Gaseous Emission-Control Technologies (Air-Quality Technology) (Nazaroff & Alvarez-Cohen, Section 7.C) (Mihelcic & Zimmerman, Section 12.

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1 Gaseous Emission-Control Technologies (Air-Quality Technology) (Nazaroff & Alvarez-Cohen, Section 7.C) (Mihelcic & Zimmerman, Section 12.7) Two different aroaches: - Pollution Prevention at the source the better alternative - Treatment of fumes as they are formed the classical aroach (Nazaroff & Alvarez-Cohen, Table 7.C.2, age 444) 1

2 Examle of stes taken to minimize air ollution from gasoline internal combustion engines 2

3 A three-way catalytic converter erforms three simultaneous tasks: 1. Oxidation of carbon monoxide (CO) to carbon dioxide 2 CO + O 2 2 CO 2 2. Oxidation of unburned hydrocarbons (HC) to carbon dioxide and water C x H 2y + [(2x+y)/2]O 2 x CO 2 + y H 2 O 3. Reduction of nitrogen oxides to nitrogen and oxygen 2NO x N 2 + x O 2 In other words: Choose your ollutant! Techniques to remove articles from an air stream (Nazaroff & Alvarez-Cohen, Table 7.C.1, age 443) 3

4 Cyclone Searators and their Design (Nazaroff & Alvarez-Cohen, ages augmented) (Mihelcic & Zimmerman, Section ) Low-ressure dro cyclone at Rochester Ashalt Plant (Victor, NY) A cyclone used in a woodsho (Lebanon, NH) htt:// Cyclone searators have been used in the United States for about 100 years, and are still one of the most widely used of all industrial gas-cleaning devices. The main reasons for the wide-sread use of cyclones are that they are inexensive to urchase, they have no moving arts, and they can be constructed to withstand harsh oerating conditions. Cyclone Design (Adated from Air Pollution Control by C. D. Cooer & F.C. Alley, 1986) Tyically, a articulate-laden gas enters tangentially near the to of the cyclone, as shown schematically in the left figure. The gas flow is forced into a downward siral simly because of the cyclone s shae and the tangential entry. Another tye of cyclone (a vane-axial cyclone see right figure) emloys an axial inlet with fixed turning vanes to achieve a siraling flow. Centrifugal force and inertia cause the articles to move outward, collide with the outer wall, and then slide downward to the bottom of the device. Near the bottom of the cyclone, the gas reverses its downward siral and moves uward in a smaller inner siral. The cleaned gas exits from the to through a vortex-finder tube, and the articles exit from the bottom of the cyclone through a ie sealed by a sringloaded flaer valve or rotary valve. 4

5 Advantages of cyclones: Low caital cost (few arts, easy to assemble) Ability to oerate at high temeratures (all metal arts) Low maintenance requirements (no moving arts). Disadvantages of cyclones: Low collection efficiencies (esecially for very small articles) cyclones used almost exclusively for articles > 5 m. High oerating costs (ower required to overcome large ressure dro). Cyclones by themselves are generally not adequate to meet stringent air ollution regulations, but they serve an imortant urose. Their low caital cost and their maintenance-free oeration make them ideal for use as re-cleaners for more exensive final control devices such as baghouses or electrostatic reciitators. In addition to use for ollution control, cyclones are used extensively in rocess industries. For examle, they are used for recovering and recycling certain catalysts in etroleum refineries, for recovering freeze-dried coffee in food rocessing lants, and for caturing saw dust in a lumber sho. Cyclones have often been regarded as low-efficiency collectors. However, efficiency varies greatly with article size and cyclone design. Advanced design work has greatly imroved cyclone erformance. Some cyclone manufacturers advertise cyclones that have efficiencies greater than 98% for articles larger than 5 microns, and others that routinely achieve efficiencies of 90% for articles larger than microns. In general, oerating costs increase with efficiency (higher efficiency requires higher inflow ressure), and three categories of cyclones are available: high efficiency, conventional, and high throughut. Tyical efficiency curves for these three tyes of cyclones are resented in the figure. 5

6 Standard Cyclone Dimensions Extensive work has been done to determine in what manner dimensions of cyclones affect erformance. In some classic work that is still used today, Sheherd and Lale (1939, 1940) determined otimal dimensions for cyclones. Subsequent investigators reorted 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 alied generally. The table on the next slide summarizes the dimensions of standard cyclones of the three tyes mentioned in the revious figure. The side figure illustrates the various dimensions used in the table. Standard cyclone dimensions Cyclone Tye High Efficiency Conventional High Throughut (1) (2) (3) (4) (5) (6) Body Diameter, D/D Height of Inlet, H/D Width of Inlet, W/D Diameter of Gas Exit, D e /D Length of Vortex Finder, S /D Length of Body, L b /D Length of Cone, L c /D Diameter of Dust Outlet, D d /D SOURCES: Columns (1) and (5) = Stairmand, 1951; columns (2), (4) and (6) = Swift, 1969; column (3) and sketch = Lale,

7 Cyclone Theory Collection Efficiency A very simle model can be used to determine the effects of both cyclone design and oeration on collection efficiency. In this model, gas sins through a number N of revolutions in the outer vortex. The value of N can be aroximated as the sum of revolutions inside the body and inside the cone: 1 c L N Lb H 2 where N = number of turns inside the device (no units) H = height of inlet duct (m or ft) L b = length of cyclone body (m or ft) L c = length (vertical) of cyclone cone (m or ft). To be collected, articles 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 t ath length / seed D N / V i where t = time sent by gas during siraling descent (sec) D = cyclone body diameter (m or ft) V i = gas inlet velocity (m/s or ft/s) = Q/WH Q = volumetric inflow (m 3 /s or ft 3 /s) H = height of inlet (m or ft) W = width of inlet (m or ft). The maximum radial distance traveled by any article is the width of the inlet duct W. The centrifugal force quickly accelerates the article to its terminal velocity in the outward (radial) direction, with the oosing drag force equaling the centrifugal force. The terminal velocity that will just allow a article initially at distance W away from the wall to be collected in time is V t W / t where V t = article drift velocity in the radial direction (m/s or ft/s). 7

8 The article drift velocity is a function of article size. Assuming Stokes regime flow (drag force = 3πμd V t ) and sherical articles subjected to a centrifugal force mv 2 /r, with m = mass of article in excess of mass of air dislaced, v = V i of inlet flow, and r = D/2, we obtain ( a) d Vt 9 D 2 V 2 i where V t = terminal drift transverse velocity (m/s or ft/s) d = diameter of the article (m or ft) = density of the article (kg/m 3 ) a = air density (kg/m 3 ) = air viscosity (kg/m.s). Substitution of the 2 nd equation into the 3 rd eliminates t. Then, setting the two exressions for V t equal to each other and rearranging to solve for article diameter, we obtain d 9 W N Vi ( a ) 1/ 2 It is worth noting that in this exression, d is the size of the smallest article that will be collected if it starts at the inside edge of the inlet duct. Thus, in theory, all articles of size d or larger should be collected with 100% efficiency. Note that the units must be consistent in all equations. One consistent set is m for d, R and W; m/s for V i and V t ; kg/m.s for ; and kg/m 3 for and a. An equivalent set in English units is ft for d, R and W; ft/sec for V i and V t ; lbm/ft.sec for ; and lbm/ft 3 for and a. 8

9 The receding equation shows that, in theory, the smallest diameter of articles 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 articles and the gas. In ractice, collection efficiency does, in fact, deend on these arameters. However, the model has a major flaw: It redicts that all articles larger than d will be collected with 100% efficiency, which is incorrect. This discreancy is the result of all our aroximations. Lale (1951) develoed a semi-emirical relationshi to calculate a 50% cut diameter d c, which is the diameter of articles collected with 50% efficiency. The exression is d c 9 W 2 N Vi ( a ) 1/ 2 where d c = diameter of article collected with 50% efficiency. Note the similarity between the last two equations. The only difference is a factor 2 in the denominator. Particle collection efficiency versus article size ratio for standard conventional cyclones 100%, all articles collected 50%, 1 article in 2 is collected Lale then develoed a general curve for standard conventional cyclones to redict the collection efficiency for any article size (see side figure). If the size distribution of articles is known, the overall collection efficiency of a cyclone can be redicted by using the figure. d c Theodore and DePaola (1980) then fitted an algebraic equation to the curve, which makes Lale s aroach more recise and more convenient for alication to comuters. The efficiency of collection of any size of article is given by O%, no article collected where j 1 1 d / d 2 j = collection efficiency of articles in the jth size range (0 < j < 1) d j = characteristic diameter of the jth article size range (in m). c j 9

10 The overall efficiency, called erformance, of the cyclone is a weighted average of the collection efficiencies for the various size ranges, namely j M m j where = overall collection efficiency (0 < < 1) m j = mass of articles in the jth size range M = total mass of articles. Examle of Cyclone Analysis Given: Conventional tye (standard roortions) D = m Flow rate = Q = 150 m 3 /min Particle density = = 1600 kg/m 3 Particle size distribution as follows: Question: Particle size % mass in that size range (d ) (m/m) 0-2 m % 2-4 m 9.0% 4-6 m 10.0% 6-10 m 30.0% m 30.0% m 14.0% m 5.0% m % 100% What is the collection efficiency? 10

11 Solution 1 N L H b Lc 6 2 V i Q WH Q D m/min = 20 m/s d c W 2 NV ( ) D V ( ) 9 6 i a i a m = 5.79 m Size range (in m) Average size d (in m) Collection efficiency Mass fraction m/m Contribution to erformance x m /M % % % % % % % % % % % % % % % % % Two distinct aroaches in cyclone analysis 1. Performance analysis The cyclone exists All dimensions and characteristics are known. The engineer calculates its collection efficiency (). 2. Design analysis The cyclone needs to be chosen for a given task. A target erformance () is imosed. Tyically, the engineer knows the amount of air flow (Q), & characteristics of articles (range of d s, mass fractions m j /M). The engineer needs to determine the tye of cyclone needed (conventional, high-throughut, ) the required size of the device (diameter D). Note: Design analysis usually necessitates an iterative aroach. (Successive guesses until the required erformance is achieved.) 11