Preventing Pollution: Choices During Process Synthesis - Chapter 9. David R. Shonnard Department of Chemical Engineering

Transcription

1 Preventing Poution: Choices During Process Synthesis - Chapter 9 David R. Shonnard Department of Chemica Engineering 1 Chapter 9 (a): Outine After the Input-Output structure is estabished, an environmenta evauation during process synthesis can identify arge sources of waste generation and reease; directing the attention of the designer to poution prevention options within the process Educationa goas and topics covered in the modue Poution prevention strategies for process units - Materia seection and chemica reactors Poution prevention strategies for process units - Separations, Storage Tanks, Fugitive Sources, Separative Reactors, and Safety Concerns 2

2 Educationa goas and topics covered in Chapter 9 Students wi: become famiiar with practica poution prevention strategies for process units 3 Chapter 9: Important issues regarding poution prevention for unit operations 1. Materia seection: fue type, mass separating agents (MSAs), air, water, diuents, heat transfer fuids 2. Operating conditions: temperature, pressure, mixing intensity 3. Energy consumption: high efficiency boiers, operation of units to minimize energy usage 4. Materia storage and fugitive sources: storage tank choices and equipment monitoring and maintenance 5. Waste generation mechanisms: understanding this wi ead to poution prevention strategies 4

3 Chapter 9: Poution prevention through materia seection - fue type Exampe Probem: Cacuate the annua uncontroed SO 2 emissions to satisfy a steam energy demand of 10 8 Btu/yr with a boier efficiency of.85 assuming Fue Oi #6, #2, and Natura Gas. #6 Fue Oi #2 Fue Oi Natura Gas Emission Factor, EF (b/10 3 ga) 157S 143S 0.6 b/10 6 scf Sufur Content, S % Heating Vaue, HV 1050x10 6 (Btu/10 3 ga) 1.48x x10 8 Btu/10 6 scf Annua Emission, E (b SO 2/yr) E i (b i /yr)= EF av (b i /103 ga) ED(Btu / yr) HV(Btu / 10 3 ga) BE E i (b i /yr)= EF(b i /106 scf ) ED(Btu / yr) HV(Btu / 10 6 scf) BE 5 Chapter 9: Poution prevention through materia seection - reactor appications 1. Cataysts: that aow the use of more environmentay benign raw materias - e.g. ess hazardous raw materias that convert wastes to usabe products and feedstocks products more environmentay friendy - e.g. RFG / ow S diese fue 2. Oxidants: in partia oxidation reactions repace air with pure O 2 or enriched air to reduce NOx emissions 3. Sovents and diuents : repace toxic sovents with benign aternatives for poymer synthesis repace air with CO 2 as heat sinks in exothermic gas phase reactions 6

4 Chapter 9: Poution prevention for chemica reactors 1. Reaction type: series versus parae pathways irreversibe versus reversibe competitive-consecutive reaction pathway 2. Reactor type: issues of residence time, mixing, heat transfer 3. Reaction conditions: effect of mixing on yied and seectivity 7 Chapter 9: Poution prevention for chemica reactions 1st Order Irreversibe Parae Reactions 1.0 R R k p P k w W kp/kw = 100 [P]/[R]o 0.7 kp/kw = kp/kw = [W]/[R]o 0.1 kp/kw = kp/kw = τ = (k p+k w) t High Conversion t > 5(k p + k w ) -1 High Seectivity k p >> k w Seectivity Independent of residence time 8

5 Chapter 9: Poution prevention for chemica reactions 1st Order Irreversibe Series Reactions R k p P k w W [R]/[R]o [P]/[R]o [W]/[R]o k p/k w = 10 k p/k w = 100 k p/k w = 1 k p/k w = 2 High Conversion t > 5 k p k p/k w = 2 High Seectivity k p >> k w k p/k w = 1 k p/k w = 10 k p/k w = 100 Seectivity dependent on residence time τ = kp t 9 Poution prevention for chemica reactions Reversibe Series Reactions CH 4 + H 2 O CO + 3H 2 Steam reforming of CH 4 CO + H 2 O CO 2 + H 2 R = CH 4 P = CO W = CO 2 Separate and recyce waste to extinction 10

6 Chapter 9: Poution prevention - reactor types 1. CSTR: not aways the best choice if residence time is critica 2. Pug fow reactor: better contro over residence time temperature contro may be a probem for highy exothermic reactions 3. Fuidized bed reactor : if seectivity is affected by temperature, tighter contro is possibe 4. Separative reactors: remove product before byproduct formation can occur: series reactions 11 Chapter 9: Poution prevention - mixing effects B o A o CSTR Irreversibe 2nd order competitive-consecutive reactions A + B k 1 P 1 P + B k 2 W Y/Y exp Y exp = R 1 A = A A o (k 2 / k 1 1) A o A o k 2 / k Y = yied = P/A o Y exp = expected yied τ = mixing time scae Increased mixing wi increase observed yied E-05 1.E-04 1.E-03 1.E-02 1.E-01 (k 1 B o τ)(a o/b o) 12

7 Chapter 9 (a): Poution prevention - other reactor modifications 1. Improve Reactant Addition: premix reactants and cataysts prior to reactor addition add ow density materias at reactor bottom to ensure effective mixing 2. Cataysts: use a heterogeneous catayst to avoid heavy meta waste streams seect cataysts with higher seectivity and physica characteristics (size, porosity, shape, etc.) 3. Distribute fow in fixed-bed reactors 4. Heating/Cooing: use co-current cooant fow for better temperature contro use inert diuents (CO 2 ) to contro temperature in gas phase reactions 5. Improve reactor monitoring and contro 13 Chapter 9: Separations - Potentia probems / opportunities Feed Materias Chemica Reactor(s) Energy Separation System Air Emissions Products Wastewater Unreacted Feed Materias & Sovents Soid Waste Liquid Waste Poution Prevention Strategies Correct choice of separation technoogies Choice of mass separating agents Design heuristics to prevent poution Carefu contro of system parameters during operation 14

8 Chapter 9: Separations - Common sense soution to distiation Exampe Probem Energy Savings in Distiation: Side Stream Case Feed F = 100 moes/hr x F = 0.4 moe fraction (1/2 vapor, 1/2 iquid) V V Q C Distiate D X D = 0.8 Side Stream S = 20 moes/hr X S = 0.65 Q R Bottoms B X D = 0.02 Energy Savings Of about Soution: 30% Coumn Design L/D D V Q R (ca/hr) Side Stream x10 4 No Side Stream x Storage Tank Emissions: Sovent Mass Cacuation Cacuate net emissions reduction for appication of new paint to existing fixed roof tank Od dark paint in poor condition: b/yr emitted New white paint: b/yr 50% (vo) sovent in paint, 100 sq. ft./ga of paint, sovent density = 6 b/ga Dark Paint: Basis 10 years Tota VOC Emission = (10 yr) (509.7 b/yr) = 5,097 b New White Paint: Basis 10 years Surface Area of a 16 ft dia and 10 ft ta Tank = ft 2 (703.7 ft 2 )(6 b/ga) Tota Sovent Emission = (10 yr) (337.6 b/yr) = 3,418.2 b (100 ft 2 /ga) 16

9 Chapter 9: Fugitive Sources - poution prevention techniques Aen, D.T. and Shonnard, D.R., Green Engineering: Environmentay Conscious Design of Chemica Processes, Prentice-Ha, Upper Sadde River, NJ, Chapter 9: Separative reactors key features Combine chemica reaction with separation in a singe unit Reduce waste generation» Shift chemica equiibrium to favor product formation by removing products from reaction mixture» Remove desired product in series reactions thereby reducing waste generation via secondary reactions Maximize product yieds Separation technoogies used» Distiation» Adsorption» Membranes 18

10 Chapter 9: Separative reactors - with adsorption Simuated Countercurrent Movingbed Chromatographic Reactor CH 4 + O 4 in stoichiometric amts Desired Reactions 2CH 4 + 1/2O 2 C 2 H 6 + H 2 O 2CH 4 + O 2 C 2 H 4 + 2H 2 O Reactors (1,000 ºK) Chromatographic Coumns (coo) Waste Reaction CH 4 + 2O 2 CO 2 + 2H 2 O Yieds of product from CH 4 increase from <20% to > 50% Aen, D.T. and Shonnard, D.R., Green Engineering: Environmentay Conscious Design of Chemica Processes, Prentice-Ha, Upper Sadde River, NJ, Chapter 9: Separative reactors - with membranes membrane byproduct reactant 2 reactants product reactant 1 product A Product/byproduct remova mode B Reactant addition mode of operation CH3CH2 C6H6 CH2CH C6H6 + H2 Dehydrogenation of ethybenzene to styrene reaction Yieds of product increase 15% and seectivity increases 2-5% 20

11 Chapter 9: Safety concerns with poution prevention Poution prevention appications may make a chemica process more compex, thereby increasing safety concerns. Exampe probem Safety concerns for storage tank poution prevention roof PC N 2 40 psig LC foating roof LC foating roof 1 psig LC coumn LC Externa Foating-Roof Tank Interna Foating-Roof Tank Aen, D.T. and Shonnard, D.R., Green Engineering: Environmentay Conscious Design of Chemica Processes, Prentice-Ha, Upper Sadde River, NJ, Tabe Limited HAZ-OP Anaysis of storage tank poution prevention Guide Words Deviation EFRT IFRT Possibe Cause Consequences NO Inet pump fais to stop or Outet pump fais to start X X 1. Leve gauge fais 2. Pump fais 1. Touene spis out top of tank 2. Soi and ground water contamination 3. Exposure to site personne MORE LESS AS WELL AS Inert N2 fais to stop Inert N2 insufficient Water in tank PART OF Inert N2 insufficient REVERSE Pumps reverse OTHER THAN Another inerting gas EFRT Externa Foating-Roof Tank IFRT Interna Foating-Roof Tank X X 1. Pressure contro fais 1. N2 suppy interruption X X 1. Foating roof eak in EFRT 2. Externa roof ead in IFRT X 1. Covered under LESS 1. Overpressure of storage tank and tank roof faiure 2. Tank rupture and spi of touene 1. fammabe mixture of Touena + air 1. Contamination of touene product and generation of additiona waste 1. Covered under LESS X X 1. Impossibe 1. Leve contro faiure and spi of touene X 1. Another gas is used 1. If O2 is used by mistake, a fammabe mixture is created. Aen, D.T. and Shonnard, D.R., Green Engineering: Environmentay Conscious Design of Chemica Processes, Prentice-Ha, Upper Sadde River, NJ,

12 Chapter 9: Summary/Concusions 1. Poution prevention for chemica reactions and reactors: materia choices for reactions reactor types (CSTR, pug fow, fuid bed, etc.) reaction conditions (temperature, mixing, concentration effects) 2. Poution prevention for within other unit operations Separation units Materia choices can affect energy consumption Storage tanks - some types are inherenty ess pouting Fugitive sources - some types of equipment emit ess VOCs Separative reactors can increase reaction yied and seectivity Safety concerns originate from the increase in compexity of poution prevention design modifications 23