Adiabatic reactor

Isothermal operation can be approached closely in small-scale, experimental reactors, where practical and economic issues are not of great concern. In designing an experimental PFR to operate isothermaUy, several tricks can be employed that are not usually feasible on a larger scale. The object of these tricks is to increase the value of UA to the point that the required value of AT is very small. In that case, the temperature of the reactor contents approaches the temperature of the heat-transfer medium. The temperature of the heat-transfer medium is kept nearly constant, e.g., by using a large flow rate. 

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The most common approach to maintaining the desired reaction temperature is to operate with a significant excess of phenol in the reactor. An adiabatic reactor fed with 2 moles of phenol and 1 mole of isobutylene at 40°C would reach about 180°C if all the isobutylene formed PTBP. The selectivity towards the desired mono alkylphenol product almost always improves as the phenol to alkene mole ratio increases. These gains must be weighed... 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

Fig. 3. Multiple fixed-bed configurations (a) adiabatic fixed-bed reactor, (b) tubular fixed beds, (c) staged adiabatic reactor witb interbed beating (cooling),...

The reactor combinations for the two reactors in series consist of two fixed-beds for the Arco process an expanded bed followed by a catalytic distillation reactor for lEP a fixed-bed followed by a catalytic distillation reactor for CDTECH and two fixed-beds for Phillips. The Huls process uses an adiabatic reactor for the second reactor. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Chlorine free radicals used for the substitutioa reactioa are obtaiaed by either thermal, photochemical, or chemical means. The thermal method requites temperatures of at least 250°C to iaitiate decomposition of the diatomic chlorine molecules iato chlorine radicals. The large reaction exotherm demands close temperature control by cooling or dilution, although adiabatic reactors with an appropriate diluent are commonly used ia iadustrial processes. Thermal chlorination is iaexpeasive and less sensitive to inhibition than the photochemical process. Mercury arc lamps are the usual source of ultraviolet light for photochemical processes furnishing wavelengths from 300—500 nm. 

Figure 1.3.2 gives another perspective for scale-down to recycle reactor studies. In this actual case, after preliminary studies in a recycle reactor, a 5-stage adiabatic reactor was envisioned (Betty 1979.) Scaling down the proposed commercial reactor, a 3 diameter tube was designed with elaborate temperature compensation (heating and insulation) for pilot-plant studies (Betty 1968, 1969.) Small squares in the proposed reactor represent side views of cylindrical catalyst cutouts for the recycle reactor... 

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. 

Control of pollutants by oxidation is another exothermic process in which high conversion is the most important performance criterion. Interest in efficiency is limited to minimize byproduct formation the byproducts can be more damaging and more refractory than the original pollutants were. Commercially, most adiabatic reactors used for pollution control are of the least expensive construction. 

Figure 7.4.3 Single-stage adiabatic reactor system. ...

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. 

Figure 8.2.2 Conceptual scheme for a tray-type adiabatic reactor.

The original van Heerden diagram, as presented in his paper of 1953, was constructed for an adiabatic reactor case. In that case, at fixed feed temperature, there was a different slope (representing heat removal rate) for each feed rate. There was also a different heat generation versus temperature... 

For adiabatic reactors one example was presented by Berty et al (1968) on a six-stage adiabatic reactor system that had intercoolers between the stages. Every adiabatic stage is always sensitive or unstable but the full six-stage... 

Figure 9.7.1 Staged adiabatic reactor with intercoolers. ...

The need to keep a concave temperature profile for a tubular reactor can be derived from the former multi-stage adiabatic reactor example. For this, the total catalyst volume is divided into more and more stages, keeping the flow cross-section and mass flow rate unchanged. It is not too difficult to realize that at multiple small stages and with similar small intercoolers this should become something like a cooled tubular reactor. Mathematically the requirement for a multi-stage reactor can be manipulated to a different form ... 

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... 

Figure 12. Schematic flow sheet for methanation system using equilibrium-controlled adiabatic reactors...

Interesting features of this process include the potential for one-stage methanation to completion without the need for gas recycle. This feature was cited by Chem Systems, but, according to Rheinpruessen-Koppers work on the Fischer-Tropsch (52, 53), gas recycle was necessary with high H2 CO ratios. Drawbacks include such factors as catalyst attrition (48, 50), and low volume productivities of the methanator (less than one-tenth that reported for fixed bed adiabatic reactors) (48, 50, 52, 53, 61). 

The scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

Dr. Moeller I think to answer this question now is a bit difficult. It s just a mechanical problem of the maximum temperature the recycle compressor can handle. So, in the end, we will go to the inlet temperature to the compressor in the range of the inlet temperature to the reactor. So what we are endeavoring to attain is a simple reaction system consisting of an adiabatic reactor in series with waste heat boilers and nothing more than one recycle compressor. These compressors are used in the chemical industry with no problem in operation. So, in the end, you can go to hot recycle with an inlet compressor temperature the same as the inlet reactor temperature. All the heat from... 

This you cannot do in an adiabatic reactor unless you go to extremely high mixing ratios of fresh feed and recycle gas. In summary, it is a question of selectivity, which is the reason for using the isothermal reactor for Fischer-Tropsch. An adiabatic reactor with a waste heat boiler is cheaper than an isothermal feactor, and hence it is used for methanation. 

The first two of these methods are preferred when heat transfer is important. The third method is cheaper for adiabatic reactors. 

Example 5.6 Hydrocarbon cracking reactions are endothermic, and many different techniques are used to supply heat to the system. The maximum inlet temperature is limited by problems of materials of construction or by undesirable side reactions such as coking. Consider an adiabatic reactor with inlet temperature Tm. Then T z) < T, and the temperature will gradually decline as the reaction proceeds. This decrease, with the consequent reduction in reaction rate, can be minimized by using a high proportion of inerts in the feed stream. 

FIGURE 5.4 Concentration profiles for an endothermic reaction in an adiabatic reactor. 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

Solution A program for evaluating the adiabatic reactor is given below. Subroutine Reactor solves the simultaneous ODEs for the concentrations and temperature. The equation for temperature includes contributions from both reactions according to the methods of Section 5.2. 

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