Article pubs.acs.org/IECR
Gas−Liquid Reactive Crystallization Kinetics of Hydromagnesite in the MgCl2−CO2−NH3−H2O System: Its Potential in CO2 Sequestration Daoguang Wang† and Zhibao Li*,† †
Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China ABSTRACT: The reactive crystallization kinetics of hydromagnesite in the MgCl2−CO2−NH3−H2O system, which plays an important role in a new approach to sequester CO2 with saline sea/lake brines, was systematically investigated by the MSMPR (mixed-suspension-mixed-product removal) crystallization method. The temperature effect on the crystallization of magnesium carbonate hydrates in the system was first investigated batchwise. The optimum temperature of 353.2 K for precipitation of the regular spherical hydromagnesite with good filterability was selected for kinetic study. The supersaturation of hydromagnesite was experimentally tested, and the activity coefficients of aqueous species were strictly calculated by the Pitzer model embedded in the Aspen Plus platform. The nucleation and growth rate and agglomeration kernel were determined by moments analysis based on the PSD (particle size distribution) of crystals in volume coordinates. The corresponding kinetic parameters in three empirical equations were then estimated by linear regression. The resulting volume growth rate order of 2.30 means that surface integration is dominant in the volume growth of hydromagnesite. A simplified process for the new sequestration approach was finally constructed. The value of hydromagnesite as carbonated product was assessed considering the scale-up and costs of such a process. as an important constituent of seawater (about 1.3 g·L−1), and, moreover, a greater proportion of CO2 is found in Mg carbonates than in Ca carbonates. This method may be widely applicable in countries like China rich in sea/lake saline brine resources containing considerable amounts of magnesium.16 Ferrini et al.3 have proposed a variant of the Solvay process which would produce carbonates focused on nesquehonite from a chloride-rich reactant. In their work, ammonium chloride solution was obtained after separation by filtration and was subsequently subjected to regeneration of ammonia by activated carbon (AC) with the byproduct hydrochloric acid. However, the decomposition method with activated carbon may be far from commercial application.4 In the present work, a novel variant of the combination process focused on the precipitation of hydromagnesite simultaneously with the recovery of salable ammonium chloride17 is proposed to sequester CO2. The general chemical reaction for the novel process involving the precipitation of hydromagnesite from the gas−liquid reactive system of MgCl2−CO2−NH3−H2O can be expressed as
1. INTRODUCTION The rising level of CO2 in the atmosphere, mainly resulting from the combustion of fossil fuels, and its deleterious impact on the climate continues to raise concerns. Since alternative energy sources are not likely to replace fossil fuels soon, and since the atmospheric concentration of CO2 is expected to triple by the end of 21st century with the continued growth of emerging densely populated developing countries, it is necessary to develop effective methods of sequestering CO2.1−3 Numerous approaches to CO2 sequestration, including ocean, terrestrial, geological, chemical, and biological options, are currently being studied, and some commercial scale processes have been demonstrated.1,3−12 However, the implementation of these methods is limited because of either their extensive energy cost or unsteady behavior of CO2.10 Mineral sequestration via reaction of CO2 with Mg−Ca silicate rocks in aqueous solutions11,12 offers an attractive option for the permanent and safe storage of CO2 in a solid form because the resulting neoformations are thermodynamically stable at ambient temperature.13 This method, first proposed by Seifritz,14 requires cations, i.e. Mg and Ca in silicates as olivine and serpentine-group minerals, to neutralize CO2 through formation of carbonates. Unfortunately, the industrial extraction of Ca and Mg from silicate minerals requires expensiveprocessing, which contributes to the problem rather than to the solution.15 Furthermore, this option is not at all practical in many countries owing to the paucity of exposed basic and ultrabasic rocks.3 An attractive alternative involves the interaction of ions in aqueous solution with CO2.3 The sources of Ca and Mg in such a process could be sea-lake brines,10 which result in the precipitation of carbonates proceeding much more rapidly than when the cations are locked in a silicate structure. Mg carbonates are the focus herein since Mg is readily available © 2012 American Chemical Society
MgCl2 + CO2 + NH3 + H 2O → hydromagnesite + NH4Cl
(1)
Following reaction 1, hydromagnesite is separated by filtration, leaving behind an ammonium chloride-water mixture. The recovery of valuable byproduct of ammonium chloride can Received: Revised: Accepted: Published: 16299
August 24, 2012 November 26, 2012 November 26, 2012 November 26, 2012 dx.doi.org/10.1021/ie302271u | Ind. Eng. Chem. Res. 2012, 51, 16299−16310
Industrial & Engineering Chemistry Research
Article
Figure 1. Flow diagram of the experimental equipment: a. mass flow controller; b. peristaltic pump; c. MSMPR crystallizer; d. stirrer; e. sampling; f. pH-thermometer; g. absorber flask (10% H2SO4); h. desiccant; i. mass flow controller.
2. EXPERIMENTAL SECTION 2.1. Chemical Agents. Magnesium chloride hexahydrate (98%, Beijing Chemical Plant) of analytical grade was used without further purification in the experiments. A series of magnesium chloride solutions, concentrations ranging from 0.75 to 1.5 mol·L−1 with an interval of 0.25 mol·L−1, were prepared by dissolving magnesium chloride hexahydrate in double distilled water (conductivity
