Technology
Introduction
Vertical shell and tube heat exchangers applying a fluidized bed of solid particles in the tubes can operate with a clean surface where conventional heat exchangers may suffer from severe fouling in their tubes in a matter of weeks, days or even hours. These solid particles, made of either glass, ceramic or metal (cut metal wire) with diameters of 1.5 to 5 mm, create a mild scouring effect on the wall of the heat exchanger tubes. This scouring action removes the fouling deposits from the tube wall at an early stage of formation and keeps the heat transfer surface clean and thus a constant heat transfer coefficient is maintained. Besides the scouring effect, the particles enhance the heat transfer at lower liquid velocities and reduce pressure drop compared to conventional heat exchangers. Zero-fouling is guaranteed when the rate of removal of deposits by the particles exceeds the rate of precipitation of deposits.
Operating Principle
The operating principle of the self-cleaning fluidized bed heat exchangers is based on the circulation of solid particles through the tubes of a vertical shell and tube heat exchanger. The fouling liquid flows upward through the tube bundle of the heat exchanger that incorporates specially designed inlet and outlet channels. In the inlet channel the solid particles are fed to the fluid using a proprietary distribution system to ensure a uniform division of particles over all the tubes. The particles are fluidized by the upward flow of liquid, where they create the mild scouring effect on the wall of the heat exchanger tubes, thereby removing any deposit at an early stage of fouling formation. After the tube bundle the particles disengage from the liquid in the separator and are returned to the inlet channel through an external down comer, and the cycle is repeated.
To control the amount of particles fed to the inlet, a part of the inlet flow to the heat exchanger is used to push the particles from the down comer into the inlet channel. Changing the amount of particles is one of the parameters to influence the cleaning mechanism. Other parameters are particle size and material and the fluid velocity.
With this self-cleaning heat exchanger many types of fouling deposits can be effectively handled, whether hard or soft, originating from biological, crystallization, chemical or particulate fouling mechanism, or a combination of these. A wide variety of fluids can be handled ranging from aqueous solutions, to oils and slurries.
Video about the Technology
Watch the video presentation about the operating principle of the self-cleaning heat exchanger.
Advantages of the self-cleaning heat exchanger technology
Enhanced productivity
Sustainable
Better energy performance
In a self-cleaning heat exchanger the tubes remain clean and therefore the heat transfer can kept constant which improves the energy performance:
- A shell and tube heat exchanger to cool quench water had a decrease in heat transfer coefficient of 50% within 20 days of operation. With a self-cleaning heat exchanger, the heat transfer coefficient remained constant.
- Because of the lower fluid velocities and shorter tube length in most applications the pumping power required is reduced. In the apllication mentioned above the reduction was more than 50%.
- Applying the self-cleaning fluidized bed technology for an evaporator that concentrates waste water allows to reach higher solid concentrations in the recirculation flow without incurring in fouling problems as compared to falling film evaporators. This results in a higher water recovery and a drastic reduction of the blow down. When adding a spray dryer to come to Zero Liquid Discharge the overall energy consumption of the self-cleaning unit combined with the spray dryer is only 60% of the energy consumption of the falling film unit combined with the spray dryer. Main reason for this is that with the technology of Klaren International the waste water can be further concentrated meaning a smaller volume flow as blow down requiring less energy for spray drying.
- A shell and tube heat exchanger uses water to recover heat from an exothermic reaction. The heat is used elsewhere in the process. Fouling on the chemical component side of the heat exchanger causes a decrease on heat transfer coefficient which results in a decreasing heat recovery capacity. As the fouling layer develops an increasing amount of make-up steam has to be used to further heat the process elsewhere. With a self-cleaning heat exchanger the heat recovery form the exothermic reaction remains constant and no steam has to be used.
Enhanced productivity
A self-cleaning heat exchanger needs not to be taken out of production for cleaning, therefore the capacity remains constant and in some cases the production yield can be increased, which enhances the productivity:
- With fouling of heat exchangers the period between two cleanings can range from a year to 4 hours. In most cases cleaning means a stop in operation with as a consequence a loss of production. If a stop in operation is not allowed, it is usually decided to install redundant equipment. Choosing for redundancy means an additional investment.
- It is known that in case of fouling in crude oil pre-heaters, installed upstream the refinery furnace, the required inlet temperature of the distillation column cannot be met and thus the throughput needs to be reduced to meet the required inlet temperatures. Obviously, reducing the throughput directly affects the production output and the revenues of the refinery. With a self-cleaning heat exchanger there is no downtime for cleaning nor is it required to install redundant equipment and throughput can remain constant.
Sustainable
When using the self-cleaning heat exchanger technology chemicals are not required as online additives or needed for cleaning purposes. Therefore, there are no hazardous waste streams from cleaning which makes the heat exchanger more sustainable:
- All MSF plants for sea water desalination use a chemical additive to suppress fouling or scaling of sea water in the condenser section or the top brine heater. Already in the late 1970’s at the Island of Texel a MSF plant with a vertical orientation has been built and operated that applied a fluidized bed in the tubes of the condenser and final heater and could run without the need of chemicals to prevent scaling.
- PTA coolers face a major issue of fouling. A major PTA producer has to clean the PTA coolers every 6 hours. Cleaning of these PTA coolers is done using chemical solutions i.e. caustic solution (5% w/w caustic @ 80 deg C). After cleaning they have to dispose the caustic solution which contains valuable terephthalic acid crystals. Replacing the existing PTA coolers with the self- cleaning coolers will enable the PTA producer to skip the caustic cleaning of the heat exchanger and thus there will be no disposal of the cleaning solution.
Compact design
Because of a constant heat transfer in the self-cleaning heat exchanger, over dimensioning of the heat transfer surface is not required which results in a more compact design of the heat exchanger:
- With a heat exchanger application known to foul, the required surface of the heat exchanger is normally calculated by adding a fouling factor to the heat transfer coefficient. Fouling factors can easily be 2 to 5 *10-4 m2K/W. Starting with an overall heat transfer coefficient of 2000 W/m2K, the heat transfer over time will drop to 1500 W/m2K or in the worst case to only 1000 W/m2K. So, for the same heat capacity, the heat exchanger needs an overdesign of 30 to 100% to meet the required capacity over time. With the self-cleaning heat exchanger overdesign is not required leading to a more compact design.
- Besides the scouring effect of the particles, the fluidized bed in the fluid stream has an additional effect. The chaotic movement of the particles also breaks down the boundary layer and thereby strongly improves the heat transfer with a factor of around 4 at the same fluid velocity. Because the self-cleaning heat exchanger normally applies lower velocities the heat transfer improves with around 20% compared to that of a regular shell and tube heat exchanger.
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