Reaction external heat transfer

The existence of three steady states, two stable and one metastable, is common for exothermic reactions in stirred tanks. Also common is the existence of only one steady state. For the styrene polymerization example, three steady states exist for a limited range of the process variables. For example, if Ti is sufficiently low, no reaction occurs, and only the lower steady state is possible. If Tin is sufficiently high, only the upper, runaway condition can be realized. The external heat transfer term, UAextiTout — Text in Equation (5.28) can also be used to vary the location and number of steady states. 

GP 11] [R 19] For an autothermal reactor, i.e. a device with neither internal nor external heat transfer, steep temperature profiles along the flow axis were found [9]. Via an inspection window, glowing of the front zone of the wire reactor was observed, indicating complete conversion within a few mm reaction passages. The... 

The simulation of reacting flows in packed tubes by CFD is still in its earliest stages. So far, only isothermal surface reactions for simplified geometries and elementary reactions have been attempted. Heterogeneous catalysis with diffusion, reaction, and heat transfer in solid particles coupled to the flow, species, and temperature fields external to the particles remains a challenge for the future. 

In the case of exothermic or endothermic reactions, scale-up may impair conditions for heat input or removal because the ratio of the heat transfer surface area to the reactor volume is reduced. Identical conditions for heat transfer in both the model and full-scale plants may be achieved in exothermal reactions if both have the same thermal stability coefficient. This requirement is obtained by introducing external heat exchangers. Alternatively, a reactor with a strong exothermic reaction can be divided into several small size reactors. In this manner, the ratio of the external heat transfer surface area to the reactor volume is increased, thereby avoiding an excessive temperature rise in the reactor. 

Criterion Biot determines the ratio of intensity of external heat exchange processes (numerator) and effective thermal conductivity of a hydride layer (denominator). To carry out frontal chemical reactions of hydrogen sorption -desorption, small numbers Biot (Bi<0.1) are preferable. Number Bi can be decreased by several ways 1) decreasing of the characteristic layer size 2) decreasing of intensity of an external heat transfer (but time of non-stationary processes is growing) 3) increasing of effective hydride bed thermal conductivity. 

So far it has been assumed that both reactions are first order and the pellet can be treated as isothermal. It may be obvious to note that under nonisothermal conditions the ratio of the intrinsic activation energies and, if necessary, the ratio of the external heat transfer coefficients will also affect the apparent selectivity of the catalyst. In addition, if the kinetic orders of the two reactions are different, this will also influence selectivity. 

Note that criterion 7.186 requires a knowledge of the apparent activation energy for the reaction. Criterion 7.186 holds whether internal diffusion limitations exist or not. When the criterion concerning external heat transfer is compared to criterion 7.167 concerning radial heat transport limitations through the bed, it can been seen that the latter are more critical unless ... 

An experimental test to verify the absence of significant concentration gradients inside the catalyst pellet is based on the inverse proportional relation between the effectiveness factor and the pellet diameter for strong internal diffusion limitations. Hence, a measured rate which is independent of the pellet size indicates that internal diffusion limitations can be neglected. Care should be taken to avoid artifacts. External heat transfer effects also depend on pellet size and for exothermic reactions might compensate the internal diffusion limitations. If the catalyst pellet consists of a support with an non-uniformly distributed active phase, crushing and sieving to obtain smaller pellets is hazardous. 

In the foregoing illustration the temperature rise at the catalyst surface had a beneficial effect on selectivity. This is because the activation energy for the desired reaction was greater than that for the reaction producing by-product C. If were less than E2, external heat-transfer resistance would have reduced the selectivity for exothermic reactions. 

Analogously, resistances to heat transfer can occur between bulk and catalyst as well as within the catalyst pellet. The first of these, external heat transfer limitation is observed occasionally, particularly in stagnant or slow flowing liquids. Significant resistance to heat transfer within a pellet internal resistance to heat transfer is only observed for highly endo- or exothermic reactions. This is because the heat is more easily distributed across the pellet by means of conduction through the solid, than by convection in the pores. 

Thermal effects are often the key concern in reactor scaleup. The generation of heat is proportional to the volume of the reactor. Note the factor of V in Equation 5.31. For a scaleup that maintains geomedic similarity, the surface area increases only as Sooner or later, temperature can no longer be controlled by external heat transfer, and the reactor will approach adiabatic operation. There are relatively few reactions where the full adiabatic temperature change can be tolerated. Endothermic reactions will have poor yields. Exothermic reactions will have thermal runaways giving undesired byproducts. It is the reactor designer s job to avoid limitations of scale or at least to understand them so that a desired product will result. There are many options. The best process and the best equipment at the laboratory scale are rarely the best for scaleup. Put another way, a process that is less than perfect at a small scale may be better for scaleup precisely because it is scaleable. 

To conclude, an overall summary of calculations based on the above results indicates that the usual order of events as transport limitations occur is to begin with no limitations—chemical reaction controls throughout the pellet. Next, internal pore diffusion begins to have an effect, followed by external heat transfer... 

Rate of generated heat due to the chemical reaction Rate of external heat transferred (supplied or received) Work done by system... 

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... 

The general problem of diffusion-reaction for the overall effectiveness factor D is rather complicated. However, the physical and chemical rate processes prevailing under practical conditions promote isothermal particles and negligible external mass transfer limitations. In other words, the key transport limitations are external heat transfer and internal mass transfer. External temperature gradients can be significant even when external mass transfer resistances are negligibly small. 

An interesting feature of packed bed membrane reactors is the possibility to operate them in a reverse flow mode, integrating the reaction and separation with the recuperative heat exchange inside the reactor. This operational mode is quite interesting for partial oxidation of methane as indicated by Smit et al. [10]. In fact, as stated by the authors, in conventional POM systems air and CH4 feed streams have to be preheated to the reaction temperature, while the POM reaction being only slightly exothermic, the external heat transfer between feed... 

Micro and macro thermal gradients are generated within or outside the pellets by the heat of reaction. This suggests that scaling up of Trickle Bed Reactors for exothermic reactions must be done with caution to avoid temperature excursions which could cause excessive vaporization of the liquid outside and inside the pellets and give rise to local hot spots, an increase in the heat release rate and a decrease of external heat transfer coefficients. 

Only isothermal pellets and internal effectiveness factors have been treated so far. Limitations on internal heat transfer, external mass transfer, and external heat transfer can all affect the reaction rate. Consider a pellet placed in a fluid medium. Assuming constant physical properties, one-dimensional mass and heat balances yield for a slab-like pellet ... 

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