Contents1) Process Classification2) Balances on Single Unit Process

3) Balance on Multiple Unit Processes

4) Recycle and Bypass

5) Balances on Reactive Process System

6) Product Separation & recycle

General Balance Equation General Procedure in Material Balance Calculation Flowcharts Degree of Freedom Analysis Sources of Balance Equations

Stoichiometry Limiting & Excess Reactant Fractional Conversion Extent of Reaction Chemical Equilibrium Multiple Reaction, Yield & Selectivity Methods Used to Solve Balance on Reactive Process

7) Purge Stream

Recycle Normally in chemical reaction, some of

unreacted reactant also found in the product.

This unreacted reactant can be separated and recycle back to the reactor.

Product

Recycle Stream

Fresh Feed(raw material)

Reactor Separator

Purpose of Recycle

1) Recovery of catalyst – catalyst is very expensive.

2) Dilution of process stream – typically for slurry solution.

3) Control of process variables – especially for the reaction that release heat, heat can be reduce by lowering the feed concentration.

4) Circulation of working fluid such as in refrigerator system.

Example: Material balance on an air conditioner.

Fresh air containing 4.00 mole % water vapor is to becooled and dehumidified to a water content of 1.70 mole% H2O. A stream of fresh air is combined with a recyclestream of previously dehumidified air and passesthrough the cooler. The blended stream entering the unitcontains 2.30 mole % H2O. In the air conditioner, some ofthe water in the feed stream is condensed and removedas liquid. A fraction of the dehumidified air leaving thecooler is recycled and the remainder is delivered to aroom. Taking 100 mol of dehumidified air delivered tothe room as a basis of calculation, calculate the moles offresh feed, moles of water condensed and moles ofdehumidified air recycled.

Solution:

Exercise (An Evaporative Crystallization Process)

The flowchart of a steady state process to recover crystalline potassiumchromate (K2CRO4) from an aqueous solution of this salt is shown below:

Forty-five hundred kilograms per hour of a solution that is one-third K2CrO4 by massis joined by a recycle stream containing 36.4 % K2CrO4, and the combined stream isfed into an evaporator. The concentrated stream leaving the evaporator contains49.4 % K2CrO4; this stream is fed into a crystallizer in which it is cooled (causingcrystals of K2CrO4 to come out of solution) and then filtered. The filter cake consistsof K2CrO4 crystals and a solution that contains 36.4 % K2CrO4 by mass; the crystalsaccount for 95 % of the total mass of the filter cake. The solution that passesthrough the filter, also 36.4 % K2CrO4, is the recycle stream.

1. Calculate the rate of evaporation, the rate of production ofcrystalline K2CrO4, the feed rates that the evaporator andthe crystallizer must be designed to handle, and therecycle ratio (mass of recycle)/(mass of fresh feed).

2. Suppose that the filtrate were discarded instead of beingrecycled. Calculate the production rate of crystals. Whatare the benefits and costs of the recycling?

Bypass Fraction of the feed to a process unit is diverted

around the unit and combined with the output stream from the unit.

One use of bypass is to obtain precise control of the output stream

Product

Bypass Stream

Fresh Feed(raw material)

Process Unit

Balance on Reactive Processes System

Balance on Reactive Processes System

Stoichiometry.

Conversion (X), limiting and excess reactant

Chemical equilibrium.

Multiple Reaction, Yield and Selectivity.

Balance on Reactive System.

Rate of reaction (r).

Reactor

Ammonia Reactor Acid Nitric Reactor

Bioreactor/ fermenter

Reactor or bioreactor: equipment in which chemical or biochemical reactions occur.

RATE OF REACTION (r)

• Obtained from stoichiometric balance of chemical reactions.

• Stoichiometry = relative proportions of chemical components participating in a chemical reaction.

• Stoichiometric equation of chemical reaction: showing the relative number of molecules/moles of components participating in the chemical reaction.

• Reactants– components that react with each other in a chemical reaction.

• Products – components that are produced by a chemical reaction.

• Chemical reactor- equipment in which chemical reactions occur.

• r: Net rate of generation of component (c) in units of moles of component per unit time.

Stoichiometry Equation

Stoichiometric coefficient (α)

Stoichiometric coefficient (α)

2 SO2 + O2 2 SO3

Reactant Product

Stoichiometric coefficient (α)

(components that react with each other in a chemical reaction)

(components that are produced by a chemical reaction).

Note: All the elements must be balanced to determine the α for the

reactants and products. α𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 -ve and α𝑝𝑟𝑜𝑑𝑢𝑐𝑡 +ve.

Stoichiometry Stoichiometric ratio: ratio of species stoichiometry coefficients

in the balanced reaction equation can be used as a conversion factor to calculate the amount of particular reactant (or product) that was consumed (produced).

2 mol SO3 generated

2 mol SO2 consumed

2 mol SO2 consumed

1 mol O2 consumedor

2 SO2 + O2 2 SO3

Test YourselfC4H8 + 6 O2 4 CO2 + 4 H2O

1) Is the stochiometric equation balance?

2) What is stochiometric coefficient for CO2

4) What is stochiometric ratio of H2O to O2 including it unit

3) How many lb-moles of O2 reacted to form 400lb-moles CO2

5) 100 mol/min C4H8 fed into reactor and 50% is reacted. At what rate water is formed?

• Yes

• 4

• 4 mol H2O generated/ 6 mol O2 consumed

• 600 lb-moles O2 reacted

• 200 mol/min water generated

CONVERSION (X) & LIMITING REACTANT Common measure of course of reaction is the fractional conversion / conversion of the

limiting reactant.

Conversion links the outlet flow rate with the inlet flow rate of the same component = additional independent equation.

Limiting reactant=reactant that has the lowest value of reaction rate:

Note: Here, component (c) refer to limiting reactant.

Reactant with the lowest value for nic (-k) finishes first

The limiting reactant finishes first if the reaction is left to react by itself.

If the reaction is left to react, the rate of reaction r increases to reach the value of rlimiting

when nok = 0.

Reactant with the lowest value for Nik/(-k) finishes first.

Other reactants= excess reactant

Percentage of Excess component, Ec:

Note: Here, component (c) refer to excess reactant.

Example:

H2 is excess reactant and C2H2 is limiting reactant

% excess of H2 = (50-40)/40 * 100% = 25 %

C2H2 + 2H2 C2H6

Inlet condition: 20 kmol/h C2H2 and 50 kmol/h H2

What is limiting reactant and fractional excess?

Solution:

k

ikLimiting

Nr

Only consider reactant components:

MATERIAL BALANCE WITH SINGLE REACTION

• Lets says: Ammonia synthesis reaction : N2 + 3H2 2NH3

• If rate of reaction of nitrogen = -rN2 (negative: nitrogen is used)

• Rate of reaction of hydrogen = -rH2 = (H2 /N2)(-rN2 )= (-3/-1)(-rN2)

• Rate of reaction of ammonia = rNH3 = (NH3 /N2)(-rN2 )= (2/-1)(-rN2)

• Then:

Rate of reaction r is fixed for a given reaction stoichiometric equation

• Rate of reaction of component:

• Thus, COMPONENT MATERIAL BALANCE FOR REACTING SYSTEMS Molar:

COMPONENT MATERIAL BALANCE FOR REACTING SYSTEMS

Molar:

Rate of chemical reaction of component c, rc.

Lets say for the SO3 synthesis reaction, 15 mole h1 O2 40 mole h-1 SO2 and 0 mole h-1 SO3 is fed into a reactor. If the flow rate out of O2 is 8 mole h-1 calculate the flow rates of other components.

niO = 15 mole h-1

niSO2 = 40 mole h-1

niSO3 = 0 mole h-1

noO =8 mole h-1

noSO2

niSO3

Reactor SO3

O2 + 2SO2 2 SO3

Choose component mole balance that has most information to get r :

Thus, substituting r in SO2 & SO3 balances:

Example 1:

Solution:

O2 mole balance: 1hmole7 rThen

SO2 mole balance:

SO3 mole balance:

Test Yourself2 C2H4 + O2 2 C2H4O

The feed to a reactors contains 100kmol C2H4 and 100kmol O2.

1) which is limiting reactant?

2) Percentage of excess?

3) O2 out?

4) C2H4 formed?

5) rate of reaction?

6) If fractional conversion for limiting reactant is 50%, what is outlet composition and rate of reaction?

7) If reaction proceed to a point where 60kmol O2 left, what is fractional conversion for C2H4? Fractional conversion of O2 and rate of reaction?

C2H4

{(100-50)/50 }x100%=100%

50kmol

100kmol C2H4

50kmol

50kmol C2H4; 75 kmol O2 and 50 kmol C2H4O rate of reaction = 25 kmol;

fC2H4=0.8 fO2=0.4 Rate of reaction = 40 kmol

1) The reaction between ammonia and oxygen on Pt catalyst produces nitro oxide and water. The stoichiometric equation is given by

4NH3 + 5O2 4NO + 6H2O

Under certain conditions, conversion of NH3 into NO (N) can achieve 90% at ammonia flow rate NH3 40 mole h-1 and O2 60 mole h-1 . Calculate the other flow rate.

Test yourself:

Answers: noNH3 = 4 mole h-1

noO2 = 15 mole h-1

noN = 36 mole h-1

noH2O = 54 mole h-1

2) If the reaction in Example 3.6 achieves 80% conversion with equimolar ammonia and oxygen feed that is fed at 100 mole h-1. Calculate the flow rate out of all components.

Stoichiometric equation is given by: 4NH3 + 5O2 4NO + 6H2O

Answers: noNH3 = 18 mole h-1

noO2 = 10 mole h-1

noN = 32 mole h-1

noH2O = 48 mole h-1

3) Acrylonitrile (C) is produced by the following reaction:

C3H6 + NH3 + (3/2)O2 C3H3N + 3H2O

The feed contains 10% mole propylene (P), 12% mole ammonia (A) and 78% mole air. Conversion of limiting reactant is 30%. By choosing 100 mole h-1 feed as the basis, determine the limiting reactant, fractional excess of other reactants and flow rate out of all components.

Answers:

1hmole6261 .iNoN NN 1hmole93112 oAN

1hmole88113513816 ...oON

1hmole3310 oCN

1hmole9330 oWN

201101

110112.

)(

)(

PiPA

PiPAiAA

N

NNE

092011051

110513816

2

.).(

).(.

PiPO

PiPOiOO

N

NNE

12

1

12

W

iWN

10

1

10

P

iPN

9210

51

3816.

.

.

O

iON

Propylene is limiting reactant:

Fractional Excess:

Output flowrate:

1hmole31

1030

.

P

iPP NXr

Chemical Equilibrium For a given set reactive species and reaction condition, two fundamental question

might be ask:1. What will be the final (equilibrium) composition of the reaction mixture? –

chemical engineering thermodynamics2. How long will the system take to reach a specified state short of equilibrium?

– chemical kinetics

1) Irreversible reaction• reaction proceeds only in a single direction (from reactants to products).• the concentration of the limiting reactant eventually approaches zero.

2) Reversible reaction• reactants form products for forward reaction and products undergo the

reverse reactions to reform the reactants.• Equilibrium point is a rate of forward reaction and reverse reaction are

equal

Note:However, the discussion to get the chemical

equilibrium point is not covered in this text- learn in chemical engineering thermodynamic

Two types of reaction:

Product Separation & Recycle

75 mol B/min100 mol A/min75 mol A/minReactor

Product

SeparationUnit

25 mol A/min75 mol B/min

25 mol A/min

Overall Conversion =Reactant input to Process – reactant output from Process

Reactant input to Process

Single Pass Conversion =Reactant input to Reactor – reactant output from Reactor

Reactant input to Reactor

Purging

ProductFresh Feed

(raw material)Reactor

Product Separation

Unit

Recycle Purge

To prevent any inert or insoluble substance build up and accumulate in the system

Purge stream and recycle stream before and after the purge have a same composition.