FLUID FLOW AND MIXING IN BIOREACTOR

Made by-
Vikash Shashi
Venishetty Vivek
K. Srinivas Naik

Contents
 Fluid Flow in Bioreactors.
 Mixing in CSTR (Continuous Stir Reactor).
 Mixing in Bubble Column Reactor.
 Mixing in Airlift Reactor.
 Mixing in Packed Bed Reactor.
 Mixing in Trickle bed Reactor.
 Mixing in Fluidised Bed Reactor.

INTRODUCTION
 A fluid is a substance which undergoes
continuous deformation
when subjected to a shearing force .
 A simple shearing force which causes thin
parallel plates to slide over each other,
as in a pack of cards.
 Fluids in bioprocessing often contain suspended
solids , consist of more than one phase , and
have non Newtonian properties .
 A shear force must be applied to produce
fluid flow.

 Two physical properties are used to classify fluids .
VISCOSITY and DENSITY.
 DENSITY : Compressible fluids and Incompressible fluids .
 VISCOCITY : an ideal or perfect fluid is a hypothetical liq. Or
gas which is incompressible and has zero viscosity.
Inviscid fluids and viscid fluids.
 fluids can also be classified further as Newtonian and
non Newtonian .
 NEWTONIAN FLUIDS : which obeys the newton’s laws of
viscosity i.e.
t = mdv/dy ; where t = shear stress, m = viscosity of fluid,
dv/dy = shear rate, rate of strain or velocity gradient.
 NON NEWTONIAN FLUIDS : which do not obey the
Newton's law of viscosity.

FLUIDS IN MOTION
 When a fluid flows through pipe or over a solid object ,
the velocity of the fluid varies depending on position.
 One way of representing variation in velocity is
streamlines,
which follow the flow path. Constant velocity is shown by
equidistant spacing of parallel streamlines . As in fig. 1.
 Where as in fig . 2 there is a reducing space between
the streamlines indicates that velocity at top and bottom
of the object is greater than at the front and back.
 Therefore ,
slow fluid flow is called STREAMLINE or LAMINAR
FLOW.
And in fast motion, fluid particles cross and recross the
streamline and the motion is called as TURBULENT
FLOW.

REYNOLDS NUMBER
 A parameter used to characterise fluid flow .
 For full flow in pipes with cross section , Reynolds number Re is :
Re = Duρ/μ ; where D is pipe diameter ,
u is the average linear velocity of the fluid,μ is fluid viscosity.
 For a stirred vessel there is another definition of the Reynold no.
Rei = Ni Di² ρ / μ ; where Rei is the impeller Reynolds no. ,
Ni is the stirrer speed , ρ is the fluid density , Di is the impeller
diameter.
 The Reynolds no. is a dimensionless variable .
 Reynolds no. is named after OSBORNE Reynolds , who published in
1883
a classical series of papers on the nature of flow in pipes.

NON NEWTONIAN FLUIDS
 Most slurries , suspensions
and dispersions are non
Newtonian .
 Many fermentation
processes involve materials
which exhibit non
Newtonian behaviour , such
as starches, extracellular
polysaccharides ,and culture
broth containing cell
suspensions or pellets.

FLUID FLOW AND MIXING IN BIOREACTOR

HYDRODYNAMIC BOUNDARY
LAYERS
 The part of the fluid where flow is affected by the solid is called the
‘’ boundary layer ‘’.
 Contact between moving fluid and the plate causes the
formation of the boundary layer beginning at the leading
edge and developing on both top and bottom of plate.
 When fluid flows over a stationary object , a thin film of
fluid in contact with the surface adheres it to prevent
slippage over the surface. fluid velocity at the surface
of the plate in fig 7.3 b is therefore zero.
 When a part of flowing fluid has been brought to rest ,
the flow of adjacent fluid layers is slowed down by the
action of ‘ viscous drag ‘ .
 Compared with velocity uB in the bilk fluid , velocity in the
boundary layer is zero at the plate surface but increases
with distance from the plate to reach uB near the outer limit
of boundary layer .

BOUNDARY LAYER SEPERATION
 What happens when contact is
broken between a fluid and a solid
immersed in
the flow path ?
 when fluid reaches the top or
bottom of the plate its momentum
prevents it from
making the sharp turn around the
edge . As a result fluid separates from
the plate
and proceeds outwards into the
bulk fluid .

Stir Tank Reactor
The continuous stirred-tank
reactor, also known back mix
reactor, is a common ideal
reactor type in chemical
engineering.
A CSTR often refers to a model
used to estimate the key unit
operation variables when using a
continuous agitated-tank reactor
to reach a specified output.

Mixing Method
 Mixing method:
Mechanical agitation
• Baffles are usually used
to reduce vortexing
• Applications: free and
immobilized enzyme
reactions
• High shear forces may
damage cells
• Require high energy
input

 An ideal CSTR has complete back -mixing
resulting in a minimisation of the substrate
concentration, and a maximisation of the
product concentration, relative to the final
conversion, at every point within the reactor
the effectiveness factor being uniform
throughout. Thus, CSTRs are the preferred
reactors, everything else being equal, for
processes involving substrate inhibition or
product activation. They are also useful
where the substrate stream contains an
enzyme inhibitor, as it is diluted within the
reactor. This effect is most noticeable if the
inhibitor concentration is greater than the
inhibition constant and [S]0/Km is low for
competitive inhibition or high for
uncompetitive inhibition, when the inhibitor

Bubble column Bioreactor
A bubble column reactor is an apparatus
used for gas-liquid reactions first
applied by Helmut Gerstenberg.
It consists of vertically arranged
cylindrical columns. The introduction
of gas takes place at the bottom
of the column and causes a turbulent
stream to enable an optimum gas
exchange.
In BCR, gas & liquid reactants are
compacted in presence of finely
dispersed catalyst are used in different
applications from fermentations to
production of chemicals &
pharmaceuticals. They have high
volumetric productivity & excellent heat

Mixing
 Bubble column reactors are widely used to
carry out multi-phase reactions. Mixing and
transport processes are the key issues in the
design of bubble columns, especially for
processes involving multiple reactions where
selectivity to the desired product is important.
Under such circumstances, liquid phase
mixing often decides the reactor performance.
The local flow field and turbulence governs the
fluid mixing and is interrelated in a complex
way with the design and operating parameters.

Mixing
 Both axial and radial mixing are possible in
bubble column reactor. Mixing in axial
direction is a function of aeration rate,
geometry of the column and the properties
of the fluid. Rising gas bubbles carry
elements of circulating fluid in bubble wakes
produce axial mixing. Because bubble rises
faster than the liquid, a certain amount of
liquid is carried forward faster than the bulk
flow of the liquid. This produces mixing in
the axial direction.
 For tubular reactors, axial mixing is usually
several times higher than radial mixing.
Thus, for most practical purposes, attention
is focused only on axial mixing.
 In case of radial mixing, bubbles may
impinge on the walls of the reactor and
break consequently with improvement of
mass transfer.

Airlift reactor
Air-lift bioreactors are similar to
bubble column reactors, but differ
by the fact
that they contain a draft tube.
The draft tube is always an inner
tube
or an external tube .This kind of
air-lift bioreactor is called "air-lift
bioreactor
with an external loop” which
improves circulation and oxygen
transfer and
equalizes shear forces in the
reactor.

Mixing
 Mixing method: Airlift
• In these reactors mixing circulation and
aeration is performed by gas injection and if
needed by additional external liquid
circulation to obtain the required mixing
pattern. The figure, gives an example of a
possible configuration. This usually results in
less shear for a given quality of mixing than in
stirred tanks. Air lifts give more vigorous
recirculation for the same air flow, but often
lower oxygen transfer rates than bubble
columns. To limit shear, small bubbles can be
used in aeration, but depending on conditions
this may cause excessive foaming and
requires more energy for their generation at
porous distributors.

Packed-bed reactor
 Packed-bed reactors are
used with immobilized or
particulate biocatalysts.
 Medium can be fed either
at the top or bottom and
forms a continuous liquid
phase. The advantage of
using a packed bed reactor
is the higher conversion per
weight of catalyst than
other catalytic reactors. The
conversion is based on the
amount of the solid catalyst
rather than the volume of
the reactor.

Mixing
 In packed bed reactors, cells are
immobilized on large particles. These
particles do not move with the liquid.
Packed bed reactors are simple to
construct and operate but can suffer
from blockages and from poor oxygen
transfer.
Continuous packed bed reactors are
the most widely used reactors for
immobilized enzymes eg.
Amiloglucosidase and immobilized
microbial cells. In these systems, it is
necessary to consider the pressure
drop across the packed bed or column,
and the effect of the column
dimensions on the reaction rate.

Trickle-bed reactor
The trickle-bed reactor is another
variation of the packed bed reactors.
Liquid is sprayed onto the top of the
packing and trickles down through
the bed in small rivulets.
It is considered to be the simplest
reactor type for performing catalytic
reactions where a gas and liquid
(normally both reagents) are present
in the reactor and accordingly it is
extensively used in processing plants.

Mixing
In a trickle bed reactor the liquid and gas phases flow
concurrently downwards through a fixed bed of catalyst
particles while the reaction takes place. In certain cases,
the two-phases also flow concurrently upwards. The
concurrent upward flow operation provides better radial
and axial mixing than the downward flow operation, thus
facilitating better heat transfer between the liquid and
solid phases. This is highly useful in exothermic reactions
where heat is required to be removed continuously from
the reactor. However, due to higher axial mixing in the
upward flow operation, the degree of conversion, a
crucial factor in the operation is preferred. Because of
lower axial mixing, better mechanical stability and less
flooding is achieved , thus facilitating processing of
higher flow rates and increased reactor capacity.

Flow Regimes
 Trickle bed reactors operate in a variety of
flow regimes ranging from gas-continuous
to liquid-continuous patterns. They usually
fall into two broad categories referred to as
low interaction regime (trickle flow regime)
and high interaction regime (pulse, spray,
bubble and dispersed bubble flow regimes).
The low interaction regime is observed at
low gas and liquid flow rates and is
characterized by a weak gas-liquid
interfacial activity and a gravity-driven
liquid flow. High interaction regime is
characterized by a moderate to intense gas-
liquid shear due to moderate to high flow
rate of one or both of the fluids. As a result,
various flow patterns arise depending on the
gas and liquid flow rates and the physical
properties of the liquid.
Schematic diagram of the trickle flow

Fluidized bed reactor
Fluidized bed reactor (FBR) is a type
of reactor device that can be used to
carry out a variety
of multiphase chemical reactions. In this
type of reactor, a fluid (gas or liquid)
is passed through a granular solid
material (usually a catalyst possibly
shaped as tiny spheres) at high
enough velocities to suspend the solid
and cause it to behave as though it were
a fluid. This process, known
as fluidization, imparts many important
advantages to the FBR.

Mixing
 The solid substrate (the catalytic material
upon which chemical species react) in the
fluidized bed reactor is typically supported
by a porous plate, known as a
distributor. The fluid is then forced through
the distributor up through the solid material.
At lower fluid velocities, the solids remain in
place as the fluid passes through the voids
in the material. As the fluid velocity is
increased, the reactor will reach a stage
where the force of the fluid on the solids is
enough to balance the weight of the solid
material. This stage is known as incipient
fluidization and occurs at this minimum
fluidization velocity. Once this minimum
velocity is surpassed, the contents of the
reactor bed begin to expand and swirl
around much like an agitated tank or boiling
pot of water. The reactor is now a fluidized

References
 NChE Journal (Vol. 21, No. 2)
 Wikipedia
 Braz. J. Chem. Eng. vol.31 no.1 São
Paulo Jan./Mar. 2014
 Trans IChemE, Part A, Chemical Engineering
Research and Design, 2004, 82(A10): 1367–
1374
 N. Kantarci et al. / Process Biochemistry 40
(2005) 2263–2283

FLUID FLOW AND MIXING IN BIOREACTOR

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