Axial temperature profiles reactors

With decreasing cell size, the temperature difference between the wall of the cell and the eatalyst partiele in the cell would decrease to zero. For sufficiently small cell dimensions, we may assume the two temperatures are the same. In this case, the heat conduction through the wall becomes dominant and affects the axial temperature profile. As the external heat exchange is absent and the outside of the reactor is normally insulated, the temperature profile is flat along the direction transverse to the reactant flow, and the conditions in all channels are identical to each other. The energy balance is... 

A main objective of the work of Hardt et al. was to study the influence of heat transfer on the achievable molar flux per unit reactor volume of the product species. They compared unstructured channels to channels containing micro fins such as shown in Figure 2.31. Heat transfer enhancement due to micro fins resulted in a different axial temperature profile with a higher outlet temperature in the reaction gas channel. Owing to this effect and by virtue of the temperature dependence... 

The growth rate is quite sensitive to the axial temperature profile. An axial temperature profile that increases along the reactor because it improves the deposition uniformity is commonly used in industry. The temperature of each successive zone in the furnace (defined by the furnace elements in Figure E14.5a) can be adjusted by voltage applied to variac heaters. The zone temperatures are assumed constant within each zone, T-,j = 1,..., ntv where ntz is the number of temperature zones to be used,... 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

Fig. 11. Axial temperature profile in cooled fixed-bed catalytic reactor.

Figure 4 shows the effect of a 10% drop in the inlet gas temperature (from 573 to 515.7 K) on the axial temperature profiles within the reactor for standard type I operating conditions. Although the inlet gas temperature is reduced, the cooling jacket temperature remains unchanged (Tw = 573 K),... 

A detailed experimental exploration of temperature profiles in the reactor packed with the CuO catalyst showed near at the extinction boundary three steady-state axial temperature profiles which were easily reproducible (Fig. 19). There is no simple explanation of these effects so far 55). 

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

Figure 10. Axial temperature profiles in a wall cooled reactor. Conditions yC,He = 0.005 T — 613 K and Rep = 20. Key , measured points -------, calculated with uniform flow and---, calcu-...

Figure 11. Axial temperature profiles in a wall cooled reactor. Conditions and key are the same as in Figure 10.

The catalytic CO oxidation by pure oxygen was selected as a model reaction. The Pt/alumina catalyst In the form of 3.4 mm spherical pellets was used. The CO used In this study was obtained by a thermal decomposition of formic acid In a hot sulphuric acid. The reactor was constructed by three coaxial glass tubes. Through the outer jacket silicon oil was pumped, while air was blown through the inner jacket as a cooling medium. The catalyst was placed in the central part of the tube. The axial temperature profiles were measured by a thermocouple moving axially in a thermowell. Gas analysis was performed by an infrared analyzer or by a thermal conductivity cell. [7]. 

A detailed experimental study of operating conditions in a nonadiabatic fixed bed reactor revealed that for certain inlet conditions oscillatory or erratic behavior of temperature profiles can be observed [23]. To follow this phenomenon local thermocouple temperature reading and axial temperature profiles were monitored. The results of measurements are reported in Fig. 3. 

Although two reactors are shown in Figure 1, they were not used simultaneously. The reactor shown in the center was the fixed bed reactor which is of primary interest in this contribution. It consisted of a 12.7 mm diameter X 250 mm long steel tube packed with 40/50 mesh catalyst (0.3 mm average particle diameter). The reactor was heated by a nichrome wire coil and was well insulated. The coil spacing was adjusted and was packed in insulation with the intent of making the reactor crudely adiabatic. A variac controlled heater on the reactor inlet and a thermocouple sensor kept the feed to the reactor at the nominal reaction (or feed inlet) temperature of 400°C. The tube of the fixed-bed, reactor was fitted with 12 thermocouples to record the axial temperature profile in the bed (Figure 1). 

Heat transfer studies on fixed beds have almost invariably been made on tubes of large diameter by measuring radial temperature profiles (1). The correlations so obtained involve large extrapolations of tube diameter and are of questionable validity in the design of many industrial reactors, involving the use of narrow tubes. In such beds it is only possible to measure an axial temperature profile, usually that along the central axis (2), from which an overall heat transfer coefficient (U) can be determined. The overall heat transfer coefficient (U) can be then used in one--dimensional reactor models to obtain a preliminary impression of longitudinal product and temperature distributions. 

Some measured axial temperature profiles are displayed in Fig. 2 at various air flow rates. The data are plotted as ln0vs. bed depth z, where 0=Tg-T(z), and z is the axial distance measured from the top of the reactor tube (Fig. 1). At bed depths between 20 and 30 cms. radial temperature and velocity profiles become fully developed and all the plots become linear. The overall heat transfer coefficient (U) can then be obtained simply from the slope of the lines, since... 

In practice the heat effects associated with chemical reactions result in nonisothermal conditions. In the case of a batch reactor the temperature changes as a function of time, whereas an axial temperature profile is established in a plug flow reactor. The application of the law of conservation of energy, in a similar... 

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. 

Figure 17 compares the actually measured axial temperature profile with the theoretical one in the simulation of the first reactor of a catalytic reformer by such a microreactor. It can be seen that the deviations from the theoretical profile are within a few degrees centigrade, even in this demanding test case where the reaction heat is only of the order of 5 W at a temperature level of about 500 °C. 

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. 

Theonly important current application of tubular reactors in polymer syntheses is in the production of high pressure, low density polyethylene. In tubular processes, the newer reactors typically have inside diameters about 2.5 cm and lengths of the order of I km. Ethylene, a free-radical initiator, and a chain transfer agent are injected at the tube inlet and sometimes downstream as well. The high heat of polymerization causes nonisothermal conditions with the temperature increasing towards the tube center and away from the inlet. A typical axial temperature profile peaks some distance down the tube where the bulk of the initiator has been consumed. The reactors are operated at 200-300°C and 2000-3000 atm pressure. 

Catalytic tests were made using a quartz continuous flow reactor (5 mm inner diameter) loaded with 0.5 g of catalyst in the form of small pellets (0.1-0.3 nun range). The feed composition was 3% toluene with a stoichiometric CVtoluene ratio (1.5). The space-velocity was set to 14500 h to avoid diffusional limitations. Tests were made in the 250-550°C range, but above 500°C severe deactivation of the catalysts occurs. Before the tests the catalysts were conditioned at 450°C for 6 h in the presence of standard feed. The axial temperature profile was determined by a thermocouple inserted in the catalytic bed. Preliminary experimental tests were made to ensure the absence of mass and heat diffusional limitations on the reaction rates. 

The introduction of the catalyst presents one of the main problems in using MSRs for heterogeneously catalyzed reactions. There are some examples of reactors that are constructed directly from the catalytically active material. Kestenbaum et al. [145] used silver foils for the construction of a microchannel reactor for the partial oxidation of ethene to oxirane. A similar concept was proposed by Fichtner et al. [91,146], These authors used a microstructured rhodium catalyst for the partial oxidation of methane to syngas. This reaction can be considered as a coupling of the exothermic oxidation and the endothermic reforming of methane, which occur at different reaction rates. In such a case, the formation of a pronounced axial temperature profile can be avoided through the use of a material with high thermal conductivity. The reactor... 

Axial Temperature Profile in Adiabatic Fixed-bed Reactor... 

J Hanika, K Sporka, V Ruzicka, J Hrstka. Measurement of axial temperature profiles in an adiabatic trickle bed reactor. Chem Eng J 12 193-197,1976. 

In this way, if the rate equation is already known, rates can be calculated at several temperatures from one scan. In effect, each ramping produces a traverse of many isothermal operating lines, as is usual in scanning operations. The difference in this case is that we know the axial temperature profile for each increment, and we know it is isothermal. This allows us to treat each measurement as if it were the output of a conventional isothermal reactor, despite the feet that we were in feet slowly ramping the temperature. Even in this simple case the TS procedures described above will allow the data of several rampings to be used to construct the multitude of the actual (they may in feet not be ideally isothermal) operating lines required for the search for an unknown rate equation. 

Hanika J., K. Sporka, V. Ruzicka and J.H. Rstka Measurement of Axial Temperature Profiles in an Adiabatic Trickle Bed Reactor . Chem Engn J. 12, 193 (1976). 

---