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.

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

dilution has more effect than the substrate

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

transfer properties.

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

bed.

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