Exothermic reactions, temperature dependence

Hicks (H6) and Frazer and Hicks (F3) considered the ignition model in which exothermic, exponentially temperature-dependent reactions occur within the solid phase. Assuming a uniformly mixed solid phase, the one-dimensional unsteady heat-flow equation relates the propellant temperature, depth from the surface, and time by the nonlinear equation ... 

The study of kinetics at high temperatures should be begun precisely from the determination of the minimum reaction time corresponding to disruption of the combustion regime ( quenching, cf. [4]). Plotting the concentration of the initial substance in the reaction products as a function of the reaction time, we obtain for a typical exothermic reaction a dependence of the form in Fig. 7. 

The temperature rise of an exothermic reaction is dependent on three factors the heat of the reaction, the heat capacity of the system, and the heat loss of the system The temperature rise of a reaction in a system with no heat loss, the adiabatic temperature rise ( N T), is dependent on the heat of the reaction and the heat capacity of the system, and independent of scale To determine the adiabatic temperature rise of this system, the CDMP/sulfuric acid solution, prewarmed to 30°C, was added all at once to a dewar flask containing the nitric acid/sulfuric acid solution which was also prewarmed to 30°C We observed a temperature rise of 17°C over a period of 4 minutes, with a temperature drop of 1 5°C over the next 4 minutes (Figure 3) We therefore estimated the AT to be about 18.5°C Since this temperature rise was, in theory, independent of scale, we could predict that the large scale nitration reaction would not rise to a temperature of exothermic activity Based on these results, we considered this revised nitration procedure to be safe upon scale up to the pilot plant ... 

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]. 

Reaction 1 is highly exothermic. The heat of reaction at 25°C and 101.3 kPa (1 atm) is ia the range of 159 kj/mol (38 kcal/mol) of soHd carbamate (9). The excess heat must be removed from the reaction. The rate and the equilibrium of reaction 1 depend gready upon pressure and temperature, because large volume changes take place. This reaction may only occur at a pressure that is below the pressure of ammonium carbamate at which dissociation begias or, conversely, the operating pressure of the reactor must be maintained above the vapor pressure of ammonium carbamate. Reaction 2 is endothermic by ca 31.4 kJ / mol (7.5 kcal/mol) of urea formed. It takes place mainly ia the Hquid phase the rate ia the soHd phase is much slower with minor variations ia volume. 

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. 

For an endothermic reaction AH/ is positive for an exothermic reaction it is negative. The temperature dependence of AH/ is given by dAHf... 

Most chemical reactions are greatly affected by temperature. The previous chapters discussed reactions at isothermal condition, however, industrial reactors often operate under non-isothermal condition. This is because chemical reactions strongly depend on temperature, either absorbing (i.e., endothermic) or generating (i.e., exothermic) a large amount of heat. 

The evolution of a. star after it leaves the red-giant phase depends to some extent on its mass. If it is not more than about 1.4 M it may contract appreciably again and then enter an oscillatory phase of its life before becoming a white dwarf (p. 7). When core contraction following helium and carbon depletion raises the temperature above I0 K the y-ray.s in the stellar assembly become sufficiently energetic to promote the (endothermic) reaction Ne(y,a) 0. The a-paiticle released can penetrate the coulomb barrier of other neon nuclei to form " Mg in a strongly exothermic reaction ... 

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. 

With a reaction enthalpy of A RH = -170 kJ/g mol the sulfonation with S03 is strongly exothermic. As the color of the acid is dependent not only on the residence time but also to a considerable extent on the reaction temperature, it is necessary to have an effective thermal dissipation. This applies to all of the reactors listed in Table 13. The falling film reactors, of which there are various designs, have the advantage that a very short residence time can be realized [152]. 

We can see from Table 9.2 that the equilibrium constant depends on the temperature. For an exothermic reaction, the formation of products is found experimentally to be favored by lowering the temperature. Conversely, for an endothermic reaction, the products are favored by an increase in temperature. 

Most of the chemical reactions in the process industry are temperature dependent. They are either exothermic or endothermic. As a consequence, it is often necessary to remove the heat generated by an exothermic reaction to control the reaction temperature and to avoid thermal runaway reactions or to suppress endothermic by-product reactions, for instance [8]. 

When sodium hydride was heated to 50°C with DMF, this gave rise to an exothermic reaction the temperature then reached 75 C. An external cooling was then carried out, but was not successful. The reaction went out of control and the reactor s content overflowed. A test that was carried out on a small scale showed that the reaction started at 26-50 C, depending on the DMF s degree of drying and then accelerated. DMA s behaviour is the same the reaction starts at a temperature of 40°C. 

Figure 3.3 The temperature dependence of AC0, AH0, and T AS0 over large temperature ranges the behaviour (A) for negative AS0, (B) positive AS0 and (C) variation of In K with 1/7" for exothermic and endothermic reactions.

As a vessel of a given shape increases in size, both the surface area and the volume increase, but they do not increase at the same rate. For a sphere the surface area is a function of the diameter squared and the volume is a function of the diameter cubed. This is also true for a cylinder whose height is a multiple of its diameter. The polymerization of styrene is an exothermic reaction. The amount of energy released at any time is dependent on the volume of the reactor, and the rate of removal of that heat is dependent on the surface area. Unless the heat is removed, the temperature will rise and the reaction rate will increase. The result will be an uncontrolled reaction that not only may ruin the batch but could also damage the reactor and might cause a tire or explosion to occur. 

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