5 , combines the concepts of both the GSP2 and RSV ret- rofits previously described, requiring a new heat exchanger and an absorber column with pumps to ‘extend’ the exist- ing column. The absorber bottoms pumps are required to transfer liquids from the bottom of the absorber to the top of the existing demethaniser. As with GSP2, higher feed gas flow rates are possible because the feed gas is split into two streams. The first por- tion flows to the existing equipment at approximately the same design flow rate. The remainder of the feed gas flows to a new heat exchanger, where it is cooled, condensed, and subcooled with cold residue gas. It is then flashed to the existing column through the original top reflux piping. The original separator vapour reflux stream is separately cooled and condensed in the new heat exchanger before feeding the middle of the new absorber. As described in the RSV retrofit, a new reflux stream is provided by recycling a portion of the compressed residue gas stream. This lean gas stream is cooled, completely con- densed, and then subcooled in the new heat exchanger. It is then flashed to the absorber and fed as the top reflux stream. RSV2 is capable of increasing plant throughput by as much as 20% above nameplate capacity by bypassing additional flow around the existing equipment. At the same time, it achieves higher ethane and propane product recov- eries than the original GSP whether operating in ethane recovery or ethane rejection mode. Equipment considerations for retrofits Retrofitting any existing plant to improve performance and/ or throughput requires careful analysis. In addition to com- pleting simulations of the existing process and proposed configuration, the capabilities of the existing equipment must be confirmed. The following shortlist identifies some of the areas to review: • Metallurgy: To improve product recovery in a cryogenic NGL recovery plant, the selected retrofit process may require colder operating temperatures compared to the original design. All existing piping and equipment minimum design temperatures must be checked to ensure safe oper- ation under the new conditions • Design pressures: The selected retrofit process may allow the NGL recovery plant to achieve the desired prod- uct recoveries while operating the low-pressure section of the plant (including the demethaniser) at a higher pressure. The proposed retrofit must be designed with a safe margin from the pressure limitations of the existing equipment • Pumps: Higher product recoveries and certainly increased plant throughput will require greater pumping capacity in the product pumps. Existing pumps may be modified, or additional pumps may be installed to handle the higher liquid flows • Separators: With potentially higher vapour and liquid flow rates, the capability of all separators must be con- firmed at the new conditions • Column capacity: The ability of the existing column must be reviewed for the new conditions. Both the vapour and liquid flow rates and fractionation properties of each stage must be considered, and the column internals may require
New heat exchanger
Subcooler
Absorber
Residue gas
Pump
Residue gas compressor
Expander
Inlet gas
Demethaniser
Bottom product
Figure 5 RSV2 retrofit
continuing to process its design flow rate. The additional flow above nameplate capacity bypasses the existing inlet feed gas-cooling section and is cooled and condensed using cold residue gas in a new multi-pass heat exchanger. This new exchanger, which may either supplement or replace the existing subcooler, provides a similar functionality to the subcooler. The high-pressure, condensed feed gas 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 bypass stream is then fed to the original top reflux point in the existing demethaniser. A new absorber column is added to ‘extend’ the existing fractionation column. Absorber bottoms pumps transfer the liquids leaving the bottom of the absorber to the top of the existing demethaniser, and the existing column over- head vapour is routed to the bottom of the new absorber. The top reflux for the absorber is provided by the original separator vapour stream that is cooled and condensed in the new heat exchanger. This provides the leanest possible reflux for the GSP process, which is preferable to using the richer feed gas to reflux the top of the absorber. GSP2 can increase the plant throughput by as much as 20% above nameplate capacity while maintaining or improving the original GSP plant product recoveries. RSV2 retrofit When the retrofit objectives include both higher product recoveries and additional throughput, the proprietary RSV2 process⁹ can be considered. This option, shown in Figure
6
Revamps 2023
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