Water permeability constant

All symbols are defined at the end of the paper. Equation 10 defines the pure water permeability constant A for the membrane which is a measure of its overall porosity eq 12 defines the solute transport parameter D /K6 for the membrane, which is also a measure of the average pore size on the membrane surface on a relative scale. The Important feature of the above set of equations is that neither any one equation in the set of equations 10 to 13, nor any part of this set of equations is adequate representation of reverse osmosis transport the latter is governed simultaneously by the entire set of eq 10 to 13. Further, under steady state operating conditions, a single set of experimental data on (PWP), (PR), and f enables one to calculate the quantities A, Xy 2> point... 

Membrane Specifications. At a specified operating temperature and pressure, a cellulose acetate membrane is completely specified in terms of its pure water permeability constant A and solute transport parameter D /k6 for a convenient reference solute such as sodium chloride. A single set of experimental data on (PWP), (PR), and f at known operating conditions is enough to obtain data on the specifying parameters A and (DAM/X6)jjg(. 2 at any given temperature and pressure. 

Figure 16. Decrease of separation (or increase of solute permeabilities) of seawater reverse osmosis desalination at several concentration levels of NaOCl. Initial membrane constants pure water permeability constant = 97.0 nmol m Pa s and the solute permeability constant for NaCl = 0.9 X 10 cm s . Operational conditions k = 7.(97 X 10 cm s Ap = 6.0 MPa, and T = 25°C.

Reverse-Osmosis Experiments. All reverse-osmosis experiments were performed with continuous-flow cells. Each membrane was subjected to an initial pure water pressure of 2068 kPag (300 psig) for 2 h pure water was used as feed to minimize the compaction effect. The specifications of all the membranes in terms of the solute transport parameter [(Dam/ 6)Naci]> the pure water permeability constant (A), the separation, and the product rate (PR) are given in Table I. These were determined by Kimura-Sourirajan analysis (7) of experimental reverse-osmosis data with sodium chloride solution at a feed concentration of 0.06 m unless otherwise stated. All other reverse-osmosis experiments were carried out at laboratory temperature (23-25 °C), an operating pressure of 1724 kPag (250 psig), a feed concentration of 100 ppm, and a feed flow rate >400 cmVmin. The fraction solute separation (/) is defined as follows ... 

Examination of equation 12 shows that water separation rate increases with the Water Permeability Constant K. Unfortunately, the salt flux across the membrane also does, resulting in a more salty product. An approximation for this salt flow is... 

Permeability constants for membranes must be determined experimentally for the particular type of membrane to be used. For cellulose acetate membranes, typical water permeability constants A , range from about 1 x 10to 5 X 10" kg solvents/s m -atm (Al, M3, Wl). Values for other types of membranes can differ widely. Generally, the water permeability constant for a particular membrane does not depend upon the solute present. For the solute permeability constants of cellulose acetate membranes, some relative typical values are as follows, assuming a value of = 4 X 10 m/s for NaCl 1.6 x 10 m/s (BaClz), 2.2 X 10" (MgClz), 2.4 x 10 (CaClj), 4.0 X 10 (Na2S04>, 6.0 x 10 (KCl), 6.0 x 10 (NH4CI) (Al). 

Table IV. Water Permeability Constant K and Activation Energy E for Membranes...

A is often referred to as the water permeability constant. Note that if the soluble spedes rejection is not complete, the osmotic pressure difference across the membrane must be related to the osmotic pressure difference between the feed and the permeate streams, instead of that between the feed and pure water. Another factor commonly reported for membrane performance is the salt or solute rejection efficiency (ratio), R, defined as... 

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