recovering valuable ethane and propane that exit the top of the existing demethaniser. The resulting RSV retrofit plant can recover as much as 99% of the ethane and 100% of the propane, or it can reject ethane, recovering less than 2% of the ethane while main- taining greater than 99% propane recovery. To achieve these recovery levels, the RSV process usually requires 10-15% more compression power compared to the base GSP plant, which is limited to 90-94% ethane recovery. Therefore, a ret- rofit will typically require either a separate recycle gas com - pressor or additional sales gas compression unless surplus compression power was originally installed. Retro-Flex retrofit The proprietary UOP Retro-Flex technology 6 was co-devel- oped in partnership with SME Products, LP specifically to increase the recoveries of propane and heavier components from GSP plants. It is applicable to both ethane recovery and ethane rejection modes of operation and does not require additional compression. Retro-Flex is considered a ‘bolt-on’ retrofit, with all new equipment located on a skid adjacent to the existing plant. This minimises the number of tie-in points and downtime. Figure 3 shows a Retro-Flex retrofit for a GSP system operating in ethane rejection mode. The cold flashed stream from the GSP subcooler, which normally feeds the top of the column (shown dashed), is instead routed to the cold refluxing module (CRM) for processing. The residue gas stream that normally flows from the column overhead directly to the subcooler (shown dashed) is instead routed to the CRM and then subsequently routed to the subcooler. The resulting condensed liquids are pumped to the top of the existing column as top reflux. For a standard GSP plant, the Retro-Flex installation requires only four tie-in points, shown with circles in Figure 3. By installing appropriate isolation valves, the installation downtime can be minimised, and the CRM module can be easily bypassed if desired, allowing the plant to revert to the original GSP section. Typically, no modifications to the column or other existing equipment are required. The CRM and pumps are mounted on a single module for easy instal- lation, and all equipment is accessible for monitoring and maintenance. In ethane rejection mode, a Retro-Flex retrofitted GSP plant provides a significant propane recovery improvement compared to the original plant performance. Retro-Flex can achieve greater than 99% propane recovery even when rejecting almost all the ethane, whereas the GSP process is typically limited to 85-94% propane recovery while reject- ing ethane. In ethane recovery mode, Retro-Flex achieves nearly 100% propane recovery, while GSP is typically lim- ited to less than 99% propane recovery. It is important to note that the maximum Retro-Flex ethane recovery is gen- erally limited to levels achievable by GSP, i.e., 90-94%. If ultra-high ethane recovery (greater than 98%) is desired, the Retro-Flex Plus technology⁷ should be consid - ered for the retrofit. In this process, a small reflux compres - sor is added to the standard Retro-Flex retrofit package to compress a portion of the column overhead vapour before
CRM
Subcooler
Residue gas
Residue gas compressor
Expander
Inlet gas
Demethaniser
Bottom product
Figure 3 Retro-Flex retrofit
condensing it in the CRM. The additional cooling and rec - tification of this stream allows Retro-Flex Plus to capture more of the unrecovered ethane. In ethane rejection oper- ation, the reflux compressor is shut down, and the retrofit design reverts to the Retro-Flex technology. A Retro-Flex retrofit offers a convenient upgrade option for GSP plant operators who want to improve their propane recovery levels, either in ethane recovery or rejection modes of operation. Moreover, with only four tie-in points, it requires minimal downtime and has small plot space requirements. GSP2 retrofit The standard RSV and Retro-Flex retrofits rely on existing equipment for all the feed gas processing. When through- put greater than nameplate capacity is desired, special con- sideration must be given to processing the additional feed gas without exceeding the existing equipment limitations. One such technology is the proprietary UOP Ortloff GSP2 process⁸ shown in Figure 4 . With GSP2, the feed gas is split into two streams, usually with the existing plant
New heat exchanger
Subcooler
Absorber
Residue gas
Pump
Residue gas compressor
Expander
Inlet gas
Demethaniser
Bottom product
Figure 4 GSP2 retrofit
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