Adiabatic reactor conditions

Peclet number independent of Reynolds number also means that turbulent diffusion or dispersion is directly proportional to the fluid velocity. In general, reactors that are simple in construction, (tubular reactors and adiabatic reactors) approach their ideal condition much better in commercial size then on laboratory scale. On small scale and corresponding low flows, they are handicapped by significant temperature and concentration gradients that are not even well defined. In contrast, recycle reactors and CSTRs come much closer to their ideal state in laboratory sizes than in large equipment. The energy requirement for recycle reaci ors grows with the square of the volume. This limits increases in size or applicable recycle ratios. 

The unit was built in a loop because the needed 85 standard m /hour gas exceeded the laboratory capabilities. In addition, by controlling the recycle loop-to-makeup ratio, various quantities of product could be fed for the experiments. The adiabatic reactor was a 1.8 m long, 7.5 cm diameter stainless steel pipe (3 sch. 40 pipe) with thermocouples at every 5 centimeter distance. After a SS was reached at the desired condition, the bypass valve around the preheater was suddenly closed, forcing all the gas through the preheater. This generated a step change increase in the feed temperature that started the runaway. The 20 thermocouples were displayed on an oscilloscope to see the transient changes. This was also recorded on a videotape to play back later for detailed observation. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

The heats of these reactions (2, 3) (Figure 1) indicate that all the reactions are exothermic over the cited range of conditions. For example, the heat liberated under typical reaction conditions for the conversion of CO to methane is 52,730 cal/mole CO that for carbon dioxide is 43,680 cal/mole. Such high heats of reaction cannot be absorbed by the process stream in an adiabatic reactor unless the CO and/or C02 conversion is limited to less than about 2.5 moles/100 moles feed gas. Since... 

The time necessary to accomplish this exothermic reaction under adiabatic operating conditions is only an extremely small fraction of that necessary for isothermal operation. In fact, the times necessary to fill and drain the reactor and to heat it to a temperature where the rate becomes appreciable will be greater than that necessary to accomplish the reaction. Thus,... 

The RC1 reactor system temperature control can be operated in three different modes isothermal (temperature of the reactor contents is constant), isoperibolic (temperature of the jacket is constant), or adiabatic (reactor contents temperature equals the jacket temperature). Critical operational parameters can then be evaluated under conditions comparable to those used in practice on a large scale, and relationships can be made relative to enthalpies of reaction, reaction rate constants, product purity, and physical properties. Such information is meaningful provided effective heat transfer exists. The heat generation rate, qr, resulting from the chemical reactions and/or physical characteristic changes of the reactor contents, is obtained from the transferred and accumulated heats as represented by Equation (3-17) ... 

Some of the most generally useful isothermal and adiabatic reactors and a few other devices, all usable under high vacuum conditions, will be described in the following sections. 

Principles of the adiabatic reactor method have been discussed elsewhere [67,68], Under adiabatic conditions, assuming constant heat capacity, constant heat of reaction, and homogeneous reaction, temperature rise data yields fractional conversion, X [68] ... 

KINPTR s real-time activity kinetics determine the adiabatic reactor inlet temperature required to make a target octane. The accuracy of KINPTR s reactor inlet temperature predictions is shown in Fig. 30 for a wide range of process conditions. The average deviation is + 3.8 K with no significant bias. This degree of accuracy is very reasonable considering the sensitivity of catalyst activity to start-up conditions and initial catalyst state (e.g., chloride added). 

In many situations, the monolith reactor can be represented by a single channel. This assumption is correct for the isothermal or adiabatic reactor with uniform inlet flow distribution. If the actual conditions in the reactor are significantly different, more parallel channels with heat exchange have to be simulated (cf., e.g. Chen et al., 1988 Jahn et al., 1997, 2001 Tischer and Deutschmann, 2005 Wanker et al., 2000 Young and Finlayson, 1976). In this section we will further discuss effective single channel models. 

It cuts. the axis at 0ad = 4 as 1/tn tends to zero (adiabatic limit). We have already seen that this is the condition for transition from multiple stationary states (hysteresis loop) to unique solutions for adiabatic reactors, so the line is the continuation of this condition to non-adiabatic systems. Above this line the stationary-state locus has a hysteresis loop this loop opens out as the line is crossed and does not exist below it. Thus, as heat loss becomes more significant (l/iN increases), the requirement on the exothermicity of the reaction for the hysteresis loop to exist increases. 

Thermal data where reactions are detected at lower temperatures are obtained from test runs on an ARC or other more sensitive calorimeters. In the ARC, the temperature is raised stepwise and at a much slower effective rate than with the DSC. The ARC is nearly adiabatic and, thus, more nearly approaches plant reactor conditions. Another important advantage is the fact that the reaction pressure is monitored and recorded in the ARC. 

The cooling failure is not considered here, as the adiabatic reactor is designed to work without cooling. If the conditions listed above are fulfilled, the adiabatic batch reactor is inherently safe as far as the charge is guaranteed. The reaction course is not affected by any eventual cooling failure or breakdown of utilities. The batch reactor can be made safe only if it is designed for adiabatic conditions. 

When (2.25) is integrated from the initial condition t = 0 and Ca = Cao to t —> oo and Ca -> 0 in the case of adiabatic reactor (US = 0), the adiabatic temperature rise A rad = 7 id - To is obtained, which represents a useful measure of practical utility of the system reactivity in terms of the maximum temperature obtainable when chemical energy is entirely transformed into sensible heat. 

Tubular reactors often have high-temperature limitations because of the occurrence of undesirable reactions, catalyst degradation, or materials of construction. This means that the maximum temperature anywhere in the reactor cannot exceed this limit. An exothermic reaction in an adiabatic reactor produces a maximum temperature at the exit under steady-state conditions. An exothermic reaction in a cooled reactor can... 

There are five fundamental differences between CSTRs and tubular reactors. The first is the variation in properties with axial position down the length of the reactor. For example, in an adiabatic reactor with an exothermic irreversible reaction, the maximum temperature occurs at the exit of the reactor under steady-state conditions. However, in a cooled tubular reactor, the peak temperature usually occurs at an intermediate axial position in the reactor. To control this peak temperature, we must be able to measure a number of temperatures along the reactor length. 

A maximum reactor temperature of 500 K is used in this study. This maximum temperature occurs at the exit of the adiabatic reactor under steady-state conditions. Plug flow is assumed with no radial gradients in concentrations or temperatures and no axial diffusion or conduction. 

Adiabatic reactor Vessel that is well insulated to minimize heat transfer and has an increase or decrease in temperature from the initial inlet conditions due solely to the heats of reaction. 

As a consequence of this explanation the reaction runaway to total methanation is not a necessary condition for the observed phenomenon. Any simple exothermic two phase reaction in an adiabatic reactor ought to show the same behaviour provided that one phase with a high throughput is used to carry the heat out of the reactor and the flow is suddenly reduced. This will be shown in the following simulation results. Due to problems with the numerical stability of the solution (see Apendix) only a moderate reaction rate will be considered. Reaction parameters are chosen in such a way that in steady state the liquid concentration Cf drops from 4.42 to 3.11 kmol/m3 but the temperature rise is only 3°C (hydrogen in great excess). At t = 0 the uniform flow profile... 

The vinyl acetate reaction is even more sensitive. With S = 44.2 for the side reaction alone it would be virtually impossible to try to prevent an adiabatic reactor from reaching full conversion on oxygen by mere control of the inlet temperature. Small changes in the inlet conditions would quickly amplify down the reactor, forcing complete conversion, adiabatic temperature rise, and destruction of the catalyst, if we were lucky in the best scenario. We must apply external cooling to the vinyl acetate reactor. 

Alkyladon t es place in the vapor phase, in the presence of a gaseous eminent (nitrogen or hydrogen) around 475 C, at adiabatic reactors, with catalyst, around 450 to 500, and in the presence of steam. The blend of methylstyrenes currently commerdaitzed by Dow results from the liquid phase alkylation of toluene with aliuninom diloride. 

Of all the data which can be obtained from an isothermal alkylation reactor, only the kinetic and thermodynamic data can be applied to an adiabatic reactor. If the process or reaction is divided into alkylation and transalkylation sections, then one can select conditions for the process to give high catalyst life times and produce a pure product at high efficiencies. 

The third and fourth condition are fulfilled by Tarhan [25]. Axial dispersion is fundamentally local backmixing of reactants and products in the axial, or longitudinal direction in the small interstices of the packed bed, which is due to molecular diffusion, convection, and turbulence. Axial dispersion has been shown to be negligible in fixed-bed gas reactors. The fourth condition (no radial dispersion) can be met if the flow pattern through the bed already meets the second condition. If the flow velocity in the axial direction is constant through the entire cross section and if the reactor is well insulated (first condition), there can be no radial dispersion to speak of in gas reactors. Thus, the one-dimensional adiabatic reactor model may be actualized without great difficulties. ... 

The study was carried out in relatively small isothermal reactors without recycle, constructed fortesting and comparison of different catalysts and feedstocks. Detailed information about catalyst performance under different conditions can be efficiently obtained under very controlled conditions in such equipment (/). However, exact predictions of the performance of a commercial reformer unit consisting of 3-4 adiabatic reactors will need detailed kinetic and reactor modeling, which is not included in this paper. 

Solve reactive-system energy balance problems for (a) the heat transfer required for specified inlet and outlet conditions, (b) the outlet temperature corresponding to a specified heat input (e.g., for an adiabatic reactor), and (c) the product composition corresponding to a specified heat input and a specified outlet temperature. 

In another important class of problems, the input conditions, heat input, and product composition are specified, and the outlet temperature is to be calculated. To solve such problems, you must evaluate the enthalpies of the products relative to the chosen reference states in terms of the unknown final temperature, and then substitute the resulting expressions into the energy balance Q = A//, or Af/ = 0 for an adiabatic reactor) to calculate Tout-... 

Sometimes the feed conditions and heat input to a reactor are specified (as in an adiabatic reactor) and the outlet temperature, is to be determined. The procedure is to derive expressions for the specific enthalpies of the reactor outlet species in terms of Tout substitute these expressions into the summation Xom in the expression for Mi substitute in turn for H(Taax) in the energy balance, and solve the resulting equation for Tout-... 

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