**5. New installations equipped with self-cleaning fluidised bed heat exchangers**

#### **5.1 Multi-Stage Flash / Fluidised Bed Evaporator (MSF / FBE); most promising tool for thermal seawater desalination**

As the development of this heat exchanger began in the early 70s with the application of stationary fluidised bed condensers in MSF evaporators, we like to begin this paragraph with what we may consider 'the origin of the fluidised bed heat exchange technology' as developed for seawater desalination, and referred to as Multi-Stage Flash/Fluidised Bed Evaporator or MSF/FBE. In the example below, we compare this evaporator with a conventional MSF. This comparison shows that the advantages of the self-cleaning fluidised bed heat exchange technology for the MSF are responsible for a much wider range of improvements than non-fouling heat exchange only.

A picture of the conventional MSF and its corresponding temperature diagram is shown in Fig. 9. The principle of this MSF can be best described as a large counter-current heat exchanger, where the cold feed is heated by the condensing vapour in the heat recovery section and the external heat supply takes place in the final heater. After leaving the final heat exchanger at its highest temperature, the liquid flashes through all stages, by way of

and, therefore, the advantages of this heat exchange technology does reach much further

Now we have been informed about the remarkable effects of scouring particles on the heat transfer film coefficients at very low liquid velocities, low pumping power requirements and their potential to remove fouling, one might wonder what the consequences are of the scouring action of the particles with respect to wear and /or material loss of the heat exchanger tubes and the particles. After many years operating experiences we have come to the conclusion that only in case of the formation of a weak corrosion layer on tube and/or particle material, the scouring action of the particles may cause material loss due to the removal of this corrosion layer. For applications where corrosion of metal surfaces does not

In a US plant, after one year of operation, the cleaning particles made of chopped stainless steel wire lost 2.5 % of weight. This is caused by the rounding-off effects of the sharp edged cylindrical particles. In the second year, being substantially rounded-off already during the first year of operation, the weight loss of the particles dropped to less than 0.5%. Because the smooth stainless steel tube wall is not subjected to metal loss as a result of rounding-off

Similar experiences have been obtained in Japan with stainless steel tubes and particles. Again, after one year of operation a weight loss of approx. 2% was measured. In the second year, this weight loss was negligible. Fig. 8 shows the rounding-off effects of chopped stainless steel wire as a function of operating time. The loss of approx. 2% in the first year of operation as mentioned above can also be avoided by using particles which have already

**5.1 Multi-Stage Flash / Fluidised Bed Evaporator (MSF / FBE); most promising tool for** 

As the development of this heat exchanger began in the early 70s with the application of stationary fluidised bed condensers in MSF evaporators, we like to begin this paragraph with what we may consider 'the origin of the fluidised bed heat exchange technology' as developed for seawater desalination, and referred to as Multi-Stage Flash/Fluidised Bed Evaporator or MSF/FBE. In the example below, we compare this evaporator with a conventional MSF. This comparison shows that the advantages of the self-cleaning fluidised bed heat exchange technology for the MSF are responsible for a much wider range of

A picture of the conventional MSF and its corresponding temperature diagram is shown in Fig. 9. The principle of this MSF can be best described as a large counter-current heat exchanger, where the cold feed is heated by the condensing vapour in the heat recovery section and the external heat supply takes place in the final heater. After leaving the final heat exchanger at its highest temperature, the liquid flashes through all stages, by way of

effects, the material loss of the tubes should be much less than 0.5% per year.

been rounded-off mechanically directly after their fabrication (chopping) process.

**5. New installations equipped with self-cleaning fluidised bed heat** 

than solving heat exchanger fouling problems only.

play a role, we present the following examples:

**4.5 Wear** 

**exchangers** 

**thermal seawater desalination** 

improvements than non-fouling heat exchange only.

Fig. 8. Rounding-off effects of 2 mm stainless steel particles as a function of operating period.

Fig. 9. Principle of conventional MSF and its temperature diagram.

Self-Cleaning Fluidised Bed Heat Exchangers

specific heat consumption.

its steel weight.

distillate production of 500 m³/d.

and [9].

as:

for Severely Fouling Liquids and Their Impact on Process Design 567

As the result of the vertical layout of the MSF/FBE and the flashing down flow in the flash chambers with a height for each chamber of approx. 0.4 m, we are able to add a number of interesting improvements to the MSF/FBE in comparison with the conventional MSF, such






Above, we have clearly shown that *the vertical layout of the MSF/FBE*, as the result of the integration of the vertical stationary fluidised bed condenser with the flash chambers, increases the advantages of this evaporator too such an extent, that this evaporator may be considered as the most promising tool for thermal seawater desalination in the future. Fig. 11 shows an MSF/FBE demonstration plant operating on natural seawater for a

For more information about this fascinating evaporator, one is referred to the Ref. [8]

between 0 and 100 %, while still maintaining an excellent distillate quality.

reduce the specific heat consumption of the evaporator.

specific heat consumption of the evaporator.

openings in the bottom or intersection walls of the stages, and a gradual drop in saturation temperature takes place resulting in a partial evaporation of the liquid in each flash chamber. The flash vapour flows through the water-steam separators and finally condenses on the condenser surfaces, which are cooled by the colder incoming feed. The distillate is collected at the bottom of each stage and cascades down in the same way as the liquid in the flash chambers to the next stage. The plant has to be completed with pumps for the removal of the concentrated liquid or brine and distillate out of the coldest stage and for the feed supply. Dissolved gases and in-leaking non-condensables are removed from the feed by a vacuum line connected to a vacuum pump. The installation of the great length of horizontal condenser tubes in a conventional MSF requires the installation of several vessels in series.

The principle of the MSF/FBE is not much different from a conventional MSF, although, we have already shown that the total length of the vertical condenser tubes passing through all stages can be much shorter for an MSF/FBE than for a conventional MSF. This makes it possible to install all condenser tubes and flash chambers in only one vessel of limited height as is shown in Fig. 10.

Fig. 10. Principle MSF / FBE.

openings in the bottom or intersection walls of the stages, and a gradual drop in saturation temperature takes place resulting in a partial evaporation of the liquid in each flash chamber. The flash vapour flows through the water-steam separators and finally condenses on the condenser surfaces, which are cooled by the colder incoming feed. The distillate is collected at the bottom of each stage and cascades down in the same way as the liquid in the flash chambers to the next stage. The plant has to be completed with pumps for the removal of the concentrated liquid or brine and distillate out of the coldest stage and for the feed supply. Dissolved gases and in-leaking non-condensables are removed from the feed by a vacuum line connected to a vacuum pump. The installation of the great length of horizontal condenser tubes in a conventional MSF requires the installation of several vessels in series. The principle of the MSF/FBE is not much different from a conventional MSF, although, we have already shown that the total length of the vertical condenser tubes passing through all stages can be much shorter for an MSF/FBE than for a conventional MSF. This makes it possible to install all condenser tubes and flash chambers in only one vessel of limited

**Steam**

Distillate (interstage)

**Condensate**

Outlet channel

Recovery section (fluidised bed (FB) condensers in series)

Inlet channel

height as is shown in Fig. 10.

**Discharge (Brine)**

Flash chambers in series

Final fluidised bed (FB) heater

Stage - 1

Stage - n

**Distillate**

Fig. 10. Principle MSF / FBE.

**Feed (Natural seawater)**

As the result of the vertical layout of the MSF/FBE and the flashing down flow in the flash chambers with a height for each chamber of approx. 0.4 m, we are able to add a number of interesting improvements to the MSF/FBE in comparison with the conventional MSF, such as:


Above, we have clearly shown that *the vertical layout of the MSF/FBE*, as the result of the integration of the vertical stationary fluidised bed condenser with the flash chambers, increases the advantages of this evaporator too such an extent, that this evaporator may be considered as the most promising tool for thermal seawater desalination in the future. Fig. 11 shows an MSF/FBE demonstration plant operating on natural seawater for a distillate production of 500 m³/d.

For more information about this fascinating evaporator, one is referred to the Ref. [8] and [9].

Self-Cleaning Fluidised Bed Heat Exchangers

for over a year without any operating problems:

required every 10 weeks for a costly cleaning operation.

recovery column.

fouling.

for Severely Fouling Liquids and Their Impact on Process Design 569

(downcomer) tubes according to the design shown in Fig. 4. The process liquid circulated at a constant flow of 160 m³/h, is raised from about 120 to 150 °C with condensing steam at the shell-side. Back pressure is maintained on the process side of the exchanger to prevent vaporisation which would interfere with the fluidisation of the particles. Upon discharge from the exchanger, the heated liquid flashes across a control valve into the base of the

Comments by the operators in September 1992 after the heat exchanger had been in service

There have been no shutdowns for cleaning tubes and no process upsets, and maintenance has been nil. This is a significant cost cutting result from the higher recovery of acetic acid and the more concentrated residue in the bottoms. The self-cleaning fluidised bed heat exchanger appears capable for at least a full year between turnarounds. A sample of chopped metal wire particles taken from the unit after several months of operation indicated only normal rounding off of the edges. Under the new system, the reboiler circulation rate has been constant, thus providing uniform tower operation and more total throughput. If the alternative falling film evaporator had been installed, a shutdown would have been

Today, July 2011, twenty years after the heat exchanger has been put in service and after 150 000 operating hours, the heat exchanger is still in operation using the same shiny tubes

Fig. 12. Self-cleaning fluidised bed heat exchanger at chemical plant eliminates reboiler

and to entire satisfaction of the operators. For more information, see Ref. [2].

Fig. 11. MSF / FBE evaporator, Isle of Texel.
