2008 international ansys conference...– main reactions: zero order with gaseous reactant (beyond a...

© 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

2008 International ANSYS Conference

Improving Multiphase Reactors

Vivek V. Ranade*Tridiagonal Solutions Pvt. Ltd.100 NCL Innovation Park, Pune, Indiawww.tridiagonal.co.in* On leave from National Chemical Laboratory, Pune, India

http://www.tridiagonal.co.in/


Industrial Reactors

• What transformations are expected to occur?• How fast these transformations will occur?• What is the best way to carry out these

transformations?


Reactor Performance

• Performance Matrix • Performance Controlling Processes

– Mixing of reactants– Heat transfer– Contacting of multiple

phases– Mass transfer– Chemical reactions– Phase changes


Improving Reactor Performance

• RAPIDTM Program*: Reactor Analysis and Performance Improvement Diagnostics

– Critical analysis of laboratory and plant data– Mathematical modeling– Identify performance controlling step(s)– Evolving ideas for improvement– Computational evaluation of evolved ideas– Fine tuning and implementation

* Proprietary program of Tridiagonal Solutions Pvt. Ltd.


The RAPIDTM Methodology


Simulating Reactor Performance

• Link reactor performance to reactor hardware and operating protocols

– Solution of mass, momentum & energy conservation equations: CFD

Concentration profiles over time


Modeling and Simulation

• Uncertainties / Limitations– Inadequacies of the underlying mathematical model &

input data• Turbulence/ multiphase flows/ complex rheology/ reactions

– Inaccuracies of the numerical techniques– Computational constraints– Interpretation of results

• Despite the Limitations, Computational Modeling has Enormous Potential !

Necessary to develop appropriate methodology to harness this potential


Harnessing Power of Modeling

• Need to Develop & Use Multi-scale Modeling Capabilities

• Need to Use Multiple Models & Modeling Technologies

• Tridiagonal’s Approach of Compute for Innovation & Development

Tools+


Case Study: G-L-S Reactor

• Reactions– In presence of gaseous reactant

– Main reactions: zero order with gaseous reactant (beyond a critical concentration)

– Undesired reactions: NOT zero order– ‘B’ precipitates due to low solubility– Influenced by temperature and pH

A B C (desired product)

D (undesired products) E


G-L-S Reactor

• Need to understand and quantify time scales of different reactions and processes

– Concentration of gaseous reactant• Mass transfer: dispersion of gas, hold-up, (P/M)

– Dissolution of solids• Suspension/ solid-liquid mass transfer

– Temperature• Heat transfer: direct contact, jacket

– pH• Mixing

Need to quantitatively

understand multi-phase fluid dynamics

of GLS reactor


CFD Model of Existing Reactor

• Eulerian-Eulerian Approach– Single effective bubble size based on estimates of

interfacial area– Inter-phase coupling: need to include impact of

impeller generated turbulence• Drag coefficient as a function of Re and (dp/λ)

• Turbulent Flow– Two equation (dispersed phase) models

• Multiple Reference Frame approach


• Semi-batch reactor operation– Quantify Influence of

• Gas flow rate• Power per unit mass/ tip speed• pH• Addition rates• Operating temperature

Kinetics via Laboratory Experiments

Obtained simplified kinetic model0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25 30Time (hr)

Key

reac

tant

con

cent

ratio

n (m

ol/li

t)

Plant dataModel


Scale-up

• Translating findings from the laboratory reactor (5 lit) to the plant reactor (> 50 m3)– Develop and use CFD model to understand gas-liquid-solid flow in

industrial stirred vessel– Determine how key time and length scales change with increase in

reactor volume– Evolve ideas for improving performance

• Configuration, size and RPM of impeller(s)• Location of feed nozzles: gas, alkali and diluents• Manipulate heat transfer for desired temperature profile

– Evaluate and fine tune the recommendation

• Using GAMBIT and FLUENT


Reactor Configuration


Indirect Validation

• Power dissipation with and without gas • Pumping numbers with and without gas• Mixing

– pH measurements at different locations after alkali addition

• Overall gas hold-up

• Accepted CFD model with identified– Drag coefficients (on bubbles and solids), effective

bubble size, numerical parameters and strategies


Gaining Insight via CFD I


Gaining Insight via CFD II

• Understanding mixing for manipulating pH

• Influence of alkali addition nozzle (size/ location) and influence of reactor scale was investigated


Gaining Insight via CFD III

• Influence of sparger (size/ location) and influence of reactor scale was investigated


Improving Mixing

• Alkali addition sequence and mixing (prevailing pH) had significant influence on– Yield of desired product– Overall rate– Foaming

• CFD Model was used to improve mixing by– Manipulating impeller locations/ RPM– Designing foam breaker to ensure trouble free

operation


Improving Mass Transfer

• Gas-liquid (mainly for the first step) and solid-liquid (for the second step) mass transfer were important. Using developed CFD models:

– Identified roles of different impellers & manipulated locations and clearance between the two impellers

– Modified configuration and location of gas sparger– Special care was taken to handle accumulation of

precipitated solids on foam surface


Realizing Performance Enhancement

• Dilution strategy and pH profile was manipulated to realize– Significant reduction in batch time– Some increase in yield of desired product

• CFD model was used to identify most attractive alternative to realize performance enhancement with minimum investment for retrofitting


Summary

• CFD models and simulations provided valuable insight in understanding relevant time scales & rate controlling steps

• CFD simulations allowed virtual evaluation of alternative ideas to identify most effective solution for practical implementation

Tremendous Potential for Realizing Performance Enhancement in Practice by Judicious Use of CFD

2008 international ansys conference...– main reactions: zero order with gaseous reactant (beyond a...

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