Chemical Processes What is Engineering? July 8, 2009 Shampoo Soap Toothpaste Dyes Gasoline Decaffeinated Coffee Sugar Cosmetics Paint Food additives Hydrogen Fertilizer Polymers Pharmaceuticals Chemical Engineering Look around you nearly everything you see has parts designed by chemical engineers! Chemical Engineering Chemists vs Chemical Engineers: Chemists determine reactions to make new compounds in a test tube. Chemical engineers have backgrounds in chemistry AND fluid dynamics, heat transfer, materials science Chemical Engineers design processes to make compounds at a rate of 1000 L/min that are efficient and dont explode. If that isnt reason enough In the United States: 170 Major Chemical Companies $400 Billion a year Employs more than a million workersChemical Engineering Chemicals: Raw Materials Small and Simple Helium (He) Ammonia (NH 3 ) Hydrogen Flouride (HF) Trinitrotoluene ( C 6 H 2 (NO 2 ) 3 CH 3 ) Large and Complicated Insulin C 257 H 383 N 65 O 77 S 6 Large and Simple Polyvinyl Chloride (-CH 2 -CHCl-) n Chemicals: Measure in Moles Moles are the SI unit for the amount of a substance. Formally, amount of a substance that contains the same number of entities as there are atoms in exactly 12 grams of Carbon-12 (6.022 x ). Converting mass percent to mole percent: Convert 20% C 2 H 5 OH, 80% H 2 O by mass into mole percents Efficient Engineering $ is the most important parameter in engineering! Consider sulfuric acid production. In 1997, 160 million tons consumed; cost $8 billion to produce. 2S + 3O 2 + 2H 2 O 2H 2 SO 4 If improve efficiency of process by 1% (better mixing, improved reactor design, etc), save $80 million. Chemical Engineering: Unit Operations Chemical Engineering: Two Processes Chemical Engineering Reactions (Producing chemicals on from raw materials on large scale) Separations (Producing chemicals by isolating from a mixture) Need to understand: Transport (flow & mixing of molecules) Thermodynamics (energy & heat) Material & Energy balances (conservation laws) Balance Equation: Input + Generation Output = Accumulation Control Volume Material and Energy Balances For non-reacting systems Generation = 0 For systems operated at steady state (flow rate in = flow rate out): Accumulation = 0 Balance equation reduces to: Input = Output Material and Energy Balances Material Balances How quantify what happening in a chemical process. We can solve for the flow rates and the mole fractions in the exit stream Mixing Process: No reactions No accumulation 100 moles/min 5% C 2 H 5 OH 40% CH 3 OH 45% H 2 O 20 moles/min 40% C 2 H 5 OH 60% H 2 O ? moles/min ?% C2H5OH ?% CH3OH ?% H2O mole fraction of i in a liquid stream: x i mole fraction of i in a vapor (gas) stream: y i Material Balances What are the input streams and output streams? How do we lose weight? Reaction Engineering Raw Materials Energy Catalysts Reactor Products Raw Materials Byproducts Energy Catalysts Create desired compounds from raw materials via chemical reactions. Cost considerations Byproducts: Could be expensive to dispose of Could be valuable sell in addition to product Purifying product Reaction Engineering Catalytic Converter Digestive tract reactions Possible Problem with Exothermic Reactions Reactor Water Bath A+B->C Energy Produced by reaction is proportional to reactor volume L 3 Energy Removed is proportional to surface area L 2 L Possible Scale up Problem Reaction Engineering Making Epoxy Separation Problem We have a ton of material composed of sawdust, iron filings, 1 diameter marbles, 2 marbles diameter, and salt crystals. Design a process to separate these components into pure form. Separations Engineering Molecular Property Boiling Point Freezing Point Particle size Affinity to a stationary phase Density Selective affinity to solid particles Separation Process Evaporation Distillation Crystallization Extraction Filtration Chromatography Centrifuge Absorption Exploit differential properties of mixture components. Produce desired product by isolating it from a mixture. Boiling/Freezing Points Temperatures at which substances change phase Boiling: liquid vapor Freezing: liquid solid Depends on atmospheric pressure (1 atm): T b (H 2 O) = 100 C T f (H 2 O) = 0 C Density How closely together atoms are packed together. Liquid and solid atoms are packed together ~1000x vapor atoms! fluid vapor vs. Volatility Tendency for atoms or molecules in a liquid to evaporate and become a vapor. Always have atoms or molecules leaving liquid phase for vapor phase and vice versa. When rate of liquid vapor equals vapor liquid, system at equilibrium Mixtures of different volatilities y i x i (more volatile) (less volatile) Surface Tension Force needed to stretch a film of atoms/molecules on surface of a liquid. Surface Spheres minimize surface area, maximize molecular interactions Viscosity Measure of how easy it is to stir a substance Increasing viscosity Harder to stir, more viscous, more energy needed to move substance Molecular Shape Size, shape, polarization Determines molecule behavior (boiling point, viscosity, etc) because of effect on intermolecular interactions Solubility Ability of one substance to dissolve into another Can be: Solids into liquids (sugar into iced tea) Gases into liquids (CO 2 into water carbonated beverages) Liquids into liquids (oil not dissolving into water) SubstanceSolubility (grams) O2O NH 3 53 Salt36 Solubility in 100 grams of water at 1 atm Separations Engineering Molecular Property Boiling Point Freezing Point Particle size Affinity to a stationary phase Density Selective affinity to solid particles Separation Process Distillation Evaporation Extraction Filtration Chromatography Centrifuge Absorption Exploits Differences of Material Properties of mixtures components Produce desired product by isolating it from a mixture. Distillation Utilizes differences in boiling points in liquids. Work horse of chemical engineering. Used extensively in petroleum industry to refine crude oil. Consider liquid mixture of ethanol (T b = 78.5 C) and water (T b = 100 C): Distillation Column T = 90 C Liquid (Feed Mixture) x EtOH,f x H2O,f Vapor (Top Product) y EtOH, t y H2O, t Liquid (Bottom Product) x EtOH, b x H2O, b Ethanol more volatile, therefore: y EtOH, t > x EtOH, f > x EtOH,b y H2O, t < x H2O, f < x H2O, b x i = liquid mole faction y i = vapor mole fraction Distillation: McCabe Thiele Diagram Represent vapor liquid equilibrium data for more volatile component in an x-vs-y graph (McCabe Thiele Diagram): Pressure constant, but temperature is changing! (from experimental data) Reference line (y = x) * xFxF y1y1 Distillation: McCabe Thiele Diagram Separator Distillation Equilibrium Stages have vapor and liquid phases in equilibrium with each other (Distillation Column) Engineering Design Problem: How many stages needed in order to get the desired concentration of the volatile component at the top of the column? (In most cases, y i x i ) Distillation: McCabe Thiele Diagram N = 5 Benefits Applicable for many liquid systems Technology is well developed High Throughput Drawbacks High heating and cooling costs AzeotropesDistillation Separations limitation Due to molecular interactions. Composition of vapor equal to composition of liquid mixture. Distillation: Azeotrope Azeotrope Batch distillation apparatus only one equilibrium stage! Batch Distillation Evaporation Utilizes differences in volatilities in a mixture. Desired product is the less volatile one. Remove other components from mixture by vaporization. Example: evaporation of sea water (undesired) to get salt (desired). Sometimes add volatile components to give a temporary property. When no longer needed, volatile component evaporates. Gas Absorption/Desorption Utilizes differences in solubility of components. Transfer of a vapor to/from a liquid from a gas phase. Absorption Example: Burn coal, sulfur is a byproduct dont want entering atmosphere. Pass vapor exhaust stream through liquid water, SO 2 absorbs and is removed from vapor stream. Called scrubbing Extraction Also utilizes differences in solubility of components. Use one liquid to remove a component from another liquid. Extracted component must be soluble in both liquids! Two liquids being transferred between must be immiscible (oil & water)! More dense liquid with desired product Less dense liquid with higher solubility for desired product Solvent + desired Filtration Utilizes differences in physical properties (size and shape) Separating a solid from a liquid or a gas. Example: Passing spaghetti and water through a colander to get just spaghetti (desired product) Chromatography Utilizes varying degrees of affinity for a solvent and chromatography material. Good for biology applications because is a gentle process (no high temperatures or harsh chemicals) Consider paper chromatography: Solvent & solute travel up absorbent paper. Molecules that have a higher affinity for solvent will travel further up paper! Separating Uranium 235 from Uranium 238 Impossible Separation #1 Separating (R)Thalidomide from (L)Thalidomide Impossible Separation #2 Molecule has exact same boiling point, density, freezing point, and only is slightly different in geometry Chemicals are produced by reactions or separations The driving force for separations of mixtures are differences in component properties Systems can be analyzed by mass and energy balances Distillation is the workhorse of separationsConclusions Three Parts: Energy Transfer Chromatography Batch Distillation (One equilibrium stage) Chemical Processes Laboratory Want efficient transfer and conversion of energy ($$) In lab, will be examining energy transfer in the form of heat: warming a pot of water with a hot plate what is the efficiency of energy transport from electricity to the water? Energy Transfer Separation technique that takes advantage of varying affinities of solutes for a given solvent traveling up a filter paper. Solutes: colored dyes Solvents: water, methanol, 2-propanol Measure the distance traveled by the solutes and solvents! **Methanol and 2-propanol are poisons! Wear safety goggles, do not ingest or inhale and rinse skin immediately if spilled.Chromatography Using distillation to separate a liquid mixture of ethanol and water Ethanol is the more volatile material (it will boil first) Take samples of distillate with time to determine the concentration of ethanol in the mixture! **Ethanol is a poison! Wear safety goggles, do not ingest or inhale and rinse skin immediately if spilled.Distillation Bilinear Interpolation % 10C 15C 20C25C30C35C40C %Alcohol by weight What is the mass fraction of alcohol at a temperature of 22C and a specific gravity of Temperature