Could the energy cost of using supercritical fluids be mitigated by using CO2 from carbon capture and storage (CCS)?

Article abstract of DOI:10.1039/c1gc15503b

This article explores the possibility of utilising supercritical CO ₂ obtained from carbon capture and storage (CCS) as a solvent and examines the hydrogenation of isophorone to 3,3,5-trimethylcyclohexanone using supercritical CO₂ with added N₂, CO or H₂O to emulate the contaminants expected in CO₂ from CCS. None of the impurities appear to cause insuperable problems in the hydrogenation of isophorone when present at concentrations likely to be found in CO₂ from CCS. N₂ introduces modest changes in phase behaviour at some pressures, while CO and H₂O reduce the activity of the catalyst. However, the activity can be largely regained by increasing the reaction temperature.

Full text of DOI:10.1039/c1gc15503b

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Volume 13 | Number 10 | October 2011 | Pages 2589–2976
ISSN 1463-9262
COVER ARTICLE
Poliakoff et al.
Could the energy cost of using
supercritical fluids be mitigated by
using CO2 from carbon capture and
storage (CCS)?
1463-9262(2011)13:10;1-9

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PAPER
Could the energy cost of using supercritical fluids be mitigated by using
CO2 from carbon capture and storage (CCS)?
James G. Stevens,a Pilar Go´mez,b Richard A. Bourne,a Trevor C. Drage,c Michael W. Georgea and
Martyn Poliakoff*a
Received 4th May 2011, Accepted 28th July 2011
DOI: 10.1039/c1gc15503b
This article explores the possibility of utilising supercritical CO2 obtained from carbon capture
and storage (CCS) as a solvent and examines the hydrogenation of isophorone to
3,3,5-trimethylcyclohexanone using supercritical CO2 with added N2, CO or H2O to emulate the
contaminants expected in CO2 from CCS. None of the impurities appear to cause insuperable
problems in the hydrogenation of isophorone when present at concentrations likely to be found in
CO2 from CCS. N2 introduces modest changes in phase behaviour at some pressures, while CO
and H2O reduce the activity of the catalyst. However, the activity can be largely regained by
increasing the reaction temperature.
reactions should preferably be run at ambient temperature and
Introduction
pressure.
Supercritical CO2 (scCO2) has proved to be an effec-
In recent years, our research group has explored a variety
tive alternative solvent for a range of continuous catalytic
of strategies to offset the energy costs of using high pressure
reactions,1,2 including hydrogenation,3,4 oxidation5 and acid-
CO2 by providing additional benefits, for example, in tandem
catalysed reactions.6 In the case of hydrogenation, some reac-
reactions10,11 or the energy efficient separation of products.12
tions have already been scaled up to commercial scale plants.7,8
However, there may be a completely new opportunity if cur-
The Thomas Swan plant8 hydrogenated isophorone to 3,3,5-
rent plans for carbon capture and storage (CCS) are indeed
trimethylcyclohexanone (TMCH) (Scheme 1) on a scale equiv-
implemented on a large scale.13
alent to 1 000 tons p.a. The process was technically successful;
CCS is considered to be a promising route to achieving large
TMCH was produced with a selectivity high enough to render
cuts in CO2 emissions, particularly from coal- and natural gas-
downstream purification unnecessary. However, compressing
fired power plants.14 It is expected that, as CCS is implemented,
CO2 is energy intensive and sharply rising energy costs quickly
very considerable quantities of pressurised liquid CO2 will be
made the process uncompetitive. This problem is highlighted
by the Sixth Principle of Green Chemistry,9 which advises that
available. In the UK, it is anticipated that a typical large power
station might output CO2 compressed to ca. 100 bar at a rate
>0.5 tons per second. By comparison, the Thomas Swan plant8
had CO2 flowing through the reactor at a rate of <0.5 tons per
hour. Thus, in theory at least, the power station could provide
enough CO2 in only 90 min to run the Thomas Swan plant for a
whole year without any need to recycle the CO2.
The key point is that, in the near future, there will be an
Scheme 1 The hydrogenation of isophorone to trimethylcyclohexanone
abundance of piped high pressure CO2, with much of the
(TMCH). The original reaction4 and all those described here used JM
compression cost already being supplied at the power station.
Type 31 catalyst, 2% Pd on SiO2/Al2O3.
This would lead to a reduction in the large energy burden
currently associated with compressing CO2 for use as a ‘green’
solvent. Conceptually, CO2 could be ‘tapped off’ either from
aSchool of Chemistry, The University of Nottingham, Nottingham, NG7
the CO2 transport network or directly from the capture plant,
used as a reaction solvent and then fed back into the large
scale compression process with only a marginal energy cost.
This would require some complex engineering but there are
potentially more serious problems from a chemical point of view.
The huge scale of CCS activities means that the captured CO2
2RD, UK. E-mail: Martyn.Poliakoff@nottingham.ac.uk; Fax: +44
(0)115 951 3508; Tel: +44 (0)115 951 3520
bChemical Engineering Department, Faculty of Chemistry, Complutense
University of Madrid, Avda Complutense, s/n, 28040, Madrid, Spain.
E-mail: pilar.gomez@quim.ucm.es
cDepartment of Chemical and Environmental Engineering, The
University of Nottingham, Nottingham, NG7 2RD, UK
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Table 1 The composition of the gas mixture used for each of the
will have a lower purity than most commercial grades of CO2.
This paper reports the results of a preliminary study to assess
the effects on supercritical hydrogenation of the most probable
impurities in CCS CO2.
temperature ramps
Compositiona (mol%)
Entry
Isophorone
H2
N2
CO2
Strategy
1
2.78
2.65
2.52
2.78
2.64
2.49
4.86
4.64
4.41
4.86
9.74
14.63
—
92.36
88.08
83.76
92.36
87.63
82.88
2b
3c
4
4.64
9.31
—
—
—
CCS is likely to be carried out using one of three pro-
cesses: pre-combustion, post-combustion or oxy-combustion
capture. These processes have been reviewed by Davison and
Thambimuthu,15 indicating typical contaminants including up
to 4.1 vol% N2/Ar/O2 in CO2 from oxy-combustion, 0.4 vol%
CO and 2.0 vol% H2 from pre-combustion processes, and
potentially small volumes of water in all of the processes.16† Since
we are considering hydrogenation here, contamination of the
CO2 feed by H2 is not problematic and could even be beneficial.
Therefore, this study focuses on the effects of N2, H2O and CO
on the scCO2 used for the hydrogenation of isophorone.
5
6
a Gas composition confirmed by on-line GC. b Equivalent to using
5 mol% N2/CO2. c Equivalent to 10 mol% N2/CO2.
We have used an automated continuous reactor11 described
in the Experimental section; this has the capability of dosing
N2, CO or H2O into a mixture of CO2, H2 and isophorone.
The temperature of the catalyst bed is ramped under computer
control and the composition of the downstream product mixture
is monitored using on-line GLC. We have previously demon-
strated that the phase behaviour of a reaction mixture can have
a significant effect on the reaction outcome.8,17,18 Therefore, in
this paper, we have used the “pressure-drop” method, recently
developed in Nottingham,19 to establish rapidly the phase
state of the particular reaction mixtures. The pressure-drop is
better suited to continuous reactions and is much quicker than
conventional variable volume view cell measurements;20 further
details are given in the Experimental section.
Fig. 1 Plot showing how the yield of TMCH is unaffected by addition
of 10 mol% N2 at a pressure of 12.00 MPa as the external temperature
is ramped repeatedly: (ꢀ), 5% H ; ( ), 4.7% H , 4.7% N ; (᭡), 4.5%
᭹
2
2
2
H2, 9.6% N2; (᭢), 10% H2; (᭜), 15% H2. Conditions: 12.0 MPa, 0.1 mL
Results and discussion
min-1 isophorone, 1.0 mL min-1 CO2, 2 wt% Pd/Type 31, 40 to 130 ◦
at 0.3 ◦C min-1.
C
Effect of nitrogen
The conversion of isophorone to TMCH was investigated across
a range of temperatures, using fixed flow rates of CO2 and
isophorone; differing amounts of N2 were added into the CO2
stream in order to explore the effect of using N2-contaminated
CO2. The result of higher concentrations of H2 was also
explored. The compositions of the gas mixtures used are shown
in Table 1.
The results of six temperature ramps at 12.0 MPa are shown
in Fig. 1. The first three gas compositions (5% H2, 5% H2 + 5%
N2 and 5% H2 + 10% N2) all give very similar yields of TMCH
across the entire temperature range, indicating that the addition
of N2 has almost no effect on the hydrogenation of isophorone at
this pressure. The ramp was then repeated without N2; the result
was almost identical to the first run (with 5% H2), demonstrating
that there had been no loss of catalytic activity as a result of the
previous temperature ramps. As might be expected, when the
concentration of H2 was increased (as in ramps 5 and 6), the
yield of TMCH was higher at lower temperatures.
Similar experiments were carried out by adding N2 at three
further pressures: 8.0, 16.0 and 20.0 MPa, and two effects were
consistently observed: (i) there was no loss of catalytic activity
during the course of the experiments and (ii) an increase in
hydrogen concentration led to higher yields of TMCH at lower
temperatures. Fig. 2 shows that the yields of TMCH were similar
at 8.0 and 12.0 MPa, and were completely unaffected by the
addition of N2 to the system. At 16.0 MPa, the yield of TMCH
was slightly elevated at lower temperatures, but this may have
been due to slight differences in catalyst packing. Much more
significantly, the addition of N2 reduces the yield of TMCH at
any given temperature (see Fig. 2c). By way of contrast, N2 had
no effect on the yield of TMCH at 20.0 MPa but the yield was
higher at any given temperature than under lower pressures, with
>98% yield of TMCH >82 ◦C (compared to >94 ◦C, >112 ◦C
and >109 ◦C for 8.0, 12.0 and 16.0 MPa, respectively).
Thus, the addition of N2 to the system has little effect on the
yield of TMCH, even at much higher concentrations of N2 than
those anticipated by the draft specifications for CCS CO2 in the
UK.16 A subtle difference was observed at 16.0 MPa, almost
certainly due to the phase behaviour of the system, and this is
discussed in more detail below.
† Other minor impurities, e.g. SO2, are anticipated for CCS CO2.
Although their levels are expected to be much lower than those of N2,
CO and H2O, their effects on supercritical reactions will need to be
established.
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Fig. 2 Plot showing the effect of adding N on the yield of TMCH at different pressures: (ꢀ), 5% H ; ( ), 4.7% H , 4.7% N ; (᭡), 4.5% H , 9.6%
᭹
2
2
2
2
2
N2. Conditions: 0.1 mL min-1 isophorone, 1.0 mL min-1 CO2, 2 wt% Pd/Type 31, 40 to 130 ◦C at 0.3 C min-1 at (a) 8.0 MPa, (b) 12.0 MPa, (c)
16.0 MPa and (d) 20.0 MPa. Note that only at 16.0 MPa did the addition of N2 have any effect.
◦
Table 2 Results of adding water to the system at 0.01 mL min-1 at
Effect of water
both 60 and 100 ◦C. 0.1 mL min-1 isophorone, 1.0 mL min-1 CO2, 1.75
equivalents of H2, 12.0 MPa
Water is also a potential contaminant of CCS CO2, and to test
possible effects on the reaction, water was pumped into the
system using an additional HPLC pump. At 60 ◦C, the addition
of water leads to a dramatic decrease in the yield of TMCH
(Table 2). When the water flow was stopped, the TMCH yield
returned to “pre-water levels” after 240 min. The effect is much
TMCH (%)a
Time water flowing/min
60 ◦
C
100 ◦
C
0
40
80
63
55
2
99
89
91
◦
reduced at 100 C, with less than a 10% reduction in the yield
following the addition of water to the system flow. This suggests
that H2O is unlikely to be a serious problem for this reaction if it
is present in CCS CO2 at the levels used here (2.3 mol%, which
is well above the anticipated maximum of 500 ppm).21
We have already demonstrated that, in the hydrogenation of
2-methylfuran in scCO2, higher levels of H2O can be successfully
removed by phase separation prior to hydrogenation.10
a Composition calculated by on-line GC.
into the poisoning effect of CO on palladium catalysts.22,23
However, we found that the effect could be largely ◦overcome
by increasing the temperature of the reactor to 260 C, where
a 99% of yield of TMCH was achieved. The high temperatures
led to a small decrease in the selectivity with ca. 1% of other
compounds being generated; possibly the selectivity could be
improved by further reaction optimisation. Although running
reactions at high temperature is discouraged by the Sixth Prin-
ciple of Green Chemistry,9 temperature is less important here
because the hydrogenation of isophorone is highly exothermic.
Therefore, with appropriate heat exchange, running the reaction
at high temperature will not necessarily be energy intensive.
Effect of carbon monoxide
Rubin et al. have reported16 that CO concentrations of up to
0.4 mol% may be found in pre-combustion capture sources of
CO2. This is potentially a serious problem because CO is a
well known poison of hydrogenation catalysts. Indeed, Fig. 3
shows that the addition of 0.5 mol% CO drastically reduced
the activity of the catalyst, as expected from previous studies
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Fig. 3 Plot showing the effect of 0.5 mol% CO on the yield of
TMCH across a range of temperatures under standard conditions. Mol.
percentages isophorone : H2 : CO : CO2 used (---) 2.8 : 4.9 : 0 : 92.4 and
Fig. 4 Phase diagram constructed from pressure drop measurements
taken over the range 10 to 20 MPa and 30 to 200 ◦C; the first and
second transitions have been joined by lines to aid visualisation (see
Table 3 for numerical data). Lines represent the phase boundary for: ꢀ,
isophorone + H2 + CO2; ꢁ, isophorone + H2 + CO2 with N2 (based on
(
) 2.8 : 4.9 : 0.5 : 91.6, respectively.
Alternatively, a number of routes could be envisaged to remove
the CO from the CO2 stream. However the simplest solution
might be to couple the chemical plant with the most appropriate
capture technology, e.g. post-combustion capture, which pro-
vides the highest purity CO2, free from both CO and H2S.16,21 In
a different context, we have shown10 that 2-methylfuran can be
successfully hydrogenated over Pd in CO2 saturated in water at
room temperature and containing CO. In the present context,
this shows that a hydrogenation reaction can be carried out in
the presence of both H2O and CO, and that our conclusions are
not restricted to the hydrogenation of isophorone.
5 mol% N in CO ); , TMCH + H + CO ; , TMCH + H2 + CO2
᭹
2
2
2
2
᭺
with N2 (based on 5 mol% N2 in CO2).
multi-phase at >86 ◦C; however, at this temperature, the system
is already undergoing >99% conversion. This suggests that, at
20.0 MPa, the reaction proceeds under a largely single-phase
regime. The addition of N2 to the system does not affect the
phase behaviour significantly.
At 16.0 MPa, significant differences were found for the
reaction with and without N2 (see Fig. 2). To assess further
the phase behaviour of the reaction system, the pressure drop
equipment was coupled with the catalytic reactor. This allows
phase behaviour to be investigated under genuine reaction
conditions. Fig. 5 shows the results of the pressure drop of the
product mixture as the reactor temperature is increased.
Effect of nitrogen on the system phase behaviour
The anomalous behaviour of N2 at 16.0 MPa prompted us
to examine the phase envelope of the quaternary system
[isophorone + CO2 + H2 + N2]. The phase diagram (Fig. 4)
was constructed from fixed pressure measurements (from 8.0
to 20.0 MPa) for isophorone over the temperature range used
for the previous hydrogenation tests (30 to 200 ◦C). Initially, the
phase boundary was measured for [isophorone + H2 + CO2] with
the same flow rates as used during the hydrogenation experiment.
Our data were in good agreement with those reported by
Sokolova et al.,8 who used more conventional measurement
techniques. Phase boundaries were then measured for TMCH
with the appropriate amount of hydrogen remaining in the CO2
at 100% conversion across the same temperature and pressure
range. Measurements were also carried out for both systems
with the addition of N2.
Fig. 4 shows that the addition of N2 causes an expansion
of the multi-phase region for both the starting reagents and
the product. Also, the product stream has a higher solubility
than isophorone in CO2, with only a relatively small multi-phase
region. This was initially thought to be due to the decrease in the
concentration of H2, which can act as an anti-solvent; however,
TMCH was tested with the same amount of H2 as under the
starting conditions and was found to have a smaller multi-phase
region than that of isophorone.
Fig. 5 Pressure drop data, displayed with yield of TMCH from the
reactor: (black line), without N2; (grey line), with added N2. Reaction
conditions: 0.1 mL min-1 isophorone, 1.0 mL min-1 CO2, org. : H2 ratio
of 1 : 1.75, 16.0 MPa, 2% Pd/Type 31.
These phase data provide a plausible explanation for the
different results observed with and without N2. At 12.0 MPa
(and at 8.0 MPa), the mixture is in a multi-phase region under
almost all temperature conditions of interest, both with and
By way of contrast, at 20.0 MPa, the product mixture is a
single phase across all temperatures. The starting mixture is
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Table 3 Phase transition data from the pressure drop method
Isophorone : H2 : N2 : CO2
2.8 : 4.9 : 0 : 92.4
Pressure/MPa
T
trans 1/◦C
Ttrans 2a/◦C
TMCH : H2 : N2 : CO2
2.9 : 2.1 : 0 : 95.0
Pressure/MPa
T
trans 1/◦C
Ttrans 2a/◦C
10
12
14
16
18
20
37
41
47
62
73
86
—
—
—
—
198
196
10
12
14
16
18
20
38
54
70
192
190
180
88
170
Noneb
Noneb
Noneb
Noneb
2.7 : 2.0 : 4.8 : 90.5
10
12
14
16
18
20
33
34
35
43
56
68
—
—
—
210
204
200
2.7 : 4.6 : 4.6 : 88.1
10
12
14
16
18
20
36
42
54
72
190
188
184
172
157
noneb
94
noneb
a Where no temperature is given, the second transition (Ttrans 2) was not observed over the tested temperature range. b No transition as this pressure is
above the multi-phase envelope.
without N2. At 12.0 MPa, the 100% conversion system with
TMCH and a small quantity of H2 enters a multi-phase region
at 54 C under N2-free conditions. The phase boundary of the
to TMCH, favouring single-phase conditions. The temperature
and conversion will eventually be balanced by the materials in
the reactor becoming multi-phase along its entire length. From
Fig. 5, we can determine that this occurs at 88 ◦C. In the system
containing N2, the reagent mixture becomes multi-phase at only
43 ◦C. For the 100% conversion mixture, the phase transition
occurs at 72 ◦C. However, Fig. 5 shows that, with N2, the reactor
outlet is multi-phase at 60 ◦C and indeed the entire reactor bed is
multi-phase >60 ◦C. This is probably related both to the reduced
solubility of isophorone and TMCH in the CO2 + N2 system,
and to the lower conversion of isophorone in this transitional
temperature range.
Thus, phase measurements provide a rationalization of the
differences observed between the ramps carried out at 16.0 MPa
and at the other pressures. At 16.0 MPa, the addition of N2
appears to change the behaviour of the reaction, with a clear
difference from that without N2. The addition of N2 to the
system at 16.0 MPa causes the system to change from being
predominantly single-phase (without N2) to predominantly
multi-phase. The phase behaviour of the systems at the other
pressures is unchanged by the addition of N2, either remaining
predominantly multi-phase (as at 8.0 and 12.0 MPa) or remain-
ing largely single-phase (as at 20.0 MPa).
◦
starting reagents occurs at 42 ◦C, where there is little conversion.
Therefore, the reaction occurs under predominantly multi-phase
conditions across the entire temperature range. The addition of
N2 has little effect because the system already exists in a multi-
phase regime.
At 16.0 MPa◦, the starting reagents without N2 are multi-
phase above 62 C. For 100% conversion, the system becomes
◦
multi-phase at 88 C. The reactor will be entirely single-phase
<62 ◦C and multi-phase >88 ◦C. As the temperature increases
from 62 to 88 ◦C, the system will become increasingly multi-
phase. Initially, the reactor will be under multi-phase conditions
at the top, becoming single-phase as the isophorone is converted
to the more CO2-soluble TMCH, as shown pictorially in Fig.
6. As the temperature is increased at constant pressure, the
solubility of the compounds is reduced; however, the lower
solubility of isophorone is balanced by the increasing conversion
Conclusions
In this paper, we have reported what we believe to be the
first attempt to investigate the chemical problems that might
arise when using CO2 from CCS as a solvent for continuous
supercritical hydrogenation. The results are encouraging. None
of the three impurities examined, N2, CO or H2O, appear to
cause insuperable problems in the hydrogenation of isophorone
when present at concentrations likely to be found in CO2 from
CCS. N2 introduces modest changes in phase behaviour at some
pressures, while CO and H2O reduce the activity of the catalyst.
However, the activity can be largely regained by increasing the
reaction temperature.
Obviously, these conclusions are preliminary and only apply
to one reaction.† However, if they can be extended equally
successfully to other reactions, they will open up the possibility
Fig. 6 Schematic representation of the phase behaviour along the
reactor length with and without N2 at 16.0 MPa and at different
temperatures.
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of transforming captured CO2 from a waste product into a
resource. At the same time, one would be harnessing the energy
already spent on capture and compression of the CO2, thereby
bringing supercritical reactions much closer to the spirit of the
Sixth Principle of Green Chemistry.
In a typical experiment, the reactor (156 mm long ¥ 3.525 mm
i.d.) was filled with 0.60 g of 2% Pd Type 31 catalyst (Johnson
Matthey) and sealed into the apparatus with 20 mm of sand on
top of the catalyst to act as a pre-heater. After the equipment
had stabilised at the required pressure and flow rate of CO2, the
catalyst was pre-reduced with H2 at 200 ◦C. The reactor was then
heated to the reaction temperature and the substrate pumped
into the system. Experimental parameters were programmed
into the pumps, back pressure regulator and GC. Unless
otherwise stated, the standard reacti◦on conditions were: flow of
CO2 1.0 mL min-1 (pumphead at -5 C) and flow of isophorone
0.1 mL min-1. The GC was fitted with a HP-5 column (30 m,
i.d. 0.32 mm, film thickness 0.25 mm) operated isothermally
at 60 ◦C. Quantification was performed by integration of the
peak areas; response factors and conversions were calculated
by the internal normalisation method. In those hydrogenation
experiments involving a temperature ramp, the rate of heating
Experimental
Reagents
Isophorone (Acros, 98%), CO2 (Cryoservice, food grade), H2
(BOC, 99.995%) and CO (BOC, CP grade) were used as supplied.
Automated continuous flow apparatus and reaction procedure
CAUTION! The experiments described in this paper involve the
use of relatively high pressures and require equipment with the
appropriate pressure rating.
◦
was 0.3 C min-1, comparable to the equilibration rate of the
reactor.
All experiments were carried out on an automated continuous
reactor with on-line GC analysis Fig. 7. This reactor, described
in detail previously,11 is designed to record the effect on product
yield of varying a single reaction parameter (e.g. temperature,
pressure, flow rate, etc.). In this work, on-line analysis allows the
constant monitoring of the product stream over long periods of
time, while a second column with a TCD detector permits the
simultaneous monitoring of the gas mixture (CO, N2 and CO2).
Phase behaviour measurements
Pressure drop measurements were collected using calibrated
pressure transducers at the inlet and outlet of a reactor (100 mm
long ¥ 11.3 mm internal diameter) filled with 0.60 g of 600
grit (ca. 20 mm) silicon carbide, using glass balls as a pre-
heating zone, using the system previously developed by Akien
et al.19 (Fig. 8). In each experiment, the outlet pressure was
fixed using the BPR. As the temperature was increased during
the experiment, the pressure drop across the silicon carbide
varied, resulting in a varying inlet pressure. The inlet and outlet
Fig. 7 Schematic of the automated supercritical fluid continuous flow
apparatus. The components are labelled as follows: CP, chilled CO2
pump; HD, H2 dosing unit; M, mixer; OP, organic pump; PH, pre-
heater and mixer; R, reactor; HP-SL, high pressure sample loop; BPR,
back pressure regulator; GLC, gas liquid chromatograph; W, organic
waste.
Fig. 8 Schematic of the reactor used for pressure drop measurements,
showing the inlet pressure transducer, reactor bed filled with glass balls
followed by silicon carbide (SiC), the outlet pressure transducer and the
back pressure regulator (BPR). The inlet pressure, outlet pressure, and
R
internal and external temperatures are all recorded via a Picologꢀ data
recorder.
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R
pressures were recorded using a Picologꢀ data recorder as
Notes and references
the temperature was ramped. The pressure difference was then
plotted against temperature as shown in Fig. 9.
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Fig. 9 Typical pressure drop measurement showing the tran◦sitions at
70 and 180 ◦C as the reactor is ramped from 50 to 230 ◦C at 1.0 C min-1.
16.0 MPa, 0.1 mL min-1 isophorone, 1.0 mL min-1 CO2, org. : H2 ratio
of 1 : 1.75 equivalent to isophorone : H2 : CO2 mol% of 2.8 : 4.9 : 92.4.
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A typical pressure drop experiment was carried out at a fixed
pressure, with the temperature gradually increased across a wide
range, as shown in Fig. 9. When the system goes from single-
phase to multi-phase regimes (or vice versa), a discontinuity
is observed in the plot of pressure drop vs. temperature.19
This discontinuity is due to a change in the fluid properties
(particularly density) between a single gas phase, single liquid
phase or mixed phase systems.
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Acknowledgements
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Report POL-O-2007-138-A, 2008.
We thank the EPSRC, Thomas Swan & Co. Ltd., the EPSRC
DICE project and Complutense University of Madrid (Project
number: S2009AMB1588) for support. M. W. G. gratefully
acknowledges receipt of a Royal Society Wolfson Merit Award.
We also thank Mr R. A. Skilton and Dr G. R. Akien for scientific
assistance with the pressure drop work, and Mr M. Dellar, Mr
M. Guyler, Mr R. Wilson and Mr P. Fields for their technical
support at the University of Nottingham.
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This journal is
The Royal Society of Chemistry 2011
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