(271d) Increased Operational Performance of Thermosiphon Reboilers through Turbulence Promotors and Structured Tubes

Vertical thermosiphon reboilers (TSR) with tube-side boiling are widely used in the refining, petrochemical and process industry as effective, reliable and economic column reboiler and single stage evaporator. Their pumpless operation not only reduces investment and operational costs, TSRxs are also preferrentially employed for components sensible to mechanical stress, e.g. monomers with a tendency to polymerize when pumped repeatedly (styrene, acrylic acid, acrylates) or polymers with a tendency to crack.

Their operation is based on an equilibrium of heat transfer and fluiddynamic conditions. This equilibrium is established through the continuous partial evaporation of fluid flowing through the tubes thus generating a two phase flow with lower overall density than the feed fluid. The circulation rate balances heat transfer intensity and pressure drop. Due to the hydrostatic suppression of boiling at the lower tube inlet single phase heat transfer is found in the sensible heating zone. After reaching saturation temperature at the local pressure conditions boiling starts. In the subsequent boiling zone the bulk fluid temperature drops for single components as well as for tight-boiling mixtures due to the reduction of hydrostatic pressure. For wide boiling mixtures a continuous increase in boiling temperature may be found due to the concentration of high-boiling components. This operation characteristics requires a minimum driving temperature difference ΔT_min between heating and evaporation side which limits the application of this equipment type. The actual value of ΔT_min is influenced by the operating pressure, product viscosity and composition, apparent liquid head in the column and further parameters. In fatal cases circulation breaks down resulting in a non-operability and potential product damages.

The influence of turbulence promotors and structured tubes on heat transfer and operational behavior of tubular thermosiphon reboilers was investigated experimentally as well as theoretically. The experiments were performed at a single tube with geometry do x s x l = 20 x 2 x 1500 mm³. Water and a water/glycerol mixture were used to represent column reboiler as well as evaporator functionality. Condensation of water or organic vapors was used for heating. Temperature difference between heating and evaporation side, apparent liquid head and total pressure were varied in a wide range. Fluiddynamic as well as heat transfer characteristics were monitored for the plain stainless steel tube, the tube equipped with a hiTRAN® insert as well as a structured copper tube.

For water the application of the hiTRAN® insert reduces the circulating flow by about 40 % while especially for low driving temperature differences a significant stabilisation of the circulation is achieved. No enhancement of heat transfer could be found. For the glycerol/water mixture heat transfer is enhanced up to 15 % especially at large values of the apparent liquid head. For small driving temperature differences a stable and finite circulation is found for the insert while circulation breaks down for the plain tube. For the structured copper tube a significant increase in heat transfer was found allowing a reduction of the required ΔT_min down to 8 K for the water/glycerol mixture. Based on the experimental results and their theoretical evaluation advantageous applications for turbulence promoters and structured tubes may be identified. This broadens the window for potential applications of thermosiphon reboilers in a variety of processes.