吸波材料辅助的液体物料微波冷冻干燥多物理场耦合模型

doi:10.11949/0438-1157.20190281

摘要/Abstract

摘要：

为了研究吸波材料辅助微波加热对传统冷冻干燥过程的强化作用，建立了多孔介质温度、浓度和电磁场耦合的多相传递模型；以烧结的碳化硅(SiC)为吸波材料、以甘露醇水溶液为待干料液进行了微波冷冻干燥实验，并测定了甘露醇固体的介电特性。模拟和实验结果均表明，吸波材料对初始非饱和多孔物料微波冷冻干燥具有显著的强化作用。初始非饱和样品微波冷冻干燥时间比传统冷冻干燥缩短了18%，比常规饱和样品传统冷冻干燥缩短了30%。模拟结果与实验数据吻合良好。这表明提出的新型干燥方法确实能够实现过程传热传质的同时强化。通过考察样品内部温度、饱和度和电场强度的实时分布，分析了微波冷冻干燥过程的传热传质和电磁波传播与耗散机理。在微波冷冻干燥过程中，初始非饱和样品累计吸收的辐射能和微波能的总和与传统冷冻干燥相当。这说明，该干燥方法只是提高了能量效率，从而大幅缩短了冷冻干燥时间。

关键词: 微波, 吸波材料, 损耗因子, 饱和度, 干燥, 传热, 传质

Abstract:

To investigate the enhanced effect of wave-absorbing material assisted microwave heating on traditional freeze-drying, a multiphase transport mathematical model conjugating temperature field, concentration field and electromagnetic field was developed and numerically solved. The sintered silicon carbide (SiC) was used as the absorbing material and the mannitol aqueous solution was used. Microwave freeze-drying experiments of mannitol aqueous solution were carried out, and dielectric properties of mannitol solid powder were measured. Both numerical and experimental results show that using wave-absorbing material as the supporting pad of sample can effectively enhance the microwave freeze-drying process. Under the typical operating conditions, microwave freeze-drying time of the initially unsaturated material can be 18% and 30%, respectively, shorter than the traditional freeze-drying process of the initially unsaturated and conventionally saturated samples. Excellent agreements were received between experimental measurements and model predictions. This indicates that the proposed freeze-drying method can achieve the simultaneous enhancement of heat and mass transfer. Based on the profiles of temperature, ice saturation and electrical field strength, mechanisms of heat and mass transfer, as well as electromagnetic wave propagation and dissipation inside a sample were analyzed during drying. The accumulatively absorbed energy of radiation and microwave during drying was almost the same as those of traditional freeze-drying of the initially saturated and unsaturated samples. This demonstrates that the proposed method only increases the energy utilization so as to largely reduce the freeze-drying time.

Key words: microwave, wave-absorbing material, loss factor, saturation, drying, heat transfer, mass transfer

中图分类号:

TQ 028.5

杨菁, 王维, 张朔, 宋春芳, 唐宇佳. 吸波材料辅助的液体物料微波冷冻干燥多物理场耦合模型[J]. 化工学报, 2019, 70(9): 3307-3319.

Jing YANG, Wei WANG, Shuo ZHANG, Chunfang SONG, Yujia TANG. Multiphysics conjugated model for freeze-drying of liquid solution assisted by wave-absorbing material[J]. CIESC Journal, 2019, 70(9): 3307-3319.

图/表 14

图1 微波干燥室物理模型

Fig.1 Physical model of microwave drying chamber

图2 样品组件

Fig.2 Schematic diagram of sample mold

图3 多功能微波冷冻干燥系统流程图

Fig.3 Flow diagram of multifunctional microwave freeze drying system

图4 甘露醇相对介电常数与损耗因子

Fig.4 Dielectric constant and loss factor of mannitol

表1 模拟输入参数

Table 1 Input parameters used in simulation

Parameter	Value	Ref
c _b /(J/(kg·K))	1200	default^①
c _i /(J/(kg·K))	1930	[42]
c _s /(J/(kg·K))	1310	[43]
c _v /(J/(kg·K))	1886	[42]
e _b	0.5	default^①
e _i	0.97	[24]
e _s	0.6	[35]
ΔH /(J/kg)	2.839×10⁶	[43]
K _r/s^-1	300(S ₀=1.0)/30(S ₀=0.28)	[25]
?	0.8798(S ₀=1.0)/0.9631(S ₀=0.28)	Exp.
ε _r,b '	-12.36+0.087T	[33]
ε _r,i '	3.2	[44]
ε _r,s '	2.42894+0.00385T(℃)	Exp.
ε _r,v '	1	[45]
ε _r,b″	27.99	[46]
ε _r,i″	0.003	[44]
ε _r,s″	0.22+0.00038T(℃)	Exp.
ε _r,v″	0	[45]
λ _b /(W/(m·K))	450(300/T)^0.75	default^①
λ _i /(W/(m·K))	2.22	[42]
λ _s /(W/(m·K))	2.64	[47]
λ _v /(W/(m·K))	0.022	[42]
μ _r	1	—
μ _v/(kg/(m·s))	0.011(T/273)^1.5/(T+961)	[48]
ρ _b /(kg/m³)	3200	default^①
ρ _i/(kg/m³)	913	[42]
ρ _s /(kg/m³)	1489	[49]

图5 不同冷冻干燥过程的模拟与实验干燥曲线比较

Fig.5 Comparisons between experimental and predicted drying curves of different processes

图6 初始非饱和样品组件内部温度分布

Fig.6 Temperature distributions in initially unsaturated sample mold

图7 初始非饱和样品内部饱和度分布

Fig. 7 Saturation distributions in initially unsaturated sample

图8 0 s时波导、微波腔体以及样品组件表面的电场强度分布

Fig.8 Electric field strength distribution on surfaces of waveguide, microwave cavity and sample mold at 0 s

图9 0 s时波导、微波腔体以及样品组件内部的电场强度分布

Fig.9 Electric field strength distribution inside waveguide, microwave cavity and sample mold at 0 s

图10 初始非饱和样品内部电场强度分布

Fig. 10 Electric field strength distributions in initially unsaturated sample

图11 底盘内部电场强度分布(t=500 s)

Fig. 11 Electric field strength distribution in supporting pad (t=500 s)

图12 0 W和1 W功率下两种样品组件吸收的能量

Fig.12 Energies absorbed by two sample molds at 0 W and 1 W

图13 初始非饱和样品在不同功率下的干燥曲线

Fig.13 Drying curves of initially unsaturated samples at different microwave powers

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