Article pubs.acs.org/crystal
Solubility and Growth Kinetics of Ammonium Bicarbonate in Aqueous Solution Daniel Sutter and Marco Mazzotti* Institute of Process Engineering, ETH Zurich, Sonneggstrasse 3, 8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: The formation of solid compounds in the CO2−NH3−H2O system is an important challenge in the continuous operation of ammonia-based CO 2 capture processes. As the concentration of CO2 in the solvent increases along the absorption column, the formation of solidsmost importantly ammonium bicarbonate (NH4HCO3)may lead to clogging of the packing. If properly controlled, the formation of solids and their separation from the solution offer potential for improvements in the energy penalty of the capture process. This work introduces an experimental setup for seeded growth measurements in this complex system with highly volatile solutes. The crystal growth rate parameters of NH4HCO3 are determined from such experiments using a population balance equation model.
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INTRODUCTION Solid formation in the CO2−NH3−H2O system received significant research interest in the 1920s to 1940s, with a focus on the exact composition of the intermediate solid compounds and their solubility.1−6 The experimental investigation of this system is complicated by the evaporation of CO 2 and NH 3 from the solution as well as by the decomposition of the salts,2,3,7 which may explain the variation in the available data.2 More recently, the development of ammonia-based CO2 capture processes, also known as the chilled ammonia process (CAP), sparked intensified interest in the thermodynamics and kinetics of this system. Such processes allow capturing CO2 from flue gas streams of large CO2 point sources, e.g., coal and natural gas fired power plants or cement plants, in the context of CO2 capture and storage (CCS). The CCS system of technologies enables drastic and rapid reductions of the global CO2 emissions while ensuring a safe and reliable, yet flexible, supply of energy through the utilization of fossil fuels. Thus, it represents an important option in the portfolio of measures required to mitigate climate change and a valuable bridging technology that can support the shift toward a sustainable energy supply.8 In the CAP, a cold ammonia solution absorbs CO2 from the flue gas in an absorption tower that contains a packing material which increases the interfacial contact area. The resulting CO2rich solution releases CO2 with high purity upon heating, and the (partially) regenerated ammonia solution can be recycled to the absorption section. At several points in the process flowscheme, the liquid composition may exceed the solubility of the intermediate compounds at the relevant temperature. Solid formation can lead to clogging of important instrumenta© XXXX American Chemical Society
tion or entire process units and force a plant shutdown. The criticalities for solid formation in the entire process have been analyzed recently.9 On the other hand, the formation of solids can increase the CO2 uptake capacity of the solvent with beneficial effects on the specific energy demand per kg of CO2 captured. We have developed the so-called controlled solid formation-chilled ammonia process (CSF-CAP), that successfully reduces the energy demand by exploiting solid formation.10 Besides a good understanding of the system thermodynamics, the rate-based modeling and detailed design of the solid handling equipment require information about the kinetics of solid formation. This work aims at measuring the growth rate of ammonium bicarbonate (NH4HCO3, in the following also “BC”) in aqueous solution. At the conditions typically prevailing in the CAP, ammonium bicarbonate is the most likely solid to be formed. An experimental setup enabling seeded growth measurements was developed. By focusing on the binary system BC−H2O, some of the experimental challenges related to the continuous in situ measurement of the liquid composition could be resolved. A calibration procedure for infrared spectroscopy that treats BC as a single solute (despite the speciation reactions occurring) was established, and a syringe-based injection system enables the addition of seeds into a closed and pressurized system. The rate constants of crystal growth were regressed on the basis of seeded growth measurements and a model that couples a population balance equation (PBE) with the mass balance of the system. Received: December 1, 2016 Revised: March 17, 2017
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DOI: 10.1021/acs.cgd.6b01751 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
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Article
Thomsen and Rasmussen11 developed a detailed model for the CO2−NH3−H2O system on the basis of their extended UNIQUAC equation and the Soave−Redlich−Kwong equation of state. The model was updated with an extended database and improved parameter fitting by Darde et al.12 This work applies the updated model by Darde et al.,12 whose essentials are summarized in Figure 2.
THERMODYNAMICS The binary system BC−water represents a special case of the three-component system CO2−NH3−H2O, a complex reactive system. Four different solid intermediate compounds can be formed, and all three components are volatile and undergo significant evaporation under a broad range of conditions. Including the self-ionization of water and the formation of ice, there are in total five coupled reaction equilibria, five solid− liquid equilibria (SLE), and three vapor−liquid equilibria (VLE) to be considered. Figure 1 highlights the BC−water
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EXPERIMENTAL SECTION
Experimental Setup and Materials. The CO2−NH3−H2O system is difficult to explore experimentally due to the high vapor pressure of CO2 and ammonia. Many state-of-the-art experimental techniques for crystallization research have been developed for the relatively large organic molecules typically encountered in the pharmaceutical industry. These methods neglect the evaporation of the precipitating substance and can thus not be applied to the CO2− NH3−H2O system directly. The experimental setup developed for our investigation aims at enabling in situ measurements in a closed and gastight system that minimizes effects of the VLE on the liquid composition. It consists of a jacketed chromium steel reactor (Premex Reactor AG, Switzerland; CC230 thermostat, Huber, Switzerland) with a flat lid and a circular Kalrez sealing. The design of the reactor allows maximizing the liquid filling level in order to keep the volume fraction of the gas phase below 1%, so as the vapor phase and its changes in composition can be neglected in the overall mass balance. In-situ measurements are enabled by custom-made pressure connectors in the lid of the reactor for inserting immersion probes for focused beam reflectance measurement (FBRM; Lasentec, Redmont, WA) and attenuated total reflectance - infrared spectroscopy (ATR-FTIR; ReactIR 45m with DiComp probe, Mettler-Toledo, Greifensee, Switzerland), as well as a Pt100 temperature probe. Furthermore, the reactor is equipped with a pressure sensor, a rupture disk, and a magnetically coupled 4-bladed stirrer with 45° pitched paddles. The pressure rating of the reactor is 6 bar. For a schematic illustration of the reactor setup, see Figure 3. In addition to the online measurements, particle size distributions of the seeds were determined by Coulter counter measurements (Coulter Multisizer 3, Beckmann, Nyon, Switzerland). The Coulter principle is based on measuring the variation in the electric conductance when the particles pass through a precisely defined aperture. Hence, sufficient conductivity of the solution has to be guaranteed while avoiding the evaporation of ammonia or CO2. A 5% wt ammonium thiocyanate in 2-propanol solution, saturated with ammonium bicarbonate, was found to fulfill these requirements. For the measurement, dry particles were suspended in this electrolyte solution. Moreover, optical microscopy (Zeiss Axioplan, Feldbach, Switzerland) was applied to determine the particle shape and to check for traces of attrition and agglomeration.
Figure 1. Isothermal phase diagram for the CO2−NH3−H2O system at 20 °C and 1 bar in weight fractions. Construction and use have been described in detail elsewhere.9,13 The dashed orange line indicates the binary system studied in this work.
binary studied in this work in the underlying ternary system. The binary system exhibits a eutectic at −3.9 °C,2 which could be confirmed in this work and effectively limits the feasible operating range for the experimental study. The formation of ice at temperatures below the eutectic leads to a distinct increase in pressure, such that it can be easily detected. Because of the complete solidification of the solution, experiments can however not be extended below the eutectic temperature.
Figure 2. Graphical illustration of the thermodynamic system and model. B
DOI: 10.1021/acs.cgd.6b01751 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
The FBRM indicates when nucleation occurs, with the consequence that subsequent measurements are discarded from the calibration set. The indication of nucleation by the FBRM represents an approximation, since the nuclei have to grow to an observable size and quantity, as with every available tool for particle detection. Here, the applicability of this approximation is supported by the fact that only a small amount of solute should be consumed by the nucleation of particles before they reach the detection limit. Second, IR measurements in suspensions at low temperature could be added to the calibration set after determining the liquid phase composition through titration. Therefore, the cooling ramp described above was continued to temperatures between −3.5 and 0 °C. After several hours of equilibration, the stirrer was switched off, and a liquid sample was taken after the solids had settled. At such low temperatures, the pressure in the reactor is around 1 bar. Hence, the sample could be taken through an empty connector port in the lid, which can be opened to insert a pipet. After that, the bottom outlet of the reactor was opened, the suspension was filtered and washed as described above, and the mass of solid was determined. The material balances for CO2 and NH3 can be closed with
