(737h) Alkaline Crystallization of CaCO3 in a Direct Air Capture Process

Optimization paths for reduction of fine CaCO₃ particle generation are identified to improve calcium retention within an alkaline fluidized bed pellet reactor. This work focuses on the development of knowledge on the temperature effects on the CaCO₃ crystallization in a system with a high pH solution. The crystallization morphologies of CaCO₃ formed were also studied. The development of a lab scale fluidized bed pellet reactor was accomplished following the CaCO₃ crystallization reactor design within Carbon Engineering’s patented carbon capture process. The lab scale device allows temperature dependence and crystalline morphologies to be investigated in a flow environment analogous to the process used by Carbon Engineering.

The Direct Air Capture process used by Carbon Engineering contains a fluidized bed pellet reactor that operates at a pH of 14 or higher. This reactor is fluidized by a CO₃^2- rich KOH solution and is fed with hydrated lime, Ca(OH)₂, to regenerate OH^- and produce crystalline CaCO₃. The Ca²⁺ and CO₃^2- ions precipitate onto the CaCO₃ pellets the reactor is seeded with initially. Fine particles are generated within the reactor when these ions precipitate in solution rather than at the surface of a pellet.

Seeded CaCO₃ crystallization has been well studied in the water softening industry [1] where

pellet fluid dynamics and bed growth are described with a Richardson-Zaki model [2]. Description of bed characteristics by this model employs empirical constants within the drag equation for the pellets in this reactor type. In our earlier work we adapted this model with new empirical constants for the growth of CaCO₃ in a solution with high pH [3]. This earlier model assumes spherical pellets for the surface area (SA) calculation. However, scanning electron microscopy (SEM) images have proven the seed pellets used in this fluidized bed are not spherical but become more round during growth. Hence, the assumption of spherical particles is over simplified and cannot be made in the context of our study.

The development of a kinetic model for the crystallization rate for a fluidized bed used in a CO₂ direct air capture process is investigated similarly from the equations developed for the water softening industry. The crystallization rate expression found by van Schagen et al. [1][2] is applied for this purpose and kinetic parameters are used. For crystallization, knowledge about the transport of supersaturated liquid to the pellet and the crystallization of supersaturated carbonate solution on the pellet are needed. The former depends on the flow pattern in the fluidized bed [1] and earlier work has determined this is comparable to the water softening industry reactors when new empirical constants are substituted [3]. The latter is temperature dependent [1].

Work over a range of pH and temperatures acknowledges that higher pH solutions lead to smaller crystals and suggests a codependence of crystal structure on pH and temperature [4].

Various crystallization morphologies are possible for CaCO₃ including calcite, vaterite, aragonite, ikaite. In stirred beaker experiments run at a pH of 8.2 most crystallization at 60°C is vaterite. At 90°C aragonite is found with vaterite and in cooler systems (30°C) product is primarily calcite [5]. Investigation of morphology provides for insight into surface area available.

A lab scale fluidized bed apparatus consists of a 2-inch schedule 40, 10-foot-long clear PVC pipe mounted vertically on the wall. Turbidity probes were used for measuring fine particles in the influent and effluent flows. A 20 L settling tank ensures that precipitated fines did not recirculate. Within the setting tank a baffle was used to allow optimized settling while minimizing settling tank volume required.

The large-scale reactor runs continuously in-line with the other reactors that make up the direct air capture process. However, to investigate individual effects on the pellets, this lab scale version runs in batches and in isolation. The lab scale reactor was operated in batch mode over a testing time of 8 h. Earlier work allows us to predict how changes in flow parameters for the lab scale system will be observed in scale up [3].

Temperatures between 20 and 50°C were investigated to determine their effects on pellet growth within a fluidized bed environment. To develop full knowledge of alkaline CaCO₃ growth and crystallization, stirred beaker experiments were run at temperatures up to 80°C.

An empirical kinetic parameter for the crystallization rate expression was found and an optimum process temperature is determined as a result. Crystalline morphologies of CaCO₃ at high pH were characterized over the range of 20 to 80°C. An analysis of the available surface area in the reactor based on the grown pellets was also studied.

[1] K. van Schagen, L. Rietveld, R. Babuška, and E. Baars, “Control of the fluidised bed in the pellet softening process,” Chem. Eng. Sci., vol. 63, no. 5, pp. 1390–1400, 2008.

[2] K. M. Van Schagen, L. C. Rietveld, and R. Babuška, “Dynamic modelling for optimisation of pellet softening,” J. Water Supply Res. Technol. - AQUA, vol. 57, no. 1, pp. 45–56, 2008.

[3] L. Burhenne, C. Giacomin, T. Follett, J. Ritchie, J. S. J. McCahill, and W. Mérida, “Characterization of reactive CaCO3 crystallization in a fluidized bed reactor as a central process of direct air capture,” J. Environ. Chem. Eng., vol. 5, no. 6, pp. 5968–5977, 2017.

[4] Y. F. Ma, Y. H. Gao, and Q. L. Feng, “Effects of pH and temperature on CaCO3 crystallization in aqueous solution with water soluble matrix of pearls,” J. Cryst. Growth, vol. 312, no. 21, pp. 3165–3170, 2010.

[5] R. Ševčík, M. Pérez-Estébanez, A. Viani, P. Šašek, and P. Mácová, “Characterization of vaterite synthesized at various temperatures and stirring velocities without use of additives,” Powder Technol., vol. 284, pp. 265–271, 2015.