Intermediate-thermostat

Temperature in base thermostat Temperature in intermediate thermostat Temperature in measuring kettle... 

Fig. 2.1 Principle of accurate determination of thermal reaction power during an isothermal, discontinuous reaction [based on the same measuring principle of the calorimeter, this system of intermediate thermostat—controlled heater, base thermostat (controlled heat sink)—was replaced recently [54] by a new type of intermediate thermostat metal, bordering controlled Peltier elements, thermostat (controlled heat sink)]...

The base thermostat displays the lowest constant temperature Ts, the intermediate thermostat the medium temperature Ts + ATi, and the measuring kettle the highest temperature T + AT] + AT2. 

From the measuring kettle heat flows to the intermediate thermostat and from there to the base thermostat. In controlled equilibrium, the following applies with respect to the balance of heat power ... 

Sum of all heat powers heat flow out of in intermediate thermostat intermediate thermostat k-F)2 - AT2+Pi+Psti = ik F) Tl. 

The reason and the necessity for using an intermediate thermostat in addition to the measuring kettle to estimate the exact thermal reaction power q t) appears to the naked eye as follows. 

Equation (2.5) shows the only thing that the solely course of the electric heat power Px t) in the measuring kettle versus time does not allow for the estimation of the thermal reaction power q t) It can be determined only when the course of a reference power, the baseline Pb(0> is known. The intermediate thermostat contributes essentially to its registration. 

To maintain ATi = const, the change in electric heating power of the intermediate thermostat 6pi must be... 

In the first case is enlarged by the amount of increased heat flow from the measuring kettle into the intermediate thermostat 8 k F)2 AT2, ... 

Therefore, in the case of a non-compact calorimeter (Fig. 2.11), tmly a part of the intermediate thermostat encloses the measuring kettle in the form of a hollow jacket. This hollow jacket is connected to the central part of the intermediate thermostat by a thermally insulated pipeline. The central part of the intermediate thermostat is immersed in the base thermostat. The thermostat liquid circulates turbulently via a pipeline through the hollow jacket and lid. For reactions under a pressure of up to 10 bar, the measuring kettle with a hollow jacket consists of glass (Fig. 2.12). A simple visual observation of the reaction mixture is possible. 

Material of measuring kettle/intermediate thermostat Glass, HC4, V4A... 

In addition to the electromotor without armature retroaction, the use of the intermediate thermostat is necessary to determine the baseline by analogous measurement and ultimately to determine the thermal reaction power by physical means in the classic working manner, i.e. by simple potentiometric addition and multiplication of measured quantities. The modem method of recording measured quantities and proceeding on the basis of digital electronics makes it possible—with limitations—to neglect the intermediate thermostat and to determine an adequately precise course of the baseline proceeding from one point in time to another as follows. 

The temperature of the intermediate thermostat is maintained as constant as is, for that reason, the temperature difference T — T by means of the control system R1, which controls the heating power pi. The heat balances of the measuring kettle respectively intermediate thermostat are... 

Such overstepping of the limits of power supplies can be avoided if one gives up on the idea of measuring accuracy using a simplified design, conveniently a non-compact calorimeter without use of an intermediate thermostat, as follows. 

The reaction mixture is separated from the filling of the intermediate thermostat by a wall of finite heat capacity... 

The heat flows due to the change in temperature simultaneously firom the reaction mixture and the filUng of the intermediate thermostat into the wall are therefore different... 

The mean temperature of the wall changes is temporarilly delayed compared with the changing temperature of the measuring kettle and the intermediate thermostat... 

Heat-transfer coefficient intermediate thermostat —> outside wall of measuring kettle... 

As a rule, the amount of heat capacity of the measuring-kettle wall Cm cannot be neglected, especially with a pressure kettle. Therefore, the heat contents of the intermediate thermostat and the reaction mixture contribute substantially to the change in temperature within the wall of the measuring kettle, which occurs in the ratio of the transfer coefficients (a)im and (a)2m> in which generally (a)im is smaller than (a)2m-Due to the heat loss from the reaction mixture into the measuring-kettle wall deviate... 

In this situation, e.g. heat is briefly more quickly released than heat—which up to now has been relevant for the maintenance of thermal equilibrium— flows simultaneously from the measuring kettle into the intermediate thermostat. But the momentarily changed temperature difference T2(t)-F... 

When the time interval of the isothermal situation is small compared with the previous non-isothermal phase, for the sake of simplification the measurements are to be performed from the very beginning in non-isothermal conditions, i.e. without heat compensation, hence with the electric heating power turned off,p2 = 0. When the reaction induces only a small change in the heat transfer coefficient, it can be measured without use of the intermediate thermostat (Fig. 2.31). 

The heat-transfer coefficient (k-F)i and the effective heat capacity Ci of the filling in the intermediate thermostat are constant during the reaction. k F)i, C and C2 can be found using an analogous procedure to that just illustrated. The stirring powers pst2 and psti must be measured (Sect. 6.3). 

The heat kF)2-AT2 flows from the measuring kettle into the intermediate thermostat owing to the temperature difference AT2, and the heat (k F)i ATi flows from the intermediate thermostat to the base thermostat owing to the temperature difference ATi. qui is the rate of physicochemical heat release by mixing, and following the balance of heat powers in equilibrium of control the following conditimis apply ... 

The heat-transfer coefficient (k F)i and the stirring power psti of the intermediate thermostat are not influenced by the reaction within the measiuing kettle they are to a certain extent apparatus constants. ATi remains constant because pi changes, regulated by a control procedure, opposite and equal to the change in the heat flow (k F)2 AT2 from the measuring kettle into the intermediate thermostat. 

A reflux condenser (condenser-kettle embedded in an intermediate thermostat) within a base thermostat is combined via a thermally insulated pipe with the measuring kettle of a calorimeter for a discontinuous reaction (Fig. 2.35). Provided that in the measuring kettle evaporable components are generated by a not-too-fast chemical conversion, between the measuring kettle and the condenser there exists with regard to the evaporable components a quasi-continual equilibrium. ... 

However, under certain conditions, this measurement can be carried out indirectly by measuring the rate of heat release from the condensation of vapour in the condenser, which is recorded by the condenser/ intermediate thermostat (Fig. 2.36). The conditions for equality of the thermal reaction power and the rate of heat release by condensation are as follows ... 

The time constant of the reaction must be large compared to the time constant of the heat transfer from the condenser to the condenser/intermediate thermostat ... 

The time constant of the reaction must be large compared to the time constant for the adjustment of the thermal equihbrium in the coupled measuring kettle/ condenser system. In this way, the power compensation in the condenser/ intermediate thermostat occurs on the basis of quasi-continual thermal equilibrium. In that case, the amoimt of substance leaving the measuring kettle as vapour in accordance with the released heat of the reaction per unit of time corresponds to the amount of condensate in the condenser per unit of time. 

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