Manipulating the Reactor Status

Finally, one can easily obtain the properties of the stirring system (rotational speed and torque) and compute the stirring power from their product, but these variables are only indirectly linked to the reactor status through the system density. 

An offline measurement apparatus is usually not directly mounted on the reactor, but is fed with samples withdrawn from it manually or automatically. This is the typical case of chromatography, a widely used measurement device for gas and liquid composition. Both gas and liquid chromatographies are based on the separation of the sample by means of selective adsorption on a solid substrate posed in a fixed bed column, and on the detection of the change of a suitable property of the (gas or liquid) carrier, usually thermal conductivity. 

It appears that each component is characterized by a retention time (which also depends on the substrate and on the column length and temperature) and by a relationship between its amount and the thermal conductivity of the modified carrier. Therefore, not only a calibration curve is required for any component, but also the operating conditions must be optimized in order to obtain a sharp separation among the different components. 

In conclusion, a complete analysis of a complex mixture can require very long times (from a few minutes up to many hours) thus, such a measuring apparatus is not suited for online measurements to be used in reactor control. On the contrary, in the laboratory, chromatography is very often the preferred method of analysis of complex mixtures, since these more accurate data can be used to identify the reaction mechanism and the relevant kinetic parameters. 

This brief overview of offline measurements can be concluded by considering the measurements of the heat released by chemical reactions, which can be obtained via calorimetric measurements [7, 18]. The most diffused industrial calorimeters are the so-called reaction calorimeters, basically consisting in jacketed vessels in which the reaction takes place and the heat released is measured by monitoring the temperature of the fluid in the jacket. A class of alternative instruments are the scanning calorimeters (differential or adiabatic), in which the analysis is performed by linearly increasing the sample temperature with respect to time, in order to test the reactivity of potentially unstable chemical systems in a proper temperature range by measuring the released heat. 

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