The membrane test cell (MTC) is an automated laboratory apparatus that applies a known osmotic potential across a membrane and measures the kinetics of the resulting transport of solvents across the membrane as a function of time. Data acquired by use of the MTC should prove especially helpful in designing industrial processes that rely on membrane separation techniques. Examples of such processes include desalination, recovery of designated chemicals from process streams, and some recycling operations.
The MTC includes two fluid chambers separated by the membrane under test, volumetric flow sensors connected to the chambers, a fluid-manipulation subsystem consisting of pumps and valves, a computer, and electronic circuits that serve as control and data-acquisition interfaces. The chambers, the fluid-manipulation subsystem, and the associated plumbing are designed to contain the fluids of interest and to enable automated filling and draining of the chambers without trapping of bubbles. The chambers can be taken apart at the membrane seal to replace the membrane. The computer runs software that controls the fluid-manipulation subsystem and acquires kinetic data from the volumetric flow sensors for characterization of functionality of the membrane.
At the beginning of a test, the fluid-manipulation subsystem introduces solutions of differing chemical potential (known different compositions) into the chambers. This filling operation is performed quickly in order to enable precise definition of the starting time. A solute-outflow sensor signals when the solute cell (one of the chambers) is full and initiates the data-acquisition functions. The fluid in the solute cell is stirred during the test to prevent stratification and thereby provide a relatively constant chemical potential to drive transport through the membrane. The volumetric flow sensors measure the relative volumes of fluids in the chambers as functions of time. The volume-vs.- time data, which are directly related to the kinetics of osmosis, are recorded in a data file in the computer and displayed on the computer screen. The volume measurements can be differentiated with respect to time to determine flow rates as a function of time. The volume-vs.-time data can also be processed to determine the permeability and flow characteristics of the membrane as functions of known driving potential and of time.
A new membrane can be characterized by use of the MTC and thereafter the membrane can be tested at intervals throughout its functional lifetime to identify gradual changes in its kinetics. Such changes can be indicative of fouling, leakage, and other effects associated with deterioration.
A collaboration among Lockheed Martin Astronautics, NASA, the National institute of Standards and Technology (NIST), and the Federal Bureau of Reclamation began in 1997, when a prototype MTC was developed and characterized. Results from experiments on the prototype MTC have shown that the MTC is useful for characterizing the performances of membranes and can be used to differentiate among various deterioration processes.
This work was done by Larry W. Mason of Lockheed Martin and John Peleigrino of NIST for Marshall Space Flight Center. For more information, contact the Marshall Commercial Technology Office at 256-544-2615.