Abstract:

     This report is concerned about cyclone efficiency. The method cyclone is used to remove particles from gaseous waste streams in the experiment. Based on the literature search, more knowledge has been known about particle collecting and particle sizing. Qualitative method is used to describe the particle size distribution using the ELPI device. The cyclone efficiency results are analyzed with graphs and discussion. Comments on the most appropriate methods of control of particles of the size are given at last in the section of discussion.

Introduction:

Background:

Air pollution is man-made harmful materials and particulars in the air, which produce disadvantageous effects to people’s health. It is a big problem, however, public was not focused on air pollution until 1969. Before that time, the air pollution increased dramatically. We impossibly solve the air pollution problems by blowing away the poisonous air and the particles in the air. The most important thing is whether we could control them by reducing pollutant emissions. Air pollution problem is not a single problem, but a series of related problems. The overall air problem basically includes the following form, emissions, transport, dilution, and modification in the atmosphere, which effects on people, property, and the environment. We are unlikely to find a good and easy way to solve all these problems. Instead, we will have to make some steps to achieve the goals such as doing some research on particulate removal from gaseous waste streams so that we can improve the air quality.

Name of Technology: Cyclones

This type of technology is a part of the group of air pollution controls collectively referred to as “pre-cleaners,” because they are oftentimes used to reduce the inlet loading of particulate matter (PM) to downstream collection devices by removing larger, abrasive particles. Cyclones are also referred to as cyclone collectors, cyclone separators, centrifugal separators, and inertial separators. In applications where many small cyclones are operating in parallel, the entire system is called a multiple tube cyclone, multi-cyclone, or multi-clone.

Type of Technology:

Removals of PM by centrifugal and inertial forces are induced by forcing particulate-laden gas to change direction.

Applicable Pollutants:

Cyclones are used to control PM, and primarily PM greater than 10 micrometers (Fm) in aerodynamic diameter. However, there are high efficiency cyclones designed to be effective for PM less than or equal to 10 Fm and less than or equal to 2.5 Fm in aerodynamic diameter (PM10 and PM2.5). Although cyclones may be used to collect particles larger than 200 Fm, gravity settling chambers or simple momentum separators is usually satisfactory and less subject to abrasion.

Achievable Emission Limits/Reductions:

The collection efficiency of cyclones varies as a function of particle size and cyclone design. Cyclone efficiency generally increases with particle size and/or density, inlet duct velocity, cyclone body length, number of gas revolutions in the cyclone, ratio of cyclone body diameter to gas exit diameter, dust loading, and smoothness of the cyclone inner wall. Cyclone efficiency will decrease with increases in gas viscosity, body diameter, gas exit diameter, gas inlet duct area, and gas density. A common factor contributing to decreased control efficiencies in cyclones is leakage of air into the dust outlet.

Control efficiency ranges for single cyclones are often based on three classifications of cyclone, i.e., conventional, high-efficiency, and high-throughput. The control efficiency range for conventional single cyclones is estimated to be 70 to 90 percent for PM, 30 to 90 percent for PM10, and 0 to 40 percent for PM2.5. High efficiency single cyclones are designed to achieve higher control of smaller particles than conventional cyclones.

Summary:

This experiment is focused on the sizing of airborne dust or sprays and with one specific method of particulate removal from gas streams (a cyclone). Our major aims are to assess possible dust hazards when handling powders, the efficiency of filters and other arresters and assess the properties of aerosol.

Literature Search:

Control of primary particulates:

     Most of the fine particles in the air are secondary particles. Many of the primary particles are more toxic than most secondary particles. Though primary particles are generally larger than secondary particles, many primary particles are small enough to be respirable and are thus of health concern.

     The first three kinds of control devices are gravity settlers, cyclone separators, and electrostatic precipitators. All function is driving the particles to a solid wall, where they adhere to each other to form agglomerates that can be removed from the collection device and disposed of.

Choosing a Collector:

Gravity settling chambers, cyclones, and ESPs work by driving the particles to a solid wall where they form agglomerates that can be collected. These three devices have similar design equations.

Filters and scrubbers divide the flow. They have different design equations from wall collection devices and from each other.

Both surface and depth filters are used for particle collection. Surface filters are used to collect most of the particles in a heavily laden gas stream. Depth filters are mostly used for the final cleanup of air or gas that must be very clean or for fine liquid drops, which coalesce on them and then drop off.

To collect small particles, a scrubber must have a very large relative velocity between the gas being cleaned and the liquid drops. For this reason co-flow scrubbers are most often used. The venture scrubber is the most widely used type of co-flow scrubber.

Particle Size Analysis:

In many powder and material processing operations, particle size and size distribution play an important role in determining the bulk properties. Describing the size distribution of the particles which make up a powder is therefore central in characterizing the powder. In a number of industrial applications, a single number will be required to characterize the particle size. This can only be done accurately and easily with a mono-sized distribution of spheres or cubes. Real particles with shapes which require more than one dimension to describe and real powders with particles in a range of sizes mean that in practice the identification of single number to describe the size of the particles is far from straightforward.

Separation of Particles from a Gas: Gas Cyclones

Gas Cyclones – Description:

Cyclones are extensively used for removing particles from gas stream. The most common type of cyclone is the reverse flow type. Inlet gas is brought tangentially into the cylindrical section and a strong vortex is created inside the cyclone body. Particles in the gas are subjected to centrifugal forces which move them radially outwards, against the inward flow of gas and towards the inside surface of the cyclone on which the solids separate. The direction of flow of the vortex reverses near the bottom of the cylindrical section and the gas leaves the cyclone via the outlet in the top. The solids at the wall of the cyclone are pushed downwards by the outer vortex and out of the solids exit. Gravity has little effect on the operation of the cyclone.

Efficiency of Separation:

     Consider a cyclone to which the solids mass flow rate is M, the mass flow discharged from the solids exit orifice is Mc (known as the fine product). The total material balance on the solids over this cyclone may be written: Total: M=Mf+Mc and the ‘component’ material balance for each particle size x (assuming no breakage or growth or particles within the cyclone) is: Component: M (dF/dx) = Mf (dFf/dx) + Mc (dFc/dx) where, dF/dx, dFf /dx and dFc/dx are the differential frequency size distributions by mass (i.e. mass fraction of size x) for the feed, fine product and coarse product respectively. F, Ff and Fc are the cumulative frequency size distributions by mass (mass fraction less than size x) for the feed, fine product and coarse product respectively.

Experimental:

     Remove and reweigh the filter and the hopper. Calculate the efficiency of cyclone. The instrument EPLI which is widely used for determining near real-time measurements was used to do this experiment. It can measure particles within the size range 30nm up to 10um on the basis of their aerodynamic diameter. In this experiment, two kinds of particles, MgO, and fly ash particles were measured by the demonstrator. And four groups of data were obtained in this experiment, the first two groups are for fly ash particles and the second two groups are for MgO.

Results:

During operation particles are drawn through a charger where they will receive a charge, before passing into the impactor which contains a number of stages, each one connected to a multi-channel electrometer. Depending on the aerodynamic size of the particles they will be impacted on the different stages. The current values obtained from the different stages are converted to a size distribution, a graph of which is shown in Figure 1. Three kinds of particle have different peak value of number concentration. The peak value of MgO is around 2000 particles/cm3, fly ash particle is about 1500 particles/cm3, and Atmospheric particle is above 2500 particles/cm3. The peak of fly ash particles is lower than MgO particles’.

Discussion:

It can be seen from Figure 1 that there are clearly overlapping size distributions present in the sample of Atmospheric particles, MgO particles and fly ash particles. Real samples always contain particles with more than one source. Atmospheric dust might for example contain pollen as well as pollutants. The size distribution will then have two peak values.

In this experiment, fly ash particles can be found a bit larger than MgO particles. And the color of fly ash is darker than MgO, MgO is white. In addition, it can be seen in figure 1 that the start point of MgO particles is much higher than fly ash particles. Because of influence of the distribution and size, the cyclone efficiency of fly ash particles is a little bit higher than MgO particles in average.

From Figure 1, it also can be seen that the number concentration of MgO particles firstly reach its peek value, and then the fly ash particles get its peak. Atmospheric particles obtain its peak value at almost the same time as fly ash particles. In the other hand, the distribution of the total number of MgO and fly ash particles is different, it can be seen from Figure 1 that as the diameter of MgO increase, the total number firstly increase, then go down, and then go up again. But the curve of fly ash does not have this phenomenon. Its curve only has one wave crest and compared with MgO, the number concentration of fly ash first get the value 0. In addition, it is worthy to note that the atmospheric particles’ curve almost has the same shape as fly ash’s curve. The only big difference is that the atmospheric particle’s curve has a much higher peak value of number concentration.

In the experiment, the particles escaping from the cyclone in the process might lose, which could influence the cyclone efficiency.

Also, from this experiment, it can be found that there are some advantages of using cyclone method to calculate the efficiency. The first one is that temperature and pressure limitations are only dependent on the materials of construction; the second one, dry collection and disposal; and the third one is relatively small space requirements. However, there are some disadvantages of using cyclone. Firstly, it is unable to handle sticky or tacky materials. Secondly, high efficiency units might experience high pressure drops.

In the industry, cyclone is normally for relatively big particles, and ESP or fabric filter is for smaller particles. Because in the atmosphere the toxic particles are generally smaller than fine particles, ESP and fabric filter should be used more than other instrument. The electrostatic precipitator (ESP) is like a gravity settler or centrifugal separator, but electrostatic force drives the particles to the wall. It is effective on much smaller particles than the previous two devices.