Chapter 1 (Part 2) Process Variables

8/15/2019 Chapter 1 (Part 2) Process Variables

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Chapter 1(Part 2)

Processes and Process

Variables


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Evaluation of performance of processoperation requires the knowledge of theamounts composition conditions of

materials that enter and leave eachprocess units!

"his chapter presents methods ofcalculating variables that characteri#ethe operation of processes andindividual

process units!

Processes and Process Variables


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Process

Process$ an% operation that cause a ph%sical orchemical change in a substance! Can consist of

several process unit!

Process streams connecting process units andform the process &ow sheet!

Chemical engineer is responsible to design andoperate the process!

Process

'nitnput*eed +utputProduct

Process

'nit +utputProduct


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,

Process Variables???

- "he quantities used to describe a processand these must be measured andcomputed

PROCESS .Feed/

0P'" .Products/

+'"P'"

- "o design or anal%#e a process we need to knowthe amounts compositions and condition ofmaterials entering leaving and within theprocess!


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Processes and

Process Variables

Density and

Specific Gravity

Flow rate

Chemical

Composition

Pressure

Temperature


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ensit% 3peci4c Volume

ensit% (ρ) mass per unit volume of a substanceensit% of a substance can be used as a conversion

factor to relate the mass and the volume of thesubstances!

'nit5 gcm67 kgm67 lbmft6!

3peci4c Volume volume per unit mass of a substance inverse of densit% 'nit5 cm6g7 m6kg7 ft6lbm


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"he densit% of CCl, is 1!898 gcm67 what isa) :ass of 2; cm6 of CCl,

b) Volume of

20 cm3 1.595 g= 31.9 g

cm3

6.20 lbm 454 g cm3

= 1760 cm3

1 lbm 1.595 g

Try This…


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Specifc Gravity3peci4c =ravit% (3=)>atio of the densit% (ρ) of a substance to the densit%

of a reference (ρref ) substance at a speci4c condition5

ensit% of water at ,?C is used as a referencedensit%7ρref =ρ@2+(l) (,?C) A 1!;;; gcm6

A 1;;; kgm6

A


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B liquid has a 3= of ;!8;! *inda) ensit% in gcm6

b) ensit% in lbmft6

c) :ass of 6 cm6 of this liquidd) Volume occupied b% 1 g of this liquid

Try This…


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3olution

a) ρ =0.5 1 g

= 0.5 g/cm3cm3

b) ρ =0.5 62.43 lbm

= 31.215 lbm

/ft3

ft3

c) 3 cm3 0.5 1 g

= 1.5 gcm3

d) 18 g cm3

= 36 cm30.5 g


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Flow Rate

Continuous process involve movement ofmaterials from one point to another withcertain rate!

*low rate$ the amount of material thatmoves into or out of a process unit perunit time

*low rate can be eDpressed as 5◦ :ass &ow rate (masstime) A◦ Volumetric &ow rate (volumetime) A◦ :olar &ow rate5 (molestime )

mQ


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12

Relat!o" # $ea%!e%t to mea%&'e) to

(f eloc!t*+ v !% mea%&'ed+ # t,e" calc&lated a%

m

Qm ρ =

AQ ν =

Flow RateFlow Rate


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-,e ma%% flow'ate of ",ea"e $ρ=0.659g/cm3) !" a !e !% 6.59 g/%. ete'm!"e t,eol&met'!c flow'ate of t,e ",ea"e

Try This…


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*low meter is a device mounted in aprocess line that provides a continuousreading of the &ow rate in the line!

"wo commonl% used &ow meter arerotameter and ori4ce meter!


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Flow meterFlow meter

Rotamete'

'!f!ce mete'$ba%ed o" 'e%%&'e

d'o)


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ChemicalComposition

Moles and

Molecular Weiht

Mass and

Mole Fractions

!verae

Molecular Weiht

Concentration

Parts per Million "ppm#

$ Part per %illion "ppb#

Chemical CompositionChemical Composition


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ole% olec&la' e!g,t

Atomic weight - mass of an atom based on carbonisotope 12C.

Molecular weight - Sum of the atomic weights ofatoms that represent a molecule of the compound.

Eg.: !"gen atomic weight #$ % 1&.' g(g-mol thus) the 2 M* % 1&.' + 1&.' % ,2.' g(g-

mol .

nit: g(mol) g(mol) and lbm(lbmole


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ole% olec&la' e!g,tco"t.

t 1 mol of a %&b%ta"ce+ !t% ma%% !" g'am !% e&al to !t% molec&la'we!g,t.

amle: ; ,a% molec&la' we!g,t of 28g/mole.

-,e'efo'e


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19

amle :

34 g of >?3 !% e&!ale"t to ole%

$ >?3 = 17.0 g/mol)

" >?3 = 34 g/ $17.0 g/mol) = 2 mol

&'( )bmol


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a%% a"d ole F'act!o"

Process input or output streams can contain mi!tures ofli/uids or gases) solutions of one or more solutes in asol0ent.

need mass fraction and mole fraction to define thecompositions:


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a%% a"d ole F'act!o"

:ass fraction

nit: g A(g total g A(g total lbm A(lbm total

:ole fraction

'nit5 kmol Bkmol total7 lb$moles Blb$moletotal

masstotal

Aof mass= A x

molestotal

Aof moles= A y


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B miDture of gases has the following masscomposition5

+2 1


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, steps to convert from mass fractions to molesfractions5


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3olution

@a%!%: 100g of m!t&'e

;omo"e"ta%%

F'act!o" a%% ole% ole F'act!o"

! ! m! ! "! *!

2 0.16 16 32 0.500 0.152

; 0.04 4 28 0.143 0.044

;2 0.17 17 44 0.386 0.118

>2 0.63 63 28 2.250 0.686

-otal 1.00 100 3.279 1.000


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Bverage :olecular Geight

e'age olec&la' e!g,t

Mean molecular weight of a mi!ture #g(mol) lbm(lbmole$.

f " i is the mole fraction of the component i of the mi!ture

and Mi is the molecular weight:

f !i is the mass fraction of the component i of the mi!ture

and Mi is the molecular weight:

∑=++= com0o"e"tall!!2211 *.....**

∑=++=como"e"tall !

!

2

2

1

1

4

/.....

4

/

4

/

4

1

4


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2<

E!ample:

3etermine the a0erage molecular weight of refiner" waste gas has the followinganal"sis b" 0olume CH4- 78.0%, C2H6-10.0%, C3H8- 8.0%, C4H10-4.0%

• Strategy:

4 5ASS: 1'' mol refiner" gas

4 nit: 6g(6mole) g(mole

Vol.% kmol MW kg

CH4

C2H6

C3H

C4H!"

#$"

!"$"

$"

4$"

#$"

!"$"

$"

4$"

!6$"

3"$!

44$!

%$!

!24

3"!

3%2

232

!""$"" !""$"" 2!33

∴Mavg = 2133/100= 21.33


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2F

ConcentrationsConcentrations

%

mixtureof Volume

Aof masscA =

mixtureof Volume

Aof molesC

A =

Liter inmixtureof Volume

Aof molesMolarity =


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Iou have ;!;2 molar solution of 0a+@ (;!;2mol 0a+@ 1 J of solution)

@ow man% moles in 8 J of solutionsH

;!;2 mol 0a+@ D 8 J A ;!1 mol 0a+@ 1 J

Try This…


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Aa't% e' !ll!o" $m)Aa't% e' @!ll!o" $b)

-o e'e%% t,e co"ce"t'at!o"% of t'ace %ec!e% !" m!t&'e%of ga%e% o' l!&!d%.

a* 'efe' to ma%% 'at!o% $&%&al fo' l!&!d%) o' mole 'at!o%

$&%&al fo' ga%e%). ?ow ma"* a't% $!" g'am o' mole%) of t,e %ec!e% a'e

'e%e"t e' m!ll!o" o' b!ll!o" a't% of t,e m!t&'e.

m!= *! 106

b! = *! 109


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Pressure

'e%%&'e !% t,e 'at!o of a fo'ce to t,e a'ea o" w,!c,t,e fo'ce act% $A= F/).

A'e%%&'e &"!t%: >/m2+ d*"e%/cm2+ lbf /!"2+ %!+ Aa.


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Hydrostatic PressureHydrostatic Pressure

- n term of @eight h(m) of column of &uid and has a

cross sectional area A (m2

) !- "he &uid has a densit% of K (kgm6) that a pressure

P;(0m2) is eDerted on the upper surface of the

column!

- "he pressure P at the base of the column is b%de4nition the force eDerted on the base divided b%the area A that force is the weight of the columnplus an% force acting on the top!

gh A F P

g Ah g m F

mg ma F

ρ

ρ

=∆=∆

=∆=∆

==

/

)()(

ghPP 0 ρ+=


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62

Pressure HeadPressure Head

- Ghen pressure is eDpressed in terms of aheight of &uid it is called &uid LheadM

- 'suall% water or mercur% is used

- @ead units are mostl% used for ver% lowpressures and eDpressed as Lmm @gL or

Lin @2+N- Converting between forcearea and head

units )( fluid head of gP area force

P h fluid →=

ρ

h i l ! "


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Atmospheric, Asolute ! "au#ePressure

"he atmospheric pressure can be thought ofas the pressure at the base of a column of&uid (air) located at the point measurement(e!g! at sea level)!

B t%pical value of the atmospheric pressure atsea level F


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Bbsolute pressures$ absolute value of the force perunit area eDerted on a surface b% a &uid!n a perfect vacuum the absolute pressure is#ero!

=auge Pressure $the pressure relative to the ambientmeasuring or atmospheric pressure at measurementpoint!

>elationship between absolute pressure and gaugepressure is5

Atmospheric, Asolute ! "au#eAtmospheric, Asolute ! "au#e

Pressure$Pressure$

atmopheric gaugeabsolute P P P +=


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Fluid Pressure Measurement

Common pressure$ measurement devices$Oourdon gauge and monometer!

:onometer$ '$shaped tube partiall% 4lled

with &uid of known densit%!

:anometer gives the measurement ofpressure in pressure dierence ∆P (P1$P2)!


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Pressure test equipment

:anometer Pressure gauge

2-36


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-eme'at&'e of a %&b%ta"ce !" a a't!c&la' %tate ofagg'egat!o" $%ol!d+ l!&!d+ o' ga%) !% a mea%&'e of t,eae'age !"et!c e"e'g* o%%e%%ed b* t,e %&b%ta"cemolec&le%.

Bome teme'at&'e mea%&'!"g de!ce%: 'e%!%ta"cet,e'momete'+ t,e'moco&le+ *'omete' a"dt,e'momete'.

TemperatureTemperature


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-,e follow!"g 'elat!o"%,! ma* be &%ed toco"e't a teme'at&'e e'e%%ed !" o"edef!"ed %cale &"!t to !t% e&!ale"t !" a"ot,e'<

Temperature%cont$Temperature%cont$

( ) ( )

( ) ( )

( ) ( )

( ) ( ) 32!1

!1

"#!$%&

1%!2#3

+=

=

+=

+=

C T F T

K T RT

F T RT

C T K T

…


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69

…

Temperature inter&alTemperature inter&al

the conversion actor !se" are1.# o$ 1.# oR 1 o$ 1oC

1oC 1 % 1 oR 1%

CAD :

1) F!"d t,e "&mbe' of celc!&% deg'ee% betwee" 32 oF a"d 212 oF

∆- $o;) = $212 E 32) 1o; = 100o;1.8 oF

2. F!"d t,e teme'at&'e of 32oF !" o;

-$oF) = 1.8 -$o;) 32

32 = 1.8 -$o;) 32 = 0 o;


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Consider the interval from 2;Q* to ;Q*

a) Calculate the equivalent temperature inQC and the interval between them!

;33.36.7)$26.6--G-

;26.6;1.8

3280F)$80-

;6.7;1.8

3220F)$20-

1.8

32F)-$;)-$

12

2

1

°=−−=−=

°=°

−=°

°−=°

−=°

−°=°

Try This…


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Chapter 1 (Part 2) Process Variables

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