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

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Article Summary

The 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 capacity, 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 propane 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 effectively 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 maintaining 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 ensuring 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 turboexpander was a significant improvement over the previous standard, the ‘refrigerated J-T plant’, providing higher product 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 subcooled process, or GSP (see Figure 1).

Compared to the simple expander plant, the key feature 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 residue 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 hydrocarbon 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 destination for an ethane product or where market conditions made ethane more valuable for its heating value in the residue gas.

The GSP technology was a great improvement and remained the best available NGL recovery technology for many years, but it was not without limitations. In particular, the source of the top reflux stream was still essentially feed gas, containing significant fractions of ethane and propane. In ethane recovery mode, ethane recovery is limited to approximately 90-94%, with a propane recovery level of around 99%. Recoveries beyond these levels are technically achievable, but the required additional compression is not economically viable. In addition, market conditions in recent years have, at times, made ethane more valuable in the residue gas product for the reasons previously mentioned, forcing operators to switch to ethane rejection mode. Unfortunately, the ‘standard’ GSP process typically loses 5% to 15% of the propane to the residue gas stream when configured for full ethane rejection, i.e., less than 2% ethane recovery.

Process retrofit feasibility
The limited performance of the ‘standard’ GSP technology provides many opportunities for retrofitting an older facility with an enhanced, more efficient technology, especially given the large number of GSP installations worldwide. The goals for any retrofit will depend on the specifics of each project. As previously mentioned, potential benefits include higher product recoveries, improved operational flexibility, and increased plant throughput. These enhanced retrofit technologies can achieve greater than 99% ethane recovery and 100% propane recovery in ethane recovery mode or maintain greater than 99% propane recovery while rejecting ethane, with equal or only slightly more compression power.

Today’s owners/operators are also frequently faced with the challenge of processing additional gas supply while minimising capital expenditures. A retrofit can provide a relatively inexpensive alternative to building a new plant to achieve higher production targets.

Once the goals of a retrofit are identified, a study is recommended to select the best process to achieve the stated goals. This study should include a simulation of the original process design and a simulation of the current plant that may include updates to equipment or possible deficiencies that may or may not be corrected with the retrofit.

When an accurate representation of the equipment performance has been completed, simulations of various retrofit alternatives can be developed. These simulations should consider all limitations of the current equipment, such as exchanger performance, rotating equipment capabilities (using available power and performance curves), and rating of separators and columns at the proposed retrofit conditions.

Finally, additional external restrictions must be considered. These may include the available plot space and location for new equipment, time allotted for a plant shutdown to make tie-ins and install equipment, and available funding for the project. A detailed analysis will identify the best technology for the plant retrofit.

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