Separation and Capture of Carbon Dioxide from CO2/H2 Syngas

Mixture Using Semi-Clathrate Hydrates

Jebraeel Gholinezhad, Antonin Chapoy*, Bahman Tohidi

Centre for Gas Hydrate Research, Institute of Petroleum Engineering, Heriot-Watt

University, Edinburgh EH14 4AS, Scotland, U.K.

The relation between anthropogenic emissions of CO2 and its increased levels in the

atmosphere with global warming and climate change has been well established and

accepted. Major portion of carbon dioxide released to the atmosphere, originates from

combustion of fossil fuels. Integrated Gasification Combined Cycle (IGCC) offers a

promising fossil fuel technology considered as a clean coal-based process for power

generation particularly if accompanied by precombustion capture. The latter includes

separation of carbon dioxide from a synthesis gas mixture containing 40 mole% CO2 and

60 mole% H2.

A novel approach for capturing CO2 from the above gas mixture is to use gas hydrate

formation. This process is based on selective partition of CO2 between hydrate phase and

gas phase and has already been studied with promising results. However high-pressure

requirement for hydrate formation is a major problem.

We have used semiclathrate formation from tetrabutylammonium bromide (TBAB) to

experimentally investigate CO2 capture from a mixture containing 40.2 mole% of CO2

and 59.8 mole% of H2. The results shows that in one stage of gas hydrate formation and

dissociation, CO2 can be enriched from 40 mole% to 86 mole% while the concentration

of CO2 in equilibrium gas phase is reduced to 18%. While separation efficiency of

processes based on hydrates and semi-clathrates are comparable, the presence of TBAB

improves the operating conditions significantly. Furthermore, CO2 concentration could be

increased to 96 mole% by separating CO2 in two stages.

Keywords: gas hydrates, TBAB, gas separation, CO2 capture, hydrogen

* Corresponding author. Tel.: +44 0 131 451 3797; fax: +44 0 131 451 3127.

E-mail address: Antonin.Chapoy@pet.hw.ac.uk

1. Introduction

It is now widely accepted that anthropogenic CO2 emissions from burning of fossil fuels

are largely responsible for the rapid rise in global temperatures recorded over the past

century. Worldwide concerns over the threat of global warming have led industrialised

countries to agree to reduce carbon emissions. In order to meet this goal, the research

communities are trying to develop new solutions for reducing CO2 (and other greenhouse

gas) emissions from existing and new fossil fuel usage.

Integrated Gasification Combined Cycle (IGCC) is rapidly emerging as one of the most

promising technologies in power generation, hydrocarbons (gas, coal and oil) are broken

down to yield a high value natural gas type fuel also called synthesis gas (Barchas and

Davis, 1992). This synthesis gas can be converted (by a shift reaction) into a mixture of

CO2 and H2. A typical synthesis gas from an IGCC power station consists approximately

of a 40 mole% CO2 and 60 mole% H2 mixture at a total pressure of 2.5 to 5 MPa (Klara

and Srivastava, 2002). The separation of carbon dioxide from this mixture, also called

“pre-combustion capture”, is an essential element of these processes. The resultant stream

of H2 could be used in fuel cells or in a gas turbine and the carbon dioxide could be

geologically sequestered. There is an ongoing effort to reduce operating cost of an IGCC

plant, particularly for CO2 capture.

A novel method for CO2 capture and storage is to use gas hydrates (Spencer, 1997; Aaron

and Tsouris, 2005; Linga et al., 2007a; Kumar et al., 2009; Zhang et al., 2009). The basis

for the separation is the selective partition of CO2 between the hydrate phase and the

gaseous phase. The kinetics of hydrate formation in the system CO2/H2-water was studied

by Linga et. al. at 273.7 K and 7.5 MPa (Linga et al., 2007a). However, the 7.5 MPa

operating pressure was higher than the typical pressure of the fuel gas. There is an

ongoing interest in using additives to reduce the operating pressure. Several additives

have been used for this purpose including tetrahydrofuran, hereafter THF (Lee et al.,

2009), propane (Kumar et al., 2009), cyclopentane (Zhang et al., 2009).

Recently, Tetra-n-Butyl Ammonium Bromide (TBAB) has attracted significant attention

as a suitable additive for different hydrate applications such as hydrogen storage (Chapoy

et al., 2007; Struzhkin et al., 2007; Tohidi et al., Int. patent, 2006), air conditioning

(Darbouret et al., 2005; Delahaye et al., 2008; Daitoku and Utaka, 2009) and CO2 capture

(Kang and Lee, 2000; Klara and Srivastava, 2002; Aaron and Tsouris, 2005). This is due

to the fact that small gas molecules (CH4, CO2) could be encaged into the dodecahedral

cavities (S-cage, 512) of TBAB hydrate at favourable pressure and temperature conditions

(Shimada et al., 2005; Arjmandi et al., 2007). TBAB forms semi-clathrates that could

offer greater benefits for CO2H2 separation: operating pressures could be lowered

significantly and therefore less cooling being required for hydrate formation.

TBAB forms semi-clathrate hydrates with water at ambient conditions, i.e. at 1 atm and

12 oC (Jeffrey and McMullan, 1967; Dyadin and Udachin, 1984; Oyama et al., 2005). If a

gas or mixture of gases is associated with these hydrates, the thermodynamic stability of

the mixture becomes even more favourable (Hashimoto et al., 2006; Arjmandi et al.,

2007; Li et al., 2007). Li et. al. measured the dissociation conditions for the system

CO2/H2-TBAB-water for a 39.2% CO2 and 60.8% H2 gas mixture with different

concentrations of TBAB (Li et al., 2009). They found that with the addition of 5 wt%

TBAB, the equilibrium hydrate formation pressure for the typical gas mixture reduces

from 11.01 MPa at 278.75 in the case of pure water to 0.85 MPa at the same temperature

which shows an approximate reduction of 92.3% in the equilibrium pressure. TBAB

semi-clathrates have more stable structures than normal hydrates and forms at lower

pressures. However, no data are given by Li et al. on the separation efficiency and gas

compositions after hydrate formation.

In this study, we have used TBAB to investigate the capture efficiency of CO2 from

CO2/H2 mixture and the kinetics of gas uptake by semi-clathrates formed in the systems

CO2/H2-TBAB-water for TBAB concentrations of 5% and 10% in aqueous solutions. The

results have been compared with systems in which other additives have been used.

2. Experimental section

2.1. Materials

Tetra-n-butylammonium bromide, 50 weight percent in water, was purchased from

Aldrich. Deionised water was used to dilute the tetrabutylammonium bromide solution to

different desired mass fractions in the experiments. Hydrogen was purchased from BOC

gases with a certified purity greater than 99.995 vol. %. Carbon dioxide (> 99.9 % purity)

was purchased from Air Product Ltd.

2.2. Apparatus

The setup used in this work, is shown in Figure 1 The equilibrium cell has an inner

volume of 527 ml and a maximum working pressure of 40 MPa and is hosted in a cooling

jacket. The cell temperature is measured by means of platinum resistance thermometer,

PRT within a precision of 0.01 K. The pressure is measured by a DRUCK pressure

transducer within a precision of 0.008 MPa for the complete operating range of 0-138

MPa. The cell temperature is controlled by a programmable cryostat, which can be kept

stable to within 0.02 K. The stirrer is used to agitate the test fluids. A Varian CP-3800

gas chromatograph (GC) equipped with a thermal conductivity detector, TCD was used

to determine the composition of the gas samples. The temperature and pressure are

continuously monitored and recorded by a computer. The piston vessel containing the gas

mixture is connected to the cell while a pump is used to keep the pressure constant.

2.3. Procedures

In a typical separation experiment, the cell was first vacuumed and a known amount of

TBAB aqueous solution (5 wt% or 10 wt%) was then loaded into the cell to occupy up to

50% of its volume. A certain amount of the CO2/H2 gas mixture was injected to reach the

desired pressure. A piston vessel was kept connected to the cell to maintain the pressure

by injecting water to the bottom of piston vessel with a high-precision Quizix pump. The

number of moles of gas in the cell and the vessel can be determined and controlled by the

amount of fluid injected behind the piston. During the experiment, temperature and

pressure in the system were recorded. The stirrer was started to operate with the speed of

600-700rpm. The cap gas was sampled and compositionally analyzed by GC in order to

determine the amount of gas dissolved in the liquid at equilibrium before hydrate

formation.

When loaded and equilibrated, the cell temperature was lowered until clathrate formation

began. This was confirmed by a rapid rise in the cell temperature as well as an increase of

the rate of fluid injection. When there was no further clathrate formation and the injection

of pump fluid was stopped, the gas in equilibrium with hydrates was sampled and

analyzed by GC in order to determine its composition. The residual gas was rapidly

vented to the atmosphere and hydrates were dissociated by increasing the temperature of

the cell. The gas released from hydrates (including the dissolved gas) was collected,

sampled and analyzed by GC, thus giving the composition of the gas in hydrate.

3. Results and Discussion

The separation tests were carried out with 5 wt% and 10 wt% TBAB aqueous solutions

and volumetric calculations were performed for each concentration. The number of moles

of gas consumed at any time during hydrate formation (nH) can be calculated by the

following equation:

tGG

tGGHzRT

PV

zRT

PVnnn

,0,

,0,

(1)

where nG,0 and nG,t are the number of moles of free gas in the cell before and after hydrate

formation, respectively. P, T, V, z, and R are the system pressure (constant in these

experiments), the system temperature, the volume of the cell occupied by free gas, the

gas compressibility factor and the universal gas constant, respectively. Figures 2 and 3

show the changes in temperature and moles of gas consumed during hydrate formation in

5 and 10 wt% TBAB aqueous solutions, respectively. The pressure is maintained constant

at 3.8 MPa during the process. The trend of gas consumption was similar in both cases.

Initially there is a short period of slow gas uptake which could be mainly due to the

reduction in the system temperature. Then there is a sharp increase in the amount of gas

consumed (trapped in the cages of clathrates) due to hydrate formation. Hydrates

continue to form slowly before it stops and no more gas is consumed.

Figure 4 compares the gas uptake of the two cases. The amounts of gas consumed at the

end of the process are 0.0502 and 0.08604 moles for the 5 wt% and 10 wt% TBAB

aqueous solutions, respectively.

Parameters by which the separation efficiency can be evaluated are: CO2 mole fractions

in different phases, CO2 Recovery (R) and CO2 Separation Factor (S). The CO2 recovery

or split fraction is calculated as (Linga et al., 2007b):

Feed

CO

H

CO

n

nR

2

2 (2)

where Feed

COn2

is defined as number of moles of CO2 in feed gas (feed gas is the sum of the

number of moles of gas present in the gas phase of the cell at the start of the experiment

and the number of moles of gas consumed during hydrate formation from the supply

vessel); and H

COn2

is the number of moles of CO2 in the hydrate phase at the end of the

experiment which accounts for the dissolved gas as well.

The separation factor is determined as (Linga et al., 2007b):

22

11

/

/

/

/

22

22

yx

yx

nn

nnS

gas

H

gas

CO

H

H

H

CO (3)

where gas

Hn2

= number of moles of H2 in the gas phase at the end of the experiment;

H

Hn2

= number of moles of H2 in the hydrate phase; and gas

COn2

= number of moles of CO2

in the gas phase at the end of the experiment.

In addition, x1 and y1 are concentrations of CO2 in hydrate and gas phases at the end of

the experiment, respectively. x2 and y2 are concentrations of H2 in hydrate and gas phases

at the end of the experiment, respectively.

The bar chart in Figure 5 shows the values obtained for CO2 mole fraction in the feed gas

before hydrate formation and in the hydrate phase and gas phase in equilibrium after

semi-clathrate formation. For comparison, results on other systems including CO2/H2

(Linga et al., 2007b), CO2/H2/C3H8 (Kumar et al., 2009), CO2/H2/Cyclopentane (Zhang et

al., 2009) and CO2/H2/THF (Lee et al., 2009) are also given in Figure 5. CO2

concentrations in hydrate phase in all cases seem to be comparable. In other words, for a

single stage hydrate process, the CO2 content of the gas mixture can be enriched from

40% to about 86%. The values of CO2 recovery and separation efficiency for the system

CO2/H2/TBAB in our experiments as well as CO2/H2 systems (Linga et al., 2007b) and

CO2/H2/C3H8 (Kumar et al., 2009) are shown in Table 1. CO2 recovery for the tests with

TBAB is found to be 0.41 and 0.47 for 5 wt% and 10 wt% TBAB aqueous solutions,

respectively. This indicates a better capture of CO2 in the hydrate crystals formed from

10 wt% TBAB. Separation factor is also higher for the 10 wt% solution (28.0 compared

to 15.7 for the 5 wt% solution). However, it is not enough to conclude that the recovery

can be increased with increasing the TBAB concentration. It has also been reported that

with the increase of pressure, the separation efficiency decreases due to the fact that at

higher pressures more H2 molecules occupies the hydrate cages, hence reduces the

amount of CO2 that can be captured (Fan et al., 2009). Therefore, it is recommended to

carry out more experiments in the future to seek the optimum pressure as well as the

optimum TBAB concentration for these separation tests.

Although the separation efficiency of these experiments are not very different from the

results of other workers that have used alternative additives, the dissociation pressure for

TBAB semi-clathrates at a given temperature (as mentioned before) are much lower than

other systems (Li et al., 2009).

Material balance calculations were performed to calculate the difference between the

amount of initial gas and the final gas at different phases. The value of error associated

with the experiments hence was found to be less than %2 .

A series of experiments simulating two-stage capture were conducted by taking the gas

mixture coming out of hydrates in the first stage (CO2-rich stream) as feed for the second

stage. In these experiments, a mixture of CO2/H2 with a typical composition of 80.5

mole% CO2 and 19.5 mole% H2 were prepared and injected into the cell containing

TBAB aqueous solution with 5 wt% TBAB. The operating pressure and temperature

were 3.31 MPa and 276.7 K, respectively. The results of this experiment are presented in

Table 2. It can be seen that the CO2 content of the gas in hydrate phase is as high as 96

mole%. This demonstrates that with two-stage hydrate formation, CO2-rich stream could

be enriched further to more than 95 mole% of CO2 which could ultimately be

geologically sequestered or utilized for enhanced oil recovery projects. It has been

suggested to use an integrated membrane separation unit to separate H2 and CO2 from the

remaining gas mixtures above hydrate (CO2-lean stream) (Linga et al., 2007b).

4. Conclusions

In this work we used semi-clathrate hydrate formation for capturing carbon dioxide from

simulated syngas systems. The results showed that with the addition of 5 wt% TBAB, the

CO2 recovery and separation factor are not compromised compared to the system without

TBAB while the thermodynamic conditions are more favourable in the presence of

TBAB. The corresponding values for CO2 recovery and separation factor were found to

be 0.41 and 15.7, respectively. These values were 0.47 and 28 in the case of addition of

10 wt% TBAB. CO2 concentration reached 86 mole% in the hydrate phase in one stage,

while it can be enriched to 96 mole% with two stage hydrate formation and CO2 capture.

Acknowledgements

This work is part of a project supported by the UK Engineering and Physical Science

Research Council (EPSRC Grant EP/E04803X/1), which is gratefully acknowledged.

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Table 1. CO2 recovery and separation factor in hydrate formation process in different

systems

Clathrate type operating P, MPa operating T, K R S

CO2/H2 a 7.5 273.7 0.42 98.7

CO2/H2/5wt% TBAB 3.9 273.5 0.41 15.7

CO2/H2/10wt% TBAB 3.8 273.9 0.47 28.0

CO2/H2/C3H8 b 3.8 273.7 0.48 27.8

a data from Linga et. al. (Linga et al., 2007) b data from Kumar et. al. (Kumar et al., 2009)

Table 2. Results of second stage of CO2 separation

CO2 (80.5%)/H2 (19.5%)/5wt% TBAB

CO2 in Feed, mol% 80.5

CO2 in gas phase, mol% 61.6

CO2 in hydrate phase, mol% 96.0

CO2 Recovery, R 0.35

Separation factor, S 15.0

Figure 1: Schematic diagram of the experimental apparatus used in this work for CO2

capture via semi-clathrate hydrate formation

0.0000

0.0100

0.0200

0.0300

0.0400

0.0500

0.0600

0 10 20 30 40 50

Time, min

gas c

on

su

med

, m

ol

269.15

270.15

271.15

272.15

273.15

274.15

275.15

Tem

pera

ture

, K

moles of gas consumed

Temperature profile

Figure 2: Gas consumption during semi-clathrates formation in the system with 5 wt%

TBAB in the aqueous solution; : total moles of gas consumed; : changes in the cell

temperature

0.0000

0.0100

0.0200

0.0300

0.0400

0.0500

0.0600

0.0700

0.0800

0.0900

0.1000

0 10 20 30 40 50 60

Time, min

ga

s c

on

su

me

d,

mo

l

271.15

272.15

273.15

274.15

275.15

276.15

277.15

278.15

279.15

280.15

281.15

Te

mp

era

ture

, K

moles of gas consumed

Temperature profile

Figure 3: Gas consumption duing semi-clathrates formation in the system with 10 wt%

TBAB in the aqueous solution; : total moles of gas consumed; : changes in the cell

temperature

0.0000

0.0100

0.0200

0.0300

0.0400

0.0500

0.0600

0.0700

0.0800

0.0900

0.1000

0 10 20 30 40 50 60 70

time, min

ga

s c

on

su

me

d,

mo

l

TBAB 5wt%

TBAB 10wt%

Figure 4: comparison of gas uptake by semi-clathrates in the system with 5 wt % and 10

wt% TBAB in aqueous solution; : total moles of gas consumed with TBAB

concentration of 5 wt% ; : total moles of gas consumed with TBAB concentration of 10

wt%.

39.2 4038.1

40 39.9

86.5 8683.6 84

88

20.318

24.1

0

10

20

30

40

50

60

70

80

90

100

CO2/H2 CO2/H2/TBAB CO2/H2/C3H8 CO2/H2/CP CO2/H2/THF

Carb

on

dio

xid

e,

mo

le %

Feed Gas Hydrate Equilibrium Gas

Figure 5: Comparison of CO2 content in the feed and different phases after clathrate

formation in; CO2/H2: data from Linga et. al. (Linga et al., 2007); CO2/H2/TBAB: this

work; CO2/H2/C3H8: data from Kumar et. al. (Kumar et al., 2009); CO2/H2/CP: data from

Zhang et. al. (Zhang et al., 2009); CO2/H2/THF: data from Lee et. al. (Lee et al., 2009).

No reported CO2 concentration in the equilibrium gas after hydrate formation for the last

two systems.