Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR THE RE~lOVAL OF
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SULF~IR OXID' FROIl A GAS
FIELD OF T~E II~VENTION
The field of art to which the clairned invention pertains
is the removal of sulfur oxide from a gas, particularly a flue gas.
BACKGROUND INFOR~lATION
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Oue to the ever increasing concern about air pollution,
great efforts have been expended in recent years toward the develop-
ment of processes to reduce the pollutants introduced into the atmo-
sphere from various industrial operations. One of the most onerous
of these pollutants is sulfur dioxide which is present in the stacks
of flue gases from various operations. In one such operation, the
fluidized catalytic cracking (FCC~ process, sulfur cormpounds con-
tained in the hydrocarbon feedstock result in sulfur containingmaterial to be deposited on the FCC catalyst along with the carbo-
naceous material and thereby cause the generation of sulfur dioxide
in the FCC regeneration section wnen the sulfur is burned off the
catalyst along with the carbon deposits. This sulfur dioxide be-
comes a part of the regenerator flue gas and thus a pollutant whenthe flue gas eventually finds its way into the atmosphere.
There are many methods known to the art for removal of
sulfur dioxide from stack or flue gases. There is, for example,
the wet scrubbing process in which the sulfur dioxide reacts with
an appropriate reactant contained in an aqueous solution or slurry
Y~
sprayed into the flue gas, the sulfur thereby being remoYed fro~
the systern as a compound contained in the liquid phase. In
another process the flue gas is passed through a fixed solid bed
cQntaining a sulfur "accep~or" with which the sulfur dioxide reacts
S and on which the sulfur is retained in the sulfate form, thereby
being removed from the flue gas.
The basic prior art process for removal of sulfur dioxide
from flue gas highly Dertinent to the present invention is that
disclosed in U.S. Patent 4,071,436 to Blanton, Jr., et al. This
reference discloses alumina and/or magnesia particles used to react
with sulfur dioxide in an FCC regeneratcr flue gas to form a solid
compound. ~hen reacted with hydrocarbons in a reducing environment,
such as the reactor, the sulfur is released from the acceptor.
Hydrogen-containing gas is stated to be a less preferred reducing
medium. This reference further states that it is preferred that
materials such as lithium not be present in the particulate solid
used for removal of the sulfur dioxide, since they simply form a
nonregenerable sulfur-containing solid.
Other references having similar teachings as Blanton, Jr.,
et al. but not as relevant or no more relevant to the present inven-
tion are 4,153,535 to Vasalos et al.; 4,153,534 to Yasalos; 4,204,945
to Flanders et al.; 4,243,556 to Blanton, Jr.; 4,252,635 to Blanton,
Jr.; 4,300,997 to Meguerian et al. and 4,325,811 to Sorrentino.
The last mentioned reference also teaches the use of a reducing
zone, separate from the reactor and regenerator, in which the sulfur
laden acceptor is relieved of sulfur by reduction with hydrogen or
a hydrocarbon gas.
~he present invention is based on the discovery of an
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acceptor of a particular composition which has unique capabilities
with regard to the disposition of sulfur oxides in a flue gas.
SUMMARY OF THE INYENTION
It is the primary objective of the present invention to
provide a process which is not only capable of selectively removiny
sulfur oxide from a gas by means of an acceptor, but which also maximizes
the proportion of the sulfur subsequently recovered from the acceptor
by reduction with hydrogen which is in the form of hydrogen sulfide.
Accordingly, in one major embodiment, the present invention
comprises a process for removing sulfur oxides from a gas stream which
comprises: (a) contacting the gas stream with an acceptor at acceptance
conditions selected to react and retain at least a portion of the sulfur
oxides in the acceptor, the acceptor comprising a lithium doped mixture
of magnesiurn and aluminum oxides; and thereafter (b) removing the
retained sulfur oxides from the acceptor by contacting the resulting
acceptor with hydrogen at reduction conditions.
Other embodiments of the invention encompass details about
acceptor cornposition, flow schemes and reaction conditions, all of which
are hereinafter disclosed in the following discussion of each of the
facets of the invention.
DESCRIPTION OF THE INVE~TION
I have made the surprising and unexpected discovery that
a lithium doped mixture of magnesium and aluminum oxides is not only
an excellent acceptor, as far as removing sulfur oxide from a gas
2~ is concerned, but is also readily regenerable, i.e., the accepted
sulfur oxide is easily removed by reduction with hydrogen. This is in
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contradistinction to the express teaching of aforementione~ U.S.
Patent 4,071,~36(co'lumn 13) that lithiu~, among otner materials~
not be present in a particulate solid used for removing sulfur
compounds from gases, sînce it simply forms a nonregenerable sulfur-
containing solid. I have also made the equally sur~rising and justas important discovery ~ at when reduced with hydrogen the sulfur-
containing acceptor of the process of my invention produces almo~t
exclusively hydrogen sulfide gas as opposed to the much less desir-
able free sulfur or sulfur dioxide as produced by the acceptors of
known processes.
It is important to avoid the production of free sulfur or
sulfur dioxide in the course of reducing or regenerating acceptors.
Free sulfur would have the tendency to plug the process equipment
and the production of sulfur dioxide would, of course, defeat the
purpose of the sulfur oxide removal from the gas. The high purity
hydrogen sulfide obtained by the process of the present invention
may be disposed of in many ways, including direct sale as a valuable
industrial chemical.
The term "lithium doped" as used herein may ~ given a
~0 very broad construction, since it is not important exactly in what
manner the lithium is incorporated with the mixture of magnesium and
aluminum oxides. Thus, lithium doping may be accomplished by only known
methods, e.g., by adding a lithium salt to an aqueous sol of magnesium and
aluminum salts prior to forming the sol into a gel for extrusion
or sphere formation, or magnesia and alumina mixture extrudates
or spheres may be impregnated with an aqueous solution of a lithium
salt, dried and calcined. The amount of lithium added by whatever method of
preparation is used should preferably result in an acceptor containing
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about 0.5 wt. % to about 1.0 wt. % lithium on the basis of the free metal.
The weight ratio of magnesium oxide to aluminum oxide in the acceptor
is preferably from about 0.15:1.0 to about 0.25:1Ø
The process of the present invention is best employed
in a dry particle scrubber. More particularly, the acceptor is
preferably in the form of spheres or extrudates packed in a fixed
bed contained in a contacting zone or s~rubbing chamber, although
a fluidized bed of small particles of acceptor would also
be effective. The bed of acceptor may be divided into two or more
portions, with each portion being in an isolated contacting zone.
Each such isolated zone may then be cycled between the acceptance
step of the invention where the sulfur oxides combined with the
acceptor and the removal step of the invention where the sulfur
oxides are removed from the acceptor by contact with hydrogen at
reduction conditions. Such cycling may be accomplished by a sys-
tem of valves and automatic controllers, the details of which would
be familiar to those skilled in the art. The acceptance step con-
ditions may include a temperature from about 540C to about 760C,
while the removal or reduction step conditions would ideally include
a temperature in excess of about 730C.
The process of the present invention would be particularly
useful for treating the flue gas from a fluid catalytic cracking
unit (FCC) regenerator. It is not unusual for such flue gas to be
high in sulfur oxide content due to the high sulfur content frequently
found in low quality, high sulfur FCC feedstocks, the use of which is
becoming increasingly common. Associating the process of the present
invention with the FCC process would have the further advantage
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that a source of reducing hydrogen would be readily available since
hydrogen is one of the FCC reacticn products and could be supplied
from the standard FCC gas concentration or treatment facilities in
a quantity and purity quite adequate for use in the process of the
present invention. Unlike some of the aforementioned background
processes the acceptor used by the present inve~tion could not be
mixed and circulated with the FCC catalyst because the lithium
compounds would tend to migrate from the acceptor to the c~talyst
which would be deleterious to the performance of both.
The following nonli Mi ting examples are presented to illus-
trate the remarkable capability of the process of the present in-
vention to not only achieve complete acceptance of S02 from a gas,
but also to release the retained sulfur in the course of reduction
with hydrogen in the desirable form of H2S.
EXAMPLE I
Various samples of aluminum and magnesium oxide mixtures
were obtained, two doped with sodium and two doped with lithium.
The latter two samples were prepared for use in the process of
the present invention by adding lithium nitrate to a solution of
aluminum and magnesium nitrates, gelling the solution by adding
sufficient ammonium hydroxide to it to raise the pH to 8.5,
filtering out the aluminum/magnesium co-gel, drying the gel at
150C for 3 hours and calcining the nel at 595C for 6 hours.
This preparation was used for obtaining the small quantities of
sample needed for the laboratory analysis. Commercial quantities
of acceptor would be obtained by spray dryir,g or oil droppinq
and aging the co-gel by methods known to the art.
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Follo~ing is a summary of the compositions of the above
acceptors and performance (averaged over five cycles), when used
to remove sulfur dioxide from a typical flue gas at 730 C
followed by reduction with hydrogen at 730C.
Acceptor Composition Reduction
Gesignation ,. Na ,~ Li ~llg Al ~ Acceptance ' H S SO ~ S
~ 2- 2 - -~
A 2.1 - 19. ~ 33 8 10O 75.3 1.022.7
B 1.2 - 17.3 33.2 100 80.0 020.0
C -0.69 19.2 33.8 100 96.4 0 4.6
D -0.56 17.9 34.6 100 96.8 0 3.2
The remarkable effect of the process of the present inven-
tion is clearly apparent from the above data. The difference in thepercent hydrogen sulfide produced during reduction between the sodium
doped and lithium doped acceptor is testimony to the advantageous use
of the present invention.
Data for the performance of above samples A and C over five
15 cycles are as follows:
% H S
Acceptors ~y~ 22 3 4 5
A 95.8 82.8 70.9 65.3 66.2
C 90.7 95.2 99.8 96.5 g9.8
The last above data demonstrates that the advantageous
effect of the present invention remains high and even improves over
20 subsequent cycles in contradistinction to the process using the
sodium doped acceptor. The high H2S selectivity observed for cycle -1
of acceptor A was due to the low recovery of free sulfur from the
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acceptor during the reduction step of that cycle. The sulfur re-
covery in the first cycle is al~Jays lo~l for sodium doped acceptors.
Presumably part of the sodium retains sulfur tenaciously with such
acceptors.
EXAMPLE _
For the purpo~e of further comparison of the process of
the present invention and a process known to the art~ the same
above test procedure was applied to an Al203 acceptor as disclosed
in Blanton, Jr., et al. The results obtained are as follows:
Reduction
Composition ~O Acceptance C/J H2S /~ S02 ~v Sx
Al23 40 5 33.4 26.l
The low H2S selectivity and even low degree of acceptance
of the Blanton, Jr., et al. acceptor is further evidence of the
advantageous effect of the present invention.
EXAMPLE III
Thermogravimetric analysis was conducted on Al203 and the
Li-MgO-Al2O3 of the present invention to determine the maximum S02
uptake capacity of each, as one would prefer an acceptor with not
only a high S02 acceptance efficiency ~acceptance rate) but also a
high capacity (total SO2 adsorbed for saturation of the acceptor).
The latter is of importance particularly in the present process,
because a high capacity means a longer duration before the acceptor
needs to be regenerated which also means the frequency of cycling
between acceptance and reduction will be minimal.
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The following results were obtained:
Net weight increase from SO at 730C
(c, of the initial sam~le ~le~ght) _ _
Li.MgO-A1203 50
2 3 3.
The superiority of the acceptor of the present invention
S with regard to 52 uptake capacity is thus also dernonstrated.
