Chemical Technology May 2016 low res1

Chemical Technology •May 2016

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

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.