Revamps 2023 Issue

replacement with internals better suited for the retrofit operating conditions • Turboexpander: Retrofit operating conditions can be very different from the design point for the expander and should be reviewed by the turboexpander vendor. The expected operating efficiencies must be confirmed and incorporated into the retrofit simulation. In some cases, the expander and/or compressor wheel may need to be replaced for more efficient operation under the new conditions • Heat exchangers: The existing heat exchangers must be re-rated for the new conditions. This includes the heat transfer capability and any change in pressure drops at the new conditions • Bottom reboiler: The bottom reboiler ultimately provides the heat required to meet the liquid product specification. Higher recovery levels will require more heat input, man - dating a review of both the exchanger performance and the heating medium system (steam, hot oil). In cases where the original plant was not designed for ethane rejection but it is now desired, a new bottom reboiler with an external heat source will be required • Residue gas compression: With any increase in plant throughput, additional residue gas compression will likely be required. The same is true when upgrading to a technol - ogy that requires a residue recycle reflux stream. Potential options include a new parallel compressor to handle the higher flow rates, or simply a separate recycle gas com - pressor if plant throughput is not increased • Other units: In cases where increased throughput is desired, the effect on other units must be considered. This includes any inlet treating, such as amine units and dehy - dration, as well as downstream fractionation. These units may require debottlenecking to handle the extra flows. A careful review of existing equipment is as important as designing new equipment in any retrofit and is required to ensure a successful project. Case study 1 The Dover Hennesey Gas Plant, operated by Mustang Gas Products, LLC, was commissioned in 1978. It was designed to process 47 MMSCFD using a simple expander plant pro - cess, with the expander feeding the top of the demethaniser. Although plant performance was low compared to process technologies available today, the plant provided reliable operation for more than 40 years. Prior to the retrofit, the original design recovered 72.0% of the ethane and 98.4% of the propane. Retrofit goals were identified as follows: • Increase processing capacity to 80 MMSCFD • Improve product recoveries, with desired ethane recov - ery greater than 90% • Add the ability to reject ethane, with less than 10% ethane recovery level and maximum propane recovery. Given the desired ethane recovery level, a simple GSP retrofit would have sufficed. However, the requirement to maximise propane recovery while rejecting ethane demanded a different process. The chosen option was to upgrade this plant to GSP technology while also including

Figure 6 Absorber with CRM, left, and existing column, right

a Retro-Flex module to maintain the required propane recoveries. Since the original design only included the ability to recover ethane, the retrofit had to include equipment to accommodate ethane rejection operation. A new bottom reboiler heat exchanger was added to provide this capabil - ity, using hot oil as the heat medium. In addition, problems were identified in several of the existing equipment items, requiring some modifications and replacements: • The gas/gas exchanger was exhibiting very high-pressure

Dover Hennessey retrofit results

Pre-retrofit Retrofit performance Performance Predicted Actual

Ethane recovery mode Ethane recovery, % Propane recovery, % Ethane rejection mode Ethane recovery, % Propane recovery, % Plant capacity, MMSCFD Residue compression, hp

72.0 98.4

92.9

95.0

100.0

N/A N/A

<10.0 >99.0

<5.0

>99.0

5,400

5,400 1,200

C₃ refrigeration, hp

700

Table 1

Revamps 2023