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This paper gives introduction to cyclone separators, reasons for its popularity and objective of using cyclone separator at micro level.

Introduction

Chemical processes consist of reaction stages and/or separation stages in which the process streams are separated and purified. Such separations involve physical principles based on differences in the properties of the constituents in the stream. Heterogeneous mixtures consist of two or more phases which have different composition. These mixtures consist of components that do not react chemically and have clearly visible boundaries of separation between the different phases. Components of such mixture can be separated using one or more appropriate techniques. These separation processes include Gas-Liquid (vapor-liquid) separation, Gas-Solid separation (vapor-solid), Liquid-Liquid separation (immiscible), Liquid-Solid, and Solid-Solid separation etc. This separation can be done by exploiting the differences in density between the phases. Gravitational force or centrifugal force can be used to enhance the separation. The separation units can be either horizontal or vertical. The main techniques used to separate the phases, and the components within the phases, are discussed in details. The principle methods for the separation of such mixtures could be classified as:

1. Cyclone separator, 2. Gas-Liquid separator, 3. Liquid-Liquid separator

4. Gravity separator, 5. Centrifugal separator, 6. High speed tubular centrifuge 7. Scrubbers 8. Electrostatic precipitator, 9. Hydro cyclone

Cyclone Separator

Cyclone separators provide a method of removing particulate matter from air or other gas streams at low cost and low maintenance. Cyclones are somewhat more complicated in design than simple gravity settling systems, and their removal efficiency is much better than that of settling chamber. Cyclones are basically centrifugal separators, consists of an upper cylindrical part referred to as the barrel and a lower conical part referred to as cone (figure 5.1). They simply transform the inertia force of gas particle flows to a centrifugal force by means of a vortex generated in the cyclone body. The particle laden air stream enters tangentially at the top of the barrel and travels downward into the cone forming an outer vortex. The increasing air velocity in the outer vortex results in a centrifugal force on the particles separating them from the air stream. When the air reaches the bottom of the cone, it begins to flow radially

inwards and out the top as clean air/gas while the particulates fall into the dust collection chamber attached to the bottom of the cyclone.

Figure. 1

Types of Cyclone

Three different types of cyclone are shown in figure 2. First figure i.e. 2a shows a cyclone with a tangential entry. These types of cyclones have a distinctive and easily recognized form and widely used in power and cement plants, feed mills and many other process industries.

Figure 2b shows the axial entry cyclones, the gas enters parallel to the axis of the cyclone body. In this case the dust laden gases enter from the top and are directed into a vortex pattern by the vanes attached to the central tube. Axial entry units are commonly used in multi cyclone configuration, as these units provide higher efficiencies. Another type of larger cyclonic separator shown in figure 2c is often used after wet scrubbers to trap particulate matter entrained in water droplets. In this type, the air enters tangentially at the bottom, forming vertex. Large water droplets are forced against the walls and are removed the air stream.

Cyclone collectors can be designed for many applications, and they are typically categorized as high efficiency, conventional (medium efficiency), or high throughput (low efficiency). High efficiency cyclones are likely to have the highest-pressure drops of the three cyclone types, while high throughput cyclones are designed to treat large volumes of gas with a low-pressure drop. Each of these three cyclone types have the same basic design. Different levels of collection efficiency and operation are achieved by varying the standard cyclone dimensions.

Figure 2

Different Cyclone Model

In the agricultural processing industry, 2D2D (Shepherd and Lapple, 1939) and 1D3D (Parnell and Davis, 1979) cyclone designs are the most commonly used abatement devices for particulate matter control. The D’s in the 2D2D designation refer to the barrel diameter of the cyclone. The numbers preceding the D’s relate to the length of the barrel and cone sections, respectively. A 2D2D cyclone has barrel and cone lengths of two times the barrel diameter, whereas the 1D3D cyclone has a barrel length equal to the barrel diameter and a cone length of three times the barrel diameter. The configurations of these two cyclone designs are shown in figure 2. Previous research (Wang, 2000) indicated that, compared to other cyclone designs, 1D3D and 2D2D are the most efficient cyclone collectors for fine dust (particle diameters less

than 100 μm). Mihalski et al (1993) reported “cycling lint” near the trash exit for the 1D3D and 2D2D cyclone designs when the PM in the inlet air stream contained lint fiber. Mihalski reported a significant increase in the exit PM concentration for these high efficiency cyclone designs and attributed this to small balls of lint fiber “cycling” near the trash exit causing the fine PM that would normally be collected to be diverted to the clean air exit stream. Simpson and Parnell (1995) introduced a new low-pressure cyclone, called the 1D2D cyclone, for the cotton ginning industry to solve the cycling-lint problem. The 1D2D cyclone is a better design for high-lint content trash compared with 1D3D and 2D2D cyclones (Wang et al., 1999). Figure 3 illustrates the configuration of 1D2D cyclone design.

Similarly, cyclone efficiency will decrease with increases in the parameters such as gas viscosity; cyclone body diameter; gas exit diameter; gas inlet duct area; gas density; leakage of air into the dust outlet. The efficiency of a cyclone collector is related to the pressure drop across the collector. This is an indirect measure of the energy required to move the gas through the system. The pressure drop is a function of the inlet velocity and cyclone diameter. Form the above discussion it is clear that small cyclones are more efficient than large cyclones. Small cyclones, however, have a higher pressure drop and are limited with respect to volumetric flow rates. Another option is arrange smaller cyclones in series and/or in parallel to substantially increase efficiency at lower pressure drops. These gains are somewhat compensated, however,

by the increased cost and maintenance problems. Also, these types of arrangements tend to plug more easily. When common hoppers are used in such arrangements, different flows through cyclones can lead to entrainment problems. A typical series arrangement is shown in figure In such arrangements large particle can be arrested in the first cyclone and a smaller, more efficient cyclone can collect smaller particles. Due to that it reduces dust loading in the second cyclone and avoids problems of abrasion and plugging. Also, if the first cyclone is plugged, still there will be some collection occurring in the second cyclone. The additional pressure drops produced by the second cyclone adds to the overall pressure drop of the system and higher pressure can be a disadvantage in such series system design. Cyclone efficiency can also be improved if a portion of the flue gas is drawn through the hopper. An additional vane or lower pressure duct can provide this flow. However, it may then become necessary to recirculate or otherwise treat this as purge exhaust to remove uncollected particulate matter.

CLASSICAL CYCLONE DESIGN (CCD)

The cyclone design procedure outlined in Cooper and Alley (1994), hereafter referred to as the classical cyclone design (CCD) process, was developed by Lapple in the early 1950s. The CCD process (the Lapple model) is perceived as a standard method and has been considered by some engineers to be acceptable. However, there are several problems associated with this design procedure. First of all, the CCD process does not consider the cyclone inlet velocity in developing cyclone dimensions. It was reported (Parnell, 1996) that there is an “ideal” inlet velocity for the different cyclone designs for optimum cyclone performance. Secondly, the CCD does not predict the correct number of turns for different type cyclones. The overall efficiency predicted by the CCD process In order to use the CCD process, it is assumed that the design engineer will have knowledge of (1) flow conditions, (2) particulate matter (PM) concentrations and particle size distribution (PSD) and (3) the type of cyclone to be designed (high efficiency, conventional, or high throughput). The PSD must be in the form of mass fraction versus aerodynamic equivalent diameter of the PM. The cyclone type will provide all principle dimensions as a function of the cyclone barrel diameter (D). With these given data, the CCD process is as follows:

Standard Cyclone Dimensions

Extensive work has been done to determine in what manner dimensions of cyclones affect performance. In some classic work that is still used today, Shepherd and Lapple (1939,

1940) determined “optimal” dimensions for cyclones. Subsequent investigators reported similar work, and the so-called “standard” cyclones were born. All dimensions are related to the body diameter of the cyclone so that the results can be applied generally. The table on the next slide summarizes the dimensions of standard cyclones of the three types mentioned in the previous figure. The side figure illustrates the various dimensions used in the table.

The first step of CCD process is to calculate the number of effective turns. The number of effective turns in a cyclone is the number of revolutions the gas spins while passing through the cyclone outer vortex. A higher number of turns of the air stream result in a higher collection efficiency. The Lapple model for Ne calculation is as follows:

where

N = number of turns inside the device (no units)

H = height of inlet duct (m or ft)

Lb = length of cyclone body (m or ft)

Lc = length (vertical) of cyclone cone (m or ft).

Based on equation the predicted numbers of turns for 4 cyclone designs were calculated. 1D2D, 2D2D, and 1D3D cyclones are the cyclone designs shown in figures 2 and 3. These three cyclone designs have the same inlet dimensions (Hc and Bc), referred to as the 2D2D inlet. The 1D3D cyclone is a traditional 1D3Dt cyclone design, which has the same design dimensions as 1D3D .

. It has been observed that the Lapple model for Nproduces an excellent estimation of the number of turns for the 2D2D cyclone designs. However, this model cyclones in figure 2 except the inlet dimensions. The 1D3Dt cyclone has an inlet height equal to the barrel diameter (Hc = Dc) and an inlet width of one eighth of the barrel diameter (Bc = Dc/8). Table 1 gives the comparison of the predicted Ne vs. the observed Ne fails to give an accurate estimation of Ne for the cyclone design other than 2D2D design. This observation indicates a limitation for the Lapple model to accurately predict the number of effective turns. The Ne model is valid only for 2D2D cyclone designs, which was originally developed by Shepherd and Lapple (1939).

Cut point Diameter

The second step of the CCD process is the calculation of the cut-point diameter. The cut-point of a cyclone is the aerodynamic equivalent diameter (AED) of the particle collected with 50% efficiency. As the cut-point diameter increases, the collection efficiency decreases.

Where, dp = diameter of the smallest particle that will be collected by the cyclone

· = gas viscosity (kg/m. s) W = width of inlet duct (m)

Vi = inlet gas velocity (m/s)

ρp = particle density (mkg3)

pa = Density of fluid

It is worth noting that in this expression, dp is the size of the smallest particle that will be collected if it starts at the inside edge of the inlet duct. Thus, in theory, all particles of size dp or larger should be collected with 100% efficiency. The preceding equation shows that, in theory, the smallest diameter of particles collected with 100% efficiency is directly related to gas viscosity and inlet duct width, and inversely related to the number of effective turns, inlet gas velocity, and density difference between the particles and the gas.

To be collected, particles must strike the wall within the amount of time that the gas travels in the outer vortex. The gas residence time in the outer vortex is

The maximum radial distance traveled by any particle is the width of the inlet duct W. The centrifugal force quickly accelerates the particle to its terminal velocity in the outward (radial) direction, with the opposing drag force equaling the centrifugal force. The terminal velocity that will just allow a particle initially at distance W away from thewall to be collected in time is

where Vt = particle drift velocity in the radial direction (m/s or ft/s).

Fractional Efficiency Curve

The third step of CCD process is to determine the fractional efficiency. Based upon the cut-point, Lapple then developed an empirical model for the prediction of the collection efficiency for any particle size, which is also known as fractional efficiency curve:

dpj= collection efficiency of particles in the jth size range (0 < nj < 1) dpj = characteristic diameter of the jth particle size range (in microns).

Pressure Drop ( P)

Cyclone pressure drop is another major parameter to be considered in the process of designing a cyclone system. Two steps are involved in the Lapple approach to estimation of cyclone pressure drop. The first step in this approach is to calculate the pressure drop in the number of

There is one problem associated with this approach. “The Lapple pressure drop equation does not consider any vertical dimensions as contributing to pressure drop” (Leith and Mehta, 1973). This is a misleading in that a tall cyclone would have the same pressure drop as a short one as long as cyclone inlets and outlets dimensions and inlet velocities are the same. It has been considered that cyclone efficiency increases with an increase of the vertical dimensions. With the misleading by Lapple pressure drop model.

one could conclude that the cyclone should be as long as possible since it would increase cyclone efficiency at no cost in pressure drop (Leith and Mehta, 1973). A new scientific approach is needed to predict cyclone pressure drop associated with the dimensions of a cyclone.

Where

Hv = pressure drop, expressed in number of inlet velocity Heads

K = constant that depends on cyclone configurations and

Operating conditions (K = 12 to 18 for a standard tangential-entry cyclone)

Material of Construction

Material used for fabrication of cyclone separator is iron 26 Gauge of thickness. Material is available on market easily .

Standard Cyclone Dimension (Lapple Dimesion)

Conventional Dimensions

Cyclone Separator Dimesions

Blower Calculations

Volumetric flow rate of Blower=0.058 m3/s

Velocity of Air inlet Duct = Vi= Q/WH

· 0.058/0.0762 x 0.1524

· 5.272 m/s

· 1/2x1.22x2.77 P = 16941 pa

Number of Effective turn

= 1 [ + 2 ]

Ne = 1/0.1524[0.609+0.609/2]

Ne = 6

Δt= πDN/Vi

· 3.14x0.3048x6/5.27

· 1.08 Sec

Particle Drift Velocity

Vt=W/ t

= 0.0762/1.08=0.07055 m/sec

Terminal Drift Transverse Velocity

Vt = ( − ) ^2 ^2/9µ

· (1602-1.22)x(0.0004)2x(5.27)2/9x0.0000183x0.3048

· 226830.16 m/sec

Cut point Diameter

dpc =[9µ /2 ( − )] ½

=[9x0.0000183x0.0762/2x3.14x6x5.27x(1602-1.22)]1/2

dpc = 6.28 µm

Pressure Drop

ΔP= α Vi2 /2

· =16 HW/De2 =16x0.1524x0.0762/0.0232=9.29

ΔP =9.29 x1.22x27.772/2=157.38 Pa

Power Requirement

· 0.058x157.38

W = 9.12 J/sec

Outlet Gas Velocity

Vo =Q/ ri2

· 0.05806/3.14x0.005806

· 3.184m/sec

ηj = 1/1+(dpc/dpj)2

ηj = 1/1+(dpc/dpj)2

= 1/1+(6.28 µm/1 µm)2

= 0.02

ηj = 1/1+(dpc/dpj)2

= 1/1+(6.28 µm/2 µm)2

= 0.09

ηj = 1/1+(dpc/dpj)2

= 1/1+(6.28 µm/3µm)2

= 0.1

ηj = 1/1+(dpc/dpj)2

= 1/1+(6.28 µm/5µm)2

= 0.38

ηj = 1/1+(dpc/dpj)2

= 1/1+(6.28 µm/9µm)2

= 0.67

ηj = 1/1+(dpc/dpj)2

= 1/1+(6.28 µm/15µm)2

ηj = 1/1+(dpc/dpj)2

· 1/1+(6.28 µm/25µm)2

· 0.940

ηj = 1/1+(dpc/dpj)2

· 1/1+(6.28 µm/50µm)2

· 0.984

Graph dj v/s ηj

1.2

1

0.8

0.6

0.4

0.2

0

0 10 20 30 40 50 60

Overall separation efficiency

Mf=92+10

= 92/100=1-8/100=92/92+80

=0.92x100

=92%

Cost Estimation

Cost of cyclone Body=150$

Conclusion

A prominent problem in calculating the efficiency of cyclone is the effect of flow characters in cyclone. In big cyclones the flow is turbulent and friction factors assumed give good results. This is not true for small cyclones. The flow in small cyclones can be laminar or even transitional. In such case the operational conditions, like velocity, temperature, pressure, viscosity and cyclone diameter, may be of significant importance and their effect changes from cyclone to cyclone. In laminar flow, operating parameters influence cyclone efficiency more than turbulent case. This makes the prediction of efficiency and pressure drop very difficult especially in small cyclone. Most of the models depend on empirical or semi-empirical equations. The models calculate efficiency and predict the cutoff size which corresponds to 50% efficiency. According to Wang et al. cyclone performance is function of geometry and operating parameters of cyclone, as well as particle size distribution of the entrained particulate matter. Several models have been proposed to predict the efficiency of cyclone. It is widely agreed amongst the scientists that cyclone performance is definitely affected by operating parameters and hence they should be included in the modeling. Many theories account for density, gas velocity, viscosity and particle diameter. As far as effect of geometry is considered there is difference in approach for various scientists. Some consider all the geometric.

parameters where as some consider only few important parameters like inlet and outlet diameter and height in their models. As mentioned, most of the theories consider cut size “d50”, which corresponds to diameter of particle where 50% of particles smaller and 50% of particles greater that that size will be collected. Two most common approaches for calculating efficiency are Force Balance Theory [Lapple] which assumes that terminal velocity is achieved when drag fore and centrifugal force equal each other and the Static Particle Approach [Barth] which considers simple force balance where forces acting on particle are balanced. Various other complicated theories have been proposed but the essentially have their base in one of the two theories.

REFERENCES

· Barth W. 1956. Design and layout of the cyclone separator on the basis of new investigations. Brennstoff-Warme-Kraft 8: 1-9.

· Cooper, C.C. and G.C Alley. 1994. Air Pollution Control; A Design Approach. Prospect Heights, Ill.: Waveland Press, Inc.

· First, M.W., 1950. Fundamental Factors in the Design of Cyclone Dust Collectors. Ph.D. dissertation. Cambridge, Mass.: Harvard University.

· Hinds, William C., 1999. Aerosol Technology. New York: John Wiley & Sons

· Kaspar, P., K.D. Mihalski and C.B. Parnell, Jr. 1993. Evaluation and development of cyclone design theory. In Proc. 1993 Beltwide Cotton Production Conferences. New

Orleans, La. National Cotton Council.

· Lapple, C. E. 1951. Processes use many collector types. Chemical Engineering 58

(5):144-151

· Leith, D. and W. Licht, 1972. The collection efficiency of cyclone type particle collectors – A new theoretical approach. AIChE Symposium Series 126, 68: 196-206

· A.J. Hoekstra, J.J. Derksen, H.E.A. Van Den Akker

An experimental and numerical study of turbulent swirling flow in gas cyclones

· L.Y. Hu, L.X. Zhou, J. Zhang, M.X. Shi

Studies on strongly swirling flows in the full space of a volute cyclone separator

· Shepherd, C.B. and C.E. Lapple. 1940. Flow pattern and pressure drop in cyclone dust collectors

· Bayless, G. Kremer and B. Stuart. 2006. CFD simulation of the influence of temperature and pressure on the flow pattern in cyclones. Ind. Eng. Chem. Res. 45:7667–7672.