**2.1 Consequences of heat exchanger fouling**

It can be stated that a general solution to heat exchanger fouling still does not exist. This is not surprising, as knowledge of underlying mechanisms of the fouling process remains limited. Moreover, fouling in heat exchangers often concerns different types of heat exchangers, each with its own unique characteristics. Also, there are large differences in physical properties of the fluids to be applied in the exchangers. The consequences of heat exchanger fouling are:

Self-Cleaning Fluidised Bed Heat Exchangers

Crude storage

Reflux

column

Fig. 1. Simplified flow diagram of a crude oil preheat train.

Heavy

gas

oil

Ex4

Gas

oil

Residu

Flow Density Spec.Heat Viscosity Thermal cond.

Fig. 2. Temperature diagram crude oil preheaters in simplified flow diagram shown in

100

Fig. 1.

130 144

163.3 184.7

243.3

250 260

210

255

274

161 182

350

380

271

Temperature in °C

T ambient

Furnace

380°C 271°C

for Severely Fouling Liquids and Their Impact on Process Design 553

Heat exchangers

Distillation Ex8 Ex7 Ex4

Density Spec.Heat Viscosity Thermal cond.

Ex1 Ex2 Ex3

Desalter

Ex6 Ex5

Residu

Furnace

130°C

Preheat exchanger train

: 800 kg/m³ : 2,500 J/(kg·K) : 2 cP : 0.1 W/(m·K)

Average values hydrocarbons

> Crude oil

: 660 m³/h : 750 kg/m³ : 2,500 J/(kg·K) : 1 cP : 0.1 W/(m·K)

Crude oil

Gas oil pump around

Heat exchangers crude oil preheat exchanger train upstream desalter

Ex5 Ex6 Ex7 Ex8


Over sizing of heat transfer equipment has become an accepted approach to increase the period of time necessary to reach the fouled state. The equipment is then cleaned (chemically or mechanically) to return the heat transfer surface to a near clean condition with recurring maintenance cost and the possibility of cleaning solution disposal problems. Later in this chapter, it will be shown that there are existing cases where over sizing of heat transfer surface can involve the installation of two to five times the surface required for the clean condition. Also, in these severe cases it may be necessary to carry out the cleaning procedure every two or three days, resulting in excessive downtime, maintenance costs and solution disposal problems. Sometimes the fouling problems are so severe that heat transfer performance reduces to almost zero in a matter of hours.

Experience has shown that the alternatives to recurring fouling problems associated with the cooling or heating of a severe fouling liquid are certainly limited. In the case of cooling applications unsuccessful attempts to recover energy from hot waste streams may lead to the total abandonment of an otherwise promising energy management program.

Frequently the only acceptable approach to heating severe fouling liquids will involve direct steam injection. This results in a loss of condensate and the dilution of the process stream, which often requires costly reconcentration later in the process. However, heating by direct steam injection does offer a unique opportunity to define the actual cost of fouling in terms of lost condensate and the subsequent cost of water removal. In the next paragraph we will pay attention to the very high cost of heat exchanger fouling on a global scale, and for one process in particular.

#### **2.2 Cost associated with heat exchanger fouling**

The heat exchangers in a crude oil train of a refinery for the distillation of crude oil in lighter fractions are often subject to severe fouling, and do represent globally a very high level of cost. In this sub-paragraph we like to explain this particular example in a nutshell. For a much more detailed explanation one is referred to Ref. [5].

Fig. 1 gives an schematic impression of the heating of crude oil in a crude oil train downstream the desalter and in the furnace, where after the oil is cracked in much lighter fractions in the distillation column. Fig. 2 gives an impression of the temperatures in this very much simplified example.

Fouling of the crude oil heat exchangers downstream the desalter, Ex4 up to and inclusive Ex8, is shown at first instance by a drop of the inlet temperature 271 °C of the crude oil in the furnace, which means that more heat has to be supplied into the furnace to meet the required outlet temperature 380 °C of the crude oil entering the distillation column. This, of course, a phenomenon caused by fouling of the heat exchangers, does requires extra fuel (i.e. extra energy) to be burned in the furnace to keep the distillation facility in operation. At a certain moment, the inlet temperature of the crude oil in the furnace has dropped to such an

Over sizing of heat transfer equipment has become an accepted approach to increase the period of time necessary to reach the fouled state. The equipment is then cleaned (chemically or mechanically) to return the heat transfer surface to a near clean condition with recurring maintenance cost and the possibility of cleaning solution disposal problems. Later in this chapter, it will be shown that there are existing cases where over sizing of heat transfer surface can involve the installation of two to five times the surface required for the clean condition. Also, in these severe cases it may be necessary to carry out the cleaning procedure every two or three days, resulting in excessive downtime, maintenance costs and solution disposal problems. Sometimes the fouling problems are so severe that heat transfer

Experience has shown that the alternatives to recurring fouling problems associated with the cooling or heating of a severe fouling liquid are certainly limited. In the case of cooling applications unsuccessful attempts to recover energy from hot waste streams may lead to

Frequently the only acceptable approach to heating severe fouling liquids will involve direct steam injection. This results in a loss of condensate and the dilution of the process stream, which often requires costly reconcentration later in the process. However, heating by direct steam injection does offer a unique opportunity to define the actual cost of fouling in terms of lost condensate and the subsequent cost of water removal. In the next paragraph we will pay attention to the very high cost of heat exchanger fouling on a global scale, and for one

The heat exchangers in a crude oil train of a refinery for the distillation of crude oil in lighter fractions are often subject to severe fouling, and do represent globally a very high level of cost. In this sub-paragraph we like to explain this particular example in a nutshell. For a

Fig. 1 gives an schematic impression of the heating of crude oil in a crude oil train downstream the desalter and in the furnace, where after the oil is cracked in much lighter fractions in the distillation column. Fig. 2 gives an impression of the temperatures in this

Fouling of the crude oil heat exchangers downstream the desalter, Ex4 up to and inclusive Ex8, is shown at first instance by a drop of the inlet temperature 271 °C of the crude oil in the furnace, which means that more heat has to be supplied into the furnace to meet the required outlet temperature 380 °C of the crude oil entering the distillation column. This, of course, a phenomenon caused by fouling of the heat exchangers, does requires extra fuel (i.e. extra energy) to be burned in the furnace to keep the distillation facility in operation. At a certain moment, the inlet temperature of the crude oil in the furnace has dropped to such an

the total abandonment of an otherwise promising energy management program.

Loss of energy,

process in particular.

very much simplified example.

excessive maintenance cost,

 loss of production or reduced capacity operation, over sizing and / or redundancy of equipment,

hazardous cleaning solution handling and disposal.

performance reduces to almost zero in a matter of hours.

**2.2 Cost associated with heat exchanger fouling** 

much more detailed explanation one is referred to Ref. [5].

Fig. 1. Simplified flow diagram of a crude oil preheat train.

Fig. 2. Temperature diagram crude oil preheaters in simplified flow diagram shown in Fig. 1.

Self-Cleaning Fluidised Bed Heat Exchangers

for Severely Fouling Liquids and Their Impact on Process Design 555

that uses a circulating fluidised bed. This section pays attention to both principles of which the circulating concept is more widely applicable in comparison with the stationary type.

In principle such a stationary fluidised bed heat exchanger consists of a large number of parallel vertical tubes, in which small solid particles are kept in a stationary fluidised condition by the liquid passing up the tubes. The solid particles regularly break through the boundary layer of the liquid in the tubes, so that good heat transfer is achieved in spite of comparatively low liquid velocities in the tubes. Further, the solid particles have a slightly abrasive effect on

Fig. 3 shows a heat exchanger with a stationary fluidised bed, which means there is no change in position of the particles as a function of time. The inlet channel contains a fluidised bed and a flow distribution system which is of utmost importance to achieve stable operation of all parallel exchanger tubes, or said otherwise: Equal distribution of liquid and solid particles over all the tubes. This exchanger is characterised by the use of glass beads with diameters of 2 to 3 mm and very low liquid velocities in the tubes. The glass beads are fluidised along the tubes and form a shallow fluidised bed layer in the outlet channel. This

Outlet fouling liquid

Outlet channel

Cleaning solids and liquid

Heat exchangers tubes with upward flow and stationary fluidised bed of cleaning solids

Liquid

Shell

Liquid

Inlet fouling liquid

Fig. 3. Self-cleaning heat exchanger with stationary fluidised bed of cleaning solids.

Distribution plate

Inlet channel

Tube plate

Tube plate

the tube wall of the exchanger tubes, removing any deposit at an early stage.

exchanger is only suitable for operation on constant flow.

Inlet

Outlet

extent that the heating capacity of the furnace is insufficient to meet the required outlet temperature of the crude oil. This temperature can only be maintained by reducing the throughput of crude oil through the heat exchanger train, which, however, also reduces the production capacity of the refinery. This example shows very clearly that fouling of heat exchangers does cost extra energy and may also reduce the production capacity of an installation. For our crude oil preheat train, both facts, including the maintenance cost, increases the refining cost for each barrel of crude oil. What are these costs on a global scale?

At this moment (2011), the global production of crude oil amounts to approx. 85 million barrels per day (bpd). Table 1 has been derived from information given in Ref. [5], and gives an impression about the annual fouling cost for the crude oil being processed in the crude oil preheater trains of all refineries in the world as a function of the price per barrel crude oil.


Table 1. Fouling costs crude oil trains as a function of crude oil price.

It is assumed that for a crude oil price of US\$ 60 / barrel, the total fouling cost in crude oil preheat trains processing the global crude oil production of 85 million bpd represents approx. 10 % of the worldwide fouling costs in heat exchangers, which costs include all kind of heat exchangers for both liquids and gases. From this statement and the numbers presented in Table 1, it can be concluded that the total cost the world has to pay annually for fouling of heat exchangers amounts to approx. US\$ 125 billion. In Ref. [1], Garrett-Price used a different approach and concluded that the fouling of heat exchangers do cost an industrialised nation approx. 0.3 % of its Gross National Product (GNP). If we apply this rule to the GNP of the whole world (2007) of US\$ 55 000 billion, then we find for the global fouling cost US\$ 165 billion. This is higher than US\$ 125 billion, but, very likely, because not all countries can be considered as sufficiently industrialised.

It is evident that the often excessive costs of heat exchanger fouling have led to a number of initiatives to develop some additional alternative solutions, often derived from research into the various fouling mechanisms. Over a period of forty years, the principal author Dr. Ir. Dick G. Klaren has participated in the development of one of the more promising alternatives: The self-cleaning or non-fouling fluidised bed heat exchanger. During this period the concept was taken from a laboratory tool to a fully developed heat transfer tool, which is now used to resolve severe fouling problems in a range of applications throughout the process industries.

## **3. Principle of the self-cleaning fluidised bed heat exchanger**

Over the past 40 years, the principle of the fluidised bed heat exchange technology evolved from a type that applied a stationary fluidised bed into a more widely applicable concept

extent that the heating capacity of the furnace is insufficient to meet the required outlet temperature of the crude oil. This temperature can only be maintained by reducing the throughput of crude oil through the heat exchanger train, which, however, also reduces the production capacity of the refinery. This example shows very clearly that fouling of heat exchangers does cost extra energy and may also reduce the production capacity of an installation. For our crude oil preheat train, both facts, including the maintenance cost, increases the refining cost for each barrel of crude oil. What are these costs on a global scale? At this moment (2011), the global production of crude oil amounts to approx. 85 million barrels per day (bpd). Table 1 has been derived from information given in Ref. [5], and gives an impression about the annual fouling cost for the crude oil being processed in the crude oil preheater trains of all refineries in the world as a function of the price per barrel crude

> **Crude oil price in US\$ per barrel \$ 45 \$ 60 \$ 75 \$ 90**

dollars 10.9 12.5 14.1 15.7

It is assumed that for a crude oil price of US\$ 60 / barrel, the total fouling cost in crude oil preheat trains processing the global crude oil production of 85 million bpd represents approx. 10 % of the worldwide fouling costs in heat exchangers, which costs include all kind of heat exchangers for both liquids and gases. From this statement and the numbers presented in Table 1, it can be concluded that the total cost the world has to pay annually for fouling of heat exchangers amounts to approx. US\$ 125 billion. In Ref. [1], Garrett-Price used a different approach and concluded that the fouling of heat exchangers do cost an industrialised nation approx. 0.3 % of its Gross National Product (GNP). If we apply this rule to the GNP of the whole world (2007) of US\$ 55 000 billion, then we find for the global fouling cost US\$ 165 billion. This is higher than US\$ 125 billion, but, very likely, because not

It is evident that the often excessive costs of heat exchanger fouling have led to a number of initiatives to develop some additional alternative solutions, often derived from research into the various fouling mechanisms. Over a period of forty years, the principal author Dr. Ir. Dick G. Klaren has participated in the development of one of the more promising alternatives: The self-cleaning or non-fouling fluidised bed heat exchanger. During this period the concept was taken from a laboratory tool to a fully developed heat transfer tool, which is now used to resolve severe fouling problems in a range of applications throughout

Over the past 40 years, the principle of the fluidised bed heat exchange technology evolved from a type that applied a stationary fluidised bed into a more widely applicable concept

oil.

Fouling costs in billion US

Table 1. Fouling costs crude oil trains as a function of crude oil price.

all countries can be considered as sufficiently industrialised.

**3. Principle of the self-cleaning fluidised bed heat exchanger** 

the process industries.

that uses a circulating fluidised bed. This section pays attention to both principles of which the circulating concept is more widely applicable in comparison with the stationary type.

In principle such a stationary fluidised bed heat exchanger consists of a large number of parallel vertical tubes, in which small solid particles are kept in a stationary fluidised condition by the liquid passing up the tubes. The solid particles regularly break through the boundary layer of the liquid in the tubes, so that good heat transfer is achieved in spite of comparatively low liquid velocities in the tubes. Further, the solid particles have a slightly abrasive effect on the tube wall of the exchanger tubes, removing any deposit at an early stage.

Fig. 3 shows a heat exchanger with a stationary fluidised bed, which means there is no change in position of the particles as a function of time. The inlet channel contains a fluidised bed and a flow distribution system which is of utmost importance to achieve stable operation of all parallel exchanger tubes, or said otherwise: Equal distribution of liquid and solid particles over all the tubes. This exchanger is characterised by the use of glass beads with diameters of 2 to 3 mm and very low liquid velocities in the tubes. The glass beads are fluidised along the tubes and form a shallow fluidised bed layer in the outlet channel. This exchanger is only suitable for operation on constant flow.

Fig. 3. Self-cleaning heat exchanger with stationary fluidised bed of cleaning solids.

Self-Cleaning Fluidised Bed Heat Exchangers

advantage in comparison with the configuration shown in Fig. 4.

3

1B = Control flow

4

1A 1B

Fig. 5. Self-cleaning heat exchanger with external circulation of cleaning solids.

1

Inlet fouling liquid

Control channel

Downcomer with downward flow of cleaning solids

Cleaning solids

Liquid + cleaning solids

Support

1A = Main flow

Inlet channel with distribution sytem

Outlet

Heat exchanger

Outlet channel

Inlet

for Severely Fouling Liquids and Their Impact on Process Design 557

The heat exchanger shown in Fig. 5 applies an 'externally circulating' fluidised bed. In this heat exchanger the liquid en particles flow from the outlet channel into an external separator where the particles are separated from the liquid, where after the particles flow from the separator into the inlet channel through only one downcomer and control channel For hydraulic stability reasons, this heat exchanger has the advantage that it only uses one downcomer, and the flow through this external and accessible downcomer can be monitored, influenced and varied by the control flow through line 1B. This flow only represents approximately 5 % of the feed flow through line 1 and shutting off this flow makes it possible to use the particles intermittently. This configuration also makes it possible to revamp existing severely fouling vertical conventional heat exchangers into a self-cleaning configuration as will be presented later in this chapter; this is a major

> Liquid + cleaning solids

Separator Liquid

> Outlet fouling liquid

2

Fig. 4 shows a heat exchanger with an 'internally circulating' fluidised bed. In this heat exchanger the liquid and particles flow through the tubes from the inlet channel into the widened outlet channel, where the particles disengage from the liquid and are returned to the inlet channel through multiple downcomer tubes, which are uniformly distributed over the actual heat exchanger or riser tubes. Now, the particles in the tubes experience a change of position with time. This heat exchanger can also use higher density materials like chopped metal wire as particles with dimensions up to 4 mm, and normally operates on higher liquid velocities in the tubes than the exchanger with the stationary fluidised bed. Depending on the design, this exchanger can also operate on a varying flow and in case of chopped metal wire particles; this exchanger represents the ultimate tool for handling the most severe fouling problems in liquid heat transfer.

Fig. 4. Self-cleaning heat exchanger with internal circulation of cleaning solids.

Fig. 4 shows a heat exchanger with an 'internally circulating' fluidised bed. In this heat exchanger the liquid and particles flow through the tubes from the inlet channel into the widened outlet channel, where the particles disengage from the liquid and are returned to the inlet channel through multiple downcomer tubes, which are uniformly distributed over the actual heat exchanger or riser tubes. Now, the particles in the tubes experience a change of position with time. This heat exchanger can also use higher density materials like chopped metal wire as particles with dimensions up to 4 mm, and normally operates on higher liquid velocities in the tubes than the exchanger with the stationary fluidised bed. Depending on the design, this exchanger can also operate on a varying flow and in case of chopped metal wire particles; this exchanger represents the ultimate tool for handling the

Outlet fouling liquid

Outlet channel

cleaning solids

Distribution plate

Inlet channel

Tube plate

Shell

Inlet fouling liquid

Fig. 4. Self-cleaning heat exchanger with internal circulation of cleaning solids.

Tube plate

Cleaning solids and liquid

Risers (heat exchangers tubes) with upward flow of liquid and

Downcomer with downward flow of liquid and cleaning solids

Liquid

most severe fouling problems in liquid heat transfer.

Inlet

Outlet

The heat exchanger shown in Fig. 5 applies an 'externally circulating' fluidised bed. In this heat exchanger the liquid en particles flow from the outlet channel into an external separator where the particles are separated from the liquid, where after the particles flow from the separator into the inlet channel through only one downcomer and control channel For hydraulic stability reasons, this heat exchanger has the advantage that it only uses one downcomer, and the flow through this external and accessible downcomer can be monitored, influenced and varied by the control flow through line 1B. This flow only represents approximately 5 % of the feed flow through line 1 and shutting off this flow makes it possible to use the particles intermittently. This configuration also makes it possible to revamp existing severely fouling vertical conventional heat exchangers into a self-cleaning configuration as will be presented later in this chapter; this is a major advantage in comparison with the configuration shown in Fig. 4.

Fig. 5. Self-cleaning heat exchanger with external circulation of cleaning solids.

Self-Cleaning Fluidised Bed Heat Exchangers

bed exchanger, is composed as follows:

superficial velocity Ul,s related to the porosity ε of the bed

the range 0.9 < ε 1.0 , the following equation is suggested:

Ub,w as the liquid velocity used in the Reynolds number.

coefficient and relevant parameters for the stationary fluidised bed bed.

plant is anyhow necessary to demonstrate the non-fouling operation.

(stationary) fluidised bed moving along the tube wall

 

bed, which then yields:

where:

Reynolds number.

for Severely Fouling Liquids and Their Impact on Process Design 559

For Ub,w = 0, the circulating fluidised bed satisfies the conditions of a stationary fluidised

The heat transfer coefficient αw,l between the wall and the liquid of a circulating fluidised

*w,l l c*

αl = wall-to-liquid heat transfer coefficient of a stationary fluidised bed with a

αc = wall-to-liquid heat transfer coefficient for forced convection in a tube, taking into account a liquid velocity Ub,w, which actually corresponds with the velocity of the

For the heat transfer coefficient αl one is referred to Ruckenstein, Ref. [11], as long as superficial liquid velocities are calculated from porosities (ε) lower than 0.9. For porosities in

> 0.9 ( *l l = 1.0 l l = = 1.0 (1- ) = + <sup>|</sup> | | 1 - 0.9 )*

The heat transfer coefficient αl|<sup>ε</sup> <sup>=</sup> 1.0 is calculated using the equation of Dittus and Boelter taking into account the liquid velocity in the tube which corresponds with the terminal falling velocity on one single particle in the tube, i.e. ε = 1.0, as the liquid velocity used in the

The heat transfer coefficient αc is also obtained using the equation of Dittus and Boelter with

Fig. 7 shows the wall-to-liquid heat transfer coefficients in an exchanger with a circulating fluidised bed as a function of the various process parameters using 2.0 mm glass particles. It should be noticed that in Fig. 7 the curve Us = Ul,s shows the relation between heat transfer

It should be emphasised that this heat transfer correlation is only an attempt to produce some approximate numbers for the overall heat transfer coefficients for any preliminary design. The real numbers which should be used in the performance guarantee of the heat exchanger follow from experimental operation of a representative pilot plant. Such a pilot

 

 

 *= +*  

where Ul,s follows from the theory presented by Richardson and Zaki, Ref. [10].

*UU U s b,w l,s = +* (1)

*U U s l,s <sup>=</sup>* (2)

(3)

(4)
