Pellet runaway

Thermal Behavior of Catalyst Particles and Pellet Runaway... 

In the first instance, 6g should be lowered to prevent pellet runaway. This may lead to too low a temperature and hence too low a conversion. In this case the reactant concentration in the feed may be decreased to increase An, so that 0t can be increased again to achieve sufficiently high reaction rates. 

This reasoning also indicates the importance of gas flow rates. If, for example, in a tube of a multitubular reactor, a deposit is formed that decreases the gas velocity because of plugging, at some point pellet runaway will occur irreparably and, consequently, full runaway of that tube [12]. 

The goo is then pumped to the top of a vertical, jacketed tower with internal temperature-regulating coils. The vessel is kept full of the styrene/PS mixture. A temperature gradient (change) of 280°F at the top and 400°F at the bottom is maintained. The temperatures are controlled to prevent runaway, but to permit 95% conversion of styrene to PS. As the polystyrene molecules grow, they sink to the bottom of the vessel and can be drawn off The residence time in this vessel is three to four hours. The molten PS is extruded to strands, chopped into pellets, and bagged. 

High conversions maximize production and eliminate any potential for runaway reactions. A hydrocarbon solvent is used to keep the contents of the reactor in solution and also aids in heat removal. The solvent is flashed and recovered, along with the energy captured from the heat of reaction, and circulated back to the reactor. Molten polymer is sent to a simple extruder and pelletizer assembly. 

Figure 4.6 Dimensionless pellet temperature 0p versus 0 for An -3 and 0, = 0.3. Note the hysteresis, runaway, and blowout.

Introspection will show that this sensitivity is influenced by heat and mass transfer between pellet and fluid. Therefore the treatment of parametric sensitivity and runaway will be in two parts runaway in pseudohomogeneous reactors and runaway in heterogeneous reactors. 

As the catalytic reaction taking place inside the pellets is usually accompanied by heat effects, the particle-liquid heat transfer coefficient becomes a fundamental ingredient to be estimated for the assessment of the efficacy of the heat withdrawal from the particle level away to the reactor wall leveL In particular, when highly exothermic reactions are in play, impediment of liquid replenishment over the dried spots on the catalyst surface may favor inception of hot spots that are responsible for reactor runaway. As a result, evacuation of heat across the liquid-covered pellet spots becomes a critical issue. Not many studies in literature deal with particle-liquid heat transfer rates in three-phase fixed-bed reactors. The main reason is probably the difficulty to find an accurate experimental method. The following current trends emanate from the analysis of the particle-liquid heat transfer two-phase downflow fixed-bed literature (i) the transition from trickle to pulsing flow is accompanied by a... 

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