Process for selective oxidation of hydrogen sulfide to elemental sulfur

A process for selectively converting H.sub.2 S to S, utilizing a porous catalyst having the atomic proportions covered by the formula Fe.sub.A Zn.sub.B, wherein A has a value of 0.5 to 10 and B has a value of 1 to 2. The catalyst contains substantially no chromium. A gas stream comprising hydrogen sulfide and oxygen is passed over the catalyst at a temperature above the dew point of sulfur and up to about 300.degree. C.

CROSS-REFERENCES 
The present application is related to U.S. patent application Ser. No. 
08/443,252 (now U.S. Pat. No. 5,603,913), filed on the date as this 
application and entitled "CATALYSTS AND PROCESS FOR SELECTIVE OXIDATION OF 
HYDROGEN SULFIDE TO ELEMENTAL SULFUR". 
BACKGROUND 
The claimed invention relates to novel catalysts for use in processes for 
the selective oxidation of hydrogen sulfide to form elemental sulfur. 
As is known, many gases, both natural and industry generated, contain 
hydrogen sulfide (H.sub.2 S). For example, the H.sub.2 S content of 
natural hydrocarbon gases can be up to 25%. Hydrotreater gases, synthesis 
gases from coal gasification, and the like also contain H.sub.2 S. It is 
very important to convert the H.sub.2 S into sulfur for many reasons. 
One reason is that the presence of hydrogen sulfide in a gas, even in very 
small quantities, decreases the value of the gas, often making the gas 
valueless. This is because H.sub.2 S has a noxious smell, is highly 
corrosive, is an extremely strong poison for most living things, including 
humans, and is a poison for many catalysts. 
Hydrogen sulfide conversion into elemental sulfur (S.sub.x) can be carried 
out by two different principle methods: 
(a) Decomposition, according to reaction: 
EQU H.sub.2 S.fwdarw.H.sub.2 +1/x S.sub.x ( 1) 
(b) Oxidation, according to reactions: 
EQU H.sub.2 S+1/2O.sub.2 .fwdarw.H.sub.2 O+1/x S.sub.x ( 2) 
EQU H.sub.2 S+1/2SO.sub.2 .fwdarw.H.sub.2 O+3/2x S.sub.x ( 3) 
In the second method, in addition to O.sub.2 and SO.sub.2, other oxidants 
can be used, such as H.sub.2 O.sub.2, NO.sub.x, and the like. 
From a practical point of view, the most attractive way of sulfur 
production from hydrogen sulfide is selective oxidation by using oxygen 
from air, according to reaction: 
EQU H.sub.2 S+1/2O.sub.2 .fwdarw.H.sub.2 O+1/x S.sub.x ( 4) 
Reaction (4) is thermodynamically possible over a very wide range of 
industrially acceptable temperatures. Without a catalyst, however, the 
rate of reaction is low and the reaction is noticeable only at 
temperatures higher than 300.degree. C. However, reaction (4) is 
accompanied by conversion of H.sub.2 S to sulfur dioxide at temperatures 
greater than about 300.degree. C. by the reactions: 
EQU H.sub.2 S+3/2O.sub.2 .fwdarw.H.sub.2 O+SO.sub.2 ( 5) 
EQU 1/x S.sub.x +O.sub.2 .fwdarw.SO.sub.2 ( 6) 
Further, sulfur dioxide can form according to the reverse Claus reaction: 
EQU 3/2x S.sub.x +H.sub.2 O.fwdarw.H.sub.2 S+1/2SO.sub.2 ( 7) 
Thus, in order to selectively form elemental sulfur, H.sub.2 S oxidation 
should be conducted at temperatures less than about 300.degree. C. 
However, this is only possible by the use of suitable catalyst. A 
preferred catalyst should not promote the reverse Claus reaction (reaction 
(7)) to minimize the formation of SO.sub.2. If a solid catalyst is used, 
the process temperature should be at least 180.degree. C., in order to 
prevent condensation of formed sulfur on the catalyst. Condensed sulfur 
blocks the catalyst surface, thereby reducing H.sub.2 S oxidation rate. 
In sum, to carry out oxidation of selective H.sub.2 S to S, catalysts 
showing-high activity at temperatures 180.degree.-300.degree. C. are 
required. In addition to high activity, it is desirable that the catalysts 
possess high selectivity, because in the reaction medium, which contains 
H.sub.2 S and O.sub.2, in addition to undesirable side reactions (5) to 
(7) above, other undesirable side reactions which decrease H.sub.2 S 
conversion into S are thermodynamically possible. These other undesirable 
side reactions include: 
EQU H.sub.2 S+2O.sub.2 .fwdarw.H.sub.2 O+SO.sub.3 ( 8) 
EQU 1/x S.sub.x +3/2O.sub.2 .fwdarw.SO.sub.3 ( 9) 
These reactions usually take place only at temperatures higher than about 
400.degree. C. 
For achievement of highly selective oxidation of H.sub.2 S into S.sub.x by 
use of a solid catalyst, preferably the catalyst contains as few small 
pores and as many large pores as possible. This structure allows molecules 
of formed sulfur to leave catalysts pores rapidly and thereby avoid 
reactions (6) and (7). Since a catalyst's surface is generally made up of 
its pores, catalysts with large pores usually do not have a large specific 
surface since specific surface is inversely proportional to pore diameter. 
Different methods for preparing catalysts with large pores and accordingly 
small specific surface are known in heterogeneous catalysis. For example, 
USSR Inventors Certificate 871813 (1981) (which is incorporated herein by 
reference) discloses an iron oxide based catalyst having specific surface 
1-2 m.sup.2 /g and average pore diameter 2500-2900 .ANG. for use as a 
H.sub.2 S oxidation catalyst. USSR Inventors Certificate 967551 (1982) 
(which is incorporated herein by reference) also discloses a catalyst in 
which an active compound is applied on an inert carrier having a specific 
surface of 1.5-2.0 m.sup.2 /g and an average pore diameter of 2500-3000 
.ANG.. U.S. Pat. Nos. 4,818,740 and 5,037,629 disclose catalysts prepared 
by depositing oxides of iron or oxides of iron and chromium on carriers 
having large pores and small specific surface for the selective oxidation 
of H.sub.2 S to S. 
The pore structure of a catalyst allows the active components of the 
catalyst to perform effectively. The catalyst pore structure however, by 
itself, cannot provide high activity and selectivity. These are effected 
by the chemical and phase composition of the catalyst. Thus, to provide an 
effective catalyst, chemical and phase composition must be optimized. 
However, the level of knowledge of chemistry and catalysis does not allow 
the prediction of a catalyst composition for a given reaction. 
Analysis of periodical and patent literature, reveals that oxides of iron, 
aluminum, vanadium, titanium, and other metals have been suggested for 
selective oxidation of H.sub.2 S to S. Such oxides display catalytic 
activity for H.sub.2 S oxidation, but they have not found wide application 
in selective oxidation processes because of their disadvantages. Iron 
oxide as a catalyst for H.sub.2 S oxidation was suggested by Claus about 
100 years ago. However, the form of oxides proposed by Claus did not 
achieve high selectivity. In USSR Inventors Certificate 871873 iron oxide 
with small specific surface, reduced by calcination at a high temperature 
to turn Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4, is disclosed as being more 
selective than the iron oxide used by Claus. Use of a catalyst containing 
iron oxide is described in U.S. Pat. Nos. 4,576,925 and 4,519,992, as well 
as U.K. Patents Nos. 2,164,867A and 2,152,489A, all of which are 
incorporated herein by reference. 
Aluminum oxide (A1.sub.2 O.sub.3) is also mentioned as a catalyst for 
H.sub.2 S oxidation, but has the disadvantage that it is catalytically 
active in the reverse Claus reaction (7). In addition, it is not stable 
and can lose its activity quickly because of surface sulfation. 
Vanadium oxide, which is used in catalyst compositions for the Selectox 
process described in U.S. Pat. No. 4,311,683, has the disadvantage that it 
is very active for reactions (6) and (7), and therefore does not have a 
high selectivity for H.sub.2 S conversion to S. 
Titanium oxide as a catalyst for H.sub.2 S oxidation to S has also been 
suggested. However, this oxide is catalytically active not only in 
reaction (4), but also for reaction (7). Thus, it can be used for 
selective oxidation of H.sub.2 S by oxygen only for low water content 
reaction mixtures. 
Heterogeneous catalysts containing iron and chromium oxides for H.sub.2 S 
oxidation to S have been described, for example, in U.S. Pat. Nos. 
4,818,740 and 4,576,925. More complex catalysts comprising three or more 
metal oxides have been described, for example, in UK Patent No. 2164867A. 
In addition to iron and chromium oxides, one of several oxides of the 
following metals were added in a quantity of 1.5-25% by weight: cobalt, 
nickel, manganese, copper, zinc and titanium. Although the addition of 
zinc and titanium oxide can improve the properties of an iron oxide based 
catalyst, nevertheless these catalysts display noticeable activity in the 
reverse Claus reaction and in the oxidation of sulfur to SO.sub.2. 
Accordingly, there is a need for a highly efficient and highly selective 
catalyst that is effective in converting hydrogen sulfide to sulfur at 
temperatures above the sulfur dew point to about 300.degree. C. 
SUMMARY 
The present invention is directed to a process for the selective oxidation 
of hydrogen sulfide to sulfur at a temperature above the sulfur dew point 
and up to about 300.degree. C., i.e., no more than about 300.degree. C. In 
the process, a gas stream comprising hydrogen sulfide and oxygen, 
preferably from air, is continuously passed over a solid porous catalyst. 
The catalyst is formed of oxides of (a) iron and (b) zinc. The catalyst 
has the atomic proportions covered by the formula Fe.sub.A Zn.sub.B 
wherein A has a value of 0.5 to 10, and B has a value of 1 to 2. 
Preferably A is from 1 to 5 and B is 1. The catalyst can comprise iron and 
zinc in an atomic ratio of 2:1, or in an atomic ratio of 3:1. The catalyst 
contains substantially no chromium for environmental reasons, and 
preferably consists essentially of oxides of Fe and Zn. 
DESCRIPTION 
The H.sub.2 S conversion process of the present invention utilizes a 
catalyst at a temperature above the condensation temperature of sulfur, 
typically greater than about 180.degree. C., up to a temperature of about 
300.degree. C. Selective oxidation occurs by continuously passing a gas 
stream comprising hydrogen sulfide and oxygen, normally provided from air, 
over the catalyst at a space velocity of 1000 to 6000 hr.sup.-1 or more. 
The feed gas typically contains at least 0.1%, by volume, H.sub.2 S and no 
more than about 50%, by volume, H.sub.2 S. The temperature preferably is 
maintained below about 300.degree. C. to ensure that conversion of 
hydrogen sulfide to sulfur is maximized. An inert gas coolant such as 
nitrogen can be used. Water content has little impact on the level of 
conversion of hydrogen sulfide. In all instances it has been observed that 
the level of hydrogen sulfide conversion is in excess of about 95 percent 
with approximately 92 to 96 percent of the sulfur in H.sub.2 S present in 
a gas selectively converted to elemental sulfur. 
Substantially any gas containing H.sub.2 S can be treated using this 
process. For example, a process according to this invention can be applied 
to direct conversion of hydrogen sulfide to elemental sulfur, used in the 
last catalytic stage of a Claus unit, or used to process tail gas streams 
discharged from a Claus plant to convert residual hydrogen sulfide in such 
gas streams, after all of the sulfur has been hydrogenated to hydrogen 
sulfide, to elemental sulfur. The process can also be used to treat a 
primary gas from an amine unit. 
The iron/zinc catalyst used in the present invention can be prepared by 
many procedures, using different initial compounds, containing iron and 
zinc. Conditions are chosen so that zinc ferrites can be easily achieved 
by calcination of intermediates during the last stage of catalyst 
preparation at temperatures from about 600.degree. C. to about 
1000.degree. C. Several hours are enough. Higher temperatures and very 
long calcination can result in catalyst sintering. 
The catalysts of this invention are normally prepared by forming an aqueous 
solution of soluble salts of the metals to be combined. A base is added to 
cause precipitation of the salts in the hydroxide form. The precipitate is 
then partially dried and formed into desirable catalyst shape and 
converted to the corresponding oxides by calcination. Calcination normally 
occurs with the temperatures from 600.degree. C. to 1000.degree. C. The 
formed catalysts have a surface area of about 1 to 5 m.sup.2 /g with at 
least 90% of pore diameters being greater than about 500.degree. .ANG.. 
Pores are attributed to the lattice work of the formed oxides. Preferably, 
deposition onto a carrier is not resorted to so that the entire catalyst 
is made up of the catalytically active metals. However, a carrier can be 
used, such as described in USSR Inventors Certificate 871,813.

Examples of catalyst preparation used in a selective oxidation process are 
shown below: 
EXAMPLE 1 
A catalyst was prepared from iron and zinc oxides in quantities 
corresponding to the following atomic ratio: Fe.sub.2 O.sub.3 :ZnO=1:1 was 
used. The oxides were thoroughly ground to a powder and mixed in a ball 
mill. Water was added with mixing. The prepared paste was deposited on a 
gypsum board, which was covered by cotton cloth and left on the board for 
24 hours at room temperature for dewatering. The paste, with a moisture 
content of 32-33%, was shaped by a screw-extruder. The resultant 
extrudates, having a diameter of 4 mm, were cut into parts with a length 
of 4-6 mm, dried at 130.degree.-140.degree. C. for 4-5 hours, then 
calcined for 3.5 hours at 850.degree. C. 
EXAMPLE 2 
A catalyst was prepared according to Example 1, but in quantities 
proportional to the atomic ratio of Fe:Zn=3:1. 
EXAMPLE 3 
Catalysts, prepared as described in Examples 1 and 2 were used for H.sub.2 
S oxidation. For this purpose, the catalysts were loaded into a 
once-through reactor with electric furnace heating. A gas mixture, 
consisting of specified quantities of H.sub.2 S, O.sub.2 and water vapor 
passed through the reactor. Nitrogen was used as a diluent. In order to 
examine the effect of individual gas mixture components different amounts 
of H.sub.2, CH.sub.4 and other saturated hydrocarbons, CO.sub.2 and others 
were added. Gas was passed through the reactor at a space velocity of 
3000-5000 per hr. H.sub.2 S concentration was varied in a range of 1-3%; 
H.sub.2 O vapor content varied in a range of 3-30% vol. 
The results of catalyst activity determination are shown in Table 1. Tests 
were carried out at O.sub.2 :H.sub.2 
TABLE 1 
__________________________________________________________________________ 
Catalyst Process Condition 
Total 
Composition 
Space Conversion 
S 
Example 
mol. % Temp 
Velocity 
H.sub.2 S 
of H.sub.2 S, 
Recovery, 
No. Fe.sub.2 O.sub.3 
ZnO 
(.degree.C.) 
(Hr.sup.-1) 
Concentration 
% % 
__________________________________________________________________________ 
1 50 50 250 3000 2.0 96.2 93.4 
1 50 50 270 3000 2.0 97.6 93.3 
2 60 40 230 3000 2.0 97.0 93.1 
2 60 40 250 5000 2.0 96.3 93 
__________________________________________________________________________ 
The addition of H.sub.2, saturated hydrocarbons, or CO.sub.2 to the 
reaction mixture had little effect on total conversion or sulfur recovery. 
Although the present invention has been described in considerable detail 
with reference to certain preferred versions thereof, other versions are 
possible. Therefore, the scope of the appended claims should not be 
limited to the description of the preferred versions contained herein.