The mass transfer coefficient for oxygen reacting with a carbon particle in a fluidized or packed bed
The mass transfer coefficient for oxygen reacting with a carbon particle in a fluidized or packed bed
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DOI：
10.1016/s0010-2180(99)00178-9
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发表时间：
2000-06
影响因子：
4.4
通讯作者：
A. Hayhurst
中科院分区：
文献类型：
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作者：
A. Hayhurst
重新考虑了单碳或多孔煤焦颗粒燃烧的简单模型，因为例如在流态化床中的燃烧是使用这样的模型来分析的。首先，考虑O2朝向颗粒的等摩尔反扩散，以及唯一产物CO2的反扩散。其次，CO被认为是燃烧的唯一产物(就像在流态化床中一样)，因此化学要求这样一个燃烧颗粒附近的气体净流量不为零。这就得出了一个普遍的结论：给出有效传质系数Kg的Sherwood数(dkg/D)取决于在直径为d的碳球上发生的反应的化学计量比。重要的参数实际上是ShEMCD，即在反应粒子附近有等摩尔反向扩散(反应物和产物)的Sherwood数。因此，对于在单个孤立的球形颗粒上流动的空气，由著名的公式ShEMCD=2.0+0.69Re1/2Sc1/3给出ShEMCD，它与该粒子发生反应。一般来说，实际的舍伍德数(给出千克)不等于ShEMCD；比率(Sh/ShEMCD)取决于(I)化学反应引起的(流体中)摩尔数的变化和(Ii)流体中反应物的浓度。因此，如果碳在Cs+1/2 O2→CO中氧化，其效果是将来自ShEMCD的kgas减少一个系数(1+y)logm，即(1+yb)和(1+ys)的对数平均值，其中yband ys分别是主体流体和固体表面O2的摩尔分数。在这种特殊情况下(Sh/ShEMCD)=1/(1+y)logm。如果CO在燃烧的碳颗粒周围氧化，了解传质膜的厚度是很重要的。对于流态化或填充床，等摩尔传质的一般经验关联式Shemcd=Sho+αRe1/2(α为常数)可以重写为Shemcd=Sho{1+(d/2)/δ}，其中δ是传质膜的平均厚度。这意味着对于单个孤立的球体与流动流体中的物种发生反应，shemcd=2+d/δ=Nu。因此，强迫对流的作用是通过用Re>；O将δ从Re=O时的无穷大降低到有限值来增加δ。最后，计算了ShEMCD的大小，并与碳球和液滴燃烧的两膜模型的厚度进行了比较。
Simple models for the burning of either a single carbon or a porous coal char particle are reconsidered, because combustion in, for example, a fluidized bed is analyzed using such models. First, equimolar counterdiffusion of O2towards the particle and also of the sole product, CO2, away from it is considered. Next, CO is considered to be the only product of combustion (as in a fluidized bed), so the chemistry requires a nonzero net flux of gases near such a burning particle. This leads to the general conclusion that the Sherwood number (dkg/D), giving kg, the effective mass transfer coefficient, depends on the stoichiometry of the reactions occurring at a carbon sphere of diameter d. The important parameter is in fact ShEMCD, the Sherwood number for there being equimolar counterdiffusion (of reactants and products) near the reacting particle. Thus ShEMCDis given by the well-known statement ShEMCD= 2.0 + 0.69 Re1/2Sc1/3for air flowing over a single isolated spherical particle, with which it reacts. In general, the actual Sherwood number (which gives kg) does not equal ShEMCD; the ratio (Sh/ShEMCD) is shown to depend on (i) the change in the number of moles (in the fluid) caused by the chemical reaction and (ii) the concentration of reactant in the fluid. Consequently, if carbon oxidizes in Cs+ 1/2 O2→ CO, the effect is to diminish kgas derived from ShEMCDby a factor (1 + y)logm, the logarithmic mean of (1 + yb) and (1 + ys), where yband ysare the mole fractions of O2in the bulk fluid and at the solid’s surface, respectively. In this particular case (Sh/ShEMCD) = 1/(1 + y)logm. If the CO oxidizes around the burning carbon particle, it is important to know the thickness of the mass transfer film. For a fluidized or packed bed, the general empirical correlation for equimolar mass transfer ShEMCD= Sho+ αRe1/2(Shoand α are constants) can always be rewritten as ShEMCD= Sho{1 + (d/2)/δ}, where δ is the mean thickness of the mass transfer film. This means that ShEMCD= 2 + d/δ = Nu for one single isolated sphere reacting with a species in a flowing fluid. Thus the effect of forced convection is to increase ShEMCDby reducing δ from infinity at Re = O to a finite value with Re > O. Finally, the magnitude of δ is calculated and compared with the thickness of a two-film model for the combustion of a carbon sphere and also of a liquid droplet.