Kinetics of sodium bicarbonate decomposition

Abstract

A thermal gravimetric method was used to measure rates of decomposition of NAHCO, particles. Such decomposition produces a highly porous Na,Co, that reacts with SO, rapidly and completely at moderate temperatures. Hence, NaHCO, decomposition provides a reactant with attractive features for SO, removal. The rapid rate of decomposition combined with the high heat effect prevented determining intrinsic rates by constant temperature runs when the temperature level was above 400 K. However, rising-temperature runs, which allowed time for heat transfer to equilibrate temperatures of the thermocouple and particles, gave reliable results at high temperatures. The activation energy was 102 kJ/mol. Porosimeter data verified the large increase in pore volume (from 0.03 to 0.39 x lo-, m3/kg) on converting the NaHCO, particles to Na,CO,.First-order kinetics were observed up to high conversions, after which the apparent order decreased. However, the sodium bicarbonate could be completely converted to Na,CO,. SCOPEIt has been well established that thermal decomposition of NaHCO, particles can produce a highly porous Na,CO, product that reacts rapidly with SO, in waste gases (Bares et at., 1970; Marecek et al., 1970 Our objective was to determine the rate of decomposition of a sample of NaHCO, particles at temperatures from 373 to 473 K over the complete range of conversion. A TGA apparatus was employed since this allowed very small sample masses to be used (leading to accurate rates) and the effect of heat and mass of transfer processes could usually be eliminated. Porevolume data also were obtained on NaHCO, and Na,CO, particles. CONCLUSIONS AND SIGNIFICANCEThe rapid rate of decomposition of NaHCO, combined with a large, endothermic heat effect required careful choice of operating conditions to obtain rates unaffected by heat transfer. At temperatures above 400 K this required the TGA experiments to be conducted by the rising-temperature method. The results showed that intrinsic kinetics were first-order in mass of NaHCO, up to high conversions. The retardation of the rate beyond this point may be due to changes in the AIChE JournalSeptember 1986 nature of the remaining Na,CO,, or the structure of the Na,CO, product surrounding the unreacted Na,HCO,. However, the rate remained high at high conversions so that the NaHCO, was completely converted to the carbonate. The data also showed that the rate decreased when the decomposition temperature was reduced from 473 to 373 K. Corresponding pore-volume data indicated that the surface area was greater for particles decomposed at 373 K.

Significance of the kinetics of thermal decomposition of NaHCO3 evaluated by thermal analysis

“…It seems meaningful to compare the present results with those obtained under the condition comparable with the present one, in particular with a smaller sample size. The values of E obtained compare with those reported by Van Dooren and Mueller [13] and Hu et aL [1].…”

“…As for the kinetic obedience, the first order (F1) law was observed by Hu et al [1]. This is not necessarily contradictory to the present result because the F 1 law was not markedly inferior and the exponent m in the A m function was extended to nonintegral values between unit and four to find the better linearity [10].…”

“…This reaction proceeds in a single step to yield NazCO3, which is well known as one of the main stages in the Solvay process. 2NaHCO 3 -* Na2CO 3 + H2O + CO 2 There are some discrepancies among experimental results reported earlier as well as their interpretation for the kinetics of the decomposition of NaHCO 3 [1][2][3][4]. This implies that the kinetics of the reaction is greatly influenced, as is usually the case with solid-state reactions, by the sample and measuring conditions.…”

Kinetics of the sodium carbonate–sulfur dioxide reaction

“…Active sodium carbonate is obtained by the decomposition of sodium bicarbonate (Wang Hu et al, 1986;Subramanian et al, 1972). The overall kinetics of the reaction of SO, with Na2C0, has also been investigated.…”

Reaction kinetics of hydrogen chloride with sodium carbonate

“…The BET surface areas and the pore specific volumes of Na 2 CO 3 , measured with a Carlo Erba Sorpotomatic 1900, were 3.38×10 3 m 2 kg −1 and 3.11 × 10 −4 cm 3 g −1 , and 3.78 × 10 3 m 2 kg −1 and 6.21 × 10 −3 cm 3 g −1 for the 120 and 209 m sorbent, respectively. As reference, Hu et al (1986) reported, for particles of NaHCO 3 of 125 m diameter decomposed at 100 • C to Na 2 CO 3 , a specific surface area of 3.3 × 10 3 m 2 kg −1 .…”