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LECTURE 4MASS BALANCE
The accounting of all mass in an industrial chemical process is referred to as a mass (or material) balance.
USES ‘day to day’ operation of process for
monitoring operating efficiency Making calculations for design and
development of a process i.e. quantities required, sizing equipment, number of items of equipment
SAMPLE PROBLEM – BATCH MIXING PROCESS
200 kg of a 40% w/w methanol/water solution is mixed with 100 kg of a 70% w/w methanol/water solution in a batch mixer unit.
What is the final quantity and composition?
SAMPLE PROBLEM:
Total initial mass = total final mass = 300 kg
Initial methanol mass = final methanol mass
80 + 70 = final methanol mass = 150 kgTherefore final composition of batch is
(150/300) x 100 = 50 % by wt.
SAMPLE PROBLEM 2
1000 kg of 8% by wt. sodium hydroxide (NaOH) solution is required. 20% sodium hydroxide solution in water and pure water are available. How much of each is required?
BATCH PROCESSES
Batch processes operate to a batch cycle and are non-steady state. Materials are added to a vessel in one operation and then process is carried out and batch cycle repeated. Integral balances are carried out on batch processes where balances are carried out on the initial and final states of the system.
Paul Ashall, 2008
BATCH CYCLE
Sequence of operations/steps repeated according to a cycle
Batch cycle time Batch size
SIMPLE BATCH REACTION CYCLE
3 steps
Start cycle t=0 t, finish cycle
Add reactants etc reaction Empty reactor
Next cycle
CONTINUOUS PROCESSES
These processes are continuous in nature and operate in steady state and balances are carried out over a fixed period of time. Materials enter and leave process continuously.
LAW OF CONSERVATION OF MASS
When there is no net accumulation or depletion of mass in a system (steady state) then:
Total mass entering system = total mass leaving system
or total mass at start = total final mass
GENERAL MASS BALANCE EQUATION
Input + generation – output – consumption = accumulation
Notes: 1. generation and consumption terms refer only to generation of products and consumption of reactants as a result of chemical reaction. If there is no chemical reaction then these terms are zero.
2. Apply to a system3. Apply to total mass and component mass
DEFINITIONS System – arbritary part or whole of a
system Steady state/non-steady state Accumulation/depletion of mass in
system Basis for calculation of mass balance
(unit of time, batch etc) Component or substance
SAMPLE PROBLEM
1000 kg of a 10 % by wt. sodium chloride solution is concentrated to 50 % in a batch evaporator. Calculate the product mass and the mass of water evaporated from the evaporator.
Paul Ashall, 2008
MIXING OF STREAMS
F1
F2
F3
F4
EXAMPLE
Calculate E and x
Fresh feed 1000kg, 15%by wt sodium hydrogen carbonate
Recycle stream 300 kg, 10% satd. soln.
evaporator feed E, composition x%
FLOWSHEETS Streams Operations/equipment sequence Standard symbols
PROCESS FLOW DIAGRAMS Process flow diagram
TYPICAL SIMPLE FLOWSHEET ARRANGEMENT
reactorSeparation & purificationFresh feed
(reactants, solvents,reagents, catalysts etc)
product
Recycle of unreacted material
Byproducts/coproductswaste
SAMPLE PROBLEM
A 1000 kg batch of a pharmaceutical powder containing 5 % by wt water is dried in a double cone drier. After drying 90 % of the water has been removed. Calculate the final batch composition and the weight of water removed.
SAMPLE PROBLEM – BATCH DISTILLATION
1000 kg of a 20% by wt mixture of acetone in water is separated by multistage batch distillation. The top product (distillate) contains 95% by wt. acetone and the residue still contains 2% acetone. Calculate the amount of distillate.
USE OF MOLAR QUANTITIES
It is often useful to calculate a mass balance using molar quantities of materials and to express composition as mole fractions or mole %.
Distillation is an example, where equilibrium data is often expressed in mole fractions.
MOLAR UNITS
A mole is the molecular weight of a substance expressed in grams
To get the molecular weight of a substance you need its molecular formula and you can then add up the atomic weights of all the atoms in the molecule
To convert from moles of a substance to grams multiply by the molecular weight
To convert from grams to moles divide by the molecular weight.
Mole fraction is moles divided by total moles Mole % is mole fraction multiplied by 100
Paul Ashall, 2008
MOLAR UNITS
Benzene is C6H6. The molecular weight is (6x12) + (6x1) = 78
So 1 mole of benzene is 78 grams1 kmol is 78 kg
SAMPLE PROBLEM – BATCH DISTILLATION
1000 kmol of an equimolar mixture of benzene and toluene is distilled in a multistage batch distillation unit. 90 % of the benzene is in the top product (distillate). The top product has a benzene mole fraction of 0.95. Calculate the quantities of top and bottom products and the composition of the bottom product.
Paul Ashall, 2008
STRATEGY FOR ANALYZING MATERIAL BALANCE PROBLEM
1. Read the problem and clarify what is to be accomplished.
A train is approaching the station at 105 cm/s. A man in one car is walking forward at 30 cm/s relative to the seats. He is eating a foot-long hot dog which is entering his mouth at the rate of 2 cm/s. An ant on the hot dog is running away from the man’s mouth at 1 cm/s. How fast is the ant approaching the station?
Paul Ashall, 2008
2. Draw a sketch (flow diagram) 3. Label. Assign symbols to each variable.4. Put down the known values5. Select the basis.6. List the symbols7. Write down independent equations8. Count the numbers9. Solve the equations10. Check the answer.
Paul Ashall, 2008
MASS BALANCE PROCEDURES
Process description Flowsheet Label Assign algebraic symbols to unknowns
(compositions, concentrations, quantities) Select basis Write mass balance equations (overall, total,
component, unit) Solve equations for unknowns
Paul Ashall, 2008
MATERIAL BALANCE NOT INVOLVING REACTIONS
SAMPLE PROBLEM – BATCH DISTILLATION
1000 kg of a 20% by wt mixture of acetone in water is separated by multistage batch distillation. The top product (distillate) contains 95% by wt. acetone and the residue still contains 2% acetone. Calculate the amount of distillate.
MASS BALANCE –FILTRATION / CENTRIFUGATION
feed suspension
wash water/solvent
solid
waste water filtrate
Filtration
F1
5000 kg DM water
Impurity 55 kgWater 2600 kgAPI 450 kg Water 7300 kg
Impurity 50 kgAPI 2kg
Water 300 kgAPI 448 kgImpurity 5 kg
Mass balance - drier
feed product
water/evaporated solvent
MASS BALANCE – EXTRACTION/PHASE SEPARATION
A + B
S
A + B
S + B
A – feed solvent; B – solute; S – extracting solvent
Paul Ashall, 2008
Example (single stage extraction; immiscible solvents)
E1
feed
solvent
raffinate
extract
Paul Ashall, 2008
The next step in the synthesis of aspirin in water is to extract the aspirin with chloroform in a batch process. A 195-kg particular batch containing 0.11 kg aspirin/kg water is extracted with 596 kg chloroform. Extraction coefficient is give by y=1.72x where y = kg aspirin/kg chloroform and x = kg aspirin/kg water in raffinate. Calculate amount and composition of extract and raffinate
F = 195 kg; xf = 0.11 kg API/kgwaterS = 596 kg chloroformy = 1.72x, where y is kgAPI/kg chloroform in extract and x is kg
API/kg water in raffinate.
Total balance 195 + 596 = E + RAPI balance 19.5 = 175.5x1 + 596y1
19.5 = 175.5x1 + 596.1.72x1
x1 = 0.0162 and y1 = 0.029R is 175.5 kg water + 2.84 kg APIand E is 596 kg chloroform + 17.28 kg API
Note: chloroform and water are essentially immiscible
Paul Ashall, 2008
MASS BALANCE– ABSORPTION UNIT
feed gas stream
feed solvent
waste solvent stream
exit gas stream
Paul Ashall, 2008
SAMPLE PROBLEM- CRYSTALLIZER
A crystallizer contains 1000 kg of a saturated solution of potassium chloride at 80 deg cent. It is required to crystallise 100 kg KCl from this solution. To what temperature must the solution be cooled?
T deg cent Solubility gKCl/100 g water
80 51.1
70 48.3
60 45.5
50 42.6
40 40
30 37
20 34
10 31
0 27.6
At 80 deg cent satd soln contains (51.1/151.1)x100 % KCl i.e. 33.8% by wt
So in 1000 kg there is 338 kg KCl & 662 kg water.Crystallising 100 kg out of soln leaves a satd soln
containing 238 kg KCl and 662kg water i.e. 238/6.62 g KCl/100g water which is 36 g KCl/100g. So temperature required is approx 27 deg cent from table.
Paul Ashall, 2008
MASS BALANCES– MULTIPLE UNITS
Overall balance Unit balances Component balances
Paul Ashall, 2008
Multiple units
E – evaporator; C – crystalliser; F – filter unitF1 – fresh feed; W2 – evaporated water; P3 – solid product; R4 – recycle of
saturated solution from filter unit
R4
E C FF1
W2
P3
Paul Ashall, 2008
MASS BALANCE PROCEDURES
Process description Flowsheet Label Assign algebraic symbols to unknowns
(compositions, concentrations, quantities) Select basis Write mass balance equations (overall, total,
component, unit) Solve equations for unknowns
Paul Ashall, 2008
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 + dDi.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
Reactor mass balances
Paul Ashall, 2008
Example – aspirin synthesis reaction
Paul Ashall, 2008
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)
Paul Ashall, 2008
continued
A reactant is in excess if it is present in a quantity greater than its stoichiometric proportion.
% excess = [(moles supplied – stoichiometric moles)/stoichiometric moles] x 100
Paul Ashall, 2008
Example – aspirin synthesis
Paul Ashall, 2008
Conversion Fractional conversion = amount reactant
consumed/amount reactant supplied % conversion = fractional conversion x 100
Note: conversion may apply to single pass reactor conversion or overall process conversion
Paul Ashall, 2008
Yield
Yield = (moles product/moles limiting reactant supplied) x s.f. x 100
Where s.f. is the stoichiometric factor = stoichiometric moles reactant required per mole product
Paul Ashall, 2008
Example – aspirin synthesis
Paul Ashall, 2008
Selectivity
Selectivity = (moles product/moles reactant converted) x s.f. x100
ORSelectivity = moles desired product/moles
byproduct
Paul Ashall, 2008
ExtentExtent of reaction = (moles of component leaving
reactor – moles of component entering reactor)/stoichiometric coefficient of component
Note: the stoichiometric coefficient of a component in a chemical reaction is the no. of moles in the balanced chemical equation ( -ve for reactants and +ve for products)
Paul Ashall, 2008
ExamplesA Bi.e. stoichiometric coefficients a = 1; b =
1100 kmol fresh feed A; 90 % single pass
conversion in reactor; unreacted A is separated and recycled and therefore overall process conversion is 100%
reactor separationFR
P
Discussion - Synthesis of 3,3 dimethylindoline
Discussion - Aspirin synthesis
Paul Ashall, 2008
References Elementary Principles of Chemical
Processes, R. M. Felder and R. W. Rousseau, 3rd edition, John Wiley, 2000