Scholarly article on topic 'A novel reactive 4-diethylamino-2-butanol solvent for capturing CO2 in the aspect of absorption capacity, cyclic capacity, mass transfer, and reaction kinetics'

A novel reactive 4-diethylamino-2-butanol solvent for capturing CO2 in the aspect of absorption capacity, cyclic capacity, mass transfer, and reaction kinetics Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Teerawat Sema, Abdulaziz Naami, Zhiwu Liang, Guangying Chen, Ruimin Gao, et al.

Abstract In the present work, the performance of a novel reactive solvent, 4-diethylamino-2-butanol (DEAB), was comprehensively investigated and compared with the conventional solvents (e.g., MEA and MDEA) in terms of CO2 absorption capacity, cyclic capacity, mass transfer performance, and absorption kinetics. The results showed that DEAB can be considered as alternative promising solvent for capturing CO2 because its outstanding performance on much higher absorption capacity, higher cyclic capacity, and higher mass transfer performance; comparing with those of MEA and MDEA. In addition, the CO2 absorption kinetics of DEAB was found to be faster than that of conventional tertiary MDEA, but slower than that of conventional primary MEA.

Academic research paper on topic "A novel reactive 4-diethylamino-2-butanol solvent for capturing CO2 in the aspect of absorption capacity, cyclic capacity, mass transfer, and reaction kinetics"

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Energy Procedia 37 (2013) 477 - 484

GHGT-11

A novel reactive 4-diethylamino-2-butanol solvent for capturing CO2 in the aspect of absorption capacity, cyclic capacity, mass transfer, and reaction kinetics

Teerawat Semaab, Abdulaziz Naamib, Zhiwu Liangab*, Guangying Chena, Ruimin Gaoa, Raphael Idemab, Paitoon Tontiwachwuthikulab*

aJoint International Center for CO2 Capture and Storage (iCCS), Department of Chemical Engineering, Hunan University,

Changsha, 410082, PR China

bInternational Test Centre for CO2 Capture (ITC), Faculty of Engineering and Applied Science, University of Regina, Regina, _Saskatchewan, S4S 0A2, Canada_

Abstract

In the present work, the performance of a novel reactive solvent, 4-diethylamino-2-butanol (DEAB), was comprehensively investigated and compared with the conventional solvents (e.g., MEA and MDEA) in terms of CO2 absorption capacity, cyclic capacity, mass transfer performance, and absorption kinetics. The results showed that DEAB can be considered as alternative promising solvent for capturing CO2 because its outstanding performance on much higher absorption capacity, higher cyclic capacity, and higher mass transfer performance; comparing with those of MEA and MDEA. In addition, the CO2 absorption kinetics of DEAB was found to be faster than that of conventional tertiary MDEA, but slower than that of conventional primary MEA.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: CO2; absorption capacity; cyclic capacity; kinetics; mass transfer

1. Introduction

The absorption of carbon dioxide (CO2) into reactive solvents is one of the most promising technologies for capturing CO2 due to its maturity, cost effectiveness, and capability of handling large amounts of exhaust stream [1-4]. One of the key parameters for this technology is to use an effective solvent, which should be fast reaction kinetics, high mass transfer performance, high absorption capacity,

* Corresponding author. Tel.: +86-136-1848-1627; fax: +86-731-8857-3033. E-mail address: zwliang@hnu.edu.ca and paitoon@uregina.ca.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.133

low energy requirement for regeneration, low degradation rate, low corrosiveness and low solvent lost due to vaporization [5-8]. At International Test Centre for CO2 Capture (ITC), 9 novel reactive solvents have been developed based on a systematic modification of the structure of amino alcohols by an appropriate placement of the substituent, especially hydroxyl function, relative to the position of the amino group, in order to promote CO2 capture performance [9]. Among the 9 solvents, 4-diethylamino-2-butanol (DEAB), which is considered as a tertiary amine, shows the best performance in terms of absorption capacity [10]. By comparing the boiling point at 10 mmHg of MEA (710C), DEAB (1100C), and MDEA (1280C), it can be seen that the volatility of DEAB is higher than that of MEA and slightly lower than that of MDEA [11]. In addition, it was also found that the viscosity at 250C of DEAB (2.8 mPa.s) is lower than those of MEA (19.4 mPa.s) and MDEA (102.7 mPa.s), respectively [12]. Bearing in mind that the lower the solvent viscosity, the lower operation cost for pump and circulation system; therefore, DEAB seems to be a potentially good solvent not only in terms of the solvent volatility, but also the liquid circulation cost.

In the present work, the performance in terms of CO2 absorption capacity, cyclic capacity, mass transfer and reaction kinetics of a novel DEAB solvent was investigated and compared with the conventional amines such as MEA and MDEA. The CO2 absorption capacity and cyclic capacity were measured in an absorption cell under atmospheric pressure. The mass transfer performance was measured in a 1 inch lab-scale absorption column packed with high efficiency DX structured packing. Lastly, the reaction kinetics data were obtained from a laminar jet absorber.

2. Materials and methods

2.1 Chemicals

DEAB was synthesized according to the procedure described by Tontiwachwuthikul et al. [9] in the solvent synthesis laboratory in the International Test Centre for CO2 Capture (ITC) at the University of Regina. The purity of synthesized DEAB was determined by GC-MS and found to be in the range of 9193%. MEA and MDEA were obtained from Fisher Scientific, Canada with purities of >99%. For the absorption experiment in a laminar jet absorber, 99.9% CO2 was supplied by Praxair Inc, Canada. For the mass transfer experiment in packed column, premixed 15% CO2 balanced with nitrogen (N2) was also obtained from Praxair Inc, Canada. All materials in this study were used as received without further purification.

2.2 CO2 absorption capacity

The apparatus and experimental technique used for determining the equilibrium solubility of CO2 were similar to those in the work of Tontiwachwuthikul et al. [9]. The main features are a saturation cell, an absorption reactor, a mass flow meter, and a water bath with a temperature controller. For each experiment, the saturated cell and the absorption reactor were immersed in a water bath with the temperature controller. The CO2 concentration in the mixed gas stream was measured by a portable infrared (IR) CO2 gas analyzer. The liquid sample was then taken for the measurement of CO2 loading using the acidification technique [13].

2.3 Cyclic capacity

Cyclic capacity is defined as the difference of moles of CO2 absorbed in the solution per unit volume of solution in the absorption step and that in the regeneration step. In other words, it can be defined as the

difference between the CO2 loading in solution under absorption conditions and that under regeneration conditions, multiplies by the initial molar amine concentration [5].

2.4 CO2 absorption rate

The CO2 absorption rate was measured using the laminar jet absorber. A detailed description of the laminar jet absorber and its operation can be seen in [14]. But briefly, the amine solution was degassed by spraying it into a vacuum, and then the degassed amine solution was passed through the temperature-controlled water jacket in order to reach the desired temperature. The degassed amine solution was then passed through the jet nozzle in order to continuously generate a smooth-surfaced rod-like jet in the absorption chamber. The soap-film meter was used to measure the rate of absorption ( RA ). The two dimensional microscope was used to measure jet height (h) and jet diameter (d). Finally, the discharged liquid was collected and liquid flow rate (L) was measured.

2.5 Mass transfer in packed column

The mass transfer occurs when a component, A, in a gas phase transfers across a gas-liquid interface into a liquid phase. Based on film theory, at a steady-state condition, combining mass flux and material balance equations, overall mass transfer coefficient (KGav) can be defined as presented in Equation 1. Detail description on determination of KGav can be found in our previous work [15].

( G Y dY \

KGav = , Gi * , ^ (1)

^ PVa,G - y A dZ )

3. Results and discussion

3.1 CO2 absorption capacity of aqueous DEAB solution

The experimental CO2 absorption capacity of aqueous DEAB solutions was measured at a temperature of 313 K and a CO2 partial pressure range of 10-100 kPa. The results are presented in Fig 1. By comparing the CO2 absorption capacity of DEAB with those of conventional amines, such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ), it was found that DEAB has very high absorption capacity, which is competitive with PZ, and higher than those of AMP, MDEA, MEA, and DEA, respectively. It can be implied from this observation that the absorption of CO2 using aqueous DEAB solution does not need a high concentration of DEAB to achieve the same CO2 recovery efficiency as conventional amines.

3.2 Cyclic capacity of aqueous DEAB solution

It was discussed in Astarira et al. [5] and Maneeintr et al. [10] about the cyclic capacity concept that cyclic capacity is crucial to CO2 absorption process. At the same solvent volume, the higher cyclic capacity solvent can carry higher amount of absorbed CO2 than the lower cyclic capacity solvent. It can be implied from this concept that for high cyclic capacity solvent, low liquid circulation rate is applicable, which means (i) low operating cost on pumping liquid and (ii) small volume of solvent can be used in packed column. By comparing the cyclic capacity of DEAB with those of MDEA and MEA, it was found

that the cyclic capacity of DEAB is higher than those of MDEA and MEA, respectively, as shown in Fig 2. Therefore, DEAB requires a lower liquid circulation rate and volume of solvent than MDEA and MEA, respectively.

0 U.I 0.2 0,3 0,4 0.6 0,6 0.7 0.3 0,9 1 CD;, loading (mo) CO>fmol amine}

Fig. 1. Equilibrium solubility of CO2 in aqueous solutions of 2 M MEA, 2 M DEA, 2 M MDEA, 2 M AMP, 2 M DEAB, and 2 M PZ [11]

2MDEAB 2 M MDEA 2 M MEA

Fig 2. Cyclic capacity of aqueous solutions of 2 M DEAB, 2 M MDEA, and 2 M MEA at CO2 partial pressure of 15 kPa and temperature range of 313-353 K

3.3 Absorption kinetics of aqueous DEAB solution

The CO2 absorption rate in aqueous DEAB solutions was measured using a laminar jet absorber at atmospheric pressure over a temperature range of 298-318 K, DEAB concentration range of 1-2 M, CO2 loading range of 0.001-0.18 mol CO2/mol amine, and contact time range of 0.0044-0.018 s. The CO2 absorption rate results under these different operating conditions were treated using comprehensive

numerically solved reaction kinetics model. In this approach, series of partial differential-non linear algebraic equations were solved simultaneously. At the end of the process the reaction rate constant was extracted. The details on mathematical modeling and solution finding can be found in our previous works [18,19].

The reaction rate constant of DEAB (kDEAB) at various temperatures can be correlated using Arrhenius equation as:

kDEAB = (4.01 x1013 )(2)

The predicted reaction rate constant of DEAB at 298 K compares with that of MDEA, AMP, DEA, MEA, and PZ are shown in Table 1. By comparing kDEAB and kMDEA (both DEAB and MDEA are considered as tertiary amine), it can be seen that the kDEAB is much higher than kMDEA, which means that the reaction kinetics of CO2 absorption into DEAB is much faster than that into MDEA. Moreover, it can also observe that the kDEAB is competitive with kAMP and kDEA. It is generally accepted that AMP and DEA are considered to be reactive with CO2 since they are primary sterically hindered amine and secondary amine, respectively [7]. However, the kDEAB is much smaller than kMEA and kPZ, since MEA is primary amine, which is very reactive with CO2, and PZ is very fast accelerator. Therefore, it can be summarized that the CO2 absorption reaction with DEAB is faster than that with MDEA, also comparable with those with AMP and DEA, but slower than those with MEA and PZ, respectively.

Table 1. Reaction rate constant of CO2 absorption into aqueous solutions of MDEA, AMP, DEA, DEAB, MEA, and PZ

Amine k (m3/kmol s) References

MDEA 6/71 [16]

AMP 473 [17]

DEA 412 [18]

DEAB 429 This work

MEA 5,939 [19]

PZ 65,460 [20]

3.4 Mass transfer performance of aqueous DEAB solution

The mass transfer performance of CO2 absorption into aqueous solutions of MEA, MDEA, and DEAB was investigated in 1 inch absorption column packed with DX structural packing at atmospheric pressure, amine concentration of 2 M and temperature of 298 K. The mass transfer performance characteristic was evaluated in terms of CO2 concentration profile and overall volumetric mass transfer coefficient (KGav). The lower CO2 concentration profile indicates the higher mass transfer performance. For the KGav, the higher KGav, the higher mass transfer performance in packed column. It can be seen from Fig 3 that the CO2 concentration profile of MEA is lower than those of DEAB and MDEA, respectively. In addition, the KGav of MEA is higher than those of DEAB and MDEA, respectively, as presented in Fig 4. In can be implied from the results in Figs 3 and 4 that the mass transfer performance of CO2 absorption of MEA is higher than those of DEAB and MDEA, respectively. This is because of the reaction kinetics of CO2 absorption into aqueous solutions of MEA is faster than those of DEAB and MDEA as shown in Table 1.

In the present work, the performance of novel DEAB solvent in terms of (i) CO2 absorption capacity, (ii) cyclic capacity, (iii) reaction kinetics of CO2 absorption, and (iv) mass transfer of CO2 absorption in

packed column were comprehensively investigated and compared with the conventional amines (e.g. MEA and MDEA). The results show that DEAB has much higher absorption capacity, higher cyclic capacity, and higher mass transfer performance than that of conventional MEA and MDEA as can be seen in Figs 1-4 and Table 1. In the case of reaction kinetics, the reaction of DEAB can be ranked as: PZ>>MEA>DEAB~AMP~DEA>MDEA. A low solvent concentration used for capturing CO2 can be applied due to the high absorption capacity of DEAB. The high cyclic capacity of DEAB can directly reduce the circulation rate in the packed column. Also, the high reaction kinetics and mass transfer performance of DEAB can directly shorten the height of the absorption column, which leads to a reduction of capital cost and long-term operating costs for capturing CO2. Based on the outstanding performance of DEAB on reasonable solvent lost due to vaporization , low solvent viscosity, high cyclic capacity, high absorption capacity, fast reaction kinetics, and high mass transfer performance, it can be inferred that DEAB has a good potential to be used as the alternative solvent for capturing CO2. Especially, comparing DEAB with conventional MDEA, since both of them are considered to be tertiary amine. However, several characteristics of DEAB (e.g., solvent degradation, corrosion, calorimetric property, and heat requirement for solvent regeneration) need to be further investigated in order to effectively use DEAB for capturing CO2.

0.0 .;.[: 6.0 >.: 12.0 15.0

(TOj t'onc£ntratjon %

Fig 3. CO2 concentration profiles of CO2 absorption into aqueous solutions of 2 M MEA, 2 M MDEA, and 2 M DEAB at atmospheric pressure, temperature of 298 K, gas flow rate of 17.8 kmol/m2 hr, liquid flow rate of 5 m3/m2 hr, and initial CO2 loading of 0.14 mol C02/mol amine

0.03 - - 0.1

0.02 -I-,-,-,-,-,-1- 0

0 0.05 0.1 0.15 0,2 0.25 0,3 Lead Loading, mol COv'mol amine

Fig 4. Overall volumetric mass transfer coefficient (KGav) of aqueous solutions of 2 M MEA, 2 M MDEA, and 2 M DEAB at atmospheric pressure, temperature of 298 K, gas flow rate of 17.8 kmol/m2 hr, liquid flow rate of 5 m3/m2 hr, initial CO2 loading of 0.14 mol CO2/mol amine, and mole ratio of CO2 in gas phase of 0.14

4. Conclusion

The performance of DEAB, which is the novel reactive solvent developed by ITC, was comprehensively investigated in the present work. It was observed that DEAB has a great potential to be an alternative solvent for capturing CO2 because of its reasonable solvent lost due to vaporization, low solvent viscosity, high cyclic capacity, high absorption capacity, fast reaction kinetics, and high mass transfer performance.

Acknowledgements

We acknowledge the research support over the past many years of the Industrial Research Consortium - Future Cap Phase II of the International Test Centre for CO2 Capture (ITC) at the University of Regina. We also acknowledge the research support from the followings organizations: Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), Saskatchewan Ministry of Energy & Resources, Western Economic Diversification, Saskatchewan Power Corporation, Alberta Energy Research Institute (AERI) and Research Institute of Innovative and Technology for the Earth (RITE). In addition, we acknowledge the financial support from National Natural Science Foundation of China (NSFC No. 21276068), the Ministry of Science and Technology of the P R China (MOST No. 2012BAC26B01), the Ministry of Hunan Provincial Science and Technology (No. 2010SK2001), China's State "Project 985" in Hunan University Novel Technology Research & Development for CO2 Capture and National Students Innovation Training (SIT).

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