Enterprise Products Operating LP and partner,Marathon Oil Co., conducted a study for the design andconstruction of a second 300 MMcfd NGL recovery trainat the Neptune plant in St. Mary Parish, La. The compa-nies selected IPSI LLC’s Enhanced NGL RecoveryProcessSM after comparing available NGL recovery tech-nologies for the new train.

IPSI’s technology replaces conventional propanerefrigeration with a self-refrigeration and stripping gaspackage that allows up to 20% more processing capacitywith the same residue gas compression horsepower as inthe existing train. A novel demethanizer reflux design wasalso incorporated into the unit.

The two enhancements should allow ethane recoveriesof 90+% and maintain the flexibility to maximize propanerecovery during ethane-rejection mode. The new train isscheduled to start up by the end of 2003.

Neptune Gas PlantEnterprise and Marathon started up a cryogenic NGL

recovery plant at St. Mary Parish in February 2000 (Fig. 1).This plant, Neptune I, processes Gulf of Mexico naturalgas produced by Shell Oil Co., Marathon, and others.

Neptune I uses a standard gas subcooled process (GSP)to treat 300 MMcfd of 2.8-gpm (gal/Mcf) gas. A SolarTurbines Inc. C-406 centrifugal compressor, driven by aMars 100 gas turbine, recompresses the residue gas.

When the plant started up in February 2000, the inletgas was significantly richer than the design anticipated(4+ gpm vs. 2.8 gpm). To improve ethane and propane

recoveries, mechanical refrigeration was later added tomake Neptune I a refrigerated GSP plant.

At times, the plant has processed inlet gas volumes ofup to 330 MMcfd with an ethane recovery of 70-80%.

To process newer discoveries from the SouthernCanyon of the Gulf of Mexico, another nominal 300MMcfd train, Neptune II, is currently under construction(Figure 2). Technip-Coflexip, which also built the firstunit, is the contractor for Neptune II.

Design RequirementsInlet gas to the Neptune plants is projected to average

more than 4 gpm for many years after startup. To selectthe optimum process for Neptune II, we evaluated refrig-erated GSP, which Neptune I uses, and other proprietaryNGL recovery processes.

Technology suppliers were asked to provide informa-tion on component recoveries, any refrigeration or othercompression requirements, and an estimated incrementalcost of the process compared to a base GSP plant.

For each technology provider, we set the same com-mon design parameters:

For ease of maintenance and savings on common spareparts, use the same recompressor and gas turbine asused in Neptune I.

Ethane recoveries of 90+%, and an indication of theadditional refrigeration and compression horsepowerrequired.

Ability to reject ethane with approximately 90% C3

recovery.

Process OptionsSince the early 1980s, the Ortloff Engineers’ GSP (OGJ,

May 3, 1982, pp. 281-90) was the preferred process forhigh ethane-recovery NGL plants.1

The main feature of this process is that some of the feedgas is condensed, subcooled, and flashed to the demetha-nizer as a top feed. This top feed acts as a reflux, whichhelps the GSP tolerate more feed CO2 and providesgreater ethane and propane recoveries for the same horse-power as the older designs.

Another advantage of the GSP design is that the plantcan also operate in ethane-rejection mode while still main-taining significant propane recovery. All of the current“state-of-the-art” NGL recovery technologies are basicallyenhancements of the GSP concept.

Authors: Pervaiz Nasir, Enterprise Products Operating, LP; WelbySweet, Marathon Oil Company; and Douglas Elliot, Roger Chen, R.J. Lee, IPSI LLC

Enhanced NGL Recovery ProcessSM Selected forNeptune Gas Plant Expansion

Published in the Oil & Gas Journal, July 21, 2003,and presented at the 82nd Annual GasProcessors Association Convention, March 9-12, 2003

This photo, taken on Mar. 18, 2003, shows the Neptune II plant under construction inthe foreground. Photo courtesy of Enterprise Products Operating LP.

Figure 1

Figure 3 is a schematic diagram of Neptune I, whichuses a refrigerated GSP design.

We also evaluated one other well-known and efficientprocess from a leading technology supplier for NeptuneII. The process requires a propane refrigeration systemand can provide greater than 95% ethane and almost 100%propane recoveries.

In addition, during the ethane rejection mode, propanerecoveries are significantly higher than what can beachieved with other processes.

The IPSI stripping gas process generates internalrefrigeration by expanding a liquid stream from thedemethanizer. Flashed vapor is compressed and returnsto the demethanizer as stripping-enriching gas while theflashed liquid combines with the demethanizer product.

This design’s advantage versus conventional propanerefrigeration is that no separate propane chiller, surgetank, or economizers are required. This makes the capital

investment for the IPSI refrigeration system lower than apropane refrigeration system.

A second subcooled reflux stream to the demethanizerfurther enhances the IPSI process. This second refluxstream uses the liquid from the cold separator that is sub-cooled by heat exchange with the demethanizer overheadstream. This second reflux forms the second feed to thecolumn’s top.

Process SelectionDuplicating the Neptune I plant was the cheapest cap-

ital investment option. By copying an existing plant, wewould have saved at least half of the detailed engineeringman-hours required for the Neptune II design.

Another advantage was that the plant operators werealready fully familiar with the process and would haverequired minimum training to become familiar withNeptune II’s startup and operation. The disadvantage wasthe relatively higher compression costs for the same orlower recoveries.

The other competing processes can attain high ethaneand propane recoveries that are not possible with a refrig-

erated GSP design; however, these processes require morerecompression horsepower, which results in higher capi-tal cost and plant fuel consumption.

We could not economically justify the incrementalinstallation and operating costs for the additional horse-power due to our projected ethane and propane margins.For the Neptune II design, the enhanced IPSI process pro-

vided the optimum solution.The advantages of thisprocess design are that it:

Uses the least totalhorsepower to achieve thesame recoveries as compet-ing processes, and thereforehas the lowest compressioncosts.

Has the lowest up-frontcapital cost.

Produces the highest netpresent value to plant own-ers and gas producers.

Table 1 summarizes theprocess comparisons.

2

The Neptune II unit, currently under construction, will double the plant’s capacity.Photo courtesy of Enterprise Products Operating LP.

INLET GAS

FUEL GAS

NEPTUNE I FLOW SCHEME

COLDSEPARATOR

DEMETHANIZER

NGL TO SALES

RESIDUE GAS

C3 CHILLER

NGL SURGETANK

NGL PUMP

Figure 3

Table 1

Figure 2

In the IPSI process (Figure 4), compressed strippinggas vapor provides all thedemethanizer bottom reboilerduty for controlling C1:C2 orCO2:C2 ratios in the recoveredNGLs.

We decided to install an elec-tric power cogeneration plant atNeptune. The power generatoruses waste heat from the two gasturbines’ exhausts to generatemost of the power required bythe processing trains.

Since the IPSI Process doesnot require an external heatsource for the demethanizerreboiler, the turbine waste heat isused to generate an additional15% power versus the otherprocess designs. Because thedecision for cogeneration wasnot made at the time of process selection, this incrementalpower benefit was not included in the comparison. Onlythe IPSI Process, however, could maximize power recovery.

IPSI Enhanced ProcessSM

IPSI developed the enhanced process2 to achieve highrecovery levels of ethane and heavier hydrocarbons,reduce residue gas horsepower, and avoid the need for anexternal refrigeration system.

The process uses a slipstream from the demethanizer’sbottom as a mixed refrigerant. The mixed refrigerant istotally or partially vaporized, providing refrigeration forinlet gas cooling. Other processes accomplish this using amore costly external refrigeration system that includescompressors, condensers, refrigerant accumulators, econ-omizers, and refrigerant storage.

Vapor generated from the self-refrigeration cycle isrecompressed and recycled back to the tower’s bottom,where it serves as stripping gas. This eliminates the needfor inlet gas cooling via external refrigeration, and alsohelps the demethanizer operation by:

Lowering the temperature profile, which permits bet-ter energy integration for inlet gas cooling via sidereboilers, resulting in reduced heating and refrigera-tion requirements.

Reducing or eliminating the need for external reboilerheat, which saves fuel.

Enhancing the relative volatility of key components attypical pressures, which improves separation efficien-cy and NGL recovery. It alternatively allows increasedtower pressure for typical recoveries, reducing residuegas compression requirements.

The self-refrigeration design can augment to almostany leading NGL-recovery technology. It enhances theoperational efficiency and reduces capital and operatingcosts of those processes.

In the Neptune II plant, IPSI’s stripping-gas refrigera-tion combines with the standard GSP design. The processis further improved via a second reflux stream to thedemethanizer. This second reflux stream is generated bysubcooling and flashing the cold separator liquids above asecond packed section in the demethanizer.

The advantages of the second reflux are:Increased ethane recoveries with no additional horse-power.

More total reflux to the top section of the demethaniz-er. For a fixed amount of total reflux, the process canreduce the GSP split and send more flow to theexpander to generate more horsepower. This allows alower demethanizer pressure, which further improvesrecoveries.

Figure 4 shows the enhanced IPSI design.

Gas Chilling, TurboexpanderDry feed gas from the dehydration unit splits into two

streams. Exchange with the cold residue gas (overheadproduct from the demethanizer) chills one of the streamsin the warm gas-gas exchanger. The gas-liquid exchangerschill the other inlet gas stream.

The chilled inlet gas streams from the two exchangerscombine and flow to the warm separator. Liquid from theseparator subcools in the cold gas-gas exchanger, andflashes above the demethanizer’s second packed section.

Vapor from the separator splits into two streams. Mostof the vapor goes to the turboexpander. Expansion of this

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INLET GAS

FUEL GAS

Figure 4IPSI FLOW SCHEME FOR NEPTUNE II

COLDSEPARATOR

DEMETHANIZER

NGL TO SALES

RESIDUE GAS

NGL SURGETANK

NGL PUMP

NGL FROMNEPTUNE I

vapor stream produces refrigerationand cools the gas and also generatesmechanical work to drive a boostercompressor on the same shaft.

The expanded stream feeds to apacked section in the demethanizer forfurther fractionation. Remaining vaporcondenses in the cold gas-gas exchangerusing refrigeration from the cold residuegas. The condensed stream feeds to thedemethanizer’s top packed section asthe top reflux.

Stripping Gas RefrigerationStripping gas is the main feature of

the IPSI design. A side liquid draw fromthe bottom section of the demethanizerexpands (decreasing its temperature) toprovide refrigeration for the inlet gas.The partially vaporized stream from the lower-side reboil-er then feeds the stripping gas separator.

The stripping-gas compressor boosts the pressure ofthe vapor from the stripping gas separator. This hot com-pressed stream provides the bottom reboiler duty for thedemethanizer.

The stripping gas cooler ensures that the demethaniz-er receives adequate heat input for meeting the C1:C2 orCO2:C2 ratio in the bottom NGL stream. Liquid from thestripping gas separator, which also meets the NGL prod-uct specifications, pumps back to the demethanizer’s bot-tom.

Demethanizer, Residue Gas CompressionThe demethanizer strips methane out of the ethane and

heavier hydrocarbons recovered as NGL. Residue gasfrom the demethanizer’s top flows through the cold gas-gas exchanger and the warm gas-gas exchanger.

Warm residue gas from the warm gas-gas exchangerfeeds the compressor side of the expander-compressor(booster compressor).

Compressed residue gas from the booster compressordischarge further compresses to final delivery pressurevia the residue gas compressor. A small portion of theresidue gas is withdrawn from the out-let of the booster compressor for use asplant fuel gas.

Process Performance,Comparison

When we compared the threeprocesses, we found that the IPSI strip-ping gas refrigeration design offersthese advantages:

Greater than 90% ethane recoverywith lower total compression horse-power and capital cost. The typicalGSP design with no refrigeration

would achieve about 64% ethane recovery with theNeptune II design composition. External propanerefrigeration provides a higher ethane recovery withthe GSP design. Propane refrigeration horsepowerrequired for a 90+% ethane recovery is 40% higherthan that for stripping gas refrigeration.

Fully integrated process refrigeration with improvedreliability and separation efficiency. We compared theIPSI stripping gas design and conventional propanerefrigeration design based on the feed gas compositionand conditions in the Neptune II design. For bothdesigns, ethane recovery level and, therefore, demeth-anizer pressure were the same.

A tray-to-tray differential temperature profile (Figure 5) shows that the IPSI design has a lower temper-ature in the column’s middle (Trays 11-15) than externalpropane refrigeration. This is usually where the sidereboiler is located

A lower column temperature means better heat inte-gration. Because the ethane recoveries are the same, thetop and bottom temperatures are also almost identical inboth designs. Figure 6 compares the relative volatility ofmethane-ethane for both designs.

4

-3 0

-2 0

-1 0

0

10

20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Stage

Tem

pera

ture

Diff

eren

ce, °

F

Figure 5DEMETHANIZER TEMPERATURES

-10

-50

510

15

10 15 20Stage

Figure 6RELATIVE VOLATILITY

Diff

eren

ce in

re

lativ

e vo

latili

ty, %

50

The IPSI design shows better relative volatility fortrays with lower temperatures in Figure 5. Better relativevolatility means an easier methane-ethane separation.

Although the plant design is based on a 300-MMcfdfeed gas rate at design composition, it can operate at350 MMcfd at a lower recovery level while producinggas and liquid products within specification.

The stripping gas system can also work in the ethane-rejection mode. The temperature profile in the col-umn’s bottom section is normally too high for integra-tion with feed-gas cooling. The stripping gas design,however, can still provide refrigeration and achieve ahigh propane recovery even when in ethane-rejectionmode.

The stripping-gas self-refrigeration system providesoverall energy integration and completely replaces thepropane refrigeration system. Refrigerant supply andmakeup is no longer a concern.

The stripping gas refrigeration system can handle vari-ations in feed gas composition. Simulations with fixeddesign equipment sizes show that the variation ofethane recovery is within 2% for feed gas compositionsof 3.1 to 4.5 gpm.

The system does not require an external heat source forthe demethanizer reboiler.

The stripping-gas package can be used to revamp mostexisting plants to provide additional refrigeration andhigher recoveries or additional capacity.

References1. Elliot, D. G., Chen, R.J.J., Brown, T.S., Sloan, E.D.,

and Kidnay, A.J., “The Economic Impact of FluidProperties on Expander Plants,” Proceedings of the 70thAnnual GPA Convention, Gas Processors Association,Tulsa, 1991, pp. 59-64.

2. Yao, J., Chen, J.J., and Elliot, D.G., “EnhancedNGL Recovery Processes, US Patent No. 5992175, Nov. 30,1999.

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