Claus tail gas recovery

An improved Claus tail gas recovery process involving stoichiometric combustion of a hydrocarbon gas when the hydrocarbon gas composition is not constant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of this invention relates to an improved process of recovering 
the sulfur from Claus tail gases. 
2. Prior Art 
The recovery of elemental sulfur by the Claus reaction is well known and 
various processes using this reaction are in commercial use. The Claus 
reaction involves obtaining elemental sulfur from a gas that contains 
sulfur dioxide and hydrogen sulfide according to the reaction: 
EQU 2 H.sub.2 S + SO.sub.2 .fwdarw. 2H.sub.2 O + 3 S 
in the Claus process one strives to obtain as good as possible a 
stoichiometric ratio of hydrogen sulfide to sulfur dioxide of 2 moles 
hydrogen sulfide to 1 mole of sulfur dioxide in order to make the yield of 
sulfur according to the Claus reaction be as large as possible. Usually 
the reaction is carried out in steps, namely one thermal step, whereby a 
great portion of the sulfur is formed and is condensed out by cooling of 
the gas, followed by two or more catalytic steps at elevated temperature, 
with intermediate cooling of the gas for condensation of the sulfur 
formed. 
The Claus reaction, however, never proceeds quite completely. Even if the 
ratio of the gases employed therein is substantially stoichiometric, the 
Claus tail gas still will contain some sulfur dioxide and/or hydrogen 
sulfide. It is also possible that some carbonyl sulfide and/or carbon 
disulfide can form in the course of the Claus reaction and appear in the 
tail gas. The result is that the effluent or tail gas stream from such a 
reaction can contain substantial quantities of sulfur compounds. 
It has been recognized that emitting this tail gas to the atmosphere can 
present environmental problems because of the sulfur content. 
A known method for recovering the sulfur content of such a tail gas stream 
involves enriching this effluent gas stream with a source of hydrogen to a 
level which is at least equal to the stoichiometric amount of hydrogen 
required to convert the contained sulfur dioxide and other sulfur 
compounds to hydrogen sulfide and catalytically hydrogenating essentially 
all of the contained sulfur compounds to hydrogen sulfide at a temperature 
from about 300.degree. to about 800.degree. F., and treating the 
hydrogenated gas stream to remove hydrogen sulfide. 
A variation of this method involving specific effluent gas streams is 
disclosed in U.S. Pat. No. 3,752,877 to Beavon issued Aug. 14, 1973, 
incorporated herein by reference. 
Such a process can be desirable because the hydrogen sulfide can be 
conveniently collected employing conventional procedures and recirculated 
to the Claus reactor. 
In such a process, the tail gas and hydrogen must be heated to a 
temperature sufficient to effect the conversion of sulfur compounds to 
hydrogen sulfide. 
A very advantageous method of supplying this heat entails combusting a 
hydrocarbon gas and mixing the hot combustion products directly into or 
with the tail gas and hydrogen, for example, via an in-line burner. 
Heretofore, however, this method of supplying heat has not provided good 
process results. Adverse results have included carbon formation on the 
catalyst, and formation of undesirable sulfur-containing reaction 
products, i.e., sulfur reaction products other than hydrogen sulfide. 
A more desirable process would include in-line heating without the adverse 
results mentioned above. 
SUMMARY OF THE INVENTION 
It has now been found that even small variations in hydrocarbon gas 
composition resulting in non-stoichiometric combustion in an in-line 
heater can significantly effect a tail gas recovery process adversely. 
In summary, this invention provides an improvement in the process of 
recovering the sulfur content of a Claus tail gas by providing for 
stoichiometric combustion of hydrocarbon gas when the hydrocarbon gas 
composition is not constant. More particularly, this improvement comprises 
providing substantially stoichiometric combustion by continuously changing 
the amount of oxygen-containing gas in response to changes in hydrocarbon 
gas composition to maintain substantially stoichiometric combustion.

DETAILED DESCRIPTION OF THE INVENTION AND ITS PREFERRED EMBODIMENTS 
It has now been found that in a process for reducing the sulfur content of 
a Claus tail gas stream involving (1) combining the tail gas stream with a 
hydrogen source in an amount at least equal to the stoichiometric amount 
required to convert the sulfur content to hydrogen sulfide, (2) raising 
the temperature of the combined stream to hydrogenation temperature by 
combusting an amount of hydrocarbon gas and oxygen-containing gas to 
provide hot gaseous combustion products and mixing the hot combustion 
products with the combined stream, (3) introducing the heated mixture to a 
catalytic reaction zone to convert substantially all of the sulfur content 
to hydrogen sulfide, and (4) treating the hydrogenated gas stream to 
remove hydrogen sulfide; that a substantial improvement is obtained by 
adjusting the amount of oxygen-containing gas in response to changes in 
hydrocarbon gas composition to maintain substantially stoichiometric 
combustion. 
The effluent or tail gas from a typical Claus reactor can contain a variety 
of sulfur compounds which, if incinerated to sulfur dioxide would emit to 
the atmosphere from about 15,000 to 30,000 parts of sulfur dioxide per 
million parts of dry gas. 
In the practice of this invention this tail gas stream is combined with 
sufficient hydrogen such that substantially all of the sulfur compounds 
are converted to hydrogen sulfide. Sulfur appears in a typical effluent 
tail gas in one or more of the following forms: COS, CS.sub.2, SO.sub.2, 
H.sub.2 S, S.sub.2, S.sub.4 and S.sub.6. The relative amounts contained in 
an effluent gas stream can be readily determined by analytical procedures. 
From this analysis the hydrogen requirement for hydrogenating the sulfur 
compounds to hydrogen sulfide can be computed. Generally a hydrogen 
concentration of an amount required sufficient for the reaction 
EQU SO.sub.2 + 3H.sub.2 .fwdarw. H.sub.2 S + 2H.sub.2 O 
will result in a significant dimunition of SO.sub.2 content for air 
pollution purposes. While this represents the minimum amount of hydrogen 
required for the reaction, it is preferred to provide the gas stream with 
hydrogen to a level of from about 1.25 to 2.0 times that of the 
stoichiometric amounts required for this reaction as this will serve to 
convert substantially all of the sulfur species present to hydrogen 
sulfide, with sulfur, for example, being converted by the reaction 
EQU S.sub.x + H.sub.2 .fwdarw. .sub.x H.sub.2 S 
wherein x is the interger 2, 4, 6 or 8. 
The hydrogen required for the reaction may be obtained from any convenient 
source, including the hydrogen which is present in the effluent gas stream 
as free hydrogen or available from a donor, such as carbon monoxide, which 
will react with water in the presence of a catalyst to yield hydrogen and 
carbon dioxide. 
Molecular hydrogen is preferred, whether contained in the effluent gas 
stream or externally generated. For example, hydrogen may be economically 
and continuously produced for use in the process of this invention by 
concurrent reaction of a low cost hydrogen donor, such as methane or 
carbon monoxide, in a hydrogen generator, such as a steam reformer, where 
the donor undergoes hydrogen-producing reactions such as: 
EQU CH.sub.4 + H.sub.2 O .fwdarw. CO + 3H.sub.2 (1) 
EQU co + h.sub.2 o .fwdarw. co.sub.2 + h.sub.2 (2) 
at temperatures generally from about 1400.degree. F. to about 1600.degree. 
F. for reaction (1) and generally from about 400.degree. F. to about 
800.degree. F. for reaction (2) above. The crude hydrogen output stream 
from the hydrogen generator can then be combined with the tail gas stream 
and, where desired, may be used to supply part of the heat to raise the 
enriched effluent gas stream to hydrogenation temperatures. 
The hydrogen enriched tail gas stream is then raised to hydrogenation 
temperatures, for example, from about 300.degree. F. to 800.degree. F. 
In accordance with this invention, this temperature is achieved by in-line 
heating, i.e., combusting an amount of hydrocarbon gas and 
oxygen-containing gas, and mixing the hot gaseous combustion products with 
the hydrogen enriched tail gas stream. 
Examples of typical hydrocarbon gases suitable for use in this invention 
are paraffinic gases such as methane, ethane, propane, butane and pentane; 
and olefinic gases such as ethylene, propylene, butenes and pentenes. 
Often hydrocarbon gases are available only as non-constant mixtures of two 
or more of such gases. 
The most suitable oxygen-containing gas is air and it can be applied in 
suitable amounts by a commercial blower. 
A preferred known practice heretofore was to maintain a substantially 
constant selected hydrogenation temperature by sensing the temperature in 
the hydrogenation reactor and adjusting the amount of hydrocarbon gas 
(supplied, for example, to an in-line heater) to the selected temperature. 
In turn, the amount of oxygen-containing gas was adjusted in response to 
changes in amounts of hydrocarbon gas according to a pre-selected ratio. 
Only so long as the hydrocarbon gas composition remains constant, however, 
could the resulting combustion be stoichiometric. 
In the process of this invention, the amount of oxygen-containing gas, 
preferably air, is carefully supplied in order to achieve substantially 
stoichiometric combustion even when the hydrocarbon gas composition is not 
constant. Stoichiometric combustion is maintained by analyzing, preferably 
continuously analyzing, the hydrocarbon gas composition and adjusting the 
amount of oxygen-containing gas in response changes in hydrocarbon gas 
composition in order to maintain substantially stoichiometric combustion. 
In practice, this can be accomplished in a variety of ways. 
For example, if the hydrocarbon gas is a complex mixture of paraffinic and 
olefinic gas, the hydrocarbon gas can be analyzed employing conventional 
gas chromatograph techniques, the amount of oxygen necessary for 
stoichiometric combustion can be determined, and the speed of a blower 
supplying air to a combustion zone can be adjusted, or more preferably a 
control valve in an air supply line can be adjusted, to supply the amount 
of air necessary for stoichiometric combustion. 
More preferably, a hydrocarbon gas will be employed which is comprised of 
paraffinic hydrocarbons or olefinic hydrocarbons. When the hydrocarbon gas 
employed is either paraffinic or olefinic hydrocarbons, the specific 
gravity of the hydrocarbon gas (which is related to its composition) can 
be related to the amount of oxygen necessary for stoichiometric 
combustion. 
Referring now to FIG. 1, it can be seen that the ratio of air to paraffinic 
hydrogen gas necessary for stoichiometric combustion is a linear function 
of the specific gravity of the hydrocarbon gas. 
In a preferred embodiment, the specific gravity of the paraffinic or 
olefinic hydrocarbon gas is continuously measured and the amount of 
oxygen-containing gas supplied for in-line burning is adjusted in response 
to changes in hydrocarbon gas specific gravity to maintain substantially 
stoichiometric combustion. 
A variety of methods and apparatuses are known for measuring the specific 
gravity of hydrocarbon gases. For example, U.S. Pat. No. 3,855,845 to 
Homolka issued Dec. 24, 1974 discloses a suitable apparatus involving 
pressure transducers and an electrical output. 
A preferred method for use in this invention is known as the "telezometer" 
method. (See, Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 7, p. 
78, copyright 1951). In this method, a measurement is made of the 
difference of the torques conveyed to two free rotors by streams of the 
test gas and a reference gas (usually air), respectively. The gas streams 
are driven by matched impellers, turned at the same speed in reservoirs of 
the two gases which are maintained at equal temperature and pressure. 
A commercially available device for continuously measuring the specific 
gravity of gases in this manner is a specific gravity analyzer known as 
"Ranarex Gas Gravitometer" sold by the Permutit Company, division of 
Sybron Corporation. The output of the specific gravity analyzer can be 
transmitted, for example, pneumatically or electronically, to a suitable 
ratio controller which can control, for example, a control valve in an air 
supply line to automatically govern the combustion air flow rate to an 
in-line heater to maintain stoichiometric combustion. 
After being heated to hydrogenation temperature, the combined gas stream 
flows to a catalytic reactor where sulfur dioxide and other sulfur 
compounds are essentially completely hydrogenated to hydrogen sulfide. 
Useful catalysts are those containing metals of Groups Va, VIa, VIII and 
the rare earth series of the Periodic Table defined by Mendeleeff and 
published as the "Periodic Chart of the Atoms" by W. N. Welch 
Manufacturing Company and incorporated herein by reference. The catalysts 
may be supported or unsupported, although catalysts supported on a silica, 
alumina or silica alumina base are preferred. The preferred catalysts are 
those containing one or more of the metals, cobalt, molybdenum, iron, 
chromium, vanadium, thoria, nickel, tungsten and uranium. 
In the catalytic hydrogenation effective conversion can be realized at a 
space velocity of about 700 to about 3000, preferably from about 1000 to 
about 2000, cubic feet (calculated at standard conditions) per hour per 
cubic feet of catalyst. 
Hot reactor effluent gases are cooled, preferably in two steps: first, in a 
stream generator to produce steam, and secondly, in a quench tower using 
water. 
The cooled hydrogenated tail gas, which contains the balance of the 
hydrogen sulfide formed in the hydrogenation reaction, is then passed 
through an extraction zone for recovery of hydrogen sulfide. Any number of 
extraction methods known to those skilled in the art are feasible for this 
step with absorption methods being preferred. For instance, the cooled 
hydrogenated tail gas may be passed through alkaline absorption solutions 
which are continuously regenerated by oxidation to produce elemental 
sulfur using catalysts such as sodium vanadate, sodium anthraquinone 
disulfonate, sodium arsenate, sodium ferrocyanide, iron oxide, iodine and 
like catalysts. 
A convenient alternative is to use absorption solutions containing amines, 
sulfonates, potassium carbonates which can preferentially absorb hydrogen 
sulfide and be continuously regenerated by steam stripping to produce 
hydrogen sulfide which can be returned to the Claus reactor. 
The following example more specifically illustrates a preferred embodiment 
of the invention. 
EXAMPLE 
A Claus sulfur plant tail gas having the following composition of sulfur 
compounds in moles per hour was treated: 
______________________________________ 
CONSTITUENT Moles/Hour 
______________________________________ 
H.sub.2 S 13.86 
SO.sub.2 6.93 
S.sub.6 0.05 
S.sub.8 0.10 
COS 0.40 
______________________________________ 
Referring now to FIG. 2, this tail gas is introduced via line 1 to a stream 
of hydrogen from line 2, the amount of hydrogen being sufficient to 
convert all of the sulfur constituents to hydrogen sulfide. This combined 
tail gas stream is sent via line 3 to an in-line heater 4. A paraffinic 
hydrocarbon gas is introduced to the in-line heater 4 via line 5. 
Hydrocarbon gas controller 20, a control valve responsive to temperature 
sensor 19, governs the amount of hydrocarbon gas delivered. The 
hydrocarbon gas controller 20 is equipped to generate a signal 
corresponding to the amount of hydrocarbon gas supplied to in-line heater 
4. The specific gravity of the hydrocarbon gas passing through line 5 is 
continuously analyzed by a specific gravity analyzer 6, for example, a 
Ranarex Gas Gravitometer equipped to generate an electronic signal 
corresponding to the specific gravity of the gas. This signal biases the 
signal from the hydrocarbon gas controller and the resulting signal is 
sent to a conventional ratio controller 21 which sets combustion air 
controller 7, a control valve, governing the air flow from a combustion 
blower 8 such that the ratio of air to hydrocarbon gas corresponds to a 
ratio as shown in FIG. 1 for stoichiometric combustion. The air is 
conveyed by line 9 to in-line heater 4 where it is introduced to the 
hydrocarbon gas under combustion conditions. Stoichiometric combustion 
occurs, eliminating the hydrocarbon gas and oxygen, and producing hot 
gaseous combustion products. These hot gaseous combustion products mix 
with the combined tail gas stream from line 3 raising the temperature to 
about 700.degree. F. This hot gaseous stream passes through line 10 to 
catalytic reactor 11 where substantially all of the sulfur is hydrogenated 
to hydrogen sulfide. This hydrogenated gas stream passes via line 12 to a 
cooling zone 14 which comprises several stages. The cooled gas passes from 
the cooling zone 14 via line 15 to a hydrogen sulfide absorber 16 where 
hydrogen sulfide is selectively absorbed. The non-absorbed gases, 
substantially free of hydrogen sulfide, are vented to the atmosphere. Even 
when the hydrocarbon gas composition fluctuates, the ratio of hydrocarbon 
gas to air is always such that stoichioometric combustion occurs. The 
result is that the process is substantially improved in that good results 
are continuously obtained; i.e., the hydrogenation catalyst is not 
adversely deactivated by carbon deposits, and undesirable sulfur reaction 
products (non-hydrogen sulfide) are minimized.