Characteristics of the reactive distillation column with a novel internal

A new catalyst loading method, in which catalyst particles are packed in a reactor with a novel internal, has been studied by measuring pressure drop, RTD, liquid holdup and mass transfer. It can be inferred from experimental results that the internal changes the conventional gas and liquid counter-current flow into a new cross-current flow, so the problems of excessive pressure drop and “flooding” are avoided. When pressure drop, liquid holdup, RTD and mass transfer coefficient are similar, the catalyst loading fraction of the new method is much higher than that of conventional methods. Furthermore, the reactive distillation reactor with the novel internal has advantages such as simple structure, low operating cost, and convenience for installation and removal of the catalyst.     

Axial dispersion in a rectangular bubble column

This paper presents the results of an experimental study on the gas holdup and the liquid phase axial dispersion coefficient in a narrow packed and unpacked rectangular bubble column. In both cases the gas and liquid flow rates were varied and the data were obtained by employing standard tracer technique. The gas holdup and the axial dispersion coefficient for both the packed and unpacked columns were found to be dependent on the gas and liquid flow rates. For given gas and liquid velocities and a given packing size in the case of the packed column, the rectangular column gave significantly higher dispersion coefficients than a cylindrical column of the equivalent cross sectional area. This result agrees very well with the one predicted by the velocity distribution model. The correlations for the Peclet number, the axial dispersion coefficient, and the fluid holdup for both the unpacked and packed bubble columns are presented.     

Radioisotope tracer study in trickle bed reactors

The residence time distribution (RTD) of liquid phase in trickle bed reactors has been measured for air‐water system using radioisotope tracer technique. Experiments were carried out in a glass column of internal diameter of 0.152 m packed with glass beads and actual catalyst particles of two different shapes. From the measured RTD curves, mean residence time of liquid was calculated and used to estimate liquid holdup. The axial dispersion model was used to simulate the experimental data and estimate mixing index, ie. Peclet number. The effect of liquid and gas flow rates on total liquid holdup and Peclet number has been investigated. Results of the study indicated that shape of the packing has significant effect on holdup and axial dispersion. Bodenstein number has been correlated to Reynolds number, Galileo number, shape and size of the packing.     

Measurements and modeling of heat generation in a trickling biofilter for biodegradation of a low concentration volatile organic compound (VOC)

The experimental and theoretical heat generation behavior of a trickling biofilter treating toluene is discussed. The experimental results show that the temperature of the packed bed has a significant effect on the purification performance of the trickling biofilter and that an optimal operation temperature exists between 30 and 40 °C. During the gas–liquid co-current flow, the temperature in the packed bed gradually rises along the direction of the gas and liquid flow due to the exothermic biodegradation of toluene. The temperature rise between the inlet and outlet of the trickling biofilter increases with an increase in the gas flow rate and inlet toluene concentration. In addition, a larger liquid flow rate leads to a smaller temperature rise. The heat generation process occurring in the trickling biofilter is modeled by representing the packed bed as an equivalent set of parallel capillary tubes covered by the biofilm. The temperature profile in the packed bed during the liquid–gas co-current flow is analyzed by simultaneously solving the problem of gas–liquid two-phase flow and heat and mass transfer through the liquid film and biofilm. It is shown that the model agrees well with the experimental data, predicting the variations of the temperature rise between the inlet and outlet of trickling biofilter with the increasing gas and liquid flow rates.     

Radial liquid distribution in cocurrent two-phase downflow in packed beds

Radial liquid distribution was measured experimentally for cocurrent, two-phase downflow in packed beds. The effects of bed length, water and air flow rates, and type of packing were determined. The experimental data were obtained in the gas-continuous, transition and pulsing trickling-flow regimes. For all finite air rates, the liquid velocity profiles were approximately flat with the maximum average velocity occurring at the center of the packed column. Increasing the air rate increased the center liquid velocity. The gas rate effect was more pronounced in shorter beds. At higher gas rates the liquid rate had less effect on the radial liquid distribution than at lower gas rates. Operation at higher liquid rates resulted in a flatter radial liquid veilocity profile. It was observed that the bed of pellets operated at high liquid rate and low gas rate was unstable. Increasing the bed height increased the stability of the system and a better liquid distribution was obtained. The effects of water flow rate, bed length, and packing type on the shape of the liquid velocity profiles were minor.     

鼓泡塔反应器分布板的开孔率和液体返混

考察鼓泡塔内鼓泡现象,发现气流经过小孔的速度（称为小孔气速）需大于某一临界值。计算并比较了气流通过多孔板的干板压力降和表面能损耗,提出计算此临界气速的公式和确定最大开孔率的方法。 用脉冲注入法测定鼓泡塔液体停留时间分布,提出非整数串连混合器模型,推导出其停留时间分布曲线方程式,计算结果与实验数据相符。并验证了Towell的轴向分散系数的计算方法。     

Experimental Study on a Co-current Gas–Liquid Down Flow Contactor with Gas Entrainment by a Liquid Jet

Two-phase flow co-current vertical downflow reactor with gas entrainment by a liquid jet is investigated in an air–water system. Experiments are carried out in order to clarify the flow behavior of the reactor under various conditions. Gas entrainment flow rates and gas holdup are quantified experimentally and their dependency on the liquid jet flow rates are shown. The experimental program also included determination of liquid phase residence time distribution (RTD) characteristics for different liquid jet flow rates. The result of the analysis of the liquid phase RTD curves justified the tank-in-series model flow for the liquid phase. On the basis of these analyses, the reactor hydrodynamics are modeled by the tank-in-series model including dead zones. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. The volumetric mass transfer coefficient k_La_Lis determined experimentally by a “gasing out” method. The interfacial area is deduced from the bubble diameter measurements which are determined by visualization experiments.     

Hydrodynamic properties of a novel ‘open wall’ monolith reactor

The hydrodynamic behavior of high-porosity ceramic acicular mullite (ACM) monoliths operated in gas–liquid Taylor flow was investigated by cold-flow tracer residence time distribution (RTD) experiments and compared with the hydrodynamic behavior of classical cordierite monoliths in terms of static liquid fraction, maldistribution and mass exchange between the dynamic liquid zone and the static liquid zone. The piston-dispersion-exchange (PDE) model was successfully fitted to the experimental data. The multiphase fluid mechanics were indeed different for ACM and cordierite monoliths: in contrast to the classical cordierite system, the mass exchange between static liquid and dynamic liquid was much higher and the liquid phase could access the open volume of the permeable ACM monolith wall. On the other hand, the larger velocity maldistribution, larger static liquid fraction and the liquid interaction between adjacent ACM microchannels resulted in less plug flow behavior.     

Experimental studies of liquid flow maldistribution in a random packed column

Liquid flow distribution has been a major concern when scaling up random packed columns. This study concerns the measurements of liquid flow distribution in a large scale column packed with 25.4 mm stainless steel Pall rings. The liquid flow distribution was studied with packed bed height from 0.9 to 3.5 m, liquid flow rate from 2.91 to 6.66 kg/m²·s, and gas flow rate from 0 to 3.0 kg/m²·s. In addition, three systems, water/air, aqueous detergent solution/air and Isopar/air, were used to study the effect of liquid physical properties on liquid flow distribution, and two different liquid distributors were employed to test the effect of liquid distributor design. It was found that liquid flow distribution was strongly influenced by liquid distributor design, packed bed height, gas flow rate and liquid viscosity, slightly influenced by liquid flow rate, but not by liquid surface tension.     

The effect of liquid maldistribution on the performance of a packed tower absorber

A study of the effect of liquid maldistribution on the performance of a gas absorber is reported. The experimental work was carried out using the carbon dioxide-water system in a simulated packed column consisting of an ordered array of spheres. This device has been shown to closely approximate the behaviour of an actual column and permits the artificial manipulation of the liquid distribution. The results indicate that maldistribution causes only a small decrease in the mass transfer coefficient at low liquid flow rates, and has almost no effect at higher flows. A proposed iterative procedure for calculating the effect is presented.     

Investigation of pressure drop in a cocurrent downflow three-phase moving bed

Based on a self-established cold-flow experimental device, the pressure drop in a cocurrent downflow three-phase moving bed was investigated under a wide range of gas, liquid, and solid flow rates during dynamic and steady-state operation. The results showed that for the startup of the bed, since the first bed layer packed by fall-falling of particles had lower voidage, it would take at least one bed volume time to make the voidage in the bed reach the steady-state. Under steady-state conditions, the pressure drop increased with the increase of gas and liquid mass flow rates, liquid viscosity, and decreased with the increase of solid flow rate. Furthermore, it was found that the liquid distribution became more uniform due to particle movement. The experimental data obtained in this study was used to develop a correlation to predict the pressure drop in a three-phase moving bed with an average relative error of 9.32%.     

Study on a catalytic distillation column with a novel internal

A novel internal for holding a catalyst in a catalytic distillation column was studied and its feasibility was verified by a model reaction, which is the alkylation of benzene with propylene over Hβ zeolite catalyst. The novel internal enlarges the void of the catalyst bed and provides the gas and liquid flow channels. It was proved that the catalytic distillation column with the novel internal has such advantages as simple structure, low operating cost, convenience for loading and unloading catalyst, and large catalyst loading fraction. It was found that the operational capability of the column with the novel internal whose volume fraction is about 30% is similar to that of a column filled with catalysts mixed with Cannon rings whose volume fraction is about 80%. It was also found that compared with the fixed-bed bubble reactor, the selectivity of cumene and the conversion of benzene are significantly improved in the CD (Catalytic Distillation) column with the internal.     

倒锥形填料筐精馏塔板的性能研究

提出了一种新型高效塔板——倒锥形填料筐新型精馏塔板(RTPLT),在冷模塔内对该塔板的流体力学性能和塔内液体停留时间分布情况进行了研究。由实验数据关联得到了干板压降、湿板压降和漏液率的经验式。结果表明:RTPLT具有塔板压降低、雾沫夹带小、处理能力大等优点,其塔内液体停留时间分布的主要因素是液体流量。     

隔壁塔内半圆填料层中壁流与返混特性研究

在填料塔三参数壁流模型的基础上,提出了应用于隔壁塔半圆型填料层的壁流模型。通过实验对模型参数进行了估计。模型计算值与实验值吻合较好。在内径为580 mm、高2800 mm的半圆冷模塔内,常温常压下以空气-水为介质,测取了散堆填料的壁流曲线,分析了气液速率变化对壁流的影响。以氯化钠溶液为示踪剂,通过脉冲注入扰动响应技术获得停留时间分布曲线。发现气液速率变化均对停留时间分布产生不同程度影响。同时比较了同一填料层高度塔壁区和整体区的平均停留时间,在壁流充分发展阶段,塔壁区的平均停留时间明显小于整体区。从另一方面量化了壁流效应的影响。应用线性回归得到单相流和气液逆流时计算Pe_d的关联式。对于新型填料及塔内件的开发、改进隔壁塔的设计方法,从而实现安全有效放大具有参考意义。     

填料塔旋转液体分布器的研究

本文介绍了一种填料塔旋转液体分布器。推导出了最佳开孔位置和转速的计算式。得出了转速、液体流量、开孔角度、气体流量对分布的影响。并给出了计算实例。     

Gas/liquid dispersion in a sequential split micromixer

Two different types of split–recombine micromixers have been tested for a gas/liquid dispersion. Mixing performances of two micromixers have been compared in terms of a mixing efficiency. The effects of liquid flow rates, the ratio of gas/liquid flow rate, bubble sizes, and bubble size distributions have been investigated. The sequential split micromixer and the caterpillar micromixer showed a consistent mixing performance at various flow rates. The sequential split micromixer has shown the smaller and narrower bubble size distribution than that of the caterpillar micromixer at a higher flow rate of 1.8 L/h.     

Impact of gravity on the bubble‐to‐pulse transition in packed beds

A model based on two‐phase volume‐averaged equations of motion is proposed to examine the gravity dependence of the bubble‐to‐pulse transition in gas‐liquid cocurrent down‐flow through packed beds. As input, the model uses experimental correlations for the frictional pressure drop under both normal gravity conditions and in the limit of vanishing gravity, as well as correlations for the liquid‐gas interfacial area per unit volume of bed in normal gravity. In accordance with experimental observations, the model shows that, for a given liquid flow, the transition to the pulse regime occurs at lower gas‐flow rates as the gravity level or the Bond number is decreased. Predicted transition boundaries agree reasonably well with observations under both reduced and normal gravity. The model also predicts a decrease in frictional pressure drop and an increase in total liquid holdup with decreasing gravity levels. © 2013 American Institute of Chemical Engineers AIChE J 60: 778–793, 2014     

Experimental and CFD analysis of two-phase cross/countercurrent flow in the packed column with a novel internal

The pressure drop and liquid hold-up for the G-L cross/counter-current flow in a packed column with a novel internal was simulated using a Eulerian/Eulerian two-fluid model solved by a commercial CFD software CFX4.4. Simulation results are in good agreement with the experimental data of pressure drop. The internal significantly increases the gas radial velocity and lower the gas axial velocity, which lowers the pressure drop and improves operational flexibility. To minimize bypass flow caused by the internal, optimum baffle thickness and width of the internal's passage are proposed.     

Local measurements of void fraction and liquid holdup in packed columns using X-ray computed tomography

A ‘tailor made’ computed X-ray tomographic scanner has been developed as a tool for the analysis of the distribution of gas, liquid and solid phases in packed columns. The very good spatial resolution of the scanner has first been assessed by the imaging of objects of known shape and size, called ‘physical’ phantoms. Images have then been realized on a 0.6 m diameter and 2 m height column packed with Cascade Mini-Ring 1A packing elements, which is a random polypropylene packing designed to be used in absorption columns. The solid phase distribution, leading to the void fraction distribution, has been analyzed on cross-section images of the dry packed column (without any liquid flowrate). The measured value of the bed void fraction is equal to the value provided by the manufacturer. The analysis of the axial profile of void fraction shows that the ‘end effect’ may be neglected, whereas the analysis of the radial profile evidences the existence of a non negligible ‘wall effect’. Images have then been carried out on the packed column irrigated by a liquid flowrate ranging between 0 and 6000 l h⁻¹ (0–6.10⁻³ m s⁻¹). Hold-up values have been measured in different cross-sections of the column and averaged in order to obtain the total hold-up value in the bed. The computed values are in very good agreement with those reported in the literature for similar packing. The dependence of the liquid hold-up on the liquid superficial velocity can be expressed in terms of a power law. The fitted value of the exponent, equal to 0.65, is in the range of exponent values found in correlations of the literature.