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Intensified and high performances thermosyphon reboilers : application to the oil and gas processing industries

Exploitable results

In the frame of two EU projects (JOU2-CT94-0362, JOE-CT97-0061) various enhanced heat exchanger tubes with structured surfaces were tested in the pool-boiling mode with propane as working fluid. The experiments were carried out with single tubes and two tubes inline. Saturation temperatures in the range from 253K to 293K and heat fluxes up to 140kW/m² were employed. The experimental results are summarised in a databank, containing the raw data, diagrams, comparisons with correlations from the literature and a correlation generated at IKE, videos to visualise the boiling phenomena and publications of IKE published in the frame of these projects. All these information and results from the project partners were used to identify the best surface structure for the design of an industrial heat exchanger prototype. A compact heat exchanger was manufactured and works now successfully at a petrochemical plant of an industrial partner of the second project. The knowledge gain is this project will benefit to the scientific and engineering communities. The experimental results of the best tested surface show improvements of the heat transfer coefficient of factors up to 3 compared with the results of a smooth reference tube. These improvements were found for heat fluxes in the range up to 50 kW/m². For higher heat fluxes the slope of the heat transfer coefficient of the structured surfaces decreases, caused by a partial deactivation of the subsurface structures by vapour which cannot leave the internal channel fast enough through the small openings to the surrounding liquid. In general, the structured surfaces show impressive improvements in heat transfer, especially in the heat flux ranges (10kW/m² to 50kW/m²), which are relevant for industrial applications.
A new design method for reboilers, with simulation of flow and heat transfer in the bundle, has been implemented in the solver of the handbook software. The efficiency of model and numerical method of the solver has been tested by comparisons with experimental data from the literature and calculations with a two-phase flow three-dimensional software. Compared with existing tools, this solver better calculates the recirculating flow in the bundle and so, the heat transfer coefficient distribution. Modelling has been realised a three stages: - Local modelling of two-phase flow across tube bundles. - 2D modelling of kettle reboilers. - 1D model for design of reboilers. These models have been developed and validated within this project. The 2D model allows analysis of new design of kettle reboilers and is unique. This model has been used to simulate industrial reboilers of large size such as those used in LNG plants. For the 1D model, a user interface has been realised and the software is available on a CD. These models will be implemented in the Greth technical handbook, which is provided to the Greth members (115 companies).
A new enhanced boiling tube for the process industry has been developed. This new tube has heat transfer performance that is up to three times higher than a comparable plain tube. Through a complete set of testing: single tube, pilot unit 1m² and full scale unit 360 m², the heat transfer performance has been measured and checked using hydrocarbons as process fluids. Tests have been performed to check the manufacturing capacity of these new enhanced boiling tubes. Thereby it is confirmed that the tube structure can be fabricated with the given tools and that the tube dimensions required for industrial units can be achieved (10 m long U-bend tubes). This new boiling tube can be used for retrofit and new design of reboilers with clean hydrocarbons. New applications using mixtures will be evaluated.