mass balance of glaciers and ice caps

‘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

Module 9001 Mass Balance

The accounting of all mass in a

chemical/pharmaceutical process is referred

to as a mass (or material) balance

• ‘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

Simple example – batch mixing

Total initial mass = total final mass = 300 kgInitial methanol mass = final methanol mass

80 + 70 = final methanol mass = 150 kg

Therefore final composition of batch is

(150/300) x 100 = 50 % by wt

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?

Integral balances are carried out on batch

processes where balances are carried out on

the initial and final states of the system.

Simple batch reaction cycle

3 steps

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

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 system

3 Apply to total mass and component mass

• 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

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

Mixing of streams

F1

F2

F4

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%

• Streams

• Operations/equipment sequence

• Standard symbols

• Process flow diagram

• PID

Typical simple flowsheet

Exercise – 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 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

• 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

Exercise – 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

Mass balance - crystalliser

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 solution be cooled?

T deg cent Solubility

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

Mass balance filtration/centrifuge

feed suspension

wash water/solvent

solid

Mass balance - drier

water/evaporated solvent

Mass balance – extraction/phase

Example (single stage extraction;

F = 195 kg; xf = 0.11 kg API/kgwater

S = 596 kg chloroform

y = 1.72x, where y is kgAPI/kg chloroform in extract and x is kg

API/kg water in raffinate.

Mass balance – absorption unit

feed gas stream

feed solvent exit gas stream

Mass balances – multiple units

• Overall balance

• Unit balances

• Component balances

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)

A mass balance and tracking of usage of a solvent

used in an API production process is required for a Pollution Emission Register (PER)

Discuss and outline in general terms how you would

do this.

• Refers to quantities of reactants and

products in a balanced chemical reaction

Reactor mass balances

Example – aspirin synthesis

reaction

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)

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

Example – aspirin synthesis

• Fractional conversion = amount reactant

consumed/amount reactant supplied

• % conversion = fractional conversion x 100

Note: conversion may apply to single pass reactor

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

Example – aspirin synthesis

Extent 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

i.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%

F

R

Discussion - Synthesis of 3,3

dimethylindoline

Discussion - Aspirin synthesis

• Elementary Principles of Chemical

Processes, R M Felder and R W

Rousseau, 3rd edition, John Wiley, 2000