bubble column reacter

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Bubble column reactors

• Basudha Maurya
• vasundhre_at_gmail.com
• Department of Chemical Engineering
• MNNIT, Allahabad

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Topics Covered

• Introduction to Heterogenous fluid-fluid reaction
• Bubble column fundamentals
• The general rate expression
• Design Equation
• Axial dispersion model
• Designing factors
• Mixing and RTD
• Practical Examples, Advantages
• and Disadvantages

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Heterogenous Fluid-fluid reactions

• Reasons for the use of Fluid-fluid reactions
• 1. the product of reaction may be a desired
  material.
• 2. to take place to facilitate the removal of
  an unwanted component from a fluid.
• 3. to take place to facilitate the removal of
  an unwanted component from a fluid.

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Heterogenous Fluid-fluid reactions

• Factors determining how to approach
• 1. The Overall Rate Expression. Since
  materials in the two separate phases must contact
  each other before reaction can occur, both the
  mass transfer and the chemical rates will enter
  the overall rate expression.
• 2. Equilibrium Solubility. solubility of the
  reacting components will limit their movement
  from phase to phase and it will determine
  whether the reaction takes place in one or both
  phases.
• 3. The Contacting Scheme. semibatch and
  countercurrent contacting schemes predominate but
  batch contacter are also used

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Type Of fluid-fluid Reacter
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Contacting patterns for g/L contactors.
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Bubble column reactors

• Why bubble column reactors?
• -Broad range of potential applications in
  the chemical, petrochemical and biochemical
  industries.
• What is bubble column reactors?
• -Bubble columns are devices in which gas, in the
  form of bubbles, comes in contact with liquid.
• -The purpose may be simply to mix the liquid
  phase.
• -Substances are transferred from one phase to the
  other

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Bubble column reactors
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Bubble column reactors

• -Simple vertical cylindrical vessels with intense
  contact betweenn the two phases.
• -The gas phase is dispersed into the liquid
  phase using specific gas distributorss at the
  bottom of the column.
• -The net liquid flow may be co-current or
  counter-current to the gas flow direction or may
  be zero.
• -Spargers, like porous plates, generate
  uniform size bubbles and distribute the gas
  uniformly at the bottom of the liquid pool.

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Type of Bubble Columns

• A).Simple bubble column B) Cascade bubble
  column with sieve trays C) Packed bubble
  column D) Multishaft bubble column E) Bubble
  column with static mixers

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Contacting Patterns
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Gas Distributions

• The gas is dispersed to create small bubbles and
  distribute them uniformly over the cross section
  of the equipment to maximize the intensity of
  mass transfer.
• The formation of fine bubbles is especially
  desirable in coalescence-hindered systems and in
  the homogeneous flow regime.
• In principle, however, significant mass transfer
  can be obtained at the gas distributor through a
  high local energy-dissipation density.

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Static Gas Spargers
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Dynamic Gas Spargers
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Concentration profile

• There is a concentration drop around the
  spherical bubble because it is migrating outward,
  
• At the planar gas-liquid interface there should
  be a discontinuity in CA at the interface due to
  the solubility of species A and a consequent
  equilibrium distribution between phases.

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Gas Holdup

• Gas holdup is one of the most important operating
  parameters because it not only governs phase
  fraction and gas-phase residence time but is also
  crucial for mass transfer between liquid and gas.
  
• Gas holdup depends chiefly on gas flow rate, but
  also to a great extent on the gas  liquid system
  involved.

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Gas Holdup

• Gas holdup is defined as the volume of the gas
  phase divided by the total volume of the
  dispersion     
• The relationship between gas holdup and gas
  velocity is generally described by the
  proportionality
• In the homogeneous flow regime, n is close to
  unity. When large bubbles are present, the
  exponent decreases, i.e., the gas holdup
  increases less than proportionally to the gas
  flow rate.

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Interphase Forces

• Drag force
• -Resultant slip velocity between two phases.
• Virtual mass force
• -Arising from the inertia effect.
• Basset force
• -Due to the development of a boundary layer
  around a bubble.
• Transversal lift force
• -Created by gradients in relative velocity
  across the bubble diameter, may also act on the
  bubble.

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Interface behaviour for the liquid-phase reaction

• Case A Instantaneous reaction with low C,
• Case B Instantaneous reaction with high CB
• Case C Fast reaction in liquid film, with low CB
  
• Case D Fast reaction in liquid film, with high
  C,
• Case E and F Intermediate rate
• with reaction in the film and in
• the main body of the liquid
• Case G Slow reaction in main
• body but with film resistance
• Case H Slow reaction, no mass
• transfer resistance

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Conti
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The General Rate Equation

• Assumptions
• - gaseous A is soluble in the liquid but that
  B does not enter the gas.
• -solubility of gasous phase obeys Henerys Law
  
• -reaction may occur in the liquid film , in
  the bulk liquid or in both
• -Whitmans two fim theary applicable

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The General Rate Equation

• The overall rate expression for the reaction
  will have to account for the mass transfer
  resistance (to bring reactants together) and the
  resistance of the chemical reactions step.
• Considers the following second-order reaction

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Conti
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Conti

• Gas film, liquid film and the main body of liquid
  act as resistance in series.
• For these three steps we can write

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Conti
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Conti

• Hence the overall rate expression is
• Where

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The Rate Equation for Straight Mass Transfer
(Absorption)

• No chemical reaction takes place
• Also known as Physical Absorption
• Here we have two resistances in series, of the
  gas film and of the liquid film.
• The final rate
• equation is as follows

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Design Equation For Bubble Column Reactor

• We assume that the rate is fast enough so that
  no unreacted A enters the main body of the
  liquid. This assumes that the Hatta modulus is
  not very much smaller than unity.
• Here we must make two accountings, a differential
  balance for the loss of A from the gas because G
  is in plug flow, and an overall balance for B
  because L is in mixed flow.
• Focusing on rising gas, we have
  
• 
  
  
  
  
• 
  
  ..(i)
• Where moles A/mole
  inert in the gas
  

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Conti

• For the liquid as a whole and for the gas as a
  whole, a balance about the whole reactor gives
• 
  ..(ii)

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Conti

• Integrating Eq.(i) along the path of the bubble
  and also using Eq.(ii) gives

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Axial Dispersion in Bubble Column Reacter

• Most realistic model is that of dispersed
  plug-flow in both phases but this is also the
  most complicated model.
• simple axial dispersion is reasonably well in
  practice.
• Because the residence time of the liquid phase
  in the column is usually much longer than that of
  the gas, hence liquid phase will be well-mixed
  even when the gas phase is not.
• In our case reaction is very fast and occurs
  wholly within the liquid film surrounding the
  bubbles and concentration in the bulk of the
  liquid of the species A is zero, and the mixing
  pattern in the liquid has therefore no influence
  on the rate of transfer.

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Axial Dispersion in Bubble Column Reacter Model
Development

• Assumptions
• - The fluid velocity and the concentrations
  of any dissolved species are assumed to be
  uniform at any cross-section of the pipe,
• -Mixing or dispersion in the direction of
  flow (i.e. in the axial r-direction) is taken
  into account
• - fluid have a constant density so that the
  mean velocity u is constant.

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Conti

• The axial mixing is described by exactly the same
  way as in molecular diffusion.
• where DL is the dispersion
• coeficient in the longitudinal
• direction.
• Fluid near the centre of
• the pipe travels more quickly
• than that near the wall, the
• overall result being mixing in
• the axial direction.

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Conti

• Axial dispersion is caused primarily by
  differences in velocity at different radial
  positions rather than by turbulent eddy.
• system is not in a steady state with respect to
  the tracer distribution, the concentration will
  vary with both z the position in the pipe and, at
  any fixed position, with time.
• Consider a material balance between z and (z
  dz), in a time interval dt unit area of
  cross-section.

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Conti
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• For very fast steady state gas-liquid reaction,
  the reactant A is transferred thus removed from
  the gas phase at a rate which is proportional to
  the concentration of A in the gas, i.e. as in a
  homogeneous first-order reaction.
• so we have
• Reaction vessel is considered to be closed,
  i.e. reaction is assumed to be confined to the
  reaction vessel itself

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Conti

• Boundary conditions
• - rate of transfer is made up of two
  contributions, the convective flow
  diffusion-like dispersive flow
• 1. Across the plane at the inlet to the reactor,
  these two fluxes must be equal which gives

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Conti

• 2. Now consider the outlet pipe from the reactor
• fluid leaving the reactor must have the same
  concentration of reactant as the fluid just
  inside the outlet plane, which yields
• To solve the differential on substituting
• in the diff eq.() we get

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Conti
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Axial Dispersion in Bubble Column Reacter

• For bubble column two-phase gas-liquid system,
• Where Gas hold-upfraction of the tube
  cross-sectional area occupied by the gas, i.e.
  the region in which gas dispersion occurs.
• Empirical equation for the gas-phase dispersion
  coefficient is

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Conti

• Empirical equation for the gas-phase dispersion
  coefficient is
• For two-phase bubble column for steady-state
  conditions
• reactant A at the exit inlet of the column
• where

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Conti

• The rate of transfer per unit volume of
  dispersion is thus
• From Henry law,
• From the ideal gas law the gas-phase
  concentration
• By using above eq. we have

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Factors to be considered for the Design Of
Bubble Column Reactor

• (a) Contacting pattern. Bubble tanks approximate
• plug G/mixed L.
• (b) kg and kl. For liquid droplets in gas kg is
  high, k, is low. For gas bubbles rising in liquid
  kg is low, k, is high.
• (c) Flow rates. more flexible in that they work
  well in a wider range of Fl/Fg values.
• (d) If the resistance is in the gas film
  dominates stay away from bubble contactors.

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Conti

• (e) Solubility.For gases of low solubility in the
  liquid, thus high H value liquid film controls
• (f) Reaction lowers the resistance of the liquid
  film,
• so
• -For absorption of highly soluble gases,
  chemical reaction is not helpful.
• -For absorption of slightly soluble gases,
  chemical reaction is helpful and does speed up
  the rate.
• (g) Kinetic Constant of the Reaction. The
  kinetics of the reaction need to be known or
  measured which may be affected by temperature.

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Mixing in bubble phases and Residence time
distribution

• If an isolated bubble rises in the reactor, then
  the flow pattern in this phase is clearly
  unmixed, and this phase should be described as a
  PFTR.
• Possible flow patterns
• 1. In simplest case ,an isolated bubble which
  rises a clearly unmixed situation.

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Conti

• 2. 2. If bubbles flow upward, but continuously
  break up and coalesce, the residence time
  distribution of the species in this pase is
  narrow or roughly that of a PFTR,
• 3. If the bubble is in a continuous phase which
  is being stirred, then in the limit of very rapid
  stirring, the residence time distribution will be
  same as in CSTR.
• 4. However with stirring and coalescence and
  breakup, both effects tend to mix the contents of
  the bubbles or drops

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CFD Modeling of Bubble Columns

• Eulerian-Lagrangian approach
• -To simulate trajectories of individual
  bubbles (bubble-scale phenomena)
• Eulerian-Eulerian approach
• -To simulate the behavior of gas-liquid
  dispersions with high gas volume fractions (e.g.
  to simulate millions of bubbles over a long
  period of time)

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High Aspect-Ratio Bubble Columns

• Bubble column with a low aspect ratio or a
  single- impeller agitated tank behaves
  essentially as a well-mixed reactor
• A much more desirable situation is the gas to be
  in a state of plug flow.
• To approximate to plug flow, the logical
  development is to use a bubble column which is
  tall in relation to its diameter.
• if the same volume of liquid is contained in a
  tall, smaller diameter column, there are two
  advantages
• 1. probability that the gas (and possibly
  the liquid) will be more nearly in a state of
  plug flow.

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Conti
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Conti

• 2.Assuming that gas is supplied at the same
  volumetric flowrate
• -superficial gas velocity through the column
  will
• be increased.
• -specific interfacial area and the gas
  hold-up also
• increase with superficial gas velocity
• -which increase in the rate of reaction per
  unit
• volume of dispersion.
• One disadvantage of a tall column is the cost of
  compressing the gas to overcome the additional
  hydrostatic head.

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• Practical examples of reactions taking place
  in bubble column reactor
• oxidation reactions (e.g. oxidation of
  cyclohexane to adipic acid, partial oxidation of
  ethylenee to acetaldehyde, oxidation of
  n-parrafins to sec-alcohols)
• hydrogenation reactions (e.g. saturation of fatty
  acids, hydrogenation of glucose to sorbitol)
• chlorination reactions (production of aliphatic
  and aromatic chlorinated compounds)
• hydrotreating and conversion of petroleum
  residues
• Fermentation (production of ethanol and mammalian
  cells)
• biological waste water treatment
• oxidesulfurization of coal
• oxichlorination of ethylene to dichlorethane
• Fischer-Tropsch synthesis
• methanol synthesis
• polymerisation of olefins

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• Advantages offered by bubble column reactor
• efficient contact between the phases, the gas
  and the liquid, and eventually the third phase,,
  the solid catalyst
• high liquid hold up, recommended for reactions
  taking place in the liquid phase (as the casee of
  bubble columns)
• reasonable inter-phase mass transfer rates at low
  energy input
• limitation of pressure drop
• easy temperature control
• little maintenance due to the simple construction
  
• lack of moving parts
• high adaptability for a specific process
• no serious erosion and plugging problems due to
  the catalyst
• low costs of construction and operation

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• Disadvantages of bubble column reactor
• considerable degree of backmixing in both the
  liquid and the gas phase
• short gas phase residence time
• higher pressure drop with respect to packed
  columns
• rapid decreasing of interfacial area above values
  of the aspect ratio greater than, say 12, due to
  the increased rate of coalescence
• Scale up is still poorly understood

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References

• Coulson Richardson's CHEMICAL ENGINEERING
  VOLUME 3 THIRD EDITION Chemical Biochemical
  Reactors Process Control , J. F. RICHARDSON
  ,Department of Chemical Engineering,University of
  Wales Swansea and D. G. PEACOCK ,The School of
  Pharmacy,London
• Chemical Reaction Engineering Third
  Edition,Octave LevenspielDepartment of Chemical
  Engineering,Oregon State University
• THE ENGINEERING OF CHEMIlCAL REACTIONS,LANNY D.
  SCHMIDT,University of Minnesota
• Scaling up bubble column reactors ,M.I.
  Urseanu ,Vant Hoff Institute for Molecular
  Sciences (HIMS)
• Bubble Coulmn,Quak Foo Lee ,Department of
  Chemical and Biological Engineering,The
  University of British Columbia

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• Thank You