**6.3 Evaporator for concentration of very viscous severely fouling slurry**

In one of the Scandinavian countries, a production plant of a proprietary product operates a very large MVR evaporator for the concentration of a slurry up to approx. 70 % solids. Even at a temperature of 100 °C this slurry, which behaves non-Newtonian, has a very high viscosity varying between 50 and more than 200 cP. This very large shell and tube heat exchanger suffers from severe fouling which sometimes requires one month (!!) of mechanical cleaning after only three months (!!) of operation. In Fig. 20, the test plant in parallel with the existing evaporator is shown and the dimensions of the existing installation

Self-Cleaning Fluidised Bed Heat Exchangers

Outlet channel

Inlet

Cooler

Existing inlet channel

Inlet channel extension with distribution system

Outlet

for Severely Fouling Liquids and Their Impact on Process Design 577

Particles

~8 500

Fig. 19. Revamped into self-cleaning configuration of Fig. 18.

Ø 1 700

Catcher / separator

> Circ. pump 4 500m³/h @ 3 mwc

Crystalliser

Ø 3 500

Ø 600

Downcommer, with packed bed flow of cleaning particles

Feed

Liquid+ crystals (suspension)

Discharge

Fig. 18. Conventional cooling crystalliser.

Ø 3 500

Circ. pump 4 500m³/h @ 3 mwc Discharge

Crystalliser

Liquid+ crystals (suspension)

Feed

Inlet

Outlet channel

Ø 1 700

~8 500

Outlet

Inlet channel

Fig. 18. Conventional cooling crystalliser.

Cooler

Fig. 19. Revamped into self-cleaning configuration of Fig. 18.

Self-Cleaning Fluidised Bed Heat Exchangers

Liquid (slurry)

Ø 4 000

**Steam from fan Condensate**

Circ. pump 1 150 m³/h (existing)

Vapour

by M.P. steam requires cleanings every four months.

for Severely Fouling Liquids and Their Impact on Process Design 579

very high liquid velocity in the tubes of 4.5 m/s and heated by L.P. steam experiences severe fouling still requiring cleanings every two months, while the thermal syphon reboiler heated

Internal (new) Slurry

12 000

Particles

Separator (new)

Downcomer (new)

Control channel (new)

Ø 1 700 Ø 250

Existing inlet channel (but modified)

Fig. 21. Existing evaporator revamped into self-cleaning configuration.

The solution we are proposing to solve this problem is quite unique and explained in Fig. 23. As a matter of fact, we have increased the tendency of fouling in the preheater due to the precipitation of solids by increasing the outlet temperature of the preheater. This can be realised by adding M.P. steam to the shell of the preheater instead of L.P. steam. As a result of this temperature increase, the preheater will also partly contribute to the degassing

2 500 Tubes 22 x 1.0

Existing

Existing outlet channel (sligthly modified)

Slurry + particles

Flash vessel **Steam to fan**

give a good impression about its size, although provided with relatively small diameter tubes with an ID of only 20 mm.

Fig. 20. Existing evaporator and test installation.

The proposal for the revamp of this installation is shown in Fig. 21 and uses a maximum of the very large existing components, including the circulation pump. The first series of experiments with the test installation are promising and shear-thinning effects caused by the increased turbulence of the slurry induced by the action of the fluidised particles are reducing the viscosity of the slurry substantially and have produced heat transfer coefficients or k-values between 1 000 and 2 000 W/(m²·K) depending on the concentration of the slurry without fouling. These coefficients should be compared with the clean heat transfer coefficients of approximately 600 W/(m²·K) for the conventional heat exchanger which, in a couple of months, reduces to only a fraction of its clean value due to by fouling.

This potential revamp reflects the benefits of recent developments which make it possible to operate a self-cleaning fluidised bed heat exchanger on a very viscous slurry and use rather large stainless steel particles (2.5 mm) in small tube diameter with an ID of only 20 mm.

#### **6.4 Combination of preheater and thermal siphon reboiler**

A chemical plant in the United States operates the preheater in series with the thermal syphon reboiler shown in Fig. 22. The 8-pass preheater with tubes with an O.D. of 25 mm, a

give a good impression about its size, although provided with relatively small diameter

**Feed**

**H.P. vapour**

**L.P. vapour**

Fan

**Discharge Condensate**

Fig. 20. Existing evaporator and test installation.

**6.4 Combination of preheater and thermal siphon reboiler** 

Liquid (slurry)

Ø 4 000

Circ. pump 1 500 m³/h

The proposal for the revamp of this installation is shown in Fig. 21 and uses a maximum of the very large existing components, including the circulation pump. The first series of experiments with the test installation are promising and shear-thinning effects caused by the increased turbulence of the slurry induced by the action of the fluidised particles are reducing the viscosity of the slurry substantially and have produced heat transfer coefficients or k-values between 1 000 and 2 000 W/(m²·K) depending on the concentration of the slurry without fouling. These coefficients should be compared with the clean heat transfer coefficients of approximately 600 W/(m²·K) for the conventional heat exchanger which, in a couple of months, reduces to only a fraction of its clean value due to by fouling. This potential revamp reflects the benefits of recent developments which make it possible to operate a self-cleaning fluidised bed heat exchanger on a very viscous slurry and use rather large stainless steel particles (2.5 mm) in small tube diameter with an ID of only 20 mm.

A chemical plant in the United States operates the preheater in series with the thermal syphon reboiler shown in Fig. 22. The 8-pass preheater with tubes with an O.D. of 25 mm, a

**Steam from fan Condensate**

Tie-in L.P. of pump

Tie-in H.P. of pump

2 500 Tubes 22×1.0 mm

Flash vessel

12 000

Ø 1 700

Vapour

tubes with an ID of only 20 mm.

very high liquid velocity in the tubes of 4.5 m/s and heated by L.P. steam experiences severe fouling still requiring cleanings every two months, while the thermal syphon reboiler heated by M.P. steam requires cleanings every four months.

Fig. 21. Existing evaporator revamped into self-cleaning configuration.

The solution we are proposing to solve this problem is quite unique and explained in Fig. 23. As a matter of fact, we have increased the tendency of fouling in the preheater due to the precipitation of solids by increasing the outlet temperature of the preheater. This can be realised by adding M.P. steam to the shell of the preheater instead of L.P. steam. As a result of this temperature increase, the preheater will also partly contribute to the degassing

Self-Cleaning Fluidised Bed Heat Exchangers

Outlet channel (modified)

M.P. steam

Existing bundle converted to 1-pass

Condensate

Inlet channel with distribution system (modified)

> Feed pump (existing)

series with thermal syphon reboiler.

Increased autoclave production capacity.

Reduced acid consumption.

evaporation in the tubes.

summarise below:

**plants** 

Downcomer

Separator

Liquid + particles

Gasses

Control channel

Circ. pump (new)

for Severely Fouling Liquids and Their Impact on Process Design 581

Condensate

Fig. 23. Conventional preheater revamped into self-cleaning configuration and operating in

For the proposed solution of this problem, we have introduced the concept of evaporation of a fraction of the liquid creating a mixture of liquid, vapour and particles in the tubes. We know that this is possible if certain design criteria are taken into account. Consequently, with this example, we have presented the possibility that our self-cleaning heat exchange technology can also be applied for applications where we even experience boiling or

**6.5 Self-cleaning fluidised bed heat exchangers in existing 'directly heated' HPAL** 

There exist a strong drive to apply indirect heating (i.e. using heat exchangers) in High Pressure Acid Leach (HPAL) plants for the extraction of nickel and cobalt from laterite ore slurry, because of the benefits of indirect heating in comparison with direct heating (i.e. using steam injection or slurry / vapour mixing condensation), which benefits we

M.P. steam

Thermal syphone reboiler (existing)

Column Liquid

> Liquid to process

Gasses

Fig. 22. Conventional preheater in series with thermal syphon reboiler.

of the liquid which is normally done in the reboiler. Above goals have been realised by revamping the existing 8-pass horizontal conventional heat exchanger into a vertical singlepass self-cleaning fluidised bed configuration using stainless steel cleaning particles with a diameter of 2.5 mm and also installing an extra circulation pump to maintain sufficient velocity in the tubes of our single-pass configuration for circulation of the cleaning particles. Although, we have indeed increased the tendency for fouling, we expect that the introduction of our self-cleaning technology will keep the preheater clean.

The separation of the gasses from the mixture of liquid and particles takes place in the widened outlet channel of the preheater, these gasses are fed into the reboiler and evenly distributed over all the tubes of the reboiler where they contribute to the (natural) circulation effect of this thermal syphon reboiler.

Considering the fact that a substantial fraction of the totally required degassing is not done anymore in the reboiler, the heat load of the reboiler can be reduced, which reduces the condensing steam temperature, the tube wall temperature and, consequently, the fouling of the reboiler.

The advantage of this approach is the revamp of the conventional preheater into a selfcleaning configuration at an increased heat load. An experiment with a single-tube selfcleaning pilot plant in parallel with the existing severely fouling preheater should demonstrate the non-fouling performance of the self-cleaning heat exchange technology. If this is indeed the case, then, we have not only solved the fouling problem of the preheater at an even higher heat load, but also reduced the fouling of the conventional thermal syphon reboiler.

M.P. steam

L.P. steam

Feed pump

the reboiler.

reboiler.

8-pass preheater

Condensate

circulation effect of this thermal syphon reboiler.

Fig. 22. Conventional preheater in series with thermal syphon reboiler.

introduction of our self-cleaning technology will keep the preheater clean.

of the liquid which is normally done in the reboiler. Above goals have been realised by revamping the existing 8-pass horizontal conventional heat exchanger into a vertical singlepass self-cleaning fluidised bed configuration using stainless steel cleaning particles with a diameter of 2.5 mm and also installing an extra circulation pump to maintain sufficient velocity in the tubes of our single-pass configuration for circulation of the cleaning particles. Although, we have indeed increased the tendency for fouling, we expect that the

The separation of the gasses from the mixture of liquid and particles takes place in the widened outlet channel of the preheater, these gasses are fed into the reboiler and evenly distributed over all the tubes of the reboiler where they contribute to the (natural)

Considering the fact that a substantial fraction of the totally required degassing is not done anymore in the reboiler, the heat load of the reboiler can be reduced, which reduces the condensing steam temperature, the tube wall temperature and, consequently, the fouling of

The advantage of this approach is the revamp of the conventional preheater into a selfcleaning configuration at an increased heat load. An experiment with a single-tube selfcleaning pilot plant in parallel with the existing severely fouling preheater should demonstrate the non-fouling performance of the self-cleaning heat exchange technology. If this is indeed the case, then, we have not only solved the fouling problem of the preheater at an even higher heat load, but also reduced the fouling of the conventional thermal syphon

Condensate

Thermal syphone reboiler (existing)

Column Liquid

> Liquid to process

Gasses

Fig. 23. Conventional preheater revamped into self-cleaning configuration and operating in series with thermal syphon reboiler.

For the proposed solution of this problem, we have introduced the concept of evaporation of a fraction of the liquid creating a mixture of liquid, vapour and particles in the tubes. We know that this is possible if certain design criteria are taken into account. Consequently, with this example, we have presented the possibility that our self-cleaning heat exchange technology can also be applied for applications where we even experience boiling or evaporation in the tubes.

#### **6.5 Self-cleaning fluidised bed heat exchangers in existing 'directly heated' HPAL plants**

There exist a strong drive to apply indirect heating (i.e. using heat exchangers) in High Pressure Acid Leach (HPAL) plants for the extraction of nickel and cobalt from laterite ore slurry, because of the benefits of indirect heating in comparison with direct heating (i.e. using steam injection or slurry / vapour mixing condensation), which benefits we summarise below:


Self-Cleaning Fluidised Bed Heat Exchangers

metals from laterites.

Flash stage nr. 1 165 °C

Atm. mixing condenser

Flash vapour

heated configuration.

260 °C

Flash stage nr. 2 212 °C

series for the conventional shell and tube heat exchanger.

for Severely Fouling Liquids and Their Impact on Process Design 583

Particularly, the high heat transfer coefficient and low slurry velocity do affect the total length of the installed heat exchange tubes. This follows from the Equation (5) for the tube length presented in paragraph 0 of his chapter, after substitution of the design and process parameters. As a consequence, the number of shells in series for the self-cleaning fluidised bed heat exchanger is a fraction (just one) in comparison with the large number of shells in

For this HPAL application, the scope of the benefits already mentioned at the beginning of this sub-paragraph increases when indirect heating is not only applied to the highest temperature stage of the installation but to all stages. It is not surprising that all major mining companies show much interest in the self-cleaning fluidised bed heat exchange technology for an even greater variety of applications than only HPAL for the extraction of

Bypass for direct

Autoclave 230 °C

Indirect

Spray tower final heating (mixing condenser)

185 °C

HEX-1

Cold water Hot process water Slurry discharge Fresh slurry

98% Sulphuric acid

2nd Stage spray tower (mixing condenser) incl. heat exchanger

1st Stage spray tower (mixing condenser) incl. heat exchanger

HP Pos.displ. feed pump

Condensate

Condensate

Condensate

Steam, 275 °C

Bypass for direct Indirect

Heat exchanger

HEX-2 (HEX-3)

Bypass for direct

Indirect

Agitators

65 °C

Atm. flash stage 100°C

Slurry storage tank

Flash vapour stage nr. 1, 170 °C

Flash vapour stage nr. 2, 215 °C

127 °C

Fig. 25. HPAL plant for laterite nickel employing direct heat transfer revamped into indirect


Poor heat transfer and hydraulic performance of conventional shell and tube heat exchangers have worked against the introduction of indirect heating in HPAL plants. We believe that self-cleaning fluidised bed heat exchangers offer a much better option, and in the example below, we introduce a 'directly heated' HPAL plant which is retrofitted into an 'indirectly heated' configuration using two different kinds of heat exchangers.

Fig. 24 shows the flow diagram, including relevant temperatures, of a 'directly heated' HPAL plant. Fig. 25 shows the above flow diagram, but, now extended in such a way that direct heating can be fully replaced by indirect heating. Now, for the high temperature end of the installation shown in Fig. 25, we have engineered these two different kinds of indirect heating solutions. One of the indirect heating solutions uses conventional shell and tube heat exchangers and the other self-cleaning fluidised bed heat exchangers. Table 4 compares both indirect heating solutions. The advantages in favour of the self-cleaning fluidised bed configuration are very convincing and we like to emphasise these advantages:


Fig. 24. HPAL plant for laterite nickel employing direct heat transfer.

Poor heat transfer and hydraulic performance of conventional shell and tube heat exchangers have worked against the introduction of indirect heating in HPAL plants. We believe that self-cleaning fluidised bed heat exchangers offer a much better option, and in the example below, we introduce a 'directly heated' HPAL plant which is retrofitted into an

Fig. 24 shows the flow diagram, including relevant temperatures, of a 'directly heated' HPAL plant. Fig. 25 shows the above flow diagram, but, now extended in such a way that direct heating can be fully replaced by indirect heating. Now, for the high temperature end of the installation shown in Fig. 25, we have engineered these two different kinds of indirect heating solutions. One of the indirect heating solutions uses conventional shell and tube heat exchangers and the other self-cleaning fluidised bed heat exchangers. Table 4 compares both indirect heating solutions. The advantages in favour of the self-cleaning fluidised bed

 Shear-thinning of the non-Newtonian highly viscous slurry due to the increased turbulence of the slurry induced by the fluidised particles, which reduces the viscosity

127 °C

185 °C

Spray tower final heating (mixing condenser)

Autoclave 230 °C

> Cold water Hot process water Slurry discharge Fresh slurry

98% Sulphuric acid

2nd Stage spray tower (mixing condenser)

1st Stage spray tower (mixing condenser)

HP Pos.displ. feed pump

Steam, 275 °C

of the slurry experienced by the fluidised bed by a factor 4 to 5 or even more.

non-fouling due to the scouring action of the fluidised particles on the tube wall.

65 °C

Fig. 24. HPAL plant for laterite nickel employing direct heat transfer.

Atm. flash stage 100 °C

Slurry storage tank

Flash vapour stage nr. 2, 215 °C

260 °C Agitators

Flash vapour stage nr. 1, 170 °C

Reduced neutralizing agent consumption.

High heat transfer coefficients,

low pressure drops in the tubes, and

low slurry velocities,

Flash stage nr. 1 165 °C

Atm. mixing condenser

Flash vapour

Flash stage nr. 2 212 °C

Recovery of demineralised condensate and process condensate.

'indirectly heated' configuration using two different kinds of heat exchangers.

configuration are very convincing and we like to emphasise these advantages:

Particularly, the high heat transfer coefficient and low slurry velocity do affect the total length of the installed heat exchange tubes. This follows from the Equation (5) for the tube length presented in paragraph 0 of his chapter, after substitution of the design and process parameters. As a consequence, the number of shells in series for the self-cleaning fluidised bed heat exchanger is a fraction (just one) in comparison with the large number of shells in series for the conventional shell and tube heat exchanger.

For this HPAL application, the scope of the benefits already mentioned at the beginning of this sub-paragraph increases when indirect heating is not only applied to the highest temperature stage of the installation but to all stages. It is not surprising that all major mining companies show much interest in the self-cleaning fluidised bed heat exchange technology for an even greater variety of applications than only HPAL for the extraction of metals from laterites.

Fig. 25. HPAL plant for laterite nickel employing direct heat transfer revamped into indirect heated configuration.

Self-Cleaning Fluidised Bed Heat Exchangers

the development of completely new processes.

**7. Final remarks** 

fouling problems only.

**8. References** 

New Yersey.

September.

October.

March.

Cayman, June.

Processing, August.

for Severely Fouling Liquids and Their Impact on Process Design 585

We have given an indication about the cost of fouling of heat exchangers on a global scale and we have shown that the self-cleaning fluidised bed heat exchange technology can play a significant role in battling these fouling cost, and does have even more potential that solving

Particularly, the latter aspect has caught the attention of an increasing number of very large companies which are very much interested to implement the self-cleaning fluidised heat exchange technology for the upgrading of their existing proprietary processes, or even for

Garrett-Price, B.A., et al. (1985). *Fouling of Heat Exchangers*, Noyes Publications, Parkridge,

Gibbs, R. & Stadig, W. (1992). *Fluidized bed heat exchanger eliminates reboiler fouling*, Chemical

Klaren, D.G. (2000). *Self-Cleaning Heat Exchangers: Principles, Industrial Applications and* 

Klaren, D.G. & de Boer, E.F. (2004). *Case Study Involving Severely Fouling Heat Transfer: Design* 

Klaren, D.G., de Boer, E.F. & Sullivan, D.W. (2007). *Consider low fouling technology* 

Klaren, D.G., de Boer, E.F. & Crossley, B. (2008). *Reflections on Indirect Heating of Laterite Ore* 

Klaren, D.G. & de Boer, E.F. (2009). *Revamping existing severely fouling conventional heat* 

Klaren, D.G. and de Boer, E.F. (2010). *Multi-Stage Flash / Fluidized Bed Evaporator (MSF / FBE):* 

Klaren, D. G. (2010). *Design, Construction and Operating Features of Multi-Stage Flash / Fluidized* 

Richardson, J.F. and Zaki, W.N. (1954). *Sedimentation and fluidization: Part 1*, Trans. Inst.

ALTA 2008, Perth, Western Australia, June.

Cleaning-2009, Schladming, Austria.

Beach, California, USA, November.

Chem. Eng., vol. 28, p. 35, 1954.

*Operating Installations*, Industrial Heat Transfer Conference, Dubai, UAE,

*and Operating Experience of a Self-Cleaning Fluidized Bed Heat Exchanger and its Comparison with the Newly Developed Compact Self-Cleaning Fluidized Bed Heat Exchanger with EM Baffles*, Presented at the Fachveranstaltung: Verminderung der Ablagerungsbildung an Wärmeübertragerflächen, Bad Dürkheim, Germany,

*for 'dirty' heat transfer services*, Hydrocarbon Processing, Bonus Report,

*Slurry in HPAL Plants Using Self-Cleaning Fluidized Bed Heat Exchangers*, Presented at

*exchangers into a self-cleaning (fluidised bed) configuration: New developments and examples of revamps*, International Conference on: Heat Exchangers Fouling and

*A resurrection in Thermal Seawater Desalination?*, CaribDA Conference, Grand

*Bed Evaporators (MSF/FBE) for very large Capacities*, IDA Conference, Huntington

For more information about the performance and the potential of HPAL plants equipped with self-cleaning fluidised bed heat exchangers, one is referred to Ref. [6].


Table 4. Comparison significant parameters for indirect heating of high temperature stage of HPAL plant of Fig. 25.
