Kinetics and transport effects in the dehydration of crystalline potassium carbonate hydrate

Abstract

The reaction kinetics and physical transport processes governing the thermal dehydration of solid KzCOy3/2HzO particles were investigated. Isothermal reaction rate data were gathered using a thermogravimetric balance in which narrowly-sized KzC03*3/2HzO crystals were dehydrated under a water vapor atmosphere at different pressures and temperatures. The magnitudes of the heat and mass transfer resistances external to and within the solid product were estimated from solutions of the relevant pseudosteady-state transport equations. In the temperature range 320 to 358 K, the vacuum dehydration of K~COs*3/2HzO crystals smaller than 710 pm (-25 +30 mesh) are accurately modeled by the spherical shrinking-core equation for the chemical rate control regime. In the presence of water vapor, external heat transfer to the particles was sufficient to prevent significant self-cooling; heat and mass transfer resistances within the particles were negligible. The activation energy for K2C03*3/2H20 dehydration is approximately 91 kJ/mol in vacuum; the reaction becomes extremely slow at relative pressures SCOPEOf the literature reports concerned with the dehydration of solid inorganic salt hydrates, a majority are based on information from experiments done in vacuum or in dry atmospheres; as a consequence, the dehydration reaction is most often treated as an irreversible thermal decomposition. The possible application of salt hydrates as absorbents in chemical heat pumps (Stanish and Perlmutter, 1981) provides an incentive to investigate the dehydration of these materials in greater detail and suggests that this process should be treated instead as one part of a reversible gas-solid reaction. This paper reports the results of experimental rate studies of the dehydration of KzC03*3/2H20, a salt hydrate that holds promise as a heat pump absorbent. Experimentally-measured particle reaction data are interpreted in terms of a shrinkingcore gassolid reaction model. The dehydration rate dependence on ambient water vapor pressure is of primary interest and is quantitatively determined for temperature in the range 338 to 358 K. Internal and external heat and mass transfer resistances are estimated for the conditions employed and their roles in the overall particle hydration process are evaluated. CONCLUSIONS AND SIGNIFICANCEThe dehydration of small crystals of KzCO3*3/2HzO may be accurately modeled by the spherical shrinking-core equation for the chemical rate control regime. In the presence of water vapor, heat transfer rates to the particles are sufficiently high to prevent significant self-cooling. Under vacuum conditions, self-cooling becomes important; particle temperature corrections were calculated from a heat balance equation that takes account of the radiation heat transfer to the solid particles and supporting sample pan. In either atmosphere, heat transfer resistance within the particle is negligible. Diffusional resistances within the product layer are not important for particles smaller than about 710 pm (-25 +30 mesh), but are...

Characterization of potassium carbonate salt hydrate for thermochemical energy storage in buildings

“…17 (b) shows that as pressure ratio increases, the reaction rate decreases. According to the literature [30] , similar experimental results have been observed, that is, when the pressure ratio ( P w / Peq ) approaches to 1, reaction rates becomes closer to zero. Analogous to the case of hydration, it can be concluded that the pressure function is also a driving force for dehydration.…”

“…It is a single step reaction leading to a simple kinetic mechanism. The only stable states of hydration reported in literature [5,24,30,31] and experimentally observed in the present work are the anhydrate and sesquihydrates forms.…”

Thermodynamic analysis of dehydration of K2CO3·1.5H2O

“…Early studies on the dehydration of K 2 CO 3 •1.5H 2 O have suggested the existence of monohydrate [14,15] or hemihydrate [16]; however, this was never fully confirmed [17,18]. The study by Stanish et al [18] describes the dehydration process of K 2 CO 3 •1.5H 2 O as a single step process of which the subsiding speed can be explained with the shrinking core model. This model describes reaction progress from outside of a particle towards its core, during which a shell of reacted material is formed, often causing diffusion issues.…”

Kinetics of hydration‐dehydration reactions considered as solid transformations

“…18. Using the rates reported for KzC0r3/zHz0 crystals by Stanish and Perlmutter (1983, Figure 6), a least-squares fit yields the activation energy t[J = 1.54 X lO-19J (with a standard deviation u = 0.05 X lO-19J) and the rate constant k, = 6.65 X 108 mK1/2/s.…”

“…to emphasize the linearity to be expected between the LHS and the grouping l/T3[ln(P/P,q)]2. This prediction is tested in Figure 1 for K2C0r3/2Hz0 at three different temperatures (Stanish and Perlmutter, 1983, Figure 5). Using the least-squares criterion, the best straight line through the origin gives the results T?…”