Climbing Film

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.

2

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 3

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 1.3.1 Film Transfer Coefficient 1.3.2 Boiling of a Submerged Surface 1.3.3 Maximum Head Flux 1.3.4 Forced Convection 1.3.5 Variation of Heat Flux with Length of Tube

6 8 10 12 12 14

CHAPTER TWO 2.0 2.1 2.2 2.3 2.4 2.5

Experimental Work Experimental Procedure at Atmospheric pressure 2.1.1 Start-up procedure 2.1.2 Shutdown Operating under Reduced Pressure 2.2.1 Start-up procedure 2.2.2 Shutdown Experiment I Experiment II Experiment III

15 15 15 16 16 17 17 17 18 19

CHAPTER THREE 3.0 3.1

Results Discussion of Results

20 21

CONCLUSION

22

RECOMMENDATION

23

BIBLIOGRAPHY

24

NOMENCLATURE

25

APPENDICES

26

LIST OF FIGURES 1.

Different types of Evaporators

3 4

2.

Climbing film evaporator

5

3.

Variation of heat transfer coefficient with liquid height.

4.

Variation of heat flux with temperature difference. 11

5.

Nature of two phase flow in an evaporator 13

5

8

LIST OF TABLES 1.

Computed values of water removed from 10% Glycerol in solution at constant pressure of 30 psig

2.

20

Computed values of water at different steam pressures using the same feed rate operation at atmospheric pressure

3.

20

Evaporative efficiency at different steam pressure for operation at atmospheric pressure

20

6

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

7

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. 8

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.

9

10

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 fiveliter condensate receivers are used to enable condensate to be removed under reduced Figure types ofemptied Evaporators pressure. The lower one1:can be Different isolated, vented, and the vacuum reapplied without interrupting the working of the plant.

11

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T

Figure 2:

Climbing Film Evaporator

12

1

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 

1 1 x 1    Rs h1 A1 kA h0 A01

2 ………………………..

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)

T

-

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)

x

-

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 T is very important. Difficulties usually arise I determining the correct value of T . 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 T 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. 13

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.

14

Heat transfer Coefficient (kW/m2K)

Falling film

Climbing film evaporator

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

TS  TB 1 x 1    Rs h1 A1 kA h0 A01

15

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

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)

x

-

the thickness of the tubes (m)

(KW/m2K)

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

T 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

16

decreases with increase in T . To dissipate heat, the surface temperature must rise to a point E, which will bring about increase in heat transfer characteristics. a

b

C

c

Heat Flux T = Temperature Difference (Tsurface – Tbulk) a=Natural Convection b=Nucleate Boiling c=Transition Boiling d=Film Boiling

Figure 4:Variations of Head Flux with Temperature Difference

17

d

E

The heat transfer coefficient in nucleate boiling region, hD can be calculated using the equation below:

 C p l   hb d   0 . 225    k     k 

0.69

 qd     l 

0.67

 pd   6   

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

q max

Pv     L   v      2 24  v 

1

4

  L  v    v  

1

2

Where 

-

the latent heat of vaporization (KJ/kg)

v

-

density of the vapor (kg/m3)

l

-

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.

18

Natural convection heating circulation line indicated

Bubble formation due to reduction in hydrostatic head

Fully developed slug flow showing liquid slippage around vapor slug

Slug formation due to bubble

Breakdown of slugs at high vapor rates

Annular flow climbing film

Figure 5:

The nature of two phase flow in an Evaporator

19

1.3.5 Variation of Heat Flux with Length of Tube

20

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.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

21

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. 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

22

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 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 interconnecting 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 23

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 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.

24

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 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

25

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) 442 724 864

Table 2:

Water Removed (ml) 820 340 180

Time (min) 10 10 7.5

Percentage Water Removed 18.93% 4.79% 2.83%

Computed values of water removed at different steam pressure using the same feed rate operating at atmospheric pressure.

Steam pressure (psig) 10 20 30 Table 3:

Feed Rate (cm3/min) 367 367 367

Time (min) 15 15 15

Water Removed (ml) 445 505 445

Percentage of water removed 8.08% 9.17% 8.08%

Evaporative Efficiency at Different steam pressure for operation at atmospheric pressure/ Steam Pressure (psig) 10 20 30

3.1

Evaporative Efficiency 485.27 88.96 50.16

Discussion of Results 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.

26

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.

27

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.

28

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.

29

BIBLIOGRAPHY 1.

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

4.

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

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.

30

NOMENCLATURE

Symbol

Definition

SI Units

m2

A

Heat transfer surface area

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

o

U

Overall heat transfer coefficient

W/m2K

H

Enthalpy per unit mass of vapor

J/kg

G

Acceleration due to gravity

m/s2

K

31

APPENDICES A.

Raw Data

A –1. Experiment 2; at constant pressure of 30 psig Feed Rate (cm3/min) 442 724 864

Time (min) 10 10 7.5

Volume of pdt concentrate (ml) 3600 6900 6300

Vapor Condensate 658 660 220

A – 2: Data for constant feed rate using pure water, operating at atmospheric pressure. Steam pressur e (psig) 10 20 30

Fees rate (cm3/min)

Time (min)

367 367 367

15 15 15

Product concentrate (ml) 5050 5000 5050

Vapor condensat e (ml) 150 255 520

32

Feed temp. (oC) 31 31 31

Steam condensat e (ml) 760 1320 1700

Vapor temp. (oC) 96 98 99.7

A – 3: Physical Properties and Other data for glycerol Tube length Tube diameter Critical pressure Critical temperature Density Density Latent heat Vapor thermal conductivity Vapor viscosity Liquid viscosity Heat capacity

20 ft 2 inches Pc = 66.9 bar Tc = 726.0 oC L = 1021 kg/m3 v = 1.31 kg/m3  = 2704 KJ/kg Kv = 0.4 w/moC k = 2.0 X 10-3 NS/m2 L = 0.4 X 10-3 NS/m2 Cp = 2405.376 J/kgoC

B.

Sample Calculation

i.)

Calculation of water removed Input =

Output

Feed

concentrated product + water removed

=

30.48 m 0.0508 m

For experiment 2, At feed rate of 442 cm3/min Volume of feed

ii.)

=

442 X 10

=

4420cm3

Volume of product concentrate

=

3600 ml

Water removed

=

4420 – 3600

=

820 ml.

Percentage water removed Water in feed Percentage of

=

90% w/w

=

18 90  98 100

=

0.16

% of water removed =

100 – 0.16 33

= iii.)

99.84%

Calculation of Evaporation Efficiency

E 

weight of water evaporated  100  weight of steam condensed - weight of steam used in raising water   F

F 

volume of product concentrat e  DrynessFra ction volume of feed

at 10 psig

=

0.72 bar

weight of steam evaporated  445cm 3

=

1g cm 3

=

0.445 kg

=

0.760 kg

weight of steam used in raising temp 

W w Cp w Tw s

weight of steam condensed  760 

=

at 0.72 bar,

445g 1g cm 3

760g

s

=

2283KJ/kg

Ww

=

5505g =



E 

5.505kg

5.505  4.2  (98 - 31)  0.66kg 2283

0.445  100  485.27 (0.76 - 0.66)(0.91 7)

34