Preheating reactor temperature profiles

Table 9.1 Reactor temperature profiles as a function of preheating method.

A final mode of heat transfer in tubular reactors is the feed-cooled reactor, where the hot products from the reactor are cooled by the feed before it enters the reactor. As shown in Figure S-22, the cold feed in the jacket is preheated by the reaction in the inner tube or a heat exchanger is used for this purpose before the reactor. Temperature profiles for feed cooling are shown in Figure 5-22. 

It was observed that higher temperatures were reached in the reactor on changing the preheating method, the effect being more pronounced at 02 CH4 feed ratios lower than 0.7, due to the minor extent of the oxidation reaction. However, the trend of the temperature profile along the reactor was unchanged irrespective of the means of reactant preheating. 

Figure 17.23. Representative temperature profiles in reaction systems (see also Figs. 17.20, 17.21(d), 17.22(d), 17.30(c), 17.34, and 17.35). (a) A jacketed tubular reactor, (b) Burner and reactor for high temperature pyrolysis of hydrocarbons (Ullmann, 1973, Vol. 3, p. 355) (c) A catalytic reactor system in which the feed is preheated to starting temperature and product is properly adjusted exo- and endothermic profiles, (d) Reactor with built-in heat exchange between feed and product and with external temperature adjustment exo- and endothermic profiles.

Armed with substantial empirical experience regarding the operating of a reactor we know, for example, that the fuel used for heat must be consumed before coming into contact with the feedstock that the ratio of air to fuel and their rates control the temperature profile in the zones of the reactor, and that the molecular structure of the oil feedstock, the preheat temperature of the oil, i.e., whether it enters the reactor as a liquid or a vapor, the oil rate, and this rate as a ratio to the air/fuel rate all have a bearing on the reactor process, its control, and the properties of the product produced. We also know that the mechanical means employed for injection of the feedstock must be kept constant as well as the reactor geometry and all of the rates and ratios mentioned above. 

Design data are 1kg water per hour, P<300 bar and T<600 °C kept constant by a fluidized sandbath in which a 6 m tubular reactor coil with an inner diameter of 2 mm is submerged. 33 thermocouples measure the reaction temperature profiles. Water, organic material and the pressurized air can be preheated. 

The liquid is initially distributed by means of four small 1/16" pipes. An inert bed, realized with inert alumina pellets, ensured preheating, liquid saturation and an uniform distribution of both fluids. The reactor is provided with an axial thermocouple well. A sliding thermocouple can be moved along the bed axis allowing to measure the axial temperature profile and to check the isothermal operation of the reactor. 

Catalyst activity was usually measured in a bench test assembly (Figure 1). The reactor included a preheat section containing tabular alumina just above (upstream from) the 30 cm3 of catalyst in the center of the reactor. Water was pumped by a minipump (Milton-Roy) to the steam generator. From a three-temperature profile around the catalyst bed, it was determined that the midpoint data were most useful and reliable. The analytical equipment consisted of an infrared device (Mine Safety Appliances) for carbon monoxide, a flame ionization detector (Beckman) for hydrocarbons, and a paramagnetic oxygen analyzer (Beckman). The entire assembly except for Telex printer and computer is pictured in Figure 2. 

From the figure, it is reasonable to expect that the ideal arrangement for many adiabatic reactors connected in series thus follows a falling temperature profile— by first preheating the feed to the first PFR to a high temperature, and then... 

The feed heater is used to increase the feed temperature and control the reactor inlet temperarnre. Although the heater efficiency depends on how it is designed and operated, the heater duty is determined by feed preheating requirement. The feed heater outlet temperature can also be a function of the reaction temperature profile—that is, an ascending temperature profile will lower the heater outlet temperature. For a given feed preheat, the heater duty is mainly a function of the heat of reaction and heat recovery. A process engineer can determine the ways to maintain process heat recovery, heater efficiency, and heat flux. 

The effluent of the second reactor is again preheated before it enters the third bed/reactor. Here, primarily, dehydrocyclization takes place, which is kinetically the least favorable reaction. The third reactor therefore requires the largest volume, 55-65%, of the total charge. Typical temperature profiles are shown in Figure 6.9.6... 

Using a flow reactor lined inside with alumina (Alsint) and equipped with five sequentially mounted thermocouples, the authors of [67] measured the temperature profiles (Fig. 3.49), which turned out to be qualitatively consistent with the results of kinetic simulations. In the absence of an oxygen conversion, the preheated reactants cooled down monoton-ically, whereas when the reaction occurred, an induction period followed by rapid heating was observed. A specific feature of these experiments was the instability and incessant fluctuations of the temperature profile. For example, the maximum heat-up temperature fluctuated up and down by 5 °C. It was also pointed out that the results depended on the spatial position of the maximum heat-up zone, with the best results being obtained when the reaction zone was as close to the water-cooled quenching unit as possible. 

This starting condition need not be isothermal along the reactor but it is best to start with an isothermal profile and zero-conversion at the inlet. The simplest way of achieving this is to allow an equilibration time before each run, with the feed entering at a constant and low-enough temperature. The feed temperature and composition as well as the external temperature of the reactor should that be held constant until the system reaches a steady-state. We assume that the feed undergoes no conversion in the preheater, before it enters the reactor, under any of the experimental conditions encountered. Two more conditions are required. 

Figure 10.10a shows propane conversion contours obtained from 2D CFD calculations for catalytic propane combustion in a non-adiabatic microchannel for the conditions mentioned in the caption [23]. Unlike the homogeneous combustion case, the preheating and combustion zones in catalytic microburners overlap since catalytic reactions can occur on the hot catalyst surface close to the reactor entrance. Figure 10.10b shows a discontinuity in the Nu profile, similar to the homogeneous combustion problem. In this case, it happens at the boundary between the preheat-ing/combustion zone and the post-combustion zone. At this point, the bulk gas temperature (cup-mixing average) and wall temperatures cross over and the direction... 

A compressor provided dry air, which was preheated and mixed with propane (Grade 3.5 purity) in two sequential static mixers. The preheated propane/air mixture was driven into the reactor through a 50-mm long inert rectangular honeycomb section that provided a uniform inlet velocity profile. The reactor inlet temperature was monitored with a thermocouple placed downstream of the honeycomb section. The high-pressure vessel was fitted with two 350-mm long and 35-mm thick quartz windows (see Fig. 2.2) which maintained optical accessibility from both reactor sides. Two additional quartz windows located at the exhaust section of the vessel and the reactor outlet provided a counterflow optical access for the LIF experiments. Apart from propane/air, experiments with propane/ air/oxygen mixtures have also been carried out... 

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