Note: Descriptions are shown in the official language in which they were submitted.
CRR062091 PATEN~
0092G Dkt. Mo. 91A251
2~69380
METHOD ~OR REMOVING PERMA~ENT GA~E~ AND LIGHT
HYDROCAR~ONS FROM WASTE AND PROCESS GAS STREAMS
AND PETROCHEMICAL PROCESSES
BACKGROUND OF THE TNVENTION
This invention relates to the :removal of certain components
from gas streams, and more particularly to the removal of
valuable or environmentally unacceptable components from waste
gas streams prior to relea5e of the waste gas streams to the
atmosphere.
Waste gas streams fron~ chemical manufacturing or processing
plants usually contain light gas elements or compounds which
are not incompatible with the environment and thus can be
released to the atmosphere. For e~ample, o~ygen, nitrogen,
argon~ water vapor and carbon dioxide are commonly present in
significant quantities in waste gas streams. It is also not
uncommon for waste gas streams to contain trace amounts of
hydrogen and helium. Since these gases are naturally occurring
components of the atmosphere, they are environmentally
acceptable and may be released to the atmosphere in reasonable
quantities.
It often happens, however, that chemical plant waste gas
streams contain higher boiling components which cannot be
released to the atmosphere in concentrations above a specified
tolerance limit. In such cases, the environmentally
unacceptable gas components must be removed from the waste gas
stream or converted to relatively harmless compounds, such as
carbon dio~ide, prior to release of the waste gas stream to the
atmosphere.
CRR 0 6 2 0 91 PATENT
00921~; Dkt-2 ~q~9
-- 2 --
One method of accomplishing the objective of rendering
waste gas streams environmentally acceptable prior to releasing
them to the atmosphere, is to burn the yases. This method can
be employed when substantially all of the harmful components
are removed from the gas streams and no other harmful gaseous
substance are produced by incineration of the stream. For
instance, when the gas stream originally contains only
hydrocarbons as heavy gases, the gas stream may be burned,
provided that combustion is sufficiently complete that the
combustion by-products constitute substantially only water
vapor and carbon dioxide.
Combustion of waste gas streams is not always a suitable
method for the elimination of environmentally unacceptable
components from the stream. For example, when the stream
contains significant quantities of hydrocarbon, combustion of
the stream will produce large quantities of CO2, the release
of which to the environment will additionally burden an already
overta~ed ecosystem. Furthermore, it often happens that the
gas stream contains components which are not combustible, or
which are combustible, but upon burning produce noxious
combustion by-products. For example, many chlorinated
hydrocarbons are not flammable, and others, although flammable,
produce chlorine-containing by-products which themselves are
not environmentally acceptable. It can readily be appreciated,
therefore, that when a waste gas stream contains components
which cannot be safely removed or destroyed from the stream by
combustion, other techniques must be employed.
A commonly used technique for removing heavy gas components
from waste gas streams is to separate the components by
fractional distillation. Thus, high boiling gas components can
often be separated from low boiling gases, such as o~ygen,
nitrogen and argon, by cooling and compressing the gas stream
to a temperature below the boiling point of the high boiling
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CRR 0 6 2 0 91 PATENT
0092G Dkt . ~o . 91A251
20~9~0
components at the e~isting pressure, and subsequently
separating the components by means of distillation. When the
gas mi~ture contains only the light gases mentioned above,
a.e., nitrogen, oxygen, argon, etc., and the higher boiling
compounds that are to be removed, i.e., ~ubstituted
hydrocarbons ~nd C3 and higher unsubstituted hydrocarbons, a
clean separation of the high boiling gases from the light gases
can usually be effected by fractional distillation~ However,
when methane or any of the C2 hydrocarbons are present in the
gas mixture in addition to the low boiling and high boiling
gases, it is o~ten difficult to obtain a clean separation by
distillation. During the course of the distillation, the C
and C2 hydrocarbons, because of their low boiling points,
distill off with the light gases. However, due to the
solubility of the high boiling gas components in the light
hydrocarbons, some of the high boiling compounds are also
distilled off with the light gases. Thus, conventional
fractional distillation cannot be effectively employed to
separate high boiling hydrocarbons and substituted hydrocarbons
from low boiling gases in gas mi~tures when the gas mi~tures
also contain methane and/or C2 hydrocarbons.
Because of increasingly stringent environmental
regulations, and because of the economic ad~antages offered by
recovering and recycling high boiling hydrocarbons, there is a
continuing effort to find improved methods for the removal of
high boiling hydrocarbons from waste gas streams which
additionally contain methane and/or one or more C2
hydrocarbons and low boiling environmentally acceptable gases.
The present invention provides an effective and efficient
method for accomplishing this result~
~IMARY OF THE INVENTION
According to the invention, a gas mi~ture containing one or
more high boiling gases, one or more low bo;ling gases and one
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CRR062091 PATENT
0092G Dkt. No. 91A251
4 2 ~ 8 0
or more light hydrocarbons (all defined below) is subjected to
adsorptive separation to produce an unadsorbed product gas
stream that is enriched in low boiling gases and light
hydrocarbons but substantially free of high boiling gases, and
a desorbed gas product stream enriched in high boiling gases.
The two product gas streams can then be further treated, if
desired, to separate the various components of each of the gas
streams.
According to the process asplect of the invention, gases
having boiling points ~reater than about -80~C are separated
from a gas mixture containing one or more gaseous components
having boiling points in the range of about -80C to about
50C, one or more hydrocarbons having boiling points between
about -170C and about -80C, and one or more gaseous
components having boiling points below about -170C. This is
accomplished by passing the gaseous mixture through a bed of
adsorbent which adsorbs the gaseous components having boiling
points greater than about -80C more readily than it adsorbs
gaseous components having boiling points lower than about
-80C, thereby producing an unadsorbed gaseous product stream
which is enriched in those components having boiling points
less than about -80C and a desorbed gaseous product stream
that is enriched in components having boiling points greater
than about -80C.
In a preferred embodiment of the process of the invention,
the adsorbent is activated carbon, silica gel, zeolites, carbon
molecular sieves and mixtures of these.
In another preferred embodiment, the unadsorbed gaseous
product stream is subjected to one or more additional
adsorptive separations to recover methane and C2 hydrocarbons
from the unadsorbed gaseous product stream.
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CRR062091 PATENT
0092G Dkt. No. 91A251
2~380
In yet another preferred embodiment, applicable to the
separation of gaseous mixtures which contain, in addition to
low boiling gases and light hydrocarbons, two or more gaseous
components having boiling points in the range of about -80C to
about 50C, the invention comprises subjecting the high boiling
gas-containing desorbed gaseous product stream to one or more
fractional distillatisn steps to separately r~cover ~ome or all
of the components of this stream.
In another preferred embodiment, hydrogen and/or carbon
mono~ide present in the waste gas stream are o~idized to water
and carbon dioxide, respectively.
In a most preferred embodiment, the process of the
invention is applied to the recovery of two or more gases
having boiling points in the range of about -80C to about
50C, one or more hydrocarbons selected from methane, ethane,
~thene and ethyne, and one or more light gaseous components
selected from oxygen, nitrogen, argon, hydrogen and carbon
mono~ide. According to this aspe~t of the invention, the
gaseous mi~ture is subjected to a first adsorptive separation
in a bed comprising an adsorbent selected from activated
carbon, silica gel, zeolites, carbon molecular sieves or
mi~tures of these, thereby producing an unadsorbed gaseous
product stream enriched in low boiling gases and light
hydrocarbons, but suostantially depleted of high boiling gases,
and a desorbed gaseous product stream enriched in high boiling
~ases. The unadsorbed gaseous product stream is then subjected
to a second adsorptive separation, thereby recovering an
unadsorbed gas stream enriched in low boiling gases and a
desorbed gaseous product enriched in one or more of methane,
ethane~ ethene and ethyne. Also, according to this embodiment,
the desorbed gaseous product enriched in the high boiling
gaseous components can be subjected to one or more fractional
distillation steps to separately recover the components of the
high boiling gas mi~ture.
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CRR062091 PATENT
0092G Dkt. No. 91A251
2~93~
-- 6 ~
BRIEF D~SCRIPTION OF THE DRAWING
Fig. 1 broadly illustrates, in a schematic diagram,
apparatus iD which the process of the invention can be carried
out.
Fig. 2 illustrates, in a schematic diagram, apparatus in
which a preferred embodiment of the invention can be carried
out.
DETAILED DES~R.IPTION OF T~E INVENTION
The invention provides an effective and efficient method
for recovering high boiling gases from a gaseous mi~ture
comprised of one or more hiyh boiling gases, one or more low
boiling gases and one or more light hydrocarbons. To simplify
the description of the invention, these terms are specifically
define~ as follows:
The term Uhigh boiling gas , is used herein to denote an
organic or inorganic gaseous element or compound which, in the
liquid state, has a normal boiling point above about -80C.
E~amples of high boiling gases are unsubstituted hydrocarbons
having three or more carbon atoms, halogenated hydrocarbons,
nitrogen-substituted hydrocarbons, o~ygen-substituted
hydrocarbons and sulphur-substituted hydrocarbons.
The term ~low boiling gas~ is used to denote a gaseous
element or compound which, in the liquid state, has a normal
boiling point below about -170C. Typical of the low boiling
gases are the atmospheric gases such as o~ygen, nitrogen,
argon, helium and hydrogen.
The term "light hydrocarbon~ is used to denote a
hydrocarbon having a normal boiling point between about -170C
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CRR 0 6 2 0 9 1 PATENT
0092G Dkt. No. 91A251
7_ 21~380
and -80C. Hydrocarbons having boiling points in this range
include methane, ethane, ethene and ethyne.
The ~normal boiling point~ of an element or a compound is
the boiling point of the element or compound at standard
pressure, i.e. 760 mm barometric pressure.
According to a main embodiment of the process of the
invention, the gas mi~ture being treated is initially subjected
to a high boiling gas adsorptive separation in an adsorption
bed which contains an absorbent that strongly adsorbs high
boiling gases, thereby producing an unadsorbed gaseous product
that is enriched in low boiling gases and light hydrocarbons
but substantially depleted in high boiling gases, and an
desorbed gaseous product that is enriched in high boiling gases.
There are a number of options available for treating the
unadsorbed gas product stream obtained from the rough cut
adsorptive separation step. One option, available when the
unadsorbed gas product stream does not contain environmentally
objectionable non-atmospheric gas components, e.g. carbon
monoxide and light hydrocarbons, at concentrations above their
ma~imum allowable limits, is to vent this stream to the
atmosphere.
A second option is to incinerate this stream to burn carbon
monoside, hydrogen and light hydrocarbons. This option is
feasible only when the concentrations of these gases in the
unadsorbed gaseous product stream are below the concentration
levels that would make recovery of these components
economically worth while.
A third alternative, worthwhile when the unadsorbed gaseous
product stream contains significant concentrations of light
hydrocarbons, is to subject this gas stream to a second
adsorptive separation to recover the light hydrocarbons from
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CRR062091 PATENT
0092G Dkt. No. 91A251
2~938~
the unadsorbed gas product ~tream. This is accomplished by
introducing the unadsorbed gaseous product stream from the high
boiling gas adsorptior zone into a second adsorption zone
containing an adsorbent that adsorbs the light hydrocarbons
conta~ned in the gas stream more strongly than it adsorbs the
low boiling gases, thereby producing a ~econd unadsorbed
gaseous product stream that is enriched in low boiling gases
but depleted in light hydrocarbons and a desorbed gas product
stream which is enriched in light hydrocarbons. The unadsorbed
gaseous product stream from the light hydrocarbon adsorption
section can be discharged to the atmosphere and the desorbed
gaseous product stream can be used as fuel or as feed for a
chemical process.
The high boiling gas components that were adsorbed in the
high boiling gas adsorption section are desorbed during
regeneration of the adsorbent. The desorbed high boiling gases
can be incinerated if the resulting combustion gases do not
contain components that are environmentally objectionable.
Alternatively, the high boiling components of this stream may
be recovered or otherwise disposed of, if desired. When the
desorbed gas stream from the high boiling gas adsorption
section contains more than one component this stream may be
suhjected to one or more fractional distillation steps to
recover some or all of the components of the gas stream.
The invention can be better understood from the attached
drawings. Only the principal components necessary for
practicing the invention, together with connecting fluid
transfer lines, are included in the drawings. Valves and
auxiliary equipment that are not necessary for an understanding
of the invention have been omitted.
Turning now to the drawings, Fig. 1 illustrates, in a
schematic diagram, a system in which the process of the
invention can be carried out. The major components included in
CRR062091 PATENT
0092G Dkt, No. 91A251
- 9 2~9380
the equipment train shown in Fig. 1 are a high boiling gas
adsorption ~ection, 4, a light hydrocarbon adsorption section,
12, and a cryogenic distillation ~ection, 28. Adsorption
section 4 is provided with a feed gas line, 2, an unadsorbed
gas product line, 6 a~d a desorbed product gas line, 22.
Unadsorbed product ~as line 6 joins vent line B and hydrocarbon
adsorption section feed line 12. A low boiling gas product
line 14 exits adsorption section 12 and connects to vent line
16 and rough cut adsorption zone purge gas line 18. Adsorption
section 12 is also provided with a desorbed light hydrocarbon
product gas line 20. Desorbed high boiling gas line 22
connects high boiling gas adsorption section 4 to both waste
gas disposal line 24 and cryogenic distillation section feed
line 26. Feed line 26 is, in turn, connected to cryogenic
distillation section 28, which is provided with light product
discharge line 30 and heavy product discharge line 32.
The high boiling gas adsorption section may comprise a
single adsorption unit, but it generally comprises a battery of
two or more adsorbers arranged in parallel and operated out of
phase with each other so that unadsorbed product gas and
desorbed product gas are continuously produced during the
operation of this section. In other words while one adsorber
of this section is in the adsorption mode another is in the
depressurization mode, a third is in the desorption mode etc.
Similarly, the light hydrocarbon adsorption section may
comprise a single adsorber but ~enerally comprises a battery of
two or more adsorption units, likewise operated out of phase to
provide a continuous flow of unadsorbed and desorbed product
gases. The number, arr~ngement and design of the adsorbers in
each of these adsorption section6 is a matter of choice and
forms no part of this invention. Similarly, the operation of
the sdsorption sections in a manner to produce a continuous
flow of product streams is well known and likewise forms no
part of the present invention.
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CRR O 6 2 0 91 PATENT
0092G Dkt . No . 91A251
2~69380
-- 10 --
Fractional distillation section 28 may comprise a single
distillation column or a battery of two or more columns
arrange~ in series, depending on the number of components in
the unadsorbed product gas stream and the degree of product
separation desired. This section may be designed to operate at
low temperatures, i.e. un~er cryogenic conditions, or at higher
temperatures or at combinations of these conditions. The
design and operation of fractional distillation sections
suitable for use in the system of the invention are well known
and form no part of the invention.
In practicing the process of the invention in the apparatus
of Fig. 1, a feed gas mixture comprised of one or more high
boiling gases, one or more light hydrocarbons and one or more
low boiling gases is introduced into adsorber 4 via feed line
2. The high boiling gas components of the mixture are adsorbed
onto the adsorbent in adsorber 4, while a large percentage of
the low boiling gases and light hydrocarbons ~ntering adsorber
4 pass through the adsorbent bed and e~it adsorber 4 through
line 6 as the unadsorbed gas product stream. If the unadsorbed
gas product stream leaving adsorber 4 contains only trace
amounts of light hydrocarbons, this stream may be incinerated
or vented to the atmosphere. In this case, these gases exit
the system through vent line 80 If, on the other hand, it is
desired to subject the unadsorbed gas product stream leaving
adsorber 4 to further adsorpti~e separation, this stream is
introduced into adsorber 12 through line 10.
As the unadsorbed gas product stream e~iting high boiling
gas adsorption section 4 passes through hydrocarbon adsorption
section 12, the light gas components o the ~tream, comprised
principally of low boiling gases, pass through the adsorbent
bed, exit section 12 through unadsorbed gas product line 14 and
leave the system through vent 16. If this gas stream does not
contain environmentally objectionably high concentrations of
hydrocarbon ~ases and carbon mono~ide it may be vented directly
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CRR 0 6 2 0 91 PATENT
0092G Dkt. No. 91A251
11 2069~80
to the atmosphere. If, howe~er, it contains these components
in higher than allowable concentrations, this gas stream must
be further treated to remove the e~cess amounts.
The light hydrocarbons entering adsorption ~ection 12 are
adsorbed onto the adsorbent. These components are removed from
the a~sorbent bed during desorption of the bed and are
discharged from the system through line 20. As noted ~bove,
the desorbed gases can be incinerated or reco~ered for use in
chemical process operations.
The adsorption process conducted in each adsorber of
adsorption section 4 is permitted to continue until the high
boiling gas front in the adsorber reaches the desired point.
At this point, which is prior to breakthrough of the
component(s) being adsorbed through the product end of the
adsorber in operation, the adsorption step in that adsorber is
terminated and the adsorbed gases are desorbed from the bed by
depressurization of the adsorber. During the desorption ~tep,
the high boiling gases leave adsorber 4 through line 22.
Adsorber 4 can be purged with low boiling gas product from
adsorber 12 via purge line 18. If the high boiling gases have
little value and do not contain components which, upon
combustion, result in the production of gas products that are
harmful to the atmosphere, they may be discharged to an
incinerator through line 24. hikewise, if this gas stream
requires additional treatment (other than fractional
distillation) or is comprised of a single high boiling
component, it may be removed from the system for further
processing or storage through line 24.
If the desorbed gas stream in line 22 contains more than
one high boiling gas component, the stream components can be
separated in fractional distillation unit 28, which, if
desired, can be operated under cryogenic conditions. When this
option is esercised, the desorbed gas stream enters fractional
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CRR062091 PATENT
0092G Dkt. ~o. 91A251
- 12 - ~938~
distillation unit 28 through feed line 26, and the light and
heavy product components leave unit 28 through lines 30 and 32,
respectively. As indicated above, if the high boiling gas
stream contains more than two components, multiple,
serially-connected distillation UllitS may be used to effect
recovery of the various components of the gas stream.
In some cases the desorbed gas stream from the high boiling
gas adsorption sect;on may contain undesirably high
concentrations of the low boiling gas components of the gas
stream. In such cases, a portion of the desorbed gaseous
product stream can be recycled to the feed stream. This will
result in a reduction in the overall concentration of low
boiling gases in the desorbed product stream.
Fig. 2 illustrates a simplified version of a preferred
embodiment of the invention. The system of Fig. 2 is designed
for the continuous ~eparation of high boiling gases and light
hydrocarbons from low boiling gases contained in a gas stream
comprised of these components. The equipment components of the
system illustrated in Fig. 2 include a feed gas compressor 104,
a rough cut adsorption zone 106, a light hydrocarbon adsorption
zone 110, and a fractional distillation zone 124.
In practicing the process of the invention in the system
illustrated in Fig. 2, the feed gas mi~ture, which typically
contains the high boiling gases, the low boiling gases and one
or more light hydrocarbons selected from methane, ethane,
ethene and ethyne, is introduced into the system through feed
line 102, ;s pressurized to the desired pressure in feed gas
compressor 104 and is introduced into adsorption zone 106. The
high boiling gas components of the mi~ture are adsorbed onto
the bed contained in adsorber 106, while the unadsorbed gas
stream, enriched in low boiling gases and light hydrocarbons
passes through the adsorption bed and e~its the adsorption zone
thxough product line 108. The unadsorbed gas stream ne~t
enters light hyclrocarbon adsorption zone 110 wherein the C2
CRR062091 PATENT
0092G Dkt. No. 91A251
2~6~80
- 13 ~
hydrocarbons and perhaps some or all of the methane contained
in the gas stream are adsorbed onto the adsorbent in a~sorber
110 and the unadsorbed gas stream, enriched in low boiling
gases, esits the adsorber and le!aves the ~ystem through line
~12.
When the beds in the adsorption modes in zones 106 and 110
become saturated with adsorbed gas, the ~eds are switched in
the well known manner and the ~aturated beds are then
regenerated, thereby producing the desorbed gas streams.
During the regeneration modes it may be desirable to purge the
beds with the unadsorbed product gas from adsorption zone 110,
which is comprised substantially of low boiliny gases. This
can be accomplished by passing the low boiling gas stream
produced in adsorption zone 110 through lines 116, 11~ and 120
and then counter-currently through adsorption zones 106 and 110.
The desorbed gas stream leaving adsorption zone 110,
comprised predominantly of light hydrocarbons, can be
incinerated or used in other chemical processes.
The desorbed gas stream leaving adsorption zone 106 during
desorption of the adsorbers in this zone is cornprised
predominantly of high boiling gases. This stream passes
through line 122 and enters fractional distillation unit 124,
which separates the gas com~onents into a light gas product and
a heavy gas product. The light gas producit leaves unit 124
through line 126 and the heavy product leaves unit 124 through
line 128. ~f the high ~oiling gas stream contains more than
two components these may ~e separat~ly recovered by means o~
addition~l fractional distillation units arranged in series
with unit 124.
The invention is further illustrated in the following
hypothetical e~ample wherein, unless otherwise indicated,
parts, psrcentages and ratios are on a volume basis.
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t::RR062091 PATENT
0092G Dkt . No . 91A251
~ 14 - 2~693~0
EXAMPLE 1
~ gas stream having the composition indicated in TABLE 1
is treated by the process of the invention in a simulated run
to remove methyl chloride ~rom the stream. The equipment train
used in the simulated run is æimilar to the equipment train
illustrated in Fiy. 2 and ~ompris~es a first set of adsorbers
containing silica gel and a second set of adsorbers containing
activated carbon. The $eed stream entering each s0t of
adsorbers is at a pressure in the range of about 20 - 25 psia.
The gases adsorbed in each set of adsorbers is desorbed at an
absolute pressure of about 200 millibars. About 61% of the
desorbed gas stream from the first set of adsorbers is recycled
to the feed stream. The remainder of the desorbed gas stream
from the first set of adsorbers is discharged from the system
as desorbed product. The projected molar concentrations of the
components in each stream is tabulated in the TABLE.
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CRR062.091 PATENT
0092~ Dkt. No. 91A251
2~38~
- 16 -
The above e~ample illustrates the projected efficiency of
the invention for removing hi~h boiling gaseous components from
a gas stream. In the process depicted in EX~MP~E 1, all of the
me~hyl chloride in the feed stream will be adsorbed in the
first ~et of adsorbers, while the non-adsorbed product from the
second set of adsorbers will contain 16.9 moles of nitrogen
(about 85.4~ of the nitrogen in the feed stream~.
Although the invention has been described with particular
reference to the illustrated embodiments and a specific
example, variations o these are contemplated. For example,
other gas separation techniques, such as membrane separation
and absorption can be used in the invention in combination with
the adsorption steps depicted in the drawings. Furthermore,
the efficiency of the disclosed process can be increaæed by
conducting multiple adsorption steps in series and/or in
parallel and by recycling any of the product streams from the
various adsorbers. The scope of the invention îs limited only
by the breadth of the appended claims.
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