Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 33

Fig. 5 Process diagram of steam explosion. Batch process (a), and continuous mode (b)

However, the sugar yield of the pretreatment is improved at low temperature
and decreased holding time, whereas the development of cellulose more accessible
for cellulase attack requires more severe conditions, in which sugar loss
is inevitable. Therefore, a trade-off between these two contradictory factors is
needed. The comprehensive impact of temperature and holding time can be
evaluated quantitatively by the severity factor R 0 , which is calculated using the
correlation R 0 =log[te (T-100)/14.75 ], where T and t represent temperature (C) and
holding time (min), respectively, and 14.75 is the active energy of the reaction [ 19 ].
Both batch and continuous processes have been developed for the SE process,
as illustrated in Fig. 5 . The batch process is simple. Humidified feedstock is fed
through a screw feeder into the reactor, which is then pressurized by saturated
steam and maintained for a period of time. After the reaction, the material is
discharged into the explosion tank at atmospheric pressure, in which volatile
components are separated, and pretreated biomass is left for washing to collect
sugars released by the hydrolysis of hemicelluloses. To overcome the disadvantage
of the discontinuity, multiple reactors can be operated alternately. By contrast, the
continuous system is more productive and effective, but the design of the reactor
and discharger is more complicated due to the high solid content of the feedstock
as well as the high pressure required by the pretreatment.
LHW is another hydrothermal pretreatment which can enhance sugar extraction
[ 20 ]. As illustrated in Fig. 6 , slurry is pre-heated via a heat exchanger, which not
only saves steam consumption for heating the slurry, but also cooling water to cool
down the pretreated material. The pre-heated slurry is further heated by steam
via another heat exchanger, and passes through the reactor for pretreatment.
Theoretically, the reactor should be operated at plug flow. Therefore, tubular