Petroleum refinery streams are typically desulfurized by the Claus process. In the Claus process, elemental sulfur is produced by reacting H.sub.2 S and SO.sub.2 in the presence of a catalyst. The Claus system uses a combustion chamber which, at 950.degree.-1,350.degree. C., converts 50 to 70% of sulfur contained in the feed gas into elemental sulfur. Sulfur is condensed by cooling the reaction gas to a temperature below the dew point of sulfur, after which the remaining gas is heated and further reacted over a catalyst. Normally, the gas passes through at least two such Claus catalyst stages.
The different stages of the process may be represented by the following equations: EQU H.sub.2 S+3/2 O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O (I) EQU 2 H.sub.2 S+SO.sub.2 .fwdarw.3 S.sub.n +2 H.sub.2 O (II)
The overall reaction is: EQU 3 H.sub.2 S+3 O.sub.2 .fwdarw.3 S.sub.n +3 H.sub.2 O (III)
Below 500.degree. C, the symbol n has a value of approximately 8.
The final Claus exhaust gas still contains small amounts of H.sub.2 S, SO.sub.2, CS.sub.2, carbon oxysulfide, CO, and elemental sulfur in the form of a vapor or mist. The exhaust gas can be subjected to post-combustion to convert substantially all sulfur species to sulfur oxides, for example, SO.sub.2 and SO.sub.3, which are then emitted into the atmosphere.
Sulfur emitted as sulfur oxides ("SO.sub.x ") into the atmosphere with the exhaust gas may amount to 2-6% of the sulfur contained in the feed gas in the form of H.sub.2 S. In view of air pollution and the loss of sulfur involved, further purification is imperative.
Claus aftertreatments have been developed. These are carried out after the last Claus stage or after the post-combustion. These aftertreatments include, for example, dry and liquid phase processes for catalytic conversion of H.sub.2 S and SO.sub.2 to elemental sulfur, catalytic hydrogenation and hydrolysis of sulfur compounds into H.sub.2 S for further processing, and oxidation of all sulfur compounds into SO.sub.x for further processing by adsorption in dry processes or absorption in wet processes.
The dry processes typically involve a gas-solid reaction. Gas-solid reactions such as these are often limited by the intra-particle mass transfer rate. This requires the use of small particles in order to achieve a sufficient degree of reaction within a reasonable reactor length. Increasing the reactor length provides the required degree of conversion, but results in a proportionately higher pressure drop, and is often undesirable due to other process restrictions. In the case of waste-gas clean up processes like those mentioned above, the available pressure drop is frequently small, and the costs involved in the addition of an extra blower in the system to provide a large pressure head for the packed bed system may be significant.
In view of the foregoing, improving gas-solid reaction system methods and apparatus is desirable. In particular, it would be beneficial to provide a mechanism for achieving high conversion efficiencies while maintaining a relatively low pressure drop through the packed bed in a dry process. This can be achieved by providing an improved conversion potential catalyst bed design that maintains a relatively low pressure drop.
It is, therefore, an object of the present invention to provide an improved method and apparatus for gas-solid reactions to remove contaminants such as sulfur oxides and/or nitrogen oxides from waste gas streams.
It is a further object of the present invention to remove the above-described contaminants without incurring an unacceptable pressure loss in the system.