Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND APPARATUS FOR THE LIQUEFACTION OF CARBON DIOXIDE
The present invention relates to a process for the production of liquid carbon
dioxide
and apparatus for use in said process.
Carbon dioxide (CO2) is a gas produced as a by-product in large quantities in
certain
industrial operations, e.g. the manufacture of ammonia, or power generation by
coal
or gas power plants. Release of this by-product into the atmosphere is
undesirable
environmentally as it is a greenhouse gas. Much effort has thus been made
towards
the development of techniques for the disposal of CO2 in a way other than
simple
release to the atmosphere. One technique of particular interest is to pump the
CO2
into porous sub-surface strata (i.e. rock), e.g. down an injector well in an
oil field.
Subsurface disposal can be sirnply into porous strata or beneficial advantage
of the
subsurface disposal can be realised if the stratum into which it is disposed
is
hydrocarbon-bearing as the injected CO2 serves to drive hydrocarbon (e.g. oil
or
gas) in the stratum towards the producer wells (i.e. wells from which
hydrocarbon is
extracted). Injection of COa is thus one standard technique in late stage
reservoir
management for achieving enhanced recovery of hydrocarbons.
The quantities of carbon dioxide involved when disposal is by subsurface
injection
are immense, generally of the order of millions of tonnes. This poses problems
in
terms of transporting the CO2 from the site at which it is created to the site
at which
it is injected, especially where the injection site is offshore. Carbon
dioxide at
ambient temperatures and pressures is gaseous and, if transported batchwise,
such
voluminous containers are required that the process would be unfeasible. While
transport by pipeline might in some circumstances be feasible, the required
infrastructure is expensive. It is therefore desirable to transport the carbon
dioxide,
especially to offshore injection sites, batchwise in liquid form.
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Transport of liquid carbon dioxide is however not a problem- or expense-free
exercise. If the liquid COZ is not refrigerated, the pressures required to
maintain it in
the liquid state are high (60-80 bar A) making the required wall thicknesses
of the
pressurized containers high and making such containers for large scale
unrefrigerated liquid CO2 transportation immensely expensive. Transport of
liquid
COZ at sub-ambient temperatures reduces the required pressures and required
container wall thicknesses but is expensive since refrigeration is required
and, as
carbon dioxide has a solid phase, there is a risk that solid carbon dioxide
can form.
Solid carbon dioxide formation makes CO2 transfer by pumping problematic and,
due to the risks of pipe or valve blockage, potentially dangerous.
Thus in balancing the economies of refrigeration and container cost and
avoiding the
risk of solid CO2 formation, in any given circumstances there will generally
be a
temperature and pressure which is optimal for the liquid CO2 in the
containers, e.g. a
temperature which is below ambient and a pressure which is above ambient but
still
sub-critical (the critical point of CO2 is 73.8 bar A). Typically for large
scale liquid
CO2 transport the optimum temperature is likely to be in the range -55 to -45
C and
the pressure is likely to be 5.5 to 7.5 bar A, i.e. corresponding to the
position in the
phase diagram for CO2 which is just above the triple point in terms of
temperature
and pressure. The triple point for CO2 is 5.2 bar A and -56.6 C. Lower
temperatures and pressures raise the risk of dry ice formation; higher
pressures
require more expensive containers; and lower pressures raise the risk of gas
or solid
formation.
While small scale production (e.g. currently typically 0.1 tonne/year) of
liquid
carbon dioxide is relatively trivial, generally involving two, three or four
cycles of
compression and cooling/expansion, bulk production at the level of millions of
tonnes is by no means trivial since, starting with a gas which is, or is maj
oratively,
carbon dioxide at or near ambient temperature and pressure, transforming this
starting material to liquid carbon dioxide at the temperatures and pressures
that are
desirable for bulk transport involves significant pressurization and energy
removal.
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We have now found that production of liquid carbon dioxide in bulk and at
temperatures
and pressures desirable for bulk transport may be effected in an
environmentally friendly
and efficient manner by producing liquid or dense fluid (i.e. super critical)
carbon dioxide
at temperatures and pressures above the desired values, expanding it to
generate liquid
carbon dioxide at the desired values and cold gaseous carbon dioxide which is
recycled
into the compression and cooling/expansion cycles bringing down the mean
enthalpy of
the CO2 flow through those cycles. In this way no expensive coolant is
required and COZ
release into the atmosphere may be avoided.
Thus viewed from one aspect the invention provides a process for the
production from a
feed gas which comprises carbon dioxide of liquid carbon dioxide at a desired
temperature
and pressure which temperature is below ambient, above the triple point
temperature for
carbon dioxide and below the critical point temperature for carbon dioxide and
which
pressure is above ambient, above the triple point pressure for carbon dioxide
and below
the critical point pressure for carbon dioxide, said process comprising:
feeding said feed
gas into the entry port of a liquefaction apparatus having a flow path from
said entry port
to an exit port connected to an expansion chamber; flowing said gas as a fluid
along the
flow path through said apparatus and subjecting said fluid to a plurality of
compression
and cooling cycles whereby to generate liquid or super-critical carbon dioxide
having a
temperature and pressure above said desired temperature and pressure; removing
water
from said fluid after at least one compression cycle and before the final
compression
cycle; passing said liquid or super-critical carbon dioxide through said exit
port into said
expansion chamber whereby to generate in said chamber gaseous carbon dioxide
and
liquid carbon dioxide at said desired temperature and pressure; and recycling
said gaseous
carbon dioxide into fluid flowing through at least one of said compression and
cooling
cycles; and optionally withdrawing said liquid carbon dioxide at said desired
temperature
and pressure from said expansion chamber.
One or more of the compression and cooling cycles, preferably all such cycles,
may
additionally involve an expansion step which will of course further cool the
fluid. It is
especially preferred that the fluid flowing to each compression step is
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monophasic, i.e. gaseous or dense fluid (super-critical); however it is
optional
whether the product of the final compression and cooling step comprises liquid
carbon dioxide or dense fluid carbon dioxide.
If desired the expansion chamber may be detachable from the liquefaction
apparatus
and may thus serve as the transport vessel for the liquid carbon dioxide.
Preferably
however the expansion chamber has a liquid removal port through which the
liquid
carbon dioxide may be withdrawn into a transport vessel. The expansion chamber
may be any component suitable for expansion, such as an expansion valve and
the
like.
The gaseous carbon dioxide which is recycled is preferably passed through one
or
more heat exchangers to draw energy from the fluid flow before being returned
into
the fluid flow at an upstream point.
Since the feed gas may contain impurities, e.g. water, nitrogen, etc., it is
desirable
that the fluid flow be subjected to one or more, treatments to remove these.
Depending on apparatus design, these removal steps may cause some
consequential
removal of carbon dioxide from the apparatus other than as liquid CO2. Careful
design however can result in only minimal such non-liquid carbon dioxide
removal.
In general, at least two (e.g. 2 to 8, preferably 4) compression steps will be
required
to transform the fluid into liquid or super-critical carbon dioxide. It is
preferred to
effect water removal after at least one compression step and before the final
compression step, e.g. between the second and third compression steps,
typically
after the cooling step following the prior compression step. It is especially
preferred
to effect water removal before each compressor step. Desirably the CO2 gas is
dried
to ppm level by adsorption after the last separator.
Water should be removed to avoid hydrates, freezing of water, corrosion and
droplets of water in the compressor feed. The solubility of water in CO2 gas
decreases with higher pressure and lower temperatures. Water can be removed in
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several ways, e.g. using separators or by passage through a water absorbent or
adsorbent bed or filter. Preferably most of the water is removed in
separators, after
each compression and cooling step.
5 For water removal by condensation and separators, the CO2 gas with liquid
contaminants (e.g. water and also other liquids such as liquefied heavy
hydrocarbons) enters a separator where the condensed liquids are drawn off fT
om the
base of the separator and the CO2leaves the top of the separator in gaseous
form.
Desirably the dried gas leaving a separator or separators is led through an
adsorption
unit before passing to the next compression step. In order to permit
continuous
operation, it is desirable to have two or more such adsorption units arranged
in
parallel so that one may be regenerated (for example by passing hot gas
throughit)
while another is in use. The gas used for regeneration will typically be
gaseous
carbon dioxide which is being recycled. The hot, moist carbon dioxide leaving
the
unit being regenerated may desirably be recycled into the fluid at an upstream
point,
e.g. between the first and second compression steps, preferably between a
compression step and the subsequent cooling steps.
Especially preferably the last free water is removed in a separator before the
last
compressor step at a pressure between 20 and 40 bar and at a temperature close
to
the hydrate formation curve, that is, between 10 C and 15 C. Desirably the CO2
gas
is dried to ppm level by adsorption after the last separator.
Where the feed gas contains further gases that, at ambient temperature,
undergo a
phase change to liquid phase at a temperature lower than that of carbon
dioxide, e.g.
gases such as nitrogen, oxygen, methane or ethane, these gases are desirably
removed prior to the last expansion.
For such feed gases it is therefore desirable that the liquefaction process
include a
step in which such "volatiles" are removed. This preferably occurs following a
compression or cooling step which generates liquid CO2, or more preferably a
fluid,
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which consists of as much gas as is to be removed in the removal step and the
rest in
the liquid phase. If heat is rejected at pressures higher than the CP in the
supercritical phase, the removal of volatiles will be done after the first
expansion
step, where the fluid is in the two phase region under the CP with a low gas
fraction.
The removal of volatile components may be done in a separation column after
heat
rejection close to the dew point line. At transport pressures of 6-7 bar A
only small
fractions of volatiles, typically 0.2-0.5 mole % can be included in the
product to
ensure that dry ice is not formed. If more volatiles are present in the feed
they
should be removed. A separator tank could be used; however, a separator column
is
preferably used to avoid venting of large quantities of CO2 to the atmosphere.
The
cooling in the condenser is provided by vaporisation of liquid CO2 at
intermediate
pressure stages or from the product tank. As a rule of thumb the loss of CO2
will be
equal to the amount of volatiles in the feed.
To further enhance volatile removal, some or all of the liquid CO2 withdrawn
from
the separator column may be warmed (e.g. in a reboiler) and returned into this
separator column. The reboiler may alternatively be integrated in the
separator
column.
The cooling units arranged to cool the fluid flow may use recycled carbon
dioxide as
the cooling fluid. However the cooling units in at least the first compression
and
cooling steps conveniently use an externally sourced fluid, typically water,
e.g. sea,
river, or lake water or ambient air.
The apparatus used in the process of the invention preferably comprises gas
tight
conduits joining the various operating units, i.e. compressors, coolers,
heaters, heat
exchangers, etc. and provided with appropriate valves. Ideally the flow path
has
only one entrance port (for the feed gas) and only one exit port (for the
liquid C02);
however exit ports for water or volatiles removal will be present in certain
embodiments.
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The feed gas for the process of the invention is preferably majoritively
carbon dioxide (on a
molar basis), e.g. 55 to 100% mole COZ or 70 to 95% mole C02, especially at
least 70% mole
C02, more especially at least 90% mole C02, particularly up to 95% mole COZ.
More
preferably the feed gas contains less than 0.5 mole % of volatile components
and less than 0.1
mole % of water. Preferably the water content is not in excess of 50 ppm by
weight. As
mentioned earlier, the carbon dioxide produced as a by-product in ammonia
production or the
carbon dioxide captured from coal or gas power plants is particularly
suitable.
Viewed from a further aspect the invention provides an apparatus for carbon
dioxide
liquefaction, the apparatus being arranged to produce liquid carbon dioxide at
a desired
temperature and pressure which temperature is below ambient, above the triple
point
temperature for carbon dioxide and below the critical point temperature for
carbon dioxide
and which pressure is above ambient, above the triple point pressure for
carbon dioxide and
below the critical point pressure for carbon dioxide, the apparatus comprising
a flow channel
for carbon dioxide passage from an inlet port to an outlet port, said channel
comprising a
plurality of compressors and coolers arranged in series, and a separator for
removal of water
at a location after at least one compressor and before the final compressor,
with an expansion
chamber in said flow channel downstream of the final compressor and cooler and
with a
recirculation channel arranged to return gaseous carbon dioxide from said
expansion chamber
into said flow channel upstream of said final compressor and cooler.
The apparatus of the invention is conveniently provided with the further
structural
components discussed above in connection with the process of the invention.
Embodiments of the invention will now be discussed further by way of
illustration and with
reference to the following non-limiting Examples and the accompanying
drawings, in which:
Figure 1 shows a schematic of an embodiment that does not form a partof the
claimed
invention; and
Figure 2 shows a schematic of a preferred embodiment of the apparatus of the
invention.
Figure 1 is a schematic of the main elements of the apparatus. Feed gas
containing 100 mole
% carbon dioxide is supplied from a source (not shown) to the input port
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of conduit 1. The gas is fed to a first compressor 2 and then to a first
intermediate
cooler 4 via conduit 3. Second stage compression and cooling is performed by
second stage compressor 5 and cooler 7 (connected by conduit 6) and the final
stage
of compression is achieved using compressor 8 and cooler 9. Heat is extracted
in
each of the coolers 4, 7 and 9 using ambient air or water (conduits not shown)
as the
cooling medium.
The fluid output fiom the last compression stage is communicated to a first
input
l0a of heat exchanger 10. The first output l Ob of heat exchanger 10 is
connected to
first input 13 a of a second heat exchanger 13. In addition, the first output
10b is
connected via conduit 12 and expansion valve 11 to the second input l Oc of
heat
exchanger 10. The expansion valve 11 is arranged to expand and cool the first
output l Ob from heat exchanger 10. This acts to cool the fluid flowing
between 10
and l Ob. The recycled carbon dioxide gas flowing between the third input 10e
and
l Of will also cool the fluid flowing l 0a to l Ob. The second output l Od is
connected
to conduit 6 between compressor 5 and cooler 7 whereby to recycle the gas
drawn
off down conduit 12.
The first output l Ob from heat exchanger 10 passes through a further heat
exchanger
13 and to expansion valve 14. The fluid is then expanded to the transport
pressure
by expansion valve 14 and fed into the separator 15. The gas phase (or flash
gas) is
returned via conduit 16 and heat exchangers 13 and 10 respectively to the
conduit 3
arranged between the first compressor 2 and first cooler 4. The arrangement of
the
two heat exchangers 10 and 13 acts to cool the flow of fluid passing between
10a,
lOb, 13a and 13b because the flash gas in conduit 16 and the expanded supply
gas in
conduit 12 will be at a lower temperature. This increases the efficiency of
the
process.
The liquid phase separated in separator 15 is output via output 17 to a
storage or
transport vessel (not shown).
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Expansion of pressurised fluids as mentioned above may conveniently involve
use
of a Joule-Thompson valve. Alternatively, an expansion turbine may be used for
expansion of the pressurised fluids as mentioned above. This will increase the
energy efficiency of the process.
Referring to Figure 2, feed gas is delivered into the inlet port of conduit 18
in the
apparatus and thence into separator 20 which serves to condense water which is
removed through conduit 21. The gas then passes, via conduit 22, to the first
stage
compressor 23 and to first stage intermediate cooler 24. This first stage of
water
removal, compression and intermediate cooling is repeated as shown in Figure 2
by
separator 25, second compressor 26 and second cooler 27. The output of the
second
intermediate cooler 27 is passed through a heat exchanger 28 via conduit 29
where
the temperature of the feed gas is further reduced by heat exchange with
gaseous
carbon dioxide recycled from downstream in the apparatus.
Intermediate coolers 24 and 27 reject heat to sea water.
The feed gas flows from heat exchanger 28 to separator 30 via conduit 31.
Water
removed in separators 25 and 30 is returned to the first separator 20 via
conduits 32
and 33.
Water is removed from the feed gas by means of the three separators 20, 25 and
30
by condensation. It is highly desirable to remove water from the feed gas to
avoid
hydrate formation and corrosion which can occur if significantly more than 50
ppm
(wt.) water is present. Removal of water also increases the efficiency of the
process.
Feed gas is then fed from the third separator 30 via conduit 34 to one of two
water
adsorption units 35a and 35b where the water content is reduced still further
to
approximately 50 ppm.
At any one stage, one water adsorption unit is in use while the other is being
regenerated (dried) by hot carbon dioxide gas from conduit 36. The moist
carbon
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dioxide from the unit being regenerated is recycled into the conduit after the
first
compressor 23 through conduit 37.
Feed gas, with a water content of approximately 50 ppm or less is fed via
conduit 3 8
5 to the final stage compressor 39 and cooler 40. The feed gas leaves
compressor 39
at the maximum pressure of the process (39 being the fmal compression stage)
and
is cooled by cooler 40 which rejects heat to sea water.
The liquid CO2 then passes via conduit 41 to the removal of volatiles column
where
10 the volatiles are removed by distillation. The volatiles are removed in the
top of the
column leaving the bulk of the CO2 in the liquid phase. Liquid carbon dioxide
is
drawn off through conduit 43. In order to enhance the removal of volatiles a
re-
boiler 44 is attached at the bottom of the column. The re-boiler provides heat
in the
bottom of the column to boil off volatiles, and thereby enhance the separation
of
volatiles from the CO2. To enhance the recovery of CO2 in the volatile rich
gas
stream at the top of the column a condenser is placed in the top of the
column. The
required cooling duty for the condenser is provided by vaporisation of liquid
CO2 at
intermediate or product pressure.
The remaining liquid carbon dioxide passes through heat exchanger 45 to
expansion
unit 46 which generates cold carbon dioxide gas and carbon dioxide liquid. The
liquid is directed via conduit 47 and heat exchanger 48 into the final
expansion tank
49 in which it is the desired temperature and pressure. The gas is split, part
flowing
via conduit 50 back through heat exchanger 45 and thence via conduit 51 to
heat
exchanger 28 and part via conduit 52 through heat exchanger 53 and thence via
conduits 54 and 51 to heat exchanger 28. Heat exchange 53 serves as a
condenser
for column 42.
The gas formed in the final expansion tank 49 is fed via heat exchangers 48,
28 and
55 to a heater 56 at which it is heated to a temperature sufficient to
regenerate the
water absorption units 35a and 35b.
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The liquid carbon dioxide in expansion tank 49 may be drawn off via conduit 57
to a
transport vessel.
In the embodiment shown in Figure 1, the pressure and temperature before and
after
compressor 2 are preferably 5 bar A/25 C and 11 bar A/25 C. The pressure and
temperature in expansion tank 15 is preferably 6.5 bar A/-50 C.
In the embodiment shown in Figure 2, the pressures and temperatures at the
sites
marked A, B, C, D, etc. are preferably as set out in Table 1 below:
Table 1
Flow location Pressure (bar A) Temperature ( C)
A 1.1 25
B 1.1 25
C 5 140
D 4.5 20
E 4.5 20
F 20 140
G 19.5 20
H 19.5 10
I 19.5 10
J 19.5 10
K 60 180
L 60 20
M 60 18
N 60 -15
0 21 -20
P 21 -20
Q 21 -22
R 6.5 -50
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S 6.3 -27
T 6.1 -5
U 5.9 200
V 5.7 400
W 5.5 200
X 20.5 -22
The following three Examples refer to alternative ways in which the process
can be
operated with respect to heat rejection above or below the critical point of
the feed
gas.
Example 1 - Heat rejection to sea water/atmosphere below the critical point
The carbon dioxide is compressed from the supply pressure of 1 bar to a
maximum
pressure of approximately 60 bar in 3 compression stages. Between each
compression stage the feed gas is cooled using sea water or atmospheric air.
The
fully pressurised feed gas, i.e. the output from the final compressor, is
condensed
with a heat exchanger again using sea water. The condensed feed gas is
expanded to
the transport pressure using an expansion valve and communicated to the flash
tank
or separator. In the separator the liquid phase is removed and forwarded to a
transport or storage vessel and the gas phase is returned to the compression
stage.
Example 2 - Heat rejection to an external cooling circuit below the critical
point
The feed gas is compressed from the supply pressure of 1 bar to a maximum
pressure of approximately 25 bar in 2 compression stages. The intermediate
cooling
(between compression stages) is achieved using sea water or atmospheric air.
The
pressurised feed gas is then condensed using a heat exchanger connected to an
external cooling circuit. The condensed feed gas is then expanded using an
expansion valve to the transport pressure and communicated to a flash tank or
separator. In the separator the liquid phase is removed and forwarded to a
transport
or storage vessel and the gas phase is returned to the compression stage.
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Example 3 - Heat rejection to sea water/atmosphere above critical point
The feed gas is compressed from the supply pressure of 1 bar to a maximum
pressure of approximately 85 bar (i.e. above the critical pressure of 73.8
bar) in 4
compression stages. The intermediate cooling (between compression stages) is
effected using sea water or atmospheric air. The pressurised feed gas is then
cooled
in the super-critical phase using sea water or atmospheric air. The
pressurised fluid _
is then expanded from the supercritical phase into the two-phase region to the
transport pressure using an expansion means and communicated to a flash tank
or
separator. In the separator the liquid phase is removed and forwarded to a
transport
or storage vessel and the gas phase is returned to the compression stage.
