Concentration of milk and dairy products by means of a forward osmosis-membrane distillation hybrid process

The concentration of liquid foods such as fruit-vegetable juices and milk by removing its water content is an important chemical process in terms of concentration of food nutrition, shelf-life and stability extension, decrease of the bacterial activities and cost reduction of transportation and storage. Milk is indisputably a crucial nutrient in human life and consists of 85-90% water. This reflects that the cost for milk storage and transportation is mostly used for water transportation and storage. The concentration of milk by removing its water content at certain rates has several advantages in terms of energy and transport costs as well as shell-life extension. Concentrated milk can be used for direct consumption as a milk supply in remote areas with a shortage of milk while in the areas where milk is available condensed milk meets the area of usage such as pastry, bakery and coffee cream. As the condensed milk alone may be a commercial product, eliminating the water content in milk in certain proportions is also an essential process during cheese, yogurt and milk powder production. Whey and milk should be concentrated up to a certain amount before they are sent to spray dryer during milk and whey powder production processes. In dairy industry generally milk is concentrated by evaporators via thermal treatment, but this process is energy -intensive and costly operation, and a loss of a portion of valuable components of milk occurs because of the heat treatment. In this study, concentrated milk production is carried out using membrane processes including membrane contactors. Membrane technology used in the removal of water in milk is forward osmosis. Forward osmosis is the transfer of the water molecules through a selective permeable membrane towards to lower water activity solution because of the osmotic pressure difference across the membrane. In this process, there is a feed solution to be concentrated on one side of the membrane while there is a concentrated salt solution on the other of the membrane to provide the osmotic pressure differential required for the passage of the water. In forward osmosis, brine solution is diluted in time during the process because of the water transfer from feed solution to brine solution. This dilution causes the water activity difference between two sides of the membrane contactor and consequently the driving force to decrease. This results in a decrease of water flux in time. In order to prevent this situation, a membrane distillation process has been added to the forward osmosis process to concentrate the brine solution simultaneously. Membrane distillation was used as direct contact membrane distillation and vacuum membrane distillation configurations. The driving force of direct contact membrane distillation is temperature difference and the driving force of vacuum membrane distillation is the pressure difference. Thus, forward osmosis is combined with direct contact membrane distillation and vacuum membrane distillation processes separately as hybrid systems. Two different hollow fiber membrane contactors were used based on hydrophobic microporus polypropylene (PP) fibers. In this study, concentration process was performed until the 9% total solid content of milk has reached to 20% total solid content. As brine solution CaCI2 solution was used in forward osmosis process. In forward osmosis, the effect of several parameters were investigated such as brine solution concentration, temperature, feed solution velocity, brine solution velocity. As optimum conditions 4 M CaCI2 solution, 25°C temperature of brine and feed solution, 1600 mL.min-1 velocity of brine and feed solution, 1:2 feed:brine solution volume ratio was determined using MEMBRANA Liquicel® Minimodule (Germany, GmbH) and these conditions were designated as the center point for the experimental study. For MICRODYN® - MD 020 CP 2N (Almanya, GmbH) membrane contactor, 500 mL.min-1 velocity of brine and feed solution, 4 M CaCI2 solution, 30°C temperature of brine and feed solution were determined as optimum condition. In forward osmosis, fluxes are obtained with two different membrane contactors were between 92.8-814.3 mL.m-2.h-1 and concentration process was completed between 70 and 200 min indicating a fast operation. In a similar way, concentration of milk was carried out using membrane distillation process and the effect of temperature and velocity parameters were investigated in this process. In membrane distillation using two different membrane contactors, fluxes were obtained between 3.84-227.5 mL.m-2.h-1. After forward osmosis-membrane distillation process was applied as hybrid process, and in this process concentration of milk was carried out in forward osmosis part of the process while the concentration of brine was being carried out in the membrane distillation part of the system simultaneously. The effect of temperature parameter on the system performance was investigated in hybrid process. Applying hybrid process, concentration of brine solution in hybrid process was found to be higher than the concentration of brine solution in forward osmosis during the concentratin process. At the end of the hybrid process experiments, brine solution was 15% more concentrated in comparison to forward osmosis experiments. Thus, fluxes of hybrid process were about 1.6 times higher than the fluxes of forward osmosis. A theoritical model was proposed for mass transfer in forward osmosis process. The model data was compatible with the experimental data. This theoretical model can be used for the design of these type of processes. In this study concentration of milk, which is an essential step in dairy industry, was performed more economically using membrane based processes and proposed as an alternative process to conventional methods. Keywords: Milk, forward osmosis, membrane distillation, hybrid system