Temperature time profile, reactor exit

Figure 6. Reactor exit temperature-time profile. Commercial test-run...

The data in Table II pertaining to pyrolysis conditions shows that all four feedstocks were pyrolyzed under substantially similar conditions, namely steam-to-hydrocarbon weight ratios of 0.9 0.1, residence times of 0.3 sec, reactor exit pressures of 2.0 bar absolute, and reactor exit temperatures of 835°C. Care also was taken to maintain identical axial temperature profiles in the reactor for each of these runs. No unambiguous measure of substrate conversion during pyrolysis is possible for distillate feedstocks of the type used in the present experiments in terms of the empirical kinetic severity function of Zdonik et al. (5), all of the present experiments were conducted at a severity of about 2. 

In the reference test, the low reactor exit temperature at the constant plant-energy input conditions indicates the expected higher heat losses in a short-duration reactor. The corresponding lower overall temperature profile through the test-reactor length reduces the process kinetic time-at-temperature. The associated gas-phase chemical kinetics at the lower residence times are believed to be responsible for the slight discrepancies in the reference test gas yields. Also, the "true enthalpy used for cracking is lower than that indicated by the measured reactor temperature. 

Pyrolysis experiments were conducted in the pressure, temperature, and residence time range of commercial interest, under 50 weight percent steam in the feed stream. Figure 1 presents the temperature and pressure profiles kept throughout the experiments. The six experiment series were made using two different temperature-profile shapes (I. and II.) and 820°C and 850 C reactor exit temperatures, respectively. 

Numerous reactions are performed by feeding the reactants continuously to cylindrical tubes, either empty or packed with catalyst, with a length which is 10 to 1000 times larger than the diameter. The mixture of unconverted reactants and reaction products is continuously withdrawn at the reactor exit. Hence, constant concentration profiles of reactants and products, as well as a temperature profile are established between the inlet and the outlet of the tubular reactor, see Fig. 7.1. This requires, in contrast to the batch reactor, the application of the law of conservation of mass over an infinitesimal volume element, dV, of the reactor. In contrast to a batch reactor the existence of a temperature profile does not allow us to consider the mass balances for the reacting components and the energy balance separately. Such a separation can only be performed for isothermal tubular reactors. 

FIGURE 9.29 Reactor with a periodic flow reversal (autothermal fixed bed reactor for catalytic combustion of VOCs). (a) Reactor configuration, (b) temperature profiles at the time of flow reversal, and (c) exit temperature versus time. 

Figure 13.16 shows two possible thermal profiles for endothermic plug-fiow reactors. This time the temperature falls for low rates of heat addition and/or high heat of reaction. The temperature rises for the reverse conditions. Under conditions between the profiles shown in Fig. 13.16, a minimum can occur in the temperature profile at an intermediate point between the inlet and exit. 

There is no constant of integration due to the boundary condition that both AG/T and A(l/7 ) are zero at equilibrium. However, AH will be temperature-dependent most of the time. For example, in producing ammonia from hydrogen and nitrogen, the goal is to maximize the output of ammonia at the exit. An approximately constant AT between the optimal path and the equilibrium temperature provides the optimal temperature profile, which reduces the exergy loss by approximately 60% in the reactor. The equipartition of forces principle for multiple, independent rate-controlled reactions and multiphase and coupled phenomena, such as reactive distillations, may lead to the improved use of energy and reduced costs (Sauar et al., 1997). 

The main problems related to the use of the tubular-flow reactor are caused by the deviations from ideal flow conditions, entrance and exit effects, heating and cooling rates and effects of heat of reaction on the temperature profiles and on the temperature difference between the reactants and the reactor wall. The heat transfer limitations can be very significant if the reactor diameter is not very small and if temperatures above 600°C are employed. Another problem arises from the difficulty in the evaluation of the actual reaction time, because of the increase in volume of the reacting mixture with temperature and conversion. 

Figure 7.17c and d show the predicted dynamic liquid molar concentration profiles of sulfur along the commercial catalytic bed at different times ranging from 60 to 1700 s for an inlet reactor temperature of 340°C. The dynamic simulation was carried out at the same reaction conditions than those employed for the simulation of the bench-scale reactor. The value of sulfur concentration reported at the exit of the isothermal bench-scale reactor is represented by symbol o . The profiles with a pronounced reduction of sulfur concentration in the first section of the reactor have already been reported by Jimenez et al. (2007). They attributed those sulfur concentration shapes in the catalytic bed to the kinetic model considered and to the operating conditions simulated. Also, the bench-scale experimental sulfur concentration value was higher than that predicted for the commercial reactor because of the increasing catalytic bed temperature observed in the liquid phase of the adiabatic reactor. 

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