Heat integrated distillation using a plate-fin heat exchanger

Heat pumps can be used to efficiently reduce the energy requirements in distillation columns by 50-80%. In conventional methods like Vapor Compression (VC) and Vapor Recompression (VRC) the required temperature lift generated by the compressor is determined by the temperature difference between reboiler and condenser. This limits the number of applications as economic temperature lifts are typically restricted to 30 0C. A way to circumvent this limitation is not to recompress the vapor from the top, as in a VRC, but extract the vapor at a lower stage, recompress this vapor and transfer the heat via internal heat exchangers from the rectifier to the stripper stages. <p> Heat integration in distillation columns (HIDiC) by means of a compact plate-fin heat exchanger (PFHE) further reduces the energy consumption because of the low pressure drop and the small minimum approach temperature, which can be in the order of 1 K due to the high specific surface area of 1,000-1,500 m2/m3. In a previous study (Hugill et al, 2007) it was shown how the PF-HIDiC concept can be converted into an industrial application leading to substantial savings in utility and investment cost. <p> For the model system cyclohexane/n-heptane the performance of a 1 meter long PF-HIDiC has been quantified with respect to hydrodynamics, separation and heat transfer. The experimental number of equilibrium stages of 1.6 is lower than 2-2.5 as predicted by the Delft model. However, the main objective, achieving separation in a PF heat exchanger was fulfilled. The heat transfer coefficient was found to be equal for the stripping and the rectifying mode; a value of 145&#177;11 W/m2/K was found, which is 25% lower than the design value. No influence of the operation mode on the separation efficiency was observed. A pressure drop of up to 5 mbar/m was found, which is higher than predicted. Flooding was not observed at any of the process conditions. <p> Special attention was given to the design and manufacturing of the liquid distributor because of the strong effect of maldistribution on the separation efficiency. In a separate set-up the distributor was tested for the model system n-decane/N2. Within its design limits of an F-factor between 1.0 and 2.8 Pa0.5 the maldistribution fraction, fav=0.12&#177;0.03.