Advancements in retrofits for existing NGL recovery plants
Better plant performance is achieved using proven retrofit techniques to upgrade a gas subcooled process or older plant with more recent retrofit technology
Michael Pierce, Scott Miller, John Wilkinson and S Allen Erickson Honeywell UOP Gerry Wooten Mustang Gas Products, LLC Raj Patel Brazos Midstream
T he processes gas plant owners/operators use to recover hydrocarbon liquids from natural gas streams have undergone continual improvements over many decades. Gas plants today are required to maximise profits by optimising both the recovery of natural gas liquids (NGL) based on market prices and plant throughput to satisfy their delivery contracts. Given recent market volatility, the ability to vary plant operation has become increasingly important. NGL recovery plants designed with built-in flexibility can maximise plant throughput, even above nameplate capac- ity, while maintaining maximum product recovery. This gives them an economic advantage over standard designs. Unfortunately, many standard cryogenic NGL recovery plants were constructed using technologies with limited product recovery capabilities and flexibility. These plants are unable to effectively achieve maximum ethane and pro- pane recovery or adjust operations without losing valuable product, in particular propane, into the residue gas stream. To stay competitive, owners/operators of older plants often need to retrofit their plants. Several new process technologies are available to effec- tively upgrade standard plants. These recent advances can provide higher ethane and propane recoveries, and many offer fully flexible ethane recovery levels, ranging from ultra-high recovery to almost full rejection while maintain- ing high propane recovery. Other technologies include the ability to process higher feed gas flow rates with minimal modifications to existing equipment. Identifying the key performance goals of a proposed retrofit is critical to ensur - ing a successful revamp. The ‘standard’ plant For many years, the ‘standard’ NGL recovery plant included a turboexpander, with the expanded feed gas providing top reflux to a fractionation column. This became known as a ‘simple expander plant’. The integration of the turbo- expander was a significant improvement over the previous standard, the ‘refrigerated J-T plant’, providing higher prod- uct recovery with less compression power. In the 1970s, The Ortloff Corporation improved on the simple expander plant by generating an additional reflux stream to feed the top of the fractionation column. 1 ,2 The
source of the reflux stream was either a portion of the separator vapour or the separator liquids or a combination of the two. The higher product recovery levels and lower power requirements quickly made this the next ‘standard’ process technology, commonly known as the gas sub- cooled process, or GSP (see Figure 1 ). Compared to the simple expander plant, the key fea - ture of GSP was the addition of a reflux stream created by condensing and subcooling a portion of the feed gas. This reflux stream captured more of the valuable ethane and heavier hydrocarbons that were otherwise lost to the res- idue gas stream from the top of the fractionation column. Although the GSP technology required an additional heat exchanger and an additional fractionation section at the top of the column, the product recoveries were significantly improved without requiring more compression power. This allowed operators to recover ethane and heavier hydrocar- bon components as the NGL product while also producing a mostly methane residue gas stream. The process could also be configured to reject the ethane into the residue gas stream while recovering only the propane and heavier components as a liquid product. The latter ‘ethane rejection mode’ of operation was useful in locations without a destina- tion for an ethane product or where market conditions made ethane more valuable for its heating value in the residue gas.
Subcooler
Residue gas
Residue gas compressor
Expander
Inlet gas
Demethaniser
Bottom product
Figure 1 Gas subcooled process (GSP)
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Revamps 2023
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