Monitoring and Control of Secondary Drying

Other methods proposed in the past to monitor secondary drying are based on the estimation of the solvent flow rate that can be obtained, for example, from the PRT. This technique can be tracked back to the early 1960s, as was proposed by Kan (1962) for detection of the drying endpoint. In fact, the mass flow rate of water (or solvent) Mw can be calculated from the slope of the curve of chamber pressure vs. time at the beginning of the PRT  

The loss of water (or solvent) during the time interval elapsed between two consecutive PRTs can be estimated assuming a constant value of the flow rate Mw in that interval that is  

Willemer (1991) proposed to relate the pressure increase as a consequence of the PRTduring secondary drying to the residual moisture content using this measure to stop secondary drying. An improved approach is that proposed by Oetjen (2001) using two successive measurements of desorption rate, calculated from the water flow rate M  

Fissore et al. (2008b, 2011b) proposed an innovative approach to determine the residual water content of the dried product vs. time and to give a reliable estimation ofthe time that is necessary to complete secondary drying, that is, to fulfill the requirement on the final water content ofthe product. The method uses the values of water (or solvent) desorption rate that can be calculated from the PRT (Eq. 4.26) and a mathematical model that describes the change with time ofthe residual water content in the dried product. To this purpose it is required to model the dependence of the desorption rate of water (or solvent) from the residual water content in the product. Various models have been proposed in the literature the desorption rate can be assumed to be proportional to the residual water content, or to the difference between the residual water content and the equilibrium value. It is possible to use the exact mechanism, if it is known. Otherwise the first-mentioned model, that is much simpler and that has been demonstrated to describe adequately the process (Liapis and Bruttini, 1995) can be applied  

The time evolution of the residual moisture content X, in kg of water per kg of dried mass, is given by the integration of the following differential equation  

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