CVI工艺过程中气体传质模型与计算

在建立碳布叠层体心立方模型、碳布叠层立方模型和纤维束叠层模型典型预制体的宏观孔隙结构几何模型和气体传质数学模型的基础上 ,研究了CVI(chemicalvaporinfiltration)工艺过程中孔隙网络结构对气体比渗透率和材料致密度的影响。结果表明 :孔隙网络的结构与形状是决定反应气体在孔隙中的比渗透率和部件最终致密度的主要因素之一。

固态发酵过程中气体传质控制策略的研究进展

固态发酵在一些生物制品的生产及生物处理过程中具有明显的优势,特别是近年来随着对固态发酵理论模型和发酵设备的研究不断深入,固态发酵的应用范围越来越广。对于好氧的固态发酵来说,氧气的传质效率低仍然是工艺优化和提高产量的主要限制因素,同时它也影响了固态发酵生产规模的扩大。该文分析了固态发酵过程中气体传质的原理及过程,并对此提出了一些提高固态发酵过程中气体传质效率的策略,如控制固体基质特性、选择合适的搅拌通风方式等,最后展望了固态发酵技术的发展前景。

三维炭纤维预制体孔隙模型与气体传质计算

通过建立三维正交结构炭纤维预制体宏观孔隙网络结构几何模型和化学气相渗透(CVI)工艺过程中反应气体在孔隙网络中的数学传质模型,研究预制体的孔隙网络结构对CVI工艺过程中气体的比渗透率和复合材料最终致密度的影响。研究结果表明:三维正交结构炭纤维预制体的宏观孔隙结构在空间上可以看成是由呈周期性排列的体心立方"晶胞"构成;X,Y和Z3个方向纤维束直径大小及比例关系决定三维正交结构炭纤维预制体宏观孔隙网络的结构和形状,也是决定反应气体在孔隙中的比渗透率和复合材料最终致密度的重要因素;当X,Y和Z3个方向纤维束直径的比例关系一定时,不同预制体在CVI增密过程中比渗透率随孔隙率变化的趋势相同;复合材料的最终致密度一定;比渗透率随纤维束直径增大而略有增大。

填料塔气体传质膜系数测定实验的数据处理

在填料塔气体传质膜系数测定实验中 ,最难的和工作量最大的就是数据的处理与图表的制作。在简介“气体传质膜系数测定”实验原理与常规处理方法的基础上 ,重点探讨了Excel 2 0 0 0在化工实验数据处理与图表制作中的运用。

高挥发性液体传质速率机理和实验研究

为研究高挥发性液体传质速率计算方法,建立矩形风道,在风道中制造不同风速空气流,对103.9 mm液面直径的乙醚液面进行气体与液体(简称气液)传质质量损失的实验研究,分析不同温度对气液传质速率的影响。以实验结果为依据,根据麦克斯韦速率分布理论,结合Mackay等[12]提出的低于沸点温度的气液传质速率计算模型进行量纲分析,提出新的温度修正项,对计算模型进行修正。液面实验结果表明:随着温度的降低,气液传质速率减小;随着风速的减小,气液传质速率减小。通过实验值与计算值的对比得到温度修正项系数为0.559,计算方法中给出气液传质速率与温度、风速0.78的次幂呈正比。为验证气液传质速率计算模型的准确性,进行液柱下流气液传质传热实验,使不同直径乙醚液柱通过2.0 m/s空气流,测量实验前后液体温度变化。根据修正后气液传质速率计算模型计算液柱下流过程中气液传质能量传递导致的温度变化情况,与实验结果进行对比。液柱下流实验结果表明:计算结果与实验测量值对比误差小于等于3.81%,修正后气液传质速率计算模型误差较小,计算精度高。

流道构型对PEMFC综合性能的影响

为探究流道构型对质子交换膜燃料电池的性能影响,运用多物理场耦合软件COMSOL对比分析了阶梯流道与直流道、波浪流道三种流道在反应气体流速、阴极压降、电池产热、反应气体传质、极化曲线等方面的性能差异性。结果表明:阶梯流场最高阴极反应气体流速约为其余两种流场的1.65倍,压降约为其余两种流场的4倍;阶梯流道温度分布梯度多于其余两种流道,传质能力与波浪流道接近且强于直流道;三种流道峰值功率密度依次为:0.5219、0.5260与0.5298W/cm^(2),但考虑到三种流场的加工难易程度,阶梯流场综合性能优于其余两种流场。

整体毡纤维体积含量对CVD增密的影响

将三组不同纤维体积含量的整体毡采用等温CVD进行沉积热解炭增密,结合CVD沉积过程中整体毡内气体传质数学模型,研究了整体毡的纤维体积含量对CVD增密过程的影响。结果表明纤维体积含量低的整体毡沉积时增重率高;纤维体积含量高的整体毡容易获得较高密度的C/C复合材料;纤维体积含量超过35％的整体毡经过300h的化学气相沉积,坯体的体积密度能达到1.52g/cm^3。

动植物细胞或组织生物反应器悬浮培养过程中的通气方式

通气在动植物细胞或组织生物反应器培养过程中起着至关重要的作用,而同时通气过程所产生的机械损伤力亦可对细胞造成直接的伤害,因此,通气方式是动植物细胞或组织生物反应器培养过程设计与工程放大的关键技术之一。本文综述了动植物细胞或组织生物反应器悬浮培养过程中三种主要通气(异养培养时又称供氧)方式的结构特点,及其对气液传质、生物量、代谢产物量和细胞损伤的影响,以及改进的新型通气方式和几种通气方式的融合并用。

Design of a Silicon Beam Resonator for a Novel Gas Sensor

A new silicon beam resonator design for a novel gas sensor based on simultaneous conductivity and mass change measurement is investigated. High selectivity and sensitivity in gas detection can be obtained by measuring the charge-to-mass ratio of gas molecules. Structures of silicon beam resonators are designed, simulated, and optimized. This gas sensor is fabricated using sacrificial layer microelectronmechanical system technology, and the resonant frequency of the microbeam is measured.

Kinetic Mass Transfer Between Non-aqueous Phase Liquid and Gas During Soil Vapor Extraction

The mass transfer between non-aqueous phase liquid(NAPL) phase and soil gas phase in soil vapor extraction(SVE) process has been investigated by one-dimensional venting experiments. During quasi-steady volatilization of three single-component NAPLs in a sandy soil, constant initial lumped mass transfer coefficient (λgN,0) canbe obtained if the relative saturation (ξ) between NAPL phase and gas phase is higher than a critical value (ξc), andthe lumped mass transfer coefficient decreases with ξ when ξ＜ξc. It is also shown that the lumped mass transfercoefficient can be increased by blending porous micro-particles into the sandy soil because of the increasing of theinterfacial area.

Connection Between Liquid Distribution and Gas-Liquid Mass Transfer in Monolithic Bed

With a particular focus on the connection between liquid flow distribution and gas-liquid mass transfer in monolithic beds in the Taylor flow regime, hydrodynamic and gas-liquid mass transfer experiments were carriedout in a column with a monolithic bed of cell density of 50 cpsi with trio different distributors （nozzle and packed bed distributors）. Liquid saturation in individual channels was measured by using self-made micro-conductivity probes. A mal-distribution factor was used to evaluate uniform degree of phase distribution in monoliths. Overall bed pressure drop and mass transfer coefficients were measured. For liquid flow distribution and gas-liquid masstransfer, it is found that the superficial liquid velocity is a crucial factor and the packed bed distributor is better than the nozzle distributor. A semi-theoretical analysis using single channel models shows that the packed bed distributor always yields shorter and uniformly distributed liquid slugs compared to the nozzle distributor, which in turn ensures a better mass transfer performance. A bed scale mass transfer model is proposed by employing the single channel models in individual channels and incorporating effects of non-uniform liquid distribution along the bedcross-section. The model predicts the overall gas-liquid mass transfer coefficient wig a relative error within ＋30%.

The Analysis of Gas Absorption into a Stationary Droplet in Atmospheric Environment

Absorption of gaseous species into stationary droplets is a fundamental interest of mass transfer between liquid droplets and ambient gas, which plays a key role in atmospheric environment control and many industrial applications. In this paper, two different considerations including equilibrium and non-equilibrium relations at the interface are used to analyze and predict the absorption time for a physical absorption at a relatively low solubility of gas. For the equilibrium pattern, in the beginning period of absorption, the mass transfer rate is considerably rapid and afterward becomes slower and slower and finally comes to almost zero as the droplet concentration closes to the saturated value. Differently, when the non-equilibrium model is adopted, the interfacial concentration increases gradually with the bulk concentration of liquid droplet, and the absorption rate mildly decelerates with the increase of bulk one throughout the process, which leads to a longer absorption time. Based on the diffusion equation of species, the concentration distribution within the droplet at different times is computed. A solution for CO2 absorption into a small water droplet is given.

The Mechanism of Interfacial Mass Transfer in Gas Absorption Process

Based on the method of molecular thermodynamics, the mass transfer mechanism at gas-liquid interface is studied theoretically, and a new mathematical model is proposed. Using laser holographic interference technique, the hydrodynamics and mass transfer characteristics of CO2 absorption are measured. It is shown that the calculated results are in good agreement with the experimental data.

A Three-dimensional Heterogeneous Mass Transfer Model for the Absorption of Gas into Multiphase System

A three-dimensional heterogeneous mass transfer model was proposed to investigate the enhancement of dispersed particles on gas absorption. The strategy to calculate local and overall enhancement factors is proposed. Instead of a global grid, the composite overlapping grid is adopted, which simplifies the setup and solution of the three-dimensional model equations. It is found that dispersed particle hold-up, particle size, liquid-solid partition coefficient of transported component, characteristic contact time, and the shortest distance between particles and gas-liquid interface have major influence on absorption enhancement factor. The particle-particle interaction on gas absorption and the lateral diffusion of transported component in liquid film were studied with the multi-particle simulation. The proposed model predicted the experimental data from the literature reasonably well.

Mass Transfer Enhancement of Gas Absorption by Adding the Dispersed Organic Phases

Mass transfer enhancement of gas absorption by adding a dispersed organic phase has been studied in this work. Various dispersed organic phases （heptanol, octanol, isoamyl alcohol, heptane, octane, and isooctane） were tested respectively in the experiment. According to the theoretical model and experimental data, the overall volumetric mass transfer coefficient and enhancement factor were obtained under different dispersed organic phase volume fraction and stirring speed. The experimental results indicate that gas-liquid mass transfer is enhanced at different level by adding a dispersed organic phase. The best performance of enhancement were achieved with the dispersed organic phase volumetric fraction of 5% and under an intermediate stirring speed of 670 r·min^-1. Among the organic phases tested in the experiment, alcohols show better performance, which gave 20% higher enhance-ment of overall volumetric mass transfer coefficient than adding alkanes.

Removal of Phenol from Dilute Solutions by Predispersed Solvent Extraction

Predispersed solvent extraction (PDSE) is a new method for separating solutes from aqueous solution by solvent extraction and one which has shown promise for extraction from extremely dilute solution very efficient and very quick. The use of colloidal liquid aphrons in predispersed solvent extraction may ameliorate the problems such as emulsion formation, reduction of interfacial mass transfer and low interfacial mass transfer areas in solvent extraction process. In present paper, colloidal liquid aphrons are successfully generated using kerosene as a solvent, tributyl phosphate(TBP) as an extractant, sodium dodecyl benzene sulphate(SDBS) as surfactant in aqueous phase and Tween-80 in oil phase. Extraction of phenol from dilute solution was studied by using colloidal liquid aphrons and colloidal gas aphrons in a semi-batch extraction column. It has been found that the PDSE process is more suitable for extraction of dilute solutions. It has also been discovered that the PDSE process has a great advantage over traditional single-stage extraction process.

Viscoelastic Analysis of a Surface Acoustic Wave Gas Sensor Coated by a New Deposition Technique

An analysis of the response of surface acoustic wave sensors coated with polymer film based on new coating deposition （self-assemble and molecularly imprinted technology） is described and the response formulas are hence deduced. Using the real part of shear modulus, the polymer can be classified into three types： glassy film, glassy-rubbery film and rubbery film, Experimental results show that the attenuation response is in better consistence with the simulation than in Martin＇s theory, but the velocity response does not accord with the calculation exactly. Maybe it is influenced by the experimental methods and environment. In addition, simulations of gas sorption for polymer films are performed. As for glassy film, the SAW sensor response increases with increasing fihn thickness, and the relationship between the sensor response and the concentration of gas is pretty linear, while as for glassy-rubbery flint and rubbery film, the relationship between the sensor sensitivity anti concentration of gas is very complicated. The ultimately calculated results indicate that the relationship between the sensor response and frequency is not always linear due to the viscoelastic prooerties of the polymer.

Gas–Liquid Mass Transfer Characteristics in a Gas–Liquid–Solid Bubble Column under Elevated Pressure and Temperature

abstract The volumetric mass transfer coefficient kLa of gases （H2, CO, CO2） and mass transfer coefficient kL on liquid par-affin side were studied using the dynamic absorption method in slurry bubble column reactors under elevated temperature and elevated pressure. Meanwhile, gas-holdup and gas-liquid interfacial area a were obtained. The effects of temperature, pressure, superficial gas velocity and solid concentration on the mass transfer coeffi-cient were discussed. Experimental results show that the gas-liquid volumetric mass transfer coefficient kLa and interfacial area a increased with the increase of pressure, temperature, and superficial gas velocity, and decreased with the slurry concentration. The mass transfer coefficient kL increased with increasing superficial gas velocity and temperature and decreased with higher slurry concentration, while it changed slightly with pressure. Ac-cording to analysis of experimental data, an empirical correlation is obtained to calculate the values of kLa for H2 （CO, CO2） in the gas-paraffin-quartz system in a bubble column under elevated temperature and elevated pressure.

Determination of Absorption Capacity and Mass Transfer Coefficient of Several Oils as Solvents for Biomass Gasification Cleaning System

With the aim to find an absorbing liquid suitable for the gas cleaning system in the application of gasification unit in a remote area, three types of oil were tested using a bubble column and a wetted wall column. Air streams containing toluene vapour with flowrates of 13.6 mL-min1 or 27.6 mL.min^-1 were bubbled through a 50 mL static oil in a glass tube at a temperature of 30 ℃ or 60 ℃. In experiments using the wetted wall column, air streams containing toluene were contacted with a falling thin film ofoil on the outer wall of a column with a diameter of 6.4 cm and two different contacting heights of 60 cm or 80 cm. Toluene concentrations in the air stream were adjusted in the range of 700-3000 ppm corresponding to a typical tars concentration in the producer gas. The phase equilibrium of toluene was represented as values of 1/tl of 326, 220 and 182 respectively for lubrication oil, palm oil and sunflower oil （H is Henry＇s constant with the toluene concentrations in g.L^-1 for liquid phase and g.Nm^-3 for gas phase）. From experiments using the bubble column, it was found that the overall mass transfer coefficient （Kc, a） was in the order of 10.3 cm3-min^-1 and the overall liquid phase mass transfer coefficient （KLa）was about 10.3 cm3.min^-1. Although lubrication oil had a slightly better absorption capacity than the other two tested oils, it had a lower mass trasfer coefficient than that of palm oil. All three proposed oils had a much better absorption capacity and absorption rate than that of water used conventionally as a scrubbing liquid in a small biomass gasification plant.