Assessment of air gap membrane distillation for milk concentration

Abstract

Multi-effect evaporation is the state of the art for concentration of liquid food products to high solid content. Membrane technology with reverse-osmosis and membrane distillation offer an alternative. For the concentration of milk, a reverse osmosis and air-gap membrane distillation network was modelled and optimized. Fouling dynamics and scheduling are taken into account. Reverse osmosis is favourable until its maximum achievable concentration. Air gap membrane distillation is, despite the low operational temperatures, energy intensive for the concentration of milk. A large recirculation flow to keep sufficient cross flow has to be heated and cooled, and the costs for heating and cooling dominate the total costs for product concentration. Moreover, fouling increases the energy requirements. The optimal system for air gap membrane distillation has only one stage operating at a high concentration and relative low flux. Applying multiple stages reduces the investment costs due to smaller units, but the heating and cooling costs increase. Major opportunities to improve the performance of air gap membrane distillation for concentration of milk are: 1) increase the cold and hot side temperatures to their maximum acceptable values, 2) develop spacers that allow lower linear flow velocities in the system and thus lower recirculation rates, and 3) make use of available waste heat.