Lect_4CBB 30902- Mass Balance

8/19/2019 Lect_4CBB 30902- Mass Balance

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The accounting of all mass in a

chemical/pharmaceutical process is referred to

as a mass (or material) balance.


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• ‘day to day’ operation of process for

monitoring operating efficiency

• Making calculations for design anddevelopment of a process i.e. quantities

required, sizing equipment, number of items

of equipment


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200 kg of a 40% w/w methanol/water solution ismixed with 100 kg of a 70% w/w

methanol/water solution in a batch mixer unit.

What is the final quantity and composition?


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MIXER200 kg

40% CH3OH/H2O

100 kg

70% CH3

OH/H2

O

OUTPUT?


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MIXER200 kg

40% CH3OH/H2O

100 kg

70% CH3OH/H2O

Total Mass in = Total Mass Out200 kg + 100 kg = 300 kg CH3OH/H2O

300 kg


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MIXER200 kg

40% CH3OH/H2O

100 kg

70% CH3OH/H2O

CH3OH Mass in = CH3OH Mass Out(40/100) x 200 kg + (70/100) x 100 kg = 150 kg

300 kg

150 kg CH3OH


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MIXER200 kg

40% CH3OH/H2O

100 kg

70% CH3OH/H2O

Final composition of CH3OH= (150 kg / 300 kg) x 100

= 50% CH3OH/H2O (w/w)

300 kg

50% CH3OH/H2O


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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 arecarried out on batch processes where balances

are carried out on the initial and final states of

the system.


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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.


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When there is no net accumulation or depletion

of mass in a system (steady state) then:

Total massentering

system

Total massleaving

system


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Notes:

Generation and consumption terms refer only togeneration of products and consumption of

reactants as a result of chemical reaction.

In Gen Out Con Acc


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• Streams

• Operations/equipment sequence

• Standard symbols


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Fresh feed

- Reactants

- Solvents

- Reagents

- Catalysts

Product

Recycle of unreacted material

By-products/co-productsWaste

Separation &purificationReactor


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It is often useful to calculate a mass balance

using molar quantities of materials and to

express composition as mole fractions or mole %.


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• A mole is the molecular weight of a substanceexpressed in grams

• To get the molecular weight of a substance you need itsmolecular 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 molecularweight.

• Mole fraction is moles divided by total moles

• Mole % is mole fraction multiplied by 100


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Example

Benzene is C6H6. The molecular weight is:

(6x12) + (6x1) = 78

So 1 mole of benzene is 78 grams.

1 kmol is 78 kg.


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A crystalliser 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 solutionbe cooled?


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T deg cent Solubility g KCl/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


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At 80 deg cent, saturated solution contains:(51.1/151.1) x 100 KCl i.e. 33.8% by wt.

So in 1,000 kg there is 338 kg KCl & 662 kgwater.

Crystallising 100 kg out of solution leaves a

saturated solution containing 238 kg KCl and662kg water i.e. 238/662 g KCl/100g water

which is 36 g KCl/100g. So temperature required

is approx 27 deg cent from table.


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feed suspension

wash water/solvent

solid

waste water filtrate


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F1

5000 kg water

Impurity 55 kg

Water 2600 kg

API 450 kgWater 7300 kg

Impurity 50 kg

API 2kg

Water 300 kg

API 448 kg

Impurity 5 kg


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A + B

S

A + B

S + B

A – feed solvent; B – solute; S – extracting solvent


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E1

feed

solvent

raffinate

extract


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feed gas stream

feed solvent

waste solvent stream

exit gas stream


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• Overall balance

• Unit balances

• Component balances


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E – evaporator; C – crystalliser; F – filter unit

F1 – fresh feed; W2 – evaporated water; P3 – solid product;

R4 – recycle of saturated solution from filter unit

R4

E C FF1

W2

P3


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• 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


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• Stoichiometric quantities

• Limiting reactant

• Excess reactant

• Conversion

• Yield

• Selectivity

• Extent of reaction


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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


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• 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)


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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


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• 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


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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


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Selectivity = (moles product/moles reactant

converted) x s.f. x100

OR

Selectivity = moles desired product/moles

byproduct


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Extent of reaction = (moles of componentleaving reactor – moles of component enteringreactor)/stoichiometric coefficient ofcomponent

Note: the stoichiometric coefficient of acomponent in a chemical reaction is the no. ofmoles in the balanced chemical equation (-vefor reactants and +ve for products)


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A Bi.e. stoichiometric coefficients a = 1; b = 1

100 kmol fresh feed A; 90 % single pass conversion in

reactor; unreacted A is separated and recycled and

therefore overall process conversion is 100%

reactor separationF

R

P


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Lect_4CBB 30902- Mass Balance