Packed fluidized bed

Particle diameter is a primary variable important to many chemical engineering calculations, including settling, slurry flow, fluidized beds, packed reactors, and packed distillation towers. Unfortunately, this dimension is usually difficult or impossible to measure, because the particles are small or irregular. Consequently, chemical engineers have become familiar with the notion of equivalent diameter of a partiele, which is the diameter of a sphere that has a volume equal to that of the particle. 

The above discussion and examples illustrate that reactor scale models have not advanced much during the past decades and this hinders our ability to reduce risk in implementation of new more efficient catalytic technologies. To make progress, it is necessary to develop improved and more accurate descriptions of flow and mixing in typical multiphase reactors. Multiphase reactors frequendy encountered in practice (Fig. 1.10) such as risers, bubble columns, fluidized beds, packed beds, and stirred tanks are opaque so that not all flow visualization tools (Table 1.6) are suitable. 

Thermal heat conductivity of gas phase in fluidized bed/ packed bed at static state (kW/m K)... 

The techniques actually available for SO2 removal from flue gases may be broadly classified into wet and dry systems. The wet methods employ a slurry or solution of some absorbent for SO2, whereas the dry systems use dry particles of absorbent in such gas-solid contacting devices as fluidized beds, packed beds, and entrainment reactors. It is with these latter methods that we are concerned here, although wet methods appear to be at least as satisfactory as the dry systems. 

The basic concepts of a gas-fluidized bed are illustrated in Figure 1. Gas velocity in fluidized beds is normally expressed as a superficial velocity, U, the gas velocity through the vessel assuming that the vessel is empty. At a low gas velocity, the soHds do not move. This constitutes a packed bed. As the gas velocity is increased, the pressure drop increases until the drag plus the buoyancy forces on the particle overcome its weight and any interparticle forces. At this point, the bed is said to be minimally fluidized, and this gas velocity is termed the minimum fluidization velocity, The bed expands slightly at this condition, and the particles are free to move about (Fig. lb). As the velocity is increased further, bubbles can form. The soHds movement is more turbulent, and the bed expands to accommodate the volume of the bubbles. 

Fig. 1. Fluidized-bed behavior where U is the superficial gas velocity and is the minimum fluidization velocity (a) packed bed, no flow (b) fluid bed,...

Contactive (Direct) Heat Transfer Contactive heat-transfer equipment is so constructed that the particulate burden in solid phase is directly exposed to and permeated by the heating or cooling medium (Sec. 20). The carrier may either heat or cool the solids. A large amount of the industrial heat processing of sohds is effected by this mechanism. Physically, these can be classified into packed beds and various degrees of agitated beds from dilute to dense fluidized beds. 

Carbon should be prewetted prior to being placed in the test columns. Backwashing the carbon at low rates (2.5 m/hr) does not remove the air. Rates that would expand the bed 50 percent or 15-30 m/hr, are required. The liquid used for prewetting can either be water, if it is compatible with the liquid to be treated, or a batch of the liquid to be treated which has been purified previously. There are three types of carbon systems (1) fixed beds, (2) pulse beds, and (3) fluidized beds, and these can be used singly, in parallel, or in combination. The majority of systems are either fixed or pulse beds. The two basic types of adsorbers which can be designed to operate under pressure or at atmospheric pressure are the moving or pulse bed and the fixed bed. Either can be operated as packed or expanded beds. 

Equation 6-108 is also a good approximation for a fluidized bed reactor up to the minimum fluidizing condition. However, beyond this range, fluid dynamic factors are more complex than for the packed bed reactor. Among the parameters that influence the AP in a fluidized bed reactor are the different types of two-phase flow, smooth fluidization, slugging or channeling, the particle size distribution, and the... 

Packed beds An absorption separator that employs a fluidized bed of plastic spheres constrained betv/een horizontal screens. 

The adsorption process is normally performed in a column. The column is run as either a packed- or fluidized-bed operation. The adsorbent, after it has reached the end of its useful life, can either be discarded or regenerated. For further information, the reader is directed to the literature. ... 

Fluidized bed when packed beds are operated in upflow mode, the bed expands at high flow rates channelling and clogging of the bed are avoided. Normal application is waste-water treatment and the production of vinegar. 

Compact bed IX designs, such as the Schwebebett, use packed-bed and fluidized-bed functions to eliminate channeling. 

The value for is conservatively interpreted as the particle diameter. This is a perfectly feasible size for use in a laboratory reactor. Due to pressure-drop limitations, it is too small for a full-scale packed bed. However, even smaller catalyst particles, dp 50 yum, are used in fluidized-bed reactors. For such small particles we can assume rj=l, even for the 3-nm pore diameters found in some cracking catalysts. 

Membrane reactors are defined here based on their membrane function and catalytic activity in a structured way, predominantly following Sanchez and Tsotsis [2]. The acronym used to define the type of membrane reactor applied at the reactor level can be set up as shown in Figure 10.4. The membrane reactor is abbreviated as MR and is placed at the end of the acronym. Because the word membrane suggests that it is permselective, an N is included in the acronym in case it is nonpermselective. When the membrane is inherently catalytically active, or a thin catalytic film is deposited on top of the membrane, a C (catalytic) is included. When catalytic activity is present besides the membrane, additional letters can be included to indicate the appearance of the catalyst, for example, packed bed (PB) or fluidized bed (FB). In the case of an inert and nonpermselective... 

Another important challenge is to enhance the reliability of the design and scale up of multi-phase reactors, such as fluidized bed reactors and bubble-colunms. The design uncertainty caused by the complex flow in these reactors has often led to the choice of a reactor configuration that is more reliable but less efficient. An example is Mobil use a packed-bed reactor for the methanol to gasoline process in New Zealand, even though a... 

Bavarian and Fan [3, 4] reported a similar phenomenon occurring in a three-phase fluidized bed. In their case, the hydraulic transport of a packed bed occurred at the start-up of a gas-liquid-solid fluidized bed. Although the cause was different from the case reported in the present study, similar phenomena were observed in both cases. 

Adsorption equilibrium of CPA and 2,4-D onto GAC could be represented by Sips equation. Adsorption equilibrium capacity increased with decreasing pH of the solution. The internal diffusion coefficients were determined by comparing the experimental concentration curves with those predicted from the surface diffusion model (SDM) and pore diffusion model (PDM). The breakthrough curve for packed bed is steeper than that for the fluidized bed and the breakthrough curves obtained from semi-fluidized beds lie between those obtained from the packed and fluidized beds. Desorption rate of 2,4-D was about 90 % using distilled water. 

Fig. 3 shows the breakthrough curves for the packed, semi-fluidized, and fluidized beds. It is seen that the breakthrough curve obtained from the semi-fluidized bed lies between those obtained from the packed and fluidized beds, since the semi-fluidized bed possesses the features of both the fluidized and packed beds. This figure also shows that the shape of the breakthrough curve for the packed bed is steeper than that for the fluidized bed. 

The shape of the breakthrough curve for a packed bed is steeper than that for the fluidized bed and the breakthrough curves obtained from semi-fluidized bed lies between those obtained from the packed and fluidized beds. 

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