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DISTRIBUTION OF RESIDENCE TIMES
FOR REACTORS
PART I
BY PUAN AZDUWIN BINTI KHASRI
10 DECEMBER 2012
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NON-IDEAL REACTOR
Deviationsfrom idealreactor behavior.
EXAMPLE:
Tank Reactors: inadequate mixing, stagnantregions, bypassing or
short-circuiting
Tubular Reactors: mixing in longitudinal
direction, incomplete mixing in radialdirection, by-passing
(especially in Fixed bed
reactors)
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NON-IDEAL REACTOR-3 CONCEPT
RTD -The distribution of residence
times in the system
MIXING - The quality of mixing
MODEL - The model used to
describe the system
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RESIDENCE-TIME DISTRIBUTION (RTD)
The residence time is how long a particle staysin the reactor
once entering.
RTD-to characterize the mixing and flow
within reactors and to compare the behaviorof real reactors to
their ideal models.
RTD can be determined by twoexperimental
method:1. Pulse input experiment
2. Step input experiment
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1. PULSE INPUT EXPERIMENT
In a pulse input, an amount of tracer N0is suddenlyinjected in
one shot into the feedstream entering thereactor in as short a time
as possible. The outletconcentration is then measured as a function
of time.
We find the RTD function, E(t),
from the tracer concentration C(t)
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1. PULSE INPUT EXPERIMENT
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2.STEP TRACER EXPERIMENT
TheRTD function E(t) can be determined
directly from a pulse input, the cumulative
distribution F(t)can be determined directly
from a step input.
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INTEGRAL RELATIONSHIP
The cumulative
RTD function F(t)
Cumulative distribution
curve, F(t).RTD Function, E(t).
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MEAN RESIDENCE TIME AND
VARIANCE
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NORMALIZED RTD FUNCTION
A normalized RTD () is used instead of the function E(t).;
A dimensionless function E() can be defined as;
The purpose : the flow performance inside reactors
of different sizes can be compared directly.
If the E() is used,All perfectly mixed CSTRs have
numerically the same RTD. If the simple function E(t)
is used,numerical values of E(t) can differ
substantiallyfor different CSTRs
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For a perfectly mixed CSTR,
The value of E(t) at identical times can be quite different for
two
different volumetric flow rates v1 and v2.
The same value of E() is the same irrespective of the size of
a
perfectly mixed CSTR.
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RTD IN IDEAL REACTORS
1.RTDs in Batch and Plug-Flow Reactors
2.Single-CSTR RTD3.Laminar Flow Reactor (LFR)
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1. RTDs IN BATCH AND PLUG-FLOW
REACTORS
E(t) for a plug flow reactor
The Dirac delta function has
the following properties:
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2. SINGLE-CSTR RTD
E(t) and E()
for a CSTR
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3.LAMINAR FLOW REACTOR (LFR)
E(t) for a laminar flow reactor
Normalized RTD function for alaminar flow reactor
