Mass transfer and substrate utilization in biofilm reactors

Mass transfer within microbial films is described using Monod-type biological kinetics in terms of properties of packing material and the feed solution. For this purpose, computer techniques have been developed for the numerical evaluation of the normalized biofilm mathematical model. A second-order partial differential equation, describing the mechanisms of dispersion phenomena inside the liquid layer, was then solved to determine the mass transfer coefficient. The application of the theory to earlier experimental data has also been demonstrated.     

气体喷射-气液旋流式冷氢箱的传质特性

开发了一种具有多重旋流结构的气体喷射-气液旋流式冷氢箱,并通过大型冷模实验对其进行传质性能测试。在气体喷射速度10～80 m·s^-1和液体进口速度0.2～0.7 m·s^-1条件下,采用氧吸收法测量了液相体积传质系数k_La,采用空气-Na₂SO₃溶液化学吸收法测量了相界接触面积a。结果表明:k_La和a均随气体喷射速度和液体进口速度增大而增大,其中受气体速度变化影响更为明显;由测定的传质参数数值可知新型冷氢箱传质效果较传统冷氢箱大幅提高,与机械输入设备处于相同量级,具有优异的气液混合性能。还采用量纲分析法对数据进行了归纳拟合,得到传质参数与气相Reynolds数、液相Weber数之间的关联式,可以较好地关联预测冷氢箱的传质性能。     

旋流气液两相强化吸收CO2的传质性能

为强化二氧化碳的吸收过程,采用一类旋流逆向气液多级接触的方式,以NaOH溶液为吸收剂,研究其与大跨度浓度CO₂(2.5%～15%)接触反应的传质性能。分别探讨了吸收剂浓度、吸收剂流量、烟气CO₂浓度、烟气流量及反应温度对气相总体积传质系数(K_ga)的定量影响。结果表明,在实验条件下,其K_ga可达(4.53×10^-5)～(9.22×10^-5)kmol·m^-3·s^-1·kPa^-1。与双级直流喷雾和单级旋流喷雾相比,旋流逆向气液多级接触能够有效强化大跨度浓度CO₂的吸收过程。K_ga随吸收剂浓度、流量和反应温度的增加而增加,随CO₂浓度增加呈现先增加后减小(CO₂浓度大于5%)的非线性关系,随气体流量增加先增加后趋于稳定。     

Mass transfer in the entrance region for axial and swirling annular flow

Rates of mass transfer to the inner core of an annular flow system have been determined for the mass transfer entry region using the limiting current density method. Both in laminar and turbulent flow, the hydrodynamic and concentration boundary layers were not fully developed. The variation of the mass transfer coefficients with length of core section has been demonstrated, and the data for swirling flow correlated by the equation for 1500<Re < 14000, 1500 < Sc < 6200 and 1.75 < L/D^e < 10.14. The data for axial flow in the entry region have been correlated by the equation. for 1800 < Re < 12500 and Sc = 2604. Mass transfer enhancement in the entry region due to swirl when compared to axial flow-systems with a jetting transverse inlet is only noticeable for Re > 6000, but is very significant compared to systems with fully developed boundary layers in axial or swirling flow.     

液-液微尺度混合体系的传质模型

针对微尺度液-液混合体系,考察了流量对膜分散萃取过程的影响,并根据传质过程方程,计算了各种条件下的传质系数和传质速率;采用现有的传质模型分别计算分散相和连续相的分传质系数,然后根据传质阻力的加合性得到总传质系数;应用理论传质系数计算传质效率,与实验值进行了比较.研究结果表明,在微尺度混合条件下,直接影响传质系数的因素是停留时间和液滴直径,传质系数随着停留时间的减小而增大.膜分散萃取的传质系数可以达到1.2×10^-4m&#8226;s^-1,比传统的萃取方式大10～100倍;不能像塔式萃取设备一样,用简单地忽略某一相的传质阻力或用总体平均的简化计算公式来计算微尺度混合的传质性能;考虑滴内滴外传质系数,并考虑时间的影响,利用现有公式分别计算滴内滴外传质系数,并采用阻力加合,可以较为准确地计算微混合条件下的总传质系数,计算值与实验值符合很好.     

Mass and heat transfer behavior of oscillating helical coils in relation to heterogeneous reactor design

Rates of mass and heat transfer at vibrating helical coils were studied by the electrochemical technique with the object of using helical coils as heat exchanger/reactor for conducting liquid–solid diffusion controlled reactions. Variables studied were frequency and amplitude of vibration, tube diameter, and superimposed axial flow velocity. The data for vibrating coil (batch reactor) were correlated for 59 < < 4965; Sc = 2314 by the equation: The data were found to be consistent with the analogy model. For vibrated helical coils with superimposed axial flow, the data were correlated by the equation: Importance of the present results in the design and operation of heterogeneous reactors used to conduct diffusion controlled exothermic reactions involving heat sensitive materials was pointed out. Also the importance of the present results in the design and operation of shell and helical tube heat exchangers of improved performance was highlighted. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3141–3149, 2017     

包膜尿素控制释放过程传质模型

根据质量守衡定律和F ick扩散定律建立了包膜尿素控制释放过程的传质模型,并用3种新颖的石蜡-松香混合物包膜尿素在蒸馏水中的释放数据和扫描电镜照片对模型进行了验证。结果表明,该模型能有效地描述和预测由扩散机制控制的包膜尿素的养分释放行为,拟合的相关系数R2分别为0.997 6,0.997 3和0.998 4。模型显示,包膜尿素的养分释放速率与膜的有效扩散系数Deff成正比,与尿素粒径的平方及膜的厚度成反比。     

A heat transfer relation for swirl flow in a vortex tube

An experimental investigation was made to study the heat transfer behavior of a water-cooled vortex tube with air as the working medium. The vortex flow was generated by leading the compressed air to enter the vortex tube tangentially. The pressures of the inlet air were 300 to 500 kPa gauge. For each of these inlet pressures, the heat transfer performance of the vortex tube was studied at different cold mass flow ratios. New experimental data are presented. An empirical correlation for the prediction of the air-side Nusselt number is established. It was shown that the air-side Nusselt number for the swirl flow in this vortex tube can be up to 30 times higher than that for a flow without swirl in the same tube.     

Large-eddy simulation of swirling gas-particle flows using a USM two-phase SGS stress model

In large-eddy simulations (LES) of gas-particle flows most investigators use single-phase subgrid scale (SGS) stress models. The Interaction between the two-phase SGS stresses is not fully taken into account. In this paper, a unified second-order moment (USM) two-phase SGS stress model for the LES of gas-particle flows is proposed, in which the interaction between the two-phase SGS stresses and anisotropy of the two-phase SGS stresses is fully taken into account. The proposed model is used in the LES of swirling gas-particle flows, together with RANS modeling using the USM two-phase turbulence model. The instantaneous results exhibit the multiple recirculating gas flow structure similar to that of the single-phase swirling flows, but the particle flow structure shows less vortices. The two-phase time-averaged velocities and RMS fluctuation velocities predicted by both LES-USM and RANS-USM models are almost the same and are in good agreement with the experimental results. However, for two-phase RMS fluctuation velocities, the LES-USM results are somewhat better than the RANS-USM results.     

压力式喷嘴雾化过程气液传质性能

对压力式喷嘴雾化过程气液传质性能进行了研究,考察了CO₂-H₂O、C₂H₂-H₂O、O₂-H₂O、CO₂-paraffin系统中液体对气体的吸收速率。液体雾化压力越大,液体对气体的吸收速率越快,而液体最终浓度与液体雾化压力几乎无关。对单液滴进行受力分析,推导出了液滴运动速度与时间的关系式,并针对CO₂-H₂O体系计算了不同液体雾化压力下液滴速度的变化,结果表明,液滴速度衰减很快,在0.6 s内液滴趋近其终端速度。基于渗透模型推导出液滴内浓度变化方程,在不同气液体系中将模型计算值与实验结果进行对比,相对误差都在20%以内。     

Heat and Mass Transfer in Process of Fluidized-Bed Spray Granulation

This article presents a mathematical model of heat and mass transfer for the process of fluidized-bed spray granulation, which can be applied in the analysis of bed temperature profile, temperature and humidity of outlet gas and moisture content of particles. Effects of operation parameters on the batch granulation are investigated. The theoretical calculation agrees reasonably well with the experimental data.     

流化床喷雾造粒过程的传热传质

This article presents a mathematical model of heat and mass transfer for the process of fluidized-bed spray granulation, which can be applied in the analysis of bed temperature profile, temperature and humidity of outlet gas and moisture content of particles. Effects of operation parameters on the batch granulation are investigated. The theoretical calculation agrees reasonably well with the experimental data.Keywords fluidized-bed spray granulation, mathematical model, heat and mass transfer     

Mass transfer around bubbles,drops, and particles in uniaxial and biaxial nonlinear extensional flows

Mass transfer around spherical bubbles, drops, and solid particles in uniaxial and biaxial nonlinear extensional creeping flows at large Peclet numbers is the subject of this theoretical report. The fluid mechanics problem is governed by the viscosity ratio (λ) and the nonlinear intensity of the flow (E). The flow outside such bodies reveals a different picture than the linear case (E = 0) such as separating surfaces or closed circulations. There is a range: −1.04 < E < 0 (bubbles and drops) and −0.490 < E < 0 (particles) where the mass transfer rate is lower than the linear case. Outside these ranges, the mass transfer rate is higher than the linear case and in general it increases as |E| increases. Same mass transfer rates are expected in uniaxial and biaxial flows, except in the presence of external closed circulations where the biaxial flow overcomes the uniaxial flow. © 2018 American Institute of Chemical Engineers AIChE J, 65: 398–408, 2019     

A second-order-moment–Monte-Carlo model for simulating swirling gas–particle flows

A second-order moment (SOM) gas-phase turbulence model, combined with a Monte-Carlo (MC) simulation of stochastic particle motion using Langevin equation to simulate the gas velocity seen by particles, is called an SOM–MC two-phase turbulence model. The SOM–MC model was applied to simulate swirling gas–particle flows with a swirl number of 0.47. The prediction results are compared with the PDPA measurement data and those predicted using the Langevin-closed unified second-order moment (LUSM) model. The comparison shows that both models give the predicted time-averaged flow field of particle phase in general agreement with those measured, and there is only slight difference between the prediction results using these two models. In the near-inlet region, the SOM-MC model gives a more reasonable distribution of particle axial velocity with reverse flows due to free of particle numerical diffusion, but it needs much longer computation time. Both models underpredict the gas and particle fluctuation velocities, compared with those measured. This is possibly caused by the particle–wall and particle–particle interaction in the near-wall region, and the effect of particles on dissipation of gas turbulence, which is not taken into account in both models.     

Mass transfer in tube bundles with liquid fluidized particles

Experimental studies were conducted of overall and local tube-to-liquid mass transfer for bundles of horizontal tubes in staggered and in-line arrays in a liquid fluidized bed. The mass transfer coefficients are compared with those obtained using a fixed packing and those obtained without particles at the shell-side of the bundle.     

内冷除湿器中热质交换过程的数值模拟与分析

介绍了一种板翅式内冷型除湿器,针对其运行特点建立了其除湿过程的数学模型。分析了除湿过程中除湿器内部溶液与空气的温度场分布以及空气含湿量的分布情况,对内冷除湿与绝热除湿进行了比较,分析了内冷对于溶液温度以及传质驱动势的影响。结果显示与绝热除湿相比内冷除湿可有效抑制溶液温升,但对于传质驱动势的影响并不大,在典型工况下,其溶液出口平均温度比绝热除湿的平均温度低4.83℃,而积分平均湿差仅比绝热除湿时高出0.38 g&#8226;kg^-1,约为3.2%。     

An integral criterion for turbulence suppression in swirling flows

A numerical study of flow in rotating pipes was conducted to elucidate the relative importance of convection and turbulence. CFD (Computational Fluid Dynamics) simulations of flow inside a rotating pipe (D = 2 cm and L/D = 20) were carried out, using the Reynolds Stress Model, for four different Reynolds numbers and a range of rotation numbers. The objective was to gain a deeper understanding of the interaction between fluid forces in swirling flows. This widely‐studied model problem was used to ascertain the conditions under which computationally cheaper turbulence models such as the k‐? model should be accurate. We identified a dimensionless rotation parameter that delineates the condition at which decreasing turbulence force equals increasing convective force as rotational speed increases. This dimensionless number establishes a criterion for knowing which forces are dominant, and thereby a rational basis for choosing turbulence models that are both cost‐effective and accurate. We found a universal, critical threshold that determines when convective forces dominate over turbulence forces. This threshold determination is based on an ‘integral measure criterion’ of local forces in the radial direction. The threshold itself is defined by a dimensionless rotation number, N, based on the ratio of the circumferential and axial flow velocities. The critical value was found to be N_cr = 0.45. Above this, convection dominates; below it, turbulence dominates. This finding will facilitate selection of CFD models to optimize cost and accuracy for modelling swirling flows. For example, k‐? models suffice when N_cr < 0.45, but more complex models are required for higher values.

     

Mass transfer in structured corrugated packing

The performance of structured corrugated packing has been simulated by establishing mechanistic models for liquid distribution, liquid flow on the packing surface and mass transfer. The models were used to investigate the effect of packing height, liquid load, initial maldistribution as well as differing initial distribution and solid-liquid contact angle on the packed column performance. Wetted surface area is the primary value of interest and the simulated results compare very well with those predicted by Onda (1968). The results clearly demonstrate that the wetted surface area is a strong function of the solid-liquid contact angle. Other predicted values such as mass transfer coefficients and overall height of a transfer unit show reasonable agreement with published data.     

A mathematical model of heat and mass transfer in Yankee drying of tissue

Final dewatering in the production of dry creped tissue is performed by Yankee drying, which includes hot pressing followed by simultaneous contact and impingement drying. The present study models Yankee drying and compares simulation results to the data obtained from trials on a pilot tissue machine. It advances models published previously by the representations developed for the transport of heat in the pressing stage and for the heat transfer involved in the dehydration of the cylinder coating spray. The model predicts an average specific drying rate within 4% in the range of the experimental data used.     

螺旋管内对流传质场协同强化模拟

通过计算流体力学方法对层流直管内浓度场的模拟,发现大部分区域浓度梯度在径向比轴向高2个数量级。为了强化传质,应降低浓度梯度矢量和速度矢量的夹角,在径向产生一定的流动。对螺旋管内浓度场和速度场的计算表明,二次流横穿浓度等值线,提高了两场的协同。两种螺旋结构的比较表明,二次流随着Reynolds数的增大而增强,传质增强效果也越显著,正是二次流显著提高了速度场和浓度场的协同。所研究的螺旋结构在Re=1000~2400时,平均二次流速度最大达到主流速度的6.5%~6.8%,Sherwood数增大了4.99~6.43倍。