Wall cooled fixed-bed reactor

Steady State Axial Temperature Profiles In Wall Cooled Fixed Bed Reactors... 

O. Kalthoff and D. Vortmeyer, Ignition/Extinction Phenomena in a Wall-Cooled Fixed-Bed Reactor, Chem. Eng. Sci. 55 1637-1643 (1980). 

Depending on the mode of reactor operation (adiabatic, wall-cooled etc.), these equations have to be complemented by respective boundary conditions (values at reactor inlet, symmetry considerations in the axis of a tubular reactor, radial heat transfer at the wall), as subsequently inspected in more detail in Section 4.10.7.3 for a wall cooled fixed bed reactor. 

The model equations of the fixed bed reactor given by Eqs. (4.10.125) and (4.10.126) are still rather complicated. Thus, criteria would be helpful to decide whether and which of the different dispersion effects can be neglected. In Section 4.10.7.2, these criteria are examined. In Section 4.10.7.3, we will give deeper insight into the modeling of wall-cooled fixed bed reactors and the problems related to the modeling of radial heat transport. 

Two-Dimensional Model of a Wall-Cooled Fixed Bed Reactor Radial heat transport is an important factor in wall-cooled (or heated) reactors, particularly if we have a strong exothermic reaction with the danger of a temperature runaway. Figure 4.10.67 shows a typical radial temperature profile in a cooled tubular fixed... 

One-Dimensional Model of a Wall-Cooled Fixed Bed Reactor In some cases, it may be convenient to use a simple one-dimensional model, for example, to get an initial insight into the reactor behavior by a less complicated model. This model also takes into account A ad and aw,int> but we now introduce a mean (constant) bed temperature Tnean and an overall heat transfer coefficient of the bed, the thermal transmittance [/ted. which collects the interplay of heat conduction in the bed (A d) and the heat transfer at the wall (a i t) (Figure 4.10.68). According to this model, heat transfer from a packed bed to a heat transfer medium that cools the outer surface of the wall of a tubular reactor is given by ... 

A second criterion for the safe operation of a cooled fixed-bed reactor is obtained from Equ.(4-136). If the demand is fulfilled that the true tube diameter must not exceed the critical diameter, then this allows a distinct maximal temperature difference between maximum and wall temperature only. Mathematically this is obtained by setting d equal to dc resulting in Ci = 1 and inserting this into Equ.(4-136). 

P., 1989. Simultaneous Solution of Energy, Mass, and Momentum Equations for Wall-Cooled Fixed-Bed Chemical Reactors. Chemie Ingenieur Technik, 61(8) ... 

The reactor simulated is a wall-cooled fixed-bed catalytic reactor reported by Valstar [7] for the synthesis of vinyl acetate from acetic acid and acetylene with zinc acetate on activated carbon as catalyst as given in Sect. 7.1.2. 

Hydrogenation of benzene to cyclohexane was effected in a fixed bed reactor at 210-230°C, but a fall in conversion was apparent. Increasing the bed temperature by 10°C and the hydrogen flow led to a large increase in reaction rate which the interbed cooling coils could not handle, and an exotherm to 280°C developed, with a hot spot of around 600° C which bulged the reactor wall. 

An important class of non-isothermal fixed-bed reactors is that for which the tube wall temperature is not constant, but varies along the reactor length. Such would be the case when the cooling tubes and reactor... 

Fixed-Bed Reactors which are Cooled or Heated Through the Wall... 

Dispersion of heat can be described in a similar manner as dispersion of mass if we use an effective thermal conductivity in the axial and radial direction (kax, 2-rad)- The corresponding dimensionless Pedet numbers are Pch ad (= MsCpp oidp/Xrad) and Pch,ax (= WsCpPmoi p/ ax)- Note that the superficial fluid velocity, Us, and not the interstitial velocity, Ws/e, is used in the definition of Pejj, as the effective heat conduction (reflecting both the effective heat conduction in the gas and solid phase) is not limited to the empty space of the packed bed as in the case of dispersion of mass (see also differential equations of a fixed bed reactor in Section 4.10.7). As a rule of thumb, we can approximately use the same values for the Pedet number for dispersion of heat for high Rep numbers (>100) as for the corresponding numbers for dispersion of mass, that is, Pe r d 10 and Pe 2. Details on the radial heat dispersion, which is important for wall-cooled reactors, are given in Section 4.10.7.3. 

As a rule of thumb, axial dispersion of heat and mass (factors 2 and 3) only influence the reactor behavior for strong variations in temperature and concentration over a length of a few particles. Thus, axial dispersion is negligible if the bed depth exceeds about ten particle diameters. Such a situation is unlikely to be encountered in industrial fixed bed reactors and mostly also in laboratory-scale systems. Radial mass transport effects (factor 1) are also usually negligible as the reactor behavior is rather insensitive to the value of the radial dispersion coefficient. Conversely, radial heat transport (factor 4) is really important for wall-cooled or heated reactors, as such reactors are sensitive to the radial heat transfer parameters. 

Axial Dispersion of Heat According to Mears (1976), solution of the differential equations for the heat and mass balance (for a first-order reaction) lead to the following equation for the deviation of the axial temperature in a wall-cooled or heated fixed bed reactor from the corresponding value in an ideal plug-flow reactor ... 

One-Dimensional Fixed Bed Reactor Model It may be convenient to use a onedimensional model, where only axial gradients of temperature and concentration are considered. We now compare how accurate this approach is. Like the two-dimensional model, the one-dimensional model also takes into account X d. w,int, Xwaii, and a ex, but now we assume a constant bed temperature and an overall thermal transmittance L/ovenii that combines conduction in the bed, heat transfer at the wall, through the wall, and to the cooling medium by ... 

Both reactor types are quite complicated firom a construction tube bundles in both reactor types have to be constructed to temperature variations that occur when the process is started, reactor requires a special distributor, and multi-cyclones catalyst in the bed. The tubular reactor requires a support each pipe. Nevertheless both reactor types find wide application Here we shall consider the fixed bed reactor with wall cooling, cooling causes radial temperature gradients, that in its... 

Catalytic methanation processes include (/) fixed or fluidized catalyst-bed reactors where temperature rise is controlled by heat exchange or by direct cooling using product gas recycle (2) through wall-cooled reactor where temperature is controlled by heat removal through the walls of catalyst-filled tubes (J) tube-wall reactors where a nickel—aluminum alloy is flame-sprayed and treated to form a Raney-nickel catalyst bonded to the reactor tube heat-exchange surface and (4) slurry or Hquid-phase (oil) methanation. 

Example 4.6 Entropy production in a packed duct flow Fluid flow and the wall-to-fluid heat transfer in a packed duct are of interest in fixed bed chemical reactors, packed separation columns, heat exchangers, and some heat storage systems. In this analysis, we take into account the wall effect on the velocity profile in the calculation of entropy production in a packed duct with the top wall heated and the bottom wall cooled (Figure 4.7). We assume... 

Radial dispersion of mass and heat in fixed bed gas-solid catalytic reactors is usually expressed by radial Peclet number for mass and heat transport. In many cases radial dispersion is negligible if the reactor is adiabatic because there is then no driving force for long range gradients to exist in the radial direction. For non-adiabatic reactors, the heat transfer coeflScient at the wall between the reaction mixture and the cooling medium needs also to be specified. 

A concept for a methanol (or ethanol) fuel processor based upon steam reforming and membrane separation was presented by Gepert et td. [400]. As shown in Figure 5.33, the alcohol/water mixture was evaporated and converted by steam reforming in a fixed-bed catalyst, into which palladium capillary membranes were inserted. The retenate then entered the combustion zone, which was positioned concentrically around the reformer bed at the reactor wall. Air was fed into the combustion zone and residual hydrogen, carbon monoxide and unconverted methanol combusted therein. The sealing of the membranes at the reactor top was an issue solved by air-cooled elastomers. 

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