Note: Descriptions are shown in the official language in which they were submitted.
CA 02600027 2007-09-04
1
Description
Process for Liquefying A Hydrocarbon-Rich Stream
The invention relates to a process for liquefying a hydrocarbon-rich stream,
specifically a natural gas stream.
Natural gas liquefaction plants are laid out either as what are known as LNG
baseload plants - plants for liquefying natural gas to provide natural gas as
primary energy - or as what are known as peak shaving plants - plants for
liquefying natural gas to meet peak demands.
Larger LNG plants are usually operated with refrigeration circuits which
consist of hydrocarbon mixtures. These mixture circuits are more energy-
efficient than expander circuits and allow relatively low specific energy
consumption.
From DE-A 102 09 799 a process for liquefying a hydrocarbon-rich stream,
specifically a natural gas stream, is known in accordance with which the
liquefaction of the hydrocarbon-rich stream takes place in the heat exchange
countercurrent to a two-component refrigerant mixture stream; the one
component is a part of the hydrocarbon-rich stream to be liquefied, while the
other component is a heavy hydrocarbon, preferably propane or propylene.
Before the cooling and the expansion to provide refrigeration of these
components, the refrigerant mixture is separated into a higher boiling and a
lower boiling refrigerant fraction.
A disadvantage of the procedure described in DE-A 102 09 799 is that
providing two refrigerant components can result in relatively large
temperature differences in the heat exchangers. These temperature
differences in turn require correspondingly high compressor performance.
CA 02600027 2007-09-04
2
A similar process for liquefying a hydrocarbon-rich stream is known from US
6, 347, 531. In this process, the low-pressure refrigerant is inducted cold
through the circulating compressor. These cold-inducting compressors have
the disadvantage that in operation, in particular during start-up and shut-
down, they are more complicated to operate than compressors not inducting
cold. Furthermore, in the liquefaction process described in US 6,347,531 it is
disadvantageous that the refrigerant is partially liquefied at an intermediate
pressure, which results in greater expense for equipment.
The object of the present invention is to specify a generic process for
liquefying a hydrocarbon-rich stream, specifically of a natural gas stream,
which avoids the disadvantages of the known processes and in addition
allows a lower specific energy requirement to be realized.
To achieve this object, a generic process for liquefying a hydrocarbon-rich
stream is proposed, wherein
- the liquefaction of the hydrocarbon-rich stream takes place in the heat
exchange countercurrent to a three- or multi-component refrigerant
mixture,
- one of the components is a part of the hydrocarbon-rich stream to be
liquefied,
- one of the components is propane, propylene or a C4 hydrocarbon,
- one of the components is C2H4 or C2H6,
- the compression of the refrigerant mixture stream takes place by
means of an at least two-stage compression,
- before the cooling and the expansion of the refrigerant mixture to
provide refrigeration, the refrigerant mixture is separated into a
higher boiling and a lower boiling refrigerant fraction and
CA 02600027 2007-09-04
3
- the higher boiling and the lower boiling refrigerant fractions, after
their expansion to provide refrigeration are taken at different
pressures to compression.
Surprisingly, it has been shown that the specific expenditure of energy for
liquefaction by means of the process in accordance with the invention can be
reduced by approximately 30%. Furthermore, the temperature differences
within the heat exchanger or heat exchangers can be reduced significantly.
The result is that transient operation is easier to control.
Additional advantageous embodiments of the process in accordance with the
invention for liquefying a hydrocarbon-rich stream are:
- the refrigerant mixture is a three-component refrigerant mixture
- the refrigerant fractions are cooled separately, expanded separately to
provide refrigeration and heated separately countercurrent to the
hydrocarbon-rich stream to be liquefied
- a further component of the refrigerant mixture is nitrogen
- compression of the refrigerant mixture stream takes place by means of
an at least two-stage compression and the higher boiling refrigerant
fraction is admixed to the lower boiling refrigerant fraction at an
intermediate pressure level
- at least one C4 to C6 hydrocarbon is used as further component(s) of the
refrigerant mixture; the use of additional refrigerant components
makes sense in particular at greater liquefaction outputs above 10 t/h.
- at least one partial stream of the lower boiling refrigerant fractions is
partially condensed and the liquid fraction obtained thereby is
supercooled and expanded.
CA 02600027 2007-09-04
4
The process in accordance with the invention and additional embodiments of
said invention which represent subjects of the dependent claims, are to be
explained in what follows using the embodiment shown in the drawing.
In accordance with the procedure shown in the drawing, a dry, pre-treated
hydrocarbon-rich stream, for example natural gas, is taken to the liquefaction
process in accordance with the invention through line X and liquefied in heat
exchanger E and supercooled if required. The hydrocarbon-rich stream is, as
an example, at a pressure of between 10 and 60 bar. The liquefied and, if
necessary supercooled, hydrocarbon-rich stream is then taken through line X'
for further use. Not shown in the drawing is a separation, which may have to
be provided, of undesirable components, for example higher hydrocarbons.
For this, reference is made to the appropriate explanations in the
aforementioned DE-A 102 09 799.
The cooling and liquefaction of the hydrocarbon-rich stream X, X' takes place
in accordance with the invention in the heat exchange countercurrent to a
three or more component refrigerant mixture stream where one of the
components is part of the hydrocarbon-rich stream to be liquefied -
preferably methane - one of the components is propane, propylene or a C4
hydrocarbon and one of the components is C2H4 or C2H6.
The corresponding refrigeration circuit preferably has a two-stage compressor
unit, consisting of the compressor stages Cl and C2. An air or water cooler -
not shown in the drawing - is located in series with each compressor stage.
The refrigeration circuit further has a high-pressure extractor D. Providing
only one high-pressure separator D reduces the operating cost of the process
in accordance with the invention substantially - compared with the known
refrigerant mixture circuits.
CA 02600027 2007-09-04
In the separator D, the refrigerant mixture is separated into a lower boiling
and a higher boiling fraction. The lower boiling fraction is removed from the
separator D through line 2, cooled in the heat exchanger E, condensed and
supercooled and then expanded at the cold end of the heat exchanger E in
expansion valve b, providing refrigeration. The expanded fraction is again
taken to the heat exchanger E through line 3, evaporated and superheated
therein countercurrent to process streams to be cooled and then taken to the
first compressor stage Cl through line 4.
Following compression and cooling not shown in the drawing, the compressed
lower boiling fraction is taken to the second compressor stage C2 through line
8 - the admixture of the higher boiling fraction will be discussed in more
detail in what follows - and compressed to the desired circulation pressure
which is, for example, between 20 and 60 bar. A heat exchanger as cooler not
shown in the drawing is also located in series with the second compressor
stage C2. The refrigerant mixture cooled and partially condensed in said
cooler is taken back to the separator D through line 1.
A higher boiling liquid fraction is drawn off from the bottom of the separator
D through line 5, cooled in the heat exchanger E and then expanded in
expansion valve a to the desired intermediate pressure, providing
refrigeration. Then this fraction is taken back to the heat exchanger E
through line 6, evaporated and superheated therein countercurrent to process
streams to be cooled and then taken through line 7 to the compressor unit
ahead of its second compressor stage C2.
In accordance with an advantageous embodiment of the liquefaction process
in accordance with the invention, at least one partial stream 9 of the lower
boiling refrigerant fraction 2 can be drawn off from the heat exchanger
following cooling and partial condensation through the broken line 9, and
taken to ("cold") separator D' indicated by broken lines. The gaseous fraction
CA 02600027 2007-09-04
6
drawn off at the head of the separator D' through line 10 indicated by broken
lines, is again returned to the heat exchanger E, supercooled and expanded
for the purpose of providing the peak cold in valve b required for the
liquefaction process.
The liquid fraction drawn off from the bottom of the separator D' through the
broken line 11 is supercooled in the heat exchanger E, expanded in valve c
providing refrigeration, taken to the heat exchanger E through line 12 and
admixed to the refrigerant fraction in line 3.
Additional "cold separators" can be provided in addition to this separator D'.
They result in an improvement of the specific energy requirement of the
liquefaction process in accordance with the invention, but they make sense
only in larger liquefaction plants because of the additional expense required
for equipment.
The higher boiling fractions recovered in the separator D' and any additional
"cold separators" are preferably supercooled, expanded to the pressure of the
(first) higher boiling fraction and taken to the compressor stage to which the
(first) higher boiling fraction is also taken. This embodiment of the process
in
accordance with the invention is indicated in the drawing by the dotted line
13. Depending on the temperature profile in the heat exchanger E, admixture
to the low-pressure refrigerant stream in line sections 3 and 4 also makes
sense.
In accordance with an advantageous embodiment of the inventive process,
the liquefaction of the hydrocarbon-rich stream takes place countercurrent to
the refrigerant mixture in plate heat exchangers. Because of the process
management in accordance with the invention, process management can be
realized in a single plate heat exchanger in liquefaction plants having a
liquefaction capacity of up to 10 to 15 t/h.
CA 02600027 2007-09-04
7
The process in accordance with the invention to liquefy a hydrocarbon-rich
stream, specifically a natural gas stream, avoids all the disadvantages of the
prior art cited at the beginning.
