Packed beds reactor

The packed-bed reactor is a cylindrical, usually vertical, reaction vessel into which particles containing the catalyst or enzyme are packed. The reaction proceeds while the fluid containing reactants is passed through the packed bed. In the case of a packed-bed bioreactor, a liquid containing the substrate is passed through a bed of particles of immobilized enzyme or cells. 

Consider an idealized simple case of a Michaelis-Menten-type bioreaction taking place in a vertical cylindrical packed-bed bioreactor containing immobilized enzyme particles. The effects of mass transfer within and outside the enzyme particles are assumed to be negligible. The reaction rate per differential packed height (m) and per unit horizontal cross-sectional area of the bed (m2) is then given as (cf. Equation 3.28)  

The required height of the packed bed z can be obtained by integration of Equation 7.47. Thus, 

In the case that the mass transfer effects are not negligible, the required height of the packed bed is greater than that without mass transfer effects. In some practices, the ratio of the packed-bed heights without and with the mass transfer effects is defined as the overall effectiveness factor rj0( ), the maximum value of which is unity. However, if the right-hand side of Equation 7.47 is multiplied by /0, it cannot be integrated simply, since rj0 is a function of CA, U, and other factors. If the reaction is extremely rapid and the liquid-phase mass transfer on the particle surface controls the overall rate, then the rate can be estimated by Equation 6.28. 

Microreactors are miniaturized reaction systems with fluid channels of very small dimensions, perhaps 0.05 to 1 mm. They are a relatively new development, and at present are used mainly for analytical systems. However, microreactor systems could be useful for small-scale production units, for the following reasons  

In packed-bed reactors (PBRs), the solid particulate catalyst particles forming the bed are fixed in an enclosed volume. The 

Multiphase Catalytic Reactors Theory, Design, Manufacturing, and Applications, First Edition. Edited by Zeynep llsen Onsan and Ahmet Kerim Avci. 

The shell/tube configuration of tubular PBRs depends on the nature of the catalytic reaction. For highly endothermic reactions such as catalytic steam reforming, the reactor geometry is similar to that of a fired furnace in which the catalyst-packed tubes are heated by the energy released by the combustion of a fuel on the shell side. Catalytic steam reforming involves the conversion of a hydrocarbon to a hydrogen-rich mixture in the presence of steam  

The process is known as the conventional method of producing hydrogen for meeting the hydrogen demands of the refining and petrochemical industry. The most widely used fuel in steam reforming is natural gas, which is mostly composed of methane  

The multitubular PBR configuration is preferred when convection is not sufficient for defivering the necessary heat flux to sustain the operation. However, in most of the exothermic and endothermic reactions, the temperature of the catalyst bed can be regulated by convective external heat transfer. In 

A cold flow model is used to calculate the effect of gas leakage from the reactors with diameter of 0.19 m and length of 0.5 m 

Experiments have been carried out in a cold and hot lab-scale prototype of 5-1 OkW  

The experiments have been carried in a lOkW, unit with different coal and petcoke with a char conversion between 0.7 and 0.9. 

The experimental setup has been tested with different oxygen carriers. The CLOU and isC-CLC processes have been carried out in prototype lab scalefi. 5 kW,h and 0.5 kW,) 

The Ergun equation can be used to predict the pressure drop along the length of a packed bed given the fluid velocity, the packing size, the viscosity, and density of the fluid [2]. 

The discovery of solid catalysts led to a breakthrough of the chemical process industry. Today most commercial gas-phase catalytic processes are carried out in fixed packed bed reactors. A fixed packed bed reactor consists of a compact, immobile stack of catalyst pellets within a generally vertical vessel. On macroscopic scales the catalyst bed behaves as a porous media. The fixed beds are thus employed as continuous tubular reactors in which the reactive species in the mobile fluid (gas) phase are reacting over the catalyst surface (interior or exterior) in the stationary packed bed. Compared to other reactor types or designs utilizing heterogeneous catalysts, the fixed packed bed reactors are preferred because of simpler technology and ease of operation. 

It appears that the complete model for both mass and heat transfer contains four adjustable constants, Dr, Er, K and Xr, but Er and Xr are constrained by the usual relationship between thermal diffusivity and thermal conductivity 

there are only three independent parameters. We take these to be Dr, K, and Xr- Imperfect but generally useful correlations for these parameters are available. For a summary of published correlations and references to the original literature see Froment and Bischoff, Dixon and Cresswell, and Dixon.  

Many practical designs use packing with a diameter that is an appreciable fraction of the tube diameter. The following relationship is used to correct Dr for large packing  

Shell-and-tube reactors may have dtldp = 3 or even smaller. A value of 3 is seen to decrease u dp/Dr by a factor of about 3. Reducing the tube diameter from Qdp to 3dp will increase Dr by a factor of about 10. Small tubes can thus have much better radial mixing than large tubes for two reasons R is lower and Dr is higher. 

FIGURE 9.1 Existing data for the radial Peclet number in large-diameter packed beds, (Pe) = usdpID,) versus pd u./u. 

As the resistances for external and internal mass transfers are mainly influenced by the particle size and the specific surface area, small particles will greatly ameliorate the transport processes as the characteristic mass transfer times depend on the particle size (Eq. (11.3) and Eq. (11.4)). 

For packed bed reactors, the external mass transfer can be estimated with the following relations [9]  

The drawback of using small particles is the high pressure drop in catalytic packed bed reactors (Eq. (11.19)) and consequently the high energy consumption. The pressure drop in packed beds can be estimated with the Ergun equation [10]  

In the following two sections, possibilities to obtain high transport rates and to avoid the above-mentioned high energy dissipations using MSR and/or structured catalysts are discussed. 

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Olefin Amination (Method 6). The most recent technology for the production of lower alkylamines is olefin amination (14). This is 2eohte-cataly2ed reaction of ammonia with an olefin, eg, isobutylene, and is practiced in a packed-bed reactor system in the vapor phase. 

En2yme techniques are primarily developed for commercial reasons, and so information about immobilisation and process conditions is usually Limited. A commercially available immobilised penicillin V acylase is made by glutaraldehyde cross-linking of a cell homogenate. It can be used ia batch stirred tank or recycled packed-bed reactors with typical operating parameters as iadicated ia Table 2 (38). Further development may lead to the creation of acylases and processes that can also be used for attaching side chains by ensymatic synthesis. 

Unsteady-State Direct Oxidation Process. Periodic iatermption of the feeds can be used to reduce the sharp temperature gradients associated with the conventional oxidation of ethylene over a silver catalyst (209). Steady and periodic operation of a packed-bed reactor has been iavestigated for the production of ethylene oxide (210). By periodically varyiag the inlet feed concentration of ethylene or oxygen, or both, considerable improvements ia the selectivity to ethylene oxide were claimed. 

Problems are stiff when the time constants for different phenomena have very different magnitudes. Consider flowthrough a packed bed reactor. The time constants for different phenomena are ... 

TABLE 7-10 Material and Energy Balances of a Packed Bed Reactor... 

Some modes of heat transfer to stirred tank reacdors are shown in Fig. 23-1 and to packed bed reactors in Fig. 23-2. Temperature and composition profiles of some processes are shown in Fig. 23-3. Operating data, catalysts, and reaction times are stated for a number of industrial reaction processes in Table 23-1. 

Transient Studies in an Adiabatic Packed-Bed Reactor was the title of a publication by Berty et al (1972). This was in connection with thermal runaway of reactors. The pertinent subject will be discussed in a following chapter in which the interest is focused on how to avoid the onset of a runaway. Here the object of the experiment was to see what happens after a runaway has started. 

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed (packed-bed) reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds (i.e., layers across the reactor) or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry (e.g., ammonia synthesis) and offers several advantages over other forms of fixed 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... 

The dispersion model has been successfully employed in modeling the behavior of packed bed reactors. In this case. 

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

The alternative to batch mode operation is continuous operation. In the continuous mode there is a continuous flow of medium into the fermentor and of product stream out of the fermentor. Continuous bioprocesses often use homogenously mixed whole cell suspensions. However, immobilised cell or enzyme processes generally operate in continuous plug flow reactors, without mixing (see Figure 2.1, packed-bed reactors). 

As well as being active, the immobilised enzyme also needs to be stable (active for a long period) and the support must promote this. The support must also have appropriate mechanical characteristics it should not disintegrate if used in a stirred tank reactor it should produce even flow (without channelling) in a packed bed reactor. Hie cost of the support is also important. 

The fat modification processes can be operated either in batches using stirred tank reactors or continuously with packed bed reactors. 

The analysis of the transient behavior of the packed bed reactor is fairly recent in the literature 142-145)- There is no published reactor dynamic model for the monolith or the screen bed, which compares well with experimental data. 

FIGURE 3.2 Annular packed-bed reactor used for adiabatic reactions favored by low pressure. 

The next example applies this general procedure to a packed-bed reactor. 

The models of Chapter 9 contain at least one empirical parameter. This parameter is used to account for complex flow fields that are not deterministic, time-invariant, and calculable. We are specifically concerned with packed-bed reactors, turbulent-flow reactors, and static mixers (also known as motionless mixers). We begin with packed-bed reactors because they are ubiquitous within the petrochemical industry and because their mathematical treatment closely parallels that of the laminar flow reactors in Chapter 8. 

FIGURE 9.3 Temperature and concentration profiles at the point of maximum temperature for the packed-bed reactor of Example 9.1. 

FIGURE 9.4 Thermal runaway in the packed-bed reactor of Examples 9.1 and 9.2 Ti = 704K. 

They convert the initial value problem into a two-point boundary value problem in the axial direction. Applying the method of lines gives a set of ODEs that can be solved using the reverse shooting method developed in Section 9.5. See also Appendix 8.3. However, axial dispersion is usually negligible compared with radial dispersion in packed-bed reactors. Perhaps more to the point, uncertainties in the value for will usually overwhelm any possible contribution of D. ... 

Chapters 8 and Section 9.1 gave preferred models for laminar flow and packed-bed reactors. The axial dispersion model can also be used for these reactors but is generally less accurate. Proper roles for the axial dispersion model are the following. 

The heat and mass transfer phenomena associated with packed-bed reactors are described in... 

A review article describing the occasionally pathological behavior of packed bed reactors is... 

The first eight chapters of this book treat homogeneous reactions. Chapter 9 provides models for packed-bed reactors, but the reaction kinetics are pseudohomogeneous so that the rate expressions are based on fluid-phase concentrations. There is a good reason for this. Fluid-phase concentrations are what can be measured. The fluid-phase concentrations at the outlet are what can be sold. 

Chapter 10 begins a more detailed treatment of heterogeneous reactors. This chapter continues the use of pseudohomogeneous models for steady-state, packed-bed reactors, but derives expressions for the reaction rate that reflect the underlying kinetics of surface-catalyzed reactions. The kinetic models are site-competition models that apply to a variety of catalytic systems, including the enzymatic reactions treated in Chapter 12. Here in Chapter 10, the example system is a solid-catalyzed gas reaction that is typical of the traditional chemical industry. A few important examples are listed here ... 

Consider an observed reaction of the form A + B P + Q occurring in a packed-bed reactor. 

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