Chapter 10(new)

*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,Chapter 10,Control of Volatile Organic Compounds (,VOCs,),key,VOCs,Control by prevention,Calculation of condensation/adsorption/absorption/combustion,Why we talk about,VOCs,?,Second-most widespread and diverse class of emissions,Toxic, carcinogenic,Participate in the smog reaction,Contribute to the formation of fine particle,Global warning because of infrared absorption,VOCs,is an acronym for Volatile Organic Compounds,How serious?,Emissions Estimates: 20 million tons/year in U.S.A,U.S.A 2006 National Emission,National as well as Guangdong situation,机动车,全国,1.9,亿辆,北京近,500,万辆,上海、广州超过,200,万辆,广州、深圳、杭州、成都等超过,100,万辆,溶剂使用和生产,2010,年全国涂料产量超过,1000,万吨（包括涂料和建筑涂料）,广东省产量超过,1/3,，占全国第一位,排放总量：,3700,万,t,（曹国良等, 2010,）,1500,万,t,（叶代启等, 2010,）,10.1 Vapor Pressure, Equilibrium Vapor Content, Evaporation,Behavior of volatile liquids as a function of their vapor pressure p and,P,atm,饱和蒸气压, 10,3,kPa,温度,Temperature,t,/,Saturated water vapor pressure,/(10,3,Pa),0,0.61129,5,0.87260,10,1.2281,15,1.7056,20,2.3388,25,3.1690,30,4.2455,Evaporate at significant rates,Raoults,Law,Example 10.1,Estimate the water content of air that is equilibrium with pure water at 68 (20),Using the value read from the previous fig., we find,ppm,mg/m,3,Example 10.2,A liquid mixture of 50 mol percent benzene and 50 mol percent toluene in equilibrium with air in a closed container. At 68 , the vapor pressure of benzene and toluene are 1.45 and 0.42,psia, respectively. Estimate the content in air.,Much higher than “the criterion ”,VOLATILITY,Clausius-Clapyron,Equation,Antoine Equation,Please calculate the sat. pressure of C,2,H,6,O in 25,.,We read from P,559,that A=8.04494,B=1554.3, C=222.65.,So, p=58.7mmHg (7.7kPa).,VOCs,10.2,VOCs,Vapor Pressure 0.01,psia,(0.0007atm, 70Pa),Boiling Points 260 ,Benzene 80 ,toluene 111 ,Xylene,138-144 ,means: Organic compounds up to 12 carbon atoms,Hydrocarbon (HC),Vapour,pressure and molecular weight change during distillation,10.3,Control by Prevention,Substitution,Oil-based paints, coatings, and inks to water-based ones,Process Modification,Improve public transport,Leakage Control,Working losses,Filling or displacement losses,Emptying losses,Breathing losses,Example of clean production,江苏某化工厂清洁生产实例,生产有机化工产品，如医药中间体吡咯烷,生产工艺落后，污染严重，被要求限期治理。吡咯烷生产存在的问题：,反应工序繁杂（,174h/,批次）,物料品种多（,13,种），物耗高（,58:1,），投入产出小,废物量大，污染严重，,56.907t,废物,/t,吡咯烷,成本高，经济效益差，亏本或微利,清洁生产方案,淘汰落后生产工艺，采用国际先进的新工艺,高压氢化法,经济分析：,投入,150,万元（固定资产,90,万，流动资金,60,万）,年增加效益,612.5,万元（其中包括减少污染治理费用,4,万），投资偿还期,4,个月,减少了工序，每批产品生产时间：,37h,物耗对比：,效果,项目,实施前,实施后,削减量,工艺三废总量,/t,1423,64,1359,物耗,1448,93,1355,煤耗,1400,650,750,废气量,94.8,0,94.8,外排废水量,工艺废水,1328,0,1328,清水,1000,0,1000,COD/(mg/L),5800,56.2,5764,COD,量,/t,135,0.56,134.44,项目,实施前,实施后,项目,实施前,实施后,色泽,淡黄色,无色,含水率,/,1.0,0.5,含量,/%,85,95,合格率,/,90,100,污染物产生量对比（以年产,25,吨吡咯烷计）,10.4 Control by Concentration and Recovery,Condensation,Adsorption,Absorption,Combustion,Biofiltration,Condensation,Condensation is the conversion of compounds from the gas phase to the liquid phase by decreasing temperature or increasing pressure,.,Condensation is useful when,when the concentration of,VOCs,higher than 1%,pollutant has a high vapor pressure but recovery down to,ppm,levels is not required,as a pretreatment,Condensation,Condensation will occur,At a given temperature, if the partial pressure of a compound is greater than or equal to its vapor pressure at that temperature,If the temperature of a gaseous mixture is reduced to the saturation temperature, at which the vapor pressure equals the partial pressure of one of the constituents,Mechanism of Condensation,Control of organic emissions by condensation is limited by equilibrium partial pressure of the component,a cold surface or a cooling liquid is deployed in the gas stream to induce condensation,the partial pressure of the material remaining in the gas phase decreases rapidly and complete condensation is not possible,Example 10.9,Treat an air stream containing 0.005 mol fraction,（,0.5%,5000ppm,）,toluene, moving at 100(37.8) and 1,atm, so as to remove 99% of the toluene by cooling. To what temperature must we cool the air stream?,-,99% 1% 50ppm,p=,y,i,P,p=,x,i,p,* x,i,=1,y,i,=5010,-6,p=5010,-6,101325=5Pa,p559 A B C,6.95 1344 219,T=-60 ,90%?,-37 ,Low temperature,Types of Condensers,In,Contact Condensers,the vapor and cooling medium are brought into direct contact,cool the vapor by spraying a cold liquid, usually water, directly into the gas stream,The condensed vapor and water mixture is treated, and recovered or disposed of,Water,Vapor,Spray,Contact Condenser,Types of Condensers,In,Surface condensers,the vapor to be condensed and the cooling medium (water) are separated by a metal wall,the cooling medium flows through the tubes and vapor condenses on the outside surface.,Surface condensers are more expensive, less flexible and less efficient in removing organic compounds than contact condensers.,Condensation,American Environmental International, Inc. (AEI),Difficulties,The temperature is too low,Below the freezing tem. of material, need frequent defrosting,Water freeze,Energy wasted,Fire hazard (explosive limit),Adsorption,Adsorption is the process whereby molecules of a liquid or a gas are preferentially separated from a flow stream and collected on the surface of a solid,Adsorption is useful when,recovery of the gas is important,the gas is in very dilute concentration,Types of Adsorption,There are two types of adsorption,Physical Adsorption,The gas molecule (,adsorbate,) adheres to the surface of the solid (adsorbent) due to intermolecular attractive forces,Process is reversible,Chemisorption,The gas molecule forms a chemical bond with the solid,process is not reversible,Mechanism of Operation,Adsorption utilizes gas-solid equilibrium,Goal: shift equilibrium towards saturation,works better near the dew point of the,adsorbate,highest pressure and lowest temperature within process limits,multiple compounds in the same gas stream,at first the compounds are equally adsorbed,breakpoint - compound with the lowest boiling point begins to,revaporize,fja,Design of a adsorption device,The amount of adsorbent,Equilibrium curve,Example 10.12,Treat an air stream containing 0.001 mol fraction(0.1%,1000ppm) toluene, moving at 37.8(100), 10000m,3,/h and 1,atm,. If we use the adsorption bed to remove the pollutant, how many kg AC we must use? The bed operate 8h between regenerations.,（,a,）,AC is used only once;,（,b,）,it is regenerated to an outlet stream toluene content of 0.5 percent at 149 (300).,-,C,7,H,8, 92 1000ppm 3.6g/m,3,3.610,-3,10000=36kg/h 8h,m,tol,=368=288kg,From fig10.12, equilibrium adsorption is 0.29kg/kg,（,a,）,adsorbent needed=288/0.29=993kg,（,b,）,From fig10.12, equilibrium adsorption is 0.11kg/kg,adsorbent needed=288/0.18=2618kg,At least!,Breakthrough curve,Wurof,equation,Assume ideal breakthrough,Within breakthrough time,a,m,saturation absorption amount,kg/m,3,A,L,C,0,u,In air,In bed,then,L,实际曲线与理论曲线的比较,1,理论线,2,实际曲线,Actually,选定吸附剂和操作条件，如温度、压力、气体流速等。对于气体净化，空床流速一般取0.10.6,m/s，,可根据已给处理气量选定。,根据净化要求，定出穿透点浓度，在载气速率,Gs,一定的情况下，选取不同的吸附剂床层长度,L,1,、,L,2,、,、,L,n,，,可根据已给处理气量选定。,以,L,为横坐标，,为纵坐标，作,L -,直线，则其斜率为,K，,截距为,0,。,根据生产中计划采取的脱附方法和脱附再生时间、能耗等因素确定操作周期，从而确定所要求的穿透时间,用希洛夫公式计算所需吸附剂床层长度,L,。,Design process,由气体流速,u,，求床层截面积,A,（,m,2,）,若,A,太大，可分为,n,个并联的小床，则每个小床的截面积（,m,2,）,由床层截面积,A,或,A,可求出床层直径,D,（,圆柱形床）或边长,B,（,正方形床）。,求所需吸附剂质量。,式中：,A,吸附床横截面积，,m,2,。,考虑到装填损失，每次新装吸附剂量为（1.051.2）理论量。,核算固定床压降,p,。,若,p,值超过允许范围，可采取增大,A,或减小,Z,的办法使,p,值降低。,采用欧根（,Ergun）,方程计算（经验公式）,设计吸附剂的支承与固定装置、气体分布装置、吸附器壳体，各连接管口及进行脱附所需的附件等。,式中：,P,通过床层的压降，,Pa；,L,床层长度，,m；,气体的动力粘度，,Pa/s；,颗粒层孔隙率，%；,气体密度，,kg/m3；,u,床层进口横截面积处气体平均流速，,m/s；,d,p,吸附剂颗粒直径，,m.,EXAMPLE:,某厂产生含,CCl,4,废气，气量,Q=1000m,3,/h，,浓度为45,g/m,3,，,一般均为白天操作，每天最多工作,8h,。,拟采用吸附法净化，并回收,CCl,4,，,试设计需用的卧式固定床吸附器。,SOLVE,：,CCl,4,为有机溶剂，沸点为76.8,，微溶于水，可选用活性炭作吸附剂进行吸附，采用水蒸气置换脱附，脱附气冷凝后沉降分离回收,CCl,4,。,根据选用粒状活性炭作吸附剂，其直径为3,mm，,堆积密度,s,=300600kg/m,3,，,孔隙率,=0.330.43。,选定在常温常压下进行吸附，维持进入吸附床的气体在20以下，压力为101325,Pa。,根据,经验,选取空床流速,u=20m/min。,将穿透点浓度定为50,m,g/m,3,。,以含,CCl,4,5g/m,3,的气流在所指定的条件下进行动态吸附实验，测定不同床层高度下的穿透时间，得到如下实验数据：,以,L-,横、,-,纵，作图得一直线。,在所作直线上取两点（0.14,200,）,（0.35,651,）,于是：,0,=101min（,查图,）,据该厂生产情况，考虑每周脱附一次，床层,每周吸附6天，每天按8,h,计，累计吸附时间,48h,。,因而床层高度为：,取,L=1.4m,采用立式圆筒进行吸附，其直径为：,取,D=1.0m,所需吸附剂量,考虑到装填损失，取损失率为10%，则每次新装填吸附剂时需活性炭545726,kg。,压力降 查20,101.325,Pa,条件下空气密度,=1.2kg/m,3,此时气速,设计吸附器壳，略,Choosing an Adsorbent,The adsorbent must selectively adsorb the desired compound:,Siliceous and metal oxide adsorbents,preferentially adsorb water from gas mixtures,Activated carbon,adsorbs,nonpolar,organics from gas mixtures, even in the presence of water vapor,Activated Carbon,granular,pelletized,powdered,coconut shell,3500X magnification,honeycomb,Activated,Carbon,Fiber,Activation,raw materials,fruit pits, coconut shells,coal, peat,petroleum residues,heat to 1560,o,to 1740,o,F,product,hard, dense granules of carbon,Replacement & Regeneration,Saturation of the adsorbent,replacement,adsorbate,recovery costs exceed replacement costs,chemisorption,is occurred,recovery,physisorption,is used,resale or reuse of pollutant for profit,pollutant must be recovered before disposal,Adsorption, contd.,Continuous operation,airstream,regeneration,Cooling after,desorption,decomposition or partial oxidation,Steam,used for regeneration,Vapor- laden air,Two Unit Fixed Bed,Adsorber,Steam,Adsorber,2,Adsorber,1,Condenser,Decanter,Top-phase,liquid,Bottom-phase,liquid,Exhaust air to atmosphere,Steam plus,solvent vapors,Lower Explosive Limit,Carbon Adsorption and Solvent Recovery,活性炭吸附塔,Absorption,Absorption is the mass transfer of pollutants from the gas to the liquid phase which is driven by a concentration gradient at the gas-liquid interface,Absorption is useful when,the gas is highly soluble in the solvent,the concentration of pollutant in gas is high,pollutants are inorganic compounds,Solubilities,Technical terms:,absorbent: liquid to absorb gas contaminants (e.g. water);,absorbate,: gaseous contaminants (SO,2, H,2,S, NH,3,);,carrier gas: inert portion of gas stream (e.g. air);,interface: area where two phases contact;,solubility: capability of a contaminant dissolving in a liquid.,PROCESS DESCRIPTION,Mass transfer in an absorption process:,from bulk gas phase to interface,from interface into liquid phase,from liquid phase to bulk liquid,Turbulent water,HCl,HCl,HCl,HCl,HCl,HCl,HCl,HCl,diffusion,eddy mixing,eddy mixing,diffusion,Turbulent air,Air-water interface,Air boundary layer,Water boundary layer,The,thickness of the two boundary layers is determined by the type of absorbers. The thinner the boundary layer, the better the transfer.,yy*,吸收,y=y*,平衡,yy*,解吸,PROCESS DESCRIPTION,1) No chemical reactions for the contaminants with large solubility in absorbent:,2) Chemical reactions exist between contaminants and absorbent for less soluble contaminants:,H,2,S (gas),H,2,S (dissolved in water),NH,3,(gas),NH,3,(dissolved in water),Two types of absorption process:,NaOH,+ SO,2,+ O,2,Na,2,SO,4,+ H,2,O,HCl,(gas),HCl,(dissolved in water),VOCs,(gas),VOCs(dissolved,in water),Fig. 10.15 The flow diagram for the most common method for removing one component from a gas stream,Absorption,Absorption, also called liquid scrubbing, is governed by,the properties of the substance,the packing material,the gas and liquid mass transfer rates,thorough contact between the gas and the liquid solvent,Liquid in,Gas Out,Liquid distributor,Packing restrainer,Shell,Random Packing,Liquid,redistributor,Packing support,Gas in,Liquid out,Counterflow,packed absorption tower,Design of absorbers,In air pollution control applications,The flow rate of gas,The content of the pollutants,The required degree of removal,The basic design variables,Selective reagent,The system pressure,The flow rates of the gas and liquid,The gas velocity in the column,The amount of liquid-gas contact,Applications of Absorption,Choosing an Absorbent,chemically similar to the,absorbate,relatively nonvolatile,noncorrosive, nonflammable,inexpensive and readily available,low viscosity, low freezing point,low in toxicity, chemically stable,Packing Materials,Packing material is used to increase the surface area of the tower, which improves collection efficiency,each shape has a unique surface area and associated gas pressure drop,Raschig,ring,Pall ring,Tellerette,Berl,Saddle,Intalox,Saddle,G = carrier gas, no pollutant,L = liquid solvent,X,bottom,Y,top,Y,bottom,X,top,dh,Y+dY,Y,X,X+dX,Mechanism of Operation,Mechanism of Operation,Absorption is favored when the liquid contains less than the equilibrium concentration of the component,G= carrier gas, no pollutant,L = liquid solvent,Mole ratio in gas, Y (poll./mol carrier gas,Mole ratio in liquid, X (poll./mol liquid solvent),Treat an,airstream,containing 0.005 mol fraction,（,0.5%,5000ppm,）,toluene, moving at 37.8, 10000m,3,/h and 1,atm,. We wish to recovering 99% toluene by absorption in a suitable solvent. Estimate the required solvent flow rate.,-,Water? No HC? OK,G=10000/25.4/60=6536mol/min,L=0.0876536=569mol/min, 10.235kg/min,Column diameter?,Column height?,Henrys law H=0.07atm,y,ibottom,=0.005, y,itop,=0.00005,x,itop,=0,H,toluene,0.8-seleted by experience,Column diameter,The flooding velocity (,溢流速度,液泛速度,),u,f,Operating velocity u=0.6-0.75 flooding velocity,A=Q/u,Ka=4.0lbmol/(h,ft,3,atm) found from some table,N is the number of transfer units,Column height,y*=,x,i,H/p,H=0.007atm,EQUIPMENT DESCRIPTION,(,Venturi,Scrubbers),Characteristics:,highest collection efficiencies;,high pressure drop;,different design based on the water injection and throat type;,relatively shorter contact time between gas and liquid;,lower gas rates and high liquid-to-gas ratios for efficient absorption.,water,EQUIPMENT DESCRIPTION,(Spray tower),Characteristics:,simplest device;,low pressure drop;,low removal efficiency;,suitable for extremely soluble contaminants;,(liquid, e.g. water),EQUIPMENT DESCRIPTION,(Packed column),dirty air,clean air,Characteristics:,most common equipment;,large surface area for gas-liquid contact;,packing materials determine surface area and cost;,pressure drop from 0.25 to 1 in./ft.,APPLICATIONS,APPLICATIONS,Comparison of gas absorption efficiencies for different absorbers,Source:,Ceilcote,Air Pollution Control,APPLICATIONS,Operating costs for some typical absorbers,Source:,Ceilcote,Air Pollution Control,Combustion,Combustion alters the chemical form of the pollutant by oxidizing it to a specific end product, evolving heat in the process,Combustion is useful when,Adsorption and absorption are not feasible,the concentration of pollutant in gas is high,Combustion,In order that a flame be self-sustaining, the mixture of air and combustibles must provide enough heat to maintain the combustion temperature.,an outside source of heat energy must be provided.,inject enough fuel gas into the air-rich effluent to bring a portion of the mass into the combustible range and thereby heat the remainder to the ignition temperature.,Combustion,Residence time,varies with effluent type, method of incineration, and burner type,turbulence,Incomplete combustion,temperature, residence time, or mixing ratio,byproducts.,secondary control systems,Combustion Kinetics,C,A,: concentration of pollutant A,k,: kinetic rate constant,n,: reaction order,If pollutant concentration is much less than O,2,concentration,n,can be assumed to be 1.,Arrhenius,Equation for rate constant,E,act,: activation energy (cal/mol),R,: gas constant (1.987 cal/,gmole,K),Determination of,A,and,E,act,Example 10.17 and 10.18,Show the calculation leading to the value of k in table 10.4 for benzene at 1000 ,Estimate the time required to destroy 99.9 percent of the benzene in a waste gas stream at 1000, 1200 and 1400,Repeating the calculation at 1200 and 1400,we find 49s and 0.2s,The typical values of the operating conditions of industrial gas incinerators,(suggested by Barnes, et al),Gas velocity: 25-50 ft/s,Resident time: 0.2-1s,Temperature:,Odor control 900-1350,Oxidize HC,900-1200,Oxidize CO,1200-1450,Three basic types of combustion,Direct Flame Incineration,The first type is direct flame incineration,temperatures up to 2500,0,F,NO,x,no preheating of gas stream,no secondary energy recovery,Path of waste gas inlet flow,Waste gas inlet connection,Adjustable control louvers,Gas connection,Removable pilot assembly,Incineration chamber,Multijet,burner for direct flame afterburner,Inlet plenum,Refractory-lined ignition chamber,Multijet,burner,Direct Flame Incineration,American Environmental International, Inc. (AEI),NESTEC,USA,Thermal Incineration,The second combustion process is thermal incineration,used when concentration of,combustant,is too low for DFI,preheating,2,o,energy recovery,temperature 1000 - 1500,o,F,Clean gases to stack,Polluted waste gas,Recuperative Thermal Incinerator,Fuel,Regenerative Thermal Oxidizer,RTO,蓄热式热氧化器,Catalytic Incineration,The third combustion process is Catalytic incineration,used when concentration of,combustant,is low,catalyst lowers the residence times,preheating,temperature 600 - 1100,o,F,lower energy requirements for oxidation,Catalyst Elements,Fume entry from oven,Preheat burner,Blower,Catalytic Afterburner,Q: Technical problems associated with the catalyst?,Problems associated with chlorinated hydrocarbon,?,Heat released from,reaction absorbed by air,Reduced activation energy results in lower temperature required.,The Catalyst,Should be active at relatively low temperatures,must be chemically and physically stable in oxidizing atmospheres,Should have a high surface to volume ratio,The combustible organic compounds in the air stream react with O,2,on the surface of the catalyst to produce H,2,O and CO,2,.,Pt, Pd, Co, Cu, Cr, Mo on Alumina,Titania,or,ceramic,support,Catalytic Oxidation System,American Environmental International, Inc. (AEI),Biological oxidation,去除效率高,一般的空气污染物去除效率超过,90,投资少，运行费用低,不需要投入额外的化学品,;,化学法则需加催化剂和氧化剂等，如次氯酸盐、过氧化氢、二氧化氯等。,污染少,生物处理的产物是生物量，很容易处理。,耗能低,生物反应在常温常压下进行，能量来自微生物利用,VOCs,成分本身产生的能量。,生物处理法消耗的动力只是污染气体进入处理系统时所消耗的能量,(,正压送风或负压引风,),Biological oxidation,适宜处理的污染气体应具有的特点：,水溶性强,兼具有蒸汽压低、亨利定律常数低的特点，向介质表面微生物膜扩散速率高；,主要有无机物如,H,2,S,和,NH,3,等、醇类、醛类、酮类以及简单芳烃等有机物。,易降解,分子被吸附在生物膜上必需被降解，否则将导致污染物浓度增高，毒害生物膜或影响传质，降低生物滤器效率，或使处理完全失败。,Biological oxidation,Typically parameters:,Soil depths: 3-4 ft,Viod,volumes: 50%,Upward gas velocities: 0.005-0.5 ft/s,Gas residence times: 15 to 60s,Example :,Estimate the occupied place for a,biofilter,to treat a,airstream, 10000m,3,/h. typically devices have soil depths of 3,4 ft, void volume of 50%, upward gas velocities of 0.005 to 0.5ft/s, and the gas residence time of 15,60s.,summary,VOC,Control alternatives,Prevention,concentration, oxidation, sorption,Some of these control options can be used for non-VOC emissions.,Homework,p386 10.17,p387 10.27,p388 10.31,p389 10.38,Thank you all!,