BASIC DESIGN EQUATIONS FOR MULTIPHASE REACTORS

BASIC DESIGN EQUATIONS FOR MULTIPHASE REACTORS

2Starting Reference

1. P. A. Ramachandran and R. V. Chaudhari, Three-Phase Catalytic Reactors, Gordon and Breach Publishers, New York, (1983).

2. Nigam, K.D.P. and Schumpe, A., Three-phase sparged reactors, Topics in chemical engineering, 8, 11-112, 679-739, (1996)

3. Trambouze, P., H. Van Landeghem, J.-P. Wauquier, Chemical Reactors: Design, Engineering, Operation, Technip, (2004)

3Objectives

1.Review microkinetic and macrokinetic processes that occur in soluble and solid-catalyzed systems.

2.Review ideal flow patterns for homogeneous systems as a precursor for application to multiphase systems.

3. Derive basic reactor performance equations using idealflow patterns for the various phases.

4.Introduce non-ideal fluid mixing models.

5.Illustrate concepts through use of case studies.

4Types of Multiphase Reactions Gas-liquid without catalyst Gas-liquid with soluble catalyst Gas-liquid with solid catalyst Gas-liquid-liquid with soluble or solid catalyst Gas-liquid-liquid with soluble or solid catalyst (two liquid phases)StraightforwardComplex

Reaction TypeDegree of Difficulty

5Hierarchy of Multiphase Reactor ModelsEmpirical

Ideal Flow Patterns

Phenomenological

Volume-AveragedConservation Laws

Pointwise ConservationLawsStraightforwardImplementationInsightVery littleVery Difficultor ImpossibleSignificant

Model Type

6Macrokinetic Processes in Slurry ReactorsHydrodynamics of the multi-phase dispersion- Fluid holdups & holdup distribution- Fluid and particle specific interfacial areas- Bubble size & catalyst size distributionsFluid macromixing- PDFs of the various phasesFluid micromixing- Bubble coalescence & breakage- Catalyst particle agglomeration & attritionHeat transfer phenomena- Liquid evaporation & condensation- Fluid-to-wall, fluid-to-internal coils, etc.Energy dissipation- Power input from variouis sources(e.g., stirrers, fluid-fluid interactions,)ReactorModel

7Hydrodynamics of the multi-phase flows- Flow regimes & pressure drop- Fluid holdups & holdup distribution- Fluid-fluid & fluid-particle specific interfacial areas- Fluid distributionFluid macromixing- PDFs of the various phasesHeat transfer phenomena- Liquid evaporation & condensation- Fluid-to-wall, fluid-to-internal coils, etc.Energy dissipation- Pressure drop(e.g., stirrers, fluid-fluid interactions,)ReactorModel

Macrokinetic Processes in Fixed-Bed Reactors

8Elements of the Reactor Model

Micro or Local Analysis

Macro or Global Analysis

Gas - liquid mass transfer

Liquid - solid mass transfer

Interparticle and interphase mass transfer

Intraparticle and intraphase diffusion

Intraparticle and intraphase heat transfer

Catalyst particle wetting

Flow patterns for the gas, liquid, and solids

Hydrodynamics of the gas, liquid, and solids

Macro distributions of the gas, liquid and solid

Heat exchange

Other types of transport phenomena

9Reactor Design Variables Reactor ProcessReaction Flow = f Performance Variables Rates Patterns

Conversion Flow rates Kinetics Macro

Selectivity Inlet C & T Transport Micro

Activity Heat exchange

FeedReactorQinTinCinProductQoutToutCout

10Ideal Flow Patterns for Single-Phase Systems

Q (m3/s)Q (m3/s)

Q (m3/s)

Q (m3/s)a. Plug-Flow b. Backmixed Flow

11Impulse Tracer Response

Q (m3/s)Q (m3/s)Reactor System

tx(t)

MT t

ty(t)

Fraction of the outflow with aresidence time between t and t + dtE(t) is the P.D.F. of the residence time distribution

Tracer mass balance requirement:

12

Fluid-Phase Mixing: Single Phase, Plug Flow

Q (m3/s)

13

Fluid-Phase Mixing: Single Phase, Backmixed

Q (m3/s)Mi = Mass of tracer injected (kmol)

14Idealized Mixing Models for Multiphase ReactorsModel Gas-Phase Liquid Phase Solid-Phase Reactor Type

1 Plug-flow Plug-flow Fixed Trickle-Bed Flooded-Bed

2 Backmixed Backmixed Backmixed Mechanically agitated

3 Plug-Flow Backmixed Backmixed Bubble column Ebullated - bed Gas-Lift & Loop

15Ideal Flow Patterns in Multiphase ReactorsExample: Mechanically Agitated Reactors

VR = vG + VL + VC 1 = G + L + Cor

16First Absolute Moment of theTracer Response for Multiphase SystemsFor a single mobile phase in contact with p stagnant phases:

For p mobile phases in contact with p - 1 mobile phases:

is the partition coefficient of the tracerbetween phase 1 and j

17Relating the PDF to Reactor PerformanceFor any system where the covariance of sojourn times is zero(i.e., when the tracer leaves and re-enters the flowing stream atthe same spatial position), the PDF of sojourn times in the reactionenvironment can be obtained from the exit-age PDF for a non-adsorbing tracer that remains confined to the flowing phaseexternal to other phases present in the system.

For a first-order process:

Hp(kc) = pdf for the stagnant phase

18Illustrations of Ideal-Mixing Modelsfor Multiphase Reactors

zGL Plug-flow of gas Backmixed liquid & catalyst Batch catalyst Catalyst is fully wetted

zGL

Plug-flow of gas Plug-flow of liquid Fixed-bed of catalyst Catalyst is fully wettedStirred tank

Bubble Column

Trickle - Bed

Flooded - Bed

19

Intrinsic Reaction Rates

Reaction Scheme: A (g) + vB (l) C (l)

20

zGL

Gas Limiting and Plug-Flow of Liquid1. Gaseous reactant is limiting

2. First-order reaction wrt dissolved gas

3. Constant gas-phase concentration

4. Plug-flow of liquid

5. Isothermal operation

6. Liquid is nonvolatile

7. Catalyst concentration is constant

8. Finite gas-liquid, liquid-solid, and intraparticle gradientsKey Assumptions

21

Gas Limiting and Plug flow of liquid

Constant gas phase concentration valid for pure gas at high flow rateConcentration or Axial HeightRelative distance from catalyst particle

(Net input by convection)(Input by Gas-Liquid Transport)(Loss by Liquid-solid Transport)+-= 0(1)(2)(3)(4)Dividing by Ar.dz and taking limit dz

22

Gas Limiting and Plug flow of liquid

23

Gas Limiting and Plug flow of liquid Solving the Model Equations

24

Concept of Reactor Efficiency

Rate of rxn in the Entire Reactor with Transport EffectsMaximum Possible Rate

25

Conversion of Reactant B(in terms of Reactor Efficiency)

26Gas Limiting and Backmixed Liquid

zGL1. Gaseous reactant is limiting

2. First-order reaction wrt dissolved gas

3. Constant gas-phase concentration

4. Liquid and catalyst are backmixed

5. Isothermal operation

6. Liquid is nonvolatile

7. Catalyst concentration is constant

8. Finite gas-liquid, liquid-solid, and intraparticle gradientsStirred Tank

Bubble Column

Key Assumptions

27

Gas Limiting and Backmixed Liquid

Concentration or Axial HeightRelative distance from catalyst particleConcentration of dissolved gas in the liquid bulk is constant [f(z)] [=Al,0]Concentration of liquid reactant in the liquid bulk is constant [f(z)] [=Bl,0]A in liquid bulk: Analysis is similar to the previous case

28

Gas Limiting and Backmixed Liquid

A at the catalyst surface:

For Reactant B:(Note: No transport to gas since B is non-volatile)

(Net input by flow)(Rate of rxn of B at the catalyst surface)=

29

Gas Limiting and Backmixed LiquidSolving the Model Equations

30Flow Patterns Concepts for Multiphase Systems

A

B

A - Single phase flow of gas or liquid with exchange between the mobile phase and stagnant phase.Fixed beds, Trickle-beds, packed bubble columns

B - Single phase flow of gas or liquid with exchange between a partially backmixed stagnant phase.Semi-batch slurries, fluidized-beds, ebullated beds

31Flow Patterns Concepts for Multiphase Systems

CD

E

C, D - Cocurrent or countercurrent two-phase flow with exchange between the phases and stagnant phase.Trickle-beds, packed or empty bubble columns

E - Exchange between two flowing phases, one ofwhich has strong internal recirculation.Empty bubble columns and fluidized beds

32

Axial Dispersion Model (Single Phase)

Basis: Plug flow with superimposed diffusional transport in the direction of flow

@ z = 0

@ z = L

Let

@ = 0

@ = 1