Abstract
M.Tech. (Chemical Engineering)
Increasing pressure on industry from national and international communities for more
environmentally friendly operations has generated a drive towards the use of ‘green’
absorbents in the removal of volatile organic compounds from contaminated air streams.
The objective of this work was to investigate the use of ester solvents, especially
biodiesel esters, as suitable ‘green’ scrubbing solvents for the absorption of volatile
organic compounds (VOCs) from gaseous waste streams. The objective was
accomplished by investigating the nature of molecular interactions between ester
solvents and various families of VOC solvents, and by studying the effect of
temperature on absorption, through the analysis of infinite dilution activity coefficients.
Activity coefficients were computed with a Microsoft Excel generated spread sheet
using Modified UNIFAC Dortmund (1987) as a thermodynamic predictive model.
Results show that non-polar solutes are more soluble in long chain ester solvents whilst
polar solutes absorb easier in short chain ester solvents. The solubility of non-polar
solutes decreases with increasing ester solvent unsaturation, whilst the opposite is true
for polar solutes in esters. Short chain length non-polar solutes are more soluble than
long chain length non-polar solutes in ester solvents, whilst the opposite trend occurs for
polar solutes in ester solvents. An increase in branching of polar solutes results in the
solute behaving more like a non-polar solute. Unfortunately UNIFAC models fail to
account for functional group proximity effects, which may result in inaccurate results
for systems involving highly branched solutes. However given that the purpose of this
study was only to establish trends for generating an understanding of molecular
interactions between the solutes and the solvent, it was acceptable to assume that these
inaccuracies would be minor for the intended purpose.
It was found that activity coefficients of ester/ solute interactions generally tend towards
ideality with increasing temperature with the exception of alkanes and the nitrogen based
VOCs, as well as tetrahydrofuran and tetrahydropyran, methacrolein and organic
acid/ unsaturated ester interactions. In the case of the alkanes, tetrahydrofuran and
tetrahydropyran, high-temperature absorption is preferred whilst for methacrolein lower
temperature absorption is preferred. It was concluded that ester solvents ,especially biodiesel esters, are well suited for the
removal of non-polar to moderately polar VOC solutes from waste gas streams due to
the favourably low activity coefficients obtained for these ester/ solute interactions. The
results presented in this report are comprehensive and can assist in making informed
decisions in using biodiesel as a scrubbing solvent.