Dynamics of Laminar Mixing in Continuous Stirred Tank Reactors

Dynamics of Laminar Mixing inContinuous Stirred Tank Reactors

Paulo Arratia, Joe Kukura &Fernando Muzzio

Department of Chemical and Biochemical Engineering 98 Brett Road, Piscataway, New Jersey 08854-8058

• Segregated zones (toroidal regions) in a Newtonian fluid

(a)

(b)

Laminar Batch System

Rushton impeller Structures in the toroidal regions

• It runs at steady state with continuousflow of reactants and products.• Uniform composition and temperatureare assumed throughout the reactor.• Its exit stream has the samecomposition as the entire tank volume.• Popular when temperature control iscritical and when conversion must takeplace at a constant composition.

Continuous Stirred Tank Reactor

Acid-Base Reaction pLIF

Continuous Flow Stirred Tank Reactor

Objectives

• To investigate experimentally and computationally themixing of viscous Newtonian fluids under laminar flowconditions in a CSTR.

• To examine the effect of inlet/outlet stream positioningand Reynolds number on the dynamics of the mixingprocess.

Top View

24 cm

7.5 cm 7.5 cm

(A) (C) (P)

Experimental Set-up• Volume=15 Liters• Flow Rate = 4.75liters/min• Residence Time = 3.15 min• Fluid - Glycerin (800 cP)• Re=67 (100 RPM)• Re=134 (200 RPM)Experimental Techniques:• Acid-Base Reactions• UV-Fluorescence• pLIF• Particle Image VelocimetryComputational Fluid Dynamics

(a) (b) (c)

Continuous Stirred Tank Reactor Base Flow

• Flow pattern produced by ‘base-flow’ visualized using acid/basereaction (a,b) and UV-Fluorescence (c).• Clear formation of an asymmetric re-circulating zone at the top of thetank due to flow expansion.

CSTR Across Case

Re=67

Re=134

Time

2 minutes 12.5 minutes7 minutes

Time

2 minutes 7 minutes 12.5 minutes

CSTR Parallel Case

Re=67

Re=134

Time

2 minutes 12.5 minutes

CSTR Center Case

Re=67

Re=134

7 minutes

Continuous Stirred Tank Mixing Efficiency Plot

0

20

40

60

80

100

0 1000 2000 3000 4000 5000 6000

Revolutions

Frac

tiona

l Are

a Co

vera

ge, %

Re 67 (C)

Re 67 (A)

Re 67 (P)

Re 134 (C)

Re 134 (A)

Re 134 (P)

Efficiency

Evolution of Lower Segregated Region

0

4

8

12

16

20

0 1000 2000 3000 4000 5000 6000

Revolutions

% A

rea

Size

of L

ower

Tor

oida

l Reg

ion Re 67 (C)

Re 67 (A)Re 67 (P)Re 134 (C)Re 134 (A)Re 134 (P)

Continuous Stirred Tank - Flow AsymmetryPIV Experiments & CFD (ORCA)

Continuous Stirred Tank - Flow AsymmetryAcid/Base Experiments & CFD (ORCA)

Short Circuiting in Continuous Flow TanksDye Experiments versus Particle Tracking (CFD - ORCA)

Particle Tracking (ORCA)

Batch Continuous Dye Experiments

Short Circuiting Phenomena inContinuous Flow Stirred Tanks

Conclusions

• We provided experimental and computational evidence ofsegregated zones in CSTR’s.

• Asymmetric flow patterns were produced by carefullypositioning inlet/outlet flow.

• Counter-intuitively, toroidal regions were not destroyed athigher Re.

• A short-circuiting phenomenon was observed bothexperimentally and computationally.