Basic investigation of integrating a membrane unit into high-pressure decaffeination processing

Abstract

Abstract Within the large-scale industrial decaffeination processes using supercritical CO 2 , caffeine is stripped from the extracting agent CO 2 with water using a high-pressure washing tower. On account of the value of the product, the caffeine is recovered from the washwater with a distillation–crystallization process. A reverse osmosis membrane process appears to be very suitable to replace the energy consuming distillation step to recover the caffeine and supply a cleaned water stream to be returned to the washing tower. Although this combination of a membrane process with high-pressure extraction was suggested earlier, very little fundamental work and experimental data have been published so far. The presented study is concentrated on basic investigations of this process. The osmotic pressure of aqueous caffeine solutions was determined with cryoscopy and additional membrane experiments. Membrane test cells were used to study the maximum separation of binary caffeine–water solutions. The influence of temperature, pressure and caffeine concentration on membrane flow and rejection was investigated for commercially available RO flat sheet membranes. The results show that RO concentration of aqueous caffeine solutions is feasible with moderate pressures and temperatures up to 60°C. Additional experiments with a non-optimized plate module supplemented the investigation of separation performance with approximately 50% of the test cell flux. Flux measurements with an agitated membrane cell show that the permeation of water with high CO 2 concentrations impairs membrane performance. A reverse osmosis washwater recycling process should flash the washwater to atmospheric pressure before the membrane unit.