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by: Thelma Dickens


Thelma Dickens
GPA 3.81

David Leith

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David Leith
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This 16 page Class Notes was uploaded by Thelma Dickens on Sunday October 25, 2015. The Class Notes belongs to ENVR 754 at University of North Carolina - Chapel Hill taught by David Leith in Fall. Since its upload, it has received 21 views. For similar materials see /class/228870/envr-754-university-of-north-carolina-chapel-hill in Environment at University of North Carolina - Chapel Hill.




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Date Created: 10/25/15
Gas Absorption Below is a picture of a packed tower used to absorb inorganic gases into liquid On the right are examples of packing materials that can be used to ll the tower Gas out Liquid in D 391 39I u TA 39quotl 39 r 0 39on o lu ooquot quot c g quot a39lt 3 ma am be I 2 r3 a n c 4E O a 395 2 go 2 Q 9 O 393 39 a I c Q 4 gagg quot o 39quot 9 5919 39 c o 939 o quot s e i ux o e a d39n o 39o 395 0 vazz a a r 3 Liquid distributor Packing restraincr Shell Random packing Liquid redistributor Packing support C Gas in D Liquid out h BERL SADDLE RASCHIG RING NTALOX SADDLE FALL RING TELLERETIE Packing materials come in different sizes from about 1 cm to about 3 or 4 cm Mole Fraction of Pollutant in Liquid or Gas x or y Mass Transfer of Punutant from Bulk Gas Across Interface and Into Bulk quuid Eddy Diffusion lNTE R FAC E vquot y l 4 J y yquot 1 Overall Drivin Forceg LIQUID lt DISTANCE GAS ENVR 754 AIR POLLUTION CONTROL HOMEWORK PROBLEMS SPRING 2007 Introduction to the Course 1 Horsepower and Cost 7 Calculate the horsepower requirements kw requirements and annual cost for each of the following control devices a A cyclone that operates 8 hrday weekdays only to collect wood dust from a furniture making operation Gas ow is 1000 cfm pressure drop is 45 inches w g A venturi scrubber that collects fume from a basic oxygen furnace that operates for 30 minutes out of each siX hour batch 330 days per year Gas ow is 8000 cfm pressure drop is 80 inches w g A furnace filter in your house that collects dust from the return air duct Flow is 500 cfm pressure drop is 03 inches wg The fan operates 20 ofthe time on an yearly basis A fabric filter at a power plant that collects y ash Flow is 9500000 cfm pressure drop is 8 inches wg The plant operates 24 hours a day 345 days per year Note This ow comes from the Roxboro Steam Electric Plant near Roxboro NC about 50 miles from Chapel Hill operated by Progress Energy Suppose that after taking this course you devise a way to reduce pressure drop through each of the above devices by 20 After graduation you decide to become a consultant to advise on these matters You promise your clients that they will be able to recover the cost of your fee within siX months after taking the advice you give them Assume for the moment that taking this advice requires no new equipment that is your advice can be implemented for free For each of the four cases above how much should you charge for your fee 2 Size Distributions 7 The following results were obtained from a Bahco analysis of y ash speci c gravity 263 This device produces a cumulative size distribution by mass Aerodynamic Diameter Weight Finer Than Micrometers 9954 250 9672 149 7866 61 6655 254 6644 237 6610 197 6256 111 5431 77 3394 44 894 21 284 13 a Plot the cumulative size distribution by mass for these data on logprobability paper b In 750 kg of y ash how much will lie between 10 and 50 pm in aerodynamic diameter How much will be less than 10 pm in aerodynamic diameter c What is the mass median size d Plot a frequency distribution by mass for these data e Does this distribution t any regular pattern 3 Probe Design 7 On your rst job after graduation you are asked to determine the size distribution and concentration of particles in a horizontal duct with an inside diameter of 10 inches This information will be used to specify a piece of gas cleaning equipment Before taking the sample you nd out from the plant engineer that the average gas velocity in the duct should be 3200 feet per minute You decide to sample isokinetically at the centerline of the duct into an Andersen cascade impactor that is designed to operate at 283 me 1 cfm Your probe must have a 90 degree bend as shown in the gure below You orient the probe to be as vertical as possible to minimize collection by gravity in the probe and to lead directly into the top of the impactor The particlegenerating process is far enough upstream that you are con dent that a sample taken at the centerline will be representative of the aerosol in the whole duct Before going into the eld to take the sample you need to design a sampling probe to sample isokinetically from the duct center line 2 Part a Specify the dimensions of your sampling probe including inside diameter length location and radius of the bend As you do this consider the problem of particle loss due to the bend of your sampling probe Is it is possible to minimize collection in the bend of the probe by controlling the radius of curvature of the bend Gas passing through a sharp bend with small radius of curvature would have a small residence time in the bend which is good but would also cause high radial acceleration which is bad Develop an equation to predict particle collection in a bend or nd one in the literature Then use your equation to determine whether the probe should have a sharp bend or a gradual sweeping bend For the probe you design use your equation to plot the relationship between particle removal efficiency due to the bend in the probe against aerodynamic particle diameter Sampling Setup um Sampling Probe Hint Look for an equation in the appropriate chapter in the book by Baron and Willeke Aerosol Measurement or look for the following articles Pui D Y H F RomayNovas et al 1987 quotExperimental study of particle deposition in bends of circular cross sectionquot Aerosol Sci and Technol 7 301315 McFarland A R H Gong et al 1997 quotAerosol deposition in bends with turbulent flowquot Environ Sci Technol 31 33713377 3 4 Isokinetic Sampling 7 When you get to the plant to sample you measure the actual gas velocity at the duct centerline and nd that it is only 2830 fpm instead of the 3200 fpm expected Instead of adjusting the sample ow which you realize would change all the cut points for the impactor you elect to sample at 283 me as originally planned in the hope that the aerosol particles will all be small so that the nonisokinetic sampling that results will not cause undue bias in the sample When you return to the lab you obtain the following data from the impactor Sample ow 283 me Sample time 50 minutes Inlet velocity 3200 fpm Duct velocity 2830 fpm Data from Andersen Impactor Measurements Impactor Stage Aerodynamic Size Range um milligrams collected Preselector gt 10 4452 0 90 to 100 2024 1 58 to 90 2821 2 47to 58 1658 3 33 to 47 852 4 21 to 33 809 5 11 to 21 752 6 065 to 11 455 7 043 to 065 151 Filter lt 043 100 Your coworker whose graduate work did not include a course like this one tells you that correctng for the nonideality in sampling is probably a waste of your time because nobody ever does it Correct your data for nonisokinetic sampling and for the bend in the probe Plot the measured uncorrected cumulative size distribution by mass and the true cumulative size distribution by mass corrected for probe bend loss as well as for nonisokinetic sampling on the same piece of logprobability paper Plot the error that would result if you did not correct your size distribution data for probe loss and nonisokinetic sampling against particle diameter Determine the measured mass concentration in the duct and the true mass concentration in the duct in gm3 Was your coworker right 4 Inertial Collectors 5 Cyclone Design 7 The dust from a rockcrushing operation has the size distribution given below The concentration of dust coming from this operation is 7 gm3 and the gas ow rate is 4000 cfm The rock has a density of 25 gcm3 Particle dia pm 100 70 50 30 20 10 5 1 Mass lt stated size 72 67 52 38 25 14 9 4 A cyclone is proposed to collect the dust from the rock crushing operation To work this problem use the solver function in Excel to optimize cyclone dimensions for given pressure drop and cut diameter a Pick a cyclone diameter and cyclone height that seem appropriate b Pick a cyclone pressure drop and maximize cyclone overall efficiency using equations that relate cyclone dimensions to pressure drop and cut diameter and Solver c Repeat these steps until you develop a plot of pressure drop versus cyclone overall efficiency d Plot on the figure from part c the point that corresponds to the overall efficiency and pressure drop of a Stairmand high efficiency standard design cyclone e If the design objective is to keep the dust concentration coming from the cyclone below 500 mgm3 list the dimensions and pressure drop of the cyclone required f Compare your results from part e with the dimensions pressure drop and outlet dust concentration obtained using a Stairmand high efficiency cyclone for the same application 5 6 Performance of the Oil Screen Collector a Develop from rst principles force mass x acceleration etc an equation for the relationship between aerodynamic particle diameter and collection ef ciency for an OilScreen collector The con guration and the dimensions of this collector are on the following page along with measurements of its collection ef ciency that we made in our lab The overall size of the OilScreen is 235 by 235 Look on the mezannine ofthe Baity Lab to nd the Oil Screen we tested from which you can get additional dimensions This device is commercially sold to remove liquid mists b Use your equation to predict collection ef ciency for an OilScreen with the following characteristics Air ow is 1000 c n Mist droplets are mineral oil with a density of 081 gcm3 The Oil Screen contains two rows of collection elements in series Plot your predictions together with the data for measured ef ciency taken from experiments performed in our la Figure 2 39 39 39 mm In m my gure an I h L 39 39 39 39 inloihcpnge diayam oflh impacxion elements in the OiLScreen 5 Mlneral Oll Soluble Oll Synthetlc Fluld DOP PPP 10an Fractional Efficlency 0 90909 n 5525559 uN IO 1 P Droplet Diametermm Figure 7 E ciency vs droplet diameter for the OilScreens alone mmhwhhu L 39 I 1 1 um Error bars represent 95 con dence intervals For the three cutting uids Air ow was 1000 c 39n Filters 7 Design of Filter for Diesel Car 7 You have been asked to determine whether a ber lter is practical to collect smoke from the exhaust of diesel cars The concern is that particles in diesel exhaust contain PAH compounds that may be carcinogenic therefore these particles should be controlled if possible Objectives in your design are i The lter should be small not to exceed the size ofa typical muf er 1 meter x 30 cm x 15 cm ii The lter should have high ef ciency for small particles iii The lter should have a reasonable pressure drop that is energy consumption of the lter should not affect gas mileage to an appreciable extent iv The lter should have a reasonable lifetime The filter may be replaced as often as an oil lter or an air lter in a car is replaced Assume that glass bers are available with diameters between 05 and 100 micrometers you can specify ber diameter in your design Also assume that the exhaust ow is 100 cfm at 50 C the mass concentration of smoke from the car is 01 gm3 and the smoke particles have a density of l gcm3 The size distribution of the smoke is given below Determine a The layout for the filter ie gas passages in and out location of lter media within the lter housing etc Be creative here The layout need not be with gas ow along the axis of a long cylindrical tube Consider the pros and cons of alternative layouts in terms of ef ciency pressure drop and lter life b The fractional ef ciency curve for the lter layout you select at the design ow c The overall ef ciency for the lter d The lter lifetime in operating hours and in days assuming the car is operated one hour per day e The pressure drop across the lter when new and at the end of its lifetime f The cost of using this lter expressed in reduction of miles per gallon of fuel used Based on your results decide whether such a lter is practical for use on diesel automobiles In working this problem you will have to make some assumptions List and justify the assumptions you make 8 Size Distribution for Diesel Exhaust Smoke Size Range Fraction of Mass micrometers in this Size Range 00 02 0043 02 03 0058 03 04 0085 04 06 0180 06 08 0261 08 10 0196 10 15 0082 15 30 0052 gt 30 0043 8 Speci c Resistance of Dust Deposit 7 For coal y ash collected on fabric lters the book by Calvert and Englund see table in your notes lists a value for K2 of 117 to 251 N ming m The same reference gives the density of y ash as 23 gcm3 Assume the porosity of a dust deposit is between 075 and 085 The size distribution for y ash from a pulverizedcoal boiler is Upper bound size by mass lt stated size 3 pm 15 5 25 10 42 20 65 40 81 a Compare this handbook value for K2 with values calculated from the Kozeny Carman equation and the RudnickHappel equation b Can disagreement between your calculated K2 values and the tabulated value be due to uncertainties in the particle size distribution of the dust or the porosity of the dust deposit Explain using examples if appropriate 9 Pulse Jet Filter Performance 7 A pulsej et lter collects rock dust with bulk density of 145 lbft3 The lter now operates at a pressure drop of 7 inches of water It has 200 bags made from untreated polyester fabric each bag is 412 inches in diameter and 8 feet long The lter handles 10000 cfm of gas Each bag is cleaned once every three minutes with compressed air at 100 psi The inlet dust concentration is 15 gm3 9 Ventilation air from a new process must also be controlled Your boss suggests that this new air can also be passed through the existing pulsejet lter The new process will provide 2000 cfm of additional air and contain rock dust in concentration of 05 gm3 this dust will probably be coarser than the dust from the existing process The dust concentration of the new process may occasionally rise to 5 gm3 and the ow from the new process may sometimes be as high as 3000 cfm a Determine KZK3 for the lter as it operates now b Determine the gas ow and dust concentration in the gas after the ow streams are combined c Using the KZK3 value from a determine whether the lter can be operated after combining the ows without pressure drop exceeding 7 inches of water Consider changes in lter operating conditions if necessary d Do you agree with your boss that the air ows can be combined Explain Scrubbers for Particle Collection 10 Venturi Scrubber Design 7 The size distribution by mass of dust emitted from a rotary rock dryer without control is given below Size by mass lt Size 01 pm 001 02 005 05 02 l l 2 5 3 40 5 95 7 999 8 9999 The particles have a density of 27 gcm3 and the temperature of the gas is 200 C 8000 cfm of gas are to be cleaned The diameter of the duct leading to the venturi is 22 inches An ef ciency of 99 is required on an overall mass basis Remember that some of the water supplied to the venturi will cool and saturate the gas stream Only after the gas is saturated will water be available to collect dust particles 10 Design a venturi scrubber that will achieve the required efficiency You should specify a the water ow necessary to cool and saturate the gas stream in gallons per minute and the temperature of the gas after saturation occurs b the diameter and throat length of the venturi c the venturi throat velocity in feet per minute d the total water rate to the venturi in gallons per minute e the pressure drop required f the efficiency achieved If you wish try using Excel s solver to explore the effect of different venturi operating conditions on pressure drop for the given efficiency requirement 11 A Practical Problem 7 The bus stop directly across the street from the Public Health School is closed due to construction As a result you must walk or run in the rain from the front door of the School of Public Health to the bus stop across from the Carolina Inn a distance of 400 meters You have forgotten your umbrella Assume the raindrops are 08 mm diameter water drops that fall vertically at the rate of 2 cm water accumulation per hour After a program of vigorous weighttraining your body closely resembles a solid rectangle 15 m tall 450 mm wide and 180 mm thick You wish to evaluate alternative strategies for reaching the bus stop a Plot the volume of water your body collects vs speed with which you run Distinguish between top head hits and front body hits b How fast should you run to the station c The bus will not arrive until 20 minutes after you leave the School regardless of how fast you run The bus shelter is already filled with people so you must stand out in the rain the whole time you are there Does this change your opinion about how fast you should run to the bus stop d How would your answers to b or c change if you had remembered your umbrella 11 Electrostatic Precipitators 12 Precipitator Design 7 The y ash from a pulverized coal red furnace has a mass median diameter of 10 um and a geometric standard deviation of 35 Particle density is 25 gcm3 This y ash is emitted at the rate of 170 lbton of coal red in a ue gas stream molecular weight of28 1 of 147 x 106 ft3min at 350 F A collection system is to be designed to meet the EPA emission regulation of0 10 lbmillion BTU The coal used has a heating value of 12800 BTU1b and is red at the rate of 35 tonshour Consider the use of an electrostatic precipitator with or without a primary collector such as a cyclone ahead of it for this purpose For the precipitator a estimate the minimum collecting surface required in square meters b propose an arrangement for the plates number in parallel spacing height length and number of compartments The overall dimensions of the precipitator should be reasonably close to cubic to minimize construction costs 13 Two Stage Precipitator Optimization 7 For a twostage precipitator consider the trade off between the length of the charging section and the length of the collection section What fraction of the total precipitator length should be devoted to the charging section H int You could approach it in one of several ways One way would be to use a rigorous analytical procedure in which for example you take the derivative of ef ciency with respect to fraction as charging section Then set the derivative equal to zero and identify the location of the ef ciency maximum make sure it s not a minimum Another way would be to set up the equations in Excel and use solver or use a trial and error approach to locate the ef ciency maximum for you Control of Gases and Vapors 14 Air Condition the Superdome 7 One of the larger carbon adsorption systems is the air conditioning system for the Louisiana Superdome in New Orleans httpWWW J com site nhn The purpose of this system is to remove odors from the air inside this structure Speci cations for the building are Building area 13 acres Building height 27 stories 12 Capacity 70000 people for football Use Rate 8 hoursday 100 daysyear You determine from a literature search that a person generates one mg of odorous compounds per hour and that in a quotstressfulquot game this rate increases to 5 mghour However the literature is ambiguous on generation rate the references you nd give values that range from 002 to 10 mghr generated per person Speci cations for the carbon used in the air handling system are Bed porosity 05 disregarding interior pores Granule diameter 2 mm a What air ow is required if the concentration of pollutants is to be below the odor threshold You must decide how big a bed to specify and how often it will be regenerated b Specify the design height width thickness of an activated carbon bed adequate to remove these pollutants c Specify how often this bed will have to be regenerated Extra Credit Estimate bed pressure drop using the theory we developed for ow through dust deposits Remember that our model assumes ow through the deposit is laminar Is this a correct application of this equation If not would actual pressure drop be higher or lower than that calculated 13 15 Carbon Bed Optimization 7 You are to evaluate a carbon adsorption bed for control of CC14 The system from which the CCl4 comes operates 10 hours per day and siX days per week According to the manufacturer the carbon bed can be regenerated 100 times before it must be replaced In addition you know that CCl4 costs 650gallon Effluent ow is 1000 cfm CCl4 concentration is 2500 ppm Activated carbon costs 125p0und Cost of an adsorption system is 25000 10 per pound of carbon installed Operating costs are 4000yr for maintenance and 50 each time the bed is regenerated Design a carbon bed system to collect this pollutant You must consider how large a bed to use and how often the bed should be regenerated Specify a Amount of carbon to be used b How often the bed should be regenerated c Bed con guration height width depth d Annual operating cost for your design e Time until initial cost is repaid In preparing your answer determine the bed size that is economically quotoptimumquot that is the bed size that results in lowest annual cost 14


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