Kinetics of thermal degradation of renewably

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Kinetics of thermal degradation of renewably prepared amines useful for flue gas treatment Rahul R. Bhosale and Vijaykumar V. Mahajani Citation: J. Renewable Sustainable Energy 5, 063110 (2013); doi: 10.1063/1.4831960 View online: http://dx.doi.org/10.1063/1.4831960 View Table of Contents: http://jrse.aip.org/resource/1/JRSEBH/v5/i6 Published by the AIP Publishing LLC.

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JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 5, 063110 (2013)

Kinetics of thermal degradation of renewably prepared amines useful for flue gas treatment Rahul R. Bhosale1,2,a) and Vijaykumar V. Mahajani2 1

Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar 2 Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai, India (Received 25 December 2012; accepted 4 November 2013; published online 18 November 2013)

N-ethylmonoethanolamine (EMEA) and N-N-diethylmonoethanolamine (DEMEA) can be prepared from renewable resources and appear to be commercially attractive solvents for post-combustion CO2 capture by absorption/stripping process. In this paper, the thermal degradation of these renewably prepared amines was studied at 423 K and compared with other amines such as monoethanolamine, diethanolamine, triethanolamine, and N-methyl diethanolamine. Furthermore, an investigation of the kinetics of thermal degradation of aqueous EMEA and DEMEA was conducted by using a 600 ml high-temperature high-pressure reactor in the temperature range of 393 to 423 K and amine concentration range of 1 to 3 kmol m3, respectively. Estimation of the active solvent content of the reaction mixture samples obtained during the degradation experiments was performed using a gas chromatograph (GC) equipped with a Flame Ionization Detector (FID) and a Tenax GC column. The obtained results indicate that the rate of thermal degradation of both aqueous EMEA and DEMEA increases with the increase in the initial amine concentration and temperature. Additionally, the degradation reaction was observed to be first order with respect to the initial amine concentration. Two intrinsic kinetic power law models were formulated to describe the kinetics of the thermal degradation of aqueous EMEA and DEMEA and the kinetic parameters were predicted by using the linear least-squares regression analysis. The kinetic rate constants for the thermal degradation of these renewably prepared amines were determined (both experimentally and by the models) and on the basis of their temperature dependency, the activation energy for the degradation reaction was estimated. This work represents the first attempt towards obtaining the intrinsic kinetic data for thermal degradation of aqueous EMEA and DEMEA and formulating a kinetic model that fits the data based on the initial rate of C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4831960] degradation. V

I. INTRODUCTION

Flue gas from coal-fired power plants, cement manufacturing, refineries, etc., is considered as one of the major sources of CO2 emissions which contribute to the increasing greenhouse effect.1 Because of this adverse effect of CO2 discharge on the global environment, as well as the world’s immense dependence on fossil fuels, the development of strategies for the reduction of CO2 emissions has become increasingly important.2 In carbon sequestration science, the separation and capture of CO2 is considered as one of the highest priorities as the capture cost is expected to be up to 75% of the total costs for geological and oceanic sequestration.3 Due to its higher efficiency and lower complexity, reactive absorption of CO2 from flue gas containing

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70%–75% N2, 10%–15% CO2, 4%–5% H2O, and 3%–10% O2 by aqueous amines is considered as the most mature post-combustion CO2 capture technology.4–19 A typical aqueous amine based absorption/stripping process used for the removal of CO2 from flue gas is shown in Fig. 1. In this process, flue gas is counter-currently contacted with the aqueous amine solution in the absorber column. The CO2 reacts reversibly with the amine to form amine carbamate and the CO2 rich amine solution is then sent through the counter-current heat exchanger, where it is preheated by the lean amine solution before entering the striping column. In the stripper, the chemical equilibrium between the amine and amine carbamate is reversed and the absorbed CO2 is liberated by heating the CO2 rich amine solution up to 393 K with the help of the steam heated reboiler (temperature not exceeding 423 K). The hot lean amine solution passes through the counter-current heat exchanger where it is cooled before being recycled to the absorber. The gas leaving the stripper contains majority of CO2 with traces of water which can be dehydrated and dry CO2 can be compressed before being sequestered. In addition, as a promising alternative option for CO2 geological and oceanic sequestration, the liberated CO2 can be re-energized into CO via a metal oxide based thermochemical looping process using concentrated solar energy.20–23 The CO produced via solar thermochemical CO2-splitting can be combined with H2 derived from metal oxide based solar thermochemical water-splitting process to form solar syngas21,22,24,25 which can be further processed to liquid fuels such as Methanol, Diesel, and Kerosene via the Fischer-Tropsch process. Although the CO2-amine reactions are reversible in nature, irreversible reactions may also occur resulting in products from which the amines are not easily recovered. This phenomenon is referred to as degradation of amines which can pose the following limitations to the CO2 absorption/stripping process: •



Because of the degradation of amine during the absorption/stripping process, it is necessary to add fresh amine solvent to the absorption/stripping process at regular time intervals to keep the solvent concentration constant and to maintain the same CO2 absorption capacity throughout the process. This can represent a significant increase in the operating cost. The products formed due to the degradation of amines possess several undesirable characteristics which obstruct the effectiveness of the absorption/stripping process and cause operational problems.

FIG. 1. Process flow diagram of a typical aqueous amine based CO2 absorption/stripping process used for flue gas treatment.

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J. Renewable Sustainable Energy 5, 063110 (2013)

Contamination of the amine solvent by degradation products usually enhances the viscosity of the amine solvent which reduces the mass transfer rates during absorption and stripping steps. The high viscosity of the solvent increases the foaming tendency that may lead to severe fouling in the heat exchanger. It further reduces the mass transfer in the contactor and increases the acid content of the treated gas leaving the contactor. The degradation further increases the corrosion in the process lines. Disposal of the degradation products is one of the major environmental concerns.

Amine degradation is a major problem associated with CO2 capture from flue gas.26 In CO2 capture processes, the aqueous amines are subject to three kinds of degradation, i.e., thermal decomposition, thermal degradation in the presence of CO2 and oxidative. Thermal decomposition only occurs at temperatures higher than 473 K. In an industrial flue gas treatment plant, the highest temperatures present are in the stripping column (393 K) and in the reboiler (423 K), and hence, thermal decomposition of amines is uncommon.27,28 Thermal degradation in presence of CO2 prevails at stripper conditions via carbamate polymerization resulting in the formation of high molecular weight degradation products.29 Degradation in the presence of CO2 is favored at high CO2 loading and the degradation is more probable at the rich end of the stripper. In addition, the rate of carbamate polymerization has a high dependence on amine concentration and hence, solvents that use a lower amine concentration will have a lower rate of degradation.1,30–32 Oxidative degradation occurs due to the presence of O2 in flue gas and results in fragmentation of the amine solvent.4,33 It will most likely occur at short times and low temperature in the absorber and at longer times and high temperature in the stripper.34 A wide variety of amines such as monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), methyldiethanolamine (MDEA), diglycolamine (DGA), etc., have been reported to be the useful solvents for CO2 capture.4–19,35 Additionally, in previous investigations, several investigators have studied the thermal and oxidative degradation of various amines such as MEA,29–42 DEA,42–46 MDEA,47–52 2-amino-2-methylpropanol (AMP)1,53 N-methylmonoethanolamine (MMEA),54,55 and others. These studies were primarily focused on (a) the amount of solvent lost because of the degradation reaction, (b) identification of the degradation products in order to establish their effect on the absorption/stripping process, and (c) kinetics of the degradation reaction. Among all these previously investigated amines, MEA is the most widely used solvent for CO2 capture in industries. However, due to several limitations related to MEA such as lower CO2 loading capacity, higher solvent regeneration cost, formation of toxic products due to the oxidative and thermal degradation, reduced scrubber efficiency due to amine decomposition and corrosion in the equipment and piping,8,19 companies such as Mitsubishi with KS-1 or Cansolv with DC-103 are indeed more and more eager to replace MEA.7,55,56 As a quest to find a potential substitute for MEA, several new amine-based solvents have been investigated in the past few years.4 Among these new solvents, N-ethylmonoethanolamine (EMEA) and N-diethylmonoethanolamine (DEMEA) were observed to be promising towards reactive absorption of CO2. The properties of these amines are listed in Table I. The rate of absorption of CO2 in an aqueous EMEA is comparable to that of the aqueous MEA and higher TABLE I. Properties of EMEA and DEMEA. Properties

EMEA

DEMEA

Molecular weight (kg/kmol)

89.14

117.19

Vapor pressure (mmHg) (20  C)

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