Thermosyphon Reboiler & its type with operational parameter.

1 | P a g e U.I.C.T., N.M.U., Jalgaon.

Contents.

Chapter 1. ......................................................................................................... 6

1. Introduction to Reboiler ......................................................................... 7

1.1. Reboiler: ................................................................................................ 7

1.2. Types of Reboilers: ................................................................................ 8

1.2.1. Kettle reboilers (natural-circulation): ................................................. 9

1.2.2. Thermosyphon reboilers (natural-circulation, operates based on the

head of liquid): .......................................................................................... 10

1.2.3. Fired heaters : .................................................................................. 11

1.2.4. Forced circulation reboilers (liquid is pumped into shell): .............. 12

1.2.5. Internal Reboilers: ......................................................................... 13

Chapter 2. ....................................................................................................... 15

2. Introduction to Thermosyphon Reboilers. .......................................... 16

2.1. Thermosyphon Reboilers: .................................................................... 16

2.2. Advantages and Disadvantages: ........................................................... 18

2.2.1. Advantages: ................................................................................... 18

2.2.2. Disadvantages: .............................................................................. 18

2.3. Types of Thermosyphon Reboilers: ..................................................... 18

Chapter 3. ....................................................................................................... 19

3. Literature Review. ................................................................................ 20


Chapter 4. ....................................................................................................... 22

4. Working Principle of Thermosyphon Reboiler ................................... 23

4.1. Working Principle: .............................................................................. 23

4.2. Process Function of Thermosyphon Reboiler: ..................................... 24

4.2.1. Heat Transfer Unit: ........................................................................ 24

4.2.2. Separation Unit: ............................................................................. 25

Chapter 5. ....................................................................................................... 26

5. Classification & Working of Thermosyphon Reboiler ....................... 27

5.1. Vertical Thermosyphon Reboilers: ...................................................... 27

5.1.1. Working Principle of Vertical Thermosyphon Reboiler: ................ 29

5.1.2. Setup of Vertical Thermosyphon Reboiler: .................................... 30

5.1.3. Advantage and Disadvantage of Vertical Thermosyphon Reboiler:

34

5.2. Horizontal Thermosyphon Reboiler: .................................................... 36

5.2.1. Working Principle of Horizontal Thermosyphon Reboiler: ............ 37

5.2.2. Advantage and Disadvantage of Horizontal Thermosyphon

Reboiler: ................................................................................................... 39

Chapter 6. ....................................................................................................... 40

6. Comparison of Vertical and Horizontal Thermosyphon Reboiler. .... 41

6.1. Horizontal Thermosyphon Reboiler Vs Vertical Thermosyphon

Reboiler: ....................................................................................................... 41

6.2. Vertical Thermosyphon Reboiler Vs Horizontal Thermosyphon

Reboiler: ....................................................................................................... 42


Chapter 7. ....................................................................................................... 43

7. Fundamentals of Thermosyphon reboiler ........................................... 44

Chapter 8. ....................................................................................................... 46

8. Operational Characteristics of Thermosiphon Reboilers ................... 47

8.1. Influence of driving temperature difference: ........................................ 47

8.2. Influence of operating pressure: ........................................................... 50

8.3. Influence of pipe diameter: .................................................................. 52

8.4. Influence of pipe length: ...................................................................... 53

8.5. Influence of driving liquid head: .......................................................... 54

Chapter 9. ....................................................................................................... 56

9. Industrial Applications of Thermosyphon Reboiler. .......................... 57

Chapter 10. ..................................................................................................... 59

10. Limitation of Thermosyphon Reboiler. ............................................... 60

Chapter 11. ..................................................................................................... 61

11. Conclusion. ............................................................................................ 62

12. References ............................................................................................. 63


List of Figures.

Figure 1:Kettle reboilers ................................................................................. 9

Figure 2:Thermosyphon reboilers. ............................................................... 10

Figure 3:Fired heaters ................................................................................... 11

Figure 4:Forced circulation reboilers. .......................................................... 12

Figure 5:Internal Reboilers ........................................................................... 13

Figure 6:Heat Transfer Unit. ........................................................................ 24

Figure 7:Separation Unit ............................................................................... 25

Figure 8:Vertical thermosiphon reboilers. ................................................... 27

Figure 9:Working Principle of Vertical Thermosyphon Reboiler. ............. 29

Figure 10:Setup of Vertical Thermosyphon Reboiler .................................. 30

Figure 11:Set up of Forced Circulation Vertical Thermosyphon Reboiler. 31

Figure 12:Setup of Vertical Thermosyphon Reboiler with Fixed Liquid

Head. ............................................................................................................... 31

Figure 13:Set up of Once-Through Vertical Thermosyphon Reboiler. ...... 32

Figure 14:Set up of Once -Through Naturally Forced Vertical

Thermosyphon Reboiler. ............................................................................... 32


Figure 15:Horizontal thermosiphon reboiler. .............................................. 36

Figure 16:Working Principle of Horizontal Thermosyphon Reboiler. ....... 37

Figure 17:Comparison of Vertical and Horizontal Thermosyphon Reboiler.

......................................................................................................................... 41

Figure 18: (A) Schematic of Vertical Thermosyphon Reboiler &(B)

Characteristic Temperature Profile. ............................................................. 44

Figure 19:Specific overall heat flux versus driving temperature difference.

......................................................................................................................... 47

Figure 20:. Mass flow density versus driving temperature difference. ....... 48

Figure 21:Influence of the operating pressure on the specific overall heat flux

(left ordinate) and the mass flow density (right ordinate). .......................... 50

Figure 22:Influence of the pipe diameter on the specific overall heat flux

(left) and the mass flow density (right). ........................................................ 52

Figure 23:Influence of the pipe length on the specific overall heat flux (left)

and the mass flow density (right). ................................................................. 53

Figure 24:Influence of the liquid head on the specific overall heat flux (left

ordinate) and the mass flow density (right ordinate). .................................. 54


Chapter 1.

Introduction to Reboiler.


1. Introduction to Reboiler

1.1. Reboiler:

The transfer of heat to and from process fluids is an essential part of most chemical process.

Reboilers are heat exchangers typically used to provide heat to the bottom of

industrial distillation columns. They boil the liquid from the bottom of a distillation column to

generate vapors which are returned to the column to drive the distillation separation. The heat

supplied to the column by the reboiler at the bottom of the column is removed by

the condenser at the top of the column.

Proper reboiler operation is vital to effective distillation. In a typical classical distillation

column, all the vapor driving the separation comes from the reboiler. The reboiler receives a

liquid stream from the column bottom and may partially or completely vaporize that

stream. Steam usually provides the heat required for the vaporization.

Well a boiler is an equipment used to convert liquid into high pressure vapor. It is familiarly

known as the famous equipment "steam boiler", that produce steam from water. However, a

reboiler does the same operation, due to its involvement in the continuous process of boiling

the recycling liquid stream in its shell side. The name was given by the reason for boiling the

same liquid again and again. In most cases, liquid is boiled in a shell with the help of hot pipes

(tubes). On external surface of the tubes liquid changes its phase by observing heat (latent heat

+ sensible heat). In-turn the required high temperature of the hot tubes are maintained by

circulating low pressure or high-pressure steam inside the tubes. Based on the temperature

sensitivity of the material and rate of vapor formation boiling is done inside or outside of the

tubes.


1.2. Types of Reboilers:

The most critical element of reboiler design is the selection of the proper type of reboiler for a

specific service. Most reboilers are of the shell and tube heat exchanger type and normally

steam is used as the heat source in such reboilers. However, other heat transfer fluids like hot

oil or Dowtherm (TM) may be used. Fuel-fired furnaces may also be used as reboilers in some

cases.

Commonly used heat exchanger type reboilers are:

Kettle Reboilers.

Thermosiphon Reboilers.

Fired Heaters.

Forced Circulation type.

Internal Reboilers.


1.2.1. Kettle reboilers (natural-circulation):

Figure 1:Kettle reboilers

Kettle reboilers are very simple and reliable. They may require pumping of the column bottoms

liquid into the kettle, or there may be sufficient liquid head to deliver the liquid into the

reboiler. In this reboiler type, steam flows through the tube bundle and exits as condensate. The

liquid from the bottom of the tower, commonly called the bottoms, flows through the shell side.

There is a retaining wall or overflow weir separating the tube bundle from the reboiler section

where the residual reboiled liquid (called the bottoms product) is withdrawn, so that the tube

bundle is kept covered with liquid and reduce the amount of low-boiling compounds in the

bottoms product. The layout of the kettle reboiler is illustrated schematically in figure. Liquid

flows from the column into a shell in which there is a horizontal tube bundle, boiling taking

place from the outside this bundle. The vapor passes back to the column as shown. Kettle

reboilers are widely used in the petroleum and chemical industries; their main problems are

that of ensuring proper disentrainment of liquid from the outgoing vapor and the problem of

the collection of scale and other solid materials in the tube bundle region over long periods of

operation.


1.2.2. Thermosyphon reboilers (natural-circulation, operates based on the

head of liquid):

Figure 2:Thermosyphon reboilers.

Thermosyphon reboilers do not require pumping of the column bottoms liquid into the reboiler.

Natural circulation is obtained by using the density difference between the reboiler inlet

column bottoms liquid and the reboiler outlet liquid-vapor mixture to provide sufficient liquid

head to deliver the tower bottoms into the reboiler. Thermosyphon reboilers (also known

as calandrias) are more complex than kettle reboilers and require more attention from the plant

operators. There are many types of thermosyphon reboilers including vertical, horizontal, once-

through or recirculating.


1.2.3. Fired heaters :

Figure 3:Fired heaters

Fired heaters, also known as furnaces, may be used as a distillation column reboiler. A pump

is required to circulate the column bottoms through the heat transfer tubes in the furnace's

convection and radiant sections. The heat source for the fired heater reboiler may be either fuel

gas or fuel oil


1.2.4. Forced circulation reboilers (liquid is pumped into shell):

Figure 4:Forced circulation reboilers.

Forced circulation reboilers are similar to vertical thermosiphon reboilers, except the pump is

used for the circulation of the liquid and the hot liquid flows inside column. Usually arranged

in a Unbaffled Recirculating Circuit unless there is a critical temperature level beyond which

the process material undergoes decomposition or polymerization. If this is the case then a

preferential type column draw-off design would be recommended over the Unbaffled

Recirculation design.

For sensitive materials, precautions should be taken in the design of fired reboilers, such that

the pressure drop is reasonably low and the heat rate in the heater is such that the film

temperatures in the furnace tubes does not approach a temperature where excess fouling,

product decomposition, or polymerization can initiate. The main use of forced flow reboilers

is in services with severe fouling problems and/or highly viscous (greater than 25 cp) liquids

for which kettle and thermosyphon reboilers are not well suited. Pumping costs render forced

flow units uneconomical for routine services.

A forced circulation reboilers uses a pump to circulate the column bottoms liquid through the

reboilers. This is useful if the reboiler must be located far from the column, or if the bottoms

product is extremely viscous.

Some fluids are temperature sensitive such as those subject to polymerization by contact with

high temperature heat transfer tube walls. High liquid recirculation rates are used to reduce

tube wall temperatures, thereby reducing polymerization on the tube and associated fouling.


1.2.5. Internal Reboilers:

Figure 5:Internal Reboilers

The simplest approach is to mount the reboiler in the distillation tower itself as is illustrated in

figure. Here, boiling takes place in the pool of liquid at the bottom of the tower, the heating

fluid being inside the bundle of tubes as shown. The major problem with internal reboilers is

the limitation imposed by the size of the distillation column. This limits the size of the reboiler.

Another problem sometimes encountered is that of mounting the bundle satisfactorily into the

column. The problem of size restriction can be overcome if compact heat exchangers are used.

Thus, Plate-Fin Exchangers are used commonly as internal reboilers in the distillation towers

of air separation plant. Another form of compact heat exchanger which has been used for this

type of duty is the printed circuit heat exchanger which has an even higher heat transfer surface

area per unit volume.

Also known as stab-in reboilers or stab-in bundles, internal reboilers are another special

application of the horizontal reboiler design. The internal reboiler is usually used where the

process can be on the shell side and the reboiler surface area is small enough to fit into the

distillation column bottom sump. The process side is on the shell side and the heating medium


is on the tube side. Boiling takes place in the pool of liquid at the bottom of the tower, the

heating fluid being inside the bundle of tubes. Since the boiling liquid forms froth, which may

vary in density, controlling bottom level can be difficult. This fact can makes this type of

reboiler less attractive, particularly in foaming and vacuum services. Applications where

internal reboilers are sometimes used include:

• Batch distillation: where the tube bundle can easily be fitted into the batch drum, and periodic

cleaning can be easily accommodated.

• Very low heat duty clean services: where column diameter is large due to other

considerations, and where the reboiler tube bundle is small.


Chapter 2.

Introduction to Thermosyphon Reboilers.


2. Introduction to Thermosyphon Reboilers.

2.1. Thermosyphon Reboilers:

Thermosyphon reboilers play a wide role in the chemical industry, which provides a simple,

low maintenance design for distillation tower reboiler system. The thermosyphon reboiler

contains the two endearing qualities of the evaporator, namely mechanical simplicity, and

operation in the nucleate boiling regime with its attractive high fluxes. These reboilers require

rational design procedure as several flow patterns manifests during the heat transfer to a

flowing two-phase boiling mixture, which in-turn depends on upon the flow rates, physical

properties of the components, pipe diameter and orientation. The circulation rate, heat-transfer

rate and pressure drop all are interrelated, and hence, iterative design procedures must be used.

Thermosyphon reboilers are heat exchanger used to provide stripping section vapor for

fractional distillation columns. This type of reboiler is a very popular for use within plants. The

reason for the popularity of the thermosyphon unit are several. First, this type of exchanger

minimizes piping and ground area and does not introduce undue problem of tube side access

for cleaning. Second is the relatively low equipment cost associated with this type of

exchanger. These reboiler offers excellent rates of heat transfer.

Thermosiphon reboilers constitute one of the most widely used types of heat transfer equipment

in refineries, petrochemical, and chemical process industries where significant capital

investment is represented by reboilers, vaporizers, and evaporators. Thermosiphon reboilers

owe their popularity to excellent heat transfer rates, mechanical simplicity, and no expenditure

of power to circulate the process fluid. The boiling of liquids in a circulation system

encountered in a thermosiphon reboiler is applied also to refrigeration systems, pipe stills,

power plants, nuclear reactors, and solar energy.

Distillation is still one of the major units for separations in the chemicals and oil refining

industries. It is also one of the largest users of energy. It is only in providing more efficient

equipment in this area that energy savings will be made. It must be remembered that a

distillation column consists not only of the column itself but also of the associated reboiler and

condenser, the providers of vapor and liquid to the column. Improved design of these associated

units will yield energy savings. One way to achieve improvements is by better understanding


of their operation. A majority of the reboiler operate as thermosyphons, liquid is driven through

the heat exchanger via a density difference created by heat input to the system. At the outlet of

the exchanger there is usually a two-phase gas-liquid mixture with a lower density than the

liquid descending from the distillation column. This density difference drives the flow.

Thermosyphon reboilers have lower operating and maintenance costs than other reboiler types

due to their simplicity and the absence of a mechanical pump. They are characterized by high

heat transfer rates and low fouling tendencies, can be operated over a range of pressures and

have proven to be adequate for heavy heat duties in petroleum and nuclear industries.

Thermosyphon reboiler usage is fundamentally attractive because of the high heat fluxes. This

imply a smaller heat transfer area and hence capital expenditure and also lower process liquid

inventory compared to other reboilers. Also, horizontal thermosyphon reboilers have been

judged, through research, to be superior in thermal performance to vertical thermosyphon and

kettle reboilers. This is due to their higher circulation, local boiling temperature differences

and heat transfer rates. Notwithstanding the merits, the presence of two-phase flow initiates

complications. Researchers and designers have to consider many aspects including pressure

drop, flow regime prediction, realistic boiling curves, and flow instabilities.

Thermosyphon reboilers are extensively used for chemical engineering applications in various

industries. They comprise of 70% of evaporation duties in all process industries. The reason

for the extensive use of this type of reboiler is due to the low operating and maintenance cost,

absence of a pump and its adjunct controllers, since it works on the principle of density gradient

induced by temperature gradient along the length of the tube, no additional pump is required

and hence the energy required for pumping can be saved. Also, addition of valves and gauges

required in pumping circuits can be avoided. Thermosyphon reboilers are majorly used in

petroleum refining, petrochemical and chemical industries. 95% of the reboilers in petroleum

industries are horizontal type, 70% are vertical type in petrochemical industries and in chemical

while nearly 100% are vertical type in chemical industries. Though Thermosyphon reboilers

are widely used in various chemical process industries, there are no methods available in the

literature either for the design of thermosyphon reboiler or prediction of its performance.

Models developed so far in the literature ignore the interfacial shear stress, the compressibility

of vapor or assume one-dimensional steady-state Newtonian flows. Instability in two-phase

can affect performance which has not been addressed.


2.2. Advantages and Disadvantages:

2.2.1. Advantages:

Cheapest reboiler installation in terms of capital and operating cost.

Permits simple, compact piping arrangement.

Provides excellent thermal performance.

Most economical because no pump is required.

2.2.2. Disadvantages:

Not suitable for viscous or solid bearing fluids

More heat transfer area required for vacuum operation

Not specified for pressure below 0.3 bar

Column base must be elevated to provide the hydrostatic head required for the

thermosyphon effect.

This increases the cost of the column supporting structure.

It is having high construction cost for the installation of the column base at suitable

at suitable elevation to get thermosyphon effect.

It is not suitable for flow temperature difference process due to boiling point

elevation imposed by static head.

2.3. Types of Thermosyphon Reboilers:

There are mainly two types of Thermosyphon Reboilers.

Vertical Thermosyphon Reboilers.

Horizontal Thermosyphon Reboilers.


Chapter 3.

Literature Review.


3. Literature Review.

Thermosyphon is a method of exchanging heat based on simple principle of natural convection.

This method is commonly used in devices in which liquid circulation takes place from a heated

region to a region heaving relatively lesser temperature. Main application of this method can

be commonly seen in solar heater for domestic purposes and reboiler in petroleum industries.

Similar to heat pumps the thermal cycle of a thermosyphon system works simultaneous

evaporation and condensation. Though thermosyphon reboilers are widely used in various

chemical process industries the flow and heat transfer characteristics are not completely

understood yet.

Most of the researchers in the field of Chemical Engineering, Mechanical Engineering &

Thermal Engineering have work on the different kinds of Reboilers, Evaporators, Heat-

Exchangers also they work on the Special type of Reboilers like Thermosyphon Reboiler &

Some of them works on its Operational Characteristics, Working Principle and its Design.

Some of their researcher are discussed below.

This section highlights some of the earlier research done in this field of Thermosyphon

Reboilers. A major part of literature related to Thermosyphon Reboilers & Operational

characteristics of Thermosyphon Reboiler [2] and Thermal performance of Thermosyphon

Reboilers [3].

“Characteristics of Thermosyphon Reboilers” had been studied and invested by Stephan

Arneth and Johann Sinclair (Germany,2000) [2]. The aim was to describes the operational

characteristics of thermosiphon reboilers on the basis of an experimental and theoretical study.

The operational responses to a variation of the driving temperature difference, the operating

pressure and the liquid head in the inlet line are discussed in detail. Furthermore, the influence

of several design parameters as length and diameter of the pipes is presented. The effects of all

these parameters are explained by a simplified model that subdivides the evaporator into a

heating and an evaporation zone. The variations of the length of these two zones are decisive

for the operational characteristics of thermosiphon reboilers.

“Intensification of fluid dynamic and thermal performance of thermosiphon reboilers”

had been invested by Stephan Scholl and Fahmi Brahim (Germany,2005) [3]. They

introduced the Thermosyphon reboiler Process functions and their applications, limitations,


design option. They give idea about Process function such as Heat Transfer Unit & Separation

Unit.

Ezekiel O. Agunlejika, Paul A. Langston, Barry J. Azzopardi, and Buddhika N.

Hewakandamby (United Kingdom,2016) [4] has research on the “Sub atmospheric boiling

study of the operation of a horizontal reboiler loop: Instability. In their article, they

explained about Distillation and Chemical Processing industries, Comparison between Vertical

and Horizontal Thermosyphon Reboiler. Also, they explained Distillation and chemical

processing under vacuum is of immense interest to petroleum and chemical industries due to

lower energy costs and improved safety. To tap into these benefits, energy efficient reboilers

with lower maintenance costs are required. Here, a horizontal thermosyphon reboiler is

investigated at sub atmospheric pressures and low heat fluxes.

“No Hassle Reboiler Selection, Hydrocarbon Processing” had invested by Love D. L

(Germany,1992) [5] in their article they explained about the industrial use of vertical

thermosyphon reboiler in hydrocarbon processing. Also, they told that vertical thermosyphon

reboiler characterized by high heat transfer rate and low fouling tendencies. When designed

and operated properly, the liquids have short residence times in this reboiler type what

minimizes the risk of thermal degradation. This reboiler type is very reliable, has far lower

operating costs than other reboilers, is easy to set up and has compact dimensions.

Installing a pump in the inlet line leads to the forced circulation vertical thermosiphon reboiler.

This setup can achieve higher heat transfer rates through higher liquid circulation rates

especially at high vacuum operation, low liquid heads or small temperature differences between

the heating medium and the liquid in the reboiler. For high vacuum services, when the pressure

drop within the reboiler or the viscosity of the fluid is very high, this type of reboiler should be

preferred [6]. This can be explained by the Kister H. Z (New York,1990) [5] in there

“Distillation Operation” research article.

Like most types of reboilers and evaporators thermosiphon reboilers may be operated according

to two different process functions, As heat transfer unit or as separation unit, were explained

by U. Eiden and S.Scholl in there Chem. Eng. Book [7] “ Use of simulation in rating and

design of distillation units”.


Chapter 4.

Working Principle of Thermosyphon Reboiler.


4. Working Principle of Thermosyphon Reboiler

4.1. Working Principle:

Thermosyphon reboilers do not require pumping of the column bottoms liquid into the reboiler.

Natural circulation is obtained by using the density difference between the reboiler inlet

column bottoms liquid and the reboiler outlet liquid-vapor mixture to provide sufficient liquid

head to deliver the tower bottoms into the reboiler. Thermosyphon reboilers (also known

as calandrias) are more complex than kettle reboilers and require more attention from the plant

operators.

Thermosyphon reboilers is basically a shell and tube heat exchanger, requiring no pumps to

pump the vapor into the column back. These reboilers work on a simple principle based on

difference of densities of liquid and vapor. Recirculation of these systems is driven by the

density difference between the outlet and inlet line. In the system as the total driving force for

flow should be equal the total resistance to flow, so we can write:

𝑫𝒓𝒊𝒗𝒊𝒏𝒈 𝑭𝒐𝒓𝒄𝒆 = 𝑹𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆 𝒇𝒐𝒓𝒄𝒆 𝒂𝒈𝒂𝒊𝒏𝒔𝒕 𝒇𝒍𝒐𝒘

So as seen from the relation written above, the gravitational potential of the liquid boot is

responsible for sending the mixture of liquid and vapor back into the column. So, in

thermosyphon reboilers we don’t need to use any pump for pumping the vapor. In this reboiler

first liquid comes into the reboiler, where it come in contact of hoi fluid flowing through the

reboiler, due to which it gets heated and most of its part is vaporized & then due to reduction

in density, it raises itself and this mixture goes back to column.


4.2. Process Function of Thermosyphon Reboiler:

Thermosiphon reboilers may be operated according to two different process functions.

Heat transfer unit

Separation unit.

4.2.1. Heat Transfer Unit:

When Thermosiphon reboilers operated as heat transfer unit the reboiler generates the vapor

phase for countercurrent two phase vapor–liquid flow required for component separation in a

rectification or stripping column. Performance specification for the reboiler is given by a heat

duty and optimization aims to increase the overall heat transfer coefficient and/or reduce the

driving temperature difference. The reboiler does not have a designated separation

functionality.

Figure 6:Heat Transfer Unit.


4.2.2. Separation Unit:

Thermosiphon reboilers operated as separation units will be specified through a concentration

of volatiles in the concentrate. Typical applications may be the reduction of monomers or

oligomers from a polymer or the recycle of organic solvents from a waste stream. Optimization

will aim at reducing the volatiles concentration in the concentrate, thus leading to increased

viscosities and vacuum operation in many cases. A reboiler configuration that combines both

process functions is the falling film evaporator with divided sump

Figure 7:Separation Unit


Chapter 5.

Classification & Working of Thermosyphon Reboiler.


5. Classification & Working of Thermosyphon Reboiler

There are mainly two types of Thermosyphon Reboilers:

5.1. Vertical Thermosyphon Reboilers:

Figure 8:Vertical thermosiphon reboilers.

Of all reboiler types, vertical thermosiphon reboilers are most widely used in chemical industry.

They are characterized by high heat transfer rate and low fouling tendencies. When designed

and operated properly, the liquids have short residence times in this reboiler type what

minimizes the risk of thermal degradation. This reboiler type is very reliable, has far lower

operating costs than other reboilers, is easy to set up and has compact dimensions.

Thermosiphon reboilers can be used in a wide range of operating pressures and temperatures.

Therefore, they are used for about 70% of all evaporation duties in chemical industry.


vertical thermosyphon reboiler consists of a shell with a single-pass tube bundle. The boiling

liquid usually flows through the tubes as shown, but shell-side boiling may be used in special

situations, e.g., with a corrosive heating medium. A mixture of vapor and liquid is returned to

the distillation column, where phase separation occurs. The driving force for the flow is the

density difference between the liquid in the feed circuit and the two-phase mixture in the

boiling region and return line. Except for vacuum services, the liquid in the column sump is

usually maintained at a level close to that of the upper tube sheet in the reboiler to provide an

adequate static head. For vacuum operations, the liquid level is typically maintained at 50–70%

of the tube height to reduce the boiling point elevation of the liquid fed to the reboiler. Vertical

thermosyphon reboilers are usually attached directly to distillation columns, so the costs of

support structures and piping are minimized, as is the required plot space. The shell is also

relatively inexpensive. Another advantage is that the relatively high velocity attained in these

units tends to minimize fouling. On the other hand, tube length is limited by the height of liquid

in the column sump and the cost of raising the skirt height to increase the liquid level. This

limitation tends to make these units relatively expensive for services with very large duties.

The boiling point increase due to the large static head is another drawback for services with

small temperature driving forces. Also, the vertical configuration makes maintenance more

difficult, especially when the heating medium causes fouling on the outside of the tubes and/or

the area near the unit is congested

Vertical tube thermosiphon reboilers have an application in chemical, petrochemical and many

other allied industries as energy efficient equipment. The prediction of rates of heat transfer

and thermally induced flow (circulation rate) is the primary requirement for the design of such

equipment.


5.1.1. Working Principle of Vertical Thermosyphon Reboiler:

Figure 9:Working Principle of Vertical Thermosyphon Reboiler.

This type is illustrated in above figure. The liquid passes from the bottom of the tower into the

reboiler, with the evaporation taking place inside the tubes. The two-phase mixture is

discharged back into the tower, where the liquid settles back to the liquid pool and the vapor

passes up the tower as shown. The heating fluid (typically condensing steam) is on the outside

of the tubes. The vertical thermosyphon reboiler is less susceptible to fouling problems and in

general has higher heat transfer coefficients than does the kettle reboiler. However, additional

height is required in order to mount the reboiler.

In a process industry, the equipment is generally a 1-1 exchanger placed vertically, with upper

tube sheet close to the liquid of the bottoms in the column. The process fluid entering the

vertical tubes of heat exchanger receives the heat from the heat flux supplied. When

vaporization takes place in the tubes, the specific volume of the liquid is increased, resulting in

its upward movement while the liquid is siphoned from the adjoining cold leg. Thus, a net flow

through the circulation loop sets in. The rate of heat transfer and the liquid flow past the heating


surface interact with each other under the influence of various governing operating parameters,

such as heat flux, inlet liquid sub cooling, liquid level in the tube (submergence), and type of

fluid.

5.1.2. Setup of Vertical Thermosyphon Reboiler:

There are several setups of thermosyphon reboilers combined with distillation column are

given below.

A. Vertical Thermosyphon Reboiler:

Figure 10:Setup of Vertical Thermosyphon Reboiler

Figure (A) depicts the standard setup. The vertical thermosiphon reboiler is connected to the

column by a liquid feed line. Usually, a valve for controlling the liquid flow rate is installed in

the inlet pipe. The liquid enters the heat exchanger at the bottom and is heated and partially

evaporated inside the pipes. A vapor–liquid mixture leaves the reboiler through the exit line.

The liquid circulation is driven by the difference in static pressure between the liquid in the

inlet line and the partially evaporated fluid in the reboiler. No pumping is required for

circulation in most services. Therefore, the design of thermosiphon reboilers has to take special

care for a low pressure drop.


B. Forced Circulation Vertical Thermosyphon Reboiler:

Figure 11:Set up of Forced Circulation Vertical Thermosyphon Reboiler.

Installing a pump in the inlet line leads to the forced circulation vertical thermosiphon reboiler

shown in figure (B). This setup can achieve higher heat transfer rates through higher liquid

circulation rates especially at high vacuum operation, low liquid heads or small temperature

differences between the heating medium and the liquid in the reboiler. For high vacuum

services, when the pressure drop within the reboiler or the viscosity of the fluid is very high,

this type of reboiler should be preferred.

C. Vertical Thermosyphon Reboiler with Fixed Liquid Head:

Figure 12:Setup of Vertical Thermosyphon Reboiler with Fixed Liquid Head.


The special design for the bottom of the column in figure (C) ensures a fixed liquid level feed

to the reboiler even if the flow rate from the column varies.

D. Once-Through Vertical Thermosyphon Reboiler:

Figure 13:Set up of Once-Through Vertical Thermosyphon Reboiler.

A reboiler where the liquid from the column is heated only once is called a once-through

vertical thermosiphon reboiler, see figure (D). A short residence time of the liquid in the

reboiler can be achieved with this design. However, just a small fraction of the liquid is

evaporated.

E. Once -Through Naturally Forced Vertical Thermosyphon Reboiler:

Figure 14:Set up of Once -Through Naturally Forced Vertical Thermosyphon Reboiler.


A very sophisticated design that has the advantages of a forced circulation reboiler without the

disadvantages of a pump (risk of break down or leakage) is shown in figure (E). This once-

through naturally forced vertical thermosiphon reboiler will be installed in distillation columns

when a low boiling substance has to be separated from a high boiling mixture.


5.1.3. Advantage and Disadvantage of Vertical Thermosyphon Reboiler:

A. Advantages:

Vertical thermosyphon reboilers do not required pumping the liquid at the bottom of

the distillation column into the reboiler.

They also provide a simple, low-cost way of adding heat to the distillation process.

The main advantage of this reboiler is low fouling factor.

It has low maintenance costs.

It required low less space and piping.

It has high heat transfer rates, thus less powered is used during distillation process.

The exchanger is cheap.

Low plot area is required.

High circulation can be achieved, leading to high heat transfer coefficient and reduced

fouling.

The single pass tube-side arrangement facilitates cleaning. Mechanical cleaning can

often be performed without removing the exchanger.

The inventory of boiling fluid is relatively low.

The process fluid is on the tube-side, which is advantage for corrosive duties.

B. Disadvantages:

These reboiler have reliability issues, cannot be used where a large surface area is

needed and can be troublesome in vacuum services.

The column has to be raised to be above the boiler, requiring an increased skirt or

additional steelwork.

The performance tends to be poor under deep vacuum conditions. This is because the

extra static head between the column sump and the base of the reboiler results in

relatively large boiling point elevation. As a result, there may be long inlet zone where

boiling is suppressed, resulting in a low heat transfer coefficient.

The performance tends to be poor near critical conditions, where the liquid and vapor

have similar densities, thus giving little driving force for the recirculation.

The boiler can be unstable in operation, with circulation and vapor generation varying

markedly in a cyclic fashion, leading to column operating problems.


Being a single pass design, it is difficult to allow for differential expansion other than

by a shell bellows.

The boiler does not contribute a full theoretical stage to the separation,

Severe fouling can reduced the rate of circulation, leading to increased percentages

vaporization, increased rate of fouling and poorer separation efficiency.


5.2. Horizontal Thermosyphon Reboiler:

Figure 15:Horizontal thermosiphon reboiler.

This is a very common type of reboiler. Horizontal thermosiphon reboilers are the preferred

reboiler type in refining applications. The process side is on the shell side, and the heating

medium is on the tube side. The boiling occurs inside shell in horizontal thermosyphon. There

is recirculation around the base of the column. A mixture of vapor and liquid leaves the reboiler

and enters the base of the column where it separates. Compared to the vertical thermosiphon

reboiler, the horizontal thermosiphon reboiler generally requires less headroom but have more

complex pipework and plot space making it more expensive to install and has a higher fouling

tendency which leads to a slightly lower availability (because of outages for cleaning).

Horizontal exchangers are more easily maintained than vertical, as tube bundles can be more

easily withdrawn. They are generally better suited than vertical thermosyphons for services

with very large duties.


5.2.1. Working Principle of Horizontal Thermosyphon Reboiler:

Figure 16:Working Principle of Horizontal Thermosyphon Reboiler.

Here, the liquid from the column passes in cross flow over a tube bundle and the liquid-vapor

mixture is returned to the column as shown. The heating fluid is inside the tubes. This design

has the advantage of preserving the natural circulation concept while allowing a lower

headroom than the vertical thermosyphon type.

However, there are more uncertainties about fouling and about the prediction of the crossflow

heat transfer rates.

Horizontal thermosyphon reboilers usually employ shell sometimes used. The tube bundle may

be configured for a single pass as shown, or for multiple passes. In the latter case, either U-

tubes or straight tubes (plain or finned) may be used. Liquid from the column is fed to the

bottom of the shell and flows upward across the tube bundle. Boiling takes place on the exterior

tube surface, and a mixture of vapor and liquid is returned to the column. As with vertical

thermosyphons, the circulation is driven by the density difference between the liquid in the

column sump and the two-phase mixture in the reboiler and return line. The flow pattern in

horizontal thermosyphon reboilers is similar to that in kettle reboilers, but the higher circulation

rates and lower vaporization fractions in horizontal thermosyphons make them less susceptible

to fouling. Due to the horizontal configuration and separate support structures, these units are

not subject to restrictions on weight or tube length. As a result, they are generally better suited

than vertical thermosyphons for services with very large duties. The horizontal configuration


is also advantageous for handling liquids of moderately high viscosity, because a relatively

small static head is required to overcome fluid friction and drive the flow. A rule of thumb is

that a horizontal rather than a vertical thermosyphon should be considered if the feed viscosity

exceeds 0.5 cp.


5.2.2. Advantage and Disadvantage of Horizontal Thermosyphon Reboiler:

A. Advantages:

The exchanger is relatively cheap.

Multi-pass arrangements for the heating fluid can be used.

Removeable bundles are possible.

High circulation can be achieved, leading to high transfer coefficient and reduced

fouling.

The elevation of the column to be above the boiler is less than for a vertical unit.

Horizontal Thermosyphon reboiler are much more effective at low temperature

difference.

It is more attractive when the heat transfer area requirement is large due to machinal

consideration (e.g. Distillation column height).

Fluids with moderate viscosity boil better in horizontal thermosyphon.

The static head required for horizontal thermosyphon is less because of their high

circulation rate.

It has super thermal performance.

B. Disadvantages:

The Design Method is less developed.

Large plot area is required than vertical unit, especially if the bundled is removed.

The process fluid is on the shell side, creating potential problems with fouling or

corrosive fluids.

Mechanical cleaning of the process side can only be done by removing the bundle, and

then generally only if square pitch tube layout is used.

The boiler dose not contribute a full theoretical stage to the separation.


Chapter 6.

Comparison of Vertical and Horizontal Thermosyphon Reboiler.


6. Comparison of Vertical and Horizontal Thermosyphon

Reboiler.

Figure 17:Comparison of Vertical and Horizontal Thermosyphon Reboiler.

6.1. Horizontal Thermosyphon Reboiler Vs Vertical Thermosyphon

Reboiler:

Horizontal thermosyphon reboilers are much more effective at low temperature

differences than kettle and vertical thermosyphon units.

Vertical thermosyphons are also less attractive than horizontal type when heat

transfer area requirements are large due to mechanical considerations (e.g.

distillation column height).

Fluids with moderate viscosity boil better in horizontal thermosyphon than in

vertical units.

It is possible to use low finned and enhanced boiling tubes on the shell side of

horizontal thermosyphon reboilers.

The vertical height of the riser between the horizontal thermosyphon and the column

discharge nozzle allows for very flexible hydraulic design.


The static head requirements are lower for horizontal thermosyphon reboilers than

for vertical units. And because of their high circulation rates, the temperature rise for

boiling fluid across horizontal thermosyphon reboilers is lower than that for kettle

reboilers, this leads to higher local boiling temperature differences and higher heat

transfer rates for horizontal thermosyphon.

Their size is not limited with respect to length of tubes and weight; thus, the

requirements for high surface area are in their favor.

They handle the process fluid on the shell side; a scheme which many applications

favor, particularly where the heating fluid has fouling tendency.

They also offer easier access for mechanical cleaning of tubes by pulling the bundle.

6.2. Vertical Thermosyphon Reboiler Vs Horizontal Thermosyphon

Reboiler:

Horizontal thermosyphon reboilers have a less-sensitive operation than vertical

types.

More area can be placed in a single shell than with vertical units. Especially for

large sizes and high duties. Sizing is not limited by constructional

considerations.

May be more suitable for greater than 2:1 turndown ratios.

More suitable for wide boiling mixtures.

Piping must be carefully designed to equalize flows in all parallel branches.

More expensive if fixed tube sheet construction cannot be used because of fouling

on the shell-side.

If the available head is limited, vertical units are preferred.

More expensive due to the complicated nature of the piping and supporting

structure.


Chapter 7.

Fundamentals of Thermosyphon reboiler.


7. Fundamentals of Thermosyphon reboiler

The influence of the major operational and design parameters on heat flux and liquid circulation

rate of thermosyphon reboilers will be discussed here.

In a thermosyphon reboiler, there exists a complex mutual interaction between two-phase flow

and heat transfer. The heat transfer depends among others on the pressure, the vapor–liquid

equilibrium, the flow rates and the system properties, while the two-phase flow is primarily

influenced by the heat transfer rate and the pressure drop. The specific influence of all these

parameters on the performance of thermosyphon reboilers will be discussed in detail.

Figure 18: (A) Schematic of Vertical Thermosyphon Reboiler &(B) Characteristic Temperature Profile.

In order to understand the response of the thermosyphon reboiler to a variation of the relevant

parameters, it is helpful to divide the reboiler into two zones: a heating zone where the liquid

is heated up to its boiling point and, above that, an evaporization zone where the liquid is

partially evaporated by further heating as well as by pressure drop (flash). The principal

mechanisms are shown in figure(A). The heat transfer coefficient is much higher in the

evaporization zone than in the heating zone. Therefore, changes of the length of these two zones

have strong influence on the total heat transfer rate.


Figure(B) illustrates the principal temperature profile versus the tube length. The liquid

entering the reboiler tubes has approximately the same temperature as the liquid in the bottom

of the column. Due to the liquid head in the vertical inlet line the fluid is significantly subcooled

at the reboiler entrance. Within the heating zone the temperature rises to the boiling point which

depends significantly on the local liquid head. Boiling begins when the liquid has reached the

local boiling temperature. Here, the heating zone ends and the evaporation zone begins. Within

the evaporation zone the state of the liquid approximately follows the vapor pressure curve.

At atmospheric pressure, the length of the heating zone is typically 20–50% of the total tube

length. It increases significantly with decreasing pressure. At high vacuum services, the length

of the heating zone approaches 90% or even more of tube length. Since just the evaporation

zone drives the liquid circulation the circulation rate decreases drastically with decreasing

pressure. Eventually, the liquid circulation breaks down.


Chapter 8.

Operational Characteristics of Thermosiphon Reboilers.


8. Operational Characteristics of Thermosiphon Reboilers

The following description of the operational characteristics of thermosiphon reboilers is based

on an extensive experimental study of a single tube evaporator. In this study, the operational

and design parameters have been varied in the range of technical relevance. Furthermore, a

novel model has been developed for the simulation of the operational characteristics of

thermosiphon reboilers. The model considers two heat transfer zones only, a heating zone and,

above that, an evaporation zone. describes the operational characteristics of thermosiphon

reboilers with sufficient accuracy.

8.1. Influence of driving temperature difference:

Figure 19:Specific overall heat flux versus driving temperature difference.

Above figure shows the influence of the driving temperature difference on the specific overall

heat flux.

At low temperature differences, the specific overall heat flux rises steeply with increasing

temperature differences. Since more liquid is evaporated, the fluid velocity and, in turn, the

heat transfer coefficients rise what reduces the length of the heating zone. As the length of the


evaporization zone with enhanced heat transfer increases, the overall heat flux rises

significantly.

At higher driving temperature differences, the increase of the heat transfer rate slows down a

little. The liquid circulation through the tubes reaches its maximum at a temperature difference

of about 20–30 K and decreases thereafter, in dependence on the pipe diameter and length.

Also, the growth in length of the vaporization zone becomes smaller.

A dependence of the overall heat flux on the operating pressure of the thermosiphon reboiler is

observed. The heat flux rises with system pressure. Besides the influence of the pressure on the

system properties, this effect mainly depends on the smaller sub cooling of the liquid at high

pressures. This mechanism will be described in detail in the next section.

Figure 20:. Mass flow density versus driving temperature difference.

The data for the mass flow density are plotted versus the driving temperature difference in

above figure. The lines represent the simulation, the dots the experimental data. The mass flow

density in a thermosiphon reboiler rises sharply at small driving temperature differences. It

usually reaches its maximum at about 20 K temperature difference and decreases thereafter.

This characteristic behavior has been observed in all experiments.

At small driving temperature differences, there exists just a small density difference between

the liquid in the feed line and the two-phase mixture in the reboiler. Thus, the driving force for


the natural circulation is small. A rise of the temperature difference will evaporate more liquid

and, in turn, enhance the liquid circulation. However, the pressure drop increases significantly

at higher evaporation rates what reduces the circulation rates. At a driving temperature

difference of 20–30 K the increase in driving force for natural circulation is compensated for

by the rising pressure drop. At higher driving temperature differences, the mass flow density

decreases since the pressure drop becomes the dominant mechanism.

There is a risk of flow instabilities (oscillations) and, eventually, of burnout at very large driving

temperature differences. Heavy deviations of the average flow rates were observed in

oscillationary flow. Closing the throttling valve in the inlet line is an effective means for

suppressing these unwanted oscillations. The risk of the development of oscillations is higher

at low liquid heads and low operating pressures. Oscillations are more often observed at

operations with organic liquids than with inorganic liquids (e.g., water). Burnout is caused by

film boiling at the upper end of the pipes at very high driving temperature differences. Burnout

must be avoided since the heat transfer to a vapor is generally rather poor. Therefore, rising the

driving temperature difference above a critical value will lead to a lower vapor generation.

Thermosiphon reboilers show an inverse operation characteristic in this range of operation.


8.2. Influence of operating pressure:

The operating pressure strongly influences the performance of a thermosiphon reboiler. At low

operating pressures, the influence of the sub cooling of the liquid at the reboiler inlet is of major

importance.

This is explained for a thermosiphon reboiler of 4 m heated pipe length operated with water.

At a pressure of 0.1 bar in the bottom of the column, the pressure due to the liquid head in the

feed line is 0.5 bar. Hence, the liquid is approximately 35 K subcooled. If the same reboiler is

operated at 3 bar, the pressure at the inlet is 3.4 bar what refers to a sub cooling of 4 K only.

Thus, at low pressures an increase of the pressure is decisive for the sub cooling and, in turn,

for the heat transfer rate. Adversely at high pressures, the sub cooling of the liquid is very low.

The heating zone where the liquid is warmed up to the boiling temperature is much shorter.

Furthermore, the increased vapor content in the pipe causes a larger density difference and, in

turn, a higher circulation rate. This increases the length of the evaporization zone. Since the

heat transfer coefficient is significantly higher in the evaporization zone than in the heating

zone, higher operating pressures enhance the heat transfer rates.

Figure 21:Influence of the operating pressure on the specific overall heat flux (left ordinate) and the mass flow density

(right ordinate).

Above figure shows the influence of the operating pressure on the specific overall heat flux q˙

(left ordinate) and the mass flow density w·ρ (right ordinate). The experimental data have been

collected with toluene in a vertical thermosiphon reboiler with tubes of 50 mm in diameter and


2 m in length. The liquid head was 75% of the tube length and the driving temperature

difference was 15 K.

The heat flux as well as the mass flow density rate increase with rising operating pressure due

to the mechanisms described above.


8.3. Influence of pipe diameter:

Figure 22:Influence of the pipe diameter on the specific overall heat flux (left) and the mass flow density

(right).

The influence of pipe diameter on the specific overall heat flux and the mass flow density is

illustrated in figure . While the specific overall heat transfer rate decreases with increasing pipe

diameter, the heat transfer rate per tube rises. With increasing pipe diameter the ratio of heat

transfer area to heated pipe volume becomes smaller. Thus, the heating zone is longer and the

heat flux smaller. In other words, smaller pipes are more effective in terms of specific heat flux

than larger ones.

The specific mass flow, i.e., the mass flux related to the pipe cross section, increases with

increasing pipe diameter.

There are two major reasons for this:

• The friction caused by the fluid flow is smaller in bigger pipes.

• The pressure drop caused by acceleration is smaller in larger pipes due to a smaller vapor

content.

Both mechanisms enhance the mass flow density. Similar results as plotted in figure 6 have

been observed at several operating pressures, fluids, pipe lengths and liquid heads in the inlet

line.


8.4. Influence of pipe length:

Figure 23:Influence of the pipe length on the specific overall heat flux (left) and the mass flow density

(right).

The effect of pipe length on the specific overall heat flux and the mass flow density is illustrated

in above figure. The longer the pipes the more liquid is evaporated. This leads to a higher mass

flow rate. However, due to the higher content of vapor the pressure drop rises even more.

Therefore, the overall mass flow density decreases slightly in longer pipes.

The pipe length has little influence on the ratio of the length of the heating and the evaporization

zone provided the pipes are longer than 1 m. Hence, the specific overall exit line is fairly

independent of the pipe length and has bigger influence on the overall pressure drop at shorter

pipes. For pipes, shorter than 1 m the experiments showed a strong influence of the pipe length

on the mass flow density rate while there was almost no influence on the specific overall heat

flux.


8.5. Influence of driving liquid head:

The driving liquid head is a very important operational parameter of a thermosiphon reboiler

since it can be manipulated very easily.

Figure 24:Influence of the liquid head on the specific overall heat flux (left ordinate) and the mass flow

density (right ordinate).

Figure 8 shows the dependence of the specific overall heat flux (left ordinate) and of the mass

flow density (right ordinate) on the driving liquid head.

The mass flow density is, above a critical value, approximately a linear function of the driving

liquid head. The higher the liquid head the larger is the mass flow density. Below a critical

value of the driving liquid head the liquid circulation breaks down and, in turn, the heat transfer

is very poor.

Astonishing is the fact that the heat transfer rate is approximately independent of the driving

liquid heads. There are two competing mechanisms that are inversely changed by a variation

of the driving liquid head. At low driving heads, the heat transfer coefficients are generally low

due to the low circulation rate but the evaporation zone with enhanced heat transfer is long.

Thus, the overall heat transfer coefficient is quite high even at low driving liquid heads.

At high driving liquid heads, the heat transfer coefficients are higher in both the heating zone

and the evaporation zone due to the higher circulation rate. However, the length of the

evaporation zone with enhanced heat transfer is shorter what reduces the mean heat transfer


coefficient of the reboiler. As can be seen from figure 8, the specific overall heat flux is nearly

independent of the driving liquid head.

Thus, increase of the heat transfer coefficient by an increased driving head is compensated for

by a reduction of the length of the evaporation zone. This holds for the evaporation of water or

similar systems at a pressure of 1 bar or higher. At low system pressures, however, especially

in vacuum services, the behavior of the thermosiphon reboiler is different due to the short length

of the evaporation zone.

Besides the experimental data, figure 8 shows the influence of the liquid head on the heat

transfer and the circulation rate as predicted by three different models. All three models predict

only a small effect of the liquid head on heat transfer rates (upper lines in figure 8). However,

the influence on the circulation rate is very different in the three models.

The own experiments and data from literature showed that thermosiphon reboilers operated at

ambient or higher pressures show best performance at driving liquid heads of 80–100% of the

pipe length while reboilers operated in vacuum conditions work best with liquid heads between

50 and 70% of the pipe length.


Chapter 9.

Industrial Applications of

Thermosyphon Reboiler.


9. Industrial Applications of Thermosyphon Reboiler.


industries. They comprise of 70% of evaporation duties in all process industries. The reason

for the extensive use of this type of reboiler is due to the low operating and maintenance cost,

absence of a pump and its adjunct controllers, since it works on the principle of density gradient

induced by temperature gradient along the length of the tube, no additional pump is required

and hence the energy required for pumping can be saved. Also, addition of valves and gauges

required in pumping circuits can be avoided. Thermosyphon reboilers are majorly used in

petroleum refining, petrochemical and chemical industries. 95% of the reboilers in petroleum

industries are horizontal type, 70% are vertical type in petrochemical industries and in chemical

while nearly 100% are vertical type in chemical industries. Though Thermosyphon reboilers

are widely used in various chemical process industries, there are no methods available in the

literature either for the design of thermosyphon reboiler or prediction of its performance.

Models developed so far in the literature ignore the interfacial shear stress, the compressibility

of vapor or assume one-dimensional steady-state Newtonian flows. Instability in two phase can

affect performance which has not been addressed.

In industry, the advantages of operating such equipment under vacuum, such as in low pressure

distillation include: higher thermodynamic efficiency; reduced energy consumption;

processing of heat sensitive materials at low temperature and achieving better separation. The

low temperatures will allow cheaper materials of construction to be used. Nowadays, many

applications in distillation are looking to use sub atmospheric pressure operation to lower

energy costs and improve safety. Distillation under vacuum is also a commonly desired process

in the chemical industry for extraction of essential oils, deodorization of vegetable oils and

purification and drying of chemicals. This is because there are favorable advantages over

atmospheric pressure distillation which include:

Use of lower process temperatures as a result of reduction in boiling points and hence shorter

time of thermal exposure of the distillate so that thermally sensitive substances, like vitamin

and hormones, can be processed easily.

Reduction of energy consumption as a result of lowered boiling point.

Increase in relative volatility of materials resulting in higher production rates.


Change in position of the azeotropic point enables separation of hard-to-separate

materials.

Reduction of oxidation losses of the feed stock.

Reduction in stripping steam requirements for de-odourisation process of oil due to

increased specific volumes (of steam), enhanced agitation and stirring of the oil.

However, vacuum operation makes the thermosyphon system more susceptible to instabilities

due to lowered system pressure and this initiates oscillatory flow. The improved vaporization

rate results in high vapor mass flux., makes the sub atmospheric pressure boiling systems prone

to instability. These instabilities are magnified by decreasing: system pressure; mass flow rate;

inlet resistance and inlet sub cooling and by increasing: riser height.

Typical applications may be the reduction of monomers or oligomers from a polymer or the

recycle of organic solvents from a waste stream. Optimization will aim at reducing the volatiles

concentration in the concentrate, thus leading to increased viscosities and vacuum operation in

many cases. A reboiler configuration that combines both process functions is the falling film

evaporator with divided sump

Classical applications of thermosyphon reboiler are for pure or well-defined mixtures and

uncritical evaporation behavior, i.e. no foaming or liquid/liquid phase separation. Typical

systems for these applications are refrigerants, low-chain hydrocarbons, water or ammonia.


Chapter 10.

Limitation of Thermosyphon Reboiler.


10. Limitation of Thermosyphon Reboiler.

Fouling is a major concern amongst all heat exchangers. Foulants such as corrosion products

and dirt form scale on heat transfer surfaces or block the tubes by forming a plug. In case of

vertical thermosyphon reboilers excessive circulation may occur when reboiler sump level is

high and cannot be lowered. Insufficient circulation might occur due to plugging of tubes and

insufficient liquid head which may lead to poor heat transfer and possible tube over-heating.

Surging may occur if the reboiler temperature difference is small and column pressure is not

controlled. When the column pressure rises, it increases the bottom pressure. Boiling decreases

or stops which results in bottom liquid level to build up. Dumping will occur, causing the

pressure to fall. This in turn increases the boiling and the pressure increases.

Oscillations have been identified which cause instabilities in reboiler. These oscillations may

be caused by pressure drop limitation in the reboiler outlet or outlet piping system. The

generated vapor cannot find its way out in sufficient quantity and some accumulates as a pocket

near the top of the reboiler. Expansion of the vapor pocket can momentarily reverse the process

flow, leading to a drop in pressure, which in turn causes liquid to rush back in. Thermosyphon

failure might be caused by low heat fluxes. This is common at start-up of a multi component

mixture with negligible reboiler temperature difference. If flow is not adequately started, the

reboiler may only vaporize some of the relatively lighter components in the liquid and leaving

behind heavy liquid.


Chapter 11.

Conclusion.


11. Conclusion.

Conclusions made from this study; In a thermosiphon reboiler, there exists a complex mutual

interaction between heat transfer and two-phase flow.

Decisive for the operational characteristic of a thermosiphon reboiler is the length of the heating

and the evaporation zone, respectively. Since the values of the heat transfer coefficients are

much higher in the evaporation than in the heating zone the overall heat transfer rate of the

reboiler is governed by the length of the evaporation zone.

The key point for the modelling of the operational characteristics of thermosiphon reboilers is

the correct description of the liquid circulation rate that depends significantly on the pressure

drop and the vapor content in the evaporator.


industries. They comprise of 70% of evaporation duties in all process industries.

Thermosyphon reboilers are majorly used in petroleum refining, petrochemical and chemical

industries. 95% of the reboilers in petroleum industries are horizontal type, 70% are vertical

type in petrochemical industries and in chemical while nearly 100% are vertical type in

chemical industries.

Fouling is a major concern amongst all heat exchangers. Foulants such as corrosion products

and dirt form scale on heat transfer surfaces or block the tubes by forming a plug. In case of

vertical thermosyphon reboilers excessive circulation may occur when reboiler sump level is

high and cannot be lowered. Insufficient circulation might occur due to plugging of tubes and

insufficient liquid head which may lead to poor heat transfer and possible tube over-heating.


12. References

[1] R. K. Sinnott., "Coulson & Richardson's Chemical Engineering.," in Chemical

Engineering Design.Volume-6, 4th Edition., Oxford, UK., Elsevier., 2005, pp. 728-743, .

[2] J. S. Stephan Arneth, "Characteristics of Thermosiphon Reboilers.," International Journal

of Thermal Sciences, Elsevier., vol. 40, no. 4, pp. 385-391, 2000.

[3] F. B. Stephan Scholl, "Intensification of fluiddynamic and thermal performance of

thermosiphon reboilers.," Applied Thermal Engineering, Elsevier., vol. 25, no. 16, pp.

2615-2629, 2005.

[4] P. A. L. B. J. A. B. N. H. Ezekiel O. Agunlejika, "Subatmospheric Boiling Study of the

Operation of a Horizontal Thermosyphon Reboiler Loop: Instability.," Applied Thermal

Engineering, Elsevier., vol. 109, no. Part-A, pp. 739-746, 2016.

[5] D. Love, "No Hassle Reboiler Selection.," Hydrocarbon Processing., vol. 71, no. 10, pp.

41-47, 1992.

[6] H. Z. Kister., Distillation Operations., New York.: McGraw-Hill., 1990.

[7] S. S. Ulrich Eiden, "Use of Simulation In Rating and Design of Distillation Units.,"

Computers &. Chemical Engineering., vol. 24, no. Supplement, pp. S199-S204., 1997.

[8] J. F. R. J. M. Coulson, "Coulson and Richardson's Chemical Engineering.," in Fluid Flow,

Heat Transfer and Mass Transfer. Volume-1, 6th Edition., Swansea., Elsevier., 1999, pp.

494-496.

Thermosyphon Reboiler & its type with operational parameter.

Engineering

Transcript of Thermosyphon Reboiler & its type with operational parameter.