Reactor exothermic reactions

To approach the problem more quantitatively, let us focus attention on the hot spot temperature, that is, the maximum of the temperature profile along the length of the reactor (exothermic reactions are the subject of examination here). At this point, dT/dz) = 0, so the rate of heat generation is equal to the rate of heat removal. From equation (4-132)... 

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 ... 

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable fall for endothermic reactors, then this is the option normally chosen. If this is the case, then the feed stream to the reactor requires heating and the efiluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) and a hot stream (the reactor efiluent). The heat of reaction appears as elevated temperature of the efiluent stream in the case of exothermic reaction or reduced temperature in the case of endothermic reaction. 

The thermal profile through the reactor will in most circumstances be carefully optimized to maximize selectivity, extend catalyst life, etc. Because of this, direct heat integration with other process streams is almost never carried out. The heat transfer to or from the reactor is instead usually carried out by a heat transfer intermediate. For example, in exothermic reactions, cooling might occur by boiling water to raise steam, which, in turn, can be used to heat cold streams elsewhere in the process. 

If the reactor can be matched with other process streams (which is unlikely), then the reactor profile should be included in the heat integration problem. This would be a hot stream in the case of an exothermic reaction or a cold stream in the case of an endothermic reaction. 

The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

A typical flow diagram for pentaerythritol production is shown in Figure 2. The main concern in mixing is to avoid loss of temperature control in this exothermic reaction, which can lead to excessive by-product formation and/or reduced yields of pentaerythritol (55,58,59). The reaction time depends on the reaction temperature and may vary from about 0.5 to 4 h at final temperatures of about 65 and 35°C, respectively. The reactor product, neutralized with acetic or formic acid, is then stripped of excess formaldehyde and water to produce a highly concentrated solution of pentaerythritol reaction products. This is then cooled under carefully controlled crystallization conditions so that the crystals can be readily separated from the Hquors by subsequent filtration. 

The reaction is exothermic reaction rates decrease with increased carbon number of the oxide (ethylene oxide > propylene oxide > butylene oxide). The ammonia—oxide ratio determines the product spht among the mono-, di-, and trialkanolamines. A high ammonia to oxide ratio favors monoproduction a low ammonia to oxide ratio favors trialkanolamine production. Mono- and dialkanolamines can also be recycled to the reactor to increase di-or trialkanolamine production. Mono- and dialkanolamines can also be converted to trialkanolamines by reaction of the mono- and di- with oxide in batch reactors. In all cases, the reaction is mn with excess ammonia to prevent unreacted oxide from leaving the reactor. 

Heat Recovery and Feed Preheating. The objective is to bring the reactants to and from reaction temperature at the least utihty cost, and to recover maximum waste heat at maximum temperature. The impact of feed preheating merits a more careful look. In an exothermic reaction, preheated feed permits the reactor to act as a heat pump, ie, to buy low and sell high. The most common example is combustion-air preheating for a furnace. 

In cases where a large reactor operates similarly to a CSTR, fluid dynamics sometimes can be estabflshed in a smaller reactor by external recycle of product. For example, the extent of soflds back-mixing and Hquid recirculation increases with reactor diameter in a gas—Hquid—soflds reactor. Consequently, if gas and Hquid velocities are maintained constant when scaling and the same space velocities are used, then the smaller pilot unit should be of the same overall height. The net result is that the large-diameter reactor is well mixed and no temperature gradients occur even with a highly exothermic reaction. 

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

This is an exothermic reaction, and both homogeneous (radical or cationic) and heterogeneous (soHd catalyst) initiators are used. The products range in molecular weight from below 1000 to a few million (see Olefin polymers). Reaction mechanisms and reactor designs have been extensively discussed (10-12). 

Once a decision to use QRA has been made, you must decide whether frequency and/or consequence information is required (Steps 6 and 7). In some cases you may simply need frequency information to make your decision. For example, suppose you wish to evaluate the adequacy of operating procedures and safety systems associated with a chemical reactor. The main hazard of concern is that the reactor could experience a violent runaway exothermic reaction. You believe that you know enough about the severe consequences of a runaway and nothing more will be gained by quantifying the consequences of potential run-... 

The alternate possibility of building a laboratory tubular reactor that is shorter and smaller in diameter is also permissible, but only for slow and only mildly exothermic reactions where smaller catalyst particles also can be used. This would not give a scaleable result for the crotonaldehyde example at the high reaction and heat release rates, where flow and pore-ditfusion influence can also be expected. 

In the petrochemical industry close to 80% of reactions are oxidations and hydrogenations, and consequently very exothermic. In addition, profitability requires fast and selective reactions. Fortunately these can be studied nowadays in gradientless reactors. The slightly exothermic reactions and many endothermic processes of the petroleum industry still can use various tubular reactors, as will be shown later. 

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