Biobutanol - Biotechnology in China: Biofuels and Bioenergy - page 97

utilization rate. However, the capacity and selectivity for butanol are currently still
quite low, which, together with the high price of absorbents, form great obstacles
to the promotion of this technology.
(b) Liquid-liquid extraction. Liquid-liquid extraction is another efficient
recovery technology that can remove butanol from the fermentation broth in situ.
This method is built on the principle that solubilities of chemicals vary in different
solutions and distribution coefficients of the chemicals vary between two immis-
cible phases. In this case, when a water-insoluble organic extractant is mixed with
the liquid fermentation broth, butanol will be selectively concentrated in the
organic phase, because it is more soluble than in the aqueous phase. Hu et al. [ 21 ]
compared four different biodiesels (waste oil, rapeseed oil, palm oil, and KFC
waste oil) for solvent extraction, finding that when palm oil was added to the
fermentation (24 h, oil:water volume ratio is 0.4:1), the fermentation intensity
increased by 10.9% compared to the traditional fermentation, and the concentra-
tion of butanol in the organic phase reached to 6.44 g/L. Liquid-liquid extraction
has high capacity and selectivity. However, it also has some problems, such as the
toxicity of the extractant to the cell and emulsion fermentation.
(c) Pervaporation. Pervaporation is another recovery technique that relies on
membranes. Membrane selectivity is mainly used to remove liquid from the volatile
compounds of fermentation. The volatile components of fermentation broth or
organic compounds undergo selective vaporization through the membrane, and
nutrients, sugar, and microbial cells remain below. Currently, ceramic membranes,
polymer membranes, and liquid membranes have been widely used in pervaporation
separation of organic compounds (such as butanol, ethanol, etc.). Zhou et al. used
homemade silicalite-1/silicone rubber hybrid pervaporation membranes to separate
butanol, acetone, and ethanol from water [ 56 ]. The effects of acetone and ethanol on
pervaporation separation of butanol were studied at different concentrations and
temperatures, and the conclusion drawn that membrane permeability of butanol
would increase in the presence of acetone and ethanol. Luo et al. tried to separate and
concentrate butanol by pervaporation [ 33 ]. The butanol concentration increases with
the rise of temperature, permeation pressure, and cross-flow rate. The selectivity of
pervaporation for butanol is the highest at 50C. The advantages of pervaporation are
high efficiency, low energy consumption, and less investment. However, the mem-
brane needs to be regenerated, because a swelling effect makes the membrane more
permeable, resulting in less selectivity of solvents.
There are no reports by Chinese researchers involved in gas stripping, perstrac-
tion, and reverse osmosis.
4 Conclusions
In China, research on ABE fermentation covers almost every aspect of this field.
The excellent solvent-producing strains, such as EA2018 and Rh8, have been
improved by native researchers using conventional mutagenic methods. Using the