Temperature of no return

The temperature eorresponding to the equipment timeline on the time to maximum rate (TMR) plot is the temperature of no return. Above the temperature of no return, the rate of heat generation from... 

Temperature of no-return Temperature of a system at which the rate of heat generation of a reactant or decomposition slightly exceeds the rate of heat loss and possibly results in a runaway reaction or thermal explosion. 

Also, the temperature of no return can be calculated from the data obtained in an ARC experiment [123]. In a given reaction vessel with given heat transfer characteristics, the temperature of no return (Tnr) is a metastable temperature such that below Tnr, the reaction temperature will not increase since the released heat from the reaction does not exceed the rate of heat removal from the system, and above Tnr the reaction temperature will in-... 

A stable situation is represented by the heat removal line (1), provided the temperature of the system is lower than the temperature Tc, which here is equal to the temperature of no return (Tnr). Under these conditions, the cooling capacity of the system exceeds the heat that is generated and the system temperature will invariably drop to the point of intersection SI. 

FIG. 23-16 For an exothermic reaction system with heat removal, e.g., to a vessel jacket and cooling coil, the limit of stable operation is reached as the reaction temperature increases to I m (temperature of no return), beyond which the rate of heat generation, which increases exponentially with increasing temperature, exceeds the capability of the system to remove the heat of reaction (see text). 

The Accelerating Rate Calorimeter (ARC ) is another adiabatic test instrument that can be used to test small samples. The ARC with the clamshell containment design can handle explosive compounds. It is a sensitive instrument that can indicate the onset of exothermicity where the reaction mixture can be accurately simulated (HSE 2000). ARC testing results can be used in determining a time to maximum rate of decomposition, as well as in calculating a temperature of no return for a container or vessel with specific heat removal characteristics. Further information and references related to the ARC are given in CCPS (1995a) and Urben (1999). 

Figure 5 shows the third possibility for the relative positions of the heat generation and heat removal lines. This represents the critical state. That is, there is only one point of intersection between the two curves, only one point at which the heat removal rate is exactly equal to the heat generation rate. This is a point of equilibrium, but if the rate of heat generation should increase (through an impurity which acts as a catalyst, for example), or the rate of heat removal should decrease (e.g., through scale build-up or an increase in the temperature of the coolant), a runaway situation will occur. This critical point is often referred to as the Temperature of No Return, T r. (5) Note that this is the temperature of the reaction mass and not the temperature of the coolant. 

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