Ahmed Abdala

Department of Chemical Engineering

The Petroleum Institute

Abu Dhabi, UAE

In order to design nonideal reactors, we need to combine kinetics. RTD, and flow models to predict the reactor size or conversion

The models must be: ◦ Mathematically tractable

◦ Describes the nonideal reactor realistically

◦ Have no more than two parameters

Models ◦ Compartment model

◦ Dispersion model

◦ Tanks in series

◦ Convection model with laminar flow

◦ Earliness of mixing, segregation, and RTD

In compartment model, the reactor volume, V, is

divided into ◦ Active volume, Va

Plug flow region, Vp

Mixed flow region, Vm

◦ Dead or stagnant region volume, Vd

The total flow rate, v is ◦ Active flow, va

◦ Bypass flow, vb

◦ Recycle flow, vr

From the E or F curve, we can find the different volumes ◦ If the tracer amount, M

𝑀 = 𝑣 (𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐𝑢𝑟𝑣𝑒)

◦ if we only measure the output C on an arbitrary scale

M cannot be found

We must know both V and v to properly evaluate all the elements of a model, including dead spaces

If only 𝑡 𝑜𝑏𝑠 is measured, we cannot find the size of these stagnant regions and must ignore them in our model ◦ If the real vessel has dead space: 𝑡 𝑜𝑏𝑠 < 𝑡

𝑡 𝑜𝑏𝑠=𝑉𝑎𝑐𝑡𝑖𝑣𝑒

𝑣

𝑡 =𝑉

𝑣

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Ideal PFR PFR with Dead zone PFR with bypass

PL PL

Ideal MFR

MFR with Dead zone MFR with bypass

PL PL

Two PFR in parallel Two PFR with recycle and deadzone

PL

Two idea MFR in parallel

PL

The semilog plot is a convenient tool for

evaluating the flow parameters of a mixed flow

compartment.

Draw the tracer response curve on this plot

Find the slope and intercept

This gives the quantities A, B, and C

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A pulse of concentrated NaCl solution is

introduced as tracer into the fluid entering a

vessel (V = 1 m3, v = 1 m3/min) and the

concentration of tracer is measured in the fluid

leaving the vessel. Develop a flow model to

represent the vessel from the tracer output data

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The second order aqueous reaction A + B →R + S

is run in a large tank reactor (V = 6 m3) and for

an equimolar feed stream (CA0 = CB0) conversion

of reactants is 60%. Unfortunately, agitation in our

reactor is rather inadequate and tracer tests of the

flow within the reactor give the flow model

What size of mixed flow reactor will equal the

performance of our present unit?

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