Numerical simulation of flow patterns of disks in the asymmetric louvered-wall moving granular filter bed

Using the Discrete Element Method (DEM) model [P.A. Cundall, O. Strack, Geotechnique, 29 (1979) 47.], the granular solid flow pattern histories, velocity field and quasi-stagnant zone development close to the louvered walls of six kinds of 2-D asymmetrical moving granular filter beds were studied numerically. The numerical results reported here and the experimental results [J.T. Kuo, J. Smid, S.S. Hsiau, C.Y. Wang, C.S. Chou, Filtr. Sep., 35(6) (1998) 529.] provide fundamental and important information for designing moving granular bed high-temperature flue gas cleanup filters.     

The capture of aerosol in a granular moving bed

Flue gas from process plants usually contains contaminants which require scrubbing prior to discharging to atmosphere. There are various techniques used to scrub the exit gas, such as packed columns, spray scrubbers, fluidised beds, filters, etc. One of the key design parameters required in order to select and design a scrubber is the capture efficiency. This efficiency is dependent on a number of factors such as the contacting mode, feed composition and operating conditions.This paper describes an experimental technique to quantify the efficiency of liquid aerosol capture in a bed of moving particles. The experimental technique provides an effective means of generating and capturing the tracer aerosol and determination of the overall aerosol capture efficiency. The results show the influence of the superficial gas and particle velocity, bed height, as well as the aerosol concentration, on the overall capture efficiency.Three possible predictive methods are considered to describe or quantify the aerosol capture efficiency. These are a capture parameter based on the available surface area, a capture parameter based on dimensionless groupings, and a dispersion model based on aerosol particle filtration by fixed bed porous filters. Each method is applied to the experimental data to determine their effectiveness in describing the capture in the granular moving bed. The dispersion model method showed good potential in quantifying the experimental capture efficiency.     

Flow of filter granules in moving granular beds with louvers and sublouvers

It is important to develop the technology of high-temperature gas cleanup for some advanced power generation systems based on the gasification or combustion of coal or biomass. The moving granular bed filter has great potential to be developed for the hot gas cleanup. However the existence of stagnant zones in granular moving bed systems with louvered walls cause major technical problems, especially in filtration and adsorption processes. With the existence of stagnant zones, the dust particulates and fly ash coming with flue gas may plug the system and the filtration efficiency decreases remarkably.We propose a new design by placing sublouvers into the convergent channel between louvers to diminish the stagnant zones. This paper presents the results of a study of the flow patterns in a quasi two-dimensional cross-flow moving granular bed with systems of louvers and sublouvers. Silica sands were used as filter granules which were discharged from an upper hopper and circulated to the bed. An image processing system was used to record the granular flow for the investigations in flow patterns of filter granules. The velocity field and velocity distributions of filter granules are also evaluated and discussed. In this study, the design of sublouver-system moving bed has been optimized to almost completely eliminate the stagnant zones.     

Moving granular filter bed of quartz sand with louvered-walls and flow-corrective inserts

This study investigated the effect of a simple flow-corrective element, a vertical plate, on the flow patterns and wall stresses in a two-dimensional louvered-wall moving granular filter bed of quartz sand. The static wall stress distributions produced by the granular solids were measured, and compared with the theoretical prediction, using the differential slice and Runge-Kutta (order four) methods. The variations in the dynamic wall stresses over time were obtained using a two-directional pressure gauge. The results show that a vertical plate placed in each stage of the granular filter bed effectively diminished the quasi-stagnant zone adjacent to the louvered-wall, and reduced the wall pressure pulsation. For example, the ratio of the stagnant zone area in the second stage to the area of quartz sand in the second stage decreased from 50.66% (without an insert) to 27.86% (with a vertical plate). The maximum dynamic normal wall stress on the convergent section of third-stage louvered-wall (louver angle: 40°) declined remarkably from 9.95 kPa (without an insert) to 5.9 kPa (with a vertical plate), and then the reduction of the maximum dynamic normal wall stress was up to 40.7%. Furthermore, a vertical plate installed at an appropriate location satisfies the following requirements: (1) it shrinks the stagnant zone and reduces the wall stresses on the louvered wall of a moving granular filter bed; and (2) it solves the potential problem of a roof-type insert, on which the stagnant zone may sit.     

Investigation of fine granular material flow through a packed bed

Three different methods cut-off, time-of-flight, and Pulsed Field Gradient Nuclear Magnetic Resonance were used to study downstream flow of fine granular material through the fixed bed reactor. For describing the transport of solid particles within a fixed granular bed, a model has been developed. In time-of-flight and cut-off techniques the highest average velocity of filtration is observed at the lowest mass flow rate in all experimental traces, while upon the flow rate increase it tends to an asymptotic value. Experimental results obtained by pulsed field gradient nuclear magnetic resonance technique have revealed the bimodal character of particles velocities distribution. The average filtration velocity has a maximum at an intermediate mass flow rate close to the bed flooding, in contrast to the results obtained by cut-off and time-of-flight methods. The velocities measured using all three techniques were compared by converting them into dimensionless values. From the experimental results, the values of model parameters have been evaluated which allowed us to describe particle velocities within a bed.     

Temperature distribution within the moving bed of rotary kilns: Measurement and analysis

Inadequacies in the temperature measurement within the moving bed have hindered a thorough understanding of the processes occurring within rotary kilns. A new measuring system, consisting of thermocouple arrays, a radio-transmitter, a radio-receiver and a computer monitor is introduced in this paper. With it, the 3D temperatures within the moving bed, as well as the temperatures of the freeboard gas and the kiln wall, can be measured and saved automatically. Experiments with sand on a co-current pilot kiln demonstrated that, in the passive layer of the moving bed, the temperatures were approximately constant in the circumferential direction. In the radial direction, however, large temperature difference was observed within the bed near the feed end of the kiln, and the difference became smaller as the bed went progressed through the kiln. This temperature measuring system can be used to obtain data over a wide range of operating conditions for use in engineering design. The obtained results may give new thoughts in theoretical modeling of heat transfer within the moving bed of rotary kilns.     

Impact of flow-corrective insert on flow patterns in two-dimensional moving bed

This paper presents the study of the flow patterns inside a granular moving bed in a quasi-two-dimensional (2D) model of a granular moving bed filter (GMBF) vessel with vertical walls and a mass flow hopper. Fine quartz sand was chosen as the granular filter media in experiments. The related flow properties were measured by a Jenike Shear Tester. According to the flow properties of fine silica sand and Johanson's theory, a 2D moving-bed apparatus was built, with transparent walls and the flow-corrective insert to diminish the quasi-stagnant zones along the side walls. The flow patterns and velocity profiles of silica sand in the system with flow-corrective insert were studied experimentally, and the effect of the location of the flow-corrective insert on the internal flow patterns was investigated. The optimal placement of flow-corrective insert according to Johansson's theory was demonstrated to efficiently diminish the quasi-stagnant zones near the side walls. Little vertical shift of the flow-corrective insert from optimal position changed the stable and uniform mass flow pattern of moving bed into the funnel flow.     

气固并流式轴向移动床过滤器的压降特性

通过冷模实验,改变移动床表观气速、颗粒循环速率、入口气体含尘浓度等操作参数,研究了轴向移动床过滤器的压降特性和合适的操作条件,结合移动床内气固两相运动特点,修正了Ergun公式,在加尘条件下分析了床内滤饼对压降稳定性的影响。结果表明,在无尘负荷条件下(“纯”移动床操作),颗粒的循环速率由0增至2.26 kg/(m2?s)时,设备的压降减小0.03 kPa。表观气速为0.126 m/s、入口气体含尘浓度为89.10 g/m3时,移动床内滤饼形成和破损呈动态平衡,过滤500 s后,压降可稳定在0.88 kPa,此时设备具有较高的除尘性能,粉尘捕集效率可达96%以上。     

The effect of vibrating conditions on the electrostatic charge in a vertical vibrating granular bed

In this study, we carried out experiments to measure the electrostatic charge of a granular matter in a vertical shaker device. The purpose was to quantify the effect of the vibrating conditions on electrostatic charging in the granular matter. In each experimental run, 3 mm glass beads were first discharged to remove any residual charge prior to subsequently studying their electrostatic charging. The accumulative electrostatic charges of the granular materials were measured using a Faraday cage. The findings show that the vibrating conditions play an important role in the saturated electrostatic charge and time constant. The electrostatic charges of granular materials are mainly generated by the contact potential difference mechanism in the vibrating granular system. The results show that the saturated accumulation charge increases as the dimensionless vibrating acceleration increases, and decreases with increasing vibrating frequency. The time constant is small when a higher vibrating frequency is applied in the vibrating granular system. Finally, we demonstrate that the saturated accumulation charge increases linearly with the increase of the dimensionless vibrating velocity regardless of the vibrating frequency.     

The control of bed height and solids circulation rate in the standpipe of a cold flow circulating fluidized bed

Circulating fluidized beds (CFBs) are used widely in the chemical industry. Knowing or estimating the bed height in the standpipe and the solids circulation rate are essential for effective control of the system. This paper incorporates a 2-region model to calculate the bed height in the standpipe with a Kalman filter algorithm to estimate the solids circulation rate (SCR). Simulations of both the standpipe bed height and SCR were compared with experimental data and shown to give good agreement.

In addition, a neural network method was applied to model the entire cold flow CFB system and measured data sets were used to train the neurons of the network. Finally, a linear controller was applied to control both the bed height and solids circulation rate to desired set points. Simulations were performed for both positive and negative step inputs for both variables and satisfactory control was demonstrated using this controller in combination with the neutral network and Kalman estimator.     

Co-gasification of a lignite/waste-tyre mixture in a moving bed

The thermal treatment of waste-tyre by co-gasification with lignite was investigated on a commercial scale during the Lurgi gasification process. The experiments proved that this material can be treated in a mixture with lignite in the process of oxygen–steam pressure co-gasification in a moving bed, because a waste-tyre admixture improves the net calorific value of the raw gas obtained by 3% in comparison with that from the gasification of lignite alone. Further, it was found that the H₂S and CH₃SH contents in the raw gas are lower in the case of co-gasification than those from the gasification of lignite alone. Considering the very low reactivity of the char from waste-tyre and the resultant unburned carbon in the ash, the optimal content of the waste-tyre admixture in the gasified feed should not exceed 10 wt.%, whereas short-term increases of up to 20 wt.% will not cause any technological problems or significant economic losses.     

Role of bed height and amount of dust on the efficiency of sound‐assisted fluidized bed filter/afterburner

A 40‐mm sound‐assisted fluidized bed filter/afterburner for hot gas clean‐up has been characterized in terms of bed saturation time, total amount of collected particles, fraction of fine particles permanently adhered on the coarse bed particles, and efficiency of using a regeneration strategy based on mechanical (attrition) and/or chemical (combustion) action. Experiments have been carried out at ambient temperature as well as at 850°C, with and without application of sound and varying bed height and amount of dust in the gas flow. The controversial effect of the application of sound: not only enhancement of particles interactions but also increase of fines permanently adhering on bed coarse particles is presented and discussed. A simplified model has been developed to obtain rough predictions of bed height which maximize fine particles capture, bed saturation time, total amount of particles collected in the bed, fraction of fine particles loading present as adhered particles on bed particles. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Cold and hot gas filtration using a novel mobile granular bed with an inner fluidized section

Two experimental studies were conducted with a novel moving granular bed filter (NMGBF). The first one was performed on a cold and dry mock‐up module operated at room temperature and was aimed at a) proving the concept, b) establishing the operating patterns and flow profiles and c) optimizing the operating conditions. The gas flowrate and the geometry of the filter were the main variables studied. The second study was conducted on a prototype hot filter PDU (Process Development Unit) under real time operating conditions. The filter was one of the modules of a hot‐gas‐conditioning‐train in a fluidized bed gasifier. Results showed similar particles collection efficiency for hot‐ and cold‐dry filtration. The filter showed an efficiency of 99.9% over 55 h filtration time for particles larger than 1 μm, while for submicronic particles the efficiency is between 84% and 96% (weight basis). This paper presents and discusses the apparatus, the experimental methods and conditions as well as characteristic results on the two prototypes used.     

The shear force amendments on the slugging behavior of upflow Anammox granular sludge bed reactor

The granulation of Anammox sludge plays an important role in improving the efficiency of high-rate Anammox bioreactors. The present study involved the use of anaerobic granular sludge to start up the Anammox process to accomplish granulation in Anammox reactor. The accumulation of nitrogen gas and subsequent slugging behavior were observed in the upflow Anammox granular sludge bed (AGSB) reactor, resulting in deteriorated effluent quality. Based on shear force analysis of the gas column under the quasi-steady state, the liquid-induced shear force was increased by progressively shortening of hydraulic residence time (HRT) and implementing effluent recycling. It appeared to be the right strategy to eradicate the slugging behavior which disappeared completely when HRT was shortened to 1.10 h with liquid upflow velocity of 1.30 cm min⁻¹. The application of high shear stress enhanced the nitrogen removal performance to 15.40 kg-N m⁻³ d⁻¹. Thus the amendments in the liquid-induced shear force by shortening HRT may be an appropriate strategy to overcome slugging behavior of the Anammox reactor.     

炭粒床高效工业除尘装置的设计与开发

The new dust removal technical route using the carbon-granular bed filter,packed of carbon particles with appropriate grade derive from an online-process vibration sieve,to replace the traditional baggy filter had been developed successfully for capturing the micro-carbon dusts produced from pulverization of petroleum coke,and the green close loop of carbon materials is thus completed in the combined pulverizing and classifying system and pulverized carbon dust removal process.The high dust removal efficiency greater than 99%,low outlet dust concentration less than 100 mg.m-3,low pressure drop through dust filtration chamber less than 980 Pa,simple and easy design,and flexible and stable operation were achieved also with the carbon-granular bed filter in both bench and industrial scale operations.     

The influence of flow rate on the decrease in pressure beneath a conical pile

The decrease in the stress at the bottom of a sandpile exhibits the preparation dependence known to occur for granular materials. By using a grid-based tactile pressure sensor, we carefully measured the progressive development of the pressure profile at the bottom of conical piles formed by funneling rice grains onto the sensor. A significant decrease in the stress was observed at small flow rates, but it gradually disappeared at greater flow rates. This result is helpful in understanding the stress distribution within sandpiles.     

Analysis of stress and buoyancy for solids flow in the lower part of a blast furnace

A quasi-stagnant coke bed formed in the lower part of blast furnace, called ‘deadman’, is replaced by new coke over a long renewal interval. The repetition motion, floating of the hearth coke bed due to buoyancy during storage of the molten material and refilling the bottom space due to descending motion of the coke bed during the discharge, is considered to be one of the driving forces for the deadman renewal motion. The purpose of the present study is to clarify the critical condition for the hearth bed to be forced to float up due to buoyancy. Assuming deadman to be a conical body, the stresses acting at the deadman surface are obtained at first extending Walters’ theory (Walters, Chem. Eng. Sci. 28 (1973a) 13; 28 (1973b) 779) to the diverging shaft and the converging annular flow-channel between deadman and furnace wall under gas flow. In the next place, a stress field in an active state of stress is proposed to predict the distribution of solid load developed in the deadman and lower hearth part. The critical storage level of molten liquid for hearth bed floating is then derived from a force balance between the solids load, buoyancy and wall shear stress. It is found that the floating mode depends on the horizontal profile of vertical load at the critical liquid level. In the case of the load profile is uniform across the cross section, the hearth bed would start to float up leaving a solid-free liquid space with horizontally uniform height. In the case of the vertical load is high in most of the cross section, but becomes lower near the wall, two types are possible for bed motion. Thus, a diagram classifying the floating mode into three categories according to the way the hearth bed behaves at the beginning of floating is given.     

Particle segregation of binary mixture in a moving bed by penetration model

Segregation patterns of particles of different sizes in a moving bed were theoretically analyzed by a penetration model. Flow experiments with a two-dimensional hopper verified the derived equations in combination with those in the filling of the hopper. The segregating component was revealed to be more unevenly distributed inside the moving bed of wider cone angle at lower feed rate and lower initial mixing ratio as a result of prevalent segregation during heaping on the top of the moving bed. The steeper velocity profile inside the moving bed resulted in a bit more uneven distribution of the segregating component in the angular direction. In general, the segregation profile gradually became smooth during descent of the particle mixture inside the hopper due to particle penetration in the gravitational direction.     

Pyrolysis characteristics of Oriental white oak: Kinetic study and fast pyrolysis in a fluidized bed with an improved reaction system

The kinetic parameters for the pyrolysis of Oriental white oak were evaluated by thermogravimetric analysis (TGA). The white oak was pyrolyzed in a fluidized bed reactor with a two-staged char separation system under a variety of operating conditions. The influence of the pyrolysis conditions on the chemical and physical characteristics of the bio-oil was also examined. TGA showed that the Oriental white oak decomposed at temperatures ranging from 250 to 400 °C. The apparent activation energy ranged from 160 to 777 kJ mol^− 1. The optimal pyrolysis temperature for the production of bio-oil in the fluidized bed unit was between 400 and 450 °C. A much smaller and larger feed size adversely affected the production of bio-oil. A higher fluidizing gas flow and higher biomass feeding rate were more effective in the production of bio-oil but the above flow rates did not affect the bio-oil yields significantly. Recycling a part of the product gas as a fluidizing medium resulted the highest bio-oil yield of 60 wt.%. In addition, high-quality bio-oil with a low solid content was produced using a hot filter as well as a cyclone. With exception of the pyrolysis temperature, the other pyrolysis conditions did not significantly affect the chemical and physical characteristics of the resulting bio-oil.