pre-combustion capture of carbon dioxide in a fixed bed reactor using the clathrate hydrate process

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Pre-combustion capture of carbon dioxide in a fixed bed reactor using theclathrate hydrate process

Ponnivalavan Babu a, Rajnish Kumar b, Praveen Linga a,*

aDepartment of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117 576, SingaporebChemical Engineering and Process Development Division, CSIR e National Chemical Laboratory, Pune, India

a r t i c l e i n f o

Article history:Received 23 July 2012Received in revised form23 October 2012Accepted 24 October 2012Available online 3 December 2012

Keywords:Clathrate hydratesGas separationPre-combustionCarbon dioxide captureGlobal warming

a b s t r a c t

Hydrate based gas separation (HBGS) process with silica sand and silica gel as contact medium wasemployed to capture CO2 from fuel gas mixture. Gas uptake measurement at three different pressures(7.5, 8.5 and 9.0 MPa) and 274.15 K were conducted for hydrate formation kinetics and overall conversionof water to hydrate, rate of hydrate formation were determined. Water conversion of up to 36% wasachieved with silica sand bed compared to 13% conversion in the silica gel bed. Effect of driving force onthe rate of hydrate formation and gas consumption was significant in silica sand bed whereas it wasfound to be insignificant in silica gel bed. Hydrate dissociation experiments by thermal stimulation (atconstant pressure) alone and a combination of depressurization and thermal stimulation were carriedout for complete recovery of the hydrated gas. A driving force of 23 K was found to be sufficient torecover all the hydrated gas within 1 h. This study indicates that silica sand can be an effective porousmedia for separation of CO2 from fuel gas when compared to silica gel.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon dioxide emission from combustion of fossil fuelsparticularly power plants, is the major contribution to globalwarming and climate change. According to intergovernmentalpanel on climate change (IPCC), the unhindered release ofanthropogenic carbon dioxide to the atmosphere would lead toglobal warming, resulting in severe weather conditions anddamage to the ecosystem [1]. In order to reduce these environ-mental concerns, anthropogenic carbon dioxide emission has to beminimized. One such approach to reduce carbon dioxide emissionfrom a coal based power plants is to capture CO2 prior tocombustion. Such an approach is termed as “pre-combustioncapture” of CO2. Producing hydrogen through gasification of fossilfuel is an essential element of an integrated gasification combinedcycle (IGCC) power station. A pre-treated fuel gas employed for pre-combustion carbon dioxide capture is essentially a mixture ofcarbon dioxide/hydrogen mixture [1e3]. Various methods such asabsorption, adsorption, membrane separation and cryogenicdistillation are used for the separation of CO2 from the fuel gas.While absorption is an established technology, it is still not prac-tical due to the higher energy penalty during solvent regeneration.Hydrate based separation (HBS) process is one of the new

approaches for CO2 separation from fuel gas through gas hydrateformation [2,4e9]. The basis for separation is the selective partitionof CO2 component of a fuel gas mixture between the hydrate phaseand the gaseous phase upon hydrate formation [6,10].

Linga et al. [6] presented a two-stage hydrate/membrane processfor separating CO2 from a fuel gas mixture using a stirred tankreactor. The proposed process operates at 7.5 MPa and 3.8 MParespectively in two stages to produce a 99% rich stream of carbondioxide. In an attempt to lower the operating pressure of the firststage, several researchers have investigated the effect of additives onthe process efficiency and separation efficiency [11e14].The addi-tives however reduce the rate of hydrate formation [10,14]. Kumaret al. [15] investigated the phase behavior of the fuel gas with theinclusion of propane as an additive and reported that there was anequilibrium shift of about 50% to the lower side when 3.1% propanewas added. Kumar et al. [14] reported a two-stage medium pressurehydrate process with the addition of 2.5% propane as an additive forpre-combustion capture. While the addition of propane as anadditive did not compromise the CO2 recovery, the rate of hydrateformation and total gas consumption were significantly lowercompared to the processwithout additive. Lee et al. [11] investigatedthe effect of tetrahydrofuran (THF) as an additive on the fuel gasseparation using the clathrate process. They have also reported thatthe addition of THF significantly decreases the rate of hydrateformation while it does not compromise the CO2 recovery andseparation factor. An optimum concentration of 1.0 mol% THF was

* Corresponding author. Tel.: þ65 6601 1487; fax: þ65 6779 1936.E-mail address: [email protected] (P. Linga).

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Energy 50 (2013) 364e373


reported to give the best performance for the fuel gasmixturewhichwas also found to be the optimum concentration for the flue gasseparation in an earlier study [16]. Kim et al. [17] investigated theeffect of tetra-n-butyl ammonium bromide (TBAB) on the thermo-dynamics and kinetics of hydrate formation on a fuel gas mixture.Even though, the phase equilibrium shifted to a lower boundarywith the addition of TBAB, the addition of TBAB resulted in lower gasconsumption and significantly reduced the CO2 recovery (or splitfraction)which is important for the separationprocess. CO2 recoverywas reported to be between 0.11 and 0.24. In addition, the gasconsumption for hydrate formation in the presence of 1.0mol% TBABwas also found to be 50% lower compared to the presence of 1.0 mol% THF at comparable experimental pressures [11,17]. Similar resultson lowgas uptake rates and gas consumptionwas also reported by Liet al. [12] while employing TBAB as an additive for CO2 capture fromfuel gas employing the clathrate hydrate process. It is noted thatwhile the search for additives to reduce the operating conditions isan ongoing effort, onemajor challenge is that the additives reportedso far tend to reduce the hydrate formation rates, compromise CO2recovery and the process efficiency when employed for fuel gasseparation of CO2 using the clathrate process.

It is noted that all the above-literature works were based onlaboratory-scale data employing a stirred tank reactor. It is wellknown that when employing stirred vessels for hydrate formation,agglomeration of hydrate crystals creates a barrier at the gas/liquidinterface to efficient gas/water contact [18]. As a result of thisbarrier, the rate of crystallization decreases and the conversion ofwater and gas to hydrate are limited. Typical water to hydrateconversions in such stirred vessels is about 5e10%. Mori [19]concluded that there is a need to develop hydrate forming reac-tors with improved/enhanced hydrate forming efficiencies. Hencethere is an ongoing interest to find the best multi-phase reactor forthe gas hydrate applications. To overcome this gas/water contactlimitation, Linga et al. [20] employed a gas inducing impeller ina large scale demonstration of the clathrate process for post andpre-combustion capture of carbon dioxide. While an enhancementin the rate of hydrate formation, gas consumption and conversionof water to hydrate were reported, the study concluded that thecost associated with stirring would be very significant thusrendering the process not economical for CO2 capture [20].

One possible approach to enhance the kinetics of hydrateformation is to employ a fixed bed column with silica gel asa medium for employing the clathrate process for CO2 capture[21,22]. An advantage of using a fixed bed column for gas separationapplications of gas hydrates is that there is no need for powerconsumption for stirring and it provides a large surface contact areabetween gas and liquid. Adeyemo et al. [22] studied the effect ofpore diameter and particle size using three different gels for flue gasseparation and reported that the gel with pore diameter of 100 nmperformed better in terms of CO2 recovery and gas consumption.Recently, Linga et al. [23] evaluated the performance of a fixed bedcolumn employing silica sand as a medium and compared theperformance with a stirred tank reactor. A considerable enhance-ment in the rate of hydrate formation and conversion of water tohydrates in a silica sand column were reported compared to a stir-red vessel. Silica sand is abundantly available and is very cheap to beemployed for such large scale separation applications.

The objective of the present study is to investigate the effec-tiveness of silica sand and silica gel as medium in a fixed bedcolumn for the separation of CO2 fromCO2/H2 (fuel gas) gasmixturethrough hydrate crystallization. The effect of driving force on rate ofhydrate formation and the conversion of water to hydrates areevaluated. The dissociation of the hydrates is an important step inemploying the clathrate process for gas separation applications[5,6]. It is noted that the kinetics of hydrate dissociation from

hydrates formed from a fuel gas mixture has not been investigatedso far in the literature. In this study, the dissociation behavior of theformed hydrates is investigated by thermal stimulation methodand its implications on process design are discussed.

2. Experimental section

2.1. Materials

The gas mixture employed was CO2/H2 mixture containing 40%by mole CO2 corresponding to a typical composition of a fuel gasmixture from an integrated coal gasification cycle and was suppliedby Soxal Private Limited. Silica sand supplied by SigmaeAldrichwas used. It is noted that the sand used in this study supplied bySigma Aldrich was similar to the sand used by Linga et al. [23e25]and Haligva et al. [26]. Spherical Silica gel particles with porediameter of 100 nm and particle size distribution of 75e200 mmpurchased from Silicycle was used. Deionized and distilled waterwas used for the experiments.

2.2. Apparatus

A new experimental facility was developed for this work.Fig. 1 shows the schematic of the new experimental apparatus. Itconsists of a crystallizer (CR) which is a cylindrical vessel (Internaldiameter ¼ 10.2 cm, Height ¼ 15.2 cm) made up of 316 stainlesssteel. It has a volume of 1240 cm3. The crystallizer is immersed ina temperature controlled water bath. The temperature of the waterbath is controlled by an external refrigerator/chiller (PolyScience).Two Rosemount smart pressure transducers, model 3051S (Emer-son Process Management, Singapore) are employed for pressuremeasurement with a maximum uncertainty of 0.1% of the span (0e20,000 kPa). The temperature of the hydrate phase and the gasphase of the crystallizer is measured using Omega copper-constantan thermocouples with an uncertainty of 0.1 K. Seventhermocouples are located in the crystallizer with one in gas phaseand six in the silica sand bed. The various locations of the seventhermocouples placed in the crystallizer are shown in Fig. 2.Hydrate formation experiments are conducted in a semi-batchmanner at constant pressure and temperature. The gas fromvessel reservoir (R) is supplied continuously at constant pressure tothe reactor containing a fixed amount of water. A Control valve(Fisher Baumann) coupled with proportional–integral–derivativecontroller enables carrying out experiment at constant pressure.The data acquisition system (National Instruments) is coupled witha computer to record the data as well as to communicate with thecontrol valve during the experiment and the software used for thispurpose is Labview 2010 (National Instruments). The apparatus isalso equipped with a safety pressure relief valve.

2.3. Experimental procedure

2.3.1. Preparation of silica sand bedThe amount of silica sand placed in the crystallizer is 645.16 g

(Height of Silica bed ¼ 5 cm). The volume of water required to fillthe void space with water was 0.217 cm3/g, which is the interstitial,or pore volume of the bed of sand particles. 140 ml of water wasadded into the sand. The bed was setup by splitting the requiredamount of sand and water into five equal parts and placing each ina batch order to form a uniform bed and also to eliminate thepresence of any air pocket [24].

2.3.2. Preparation of silica gel bedDetailed procedure for gel preparation is given elsewhere in the

literature [22]. Briefly, the procedure is as follows. Silica gel was

P. Babu et al. / Energy 50 (2013) 364e373 365


first dried at 373 K for 24 h and then weighed to determine the dryweight of gel. 140 ml of water was added to 168.7 g of gel to obtainsaturated silica gel. The gel was then placed in centrifuge and spunat 3000 rpm for 3 min to aid dispersion of water. The crystallizerwas now charged with the silica gel saturated with water.

2.3.3. Hydrate formation procedureOnce the crystallizer bed was setup either with silica sand or

silica gel (explained in the previous sections), the thermocoupleswere positioned and the crystallizer was closed. The crystallizerwas pressurized with CO2/H2 gas mixture and depressurized toatmospheric pressure three times in order to eliminate the pres-ence of any air bubble in the system. The reactor was then pres-surized to the desired pressure and temperature was allowed toreach the desired value. This time was recorded as time zero forformation experiments and temperature and pressure data wasrecorded for every 20 s. As hydrate forms in the crystallizer, there

will be a pressure drop in the crystallizer due to gas consumptionfor hydrate formation. The pressure in the crystallizer was main-tained constant by a PID controller coupled with a computer thatallows the necessary gas to flow from the reservoir to the crystal-lizer. The pressure data of the reservoir was also recorded for every20 s. The experiment was allowed to continue until no significantchange in the reservoir pressure was observed.

2.3.3.1. Calculation of the amount of gas consumed during hydrateformation. At any time, the number of moles of the gas that hasbeen consumed for hydrate formation is the difference between thenumber of moles of gas at time ¼ 0 and the number of moles of gasat time t present in the reservoir (R). Equation (1) is used tocalculate the moles of gas consumed

�DnH;Y

�t ¼ VR

�P

zRT

�0� VR

�P

zRT

�t

(1)

R

PR PCR

GC

DAQ

Thermocouples

PC

Gas

V5

V1

V3

V4

V2

cv

CR CrystallizerR ReservoirCV Control ValvePC PC & ControllerDAQ Data Acquisition SystemPCR & PR Pressure TransmitterER External RefrigeratorGC Gas ChromatographySPV Safety Pressure ValvePG Pressure Gauge

vent

ventV6

SPV

CR

ER

PG PG

V7

V8

Fig. 1. Schematic of the apparatus for hydrate formation and decomposition.

P. Babu et al. / Energy 50 (2013) 364e373366


where the compressibility factor, z, is calculated by Pitzer’s corre-lations [27], VR is the volume of the reservoir, P and T are thepressure and temperature of the reservoir.

2.3.3.2. Conversion of water to hydrates. Conversion of water tohydrate is determined by using the following equation:

Conversion of water hydrates ð%Þ

¼ DnH;Y � hydration numbernH2O

� 100 (2)

where DnH;Y is the number of moles of gas consumed for hydrateformation at the end of the experiment determined from the gas

uptake and nH2O is the total number of moles of water in thesystem. The hydration number is the number of water moleculesper guest molecule. The hydration number used in the aboveequation is 7.09 [28].

2.3.3.3. Calculation of average rate of hydrate formation. The rate ofhydrate formation is calculated through the forward differencemethod given below

�dDnH;Y

dt

�t¼

�DnH;Y

�tþDt �

�DnH;Y

�t

Dt; Dt ¼ 5 min (3)

The average of these rates were calculated for every 30 min andreported.

2.4. Hydrate decomposition procedure

Decomposition experiments were carried out at a constantexperimental pressure. After the completion of the formationexperiment, the hydrates were allowed to decompose by heatingthe system from 274.15 K to two different temperatures of 285.15 Kand 297.15 K respectively. The hydrate crystals start to decomposeonce the temperature crosses the equilibrium phase boundaryconditions resulting in instantaneous pressure rise in the crystal-lizer. However, the crystallizer pressure was kept constant by col-lecting the decomposed gas in a reservoir by using a control valvecoupled with a PID controller. The rise in the pressure of thereservoir was recorded every 20 s. The expansion of gas due toincrease in temperature was calculated by conducting a controlexperiment with no-hydrate formation [29]. The procedure for thecontrol experiment was as follows: crystallizer bed (with water inthe porous medium) was setup and thermocouples were con-nected. The crystallizer was cooled to the experimental tempera-ture (274.15 K). The crystallizer was now pressurized to theexperimental dissociation pressure. Once the temperature stabi-lized (about 5 min), the crystallizer was heated from 274.15 K to thedesired experimental temperature (285.15 K or 297.15 K). Expan-sion of gas in the crystallizer at higher temperature was recordedfrom the pressure rise in the reservoir every 20 s. The differencebetween the hydrate experiments and the control (no-hydrate)experiment corresponds to the gas released due to decompositionof the hydrates.

2.4.1. Calculation of the amount of gas recovered during hydrateformation

At any time, the number of moles of the gas that have beenreleased/recovered from hydrate dissociation is the differencebetween number of moles of gas at time t and the number of molesof gas at time zero present in the reservoir (R). Equation (1) is usedto calculate the moles of gas recovered.

�DnH;[

�t ¼ VR

�P

zRT

�t� VR

�P

zRT

�0

(4)

where the compressibility factor, z, is calculated by Pitzer’scorrelations [27], VR is the volume of the reservoir, P and T are thepressure and temperature of the reservoir.

The percent gas recovery is calculated as a function of time forany given decomposition experiment based on the informationobtained from its formation experiments, and is calculated by thefollowing equation:

Gas recovery ð%Þ ¼�DnH;[

�t�

DnH;Y�tend

� 100 (5)

Fig. 2. Cross section and top view of the crystallizer showing the location of ther-mocouples within the crystallizer.



where ðDnH;YÞtend is the number of moles consumed for hydrateformation at the end of a typical hydrate formation experiment andðDnH;[Þt is the number of moles released from hydrate duringhydrate decomposition at any given time.

The gas recovery curves are presented as normalized recoverycurves.

Normalized gas recovery ¼�Dn[

�t�

DnH;[�tend

(6)

where ðDnH;[Þt is the number of moles of gas released from hydrateduring hydrate decomposition at any given time and ðDnH;[Þtend isthe number of moles of gas released at the end of the hydratedissociation experiment.

3. Results and discussion

3.1. Hydrate formation

The minimum pressure required to form hydrate crystals fromthe fuel gas consisting of 40% CO2/60% H2 was found to be 5.56 MPaat 273.9 K [15]. It was decided to employ liquid water for theseparation process and hence kinetic experiments were conductedat 274.15 K and three different pressures (7.5, 8.5 and 9 MPa).Table 1 summarizes the hydrate formation experimental conditionsand results, indicating induction time, moles of gas consumed andwater conversion achieved.

3.1.1. Silica sand experimentsTypical gas uptake measurement curve along with temperature

profiles of the thermocouples located in the silica sand bed isshown in Fig. 3. It is noted that the general characteristics of the gasuptake curve resembles the one obtained in a stirred tank reactor[10,30]. There are three stages during the hydrate formation. Firstly,the gas diffuses into the water present between the inter-particlespaces of the sand bed. This step can be clearly seen in figure (gasuptake from 0 to 9 h). The second step is the super saturation andnucleation step. This step involves the formation of a stable nucleior critical nuclei and is characterized by a sudden increase intemperature due to the exothermic nature of hydrate formation.There is also a sudden increase in the gas consumption along withthe temperature increase (15.43 h in Fig. 3). The final step is thehydrate growth, where hydrate formation continues to occur in thebed and is seen in Fig. 3 as gas uptake curve after 15.43 h. Theexpanded graph in Fig. 3 shows the nucleation point and thesubsequent hydrate growth. It can be seen in the expanded graph

that all the thermocouples show increase in temperature at thesame time indicating that the nucleation step is uniform and theextent of temperature increase at a particular location is propor-tional to the extent of hydrates formed in its vicinity. It is noted thatthermocouples T1, T2, T3 and T4 are all located at the same radialdistance from the reactor wall (see Fig. 2). Since the crystallizeris immersed in a constant temperature controlled bath, thetemperature of the bed is gradually restored to 274.15 K.Throughout the experiment, it can be seen that only one nucleationevent was observed. This behavior was observed for all the exper-iments performed in the presence of silica sand in this study.

It is noted that the fuel gas mixture (CO2/H2) used in this studyforms structure I [28]. It is interesting to note that the

Table 1Experimental conditions along with measured induction times and moles of gas consumed at 274.15 K.

System Exp. no. PressurePexp (MPa)

Inductiontime (min)

End of experiment Water conversionto hydrate (mol %)a

Time (h) Gas consumed (mol/mol of H2O)

CO2/H2/water/silica sand S1 9.0 0.3 40 0.0404 28.65S2 9.0 18.0 40 0.0504 35.75S3 9.0 2.3 40 0.0389 27.34S4 8.5 928.0 40 0.0424 30.09S5 8.5 439.7 40 0.0370 26.22S6 8.5 619.3 40 0.0362 25.70S7 7.5 1413.0 40 0.0231 16.35S8 7.5 511.0 40 0.0196 13.92

CO2/H2/water/silica gel G1 9.0 72.0 40 0.0185 13.04G2 9.0 1041.0 40 0.0169 11.96G3 8.5 618.3 40 0.0166 11.77G4 8.5 1.33 40 0.0140 9.88G5 7.5 14.0 40 0.0187 13.30G6 7.5 Did not nucleate for 48 h

a Hydration number of 7.09 was used for calculation [28].

Time (hr)0 10 20 30 40

Tem

pera

ture

(K)

274

275

276

277

278

279

280

Gas

upt

ake

(mol

of g

as/m

ol o

f wat

er)

0.00

0.01

0.02

0.03

0.04

T1 T2 T3 T4 T5 T6 Gas uptake

Time (hr)14 15 16 17

Tem

pera

ture

(K)

274

275

276

277

278

Gas

upt

ake

(mol

of g

as/m

ol o

f wat

er)

0.004

0.006

0.008

0.010

0.012

0.014

0.016T1 T2 T3 T4 T5 T6 Gas uptake

Fig. 3. Typical gas uptake measurement curve together with the temperature profile at8.5 MPa and 274.15 K (Experiment S4).



characteristics of hydrate formation in silica sand are dependent onthe type of guest gas involved in the formation of hydrates. Theobservations reported in the literature [24,26] for methane hydrateformation in silica sand (same sand used in this study) wascompletely different to what was observed for mixed hydrateformation of CO2/H2 in this study even though both studies were onstructure I hydrate formation. Linga et al. [24] and Haligva et al. [26]observed multiple nucleation events within the bed occurring atdifferent times and the hydrate growth was observed to happen infits and starts. This observation was recently independentlyconfirmed by a morphology study done by Jin et al. [31] in whichthey reported that methane hydrate growth in porous media wasmulti staged. For all the experiments conducted on the fuel gasmixture in this study we observed only one nucleation event.

Fig. 4 shows the effect of driving force on the hydrate growthmeasured at the three different pressures experimented at 274.15 Kin silica sand bed for first 4 h after nucleation. The gas uptakeimmediately after the nucleation or induction point (for about10 min) seems to be same for all the three experimental pressures.After 10 min, higher growth rates were observed for the experi-ment conducted at 9.0 MPa followed by 8.5 MPa, and 7.5 MParespectively. As can be seen in the figure, after 4 h of hydrategrowth, the gas consumption for 9.0 MPa experiment was 154.3%higher than the experiment conducted at 8.5 MPa and 205.7%higher than the experiment conducted at 7.5 MPa.

Fig. 5 shows the average rate of hydrate growth for all experi-ments conducted at different pressures and 274.15 K in silica sandbed. It can be seen that the rate of hydrate formation is faster as thedriving force increases and hence resulting in higher water tohydrate conversion (see Table 1). The initial rate of hydrate growthat different experimental pressures are more or less samewhile thesecond average rates and there on there is a distinct difference withrate of hydrate growth following the order 9.0 MPa > 8.5 MPa> 7.5 MPa.

3.1.2. Silica gel experimentsFig. 6 shows the gas uptake measurement curve of a formation

experiment carried out at 9.0 MPa along with the temperatureprofiles of the thermocouples located in the silica gel bed. The dataduring the first 4 h after nucleation is shown in the expandedgraph. The gas consumption profile shows two stages: nucleationand hydrate growth stage. For all the experiments conducted withsilica gel there was no dissolution phase. This is due to the fact thatall the water taken for the experiment is present in the pores of thesilica gels and we believe that the nucleation is happening at thegas/water/pore contact. Whereas for the case of hydrate formation

in silica sand, we could observe a clear dissolution phase sincewater is present between the interstitial spaces of the sand parti-cles (see Fig. 3). It can also be seen from Fig. 6 that the hydrateformation in silica gel column slows down after 3 h and reachesa plateau, which results in low gas consumption and waterconversion to hydrates (see Table 1).

Fig. 7 shows the hydrate growth curves for first 4 h afternucleation for the experiments conducted in silica gel bed atdifferent experimental pressures of 7.5, 8.5 and 9.0 MPa. It is

Time (hr)0 1 2 3 4

Gas

upt

ake

(mol

of g

as/m

ol o

f wat

er)

0.000

0.005

0.010

0.015

0.020

0.025

Wat

er c

onve

rsio

n to

hyd

rate

s (%

)

0

2

4

6

8

10

12

14

16

7.5MPa (Exp S8 )8.5MPa (Exp S5) 9.0MPa (Exp S2)

Fig. 4. Effect of driving force on the gas uptake curve in silica sand bed. Time zerocorresponds to the induction time for the experiment.

Time (hr)

0 2 4 6 8 10

Aver

age

rate

of h

ydra

te fo

rmat

ion

(mol

of g

as/m

ol o

f wat

er/h

r)

0.000

0.005

0.010

0.015

0.020

9.0 MPa8.5 MPa7.5 MPa

0.0 0.5 1.0 1.5 2.00.000

0.005

0.010

0.015

0.020

Fig. 5. Comparison of rate of hydrate formation at different pressure and 274.15 K insilica sand bed. Time zero corresponds to the induction time of the experiments.

Time (hr)0 10 20 30 40

Tem

pera

ture

(K)

273

274

275

276

277

278

279

280

Gas

upt

ake

(mol

of g

as/m

ol o

f wat

er)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

T1 T2 T3 Gas uptake

Time (hr)16 17 18 19 20

Tem

pera

ture

(K)

273

274

275

276

277

278

279

280

Gas

upt

ake

(mol

of g

as/m

ol o

f wat

er)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

T1 T2 T3 Gas uptake

Fig. 6. Typical gas uptake measurement curve together with the temperature profile at9.0 MPa and 274.15 K (Experiment G2).



interesting to note here that the experimental pressure (or drivingforce) has no effect on the hydrate growth. This was observed for allthe other experiments as well but is not shown here. This is animportant observation and is in complete contrast to what wasobserved for the experiments conducted in the presence of silicasand in this work and in the literature for fuel gas mixture con-ducted in bulk water [10]. It has been observed in the literature thatrate of hydrate growth is a function of pore diameter, which isa significant parameter for hydrate growth studies in silica gelhaving small pores [32]. When using silica gel as a medium for gas/water contact for hydrate formation, availability of water in thepores and diffusion of the hydrate forming gases into the poresplays an important role. For a completely water saturated silica gelbed most of the water is present inside the pores of the gels. Hencethe gas is readily available at the outer wall of the gel forconsumption during hydrate growth. It is important to note that insuch cases evenwith increasing the driving force, availability of gasat higher pressures may possibly result in similar growth rates andquite low water to hydrate conversion.

The average rate of hydrate formation for all experiments con-ducted in silica gel bed is shown in Fig. 8. The expanded graphshows the rate of formation for the first 2 h after nucleation. Initialrate of hydrate formation is faster and later it slows down. Similarto the gas uptake measurement curve, there is no significant effectof driving force on the rate of hydrate formation.

3.1.3. DiscussionFig. 9 shows the gas uptake measurement curve for the exper-

iment conducted at 9.0 MPa and 274.15 K in silica sand bed and insilica gel bed. It can be seen that initially, the moles of gasconsumed is same between the sand and the silica gel experiments.Thewater conversion to hydrate (%) is also presented in Fig. 9 as thesecondary y-axis. The gas uptake in the silica gel bed reachesa plateau after 3 h indicating no-hydrate formation whereas in thesilica sand bed hydrate formation continues to happen and henceresulting in a higher conversion of water to hydrates. As can be seenin the figure, the conversion of water to hydrates for the silica gelbed is 12.0% whereas for the sand bed it is 22.0% after 4 h of hydrateformation.

Diffusion rates of gases through porous materials depends onthe properties of the medium such as porosity, pore size and poreconnections [33]. In silica gel, internal pore space is the dominantcontributor to bed porosity with inter-particle spaces playinga lesser role whereas in silica sand inter-particle space is thedominant contributor to bed porosity. We believe that the resis-tance for diffusion of gas molecules into internal pores spaces isgreater than that of diffusion of gas molecules into inter-particlespaces. In silica sand, water occupies the inter-particle space.Hydrates are formed between the inter-particle spaces. As thehydrate grows, water is transferred from the vicinity by capillaryaction to the hydrate crystals for further growth, thereby providingtortuous pathways for the gas molecules inside the bed.

In silica gel, water occupies the internal pore spaces. We believethat the nucleation occurs at gaseliquidesolid interface i.e. thepore walls. Hydrate formation at the pore walls blocks the poresand hence hinders further gas diffusion into the pores for hydrateformation. Hence the gas uptake curve reaches a plateau after fewhours (see Figs. 6 and 7) from nucleation and thereby resulting inlow water to hydrate conversions. It can also be seen from Fig. 3that the temperature increase due to heat released by theexothermic reaction at nucleation point is from 274.15 to 277.15 K inthe presence of silica sand. For the case of silica gel (Fig. 6), thetemperature increase at nucleation point is from 274.15 to 279.15 K.There are two possible reasons for this, one is the fact that thetemperature increase can be related to the extent of hydrateformation and the other possibility is the heat transfer effects. Thefirst case cannot be true as the gas consumption initially afternucleation in the both the beds aremore or less same (see extendedgraphs in Figs. 3 and 6). This shows that the heat removal in a silicagel bed is slow compared to the silica sand bed. It is also noted thatthe temperature is brought back to the experimented control

Time (hr)0 1 2 3 4 5

Gas

upt

ake

(mol

es o

f gas

/mol

e of

wat

er)

0.000

0.005

0.010

0.015

0.020

Wat

er c

onve

rsio

n to

hyd

rate

s (%

)

0

2

4

6

8

10

12

14

9.0MPa (Exp G2) 8.5MPa (Exp G3)7.5MPa (Exp G5)

Fig. 7. Effect of driving force on the gas uptake curve in silica gel bed. Time zerocorresponds to the induction time for the experiment.

Time (hr)

0 2 4 6 8 10

Aver

age

rate

of h

ydra

te fo

rmat

ion

(mol

of g

as/m

ol o

f wat

er/h

r)

0.000

0.005

0.010

0.015

0.020

0.025

9.0 MPa8.5 MPa7.5 MPa

0.0 0.5 1.0 1.5 2.00.000

0.005

0.010

0.015

0.020

0.025

Fig. 8. Comparison of rate of hydrate formation at different pressure and 274.15 K insilica gel bed. Time zero corresponds to the induction time for the experiment.

Time (hr)

0 2 4 6 8 10

Gas

upt

ake

(mol

of g

as/m

ol o

f wat

er)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

Wat

er c

onve

rsio

n to

hyd

rate

s (%

)

0

5

10

15

20

Silica Sand (Exp S2) Silica gel (Exp G2)

Fig. 9. Gas uptake measurement curve at 9.0 MPa and 274.15 K in silica sand and silicagel bed. Time zero corresponds to the induction time for the experiment.



temperature of 274.15 within 1 h (see extended graph in Fig. 3) inthe silica sand bed whereas it takes about 2 h to reach 274.15 in thesilica gel column (see extended graph in Fig. 6).

3.2. Hydrate decomposition

Gas recovery curve along with temperature profile of the ther-mocouples located in the crystallizer for decomposition experi-ments in silica sand bed at 297.15 K is shown in Fig. 10. Since eachformation experiment had different amounts of gas consumptionfor hydrate formation, the recovery curves are presented asnormalized curves (Equation (5)). It can be clearly seen in the figurethat the temperature profile of the thermocouples located in thebed deviates from that of the gas phase and water bath. This ismainly due to endothermic nature of the hydrate decomposition.The gas recovery curve reaches a plateau at around 1.5 h whenheated from 274.15 K to 297.15 K. Gas recovery curve when heatedfrom 274.15 to 284.15 for silica sand bed is shown in Fig. 11. It isnoted from the figure that when heated from 274.15 to 284.15 K, ittakes about 13 h for the gas recovery curve to reach a steady state.The recovery curves for the silica gel experiments exhibited similardissociation characteristics. Fig. 12 shows the comparison ofnormalized gas recovery curves for DT of 23 K obtained from thesilica sand and silica gel experiments after a dissociation time of10 h. As can be seen in the figure, the dissociation behavior of the

hydrates formed in the gel and sand exhibited similar dissociationcharacteristics based on the gas recovery curves.

It is important to note that when the hydrate samplewas heatedfor dissociation at a constant pressure, the gas released wascollected in the reservoir. This gas released/recovered is not entirelydue to hydrate dissociation and also includes the gas released dueto thermal expansion of the gas present in the crystallizer duringheating. This gas release due to thermal expansion is significant andneeds to be accounted in order to determine the exact amount ofgas released due to hydrate decomposition. Hence control experi-ments were conducted (description given in experimental section).Fig. 13 shows the gas recovery curves for the gas released from thehydrate dissociation experiment and the control experiment. nHE(red solid line inweb version) is the typical recovery curve obtainedfrom the dissociation experiment. nNHE (blue dashed line) is therecovery curve of the gas released due to thermal expansion duringheating (control experiment performed without hydrates). nHEenNHE (black dotted line) is the gas recovery curve due to hydratedissociation. As can be clearly seen in the figure, the gas releaseddue to thermal expansion during thermal stimulation is significant

Time (hr)0 2 4 6 8

Tem

pera

ture

(K)

275

280

285

290

295

Nor

mal

ised

Gas

Rec

over

y

0.0

0.2

0.4

0.6

0.8

1.0

BathT1T2T3TgasGas recovered

Fig. 10. Normalized gas recovery profile along with temperature profile for decom-position in silica sand bed at 297.15 K (Experiment S1).

Time (hr)0 5 10 15 20

Tem

pera

ture

(K)

274

276

278

280

282

284

286

Nor

mal

ised

Gas

Rec

over

y

0.0

0.2

0.4

0.6

0.8

1.0

Tbath T1 T3 T4 Tgas Gas recovered

Fig. 11. Normalized gas recovery profile along with temperature profile for decom-position in silica sand bed at 284.15 K (Experiment S2).

Time (hr)0 2 4 6 8 10

Nor

mal

ized

gas

reco

very

0.0

0.2

0.4

0.6

0.8

1.0

S1 (9.0 MPa & Δ T = 23K) G2 (9.0 MPa & Δ T = 23K)

Fig. 12. Normalized gas recovery curves for hydrate decomposition in silica gel andsilica sand bed. The results are normalized at the dissociation time of 10 h for both theexperiments.

Time (hr)0 1 2 3 4

Gas

Rec

over

y (m

ol)

0.00

0.05

0.10

0.15

0.20

nHEnNHEnHE-nNHE

Fig. 13. Gas recovery curves for the control experiment (NHE), hydrate experiment(HE, hydrate dissociation and thermal expansion), and the calculated gas recoverycurve due to decomposition (HEeNHE) for Experiment S7.



and hence need to be accounted for when thermal stimulationapproach is employed for hydrate dissociation. The temperatureprofile of T1 for the control experiment and for a dissociationexperiment is shown in Fig. 14. The heat consumed for hydratedissociation can be clearly seen by the deviation of the temperatureprofile of the hydrate dissociation experiment from that of thecontrol experiment.

Table 2 summarizes the hydrate decomposition experimentalconditions along with the amount of gas recovered from thedecomposing hydrates after compensating for the gas released dueto thermal expansion. The experiment number listed in Table 2corresponds to the formation experiment number listed inTable 1. As it can be seen the gas recovered from the sand and gelexperiments was low in the range of 0.0398e0.1596 mol for thedissociation experiments conducted at 7.5, 8.5, and 9.0 MParespectively for the driving force (DT) of 10 and 23 K. In terms ofpercent recovery, for the dissociation experiments conducted insand at 7.5, 8.5 and 9.0 MPa respectively, the recovery was about22.2e34.3% for DT of 10 K. For DT of 23 K, the final recovery at9.0 MPa was 50.8%. For the case of silica gel, the percent recoverywas about 63.5e74.8% for DT of 10 K and at experimentalpressures of 7.5, 8.5 and 9.0 MPa respectively. For DT of 23 K, thefinal recovery at 9.0 MPa was 86.7%. The percent recovery forsilica sand are on the lower side compared to silica gelexperiments due to the fact that the gas consumption for hydrateformation was significantly higher for the sand experimentscompared to the gel formation experiments (see Table 1).

Based on the gas recovery data (Table 2), it is clear that disso-ciation of hydrate crystals at 9.0, 8.5 and 7.5 MPa respectively in the

sand/gel bed (as can be seen from the gas recovery % shown inTable 2) is not complete. A possible reason for this is the fact thatthe hydrate sample formed from a fuel gas mixture is enriched inCO2 to about 83e95% [6,22,34]. The equilibrium hydrate formationconditions for this enriched mixture is significantly lowercompared to the fuel gas mixture (40%CO2/60%H2) [15]. Hence weconducted a dissociation experiment at a reduced pressure (lowerpressure) of 2.7 MPa with a thermal stimulation (heating) of 23 K.After the pressure reduction from 9.0 to 2.7 MPa, the hydratecrystals were dissociated by heating the crystallizer from 274.15 Kto 297.15 K. Gas recovery profiles for the dissociation experimentand the control experiment are shown in Fig. 15. 99.6% of the gaswas recovered as can be seen in Table 2. The implications of thisapproach is that when employing the clathrate process for gasseparation, a combination of pressure reduction and thermalstimulation (which would be the preferred mode as waste heat orwater at room temperature can be employed for hydrate dissocia-tion) of DTof 23 K is sufficient to completely recover gas captured inhydrates. Complete recovery (99.6%) was achieved in less than 1 h.It is noted that the recovery time will shorten further as the drivingforce for thermal stimulation is increased.

4. Conclusion

Experiments were conducted at three different pressures and274.15 K in Silica sand bed and Silica gel bed separately. Waterconversion up to 36% was achieved at 9.0 MPa and 274.15 K in silicasand bed. In silica gel bed, water conversionwas low andmaximumof 13.30% was achieved. In Silica sand bed, effect of driving force onthe gas uptake was profound/significant whereas in silica gel, itshowed little difference at all formation conditions. Gas uptakemeasurement curves in silica gel bed showed increase in gas uptakeat nucleation but reached a plateau after 3 h from nucleation point.Hydrate dissociation experiments were conducted at the formationexperimental pressures for two driving forces of DT of 10 and 23 Krespectively. Only partial recovery was achieved when dissociatedat the experimental formation pressure. Hence a combination ofdepressurization and thermal stimulation was employed for thecomplete dissociation of the hydrates. A driving force of 23 K wasfound to be sufficient to recover all the gas separated as hydrateswithin 1 h. From the results, it can be concluded that silica sand canbe an effective porous media for the separation of CO2 from a fuelgas mixture in a fixed bed setup.

Time (hr)

0 1 2 3

Tem

pera

ture

(K)

272

274

276

278

280

282

284

286

T1 (HE)T1 (NHE)

Fig. 14. Temperature profile for the no-hydrate experiment (NHE) and the hydrateexperiment (HE) for Experiment S7.

Table 2Hydrate decomposition experiments along with percent recovery.

System Exp.no.

Pressure(MPa)

DT No of moles of gasrecovered (mol)

Recovery (%)

CO2/H2/water/silica sand

S1 9.0 23 0.1596 50.8S2 9.0 10 0.1065 27.2S3 2.7 23 0.3015 99.6S4 8.5 10 0.0911 27.6S5 8.5 10 0.0986 34.3S7 7.5 10 0.0398 22.2

CO2/H2/water/silica gel

G1 9.0 10 0.1017 70.7G2 9.0 23 0.1139 86.7G3 8.5 10 0.1001 74.8G5 7.5 10 0.0923 63.5

Time (hr)0 1 2 3 4 5 6

Gas

Rec

over

y (m

ol)

0.0

0.1

0.2

0.3

0.4

nHEnNHEnHE-nNHE

Fig. 15. Gas recovery curves for the control experiment (NHE), hydrate experiment(HE, hydrate dissociation and thermal expansion), and the calculated gas recoverycurve due to decomposition (HEeNHE) for Experiment S3.



Acknowledgment

The financial support from the Ministry of Education’s AcRF Tier1 (R-279-000-317-133) is greatly appreciated. Rajnish Kumar wantsto thank the Council of Scientific and Industrial Research (CSIR) forthe financial support.

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pre-combustion capture of carbon dioxide in a fixed bed reactor using the clathrate hydrate process

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