Maximum entropy estimation of the bubble size distribution in fluidized beds

This work presents a new methodology, based on the maximum entropy method, to obtain bubble characteristics in fluidized beds. The probability distributions (PDF) of bubble pierced length and velocity are obtained applying the maximum entropy principle to experimental measurements. In addition, the bubble diameter distribution has been inferred from experimental pierced length measurements. This method is applied to characterize bubbles in fluidized beds for the first time and the most general bubble geometry, a truncated spheroid, is considered. The distance between probes, s, which is the minimum pierced length that is possible to measure accurately using intrusive probes, has been introduced as a constraint in the derivation of the size distribution equation.The maximum entropy method is applied to experimental measurements of bubble characteristics carried out using optical and pressure probes in a three-dimensional fluidized bed of Geldart B particles. Results on bubble size obtained from pressure and optical probes are very similar, although optical probes provide more local information and can be used at any position in the bed. The maximum entropy principle has been found to be a simple method that offers many advantages over other methods applied before for size distribution modeling in fluidized beds.     

受网络夹点控制的装置的改造分析

本文分析了过程夹点和网络夹点的不同,指出了在改造过程中存在网络夹点,并且节能潜力受网络夹点的控制,同时提出了克服网络夹点的途径,即分流和调整换热网络结构。通过对某炼油厂润滑油加氢补充精制装置的分析。提出了不分流时克服网络夹点的方法,即同一温闰的冷流体按热溶流率CP值减小依次被加热;同一温位的热流体按CP值减小依次被冷却。最后,文章对该装置提出了采用“一改动方案”的改造方案,节能效果和经济效益显著。     

气固流化床中信息熵的实验研究

在Shannon信息理论中的信息熵与互信息概念基础上,定义了信息传输率,用以重构相空间中研究从气固流化床内不同测量点的压力脉动时间序列之间的关联。实验结果证明了流化床内不同点之间存在着、三维的信息传输关系,提出了信息是通过床内粒子的扰动传输的,并由实验数据给出了这种压力脉动信息传输的平均速率及其随气速的变化规律。     

Investigations of local pressure drop fluctuation signals in annular type fluidized bed photoreactor by continuous wavelet transform

The wavelet transform is an effective tool for studying the dynamic behavior of fluidized beds in the resolution of time variables. To understand the behavior of photocatalyst under different velocity in an annular type fluidized bed, a new analysis technique (Continuous Wavelet Transform: CWT) is applied. With the time-frequency localization characteristics embedded in wavelets, the time and frequency information of signals can be presented as a visualized scheme. By analysis of various methods for pressure fluctuation signals measured from an annular type fluidized bed, it was found that the dynamic behavior of fluidization in the annulus fluidized bed reactor was easily observed with the aid of wavelet transform.     

国内外生物质气化设备研究进展

生物质是重要的可再生能源,生物质气化技术在国内外得到了广泛应用。本文综述了国内外固定床、鼓泡流化床、外循环流化床、内循环流化床、双循环流化床的结构。固定床安装简单,但焦油较多;外循环流化床燃烧效率高,但回料装置较难控制;内循环流化床不易结焦、氢含量高且不用考虑返料问题;双流化床结构复杂但焦油量少。将对固定床和流化床进行对比,认为固定床安装简单适合农村地区,流化床应不断改进和完善,更适应工业化生产。     

An experimental study of non-premixed combustion in a turbulent fluidized-bed reactor

Although the theory of fluidized bed technology is fairly well understood, non-premixed reactions in such beds have recently garnered considerable attention, particularly with respect to combustion applications, which still present a number of practical engineering and operational problems at start-up, during operation and at shutdown. While studies of such processes in fluidized beds remain essential to validating theoretical models, young researchers often perceive them as dirty and tedious experiments due to the complex operational concerns involved. Nevertheless, the planning and implementation of experiments, the analysis of the full array of interactions between system components and bed materials, internal surfaces and measurement devices, and interpretation of results to diagnose abnormal operating conditions present operators with a range of difficult intellectual challenges. Some of the important experimental issues involved in the operation of non-premixed reactions in fluidized beds are presented and discussed. In particular, widening the range of fluidized bed system operations to include direct fuel injection can expand the range of reactant compositions beyond those normally allowed by safety constraints. Therefore, the aim of this study is to understand the behaviour of non-premixed natural gas combustion in turbulent fluidized bed using various spargers under high temperature conditions. The results of this study can also provide insights into how volatiles burn inside fluidized bed reactors.     

Mechanism of bed-wall heat transfer in a fluidized bed at high temperatures

Radiant heat transfer in a fluidized bed was investigated experimentally by means of two kinds of heat exchange pipes. It was found that the contribution due to radiant heat transfer would be insignificant at temperature levels in fluidized beds up to 1000°C.The results are interpreted in terms of a model based on the behaviour of bubbles in the fluidized bed.     

Volume contraction behaviour of binary solid-liquid fluidized beds

Contrary to the widely held notion that the total bed height of a binary solid fluidized bed will be the sum of heights of the two individual mono-component beds fluidized at the same velocity, significant negative deviations have been observed in our experimental investigation. The negative deviations, signifying a contraction of the total volume of the binary solid fluidized bed, could sometimes be as high as 25% of the actual volume. The volume contraction has been found to depend mainly upon the degree of solids mixing prevailing in the bed irrespective of whether it is fully fluidized or not. The composition of the binary solid fluidized was another important factor that influenced the contraction behaviour of the bed besides the size ratio of the two constituent solid species of the binary present in the bed.     

Magnetic Resonance Imaging of fluidized beds

This paper reviews recent developments in Magnetic Resonance Imaging (MRI) which enable it to follow particle motion in fluidized beds. Imaging with a spatial resolution of 400 μm and a temporal resolution of 1 ms is now feasible; particle velocities of order 1 m/s can be measured with good accuracy. The technique provides voidage fractions on a motion-picture basis and particle velocity fields. Limitations are: (i) the particles must contain appropriate atoms e.g. C or H; and (ii) currently the fluidized bed diameter cannot exceed 50 mm, though measurements from larger units will doubtless become available. MR studies on fluidized beds are described: results are reported for (i) air jets entering the bed (ii) bubbling and slugging beds and (iii) dispersion in a bubbling bed. The data are consistent with published measurements. Study (i) helps to resolve the longstanding puzzle about the behaviour of an air jet entering a fluidized or partly-fluidized bed, answering the question: does the entering air form bubbles or a continuous jet?     

Numerical and experimental study on multiple-spout fluidized beds

In this paper we study the effect of multiple spouts on the bed dynamics in a pseudo-2D triple-spout fluidized bed, employing the discrete particle model (DPM) and non-intrusive measurement techniques such as particle image velocimetry (PIV) and positron emission particle tracking (PEPT). A flow regime map was constructed, revealing new regimes that were not reported so far. The multiple-interacting-spouts regime (C) has been studied in detail for a double- and triple-spout fluidized bed, where the corresponding fluidization regime for a single-spout fluidized bed has been studied as a reference case. The experimental results obtained with PIV and PEPT agree very well for all the three cases, showing the good performance of these techniques. The DPM simulation results slightly deviate from the experiments which is attributed to particle–wall effects that are more dominant in pseudo-2D beds than in 3D systems. The investigated multiple-interacting-spouts regime is a fully new flow regime that does not appear in single-spout fluidized beds. Two flow patterns have been observed, viz. particle circulation in between the spouts near the bottom of the bed, and an apparent single-spout fluidization motion at a higher location upwards in the bed. These findings show that the presence of multiple spouts in a spout fluidized bed highly affect the flow behaviour, which cannot be distinguished by solely investigating single-spout fluidized beds.     

磁场生物流化床特性的研究

本文对含磁粉固定化细胞载体—聚丙烯酰胺凝胶粒子的液固、气液固流化床在均匀磁场中的流化特性进行了研究.测定了液固床的空隙率及气液固三相床的相含率,并获得了相应的关联式.本文同时研究了磁场流化床用于固定化细胞处理含酚废水,进行催化降解反应的应用效果.     

压力脉动模型及其在脉冲流化床中的应用

考虑了流化床中分布板上形成的气泡不变形增长，最后以不同尺寸分布的气泡群形式通过床层，提出了能包含Ｄａｖｉｄｓｏｎ模型、Ｆａｎ模型在内的更一般化的流化床压力脉动模型。不但把气泡行为这个重要因素引进了压力脉动模型，而且成功地把压力模型应用于脉冲气流流化如何科学地运用脉冲气流来进一步改善流况提供了理论依据。     

Study of transition velocity from bubbling to turbulent fluidisation by recurrence plots analysis on pressure fluctuations

Recurrence plot (RP) and recurrence quantification analysis (RQA) have been used for studying the dynamic behaviour of gas–solid fluidised beds. RQA variables (recurrence rate, determinism, laminarity and entropy) were calculated for the bubbling fluidised bed at different superficial gas velocities (ranging 0.1–1.5 m/s) and aspect ratios (1, 1.5 and 2). Patterns within RP changed with these various conditions, and subsequently RQA parameters changed accordingly. These changes can be potentially useful to study the bed hydrodynamic. It was shown that transition from bubbling to turbulent velocity can be determined by the variation of recurrence rate and entropy. © 2011 Canadian Society for Chemical Engineering     

气固流化床混沌动力结构突变分析

A new method called structural catastrophe analysis was applied to the analysis of pressure fluctuation time series with chaotic and fractal characteristics in gas-solids fluidized bed.In every type of fixed bed,bubbling bed,turbulent bed,fast fluidized bed and pneumatic conveying stage,pressure fluctuation signals were sampled and dynamic exponents were solved.The changes of T_mean and T_max in different types of fluidized bed were analyzed and summarized.It was concluded that T_mean and T_max can indicate different types of fluidized,and T_max can also be recognized as a new characteristic parameter in addition to correlation parameters,LE exponent and K entropy.     

Parametric modelling of time series of pressure fluctuations in gas-solid fluidized beds

Applying parametric models on time series of pressure fluctuations recorded in a fluidized bed, this paper shows that the bed dynamics can be expressed in analogy with a mechanical system of a certain degree. Thus, the pressure signal is assumed to be an output of a linear time-invariant system driven by a forcing function. The forcing function represents a number of apparently random events (e.g. formation of bubbles at the air-distributor, bubble eruptions at the surface of the bed) and may thereby be approximated as white noise. Parametric models are advocated for characterization of the dynamics of fluidized beds when, for various reasons, long data records are not available or when the quality of the recorded signal is poor. An autoregressive model (AR) of the time series is proposed, and it is shown that the order of the model identifies a mechanical equivalent of certain fluidization behaviour. The model is applied to four fluidization time series, previously investigated. The result indicates that fluidized beds behave like single second-order systems or multiple higher-order mechanical systems acting in parallel. Parametric methods are also used for estimation of power spectra of pressure fluctuations. The information obtained is presented in the form of Bode plots to accentuate the behaviour of fluidized beds as linear dynamical systems. The results are compared with the corresponding information obtained by nonparametric methods, now predominantly used. Data requirements (number of samples, sampling frequency) for the use of parametric models are discussed.     

Fluidization and slugging in large-particle systems

Square-nose slugging that occurs with large particles in relatively small-diameter fluidized beds shows certain similarities with the fluidization behaviour in a fluidized bed coal combustion system with closely packed heat-exchanger tubes. In the present investigation, square-nose slugging is studied in fluidized beds of 0.1 and 0.15 m I.D. with coarse sand and alumina particles, at ambient conditions. Recording of pressure fluctuations was used to analyse the fluidization behaviour. A remarkable change in the pressure fluctuation pattern occurs at the transition from normal fluidization to slugging: a more regular signal with a narrowed frequency spectrum is found.In the square-nose slugging regime, the pressure fluctuations seem to be caused by the disintegration of a rising solids slug, followed by the raining down of the particles. Experimental evidence for this mechanism was found in the behaviour of the magnitude of the pressure fluctuations as a function of operating variables.The frequency of square-nose slugging increases with approximately the square root of the bed diameter and appears to be independent of the type of particles used. The slug frequency decreases slightly with the gas velocity between about 0.8 and 1.8 m.s⁻¹, and is inversely proportional to the stationary bed height between 0.15 and 0.4 m.     

Internally circulating fluidized bed for continuous adsorption and desorption

An internally circulating fluidized bed has been developed for continuous adsorption and desorption. This system can be used more generally for many other reaction/regeneration operations. It helps to separate and to recover gaseous pollutants and reusable compounds (e.g. CO₂, SO₂, organic solvent vapors, along with other gases). Two fluidized beds are arranged next to each other. The partition wall in the upper and lower part of the fluidized beds has horizontal openings to let solid matter pass through. As the two beds are fluidized at different rates, the bed material starts to circulate between the two beds. The bed material doubles as an adsorbent. In the adsorption zone, polluted gas is used for fluidization; in the desorption zone, heated air or steam is used. The main differences from conventional fixed-bed adsorbers are that plants can be built in a more compact manner, higher flow rates can be achieved, and separate optimization of the two zones (adsorption zone, desorption zone) is possible.     

单组分两维射流床内空隙率分布的数学模拟

根据基本守恒方程建立了描述气－固两相流动的数学模型，将用于单相流计算的ＳＩＭＰＬＥ算法改进后，成功地开发了预测流化床内流体动力学的ＣＡＳＩＣＣ软件包。进而详细地模拟了单组分射流床空隙率分布特性，结果表明时均空隙率随轴向位置依次分为：浓相区、颗粒夹带区和稀相区。所得结论与实验观察相一致，可以为该类反应器的设计和放大提供有益的建议     

气固脉冲流化床流体力学特性的研究

在φ７０ｍｍ的流化床内，采用聚氯乙烯、玻璃珠和不规则天然刚玉等Ｂ类、Ｄ类颗粒，测定了０￣５．０Ｈｚ脉冲频率下气固流化床的基本流体力学特性，探讨了影响床层流化特性的一些主要因素，并根据实验数据对脉冲流化床的临界流化速度和临界流化压降的无因次准数式进行了关联。