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

Design Factors	1.6818	1	0.0	-1.0	1.6818-
X1 =Feed flow/Max.  Feed Flow	0.84	0.7	0.5	0.3	0.16
X2=Recycle flow/Max. Recycle Flow	0.84	0.7	0.5	0.3	0.16
X3 =Jet flow/Max. Jet Flow	0.88	0.7	0.44	0.178	0.0

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

Number of Experiments	Feed flow/Max. Feed Flow	Recycle flow/Max. Recycle Flow	Jet flow/Max. Jet Flow	Efficiency
1	-1	-1	-1	93.64
2	0	0	0	94.86
3	-1	-1	1	94.63
4	-1	1	-1	94.68
5	0	0	0	95.18
6	1-	1	1	94.99
7	1	-1	-1	95.52
8	0	0	0	94.00
9	1	1	1	90.23
10	1	1	-1	95.72
11	0	0	0	94.68
12	1	1	1	96.43
13	-1.6818	0	0	95.10

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

Number of Experiments	Feed flow/Max. Feed Flow	Recycle flow/Max. Recycle Flow	Jet flow/Max. Jet Flow	Efficiency
14	1.6818	0	0	94.25
15	0	0	0	96.17
16	0	-1.6818	0	92.03
17	0	1.6818	0	96.68
18	0	0	-1.6818	94.18
19	0	0	1.6818	96.39
20	0	0	0	94.69

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