Chemistry for Chemical Engineers

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Chemistry for Chemical Engineers

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Dr Ashleigh J Fletcher

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Chemistry for Chemical Engineers

ISBN 978-87-403-0249-3

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Contents

Contents

Stoichiometry 44

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Contents

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Two of the main distinctions between chemical engineers and other engineering disciplines are the topics

of mass and energy balances Within these two topics there are a lot of underlying chemical principles that help chemical engineers to perform calculations to determine what is happening in a system, allowing better control of a process

his book will outline the basic chemistry principles that are required by chemical engineers to understand chemical reactions and relate them to the main themes of mass and energy balances It does not serve

as a complete account of all the chemistry that is important for chemical engineering but should give a grounding, which can be supplemented by reading further into the areas discussed, if required

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Quantifying systems

Quantifying systems

Working as a chemical engineer requires a capacity to interpret data and quantities provided from diferent sources It is essential that any quantities used or calculated are recorded correctly, as the inclusion or omission of units changes the context dramatically For example 7 is a purely numerical quantity, but adding a unit, say kilograms so the measurement becomes 7 kg, conveys signiicantly more information In all working it is important to write down both numerical values and the corresponding units; as a result,

it is necessary to appreciate the relationship between certain units and have an ability to convert between quantities he properties that can be measured, such as time, length and mass, are known as dimensions and can also be composed from multiplying or dividing other dimensions, for example velocity (length/ time) Units can be treated like algebraic variables when quantities are added, subtracted, multiplied or divided but note that numerical values may only be added or subtracted if their units are the same he most common set of units that chemical engineers come into contact with are the seven fundamental S.I units of measurement, as deined in the International System of Units (the abbreviation S.I comes from the French for this classiication: Système Internationale d’Unités) he system was developed in

1960 and has been widely accepted by the science and engineering communities

he table below shows the seven base units and their corresponding abbreviations, as chemical engineers the most commonly used units will be those for amount of substance, mass, length, temperature and, importantly, time

Property Unit Abbreviated Notation

Base units of measurement according to the S.I classiication

he seven units within the S.I are referred to as base units, so for length that would be metre (m), but

these can be converted to other systems of measurement that represent the equivalent dimension, such alternative units are referred also known as base units but not S.I., so for the example of length one could use (t)

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Sometimes, quantities are calculated from several dimensions, this is very common in chemical engineering where lowrates, such as mass or volumetric lowrate are frequently used In this case the quantities are measured as mass/time (kg/s) and volume/time (m3/s); the corresponding units are a

composition of all the dimensions involved and are known as derived units.

Common derived units are listed in the table below It should be noted that these dimensions have their own unit and abbreviated notation, in addition to that from their derivation

Equivalent property Unit Abbreviated notation S.I derived units

Commonly used derived units

Note 1 N is deined as being equivalent to 1 kg m/s2 because a force of 1 N produces an acceleration of

1 m/s2 when applied to a mass of 1 kg It is, therefore, useful to remember that 1J z 1 N m z 1kg m2 s-2

in order to simplify complex units generated in some equations

he base units are not always the most useful mathematical representation of the numerical value determined and may be necessary to use other methods to simplify the quantity For example, 60 s can

be represented as 1 minute (1 min), similarly 0.000001 s could be represented as 10-6 s or 1 ms, the latter

unit (microseconds) and min are known as multiple units, and it is essential to be able to understand

not only the quantities involved in a system but also their level of scale Chemical engineers must be comfortable with the common preixes used with S.I units and other units from around the globe Commonly used preixes are given below, with their names and numerical value

Common preixes in metric system

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Converting units is an essential skill for all chemical engineers and the easiest method to use is fractional representation his keeps track of all numerical values and units throughout the conversion performed, allowing those units that cancel to be easily identiied

he equivalence between two expressions of a given quantity may be deined in terms of a ratio (expressed here in common fraction notation):

mm 10

cm 1

1 centimetre per 10 millimetres

Ratios of this form are called conversion factors Generally, when converting units, multiply by conversion

factor(s) as fractions with new units as the numerator (top) and old units as the denominator (bottom) For example, convert 100 mm into cm:

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Worked example – convert the gas constant from 8.314 J mol-1 K-1 to Btu lb-mol-1 ºC-1, using the following conversions:

1 kJ = 0.9478 Btu; 1 kmol = 2.205 lb-mol; 1 K = 1 ºC

Firstly, write out the value given in fractional format:

K mol

J 8.314

hen write out each of the required conversions in the same format, making sure that the units match and can cancel out in the working For example, if the value to be converted has J on the top line, and the conversion of 1 kJ = 0.9478 Btu is to be applied, it is irstly required that J is converted to kJ To do this, divide through by 1000 J and multiplying by kJ (as 1 kJ = 103 J = 1000 J) Ater this, kJ is now on the top line:

K mol 1000

kJ 8.314

J 1000

kJ 1 K mol

J 8.314

=

×

It is then possible to use the conversion, 1 kJ = 0.9478 Btu, directly, to arrive at:

K mol 1000

Btu 0.9478 8.314

kJ 1

Btu 0.9478 K

mol 1000

kJ

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