Bio-oil
produced from the thermochemical treatment of lignocellulosic
biomass is increasingly recognized as a potentially abundant source
of renewable chemicals and fuels. Single ring phenolics and low molecular
weight carboxylic acids are significant constituent compound groups
found in bio-oil and are important end products or intermediate commodity
chemicals. Fractionation of bio-oil using supercritical fluids (usually
with CO2 as a solvent) is a relatively new process being
investigated worldwide at both laboratory and pilot scales. Solubility
data associated with supercritical carbon dioxide (scCO2) and the many chemical compounds in the complex bio-oil mixture
are required to predict the extraction behavior of different bio-oil
compounds. This article starts with a review of the composition of
bio-oil in terms of the phenolic and low molecular weight carboxylic
acid fractions which are potentially of commercial interest. Binary
solubility data of major compounds in these bio-oil fractions with
supercritical CO2 are summarized and discussed. Results
from previously reported studies in which scCO2 is used
as a solvent to recover bio-oil fractions are reviewed and collated.
Density and temperature-based Chrastil type models are developed using
available data for the solubility in scCO2 of some of the
major bio-oil compounds. Finally, extraction of compounds from the
complex bio-oil mixture is discussed in terms of the trends predicted
by the respective individual binary solubility models.
Supercritical
fluid extraction (SFE) and fractionation of products
from a complex mixture such as bio-oil, where many compounds are present
in low concentrations, is a difficult process to model. This difficulty
arises from the uncertainty associated with those interactions between
mixture components for which fundamental vapor–liquid equilibrium
(VLE) data are not available. In this work a novel extraction and
purification concept is investigated using a predictive model developed
from VLE data of binary solute–solvent systems; solute–solute
interactions in the supercritical carbon dioxide (scCO2) phase are neglected. The predictive component of the work employs
an equation of state (EOS) model to achieve the above task. The results
of pilot plant trials utilizing a biocrude feedstock were shown to
be in good agreement with the model predictions. Aspen Plus process
simulations were developed for the extraction process which comprised
supercritical extraction and subsequent purification steps utilizing
distillation and multistage evaporation. A techno-economic analysis
of different process designs were evaluated for comparison. In particular,
distillation as the primary separation process with and without multistage
evaporation were simulated to compare the economics of supercritical
extraction to distillation. It was found from simulation results that
distillation is a very energy intensive process, and total operating
costs for it are always greater than supercritical extraction counterparts.
Combining multistage evaporation with distillation will bring the
total operating cost slightly lower than supercritical extraction
processes. However, the internal rate of return (IRR) value was similar
for both SFE and distillation combined with multistage evaporation
processes. Solvent/bio-oil (S/B) ratio will have considerable impact
on total profits of SFE process in relation to distillation combined
with multistage evaporation.
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