Chemical Engineering March 2019 - 62

two different particle densities, s =
1,490 kg/m3 and s = 2,700 kg/m3,
and two particle-size distributions
Q3(d), which are assumed to be describable
as Rosin-Rammler-Sperling-Bennett
(RRSB) functions. The
two sets of RRSB parameters are:
1. dmin = 0.2 µm, dmax = 200
µm, d' = 25.5 µm, n = 0.95,
d50 = 17.5 µm
2. dmin = 0.1 µm, dmax = 30 µm,
d' = 19 µm, n = 0.8, d50 =
12.2 µm.
In all considered cases the particle
concentration is S0 = 2 g/m3.
The volume occupied by a cyclone
is mainly determined by its diameter
and by its length. A uniflow cyclone
generally requires about the same or
even a shorter length (typically 2-3
cyclone diameters) to achieve optimum
performance than a swirl tube
[22]. Thus, only the cyclone diameter
is varied to observe the influence of
the available volume. The swirl tube
diameter Dc,ST is varied between
Dc,ST/Dc,UC = 1.0 and Dc,ST/Dc,UC
= 1.3, while keeping the uniflow cyclone
diameter constant at Dc,UC =
124 mm. The pressure drop is varied
between 500 Pa (0.073 psi) and
2,000 Pa (0.290 psi).
The results show that, under the
operating conditions considered,
a uniflow cyclone achieves a significantly
higher separation efficiency
than a swirl tube if the available volume
is restricted to about the volume
needed by the uniflow cyclone for
A
Pure gas
Inlet
Gas
outlet
Crude gas
B
Crude gas
Inlet
Solids
Solids
outlet
Gas
outlet
Pure gas
Solids
outlet
FIGURE 14. Swirl tube (standard cyclone with axial inlet) (a) and uniflow cyclone (b), applicable especially
in multicyclones [26]
separating the given gas-solids feed
and if the available pressure drop is
low, in accordance with experiments
as is shown in Ref. 26. For example,
for a pressure drop of 500 Pa, the
efficiencies can differ by up to 10%.
If the volume and pressure drop are
freely available, swirl tubes reach
higher separation rates than uniflow
cyclones, with the separation efficiencies
of both cyclone types converging
as the gas-volume flow, and consequently,
the cyclone size decreases
and the pressure drop, the particle
size or the particle density increase.
The results described here are
transferable to multicyclone systems.
In many cases multicyclones use
swirl tube cells. Analogously, multicyclones
can be made from uniflow
cyclone cells. Often thse devices are
used in space-limited applications;
for example, as third-stage separators
in fluid catalytic cracking (FCC)
processes [28, 29].
Compared to swirl tubes, the
higher efficiency per volume of uniflow
cyclones at low pressure drop is
especially advantageous in multicyclone
applications with low pressure
drop (for example purifying suction
air in combustion engines [27]).
Concluding remarks
Comparing standard cyclones with
uniflow cyclones by applying well
proven calculation models for both
cyclone types indicates higher efficiencies,
but also
higher volume
required for properly designed standard
cyclones. However, if available
volume is restricted to about the
volume needed by a uniflow cyclone
for its optimum performance, and
if the pressure drop is low, a uniflow
cyclone can be more efficient,
Solids
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Chemical Engineering March 2019

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