DSM was planning a new ammonium sulfate crystallization plant. Waste heat was available from the nearby Caprolactam plant. Waste heat from this plant needed to be upgraded to a usable temperature by using Mechanical Vapor Recompression (MVR) heat pump technology.
The challenge was to generate subatmospheric steam at this large scale. Bronswerk Heat Transfer designed and supplied falling film evaporators that made it possible to capture the waste heat in steam and upgrade to usable temperature level for use in the ammonium sulfate crystallization.
In order to apply a vapor recompression the waste heat of the Caprolactam process is Sulfuric acid that can be cooled from 110°C to 90°C. With this waste heat the customer wants to generate 0.58 bar(a) steam. Classically this would be done in Kettle Type Steam Generators, however in these vacuum conditions the liquid level will suppress the boiling. This leads to very low heat transfer coefficient and large surface area required, making the project impossible to execute.
Bronswerk Heat Transfer was consulted whether they could design a Heat Exchanger that met the project requirements with respect to size, cost and operability. We found that a Kettle Type Reboiler would require too large surface area and we needed to move away from having a high liquid level in this kind of application.
The solution we designed takes away the liquid level by applying the falling film principle. Steam is generated inside the tubes by applying only a thin film of water on the inside of the tubes. The boiler feed water recirculates with a certain ratio to ensure continuous wetting of the tubes. The complete water circulation control and distribution is integrated into the heat exchanger design.
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