Chemical Technology May 2016

Volatiles removal from solids – An introduction to devolatising by Joe Bonem, Polymers and Process Engineering Consultant

Devolatilisation is a mandatory step in the manufacturing of many commercial solids ranging from pharmaceutical products to waste streams. Excessive levels of residual solvents, monomers or other volatiles can create fires or explosions in subsequent processing or transportation steps, as well as discontinuities in product appearance. The residual solvents or monomers may also be odoriferous at very low concentrations, creating end user concerns.

P roviding adequate devolatilising facilities in- volves both knowledge of equilibrium and mass transfer limits. This study will appear in three sections over the next few months, as follows: Section 1 — In the first part I will discuss the theory and show how the equilibrium and mass transfer limitations are combined to estimate the required size of the devolatilisation equipment. While this may sound too theoretical for the prac- tising process engineer, the approach has been simplified as much as possible while still being theoretically correct. Section 2 — Here, I will discuss the determination of equilibrium and how it may be calculated or determined experimentally. The emphasis is on calculating since this is the situation that many practising engineers find they must handle, ie, laboratory facilities are not available to develop actual data.

Section 3 — This part covers the development of mass transfer coefficients from a pragmatic paradigm. In addition, it describes a spreadsheet that is helpful in performing the numerical integration. When one is faced with a problem (either design or oper- ating) that is associated with a volatiles removal operation, the engineer is often tempted to use one of two incorrect ap- proaches. He/she will assume the boiling point of the solvent and the temperature of the solid are the primary variables that determine whether the solid is sufficiently devolatised. That is, no drying occurs unless the temperature of the solid is above the boiling point of the volatile or that as long as the solid is at a temperature above the boiling point of the volatile, there will be no volatile remaining in the solid. Both of these are wrong when considering either equi- librium or mass transfer. A second approach is to recognise

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Chemical Technology •May 2016