The solid remains white but crumbles into a powder as the water is added. Calcium hydroxide also known as slaked lime is used to neutralise excess acidity, for example, in lakes and soils affected by acid rain. Calcium hydroxide dissolves in excess water to produce calcium hydroxide solution limewater , which is used to test for carbon dioxide. This is how naturally ocurring acid rain is able to chemically erode limestone resulting in cave formation. When this solution evaporates the reverse reaction occurs resulting in the formation of stalactites and stalagmites.
How does carbon dioxide react with limewater? Stefan V. Aug 5, The carbonation of lime at low temperature can then be said to occur in several steps Kalinkin et al. However, studies show high carbonation conversion of Ca OH 2 is possible at ambient conditions and, therefore, it can be concluded that the new solid phase on the surface of the sorbent particle is non-protective Nakicenovic et al.
Since these carbonation reactions Eqs. As the solubility of gases in water is greater at lower temperatures, the carbonation of Ca OH 2 at a lower temperature is enhanced Dheilly et al. The objective of this work was to investigate the use of natural lime and pelletized lime-cement particles for the direct capture of CO 2 from air at ambient conditions, with the sorbents being regenerated via temperature swing. Two particle sizes were considered and experiments were performed in a fixed bed.
Using these data, a model was developed to simulate the breakthrough curves and to extract the carbonation kinetics data for various operating conditions. The lime and cement were mixed in a mass ratio of in a mechanical pelletizer Glatt GmbH.
Water was then sprayed into the vessel continuously. The resulting pellet size was controlled by the speed of an agitator and chopper attached to the vessel. Table 2 shows some physical properties of the natural and pelletized lime sorbents.
The samples with desired particle size range were obtained by sieving. The results presented in Table 2 show that the natural lime particles have a greater bulk density while the pellets have a greater pore surface area, primarily due to the formation of mayenite during calcination Manovic and Anthony a , b. The schematic of the experimental setup is shown in Fig.
A compressor was used to circulate atmospheric air through a water bubbler and then a packed bed of sorbent. It should be noted that experiments without the presence of humidified air were also performed to show the effect of humidity for air capture with lime-based sorbents at low temperature.
The air stream exiting the bed was sent to an online gas analyzer to determine the residual CO 2 content, C , of the stream leaving the system. The column containing the bed of particles is a stainless steel tube with a height of The fixed bed height was set at 68 mm, yielding an aspect ratio of around 8 that will mitigate the impact of entrance effects. Gas flow rates of 0. The particle Reynolds number varied between 0. Before each experiment, the bed material was pre-hydrated for 3 h by passing nitrogen through the water bubbler and then the fixed bed.
An experiment was typically conducted for only 1 carbonation cycle. For the experiments performed in series of cycles, after each carbonation cycle, sorbents were calcined and prepared for the next cycle. Figures 3 and 4 show that, although the natural limestone or pelletized limestone particles are pre-hydrated for 3 h, the absence of humidity in the inlet air led to a relatively short breakthrough time Fig. The conditions for all experiments are as given in Fig.
These results are in agreement with those in the literature, showing that negligible carbonation occurs when there is insufficient humidity in the reactive system Beruto and Botter ; Dheilly et al. This phenomenon is due to the partial carbonation of sorbents in the first layers of the bed. While there is competition between CO 2 and water to react with CaO, the partial carbonation reaction on the surface of the sorbents prevents further hydration and decreases the reaction rate at the surface.
Figure 5 shows the effect of particle size and gas flow rate on the breakthrough time and breakthrough curve for both the pelletized and natural limestone. As the slopes of the breakthrough curves are similar for two different gas flow rates, the external mass transfer resistance from bulk to the surface of the particles is, as expected, negligible.
The breakthrough time using the pelletized limestone particles was shorter primarily due to the lower content of CaO as a result of the binder as the total mass loading into the bed was kept constant. Impact of particle size and gas flow rate on the breakthrough curve. Table 4 presents the carbonation conversion at the top, middle, and bottom of the bed.
This shows that a relatively uniform carbonation conversion along the bed was obtained, confirming the necessity of both moist air and pre-hydration see axial conversion variation in Table 3. Moreover, a mass balance on CO 2 in the gas phase was performed to confirm the sorbent carbonation conversion measured by TGA:. After each carbonation period, the sorbent conversion was measured by TGA and after each calcination period, the sorbent BET surface area was measured.
Table 5 shows the carbonation conversion and BET surface area after selected cycles for pelletized and natural limestone particles. Although the natural limestone sorbents have an initial carbonation conversion greater than that of the pelletized limestone sorbents, after the same number of cycles, their net decay due to sintering is greater since the pellets have mayenite, which helps better maintain the structural morphology of the particle.
Hence, fitting the breakthrough curves from this study with the previously developed surface reaction rate-limiting models is not suitable since diffusional resistance plays a more pronounced role. For non-catalytic gas—solid reactions, the unreacted shrinking core model is one of the simplest models as it does not require structural parameters of the reacting solid.
Usually, three major steps are considered in such models:. Diffusion of the gaseous reactant CO 2 through the gaseous film surrounding the particle to the particle surface; not a rate-limiting step in the present setup. Diffusion of the gaseous reactant through the product layer to the unreacted core; characterized by the effective diffusivity, D e.
Reaction of the gaseous reactant with the solid at the unreacted core surface; characterized by K s , the surface reaction rate constant. In this research, the model accounts for reactions given in Eqs. Anthony , and Christoph R. Lu , Arturo Macchi , Robert T. Symonds , and Edward J. Al-Jeboori , Paul S. Fennell , Michaela Nguyen , and Ke Feng. Kierzkowska , and Christoph R. Radfarnia and Maria C. ACS Catalysis , 2 8 , Statnick , and Liang-Shih Fan.
Symonds , Dennis Y. Lu , Vasilije Manovic , and Edward J. Arias , G. Grasa , J. Abanades , V. Manovic , and E. Church , and Andrew T. Kierzkowska , Marcin Broda , and Christoph R.
Gunugunuri , Sotiris E. Pratsinis , and Panagiotis G. The Journal of Physical Chemistry C , 50 , Arias , J. Abanades , and E. Grace , and C. Jim Lim. Belova , Tuncel M. Yegulalp , Robert J. Farrauto , and Marco J.
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