1.6 process and process variables

7/28/2019 1.6 Process and Process Variables

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1.6 Process and process

variables


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Chapter outline

Mass and volume

Flow rate

Chemical composition


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1.6.1 Mass and volume

Density, = mass (kg/m3,g/m3,lbm/ft3..)

volume

Specific volume is the inverse of density

Density of pure solids and liquids are independentof pressure and vary relatively slightly withtemperature

Eg. Liquid waterT (oC) (g/cm3)

0 0.999868

3.98 1.00000

100 0.95838


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Specific gravity, SG =

density of water at 4C is used as areference density; whereas the value isshowed below:

ref@H2O(l) (4C) = 1.000 g/cm3

= 1000 kg/m3

= 62.43 lbm/ft3

ref


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SG is a dimensionless. To get the density of a

substance, multiply the SG value to the value

ofreference density.

mean that the specific gravity of a substanceat 20C with reference to water at 4C is 0.6

Refer Table B.1 for specific gravities of selected liq. and solids

SG= 0.620

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1.6.2 Flow rate

Flow rate- the rate at which a material is transportedthrough a process line.

Can be expressed as :mass flow rate, (mass/time)

volumetric flow rate, V (volume/time)

The density of a fluid can be used to convert aknown volumetric flow rate of a process stream tothe mass flow rate of that stream or vice versa.


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The mass flow rates of process streams must be known for

many process calculations, but it is frequently more

convenient to measure volumetric flow rates than mass flow

rate. Therefore, the density is used to convert volume flow

rate to mass flow rate.

Flow meter is a device mounted in a process line that

provides a continuous reading of the flow rate in the line.

Two commonly used flow meter are rotameter and orifice

meter


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1.6.3 Chemical Composition

Moles and molecular weight

Mass and mole fractions and average

molecular weight

Concentration

Parts per million and parts per billion


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Moles and molecular weight

Atomic weight of an element is the mass of an

atom on a scale that assigns 12C

Molecular weight (MW) of a compound is the

sum of the atomic weights of the atoms that

constitute a molecule of the compound.

Eg; O2

Atomic weight of O= 16

MW of O2 = 32


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Gram-mole (g-mole or mol) = Mass

Molecular Weight

If the molecular weight of a substance is M, then thereare M kg/kmol, M g/mol, and M lbm/lb-mole of thissubstance.

The molecular weight may thus be used as a conversionfactor that relates the mass and the number of moles of

a quantity of the substance. One gram-mole of any species contains 6.02 x 1023

(Avogadros number) molecules of that species.


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Mass and mole fractions and

average molecular weight

Process streams consist of mixtures of liquids orgases, or solutions of one or more solutes in a liquidsolvents.

The following terms may be used to define thecomposition of a mixture of substances, including aspecies A.

Mass fraction: xA =

Mole fraction: yA =

Mass of ATotal mass

Moles of A

Total moles


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The percent by mass of A is 100xA, and the molepercent of A is 100yA.

Procedure to Convert from Mass Fractions to

Moles Fractions1. assuming as a basis of calculation a mass of themixture (e.g. 100 kg or 100 lbm)

2. using the known mass fractions to calculate the massof each component in the basis quantity, andconverting these masses to moles.

3. taking the ratio of the moles of each component to thetotal number of moles


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The average molecular weight (or mean molecular weight)

of a mixture, (kg/kmol, lbm/lb-mole, etc.), is the ratio of

the mass of a sample of the mixture (mt) to the number of

moles of all species (nt) in the sample.

Ifyi is the mole fraction of the ith the component of the

mixture:

Ifxi is the mass fraction of the ith component of the

mixture:

componentall

ii2211 My.....MyMyM

componentall i

i

2

2

1

1

M

x.....

M

x

M

xM


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Concentration

Mass concentration of a component of a mixture or solution

is

mass of this component = g/cm3, lbm/ft3, kg/in2

volume of the mixture

Molar concentration of a component is

the no. of moles of the component = kmol/m3, lb-moles/ft3

volume of the mixture

The molarity of a solution is the value of the molarconcentration of the solute expressed in gram-molessolute/liter solution


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Parts per million and parts per

billion

Used to express the concentrations of trace

species in mixtures of gases or liquids.

Ifyi

is the fraction of component i, then by

definition

ppmi= yix 106

ppbi= yix 10

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1.6 process and process variables

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