Membrane distillation method is a separation process that is energy efficient compared to other methods such as reverse osmosis or distillation. Membrane distillation has various advantages such as: 100% theoretic rejection of ions, colloids, biological cells, and non-volatile components; low operation temperature, low operation pressure, less chemical interactions between membrane and process fluids, less vapor gap than distillation, and simple membrane mechanical characteristics. Even though membrane distillation has many advantages, this system also has many limitations, such as the feed solution must be dilute enough to avoid membrane wetting (Lawson and Lloyd, 1997).
Membrane distillation is one of thermally based membrane processes in which the membrane is not directly involved in separation and acts as barrier between two phases. Selectivity is determined by the vapor–liquid equilibrium thus the component with the highest partial pressure has the highest permeation rate (Mulder, 1996). For example, in the case of an ethanol/water mixture where the membrane is not wetted, both components are transported through the membrane. However, since the ethanol has higher vapor pressure, the permeation rate of ethanol is always relatively higher.
The membrane used in this study is a non-porous membrane thus has similar mechanism with pervaporation. The permeate is removed as a vapor due to low vapor pressure existing on the permeate side by applying a vacuum pump. Since the membrane has dense structure, membrane wetting is not a drawback of the process. In addition the dense structure provides another advantages for beer dealcoholization which avoid the permeation of other components in the beer.
In this study, vacuum membrane distillation method with non-porous membrane is investigated during dealcoholization of beer. The vacuum condition is introduced to create a driving force for the mass transfer of ethanol which will shift the vapor–liquid equilibrium of ethanol where the evaporation of ethanol happen followed by ethanol diffusion through the membrane. Non-porous membrane can avoid the loss of important components from beer because of diffusion through the membrane.