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 necessary to be determined experimentally
Equation of chemical reaction rate
number of mols of the key component reacted
DnA=nA0-nA
Degree of conversion
nA0 nA A nA0
Influence of temperature
E A The temperature dependence of k k Ae RT
12 Influence of temperature
dependence of k on T
140 120 100 E=40kJ 1) 80 - 60 E=45kJ k(s 40 20 0 0 20 40 60 80 100
Temperature (°C)
Dependence of reaction rate on temperature
t,oC k, s-1 Calculate the Arrhenius 20 7,9 constant (preexponential 30 13 factor) and Activation Energy of chemical 40 21,6 reaction 50 34,1 60 53 ! Z - Frequency factor 70 81,7 r – steric factor 80 121,4
Zero order reaction
A → B dc r A k A dt cA
c A t dc k dt A cA c A 0 0
c A c A 0 k cA t
13 First order reaction dc r A k c k k AB A dt cA A cA A c c A dc t A k dt c cA c A 0 A 0
ln c A ln c A 0 k cA t
c a ln k cA t c a 0
ln 2 k cA t t 1 c c .e 2 A A 0 k cA
First order reaction
Second order reaction
dcA A+BC r kcAcB dt
dc r A kc c c c cA=cA0-cA,reacted dt A0 A,react B0 B,react
For cA0 = cB0 and nA = nB :
c A ,react d (c c ) t A 0 A , react k dt (c c ) 2 c A 0 A 0 A , react 0
c A 0 c A k .t.c A 0 1
14 Second order reaction
Comparison of the kinetic orders
0. order 1. order 2. order t 1/2 (s) 0.5 0.693147 1
Reversible reaction k 1 ←→ dcA dcA A+B C+D rA rA rA k1cAcB k2cC cD k2 dt dt k1 k2
2 2 rA k1cA0 cA,react k2cA,react c c c A0 c A0 A,eq k C,eq k 11/ 1 11/ 2 k2 k1
15 Reversible reaction
Subsequential reactions
k1 k2 dcA dcC A→B→C rA k1cA rC k2cB dt dt
k2 k1 cc cA0 1 exp(k1t) exp(k2t) k2 k1 k2 k1
k1 cB cA0 exp k1t exp k2t k2 k1
Subsequential reactions
k1=k2 k1< 16 Side reactions k1 A → B dcA dcC rA k1cA rC k2cA A →k2 C dt dt Batch systems rate of chemical reaction dn r V A A s dt 1 cA dc t A r A cAo Continuous reactions dc r A kc dt A This definition for the reaction rate is not propriate for continuous (flow-through) systems. In these systems the stationary state is established after some time. Composition, temperature and reaction rate are not function of time at this point. Rate of reaction is expressed by inject of key component, its conversion and volume of the reactor. 17 Continuos reactions • Plug Flow Reactor (PFR, CTR – Continuous Flow Reactor) dVr nA nA+dnA nA0 • Continuous Stirred-Tank Reactor (CSTR) nA0 nA1 Types of response and appropriate reactors Phase Phase Reactor homogeneous gas continuous homogeneous liquid continuous, batch homogeneous solid continuous, batch heterogeneous gas + liquid continuous heterogeneous gas + solid continuous heterogeneous liquid +solid continuous, batch heterogeneous liquid +liquid continuous, batch heterogeneous all continuous, batch 18