EN3004 Air Pollution Control Engineering Lecture 11 PDF

EN3004 Air Pollution Control Engineering Week 11 VOCs and HCs – Characteristic & Control Tuti Lim School of Civil and Environmental Engineering Nanyang Technological University 1 ...

EN3004 Air Pollution Control Engineering Week 11 VOCs and HCs – Characteristic & Control Tuti Lim School of Civil and Environmental Engineering Nanyang Technological University 1  Outline (I) Characteristics of VOCs & HCs ⚫ Vapour pressure, Antoine equation and Raoult’s law ⚫ Behaviors of volatile liquids (p>Patm; p=Patm; p0.01 psia at ~25°C) to significantly vaporize and enter the atmosphere. Most organic compounds used as solvents and chemical feedstock are VOCs. Examples: formaldehyde, benzene. VOCs are composed of hydrogen and carbon, and may also contain elements such as oxygen, nitrogen, sulfur, chlorine, and fluorine. O C H H formaldehyde benzene 4  What are Hydrocarbons (HCs) ✓ Hydrocarbons (HCs) are composed of only hydrogen and carbon (C-H) Examples: benzene, toluene, hexane, etc. ✓ Hydrocarbons are only slightly soluble in water. ✓ Polar VOCs, which almost all contain an oxygen or nitrogen atom in addition to carbons and hydrogens (alcohols, ethers, aldehydes and ketones, carboxylic acids, esters, amines, nitriles) are much more soluble in water. 5  Characteristics of VOCs This difference in solubilities makes the polar VOCs easier to remove from a gas stream by scrubbing with water, but harder to remove from water once they dissolve in it. Within each chemical family, the solubility decreases with increasing molecular weight. VOCs are organic compounds that can volatilize and participate in photochemical reactions once they reach the ambient air. Material with higher boiling points evaporate slowly and would not be a VOC problem. 6  Why are VOCs a concern? Some of VOCs are toxic and carcinogenic Formation of Smog, Ozone and fine particles caused by VOCs Sunlight VOC + NO + O2 O3 + NO2 VOC + SOx + NOx fine particles Smog = “Smoke” + “Fog” – A word created more than three decades ago. Today, smog refers to a noxious mixture of air pollutants that can often be seen as a haze in the air. O3, NO2 and fine particles are the major components of smog. Sick Building Syndrome ----a situation in which building occupants suffer discomfort (headache, eye, nose or throat irritation, dry cough, dizziness and nausea, fatigue, sensitivity to odors, difficulty concentrating) from being in the building that can't be linked to any other causes or specific illnesses. 7  paints , varnishes, wax cleaning supplies, Where Do VOCs Come from? pesticides Are VOC formed in the air ? ? building materials furnishings copiers and printers ⚫ Mainly from motor vehicles (from motor fuels) correction fluids glues and adhesives ⚫ Solvent usage (paint industries) permanent markers ⚫ VOC storage and transport photographic solutions ⚫ Industrial processing (Petroleum, Chemical, etc.) wood preservatives aerosol sprays ⚫ Combustion process (Forest fires, residential) Cleansers ⚫ Natural - bacterial decomposition in swamps disinfectants; moth repellents air fresheners; VOCs are lost during industrial & other processes. 8 The desirable “new car smell” comes from ……. Acetaldehyde Amines 9  Understanding Vapor Pressure ⚫ Liquids can evaporate ⚫ If the liquid is in a closed container, number of molecules vaporizing into the gas phase is balanced by an equal number of molecules re-entering the liquid phase - Equilibrium ⚫ Vapor pressure is the concentration of the material in the gas stream at this equilibrium condition. 10  Dew Point & Boiling Point ⚫ The dew point is the temperature at which a vapor begins to condense at a constant pressure. ⚫ Boiling point is the temperature at which liquid changes to vapour state – temperature at which the saturated vapour pressure between a liquid and its vapour state is equal to the external pressure on the liquid (atmospheric pressure =1 atm = 101.3 kPa = 14.7 psi = 760 mmHg) 11  Vapour Pressures vs Temperature atm=psia/14.7 mmHg=51.7 (psia) Room Temperature (25C) Water Atmospheric Pressure 1 atm = 14.7 psia 77 F ~ 25 C 212 F ~ 100 C Conversion Factor: ℉=℃ (9/5)+32 OR C=(F-32)/1.8 12 15  Vapour Pressures vs Temperature Antoine Equation: B log10 p = A − (T + C ) where p is vapour pressure, A, B and C are empirical constants p in mm Hg, T in C 13  18 14 What is boiling point when water is boiled at a mountain (1000 m above sea level)? ? a) 100 C c) 100 C Altitude Above Sea Absolute Barometer Level meter mm Hg 0 760.0 153 746.3 305 733.0 458 719.6 610 706.6 763 693.9 915 681.2 1,068 668.8 15  Raoult’s Law Raoult’s law relates the vapor pressure of components to the pi composition of the solution. y i = xi P where: yi = mol fraction (= vol% for perfect gases) of component i in the vapour total pressure P vapor xi = mol fraction of component i in the liquid pressure Pi yi pi = vapour pressure of component i P = total pressure xi If pure liquid in the container, the vapor pressure of the liquid and the vapor will have the same chemical composition as the liquid. If other gases presence, then at equilibrium, the air will be VOC saturated with vapor evaporated from liquid. solution The content of volatile liquid in the vapor mix for air pollution control can be estimated using Raoult’s law. 16  VOCs and Vapor Pressure pi yi = xi total pressure P P vapor pressure Pi yi P yi xi VOC T pi yi solution 17  Example : Estimate the water content of air that is in equilibrium with pure water at 20 °C. Solution: pi Air y i = xi P yi Pure water → xi = 1.00 (ignoring small xi amount of air dissolved in water) P = 1 atm = 760 mmHg Water B log10 p = A − A = 8.10765, B = 1750.286, C = 235.0 (T + C ) (water) pi = 17.53 mmHg = 0.023 atm yi = xi pi/P = 0.023 = 2.3% 18  How to Control VOCs? (I) Prevention ✓ Substitution ✓ Process modification ✓ Control leaks (II) Concentration and recovery ✓ Condensation ✓ Adsorption ✓ Absorption (scrubbing) (III) Oxidation ✓ Incineration ✓ Biological oxidation 19 (I) Prevention - Substitution and Process Modification ⚫ Switching from oil- to water-based paints, coatings and inks ⚫ Substituting volatile solvents with less volatile or less toxic solvents ⚫ Replacing gasoline-powered vehicles with electric powered vehicles Hydrogen-powdered car 20 (I) Prevention - Control Leaks VOC Tank Headspace Vapor out vapour (vapor space) Liquid liquid ? What would happen without a vent? When liquid is withdrawn, the container internal pressure < Patm which may lead to vacuum collapse of the vessel. 21 (I) Prevention - Leakage Control: Evaporative Emissions filling liquid During filling, liquid enter the tank and displace vapor from the tank’s headspace, which is connected by a vent to the atmosphere → displacement losses. 22 (I) Prevention - Leakage Control: Evaporative Emissions emptying liquid When liquid is withdrawn from the tank, air will flow in through the vent to fill the space due to the drop in liquid level. → emptying losses 23  Understanding Emptying losses Before : y i pi y i = xi P pi B log10 pi = A − T (T + C ) emptying T=constant liquid After: y’ i When liquid is withdrawn from the tank, air will y’ < y i i flow in through the vent to fill the space due to the p’ < pii drop in liquid level. → emptying losses During emptying, none of organic vapor will p’ i pi evaporate into the air. Some remaining organic will slowly evaporate into the air. 24 (I) Prevention - Leakage Control: Evaporative Emissions liquid (a) simple thermal expansion of the vapor and liquid in the tank (b) vaporization of VOC as the liquid temperature is raised. When the tank’s temperature changes, it must “breathe” in and out (normally out every day and in every night). → breathing losses 25  Calculation of Working Losses  For all three kinds of working losses,  volume of air - VOC mixture  concentration of  VOC emission =  VOC in that mixture   expelled from the tank   mi = Vci Where: mi = mass emission of component i ci = concentration in the displaced gas 26  RT Vmolar ,gas = Ideal Gas Law (for 1 mol) P yi → mol fraction of component i in the vapour yi  MWi → mass of component i in the volume of 1mol vapour yi  MWi yi  MWi  P ci = = Vmolar ,gas RT mi = Vci 27  yi  MWi yi  MWi  P ci = = Vmolar ,gas RT vapor pi y i = xi yi P xi VOC mi xi  pi  MWi ci = = V RT ci = concentration in the displaced gas yi = molar fraction of component i in the vapour xi = molar fraction of component i in the liquid pi = vapour pressure of component i P = total pressure, MW=molecular weight 28  (I) Prevention - Leakage Control: Floating Roof Tanks EPA ruling: 151 m3 – VOC with p>0.75 psia 75-151 m3 – VOC with p=3.9 psia @ max monthly T at the site. Sealed 29 (I) Prevention - Leakage Control: Stage 1 Control 30 (I) Prevention - Leakage Control: Stage 2 Control PHASE OUT 31 (II) Concentration and Recovery – Condensation How to create cold conditions ? ✓ by passing cold water through an indirect heat exchanger ✓ by spraying cold liquid into an open chamber with the gas stream ✓ by using a freon-based refrigerant to create very cold coils ✓ by injecting cryogenic gases such as liquid nitrogen into the gas stream. 32 What is the temperature of the cooler? ? pi y i = xi P Out gas stream 50 ppm toluene inlet gas stream 5000 ppm toluene yi = 50 ppm = 0.00005 molar fraction P2 P1, T1 Gas cooler P2, T2 Phase separator Refrigerant Recovered liquid toluene B log10 p = A − (T + C ) 33  Displacement Losses filling liquid During filling, liquid enter the tank and displace vapor from the tank’s headspace, which is connected by a vent to the atmosphere → displacement losses. 34 Example: T=0 C yi=0.2 T=20 C T=-45C yi=0.414 yi=0.024 No more water as T < 0ºC 35 (II) Concentration and Recovery – Adsorption Attachment of molecules (adsorbate) to the surface of solids (adsorbent) Adsorbent (Activated carbon) Adsorbent bed 36  Physical and Chemical Adsorptions Physical adsorption (van der Waals adsorption): weak bonding of gas molecules to the solid; exothermic (~ 0.1 Kcal/mole); reversible endothermic Chemical adsorption: chemical bonding by reaction; exothermic (10 Kcal/mole); irreversible ? If an adsorption bed is operated at a higher temperature, the efficiency will ___. (a) increase (b) decrease 37  Activated Carbons Area of a few grams of activated carbon >500 – 3000 m2/g DIFFERENT RAW MATERIALS DIFFERENT PHYSICAL FORMS MANUFACTURE OF ACTIVATED CARBON OF ACTIVATED CARBON 38 Adsorption in Pores e 39  Progression of Adsorption Front concentration C5 C6 in effluent C4 C3 C1 C2 Time C0 C0 C0 C0 C0 C040 Adsorption Processes 41  (II) Concentration and Recovery - Absorption (Scrubbing) ⚫ Absorption is transfer of a gaseous component (absorbate) from the gas phase to a liquid (absorbent) phase through a gas-liquid interface. -- Dissolution into the liquid phase ⚫ VOC (absorbate) should have high solubility in liquid (absorbent) to facilitate dissolution ⚫ Technically it is a scrubbing process Notethedifferencefrom Adsorption 42  Common VOC Absorption Equipment Large absorption stripping system uses tray columns liquid gas weir Overflow (cross flow) plate pipe cap Bubble-cap Tray 43 Common VOC Absorption Equipment Small absorption stripping system uses internal packings Common packing materials Tellerette IMTP Ring Tri Pack Pall Ring 44  Absorption (Scrubbing)- Process The stripper is operated at a higher temperature and/or a lower pressure. 45  Adsorption vs Absorption Attachment of molecules to The dissolution of molecules the surface of a solid within a collection liquid or (Interfacial) solid The absorbents in most cases The adsorbents are solids are liquid (Most absorption Mostly used in air pollution processes occur in liquid) to concentrate a pollutant that is present in dilute form in the gas stream Adsorption vs Absorption 46  (III) Oxidation to Destruct VOCs ⚫ Oxidation was used when VOC-containing gas streams are too concentrated to be discharged but not large enough to be concentrated or recovered. ⚫ VOCs can be destructed by oxidation: VOCs + O2 CO2 + H2O Sometimes NO2, SO2 and HCl can also be produced due to the presence of N, S, Cl in VOCs. What problems do you foresee? 47 ✓ Thermal incinerator (combustion) Destroys VOCs using incineration at high temperatures ✓ Catalytic incinerator Destroys VOCs using a catalytic material (typically base metal or noble metal) that oxidizes organics at lower temperatures ✓ Biological filtration or bioscrubbing 48  (III) Oxidation -Thermal Incinerator ⚫ Above auto-ignition temperature of the VOC compounds (700 -1000 C). ⚫ VOC destruction efficiencies > 95% and often > 99% ⚫ Produce NOx due to high temperatures 49  Thermal Incinerator + Scrubber Thermal oxidizers handling VOC materials that contain chlorine, fluorine, or bromine atoms generate HCl, Cl2, HF, and HBr as additional reaction products during oxidation. Thermal Incinerator Scrubber They can be used for almost any VOC compound 50  NEXT WEEK (III) Oxidation - Catalytic Incinerator ⚫ Catalytic incinerators operate at substantially lower temperatures due to the presence of the catalyst (300- 500C /500 to 1000°F. ) ⚫ Catalysts are usually base metals or noble metals (e.g., Pt, Pd) ⚫ VOC destruction efficiencies > 95% and often > 99% ⚫ Produce minor NOx due to low temperature ⚫ The refractory-lined combustion They cannot be used on sources chambers can be used to recover heat that also generate small quantities of catalyst poisons. (phosphorus, tin, and zinc) 51  Three “T”s for Combustion ⚫ Temperature — Almost all chemical reactions occur faster at higher temperatures, so if we want to have combustion occur at a desired rate we will need a sufficient temperature — 700-1000 C for direct flame incinerator; 300-500C for catalytic incinerator ⚫ Residence Time — Even if we heat the fuel and air, we still need to let them hang out together for a certain length of time to enable a reaction. — 0.5-1 s for incinerator ⚫ Turbulence — Make sure a good degree of mixing 52  Incomplete Combustion ⚫ Complete combustion is defined as complete oxidation of C to CO2 and complete oxidation of H to H2O. In the contrast, incomplete combustion is the incomplete oxidation. ⚫ Why does incomplete combustion happen? ✓ Insufficient air ✓ Three “T”s are not satisfied ⚫ What happens after the incomplete combustion of VOCs? ✓ An exhaust gas containing many of intermediate products (e.g. aldehydes, dioxins, furans) can be produced. They might be even more harmful than the input VOCs! 53 (III) Oxidation – Biofiltration/bioscrubbing  A relatively new control technology, using microorganisms immobilized in the filter media to oxidize VOCs: CxHyOzSN + O2 → CO2 + H2O + SO4-2 + NO3-2 + cell  The microorganisms are housed in a filter bed (typically size of a swimming pool).  Time to oxidize VOCs can range from 20 to 60s depending on bed type and gas treated.  Suitable for VOC-laden gas streams with low VOC concentration (95%) prior entering the biofilters to prevent filter media to dry out. 55  Actual biofilters Bohn Biofilter Envirogen's Modular Biofilter 56 Advantages & Disadvantages of Biofiltration Advantages ⚫ Low capital and operating costs ⚫ No harmful end products ⚫ No chemical handling required ⚫ No supplemental fuel required, no NOx problem Disadvantages ⚫ Compounds must be biodegradable ⚫ Large footprint - space consumption due to long residence time of 30-60 seconds ⚫ Only handles low VOC streams effectively 57 Summary: Choosing a VOC control technology 58 Guide for Choosing VOC Control Technologies Factor to be considered: Emission regulation; Flow rate of gas stream; VOC concentration; Nature of VOCs; (Low Concentration) ✓toxic to the Cost for Economical? microorganisms? recovery is high. ✓desorbed ? 59  Guide for Choosing VOC Control Technologies (High Concentration) ✓no gas stream components that would poison, mask, or foul the catalyst. ✓there are environmentally acceptable means for disposal of the collected organics. 60