In packed beds

Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

In packed beds of particles possessing small pores, dilute aqueous solutions of hydroly2ed polyacrylamide will sometimes exhibit dilatant behavior iastead of the usual shear thinning behavior seen ia simple shear or Couette flow. In elongational flow, such as flow through porous sandstone, flow resistance can iacrease with flow rate due to iacreases ia elongational viscosity and normal stress differences. The iacrease ia normal stress differences with shear rate is typical of isotropic polymer solutions. Normal stress differences of anisotropic polymers, such as xanthan ia water, are shear rate iadependent (25,26). 

N. Wakao, Heat and Mass Transfer in Packed Beds, Gordon Breach, New York, 1982. 

T. R. Melli, W. B. Kolb, J. M. deSantos, and L. E. Scriven, "Cocurrent Downflow in Packed Beds Microscale Roots of Macroscale Plow Regimes,"... 

Fig. 14. Position of water front in packed bed of adsorbent during dynamic dehydration. Conditions 50% rh 10.2 cm/s air particle size = caO.167 cm ...

Effects of Temperature on tiQ and tii The Stanton-number relationship for gas-phase mass transfer in packed beds,... 

A. Heat or mass transfer in packed bed for gases and liquids... 

S] Correlates large amount of published data. Compares number of correlations, v,. = relative velocity, uVs. In packed bed, v,. = i>s pe,./e. 

Dispersion In tubes, and particiilarly in packed beds, the flow pattern is disturbed by eddies diose effect is taken into account by a dispersion coefficient in Fick s diffusion law. A PFR has a dispersion coefficient of 0 and a CSTR of oo. Some rough correlations of the Peclet number uL/D in terms of Reynolds and Schmidt numbers are Eqs. (23-47) to (23-49). There is also a relation between the Peclet number and the value of n of the RTD equation, Eq. (7-111). The dispersion model is sometimes said to be an adequate representation of a reaclor with a small deviation from phig ffow, without specifying the magnitude ol small. As a point of superiority to the RTD model, the dispersion model does have the empirical correlations that have been cited and can therefore be used for design purposes within the limits of those correlations. 

Glaser and Thodos [Am. Jn.st. Chem. Eng. J., 4, 63 (1958)] give a correlation involving individual particle shape and bed porosity. Kunii and Suzuki [Jnt. ]. Heat Mass Tran.sfer, 10, 845 (1967)] discuss heat and mass transfer in packed beds of fine particles. 

Material balances, often an energy balance, and occasionally a momentum balance are needed to describe an adsorption process. These are written in various forms depending on the specific application and desire for simplicity or rigor. Reasonably general material balances for various processes are given below. An energy balance is developed for a fixea bed for gas-phase application and simphfied for liquid-phase application. Momentum balances for pressure drop in packed beds are given in Sec. 6. 

Intraparticle convection can also occur in packed beds when the adsorbent particles have very large and well-connected pores. Although, in general, bulk flow through the pores of the adsorbent particles is only a small frac tion of the total flow, intraparticle convection can affec t the transport of veiy slowly diffusing species such as macromolecules. The driving force for convec tion, in this case, is the... 

The axial dispersion coefficient [cf. Eq. (16-51)] lumps together all mechanisms leading to axial mixing in packed beds. Thus, the axial dispersion coefficient must account not only for moleciilar diffusion and convec tive mixing but also for nonuniformities in the fluid velocity across the packed bed. As such, the axial dispersion coefficient is best determined experimentally for each specific contac tor. 

FIG. 16"10 Sherwood mimher correlations for external mass-transfer coefficients in packed beds for e = 0.4 (adapted from Siiziild, gen. refs.). 

TABLE 16-10 Coefficients for Axial Dispersion Correlations in Packed Beds Based on Eq (16-79)... 

The allowable pressure drop influences the choice of the particle size and helps determine the column length. Equations for estimating the pressure drop in packed beds are given in Section 6. 

Peclet number for dispersion Pe = uUD where t/ is a Bnear velocity, L is a hnear dimension, and is the dispersion coefficient. In packed beds, Pe = udp/De, where u is the interstitial velocity and dp is the pellet diameter. 

Re = R nolds number, dpS UolV Sc = Schmidt number, V/D D = axial dispersion coefficient dp = Diameter of particle or empty tube = Fraction voids in packed bed Uq = Superficial velocity in the vessel. 

Numerous studies have been made of the hydrodynamics and other aspects of the behavior of gas/liquid/sohd systems, in particular of trickle beds, and including absorption and extrac tiou in packed beds. Some of the hterature is reviewed in the references at the end of this subsection. 

Gas/Liquid Mass Transfer This topic has been widely investigated for gas absorption in packed beds, usually countercurrent. One correlation for cocurrent flow in catalyst beds is by Sato et al. (First Pacific Chemical Engineering Congre.s.s, Pergamon, 1972, p. 187) ... 

Flow in empty tubes has a relatively narrow band of velocities—or Reynolds numbers from 2000 to 10000—wherein the character changes from laminar to turbulent. In packed beds, even the laminar flow does not mean that motion is linear or parallel to the surface. Due to the many turns between particles, stable eddies develop and therefore the difference between laminar and turbulent flow is not as pronounced as in empty tubes. 

As can be seen in the table above, the upper two results for heat transfer coefficients hp between particle and gas are about 10% apart. The lower three results for wall heat transfer coefficients, h in packed beds have a somewhat wider range among themselves. The two groups are not very different if errors internal to the groups are considered. Since the heat transfer area of the particles is many times larger than that at the wall, the critical temperature difference will be at the wall. The significance of this will be shown later in the discussion of thermal sensitivity and stability. 

Figure 3.2.1 illustrates the mixing in packed beds (Wilhelm 1962). As Reynolds number approaches the industrial range Rep > 100, the Peclet numbers approach a constant value. This means that dispersion is influenced by turbulence and the effect of molecular diffusion is negligible. 

The original Rate Theory which describes dispersion in packed beds evolved over a number of years, probably starting with the work of Lapidus and Amundson [6] in 1952, extended by that of Glueckauf [7] and Tunitski [8] in 1954. The final form of the equation that described dispersion in packed beds as a function of the linear... 

PM Control - In packed-bed scrubbers, the gas stream is forced to follow a circuitous path through the packing material, on which much of the PM impacts. The liquid on the packing material collects the PM and flows down the chamber... 

Total liquid hold-up in packed bed, h[ = static hold-up, hg, plus operating hold-up, ho [64, 66]. 

For vaporization in packed beds of Raschig rings and Berl saddles [66] ... 

Fig. 17.6. Experimental configuration for the integrated, primary purification of intracellular proteins from unclarified disruptates. Panel A configuration employed for the purification of G3PDH from baker s yeast. Elution was performed in packed bed mode under reversed flow. Panel B configuration for loading, wash and elution in fluidised bed mode (employed for the purification of L-asparaginase from Erwinia chrysanthemi).

The area of interest covered by this paper is limited to processes in which chemical conversion occurs, as in the processes noted above. Gas-liquid-particle processes in which a gaseous phase is created by the chemical reaction between a liquid and a solid (for example, the production of acetylene by the reaction between water and carbide) are excluded from the review. Also excluded are physical separation processes, such as flotation by gas-liquid-particle operation. Gas absorption in packed beds, another gas-liquid-particle operation, is not treated explicitly, although certain results for this operation must necessarily be referred to. 

Gas absorption in packed beds may be described as a gas-liquid-particle process involving reacting gas and liquid phases and an inert particle phase, the latter functioning mainly as a momentum-transfer medium. 

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