1. Field of the Invention
This invention lies in the field of sulfur removal and recovery, and particularly in the treatment of sour gases and other gases in the chemical process industry that contain hydrogen sulfide.
2. Description of the Prior Art
Much of the natural gas produced in the United States has a hydrogen sulfide content exceeding 4 parts per million and is therefore classified as xe2x80x9csour.xe2x80x9d Since hydrogen sulfide is an environmental hazard, sour natural gas is considered unacceptable for transport or use. Hydrogen sulfide levels are also a problem in the fabrication of fuels derived from petroleum, coal and lignite, whose sulfur content is converted to hydrogen sulfide during the conversion of these materials to gasoline, jet fuels, refinery gas, coal gas, blue-water gas and the like. In addition to the environmental hazard, hydrogen sulfide represents a loss of the sulfur value of the raw material, which if recovered as elemental sulfur would be of significant use to the chemical industry.
The traditional method for converting hydrogen sulfide in natural gases and in gaseous plant effluents is the Claus process, in which part of the hydrogen sulfide is burned in air to form sulfur dioxide and water:
2H2S+3O2xe2x86x922SO2+2H2Oxe2x80x83xe2x80x83(A)
and the sulfur dioxide thus produced is reacted with further hydrogen sulfide to form sulfur and additional water:                                           2            ⁢                          xe2x80x83                        ⁢                          H              2                        ⁢            S                    +                      SO            2                          ->                                            3              x                        ⁢                          S              x                                +                      2            ⁢                          xe2x80x83                        ⁢                          H              2                        ⁢            O                                              (        B        )            
The symbol xe2x80x9cxxe2x80x9d in Reaction B is used to denote that the elemental sulfur exists in a mixture of molecular species varying in the number of sulfur atoms per molecule.
The furnace (Reaction A) in the Claus process is operated with a fuel-rich mixture, converting only one-third of the H2S to SO2. The fuel-rich atmosphere results in the partial conversion of hydrocarbons that are present in the H2S feed to such compounds as COS and CS2, which lessen the yield of elemental sulfur and are themselves hazardous. The fuel-rich atmosphere also promotes the breakdown of aromatics to soot. For high sulfur recovery, precise control of the overall stoichiometry is needed, and this is made especially difficult when considerable amounts of CO2 and other inerts are present.
Part of Reaction B occurs in the furnace and the rest is conducted in a heterogeneous system in which the reaction mixture is gas-phase and contacts a solid activated alumina catalyst of a sort well known to those skilled in the art of the Claus process. With continued use, the alumina catalyst fouls and becomes otherwise deactivated over time. This requires plant shutdown, loss of process time, and the cost of regeneration or replacement of the catalyst, together with the associated labor costs.
A further disadvantage of Reaction B is that it is equilibrium-limited at temperatures above the dewpoint of sulfur, and despite being performed in two to four stages, the reaction leaves 2% to 5% of the H2S and SO2 unreacted. Each stage requires a separate condenser to remove the elemental sulfur, and these condensers require a large heat-exchange area and reheating of the gas leaving each but the last condenser. Furthermore, the steam generated by each condenser is low in pressure, limiting its usefulness. Additional costs are entailed in treating the tail gas in which the sulfur content must be reduced by ten to twenty times.
It has now been discovered that virtually complete conversion of hydrogen sulfide in natural gas or other gas mixtures to elemental sulfur and water can be achieved with the use of a single-stage reaction between hydrogen sulfide and sulfur dioxide, in a manner producing no reaction products other than elemental sulfur and water. The reaction
2H2S+SO2xe2x86x923S+2H2Oxe2x80x83xe2x80x83(I)
is conducted with excess H2S in the liquid phase in the presence of a homogeneous liquid-phase Claus catalyst, at a temperature above the melting point of sulfur but low enough to keep the reaction in the liquid phase, and upstream of the furnace where unreacted H2S is combusted to produce the SO2 that is consumed in reaction (I). The H2S that is combusted in the furnace is the excess H2S that passes unreacted through the liquid-phase reaction, optionally supplemented by H2S from an H2S-containing stream that bypasses the reaction (I) reactor. The SO2 in the furnace combustion gas is recycled to the reaction (I) reactor either as a gas or dissolved in a solvent, and in any case serves as the entire SO2 feed to the reaction.
In basic terms, the invention as shown in FIG. 1 proceeds as follows:
(a) In a first stage 1, an H2S-containing mixture is passed through a continuous-flow catalytic reactor where the mixture contacts SO2 in accordance with the reaction
2H2S+SO2xe2x86x923S+2H2Oxe2x80x83xe2x80x83(I)
using approximately 10% to 50% excess H2S. The H2S enters the reactor either as a gas or dissolved in an organic solvent; in most cases the H2S will enter as a gas. The SO2 likewise enters the reactor either as a gas or dissolved in an organic solvent. Regardless of the phases of the streams entering the reactor, both reactants dissolve in an organic liquid solvent flowing through the reactor, the solvent entering either with one of the two incoming reactant streams with the H2S or the SO2 dissolved in the solvent, or as a circulating stream recycled from the reactor exit. The reaction causes a major fraction of the SO2, preferably substantially all of it, to react. The term xe2x80x9ca major fractionxe2x80x9d is used herein to indicate at least half, and preferably 80-90% or more. The organic liquid solvent also contains a dissolved catalyst that promotes reaction (I). The reaction produces elemental sulfur, which is recovered from the product mixture by phase separation. The reaction is conducted at a temperature above the melting point of sulfur and below the boiling point of the solvent, preferably below the temperature at which sulfur polymerizes.
(b) In a second stage, 2, H2S, including H2S that passed unreacted through the first stage, is combusted with oxygen according to the reaction
xe2x80x832H2S+3O2xe2x86x922SO2+2H2Oxe2x80x83xe2x80x83(II)
to convert the H2S to SO2. Hydrocarbons that may accompany the H2S are combusted to CO2 and H2O whereas organic sulfur compounds that may be present additionally yield SO2.
(c) In a third stage, 3, the SO2 produced in the second stage is recovered by absorption and returned to the reactor (the first stage). The SO2 in the third stage may be recovered in the solvent used in the first stage, 1, with the first stage catalyst dissolved in the solvent. The solution containing both the SO2 and the catalyst can then be recycled in its entirety to the continuous-flow reactor (the first stage) as the SO2 feed to the reactor. Alternatively, the solvent used to recover the SO2 in the third stage, 3, may be kept separate from the solvent in the reactor of the first stage, 1, with the SO2 being stripped from it and sent to the first stage, 1, as a gas. The third stage absorption leaves a tail gas that is substantially free of H2S and SO2.
Various additional process stages upstream, intermediate and downstream of these three stages are included in any of various arrangements in preferred embodiments of the invention to enhance the flow and transfer of streams, to separate phases and control concentrations and flow rates, to separate the water formed in Reactions I and II, and to control other process parameters such as temperature and pressure. These and. other characteristics, features and advantages of the invention will be better understood from the description that follows.