i am working on gas separation based on flat sheet membranes. i was trying to measure the n2 permeability through the membranes and the value i am getting is very high. and the flow rate is also obtained in the range of 2 and above lpm. is there anything wrong because i cant find any papers that has reported permeability in this range
thanks in advance for the help
There could be pinholes and defects in the membrane during the casting. How much is the thickness of the membrane? if it is too thin I would suggest increasing the thickness to avoid those problems. It is also helpful to check the selectivity of different gases, if the selectivity is relativity low (with high permeability) that's a good indication that there might be defects.
@mohammed alqahtani.. thanks for your reply. the thickness is around 150 micrometers. i would like to know if it is ok to get the volumetric flow rate in the range of above 1 lpm for a pressure of 10 cm Hg?
150 micrometer is pretty thick membrane, I'm assuming this is a polymeric membrane. I would suggest you modify the casting procedure: polymer concentration, drying conditions, etc.
Mohammad Alqahtani thanks for the reply. yes its polymeric membrane. one more doubt.. i saw time-lag method for permeation tests. im new to the field so please pardon me if the doubt seems to be foolish. i saw the graph of downstream presure versus time. how is this graph obtained? how is the time measured ?
There are two ways to calculate the gas permeability: 1) constant pressure; 1) constant volume (the so-called time-lag method).
In the constant pressure method, the downstream of the membrane is open to the atmosphere (1 atm, the pressure does not change hence the name) and you measure the flow rate. this is good if you have high permeability and/or doing a mix gas experiments.
In the constant volume method, you have a fixed volume (e.g. small cylinder) attached downstream the membrane, at the beginning of the experiment you evacuate the volume then you close the valve (this marks the start of the experiment, t=0), the gas will start to permeate through the membrane and accumulate in the fixed volume causing the pressure to increase. The pressure is measure by a pressure transducer. One thing you need to make sure of, is that the pressure increase is due to permeation through the membrane and not a leak.
You should read the literature to understand the experimental echniques, here are few:
1. AlQahtani Mohammad, Mezghani Khaled, Thermally rearranged polypyrrolone membranes for high-pressure natural gas separation applications, Journal of Natural Gas Science and Engineering, 51 (2018) 262-270.
2. S.W. Rutherford, D.D. Do, Review of time lag permeation technique as a method for characterization of porous media and membranes
Adsorption, 3 (1997), pp. 283-312
Riya Sidhikku Kandath Valappil as you said, the membrane is having a thickness of 150 microns; it is thick and shows high hydraulic resistance to the penetrants. I do agree with Mohammad Alqahtani it is due to the presence of pinholes. You can identify defective membrane by calculating idea selectivity for any gas pair if the operational conditions are same and you are getting selectivity equal to 1, confirms both the gases are coming at the same rate. In general, the driving pressure of 10 cm Hg is too small in gas separation and the membrane having 150 micrometres won't result sich a high penetrant rate. How you are measuring the volumetric flow rate?
Mohammad Alqahtani thanks a lot for your help... it was really helpful
Shiva Prasad Nandala I am using flow meter to measure the flow meter. wat would b the range of flow rate for 10 cm Hg ? Would you please give any suggestions for decreasing the pinhole?
It depends on the surface area, which has provided in the test study. For better understanding, you may back-calculate to find the expected volumetric flow rate under test operation conditions. As per my knowledge, the dense membrane having 150 microns may have permeation coefficient ranging from 2 to 4 barrer (depends on your polymer).
Shiva Prasad Nandala and Mohammad Alqahtani thanks for the reply. im using polylactic acid. when im converting the permeability coefficeint to the units of m3/sec.m2.Pa, the value of permeability is coming around 475*10^-7 m3/sec.m2.Pa. is this ok? and also, to see if anything is wrong, i tried measuring the permeability of paper instead of membrane and the flow meter reading was almost in the same range. i checked for leaksin the set up using soap solution, but nothing was found. could you help me please. what would be wrong here. im stuck at this point for a long time. thanks in advance for the help
I suggest you, to read the following articles for better understanding.
- M. Lomax, 1980, “Permeation of gases and vapours through polymer films and thin sheet—part I”, Polymer Testing, vol. 1, no. 2, pp. 105-147, doi: https://doi.org/10.1016/0142-9418(80)90037-9.
- Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting, D1434 − 82 2015.
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