Elsevier

Chemical Engineering Journal

Volumes 181–182, 1 February 2012, Pages 694-701
Chemical Engineering Journal

Carbon dioxide absorption into unpromoted and borate-catalyzed potassium carbonate solutions

https://doi.org/10.1016/j.cej.2011.12.059Get rights and content

Abstract

Potassium carbonate based solvents have the potential for capturing CO2 from emission sources such as power stations burning fossil fuels. However, due to poor reaction kinetics a rate promoter is considered necessary to improve the rate of reaction of CO2 with the solvent. Using a characterized wetted-wall column, we have studied the reaction kinetics of CO2 into unpromoted and borate-promoted 30 wt% potassium carbonate solutions. Results presented here show that, at 80 °C, addition of small amounts of boric acid (0.2 M, 0.6 M and 1.5 M) accelerate the overall absorption process of CO2 in carbonate solvents by 3%, 10% and 29% respectively. The Arrhenius expression for the reactions CO2 + OH and CO2 + B(OH)4 are kOH [M−1 s−1] = 2.53 × 1011 exp(−4311/T [K]) and kborate [M−1 s−1] = 5.5 × 1011 exp(−6927/T [K]); and the activation energies are 35.8 kJ mol−1 and 57.6 kJ mol−1 respectively. Experiments were conducted between 40 °C and 80 °C and at a bulk partial pressure of CO2 of 90 kPa.

Highlights

► Unpromoted K2CO3 solutions have a relatively slow rate of reaction with CO2. ► Boric acid improves the rate of reaction between CO2 and K2CO3. ► A wetted wall column has been characterized and used to measure rate constants.

Introduction

It is widely accepted that the increase in the carbon dioxide (CO2) emissions to our atmosphere is the major contributor to global climate change. Numerous methods for reducing greenhouse gases to our atmosphere have been proposed including carbon capture and sequestration (CCS). Reactive absorption into a solvent is the benchmark technology for CO2 removal from the flue gas of coal fired power stations [1], [2].

The basic principle behind all solvent absorption processes is the mass transfer of a solute (e.g. CO2) from a gas stream to a liquid solvent in a gas–liquid contacting absorber. The CO2 loaded solution is subsequently fed to a high temperature and/or low pressure regenerator that releases the CO2 which can then be used for other purposes such as sequestration in the case of carbon capture and storage. The ‘lean’ solvent is returned back to the absorber. The absorption of CO2 into carbonate–bicarbonate solutions is a widely known process [3]. German patents exploring hot carbonate absorption of CO2 exist as early as 1904 [4]. In the 1950s Benson and Field developed the Benfield process which used hot potassium carbonate as a CO2 absorption solvent to reduce the costs of synthesis gas purification for the production of liquid fuel from coal [5], [6], [7].

In comparison to the benchmark industrial solvent for CO2 absorption, monoethanolamine (MEA), potassium carbonate processes require less energy as the heat of absorption is 37% that of MEA [5]. Furthermore, MEA degrades at high temperatures as well as in the presence of oxygen, forms corrosive and toxic organic products, and requires pre-treatment processes for sulphur dioxide removal [8], [9], [10]. Despite its immunity to these constraints, a key limitation for a carbonate based solvent is the slow rate of reaction with CO2. Promoters such as piperazine [11], amino acids [12], [13], [14], arsenious acid [15], [16], amine derivatives [17], [18], [19], carbonic anhydrase [20], [21], [22], and boric acid [20], [23] have been suggested to enhance the reaction kinetics. Of these, boric acid B(OH)3 is attractive because it is relatively benign and economically affordable, and is not expected to interact with species such as sulphur dioxide or oxygen often present in the flue gas from power stations [24].

In basic solutions (pH > 9), where pre- and post-combustion carbon capture normally takes place [1], [2], the reaction of CO2 + H2O forming HCO3 can be neglected [25], and the predominant equilibrium reactions are as follows (NB: in this work all species are aqueous unless otherwise stated):OH + CO2  HCO3HCO3 + OH  CO32− + H2OIt is well established that the rate-limiting reaction in chemical absorption of CO2 into carbonate solutions is the formation of the bicarbonate ion, Eq. (1) [26].

It has been reported that B(OH)3 and B(OH)4 are the predominant boron species at boron concentrations and pH range relevant to carbon capture systems [20], [27]. Using a stopped flow indicator technique, results from our previous work [20] showed that, at total boron concentrations from 4 to 12.5 mM and a temperature range 25–40 °C, B(OH)4 is the active form of boron, and that it catalyzes CO2 hydration via the same fundamental mechanism as the enzyme carbonic anhydrase:B(OH)3·H2O  B(OH)4 + H+B(OH)4 + CO2  B(OH)4CO2B(OH)4CO2 + H2O  B(OH)3·H2O + HCO3CO2 + H2O  HCO3 + H+In this study we aim to extend our previous work [20] by investigating the effect of boric acid on CO2 absorption in potassium carbonate solutions at significantly higher boron concentrations and temperatures, which are closer to the conditions encountered in industrial carbon capture systems [24], [28]. Therefore we have studied the kinetics of CO2 reacting with B(OH)4 using a wetted-wall column (WWC). This work has implications for the operation of borate-promoted potassium carbonate solvent systems for carbon capture and sequestration, and more specifically allows accurate design of absorber and regenerator units.

Section snippets

Materials and methods

The kinetics of reactions CO2 + OH and CO2 + B(OH)4 were studied using a wetted-wall column (WWC), a device that allows contact between a gas and a liquid phase with a controlled and measurable surface area for mass transfer, and thus, accurate measurements of the flux of CO2 into un-promoted and borate-promoted potassium carbonate solvents.

All chemicals employed in this study were of analytical reagent grade and used as supplied without further purification. Potassium Carbonate (≥99%, Thasco

Gas mass transfer coefficient

The gas film mass transfer coefficient, kg, in the WWC has been determined by Bishnoi and Rochelle [29] using SO2 absorption into 0.1 M NaOH. The results of these experiments have been correlated with a predicted kg, proposed by Hobler [37].Sh=1.075ReSCdh0.85The Schmidt number is defined as:Sc=μgρgDCO2where ρg [kg m−3] is the gas density and μg [pa s−1] is the gas viscosity. Also, d is the hydraulic diameter of the annulus (13 mm) and h is the height of the WWC (115 mm). The Reynolds number is:Re=ρg

Discussion

A comprehensive kinetic study on the absorption of CO2 into borate-promoted potassium carbonate under conditions resembling industrial capture plants has been presented in this work. Experimental results clearly show that borate catalyzes the overall absorption rate of CO2 into potassium carbonate. A comparison between rate constants of various amine-based promoters at 1.0 M and 25 °C is shown in Table 1. The activation energy of the reaction CO2 + B(OH)4 obtained in this study is larger than all

Conclusions

A comprehensive kinetic study on the absorption of CO2 into unpromoted and borate-promoted potassium carbonate solutions is presented in this work. Results indicate that an addition of a small amount of boric acid has accelerated the apparent pseudo-first-order rate constant, and thus, the overall absorption of CO2 into potassium carbonate solutions. B(OH)4 is found to exhibit a comparable catalytic activity to that of tertiary and hindered amine-promoters. Rate constants (kOH and kborate)

Acknowledgments

The authors acknowledge the financial support provided by the Australian Government through its Cooperative Research Centre program for this CO2CRC research project. Infrastructure support from the Particulate Fluids Processing Centre (PFPC), a special research centre of the Australian Research Council is also gratefully acknowledged.

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