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1Eng -Design Basics, Mass Balance, Kinetics.Pdf Applied chemical process Basic terms, mass balance Engineering thinking description of the industrial apparatus + description of the chemical and physical operation in the process precise formulation of the problem + design the right solution Process technology development design finish design development end of end construction of KH delivery KH Intensity Intensity year 1 Operations • Chemical processes - reactors • Mechanical processes - transport, mixing, filtration, grinding, sedimentation… • Diffusion processes - extraction, distillation, crystallization, drying • Thermal processes – coolig, warming-up, condensation, evaporation Description of the technology Maximal yield x minimum cost • Amount and composition of the products – reactants, products, waste (material balance) • Energy consumption – stream, cooling water, electrical energy, cooling air (enthalpy balance) • Industrial devices – wattage, type, dimension, output (design and check calculation) • Costs – raw materials, energy, investments, payroll (economic balance) Economic efficiency of the process • Costing and price of the product • Energy costs • Depreciation and maintenance • Cost of human work = Production costs + Corporate overhead = Complete costs + Profit = Total Price 2 Cost versus production - development of a new product 1) Old process production fall off Not simultaneously 2) Application of the modern technology Influence of capacity to the relative cost raw materials cost relative invest cost energy capacity (t/year) Type of the system – based on the exchange of matter and energy Open system It can exchange matter and energy with the surroundings during the reaction Closed system It can not exchange matter but It can exchange energy with the surroundings during the reaction Isolated system It can not exchange matter and energy with the surroundings during the reaction 3 Type of the systems – timeline point of view inputinput outputoutput continuousinput discontinousinput output output unsettled steady Generally batch periodic system state discontinous Technological scheme Technological scheme -marking apparatus A vessel with stirrer C rectifying, absorption column E heat exchanger F filter H container J vacuum pump K compressor P pump R reactor R111, A111, R121, A121,R112, P101a, P101b 4 Balance Express quantitative arrangement of things in space and time Conservation of mass Balance equations INPUT + GENERATION = OUTPUT + ACCUMULATION What we balance? – extensive variables Where we balance? – in balancing system At what time we balance? – balance period Mass balance INPUT + GENERATION = OUTPUT + ACCUMULATION . V . dnA n Ai r dV n Ae A 0 dt without chemical reaction – GENERATION = 0 steady state – ACCUMULATION = 0 INPUT = OUTPUT Balance - Definitions Initial amount – amount of variable, which is located in the balancing system at the beginning Finite amount – …at the end Input – number of variables that enter into the balancing system Output – …exit the balancing system Accumulation - amount of variable that will be added and rest in the balancing system Generation – amount of variable, which will be generate in the balancing system 5 Definitions • Stoichiometric quantities • Limiting reactant • Excess reactant • Conversion • Yield • Selectivity • Extent of reaction Stoichiometry Refers to quantities of reactants and products in a balanced chemical reaction. aA + bB cC + dD i.e. a moles of A react with b moles of B to give c moles of C and d moles of D. a,b,c,d are stoichiometric quantities Limiting reactant/excess reactant In practice a reactant may be used in excess of the stoichiometric quantity for various reasons. In this case the other reactant is limiting i.e. it will limit the yield of product(s) (Key reactant) A reactant is in excess if it is present in a quantity greater than its stoichiometric proportion. % excess = [(moles supplied – theoretic moles required)/ theoretic moles required] x 100 6 Extent of reaction (x [xi]) - is equal to the amount of product P formed (reactant A consumed) during the reaction divided by the stoichiometric coefficient of this product (reactant) ξ = ν x is equal for all compounds in the reaction Conversion (z) [zeta] or X • relative degree of conversion - fraction of the amount of a reactant that has been converted Xi = (wi,0-wi)/wi,0 = (ni,0-ni)/ni,0 = (ci,0-ci)/ci,0 mass fractions number of moles concentrations [V=const.] Note: conversion may apply to single pass reactor conversion or overall process conversion value 0-1 or 0-100 % Example of conversion A B i.e. stoichiometric coefficients a = 1; b = 1 100 kmol fresh feed A; 90 % single pass conversion in reactor; unreacted A is separated (100%) and recycled and therefore overall process conversion is 100% R F reactor separation I P 7 Selectivity (s [sigma] or S) Selectivity – ratio between the amount of desired product P obtained and the amount of key reactant A converted x stoichiometric factor (−,) υ = (−,) υ stoichiometric factor = stoichiometric coef. of limit. reactant required per stoich. coef. of product Values 0 (no P formed) to 1 (all A which was consumed was converted to P) or 0-100 % other definition: ratio between the rate of forming desired product and rate of forming undesired product ratio of the moles desired product/moles byproduct for parallel or series reaction) x s.f. Yield (h [eta] or Y) Ratio between the amount of desired product obtained and the amount that could be obtained if all of key reactant were converted to product with 100% selectivity (in other way: ratio between reactant converted to desired product and total amount of reactant) hP = sP · zA (BATCH and CSTR) hP = (moles of the product P/moles limiting reactant supplied) x s.f. s.f. is the stoichiometric factor = stoichiometric coef. of limit. reactant required per stoich. coef. of product Material balance – recommended procedure • Draw a diagram of the balance sheet, marking nodes, streams and components • Writing assumptions • Writing equations stoichiometric chemical reactions • Choice of base calculation • Select the type of balance (mass x mole) • The conversion of the input data • Writing the matrix entries • Assembly of balance equations and additional relationships • Solving systems of equations • Check for correct calculation 8 Material balance Simple example of material balance Mixture containing 30 wt. % Benzene and 70 wt. % Toluene is injecting into continuous rectification column. The distillate contains 54 wt. % Benzene and distillation residue 5 wt. % Benzene. What percentage of the benzene contained in the feed is obtained from the distillate? distillate Result: Wyield = 91.8 wt.% input residue Mole balance of chemical reactor Sulfur is burned in a stream of oxygen (air). What must be the excess air to exhaust gases from the furnace contain a quantity of SO2, which is necessary for the next step in the manufacture of sulfuric acid. % excess = [(moles supplied – S stoichiometric moles)/stoichiometric product moles] x 100 air Result: Pair = 75 molar % Mass balance of chemical reactor For manufacture of sulphuric acid we have 1 ton of pyrite ore, which contains 85 wt % FeS2. First step is oxidation to SO2 by 100% excess of air. Conversion of FeS2 is 95 %. Determine the composition of outlet gas and remaining solid product. Gas product % conversion = (amount reactant Pyrite ore consumed/amount reactant supplied) x 100 % excess = [(moles supplied – Air Solid product stoichiometric moles)/stoichiometric moles] x 100 Results: Gas : N2 = 73.36 ,O2=11.5, SO2=15.14 wt.% Slag: Fe2O3 = 73.68, FeS2=5.82, ballast=20.5 wt.% 9 Mikrokinetics and macrokinetics Microkinetics Same for any apparatus Related to the behavior of molecules described by Physical Chemistry Rate constant, diffusion coefficient … Macrokinetics Dependent on the apparatus Related to the system as a whole (the size of the reactor) described by Chemical Engineering The volume of the reactor, transfer coefficients … Classification of chemical reactions General terms: homogeneous Number of phases: heterogeneous Process: batch injection continuous isothermal Reaction conditions: adiabatic isochoric isobaric Classification of chemical reactions Kinetic terms: Not influencing the position of the chemical equilibrium Activation: • thermal activation • activation by other reaction • activation by catalyst Influencing the position of the chemical equilibrium • light activation • by electrical energy • by nuclear radiation • ultrasonic activation Reaction order: first, second…, zero, fractional or negative order Elementary reaction Process: reversible Complex reaction parallel series elementary Reaction mechanism: non-elementary 10 Mikrokinetics Chemical production rate for component A • is defined as the number of moles of A produced per unit time and volume. -3 -1 rA [molA.m .s ] Directly measurable variable Chemical rates If a reaction proceeds according to stoichiometry AA+BB CC+DD • Chemical reaction rate r r r r r A B C D A B C D • rA - chemical production rate for component A • Value of rate depends on the stoichiometric equation Equation of chemical reaction rate • Irreversible elementary reaction • r=fT(T)fc(ci,cj,…..) • Ideal system • Molecularity k – reaction rate constant molecularity r k ci reactants A B r=kcA 2 2A B r=kcA A+BC r=kcAcB 11 Equation of chemical reaction rate Real system (non elementary reactions) a a A B r=kcA a b a A+ b BC r=kcA cB Reaction order expresses non-ideality reaction rate on the concentration Ideal system – reaction order=molecularity can be fractional
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