Adiabatic reactor-design

Typical adiabatic reactor design involves finding the best values of recycle concentration, inlet temperature, reactor size, recycle flowrate, and per-pass conversion. Of course, not all of these parameters are independent. When the entire process is studied... 

With regard to application and construction, it is convenient to differentiate between fixed-bed reactors for adiabatic operation and those for nonadiabatic operation. Since temperature control is one of the most important methods to influence a chemical reaction, adiabatic reactors are used only where the heat of reaction is small, or where there is only one major reaction pathway in these cases no adverse effects on selectivity or yield due to the adiabatic temperature development are expected. The characteristic feature of an adiabatic reactor is that the catalyst is present in the form of a uniform fixed bed that is surrounded by an outer insulating jacket (Fig. 1A). Adiabatic reactor designs are discussed in Section 10.1.3.1. 

The principle of dynamic programming can be exploited in the adiabatic reactor design in two ways ... 

Example 8.3 The dynamic programming method for muitiple-bed adiabatic reactor design (Doraiswamy and Sharma, 1984)... 

Linde A.G. of Munich, Germany, has developed an isothermal Claus reactor design in which the heat of reaction is removed directly from the Claus reactor. This concept achieves sulfur recoveries comparable to the conventional adiabatic reactor design, but with fewer reaction stages and less total equipment. 

Temperature control. Let us now consider temperature control of the reactor. In the first instance, adiabatic operation of the reactor should be considered, since this leads to the simplest and cheapest reactor design. If adiabatic operation produces an unacceptable rise in temperature for exothermic reactions or an unacceptable fall in temperature for endothermic reactions, this can be dealt with in a number of ways ... 

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."... 

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... 

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 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. 

Vocabulary of Terms Used in Reactor Design. There are several terms that will be used extensively throughout the remainder of this text that deserve definition or comment. The concepts involved include steady-state and transient operation, heterogeneous and homogeneous reaction systems, adiabatic and isothermal operation, mean residence time, contacting and holding time, and space time and space velocity. Each of these concepts will be discussed in turn. 

In many respects, the solutions to equations 12.7.38 and 12.7.47 do not provide sufficient additional information to warrant their use in design calculations. It has been clearly demonstrated that for the fluid velocities used in industrial practice, the influence of axial dispersion of both heat and mass on the conversion achieved is negligible provided that the packing depth is in excess of 100 pellet diameters (109). Such shallow beds are only employed as the first stage of multibed adiabatic reactors. There is some question as to whether or not such short beds can be adequately described by an effective transport model. Thus for most preliminary design calculations, the simplified one-dimensional model discussed earlier is preferred. The discrepancies between model simulations and actual reactor behavior are not resolved by the inclusion of longitudinal dispersion terms. Their effects are small compared to the influence of radial gradients in temperature and composition. Consequently, for more accurate simulations, we employ a two-dimensional model (Section 12.7.2.2). 

It is often necessary to employ more than one adiabatic reactor to achieve a desired conversion. The catalytic oxidation of SOj to SO3 is a case in point. In the first place, chemical equilibrium may have been established in the first reactor and it would be necessary to cool and/or remove the product before entering the second reactor. This, of course, is one good reason for choosing a catalyst which will function at the lowest possible temperature. Secondly, for an exothermic reaction, the temperature may rise to a point at which it is deleterious to the catalyst activity. At this point, the products from the first reactor are cooled prior to entering a second adiabatic reactor. To design such a system, it is only necessary to superimpose on the rate contours the adiabatic temperature paths for each of the reactors. The volume requirements for each reactor can then be computed from the rate contours in the same way as for a... 

Figure 17.27. Catalyst packed adiabatic reactor, showing application of ceramic balls of graduated sizes for support at the bottom and hold-down at the top Rase, Chemical Reactor Design for Process Plants, Wiley, New York, 1977).

Studies in optimization-I The optimum design of adiabatic reactors with several beds. Chem. Eng. Sci., 12, 243-252 (1960). 

The optimal design of stagewise adiabatic reactors. Paper presented at the AIChE/ORSA Symposium on Optimization in Chemical Engineering, New York, 1960. 

Fio. 3. IS. Optimum design of a two-stage and three-stage adiabatic reactor... 

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. 

We start by studying the steady-state design and economics of a process with a single adiabatic reactor. The design considers the entire plantwide process reactor, heat exchangers, gas recycle compressor, preheat furnace, condenser, and separator. The economic objective function is total annual cost, which includes annual capital cost (reactor, catalyst, compressor, and heat exchangers) and energy cost (compressor work and furnace fuel). 

The energy requirement of the furnace is zero for this adiabatic reactor case since the reactor exit stream at maximum temperature can provide enough heat. The design of the FEHE and the amount of bypassing are determined by the reactor inlet temperature T-m. A temperature difference of 25 K is assumed for the hot end of the FEHE. The hot reactor effluent enters the hot side of the FEHE at the high-temperature limit of 500 K, so the cold-side exit stream is 475 K. If the specified reactor inlet temperature is less than 475 K, bypassing is used. A fairly low overall heat transfer coefficient of... 

A contour plot given in Figure 5.17 shows how TAC varies in a two-stage adiabatic reactor system with interstage cooling. The reactant ratio yRA/yRB is fixed at unity in this figure, so there are two design optimization variables, the inlet temperatures of the two reactors 7) and T2-... 

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