How do you calculate the osmotic pressure that is exerted on a nanofiltration membrane?
I have measured water activites on some of my samples (seawater), and calculated the (total) osmotic pressure.
But I assume that because of the low rejection coefficients for Na and Cl, they do not contribute fully to the osmotic pressure influencing the membrane?
I am trying to see how the osmotic pressure varies over time when I run seawater through the NF setup (batch process), but the osmotic pressure calculated from the water activity tells me nothing.
The osmotic pressure is a consequence of the concentrations on each side of a membrane. Since an NF membrane passes some salt, the osmotic pressure, which is really a pressure differential across the membrane, is reduced relative to what would be observed for a highly rejecting RO membrane. In order to determine the osmotic pressure differential across an NF membrane, you can use the van Hoft equation and the measured salt concentrations on each side of the membrane.
I strongly agree with Prof. Krantz. There is a dynamic change in the osmotic pressure during the batch operation (dead-end-crossflow?), which is greatly affected by the rejection of the ions from your NF membrane (loose, tight?). Considering also that seawater is being processed, other co-ions and inorganic/organic ingredients inclunded may affect the measured rejections and consequently the total osmotic pressure.
Yes I also agree with Prof. Krantz and Dr. Plakas. NF membranes usually reject most of divalent ions wherease monovalent ions are only rejected to a certain percentage. The easiest way is measuring the osmolality and calculate like that the osmotic pressure.
Thank you for all your answers.
I am running seawater through a nanofiltration membrane (NF99HF). I plan to calculate the osmotic pressure through measurement of water activity.
I think it should be possible by substracting the osmotic pressure of the permeate from the retentate (it is crossflow filtration). Then I should be able to calculate the resistance caused by fouling.
I am not sure if I can use Van't Hoffs equation because I have quite high salt concentrations, and I think Van't Hoffs works for dilute solutions?
I usually use a osmometer to determine the osmolality and then I calculate the osmotic pressure. At least you are a real measure.
Osmotic pressure is compromised at high salt concentrations due to non-ideality inherent to the system. So also does a mixture of solutes affect due to interactions. Fouling occurs when one does not strictly measure at initial conditions. Given the complexities and approximations that make the colligative property measurements per se dnon-reliable, I once resorted to tying the membrane to a funnel whose other end was connected to a Warburg manometer, with sea water (or whatever) on one side and pure distilled water on the other and ensured thermal equilibration well. You need to independently determine the osmotic pressures of solutions of the solutions and the area of the membrane first. Once you know the true initial flux by volume determination, you could compute the relevant parameters. I assume your interest is in the membrane characteristics rather than the solution per se. We got that.
the practical manner by which the net osmotic pressure of the solution of a NF / RO process is back calculation knowing the observed flux, the flux exhibited by the membrane for pure water under the same operating pressure.
osmotic pressure being essentially the solution property the mathematical value for the same would vary with the concentration of the solution which is dynamically changing in an experimental scheme and conditions.
seawater for all practical purposes is considered close to ideal as it contains only about 3-4% of solutes remaining being water. it is hardly about 0.5 molar while the mole fraction is very low ie 0.5/ 56 approximately.
in the above method of determining the osmotic pressure there could be a minor error due to fouling but this can be circumvented by making measurement at very low recoveries.
I have studied 8040 SW NF for different manufacturers. NF membrane performance is affected by the system different operating parameters, such as; feed intake type, feed temperature, flow rate, pressure, and salt concentration. The membrane performance is changeable from system to another even for the same operating conditions, which depend upon the membrane manufacturer.
Sodium (Na+), Potassium (K+), Chloride (Cl-), and total monovalent; is removed by the (NF) membranes to an ions salt rejection (SR %) values average at 46.9%, 41.2%, 52.1% and 50.4% respectively.
A sample of the membrane NF90- 400, Kw, Ks, and net driving pressure is attached.
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