Falling film evaporation

Paul Boughton

Johan van der Kamp reveals the most efficient energy optimisation for exothermal processes

As a sector that is constantly searching for sustainable solutions, the oil and gas industry regularly uses waste heat boilers to win back residual heat at high temperatures. Bronswerk Heat Transfer is an expert in this technology and it offers an innovative solution that also allows for heat recovery at low temperatures.

When one of its customers was building a new factory, it was decided to raise the temperature of the process so that the released heat could be usefully applied, that heat now being used to create ‘vacuum steam’. Steam compression then produces a higher pressure and temperature so that the steam can be incorporated in the process cycle. This reduces the process demand for external heat, and much less heat has to be removed by means of cooling towers.

The product of the exothermal process enters at a temperature of 110°C and must be cooled down to 90°C. On one hand, the steam pressure in the steam generator is set as high as possible to achieve the steam condition that requires the least possible driving power from the compressor. On the other hand, excessive steam pressure would produce too small a temperature difference between the heat source and the steam, which would result in a much larger required heat transfer surface so that the heat exchangers would become too expensive. Moreover, the process medium has a high viscosity, which results in the wall heat transfer of this medium to be low. That is why it is important to find an evaporation temperature as close as possible to 90°C, but with sufficient driving force to realise an efficient design in terms of size and price – the search for optimisation. Three solutions were studied to establish how steam can be generated in the most efficient way.

The first is the Thermosyphon reboiler. This is a vertical heat exchanger with the viscous process medium on the outside of the pipes, and the water/steam mixture inside. Alongside the vertical heat exchanger there is also a separation tank. The water level in the tank is kept above the top of the pipe plate of the steam generator, and because the tank and the heat exchanger function as communicating vessels the pipes are filled with water. The water in the pipes evaporates due to the heat supplied by the hot medium, so that the weight of the water column in the pipes reduces, causing it to rise. The mixture of steam and water flows into the separation tank, the water flows downward and the steam flows out through a connection at the top of the tank. 

The problem with this principle in this situation is ‘boiling point suppression’. For example, take an evaporation temperature of 84°C with a corresponding evaporation pressure of 0.556bar. The vertical pipe length is 6m. This 6m of extra water column produces a pressure at the bottom of the pipes of approx. 1.156bar, with a corresponding evaporation temperature of 113°C! As a consequence, the water does not boil in a large part of the pipes, but only though a low heat transfer coefficient. This adverse boiling suppression gives rise to a far from optimal heat transfer process.

The second solution assessed was a Kettle type reboiler. The advantage of this approach is that no individual separation tank is necessary. The shell has a much larger diameter than the bundle, creating an area of steam above the bundle with natural separation of steam and water. The horizontal body includes a water level that is maintained at a few centimetres above the pipe bundle. 

Here, too, the consequence is boiling point suppression. Due to the level of liquid, the water pressure in the bottom pipes of the bundle is much higher than the 0.556bar, with the result that the water here does not evaporate. The highly viscous medium now runs through the pipes and would lead to a very low heat transfer coefficient because of the low Reynolds number. The use of turbulence promoters in the pipes somewhat eases this problem – but not enough to make it a viable solution for the factory. 

Falling film evaporation was the final solution explored. This creates a situation in which no liquid level is maintained over the bundle and no boiling suppression can take place. It also involves a vertical heat exchanger with evaporating water in the pipes. This condensate flows downward from the top of the pipes as a thin film. On the way down, part of the water evaporates and is led upward as steam. Due to the thin water film, the heat transfer coefficient to the water is very high. The non-evaporated water exits the pipes at the bottom and falls into the bottom tank. A fixed water level is maintained in the tank; this level controls the supply of fresh condensate. From this bottom tank, excess water is led to the top of the pipe plate, where a special header ensures that every pipe is supplied with sufficient water and that this water is evenly distributed over the pipe wall. 

The solution is a perfect example of simplicity and robustness. The decision to use steam generators based on the falling film principle results in equipment with the highest possible steam pressure. This is the best possible economic balance between CAPEX and OPEX.

For more information visit www.engineerlive.com/iog

Johan van der Kamp is with Bronswerk Heat Transfer

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