Catalytic removal of hydrogen sulfides from liquid sulfur

The invention is a catalytic process for removing H.sub.2 S and hydrogen polysulfides from liquid sulfur. The process comprises mixing a heterocyclic compound with the liquid sulfur and removing the hydrogen sulfide from the liquid sulfur by contact with a gas which is inert to the liquid sulfur at the treating temperature.

The invention relates to a process for the quick removal of H.sub.2 S 
present in liquid sulfur in a dissolved state alone or with hydrogen 
polysulfides. The invention is also concerned with a catalytic system that 
can be used for carrying out the process. 
BACKGROUND OF THE INVENTION 
Large quantities of sulfur are produced by the CLAUS process by reacting 
the hydrogen sulfide originating, for example, from deacidification of 
natural gases or from treatment of petroleum products with sulfur dioxide 
formed, for example, by combustion of H.sub.2 S or combustion of sulfur or 
sulfur-containing compounds such as pyrites, according to the reaction: 
EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O 
A substantial fraction of the sulfur produced by the CLAUS process is 
stored in the liquid state in heat-insulated tanks, then transported in 
liquid form to the places of use by trucks, boats, trains or pipelines. 
The transportation means is provided with adequate heat-insulating means 
to permit the sulfur to remain in the liquid state during the 
transportation. 
The liquid sulfur obtained by the CLAUS process always contains, in a 
dissolved state, a small amount of H.sub.2 S and hydrogen polysulfides, 
also called sulfanes, of the formula H.sub.2 S.sub.x wherein x represents 
a number equal to or higher than 2. The sulfanes slowly decompose to 
release H.sub.2 S. The gaseous phase above the liquid sulfur in the 
storage tank or in the container used for its transportation contains, 
therefore, a certain proportion of H.sub.2 S, which, due to its toxicity 
and to its tendency to spontaneous inflammation, makes the loading and 
unloading of the containers that serve to transport the liquid sulfur 
dangerous. In order that these operations can be performed safely, the 
CLAUS liquid sulfur is generally subjected, prior to storage and 
transportation, to a degasification treatment whose purpose is to lower 
the content of free and combined H.sub.2 S of the liquid sulfur below a 
threshold fixed by the practice at 10 ppm. 
The total amount of H.sub.2 S and of sulfanes in the CLAUS liquid sulfur is 
generally between 50 and 700 ppm and generally depends on the 
concentration of H.sub.2 S in the gaseous phase above the liquid sulfur 
and on the temperature of the latter. In addition, the relative 
proportions of free H.sub.2 S and sulfanes dissolved in the liquid sulfur 
depend also on the temperature of the sulfur. 
The process of removal of the H.sub.2 S present in the liquid sulfur in 
free and combined forms includes two phases; a first phase in which the 
sulfanes are decomposed according to the reaction H.sub.2 S.sub.x 
.fwdarw.H.sub.2 S+S.sub.x-1 and a second phase in which the released and 
simply dissolved H.sub.2 S and the light sulfanes are removed from the 
liquid sulfur. 
The decomposition of the sulfanes to H.sub.2 S and sulfur is a slow 
reaction. The rate of removal of the H.sub.2 S and sulfanes is limited by 
the speed of the decomposition reaction. 
Most of the processes proposed for removing the free and combined H.sub.2 S 
dissolved in the liquid sulfur, are of the type in which a catalytic 
system consisting of ammonia or compounds that release ammonia at the 
liquid sulfur condition or compounds having a basic character in the sense 
of BRONSTEDT are added to the liquid sulfur. The catalytic system 
facilitates the decomposition of the sulfanes, the released H.sub.2 S 
being simply dissolved in the sulfur is separated from the liquid sulfur 
by any method that allows the H.sub.2 S dissolved physically to escape 
from the liquid sulfur or to be transformed in situ into sulfur under the 
action of an oxidizing gas. In particular, in the process disclosed in 
EP-0045636, the catalytic system consists of a compound selected from the 
inorganic compounds of phosphorus, urea, urea derivatives, dithionates, 
dithionites, thiosulfates, bisulfates, and bisulfites. In the processes 
described in U.S. Pat. No. 3,364,655, FR No. 2,159,691 and U.S. Pat. No. 
4,131,437, ammonia is used as the catalyst and the H.sub.2 S released by 
decomposition of the sulfanes is removed by atomization of the liquid 
sulfur (U.S. Pat. No. 3,364,655), or stripping with an inert gas (FR No. 
2,159,691), or by sweeping the open surface of the liquid sulfur by means 
of a gas such as water vapor, nitrogen, air or residual gas from a sulfur 
plant (U.S. Pat. No. 4,131,437). The use of a catalytic system of the type 
which utilizes ammonia, ammonium salts, amines and other nitrogenous 
compounds has also been suggested in FR No. 2,185,587 and U.S. Pat. No. 
3,447,903 with transformation of the H.sub.2 S formed, in situ into sulfur 
by the action of an oxidizing gas namely, air in the first case and 
SO.sub.2 in the second case, injected into the liquid sulfur. 
The above cited processes have the inconvenience of slow reaction rates 
which hinder carrying out the process continuously at the exit from a 
sulfur manufacturing process. In effect, the processes disclosed require 
several hours, for example, at least 2.5 hours when using a catalytic 
system formed of compounds such as defined in EP-0045636, or at least from 
5 to 8 hours when using ammonia as the catalytic system to obtain a liquid 
sulfur having a content of free and combined H.sub.2 S below the level 
required by the standards. 
BRIEF DESCRIPTION OF THE INVENTION 
It has now been discovered that by using the basic catalytic system of 
improved effectiveness of the present invention, it is possible to lower 
the "total H.sub.2 S" content; that is, the content of free and combined 
H.sub.2 S of the liquid sulfur below the 10 ppm threshold in shorter 
treatment times than those disclosed in the cited processes. The catalysts 
of the invention are easy to use, and do not cause discoloration of the 
treated sulfur or the formation of deposits. 
In the process of the present invention, a catalyst system is added to the 
liquid sulfur. The catalyst system contains one or more basic compounds 
and the whole is maintained under conditions adequate for removing the 
hydrogen sulfide from the liquid sulfur. The catalytic system consists 
essentially of one or more compounds selected from the group consisting of 
monocyclic and polycyclic heterocyclic compounds that contain at least one 
nitrogen heteroatom. The heterocyclic compound can additionally contain 
other heteroatoms such as sulfur and/or oxygen. The catalytic compounds 
useful in the present invention are soluble and stable in the liquid 
sulfur at the temperatures of treatment and have a boiling point above 
200.degree. C. at atmospheric pressure. 
The monocyclic and polycyclic heterocyclic compounds are preferably 
aromatic heterocyclic compounds, and most preferably include at least one 
ring of the benzenic type that contains no substituent linked to the ring.

DETAILED DESCRIPTION OF THE INVENTION 
The heterocyclic compounds useful in the catalytic system of the invention 
include several heteroatoms comprising at least one nitrogen atom. The 
heteroatoms can consist exclusively of nitrogen atoms. The preferred 
compounds comprise heterocyclic polycyclic aromatic compounds with 
condensed nuclei carrying said heteroatoms, and wherein the heteroatoms 
are distributed in the cyclic groups so that there is not more than one 
heteroatom per cyclic group. In the preferred heterocyclic compounds, it 
is desirable that two similar nitrogen heteroatoms each one being in a 
different cyclic group are separated by a carbon chain of not more than 
three carbon atoms. 
The phrase "heterocyclic monocyclic compounds" as used herein refers to 
heterocyclic compounds having any one ring, the ring having at least one 
heteratom incorporated therein. The heterocyclic monocyclic compounds can 
have substituents appended to the heterocyclic ring. The phrase 
"heterocyclic polycyclic compounds" as used herein refers to compounds 
having two or more cyclic members, which may be attached or fused, in 
which at least one of the cyclic moieties is a heterocyclic moiety having 
at least one heteroatom incorporated in the ring. Preferably, two or more 
of the cyclic moieties are heterocyclic moieties in the polycyclic 
compounds. The heterocyclic polycyclic compounds can have substituents 
appended thereto, as long as the substituents do not eliminate the 
catalytic activity of the compounds. 
Examples of heterocyclic compounds that can be used as the catalytic system 
added to the liquid sulfur in the process of the present invention include 
quinoline, isoquinoline, benzoquinoline, acridine, benzacridine, 
quinoxalines, quinazoline, phenazine, phenantridine, phenantrolines, 
naphthyridines, bipyridyls, and the like. 
The amount of catalytic system added to the liquid sulfur must, on one 
hand, be sufficient to obtain the quick and satisfactory removal of the 
H.sub.2 S and, on the other hand, not increase the amount of ash of the 
liquid sulfur. The amount of catalytic system added to the liquid sulfur 
is preferably in the range of about 5 to 120 ppm, based on the weight of 
the sulfur. 
The process according to the invention can be carried out over a broad 
temperature range above the melting point of sulfur. The process is 
generally carried out at a temperature from about 125.degree. C. to about 
180.degree. C., and preferably from about 130.degree. C. to about 
165.degree. C. 
The process according to the invention is preferably carried out under 
conditions that ensure as homogenous as possible, a distribution of the 
catalytic system in the mass of liquid sulfur and that allows the removal 
from the liquid sulfur of the hydrogen sulfide released by decomposition 
of the sulfanes and present in a dissolved state in the liquid sulfur. To 
ensure distribution of the catalytic system throughout the sulfur, the 
liquid sulfur containing the catalytic system can be subjected to 
agitation by any adequate mechanical system of agitation, the released 
H.sub.2 S naturally escaping from the mass of the liquid sulfur. The 
agitation of the liquid sulfur and the separation of the released H.sub.2 
S can also be effected by stripping with an inert gas. 
According to the invention, an inert gas is a gas which does not react with 
the constituents of the medium in which it is introduced, under the 
conditions of temperature selected for carrying out the process. The 
stripping gas is preferably nitrogen, CO.sub.2, air, water vapor, residual 
gas of a sulfur plant, or mixtures of said gases. The agitation of the 
liquid sulfur containing the catalytic system and the removal of the 
released H.sub.2 S can be effected by subjecting the liquid sulfur to 
atomization or spraying, as described in U.S. Pat. No. 3,364,655. The 
process can also be carried out by using a combination of the different 
modes of agitation of the liquid sulfur and of removal of the released 
H.sub.2 S. 
The treatment of the liquid sulfur including one or a combination of the 
above operations of agitation and gaseous contact is generally carried out 
in a heat-insulated vessel such as a metal tank or reservoir in which the 
liquid sulfur is stored. It is preferred that the space above the open 
surface of the liquid sulfur be swept with an inert gas such as CO.sub.2, 
nitrogen, air, a residual gas of a CLAUS sulfur plant, or the like. The 
H.sub.2 S containing gas is generally passed to an incineration zone. The 
inert gas sweep facilitates driving the H.sub.2 S released by 
decomposition of the sulfanes away from the mass of liquid sulfur. When 
the sweeping operation is effected by means of a residual gas from a 
sulfur plant that is, a gas containing a small amount of H.sub.2 S, the 
concentration of H.sub.2 S in the residual gas must be controlled. The 
concentration of H.sub.2 S can be controlled by dilution of the residual 
gas with an inert gas so that the concentration of H.sub.2 S is not above, 
at least toward the end of the operation, the content of gaseous H.sub.2 S 
in equilibrium, at the operation temperature, with the amount of H.sub.2 S 
permitted in the liquid sulfur. 
The catalytic system can be added to the liquid sulfur at one time at the 
beginning of the operation or by portions throughout the operation. When 
the treatment for removal of H.sub.2 S includes mechanical agitation of 
the liquid sulfur contained in a metal vessel, tank, or reservoir wherein 
it is stored, the catalytic system can be added with the liquid sulfur 
entering the vessel that serves for the treatment. If stripping is used 
for removing the H.sub.2 S from the mass of liquid sulfur, the catalytic 
system can be introduced in the stripping gas and/or added in the liquid 
sulfur feed to the processing vessel. In the case where atomization or 
spraying of the liquid sulfur is effected, such spraying generally 
includes the introduction of the liquid sulfur into a spray nozzle by 
means of a pump, including a suction conduit that plunges into the liquid 
sulfur. The catalytic system can be injected into the pump suction or 
discharge and/or into the feed of liquid sulfur to the processing vessel. 
The process according to the present invention can be carried out 
continuously or batchwise. 
FIGS. 1 and 2 of the enclosed drawing diagrammatically show two devices 
that can be used for carrying out the process according to the invention. 
The process according to the invention with atomization of the liquid 
sulfur containing the catalytic system can be carried out batchwise in an 
apparatus, as described in FIG. 1. The apparatus comprises a 
heat-insulated closed vessel 1, preferably a metal tank or reservoir 
divided into two compartments; a first compartment 2 of small volume and a 
second compartment 3 of large volume, by a partition 4 open at the upper 
part so as to form an overflow that allows the liquid sulfur to flow from 
the first to the second compartment. The bottom 5 of the first compartment 
is raised in relation to bottom 6 of the second compartment. The vessel is 
provided with an inlet 7 for sweeping gas that discharges into the first 
compartment above the level of the overflow and with an outlet 8 for 
sweeping gas provided in the upper part of the second compartment. The 
first compartment is provided with a conduit 9 for introduction of the 
liquid sulfur, said conduit being connected with line 10 for the addition 
of the catalytic system. Compartment 2 is equipped with mechanical 
agitation means 11, and the second compartment 3 is equipped with an 
atomization system 12 including a pump 13 with the inlet of suction 
conduit 14 near to a bottom of compartment 3 below the upper surface of 
the liquid sulfur and the discharge 15 is connected to an atomization 
nozzle 16 arranged in the second compartment so as to be situated above 
the upper surface 17 of the liquid sulfur contained in this compartment. 
When carrying out the process in this manner, the liquid sulfur containing 
the catalytic system is subjected to vigorous agitation in the first 
compartment 2 which leads to a substantially homogeneous distribution of 
the catalytic system in the liquid sulfur contained in the compartment and 
to initiate the decomposition reaction of the sulphanes; then the mixture 
of liquid sulfur and catalytic system flows across the overflow 4 into the 
second compartment 3 in which said mixture is subjected to spraying or 
atomization. The H.sub.2 S escaping from the mass of liquid sulfur 
contained in the two compartments is evacuated by circulation of the 
sweeping gas such as a residual gas of CLAUS sulfur plant and directed 
with said gas toward an incineration zone. The liquid sulfur is removed 
from the vessel 1 through line 51; the concentration of H.sub.2 S and 
sulfanes in the liquid sulfur has reached the required level. 
The process according to the invention with atomization of the liquid 
sulfur containing the catalytic system can be continuously carried out; 
for example, as shown in FIG. 2, by working in a heat-insulated closed 
vessel 20. The heat-insulated metal reservoir 20 is divided into at least 
three successive compartments namely, an inlet compartment 21, one or more 
intermediary compartments 31 and an outlet compartment 41, by parallel 
vertical partitions 22 and 32 which are open in the lower part to connect 
each one of the compartments with the adjacent compartments and which have 
a lesser height than the vessel so as to be able, should that be the case, 
to play the part of overflow for the liquid sulfur. The inlet compartment 
is provided with a conduit 23 for introducing the liquid sulfur. The 
conduit 23 is in fluid communication with conduit 24 for the addition of 
the catalytic system and in its upper part with an outlet 25 for a 
sweeping gas. Inlet compartment 21 is equipped with a mechanical agitation 
system 26. In addition, each one of the intermediary compartment(s) 31 and 
the outlet compartment 41 are equipped with an atomization or spraying 
system 33 and 43, said system including a pump 34 and 44 provided with 
suction conduits 35 and 45 having inlets below the level of liquid sulfur 
in the downstream part of the compartment concerned and, on the other 
hand, with discharge conduits 36 and 46 connected to atomization or spray 
nozzles 37 and 47 arranged in said compartment so as to be in the upstream 
portion of the compartment above the open surface 30 of the liquid sulfur 
it contains. The outlet compartment 41 is in addition provided with an 
inlet 48 for a sweeping gas arranged above the open surface 30 of the 
liquid sulfur. Further the pump of the atomization system includes a 
conduit 49 for removing the treated liquid sulfur. 
When carrying out the process, the liquid sulfur, to which the catalytic 
system has been added, continuously enters the inlet compartment 21 and is 
there subjected to vigorous agitation which mixes the catalytic system 
with the liquid sulfur contained in the compartment 21. The catalytic 
decomposition reaction of the sulfanes is initiated. The mixture of the 
liquid sulfur and the catalytic system then continuously and successively 
flows into the intermediary compartment(s) 31, then into the outlet 
compartment 41 in each one of which the mixture is subjected to an 
atomization or spraying. The H.sub.2 S escaping from the mass of liquid 
sulfur contained in the different compartments is continuously carried 
from the vessel mixed with the sweeping gas which can be a residual gas 
from a CLAUS sulfur plant. The gas is injected into the outlet compartment 
41 and circulates above the open surface 30 of the liquid sulfur in a 
countercurrent flow, the gas leaves the vessel by the outlet 25 provided 
for this purpose in the inlet compartment 21. The sweeping gas loaded with 
H.sub.2 S is then directed to an incineration zone. The treated liquid 
sulfur is continuously drawn off through the conduit 49 arranged to 
communicate with pump 44 of the atomization system 43. The level of sulfur 
in vessel 20 can be controlled by a level control means and control value 
in line 49, not shown. 
By way of variation, the operation of atomization of the mixture of liquid 
sulfur and catalytic system provided when the process is carried out 
discontinuously and at least one of the spraying operations provided when 
the process is carried out continuously can be replaced by a stripping 
operation of the H.sub.2 S by means of an inert gas injected into the 
liquid sulfur contained in the compartment(s) concerned of the processing 
vessel. In the device of FIG. 1, the compartment 2 can be eliminated, the 
liquid sulfur and the catalytic system being directly introduced in the 
compartment 3 that constitutes then the only compartment of the vessel. 
The invention is illustrated by the non-limiting examples that follow: 
EXAMPLES 1 TO 11 
In these examples, the H.sub.2 S contained in the liquid sulfur originating 
from a CLAUS sulfur unit was eliminated by working either in the presence 
of ammonia as the catalytic system (control Example 1) or in the presence 
of a catalytic system according to the invention. 
The work was carried out in a glass flask kept at constant temperature in a 
thermostatically controlled oil bath. The flask was provided with a first 
glass dip tube for introducing a stripping gas into the liquid sulfur, 
with a second glass tube for introducing a sweeping gas into the flask, 
the end of said tube being situated about two centimeters above the open 
surface of the liquid sulfur, and with a third pipe for injection of the 
catalyst and removal of samples of liquid sulfur; the flasks had an outlet 
for the gases that pass the gases to a trapping system for the H.sub.2 S. 
In the flask were placed 1,000 g of liquid sulfur. The sulfur was collected 
at the sulfur recovery unit condenser. The sulfur was kept at the selected 
temperature. At the beginning of the process, there were introduced in the 
flask through the dip tube, 100 1/h of a stripping gas; nitrogen (Examples 
1, 7 and 5); or air (the other examples) and through the sweeping tube, 
120 1/h of air as sweeping gas. Through the appropriated conduit there 
were evacuated 220 1/h of a gaseous effluent that was passed to the 
trapping system for the H.sub.2 S. At defined intervals, samples of liquid 
sulfur were removed and the H.sub.2 S and the sulfanes that it still 
contained were determined by iodometry or argentimetry. The end point of 
the reaction was detected by potentiometry at an accuracy of bimetallic 
electrodes. 
The result of the determination designated by "total H.sub.2 S" represented 
the sum of the contents of liquid sulfur respectively of simply dissolved 
free H.sub.2 S and of H.sub.2 S combined in the form of sulfanes. 
In control Example 1, there were effected two injections of 25 ppm ammonia 
into the liquid sulfur by addition to the stripping gas; the first one at 
the beginning of the process and the second at the end of one hour. In the 
other examples, the catalytic system was added to the contents of the 
flask at the beginning of the process in the form of a solution in a small 
quantity of liquid sulfur. This addition was effected through the pipe 
serving for removal of the samples of liquid sulfur. 
The specific conditions of operation of the different examples and the 
results obtained are shown in Table I. 
TABLE 1 
__________________________________________________________________________ 
CATALYTIC 
SYSTEM TREATMENT "H.sub.2 S TOTAL" CONTENT 
(Nature & 
Temperature 
Duration 
OF THE LIQUID SULFUR 
EXAMPLE 
Quantity) 
(.degree.C.) 
(min.) 
(ppm) 
__________________________________________________________________________ 
1 NH.sub.3 0 465 
(control) 
2 .times. 25 ppm 
140 60 284 
120 132 
240 18 
300 9.5 
2 Quinoline 0 511 
92 ppm 135 12 80 
24 27 
38 6 
48 &lt;2 
3 Quinoline 0 534 
98 ppm 150 12 150 
22 77 
40 24 
50 10 
80 &lt;2 
4 Quinoline 0 430 
100 ppm 165 12 189 
35 90 
62 71 
135 32 
200 10 
5 Quinoline 0 489 
90 ppm 150 10 156 
30 30 
52 10 
80 &lt;2 
6 Isoquinoline 0 509 
89 ppm 150 15 51 
28 21 
45 5 
50 &lt;2 
7 Phenantridine 0 340 
41 ppm 150 10 132 
26 44 
40 32 
74 10 
108 &lt;2 
8 o. Phenantroline 
0 543 
8 ppm 150 7 109 
20 17 
35 &lt;2 
9 o. Phenantroline 
0 298 
56 ppm 150 8 16 
15 &lt;2 
10 Bipyridyl-2,2' 0 432 
85 ppm 150 15 290 
70 37 
108 10 
150 &lt;2 
11 1,8-Naphtyridine 
0 583 
80 ppm 150 6 220 
20 49 
40 7.5 
__________________________________________________________________________ 
A comparison of the results of Examples 2 to 11 with those of control 
Example 1 demonstrates the effectiveness of the catalytic systems 
according to the invention which make it possible to obtain "total H.sub.2 
S" contents in the liquid sulfur below 10 ppm at the end of a duration of 
treatment substantially shorter than that needed when using a conventional 
basic catalyst such as ammonia. 
In the process of the present invention, the H.sub.2 S may be stripped from 
the liquid sulfur by countercurrent contact with a gas by a gas contact 
means. The gas contact means can be a packed, a tray or an open column.