Countercurrent Flow Of Gas Research Articles

TUM–WelChem cell model for the prediction of liquid distribution in random packed columns

AbstractLiquid maldistribution is one of the main deficiencies in random packed column design. Therefore, the knowledge of liquid distribution and its model‐based prediction is of great interest. This work aims to further develop and validate the TUM–WelChem Cell Model for random packed columns. First, cell dimension calculations and the determination of random packing element orientations are standardized. The original WelChem Cell Model applies a liquid distribution mechanism based on liquid spread factors derived by virtual 3D irrigation experiments. An extension of the model involves the implementation of liquid and gas load related distribution mechanisms, considering dispersion effects caused by liquid loading and the countercurrent gas flow. The wall flow is refined by an increase of packing porosity at the column wall. Liquid distribution profiles provided by the TUM–WelChem Cell Model are validated against experimental data and show good agreement for both uniform and point source initial liquid distribution.

Effect of wall corrugations on scalar transfer to a wavy falling liquid film

Direct numerical simulation is employed to study the effect of small-scale wall corrugations on scalar transfer through the wavy surface of a vertically falling liquid film in interaction with a strongly confined counter-current gas flow. Three wall geometries are considered: (i) a flat wall for reference; (ii) a sinusoidal corrugation typically found on structured packings in chemical engineering devices; and (iii) a heuristic design consisting of isolated semicircular bumps distanced by the wavelength of the surface waves. We consider the limiting case of a Dirichlet condition for the transported scalar (temperature or mass fraction) at the liquid–gas interface and focus on liquid-side transport. We consider convection-dominated regimes at moderate and large Péclet numbers, representative of heat and mass transfer respectively, and confront forced and noise-driven wave regimes. Our results show that sinusoidal wall corrugations increase transfer by up to 30 per cent in terms of the exchange length required to transfer a fixed amount of the transported quantity. A slightly greater intensification is achieved through the bump-shaped corrugations, which intermittently disrupt the moving-frame vortex forming within the large-amplitude solitary waves, allowing these to replenish with unsaturated liquid. However, when the velocity of the strongly confined gas flow is increased above a certain threshold, the bumps can trigger the flooding of the channel.

Flow pattern and CO2 absorption in a falling film reactor with mixed aqueous solution of ionic liquid and MEA

This work dealt with the effect of counter-current gas flow on the flow pattern transitions and CO2 absorption with the mixed aqueous solution of ionic liquid and MEA. The falling film was observed to take the form of “corner rivulet flow (denoted as CRF)”, “falling film flow with dry patches (denoted as FDP)” or “complete falling film flow (denoted as FFP)”. The flow patterns are significantly different between the cases of increasing the liquid flow rate gradually and decreasing the liquid flow rate gradually. The critical flow rate enlarged due to the increasing gas flow rate. The absorption rate RA increased with increasing liquid flow rate under the flow patterns of CRF and FDP. The utilization rate reached a peak at the critical liquid flow rate QCRF,CFF-FDP. Liquid mass transfer coefficient decreased with the increasing liquid flow rate under the flow patterns of CRF and FDP.

Characteristics of solitary waves on a falling liquid film sheared by a turbulent counter-current gas flow

We report an experimental investigation of a falling water film sheared by a turbulent counter-current air flow in an inclined rectangular channel. Film thickness and wave velocity measurements associated with visual observation are conducted to study the influence of the air flow on controlled traveling waves consisting of a large wave hump preceded by capillary ripples. First, we focus on the variation of the shape, amplitude and velocity of the waves as the gas velocity is gradually increased. We demonstrate that the amplitude of the main hump grows substantially even for moderate gas velocities, whereas modification of the wave celerity becomes significant above a specific gas velocity around 4 m/s, associated with an alteration of the capillary region. The influence of the gas flow on 3D secondary instabilities of the solitary waves detected in a previous study Kofman et al. (2014), namely rugged or scallop waves, is also investigated. We show that the capillary mode is damped while the inertial mode is enhanced by the interfacial shear. Next, the gas velocity is increased until the onset of upstream-moving patterns referred to as flooding in our experiments. At moderate inclination angles (typically < 7 ○), flooding occurs for a gas velocity around 8 m/s and is initiated at the scallop wave crests by a backward wave-breaking phenomenon preceded by the onset of ripples on the flat residual film separating two waves. At high inclination angle, a rapid development of solitons is observed as the air velocity is increased preventing the waves to turn back. Finally, at high liquid Reynolds number, sudden and intermittent events are triggered consisting of very large amplitude waves that go back upwards very fast. These “slugs” either extend over the whole width of the channel or are very localized and can thus potentially evolve towards atomization.

A numerical study of effects of counter-current gas flow rate on local hydrodynamic characteristics of falling films over horizontal tubes

A three-dimensional Computational Fluid Dynamics (CFD) model based on the VOF method is developed to investigate the hydrodynamic characteristics of falling film over horizontal tubes. The model not only considers gravity and surface tension, but also considers drag force when counter-current gas flow is imposed. For different liquid flow rates, laminar model, and k-ε turbulence model are employed to improve the model accuracy. Simulations are performed for the film Re varying from 100 to 1000 and gas flow rate ranging from 0m/s to 0.5375m/s. Based on the simulations, the gas flow effects on flow mode transitions agree well with the experimental data. In the circumferential direction, all bulk film thicknesses increase with the increased gas flow rate in the three basic flow modes — droplet, column, and sheet. In both the droplet and column flow mode, the position in which the thinnest film is located changes when gas flow is imposed. In the axial direction, the maximum film thickness is approximately twice that of the minimum in the droplet flow mode. Regular stable–crest–stable distribution can be observed in the column flow mode and the film distribution nearly has no variation in the sheet flow mode over time.

Heat Recovery Process from Packed Bed of Hot Slag Plates

Reduction of CO2 emissions from the steelmaking process is strongly required for prevention of global warming. One promising heat resource that is estimated to have great potential for energy saving is the waste heat of steelmaking slag, which has a temperature of above 1673 K in the molten state. This molten slag can be solidified in a plate-like shape by feeding it on the surface of water-cooled rolls, and the heat of the plate-like slag can be recovered easily in spite of its low heat conductivity. When these hot slag plates are packed in a slag chamber, the heat of the slag can be recovered by heat exchange with a counter current gas flow. Because the efficiency of gas-slag heat transfer changes depending on the shape of the packed slag, it is difficult to estimate the efficiency of slag heat recovery without evaluating the accuracy of the heat transfer coefficient in the bed. In this work, the effect of the slag shape on the accuracy of the heat transfer equation was evaluated by conducting both laboratory-scale and pilot-scale slag heat recovery experiments and performing a fitting analysis by using a slag packed bed heat transfer simulation model. A comparison of the experimental results and calculation results confirmed that the heat transfer coefficient can be estimated by using Johnson-Rubesin’s equation modified by a correction factor in case the packed materials are plate-like. The effect of the correction factor on the efficiency of slag heat recovery at the industrial scale was also estimated.

Heating of the liquid film flowing under gravity and gas flow

The lliquid films are applied in the various equipments intended for an intensification of heat and mass transfer processes. Heat transfer can also be intensifying by the gas flow above the liquid surface; these processes take a place at two-phase flows in mini- and microchannels. This study deals with the heat transfer in a locally heated liquid film of constant thickness h. The film is supposed to flow along the heated plate having constant temperature TW under the action of gravity and a gas flow. Influence of a gas flow on a velocity profile in the liquid is considered through shear stress on an interface. Coefficient of heat exchange and shear stress on a film surface suppose be set. Equation of the energy for a liquid film is solved by Galerkin technique which is special case of the weighed residuals method (WRM). Applying of Galerkin technique to this problem allows calculating the temperature in a liquid analytically for arbitrary a temperature profile in initial cross-section of a film. For initial temperature profile, gained from the analytical solution the temperature field is calculated. Results of these calculations are compared to results of numerical solution of the energy equation by finite-difference method. This comparison has shown the good agreement both for a falling film, and for cocurrent or countercurrent gas flow.

Dryout of counter-current two-phase flow in a vertical tube

Dryout of counter-current two-phase flow is one of key issues in chemical and/or industrial systems. The aim of the present study is to provide insights into the dryout phenomena of such counter-current two-phase flow of binary gas and liquid in a vertical tube. Experimental results suggest a significant influence of the subcooling of water and the binary gas flow rate on the dryout characteristics. The observed dryout was classified into three modes, i.e., film breakdown, dryout due to flooding and dryout due to critical flooding. At the low inlet air flow rate, dryout characteristics were affected by subcooling of water. The test section was partially overheated owing to the liquid film breakdown due to the Marangoni effect. At the moderate inlet air flow rate, the dryout is induced by the reduction on the falling liquid film due to flooding. At the high gas flow rate, most of the water was carried over due to the critical flooding. In the latter two cases, there was no influence of subcooling of water on dryout. These dryout phenomena were successfully realized in the numerical simulation based on the one-dimensional heat and mass transfer model, i.e., energy balance of a heat input, a heat release rate from the heating surface and a mass transfer rate due to phase change.

Wavy liquid film in the presence of co- or counter-current turbulent gas flow

The wavy downflow of a viscous liquid film in the presence of the turbulent gas flow was analyzed theoretically. Two-dimensional stationary running waves are calculated in a wide range of Reynolds numbers of liquid and gas. Hydrodynamics of liquid is calculated based on complete Navier-Stokes equations. The wave interface surface is considered as a small perturbation and equations in gas are linearized near the main turbulent flow. Different optimal downflow regimes are determined, and the main wave characteristics are compared in detail with and without the co- and counter-current gas flows. It is shown that at high velocities of the co-current gas flow, the calculated waves correspond to ripples observed in experiments.

Calculation and Analysis of Liquid Holdup in Lower Blast Furnace by Model Experiments

A hydromechanics experiment on the countercurrent flow of gas and liquid simulating the flow conditions in the lower blast furnace was carried out. A cold model of a packed bed with various packing materials and liquids was used to study the holdup of liquid. Correlations for static holdup, dynamic holdup, and total holdup were obtained. A good agreement was found between the calculated and experimental data. A mathematical model simulating the flow fields was applied to study the effect of liquid holdup in blast furnace. The results of the model calculation show that static holdup is the determinant of the total holdup of molten materials when the blast furnace works in stable condition. The slag phase generally reaches flooding holdup ahead of the hot metal. The radial distribution of gas flow is almost not influenced by the holdup of molten materials, but it has a greater influence on the pressure drop. The size of coke has far greater influence on static holdup than liquid properties does. The study is useful for acquiring a deeper understanding of the complex phenomena in the blast furnace and for determining appropriate operational actions under different production conditions.

Mass transfer characteristics of liquid films flowing down a vertical wire in a counter current gas flow

The wetted-wire packing, mainly consisting of a bundle of vertical parallel wires, is a promising concept for the use in separation columns. To investigate the multiphase flow inside the packing in detail and to estimate the performance of the packing, experiments on liquid films on a single vertical wire in a counter current gas flow were carried out. To get information about the interfacial area, an optical measurement of the film thickness was carried out with a digital high speed camera and image recognition tools. By measuring the evaporation of water and aqueous polyvinylpyrrolidone solutions into air, the gas-side mass transfer was determined. The liquid-side mass transfer was examined by measuring the desorption of CO 2 from water into air. The results show that the mass transfer coefficients are comparable to those appearing in common structured packings. When assuming a sufficiently high wire packing density, a specific interfacial area similar to corrugated sheet structured packings can be reached. Previous studies predicted a low pressure drop per packing height and extended capacity limits compared to common packings. In consideration of these results, the wetted wire packing therefore is shown to be suitable especially for absorption processes where a low pressure drop is favourable.

Thermocapillary instabilities in liquid columns under co – and counter-current gas flows

The thermocapillary flows in an infinite liquid column surrounded by an annular channel of gas with the axial temperature gradient are investigated. The gas is pumped through the channel parallel to the interface with a given flow rate. The solution for stationary motion is derived and possible flow regimes are analyzed depending on the Marangoni number and dimensionless gas flow rate. Linear stability analysis of these regimes is performed. It is shown that gas pumping in the same direction with respect to the thermocapillary motion on the interface has a destabilizing effect on the system. Gas pumping in the opposite direction can be stabilizing or destabilizing depending on the gas flow rate. It is shown that the motion of liquid can be completely vanished by the gas flow with a specified flow rate. The obtained results demonstrate the possibility of controlling thermocapillary flows and their stability in liquid columns (liquid bridges) by gas flows.

Two-Phase Flow Countercurrent Operation of a Trickle Bed Reactor: Hold-up and Mixing Behavior over Raschig Rings Fixed Bed and Structured Bale Packing

The hydrodynamics of countercurrent gas and liquid flows through a trickle bed reactor with unstructured (8 mm Raschig rings) and structured (Bale) packing has been investigated using the residence...

Coupling between countercurrent gas and solid flows in a moving granular bed: The role of shear bands at the walls

We extended the standard approach to countercurrent gas–solid flow in vertical vessels by explicitly coupling the gas flow and the rheology of the moving bed of granular solids, modelled as a continuum, pseudo-fluid. The method aims at quantitatively accounting for the presence of shear in the granular material that induces changes in local porosity, affecting the gas flow pattern through the solids. Results are presented for the vertical channel configuration, discussing the gas maldistribution both through global and specific indexes, highlighting the effect of the relevant parameters such as solids and gas flowrate, channel width, and wall friction. Non-uniform gas flow distribution resulting from uneven bed porosity is also discussed in terms of gas residence time distribution (RTD). The theoretical RTD in a vessel of constant porosity and Literature data obtained in actual moving beds are qualitatively compared to our results, supporting the relevance under given circumstances of the coupling between gas and solids flow.

A mathematical model to characterise effects of liquid hold-up on bosh silicon transport in the dripping zone of a blast furnace

A first principle based mathematical model has been developed to characterise the effect of total liquid hold-up on the bosh silicon distribution behaviour in the dripping zone of a blast furnace. Two specific cases of hold-up behaviour have been investigated, namely, hold-up in the absence and in the presence of counter current gas flow conditions. The model exemplifies coupled phenomenon of chemical kinetics, transport processes and liquid hold-up to characterise the silicon behaviour in the dripping zone. The present modelling investigation shows that the bosh silicon level diminishes with the enhanced liquid hold-up in the dripping zone. Further, the influence of counter current gas flow on the hold-up is not significant. However, it has been observed that the liquid phase temperature reduces with increased liquid hold-up in dripping zone under steady state operating conditions. The model predictions of bosh silicon distribution have been validated with the published literatures (bulk values) and found to be in good agreement.

Study on flow mode of combined converter for methanol synthesis from coal-based syngas

The mathematic model of combined converter with two different flow modes of gas-cooled reactor was established. The effects of gas flow mode in gas-cooled reactor on combined converter was investigated with the yield of methanol was 1 400 kt/a. The results show that if the flow mode of the cooling pipe gas and the catalytic bed gas change from countercurrent to concurrent, the catalytic bed temperature distribution does not fit the most optimum temperature curve of reversible exothermic reaction and the heat duty of heat changer in whole process increased seriously, which means that there is much more equipment investment and more operating cost. The gas flow mode of gas-cooled reactor affects the methanol yield slightly. Therefore, the countercurrent gas flow mode of gas-cooled reactor is more lucrative in the combined converter process.

Texture influence on liquid-side mass transfer

The impact on different packing material textures to liquid-side controlled mass transfer for the CO 2 absorption into silicone oil is analyzed experimentally. Typical industrially applied textures with bidirectional pyramidal and unidirectional wavy topographies are investigated for a wide range of liquid loads and viscosities. It is found that the texture design has a significant influence on the mass transfer efficiency, e.g. mass transfer intensification up to 80% can be performed by a textured surface compared with a flat inclined plate. Additionally, counter current gas flow affects the mass transfer positively as expected, but the texture impact is significantly higher. Silicone oil is widely used for fluid dynamic analysis. Here, fundamental data and procedures for mass transfer determination are provided for the first time in order to characterize present textures of column internals.

Liquid flow on a vertical wire in a countercurrent gas flow

The concept of the wire bundle packing is regarded as a potential alternative to common structured packings in the chemical industry, but until now it is still in prototype stadium and its capability has to be proven. The packing mainly consists of parallel vertical wires that are supplied with liquid separately from a special liquid distributor. To gain insight into the fluid dynamics of the gas/liquid flow, experiments with a single wire in a channel are conducted. Different methods are employed to measure local film thicknesses, liquid hold-up, liquid bead velocities, load limits and pressure drop. Due to the strongly curved surface of the wire, the film flow differs from the fluid dynamics of liquid films on plane surfaces. The film flow shows a distinctive pattern of liquid “beads” that run on a thin basis film. When the film is subject to a significant counter current gas flow the flow pattern changes, but the liquid hold-up and the interfacial area are hardly influenced. This behaviour can be observed up to the load limit where the beads disintegrate and flooding occurs. An estimation of the fluid dynamic performance of the wire packing from the single wire experimental data is made. The results indicate that the interfacial area of the fully wetted packing exceeds the dry packing area up to 60%. The estimated pressure drop is about one magnitude lower than that of a comparable corrugated sheet structured packing and the load limits are significantly higher.

Liquid load point determination in a reactive distillation packing by X‐ray tomography

AbstractIn this paper, we report on the use of X‐ray tomography to determine the liquid load point in 0.1 m diameter modular catalytic distillation packings Katapak‐SP11 and Katapak‐SP12. The liquid load point corresponds to the overall packed bed liquid load above which there is an increment in the liquid flowing outside the catalytic baskets and the catalytic baskets themselves are saturated with liquid. From tomographic images, we show that several factors affect the wetting and filling of catalytic baskets. The complex hybrid structure of catalytic packings influences the liquid distribution inside the elements. The liquid preferentially fills the external catalytic baskets because they receive the liquid not only from the packing element situated above but also from the wall wipers. Moreover, liquid hold‐up inside a catalytic basket section depends significantly on the vertical position in the packing element and on the position of the packing in the column packed bed. The counter–current gas flow speeds up the process of liquid filling of the baskets, also for low liquid loads. The non‐uniform distribution of liquid in catalytic basket which is observed experimentally makes the identification of a unique liquid load point not straightforward.

Effects of a countercurrent gas flow on falling-film mode transitions between horizontal tubes

A liquid film falling between horizontal tubes is known to take the form of droplets, jets or sheets, depending on the liquid flow rate; the form of the flow is the so-called “falling-film mode”. Although previously neglected in studies of mode transition, a countercurrent gas flow often exists in falling-film heat exchangers, and its effect on the liquid flow might be important: it could impact the flow regime, lead to local “dryout,” and decrease the heat transfer rate. Experiments are conducted to explore the effects of a countercurrent gas flow and liquid feeding length on falling-film mode transitions for a liquid flowing over horizontal tubes. The effects on mode transition are shown to depend on fluid properties and are explained in terms of unsteadiness and film thickness. In general, transition hysteresis is reduced with an increasing gas velocity. A correlation is developed to predict the countercurrent gas flow effects on falling-film mode transitions. The liquid feeding length can affect mode transitions in quiescent surroundings and when a countercurrent gas flow imposed.