**5.2 Reboiler at chemical plant; annual turnaround replaces cleaning every 4 to 5 days**

A steam-heated evaporation system at a chemical plant in the Unites States recovers a volatile organic from a heavy organic solution laden with foulants. A hard black scale, that was forming in the upper 25 % of the tubes, was forcing the plant to switch two parallel once through rising film evaporators with a clean pair every four to five days.

When asked to increase throughput and simplify operations engineers considered installing a 190 m² falling film evaporator to operate in series with the existing rising film evaporators. Although the combination system was expected to run approximately 10 weeks between cleanings, a better solution was needed and found when engineers heard of an innovative self-cleaning fluidised bed heat exchanger technology being used at another chemical plant in the United States. The final decision in favour of the self-cleaning fluidised bed heat exchanger was made after the engineers viewed a 1 m tall, transparent desktop demonstration unit with six 12 mm up flow tubes and one 12 mm down flow (downcomer) tube.

The full-size exchanger with widened outlet channel shown in Fig. 12 at the right of the distillation column contains 73 m² of heat transfer surface. It applies internal circulation of 2.0 mm chopped stainless steel wire particles and uses 51 up flow and four down flow

**5.2 Reboiler at chemical plant; annual turnaround replaces cleaning every 4 to 5 days**  A steam-heated evaporation system at a chemical plant in the Unites States recovers a volatile organic from a heavy organic solution laden with foulants. A hard black scale, that was forming in the upper 25 % of the tubes, was forcing the plant to switch two parallel

When asked to increase throughput and simplify operations engineers considered installing a 190 m² falling film evaporator to operate in series with the existing rising film evaporators. Although the combination system was expected to run approximately 10 weeks between cleanings, a better solution was needed and found when engineers heard of an innovative self-cleaning fluidised bed heat exchanger technology being used at another chemical plant in the United States. The final decision in favour of the self-cleaning fluidised bed heat exchanger was made after the engineers viewed a 1 m tall, transparent desktop demonstration unit with six 12 mm up flow tubes and one 12 mm down flow (downcomer)

The full-size exchanger with widened outlet channel shown in Fig. 12 at the right of the distillation column contains 73 m² of heat transfer surface. It applies internal circulation of 2.0 mm chopped stainless steel wire particles and uses 51 up flow and four down flow

once through rising film evaporators with a clean pair every four to five days.

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

tube.

(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 recovery column.

Comments by the operators in September 1992 after the heat exchanger had been in service for over a year without any operating problems:

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 required every 10 weeks for a costly cleaning operation.

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

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

Self-Cleaning Fluidised Bed Heat Exchangers

Total number of heat

Number of cleanings per year

exchanger.

surface.

for Severely Fouling Liquids and Their Impact on Process Design 571

Heat transfer surface m² 24 000 4 600

**Unit Conventional** 

exchangers - 24 × 1 000 m² 4 × 1 150 m²

Configuration - 3 × 50 % 2 × 50 %

Pumping power kW 2 100 840

Table 2. Comparison conventional heat exchanger versus self-cleaning fluidised bed heat

Fig. 14. Installation of 4 600 m² self-cleaning surface replacing 24 000 m² conventional

exchanger uses 9 000 kg cut metal wire particles with a diameter of 1.6 mm.

Fig. 14 shows the installation which serves two parallel production lines. In each production line two identical self-cleaning fluidised bed heat exchangers were installed handling 2 × 700 m³/h process liquid at the tube-side and 2× 2 100 m³/h cooling water at the shellside. Each exchanger applies external circulation of the particles as shown in Fig. 5, has a shell diameter of 1 200 mm, a total height of 20 m and a heat transfer surface of 1 150 m², which surface consists of 700 parallel tubes with an outer diameter of 31.75 mm. Each

The exchangers serving the first production line were put into operation in October 1998. Fig. 15 presents the trend of the overall heat transfer coefficient (k-value) after start-up till the end of April 1999. In spite of some fluctuations at the beginning, this figure shows a constant k-value of approximately 2 000 W/(m²·K). During a period of more than six months both exchangers operated continuously, with exception of a few short sops caused by interruptions in the power supply. The dotted line in Fig. 15 shows the trend of the k-

**heat exchanger** 


**Self-cleaning heat exchanger** 

#### **5.3 Quench coolers at chemical plant; the real breakthrough of the self-cleaning fluidised bed heat exchange technology**

A chemical plant in the United States cooled large quench water flows from a proprietary process in open cooling towers. This quench water released volatile organic compounds (VOCs) into atmosphere. As a consequence of environmental regulations the quench water cycle had to be closed by installing heat exchangers between the quench water and the cooling water from the cooling towers.

In August 1997, after considering other solutions using conventional shell and tube heat exchangers, plant management decided to carry out a test with a small self-cleaning fluidised bed heat exchanger and compared its performance with that of a conventional shell and tube heat exchanger, which suffered from a severe fouling deposit consisting of a tarry substance. Fig. 13 shows the results of this test, while Table 2 compares the design consequences for the self-cleaning heat exchangers and the conventional shell and tube exchangers. Plant management decided in favour of the self-cleaning fluidised bed heat exchange technology because of the above results and the dramatic savings on investment and operating cost.

Fig. 13. Overall heat transfer coefficient (k-value) and pressure drop (∆p) as function of operating time.

A chemical plant in the United States cooled large quench water flows from a proprietary process in open cooling towers. This quench water released volatile organic compounds (VOCs) into atmosphere. As a consequence of environmental regulations the quench water cycle had to be closed by installing heat exchangers between the quench water

In August 1997, after considering other solutions using conventional shell and tube heat exchangers, plant management decided to carry out a test with a small self-cleaning fluidised bed heat exchanger and compared its performance with that of a conventional shell and tube heat exchanger, which suffered from a severe fouling deposit consisting of a tarry substance. Fig. 13 shows the results of this test, while Table 2 compares the design consequences for the self-cleaning heat exchangers and the conventional shell and tube exchangers. Plant management decided in favour of the self-cleaning fluidised bed heat exchange technology because of the above results and the dramatic savings on investment

Fig. 13. Overall heat transfer coefficient (k-value) and pressure drop (∆p) as function of

**5.3 Quench coolers at chemical plant; the real breakthrough of the self-cleaning** 

**fluidised bed heat exchange technology** 

and the cooling water from the cooling towers.

and operating cost.

operating time.


Table 2. Comparison conventional heat exchanger versus self-cleaning fluidised bed heat exchanger.

Fig. 14. Installation of 4 600 m² self-cleaning surface replacing 24 000 m² conventional surface.

Fig. 14 shows the installation which serves two parallel production lines. In each production line two identical self-cleaning fluidised bed heat exchangers were installed handling 2 × 700 m³/h process liquid at the tube-side and 2× 2 100 m³/h cooling water at the shellside. Each exchanger applies external circulation of the particles as shown in Fig. 5, has a shell diameter of 1 200 mm, a total height of 20 m and a heat transfer surface of 1 150 m², which surface consists of 700 parallel tubes with an outer diameter of 31.75 mm. Each exchanger uses 9 000 kg cut metal wire particles with a diameter of 1.6 mm.

The exchangers serving the first production line were put into operation in October 1998. Fig. 15 presents the trend of the overall heat transfer coefficient (k-value) after start-up till the end of April 1999. In spite of some fluctuations at the beginning, this figure shows a constant k-value of approximately 2 000 W/(m²·K). During a period of more than six months both exchangers operated continuously, with exception of a few short sops caused by interruptions in the power supply. The dotted line in Fig. 15 shows the trend of the k-

Self-Cleaning Fluidised Bed Heat Exchangers

application, one is referred to Ref. [3].

**self-cleaning fluidised bed configuration** 

but also for very complex industrial processes.

flow, temperatures and liquid velocity in the tubes. 2. The connections to the column should be maintained.

than for the conventional severe fouling shell and tube exchanger.

Ref. [4].

**6.1 Reboiler** 

summarised as follows:

for Severely Fouling Liquids and Their Impact on Process Design 573

open. The cut metal wire particles showed a slight weight loss caused by rounding-off effects as discussed earlier. For more information about these fascinating heat exchange

In the first years of the new millennium, research and development concentrated on reducing the tube diameter of the self-cleaning fluidised bed heat exchangers in combination with rather large particles. A smaller tube diameter reduces the length of the heat exchanger tubes which creates a more compact heat exchanger with less height and, consequently, reduces the pumping power required for the process liquid. Then, we also paid attention to the installation of a novel type of baffle in the shell of the exchanger. This very innovative baffle is called the EM baffle and has been developed by Shell Global Solutions. The results of this redesign of the self-cleaning fluidised bed heat exchangers shown in Fig. 14 as far as heat transfer surface and pumping power are considered are presented in Table 3. For more information about this improved design, one is referred to

**6. Existing conventional severely fouling heat exchangers revamped into a** 

with the preferred type of flow for the self-cleaning fluidised bed heat exchanger.

Moreover, also in this paragraph, it will be shown that this approach of introducing the selfcleaning heat exchange technology in existing plants could not only be an attractive solution for straight forward rather simple heat exchange applications suffering from severe fouling,

A typical example of a conventional reboiler that is suitable for revamping is shown in Fig. 16 with the revamped configuration shown in Fig. 17. Generally speaking, the requirements specified by plant management for the majority of revamps can be

1. The same process conditions should be maintained as in the original installation, i.e.

3. The installed pumps should be used and can be used because pumping power requirements are generally lower for the self-cleaning fluidised bed heat exchangers

The idea of changing internal circulation of particles as shown in Fig. 4 into the configuration where this circulation takes place through only one external downcomer shown in Fig. 5 was proposed by engineers of Shell in the early 90s. According to these engineers, this modification would make it possible to revamp existing vertical severely fouling conventionally designed reboilers into a self-cleaning configuration. Moreover, it would be an elegant and rather low cost but also a low risk approach to introduce a new technology due to the possibility of an immediate fallback from new technology to old proven technology. This idea is not only applicable for reboilers but also for evaporators and crystallisers and the constant circulating flow required by these unit operations corresponds

Fig. 15. k-values for self-cleaning heat exchangers of first production line, as a function of operating time and compared with the performance of conventional heat exchanger.

value for conventional shell and tube exchangers as derived from the test results shown in Fig. 13. The two exchangers of the second production line were put in operation in May 1999 and showed the same trend in k-value as the exchangers of the first production line. In December 2000, this chemical plant stopped production and the exchangers, after a final inspection, were mothballed and have never been put into operation again. This final inspection did not reveal any measurable wear of the tubes. All tubes were shiny and


Table 3. Comparison conventional heat exchanger versus self-cleaning fluidised bed heat exchangers, state-of-the-art 1998 and 2005.

open. The cut metal wire particles showed a slight weight loss caused by rounding-off effects as discussed earlier. For more information about these fascinating heat exchange application, one is referred to Ref. [3].

In the first years of the new millennium, research and development concentrated on reducing the tube diameter of the self-cleaning fluidised bed heat exchangers in combination with rather large particles. A smaller tube diameter reduces the length of the heat exchanger tubes which creates a more compact heat exchanger with less height and, consequently, reduces the pumping power required for the process liquid. Then, we also paid attention to the installation of a novel type of baffle in the shell of the exchanger. This very innovative baffle is called the EM baffle and has been developed by Shell Global Solutions. The results of this redesign of the self-cleaning fluidised bed heat exchangers shown in Fig. 14 as far as heat transfer surface and pumping power are considered are presented in Table 3. For more information about this improved design, one is referred to Ref. [4].
