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Powerful revamp

Revamping fouling, conventional heat exchangers as a self-cleaning configuration
Powerful revamp

The introduction of self-cleaning, fluidised-bed heat exchange technology can benefit processes where conventional heat exchangers suffer from severe fouling. As an alternative to supplying new, self-cleaning exchangers, Klaren BV engineers investigate the option of revamping fouling, conventional exchangers as a self-cleaning configuration. Modern, self-cleaning heat exchanger technology has widened the scope for such revamps by solving a number of bottlenecks which in the past presented insurmountable problems.

Dick G. Klaren, Eric F. de Boer and Douglas W. Sullivan

Introducing a relatively new technology is still difficult to accomplish in the chemical process industries. It invariably involves certain risks for operation and production in processing plant. Retrofitting existing exchangers with self-cleaning technology is consequently a less costly avenue for establishing self-cleaning exchanger technology. Research and development work has been carried out to discover means of overcoming former limitations on retrofitting and allowing the use of existing assets to upgrade conventional, severely fouling heat exchangers to a self-cleaning configuration (Figure 1).
The original idea to revamp a conventional heat exchanger as a self-cleaning configuration came from a chemical plant in Europe, which operated a forced-circulation boiler that suffered from very severe fouling. The Klaren engineers were asked by the plant management to suggest a solution for this reboiler. Revamping the existing reboiler, which operated at a liquid velocity of 1.2 m/s, as a self-cleaning configuration with the same liquid velocity was an appealing challenge. The management appreciated the fact that the cleaning particles could be removed from the exchanger while in operation. If the revamp did not totally solve the fouling problem, the exchanger could revert to operating in the same way as before the retrofit without having to be shut down. The operator could switch between the new and old technologies without interrupting operation.
The plant management stipulated that the retrofit work should be minimal. The installed pump was to be re-used and the connection between the reboiler and the column maintained. An elegant design proposal was submitted that met all the design criteria and is shown in Figure 2. This design employs a widened outlet channel to disengage cleaning particles from the liquid. The outlet channel is connected to an external downcomer for recirculating the particles. At the bottom of the downcomer, particles pass through the control channel into the inlet channel. For this particular application, the cleaning particles consisted of chopped stainless steel wire with a diameter of 2 mm.
Although the plant was shut down before the revamp could be carried out, this example demonstrated the huge potential for revamping. It is evident that installations which operate at constant flow are best suited for revamping as a self-cleaning, fluidised-bed heat exchanger. Since these heat exchangers prefer constant flow conditions, the existing, severely fouling heat exchangers in forced-circulation evaporators, reboilers and crystallisers represent an interesting area for this kind of revamp.
The most important conditions for a successful revamp include the following:
  • The same process conditions must be maintained for the liquid velocities in the tubes as well as the inlet and outlet temperatures.
  • The connections to columns or vessels must be re-used.
  • The installed pumps should be re-used.
  • The existing inlet and outlet channels should be re-used where possible.
  • The revamp should be carried out within the available space.
  • A fallback should be possible from the new technology to the old technology.
Consequences and improvements
In the old days, when revamping with self-cleaning, fluidised-bed heat exchange technology was investigated by retrofitting into existing, severely fouling, conventional heat exchangers, the following problems were liable to occur:
  • The tube diameter of the existing conventional heat exchanger and its multi-pass tube-side design was too small; it was therefore not the preferred choice for the revamped installation.
  • The ratio of the internal tube diameter to the particle diameter, i.e. Di/dp, was too small for the type of particle considered and responsible for plugged tubes.
  • The liquid velocities in the tubes of the existing, conventional heat exchangers were too high.
However, once the difficulties had been identified, R&D work was performed to solve or at least mitigate the problems associated with retrofitting. Today, thanks to considerable R&D efforts, these problems have been overcome. The new, self-cleaning heat exchange technology can now be applied independently of the tube diameter, the liquid velocity in the tubes and the liquid viscosity, and even allows a Di/dp ratio <8 when using stainless steel, chopped-wire particles. Other improvements include the following:
  • Improved particle separator
  • Modified and improved external downcomer and control channel
  • Extremely compact, self-cleaning strainer, which can be installed in the inlet channel of the self-cleaning, fluidised-bed heat exchanger.
The compact self-cleaning strainer serves a particular purpose. Any drop in the heat transfer coefficient points to fouling of the heat exchanger tubes. Yet fouling of the heat exchanger tubes can have two completely different causes, i.e. scaling of the tube walls on the one hand, which can be handled and solved with self-cleaning, fluidised-bed heat exchange technology thanks to the scouring action of the circulating particles on the tube wall, and plugging of the distribution plate on the other; the latter occurs in the inlet channel due to large pieces of scale breaking loose from the walls of piping and vessels upstream of the heat exchanger and being carried by the flow entering the exchanger. The interruption of the flow at the distribution plate can cause an uneven distribution of particles in the tubes, leading to inadequate cleaning of all tubes. The above-mentioned compact strainer should prevent the distribution plate of a revamped heat exchanger from plugging.
Candidates for a potential revamp
A number of plants with severely fouling heat exchangers are currently considering the revamp option. The company names are withheld for confidentiality reasons, but the following processes are potential candidates. The first example concerns a cooling crystallisation plant using evaporating ammonia. This plant for sodium sulphate production has four horizontal, kettle-type, shell-and-tube heat exchangers with two passes on the tube side and evaporating (pool boiling) ammonia in the shell. Severe fouling of the exchangers due to the precipitation of Glauber’s salt means cleaning is essential every four days, resulting in production losses, excessive maintenance costs and high energy consumption.
The proposed revamp shown in Figure 4 looks quite different from the existing installation. As a part of the proposal, the revamped exchanger will be vertical rather than horizontal with pool boiling in the shell. The proposal additionally includes an evaporating falling film on the shell side in combination with a single-pass design on the tube side. This labour-intensive revamp is justified because the tubes are made of expensive Hastelloy C and local wage costs are relatively inexpensive. A further advantage of the evaporating falling film design compared to pool boiling is the reduction in the volume of ammonia in the shell by a factor of 10. Furthermore, the revamp uses glass spheres with a diameter of 4 mm in heat exchange tubes with an O.D. of 38 mm, a liquid velocity in the tubes of approximately 0.75 m/s, a screen-type separator and a fast-running downcomer. Maximum use is additionally made of existing components when it comes to the high-flow/low-head pump. Supplementary information from the client will determine whether a self-cleaning strainer is required in the inlet channel.
Reboiler in nickel refining plant
A nickel refining plant operates a reboiler, also referred to as copper boil. The existing installation has tubes with an O.D. of 25 mm and a liquid velocity in the tubes of 2.4 m/s. In spite of this high velocity, the heat exchanger suffers from severe fouling due to scale formation on the tubes as well as to tube plugging as a result of pieces of dirt or broken loose scale from upstream equipment that is entrained with the flow. The connecting line between the outlet channel of the heat exchanger and the flash column is long and winding.
A few years ago, a test with a single-tube, self-cleaning, fluidised-bed heat exchanger convincingly demonstrated that fouling due to the precipitation of scale on the tube walls can be eliminated with a self-cleaning exchanger. It should therefore not be a problem for the revamped, full-size heat exchanger. The possibility of tubes being plugged by large pieces of scale entrained with the flow and entering the exchanger still exists, however.
Figure 5 shows the existing installation after being revamped as a self-cleaning configuration. The existing outlet channel was re-used by installing an internal channel that directs the flow of liquid and particles into the separator. The liquid flow from the separator is returned to the outlet channel and from there into the column, using the same connection.
In this installation, only limited space is available for installing the self-cleaning strainer between the outlet of the large circulation pump and the inlet of the tube bundle. An altogether different solution is suggested for removing these pieces of scale. Figure 5 also shows the method planned for removing pieces of scale larger than 3 mm in a 2-stage separator, consisting of a self-cleaning strainer and a cyclone that is installed in series and placed on the suction side of the circulation pump. The existing pump can provide the low, additional pressure drop created by our 2-stage separator.
Evaporator for viscous slurry
A production plant for a proprietary product operates a very large mechanical vapour recompression (MVR) evaporator for concentrating slurry to approximately 70 % solids. Even at a temperature of 100 °C, this slurry – which exhibits non-Newtonian behaviour – has a very high viscosity that varies between 50 and more than 200 cP. This extremely large shell-and-tube heat exchanger suffers from severe fouling, which sometimes requires one month of cleaning after only three months of operation.
The first series of experiments has been promising. The shear-thinning effects caused by the increased turbulence of the slurry, which is induced by the action of the fluidised particles, reduce the slurry’s viscosity and produce heat transfer coefficients between 1000 and 2000 W/(m²·K), depending on the slurry concentration. These coefficients can be compared with the clean heat transfer coefficients of approximately 600 W/(m²·K) for the conventional heat exchanger, which is fouled to a fraction of its clean value in only a couple of months. The revamp proposal for the full-size installation shown in Figure 6 incorporates the large existing components, including the tube bundle and circulation pump. This potential revamp integrates all the benefits of recent developments and allows relatively large stainless steel particles (2.5 mm) to be used in even smaller-diameter tubes (I.D. only 20 mm) together with very high slurry viscosities.
Online information www.cpp-net.com/2110###
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