dynamics of laminar mixing in continuous stirred tank reactors
TRANSCRIPT
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
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.
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)
Short Circuiting in Continuous Flow TanksDye Experiments versus Particle Tracking (CFD - ORCA)
Particle Tracking (ORCA)
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.