Gas back-mixing characteristics and the effects on gas-solid reaction behavior and activation energy characterization

doi:10.11949/0438-1157.20200413

Abstract

Abstract:

Using the method of combining cold state research and high temperature reaction, the influence of backmixing on the reaction characteristics and kinetics in the gas-solid reaction process was explored. Firstly, the gas residence time distribution (RTD) was measured by the pulse tracer method to investigate the gas back-mixing characteristics in micro fluidized beds. Parametric influences on the gas back-mixing were analyzed with respect to bed diameter (D), superficial gas velocity (U_g) and medium particle size (d_p). The deviation degree of the gas flow in micro fluidized beds from the ideal plug flow of gas was quantitatively compared to obtain the appropriate operating parameters guaranteeing the minimal gas back-maxing or maximal approach to the ideal plug flow. On this basis, the combustion reaction of active coke was tested using the micro fluidized bed reaction analyzer to understand the variation of reaction kinetics with gas back-mixing at different operating conditions. Obvious effects of D, U_g and d_p were identified for the estimated activation energy of the combustion reaction. There was suppressed gas back-mixing by decreasing inner diameter of bed and increasing the particle size and superficial gas velocity. These make the gaseous reaction products reach the online detection instrument quickly via a nearly plug flow throughout the reactor so that the obtained activation energy is generally higher at lower gas-back mixing.

Key words: gas flow, gas back-mixing, residence time distribution, gas-solid reaction, kinetics, micro fluidized bed

摘要：

采用冷态研究和高温反应相结合的方法，探究了气-固反应过程中返混对反应特性和动力学的影响。首先利用脉冲示踪法考察微型流化床内的气体停留时间分布规律和气体返混特性，揭示床内径D、表观气速U_g、介质颗粒粒径d_p对床内气体返混程度的影响，并分析气体流动偏离平推流的程度，得到其最大程度接近平推流的操作范围。进而，选取活性焦燃烧这一典型气-固反应，分析微型流化床反应分析仪中不同程度的返混状态下等温燃烧的反应行为和活化能演变，再现了D、U_g、d_p对反应测试结果的影响，即：随着床径减小及表观气速和介质颗粒粒径的增大，反应器内产物气体经历的返混程度减小，使得产物气体近平推流的输出并被即时检测，获得更好揭示反应本质特性的反应活化能。活化能数值随返混程度的减少而增大，且更易达到稳定反应状态。

关键词: 气体流动, 返混, 停留时间分布, 气固反应, 动力学, 微型流化床

CLC Number:

HU Dandan, GENG Sulong, ZENG Xi, WANG Fang, YUE Junrong, XU Guangwen. Gas back-mixing characteristics and the effects on gas-solid reaction behavior and activation energy characterization[J]. CIESC Journal, 2021, 72(3): 1354-1363.

胡丹丹, 耿素龙, 曾玺, 王芳, 岳君容, 许光文. 返混对气-固反应特性测试和活化能表征的影响[J]. 化工学报, 2021, 72(3): 1354-1363.

Figures/Tables 9

Fig.1 Equipment for gas back-mixing measurement (a) and micro fluidized bed reaction analyzer (b)

Fig.2 Example of Eout calculation for a typical RTD test in a bed of D = 15 mm, Hs = 20 mm and Ug = 5 Umf of quartz sand

Fig.3 Generation curve of gaseous product by MFBRA and data treatment approach

Fig.4 Gas residence time distribution curves E(t) for different conditions in the fluidized bed reactors

Table 2 Characteristic parameters of RTDs in micro fluidized bed under different conditions

d_p/μm	D/mm	H_s/mm	U_g/U_mf	$t ˉ$ /s	E(t)_h/s^-1	σ_t²/s²	D_a,g/(m²·s^-1)	Pe_a,g	Δt_a/s	Δt_r/%
90	15	20	5	2.08	0.65	0.470	0.00019	18	0.210	11.1
155	10	20	5	0.64	2.44	0.030	0.00040	27	0.044	7.5
	15	20	3	1.38	1.37	0.098	0.00020	27	0.080	7.6
			4	0.85	1.84	0.054	0.00030	26	0.059	7.4
			5	0.68	2.31	0.035	0.00040	25	0.048	7.5
			6	0.58	2.53	0.030	0.00055	22	0.048	9.1
			7	0.50	2.81	0.024	0.00068	21	0.045	9.9
	20	20	5	0.70	1.79	0.063	0.00070	15	0.081	13.2
185	15	20	5	0.47	4.04	0.010	0.00035	41	0.022	4.9

Table 2 Characteristic parameters of RTDs in micro fluidized bed under different conditions

d_p/μm	D/mm	H_s/mm	U_g/U_mf	$t ˉ$ /s	E(t)_h/s^-1	σ_t²/s²	D_a,g/(m²·s^-1)	Pe_a,g	Δt_a/s	Δt_r/%
90	15	20	5	2.08	0.65	0.470	0.00019	18	0.210	11.1
155	10	20	5	0.64	2.44	0.030	0.00040	27	0.044	7.5
	15	20	3	1.38	1.37	0.098	0.00020	27	0.080	7.6
			4	0.85	1.84	0.054	0.00030	26	0.059	7.4
			5	0.68	2.31	0.035	0.00040	25	0.048	7.5
			6	0.58	2.53	0.030	0.00055	22	0.048	9.1
			7	0.50	2.81	0.024	0.00068	21	0.045	9.9
	20	20	5	0.70	1.79	0.063	0.00070	15	0.081	13.2
185	15	20	5	0.47	4.04	0.010	0.00035	41	0.022	4.9

Fig.5 Effect of temperature on conversion (a) and reaction rate (b) during active char combustion in MFBRA

Fig.6 Correlation between ln(dx/dt) and 1/T at different conversions

Table 3 Reaction activation energy at different temperatures and conversions

Conversion x	E^*/(kJ·mol^-1)
Conversion x	600—750℃	750—900℃
0.1	168.4	30.7
0.2	176.1	27.8
0.3	201.1	26.9
0.4	231.4	28.8
0.5	239.3	36.1
0.6	241.9	41.1
0.7	203.8	52.8
0.8	157.3	71.1
0.9	108.8	87.5
average	237.5	44.8

Fig.7 Variation of activation energy in active char combustion under different gas back-mixing conditions

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