Stoichiometry

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StoichiometryUnit process, Material and energy balances, Molecular units, mole fraction, Tie substance, Gas laws, Mole volume, Primary and secondary quantities, Equilibrium state, Dimensionless equations, Dimensionless formulae, Dimensionless groups.

Presented by : Dr. S. H. Majumdar

Associate Professor,GES’s, Satara College of Pharmacy, Satara

IntroductionA unit operation is any part of potentially multiple-step

process which can be considered to have a single function.

Each unit operation is based on one type of scientific principle.

Examples of unit operations include:Separation Processes Purification Processes Mixing Processes Reaction Processes Power Generation Processes Heat Exchangers 2

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Unit operations are based on both science & experience.

The theory & practice must be combined judiciously for achieving proficiency in handling equipment

Introduction

Unit Process

• The process in which several unit operations are combined in a sequence to achieve the objectives of a chemical or physical process.

Unit Process- Physical Process

• Consider the example of manufacture of common salt.

Transportation of fluids & Solids

Transfer of Heat

Evaporation

CrystallizationFiltrationDryingScreening

Paracetamol p-Aminophenol

NitrobenzeneBenzene

Unit Process-Chemical Process

• Consider the production of Paracetamol from benzene

Nitration

HNO3 / H2SO4,, 1 h

Reduction Al / H2SO4, 10 h

Acetylation

Acetic anhydride / sulphuric acid

Unit Process-Chemical Process

• In the above process, three unit operations are involved. • These are nitration, reduction & acetylation• Each unit process is in turn made of a number of unit

operations.• For example, in the nitration of benzene to nitrobenzene,

the unit operation involved are:

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Fluid Flow

Charging of nitric acid into the reactor

Heat Transfer

Cold brine is passed to reduce the temp to 150 C

Fluid Flow

Addition of Sulphuric acid

Fluid Flow

Addition of benzene in small quantities

Heat Transfer

Heating to 600C for 1 hour

Filtration

Drying Crystallization

Production of paracetamol

Unit Operation

Unit operations are largely used to conduct the physical steps such as:

1. Preparation of reactants2. Separation & purification of the products,3. Recycling of the unconverted reactants,4. Controlling the energy transfer into or out of the

chemical reactor Thus several steps are carried in a sequential order to

achieve a process efficiently & economically

Scientific Foundations• A large number of unit operations are simultaneously handled

in a chemical process.• Hence, the knowledge of the elementary physical & chemical

laws is essential for the application of scientific principles & techniques.

• Some of these laws are– Basic Laws: Laws of conservation of matter

Laws of conservations of energy.– Special Laws: Universal gas laws (Ideal gas equation)

Dalton’s law of partial pressures.

Energy Balance

Input = Output

For all forms of energy

(a) Molar Conc.

(b) Mole Fraction, X2

Material Balance

Input = output

Basic Laws

Laws of conservations

Unit Operations

Governed by Physical & chemical Laws

(a)Heat(b)Mechanical(c)Electrical(d)Chemical(e)radiation

Governed By

EnergyMatter

EstimatedConsidered

Basic Laws

• The general law of conservation can be applied to any process.

• It is employed in the engineering in the form of :

(a) Material Balance

(b) Energy Balance

Material Balance

• The law of conservation of matter states that material cannot be destroyed or created, it can be changed from one form to another.

• Input = Output

Amount of raw Material

Amount of changed material

+ Amount of unchanged material

Estimation of Material Balance

• Measuring the amount of all the components (constituents).

• Amount is expressed in concentration units; – moles/litre, – molal units, – mole fraction units, – % w/v, – % w/w etc

Estimation of Material Balancecontd.

• Mole: quantity of substance whose mass is numerically equal to its molecular weight.

Gram-moles of a substance

=Mass in gramsMolecular weight

Estimation of Material Balancecontd.

• Molality:

• Mole Fraction

Molality m =No. of moles of substance

No. of kg of medium (solvent)

No. of moles of one constituent

Total No. of moles of all constituents=

Numerical

A solution of common salt in water is prepared by adding 20 kg of salt to 100 kg of water, to make a liquid of density 1323 kg m-3.

•Calculate the concentration of salt in this solution as a –(a) weight/weight fraction, –(b) weight/volume fraction, –(c) mole fraction, –(d) molar concentration.

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Weight Fraction

Weight/volume fraction• A density of 1323 kg m-3 means that 1m3 of solution weighs 1323 kg, • But 1323 kg of salt solution contains

• And so1 m3 solution contains 220.5 kg salt.

• Weight/volume fraction

• and so weight/volume = 22.1%

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Mole Fraction

The molar concentration (M) is

220.5/58.5 = 3.77 moles in 1 m3.

Numerical

• If air consists of 77% by weight of nitrogen and 23% by weight of oxygen calculate the:(a) mean molecular weight of air,(b) mole fraction of oxygen,(c) concentration of oxygen in mole m-3and kg m-3if the total

pressure is 1.5 atmospheres and the temperature is 25°C.

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Energy Balance

• Energy is the capacity to exert a force through a distance & manifests itself in various forms.

• Law of conservation of energy states that the energy output must be same as the energy input in a chemical process.

Stoichiometry• Stoichiometry: It deals with combining weights of elements

and compounds.• It is defined as the theory of the proportions of a chemical

reaction based on which chemical species react.

• The stoichiometriequation shows relative numbers of moles of reactants in a balanced chemical reaction i.e, the ratios obtained from numerical coefficients are called the Stoichiometric ratios ,e.g,

C4H8+6O2--------------4CO2+ 4H2O• Which means one mole of Butane and 6 moles of Oxygen are

required to attain complete combustion.

Stoichiometric Equation-Balancing

Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions.

Stoichometry means carrying out of calculations based on quantitative relationship.

In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers.

For example, in a reaction that forms ammonia (NH3), exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of NH3.

N2 + 3H2 → 2NH3

Stoichiometric Equation-Balancing

• Stoichiometry can be used to find quantities such as the amount of products (in mass, moles, volume, etc.) that can be produced with given reactants and percent yield (the percentage of the given reactant that is made into the product).

• Stoichiometry calculations can predict how elements and components diluted in a standard solution react in experimental conditions.

• Stoichiometry is founded on the law of conservation of mass: the mass of the reactants equals the mass of the products.

• Reaction stoichiometry describes the quantitative relationships among substances as they participate in chemical reactions.

• In the example above, reaction stoichiometry describes the 1:3:2 ratio of molecules of nitrogen, hydrogen, and ammonia

• Composition stoichiometry describes the quantitative (mass) relationships among elements in compounds.

• For example, composition stoichiometry describes the nitrogen to hydrogen ratio in the compound ammonia: 1 mol of ammonia consists of 1 mol of nitrogen and 3 mol of hydrogen. As the nitrogen atom is about 14 times heavier than the hydrogen atom, the mass ratio is 14:3, thus 17 kg of ammonia contains 14 kg of nitrogen and 3 kg of hydrogen.

• A stoichiometric amount or stoichiometric ratio of a reagent is the optimum amount or ratio where, assuming that the reaction proceeds to completion:– All of the reagent is consumed,– There is no shortfall of the reagent,– There is no excess of the reagent.

• A non-stoichiometric mixture, where reactions have gone to completion, will have only the limiting reagent consumed completely.

Definitions

• Yield = Actual moles of desired product/Total moles produced as100%conc.

• Selectivity= Actual moles of desired product/moles of undesired products

• Conversion=Total moles reacted/Total moles fed to reactor

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Material Balance Involving Chemical Reactions and Tie Substances

DEFINITIONSa) Excess Reactants: Consider for the moment the oxidation of o-

xylene to phathalic anhydride by the reactionC8H10+ 3 O2 C8H4O3 + 3H2O

if we react exactly 3 moles of O2with one mole of C8H10 and if the reaction were 100 % complete, input / output table would looks like:

Input output C8H10 1 -

O2 3 -

C8H4O3 - 1

H2O - 3


DEFINITIONSa)Excess Reactants:Generally however, it is neither convenient nondesirable to use

reactants in exactly stoichiometric amounts, and in many cases, a large surplus of one reactant relative to another is required. In this case, we refer to this surplus as an excess of one reactant relative to another.

As a further example, suppose that in the above reaction there is a 200% excess of O2. Now, on the basis of one mole of C8H10, we see that 3moles of O2 is the maximum amount that can theoretically react, therefore a 100% excess indicator that 6 moles are preset and 200% excess that 9 moles are preset.

a) Excess Reactants:Input output

C8H10 1 -

O2 9 6

C8H4O3 - 1

H2O - 3

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DEFINITIONSb)Incomplete reactions: In addition to the fact that usually one

or more reactants are in excess, chemical reactions are generally incomplete ,i.e. the limiting reactant is not completely used up. This idea is expressed as completion percent, which is based on the limiting reactant. Retiring to the Ph.A. reaction, suppose that the reactants are in stoichiometric ratio, but the reaction is only 60% complete. Under these conditions, the input/output table will be as follows.

b)Incomplete reactions: Input output

C8H10 1 0.4

O2 3 1.2

C8H4O3 - 0.6

H2O - 1.8

-Where we note that the unreacted C8H10 must be included in the output.

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DEFINITIONS•c)Inert Substances :As a third complicating feature in material balances, inert substances

which do not react in anyway, are often present in the input streams, e.g. in most oxidation processes, air is used as a source of oxygen. Since air has 79 mol percent as nitrogen, which is an inert substance at the oxidation reaction conditions, we must account for this nitrogen in the materials balance.

Therefore, if air is used as a source of oxygen and the oxygen is in a sloichionetric ratio, the input / outputtable for the previous example is as follows:

(Rxn100%complete)


c)Inert Substances :


DEFINITIONS2)Tie Substances :When an inert substance is present in the input to a process, and

consequently, also in the output, it often constitutes what is known as a tie substance.

We can define a tie substance as a material which passes through a system from a single input stream to a single outputstream, remaining unchanged during such process. Thus it forms a tie between both input and outputstreams.


2)Tie Substances :• In the last example, we saw that nitrogen has represented a tie

substance as 28.2 moles entered in the airstream and 28.2 moles left in the output waste gas stream.

Dimensions & Measurements

• “Dimension” is characteristic of the object, condition, or event and is described quantitatively in terms of defined “units”.

• A physical quantity is equal to the product of two elements:– A quality or dimension– A quantity expressed in terms of “units”

• Dimensions– Physical things are measurable in terms of three primitive qualities

(Maxwell 1871)• Mass (M)• Length (L)• Time (T)

Note: (Temperature, electrical charge, chemical quantity, and luminosity were added as “primitives” some years later.)

Dimensions & Measurements

– Examples• Length (L)• Velocity (L/T)• Force (ML/T2)

• Units– Measurements systems--cgs, MKS, SI--define units– SI units are now the international standard (although many

engineers continue to use Imperial or U.S.)

Dimensional Analysis

• Dimensional analysis is a conceptual tool often applied in Engineering and Science to attack a complicated problem for which no formal mathematical equation could be derived.

• It is often the basis of mathematical models of real situation. • This method is intermediate between formal mathematical

development and completely empirical method.

• Dimensionless groups are generated after analysis of variables involved in a phenomenon by dimensional analysis method.

• Fluid flow when analyzed, Reynolds number is generated

“Dimension less” Quantities• Dimensional quantities can be made “dimensionless” by “normalizing”

them with respect to another dimensional quantity of the same dimensionality.

Example: speed V (m/s) can be made "dimensionless“ by dividing by the velocity of sound c (m/s) to obtain M = V/c, a dimensionless speed known as the Mach number. M>1 is faster than the speed of sound; M<1 is slower than the speed of sound.Other examples: percent, relative humidity, efficiency

• Equations and variables can be made dimensionless, e.g., Cd = 2D/(ρv2A)

• Useful properties:– Dimensionless equations and variable are independent of units.– Relative importance of terms can be easily estimated.– Scale (battleship or model ship) is automatically built into the

dimensionless expression.– Reduces many problems to a single problem through normalization

Dimensionless form of equations

Motivation: There are three important motivations for writing complex equations in dimensionless or dimensionally reduced form.

1) It is easier to recognize when to apply familiar mathermatical techniques.2) It reduces the number of times we might have to solve the equation

numerically.3) It gives us insight into what might be small parameters that could be

ignored or treated approximately.

Dimensionless variables and numbers

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REFERENCES

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THANK YOU

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Stoichiometry