Characteristic and kinetics of light calcination of magnesite in micro fluidized bed reaction analyzer

doi:10.11949/0438-1157.20190411

Abstract

Abstract:

The calcination reaction characteristics and reaction kinetics of magnesite powder in nitrogen and air atmosphere were studied by microfluidic bed reaction analyzer (MFBRA) and thermogravimetric analyzer (TGA). It was shown that the activation energy of magnesite calcination measured in nitrogen and air was quite similar, indicating that the oxygen in the air had little effect on the calcination of magnesite. The time for complete decomposition of magnesite in MFBRA was short as a few seconds in MFBRA at temperatures above 800℃, showing its reaction rate much quicker than that in TGA，which revealed the difference between TGA and MFBRA in measuring the reaction kinetics. Both activation energy and pre-exponential factor are significantly lower in MFBRA (about 125 kJ/mol, 10⁵ s^-1) than in TGA (about 200 kJ/mol，10¹⁵ s^-1). The gas product CO₂ produced by magnesite decomposition is difficult to fully flow out from the crucible of TGA and thus inhibits the decomposition reaction of magnesite. For MFBRA, it can effectively reduce the inhibition effect from the formed CO₂ that easily flows into the main gas flow surrounding the particles. It is also found that the magnesite decomposition reaction follows the nucleation and growth control model in both MFBRA and TGA. Overall, for the reaction in fluidized beds, the study verifies that MFBRA provides a reliable secure of the reaction rate and kinetic parameters for the gas-formation calcination or decomposition reactions like tested light calcination of magnesite.

Key words: fluidized bed, kinetics, magnesite, light calcination, diffusion

摘要：

通过微型流化床反应分析仪（MFBRA）和热重分析仪（TGA）对比研究了菱镁矿粉在氮气和空气气氛中的煅烧反应特性和反应动力学特性。结果表明：菱镁矿在氮气和空气中测得的表观活化能相似，说明空气中的氧气对菱镁矿的煅烧没有影响。MFBRA中菱镁矿粉在800℃以上完全分解的时间只有短短数秒，显著少于在TGA中完全分解的时间，揭示了TGA和MFBRA在测定反应动力学方面的差异。MFBRA测定菱镁矿在空气和氮气气氛中分解的表观活化能（约125 kJ/mol）和指前因子（10⁵ s^-1）显著低于TGA所得结果（约200 kJ/mol，10¹⁵ s^-1），进一步测试表明这与TGA中的反应受气氛抑制有关。即菱镁矿在TGA坩埚内分解产生的CO₂不能及时随气流排出，抑制了轻烧反应的进行，而MFBRA可有效降低外扩散对反应的影响。但两种仪器测试的反应均遵循成核与生长控制模型。对于流化床反应，证明了MFBRA测试结果具有更高的准确性，为有气体生成的煅烧或分解反应（如菱镁矿轻烧）提供了可靠的反应测试方法和仪器。

关键词: 流化床, 动力学, 菱镁矿, 轻烧, 扩散

CLC Number:

TQ 028.8

Weiwei JIANG, Wenqian HAO, Xuejing LIU, Zhennan HAN, Junrong YUE, Guangwen XU. Characteristic and kinetics of light calcination of magnesite in micro fluidized bed reaction analyzer[J]. CIESC Journal, 2019, 70(8): 2928-2937.

姜微微, 郝文倩, 刘雪景, 韩振南, 岳君容, 许光文. 微型流化床内菱镁矿轻烧反应特性及动力学[J]. 化工学报, 2019, 70(8): 2928-2937.

Figures/Tables 13

Fig.1 Schematic diagram of used MFBRA

Fig.2 CO2 formation curve of magnesite calcination in MFBRA detected using process mass spectrometry

Fig.3 Variation of conversion with time for magnesite calcination in MFBRA at different preset temperatures

Fig.4 Reaction rate varying with conversion for magnesite calcination corresponding to Fig.3

Fig.5 Variation of conversion with time for magnesite calcination in TGA at different heating rates.

Fig.6 Fitting curve of ln(dx/dt) versus 1/T for magnesite calcination reaction in MFBRA

Table 1 Activation energy of magnesite calcination reaction calculated according to isothermal model for MFBRA-measured data in N2 and air atmospheres

模型	N₂			Air
模型	E/ (kJ/mol)	lgA	R ²	E/ (kJ/mol)	lgA	R ²
G(x)= -ln(1-x)	124.45	5.61	0.997	125.09	5.69	0.996
G(x)=1-(1-x)^1/3	128.67	5.36	0.997	129.32	5.39	0.996
G(x)=1-(1-x)^1/2	123.25	5.30	0.998	121.08	5.08	0.998

Fig.7 Variation of conversion with temperature measured in TGA at different heating rates

Fig.8 Relationship of lgβ with 1/T for light calcination of magnesite power from tests in TGA

Fig.9 Relationship of activation energy with conversion for magnesite power calcination in TGA

Table 2 Fitting results using Coats-Redfern formula for literature-reported mechanism models based on TGA data

模型		β→0	β=1℃/min		β=5℃/min		β=10℃/min
模型		E/(kJ/mol)	E/(kJ/mol)	r	E/(kJ/mol)	r	E/(kJ/mol)	r
N₂	G(x)= -ln(1-x)	201.72	187.88	0.994	224.46	0.997	224.81	0.997
	G(x)=1- (1-x)^1/2	191.02	187.88	0.994	224.48	0.997	224.84	0.997
	G(x)=1- (1-x)^1/3	228.12	166.02	0.993	198.53	0.997	202.75	0.996
air	G(x)= -ln(1-x)	236.76	225.72	0.997	224.79	0.997	224.61	0.997
	G(x)=1-(1-x)^1/2	197.21	187.14	0.995	186.82	0.996	186.59	0.995
	G(x)=1-(1-x)^1/3	209.74	199.72	0.997	198.83	0.997	198.62	0.997

Table 3 Kinetic parameters of magnesite calcination in N2 and air atmospheres measured using TGA and MFBRA

Analyzer	E/(kJ/mol)	lgA	G(x)
TGA (N₂)	205.94	15.68	-ln(1-x)
TGA (air)	236.60	15.81	-ln(1-x)
MFBRA (N₂)	125.74	5.61	-ln(1-x)
MFBRA (air)	124.16	5.69	-ln(1-x)

Fig.10 Curves of magnesite decomposition under different crucibles in N2 and CO2 atmospheres in TGA

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