Method for removing hydrogen sulfide from sulfur-bearing industrial gases with claus-type reactors

Integrating an absorption-desorption step, a Claus plant, and a catalytic hydrogen reactor in a recycle loop provides a gas-desulfuration plant that operates at a substantially zero emission level. Hydrogen sulfide is removed from industrial gas streams by absorption-desorption in a liquid absorbent. The resulting foul gas is then reacted with sulfur dioxide in a Claus reaction, i.e., the reaction of hydrogen sulfide with sulfur dioxide to form elemental sulfur and water. The tail gas from this Claus reaction is then passed through a catalytic hydrogenation reactor together with a supply of hydrogen to reduce the sulfur and sulfur compounds to hydrogen sulfide. The now reduced tail gas is then recycled back to the industrial gas, upstream of the absorption-desorption step.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to a co-pending application entitled "Method 
for Substantially Complete Removal of Hydrogen Sulfide from Sulfur-Bearing 
Industrial Gases" filed concurrently with the present application in the 
names of M. O. Tarhan and D. Kwasnoski and assigned to the assignee of the 
present application. 
BACKGROUND OF THE INVENTION 
This invention relates to the removal of undesirable components from 
industrial gases and more particularly to the substantial elimination of 
hydrogen sulfide from such gases. 
Industrial gases such as coke oven gas, natural gas and various artifically 
produced fuel gases are used either by industrial plants to make useful 
products or burned in suitable combustion apparatus to produce heat. These 
gases are composed of varying mixtures of hydrogen, carbon monoxide, 
various aliphatic and aromatic hydrocarbons, hydrogen sulfide, hydrogen 
cyanide, carbonyl sulfide and other combustibles. The presence of sulfur 
compounds in such industrial gases is undesirable because of possible 
corrosion of intermediate gas transmission lines and other apparatus by 
the gases, possible contamination of chemical substances made from the 
gases, and possible discharge of undesirable concentrations of sulfur 
oxides to the atmosphere during combustion of the gases. 
In the past such industrial gases have often been treated by passing them 
through absorption-desortion processes of various types. These 
absorption-desorption processes give off so-called foul gases which are 
treated to recover the sulfur present in the gas and thus prevent its 
discharge to the atmosphere. 
A. Some Typical Absorption-Desorption Processes 
Typical absorption-desorption processes are the hot potassium carbonate 
process, the vacuum carbonate process, the amine processes, especially 
those using mono-, di-, and triethanolamine, and various other processes 
using organic solvents. The alkanolamine processes, and particularly the 
diethanolamine and monoethanolamine processes, have proven to be 
especially attractive from an industrial standpoint due to their 
attractive economics and relatively trouble-free operation. The 
monoethanolamine process in particular has proven to be very convenient 
and efficient in removing hydrogen sulfide and other sulfur compounds from 
sulfur containing gas streams. Monoethanolamine solutions can easily 
remove substantially the entire sulfur component from industrial gases so 
that the gas leaving the monoethanolamine absorber contains no more than 
10 grains of sulfur per 100 standard cubic feet of gas exhausted, i.e. 
less than 0.2288 grams of hydrogen sulfide per one standard cubic meter. 
This amount of sulfur is quantitatively very minor. 
B. Claus Process 
These absorption-desorption processes, while effective to reduce the sulfur 
content of treated industrial gas to a very low level, regenerate the acid 
components of the gases in a more concentrated form. The regenerated 
"foul" gases have to be treated in turn to reduce their sulfur content in 
some satisfactory manner. Very frequently the foul gases from the 
absorption-desorption processes have been used to produce elemental sulfur 
by some variation of the Claus process. In this process a portion of the 
sulfur, usually in the form of hydrogen sulfide, is oxidized to sulfur 
dioxide and the sulfur dioxide and remaining hydrogen sulfide are then 
reacted in a catalytic or other type converter to form elemental sulfur 
and water. There are a number of industrial variations of the process in 
which either a portion of the initial hydrogen sulfide is oxidized to 
sulfur dioxide or a portion of the final elemental sulfur product is 
subsequently oxidized to sulfur dioxide for use in the Claus reaction. 
A single Claus reactor exhibits a fairly poor conversion of sulfur based 
upon the amount of sulfur in the inlet foul gas and it is customary to use 
three or even four Claus reactors in series in order to effect recovery of 
more than 85 to 90% of the sulfur. The reason for the incomplete recovery 
of sulfur is that the reversible Claus reaction 
EQU 2H.sub.2 S + SO.sub.2 .revreaction. 3S + 2H.sub.2 O 
cannot be completed since sufficient water and sulfur vapor are always 
present in the reaction area to limit the desired reaction. The 
thermodynamic equilibrium of the traditional Claus reaction also does not 
favor completion of the reaction to the right. Carbon dioxide and water 
vapor in the feed gas are likewise diluents which shift the equilibrium of 
the reaction adversely. Hydrocarbons in the feed, furthermore, affect the 
efficiency of the reaction by increasing the formation of undesirable side 
reaction products such as carbonyl sulfide and carbon disulfide. The tail 
gas from the Claus plant may as a result of these various factors contain 
as much as 10% of the sulfur originally removed by the absorption system 
from the fuel gas. 
Thus while the Claus process is fairly efficient and has the advantage over 
other sulfur recovery processes of producing a good quality, useable 
sulfur, it does have the disadvantage that there is invariably a residue 
of gas known as the tail gas in which either sulfur dioxide or hydrogen 
sulfide, or very frequently both, as well as carbonyl sulfide and carbon 
disulfide, remain. This tail gas must be disposed of in some manner and is 
frequently at this point discharged to the atmosphere after incineration. 
While the total amount of residual sulfur compounds contained in the tail 
gas and discharged to the atmosphere is much reduced from the 
concentrations of sulfur in the original gas treated, there is still, due 
to inherent inefficiencies of the system, a residual amount of sulfur 
remaining in the tail gas which may be objectionable. The amount of this 
remaining sulfur can be decreased by subsequent processing, for example, 
by the use of several Claus-type processing reactors in series. However, 
due to the small amount of remaining sulfur compounds in the final tail 
gas, any further processing becomes more and more inefficient and 
expensive and there is a final minimum of sulfur which is almost 
impossible to remove. 
C. Some Methods for Final-Treating the Tail Gas from a Claus Reactor 
One fairly simple expedient for final treatment of sulfur containing tail 
gases has been to oxidize all the remaining sulfur to sulfur dioxide and 
then to wash the sulfur dioxide out of the gas with a simple water wash 
system. The wash water can then be used to make sulfuric acid if it is 
concentrated enough, or it can be discarded. However, the water containing 
the sulfur dioxide, if discarded to waste, represents an escape and loss 
of sulfur values. On the other hand, the amount of sulfur dioxide 
dissolved in the waste water is very frequently insufficient for really 
effective use as a source of sulfur. 
A number of other processes have been proposed as cleanup processes for 
treatment of Claus-unit tail gas. Several of these depend upon treatment 
of the tail gas so that the residual sulfur occurs as hydrogen sulfide, 
which is then converted to sulfur in a so-called Stretford unit. There are 
a number of other proposals for improved cleanup of the tail gas including 
the use of improved catalysts in the Claus reactors, wet scrubbing, 
reaction with ammonia, high temperature sulfur dioxide removal, 
concentration of sulfur dioxide by absorption, catalytic sulfuric acid 
production, and absorption-desorption type chemical removal. Some of these 
proposals are applied to the tail gas after incineration to change all of 
the contained sulfur to sulfur dioxide. While some are fairly efficient, 
at least in the laboratory, in removing the sulfur components, few if any 
of these proposals are really satisfactory on an industrial scale and none 
is completely efficient in removing sulfur compounds. 
A large number of processes have also been developed which specifically 
make use of the broad principle of recycling in order to increase the 
recovery of the sulfur compounds from a gas. For example, the tail gas 
from a Claus plant has been oxidized to convert all residual hydrogen 
sulfide to sulfur dioxide, and the sulfur dioxide has then been recycled 
back to the Claus reactor to replace a portion of the sulfur dioxide used 
in the reactor. In some proposals the sulfur dioxide has been absorbed 
from the tail gas into lime or the like and then regenerated from the lime 
and recycled into the Claus plant. A variation of this recycle arrangement 
would maintain an excess of hydrogen sulfide in the reacting gases 
resulting in an excess of hydrogen sulfide in the effluent from the 
reactor which excess is separated by any suitable means and passed to the 
sulfur burner which oxidizes hydrogen sulfide to sulfur dioxide for use in 
the Claus plant. 
Several processes have been developed in which hydrogen sulfide and sulfur 
dioxide are reacted together in a liquid reaction medium of some suitable 
composition. The liquid reaction medium may be renewed at intervals by 
stripping volatilizable gases including, for example, hydrogen sulfide 
which is then recycled to the primary reactor. Occasionally an ammonium 
salt solution such as an ammonium sulfite solution has been used as the 
reaction or absorption solution and in these cases unreacted hydrogen 
sulfide or sulfur dioxide passing from the solution may be recycled back 
to the absorption solution. It has also been proposed to recycle the 
entire tail gas stream containing both sulfur dioxide and hydrogen sulfide 
from a Claus plant back to an original coke oven gas stream to react with 
the ammonia in the coke gas. More recently it has also been proposed to 
use a so-called molecular sieve to reversibly absorb hydrogen sulfide from 
a tail gas derived from a Claus plant and recycle it back to an absorption 
step. 
D. Related Co-pending Method of Disposing of Tail Gas by Recycling 
A co-pending application filed concurrently with the present application by 
M. O. Tarhan and D. Kwasnoski, referred to above under the heading "Cross 
References to Related Applications", discloses a novel process for the 
substantially complete elimination of sulfur-bearing tail gases wherein a 
reaction loop is established which includes an absorption-desorption 
process unit and a liquid-phase sulfur reaction unit. The feed gas stream 
is first desulfurized in the absorption-desorption unit and the resulting 
foul gas is then treated in the liquid phase reaction unit. The tail gas 
from the liquid-phase reaction is recycled back to the gas entering the 
absorption-desorption apparatus thus eliminating the tail gas. An excess 
of hydrogen sulfide is maintained in the liquid phase sulfur reaction 
apparatus to insure that no sulfur dioxide is recycled. Since the 
absorption-desorption system is extremely efficient in removing hydrogen 
sulfide from the combined gas stream, substantially all of the sulfur in 
the original feed gas can be removed. Less than 10 grains of sulfur values 
per 100 standard cubic feet of gas remain in the gas stream exhausted from 
the absorber. The process can be combined with a Claus type process 
wherein the foul gas derived from the absorption-desorption system 
initially passes through one or more Claus type reaction units prior to 
passage through the liquid phase sulfur reaction unit. 
Claus units inevitably form some carbonyl sulfide if there is any carbon 
dioxide or carbon monoxide in the original foul gas. In addition, because 
of inefficiencies of the Claus reaction, which is run at high 
temperatures, there is always some sulfur dioxide left in the tail gas. 
Tarhan and Kwasnoski have found that by the use of a liquid phase sulfur 
process after a Claus type processes together with maintenance of an 
excess of hydrogen sulfide during reaction, the remaining sulfur dioxide 
is substantially completely removed from the gas stream and only hydrogen 
sulfide is recirculated back to the absorber. Hydrogen sulfide as 
explained above is substantially completely removed from the gas stream by 
alkanolamine solutions. 
E. Increasing the Efficiency of Claus-Reactor Gas Desulfurisation by the 
Inclusion of Catalytic Hydrogenation 
The present inventors have discovered that conventional Claus units, i.e. 
gas phase reaction units, can be rendered more efficient by passing the 
tail gas first through a catalytic hydrogenation reactor together with 
some hydrogen from any suitable source in order to hydrogenate all sulfur 
dioxide or carbonyl sulfide in the gas to hydrogen sulfide and that the 
tail gas may then be recycled back to the absorption-desorption system 
where the hydrogen sulfide is removed from the tail gas. The hydrogen 
sulfide passes into the foul gas which is again passed through the Claus 
plant to form elemental sulfur and water. The use of the hydrogenation 
reactor in the recycle loop removes sulfur which has been converted to 
carbonyl sulfide by the Claus thermal reactor. It also enables the use 
either of a single or a minimum number of Claus catalytic reactor units to 
process the hydrogen sulfide in the foul gas. The efficiency and economy 
of the entire process is thus considerably enhanced. 
While prior and contemporary workers have, therefore, used the principle of 
recycling in various ways in connection with sulfur removal systems for 
the desulfurization of industrial gas, no prior worker has realized that 
an absorption-desorption process through which one passes all of the gas 
to be desulfurized could be combined in a single recycle loop with a Claus 
process and a catalytic hydrogenation process in order to completely 
eliminate any tail gas and maintain a minimum sulfur content in the gases 
exhausted from the system. 
F. Prior Use of Catalytic Reduction of Claus Plant Tail Gas 
U.S. Pat. No. 3,794,710 to J. J. Merrill discloses a process in which 
sulfur plant tail gases comprising sulfur dioxide, carbonyl sulfide, 
carbon disulfide and hydrogen sulfide derived from a Claus type reaction 
are treated first in an oxidizing zone to convert carbonyl sulfide to 
sulfur dioxide and carbon dioxide and then in a hydrogenating zone to 
hydrogenate the oxidized gas stream and form a hydrogenated gas stream, 
the principle components of which will be hydrogen sulfide and carbon 
dioxide from which gas stream the hydrogen sulfide is then scrubbed to 
obtain a purified gas stream. The so-called Stretford process is the 
preferred process for scrubbing the hydrogen sulfide containing gas stream 
in order to remove the last vestiges of hydrogen sulfide. Merrill does not 
disclose the use of the hydrogenation reactor in a recycle loop including 
an alkanolamine absorber and a Claus plant. Thus Merrill uses a much more 
expensive arrangement than is used by the present Applicants. 
U.S. Pat. No. 3,848,071 to W. Groenendaal discloses a process in which tail 
gases from a Claus plant are subjected to a catalytic reduction step to 
reduce contained sulfur oxides to hydrogen sulfide. The hydrogen sulfide 
is then removed from the gas stream with either a solid absorbent or an 
absorbing solution of some suitable type. It is stated that the absorbents 
are preferably regeneratable and that any hydrogen sulfide-containing gas 
liberated during regeneration may be recycled back to the Claus plant. 
There is no disclosure by Groenendaal of the recycling of the hydrogen 
sulfide back through a single absorbent desulfurization loop through which 
all of the gas passes. Groenendaal's contribution to the art was the 
injection into the Claus feed stream of a hydrocarbon which breaks down in 
the Claus process to provide sufficient carbon monoxide and hydrogen to 
serve to reduce sulfur oxides and thus eliminates the necessity of having 
a reducing gas such as hydrogen or carbon monoxide available. There is no 
disclosure of the use of some of the original feed gas prior to absorption 
of the sulfur values as a reducing gas. 
SUMMARY OF THE INVENTION 
The problems and difficulties associated with the prior art methods of 
removing minor amounts of sulfur remaining in gases after conversion of 
the major portion of the sulfur content of the gas to elemental sulfur 
have now been obviated by the present invention. In accordance with the 
invention the principal amount of the sulfur content of a gas is absorbed 
in the form of hydrogen sulfide from a feed gas stream by passing the gas 
through an absorption-desorption type acid gas removal apparatus. The 
absorbed acid gases are then regenerated from the absorption medium and 
the hydrogen sulfide containing foul gas from the desorption step is 
treated with sulfur dioxide in the reaction zones of a Claus plant in 
substantially a stoichimetric ratio of two volumes of hydrogen sulfide 
with one volume sulfur dioxide in order to form elemental sulfur and 
water. The Claus type process operates at a high temperature and thus 
produces a tail gas containing minor amounts of sulfur as hydrogen 
sulfide, sulfur dioxide, carbonyl sulfide, and carbon disulfide. This tail 
gas is passed through a catalytic hydrogenation reactor to hydrogenate all 
the sulfur compounds back to hydrogen sulfide. The hydrogen sulfide 
containing tail gas is then recycled directly back to the original 
hydrogen sulfide containing fuel gas stream prior to its entrance into the 
absorption-desorption apparatus. Alternatively the tail gas may be 
introduced directly into the absorption-desorption step. 
By operation in accordance with the present invention it is possible to 
remove a maximum amount of sulfur components from a gas stream by the use 
of a Claus plant and still use only one or two Claus reactors. The process 
is extremely efficient and results in a minimum escape of sulfur 
containing gases to the atmosphere. 
When the process of the invention is used for the removal of sulfur from a 
coke oven or similar gas, which will usually be the primary and best use 
of the invention, it is preferable to obtain the hydrogen for use in the 
hydrogenation reactor by the use of the original coke oven gas prior to 
its entrance into the absorption-desorption system. In this manner it is 
possible to avoid the purchase or manufacture of hydrogen specifically for 
use in the tail gas hydrogenation reactor. In some cases it may be 
desirable, however, to add a little extra hydrogen from an additional 
source into the gas being passed into the tail gas hydrogenation reactor. 
It is, of course, also broadly contemplated that where the economics are 
favorable, or for other reasons, that hydrogen as such may be passed into 
the tail gas hydrogenation reactor with the tail gas in order to carry out 
the hydrogenation reaction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention in brief provides a process by which the tail gas 
from a Claus plant can be recycled to the feed point of a fuel gas 
desulfurization plant in order to obtain a very efficient removal of 
sulfur components from the fuel gas and at the same time provide a process 
from which no deleterious tail gas is emitted or discharged. The 
desulfurization plant can utilize any of a number of absorption-desorption 
processes such as those based on sodium or potassium carbonate, mono-, di- 
or triethanolamine, ammonia or various organic compounds. However, for 
maximum sulfur removal and thus minimum sulfur discharge via the 
desulfurized fuel gas, processes based on alkanolamine and particularly 
ethanolamines are preferred. 
In the process of the invention a fuel gas such as a sour coke oven gas 
enters the desulfurization plant, which is an absorption-desorption type 
plant, where hydrogen sulfide, carbonyl sulfide, carbon disulfide, and a 
portion of the carbon dioxide in the fuel gas is removed in an absorbing 
or absorbent solution. The desulfurized fuel gas, which will preferably be 
desulfurized coke oven gas, since it has been found that coke oven gas is 
particularly applicable for use in accordance with the invention, passes 
from the absorption-desorption apparatus with perhaps 2% carbon dioxide 
content by volume and less than 0.01% hydrogen sulfide content by volume. 
The desulfurized gas may then be used in any normal manner in further 
industrial synthesization processes or burned as a fuel gas. 
The sulfur compounds which have been absorbed from the desulfurized fuel 
gas are then desorbed or stripped from the absorbing or absorbent 
solution. The sulfur compounds such as hydrogen sulfide, which is the 
predominant sulfur compound, and other contaminating sulfur compounds such 
as carbonyl sulfide and carbon disulfide together with some carbon 
dioxide, leave the desulfurization plant after desorption as a foul gas 
stream which is then passed as a feed gas to a Claus sulfur recovery plant 
which operates according to the well known chemistry: 
EQU Reaction I H.sub.2 S + 3/2O.sub.2 .fwdarw. SO.sub.2 + H.sub.2 O 
EQU reaction II SO.sub.2 + 2H.sub.2 S .fwdarw. 2H.sub.2 O + 3S 
the overall reaction thus may be expressed as: 
EQU Reaction III 3H.sub.2 S + 3/2O.sub.2 .fwdarw. 3S + 3H.sub.2 O 
typically air is used to supply the oxygen to completely oxidize one third 
of the hydrogen sulfide in the foul gas stream to sulfur dioxide and 
water. The air is mixed with the foul gas stream in less than 
stoichiometric amounts and burned in a foul gas burner, after which the 
resultant sulfur dioxide and hydrogen sulfide mixture is reacted in a 
thermal reactor and a catalytic fixed bed reactor, which is packed with 
activated alumina or bauxite or other suitable catalysts, to produce 
elemental sulfur according to Reaction II above. Small quantities of 
carbonyl sulfide and carbon disulfide are also formed in the foul gas 
burner as a result of the reaction of hydrocarbons and carbon dioxide with 
sulfur compounds in the reducing atmosphere which is present during the 
partial oxidation reaction. Claus reactors, which are primarily high 
temperature reactors, are particularly liable to these side reactions. 
Reaction II above is an equilibrium reaction which cannot be driven to 
completion, i.e. it is impossible to react together all of the sulfur 
dioxide and hydrogen sulfide present in a practical system with a 
reasonable number of catalytic reactors. 
Thus, when Reaction II has been driven to a feasible degree of completion 
and the gas stream has been cooled so as to condense elemental sulfur the 
residual tail gas contains from parts per million to actual whole or 
integer percentage quantities of carbonyl sulfide, carbon disulfide, 
hydrogen sulfide, elemental sulfur vapor, and sulfur dioxide. Because of 
the high toxicity of carbonyl sulfide, carbon disulfide, and hydrogen 
sulfide relative to sulfur dioxide, it has been the practice in the past 
to incinerate the residual tail gas by mixing it with the required 
quantity of fuel gas and an excess of combustion air to insure the 
complete combustion of all sulfur compounds to sulfur dioxide which is 
then vented to the atmosphere diluted by the combustion gases. The product 
sulfur is, as mentioned above, condensed and is then passed, usually 
initially in the form of molten sulfur, from the Claus plant to some 
storage facilities from which it may eventually be used for various 
purposes. The sulfur product may be used either as elemental sulfur or it 
may be oxidized to produce sulfur dioxide and ultimately sulfur trioxide 
for use in a contact type sulfuric acid plant. 
The key feature of the present invention involves a processing scheme that 
permits the recycling of the tail gas from the Claus plant or reactors to 
the coke oven gas ahead of the desulfurization plant or apparatus. Such 
recycling has not been found feasible in the past due to the presence of 
highly corrosive and highly reactive sulfur dioxide, sulfur, carbonyl 
sulfide and carbon disulfide in the tail gas stream. However, in 
accordance with the present invention the tail gas which contains the 
sulfur dioxide, elemental sulfur, carbonyl sulfide and carbon disulfide is 
passed through a catalytic hydrogenation zone in a hydrogenating reactor 
together with a supply of hydrogen to completely hydrogenate the sulfur 
compounds to hydrogen sulfide, water and methane. The catalytic 
hydrogenation reactor may be of the usual type using a fixed bed and mild 
conditions including dilute hydrogen, ambient pressures, and temperatures 
in the range of 200.degree. to 500.degree. C (392.degree. to 932.degree. 
F). Any known hydro-treating catalysts such as sulfided cobalt molybdate 
on alumina may be used. The hydrogenated tail gas containing only hydrogen 
sulfide, methane, water, carbon dioxide and nitrogen can then be recycled 
to the feed gas of a monoethanolamine (MEA) desulurization plant without 
detrimental effects. 
In the case where the fuel gas which is being desulfurized is a coke oven 
gas or similar fuel gas containing hydrogen, it is very advantageous and 
preferable in order to obtain the required hydrogen for use in the 
hydrogenation in the hydro-treating reactor, to pass a portion of the coke 
oven gas or other fuel gas prior to desulfurization in the 
absorption-desorption apparatus into the hydrogenation reactor. The 
hydrogen that is a normal constituent of fuel gas such as coke oven gas 
can then react directly with the sulfur compounds which enter the 
hydrogenation reactor with the tail gas from the Claus plant. The 
relatively small amount of undesulfurized fuel gas used in the 
hydrogenation reactor ultimately passes from the hydrogenation reactor 
back into the feed gas line and hence into the desulfurization plant where 
it is desulfurized. Alternatively, of course, the mixture of hydrogenated 
tail gas and undesulfurized fuel gas can be passed directly into the 
absorption apparatus rather than being first mixed into the coke oven gas 
or other fuel gas which is entering the absorption apparatus. 
Implementation of desulfurization in accordance with the present invention 
very substantially reduces the cost of the Claus plant inasmuch as the 
plant need no longer be designed with a large number of reactors in series 
in order to achieve high sulfur recovery from the foul gas. For example, 
in Table IV of an article by C. B. Barry entitled "Reduced Claus Sulfur 
Emission", in Hydrocarbon Processing, page 102, April 1972, it is 
disclosed that a Claus plant receiving 40% hydrogen sulfide in its foul 
gas feed and operating with four reactors in series will have a usual 
sulfur recovery effiency of approximately 96.1%. Claus reactors, however, 
are expensive units and are tricky in operation where any sort of varying 
gas feed passes to them. With the recycle process of the present invention 
in operation, however, it no longer becomes important to achieve high 
final sulfur recoveries since unreacted sulfur compounds are no longer 
discharged from the apparatus at all. the only sulfur compounds leaving 
the apparatus are the very small amounts which pass from the desulfurizer 
with the desulfurized fuel gas and the elemental sulfur product which is 
removed from the Claus plant. 
In accordance with the present invention a single Claus reactor or at most 
two Claus reactors can be used. A single Claus reactor following the 
customary thermal reactor may effect a sulfur conversion in about the 90 
to 92% range, while a double catalytic Claus reactor installation may 
achieve a sulfur conversion efficiency of about 95%. Such efficiencies 
when operated together with the recycle arrangement of the present 
invention are quite sufficient to provide a very efficient overall sulfur 
removal. The savings in eliminating two or three Claus reactors more than 
offset the capital cost of the tail gas hydrogenation reactor of the 
invention. Furthermore, the process in accordance with the present 
invention is much more efficient than an equivalent Claus type sulfur 
recovery plant which does not use the process of the invention. The 
percent removal of sulfur from the gas is very high and since all of the 
tail gas from the Claus plant is recycled there is no exhaust of tail gas 
of any sort to the atmosphere. In addition, since all of the tail gas is 
recycled back into the system and eventually ends up again in the Claus 
reactors, it is not necessary to operate the Claus reactors at their 
maximum efficiency. The operation of the Claus reactors, therefore, is not 
as sensitive and need not be monitored as closely as is necessary in prior 
Claus systems. Recycling of the fuel gas which is used as a hydrogenation 
medium back to the original fuel gas stream in addition enables the 
efficient use of the fuel gas stream as a primary source of hydrogen for 
the hydrogenation reaction. 
The operation of the invention may be more clearly understood with 
reference to the following description of an example of the present 
invention and with reference to FIG. 1 wherein a sour coke oven gas 
containing about 2% carbon dioxide, about 0.5% hydrogen sulfide and traces 
of carbonyl sulfide and carbon disulfide enter through a feed line 11 
which is shown passing to an MEA type desulfurization plant 13. While an 
alkanolamine or ethanolamine absorbing solution such as mono-, di-, or 
triethanolamine is preferred, it will be understood that other suitable 
absorbent solutions or liquids such as aqueous ammonia, aqueous sodium or 
potassium carbonate or organic esters of a polyhydroxyalcohol such as 
propylene carbonate or glycerol triacetate may be used. As will be readily 
understood the MEA desulfurization plant 13 will comprise an absorption 
apparatus, not shown, and a desorption apparatus, also not shown. The sour 
coke over gas is desulfurized in the MEA or other desulfurization plant 
apparatus and passes from the desulfurization plant as a clean coke oven 
gas, containing about 2% carbon dioxide and less than 0.01% hydrogen 
sulfide, through desulfurized gas line 15. The sulfur compounds absorbed 
in the absorption apparatus are then desorbed in the desorption apparatus 
of the desulfurization plant and the desorbed foul gas is passed through a 
line 17 to a Claus plant shown as block 19. 
Oxygen in the form of O.sub.2 from the atmosphere is fed to the Claus plant 
through line 21. As will be well understood by those skilled in the art, 
the O.sub.2 is reacted in the Claus plant with a portion of the hydrogen 
sulfide in the foul gas entering the Claus plant from line 17 to provide 
sulfur dioxide which is then reacted in accordance with the well known 
Claus Reaction II above to form elemental sulfur and water from the 
hydrogen sulfide. Alternatively, of course, as is also well known to those 
skilled in the art, a portion of the sulfur product from the Claus plant 
could be oxidized by the O.sub.2 to form sulfur dioxide for use in the 
Claus plant. The sulfur product from the Claus plant leaves the plant 19 
through the sulfur product line 23. 
The tail gas from the Clause plant is passed through a line 25 to a tail 
gas hydrogenation reactor 27. A line 29 from a hydrogen source 31 passes 
hydrogen into the tail gas line 25 just prior to the entrance of the tail 
gas into the tail gas hydrogenation reactor 27. Preferably an additional 
line 33 which may take the form of a bleed-off line is provided as a 
connection between the feed gas line 11 and the hydrogen source 31 to 
conduct a portion of the feed gas, which in the example given is a coke 
oven gas, to the hydrogen source 31, which may be a small storage tank or 
the like for the coke oven gas. Alternatively an additional source of 
hydrogen may be maintained in the source 31 to mix with the hydrogen of 
the coke oven gas in order to increase the percentage of hydrogen in such 
gas prior to the time that it is passed to the tail gas hydrogenation 
reactor through the line 29 and tail gas line 25. It is preferable, in 
order to have as high a percentage of hydrogen in the gas from the feed 
gas line as possible, to bleed off the coke oven gas or similar gas from 
the feed line 11 for use in the hydrogenation reactor prior to the 
entrance of the gas from the recycle line 35 into the feed line 11. 
In the tail gas hydrogenation reactor 27 the tail gas from the Claus plant 
is reacted with the hydrogen from the coke oven gas or hydrogen source to 
convert the various sulfur products in the tail gas to their hydrogenated 
form in accordance with the following reaction: 
EQU Reaction IV S + H.sub.2 .fwdarw. H.sub.2 S 
EQU reaction V COS + 4H.sub.2 .fwdarw. CH.sub.4 + H.sub.2 O + H.sub.2 S 
EQU reaction VI CS.sub.2 + 4H.sub.2 .fwdarw. CH.sub.4 + 2H.sub.2 S 
EQU reaction VII SO.sub.2 + 3H.sub.2 .fwdarw. H.sub.2 S + 2H.sub.2 O 
these reactions effectively convert all the sulfur compounds, and also the 
small amount of vaporized sulfur product which passes from the Claus plant 
even after condensation of the major portion portion of the liquid sulfur 
product, to hydrogen sulfide. This hydrogen sulfide then passes through 
the recycle line 35 back to the feed gas line 11. The recycle line carries 
a gas which is completely or substantially free of sulfur dioxide and 
which contains predominantly hydrogen sulfide, methane, water, carbon 
dioxide and nitrogen. Some small amount of unreacted hydrogen and other 
minor components from the hydrogenating gas will also be present. It will 
be recognized that some of the hydrogen sulfide, methane, water, carbon 
dioxide, and possibly some small amount also of hydrogen cyanide, which 
pass through the line 35 will have been derived from the original feed gas 
in the line 11 through the bleed-off line 33 which provides hydrogen for 
use in the tail gas hydrogenation reactor 27. However, it has been found 
that it is not detrimental to pass this original coke oven gas into the 
tail gas and through the hydrogenation reactor 27 prior to the time that 
such fuel gas, i.e. the coke gas, passes through the MEA desulfurization 
plant, since the residual fuel gas which is passed together with the 
hydrogenated tail gas via line 35 back into the main fuel gas stream in 
line 11 through the line 35 will immediately be directed to the MEA 
desulfurization plant where the hydrogen sulfide will be substantially 
almost completely removed from the coke oven gas. 
If the fuel gas which is used as a hydrogenating gas is taken from some 
source other than the original feed gas line 11, the concentration of 
hydrogen in the gas will be less. However, as an alternative, if a 
somewhat lower concentration of hydrogen is acceptable, a portion of the 
desulfurized fuel gas can be bled off the clean gas line 15 for use in the 
hydrogenation reactor 27. In such an event, as well as when some external 
hydrogen source such as, for example, liquid hydrogen in the usual 
cylinders or the like, is used, there would be no additional hydrogen 
sulfide or other like compounds added to the tail gas beyond those derived 
from the Claus plant. Under such circumstances it may be practical to 
merely treat the tail gas after hydrogenation in some small apparatus, 
since only small amounts of hydrogen sulfide would be involved. It will be 
seen that the use of the recycle line 35 makes the use of the original 
fuel gas such as the coke oven gas as a hydrogen source for the tail gas 
hydrogenation reactor practical without prior treatment. Without the use 
of such recycles line 35 the original fuel gas could not be effectively 
used as a hydrogen source for a tail gas hydrogenation reactor, since the 
fuel gas, which contains substantial amounts of hydrogen sulfide, would 
then have to be treated by some further process which would require a 
fairly substantial apparatus for effective operation. The recycle 
arrangement is, however, very desirable regardless of the source of the 
hydrogen in order to close the system and prevent the escape of any sulfur 
from the system and is consquently an integral part of the present 
invention. 
In FIG. 2 are shown, by the use of a schematic apparatus type flow diagram, 
further details of the process in accordance with the invention broadly 
outlined in the flow diagram of FIG. 1. In FIG. 2 a feed gas line 51 
conducts gas such as sour coke oven gas or another hydrogen sulfide 
containing fuel gas from some source, not shown, into an MEA absorber 53. 
The fuel gas passes up through the absorber 53, which absorber may be of 
various types such as a spray apparatus or a bubble cap apparatus or other 
type absorption column, and the desulfurized gas passes out of the top of 
the absorber 53 through the line 55. The absorbent solution, which in the 
case shown is an aqueous MEA solution containing approximately 20% 
monoethanolamine, passes into the absorber from the line 57 through a 
distributer 59. The MEA solution is then collected at the bottom of the 
absorber in the reservoir portion 61 of the absorber 53 and is pumped from 
the bottom of the absorber 53 through the line 63, the pump 65 and line 67 
through heat exchanger 69 into stripping column 71 through a distributer 
73. The MEA solution, which contains hydrogen sulfide, carbon dioxide and 
other sulfur components passes down through the stripping column 71 
countercurrently with respect to hot vapors rising from the bottom of the 
stripper. The heat from these vapors causes the hydrogen sulfide and 
carbon dioxide, and other gases which may be dissolved into the MEA 
solution, to be stripped from the solution and to pass out of the top of 
the stripping column through the line 75 as a concentrated gas containing 
a large percentage of hydrogen sulfide plus other acid gases. The 
concentration of the hydrogen sulfide in line 75 may range up to as much 
as 40% of the volume of the gas at ambient saturated conditions and the 
gas may also contain other gases such as carbonyl sulfide and carbon 
disulfide in addition to a fairly large percentage of carbon dioxide. The 
partially stripped MEA solution gathers in the bottom of the stripper 71 
in a reservoir portion 77. This MEA solution is passed through the line 79 
into a reboiler 81 where a portion is vaporized and said vapors are passed 
through a line 83 back into the bottom of the stripper 71 where it rises 
up through the stripper 71. The reboiler 81 obtains its heat from steam 
supplied through a steam line 85. A second line 86 from the bottom of the 
reboiler 81 conducts stripped MEA solution via pump 87 to the line 89 
which conducts the hot stripped MEA solution through the heat exchanger 69 
and into a cooling coil 91 which serves to cool the solution before it 
passes through line 57 into the distributer 59 through which the MEA 
solution is fed into the absorber 53. 
The acidic foul gas passes through condenser 74 and line 75 to a burner 93 
which is supplied with oxygen in the form of O.sub.2 from the atmosphere 
through line 95. The oxygen is supplied in a stoichimetric amount 
sufficient to oxidize approximately one third of the hydrogen sulfide 
contained in the gas passing through the line 75 into the burner 93. The 
combined gas is then passed into a thermal reactor 97 which is positioned 
adjacent to the burner 93. In the thermal reactor the hydrogen sulifide in 
the gas plus the sulfur dioxide which has been formed in the burner 93 
partially combine in accordance with the well known Claus Reaction II 
above to form elemental sulfur and water. Some of the heat of the thermal 
reactor is used in the boiler 99 which is fed with water from line 101 to 
provide process steam through the line 103. 
The partially reacted gases pass from the thermal reactor 97 through a line 
105 which passes to a condensor 107 where the vaporized elemental sulfur 
is condensed. The gas and condensed vapor are then passed through line 111 
into a condensate reservoir 109 where the molten sulfur collects. The 
uncondensed vapors and gases pass through the condensate reservoir 109 
into the line 113 through heat exchanger 115 where a heat source such as 
hot oil from a line 119 reheats the vapors to reaction temperature and 
into a catalytic reactor 117 where additional hydrogen sulfide and sulfur 
dioxide react with each other in accordance with the well known Claus 
Reaction II above to form additional vaporized elemental sulfur and water. 
The hot gases pass through line 121 into the condensor 123 where vaporized 
sulfur particles are condensed into a liquid sulfur which is then caught 
in a condensate reservoir 125 after having passed through line 127. The 
remaining gases and vapors pass from the reservoir 125 through the line 
129 into the heat exchanger 131 where a heat source such as hot oil 
derived from line 135 again reheats the gas to reaction temperature and 
thence to a second catalytic reactor 133. The action of the catalytic 
reactor 133 is the same as the catalytic reactor 117 and thus additional 
hydrogen sulfide and sulfur dioxide which may not have reacted in the 
first stage catalytic reactor 117 reacts in the second stage reactor 133. 
The hot gases and vapors then pass from the catalytic reactor 133 through 
line 137 into a condenser 139 where the gases and vapors are cooled until 
the vaporized elemental sulfur condenses into molten sulfur which then 
passes through line 141 into the third sulfur reservoir 143. While a three 
stage Claus reactor arrangement comprised of one thermal reactor and two 
catalytic reactors has been shown for purposes of illustration, it should 
be understood that other arrangements using more or less stages and 
particularly two or even one stage could also be used. 
Liquid sulfur from the first, second and third condensate reservoirs 109, 
125 and 143 respectively passes through the line 145, 147 and 149 
respectively into a common line 151 and then through line 153 to a sulfur 
storage reservoir 155. It will be understood that the sulfur storage 
reservoir 155 will store the sulfur in a molten condition. The molten 
sulfur may then be pumped periodically to tank trucks or the like or the 
molten sulfur may be continuously pumped to some subsequent use. 
Alternatively the sulfur storage may constitute storage facilities for 
solid elemental sulfur product. In this case the sulfur will ordinarily be 
pumped through the respective lines to the sulfur storage and pumped onto 
a pile of already solidified sulfur allowing the molten sulfur to solidify 
on the pile for temporary storage. 
The uncondensed vapors from the catalytic reactor 133 and condenser 139, 
which vapors constitute the Claus plant tail gas, pass through the sulfur 
reservoir 143 and out the line 157 which leads to a catalytic 
hydrogenation reactor 159. Just prior to entrance into the catalytic 
reactor 159 the tail gas is joined by a stream of hydrogen or hydrogen 
containing gas which passes into the tail gas line 157 through a line 161 
either from a hydrogen source 163 or preferably from the original feed gas 
line by way of line 165. The line 165 will be referred to for convenience 
here as a bypass line 165. This line serves to conduct coke oven gas from 
the feed line 51 to the line 161 which conducts the gas into the tail gas 
line 157 and thence to the catalytic reactor 159. It will be understood 
that the hydrogen source 163 will serve if desired to increase the 
percentage of hydrogen in the coke oven gas in line 165 so that not so 
much coke oven gas need be fed to the catalytic reactor 159. However, it 
will also be understood that the hydrogen source 163 will preferably not 
be used at all or at most will be used only intermittently during such 
times as insufficient hydrogen may be found in the coke oven gas for 
normal hydrogenation of the tail gas from the last catalytic reactor. If 
preferred the hydrogen source 163 can be completely omitted and hydrogen 
from the coke oven gas bypass line relied upon as a source. As a further 
alternative, and particularly if the extra hydrogen source 163 is included 
for occasional or continuous enrichment of the gas stream, the 
desulfurized gas in line 55, which contains a somewhat decreased 
concentration of hydrogen, may be tapped off for use in the catalytic 
reactor 159. 
Catalytic reactor 159 may take various forms. However, as illustrated the 
catalytic reactor is a fixed bed reactor containing sulfided cobalt 
molybdate supported on an alumina substrate. In the catalytic reactor 159 
the small gas entrained particles or vapor of elemental sulfur will be 
reacted with hydrogen in the form of H.sub.2 to form hydrogen sulfide. The 
gas carbonyl sulfide will be reacted with hydrogen in the form of H.sub.2 
to form methane plus water and hydrogen sulfide, any carbon disulfide 
vapor will be reacted with hydrogen to form methane and hydrogen sulfide, 
and sulfur dioxide gas will be reacted with the hydrogen to form hydrogen 
sulfide plus water. It will readily be understood by those skilled in the 
art that sufficient hydrogen must be provided in the hydrogenating gas to 
completely hydrogenate all of the above compounds. Normally this will 
involve the use of more than a stoichiometric amount of hydrogen or 
hydrogen containing gases, for example, an excess of hydrogen of perhaps 
30 to 60% or even more. If some of the original fuel gas such as coke oven 
gas is used as the hydrogenating gas there will be no problem in using a 
very substantial excess of such gas since it is all ultimately recycled 
through the defulfurizing equipment in any event. 
It will be understood that since the Claus reaction process is essentially 
a high temperature process that the original hydrogen sulfide in the coke 
oven gas will have tended to form by reaction with the hydrocabons and 
carbon oxides additional carbonyl sulfide and carbon disulfide above and 
beyond the original carbonyl sulfide and carbon disulfide content of the 
coke oven feed gas. These carbon sulfide gases are in themselves quite 
poisonous so it is advantageous to remove them from the tail gas. As has 
been set forth above, it has frequently been past practice, because of the 
high toxicity of these gases, to incinerate the tail gas in order to 
convert the gases to the less toxic though still objectionable sulfur 
dioxide gas prior to exhaustion to the atmosphere. In accordance with the 
present invention, however, the sulfur content of these gases can now be 
reconverted into their original hydrogen sulfide form and the hydrogen 
sulfide can be recycled through the line 167 back to the original feed gas 
line 51 or alternatively directly to the hydrogen sulfide absorber 53. The 
alkanolamine absorber and particularly the MEA absorbing solution is 
particularly efficient in the removal of hydrogen sulfide from the 
original gas stream and it will be understood that substantially all of 
the hydrogen sulfide is removed from the gas stream, which includes the 
recycled gas, at this point. After absorption and desorption the hydrogen 
sulfide is again recycled through the Claus plant where a major proportion 
of it will be converted into elementary sulfur which can be removed from 
the system. 
By operation of a sulfur removal and recovery process in accordance with 
the present invention a very selective removal of all sulfur can be 
effected upon the original gas stream in a most economical and efficient 
manner without the discharge of any deleterious sulfur containing tail gas 
from the system at all. Furthermore, the process in accordance with the 
invention is a completely closed and substantially self-contained system 
which has no input except the original gas which is to be desulfurized and 
has no output except for the clean gas which is discharged for use 
elsewhere and the sulfur product which is also available for use in some 
subsequent process. 
It will be noted that the MEA solution or other absorbent solution which is 
used is continuously recycled and reused and that the hydrogen which is 
used is, in the preferred embodiment, obtained from the original coke oven 
gas or other fuel gas. The heat of the reaction in the thermal reactor of 
the Claus unit provides heat for making the process steam which is used 
for example in the reboiler 81. Since all the hydrogen sulfide which 
passes through the Claus plant unchanged is eventually recycled into the 
absorber, and since all the sulfur dioxide, carbonyl sulfide and sulfur 
disulfide which are derived from the Claus plant thermal reactor and 
catalytic reactors is hydrogenated and converted back into hydrogen 
sulfide and recycled again to the absorber, it will be seen that it is not 
as important as in many Claus reaction processes for the exact 
stoichimetric ratio of the hydrogen sulfide and sulfur dioxide to be 
maintained in the Claus plant since minor variations in the amount of 
either sulfur dioxide or hydrogen sulfide given off by the Claus reactors 
are easily absorbed by the overall system without the exhaustion to the 
environment of any deleterious sulfur containing gases in a tail gas. The 
monitoring of the process in accordance with the present invention thus 
need not be as severe or critical as in most Claus type plants in order to 
prevent possible pollution of the environment with sulfur gases. 
While a two stage catalytic reaction has been shown in the Claus reaction 
apparatus shown in FIG. 2 as an example, a single stage reaction sequence 
may also be very desirably used. This will save the expense of additional 
stages of catalytic reactors in the Claus plant. The capital expense of 
the process may, therefore, be decreased without a corresponding decrease 
in the effectiveness of the entire system. This is particularly true in 
the case where all the hydrogen for the operation of the process is 
obtained by use of the original coke oven gas as a hydrogen donor gas 
which can then be recycled back to the absorber of the alkanolamine 
desulfurizer system. 
As one example of the process of the invention, a coke oven gas containing 
2% carbon dioxide, 0.5% hydrogen sulfide and traces of carbonyl sulfide 
and carbon disulfide is passed to a MEA absorption-desorption 
desulfurization plant. In the absorber of the absorption-desorption 
apparatus substantially all of the hydrogen sulfide is absorbed leaving 
only 0.01% hydrogen sulfide in the desulfurized or clean coke oven gas. 
This gas will also contain about 1.5% carbon dioxide. The foul gas 
stripped from the desorption apparatus of the absorber-desorber system is 
passed to a Claus plant. This foul gas will contain about 40% hydrogen 
sulfide, about 50% carbon dioxide and diminishing percentages of other 
gases such as hydrogen cyanide, carbon disulfide and carbonyl sulfide. In 
the Claus plant, oxygen is first reacted with approximately one third of 
the hydrogen sulfide content of the gas and the resulting sulfur dioxide 
is then reacted with the hydrogen sulfide to form sulfur and water. The 
tail gas derived from the Claus plant will contain about 0 to 1 percent 
hydrogen sulfide, 0 to 1 percent carbonyl sulfide, 0 to 0.5 percent carbon 
disulfide and 0 to 1 percent sulfur dioxide as examples. This gas is 
combined with about 50 percent stoichiometric excess of the original coke 
oven gas based upon the hydrogen content of the coke gas and is passed 
through a tail gas hydrogenation reactor where substantially all of the 
elemental sulfur vapor, carbonyl sulfide, carbon disulfide and sulfur 
dioxide are hydrogenated to form hydrogen sulfide and other products as 
set forth above. The tail gas, which now contains about 2.5% hydrogen 
sulfide, 13% methane, 15% water, 6% carbon dioxide and which is completely 
free of any sulfur dioxide, the passes back into the feed line for the 
coke oven gas. 
By operation in accordance with the invention with a full recycle loop in 
combination with a catalytic hydrogenation step, preferably using a 
portion of the original fuel gas as the hydrogenating medium, an extremely 
efficient sulfur removal system is provided from which no tail gas sulfur 
at all can escape. The process will be found to be both efficient, 
practical, and economical in effecting substantially complete removal of 
all sulfur components from a feed gas leaving substantially no sulfur to 
be exhausted to the environment.