Chemical Technology May 2016

MINERALS PROCESSING & METALLURGY

From a practical point of view, the same theory can be used for any counter-current flow dryer. It can also be used for a co-current flow dryer or fluid bed dryer with some

that there may be an equilibrium limit and simulate the limit by using a computer programme where the properties of the volatile are used and the solid is assumed to have properties equivalent to the least volatile component in the computer programme data base. While this approach may provide an approximation of equilibrium, it does not allow for the non-ideality between the volatile and the solid. Thus reliance on this approach may cause one to believe that this estimated equilibrium can be reached in a very short time period. If one is to avoid the potential product, process, envi- ronment and safety problems related to devolatilisation, it is imperative that both equilibrium and mass transfer be considered more carefully than described above. In devel- oping the theoretically correct approach, I have chosen to use a purge bin ‘plug flow’ dryer that is shown in Figure 1.

modifications in the equations. Background and theory

In a purge bin, gas and solids flow counter-current to each other. The bed is designed to promote ‘plug flow’ character- istics for both the gas and solids. In true ‘plug flow’, each solid particle has an identical residence time. In addition, the equilibrium concentration of volatiles in the exiting solids is a function of inlet gas concentration . This is in contrast with a single stage fluidised bed where the equi- librium concentration of volatiles in the exiting solids is a function of outlet gas concentration . A well-designed purge bin is an excellent approach to reducing volatiles to a very

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Chemical Technology • May 2016