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GAS PROCESSING DEVELOPMENTS SPECIALREPORT
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acid-gas removal units. This process recycles a portion of the acid gas to a second absorber to concentrate the H2S. Fig. 2 shows the basic configuration of the double-absorption process.
The acid-gas feed stream enters the unit and is scrubbed in the first amine absorber, V-1, with a lean amine stream 2. The solvent typically consists of 40% to 50% MDEA, although other solvents, such as a sterically hindered amine can be used. The amine absorber generally consists of 12 to 18 trays. About 85% to 90% of the feed gas CO2 is rejected in stream 3. The rich-solvent stream 4 exits the bottom of the first absorber and combines with the rich solvent from the second absorber, V-2, forming stream 6.
The combined stream is pumped and heated with the lean/rich exchanger, E-1, using the heat content of the lean solvent from the regenerator, V-3. The regenerator operates at a slightly higher pressure than the absorber. This allows recycling of a portion of the acid gas without using a compressor.
The heated stream enters the top of the regenerator, which consists of 20 to 22 stripping trays and a wash section. Alterna-tively, other contacting devices, such as packing, can be used.
The acid gas in the rich solvent is stripped with heat applied at the bottom reboiler, E-2, producing overhead stream 9 and a lean solvent, stream 10.
The lean solvent is pumped and cooled in the lean/rich exchanger, E-1. The lean solvent is further cooled in E-3. Air or cooling water can be used as the cooling medium. The lean amine should be cooled as much as possible, as cooling favors the selec-tive absorption of H2S, thereby increasing the H2S selectivity. The cooled lean amine is split into two portions, stream 2 and stream 22, which are fed into the first absorber, V-1, and the second absorber, V-2, respectively.
The overhead vapor from the regenerator, stream 9, is cooled in the overhead condenser, E-4. Liquid in the stream is separated in the reflux drum, V-4. The liquid stream, which is mostly water, is pumped and used to reflux the regenerator. The enriched acid gas is split into two portions, stream 17 and stream 18. Stream 17 is routed to the second absorber, V-2, for further enrichment and stream 18 is sent to the SRU.
The flow ratio of stream 17 to stream 14 ranges from 25%
to 75%, depending on the H2S concentration in the feed gas.
For a low H2S-content feed gas, a higher flow ratio of possibly 75% may be necessary. The ratio can be reduced to less than 25% when the feed gas contains a higher H2S concentration.
For most applications, acid-gas enrichment to about 75% H2S can be achieved. In addition, over 90% of the hydrocarbons and BTX components can be rejected with the CO2 stream. The H2S enrichment and the absence of BTX and heavy hydrocarbons in the enriched acid gas are highly desirable for good performance of the Claus SRU.5,6
Furthermore, depending on the feed-gas composition and acid-gas loading of the semi-lean solvent, the overall circulation rate can be reduced by splitting the semi-loaded rich solvent stream 7 from the first absorber into two separate streams. One stream can be cooled and reused for absorption in the second absorber V-2. The other stream, consisting of semi-lean rich solvent from the first absorber, which is still unloaded in terms of its H2S content, can be fed to the lower section of the second absorber for bulk H2S removal. The remaining semi-lean solvent can then be sent to the regenerator, V-3, for solvent regeneration.
Fig. 3 shows the configuration for this option.
Integrated double-absorption acid-gas enrichment/sulfur recovery process. The double-absorption acid-gas enrichment
configuration can be integrated with the tail-gas unit to reduce the total project cost. The semi-lean solvent from the tail-gas unit, which is unloaded at the upstream absorber conditions, can be reused to reduce the overall solvent circulation while eliminating a dedicated regenerator. With this option, a single regenerator can be used to regenerate the rich-solvent streams from both the acid-gas enrichment and the tail-gas units.
Fig. 4 shows a configuration where acid-gas enrichment is integrated with the tail-gas treating unit. The combination of the enrichment unit with a tail-gas absorber processing the tail gas from a Claus unit can achieve over 99.9% total sulfur recovery even when the feed gas H2S concentration is low.
Acid gas to
incineration To sulfur plant
Feed gas
Double-absorption acid-gas enrichment process with rich-solvent splitting.
Integrated double-absorption acid-gas enrichment/sulfur recovery process.
FIG. 4
GAS PROCESSING DEVELOPMENTS SPECIALREPORT
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JANUARY 2011 HYDROCARBON PROCESSINGIn this configuration, a two- or three-stage Claus SRU is used to process the enriched acid gas. A conventional modified Claus SRU would require more than three stages and other various additional processing steps to achieve at best, 99% sulfur recov-ery. This integrated tail-gas treating configuration significantly reduces the sulfur plant energy requirement and improves sulfur-recovery efficiency while requiring less capital investment than a conventional design.
The effluent from the Claus unit, which contains trace quan-tities of H2S, SO2 and other sulfur compounds, is processed in the hydrogenation unit. The hydrogenated gas is quenched and extra water is condensed and removed prior to routing the gas to the tail-gas absorber, V-5. Lean amine is supplied from the lean-amine header. Effluent from the tail gas absorber, which contains environmentally acceptable levels of H2S, can, depending on envi-ronmental regulations, either be vented directly to the atmosphere or routed to an incinerator for disposal.6, 7
The rich amine from the tail-gas absorber is pumped and combined with the rich-amine streams from the first and sec-ond absorbers. The combined stream is heated in the lean /rich exchanger and fed to the common regenerator. Depending on the actual feed gas conditions and sulfur-recovery requirements, the semi-loaded solvent, stream 35, from the tail gas absorber, V-5, can be re-used in the second absorber, V-2. This configura-tion (opconfigura-tion 1) as shown in Fig. 5, reduces solvent circulaconfigura-tion and the solvent regeneration duty. With this configuration, the incremental amount of solvent used in the tail-gas absorber can be reduced, thus improving process economics while maintaining high sulfur-recovery efficiency.
Depending on the acid-gas composition and the semi-rich solvent loading, another option, shown in Fig. 6, can be used to further reduce the total solvent circulation rate and solvent regeneration duty. This configuration re-routes a portion of the semi-loaded rich solvent stream 7 from the first absorber, V-1, to the second absorber, V-2. The stream is cooled prior to entering the lower section of the second absorber, providing a cost-effective means for processing lean acid-gas feeds.
Optimization options. Several acid-gas enrichment process configurations and various options for integrating sulfur recovery with tail gas treating are presented here. The double-absorption process and various configuration options effectively produce acid-gas enriched in H2S from a lean acid- gas feed. Acid gas can be enriched from less than 7% to over 75% H2S. The various configurations also allow removal of hydrocarbons and BTX that are known to interfere with SRU operation. Furthermore, a CO2
stream with environmentally acceptable levels of H2S can be pro-duced from the absorbers for disposal by incineration. When inte-grated with Claus and tail-gas treating units, the process is capable of reducing the number of Claus reaction stages and can achieve over 99.9% total sulfur recovery. The double-absorption process also solves the problems of low H2S content and low acid heating value by providing a Claus plant feed with a high H2S content. HP
LITERATURE CITED
1 ZareNezhad, B. and N. Hosseinpour, Applied Thermal Engineering, Vol. 28, Issue 7, May 2008.
2 ZareNezhad, B., Hydrocarbon Processing, October 2008, pp 109–115.
3 ZareNezhad, B., Research Report 1342B, Petroleum Ministry, November 2008.
4 ZareNezhad, B., Hydrocarbon Processing, February 2009, pp. 63–72.
5 Chow, T. K., C. H. Lawrence, J. A. Gebur and V. W. Wong, Canadian International Petroleum Conference 55th Annual Technical Meeting, Calgary, Canada, 2004.
6 Clarke, D., J. Iyengar, M. Al-Khaldy and S. Summers, 51st Annual Gas Conditioning Conference, Oklahoma, 2001.
7 Chow, T. K, J. A. Gebur, and V. W. Wong, World Petroleum Congress, Second Regional Meeting, Doha, Qatar, 2003.
Integrated double-absorption acid-gas enrichment/sulfur recovery process (option 1).
FIG. 5
Integrated double-absorption acid-gas enrichment/sulfur recovery process (option 2).
FIG. 6
Dr. Bahman ZareNezhad is an academic professional mem-ber of the Ministry of Science, Research and Technology in Iran. His research activities are mainly focused on advanced oil refining and gas processing technologies, tail-gas treatment, sulfur recovery and NGL extraction processes. Dr. ZareNezhad has published several technical and research papers in international journals and has presented several technical courses regarding oil and gas industries. He has 22 years of varied experience in research, process engineering, project management and technology development, and is a consultant for several oil and gas companies. Dr. ZareNezhad holds a PhD in chemical engineering from the University of Manchester Institute of Science and Technology (UMIST) in England.