Note: Descriptions are shown in the official language in which they were submitted.
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Hydrid Turbo Expander and Refrigerated Lean Oil Absorber System
Background Information - Current Industry Practice
There are essentially three processes that are utilized to recover natural gas
liquids ("NGLs") from natural gas, depending primarily on the desired degree
of ethane recovery. These include:
1. Lean oil absorption for low ethane recovery percentages - The
objective in these applications is to minimize capital costs while
capturing most of the propane and heavier components in the natural
gas. Any ethane that's recovered tends to be incidental, generally in
the 10 to 15% range.
2. Refrigerated lean oil absorption for intermediate ethane recovery
percentages - The objective in these applications is to increase NGL
recovery percentages, including those for ethane, if ethane recovery is
desirable, while minimizing capital costs. Ethane recovery percentages
are typically in the 30°!° range. It utilizes a lean oil system
but chills the
incoming natural gas and the lean oil using mechanical refrigeration
(usually a propane system). This type of system might also be utilized
when the natural gas pressure is too low to warrant the installation of a
turbo expander as discussed in 3.
3. Turbo expander for high ethane recovery percentages ("deep cut") -
The turbo expander drops the pressure of the incoming natural gas and
captures the "cold" produced in the expansion process, thus eliminating
the need for mechanical refrigeration. Depending on the inlet pressure
and C02 content of the gas, ethane recovery percentages can bewery
high, exceeding 90%. When high efficiency operations are not required
a Joule-Thompson (JT) valve might be utilized instead of a turbo
expander; however, inlet gas pressures exceeding 1000 psi are
required to provide reasonable ethane recoveries.
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Unigue Combination of Current Technologies to Provide New Processing
S-_ sY tem
There are several unique aspects incorporated in this new processing system,
namely:
1. Removes C02 from all the natural gas fed to the system prior to NGL
recovery.
2. Splits the natural gas flow, utilizing a turbo expander on part of the gas
to provide high ethane recovery and refrigeration for a refrigerated lean
oil system for absorption of NGLs from the other part of the natural gas.
3. The flow to the turbo expander can be adjusted to control the amount
of refrigeration available to the process and the level of ethane
recovery. This flexibility is important not only in adjusting to variable
ethane compositions in the natural gas feed and demand requirements,
but also in minimizing operating costs with seasonal ambient
temperature variations.
4. The liquids from the cold separator downstream of the turbo expander,
which contain about 50% methane, can be pumped, resulting in a
significant operating cost saving versus compression, and utilized for
condensing ethane from the de-ethanizer overhead, and as chilled lean
oil for feed to the absorber. This eliminates the de-methanizer that
must be supplied downstream of the turbo expanders in current turbo
expander applications, resulting in significant capital cost savings.
5. Minimizes gas re-compression requirements with only the gas from the
cold separator downstream of the turbo expander, less internal fuel gas
requirements, requiring significant re-compression. The gas processed
through the lean oil absorber only requires about 50 psi of re-
compression,, resulting in both significant capital and operating cost
savings.
6. Allows for intermediate ethane recovery levels while eliminating
mechanical refrigeration and the accompanying relatively high
maintenance costs associated with its operation.
7. By integrating the lean oil stripper with the de-ethanizer and taking a
propane/butane (C3/C4) side-stream from this tower, the process
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eliminates the need and cost for a separate tower to recover the
natural gas condensate (C5+) which is utilized as lean oil and recycled
to the lean oil absorber.
8. The process utilizes C5+ as lean oil and does not have to purchase
and inventory an external supply of lean oil.
Description of The New Processing System
1. Inlet Natural Gas Treating
The inlet natural gas is treated for C02 removal and subsequent
dehydration. This step eliminates the potential for the formation of
hydrates or solid C02 during the subsequent processing steps.
2. Turbo Expander for Refrigeration Reguirements
After treating, part of the natural gas is routed to the Turbo Expander
where the pressure drop across the expander results in the liquefaction
of most of the NGLs. The re-compression side of the Turbo Expander
is utilized to recapture some of this pressure drop by re-compressing
the lean gas from Cold Separator 1, downstream of the Turbo
Expander, after this cold gas has been utilized in the Cold Box to cool
the inlet gas and the C5+ lean oil which are routed to the Lean Oil
Absorber. The flow to the Turbo Expander is controlled to provide
adequate refrigeration to provide the desired ethane recovery
percentage from the overall system. The NGLs from Cold Separator 1
are pumped to the De-ethanizer Overhead Condenser to provide
cooling to condense the ethane coming overhead from the De-
ethanizer. The NGLs then flow to the Lean Oil Absorber as
intermediate lean oil. The lean gas from the outlet of the re-compressor
flows to a compressor which boosts the pressure back to that of the
inlet natural gas.
3. Refrigerated Lean Oil Absorber
The remaining natural gas, not routed to the Turbo Expander, flows to
the Cold Box where it is cooled utilizing the cold gas from the Lean Oil
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Absorber and Cold Separator 1. It then flows to Cold Separator 2
where the condensed NGLs are separated from the gas and pumped
to the Lean Oil Absorber as another intermediate lean oil stream. The
gas flows to the bottom of the Lean Oil Absorber where it contacts
various lean oil streams (C5+ introduced at the top of the absorber,
NGLs from Cold Separator 2, introduced to the absorber at a lower
section in the tower and NGLs from Cold Separator 2 introduced below
that) all on a counter current basis. The C5+ lean oil has been
recovered at the bottom of the De-ethanizer and chilled in the Cold Box
prior to recycle to the absorber. The lean gas flowing overhead from
the Lean Oil Absorber is routed to the Cold Box to cool the inlet gas
and C5+ and is then re-compressed to the original inlet natural gas
pressure. This re-compression requirement is minimal (about 50 psi)
since the pressure has only been reduced by the pressure drop across
the Inlet Natural Gas Treating, Cold Box and Lean Oil Absorption
components of the process. The rich lean oil, containing the extracted
NGLs, from the bottoms of this tower flows to the De-methanizer.
4. De-methanizer
The De-methanizer, utilizing a bottoms reboiler, drives the methane
contained in the lean oil overhead, where it joins the gas from the outlet
of the Turbo Expander Re-compressor and is then compressed to the
original inlet natural gas pressure. Any gases that are not condensed in
the De-ethanizer Overhead Condenser are routed from the De-
ethanizer Reflux Accumulator to the De-methanizer for recovery of
methane. A slip stream of chilled C5+ is routed to the top of the De-
methanizer as lean oil to minimize the amount of NGLs that go
overhead with the methane.
5. De-ethanizer
The De-ethanizer, which includes a bottoms reboiler, serves several
functions including the production of specification ethane, a C3/C4 mix
as a side-stream and production of C5+ as a bottoms product and lean
oil for recycle to the Lean Oil Absorber and De-methanizer. The side-
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stream for production of a C3/C4 mix includes a reboiled Side-Stream
Stripper to drive ethane into the overhead product.
Drawin
The invention is described further in the attached Figure.
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