Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~r~ 'a'~ 9
_ ATM~NT OE ORGANIC SULFUR G~SES ESPECIALJY IN
KRAFT PULPING SYSTEMS AND P~OCESSES
BACKGROUND AND SUMMARY OF THE INVENTION
In the production of kraft paper pulp, and like
processes, it has been recognized for many years
that off gases are produced which contain a high
volume of organic sulfur compounds. Heretofore, no
significant use has been made of such off gases,
which typically occur from the digester, and from
black liquor evaporators. Recently, in the
commercialization of a method for decreasing black
liquor viscosity disclosed in U.S. patent 4,929,307
it was found that very high volumes of off gases
containing organic sulfur compounds including DMS,
methyl mercaptan, and hydrogen sulfide are
produced. Such high volumes of gas are produced
that some technique must be utilized to act on the
gases, otherwise the black liquor heating apparatus
may become a significant source of air pollution,
and a substantial volume of sulfur will be lost from
the pulping system.
From all of the various sources of off gases
containing organic sulfur compounds in kraft (and
~ulfite) pulping the concentrations o organic
sulfur compounds are much higher than are found in
other industries that have dealt with handling such
gases on a commercial basis (for example the oil
indu try). Typically in the oil industry, the
concentration of organic sulfur ~ompounds is less
than three percent, and there is virtually no water
vapor present. However in the kraft pulping
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industry, off gases from the digester, black liquor
evaporators, and black liquor heaters, typically are
in the range of a~out 10-80% (usually over 15%), and
water vapor (in the form of steam) is virtually
universally present.
According to the present invention, a method is
provided for treatment of the off gases associated
with kraft or æulfite pulping so as to change them
from a source of pollution to a source of useful
chemicals.
According to one aspect of the present
invention a method of acting on a first gas stream
consisting essentially of of gases from a kraft or
sulfite digester, off gases from a black liquor
evaporator, and mixtures thereo, i6 provided which
comprises the steps of (a) treating the gases in the
first gas stream to produce a second ga~ stream
containing primarily hydrogen sulfide and methane,
and then (b) separating the hydrogen sulfide from
the methane. Step (a) may be practiced by adding
hydrogen to the gas in the first gas stream, and
passing the first gas stream -- in the presence of
the added hydrogen -- past a hydrogen
desulfurization catalyst (e.g. one selected from the
group consisting essentially of nickel molybdenum
and cobalt molybdenum) to produce a second gas
stream. Alternatively step (a) may be practiced by
effecting substochiometrlc combustion of at least
some of the gas in the first gas stream to produce a
third gas stream, removing particulates from the gas
in the third gas stream, and passing the gas in the
third gas stream into contact with a dirty shift
catalyst (e.g. one containing oxides of cobalt and
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molybdenum, or iron and chromium) to subject it to
the water gas shift reaction namely:
H o + C0 -> H2 ~ C2
to ther~by produce the second gas stream. Step (b)
may be practiced by passing the gas in the second
gas stream into contact with white liquor from kraft
paper pulp being processed to remove the hydrogen
sulfide therefrom, and increase the sulfidity of the
white liquor.
According to another aspect of the present
invention, a method of converting a first gas stream
in~luding water vapor and over 10% (e.g. about
15-80%) by weight organic sulfur compounds, into a
second gas stream comprising primarily H2S and
methane is provided. The first gas stream is
preferably composed of off gases from kraft or
sulfite pulping processes containing methyl
mercaptan, DMS, and hydrogen sulfide, but other
sources may also be treated according to the
invention. The method comprises the steps of: ~a)
Adding hydrogen to the gas in the first gas stream.
And, (b) passing the first gas stream, in the
presence of the added hydrogen, past a hydrogen
desulfurization catalyst to produce the second gas
stream. There is also preferably the further step
(c), prior to step (a), of removing the vast
majority of the water vapor from the first gas
stream. The method may consist of only these steps
(a)-(c). Step (a) may be practiced by: (al~
Removing hydrogen sulfide from the gas in the second
gas stream to produce a third gas stream containing
primarily methane and hydrogen. (a2) Reacting the
gas in the third gas stream with steam, and then
4 ~ S3 ~r3 ~
subjecting it to the water gas shift reaction in the
presence of a catalyst to produce a fourth gas
stream containing primarily hydrogen, methane, and
carbon dioxide. And, (a3) separating the hydrogen
from the fourth gas stream, to be added to the gas
in the first gas stream, while producing a fifth gas
stream containing primarily methane and carbon
dioxide. There may also be the further step of
combusting the gas in the fifth gas stream to
provide heat to produce the steam utilized in step
(a2).
According to yet another aspect of the present
invention a method of converting a first gas stream
including water vapor, and over 10% (e.g. about
15-80%) by weight organic sulfur compounds, into a
third gas stream comprising primarily H2S and
methane is provided. This method comprises the
steps of: (a) Effecting substochiometric combustion
of at least some the gas in the first gas stream to
produce a second gas stream. (b) Removing
particulates from the gas in the second gas stream.
And, (c) passing the gas in the second gas stream
into contact with a dirty shift catalyst to subject
it to the water gas shift reaction and thereby
produce a third gas stream comprising primarily
hydrogen sulfide and methane, with some carbon
dioxide. Step (a) may be practiced to effect
substochiometric combustion of only a first part of
the gas in the first gas stream, and then there is a
further step (d) of passing a second part of the gas
in the first gas stream into the second gas stream
before the practice of step (c). Alternatively,
step (a) may be practiced to effect substochiometric
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combustion of essentially all of the gas in the
first gas stream.
It is the primary object of the present
invention to provide for the effective utilization
of gases containing high amounts of organic sulfur
compounds, particularly for producing the gas
containing hydrogen sulfide and methane from such an
off gas. This and other objects of the invention
will become clear from an inspection of the detailed
description of the invention and from the appended
claims.
BRIEF DESCRIPTION OE T~E DRA~I~GS
FIGURES lA and lB are a schematic of one
exemplary embodiment of apparatus for treating gases
containing organic sulphur compounds in accordance
with the present invention;
EIGURE 2 is a schematic of an alternative
method of treating gases containing organic sulphur
compounds; and
FIGURE 3 is a schematic like that of FIGURE 2
indicating a few process changes.
DETAILED DESCRIPTION OE THE DRAWING5
FIGURES lA and lB schematically illustrate at
10 apparatus for practicing a method of converting a
gas stream having a large amount of organic sulfur
compounds therein into a second gas stream
comprising primarily H2S and methane. Typically the
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off gas streams treated according to the invention
include water vapor and over 10% (e.g. about 15-80%)
by weight organic sulfur compounds. The organic
sulfur compounds typically present are methyl
mercaptan, dimethyl sulfide (DMS), and hydrogen
sulfide, the water vapor is in the form of steam,
and additional organic and inorganic compounds are
also present. While the apparatus 10 is useful for
the treatment of any gas stream having a high level
of organic sulfur compounds, it is preferably
utilized with -- and will be described with respect
to herein -- the treatment of off gases from kraft
~or sulfite) pulping processes in the production of
paper pulp.
In a typical kraft pulping process, a digester
12 (FIGURE lB) -- which may be a continuous digester
such as that sold by Kamyr, Inc. of Glens Falls, New
York, or a batch digester -- has a stream of off
gases 13, and black liquor is withdrawn at an
intermediate position as indicated at 14 in FIGURE
lB, while the kraft pulp produced is discharged at
15. When the black liquor in line 14 is discharged,
it may be passed directly to black liquor
evaporators, but preferably it is first subjected to
a heat treatment process at step 17 to produce off
gases in line 18. The heat treatment of block 17 in
FIGURE lA is preferably that described in U.S.
patent 4,929,307. After heat treatment at 17 the
black liquor passes to evaporators 19 which also
produce off gases in line 20, then to a conventional
recovery ~oiler 22, with conventional white liquor
manufacture at block 23, to produce white li~uor in
line 24 having conventional sulfidity.
'
According to the present invention the off
gases in line 18 -- which may include only those
from the heat treatment 17, only those from the
digester line 13, only those from the black liquor
evaporators in line 20, or a combination of two or
all of them -- are preîerably first subjected to a
drying or absorption stage (as indicated
schematically at 27 in FIGURE 1) in order to remove
a majority of the water vapor. The removal of the
water vapor increases the concentration of organic
sulfur gases. Step 27 is accomplished by drying the
gas stream by any conventional means and/or by
absorbing or adsorbing the organic portion of the
gas onto a solid, or additionally by absorbing the
off gas into one of any non-polar li~uids (e.g.
kerosene or mineral oil). Treatment may be provided
of a liquid mixture, but preferably according to the
invention further treatment is of the concentrated
gas.
The next steps in the practice of the invention
as illustrated in FIGURE 1 are to add hydrogen, and
then pass the gas in the presence of the added
hydrogen past a hydrogen desulfurization catalyst.
This is accomplished in the hydrogen desulfurization
(HDS) unit 28, with the hydrogen gas from line 29
added to the unit 28 along with the gas in line 18.
The hydrogen gas in line 29 can come from any source
(e.g. be purchased , from water electroylsis,
etc.). It is preferred that the hydrogen
desulfurization catalyst consist essentially of a
transition metal molybdenum alloy, particularly a
nickel molybdenum or cobalt molybdenum alloy. An
?~ 9
example of the decomposition reaction that takes
place in unit 28 is as follows:
CH3SCH3 2 H2 4 2
This process is effective despite the fact that the
concentration of organic sulfur compounds is
typically higher than 15%. The temperature
conditions in the unit 28 must be at least 50F over
the dew point of the gas, and are typically
390-750F, preferably about 410-700F, and the
pressure conditions are typically 150-300 psig, but
can run from atmospheric pressure to 1,000 psig.
If liquid phase processing is practiced, the
hydrogen sulfide, methane, and other sulfur free
gases may be stripped from the liguid phase.
Typically however in gaseous processing, the product
gas stream in line 30 is further acted upon. The
gas stream in line 30 contains primarily methane,
and hydrogen sulfide, with hydrogen also present.
Preferably the hydrogen sulfide is then separated
out from the gas in stream 30. While the hydrogen
sulfide may be separated as a gas and then used in
its gaseous form in conventional pulping techniques,
preferably it is fed to a scrubber 32 in which it is
brought into contact with a caustic solution. In
the preferred embodiment illustrated in the
drawings, white liquor (although green liquor may
also be advantageously utilized) from line 24 is
passed into inlet 33 of scrubber 32, the hydrogen
sulfide being absorbed in the white liquor and
thereby significantly enhancing its sulfidity. The
significantly enhanced sulfidity white liquor then
passes through the outlet 34 of the ~crubber into
line 35, to ~e used in the pulping process. The gas
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stream that remains, in line 38, contains primarily
methane, but also some hydrogen. This is
accomplished by splitting of gas flow in line 38
into lines 39 and 40 (FIGURE lA), line 39 providing
a reformer feed gas line for a methane/steam
reformer 42, while the gas in line 40 is part of the
feed gas for the pressure swing adsorption ~PSA~ :
unit 43.
A feed water line 45 is provided through heat
exchanger 46 in the hot gas exhaust conduit 47 of
reformer 42. The feed water is turned into steam in
46, which passes in line 48 to mix with the reformer
eed gas in line 39, in the line 49. In the
reformer 42, the following reaction occurs:
CnHm ~ n H20 -> n C0 ~ (n ~ m/2) ~2
The gas in line 50 thus is primarily hydrogen and
carbon monoxide. From line 50 it passes to a shift
converter 52. In the shift converter 52, the gas is
reacted over an appropriate mixture of the oxides of
iron and chromium which act as catalysts to promote
the water gas shift reaction
H 0 ~ C0 -> H2 + C2
The temperature conditions in the unit 52 are at
least 50F above the dew point of the gas, and are
typically 450-950F, and the pressure conditions are
typically 150-300 psig, but can run from atmospheric
to 1,000 psig.
The product gas mixture from the water gas
shift reaction in line 54 contains primarily
hydrogen and carbon dioxide, and this is added with
the methane from line 40 to the PSA unit 43. In ths
unit 43, conventional adsorption or absorption
methods are practiced to yield the pure hydrogen
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stream ~9, and a second stream 56 which contains
primarily methane and carbon dioxide. The methane
and carbon dioxide in line 56 may be split into a
line S7 which is used as the fuel gas for the
combustion chamber 58 of the methane/steam reformer
42 -- and thus providing the energy source for the
reformer 42. If there is any excess fuel gas, it
may be passed into storage at 60, or used as the
combustion source for the lime Xiln typically
associated with a kraft pulping plant.
While not part of the claimed method according
to the invention, it is to be understood that the
white liguor in line 35 may be very advantageously
used in pulp processing. The liquor in line 35 has
enhanced sulfidity compared to the conventional
white liguor in line 24, and may be used in
apparatus 62 (FIGURE lB) which comprises a
conventional chip feed system, and/or a conventional
impregnation vessel. The pulp slurry, with the
increased sulfidity white liquor, in line 64 passes
to the top of the digester 12 (typically a
continuous digester), while white li~uor of
conventional sulfidity passes in line 66 to be added
to a circulation loop in the digester 12. Typically
the conventional sulfidity white liquor in line 66
can be used in the MCC~ process of Kamyr, Inc. of
Glens Falls, New York.
It is noted that when white liquor with
enhanced sulfidity is utilized in the pulping
process, it can be expected that the off gases in
line 13 from digester 12 will have an increased
amount of organic sulfur compounds, thereby either
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necessitating, or making more desirable, their
treatment in the apparatus heretofore described.
In the ollowing example, a source of DMS at a
high concentration was treated over a hydrotreating
catalyst to demonstrate the feasibility of the
process heretofore described. Because conventional
testing facilities that are capable of hydrotreating
streams of DMS are not set up to act upon the exact
gas streams that will be utilized in the practice of
the invention, but in order to effectively simulate
such reactions, the DMS was processed in a blend of
25 volu~e percent DMS and 75 volume percent
kerosene. The test is reported as follows:
Example 1
The initial processing conditions selected
were: 6.0 LHSV, 150 psig and 2500 SCFB reactor gas
rate (based on total feed, not just DMS). These
conditions were sufficient to convert the great
majority of the DMS when the temperature was raised
to an average of 640F. Reducing the throughput to
3.0 LHSV resulted in a slight apparent increase in
conversion. Reducing the reactor gas rate to 1600
SCFB did not slow the conversation of DMS (in fact
it may have increased it), but reducing it to about
1000 SCFB did.
A reactor in a bench scale isothermal
hydrotreating pilot plant was loaded with 87 ccs. of
Criterion C324 nickel molybdenum hydrotreating
catalyst.
After pressure testing and gas meter
calibration checks, the unit was started up and the
catalyst sulfided using standard procedure.
Kerosene containing 4 wt% DMS was used as sulfiding
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12 ~ ,f~ g
feed. After sulfiding was completed, the
temperature was lowered to 500F and the run feed
was introduced.
The run feed was a 25/7~ volume percent blend
of DMS and kerosene. Assuming complete conversion
of the DMS with no reaction of the kerosene, then
the expected yield of ~2S would be 14.2 wt% of the
total feed and the yield of Cl would be 13.3 wt%.
If there is some reaction of the kerosene it should
not contribute more than 0.01 to 0.02 wt% to the H2S
yield. The contribution of the kerosene to the
methane yield is expected to be small.
The unit was lined out at the initial run
conditions of S00F, 6.0 LHSV, 150 psig and 2500
SCFB reactor gas rate (based on the total feed).
The conversion of DMS ~as very low at these
conditions, so the temperature was raised. Between
500F and 520F, the DMS began to convert in large
quantities, which generated a large exotherm. Once
the reactor temperatures were ~rought under control,
the reactor inlet temperature was increased as much
as possible (to about 560F) so that the average
reactor temperature was maximized at about 660F
while the hottest points were held to 734F.
Exceeding approximately 750F was avoided so as to
avoid extensive thermal cracking of the kerosene,
which could possibly cause a much greater exotherm.
At these conditions, the weight percent yields of
hydrogen sulfide and methane were approximately 13.3
and 12.5, respectiveLy.
The reactor gas rate was reduced to 1575 SCFB,
which did not reduce conversion at all. H2S and C
yields were 14.1 and 12.6, which indicate
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13
conversation is no lower and possibly even higher
(perhaps due to increased residence time in the
reactor~. The reactor gas was then reduced to 1040
SCFB which did cause the H2S yield to drop to 13.4
although the Cl yield increased to about 13.5.
The example indicated that near-complete
conversion of DMS over a hydrotreating catalyst is
feasible. The temperature at which the reaction
begins to proceed very quickly is between 510F and
520F (at the catalyst inlet). A minimum gas rate
o 1600 SCFB (based on total feed, the te~t feed
being at 75/25 volume percent blend of kerosene and
DMS) is desirable to keep the reaction rom being
limited by hydrogen availability, but additional
hydrogen may be necessary for long catalyst life.
Another embodiment according to the invention
is schematically illustrated in FIGURE 2. In this
embodiment, the off gases in line lB are split into
two streams, 71 and 72, with the stream 71 led to
the partial oxidation block 73 in FIGURE 2. That is
the gas in line 71 is reacted with a
substochiometric amount of oxygen or air from line
74 (with or without steam) to produce a gas in line
75 rich in hydrogen and carbon monoxide, and also
containing carbon dioxide, hydrogen sulfide,
carbonyl sulfide, methane, sulfur dioxide, and other
compounds including particulates (soot). Nitrogen
will also be present if the gas from source 74 is
air instead of essentially pure oxygen. The gas in
line 75 is preferably cleaned of soot by
conventional techni~ues as indicated at 76, and feed
gas in line 72 is recombined with the gas in line
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75, either before or after the soot removal block
76. Then the gas in line 75 is passed over a
special shift catalyst which can tolerate sulfur
compounds, as indicated at 77 in FIGURE 2. The
dirty shift catalyst in 77 -- which preferably
comprises an alumina catalyst containing oxides of
an cobalt and molybdenum (commercially available
from BASF under the trade name K8-11) -- promotes
the water gas shift reaction H20 ~ C0 -> H2 ~ C2
even in the presence of sulfur. The temperature and
pressure conditions in 77 are about 450-950F, and
about 150-300 psig. A normal catalyst for the water
gas shift reaction, such as oxides of iron and
chromium as described with respect tv FIGURE lA, is
poisoned by sulfur compounds, ~ut catalysts
containing oxides of cobalt and molybdenum can
effectively treat such gases. This catalyst also
promotes the hydrolysis of carbonyl sulfide into
hydrogen sulfide as follows:
COS + H2 -~ H2S ~ C2 .:
The primarily hydrogen and carbon dioxide gas
stream 79 produced can then be used to treat the
remaining sulfur containing off gases which are
added in line 72. The gases in line 72 may be added
either as illustrated in FIGURE 2, or directly to
the line 79. The gases in line 79 may also be added
to an HDS unit 28 to effect organic sulfur
decomposition, as indicated at box 80 in FIGURE 2.
The latter reaction may also be accomplished by
passing the hydrogen, carbon monoxide, and sulfur
containing off gas mixtures through several sulfur
tolerant carbon monoxide shift reactors in series.
The sulfur tolerant catalyst in the shift reactor
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can also promote the organic sulfur decomposition
reaction as follows: CH3SCH3 ~ 2 H2 ~~~ 2 CH4 ~
H2S. ThP gas mixture is then again separated into a
stream containing hydrogen sulfide and a stream
containing clean fuel gas. If this processing
scheme is employed, the final gas mixture before
separation will contain carbon dioxide, methane,
hydrogen, and hydrogen sulfide. The gas stream will
also contain nitrogen if air was used as the oxygen
source for the partial oxidation reaction.
The reaction then proceeds to the H2S recovery
stage 81. If carbon dioxide can be tolerated in the
hydrogen sulfide stream, a simple acid gas scrubbing
system may be used to separate the carbon dioxide
and hydrogen sulfide from the gas mixture. If
carbon dioxide cannot be tolerated in the hydrogen
sulfide stream, selective removal of the hydrogen
sulfide by adsorption, absorption, or membrane may
be employed. Scrubbing with a selective solvent
such as methyldiethanolamine can remove essentially
all of the hydrogen sulfide from the gas stream
while removing only a small fraction of the carbon
dioxide. The hydrogen sulfide in line 82 may be
used directly in pulping, or absorbed into white or
green li~uor as described with respect to FIGURE 1.
The gas in line 83 is primarily fuel gas, but
it may contain nitrogen if air was used as the gas
in line 74. The nitrogen may optionally be removed
-- as indicated by line box 84 -- by adsorption,
membrane separation, or other viable techniques so
as to increase the heat value of the product fuel
gas. The remaining gas may then be used as a fuel
or the carbon dioxide and remaining hydrogen may be
.
16 ~ ` q ~3
passed, at 86, over a methanation reactor containing
an appropriate nickel catalyst where the carbon
dioxide and hydrogen are converted into methane and
water vapor as follows:
C2 + 4 H2 ~~ C~4 + 2 H2
The resulting ~uel gas stream 88 can then be
dehydrated at 87, and used as a clean fuel source.
Example 2
The following table provides the summary of gas
analysis in reacting DMS with oxygen, in the
presence of nitrogen (simulating air) to demonstrate
the composition of gases from the partial oxidation
stage 73:
TAsLEI
Tille 20:00 24:00 04:00 08:00
0~ygen Rate, llhr 16.0 21.3 10.7 16.0
~ol eslhr0 . 68 0 . 900. 45 0 . 68
Nitrogen Rate l/hr 22.6 22.6 0.0 0.0
~oleslhr0,96 0.96 0.0 0.0
DMS Rate, g~/hr31.1 31.1 31.1 31.1
~oles/hr0.50 0.50 0.50 0.50
Sa~pling ~lethodBrine Heat Heat Brine
Special GC Analysis
N, C0, C 66.7 65.7 29.0 38.0
SU I 9.6 1~.4 10.0 4.0
D~ 4.9 5.7 13.4 14.4
R S 0.0 0.003 0.0 0.0
C~S 1.4 0.6 ~.5 0.7
Standard GC Analysis
C 0.9 1.8 4.2 3.4
i3-C 0.2 0.2
H 4 9.2 16.0 35.6 44.3
o2 1.4 1.7 2.9 3.1
112 55.4 50.7 0.1 0.4
c2 12.6 8,0 22.8 9.2
C0 14.9 18.7 29.8 35.8
C0 1.6 2.0 1.8 2.4
C D 2 . 5 1 . 1 2 . 4 1 . I
C22 0.2 0.06 0.1 0.1
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The following table indicates similar results
when water vapor is part of the feed to the partial
oxidation stage 73:
TABLE2
Ti~e 17:00 18:00
O~ygen Rate, 1/hr 21.3 21.3
~oles/hr 0 90 0.90
Nitrogen Rate, I/hr 17.8 17.8
ooles/hr0 75 0,75
DMS Rate, g~/hr31.1 31.1
noles/hr 0.50 0.50
Water Rate, g~/hr 9.0 9.0
roles/hr 0 50 0 50
Sa~pling MethodSaturated S04 Solution
Special ~C Analysis
N, C0, Cl 90.7 92.7
Su 0.0 0.0
DM~S 4.6 2.4
R S 0.8 0.8
C~S 4 0 4.1
Standard GC Analysis
C 0.5 0.3
'12 6.4 1.5
0 0.8 0.9
N22 46.4 64.1
C 21.1 12.8
C~ ~.7 1.6
C2 10.3 16.0
C2~ 4.3 2.4
C2 0.5 0.4
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FIGURE 3 illustrates a slightly modified form
of the process of FIGURE 2. In FIGURE 3 components
comparable to those in FIGURE 2 are shown by the
same reference numeral. The major distinction of
the method of FIGURE 3 is that the substochiometric
combustion is for the entire gas stream in line 71.
Also, block 91 is provided indicating the heat value
of the gas may be incxeased by passing it over a
sulfur tolerant direct methanation catalyst (such as
one available from Haldor Topsoe Company~ which
promotes the following reaction:
2 C0 + 2 H2 ~~ CH4 + C02
The gas mixture may then be purified and the sulfur
compounds isolated as described with respect to
FIGURE 2.
It will thus be seen that according to the
present invention it is possible to very effectively
act upon the off gases from a kraft or sulfite
digester, or black liquor evaporators, as well as
off gases from the heating of black liquor to reduce
its viscosity, which off gases contain high levels
of orqanic sulfur compounds and water vapor, the gas
typically containing over 10% ~e.g. about 15-80%) by
weight organic sulur compounds. While the
invention has been herein shown and described in
what is presently conceived the most practical and
preferred embodiment it will be apparent to those of
ordinary skill in the art that many modifications
may be made thereof within the scope of the
invention, which scope is to be accorded the
broadest interpretation of the appended claims so as
to encompass all equivalent processes and procedures.
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