
very high toxicity. For example, the toxicity of methyl bromide is
so great that it has been used for many years in the extermination
of insects in mills, warehouses, vaults, ships, freight cars, and
also as a soil fumigant for use by growers of strawberries,
tomatoes, and other crops. Other applications include treatment of
ships to remove rodents and treatment of foods such as fruits
including dried fruits, grain, flour, nuts, and tobacco products to
remove potential pests. Additionally, methyl bromide has been
successful in fumigation against various microorganisms including
fungi and bacteria. Recently, it has been advocated as the most
effective agent against anthrax (Bacillus anthracis). Its virtues
include the fact that it is not explosive, practically
nonflammable, has very high diffusivity and permeability, and has
been used safely for over 60 years.
[0003] Unfortunately, release of methyl bromide into the atmosphere
is generally accepted to cause ozone layer depletion that can
result in increased incidences of skin cancer. Thus, there is a
need for methods of disposing of methyl bromide without releasing
it to the atmosphere. In addition, there is a more general need for
methods of rapidly and economically removing volatile alkyl halides
such as methyl bromide from streams such as air or petroleum
vapors.
[0004] Prior art methods for removal of alkyl halides can be
divided into nondestructive and destructive ones. Most such methods
in either category are not satisfactory for rapid removal of alkyl
halides from gaseous streams and/or air volumes.
Non-Destructive Methods of Alkyl Halide Removal
[0005] Non-destructive methods of alkyl halide removal typically
involve either scrubbing by dissolution of the halide in an
appropriate solvent, or by adsorption onto a suitable adsorbent.
Scrubbing solvents that work by dissolution of the alkyl halide
include for example vegetable oil and marine oil (Canadian Patent
1,282,317). Examples of applicable adsorbing agents include
zeolites (U.S. Pat. No. 4,309,281), and activated carbon (J. G.
Leesch, G. Knapp, B. E. Mackey, Methyl Bromide Adsorption on
Activated Carbon to Control Emissions from Commodity Fumigations,
www.nal.usda.gov/ttic/tektran/data/000008/58/0000085839.html, also:
J. G. Leesch, G. Knapp, B. E. Mackey, J. Stored Prod. Res., 36, 65,
2000). Methods based on the use of adsorbents or scrubbing solvents
suffer from non-discriminatory scrubbing of other volatile
compounds in the gas streams, resulting in the formation of
unwanted degradation products during regeneration of the adsorbent
or scrubbing solvent.
Destructive Methods of Alkyl Halide Removal
[0006] Japanese Kokai JP 49-127,862 discloses a method involving a
reaction of methyl bromide in isopropanol with ethanolamine
dissolved in water. The method takes advantage of a nucleophilic
substitution reaction that is, however, not sufficiently rapid for
a variety of applications. Another method employs bioreactors and
specific species of a-Proteobacteria that can directly oxidize and
grow on methyl halides (L G. Miller, S. M. Baesman, R. S. Oremland,
Use of Bioreactors to Remove Methyl Bromide Following Contained
Fumigations, Proceedings of 2002 Annual International Research
Conference on Methyl Bromide Alternatives and Emissions Reductions;
also: L. G. Miller, S. M. Baesman, R. S. Oremland, Bioreactors for
Removing Methyl Bromide following Contained Fumigations, Environ.
Sci. Technol., 37, 1698, 2003). Yet another destructive method (F.
G. Belmonte, K. J. Abrams, J. P. Oppenheim, U.S. Pat. No. 6,207,120
B1; Mar. 27, 2001) proposes to heat and mix a vent gas containing
an alkyl halide with a combustible fluid, followed by catalytic
oxidation of the mixture. In U.S. Pat. No. 5,904,909, Yates
discloses a method of destroying methyl bromide, for example from a
fumigation gas stream, by adsorption or retention of that compound
on a substrate such as carbon, followed by destruction of the
adsorbed methyl bromide by treating it with thiosulfate. They state
"if methyl bromide gas is merely bubbled through an aqueous
solution of thiosulfate, decomposition of methyl bromide is very
slow and incomplete (about 20-30% decomposition after 2
hours)."
[0007] Despite these advances, there continues to be a need for
rapid and efficient means for removing alkyl halides at low and
moderate concentrations from gas streams.
SUMMARY OF THE INVENTION
[0008] In one aspect, the invention provides an apparatus for
removing an alkyl halide from a gas volume. The apparatus includes
a reactor assembly having a reaction vessel containing a single
continuous liquid phase containing water and a nucleophile, and one
or more gas dispersers having therein a plurality of holes. The
holes are between 1 and 200 .mu.m across at a widest part thereof,
and the one or more dispersers are immersed in the liquid
phase.
[0009] In another aspect, the invention provides a method of
removing an alkyl halide from a gas volume. The method
includes:
[0010] withdrawing a gas stream from the gas volume;
[0011] passing the gas stream through one or more gas dispersers
having therein a plurality of holes, the holes being between 1 and
200 .mu.m across at a widest part thereof, thereby producing gas
bubbles; and
[0012] passing the bubbles through a continuous liquid phase
contained in a reaction vessel, the liquid phase containing water
and a nucleophile, to produce a purified gas stream.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic illustration of a gas scrubbing system
according to one exemplary embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The present invention provides a method of permanently
removing alkyl halides from a gas stream or gas volume, for example
an air stream or air volume. As used herein, the terms "gas volume"
and "air volume" refer to any body of gas or air, respectively,
whether contained or not contained. According to the invention,
very fine bubbles of gas are passed through an aqueous solution of
a nucleophile, optionally containing an organic compound, which has
been found to increase the effectiveness of the scrubber. The fine
bubbles rise slowly through the aqueous solution, during which time
the alkyl halide is converted to relatively nonvolatile materials,
which may then be collected for use or merely disposed of. Methods
and devices according to the present invention may be applied on
either a batch or continuous basis.
[0015] The invention will next be illustrated with reference to the
figures, wherein the same numbers indicate the same elements in all
figures. Such figures are intended to be illustrative rather than
limiting and are included herewith to facilitate the explanation of
the present invention. The figures are not to scale, and are not
intended to serve as engineering drawings.
[0016] Attention is now drawn to FIG. 1, which shows a gas
scrubbing system according to one exemplary embodiment of the
invention. The gas scrubbing system, indicated generally at 10,
comprises a gas volume 11 containing a gas having some amount
therein of an alkyl halide, typically in the vapor phase but
possibly as droplets or as part of other suspended particles. Gas
volume 11 may be an unconfined region of gas, or it may be a gas
contained within a house or other habitable structure, a fumigation
chamber, or within a temporary enclosure such as may be constructed
from tarpaulins covering an area of crops or the like. Gas volume
11 may also be in a tank or similar gas-handling container.
[0017] A gas stream 12 is pulled from the gas volume by pump 14
through gas stream feed tube 13, and delivered to reactor assembly
16. Although a pump is shown for item 14, a fan or other air-moving
device may be used. Also, while the embodiment shown in FIG. 1
shows pump 14 being situated upstream of reactor assembly 16, the
placement of the pump is not an important design feature, and the
pump could be downstream of the reactor assembly. A purified gas
stream 18 exits from reactor assembly 16 through purified gas
stream return tube 19. By appropriate use of valves 21 and 23,
purified gas stream 18 may be recycled back to gas volume 11, or it
may be delivered to vent 25, which may lead to the atmosphere or
into a product tank or other enclosure, or into another scrubber.
By recycling purified gas stream 18 back to gas volume 11, a higher
level of alkyl halide removal may be obtained. Although a single
reactor assembly 16 is shown in FIG. 1, two or more may be used,
and they may be connected in parallel and/or in series (not shown).
If two or more reactors are used, it may be desirable to use
optional additional pumps to facilitate gas flow. For example, if
two reactors are connected in series, a second pump may
conveniently be located between the reactors, or elsewhere.
[0018] Reactor assembly 16 comprises a reaction vessel 20
containing an aqueous phase 22. Reaction vessel 20 may be of any
convenient shape and appropriate material of construction. In the
embodiment shown in FIG. 1, gas stream 12 passes into aqueous phase
22 through a gas disperser 26, for example a glass frit that
provides introduction of small bubbles of feed gas into the liquid
to enhance the overall gas-liquid mass transfer rate. Other types
of gas disperser may also be used, for example a pipe with holes in
it, or a plate with holes in it, or any other device known in the
art to convert the gas stream into small bubbles. Bubbles 28 rise
through a liquid column 24 through aqueous phase 22 until they
reach the upper surface 30 of the aqueous phase, during which time
contact is made such that the alkyl halide can rapidly be carried
into the aqueous phase 22. It is preferred that the bubbles 28 be
small, to maximize the gas-liquid surface area and thereby increase
the rate at which alkyl halide is carried into aqueous phase 22 to
the point where gas-liquid mass transfer is not the rate-limiting
step in the reaction of alkyl halide with nucleophile.
[0019] FIG. 1 does not show an agitator, although one may be used.
However, the invention does not rely on any mechanical stirring,
but takes advantage of the turbulence created in the liquid phase
due to the introduction of the gas through the small openings in
the frit. Aqueous phase 22 may be recycled or discarded when the
nucleophile (discussed below) has been depleted due to reaction
with the alkyl halide. Aqueous phase 22 may also be treated with
appropriate reagents after scrubbing has been completed, for
example to minimize toxicity or odor, and/or purified gas stream 18
may be treated for these or other purposes.
[0020] In the case where gas volume 11 comprises methyl bromide, a
wide range of concentrations of methyl bromide may be scrubbed
effectively, according to the invention. Thus for example, methyl
bromide in air having a concentration of from 1 ppm to 99% by
volume may be efficiently scrubbed. More typically, the
concentration may be from about 25 ppm to about 10% by volume, most
typically about 0.4% to about 6.0% by volume. Moreover, methyl
bromide in these various concentration ranges in gases other than
air, for example hydrocarbon gases, may be effectively scrubbed as
well, according to the invention.
[0021] In some applications, for example where alkyl halide levels
are to be reduced to an especially low level, it may be desirable
to connect two or more scrubbers in series, such that purified
gases exiting a scrubber are further purified by subsequent passage
through another. On the other hand, in some applications it may be
desired to rapidly purify a large volume of gas, in which case two
or more scrubbers may be used in parallel. Combinations of series
and parallel arrangements may also be practiced according to the
invention, using multiple scrubbers.
Gas Flow and Dispersion
[0022] The inventors have found that dispersion of an alkyl
halide-containing gas into very fine bubbles, with the bubbles
passing through a sufficient travel distance in an aqueous solution
containing an appropriate amount of a suitable nucleophile, makes
possible a surprisingly rapid reactive destruction of alkyl halides
and thereby provides fast and extensive removal of the halides from
a gas.
[0023] Suitably small bubbles may be provided by any means known in
the art, but they are conveniently provided by use of porous tubes
(spargers) having pores between 1 and 200 .mu.m across at their
widest point. More typically, the pores will be between 10 and 50
.mu.m across. It is preferable that the majority of pores be within
the range specified, but this is not an absolute requirement. The
holes are typically roughly circular, but need not be of any
particular shape. The spargers are typically situated such that
there is a relatively unobstructed or free flow of bubbles through
the aqueous phase containing the nucleophile. If the bubbles
collide with each other in a manner where they lose their integrity
and thus create larger bubbles, as is the case when the volume of
gas passed through the sparger is too great or the spargers are too
close to each other, significantly reduced removal efficiencies may
be encountered due to coalescence of smaller bubbles to form larger
ones. When this happens, it may result in less surface area for
mass transfer per unit volume of gas. The design issue becomes
providing enough gas-liquid contact area via the creation of small,
finely divided bubbles, to transfer significant amounts of reactive
alkyl halide gas to the liquid phase. One of the factors
determining the bubble size is the size of pores coming through the
sparger tubes. When the pores are too small the corresponding
pressure drop may be very large thus requiring gas compression of
large volumes of gas that dramatically increases the processing
cost.
[0024] In some embodiments, the spargers are made of polypropylene
or other inexpensive plastic materials. Such spargers typically are
quite effective and relatively inexpensive, and frequently exhibit
very good chemical resistance and are easy to install. Other
suitable materials of construction include, as nonlimiting
examples, metals and ceramic materials. Metal spargers made of
sintered metal are also available, and they can be scaled up by
using a larger number of cylindrical units. Although many spargers
are of a cylindrical shape, flat spargers are also suitable, and in
fact spargers for use according to the invention may be of any
shape.
[0025] Typically, the distance from the upper end of disperser 26
to upper surface 30 of the liquid phase is at least 6 inches and
more typically at least 12 inches. The distance is typically at
most 120 inches as more typically at most 36 inches. However,
greater liquid depths can be used, as long the pump has sufficient
capacity to introduce gas at the desired flow rate and pressure.
Thus, no real upper bound for liquid depth exists other than that
resulting from pump capability, available space, and other
practical limitations.
[0026] The inventors have found that the rate at which the gas
stream enters the scrubber affects the degree of completeness with
which the alkyl halide is consumed, with too high a rate tending to
decrease the degree of halide destruction. One suitable measure of
the rate of gas flow relative to the size of the scrubber is the
superficial gas velocity, which may be calculated by dividing the
volumetric flow rate of gas into the scrubber by the area of the
upper surface 30 of the liquid phase. An acceptable superficial gas
velocity for a given situation depends inter alia upon the type and
concentration of alkyl halide in the gas stream, the type and
concentration of nucleophile employed, the amount and type (if any)
of soluble organic compound in the aqueous phase, the size of the
bubbles produced by the gas disperser(s), the distance that the
bubbles travel through the aqueous phase, the temperature of the
aqueous phase, and the desired level of alkyl halide removal from
the gas stream. For example, scrubbing of methyl bromide from an
air stream with thiosulfate in the presence of PEG 200, using a gas
disperser having approximately 20-.mu.m pores and a 12-inch travel
of the resulting bubbles, a superficial gas velocity may typically
be at most 1.1 ft/min, and more typically will be at most 0.8
ft/min.
Aqueous Phase
[0027] Aqueous phase 22 is a single continuous aqueous liquid phase
containing a nucleophile. The single continuous aqueous phase may
include dissolved materials such as co-solvents, and of course
products formed by the nucleophilic reaction. The aqueous phase is
typically essentially free of suspended undissolved material, but
this is not required. The term "nucleophile" as used herein means
an anion or molecule having a high electron density which is
accessible for reaction with another molecule by displacement of a
leaving group, typically an anion such as halide. Due to the
presence of a good leaving group (halide anion), alkyl halides can
take part in nucleophilic substitution reactions with nucleophiles,
such reactions typically (but not necessarily) being of the
bimolecular (S.sub.N2) type.
[0028] Many neutral and anionic nucleophiles can participate in
nucleophilic substitution reactions with alkyl halide, according to
this invention. A non-limiting list of anions suitable for use as
nucleophiles according to the invention includes the following and
their derivatives: cyanide (CN.sup.-), thiocyanate (SCN.sup.-),
cyanate (OCN.sup.-), bisulfide (HS.sup.-), sulfide (S.sup.2-),
carbonate (CO.sub.3.sup.2-), bicarbonate (HCO.sub.3.sup.-),
thiocarbonates (monothio, dithio, and trithio), azide
(N.sub.3.sup.-), sulfite, bisulfite, alkyl, aryl, or aralkyl
thiolate, nitrite, nitrate, phosphates (mono and di hydrogen
phosphates plus phosphate), thiophosphates, biselenide (HSe.sup.-),
selenide (Se.sup.2-), (substituted and non-substituted)
benzenesulfonate, chloride, bromide, fluoride, iodide, thiosulfate,
chlorate, hypochlorite, malonate, carboxylates such as
trichloroacetate (CCl.sub.3COO.sup.-), dichloroacetate,
chloroacetate, terephthalate, adipate, lactate,
m-chloroperbenzoate, formate, acetate, acrylate, propionate,
butyrate, benzoate, furoate, oxalate, phthalate, hydrogen
phthalate, silicates, bromate, periodate, performate, and
phenolate, cresolate, and catecholate. Suitable neutral
nucleophiles may include for example ammonia and primary,
secondary, and tertiary amines, where the substituents on nitrogen
may be any combination of alkyl, aryl, and aralkyl groups, and
phosphines analogous to such amines. In this context, the term
"derivative" means a compound that contains one of the nucleophilic
groups listed above.
[0029] Particularly suitable nucleophiles for use according to the
invention include compounds containing sulfur or nitrogen at the
nucleophilic center. As used herein, the term "nucleophilic center"
means that atom which becomes bonded to the alkyl halide residue by
virtue of the nucleophilic reaction. Specific examples of suitable
sulfur nucleophiles include aliphatic and, preferably, aromatic
thiols and their salts, aliphatic and aromatic disulfides and
polysulfides, sulfide anion, bisulfide anion, thiosulfate anion,
sulfite or bisulfite anion, and thiocyanate anion. In one exemplary
embodiment of the invention, the nucleophile comprises at least one
of sodium sulfide and sodium bisulfide at a concentration of from
about 0.1 wt % to the saturation limit in the aqueous phase. When
sulfur nucleophiles are used, it may be advantageous to oxidize the
resulting reaction products, for example with sodium hypochlorite,
to convert them to materials having less odor.
[0030] Other suitable nucleophiles are alkoxides, carboxylates,
hydroxide, and selenium analogs of sulfur nucleophiles.
[0031] When a precursor species must be ionized to become a highly
reactive nucleophile, for example when a hydroxy compound or thiol
or carboxylic acid must be converted to the corresponding anion, a
pH-adjusting agent is used in such an amount as to ensure that the
pH is raised to a level sufficient to ionize the chemical species,
namely by removing a proton from the species and generating a
negatively charged species in the aqueous phase. The required pH is
dependent on the nature of the nucleophile, namely whether its
conjugate acid is a strong or weak acid. For example, if the
nucleophile is the anion of a weak acid, a relatively higher pH may
be required in order to produce a sufficient concentration of the
anion. Conversely, when the chemical species already exists as a
nucleophilic anion or as a neutral compound that can act as a
nucleophile, no pH-adjusting agent may be needed. When a pH
adjusting agent is needed, the particular amount of the agent or
base will vary depending on process conditions, but can be
optimized easily by altering the concentration and determining its
effect on yield, bearing in mind the ranges of excess molar
concentrations set forth above.
[0032] According to the present invention, a pH-adjusting agent (if
needed to produce suitable quantities of nucleophile) is used in an
amount sufficient to provide an excess molar concentration of base
in the range between -0.99 and 1.0, preferably between -0.25 and
0.5, more preferably between stoichiometric and 0.25, and most
preferably between 0.01 and 0.1. As used herein, the term
"stoichiometric" means the amount of base indicated by a balanced
chemical equation to be necessary to convert all of the precursor
species to the desired nucleophile. Thus, the "excess molar
concentration of base" is the amount of base actually in the system
above that which would be stoichiometrically required to neutralize
ionizable hydrogen atoms, and is expressed herein as the difference
between the actual concentration of base and the stoichiometric
concentration divided by the stoichiometric concentration. Thus, a
negative value of excess molar concentration of base contemplates
that less than the stoichiometric amount.
[0033] A suitable pH for purposes of the invention is one at which
a nucleophilic anion is present and is at least partially soluble
in the aqueous solution, typically from pH 7 to 13.5. However,
certain embodiments of the present invention may provide sufficient
amounts of nucleophile even at lower pH values, even as low as a pH
of about 1, depending on the nucleophile used.
[0034] It should be recognized that the pH as used herein refers to
the pH in the aqueous phase. The pH adjusting agent may be added to
the aqueous phase prior to contacting the gas stream, or
afterwards. Any of a number of suitable pH adjusting agents may be
used, but some typical ones are sodium hydroxide, potassium
hydroxide, magnesium hydroxide, calcium hydroxide, lithium
hydroxide, ammonium hydroxide, magnesium carbonate, calcium
carbonate, tetraalkyl ammonium hydroxides, sodium and potassium
carbonates, bicarbonates, phosphates, similar salts, and mixtures
thereof.
[0035] According to one exemplary embodiment of the invention,
aqueous phase 22 comprises from about 10 wt % to about 20 wt % of
sodium bisulfide dissolved in water. Such a composition is highly
effective in reacting with alkyl halides, for example methyl
bromide.
[0036] The aqueous phase may also contain a water-soluble organic
compound, and the presence of such compounds has been found in some
cases to increase the rate and/or completeness of alkyl halide
destruction. For example, the addition of water soluble organic
compounds such as N-methylpyrrolidone, dimethyl formamide, dimethyl
sulfoxide (DMSO), and poly(ethylene glycol) to the water phase has
been shown to improve the level of removal of methyl bromide.
Without wishing to be bound by any particular theory or
explanation, it is believed that the water soluble organic compound
increases alkyl halide solubility by decreasing the polarity of the
aqueous phase, and that this increases the rate of reaction between
the alkyl halide and the nucleophile. Water-soluble organic
compounds may constitute between 1 and 99 wt % of the aqueous
phase, more typically between 1 and 25 wt %. In some embodiments,
the organic compound is relatively nonvolatile, by which is meant
it does not boil below 125.degree. C. In some embodiments of the
invention, the organic compound is a polyglycol according to the
formula HO--(CH.sub.2CHR).sub.n--OH, wherein n is an integer from 1
to 20 and R is H or CH.sub.3. One useful example is tetraethylene
glycol.
[0037] Preferred nucleophilic reaction conditions for the
destruction of alkyl halides depend on a number of factors,
including the specific nucleophilic species used, and the organic
substrate used. In general, the time and temperature should be
selected to cause the reaction to proceed rapidly. As is well
known, the choice of temperature is dictated by the kinetics of the
reaction and the solubility of alkyl halide in the reaction medium.
Reactions that occur more slowly are preferably run at higher
temperatures. Lower reaction temperatures may however be suitable
or even preferable in some situations, provided only that the
reaction rate of alkyl halide be sufficiently fast to achieve the
desired degree of removal. Typical suitable temperatures are from
-3.degree. C. to 105.degree. C., more typically from 2 to
40.degree. C., and most typically from 5 to 30.degree. C.
[0038] The scrubber may be run at approximately atmospheric
pressure, i.e. atmospheric pressure plus the incremental additional
pressure generated by the head of liquid over the disperser. It may
also be operated at pressures well above atmospheric, and there is
no known limit to how high a pressure may be used. Higher pressures
may increase the rate of reaction, and may be especially useful in
cases where there is a relatively high concentration of alkyl
halide and a correspondingly lower level of diluent gas (e.g. air)
in the feed, since the cost of compressing and decompressing the
feed may be less in such a situation. Higher pressures may also be
beneficial when a higher scrubber reaction temperature is desired,
with the higher pressure making it possible to reduce loss of water
or other volatile components.
Examples
[0039] In the following examples, air containing a known
concentration of methyl bromide (MeBr) was scrubbed by passing into
the equipment indicated, using the indicated aqueous phase
scrubbing liquid. Measurements of methyl bromide concentration were
performed with a Neutronics MGT3 infrared analyzer (available from
Neutronics Corp, Exton, Pa.) calibrated for methyl bromide.
Sintered metal sparger elements are available from Mott
Corporation, Farmington, Conn., and Plastic sparger elements are
available from Genpore, Inc. Reading, Pa.
Example 1
[0040] Equipment:
[0041] Reactor Vessel: 13 in. by 13 in. rectangular plastic tank
fitted with one sparging element centered and entering from one
side wall, one inch from the bottom.
[0042] Sparger Element: 6-inch metal cylinder (Mott Sparger, 0.75
in. OD, Model No. 2312-A04-06-A00-02-AB)
[0043] Air Flowrate: 1.00 CFM (GF-1460, Size 4, glass ball, Gilmont
Flowmeter)
[0044] Aqueous Phase: 20 wt % ammonium thiosulfate, 10 wt %
polyethylene glycol, balance tap water. Liquid Level:
[0045] =8.25 inches above top of sparge element (without gas
flowing)
[0046] Concentration of MeBr in Feed:
[0047] 15,000 ppm Calibration gas supplied by Scott-Marin,
Riverside Calif.
[0048] Aqueous phase was charged to the plastic tank. A known
concentration (15,000 ppm) of methyl bromide gas in air under
pressure was regulated through the flowmeter and passed upwards
through the bottom of the plastic box reactor. The outlet
concentration of methyl bromide was measured for 10 minutes at an
average value of 5,600 ppm with an experimental uncertainty of
1.2%, showing a 63% removal. Polyethylene glycol used in the
aqueous phase had a molecular weight of about 400.
Example 2
[0049] Equipment:
[0050] Reactor Vessel: 13 in. by 13 in. rectangular plastic tank
fitted with one sparging element centered and entering from one
side wall, one inch from the bottom.
[0051] Sparger Element: 8.75-inch polyethylene cylinder (Genpore,
0.875 in. OD, 0.5 in ID)
[0052] Air Flowrate: 1.00 CFM (GF-1460, Size 4, glass ball, Gilmont
Flowmeter)
[0053] Aqueous Phase:
[0054] 20 wt % ammonium thiosulfate, 10 wt % polyethylene glycol,
balance tap water. Liquid Level:
[0055] =8.25 inches above top of sparge element (without gas
flowing)
[0056] Concentration of MeBr in Feed:
[0057] 15,000 ppm Calibration gas supplied by Scott-Marin,
Riverside Calif.
[0058] Aqueous phase was charged to the plastic tank. A known
concentration (15,000 ppm) of methyl bromide gas under pressure was
regulated through the flowmeter and passed upwards through the
bottom of the plastic box reactor. The outlet concentration of
methyl bromide was measured for 10 minutes at an average value of
6,200 ppm with an experimental uncertainty of 1.2%, showing a 56%
removal.
Example 3
[0059] Equipment:
[0060] Reactor Vessel: 13 in. by 13 in. rectangular plastic tank
fitted with one sparging element centered and entering from one
side wall, one inch from the bottom.
[0061] Sparger Element: 8.75-inch polyethylene cylinder (Genpore,
0.875 in. OD, 0.5 in ID)
[0062] Air Flowrate: 0.51 CFM (GF-1460, Size 4, glass ball, Gilmont
Flowmeter)
[0063] Aqueous Phase:
[0064] 18 wt % ammonium thiosulfate, 9 wt % polyethylene glycol, 9%
tetraethylene glycol, balance tap water.
[0065] Liquid Level:
[0066] =8.25 inches above top of sparge element (without gas
flowing) Concentration of MeBr in Feed:
[0067] 15,000 ppm Calibration gas supplied by Scott-Marin,
Riverside Calif.
[0068] Aqueous phase was charged to the plastic tank. A known
concentration (15,000 ppm) of methyl bromide gas under pressure was
regulated through the flowmeter and passed upwards through the
bottom of the plastic box reactor. The outlet concentration of
methyl bromide was measured for 10 minutes at an average value of
4,294 ppm with an experimental uncertainty of 1.2%, showing a 70%
removal.
Example 4
[0069] Equipment:
[0070] Reactor Vessel: 13 in. by 13 in. rectangular plastic tank
fitted with one sparging element centered and entering from one
side wall, one inch from the bottom.
[0071] Sparger Element: 8.75-inch polyethylene cylinder (Genpore,
0.875 in. OD, 0.5 in ID)
[0072] Air Flowrate: 0.51 CFM (GF-1460, Size 4, glass ball, Gilmont
Flowmeter)
[0073] Aqueous Phase:
[0074] 5.2 wt % ammonium thiosulfate, 2.6 wt % polyethylene glycol,
2.9% tetraethylene glycol, balance tap water.
[0075] Liquid Level:
[0076] =11.5 inches above top of sparge element (without gas
flowing)
[0077] Concentration of MeBr in Feed:
[0078] 15,000 ppm Calibration gas supplied by Scott-Marin,
Riverside Calif.
[0079] Aqueous phase was charged to the plastic tank. A known
concentration (15,000 ppm) of methyl bromide gas under pressure was
regulated through the flowmeter and passed upwards through the
bottom of the plastic box reactor. The outlet concentration of
methyl bromide was measured for 10 minutes at an average value of
1,952 ppm with an experimental uncertainty of 1.2%, showing a 86%
removal.
Example 5
[0080] Equipment:
[0081] Reactor Vessel: 13 in. by 13 in. rectangular plastic tank
fitted with one sparging element centered and entering from one
side wall, one inch from the bottom.
[0082] Sparger Element: 8.75-inch polyethylene cylinder (Genpore,
0.875 in. OD, 0.5 in ID)
[0083] Air Flowrate: 0.51 CFM (GF-1460, Size 4, glass ball, Gilmont
Flowmeter)
[0084] Aqueous Phase:
[0085] 9.5 wt % ammonium thiosulfate, balance tap water.
[0086] Liquid Level:
[0087] =11.5 inches above top of sparge element (without gas
flowing)
[0088] Concentration of MeBr in Feed:
[0089] 15,000 ppm Calibration gas supplied by Scott-Marin,
Riverside Calif.
[0090] Aqueous phase was charged to the plastic tank. A known
concentration (15,000 ppm) of methyl bromide gas under pressure was
regulated through the flowmeter and passed upwards through the
bottom of the plastic box reactor. The outlet concentration of
methyl bromide was measured for 10 minutes at an average value of
2,906 ppm with an experimental uncertainty of 1.2%, showing a 80%
removal.
Example 6
[0091] Equipment:
[0092] Reactor Vessel: 13 in. by 13 in. rectangular plastic tank
fitted with two sparging elements whose centerlines were 6 inches
apart and 3.5 inches from the nearest side wall, one inch from the
bottom.
[0093] Sparger Elements: Two 8.75-inch polyethylene cylinders
(Genpore, 0.875 in. OD, 0.5 in ID)
[0094] Air Flowrate: 0.76 CFM (GF-1460, Size 4, glass ball, Gilmont
Flowmeter)
[0095] Aqueous Phase:
[0096] 5.8 wt % ammonium thiosulfate, 2.9 wt % polyethylene glycol,
3.2 wt % tetraethylene glycol, balance tap water.
[0097] Liquid Level:
[0098] =11.5 inches above top of sparge element (without gas
flowing)
[0099] Concentration of MeBr in Feed:
[0100] 15,000 ppm Calibration gas supplied by Scott-Marin,
Riverside Calif.
[0101] Aqueous phase was charged to the plastic tank. A known
concentration (15,000 ppm) of methyl bromide gas under pressure was
regulated through the flowmeter and passed upwards through the
bottom of the plastic box reactor and distributed to both sparging
elements. The outlet concentration of methyl bromide was measured
for 10 minutes at an average value of 3,800 ppm with an
experimental uncertainty of 1.2%, showing a 73% removal.
Example 7
[0102] Equipment:
[0103] Reactor Vessel: 13 in. by 13 in. rectangular plastic tank
fitted with two sparging elements whose centerlines were 6 inches
apart and 3.5 inches from the nearest side wall, one inch from the
bottom.
[0104] Sparger Elements: Two 8.75-inch polyethylene cylinders
(Genpore, 0.875 in. OD, 0.5 in ID)
[0105] Air Flowrate: 1.02 CFM (GF-1460, Size 4, glass ball, Gilmont
Flowmeter)
[0106] Aqueous Phase:
[0107] 5.8 wt % ammonium thiosulfate, 2.9 wt % polyethylene glycol,
3.2wt % tetraethylene glycol, balance tap water.
[0108] Liquid Level:
[0109] =11.5 inches above top of sparge element (without gas
flowing)
[0110] Concentration of MeBr in Feed:
[0111] 15,000 ppm Calibration gas supplied by Scott-Marin,
Riverside Calif.
[0112] Aqueous phase was charged to the plastic tank. A known
concentration (15,000 ppm) of methyl bromide gas under pressure was
regulated through the flowmeter and passed upwards through the
bottom of the plastic box reactor and distributed to both sparging
elements. The outlet concentration of methyl bromide was measured
for 10 minutes at an average value of 3,250 ppm with an
experimental uncertainty of 1.2%, showing a 77% removal. All
Examples were performed at ambient temperature. When the solution
pH dropped below 9, it was brought back above this value by
addition of aqueous sodium hydroxide.
Example 8
[0113] Equipment:
[0114] Reactor Vessel: 80 in. by 132 in. rectangular plastic tank
fitted with a center spine distributor (2 in. Schedule 80 plastic
pipe) with sparger elements mounted on it, 5.5 inches apart, 2.5
inches from the bottom of the tank.
[0115] Sparger Elements: Forty 28-inch long polyethylene cylinders
(Genpore, 0.875 in. OD, 0.5 in ID)
[0116] Air Flowrate: 66 CFM (KEY Instruments, Acrylic Flowmeter,
McMaster-Carr Catalog No. 5077K22)
[0117] Aqueous Phase:
[0118] 6.0 wt % ammonium thiosulfate, 9.0 wt % polyethylene glycol,
balance water.
[0119] Liquid Level:
[0120] =23 inches above top of sparge element (without gas
flowing)
[0121] Concentration of MeBr in Feed at start of ventilation: 64
ounces per 1,000 ft.sup.3 (15,000 ppm). Methogas methyl bromide was
supplied by Ameribrom of Newark, N.J.)
[0122] Aqueous phase was charged to the plastic tank. Methyl
bromide was charged to a large empty sealed (via tarps) volume
having an enclosed volume of about 2,840 cubic feet until a
concentration of 16,200 ppm was reached (64 ounces per thousand
cubic feet). A HEPA filter capable of filtering dust particles to
less than 0.3 microns was installed on the feed line to the
scrubber. A blower fan was configured to pull fresh air into the
tarped volume and deliver air containing methyl bromide from the
tarped volume to the scrubber, where it entered the sparger
elements. Methyl bromide concentration in the air exiting the
scrubber was measured and every 20 seconds and logged into a
computer spreadsheet. The introduction of fresh air decreased the
concentration of methyl bromide in the volume in line with an
exponential decay curve. The same phenomenon, exponential decay,
was shown in the outlet concentration. The percent removal averaged
87.4% with a standard deviation of 1.7% over a time period of 31
minutes. Selected data recorded in the spreadsheet are shown
below:
TABLE-US-00001 Inlet Concentration Outlet Concentration Time of
(Tarped Volume) (Scrubber Exhaust) Day ppm ppm % Removal 17:46
16534 0 Prior to startup 17:49 16421 0 Fresh Air start 17:56 14,314
1062 92.6% 17:59 13,117 1785 85.6% 18:02 12,202 1647 86.5% 18:08
10,238 1647 83.9% 18:18 7,884 816 89.7% 18:24 6,108 754 87.7%
[0123] As can be seen by reference to the above examples, alkyl
halides such as methyl bromide can be effectively removed from gas
streams by using the devices and methods of this invention.
[0124] Although the invention is illustrated and described herein
with reference to specific embodiments, the invention is not
intended to be limited to the details shown. Rather, various
modifications may be made in the details within the scope and range
of equivalents of the claims without departing from the
invention.
* * * * *
References