FLUIDIZED BED REACTORS

Chemical Reaction Engineering-II (2170501)

Semester-VII, AY:2020-21
Presented by:
Guided by:
170110105038 - Yash Patel
170110105041 - Shubham Prajapati Dr. Mathur kumar Bhakhar
170110105042 - Manan Raichura Professor,
170110105045 - Dharmik Shah Department of Chemical
Engineering,
GCET, Vallabh Vidyanagar.

G H Patel College of Engineering And Technology

Different types of Fluidized beds:
Geldart (1973) and Geldart and Abrahamson (1978) looked at how different kinds of
solids behaved when fluidized, and came up with the following simple classification
of solids which we now call the Geldart classification.
THE BUBBLING FLUIDIZED BED-BFB
Passing gas upward through a bed of fine particles. For superficial (or inlet) gas velocities u, much in
excess of this minimum the bed takes on the appearance of a boiling liquid with large bubbles rising
rapidly through the bed. In this state we have the bubbling fluidized bed.

But the bubbling bed represents severe deviations from ideal contacting and so a wide variety of
approaches have been tried.

● Dispersion and Tanks in Series Model

● RTD models
● Two-Region models
● Hydrodynamics Flow models
Dispersion and Tanks in Series Model
The first attempts at modeling naturally tried the simple one-parameter models; however, observed
conversion well below mixed flow cannot be accounted for by these models so this approach has been
dropped.

RTD Models
The next class of models relied on the RTD to calculate conversions. But since the rate of catalytic
reaction of an element of gas depends on the amount of solid in its vicinity, the effective rate constant is
low for bubble gas, high for emulsion gas. Thus any model that simply tries to calculate conversion
from the RTD and the fixed rate constant in effect assumes that all elements of gas, both slow and fast
moving, spend the same fraction of time in each of the phases is not completely correct. Hence the
direct use of the RTD to predict conversions is quite inadequate.
Contact Time Distribution Models
To overcome this difficulty and still use the information given by the RTD, models were proposed
which assumed that faster gas stayed mainly in the bubble phase, the slower in the emulsion. Gilliland
and Knudsen (1971) used this approach and proposed that the effective rate constant depends on the
length of stay of the element of gas in the bed,

Thus k = k0tm , where m is a fitted parameter.

We find,

But the problem with this approach involves obtaining a meaningful E function to use in above Eq.
from a measured C-curve. So this approach has also been discarded.
Two-Region Models
Recognizing that the bubbling bed consists of two rather distinct zones, the bubble phase and the
emulsion phase, experimenters spent much effort in developing models based on this fact. Since such
models contain six parameters, many simplifications and special cases have been explored (eight by
1962,15 by 1972, and over two dozen to date), and even the complete six-parameter model of Fig. has
been used.
The users of this model, those dealing with FCC
reactors, claim that this model fits their data beautifully.
However, they had to choose different sets of parameter
values for each crude oil feed, in each of their FCC
(Fluid Catalytic Reactor) reactors. Also some of the
values for their parameters made no physical sense, for
example, a negative value for V1 or v2.

With this as the situation which gives a perfect fit but

predicts nothing, and brings no understanding with it,
this model is also discarded. The reason is that we have
no idea how to assign values to the parameters for new
conditions. Thus this is just a curve-fitting model.

FCC REACTOR
Hydrodynamic Flow Models
Two developments are of particular importance in this model. The first is Davidson's remarkable
theoretical development and experimental verification of the flow in the vicinity of a single rising
bubble in a fluidized bed which is otherwise at minimum fluidizing conditions. What he found was that
the rise velocity of the bubble ubr depends only on the bubble size, and that the gas behavior in the
vicinity of the bubble depends only on the relative velocity of rising bubble and of gas rising in the
emulsion ue. In the extremes he found completely different behavior, as shown in Fig.
For catalytic reactions we are only interested in fine particle beds, so let us ignore the large
particle extreme from now on.

Now, for the fine particle bed gas circulates within the bubble plus a thin cloud surrounding the
bubble. Thus the bubble gas forms a vortex ring and stays segregated from the rest of the gas in
the bed. Theory says that

The second finding on single bubbles is that every rising gas bubble drags behind it a wake of solids.
We designate this wake by a, where
THE K-L MODEL FOR BFB

Hydrodynamic type flow models can be developed to represent the BFB, based on the above two
seemingly simple findings. Let us consider the simplest of these, the K-L BFB model.

Pass an excess of gas upward through a bed of fine particles. With a large enough bed diameter we get
a freely bubbling bed of fast bubbles.

As simplifications, assume the following:

● The bubbles are all spherical, all of the same size db, and all follow the Davidson model. Thus the
bed contains bubbles surrounded by thin clouds rising through an emulsion. We ignore the upflow
of gas through the cloud because the cloud volume is small compared to that of the bubble. This is
the regime where ub >> ue.
 ● The emulsion stays at minimum fluidization conditions, thus the relative G/S velocity stays
constant in the emulsion.

● Each bubble drags up a wake of solids behind it. This generates a circulation of solids in the bed,
upflow behind the bubbles, and downflow everywhere else in the bed. If this downflow of solids
is rapid enough then gas upflow in the emulsion is impeded, can actually stop, and even reverse
itself. Such downflow of gas has been observed and recorded, and occurs when,

We ignore any upflow or downflow of gas in the emulsion.

Let u, = superficial gas velocity in the bed, m3 gas / m2 bed . s
d = diameter, m
E = fraction of voids in the bed
subscripts b, c, e, w refer to bubble, cloud, emulsion, and wake, respectively. Subscripts m, mf and f
refer to packed bed, minimum fluidization,and bubbling fluidized bed conditions, respectively
Inessence, given umf , emf , u0 , a, and the effective bubble size in the bed db , this model tells all the other
properties of the bed-flows, region volumes, interchange rates, and consequently reactor behavior.

By Using gas solid material balance, Davidson's theoretical expression for bubble cloud circulation and
the higbie theory for cloud emulsion diffusion we will get the height of the BFD is
Advantages of Fluidized Beds
● The smooth, Liquid like flow of particles allows continuous automatically
controlled operations with ease of handling.

● The rapid mixing of solids leads to nearly isothermal conditions throughout

the reactor, hence the operation can be controlled simply and reliably.

● Highly suitable for large scale operations.

● Heat and mass transfer rates between gas and particles are when compared
to the other modes of contacting.

● The internal heat exchangers require a small amount of contact surface area
due to complete distribution and ample mixing.
Disadvantages of Fluidized Beds

● The difficult to describe flow of gas, with its large deviation from plug flow
and the bypassing of the particles bubble formation represents inefficient
contacting systems.

● The rapid mixing of solids provide good mass transfer but leads to non
uniform residence times in the reactor.

● Erosion of pipes and vessels from abrasion of particles.

● For non-catalytic operations at higher temperatures the agglomeration and

sintering of fine particles.
Commercial Applications:
● Solid-Catalysed Gas-Phase Reactions:
○ Fluid Catalytic Cracking, Reforming.
○ Fischer-Tropsch Synthesis
○ Oxidation of SO2 to SO3

● Gas-Solid reactions:
○ Roasting of ores.
○ Combustion and incineration.
○ Gasification, Coking and Pyrolysis.

● Gas-Liquid-Solid:
○ Hydrotreating, Biochemical Processes etc.
Circulating Fluidized Bed Reactor (CFB)
This type of reactors are quite different from the conventional fluidized bed
reactors.
These beds allow the gas velocity to be in the turbulent region and thus it
differs in the following ways:
● Vertical upflow reactor column with high superficial gas velocities
● No distinct upper bed surface in the column
● The gas, rather than an emulsion of solid particles with the interstitial
gas, forms a continuous phase.
● There are some internal or external means to be provided to return the
large quantity of entrained particles back to its base.

The CFBs include mainly 4 types of reactors...

Turbulent Bed Reactors (TBs)
● At higher gas velocities, the BFBs transform into TBs where there is no
distinct bubble movements, much churning and violent solid movements.
● The surface of the dense bed fades and the solids are found increasingly in
the lean region above the dense beds.
● The flow of gas in the dense region is somewhere in between the BFBs and
the plug flow.
Fast Fluidized Bed Reactors (FFBs)
● At even higher gas velocity, the bed enters FF regime. Its main
characteristic is that the entrainment of solids rises dramatically at this
point in the transition.
● In the FF regime the solid movement in the lower region of the vessel
becomes less chaotic and seems to settle toa lean zone surrounded by a
denser annulus or wall zone.
Pneumatic Conveying (PC)
● At the highest gas velocities, the bed is in pneumatic transport where the
particles are well distributed in the reactor with no wall or downflow
region.
● In this case we can assume plug flow of solids and of the gas up the
vessel.
Fluidized Catalytic Crackers (FCCs)
● They are one of the most important and widely used large scale reactors.
● These reactors take long chain hydrocarbons and crack them to produce a
whole range of short chained hydrocarbons.
● These can operate at very high gas phase velocities and remain close to
plug flow.
● Nowadays researchers are working to create short contact time FCC units.
References:
● Kunni D, Levenspiel O, Fluidization engineering, Boston, MA,
Buttworth-Heinemenn,1991.
● J R Grace, C J Lim, C H Brereton, J Chaouki, Circulating Fluidized Bed
Reactor Design and Operation, University of British Columbia, Canada,
1987.
● Octave Levenspiel, Chemical Reaction Engineering, Third edition, Asian
Version, John Wiley and Sons., 2006.
THANK YOU