M O D U L E IV C O N T R O L T E C H N I Q U E S

Transcription

1 M O D U L E IV C O N T R O L T E C H N I Q U E S

2 GRAVITY SETTLING CHAMBER Gravity settling chamber is used to remove large and abrasive particles greater than 50 μ from a gas stream. This is a simple particulate collection device using the principle of gravity to settle the particulate matter in a gas stream passing through its long chamber. The primary requirement of such a device would be a chamber in which the carrier gas velocity is reduced so as to allow the particulate matter to settle out of the moving gas stream under the action of gravity.

3 This particulate matter is then collected at the bottom of the chamber. The chamber is cleaned manually to dispose the waste.

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6 The gas velocities in the settling chamber must be sufficiently low for the particles to settle due to gravitational force. Literature indicates that gas velocity less than about 3 m/s is needed to prevent re-entrainment of the settled particles. The gas velocity of less than 0.3 m/s will produce good results.

7 DESIGN PARAMETERS For 100 % efficiency t = H V t = L V H Eqn.(1) Where, H= height of settling chamber L = Length of settling chamber V t = Velocity of settling V H = velocity of flow

8 From equation 1 Settling velocity may be given by V t = V H H L = Q L W Eqn.(2) W= width of chamber (As Q = area of inlet x velocity = W.H.VH ) For streamline flow settling velocity is given by V t = g.dp2.ρ p 18. μ Eqn.(3) Equating equations 2 and 3 V H H L =g.dp2.ρ p 18. μ

9 therefore, we get, Eqn.(4) D p min is min size of particle that can be removed with 100 % efficiency For better and conservative results multiplying factor 2 can be taken

10 EFFICIENCY OF SETTLING CHAMBER Efficiency of settling chamber is given by Another equation for calculation of efficiency is

11 Gravity settling chamber with trays

12 SETTLING CHAMBER WITH TRAYS Settling trays can be used to improve removal efficiency. If, n = number of trays, then equation 4 becomes

13 ADVANTAGES 1. Low initial cost 2. Simple construction 3. Low maintenance cost 4. Low pressure drop 5. Dry and continuous disposal of collected solids 6. Can be constructed of any material 7. Temperature and pressure limitations are imposed by type of material used

14 DISADVANTAGES 1. Large space requirement 2. Only comparatively large particles can be collected

15 APPLICATIONS OF SETTLING CHAMBER Settling chambers are mostly used as pre-cleaners. They are sometimes used in the process industries, particularly in the food and metallurgical industries as the first step in dust control. Use of settling chambers as pre-cleaners can also reduce the maintenance cost of high efficiency control equipment, which is more subject to abrasive deterioration

16 PROBLEM Assume viscosity of air = 2.1 x 10-5 kg/m.s

17 SOLUTION:

18 CYCLONES Settling chambers discussed in previous lecture are not effective in removing small particles. Therefore, one needs a device that can exert more force than gravity force on the particles so that they can be removed from the gas stream. Cyclones use centrifugal forces for removing the fine particles. They are also known as centrifugal or inertial separators.

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22 The cyclone consists of a vertically placed cylinder which has an inverted cone attached to its base. The particulate laden gas stream enters tangentially at the inlet point to the cylinder. The velocity of this inlet gas stream is then transformed into a confined vortex, from which centrifugal forces tend to drive the suspended particles to the walls of the cyclone. The vortex turns upward after reaching at the bottom of the cylinder in a narrower inner spiral. The clean gas is removed from a central cylindrical opening at the top, while the dust particles are collected at the bottom in a storage hopper by gravity.

23 HOW PARTICLES SEPARATED FROM GAS? A high rate of spin Spinning gas Centrifugal force fling the dust particles to the walls of cylinder and cone The particle then slide down the wall and into the storage hopper The cleaned gas gradually spiral downward The downward cleaned gas gradually reversed its downward motion and forms a smaller ascending spiral A vortex finder tube extending downward into the cylinder aids in directing the inner vortex out of the device

24 ADVANTAGES 1. Low initial cost, 2. Simple in construction and operation, 3. Low pressure drop, 4. Low maintenance requirements, 5. Continuous disposal of solid particulate matter, and 6. Use of any material in their construction that can withstand the temperature and pressure requirements.

25 DISADVANTAGES 1. Low collection efficiency for particles below 5 10 μ in diameter, 2. Severe abrasion problems can occur during the striking of particles on the walls of the cyclone, and 3. A decrease in efficiency at low particulate concentration.

26 OPERATIONAL PROBLEMS 1. Erosion: Heavy, hard, sharp edged particles, in a high concentration, moving at a high velocity in the cyclone, continuously scrap against the wall and can erode the metallic surface. 2. Build up: A dust cake builds up on the cyclone walls, especially around the vortex finder, at the ends of any internal vanes, and especially if the dust is hygroscopic. It can be a severe problem.

27 3. Corrosion: If the cyclone is operating below the condensation point, and if reactive gases are present in the gas stream, then corrosion problems can occur. Thus the product should be kept above the dew point or a stainless steel alloy should be used.

28 APPLICATIONS OF CYCLONES 1. For the control of gas borne particulate matter in industrial operations such as cement manufacture, food and beverage, mineral processing and textile industries. 2. To separate dust in the disintegration operations, such as rock crushing, ore handling and sand conditioning in industries. 3. To recover catalyst dusts in the petroleum industry. 4. To reduce the fly ash emissions.

29 APPLICATIONS Power Plant Textile industry (Cotton, Polyester) Stone Crusher Mineral Industry Rubber Industry Material Handling Fly Ash Lead Industry Cement Industry Steel Industry

30 BAG HOUSE FILTER (FABRIC FILTER)

31 BAG HOUSE FILTER (FABRIC FILTER) Fabric filtration is one of the most common techniques to collect particulate matter from industrial waste gases. The use of fabric filters is based on the principle of filtration, which is a reliable, efficient and economic methods to remove particulate matter from the gases. The air pollution control equipment using fabric filters are known as bag houses. The high efficiency of these collectors is due to the dust cake formed on the surfaces of the bags.

32 The fabric primarily provides a surface on which dust particulates collect through the following four mechanisms: Inertial Collection (Impaction) Dust particles strike the fibers placed perpendicular to the gas-flow direction instead of changing direction with the gas stream. Interception Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size.

33 Brownian Movement Submicron particles are diffused, increasing the probability of contact between the particles and collecting surfaces. Electrostatic Forces The presence of an electrostatic charge on the particles and the filter can increase dust capture A combination of these mechanisms results in formation of the dust cake on the filter, which eventually increases the resistance to gas flow. The filter must be cleaned periodically.

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40 A bag house or a bag filter consists of numerous vertically hanging, tubular bags, 12 to 40 cm in diameter and 2 to 10 m long. They are suspended with their open ends attached to a manifold. The number of bags can vary from a few hundreds to a thousand or more depending upon the size of the bag house. Bag houses are constructed as single or compartmental units. In both cases, the bags are housed in a shell made of rigid metal material.

41 FILTER MEDIA Woven and felted materials are used to make bag filters. Woven filters are used with low energy cleaning methods such as shaking and reverse air.

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44 Felted fabrics are usually used with high energy cleaning systems such as pulse jet cleaning. While selecting the filter medium for bag houses, the characteristics and properties of the carrier gas and dust particles should be considered. The properties to be noted include: a) Carrier gas temperature b) Carrier gas composition c) Gas flow rate d) Size and shape of dust particles and its concentration

45 PROPERTIES OF BAG FABRIC

46 OPERATION OF A BAG HOUSE The gas entering the inlet pipe strikes a baffle plate, which causes larger particles to fall into a hopper due to gravity. The carrier gas then flows upward into the tubes and outward through the fabric leaving the particulate matter as a "cake" on the insides of the bags.

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48 FILTER CLEANING MECHANISMS The following mechanisms are used for cleaning the filters in a bag house: i) Rapping ii) Shaking iii) Reverse air flow (back wash) iv) Pulse jet

49 MECHANICAL CLEANING (SHAKING)

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51 PULSE JET

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53 ADVANTAGES 1. High collection efficiencies for all particle sizes, especially for particles smaller than 10 micron in diameter. 2. Simple construction and operation. 3. Nominal power consumption. 4. Dry disposal of collected material.

54 DISADVANTAGES 1. Operating limits are imposed by high carrier gas temperatures, high humidity and other parameters. 2. High maintenance and fabric replacement costs. Bag houses are difficult to maintain because of the difficulty in finding and replacing even a single leaking bag. Also as general rule, about 1/4th of the bags will need replacement every year. 3. Large size of equipment. 4. Problems in handling dusts which may abrade, corrode, or blind the cloth.

55 OPERATING PROBLEMS 1. Cleaning At intervals the bags get clogged up with a covering of dust particles that the gas can no longer pass through them. At that point, the bags have to be cleaned by rapping, shaking or by reverse air flow or by a pulse jet. 2. Rupture of the cloth The greatest problem inherent in cloth filters is the rupture of cloth, which results from shaking. It is often difficult to locate ruptures and when they re found the replacement time is often considerable.

56 3. Temperature Fabric filters will not perform properly if a gross temperature overload occurs. If the gas temperature is expected to fluctuate, a fiber material that will sustain the upper temperature fluctuation must be selected. Also, whenever the effluent contains a reactive gas like SO 2 which can form an acid whenever the temperature in the bag house falls below the dew point it can create problems. 4. Chemical attack This is another problem associated with fabric filters. The possibility of chemical attack due to corrosive chemicals present in the effluent. A proper choice of fabric filter will avoid this problem.

57 5. Bleeding This is the penetration of the fabric by fine particles, which is common in fabric filtration. It can occur if the weave is too open or the filter ratio is very high. The solution is to use a double layer material or a thick woven fabric. 6. Humidity This is a common and important problem, especially if the dust is hygroscopic. It would therefore be advisable to maintain moisture free conditions within the bag house, as a precautionary measure.

58 APPLICATIONS OF A FABRIC FILTER Fabric filters find extensive application in the following industries and operations: i) Metallurgical industry ii) Foundries iii) Cement industry iv) Chalk and lime plants vi) Ceramic industry vii) Flour mills

59 WET SCRUBBERS

60 SCRUBBERS Scrubbers are devices that remove particulate matter by contacting the dirty gas stream with liquid drops. Generally water is used as the scrubbing fluid. In a wet collector, the dust is agglomerated with water and then separated from the gas together with the water. The mechanism of particulate collection and removal by a scrubber can be described as a four-step process. 1. Transport : The particle must be transported to the vicinity of the water droplets which are usually 10 to 1000 times larger.

61 II. Collision : The particle must collide with the droplet. III. Adhesion : This is promoted by the surface tension property. IV. Precipitation: This involves the removal of the droplets, containing the dust particles from the gas phase. Collection mechanism consists of combination of many mechanisms

62 TYPES OF SCRUBBERS 1. Spray towers. 2. Venturi scrubbers. 3. Cyclone scrubbers. 4. Packed scrubbers.

63 1. SPRAY TOWER Can be used for removing both particulates and gaseous contaminants Less pressure loss. Can handle large volume of gases Gas flows upwards, entrained particles colloid with droplets sprayed across the flow passage and liquid droplets containing the particles settle by gravity to bottom of the chamber.

64 HORIZONTAL FLOW SPRAY TOWER

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66 2. VENTURI SCRUBBER

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70 Vertical Venturi Scrubber

71 Most efficient in removing particles in the range of 0.5 μ to 5 μ, that makes them specially effective in removing submicron particles associated with smoke and fumes. A coarse water spray is injected into throat, which gets atomized by high gas velocities. Liquid droplets colloid with particles in the gas stream and water particles fall down for later removal. Highly efficient for submicron removal i.e. 90% efficient.

72 3. WET CYCLONIC SCRUBBER High-pressure nozzle sprays are located in various places within the cyclone chamber generate a fine spray that intercepts particles entrained in the swirling gases. The particulate matter is thrown on to the walls by centrifugal action and then drained to the collection sump. For droplets of 100 μ size efficiency approaches 100%.

73 4. PACKED SCRUBBER

74 Packed Bed Scrubber

75 The exhaust stream being treated enters the bottom of the tower and flows upward over the packing material. Liquid is introduced at the top of the packing by sprays or weirs, and it flows downward over the packing material. As the exhaust stream moves up through the packing, it is forced to make many winding changes in direction, resulting in intimate mixing of both the exhaust gas and liquid streams.

76 ADVANTAGES 1. Low initial cost. 2. Moderately high collection efficiency for small particles 3. Applicable for high temperature installations 4. They can simultaneously remove particulates and gases 5. There is no re-entertainment of particles.

77 DISADVANTAGES 1. High power consumption for high efficiency 2. Moderate to high maintenance costs owing to corrosion and abrasion 3. Wet disposal of collected waste

78 APPLICATIONS OF SCRUBBERS They are particularly useful in the case of a hot gas that must be cooled for some reason. If the particulate matter is combustible or if any flammable gas is present, even in trace amounts, in the bulk gas phase, a scrubber is preferred to an electrostatic precipitator Scrubbers can be used when there are waste water treatment systems available on the site, with adequate reserve capacity to handle the liquid effluent. Scrubbers are also used when gas reaction and absorption are required simultaneously with particulate control.

79 ELECTROSTATIC PRECIPITATOR (ESP)

80 ELECTROSTATIC PRECIPITATORS Electrostatic precipitators (ESP) are particulate collection devices that use electrostatic force to remove the particles less than 5 micron in diameter. It is difficult to use gravity settlers and cyclones effectively for the said range of particles. Particles as small as one-tenth of a micrometer can be removed with almost 100% efficiency using electrostatic precipitators. An electrostatic precipitator is a large, industrial emissioncontrol unit. It is designed to trap and remove dust particles from the exhaust gas stream of an industrial process.

81 Precipitators are used in these industries: Power/Electric Cement Chemicals Metals Paper

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88 PRINCIPLE OF WORKING The principle behind all electrostatic precipitators is to give electrostatic charge to particles in a given gas stream and then pass the particles through an electrostatic field that drives them to a collecting electrode. The electrostatic precipitators require maintenance of a high potential difference between the two electrodes, one is a discharging electrode and the other is a collecting electrode. Because of the high potential difference between the two electrodes, a powerful ionizing field is formed.

89 Very high potentials as high as 100 kv are used. The usual range is kv. The ionization creates an active glow zone (blue electric discharge) called the corona or corona glow. Gas ionization is the dissociation of gas molecules into free ions. As the particulate in the gas pass through the field, they get charged and migrate to the oppositely charged collecting electrode, lose their charge and are removed mechanically by rapping, vibration, or washing to a hopper below.

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91 STEP BY STEP PROCESS OF REMOVING PARTICLES 1. Ionization - Charging of particles 2. Migration - Transporting the charged particles to the collecting surfaces 3. Collection - Precipitation of the charged particles onto the collecting surfaces.

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93 4. Charge Dissipation - Neutralizing the charged particles on the collecting surfaces 5. Particle Dislodging - Removing the particles from the collecting surface to the hopper 6. Particle Removal - Conveying the particles from the hopper to a disposal point

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95 ADVANTAGES 1. High collection efficiency. 2. Particles as small as 0.1 micron can be removed. 3. Low maintenance. 4. Low pressure drop ( cm of water). 5. Satisfactory handling of a large volume of high temperature gas. 6. Treatment time is negligible (0.1-10s). 7. Cleaning is easy by removing the units of precipitator from operation. 8. There is no limit to solid, liquid or corrosive chemical usage.

96 DISADVANTAGES 1. High initial cost. 2. Space requirement is more because of the large size of the equipment. 3. Possible explosion hazards during collection of combustible gases or particulate. 4. Precautions are necessary to maintain safety during operation. Proper gas flow distribution, particulate conductivity and corona spark over rate must be carefully maintained. 5. The negatively charged electrodes during gas ionization produce the ozone.

97 CLEANING OF PLATES AND DUST REMOVAL

98 APPLICATIONS OF ESPS 1. Cement factories: a) Cleaning the flue gas from the cement kiln. b) Recovery of cement dust from kilns. 2. Pulp and paper mills: a) Soda-fume recovery in the Kraft pulp mills. 3. Steel Plants: a) Cleaning blast furnace gas to use it as a fuel. b) Removing tars from coke oven gases. c) Cleaning open hearth and electric furnace gases.

99 4. Non-ferrous metals industry: a) Recovering valuable material from the flue gases. b) Collecting acid mist. 5. Chemical Industry: a) Collection of sulfuric and phosphoric acid mist. b) Cleaning various types of gas, such as hydrogen, CO 2, and SO 2. c) Removing the dust from elemental phosphorous in the vapor state. 6. Petroleum Industry: a) Recovery of catalytic dust. 7. Carbon Black industry: a) Agglomeration and collection of carbon black. 8. Electric Power Industry: a) Collecting fly ash from coal fired boilers.