**6. Existing conventional severely fouling heat exchangers revamped into a self-cleaning fluidised bed configuration**

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 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, but also for very complex industrial processes.

## **6.1 Reboiler**

572 Heat Exchangers – Basics Design Applications

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.

> **Unit Conventional heat exchanger**

length mm 25.4×1.65 / 12 <sup>000</sup> 31.75×1.65 /

Total number of heat

Tube diameter / tube

Number of cleanings per

exchangers, state-of-the-art 1998 and 2005.

year

Total heat transfer surface m² 24 000 4 600 3 332

Shell-side baffle type - segmented cross segmented cross EM Total weight particles kg n.a. 36 000 20 000 Pumping power kW 2 100 840 416

Table 3. Comparison conventional heat exchanger versus self-cleaning fluidised bed heat

exchangers - 24 × 1 000 m² 4 × 1 150 m² 4 × 833 m² Configuration - 3 × 50 % 2 × 50 % 2 × 50 %

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

> **Self-cleaning heat exchanger (1998)**


**Self-cleaning heat exchanger (2005)** 

<sup>16</sup> <sup>000</sup>15.88×1.21 / 8 <sup>700</sup>

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 summarised as follows:


Self-Cleaning Fluidised Bed Heat Exchangers

Throttle plate

Column

Liquid

Vapour

Liquid

Reboiler

Inlet channel with distribution system (new)

Circulation pump

production in approximately six months.

discussed in this paragraph can be found in Ref. [7]

Fig. 17. Conventional evaporator revamped into a self-cleaning configuration.

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

Calculations have shown that the investments necessary for the modification of the existing installation into a self-cleaning configuration will be paid back by a substantially increased

Information about the reboiler, the cooling crystallisation plant and other applications

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

Existing inlet channel

Recirculating liquid

for Severely Fouling Liquids and Their Impact on Process Design 575

Liquid

Particles (new)

Steam

Separator (new)

Existing outlet channel with new internal

Liquid + particles

Condensate

Downcomer (new)

Control channel (new)

Fig. 16. Evaporator equipped with conventional heat exchanger.


The advantages of most revamps are much lower maintenance cost, an increased production and 'smoother' operation.

## **6.2 Cooling crystallisation plant**

A 2-stage cooling crystallisation plant in Egypt produces Sodium Sulphate. The chillers of both stages suffer from severe fouling caused by heavy deposits of crystals. Shutdown of the installation every 24 hours for melting out these deposits is common. The conventional cooling crystalliser is shown in Fig. 18, while Fig. 19 depicts this installation after its revamp into a self-cleaning configuration.

Reboiler

Circulation pump

position.

and 'smoother' operation.

**6.2 Cooling crystallisation plant** 

into a self-cleaning configuration.

Fig. 16. Evaporator equipped with conventional heat exchanger.

4. As many components of the existing installation should be used in the revamped

5. The revamp must be carried out within the available space. This often means that a revamp can only be carried out when the existing installation has already a vertical

The advantages of most revamps are much lower maintenance cost, an increased production

A 2-stage cooling crystallisation plant in Egypt produces Sodium Sulphate. The chillers of both stages suffer from severe fouling caused by heavy deposits of crystals. Shutdown of the installation every 24 hours for melting out these deposits is common. The conventional cooling crystalliser is shown in Fig. 18, while Fig. 19 depicts this installation after its revamp

configuration like bundle, channels or maybe even modified channels.

Recirculated liquid

Throttle plate

Column

Vapour

Liquid

Steam

Outlet channel

Condensate

Inlet channel

Fig. 17. Conventional evaporator revamped into a self-cleaning configuration.

Calculations have shown that the investments necessary for the modification of the existing installation into a self-cleaning configuration will be paid back by a substantially increased production in approximately six months.

Information about the reboiler, the cooling crystallisation plant and other applications discussed in this paragraph can be found in Ref. [7]
