Semi-permeable membrane for use in osmosis

Abstract

The present invention concerns a semi-permeable membrane for use in osmosis consisting of one thin layer of a non-porous material (the diffusion skin), and one or more layers of a porous material (the porous layer), where the porous layer has a structure where porosity φ, thickness of the porous layer (m), and tortuosity τ, are related to one another as given by the expression: x·τ=φ·S wherein S is a structure parameter having a value equal to or less than 0.0015 meter. Further a method for providing elevated pressure by osmosis as well as a device for providing an elevated osmotic pressure and electric power is described.

1 . A semi-permeable membrane for forward osmosis or pressure retarded osmosis, the membrane comprising one thin layer of a non-porous material as a diffusion skin, and at least one layer of a porous material, wherein the porous layer has, when water wetted, a porosity φ, thickness×(m), and tortuosity τ in relation to one another as given by
     x·τ=φ·S    Equation (1)   
     where S has a value of 0.0015 meter or lower,
 the porosity φ has an average value of more than 50%, 
 the tortuosity τ is less than 2.5, 
 a water permeability of the porous layer is greater than 1×10 −11  m/s/Pa, and 
 a salt permeability of the porous layer is less than 3×10 −8  m/s. 
 
   
   
       2 . The semi-permeable membrane according to  claim 1 , wherein the membrane has a thickness of less than 150 μm. 
   
   
       3 . The semi-permeable membrane according to  claim 1 , wherein the membrane has a thickness of less than 100 μm. 
   
   
       4 . The semi-permeable membrane according to  claim 1 , comprising a diffusion film thickness of less than 60 μm. 
   
   
       5 . The semi-permeable membrane according to  claim 1 , comprising a diffusion film thickness of less than 30 μm. 
   
   
       6 . The semi-permeable membrane according to  claim 1 , wherein the porous layer of the membrane comprises hollow fibers with an outside diameter from 0.05 to 0.5 mm. 
   
   
       7 . A semi-permeable membrane for pressure retarded osmosis, the membrane comprising one thin layer of a non-porous material as a diffusion skin, and at least one layer of a porous material, wherein the porous layer has, when water wetted, a porosity φ, thickness×(m) and tortuosity τ in relation to one another as given by
     x·τ=φ·S    Equation (1)   where S has a value of 0.0015 meter or lower,   wherein the porous layer, when an amount of a first water feed stream containing salt is contacted with the diffusion skin, has properties related to a salt permeability parameter B defined by:
     B =(φ· D ·( dc/dx )/τ− J·c )·1/Δ c   s    Equation (2) 
   
     wherein:
 A is the water permeability and is greater than 1×10 −11  m/s/Pa, 
 B is the salt permeability (m/s) and is less than 3×10 −8  m/s, 
 Δc s  is the salt concentration difference over the diffusion skin (moles/cm 3 ), 
 φ is the porosity and has an average value of more than 50%, 
 x is the thickness of the porous layer (m), 
 J is the water flux (m/s), 
 C is the salt concentration (moles/cm 3 ), 
 D is the diffusion coefficient of the salt (m 2 /s), 
 τ is the tortuosity and is less than 2.5, 
 
     where the efficiency of the membrane in pressure retarded osmosis for a given value of a water permeability, A (m/s/Pa), can be expressed by an integration of Equation (2) to yield:
   Δ c   s   /c   b =exp(− d   s   ·J/D )/{1+ B ·[(exp( d   f   ·J/D+S·J/D )−exp(− d   s   ·J/D )]/ J}   
 
     wherein:
 c b  is the difference in salt concentration between a second water feed stream and said first water feed stream, wherein said second water feed stream contains less salt than said first water feed stream (moles/cm 3 ), 
 d f  and d s  are the thickness (μm) of the diffusion films on a side of the membrane contacting the second water feed stream and a side of the membrane contacting the first water feed stream, respectively, and 
 Δc s /c b  is the efficiency of the membrane in pressure retarded osmosis for a given value of the water permeability. 
 
   
   
       8 . The semi-permeable membrane according to  claim 7 , wherein the membrane has a thickness which is less than 150 μm. 
   
   
       9 . The semi-permeable membrane according to  claim 7 , wherein the membrane has a thickness of less than 100 μm. 
   
   
       10 . The semi-permeable membrane according to  claim 7 , comprising a diffusion film thickness of less than 60 μm. 
   
   
       11 . The semi-permeable membrane according to  claim 7 , comprising a diffusion film thickness of less than 30 μm. 
   
   
       12 . The semi-permeable membrane according to  claim 7 , wherein the porous layer of the membrane comprises hollow fibers with an outside diameter from 0.05 to 0.5 mm. 
   
   
       13 . A semi-permeable membrane for pressure retarded osmosis, the membrane comprising one thin layer of a non-porous material as a diffusion skin, and at least one layer of a porous material, wherein the porous layer has, when water wetted, a porosity φ, thickness×(m) and tortuosity τ in relation to one another as given by
     x·τ=φ·S    Equation (1)   where S has a value of 0.0015 meter or lower,   the porosity φ has an average value of more than 50%,   the tortuosity τ is less than 2.5,   a water permeability of the porous layer is greater than 1×10 −11  m/s/Pa, and   a salt permeability of the porous layer is less than 3×10 −8  m/s, and   wherein the membrane is configured for creating electric power through use of osmotic hydraulic elevated pressure created by said pressure retarded osmosis for driving at least one power turbine.   
   
   
       14 . The semi-permeable membrane according to  claim 13 , wherein the membrane has a thickness which is less than 150 μm. 
   
   
       15 . The semi-permeable membrane according to  claim 13 , wherein the membrane has a thickness of less than 100 μm. 
   
   
       16 . The semi-permeable membrane according to  claim 13 , comprising a diffusion film thickness of less than 60 μm. 
   
   
       17 . The semi-permeable membrane according to  claim 13 , comprising a diffusion film thickness of less than 30 μm. 
   
   
       18 . The semi-permeable membrane according to  claim 13 , wherein the porous layer of the membrane comprises hollow fibers with an outside diameter from 0.05 to 0.5 mm.