Reaction exothermicity

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 ... 

In addition to the advantage of high heat transfer rates, fluidized beds are also useful in situations where catalyst particles need frequent regeneration. Under these circumstances, particles can be removed continuously from the bed, regenerated, and recycled back to the bed. In exothermic reactions, the recycling of catalyst can be... 

Reaction temperature. For endothermic reactions. Fig. 2.9c shows that the temperature should be set as high as possible consistent with materials-of-construction limitations, catalyst life, and safety. For exothermic reactions, the ideal temperature is continuously decreasing as conversion increases (see Fig. 2.9c). 

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. 

Cold shot or hot shot. Injection of cold fresh feed for exothermic reactions or preheated feed for endothermic reactions to inter-... 

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. 

ANstreams = enthalpy change between feed and product streams AI/react = reaction enthalpy (negative in the case of exothermic reactions)... 

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. 

The highly exothermic reaction is catalyzed by strong acids sulfuric, hydrofluoric. 

Reactions in porous catalyst pellets are Invariably accompanied by thermal effects associated with the heat of reaction. Particularly In the case of exothermic reactions these may have a marked influence on the solutions, and hence on the effectiveness factor, leading to effectiveness factors greater than unity and, In certain circumstances, multiple steady state solutions with given boundary conditions [78]. These phenomena have attracted a great deal of interest and attention in recent years, and an excellent account of our present state of knowledge has been given by Arls [45]. 

Consequently, for any exothermic reaction, ArH increases with temperature as well (Fig. 5-7). 

Figure 5-7 Enthalpy as a Eunction of Temperature for the Exothermic Reaction A B.

Fit a 1500 ml. bolt-head flask with a reflux condenser and a thermometer. Place a solution of 125 g. of chloral hydrate in 225 ml. of warm water (50-60°) in the flask, add successively 77 g. of precipitated calcium carbonate, 1 ml. of amyl alcohol (to decrease the amount of frothing), and a solution of 5 g. of commercial sodium cyanide in 12 ml. of water. An exothermic reaction occurs. Heat the warm reaction mixture with a small flame so that it reaches 75° in about 10 minutes and then remove the flame. The temperature will continue to rise to 80-85° during 5-10 minutes and then falls at this point heat the mixture to boiling and reflux for 20 minutes. Cool the mixture in ice to 0-5°, acidify with 107-5 ml. of concentrated hydrochloric acid. Extract the acid with five 50 ml. portions of ether. Dry the combined ethereal extracts with 10 g. of anhydrous sodium or magnesium sulphate, remove the ether on a water bath, and distil the residue under reduced pressure using a Claiseii flask with fractionating side arm. Collect the dichloroacetic acid at 105-107°/26 mm. The yield is 85 g. 

The use of dimethylformamide (b.p. 153°) as a solvent and diluent often increases the yield materially. The vigour of the exothermic reaction which occurs with a relatively reactive aryl hahde is moderated and, furthermore, the dimethylformamide is easily removed from the reaction product since it is water soluble. Aryl hahdes which are inert under the usual Ullmann conditions do not react in the presence of dimethylformamide. 

Place 28 g. (27-5 ml.) of pure aniline and 28 g. (23 ml.) of purified methyl phosphate in a 500 ml. round-bottomed flask equipped with a reflux condenser. Heat gently at first and remove the flame when the vigorous and exothermic reaction commences. When the latter subsides. 

Prepare a solution containing about 100 g, of potassium hypochlorite from commercial calcium hypochlorite ( H.T.H. ) as detailed under -Dimethylacrylic Acid, Section 111,142, Note 1, and place it in a 1500 ml. three-necked flask provided with a thermometer, a mechanical stirrer and a reflux condenser. Warm the solution to 55° and add through the condenser 85 g, of p-acetonaphthalene (methyl p-naphthyl ketone) (1). Stir the mixture vigorously and, after the exothermic reaction commences, maintain the temperature at 60-70° by frequent cooling in an ice bath until the temperature no longer tends to rise (ca. 30 minutes). Stir the mixture for a further 30 minutes, and destroy the excess of hypochlorite completely by adding a solution of 25 g. of sodium bisulphite in 100 ml. of water make sure that no hypochlorite remains by testing the solution with acidified potassium iodide solution. Cool the solution, transfer the reaction mixture to a 2-litre beaker and cautiously acidify with 100 ml. of concentrated hydrochloric acid. Filter the crude acid at the pump. 

Method 2. Place a mixture of 126-5 g. of benzyl chloride, 76 g. of thiourea and loO ml. of rectified spirit in a 500 ml. round-bottomed flask fitted with a reflux condenser. Warm on a water bath. A sudden exothermic reaction soon occurs and aU the thiourea passes into solution. Reflux the resulting yellow solution for 30 minutes and then cool in ice. Filter off the white crystals and dry in the air upon filter paper. Concentrate the filtrate to half its original volume and thus obtain a further small crop of crystals. The yield of crude hydrochloric acid as in Method 1 the m.p. is raised to 150°, although on some occasions the form, m.p. 175°, separates. 

Girard s reagent P , C5H5NCH2C0NHNH2 C1. In a 1-htre threenecked flask, equipped as in the previous preparation, place 200 ml. of absolute ethyl alcohol, 63 g. (64 -5 ml.) of pure anhydrous pyridine and 98 - 5 g. (84 5 ml.) of ethyl chloroacetate. Heat the mixture under reflux for 2-3 hours until the formation of the quaternary salt is complete acidify a small test-portion with dilute sulphuric acid it should dissolve completely and no odour of ethyl chloroacetate should be apparent. Cool the mixture in ice and salt. Replace the thermometer by a dropping funnel, and add a solution of 40 g. of 100 per cent, hydrazine hydrate in 60 ml. of absolute ethanol all at once. A vigorous exothermic reaction soon develops and is accompanied by vigorous effervescence. The pro duct separates almost immediately. When cold, filter with suction, wash... 

Prepare a saturated solution of sodium sulphide, preferably from the fused technical sodium polysulphide, and saturate it with sulphur the sulphur content should approximate to that of sodium tetrasulphide. To 50 ml. of the saturated sodium tetrasulphide solution contained in a 500 ml. round-bottomed flask provided with a reflux condenser, add 12 -5 ml. of ethylene dichloride, followed by 1 g. of magnesium oxide to act as catalyst. Heat the mixture until the ethylene dichloride commences to reflux and remove the flame. An exothermic reaction sets in and small particles of Thiokol are formed at the interface between the tetrasulphide solution and the ethylene chloride these float to the surface, agglomerate, and then sink to the bottom of the flask. Decant the hquid, and wash the sohd several times with water. Remove the Thiokol with forceps or tongs and test its rubber-like properties (stretching, etc.). 

Many programs allow the user to input a weighting factor (i.e., to give a structure that is 70% of the way from reactants to products). This allows the application of the Hammond postulate that the transition structure will look more like the reactants for an exothermic reaction and more like the products for an endothermic reaction. 

---