Indian Journal of Chemical Technology Vol. 26, September 2019, pp. 411-417

Polyamine-promoted aqueous DEEA for CO2 capture: An experimental analysis

Akansha Verma1, Pradeep Kumar2, Ankush Bindwal2 & Subham Paul*,2

1School of Environment and Natural Resources, Doon University, Dehradun, 248 001 (Uttarakhand), India. 2Separation Processes Division, CSIR - Indian Institute of Petroleum, Dehradun, 248 005 (Uttarakhand), India.

E-mail: spaul@iip.res.in

Received 15 March 2019 ; accepted 22 July 2019

An enormous increase in global energy demand has resulted in the increasing emissions of Carbon dioxide (CO2) throughout the globe, and hence, the refinement of established technologies for the capture of CO2 is of global interest. Currently, the absorption performed with chemical solvents, particularly alkanolamines and their blends, represents the most practical option. In general, the enhancement in absorption and regeneration capacities is targeted via the blending of several alkanolamines consisting of different functionalities to seek the absorption-favoured properties of individual amines. The present work considers blending of a tertiary alkanolamine, N,N-diethylethanolamine (DEEA) with a polyamine, triethylenetetramine (TETA). This blend has been envisaged to combine the high absorption capacity of TETA with high regeneration capacity of DEEA. The concentration of DEEA and TETA in the blends is varied between 2.5-2.95 M and 0.05-0.5 M, respectively. The performance of these blends has been evaluated using an indigenous wetted-wall contactor. The CO2 partial pressure was varied between 5-15 kPa and its effect on the CO2 loading capacity and absorption flux for different compositions is evaluated in the temperature range of 30-50 . It is found that increasing the concentration of TETA increases the absorption drastically. Besides, the physical properties (density and viscosity) of these solvent systems have also been estimated.

Keywords: Carbon dioxide, Alkanolamines, Absorption, Kinetics, Wetted-wall column

A significant increase in CO2 emissions fostered by the increased energy demand over the past few decades has resulted in some critical environmental and climate issues (e.g., extreme weather and sea level rise) throughout the globe. The process industries and coal-fired power plants, being major contributors to CO2 emissions, therefore quest for the sustainable solutions to capture CO2. The captured CO2 can then be used for varying applications (e.g., production of methanol, carboxylic acids of phenols, dimethyl carbonate, and dry ice)1. The reactive absorption using an aqueous solution of alkanolamine is the most viable and essentially used technology to capture CO2 from post-combustion gas streams2.

There are few industrially effective alkanolamines (e.g., monoethanolamine (MEA), N-methyl diethanolamine (MDEA)) which are being used solely to achieve this objective. The absorption capacity of these alkanolamines can further be enhanced by using some activators (e.g., PZ, AMP). However, most of the available absorbents suffer from high energy requirements for regeneration, and their highly volatile, corrosive nature; thereby demanding novel approaches and improved absorbents for use on industrial extents2. The blending of several

alkanolamines to combine their favorable properties and arrive at an optimized absorption performance with lowered energy requirements is one of such fruitful approaches that researchers follow. To name a few, KS-1, Cansolv, Econamine FG+, and Oase Blue are the examples of such commercially available blended alkanolamines3. The polyamines, consisting of two or more functional amine groups (e.g., primary, secondary or tertiary) in their structures, have thus gained an appreciable recognition over a decade3,4. The past research on polyamines has established that their addition to primary, secondary or tertiary alkanolamines, significantly enhances mass transfer and CO2 absorption rate. Nevertheless, energy losses involved in their practical use extends the scope to investigate newer compositions with absorption-favored characteristics and tenuous energy requirements4.

A tertiary alkanolamine, N,N-diethylethanolamine (DEEA), containing two ethyl groups, was considered for this study. DEEA features high CO2 absorption and regeneration capacity as compared to conventional and most popular primary (e.g., MEA) and tertiary (e.g., MDEA) alkanolamines, which can be furthered by using a promoter5,6. Moreover, one of

INDIAN J. CHEM. TECHNOL., SEPTEMBER 2019

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the precursors for the synthesis of DEEA, ethylene oxide, can be prepared from ethanol, which is synthesized from agricultural waste residues. Thus, being a renewable absorbent, the use of DEEA appears promising for CO2 absorption process7. The present study aims to couple DEEA with a polyamine as a promoter and develops a binary solvent system with enhanced CO2 absorption rate. Based on a screening exercise to identify such polyamine promoters, we arrived at a linear tetramine viz., triethylenetetramine (TETA), comprising of two primary and two secondary amine groups. The presence of two secondary amine groups lowers the absorption heat (due to the formation of secondary carbamates) thereby reducing the energy demand and improves the absorption/desorption performance8. Nevertheless, the less volatile nature of TETA could also benefit the solvent recovery and costs involved therein9.

Based on these considerations, we formulated a few DEEA-TETA-H2O compositions to evaluate its performance for CO2 capture. Interestingly, this formulation would form two immiscible phases, CO2-rich and CO2-lean, on reaction with CO2 at high gas concentrations. Thus, subsequent regeneration of only the CO2-rich phase significantly reduces the stripper load, size, and energy requirements8,9.

In the present work, we studied the effect of various reaction variables viz., DEEA and TETA concentrations, CO2 partial pressure, and temperature on the absorption performance of the DEEA-TETA-H2O system using a wetted-wall column. The wetted-wall column offers the flexibility to conduct absorption experiments at different contact times over a wide range of temperature and pressure. Furthermore, the gas-liquid interfacial area, an essential parameter to determine mass transfer and absorption kinetics, can be estimated more accurately as compared to other methods (e.g., stirred cell reactors, stopped-flow method) that are being used to study gas-liquid reactions10. The physical properties such as density, viscosity and CO2 solubility of this solvent system were also estimated to further

determine the characteristics of the wetted-wall column and study reaction kinetics.

Experimental Section

Materials The gases CO2 (≥ 99.5%, volume fraction) and N2

(≥ 99.9%, volume fraction) were purchased from Gupta Gases and Services Pvt. Ltd., Dehradun. The absorbents, 2-(diethylamino)ethanol (≥ 99.5) and triethylenetetramine (≥ 97%) were supplied by Sigma-Aldrich Pvt. Ltd., India. These chemicals were used without any further purification. All the experiments were conducted using the deionized water (conductivity 0.054 µs) obtained through Milli-Q Ultrapure Water system (Millipore Corp., USA). The structures and a summary of basic properties (at standard conditions) of the chemicals used in this study are presented in Table 1.

Estimation of physical properties The density and viscosity are essential parameters

to calculate the film thickness in an experiment conducted on the wetted-wall column. The specific rate of absorption is a function of film thickness, and thus, an accurate estimation of these physical characteristics is highly desirable. The density of solutions was measured by using a DMA 4100M Density Meter (Anton Paar, Austria). The viscosity of the amine solutions was measured using an Ostwald Viscometer.

The solubility of CO2 in aqueous DEEA-TETA solution is an important factor to arrive at the CO2 absorption rate or flux. We used an absorption cell (see Fig. 1a) in the semi-batch mode to calculate the solubility of CO2 in various compositions of DEEA-TETA mixtures. The synthetic mixtures of CO2 and N2 were used for this purpose with a total gas flow rate of 1.20×10-5 m3/s. The flow of gas was measured and controlled using a mass flow controller (5800 Series, Brooks Instrument, US). It is to be noted that the various compositions of CO2 used in this study fall in the range of a typical flue gas composition. The concentration of CO2 in the feed and off-gas mixtures was quantified using a CO2 transmitter (NDIR,

Table 1 — The structures and a summary of basic properties of the chemicals used in this study

CAS No. Abbreviation Chemical name Structure Molecular weight

Boiling point (°C) Density (g/mL)

Dynamic viscosity (mPas)

100-37-8 DEEA 2-(Diethylamino)ethanol

117.19 163 0.88 0.70

112-24-3 TETA Triethylenetetraamine 146.23 278 0.98 45.6 7732-18-5 H2O Water 18.02 100 1.00 0.74

CARBOCAPreaching thesolvent mixmethanolic NThe standardbe noted thaexamining thOn the otherin a range gas composi

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R CO2 CAPTUR

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50ºC 2.95 M – 0.5 M 5 kPa

413

glass tube ht 0.087 m

gas was tallic tube flow from

the outer his way, a total flow

PCO2 = 0-5 of 6×10-6

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INDIAN J. CHEM. TECHNOL., SEPTEMBER 2019

414

± 1%. The wide-ranging concentration of CO2 in the experiments was measured using two online non-dispersive infrared (NDIR) CO2 transmitters (CARBOCAP GMT 221, Vaisala, India) of ranges 0-2% and 0-20%. The stable CO2 concentrations in the outlet stream were considered as an indication of attaining the steady state. The rate of CO2 absorption was thus calculated based on the difference between inlet and outlet concentrations of CO2.

Theory In a wetted wall column, the liquid film flows

down the wall under the influence of gravity. When the film has attained its terminal velocity distribution, the velocity u at any depth x beneath the surface can be expressed as,

212333

231

2 3 3L

L

V g gdu x

d V

… (1)

The velocity u is zero at the wall, i.e., at x = w (the film thickness). Thus, from Eq. (1) the film thickness w can be written as,

1

33 LVw

gd

… (2)

Therefore, Eq. (1) can be written as

2 21su u x w … (3)

where us, the velocity at the surface (x = 0) is

12333

2 3L

s

V gu

d

… (4)

And if the length of the absorption surface is h, the exposure time of a surface element to the gas can be expressed as:

21332 3

3s L

h h d

u g V

… (5)

If the gas absorbed by a unit surface area in contact time, , is expressed as Q(), the average absorption over the time () is Q()/. Thus, the total rate of absorption into the film, q, over a total exposed surface area (dh), can be related to Q() by

Q q

dh

… (6)

The absorption rate q is measured experimentally, and Q ()/ calculated from Eq. (6). The contact time is calculated from Eq. (3.5) and can be altered by altering the flow rate VL or the length h of the liquid film. Thus Q() can be determined as a function of 12.

Results and Discussion

Physical properties The density and viscosity values obtained at 30-

50ºC are presented in Table 3. An obvious decrease in density and viscosity values with increase in temperatures was observed for various solvent compositions. However, the density of the mixture was increased, and the viscosity was unaffected by increasing TETA concentration in the blends.

Preliminary study As mentioned before, we selected DEEA and

TETA combination through a screening exercise based on their CO2 loading capacity. Both these

Fig. 2 — Schematic view of a wetted-wall column assembly

Table 3 — Physical properties of aqueous DEEA-TETA mixtures at different temperatures

Temperature (°C)

DEAE + TETA Conc. (M)

Density (g/mL) Viscosity (mPas)

30 2.95 + 0.05 0.9898 0.001811 2.80 + 0.20 0.9901 0.001778 2.65 + 0.35 0.9923 0.002017 2.50 + 0.50 0.9937 0.001679

40 2.95 + 0.05 0.9847 0.001353 2.80 + 0.20 0.9848 0.001345 2.65 + 0.35 0.9871 0.001198 2.50 + 0.50 0.9887 0.001183

50 2.95 + 0.05 0.9787 0.001119 2.80 + 0.20 0.9788 0.001203 2.65 + 0.35 0.9821 0.001104 2.50 + 0.50 0.9826 0.001030

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INDIAN J. CHEM. TECHNOL., SEPTEMBER 2019

416

Effect of composition on absorption flux

The absorption flux estimation was performed with 3 M (equivalent to 30 wt% aqueous solutions) blends containing various ratios of DEEA and TETA at PCO2

= 5-15 kPa (Fig. 7). The total concentration was chosen on the basis that highly concentrated DEEA-TETA blends may cause higher viscosity with acute degradation and corrosion problems, which is highly undesirable in practical applications9. The concentrations are expressed herein molar units to

Fig. 5 — Effect of CO2 partial pressure on loading of CO2 ofvarious DEEA-TETA blends at (a) 30ºC, (b) 40ºC and (c) 50ºC

Fig. 6 — Effect of temperature on CO2 loading capacity of 23%DEEA + 7% TETA mixture (PCO2: 5-15kPa)

Fig. 7 — Effect of CO2 partial pressure on absorption flux ofvarious DEEA:TETA molar ratios at (a) 30ºC, (b) 40ºC and (c)50ºC (Total concentration of blend: 3 M)

VERMA et al.: POLYAMINE-PROMOTED AQUEOUS DEEA FOR CO2 CAPTURE

417

simplify the absorption flux calculations further. Furthermore, the CO2 partial pressures were varied between 5-15 kPa to ensure fast pseudo-first-order reaction kinetics concerning CO2. Besides, these flow rates were deployed to ensure that no gas-phase resistance was present in the system. It can be seen from Fig. 7 that an increase in TETA concentration from 0.05 M to 0.5 M almost doubles the absorption flux. This observation indicates that TETA affects the flux in a relatively higher proportion than DEEA. Li et al.9 previously reported that the CO2 reacts with the aqueous DEEA-TETA blend in the following sequence:

2 R2NH + CO2 ↔ R2NH2+ + R2NCOO- … (7)

R3N + CO2 + H2O ↔ R2NH+ + HCO3- … (8)

Thus, CO2 reacts with TETA (R2NH) first followed by the DEEA (R3N) and water. This theory justifies the higher flux values obtained with the increasing concentration of TETA.

The effect of temperature on absorption flux of 2.5 M DEEA: 0.5 M TETA is depicted in Fig. 8. Contrary to it its effect on loading of CO2 (Fig. 6), an increase in temperature lowered the absorption flux. This decrease in flux may be attributed to the inverse behavior of CO2 solubility with a rise in temperature. A decrease in CO2 solubility lessens the driving force for mass transfer at interphase. We observed that the increasing temperature from 40 to 50°C did not affect the flux significantly. This flux inhibition may be due to the stronger H bonding or the lowered movement of the molecules in the solution9. Thus, it can be said that the CO2-DEEA-TETA-H2O system perform

better at low temperatures and thus, choosing an appropriate temperature region becomes critical to achieving high absorption efficiency. Conclusion

In this work, the absorption of CO2 into blends of aqueous DEEA and TETA solvents was studied in a wetted-wall column. The absorption and cyclic performance of these solvents were compared with MDEA. The concentration of both these solvents was varied in such a proportion that does not inhibit the performance of other. The physical properties of the blends synthesized were measured, too. It was found that the changing TETA concentrations did not alter the viscosity of blends. The highest value of absorption flux (9.6 × 107 kmol/s/m2) was obtained using a DEEA:TETA composition of molar ratio 2.5:0.5 at 30 . Our experimental observation was in line with the theory that TETA is the first reactant of the blend to react with CO2 followed by the DEEA and water. The rise in temperature was found to lower CO2 loading and absorption flux due to low CO2 solubility at higher temperatures. Thus, the absorption temperature is a crucial parameter that defines the CO2-DEEA-TETA-H2O system. Acknowledgement

The author is thankful to Director, CSIR-Indian Institute of Petroleum, Dehradun for permission to conduct this work in the institute. References 1 Koytsoumpa E I, Bergins C & Kakaras E, J Supercrit Fluids,

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Fig. 8 — Effect of temperature on absorption flux of 2.5 M:0.5 MDEEA:TETA mixture (PCO2: 5-15kPa, Total concentration ofblend: 3 M)