Climbing Film Gangan

LABORATORY REPORT On CLIMBING- FILM EVAPORATOR EXPERIMENT By OGUNGBENRO, ADETOLA ELIJAH (CHE/2007/083) Submitted to DR J.A. SONIBARE IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR CHEMICAL ENGINEERING LABORATORY II (CHE 503) DEPARTMENT OF CHEMICAL ENGINEERING, OBAFEMI AWOLOWO UNIVERSITY, ILE-IFE.

JULY 2012.

OGUNGBENRO Adetola Elijah Department of Chemical Engineering, Obafemi Awolowo University, Ile- Ife. July 17, 2012. Dr J.A. Sonibare, Instructor, Climbing- Film Evaporator Experiment, Chemical Engineering Laboratory II (CHE 503), Department of Chemical Engineering, Obafemi Awolowo University, Ile- ife. Dear Sir, LETTER OF TRANSMITTAL I hereby write to present to you a Laboratory Report on Climbing- Film Evaporator Experiment, a practical work of Chemical Engineering Laboratory II (CHE 503). This report contains detailed procedures of the experimental work, results obtained and analysis, and limitations encountered in carryout in the experiment. `

I appreciate your anticipated benevolent appraisal of the report. Thanks and God bless.

Yours faithfully, …………………………………. OGUNGBENRO Adetola Elijah CHE/2007/083

TABLE OF CONTENTS Title Page

i

Letter of Transmittal

ii

Abstract

iii

Table of Contents

iv

List of Tables

vi

List of Figures List of Symbols

1.

2.

INTRODUCTION 1.1

Background

1.2

Liquid Characteristics

1.3

Single and Multiple Effect Operation

1.4

Classifications of Evaporators

1.5

The Climbing Film Evaporator

1.6

Objectives of the Experiment

THEORECTICAL REVIEW 2.1

Material and Energy Balances

2.2

Film Transfer Coefficient

2.3

Boiling of a Submerged Surface

2.4

Maximum Head Flux

2.5

Forced Convection

viii

2.6

3.

Evaporator Economy

EXPERIMENTAL WORK 3.1

Services and Materials 3.1.1

Sensing equipment

3.1.2

The vacuum

3.2

Description of the Equipment: The Climbing Film Evaporator

3.3

Experimental Procedure 3.3.1

3.3.2

3.3.3

3.3.4

Operations at atmospheric pressure 3.3.1.1

Start up procedure

3.3.1.2

Removal of products

3.3.1.3

Shut down procedure

Operations under reduced pressure 3.3.2.1

Start up procedure

3.3.2.2

Removal of products

Experimental studies 3.3.3.1

Study at 5 psig pressure

3.3.3.2

Study at 10 psig pressure

Shut- Down procedure

3.4

4.

5.

Experimental Precautions

RESULTS AND DISCUSSION 4.1

Results

4.2

Discussion of Results

CONCLUSIONS AND RECOMMENDATIONS 5.1

Conclusions

5.2

Recommendations

REFERENCES APPENDIX

LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS 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

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

ADD MORE

CHAPTER ONE

INTRODUCTION 1.1

Background Evaporation represents a gradual change of state from liquid to gas that occurs at

a liquid’s surface or below the liquid’s boiling point. A liquid’s surface area and temperature affect its rate of evaporation. Evaporation rates also depend on the type of liquid, since liquids made up of different molecules differ in the amount of attraction that exists between the molecules. Evaporation is used to separate and purify substances in many chemical and industrial processes. One of the most important industrial applications is separation of crude petroleum into gasoline, kerosene, and gas oil. In this process, called fractional distillation, crude petroleum is boiled, and the evaporated materials are cooled until they condense. These condensed vapors contain higher percentages of the more volatile (most easily vaporized) compounds necessary for producing gasoline, kerosene, and gas oil. As a result, repeating the cycle of evaporation and condensation can isolate these compounds (Williams, 2004). The vaporization of a liquid for the purpose of concentrating a solution consisting of a non-volatile solute and volatile solvent is a common unit operation in chemical processing and is performed in many ways. Evaporation is a widely used method for concentration of aqueous solutions. It is conducted by vaporizing a portion of the solvent to produce a concentrated solution or thick liquor. If the solution contains dissolved solids, the resulting strong liquor may become saturated so that the crystals are dissolved. Liquors which are to be evaporated may be classified as follows;

 Those which can be heated to high temperatures without decomposition, and those that can be heated only to a temperature of about 330 K.  Those which yield solids on concentration, in which case capital size and shapes may be important, and those which don not.  Those which at a given pressure, boils at about the same temperature as water, and those which have a much a higher boiling point.

Evaporation is achieved by adding heat to the solution to vaporize the solvent. The heat is supplied principally to provide the latent heat of vaporization and by adopting methods for recovery of heat from the vapor; it has been possible to achieve great economy in heat utilization. Whilst the normal heating medium is generally low pressure exhaust steam from turbines, special heat transfer fluids or flue gas are also used (Perry, 1973).

Evaporation differs from drying in that the residue is a liquid - sometimes a highly viscous one - rather than a solid; it differs from distillation in that the vapor is usually a single component, and even when the vapor is a mixture, no attempt is made in the evaporation step to separate the vapor into fractions; it differs from crystallization in that emphasis is placed on concentrating a solution rather than forming and building crystals.

The conditions under which evaporation is carried out in practice vary widely. The liquid to be evaporated may be less viscous than water, or it may be so viscous that it will hardly flow. It may deposit scale on the heating surface; it may precipitate in crystals; it may tend to foam; it may have a very high boiling point elevation; or it may be damaged by the application of too

high temperatures. The design of an evaporation unit requires the practical application of data on heat transfer to boiling liquids, together with an understanding of what happens to the liquid during concentration. This wide variety of problems has led to considerable variation in the types of mechanical construction used.

1.2

Liquid Characteristics ****write from HISEXCELLENCY***

1.3

Single and Multiple Effect Operation…same as 1.2

1.4

Classifications of Evaporators ****Figure of types of evaporators***

1.5

The Climbing Film Evaporator Climbing film evaporators are primarily used in industry to concentrate a solution by

removing a part of the solvent in form of vapour. These evaporators are normally operated under a vacuum, so as to lower the boiling point of the solution and aid its passage through the evaporation column. This characteristic, combined with the high heat transfer rate at low temperature and low contact time makes the climbing film evaporator ideal for concentrating heat sensitive materials (Badger and Banchero, 1955). There are two prime advantages of this apparatus. One advantage is that there is minimum contact between the fluid and the heating surface. The second advantage is that there is rapid evaporation provided by the high rate of heat transfer. Another important feature of this

type of evaporator is its low cost. The climbing film evaporator is the cheapest evaporator per unit capacity available. The climbing film evaporator can be used in production processes, pilot plant work, or as an instructional unit. Climbing film evaporators have been successfully used in processes involving antibiotics, fruit juices, milk, and many organic chemicals.

1.6

Objectives of the Experiment This experiment primarily aims to determine the effect of varying the feed flow rate and

steam pressure on the performance of the climbing film evaporator which is evaluated by heat transfer coefficient and the concentration of glycerol in the production solution. The specific objectives of this experiment are: 4

To compare the operations of the climbing film evaporator at atmospheric pressure and under reduced pressure.

5

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

6

To determine the economics of evaporation using the climbing film evaporator approach.

CHAPTER TWO

THEORETICAL REVIEW 2.1

Material and Energy Balances Consider a highly simplified diagram of an evaporator, as given in Fig 2.1, in which the

heating surface is represented by a simple coil. Let F be the kilogram of the feed to the evaporator per hour, whose solute content is x f (x is the weight fraction). Let the enthalpy of the feed per kilogram be hf. There is taken out of the evaporator L kg of thick liquor, whose composition in weight fraction of solute is x L and whose enthalpy is hL in joule per kilogram. There is also V kg of vapor having a solute concentration of y and an enthalpy of H, J/kg. In most evaporators, the vapor is pure water, and therefore y is zero. The material balance equations for this case become: Total Material Balance:

F= L + V=

(1)

Solute Balance:

F xf = L xL + Vy

(2)

In order to furnish the heat necessary for evaporation, S kg of steam is supplied to the heating surface with an enthalpy of Hs (J/kg) and there is taken out S kg of condensate with an enthalpy of hc (J/kg). One simplifying assumption usually made is that in an evaporator there is very little cooling of the condensate. This leads the assumption that the condensate will leave at the condensing temperature of the steam. The heat balance equation is: Heat in feed + Heat in steam = Heat in thick liquor + Heat in vapor + Heat in condensate + Heat lost by radiation (5.1.3)

Neglecting losses by radiation and using the relevant symbols we get the following equation: **********CONTINUE FROM MATERIAL****** ….before equatn 5.1.9 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.

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 Fig 2.2 below.

Heat transfer Coefficient (kW/m2K)

Falling film

Climbing film evaporator

Height of Liquor (m) Fig. 2.2:Variation of Heat transfer coefficient with Liquid Height

2.2

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 the form of equation (above);

q

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

(parameters as defined)

2.3

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 Fig 2.3, 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 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.

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

b

C

c

d

Heat Flux

a

T = Temperature Difference (Tsurface – Tbulk) a

=

Natural Convection

b

=

Nucleate Boiling

c = boiling = Fig. 2.3:

Transition Boiling d-film Film Boiling

Boiling Xter at a Submerged Surface

Variations of Head Flux with Temperature Difference

E

2.4

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

expressed as 1

 Pv  L   v  4   L   v  qmax      24  v 2   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)

2.5

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 Fig 2.4.

Natural convection heating circulation line indicated

Slug formation due to bubble

Breakdown of slugs at high vapor rates

Bubble formation due to reduction in hydrostatic head

Fully developed slug flow showing liquid slippage around vapor slug

Annular flow climbing film

Fig 2.4: The nature of two phase flow in an Evaporator

2.6

Evaporator Economy.

CHAPTER THREE

EXPERIMENTAL WORK

3.1 SERVICES AND MATERIALS

3.1.1

SENSING EQUIPMENT….DESCRIBE IDEAL CASE

3.1.2

THE VACUUM

3.2 DESCRIPTION OF THE EQUIPMENT: THE CLIMBING FILM EVAPORATOR

3.3 EXPERIMENTAL PROCEDURE

3.3.1

3.3.2

3.3.3

3.3.4

OPERATIONS AT ATM PRESSURE 3.3.1.1

Start up procedure

3.3.1.2

Removal of Products

3.3.1.3

Shut down Procedure

OPERATIONS under reduced PRESSURE 3.3.2.1

Start up procedure

3.3.2.2

Removal of Products

Experimental Studies 3.3.3.1

Study at 5 psig pressure

3.3.3.2

Study at 10 psig pressure

Shut- Down Procedure

3.4

4.

5.

Experimental Precautions

RESULTS AND DISCUSSION 4.1

Results

4.2

Discussion of Results

CONCLUSIONS AND RECOMMENDATIONS 5.1

Conclusions

5.2

Recommendations

REFERENCES APPENDIX