Sugar rejection membranes

Attainment of high sugar rejection at high temperature was the motlvaiton for investigating polyblend membranes. It is not difficult to obtain fructose rejections greater than 0.95 with the... 

Sugar rejection was always zero. Polyphenol rejection increased from 9% to 57% with decreasing membrane cut-off. Difference in polyphenol rejection have been observed when the must is ultr filtered in the first UF step with the BMR 100515 and after with the BMR 021006, as reported in table III. Fig.3 shows the typical behaviour of the ultrafiltrate flux observed in these experiments. A constant flux was generally obtained after two hours. Table IV shows the final must ultrafiltrate flux values. All the experiments were carried out at the same axial velocity and at the same applied pressure. Table V shows results obtained with tubular membranes (Abcor-USA). 

Table VI shows the rejection measured for total N2, polyphenols and sugars with DDS 800 and PA 300 membranes. Sugar rejections increased from about 20% with DDS 800 to 100% with PA 300. Table VII shows the rejections of various cations using the PA 300 membrane. The rejection for the majority of cations is higher than 97%. Only Cu" " and Zn" " permeate easily through the membrane. The rejections for these two cations is essentially zero. The reason is attributed to a high specific interaction of Cu and Zn with the polymeric materials forming the membranes and to Donnan equilibrium.

Low salt rejection RO membranes (e.g., R < 0.5 for NaCl) are sometimes classified as nanoporous and allow retention of sugars and large molecules while permeating small electrolytes. In this case, a hindered transport description of the process would be appropriate with the water and nonrejected electrolytes being treated as a single fluid and the rejected sugar considered the solute. 

Polyphenol and protein concentrations can be controlled without affecting the sugar content. The ultrafiltered must can be concentrated by reverse osmosis with membranes up to 100% rejection for sugars. Cu" " and Zn " cations could be extracted from the concentrated must. 

We found that a membrane being able to reject sugar up to 100 percent is a very interesting membrane for concentration of waste waters or industrial process streams. For cellulose acetate, this corresponds to 85-90% salt rejection which is usually considered being too low for desalination. I think that several membranes of polymers alternative to CA could fulfil these requirements and that such membranes have been lost in the hunt for a high degree of desalination. 

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