A CHE 503 Laboratory Report

CLIMBING FILM EVAPORATOR

By

IBIKUNLE OLUDOTUN

964387(Group 4)

submitted to

Dr. A.N. Anozie

SEPTEMBER 2003

LETTER OF TRANSMITTAL

Department of Chemical Engineering,

Obafemi Awolowo University,

Ile – Ife.

January 2, 2002.

The Coordinator,

Chemical Engineering Laboratory II (CHE 503),

Obafemi Awolowo University,

Ile – Ife.

Dear Sir,

LETTER OF TRANSMITTAL

I hereby write this letter to transmit the report of the experiment carried out on Climbing

Film, at the Unit Operations Laboratory of the Department of Chemical Engineering,

O.A.U. Ile-Ife.

The report contains detailed experimental work and results of the experiments carried out.

Thanks Sir, in anticipation of a benevolent appraisal of my report.

Yours faithfully,

IBIKUNLE, Oludotun B.

ii

ABSTRACT

The aim of this experiment was to investigate the various processing factors that

affect the operation of a climbing film evaporator, the effect of variation in the feed rate on

water removal by evaporation from the feed at constant pressure, with the view of

determining the optimum operating condition. Temperature & concentration of the liquid,

temperature and pressure of steam were some of the processing factors studied by

investigating the effect of variation of feed rate on concentration of the product, the effect

of the operating steam temperature on the rate of evaporation and steam pressure on the

thermal efficiency of the climbing film evaporator.

Water was used in this experiment, which was carried out in three stages. They are:

start-up, removal of products, re-circulation of products and shut down. A careful

measurement of the volume of product concentrate and vapor condensate was made with

respect to a varying feed rate. The feed inlet cork is opened so that the feed liquor can flow

into the unit, the liquid level is allowed to reach the steam inlet connection before the final

feed rate is set, the liquid begins to boil and the expanding bubbles begin to rise rapidly in

the tube giving the climbing film operation, the feed rate is regulated such that a good

stream of liquid and vapor enters the cyclone.

It was observed from the results that increasing the temperature difference between

liquid and condensing steam could increase the rate of evaporation to a certain limit. It was

also seen that this was done under reduced pressure and varying temperatures and therefore

concluded that operation of the climbing film evaporator under reduced pressure was more

effective, economical and safer that at constant pressure or atmospheric pressure.

TABLE OF CONTENTS

Letter of Transmittal ii

iii

Abstract iii

Table of Contents iv

List of Tables v

List of Figures vi

List of Apparatus vii

CHAPTER ONE

1.0 Introduction 1

1.1 Objectives of the Experiment 2

1.2 Description of the Equipment 4

1.3 Theory 61.3.1 Film Transfer Coefficient 81.3.2 Boiling of a Submerged Surface 101.3.3 Maximum Head Flux 121.3.4 Forced Convection 121.3.5 Variation of Heat Flux with Length of Tube 14

CHAPTER TWO

2.0 Experimental Work 152.1 Experimental Procedure at Atmospheric pressure 15

2.1.1 Start-up procedure 152.1.2 Shutdown 16

2.2 Operating under Reduced Pressure 162.2.1 Start-up procedure 172.2.2 Shutdown 17

2.3 Experiment I 172.4 Experiment II 182.5 Experiment III 19

CHAPTER THREE

3.0 Results 203.1 Discussion of Results 21

CONCLUSION 22

RECOMMENDATION 23

BIBLIOGRAPHY 24

NOMENCLATURE 25

APPENDICES 26

LIST OF FIGURES

1. Different types of Evaporators 3

iv

2. Climbing film evaporator 5

3. Variation of heat transfer coefficient with liquid height. 8

4. Variation of heat flux with temperature difference.

11

5. Nature of two phase flow in an evaporator

13

v

LIST OF TABLES

1. Computed values of water removed from 10% Glycerol in solution

at constant pressure of 30 psig 20

2. Computed values of water at different steam pressures using the same

feed rate operation at atmospheric pressure 20

3. Evaporative efficiency at different steam pressure for operation at

atmospheric pressure 20

vi

LIST OF APPARATUS

1. Climbing Film Evaporator

2. 10% weight/weight of glycerol in water

3. Steam supplied by the steam boiler

4. Thermometer

5. Vacuum pump

vii

CHAPTER ONE

1.0 Introduction

Evaporation is one of the various and the most important physical methods of

removing part or all of the solvent from a solution i.e. for the concentration of aqueous

solutions. It involves a physical separation process whereby vaporization is used for the

removal of solvent from a solution by boiling the solution in an evaporator.

Evaporators are heat transfer equipment used in processing industries for the

concentration of aqueous solutions. Examples of products that are finished with

evaporation include sugar, orange juice, milk, etc. and the choice of evaporators will be

influenced depending on the cost, space for equipment, the nature of liquid and volume of

material to be processed and also in considering this, the means to provide agitation or

circulation of the liquid must be considered, the heat transfer coefficient on the boiling

liquid side, the resistance of the separating wall and the general configuration of heat

transfer surface must all be properly considered before the choice of equipment is made.

Various types of evaporators include:

Open Kettle or Pan Evaporator which is the simplest form of evaporator is the It consists

of an open pan or kettle in which the liquid is boiled. The heat is supplied by condensation

of steam in the jacket or in oil immersed in the liquid.

Horizontal Tube Natural Circulation Evaporator, which is made up of horizontal

bundles of heating tubes. The steam enters the tubes where it condenses. The steam

condenser leaves the other end of the tubes. The vapor leaves the liquid surface and is

collected in a de-entraining device.

Vertical Type Natural Circulation Evaporator: It is made up of vertical tubes where the

liquid inside the tubes and the steam condenses outside the tube.

viii

Falling Film Evaporator: this consists of long tubes where the liquid is fed on to of the

tube and flows down the walls as a thin film.

Climbing Film Evaporators: they consist of long tubes about 10 ft long and 1 inch

nominal bore light wall heat exchangers tubing with standard buttress ends. The steam

jacket is a glass pipe, 9 ft long and 2 inches bore of a suitable wall thickness to withstand

the steam and vent connections (figure 1).

1.1 Objectives Of The Experiment

The objectives of this experiment are:

1. To compare the operations of the climbing film evaporator at atmospheric pressure

and under reduced pressure.

2. To investigate the effect of the operating steam temperature on the rate of

evaporation.

3. To investigate the effect of variations in the feed rate on the concentration of the

product.

ix

Figure 1: Different types of Evaporators

x

1.2 Description of Equipment and Operating Instructions

The climbing film evaporator, Figure 2 has a calandria tube 10 ft long and 1 inch

nominal bore light wall exchanger tubing, with standard buttress ands. The steam jacket is a

glass pipe, 9 ft long and 2 inches bore, of a suitable wall thickness to withstand the steam

and vent connections.

The vapor pipe, fitted with a thermometer pocket, leads from the calandria, via a

cyclone separator for the entrained liquid, to the 15 sg. ft. condenser. The liquid outlet from

the separator is connected directly to the concentrate receiver. This has a capacity of about

seven liters and is graduated in 50 ml increments.

To allow for recycling of the concentrate, a two-way cork connects the concentrate

receiver to the feed inlet or allows it to be emptied (not under reduced pressure). Twin five-

liter condensate receivers are used to enable condensate to be removed under reduced

pressure. The lower one can be isolated, vented, emptied and the vacuum reapplied without

interrupting the working of the plant.

Figure 2: Climbing Film Evaporator

xi

1.3 THEORY

Heat is transferred from the steam to the liquid in the annulus and the process of

evaporation in the climbing film evaporator involves the transfer of heat. The rate of heat

transfer across a given area is expressed mathematically as

Q=UA ΔT 1

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However, depending on the thickness of the surface area used for the transfer of

heat, the product of U & A can be thus defined as

UA= 11

h1 A1

+ΔxkA

+1

h0 A01

+Rs………………………..

Where

Q - the rate of heat transfer per unit time (KW)

U - the overall heat transfer coefficient (KW/m2K)

A - the heat transfer area (m2)

ΔT - the temperature difference between the steam stream and

the bulk of material (K).

hi - the inside heat transfer coefficient (W/m2 K)

ho - the outside heat transfer coefficient (W/m2 K)

Ai - the inside transfer area (m2)

Ao - the outside transfer area (m2)

Δx - thickness of the tube (m)

Rs - the overall resistance to heat transfer offered by scale

deposits on the inside and outside surface (K/W)

The determination of ΔT is very important. Difficulties usually arise I determining

the correct value of ΔT . These difficulties arise due to boiling point rise and hydrostatic

head. If water is boiled in an evaporator under a given pressure, then the temperature of the

liquid can be determined from steam tables and ΔT is readily calculated. At the same

pressure, a solution has a boiling point greater than water and the difference between its

boiling point and that of water.

2

xiii

The effect of hydrostatic head may be considered by supposing the liquor to be at

the top of the tube. Then the pressure of the liquid, which is just at the top of the tube, is

that in the vapor space and the boiling point can therefore be calculated. The liquor at the

bottom of the tube is at higher temperature corresponding to the increased pressure. Thus,

the temperature difference between the steam outside the tubes and the liquor will depend

on where boiling starts and there is no satisfactory way to determine this.

The variation of heat transfer coefficient U with liquor level is seen that after an

initial sharp rise, U falls as level of vapor is increased. The maximum point of the graph

sets a limit for maximum heat transfer per unit time and hence maximum rate of

evaporation. This relationship is shown in figure 3 below.

xiv

Falling film

Climbing film evaporator

Heat transfer C

oefficient (kW/m

2K)

Height of Liquor (m)

Figure 3:Variation of Heat transfer coefficient with Liquid Height

1.3.1 Film Transfer Coefficient

Performance of any form of evaporator depends on the value of the film coefficients

on the heating side and for the liquor, together with allowance for scale deposits and the

tube wall. The rate of heat transfer in a climbing film evaporator can be shown to be

q=T S−T B

1h1 A1

+ ΔxkA

+ 1h0 A01

+Rs

xv

Where

TS - temperature of the steam (K)

TB - temperature of bulk processing material (K)

hi - the convective heat transfer coefficient of the inside wall (KW/m2K)

ho - the convective heat transfer coefficient of the outside wall

(KW/m2K)

Ai - the inside heat transfer area (m2)

Ao - the outside heat transfer area (m2)

A - the conductive heat transfer area (m2)

Rs - resistance to heat flow due to formation of scale (K/KW)

Δx - the thickness of the tubes (m)

1.3.2 Boiling of a Submerged Surface

When heat is transferred from a heating surface to a liquid at its boiling, four

distinct regions are observed. From figure 4, it can be seen that the heat flux increases

(slowly) to increase in temperature differences (ΔT ) in range AB. In this range although

the liquid vicinity of the surface will be slightly superheated, there is no water vapor

formed and heat transfer is by natural convection with evaporation from the free surfaces.

At point B, boiling begins, over the region BC (nucleate boiling region) increases in ΔT

increases the heat flux up to point C where the surface is completely covered. Increase in

ΔT beyond C will lead to partial collapse of the nucleate boiling mechanism due to

exposure of the surface to vapor blanketing in the region CD, the average heat flux

decreases with increase inΔT . To dissipate heat, the surface temperature must rise to a

point E, which will bring about increase in heat transfer characteristics.

xvi

a b c d EC

Heat F

lux

T = Temperature Difference (Tsurface – Tbulk)

Figure 4:Variations of Head Flux with Temperature Difference

a=Natural Convectionb=Nucleate Boilingc=Transition Boilingd=Film Boiling

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The heat transfer coefficient in nucleate boiling region, hD can be calculated using

the equation below:

[ hbd

k ]=0 .225[ Cp μ l

k ]0.69

[ qdλμl ]0 .67

[ pd6 ]0 .33 [ PL−1

Pr]0 .31

1.3.3 Maximum Heat Flux

The maximum heat flux in an evaporator as defined by Zuber’s equation can be

expressed as

qmax=πλ Pv

24 [ σμ ( ρL−ρv )ρv

2 ]1

4 [ ρL−ρv

ρv ]1

2

Where

- the latent heat of vaporization (KJ/kg)

ρv - density of the vapor (kg/m3)

ρl - density of the liquid (kg/m3)

- interfacial tension (kg/m.s2)

g - acceleration due to gravity (m/s2)

1.3.4 Forced Convection

Various flow patterns are associated with forced convection depending on the vapor

rates, hydrostatic head and stage. These flow patterns are shown in figure 5.

xviii

Breakdown of slugs at high vapor rates Annular flow climbing film

Natural convection heating circulation line indicated

Bubble formation due to reduction in hydrostatic head

Slug formation due to bubble Fully developed slug flow showing liquid slippage around vapor slug

Figure 5: The nature of two phase flow in an Evaporator

xix

1.3.5 Variation of Heat Flux with Length of Tube

The properties of fluid along the tube are a distributed parameter system where the

temperature and concentration are actually functions of time and position. At steady state,

the heat flux along the length of tube can be estimated using different mathematical models.

xx

CHAPTER TWO

2.0 Experimental Work

2.1 Experimental Procedure at Atmospheric Pressure

Water was used for preliminary test evaporation. An arrangement of test liquid was

done to feed the calandria by gravity. It was ensured that a steam supply and cooling water

were available for the immediate use.

The experiment was performed in stages. These stages included start-up, running,

removal of products, re-circulation of concentrate and shutdown.

2.1.1 Start-up Procedure

All the drain corks were first closed, re-circulation control cork were also closed

with handle in the horizontal position. After closing the corks (drain and re-circulation

control), the inter-connecting between receivers of product and condensate drain value on

exit side of the calandria tube wall were all opened.

The steam

control valve was then opened slowly to allow the first steam condensate to pass out

through the drain valve. The condensate drain valve was then closed when the steam began

to blow off. The steam pressure was allowed to rise to 30 psig making sure that steam

condensate was exit off through the steam-trap. Non condensing gases were released by

opening the vent at the top of the calandria and returning it to an almost closed position so

that a mere wisp of steam was able to pass through.

xxi

The feed inlet cork was then opened so that the feed vapor could flow into the unit.

The liquid level was allowed to reach the steam inlet connection before setting the final

feed rate by use of a flow meter.

The liquid was observed to be boiling and expanding bubbles were also observed to

rise quickly giving the climbing film operation. The feed rate was regulated so that a good

stream of liquid and vapor enters the cyclone. The concentrated liquor falls from the

cyclone to the calibrated receiver. Product condensate from the heat exchanger falls to the

lower of the twin receivers.

Both product concentrate and product condensate were removed continuously at

atmospheric pressure after re-circulating the solution.

2.1.2 Shutdown

To shutdown the unit, the feed cork, steam control valve and condenser cooling

water valve were closed in sequence. After closing the cork, the feed supply from the unit

was isolated and the feed supply line was detached. The feed stock was opened and the unit

was drained off remaining feed liquor. The re-circulation cork was then opened and the

condensate drain cork was also opened to remove product condensate.

2.2 Operation Under Reduced Pressure

The procedure was almost the same as in the operation carried out under

atmospheric condition. The significant difference in operation is the availability of a

vacuum pump. The presence of a vacuum cork with a hole in it enables the vacuum to be

turned off and the receiver vented simultaneously.

2.2.1 Start-up Procedure

xxii

In addition to the start-up procedure earlier described, the vacuum line was

connected to the vacuum vent cork of the lower receiver. The vacuum pump was then

switched on and the vacuum adjusted to 26 mmHg. The rate of evaporation of the unit was

initially under estimated, but the position of the feed supply valve was then adjusted to

ensure adequate flow.

2.2.2 Shutdown

Unlike the operation of the atmospheric pressure, the concentrate and condensate

were separately removed. The vacuum cork was adjusted to give a setting, which allowed

the vacuum line to be turned off and the receiver vented simultaneously. The inter-

connecting receiver cork was closed and the lower receiver vented. The lower drain cork

was opened to empty the lower receiver. The vent was then closed and the cork opened to

the vacuum line and replacing the interconnecting cork followed this.

2.3 Experiment I

The main objective of this experiment is to investigate the effect of variations in the

feed rate on the concentration of the product.

A 10% w/w solution of glycerol in water was used. The climbing film evaporator

was started up following the procedure earlier described. It was operated at a feed rate of 4

gallons/hr, a steam pressure of 30 psig. About 5 minutes of operating was allowed for

steady conditions to be reached. In doing this, the product concentrate and condensate were

run to roast and then collected over a period of about 15 minutes. The steam pressure and

feed rate were maintained throughout the period of running. The volumes of product

xxiii

concentrate and product condensate were then measures and the percentage of the product

concentrate calculated.

The procedure was then repeated at other feed rates of 5 gallons/hr and 6 gallons/hr,

while maintaining the steam pressure constant at 30 psig in each case. The experiment was

performed at atmospheric pressure.

2.4 Experiment II

The objective of this experiment is to investigate the effect of the operating system

temperature on the rate of evaporation achieved. The operating liquor is water and the

procedure earlier described was followed in starting up the evaporator, operation was at a

feed rate of 5 g/hr at a steam pressure of 10 psig.

About 5 minutes of operation was allowed for steady conditions to be reached. In

the course of this, product concentrate and condensate were run to waste and then collected

over a period of 15 minutes. Again, the steam pressure and the feed rate were maintained

throughout the period of running. The volumes of product concentrate and product

condensate were then measured.

After operating under reduced pressure, the unit was shut down and vented before

the product concentrate would be removed. Using a second receiving vessel and applying

the operation sequence already performed by condensate could only activate removal of the

product concentrate while the unit was in action. Re-circulation of the concentrate was

carried out as performed when operating at atmospheric pressure.

The feed cork was closed and steam control was then stopped. The unit was vented

through a condensate receiver vent cork. The experiment was repeated at steam pressure 20

and 30 psig. The relative volume of product concentrate and product condensate in each

xxiv

case was recorded against the steam; the percentage of the product concentrate was then

calculated. This steam temperature was obtained from steam tables.

2.5 Experiment III

The volume of steam condensate and concentrate was collected and measured at

intervals of 15 minutes

xxv

CHAPTER THREE

3.0 Results

Table 1: Computed values of water removed from 10% Glycerol in water at constant

pressure of 30 psig

Feed Rate (cm3/min)

Time (min)Water Removed

(ml)Percentage

Water Removed442 10 820 18.93%

724 10 340 4.79%

864 7.5 180 2.83%

Table 2: Computed values of water removed at different steam pressure using the

same feed rate operating at atmospheric pressure.

Steam pressure (psig)

Feed Rate (cm3/min)

Time (min)

Water Removed (ml)

Percentage of water removed

10 367 15 445 8.08%

20 367 15 505 9.17%

30 367 15 445 8.08%

Table 3: Evaporative Efficiency at Different steam pressure for operation at

atmospheric pressure/

Steam Pressure (psig) Evaporative Efficiency

10 485.27

20 88.96

30 50.16

3.1 Discussion of Results

xxvi

It can be deduced from the analysis of results in the tables based on the computed

values that the efficiency of the climbing film evaporator reduces with increasing feed rate

i. E increase in feed rate at constant pressure results in decrease in the percentage of water

removed. This is due to decrease in heat transfer coefficient and decrease in boiling point.

Also for operations at atmospheric pressure, using different steam pressure, it can

be seen that the product concentrate initially increases with feed rate, pressure and

temperature and then suddenly drops after further increase in these parameters, which

suggests that there is an optimum feed rate for each steam pressure and temperature.

It can be seen from the tables that increase in steam pressure leads to increase in

energy losses since vapor temperature increases, this implies that energy was wasted

because the feed vapor is not needed and on a general basis, high working pressure should

be avoided.

xxvii

CONCLUSION

Several conclusions can be drawn from the results of the experiment. They are:

.1. Operating the evaporator under vacuum increases the temperature difference

between the steam and the boiling liquid and this increases the rate of evaporation.

2. Operating under reduced pressure is more effective, economical and safer that at

atmospheric pressure.

3. As the feed rate increases, the amount of water removed during evaporation

decreases.

4. The boiling temperature increases with the operating pressure of the evaporator.

5. The evaporative efficiency decreases as the steam pressure increases.

6. The quality of the products decline with temperature and length of time.

xxviii

RECOMMENDATIONS

For improved performance of the experiment on climbing film evaporator, the

following recommendations are hereby put forward:

1. Regular cleaning of the equipment – to reduce scale deposition, which affects heat

transfer.

2. Non-condensable gases should be properly vented from the steam chest and the

system.

3. The feed should be pre-heated (close to boiling point) – to increase the rate of

evaporation.

4. Reduce the pressure used in the vapor space of the evaporator – to reduce the

boiling point of water and hence the rate of evaporation.

5. Consider the sensitivity of the concentrate when choosing the temperature and

length of heating tube to prevent product degradation.

6. The pipes should be checked for steam and condensate leakages and also at the

fittings, joints and steam trap.

7. Provide adequate circulation and/or turbulence to keep coefficient from becoming

too low especially when dealing with viscous liquids.

8. Anti-foam should be added to substances that produce foam or froth during boiling.

9. Some form of de-entrainer should be added to reduce entrainment.

xxix

BIBLIOGRAPHY

1. Coulson, J.M. and Richardson, J. F. (1998). “Chemical Engineering” vol. 2, 4th

ed. Butterworth-Heinemann, Jordan Hill, Oxford.

2. Holand, F.A. (1973). “Fluid Flow for Chemical Engineers”. Edward Arnold

Inc., London.

3. McCabe, W.L. and Smith, J.C. (1976). “Unit Operations of Chemical

Engineering”, 3rd ed., McGraw-Hill Book Company, New York.

4. Perry, R.H. and Chilton, C.H. (1973). “Chemical Engineer’s Handbook”. 5 th ed.,

McGraw-Hill Book Company, New York.

5. Robinson, C.S. and Gilliland, E.R. (1950). “Elements of Fractional Distillation”,

4th ed., McGraw-Hill Book Company, New York.

xxx

NOMENCLATURE

Symbol Definition SI Units

A Heat transfer surface area m2

Cp Specific heat capacity of liquid at constant pressure J/kg K

D Liquid evaporated or steam condensed per unit time kg/s

Dc Tube Diameter m

T Temperature difference oK

U Overall heat transfer coefficient W/m2K

H Enthalpy per unit mass of vapor J/kg

G Acceleration due to gravity m/s2

xxxi

APPENDICES

A. Raw Data

A –1. Experiment 2; at constant pressure of 30 psig

Feed Rate (cm3/min) Time (min) Volume of pdt concentrate (ml) Vapor Condensate

442 10 3600 658

724 10 6900 660

864 7.5 6300 220

A – 2: Data for constant feed rate using pure water, operating at atmospheric pressure.

Steam pressure (psig)

Fees rate (cm3/min)

Time (min)

Product concentrate

(ml)

Vapor condensat

e (ml)

Feed temp. (oC)

Steam condensat

e (ml)

Vapor temp. (oC)

10 367 15 5050 150 31 760 96

20 367 15 5000 255 31 1320 98

30 367 15 5050 520 31 1700 99.7

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A – 3: Physical Properties and Other data for glycerol

Tube length 20 ft 30.48 m

Tube diameter 2 inches 0.0508 m

Critical pressure Pc = 66.9 bar

Critical temperature Tc = 726.0 oC

Density L = 1021 kg/m3

Density v = 1.31 kg/m3

Latent heat = 2704 KJ/kg

Vapor thermal conductivity Kv = 0.4 w/moC

Vapor viscosity k = 2.0 X 10-3 NS/m2

Liquid viscosity L = 0.4 X 10-3 NS/m2

Heat capacity Cp = 2405.376 J/kgoC

B. Sample Calculation

i.) Calculation of water removed

Input = Output

Feed = concentrated product + water removed

For experiment 2,

At feed rate of 442 cm3/min

Volume of feed = 442 X 10

= 4420cm3

Volume of product concentrate = 3600 ml

Water removed = 4420 – 3600

xxxiii

= 820 ml.

ii.) Percentage water removed

Water in feed = 90% w/w

Percentage of =

1898

×90100

= 0.16

% of water removed = 100 – 0.16

= 99.84%

iii.) Calculation of Evaporation Efficiency

ηE=weight of water evaporated×100( weight of steam condensed-weight of steam used in raising water )×ΔF

ΔF=volume of product concentratevolume of feed

=DrynessFraction

at 10 psig = 0.72 bar

weight of steam evaporated=445cm31g

cm3

= 445g = 0.445 kg

weight of steam condensed=760×1g

cm3

= 760g = 0.760 kg

weight of steam used in raising temp=W wCpw ΔTw

λs

at 0.72 bar, s = 2283KJ/kg

Ww = 5505g = 5.505kg

=5 .505×4 .2×(98-31 )2283

=0 .66 kg

ηE=0 . 445×100

(0 .76-0 .66)(0 .917 )=485 .27

xxxiv