55
Large-Scale Production of BNC
stirred-tank.fermenter..They.explained.this.outcome.by.the.higher.oxygen.
concentration.dissolved.and.the.lower.shear.force.environment.of.the.airlift.
bioreactor..In previous.investigations.they.clarified.that.higher.shear.caused.
by.the.impeller.in.the.stirred-tank.bioreactor.results.in.a.negative.effect.on.
BNC.production..The.effect.increases.with.increasing.stirring.speed.(Joseph.
et.al..2003;.Kouda.et al. 1996).
In.further.investigations,.Chao.et.al..(2000).realized.a.BNC.production.of.
8.7.g/l.compared.with.6.3.g/l.in.the.control.medium.using.the.addition.of.
0.1%.(w/w).agar.(water-soluble.polysaccharide)..Also,.the.length.of.the.cultivation. period. until. the. BNC. maximum. was. significantly. shortened,. to.
approximately. half. of. the. outcome. without. adding. polysaccharide.. They.
assumed.that.the.agar.addition.hindered.the.formation.of.large.BNC.clumps,.
which.resulted.in.the.increased.BNC.production.rate..Since.smaller.pellets.
are.advantageous.for.transferring.nutrients.and.oxygen.into.bacterial.cells.
located.inside.and.on.the.surface.of.the.cellulose.matrix,.faster.growth.rates.
were.achieved..They.noted.that.in.a.stirred-tank.reactor,.where.turbines.and.
baffle. plates. destroy. the. cellulose. clumps,. the. addition. of. polysaccharide.
would.have.no.influence.on.the.BNC.production.rate.
The.main.parts.of.such.an.airlift.reactor.include.a.nozzle.for.inoculation,.a.
gas.outlet,.nozzles.for.the.addition.of.acid.and.base,.sensor.nozzles,.an.inlet.
and.outlet.for.temperature-controlled.water,.a.sampling.nozzle,.a.temperature.sensor,.a.drain,.a.sight.glass,.and.a.draft.tube.(Figure 3.4).
The.application.of.airlift.bioreactors.results.in.BNC.arranged.in.elliptical.
pellets.instead.of.the.fibrous.forms.produced.by.stirred-tank.reactors..The.
pellets.are.also.more.compact.and.dense..The.cellulose.nanofibers.within.the.
pellets.are.characterized.by.a.higher.degree.of.polymerization.and.improved.
physicochemical. properties. than. fibrous. forms.. The. pellet. sizes. have. been.
found.to.decrease.with.an.increase.in.the.amount.of.substrate.in.the.feed,.but.
also.with.a.decrease.in.the.dissolved.oxygen.concentration.in.the.medium.
(Chao. et. al.. 2001a,. 2001b).. In. contrast,. the. density. of. the. pellets. increased.
with.the.increase.in.the.amount.of.substrate.used.
Wu.et.al..(1990).were.the.first.to.combine.a.bubble.column.and.a.conventional.
internal-loop.airlift.reactor..The.resulting.proposed.reactor.system.included.a.
circular,.wire.mesh.draft.tube..It.was.found.that.the.K L a depends.on.the.gas.
velocity..If.the.gas.velocity.exceeded.2.cm/s,.the.K L a of.the.proposed.reactor.
would. be. higher. than. that. of. the. original. bubble-column. or. airlift. reactor.
(Wu 1992;. Wu. and. Wu. 1991).. At. a. superficial. gas. velocity. of. 5.02. cm/s,. the.
K L a value.of.the.proposed.reactor.with.wire.mesh.draft.tube.was.found.to.be.
double.that.of.the.airlift.reactor..In.the.case.where.the.proposed.reactor.was.
used.with.a.fed-batch.culture,.an.increase.in.cell.mass.was.obtained.
Furthermore,.Cheng.et.al..(2002).developed.a.rectangular.wire.mesh.draft.
tube.that.allowed.bubble.slicing.and.subdivision..The.modified.airlift.reactor. .
was.found.to.possess.a.higher.volumetric.oxygen.transfer.coefficient.and.mixing.capability..As.a.result,.the.final.BNC.concentrations.produced.after.72 h.
of.cultivation.were.three.times.higher.than.those.produced.in.conventional.
Large-Scale Production of BNC
stirred-tank.fermenter..They.explained.this.outcome.by.the.higher.oxygen.
concentration.dissolved.and.the.lower.shear.force.environment.of.the.airlift.
bioreactor..In previous.investigations.they.clarified.that.higher.shear.caused.
by.the.impeller.in.the.stirred-tank.bioreactor.results.in.a.negative.effect.on.
BNC.production..The.effect.increases.with.increasing.stirring.speed.(Joseph.
et.al..2003;.Kouda.et al. 1996).
In.further.investigations,.Chao.et.al..(2000).realized.a.BNC.production.of.
8.7.g/l.compared.with.6.3.g/l.in.the.control.medium.using.the.addition.of.
0.1%.(w/w).agar.(water-soluble.polysaccharide)..Also,.the.length.of.the.cultivation. period. until. the. BNC. maximum. was. significantly. shortened,. to.
approximately. half. of. the. outcome. without. adding. polysaccharide.. They.
assumed.that.the.agar.addition.hindered.the.formation.of.large.BNC.clumps,.
which.resulted.in.the.increased.BNC.production.rate..Since.smaller.pellets.
are.advantageous.for.transferring.nutrients.and.oxygen.into.bacterial.cells.
located.inside.and.on.the.surface.of.the.cellulose.matrix,.faster.growth.rates.
were.achieved..They.noted.that.in.a.stirred-tank.reactor,.where.turbines.and.
baffle. plates. destroy. the. cellulose. clumps,. the. addition. of. polysaccharide.
would.have.no.influence.on.the.BNC.production.rate.
The.main.parts.of.such.an.airlift.reactor.include.a.nozzle.for.inoculation,.a.
gas.outlet,.nozzles.for.the.addition.of.acid.and.base,.sensor.nozzles,.an.inlet.
and.outlet.for.temperature-controlled.water,.a.sampling.nozzle,.a.temperature.sensor,.a.drain,.a.sight.glass,.and.a.draft.tube.(Figure 3.4).
The.application.of.airlift.bioreactors.results.in.BNC.arranged.in.elliptical.
pellets.instead.of.the.fibrous.forms.produced.by.stirred-tank.reactors..The.
pellets.are.also.more.compact.and.dense..The.cellulose.nanofibers.within.the.
pellets.are.characterized.by.a.higher.degree.of.polymerization.and.improved.
physicochemical. properties. than. fibrous. forms.. The. pellet. sizes. have. been.
found.to.decrease.with.an.increase.in.the.amount.of.substrate.in.the.feed,.but.
also.with.a.decrease.in.the.dissolved.oxygen.concentration.in.the.medium.
(Chao. et. al.. 2001a,. 2001b).. In. contrast,. the. density. of. the. pellets. increased.
with.the.increase.in.the.amount.of.substrate.used.
Wu.et.al..(1990).were.the.first.to.combine.a.bubble.column.and.a.conventional.
internal-loop.airlift.reactor..The.resulting.proposed.reactor.system.included.a.
circular,.wire.mesh.draft.tube..It.was.found.that.the.K L a depends.on.the.gas.
velocity..If.the.gas.velocity.exceeded.2.cm/s,.the.K L a of.the.proposed.reactor.
would. be. higher. than. that. of. the. original. bubble-column. or. airlift. reactor.
(Wu 1992;. Wu. and. Wu. 1991).. At. a. superficial. gas. velocity. of. 5.02. cm/s,. the.
K L a value.of.the.proposed.reactor.with.wire.mesh.draft.tube.was.found.to.be.
double.that.of.the.airlift.reactor..In.the.case.where.the.proposed.reactor.was.
used.with.a.fed-batch.culture,.an.increase.in.cell.mass.was.obtained.
Furthermore,.Cheng.et.al..(2002).developed.a.rectangular.wire.mesh.draft.
tube.that.allowed.bubble.slicing.and.subdivision..The.modified.airlift.reactor. .
was.found.to.possess.a.higher.volumetric.oxygen.transfer.coefficient.and.mixing.capability..As.a.result,.the.final.BNC.concentrations.produced.after.72 h.
of.cultivation.were.three.times.higher.than.those.produced.in.conventional.