Temperature increase adiabatic

This approach consists in measuring the adiabatic temperature increase of a sample taken at the outlet of the reactor. Samphng is made in an adiabatic vessel (Dewar vessel) and temperature is recorded until the reaction ends, that is, until an equilibrium temperature is reached. The conversion rate is thus written as... 

Test method Experimental conditions Sample mass Initial exotherm detected, K Adiabatic temperature increase, K Heat of reaction, J/g... 

The adiabatic temperature increase for an ideal gas is computed from the thermodynamic adiabatic compression equation ... 

The potential consequences of adiabatic temperature increases within a chemical plant are illustrated in the following two examples. 

Adiabatic temperature increase, energy content management 101... 

Fig. 3.18 Scenario of cooling failure with thermal runaway. ATa(j, is the adiabatic temperature rise by desired reaction. ATaa2 is the adiabatic temperature increase by the decomposition reaction. The time required for this increase is TMRad-...

A key element in the assessment of the reaction is again the thermal evaluation. A tentative decision for the assessment of the safety of normal operation can be derived from knowledge of the heat of reaction AHr from which the adiabatic temperature increase, ATadiab, can be determined. 

If the adiabatic temperature increase is known then the following estimates can be applied to the process ... 

If the adiabatic temperature increase of the reaction is less than 50 K during normal operation and the starting materials, reaction mixture or products have no thermal instabilities within a temperature range of (Tpr0cess + ATadiab) then the normal operation can be regarded as safe. The same applies when secondary decomposition reactions produce so little heat that the sum of this decomposition heat and the heat of reaction does not cause an adiabatic temperature increase of more than 50 K. 

The adiabatic temperature increase (ATadiab) is defined as the temperature increase that is established in a reactive process system when the process goes to completion without heat or mass exchange with the environment (e.g., following complete failure of cooling in a closed reactor). 

The special process feature for case 3 is a relatively high reaction enthalpy in combination with a low maximum permissible temperature Texo- An alternative safety solution would be to control both these two parameters. For example by adding a pump to the reactor and with solvent makeup the process can be made continuous (CSTR). This allows the adoption of a higher maximum permissible temperature Texo, because of the short residence time and the dilution effect, and a reduction of the adiabatic temperature increase ATadiab because of the dilution effect. Such a (drastic) process and facility change will always require an iterative safety-technical reaction PHA furthermore additional may become necessary. 

Experimental and simulation results presented below will demonstrate that barrel rotation, the physics used in most texts and the classical extrusion literature, is not equivalent to screw rotation, the physics involved in actual extruders and used as the basis for modeling and simulation in this book. By changing the physics of the problem the dissipation and thus adiabatic temperature increase can be 50% in error for Newtonian fluids. For example, the temperature increase for screw and barrel rotation experiments for a polypropylene glycol fluid is shown in Fig. 7.30. As shown in this figure, the barrel rotation experiments caused the temperature to increase to a higher level as compared to the screw rotation experiments. The analysis presented here focuses on screw rotation analysis, in contrast to the historical analysis using barrel rotation [15-17]. It was pointed out recently by Campbell et al. [59] that the theory for barrel and screw rotation predicts different adiabatic melt temperature increases. 

The specific energy from Eq. 10.13 is reported in J/g for convenience in analyzing the process. If a change in a process causes the specific energy to increase by 50 J/g, the troubleshooter can translate this to an adiabatic temperature increase of 20 °C because many unfilled resins have heat capacities near 2.5 J/(g °C). This temperature change calculation, however, is an approximation because extruders typically do not operate adiabatically. 

This power may be converted to adiabatic temperature increase rate ... 

Table 2.8 Reaction rate under adiabatic conditions with different reaction enthalpies, corresponding to an adiabatic temperature increase of 20 and 200 K.

If after the cooling failure unconverted reactants are still present in the reaction mixture, they will react in an uncontrolled way and lead to an adiabatic temperature increase. The remaining unconverted reactants are referred to as accumulated reactants. The available energy is proportional to the accumulated fraction. Thus, the answer to this question necessitates the study of the reactant conversion as a function of time, in order to determine the degree of accumulation of unconverted reactants (Xac). The concept of Maximal Temperature of the Synthesis Reaction (MTSR) was developed for this purpose ... 

Besides these purely static aspects, the dynamic behavior of an adiabatic batch reactor must also be considered. The adiabatic temperature course is a function of the thermal properties of the reaction mixture. The adiabatic temperature increase influences the final temperature as well as the rate of the temperature increase. For highly exothermal reactions, even for small increase in conversion, the increase in temperature is important (see Section 2.4.3). 

Adiabatic with Cold-Shot Cooling. Some exothermic reactions are conducted in vessels with multiple beds of catalyst, which operate adiabatically (temperature increases through the bed). At the exit of each bed, a cold stream is mixed with the hot stream leaving the bed to bring the temperature back down to the desire inlet temperature for the downstream bed. This cold stream is typically some of the feedstream that has been bypassed around the reactor feed preheating system. 

Once the onset temperature is determined, it is then possible to obtain the maximum reaction temperature by the adiabatic temperature rise and any contribution due to external heat input. The theoretical adiabatic temperature increase is... 

The exotherm starts at 150°C and ends at a final temperature of 272°C, resulting in an adiabatic temperature increase of 122°C. This value is the uncorrected measured temperature increase for both sample and vessel (bomb). The correction involves multiplication by the c )-factor. For example, if the value of the -factor from an experiment is 1.68, then the corrected adiabatic temperature is 205°C. 

Chapter 2 The adiabatic temperature increase equation 2.1 Introduction... 

Eq. (44) is obtained directly from Eq. (43), i.e., an equation ealled the adiabatic temperature increase equation. 

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