(201a) A NEW PROCESS for IMPROVED Dust Removal by Cyclone

Abstract

Many researchers have contributed to the large volume of work on improving
the efficiency of cyclones by introducing either improved design and operating
variables or by new modification in the design of the equipment [1]. In [2, 3]
an auxiliary device called post cyclone (PoC) was introduced and tested for its
ability to reduce the emissions of fines from industrial cyclones. In order to
increase the efficiency of conventional cyclones, a double cyclone has been
proposed and tested [4]. Another recent innovation for improving the collection
efficiency is the use of a centrifugal impeller inside the cyclone in place of
the immersion tube for the purpose of improved repulsion of the dust escaping
from the cyclone [5]. Also the idea of (PoC) had been tested as a recycle
stream from the outlet stream to the feed stream of the cyclone in order to
give another chance to the particles to become separated by the cyclone. In
this method an especial design chamber were used for dividing the
outlet dust included gas stream from the cyclone into two streams [6]. In the design of the chamber it was tried to take the benefits
of increasing the concentration of particle dusts in the recycle stream
compared to the cleaned out stream which are both taken from the dividing
chamber. In this way significant improvements in removal efficiency of the cyclone
were obtained. Since that the concept of this paper is an extension and
improvement of the idea of [6], for better description
of the subject the schematic diagram of the recycle cyclone in [6] is shown in
figure 1.

Figure 1:  Recycling cyclone [6]

In this paper the impact of modification is put on the design of the
dividing chamber of the idea in [6]. The dividing chamber is used for dividing a dust entrained gas
stream into two streams; one with higher particle concentration and the other
with lower concentration. The low concentration stream is considered as
cleaned-out stream from the overall dust separating process. The other one is
sent to a cyclone for dust removal and the cleaned out stream from the cyclone
is recycled back and mixed with the feed to the dividing chamber. A flow
diagram is shown in figure 2 for the proposed dust separator tested in this
paper. The main attempt in the design of the cylindrical chamber is to achieve
to more particle concentration in the recycle flow which is taken from the
chamber to be sent into the cyclone. This flow is taken from the near wall
streams from an annular cross section at the bottom of the chamber. However,
the cleaned out stream is taken from circular cross section at the bottom of
the chamber. Increasing the concentration of Particles in the output recycle
stream from the chamber is achieved by two mechanisms: one is the centrifugal
force imposed on the particles due to the tangential input of feed stream to
the chamber and the other is the jet-impingement radial flow which is applied
by a rotating tube installed inside the dividing chamber for providing radial
jet-impingement streams from its peripheral nuzzles for throwing and
concentrating the particles in zones near to the wall of the cylindrical
chamber. This process is called here jet-impingement-cyclone-recycling dust
separator. The required flow meters and blowers are also shown in this figure.
The outlet stream from the impingement nuzzles is entered from the lower part
of the nuzzle tube. The more concentrated stream flows from the annular section
in the lower section of the impingement chamber and the less concentrated
stream enters the internal cylinder duct. The impinging jets around the rotating
inner tube which are fed by clean air should be designed such that it works
more effective in throwing particles and putting them much far from the axis of
the tube. The jet-impingement-recycling-cyclone dust separating process
introduced in this paper can be considered to be much effective than a cyclone
de-duster alone provided that it is designed in a cyclone feed input process
instead of the chamber feed input design which is applied in this paper.
However, the design of chamber feed input is used in this work due to the fact
that the objective is to determine the performances of the chamber itself.

Figure 2: Jet impingement-recycling-cyclone dust separating Process
with dividing chamber feed input design

A pilot scale apparatus were designed and
built for obtaining data. Two cyclones of stairmand design with cylindrical
diameter equal to 15 cm was used with a jet-impingement diameter of 30 cm and
height equal to 165 cm. 300 nuzzles of 6 mm diameter were used around the
nuzzle tube. The apparatus was tested according to CCD (Central Composite
Design) experimental design method. For this purpose 20 experiments for
variation of three variables including: feed flow rate, recycle flow rate and
jet-impingement flow rate were conducted. By CCD experimental method optimal
operating conditions can also be explored from the data. The
following three dimension model is used for multi regression technique for
obtaining the optimum values of the three variables.

Y=B₀+B₁X₁+B₂X₂+B₃X₃+B₁₁X₁²+B₂₂X₂²+B₃₃X₃²+B₁₂X₁X₂+B₁₃X₁X₃+B₂₃X₃
             (1)

The coefficients, B_i, must be
evaluated according to the set of data. The factors X_i are the
design factors that are introduced in table 1.

Table 1:  The
coded and un-coded design factors for the three variables

The maximum values of the variables mentioned in Table 2 are as: 14, 30 and
15 cubic meters per hour, respectively.

Table 2: The results of experiments for feed from
jet-impingement chamber

Table 2: The results of experiments for feed from
jet-impingement chamber

For more
precision the experiments have been done four times for each data.  

Figure 3 ? Effect of feed flow rate on efficiency at
constant jet-impingement and recycle flow rates recycle flow rate=159.9 m³/hr
jet flow rate=76.9 m³/hr

Figure 4 ? Effect of recycle flow rate on efficiency at
constant jet-impingement and feed flow rates feed flow rate=358.0 m³/hr
jet flow rate=76.9 m³/hr

Figure 5 ?Effect of jet-Impingement flow rate on efficiency
at constant feed and recycle flow rates recycle flow rate=159.9 m³/hr
feed flow rate=358.0 m³/hr