Simulation of intermittent microwave drying of coal slime and dielectric properties

doi:10.16085/j.issn.1000-6613.2022-1054

Abstract

Abstract:

Microwave drying is an advanced technology to improve the drying efficiency of coal slime and intermittent drying technology has greater improvement potential in energy consumption and overtemperature. In microwave drying process, dielectric properties of coal slime can significantly affect the drying performance. However, there is a lack of research on the mechanism of influence of dielectric properties on microwave drying of coal slime. A multiphase porous media model was developed for intermittent microwave drying. Then the validity of the model was verified by comparing with experimental data in literature. Considering that the dielectric properties were function of temperature and moisture content, the influence of temperature and moisture content-related dielectric properties (VP) on the average temperature, vapor mass fraction, gas phase pressure and liquid water saturation of coal slime was analyzed in this paper by comparing with the conventional method (CP) which treated dielectric properties as constant. The results showed that average temperature of the CP was significantly lower than VP, and temperature rise rate did not change significantly during drying process, while average temperature of VP showed a trend of increasing sharply at first and then maintaining stability. Although the vapor mass fraction and gas pressure distribution trend of CP were the same as VP, CP was significantly lower than VP. VP could simulate the "pumping effect" during intermittent microwave drying, whereas CP could not effectively show the phenomenon. VP was more realistic than CP.

Key words: coal slime, intermittent microwave, drying, porous media, dielectric property, numerical simulation

摘要：

微波干燥是一种提高煤泥干燥效率的先进技术，其中间歇式在能耗和超温问题上具有更大的改进潜力。在微波干燥过程中，煤泥的介电性质会显著影响干燥性能，然而当前缺乏介电性质影响煤泥微波干燥的机理研究。本文首先建立了一种适用于间歇微波干燥的多相多孔介质模型，通过与实验数据对比，验证了模型的有效性。由于介电性质实际是温度和水分含量的函数，从机理上着重分析了温度、水分含量相关的介电性质（VP）对煤泥平均温度、水蒸气质量分数、气相压力及液态水饱和度的影响，通过与以往处理成常数的介电性质（CP）相对比，结果发现，CP平均温度明显低于VP，并且温升速率在干燥过程中没有显著变化，而VP的平均温度表现出先急剧升高后维持稳定的趋势。CP水蒸气质量分数和气相压力分布虽然与VP分布趋势相同，但是CP明显低于VP。VP能够模拟出间歇微波干燥期间的“泵送效应”，而CP不能有效显示此现象。因此在煤泥间歇微波干燥中VP比CP更加符合实际。

关键词: 煤泥, 间歇微波, 干燥, 多孔介质, 介电性质, 数值模拟

CLC Number:

TQ536

WANG Guangyu, MENG Jinghui, ZHANG Kai. Simulation of intermittent microwave drying of coal slime and dielectric properties[J]. Chemical Industry and Engineering Progress, 2023, 42(4): 1779-1786.

王光宇, 孟境辉, 张锴. 煤泥间歇微波干燥模拟及介电性质[J]. 化工进展, 2023, 42(4): 1779-1786.

Figures/Tables 11

参数	值或者表达式	参数	值或者表达式
液态水介电常数 $ε w'$	-0.283T+80.67^[18]	干空气介电常数 $ε a'$	1
固体基质介电常数 $ε s'$	1.88^[19]	液态水介电损失系数 $ε w ″$	0.05T+20^[18]
干空气介电损失系数 $ε a ″$	0	固体基质介电损失系数 $ε s ″$	0.1^[19]
液态水密度 $ρ w$	1000kg/m³	固体基质密度 $ρ s$	1250kg/m^3[19]
液态水定压比热容c_p_,w	4183J/(kg·K)^[20]	水蒸气定压比热容c_p_,v	2062J/(kg·K)^[20]
固体基质定压比热容c_p_,s	1250J/(kg·K)^[12]	液态水热导率k_th,w	0.644W/(m·K)^[20]
水蒸气热导率k_th,v	0.026W/(m·K)^[20]	干空气热导率k_th,a	0.026W/(m·K)^[20]
固体基质热导率k_th,s	0.46W/(m·K)^[19]	液态水黏度 $μ w$	$ρ w e x p (- 19.143 + 1540 / T)$ Pa·s^[12]
气相黏度 $μ g$	$0.017 × 10 - 3 T / 273 0.65$ Pa·s^[12]	液态水固有渗透率k_w	10^-15m^2[21]
气相固有渗透率k_g	$1 × 10 - 14$ m^2[21]	液态水相对渗透率k_r,w	(S_w)^3[22]
气相相对渗透率k_r,g	S_w<1/1.1时，取值1~1.1S_w； S_w>1/1.1时，取值0^[22]	液态水毛细扩散系数D_c	$10 - 8 e x p - 6.88 + 8 M w b$ m²/s^[13]
二元扩散系数D_va	$2.6 × 10 - 6$ m²/s^[19]	气相有效扩散系数D_eff,g	$D v a S g φ 4 / 3$ m²/s^[23]
平衡蒸气压p_v,eq	$p v, s a t T e x p (- 0.182 M d b - 0.696 + 0.232 e - 43.949 M d b M d b 0.0411 l n p v, s a t T)$ Pa^[24]	蒸发速率常数K_evap	1000s^-1[13]
环境中水蒸气分压力p_v,air	1420Pa	传质系数h_m	0.011m/s^[13]
传热系数h_T	10W/(m²·K)^[13]	周围空气温度T_air	300K
干基含水量M_db	$(c w + c v) / (1 - φ) ρ s$	湿基含水量M_wb	$M d b / (1 + M d b)$

参数	值或者表达式	参数	值或者表达式
液态水介电常数 $ε w'$	-0.283T+80.67^[18]	干空气介电常数 $ε a'$	1
固体基质介电常数 $ε s'$	1.88^[19]	液态水介电损失系数 $ε w ″$	0.05T+20^[18]
干空气介电损失系数 $ε a ″$	0	固体基质介电损失系数 $ε s ″$	0.1^[19]
液态水密度 $ρ w$	1000kg/m³	固体基质密度 $ρ s$	1250kg/m^3[19]
液态水定压比热容c_p_,w	4183J/(kg·K)^[20]	水蒸气定压比热容c_p_,v	2062J/(kg·K)^[20]
固体基质定压比热容c_p_,s	1250J/(kg·K)^[12]	液态水热导率k_th,w	0.644W/(m·K)^[20]
水蒸气热导率k_th,v	0.026W/(m·K)^[20]	干空气热导率k_th,a	0.026W/(m·K)^[20]
固体基质热导率k_th,s	0.46W/(m·K)^[19]	液态水黏度 $μ w$	$ρ w e x p (- 19.143 + 1540 / T)$ Pa·s^[12]
气相黏度 $μ g$	$0.017 × 10 - 3 T / 273 0.65$ Pa·s^[12]	液态水固有渗透率k_w	10^-15m^2[21]
气相固有渗透率k_g	$1 × 10 - 14$ m^2[21]	液态水相对渗透率k_r,w	(S_w)^3[22]
气相相对渗透率k_r,g	S_w<1/1.1时，取值1~1.1S_w； S_w>1/1.1时，取值0^[22]	液态水毛细扩散系数D_c	$10 - 8 e x p - 6.88 + 8 M w b$ m²/s^[13]
二元扩散系数D_va	$2.6 × 10 - 6$ m²/s^[19]	气相有效扩散系数D_eff,g	$D v a S g φ 4 / 3$ m²/s^[23]
平衡蒸气压p_v,eq	$p v, s a t T e x p (- 0.182 M d b - 0.696 + 0.232 e - 43.949 M d b M d b 0.0411 l n p v, s a t T)$ Pa^[24]	蒸发速率常数K_evap	1000s^-1[13]
环境中水蒸气分压力p_v,air	1420Pa	传质系数h_m	0.011m/s^[13]
传热系数h_T	10W/(m²·K)^[13]	周围空气温度T_air	300K
干基含水量M_db	$(c w + c v) / (1 - φ) ρ s$	湿基含水量M_wb	$M d b / (1 + M d b)$

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