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11.3  SELECTION OF SWEETENING PROCESS

            There are many factors to be considered in the selection of a given
            sweetening process. These include the following:
                 1. Type of impurities to be removed (H 2 S, mercaptans)
                 2. Inlet and outlet acid gas concentrations
                 3. Gas flow rate, temperature, and pressure
                 4. Feasibility of sulfur recovery
                 5. Acid gas selectivity required
                 6. Presence of heavy aromatic in the gas
                 7. Well location
                 8. Environmental consideration
                 9. Relative economics
            Figure 1 can be utilized for the initial selection of the proper process. It
            depends on the sulfur content in the feed and the desired product. Several
            commercial processes are available, as shown in the schematic flowsheet of
            Figure 2. This diagram can help in selecting the suitable process. If the
            sulfur recovery is not an option, then indirect processes can be considered.
            These processes can be classified into liquid-phase processes and dry-bed
            processes. The latter is selected if the sulfur content in the feed is low, as
            supported by Figure 1. In this case, the total sulfur to be removed should
            not be large. The dry-bed processes can be classified as iron oxide sponge
            or zinc oxide process, in this case, because the oxides are not expensive,
            the corresponding sulfide might not be regenerated on site but rather by a
            contracting company, and this might lower the operational cost and does
            not require further processing for sulfur recovery. Alternatively, the
            molecular sieve bed can be used when the bed is regenerated on site.
                 If the operating conditions require the removal of a large amount of
            H 2 S and/or CO 2 , liquid-phase processes are used. If it is required to
            selectively remove H 2 S, then physical solvents will be suitable, they also
            remove COS and CS 2 . If the feed contain high amount of heavy
                           þ
            hydrocarbons (C ), then the use of physical solvents may result in a
                           3
            significant loss of these heavy hydrocarbons because they are released
            from the solvent with the acid gases and cannot be recovered
            economically. On the other hand, if the feed contains a high amount of
            H 2 S and CO 2 and it is required to remove both of them, then chemical
            solvents such as amines or carbonates could be used. The amine processes
            offer good reactivity at low cost and good flexibility in design and
            operation. However, carbonates can be used if it is required to remove
            COS and CS 2 from the feed. The carbonate process also can be run at a
            lower utility cost.






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