Cooling of reactions

Many key industrial reactions operate at elevated temperatures. For example, formaldehyde, hydrogen cyanide, and ethylene production all occur at elevated temperatures and involve secondary reactions. In each case, reaction time is minimized and a waste heat boiler is used for rapid cooling of reaction products. 

An Inexpensive System That Enables the Cooling of Reactions Inside a Glovebox... 

Because MBR and CMR enable rapid heating and cooling of reactions, in principle they could be usefully applied to the preparation of kinetic products. A successful example with important industrial applicability is the acid-catalyzed hydrolysis of cellulose to afford practical conversions to glucose and oligosaccharides [12, 15]. Also, treatment of (S)-(+)-carvone in water for 10 min at temperatures between 180 and 250 °C, afforded 8-hydroxy-p-6-menthen-2-one as an intermediate on the pathway to carvacrol [31]. Addition of water to the 8,9 olefinic bond of carvone was regioselective and proceeded at a lower temperature than did isoaromatization (Scheme 3.11). The kinetic product was readily isolated by differential extraction from the recyclable starting ketone, affording a preparative route despite the low conversion. 

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. 

If indirect heat transfer is used with a large temperature difference to promote high rates of cooling, then the cooling fluid (e.g., boiling water) is fixed by process requirements. In this case, the heat of reaction is not available at the temperature of the reactor effluent. Rather, the heat of reaction becomes available at the temperature of the quench fluid. Thus the feed stream to the reactor is a cold stream, the quench fluid is a hot stream, and the reactor effluent after the quench is also a hot stream. 

The reaction is exothermic, and multitubular reactors are employed with indirect cooling of the reactor via a heat transfer medium. A number of heat transfer media have been proposed to carry out the reactor cooling, such as hot oil circuits, water, sulfur, mercury, etc. However, the favored heat transfer medium is usually a molten heat transfer salt which is a eutectic mixture of sodium-potassium nitrate-nitrite. 

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Then cool the reaction-mixture, filter it at the pump, leaving a black residue of selenium, and wash out the flask twice with 2x5 ml. of acetic acid, passing the washings also through the filter. Dilute the united filtrates with water, and make the solution alkaline with 10% aqueous sodium hydroxide, which precipitates the camphorquinone. Cool, filter off the yellow camphorquinone at the pump, wash with water and drain thoroughly. 

Cool the reaction-solution, and pour it into a 250 ml. beaker, washing out the flask with ca. 50 ml. of water into the beaker. Chill the solution in ice-water and add dilute hydrochloric acid with stirring until the solution is just acid when spotted externally on to Congo Red paper. The arsinic acid rapidly separates. Filter at the pump, wash well with water and drain. (Yield of crude dry product, 7-5-8 o g. m.p. 200-203°.)... 

An alternative method for isolating the n-butyl ether utilises the fact that n-butyl alcohol is soluble in saturated calcium chloride solution whilst n-butyl ether is slightly soluble. Cool the reaction mixture in ice and transfer to a separatory fimnel. Wash cautiously with 100 ml. of 2-5-3N sodium hydroxide solution the washings should be alkaline to litmus. Then wash with 30 ml. of water, followed by 30 ml. of saturated calcium chloride solution. Dry with 2-3 g. of anhydrous calcium chloride, filter and distil. Collect the di-n-butyl ether at 139-142°. The yield is 20 g. 

Into a 1 litre round-bottomed flask, fitted with a double surface condenser, place 250 g. (277 ml.) of dry ethyl acetate (1) and 25 g. of clean sodium wire (2). Warm the flask on a water bath in order to start the reaction. Once the reaction commences, it proceeds vigorously and cooling of the flask may be necessary in order to avoid loss of ethyl... 

Mix 1 g. of the nitro compound with 4 g, of sodium dichromate and 10 ml. of water in a 50 ml. flask, then attach a reflux condenser to the flask. Add slowly and with shaking 7 ml. of concentrated sulphuric acid. The reaction usually starts at once if it does not, heat the flask gently to initiate the reaction. When the heat of reaction subsides, boil the mixture, cautiously at first, under reflux for 20-30 minutes. Allow to cool, dilute with 30 ml. of water, and filter oflF the precipitated acid. Purify the crude acid by extraction with sodium carbonate solution, precipitation with dUute mineral acid, and recrystaUisation from hot water, benzene, etc. 

Cool the reaction mixture to room temperature and add gradually a solution of 75 g. of sodium hydroxide in 125 ml. of water if the mixture boils during the addition of the alkah, cool again. The hydroxide of tin which is flrst precipitated should all dissolve and the solution should be strongly alkahne the anihne separates as an oil. Equip the flask for steam distillation as in Fig. II, 40, 1, and pass steam into the warm... 

Reflux gently in a test-tube under a short air condenser 1 g. of the base with 2 5 mols or 3 0 g. (3 0 ml.) if the molecular weight is unknown of redistilled acetic anhydride for 10-15 minutes. Cool the reaction mixture and pour it into 20 ml. of cold water (CAUl ION). Boil to decompose the excess of acetic anhydride. When cold, filter the residual insoluble acetyl derivative and wash it with a little cold water. Recrystal-/ise from water or from dilute alcohol. 

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