# Lecture 2 Processes and Process Variables

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Lecture 2Processes and Process variables

Dr. A. Alim

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Processes and Process variables:Density and Specific gravity Mass and Volumetric Flow Rate Moles and Molecular Weight Concentration Pressure Temperature

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Process Variables Density and Specific Gravity

Summary: Density () = Mass / Volume Specific gravity (solids and liquids) = Density / Density of water Specific gravity (gases) = Density / Density of air Question: what are examples of units for density and for specific gravity?

Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

2005 by John Wiley & Sons, Inc. All Rights Reserved

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Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

2005 by John Wiley & Sons, Inc. All Rights Reserved

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Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

2005 by John Wiley & Sons, Inc. All Rights Reserved

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2005 by John Wiley & Sons, Inc. All Rights Reserved

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Process Variables Mass and Volumetric Flow Rate Measurements

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Process Variables Moles and Molecular Weight

A more accurate definition of atomic weight : A dimensionless ratio of the average atomic mass (average of all present isotopes) to 1/12 the atomic mass of carbon -12 Note that weight here does not mean the usual definition of weight as the force exerted by gravity we discussed earlier.Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

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Process Variables Moles and Molecular Weight

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What is the average molecular weight :

From equation 3.3-7: Av mol weight = (0.15)(32) + (0.044)(28) + (0.12)(44) + (0.69)(28) = 30.5

From equation 3.3-8: Av mol weight = 1/[0.16/32 + 0.04/28 + 0.17/44 + 0.63/28] = 30.5 Or simply: Av mol weight = total mass / total moles = 100 / 3.279 = 30.5Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

2005 by John Wiley & Sons, Inc. All Rights Reserved

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Mole fraction-Mass fraction- Volume fraction:

The process for converting mass fraction to mole fraction and vice versa can Also be extended to include conversions to and from volume fractions.Key equation for each material is : volume = mass/density Example: A mixture of A and B has 30% by mass of A (sp.gravity 0.6) and the rest is B (sp. Gravity 0.7). What are the volume fractions of A and B in the mixture?

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Mole fraction-Mass fraction- Volume fraction:

The process for converting mass fraction to mole fraction and vice versa can Also be extended to include conversions to and from volume fractions.Key equation for each material is : volume = mass/density Example: A mixture of A and B has 30% by mass of A (sp.gravity 0.6) and the rest is B (sp. Gravity 0.7). What are the volume fractions of A and B in the mixture?

Basis 100 grams of mixture: Material Mass (g) density(g/cm3) A 30 0.6 B 70 0.7 Total 100Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

volume(cm3) 50 100 150

Volume fraction 50/100 = 0.33 100/150 = 0.6715

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Process Variables Concentration

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Process Variables Pressure

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Pressure as Fluid head In addition to being expressed as force per unit area, a pressure may also be expressed as head of a particular fluid. This is the equivalent height of a hypothetical column of this fluid that would exert the given pressure at its base if the pressure at the top were zero. With P0 = 0 , we can write P = . h . (g/gc)

Therefore Ph = h = (P. gc) / ( . g)

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The BarometerAn instrument to measure atmospheric (barometric) pressure

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Pressure - is the ratio of force to the area on which the force acts; has dimensions of (Force/length2) Units in SI system = N/m2 or Pascal (Pa) Units in CGS system = dynes/cm2 Units in British system = Ibf/ft2 (psf), or more commonly Ibf/in2 (psi)

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The Bar: The Bar is a widely used metric unit of measurement for pressure and 1 bar equals precisely 100,000 Pascals. Even though Bar is not an SI unit it has been adopted as one of the most popular pressure units particularly in European countries where most pressure measurement instruments are specified with pressure ranges in bar. The value of 1 bar is a close approximation to atmospheric pressure and is often used to represent atmospheric pressure rather than standard atmosphere (101325 Pascals) which is the correct value used by the scientific and engineering community.

Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005 2005 by John Wiley & Sons, Inc. All Rights Reserved 23

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IMPORTANT : Only absolute pressure is used in thermodynamic calculations !Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

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Gage +ve

Gage ve Or Vac. Abs.

IMPORTANT : Only absolute pressure is used in thermodynamic calculations !Material primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

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Fluid Pressure Measurements Elastic elements methods - Bourdon gauge Liquid column methods Manometers Electrical methods strain gauges, transducers

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3.

1.29

2. Resistance Transducer

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Resistance Transducer

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The Manometer

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The Manometer

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DO NOT FORGET gC !!!

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Manometers: Special Cases

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Manometers: Special Cases

Note: For gases, you may assume the gas density is zeroMaterial primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

2005 by John Wiley & Sons, Inc. All Rights Reserved

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IMPORTANT : Pressure difference does not depend on manometer shape diameter, inclination,etc.The ONLY important factor is the vertical distance between fluid surfaces.

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Process Variables Temperature Is a measure of the average molecular kinetic energy. Measurements:Thermometers (Liquid expansion) Thermocouples (Resistance) Pyrometers (Radiation)

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Pyrometer

Industrial thermometer

ThermocouplesMaterial primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

41 2005 by John Wiley & Sons, Inc. All Rights Reserved

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Temperature conversion

Temperature Interval conversion

Example: The gas constant R is 1.987 cal/g mole K. Convert into Btu/lb mole oR.

Example: Water freezes at 0 oC which is the same as 1.8X0 + 32 = 32 oFMaterial primarily from Elementary Principles of Chemical Processes, Felder and Rousseau, 3rd ed. , Wiley 2005

R = (1.987 cal/g mole K).(1Btu/252 cal) .(454 g mole/1 lb mole).(1 K/1.8 oR) = 1.987 Btu/lb mole oR.

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Home Work # 1

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Home Work # 1 Thursday, June 7, 2012Problems:

2.1 2.2 2.3 2.6 2.7 2.8 2.9 2.10 2.25 Example 2.5-2 2.18 2.31 2.45 3.6 3.16 3.25 3.36 3.41 3.42 3.48 3.46

page 31 page 31 page 31 page 31 page 32 page 32 page 32 page 32 page 35 page 19 page 33 page 36 page 41 (parts a, b, and c only) page 65 page 68 page 69 page 73 page 74 page 74 page 77 page 76

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Solutions

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