Lignin
is one of the most valuable and abundant renewable resources found on Earth.
However, this polymer usually undergoes severe chemical modifications during
isolation, and these changes restrict many of its industrial applications.
Therefore, a successful isolation procedure must avoid both fragmentation and
condensation reactions.
One
of the methods commonly used to recover lignin from wood and other
lignocellulosic materials is steam explosion followed by
mild-alkaline-extraction.
Steam
Explosion technology involves treating various biomass resources.
In steam explosion the biomass is pressurized with high steam pressure
for a certain period of time, (typically 200 - 450 psig) for a period of 1 - 10
minutes and then explosively discharging the product to atmospheric pressure,
which results in a sudden decompression. This
explosive discharge changes the starting material (solid) into a fibrous mulch
by a combination of mechanical and chemical action.
In the case of wood chips, the explosion causes defibrillation of chips
into fiber bundles, and partial hydrolysis of cellulose, other carbohydrates,
lignin and volatile components.
Specifically this site deals with a two cubic foot batch digester steam explosion unit and a continuous process unit located the Thomas M. Brooks Forest Products Center of Virginia Tech under the supervision of Robert S. Wright. Several images from the unit are pictured below.
Figure 1 Figure 2 Figure 3 Figure 4
The
batch process will be described further and is pictured here.
A schematic of the continuous process is pictured here.
In
the batch unit, before any sample is fed into the steam explosion chamber, all
valves must be closed except for valve
. For each batch, the
sample would be put into the chamber through valve . After this valve is closed, steam would be released into the
chamber via valve ‚. The pressure and temperature of
the steam are controlled from a boiler. Time
is kept constant while pressure and temperature are varied in relation to
severity of steam explosion chosen. Valve
ƒ is released when a desired cooking time has been reached.
Simultaneously, an explosive expansion of the steam occurs and an
exploded sample (called steam explosion fibers, SEF) would be collected in a
container. Finally, excess steam is
released through valve
„.
Although
extensively studied and demonstrated on the pilot plant scale, steam explosion
of biomass is not currently industrially practiced. However, treatment of biomass with high pressure steam,
followed by sudden decompression, is industrially practiced in the “Masonite
process” which represents the basis for fiberboard production.
Steam
explosion of biomass is known to be a hydrolytic pretreatment; one that
increases enzyme and solvent accessibility of cellulose; one that renders
biomass separable (fractionation) into different components; and one that raises
the crystallinity of the cellulose component. (These results are based on pilot-scale studies.)
Steam explosion is unique because the technology derives from specialized equipment that can continuously inject fibrous materials into a digester pressurized at between 200 to 450 psig with high pressure steam and then explode the product out. No other equipment is able to do this.
