Exothermic reactions, temperature

A solution of 61 parts 4-chloro-l,l-di-(4-fluorophenyl)-l-butene in 400 parts 2-propanol is hydrogenated at normal pressure and at room temperature in the presence of 5.5 parts palladium-on-charcoal catalyst 10% (exothermic reaction, temperature rises to about 30°C). After the calculated amount of hydrogen is taken up, hydrogenation is stopped. The catalyst is filered off and the filtrate is evaporated. The oily residue is distilled in vacuo, yielding l-chloro-4,4-di-(4-fluorophenyl)-butane, boiling point 166° to 168°C at 6 mm pressure ... 

Non-isothermal reaction with the diffusion resistance in the pore In an exothermal reaction, temperature gradients will arise within the pellet and, consequently, the temperature of pellet will be elevated compared with its surroundings. As a result, the reaction will be faster than the isothermal counterpart. AT in the film as well in the pellet can be theoretically predicted and, consequently, the maximum AT between the outer surface (Ts) and the inside of the catalyst (T n) calculated, when there c = 0 ... 

The general method of Robertson,3 whereby toluene sulfonic acid monohydrate is added to a mixture of an alcohol and dihy-dropyran, is not recommended for this preparation since the reaction is rather exothermic. Reaction temperatures below 60°C are to be avoided for the same reason since unreacted reagents accumulate and the reaction may suddenly get out of hand with resulting boiling and colorization of the reaction mixture. 

Cyclohexylamine (l.lOg, 11 mmol) is added to a solution of (9) (R = Ph) (1.42 g, 5.0namol) in methanol (25 ml). The slightly exothermic reaction (temperature increases to 30°C) is complete in 30 min. After dilution with water the product is extracted into CH2CI2, and the solvent is dried (MgS04) and concentrated. Vacuum sublimation (twice, O.lmmllg, bath temperature SO-ldO C) gives an oil (1.1 g, 97%) which slowly solidifies, m.p. 50°C. Similarly prepared are 1-t-buty 1-5-phenyl- (82%), l-methyl-5-phcnyl- (87%), l-methyl-5-( -nitrophenyl)- (88%) and l-methyl-5-t-butylimidazoles (46%). 

For fast exothermic reactions, temperature control can be a problem. This is often solved by external circulation of part of Ihe reactor content through a heat exchanger, or by adding an internal heat exchange area. Alternatively, semi-batch operation can be applied, i.e., part of a reactant can be fed steadily over time or at certain intervals. Ihis also minimizes the occurrence of unwanted side reactions. 

Due to the exothermic reactions, temperature influences adsorption it is less efficient at high temperature. Thus, adsorption at a temperature less than 40 °C is recommended. 

PREPARATIVE TECHNIQUE Condensation (step) polymerization Exothermic reaction temperature rises to 75°C. Mixture is heated to 150°C and eventually to 200-230°C. Reaction of trichlorosilane with hexamethyldisilazane (1 > 3 mol ratio) in Ar purged flask at room temperature. ... 

The stabilization of PAN precursor fibers is highly exothermic. Reaction temperature and amount of heat generated [12] depend upon the precursor composition and on the reaction environment. Optimally, the fiber will take up 8-10 vi. % oxygen without overheating. During stabilization, scission and oxidation reactions result in an evolution of gaseous species, i.e., HCN, CO2, and H2O, and these reactions cause the fiber to shrink [13]. Comonomers exert a catalytic effect. The stabilization of the homopolymer in air is very slow, and requires several hours when run isothermally at 220-230 C. When PAN copolymers are used, the stabilization time can be reduced to less than one hour [1] [3] [13]. 

The process could be designed for a wide range of exothermic reaction temperatures. 

The sulfonation of higher fatty acid monoethanolamides with 96% sulfuric acid, 20% oleum or chlorosulfonic acid has been examined under various conditions the optimum sulfonation was found to occur at the exothermal reaction temperature. ... 

Glacial acetic acid acts as the solvent in an excess of 10-40%. Sulfuric acid (2-15% based on cellulose) catalyzes the exothermic reaction. Temperature is kept below 50 °C. A highly viscous solution of the triester is obtained. 

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