This invention relates to the alleviation of industrial air and water pollution and particularly to the removal of sulfur pollutants from industrial exhaust gases.
Industrial gases such as coke oven gas, natural gas and various artificially-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 particularly 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-desorption 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.
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 processes in particular have 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 content 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, a very small amount.
These absorption-desorption type processes, while effective to reduce the sulfur content of treated industrial gas to a very low level, do recover the acid gases in a more concentrated form. The recovered "foul" gases have to be treated in turn to remove their sulfur content in some satisfactory manner. Very frequently the foul gases from the absorption step have been used to produce elemental sulfur by some variation of the so-called Claus process. In this process a portion of the sulfur removed, 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 converter to form elemental sulfur and water.
There are a number of industrial variations of the basic Claus process in which either an initial portion of the 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. The Claus reaction may be conducted either in the gas phase by mixing the two gases in appropriate apparatus, in which case the process is usually referred to either as the Claus process or a gas phase sulfur recovery process, or the reaction may be conducted in the liquid phase. The liquid phase process is conducted by dissolving one or both of the component sulfur base reactant compounds in a liquid and then either passing the other reactant compound through the liquid or bringing together two parts of liquid in which the reactants are dissolved. Such liquid reaction medium processes, which are frequently referred to as liquid phase sulfur recovery processes, are usually run at lower temperatures than the more usual Claus type processes and have certain other advantages as well.
The Claus process and the other related processes for the recovery of elemental sulfur such as the liquid reaction medium processes are fairly efficient, but 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 frequently both remain. This tail gas must be disposed of in some manner and is usually at this point discharged to the atmosphere.
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 in the remaining 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 reactors in series, but due to the small amount of remaining sulfur components 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 impossible to remove.
One fairly simple expedient for final treatment has been to oxidize all the remaining sulfur compounds to sulfur dioxide and then wash the sulfur dioxide out of the gas with a simple water wash system. The wash water can then be discarded, or used if it is concentrated enough to make sulfuric acid. However, the amount of sulfur dioxide dissolved in the water is insufficient for really effective use as a source of sulfur, yet it is undesirable to waste the sulfur values by discarding the wash water.
The Claus process in particular exhibits a fairly poor recovery of sulfur based upon the amount of sulfur in the original 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 95% or occasionally as much as 97% of the sulfur removed from the original gas.
Some of the many liquid reaction phase processes are, on the other hand, significantly more efficient so far as the percentage of the original sulfur content which is removed from the gas as elemental sulfur is concerned. However, there is always some small amount of sulfur left in the final tail gas which it is substantially impossible to remove. One of the difficulties is that even with the best control and mixing of the two gases H.sub.2 S and SO.sub.2, it is inevitable that one or the other of the two reactant gases will be present in some degree of excess and such excess will pass unreacted through the process. It is simply impossible under industrial conditions to exactly meter the two gases together in an exact stoichiometric relationship. Usually, of course, other reacting conditions will also not be completely favorable for completion of the reaction so that a quantity of one gas or the other, and very often both, is left unreacted. Sometimes it is decided beforehand which gas is most desirable or least detrimental to have in excess and this gas is then deliberately supplied in a slight excess in order to assure that substantially none of the other gas remains at the end of the process. Usually SO.sub.2 will be preferred in the tail gas rather than H.sub.2 S because SO.sub.2 is less objectionable to the senses than H.sub.2 S in similar concentrations.
The relative amounts of hydrogen sulfide, carbon dioxide, water vapor and extraneous hydrocarbons in the foul or acid gas fed to Claus reactors have a pronounced influence on the recovery efficiency for a given number of reactors. As pointed out above there is a limit to the number of reactors which can practically be used to reduce the sulfur compounds in the tail gas. There is also a limit to the preciseness with which an operation can be run in order to keep the reaction exactly in balance and prevent decreases in the conversion of the sulfur compound gases to elemental sulfur due to variations from precise stoichiometric ratios of the reacting gases. The reversible reaction EQU 2H.sub.2 S + SO.sub.2 .revreaction. 3S + 2H.sub.2 O (1)
of the Claus process cannot, furthermore, be completed to the right of the equation because of limitations of the thermodynamic equilibrium at temperatures above 250.degree. C. at which the Claus reaction is run. Sufficient water and sulfur vapor is always present to limit the desired reaction. Carbon dioxide and additional water vapor in the feed gas are also diluents which shift the equilibrium adversely. Exact air/acid gas ratio control is also never continuously achieved even with the most sophisticated instrumentation, because of uncontrollable variations in the feed rate and composition of the gases. Hydrocarbons in the feed also affect the plant efficiently by increasing undesirable side reaction products such as COS and CS.sub.2, which are difficult to convert to elemental sulfur. The tail gas from a single Claus reactor may as a result contain as much as 10% of the sulfur originally removed by the absorption system from the fuel gas.
A number of processes have been proposed as "clean-up" processes for further treatment of Claus unit tail gas. Several of these depend upon treatment of the tail gas so that the residual sulfur values occur as hydrogen sulfide which is then converted to sulfur in a so-called Stretford unit. There are a number of other proposals for improved clean-up of the tail gas including the use of improved catalysts in the Claus reactor, wet scrubbing, reaction with ammonia, high temperature sulfur dioxide removal, concentration of sulfur dioxide by absorption, catalytic sulfuric acid production, absorption on carbon and absorption-desorption type chemical removal. Some of these proposals are applied to the tail gas after incineration to change all the sulfur values to sulfur dioxide. While some are fairly efficient, at least in the laboratory, in removing the sulfur components, many have proved impractical under industrial conditions and none is completely efficient in removing sulfur compounds.
One notable yet fairly typical example of a Claus-type process in which residual H.sub.2 S is oxidized to SO.sub.2 is shown in U.S. Pat. No. 1,915,364 issued June 27, 1933 to J. W. Harrel. Harrel separates a gas having a high content of H.sub.2 S into two portions. One portion is sent to a burner or furnace where it is burned with O.sub.2 to SO.sub.2. The SO.sub.2 is absorbed into a water solution which is then passed concurrently or countercurrently to the remainder of the H.sub.2 S gas in a reaction tower where the Claus reaction converts H.sub.2 S and SO.sub.2 to elemental sulfur and water. In order to eliminate side reactions and possible loss of sulfur an excess of H.sub.2 S is maintained in the reactor. This excess results in residual H.sub.2 S gas leaving the reactor with the solution, which H.sub.2 S is then separated from the solution and returned or recycled back to the burner or furnace where it is burned or oxidized with oxygen to form additional SO.sub.2 required for the Claus reaction. Some of this SO.sub.2 eventually escapes through a vent on the top of the absorber where the SO.sub.2 is absorbed by water, but no H.sub.2 S leaves the apparatus and pollution of the air with H.sub.2 S is thus said to be completely eliminated. The elemental sulfur formed in the Claus reaction is separated from the water suspension by any suitable method such as filtering or the like.
Harrel is fairly typical of prior processes in which excess H.sub.2 S is oxidized subsequent to the claus reaction. Many later processes have, however, attempted after oxidizing the H.sub.2 S to remove the resulting SO.sub.2 from the tail gas by the use of additional removal apparatus, none of which has proved to be really practical or efficient under industrial conditions. A number of processes for the treatment of tail gas from sulfur recovery plants for the minimization of sulfur emissions are summarized in the following articles.
(a) "Reduce Claus Sulfur Emissions" by C. B. Barry Hydrocarbon Processing April 1972 pages 102-106 PA1 (b) "Production of Clean Energy and Sulfur Recovery" by Mikio Harima Chemical Economy and Engineering Review, Vol. 6 No. 8 (No. 76), August 1974, pages 13-21 et seq.
Recycling per se in connection with sulfur recovery processes is, of course, not in itself new as illustrated by the Harrel patent noted above, and a large number of processes have 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, normal Claus reactor tail gas containing both H.sub.2 S and SO.sub.2 has been oxidized to convert all residual H.sub.2 S to SO.sub.2 and the SO.sub.2 has then been recycled back to the Claus reactor to replace a portion of the SO.sub.2 used in the reactor. In some proposals the SO.sub.2 has been absorbed from the tail gas into lime or the like and then regenerated from the lime and recycled into the Claus reactor.
Several processes have been developed in which H.sub.2 S and SO.sub.2 are reacted together in a liquid reactor medium of some suitable composition. The liquid reaction medium may be renewed at intervals by stripping gases and volatilizable components including, for example, H.sub.2 S 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 excess H.sub.2 S or SO.sub.2 passing from the solution may be recycled back to reconstitute the absorption solution. It has also been proposed to recycle the entire tail gas stream containing both SO.sub.2 and H.sub.2 S from a Claus reactor 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 adsorb H.sub.2 S from a tail gas derived from a Claus reactor and recycle it back to an absorption step.
While prior workers have, therefore, used the principle of recycling in various ways in connector with sulfur removal systems for the desulfurization of industrial gas, such prior systems have not been completely successful in eliminating exhaust of sulfur components to the environment and have in many cases been expensive to build or uneconomical to operate. Furthermore, while a great many prior workers have made use of the basic principle of recycling various sulfur compounds back to various parts of the processes for retreatment in order to increase the recovery of the sulfur compounds, none has conceived or used the novel arrangement developed by the present inventors.