chemical engineering design of a partial condenser

8/10/2019 Chemical Engineering Design of a Partial Condenser

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tw ' tube wall temperature,)*

+ ' all resistances but the gas film

A=0

q

dqU t #1$

In general, it is not possible to integrate this relationship formally using analytical expressions

for both + and &t as functions of .

The method of *olburn and ougen is generally accepted as the basis for obtaining rigorous

design of cooler-condensers. The method is tedious since it involves successive trial and error

substitutions.

The rate of transfer of sensible heat from the gas stream on the shell side of the exchanger to the

outside of the tubes is given by:

d qs

dA=h

0( t

g

tc

) #$

The rate of transfer of latent heat from the gas stream on the shell side of the exchanger to the fin

side of the tubes due to mass transfer of condensable material to the tube surface is given by:

d qL

dA=km(pgpc) #$

The total rate of heat transfer, given by the sum of #$ and #$ above, must be transferred through

the tube and water film:

q

A 'h0(tgtc) / km(pgpc ) ' +comb. (tctw) ' +&t #0$

%here:


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The fin side gas film mass transfer coefficient is obtained using the heat transfer coefficient

obtained from the above euation and the factor relationships for heat and mass transfer. The

heat transfer factor is defined as:

8h'h0

cpG(cp

k )

2 /3

(w)0.14

#9$

The mass transfer factor is defined as:

8m '

D v

kmMmP gf

G MV

#$

The mass transfer coefficient is obtained as:

"m'

w

2/3

cp Dv

k 2/3

h0Mv

cpMmPgf

#6$

%here

8h' heat-transfer ;8< factor #dimensionless$

h)' gas film heat-transfer coefficient, 2hr-m-)*

*3' =pecific heat at constant pressure, 2"g-)*

> ' mass velocity, "g2hr-m

? ' viscocity, "g2m-hr


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" ' thermal conductivity, 2hr-m-)*

8m' mass transfer 8 factor #dimensionless$

"m' mass transfer coefficient, "g2hr-mper unit of pressure

Mm' molecular weight of gas-vapour mixture #average$

M4' molecular weight of vapour

3gf ' log mean partial pressure of noncondensable gas across the film, "g2s.m

3gf ' #

p

( g)at (pg ) attc

ln(pg ) at tg

(pg ) attc

3g' partial pressure of the noncondensable gas in th main body, "g2s.m

@ ' vapour density, "g2cu.m

A4' diffusion coefficient, s.m2hr

A4' ).)177 #

3/2

P(VA

1

3+V!

1

3)2

1

MA+ 1

M!

%here:

T ' absolute temperature, )B

T ')

Can"ine, change constant to ).))76

3 ' 3ressure, atmosphere


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CHEMICAL ENGINEERING DESIGN OF A FLASH EVAPORATOR

The three principal elements involved in evaporator design are namely:

eat transfer

4apour-liuid separation and

!nergy utiliEation

The vapour-liquid separaorsare variously called bodies, vapour heads, or flash chambers. The

term flash chambers is also used to denote the minimum building bloc" of an evaporator,

comprising one heating element #the fluid with sensible heat$ and one vapour head. This type is

the choice here in this design pro8ect.

Hea ra!s"eris the most important single factor in evaporator design since the heating surface

represents the largest part of evaporator cost. !uipment costs are usually correlated as function

only of heating-surface area, materials of construction, and evaporator type. Fther things being

eual, the evaporator selected is the one having the highest transfer coefficient under operating

conditions in terms of amount of energy per hour per degree temperature per cost of installation.

4apour liuid separation may be important for a number of reasons. Most important is usually

prevention of entrainment because of value of product lost, pollution, contamination of the

condensed vapour or fouling or corrosion of the surfaces on which the vapour is condensed.

The thermodynamic efficiency of e!er#$ uili%aio!in an evaporator is very low since the

minimum energy reuirement is only eual to the heat that will be liberated if the feed were

reconstituted by mixing product and liuid solvent. *onseuently, evaporator performance is

rated on the basis of steam economy.

3roduct uality considerations may reuire low hold up time and low temperature operation to

avoid thermal degradation. The low hold up time eliminate some types of evaporator and some

types are also eliminated because of poor heat transfer characteristic at low temperature. 3roduct


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uality may also dictate special materials of construction to avoid metallic contamination or a

catalytic effect on decomposition of the product.

Glash evaporators- as the feed to evaporation ratio is increased in a forward- feed evaporator

having the feed heated by vapour blend from each effect, a point is reached where all the vapours

is needed to preheat the feed and none is available to heat the succeeding effect. Then all ther

heating surface is in the feed heaters and the evaporator themselves becomes merely flash

chambers. This heating case is called a flash evaporator

Cal&ulaio!s

The calculation of the heat and material balance on a flash evaporator is relatively easy once it is

understood that the temperature rise in each heater and temperature drop in each flash must all be

substantially eual. This euality is almost exact if the condenser from each heater is flashed to

the following heater. The steam sensory #!2=$ may be approximated from H

"

#=

1.1

A+$+ %

%here is the total temperature difference )G, between feed to the flasher

D is the approach between vapour and temperature from the flasher and the liuid leaving the

heater where the vapour condensed

is the number of stages

C is the boiling point rise in the flash

Evaporaor A&&essories

Co!de!ser


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The vapor from the effect of an evaporator is usually removed by a condenser. =urface

condensers will be employed in this design, because the mixing of condensate and condenser

cooling water is not desired.

=urface condensers use more cooling water and are so much more expensive that they are never

used where a direct contact condenser is suitable. The ratio of water consumption to vapour

condensed can be determined from the following euation:

water

vapor f&ow='v(232)

2

1

%here v' vapour enthalpy #2"g$

T ' water temperature entering and leaving the condenser,)*

Ve! s$se's

on- condensable gases may be present in the evaporator vapor as a result of lea"age air

dissolved in the feed, or decomposition reactions in the feed. Ds the non-condensable increases,

they tend to impede the heat transfer.

In any event, non-condensable gases should be vented well before their concentration reaches

1)J since gas concentration are difficult to measure the usual thing is to over-vent.


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!vaporator *osts

Capial &oss

Dpproximate selling prices of various type of evaporator are given by Kimmerman and Lavine

#165$. These prices include all auxiliary euipment that a manufacturer would normally supply

such as vapour piping barometric condenser, steam 8et, condensate flash tan"s, and in some

cases, liuor piping and pumps.

I!salled &oss

The installed cost of a number of types of evaporator is given by *hilton #1606$. The costs

include foundation steel wor", evaporator assembly, pumps, piping, insulation, painting, and a

moderate of instrumentation. It is usually impossible to estimate the effect of a change in body

material. In some cases, welded alloy bodies are cheaper than cast iron bodies.

Operai!# &oss

Fperating labour reuirement depend mainly on the proximity of the evaporator to other process

unit where occasional assistance and maintenance help can be obtained. Fccasional maintenance

labour will be reuired for the repac"ing of pumps and valve and repair of piping.

CHEMICAL ENGINEERING DESIGN OF A DISTILLATION COL(MN

Cal&ulaio! o" Mi!i'u' Nu')er o" Plaes*


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The minimum number of stages minis obtained from Gens"e euation which is,

%here LBis the average relative volatility of the light "ey with respect to the heavy"ey, and xLB

and xB are the light and heavy "ey concentrations. The suffixes d and b denote the distillate

#tops$ #d$ and the bottoms #b$,

Dverage geometric relative volatility LB' 1.51

A&ealde+$de , ./0121.3//4 , 015261 7'ol

8aer , 9 7'ol

Croo!alde+$de , :/92593.94 , ;2// 7'ol

8aer , ::92.


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*alculation of Minimum Ceflux Catio Cm:

+sing +nderwood euations

1' the relative volatility of component I ' ).67

Cm' the minimum reflux ratio,

1' concentration of component 1 ' 0299

' 1.51

' ).16)

Ny trial, ' ).5

3utting all values we get,

Cm' 1.6

Dctual Ceflux Catio, C:

The rule of thumb is:

C ' #1. ------- 1.5$ C min

C ' 1. C min

C ' 1.7


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Theoretical no. of 3lates:

>illiland related the number of euilibrium stages and the minimum reflux ratio and the no. of

euilibrium stages with a plot that was transformed by !dul8ee into the relationH

+=+

577.)minmin

1195.)

1 R

RR

N

NN

Cecall C min ' 1.6 , C ' 1.7, and min' )

Grom which the theoretical number of stages to be,

N , /9

*alculation of actual number of stages:

Dctual number of stages ' theoretical number of stages

!fficiency

Fverall Tray !fficiency !o:

!dul8ee #165$ has expressed the F


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Oa ' average relative volatility of the light "ey ' ).61

so, !o ' ).5J ' ).)5

=o,

No2 o" a&ual ra$s , /939259:6 , 69

Colu'! dia'eer

The principal factor which determines column diameter is the vapour flow-rate.

----------------------------------Eq:

where uv = maximum allowable vapour velocity, based on the gross #total$ column

cross-sectional area, m2s,

Lt ' plate spacing, m, ).7 is chosen.

4 ' density of vapour product,acetaldehyde ' ).9961g2cm' 999.61 "g2m

L ' density of liuid product, #crotonaldehyde$ / #)$ '

' Total mass2#Total volume$

Mass of crotonaldehyde formed ' 0).) "g2hr

Mass of water ' ).96 "g2hr

Aensity of water ' 1)))"g2m

Aensity of crotonaldehyde ' ).07 g2cm07 "g2m


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4olume ' Mass2#density$

4olume of water ' #).6 "g2hr$2#1)))"g2m$ ' ).)1 m2hr

4olume of crotonaldehyde ' #0).) "g2hr$2#07 "g2m$ ' ).07 m2hr

Total volume ' volume of crotonaldehyde / volume of water ' ).5)09 m2hr

Total of mass ' ).6 / 0).) ' 071.)6 "g2hr

Aensity of liuid product, L'# Total mass$2#Total volume$ ' 61.56 "g2m

Ny !, +v ' maximum allowable vapour velocity

P#-).191Q).7$/ #).9Q).7$ ( ).)09R QP#61.56-996.1$2996.1R12

).)509 Q ).0155

').)9 m2hr.

*apacity 3arameter:

Dssumed tray spacing ' 1 inch #).5 m$

The flooding velocity can be estimated from the correlation given by Gair #1671$:

B1 ' a constant obtained from a chart of liuid-vapor flow factor against " ' ).197

L' 61.56 "g2mand 4 ' ).9961g2cm

' 999.61 "g2m


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Ny this the flooding velocity +f ' ).)9 m2hr

Dssume 6)J of flooding then actual vapor velocity

4n ' ).6 Q ).)9 ' ).)9)51 m2hr

et column area used in separation is

Dn' mv24n

4olumetric flow rate of vapors ' mv

S 0o* vapor density of acetaldehyde #vapor$ ' 1.61atm ' 16177.0 2m

Mass vapor flow rate ' 9017.79 "g2hr

mv' #mass vapor flow rate 2#16177.0$

mv' ).)9 m2hr

ow, net area Dn ' mv24n' ).506 m

Dssume that downcommer occupies 15J of cross sectional Drea #Dc$ of column thus:

Dc' Dn/ Dd

%here, Dd ' downcommer area

Dc' Dn/ ).15#Dc$

Dc' Dn2 ).5

Dc').705 m


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=o Aiameter of *olumn Is

Dc'#20$A

A ' #0Dc2$

A ' ). meter

#based upon bottom conditions$

3rovisional 3late Aesign:

*olumn Aiameter Ac ' 1.051 m

*olumn *ross-sectional Drea #Dc$ ' ).705 m

Aown comer areaAd ' ).15Dc' ).)676 m

et Drea #Dn$ ' Dc- Dd').506 m

Dctive area Da' Dc-Dd' ).05) m

ole areaAhta"e 1)JAa' ).1 U ).05)

').)05 m

%eir length

Dd2 Dc' ).)676 2 ).705 ' ).15)) ).15

Grom figure 11.1 *oulson V Cichardson 7th volume 0th edition, which gives the relation

between downcomer area and weir length,

Lw2 Ac' ).)


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Lw ' 1.051 Q).)

=1.1717m

%eir length should be 7) to 5J of column diameter which is satisfactory

Ta"e

weir height, hw' 5) mm

ole diameter, dh' 5 mm

3late thic"ness ' 5 mm

%eir liuid crest

Gor a segmental downcomer the height of the liuid crest over the weir can be estimated using

%here Lw' weir length, m,

how ' weir crest, mm liuid,

Lw' liuid flow-rate, "g2s.

Maximum liuid rate WLmX ' #).96 "g2hr / 0).) "g2hr $ ' 071.56 "g2hr ' ).1 "g2s

Minimum Liuid Cate Dt 9)J turn down ratio

' ).)67 "g2sec

Cecall weir length Lw=1.1717m and L' 61.5 "g2m


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Dt Maximum rate #how$ ' .97 mm Liuid

Dt Minimum rate #how$ ' .)1 mm Liuid

Dt minimum rate hw/ how' 5) / .)1 ' 5.) mm Liuid

Grom fig 11.) #weep- point correlation#!dul8ee, 1656$$, *oulson and Cichardson 4ol.7

B ' ).1

ow *hec" %eeping: in order to calculate minimum design vapor velocity.

%here +h ' minimum vapour velocity through the holes #based on the hole area$, m2s,

dh' hole diameter, mm' 5 mm

B ' a constant, dependent on the depth of clear liuid on the plate ' ).1

Dlso recall v '999.61 "g2m. Ny that +h ' ).0)6 m2sec


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DESIGN OF REFRIGERATION S=STEM OF A COMPRESSOR.

*ompressors are typically of two types: reciprocating and rotary #screw or scroll$. =croll

compressors are limited to lower capacity halocarbon systems. Cotary screw and scroll are

increasingly popular due to lower maintenance costs. =crew compressors dominate the

refrigeration mar"et. This is mainly due to their high reliability, usually capable of operating over

5),))) hours between overhauls, and the selection of capacities of commercially available

euipment. *ommercially available motor driven capacities range from ) "ilowatts #5

horsepower$ to over 15) "ilowatts #1795 horsepower$.


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The recommended compressor for refrigeration service is a screw type compressor that comes as

a pac"age unit. The screw compressor pac"age units consist of screw compressor, motor,

coupling, oil separator, local logic controller, oil pump and filter.

=croll compressors are also positive displacement compressors. The refrigerant is compressed

when one spiral orbits around a second stationary spiral, creating smaller and smaller poc"ets

and higher pressures. Ny the time the refrigerant is discharged, it is fully pressuriEed.

D compressor is considered to be single stage when the entire compression is accomplished with

a single cylinder or a group of cylinders in parallel. Many applications involve conditions

beyond the practical capability of a single compression stage. Too great a compression ratio

#absolute discharge pressure2absolute inta"e pressure$ may cause excessive discharge

temperature or other design problems. Two stage machines are used for high pressures and are

characteriEed by lower discharge

temperature #10) to 17))*$ compared

Gor practical purposes most plant air reciprocating air compressors over 1)) horsepower are built

as multi-stage units in which two or more steps of compression are grouped in series. The air is

normally cooled between the stages to reduce the temperature and volume entering the following

stage. #ational 3roductivity *ouncil, 166$.

Ceciprocating air compressors are available either as air-cooled or water-cooled in lubricated and

non-lubricated configurations, may be pac"aged, and provide a wide range of pressure and

capacity selections.

Per"or'a!&e Assess'e! o" Re"ri#eraio! o" a &o'pressor

The cooling effect produced is uantified as tons of refrigeration #TC$.

TC of refrigeration ' )0 "*almol2hr heat re8ected ' 1759.7 "mol2hr


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The refrigeration TC is assessed as TC ' Y x *p x #T)( Ti$ 21759.7

%here Y is molar flow rate of inlet component in "mol2hr

Tiis inlet temperature in Z*

T)is outlet temperature in Z*.

Gor the condenser to be designed,

*p for Dcetaldehyde ' 6.)52mol.B

Y ' 10.9 "mol2hr #component entering the compressor$

Ti' 5)*, To' 0

)*

=ubstituting values,

TC ' 10.9 x 6.)5 x #0-5$21759.7

TC ' 1).9 "mol2hr

The above TC is also called as chiller tonnage.

Co'pressor E""i&ie!&$

=everal different measures of compressor efficiency are commonly used for the design :

volumetric efficiency, adiabatic efficiency, isothermal efficiency and mechanical efficiency.

Ddiabatic and isothermal efficiencies are computed as the isothermal or adiabatic power divided

by the actual power consumption. The figure obtained indicates the overall efficiency of a

compressor and drive motor.

Co'pressor Po>er

The power of a compressor is given byH

3 '(ork/)mo& * +n&et f&owrate

"fficienc,


23/26

%or"2Bmol ' Ki TiCn

n1(P 2P1 )

n1n 1

where K ' compressibility factor #1 for an ideal gas$,

R ' universal gas constant, .10 2B. mol

T1' inlet temperature, B

3' outlet pressure

%here T' 0)*' 17 B, T1' 5

)* ' ) B

Grom the relationship,2

1=

P2

P1 '316

308 ' 1.)5

The value of n will depend on the design and operation of the machine.

The design is more effective in a polytropic process, hence n is approximately 1.70

=ubstituting values,

%or"2Bmol ' 1 x ) x .10 x1.64

1.641 (1.025 )

1.6411.64 1

%or"2Bmol ' ).)7 "2Bmol

Inlet flowrate into the condenser ' 10.9 "mol2hr

In "mol2 sec ' 10.927)) ' ).)51 "mol 2sec

4olumetric flow rate of the compressor ' ).)51 x .0 x308

273 ' 1.6 m2s

Grom fig .7 #graph of compressor efficiency, !pagainst volumetric flow rate$ in Cichardson and

*oulson vol.7, the corresponding flow rate is 75J

Grom euristics, *ompression ratio is about the same in each stage of a multistage unit,

ratio ' #3n231$ x 12n, with n stages.

!fficiencies of reciprocating compressors: 75J at compression ratio of 1.5, 95J at .), and )-

5J at -7. !fficiency of large centrifugal compressors at suction is 97-9J. The compression

ratio is 1.5 and the compressor is not very large.


24/26

3ower '0.206* 267.73

65 ' 0.5 " 2s ' 0.5 "%

E!er#$ required per +our' 0.5 x 7)) ' )5,056.0 " ' )5.07 M

The specific power consumption "%2TC is a useful indicator of the performance of refrigeration

system. Ny measuring refrigeration duty performed in TC and the

"ilo%att inputs, "%2TC is used as a reference energy performance indicator.

Therefore, Spe&i"i& po>er &o!su'pio! 'power cons-mption

$

'84.85

10.37 ' .1

!ffectively, the overall energy consumption would be towards:

*ompressor "%

*hilled water pump "%

*ondenser water pump "%

*ooling tower fan "%, for induced 2 forced draft towers

The specific power consumption for certain TC output would therefore have to include:

*ompressor "%2TC

*hilled water pump "%2TC

*ondenser water pump "%2TC

*ooling tower fan "%2TC

The overall "%2TC is the sum of the above.

I!e#raed Par Load Value IPLV4


25/26

Dlthough the "%2 TC can serve as an initial reference, it should not be ta"en as an absolute value

since this value is derived from 1))J of the euipment[s capacity level and is based on design

conditions that are considered the most critical. These conditions occur may be, for example,

during only 1J of the total time the euipment is in operation throughout the year.

*onseuently, it is essential to have data that reflects how the euipment operates with partial

loads or in conditions that demand less than 1))J of its capacity. To overcome this, an average

of "%2TC with partial loads ie Integrated 3art Load 4alue #I3L4$ have to be formulated.

The I3L4 is the most appropriate reference, although not considered the best, because it only

captures four points within the operational cycle: 1))J, 95J, 5)J and 5J.

Gurthermore, it assigns the same weight to each value, and most euipment usually operates at

between 5) J and 95J of its capacity. This is why it is so important to prepare specific analysis

for each case that addresses the four points already mentioned, as well as developing a profile of

the heat exchanger[s operations during the year.

Lea7 qua!i"i&aio!

Gor rotary compressors, there is an easy way to estimate the amount of lea"age in the system.

This method involves starting the compressor when there are no demands on the system #when

all the air -operated, end-use euipment is turned off$. D number of measurements are ta"en to

determine the average time it ta"es to load and unload the compressor. The compressor will load

and unload because the air lea"s will cause the compressor to cycle on and off as the pressure

drops from air escaping through the lea"s. Total lea"age #percentage$ can be calculated as

follows:

Lea"age #J$ ' *100

+ t

%here T ' on-load time #minutes$


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t ' off-load time #minutes$

Lea"age will be expressed in terms of the percentage of compressor capacity lost. The

percentage lost to lea"age should be less than 1) percent in a well- maintained system. 3oorly

maintained systems can have losses as high as ) to ) percent of air capacity and power.

Gor accuracy, ta"e F V FGG times for ( 1) cycles continuously.

Gor a lea"age of 5J and a load time of 1.5 mins

Lea"age #5 J$ '1.5*100

1.5+t

t ' .5 mins

Gor the compressor capacity of 1.6 m2s ' 99.0 m2min

Lea"age capacity '1.5*77.4

1.5+28.5 ' .9 m2min

chemical engineering design of a partial condenser

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