American Oil and Gas Reporter - March 2017 - 56

SpecialReport: Gas Processing & Treating
Western Gas, a publicly traded midstream limited partnership controlled by
Anadarko Petroleum Corporation, began
the process of designing and constructing
a new facility in northern Colorado to
process Anadarko's and available thirdparty gas in 2011, when ethane recovery
was economically favorable, although
take-away capacity was limited. Weld
County was an ideal location for a new
plant, considering Anadarko holds significant producing acreage in the area,
and there are nearby pipelines and good
access for transporting equipment. The
original design called for one 200 MMcf/d
train, but quickly grew to two 300 MMcf/d
gas processing trains in response to rapid
ongoing field development.
Residue gas take-away capacity was
available on Kinder Morgan's Colorado
Interstate Gas and High Plains pipelines,
flowing either north to Wyoming or south
to Denver. However, local natural gas
liquids take-away capacity was limited.
DCP Midstream was in a similar position,
and formed a partnership with Anadarko
and Enterprise Products Partners to build
the Front Range NGL pipeline with a
connection to the Lancaster plant. Front
Range would allow Y-grade NGL products
to flow to Skellytown, Tx., connecting
to the Texas Express Pipeline that delivers
to the Mont Belvieu market hub.
Since the Front Range Pipeline could
accept all types of NGL products, operational gas processing flexibility for the
new facility would be valuable. Subsequent
to the NGL transportation projects' completion, Anadarko dropped its interests
in the Front Range and Texas Express
Pipeline to Western Gas Partners. Western

TABLE 1
Product Speciﬁcations
Residue Gas

Max higher heat value (Btu/cf)
Max inerts (mol%)

NGL Product

C1 max (LV%)
C1/C2 max (LV%)
CO2/C2 max (LV%)
H2S max

Gas's goal was to build a state-of-the-art
facility that could achieve high product
recoveries and have the flexibility to respond to changes in delivery options and
the NGL market. The product specifications are summarized in Table 1. The requirements follow a typical Y-grade specification, restricting maximum content
of methane, methane-in-ethane, carbon
dioxide-in-ethane, and hydrogen sulfide.
When rejecting ethane, the only NGL
content concern is the sulfur compounds.
However, the residue gas specifications
mandate a maximum heating value (which
can be a concern when rejecting ethane)
and a maximum CO2 content.
Facility Engineering Options
Historically, gas processing plants in
North America have not been optimized
to reject ethane while maintaining maximum propane recovery. Plants generally
have been sized with an emphasis on
ethane recovery, operating at lower throughput and relatively low propane recovery
levels in ethane-rejection mode. This is
an acceptable approach when the primary
goal is to recover ethane and rejection capability is a secondary consideration.
The gas subcooled process (GSP) design is the standard for North American

FIGURE 1A

Residue Gas
Compressor

Inlet
Gas

NGL recovery plants. Figures 1A and
1B show the GSP design for ethane rejection and recovery, respectively. For
the Lancaster plant's rich gas design
basis, the maximum propane recovery
for an open-art GSP would be approximately 90 percent when rejecting almost
all the ethane, even when using all the
horsepower required for the ethane recovery mode operation. This performance
level was not sufficient for the Lancaster
project.
GSP is effective at providing moderately high ethane recovery, but has significant limitations for ethane rejection
operation. In particular, it is difficult to
minimize the propane lost to the residue
gas stream. One option for improving
the top reflux composition is Ortloff's
Recycle Split-Vapor (RSV) technology.
In this design, a small stream of compressed residue gas is cooled and flashed
to the top of the column, theoretically allowing close to 100 percent propane recovery while rejecting ethane. Although
it requires a taller column, additional exchanger passes and additional residue
gas compression, RSV can offer flexibility
and high product recoveries. It was evaluated as an option for the Lancaster project, but rejected because of the high com-

GSP Process (Ethane Recovery Mode)
Residue
Gas

Expander/
Compressor
C3

0.5
1.5
0.35
copper strip

FIGURE 1B

GSP Process (Ethane Rejection Mode)
Residue
Gas

1,235
3.00

Demethanizer/
Deethanizer

LPG Product

56 THE AMERICAN OIL & GAS REPORTER

Residue Gas
Compressor

Inlet
Gas

Expander/
Compressor
C3

Demethanizer/
Deethanizer

NGL Product

American Oil and Gas Reporter - March 2017

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