Abstract:
This invention relates to a method and apparatus for on-line measurement of permeation characteristics (transmitting chemicals) through dense nonporous membrane. This invention is intended to facilitate various research activities, such as simultaneous analyses of diffusion coefficient, solubility coefficient, permeation rate and permeant composition, and presentation of new analysis about the permeation behavior, kinetics and so on. The object of this invention is to provide the method and apparatus for measurement of permeation characteristics of permeants in liquid, vapor or gas phase through dense nonporous membrane and more practicuraly, to provide permeation apparatus for measurement of permeation characteristics by analyzing permeation rate and permeation concentration of permeants through an on-line type dense porous membrane with time, not only for a steady state but also for an unsteady state in an accurate and reliable manner.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a method and apparatus for on-line measurement of the permeation characteristics of a permeant through dense nonporous membrane and more particularly, to a method and apparatus for on-line measurement of the permeation characteristics (permeation rate, permeate composition, etc.) of a permeant in liquid, vapor and gas phases through dense polymeric membrane. 
     According to the “solution-diffusion model”, a steady-state transport of a permeant through dense polymeric membrane is generally composed of the following three step mechanism: 
     Step 1: the sorption of permeating molecules into the membrane. 
     Step 2: diffusion through the membrane. 
     Step 3: desorption from the membrane. 
     On this occasion, the diffusion coefficient of the gas, vapor or liquid absorbed inside of the membrane is a function of the swelling state of the membrane because the molecules absorbed inside the membrane originate from a plasticization which affects the flexibility of polymer chains. 
     Namely, the diffusion coefficient varies with the membrane thickness, since the above-stated absorbed molecules cause the anisotropic swelling of the membrane in the direction from the top surface to the bottom surface. 
     The swelling state of the membrane and the solubility vary with the phases of permeants, which may be liquid, gas, or vapor, and the method for measuring permeation characteristics varies as well. 
     Since a gaseous permeant has small attraction and affinity for the membrane, it does not contribute much to the swelling state, and the permeation rate is not high due to the low solubility. 
     Therefore, for the gaseous permeants, the two methods based on the steady-state transport ( J. Appl. Polym. Sci ., 18 (1974) 351 , J. Appl. Polym. Sci ., 12 (1968) 2615) and the non-steady state transport ( J. Appl. Polym. Sci ., 26 (1957) 151 , J. Appl. Polym. Sci ., 14 (1970) 523) are used to determine the permeability and the diffusion coefficient. 
     For the evaluation of permeation characteristics such as permeability and the diffusion coefficient, the steady-state transport method known as time-lag technique is widely used. Moreover, in the steady state transport method, the permeation rate and the amount of permeation are determined indirectly or relatively by using mass spectrometer or by measuring the change of pressure in the bottom of the membrane. However, the steady state transport method does not directly measure the absolute permeation rate, and there are difficulties in analyzing permeants with high permeation rate or good affinity to a membrane. 
     The above nonsteady state transport method can be divided to sorption method and desorption method based on swelling experiments and the free-volume theory ( Adv. Polym. Sci ., 3 (1961) 1) is a key part thereof. The disadvantages of the nonsteady state transport method are that the process is complicate and the calculation results are very sensitive to the accuracy at the measurement. 
     Also, the diffusion coefficient obtained from the nonsteady state transport method is hard to apply to a steady state transport process, because the membrane has different adaptation hysteresis. 
     In addition, the above nonsteady state transport method has a disadvantage in the on-line measurement of the time-varying permeation characteristics. 
     For the vapor and/or liquid permeation, the vapor and/or liquid substance has generally large attraction with the membrane material and relatively high solubility, because when designing the membrane material, the polymer with good affinity for the vapor and liquid material is selected. Thus, new method different from the gaseous material should be chosen for the measurement of the permeation characteristics of the vapor and/or liquid material due to its high permeation rate. 
     So far, the solubility and the diffusion coefficient of the vapor and/or liquid permeants have been obtained by using the nonsteady state transport 
     However, as mentioned above, there is a problem that the adaptation hysteresis in an actual permeation process to the membrane is different from that in characteristic measurement process. 
     At present, the most prevalent method for measuring the permeation characteristics is a method to analyze the permeation rate and the permeant composition condensed for a finite time after passing through the membrane. 
     However, when the permeation rate is low, more time is required for the measurement so that the accuracy of measurement becomes poor in the analysis of the permeant after it is condensed. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to solve disadvantages and problems occurred by the conventional methods for measuring the permeation characteristics of permeants. This invention is intended to facilitate various research activities, such as simultaneous analysis of diffusion coefficient, solubility coefficient, permeation rate, and permeant composition, and presentation of new interpretation about the permeation behavior, kinetics research, and so on. 
     The object of this invention is to provide a method and apparatus for on-line measurement of permeation characteristics of permeants through dense membrane, for example, permeation rate and permeation concentration accurately and reliably, not only for a steady state but also for a nonsteady state with time. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram representing permeation apparatus for on-line measurement of permeation characteristics of permeants. 
     FIG.  2  and FIG. 3 are graphs representing results of exemplary experiments, which were performed using permeation apparatus of this invention. 
       10 : cooler 
       12 : electric heater 
       14   a , 14   b : thermostat 
       16 : feed tank 
       18 : feed pump 
       20 : permeable membrane 
       22 : heating oven 
       24 : purge gas inlet 
       26 : solenoid valve 
       28 : mass flow meter 
       30 : controller 
       32   a ,  32   b : back pressure controller 
       34 : digital display 
       36 : recorder 
       38 : gas chromatography 
       40 : secondary condenser 
       42 : condenser 
       44 : vacuumpump 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is explained in detail as set forth hereunder by referring to the attached drawings. 
     The method for measuring the permeation characteristics of a permeant through dense nonporous membrane in this invention, is characterized by: 
     a) A stage wherein a liquid feed is stored in a feed tank ( 16 ) to control to be a prescribed temperature; 
     b) A stage wherein the liquid feed in the feed tank ( 16 ) is permeated through a membrane ( 20 ) wherein moisture residues and volatile components are eliminated and the temperature is kept constant by pushing with a feed pump ( 18 ); 
     c) A stage wherein the permeant from the above membrane ( 20 ) is past through a mass flow meter ( 28 ) to generate a potential difference which is simultaneously displayed and recorded with time in a recorder ( 36 ) connected to a digital display ( 34 ) to measure the permeation rate thereof; 
     d) A stage wherein the permeant composition is measured by on-line system for the analysis of the permeant composition by passing the permeant past the mass flow meter ( 28 ) through a gas chromatography ( 38 ) equipped with a thermal conductivity detector; and 
     e) A stage wherein the permeant past the gas chromatography ( 38 ) is condensed by a condenser ( 42 ) equipped with a quick refrigerator. 
     For the measuring of the gaseous or vaporous feed, the same stages are applied by providing a gaseous or vaporous feed to a nitrogen gas inlet ( 24 ) placed on the line between the membrane ( 20 ) and the feed pump ( 18 ) connected to the outlet of the feed tank ( 4 ). 
     Before the stage wherein the permeant is transmitted into the membrane ( 20 ), the top surface of the membrane ( 20 ) is purged with nitrogen as from the nitrogen gas inlet ( 24 ) connected to nitrogen gas supplier, and at he same time, the bottom surface is kept vacuous, thereby the membrane ( 20 ) is kept dry by eliminating moisture residues and volatile components. 
     Desirably, the stage of the temperature control with a thermostat ( 14   b ) is proceeded simultaneously to prevent the permeant through downstream of the membrane ( 20 ) from condensing while the above stage is progressed. 
     More desirably, the stage proceeds wherein the recorder ( 36 ) is directly connected to the digital display ( 34 ) and the potential difference detected by the mass flow meter ( 28 ) is recorded with time, and at the same time, the potential difference is converted into the permeation rate and recorded by the established equation relating the magnitude of potential difference to the permeation rate. In this way, disadvantages of mass flow meters with restrictions of detecting very small permeation rate and detecting a change of permeation rate in small range with a digital display, can be overcome. 
     The apparatus for measuring permeation characteristics of the permeant through dense porous membrane, in accordance with this invention, is characterized by comprising: 
     a) A feed tank ( 16 ) wherein the liquid feed is stored, to which a cooler ( 10 ) and an electric heater ( 12 ) are attached for cooling and heating the liquid feed and to which a thermostat ( 14   a ) controlling the temperature of the liquid feed is connected; 
     b) A feed pump ( 18 ) connected to the outlet of feed tank ( 16 ) via a line, which pushes out the liquid feed; 
     c) A membrane ( 20 ) installed inside a cell in the heating oven ( 22 ), through which the liquid feed flows and circulates by the propulsion of the feed pump ( 18 ); 
     d) A mass flow meter ( 28 ) measuring the permeation rate, etc. of the permeant through the membrane ( 20 ), which is linked via a line wrapped by a heating band in the lower part of the cell, which include the membrane ( 20 ); 
     e) A digital display which displays the permeation rate detected by the mass flow meter ( 28 ) into the volume flow rate; 
     f) A recorder ( 36 ) which converts the change of the potential difference of the mass flow meter ( 28 ) to a change of permeation rate and records it with time; 
     g) A gas chromatography ( 38 ) connected to the outlet of the mass flow meter ( 28 ) to measure permeant composition by on-line system; and 
     h) A condenser ( 42 ) and a secondary condenser ( 40 ) for condensing permeant past the gas chromatography ( 38 ). 
     Particularly, the mass flow meter ( 28 ) can be connected directly to a high-sensitive recorder ( 36 ) to detect small potential difference generated by the permeant. 
     Additionally, the inlet of the purge gas, which is connected to a nitrogen gas supplier, is installed on the line between the feed pump ( 18 ) and the membrane ( 20 ). A back pressure controller ( 32   a ) is installed to regulate the pressure of permeants at the inlet of the purge gas. 
     Desirably, a solenoid valve ( 26 ) is installed on the line between the membrane ( 20 ) and the mass flow meter ( 28 ) to shut off the flow of the permeant automatically in an emergency. 
     More desirably, the back pressure controller ( 32   b ) is installed on the line between the mass flow meter ( 28 ) and the gas chromatography ( 38 ) to control the pressure of the downstream. 
     Moreover, in order to prevent condensing of downstream, a temperature controller ( 14   b ), which controls the temperature of the line wrapped by the heating band, is installed on the line between the gas chromatography ( 38 ) and the secondary condenser ( 40 ). 
     Hereupon, a method and apparatus for on-line measurement of the ermeation characteristics of permeants through dense porous membrane tated in this invention are explained in more detail. 
     The attached FIG. 1 illustrates the apparatus for measuring permeaton haracteristics of permeants through dense porous membrane, where the symbol ( 16 ) represents the feed tank. 
     The feed tank ( 16 ) stores the unrestricted permeants in liquid phase. An electric heater ( 12 ) and a cooler ( 10 ) for heating and cooling the stored feed are installed on the prescribed part of feed tank ( 16 ). 
     Furthermore, the feed tank is kept airtight to restrict vaporization of the liquid feed. And the upper part of the feed tank is connected to the nitrogen gas supplier. The nitrogen gas regulates the internal pressure of the feed tank less than 2 atm. 
     Moreover, the temperature controller ( 14   a ) is placed between the cooler ( 10 ) and the electric heater ( 12 ), which is able to control the temperature of the liquid feed with the precision of ±0.2° C. in the range 5˜150° C. 
     Therefore, the temperature of the liquid feed is controlled constant by a temperature controller ( 14   a ) located between the cooler ( 10 ) and the electric heater ( 12 ) inside the feed tank ( 16 ). 
     Next, a feed pump ( 18 ) is connected with a line to the outlet of permeable liquid feed. And the permeation liquid in the feed tank ( 16 ) flows out by the propulsion of the feed pump ( 18 ). 
     Subsequently, because the outlet of the feed pump ( 18 ) is linked with a line to the cell in the space including the membrane ( 20 ), the cell is in split into upper and lower spaces. 
     The cell including the membrane ( 20 ) is mounted in the heating oven ( 22 ). The temperature of the membrane ( 20 ) is kept constant with the precision of ±0.1° C. by this heating oven ( 22 ). 
     Therefore, the feed out of the feed pump ( 18 ) flows into the upper cell space of the membrane ( 20 ), and at the same time, the lower part of the membrane is vacuumed with a vacuum pump ( 44 ), thereby the feed permeates through the membrane. 
     On the other hand, the nitrogen gas inlet ( 24 ), which is connected to the nitrogen gas supplier, is installed on the line between the membrane ( 20 ) and the feed pump ( 18 ). If the feed supplied to the membrane in gas or vapor phase, it is supplied through the nitrogen gas inlet ( 24 ). 
     Before the above-mentioned feed (liquid, gas, vapor, etc) flows into the membrane ( 20 ), the membrane ( 20 ) should be kept dry. The nitrogen gas out of the supplier passes through the nitrogen gas inlet ( 24 ) and flows into the inner part of the cell. The nitrogen gas purges the top surface of the membrane ( 20 ) and eliminate moisture residues and volatile components. Thus, the membrane ( 20 ) becomes dry. 
     In addition, a back pressure controller ( 32   a ) is installed on the cell opposite to the feed supply. The back pressure controller controls the inlet pressure when the gaseous, or vaporous feed is supplied from the nitrogen gas inlet ( 24 ). 
     Next, the lower space of the cell including the membrane ( 20 ) is connected to the mass flow meter ( 28 ) with the line enclosed by the heating band. Therefore, the permeant past the membrane ( 20 ) passes through the lower parts of the cell and flows into the mass flow meter ( 28 ). 
     The digital display ( 34 ) and the recorder ( 36 ) are connected to the mass flow meter ( 28 ) in turn. The digital display ( 34 ) shows the volume speed converted from the change of potential difference, which is detected by the mass flow meter ( 28 ). And the potential difference is recorded on the recorder ( 36 ) simultaneously. 
     However, because the digital display ( 34 ) digitizes the result from the mass flow meter ( 28 ), mass flow meter may be unable to detect very small permeation rate. 
     Therefore, the high sensitive recorder ( 36 ) is linked directly with the mass flow meter ( 28 ) to detect and record very small potential difference generated by the mass flow meter ( 28 ). 
     The above-mentioned recorder ( 36 ) has a maximum sensitivity less than 0.2 mV/cm. With this sensitivity, the recorder can afford to analyze the separation membrane that has gas permeability of an order of a barrer. 
     Therefore, the potential difference detected by the mass flow meter is recorded on the recorder ( 36 ) with time. This time history is converted into the permeation rate by the equation of permeation rate vs. potential difference that is established in the standard curve (FIG.  2 ), and recorded. 
     The mass flow meter ( 28 ) has a capacity in the range of 100˜1000 SCCM. When manufacturing and installing the apparatus of this invention, the pressure difference of both the inlet and the outlet of the mass flow meter ( 28 ) is adjusted to 4-8 psi. This pressure settlement minimizes the pressure drop and regulates constantly at low pressure in the lower part of the membrane. 
     In an emergency, for example, when the membrane is torn, the pressure of the lower streamline can change abruptly. The solenoid valve is installed on the line between the lower streamline in the membrane ( 20 ) and the mass flow meter ( 28 ) to detect the emergency situation and shut off the feed flow automatically. 
     Next, the outlet of the mass flow meter is wrapped by the heating band. And the back pressure controller ( 32   b ) linked with the line settled in a fixed part is connected with the gas chromatography ( 38 ). The permeant past the mass flow meter ( 28 ) passes through the gas chromatography ( 38 ). 
     On the other hand, the solenoid valve ( 26 ) and the back pressure regulator ( 32   b ) placed on the line of both sides of the mass flow meter ( 28 ) are linked to a controller (30). The operation of the solenoid valve ( 26 ) and the back pressure regulator ( 32   b ) can be controlled at will by using this regulator (30). 
     Since a thermal conductivity detector is included in the gas chromatography ( 38 ) and an automatic 6-port valve operated by the compressed air is placed in the inlet, the permeant flows into the gas chromatography ( 38 ) via the automatic 6-port valve and the permeant composition is analyzed and graphed by the thermal conductivity detector. 
     Finally, the permeant past the gas chromatography ( 38 ) should be condensed in order to prevent from flowing into the vacuum pump, which is installed to keep the bottom of the membrane vacuumized, after the permeation process. 
     Hence, the gas chromatography ( 38 ) is connected to the secondary condenser ( 40 ) and the condenser ( 42 ), which is connected to the vacuum pump ( 44 ). 
     By the aid of the quick refrigerator with two-step freezing method, the temperature of the condenser ( 42 ) can be keep below −70° C. Thus, liquid nitrogen is unnecessary. 
     In addition, the secondary condenser ( 40 ) placed on the line between the gas chromatography ( 38 ) and the condenser ( 42 ) is used only for correction to draw a standard curve between the potential difference and the permeation rate. In this case, the accurate permeation amount per hour is measured by condensing the permeant with liquid nitrogen provided by the liquid nitrogen supplier. 
     As mentioned in detail above, permeation apparatus for measuring permeation characteristics of permeants through dense porous membrane can accomplish a gas separation process and a vapor permeation process as well as pervaporation process. 
     The experimental examples of measuring permeation characteristics of ants using the apparatus of this invention are given below. 
     The following specific examples are intended to be illustrative of this invention and should not be construed as limiting the scope of this invention as defined by the appended claims. 
     EXAMPLE 1 
     A pervaporation experiment was performed using crosslinked poly (vinyl alcohol) for the permeation membrane and water for the permeant. 
     The condensed permeant condensed during a given time in the secondary condenser was weighed using liquid nitrogen when the permeation reached the steady state. After that, the penetration rate was calculated and the potential difference displayed on the recorder was recorded. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Potential 
                   
               
               
                   
                 Membrane 
                 Measured 
                 difference 
                 Penetration 
               
               
                 Example 
                 thickness (μm) 
                 temperature (° C.) 
                 (mV) 
                 rate (g/m 2 .h) 
               
               
                   
               
             
             
               
                 1 
                 28 
                 20 
                 15.1 
                 153.3 
               
               
                 2 
                   
                 30 
                 26.5 
                 267.6 
               
               
                 3 
                   
                 40 
                 43.0 
                 421.0 
               
               
                 4 
                   
                 50 
                 68.7 
                 682.0 
               
               
                 5 
                 36 
                 20 
                 14.2 
                 147.6 
               
               
                 6 
                   
                 30 
                 23.9 
                 236.4 
               
               
                 7 
                   
                 40 
                 38.0 
                 377.3 
               
               
                 8 
                   
                 50 
                 56.0 
                 557.1 
               
               
                   
               
             
          
         
       
     
     The result (a table showing the relationship of the potential difference and the permeation rate) was obtained by performing the processes repeatedly using two membranes with different thicknesses at different temperatures. The standard curve, as shown in FIG. 2, was obtained by depicting the measured permeation rate versus the potential difference. 
     As shown in FIG. 2, they have a linear relationship regardless of the thickness of permeation membrane and the temperature. Consequently, the potential difference measured by the mass flow meter can be converted to the permeation rate. 
     EXAMPLE 2 
     A pervaporation experiment was performed for water at the temperature of 20, 30, 40, 50° C., using 36 μm thick cross-linked poly(vinyl alcohol) membrane as the permeation membrane. As a result, permeation rate with time was obtained as shown in FIG.  3 . At each temperature, it took less than 15 minutes for permeation process to reach the steady state, and the measurement could be finished within 20 minutes due to unnecessary of collecting the permeant for analysis. 
     Besides, for a verification of the accuracy and reliability of the measured permeation rate, the conventional permeation rate was measured simultaneously using the secondary condenser. This value was compared with the permeation rate displayed on the recorder of the apparatus. As a result, the following table (the comparison of the actually measured permeation rate with the permeation rate measured by the recorder at the steady state) shows that they coincided within ±2% error. 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Measured 
                 permeation rate (g/m 2 .h) 
               
             
          
           
               
                   
                 temperature 
                 Recorder 
                 Secondary condenser 
               
               
                 Example 
                 (° C.) 
                 measurement 
                 measurement 
               
               
                   
               
             
          
           
               
                 9 
                 20 
                 144.2 
                 147.6 
               
               
                 10 
                 30 
                 238.9 
                 236.4 
               
               
                 11 
                 40 
                 381.8 
                 377.3 
               
               
                 12 
                 50 
                 560.1 
                 557.1 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 3 
     After the response time was determined from each curve in FIG. 3 as by the literature ( J. Membr. Sci ., 49 (1990) 171-205 , J. Membr. Sci ., 73 (1992) 55-71), the diffusion coefficients of the permeant were calculated. The following table (the diffusion coefficients of water for the cross-linked poly(vinyl alcohol) membrane measured with the apparatus of this invention) shows the results. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Diffusion 
               
               
                   
                 Measured 
                 Response time 
                 coefficient 
               
               
                 Example 
                 temperature (° C.) 
                 (sec) 
                 (m 2 /sec) × 10 12   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 13 
                 20 
                 305.5 
                 0.72 
               
               
                 14 
                 30 
                 54.8 
                 4.00 
               
               
                 15 
                 40 
                 31.1 
                 7.04 
               
               
                 16 
                 50 
                 22.5 
                 9.73 
               
               
                   
               
             
          
         
       
     
     The literature ( J. Appl. Polym. Sci ., 59 (1996) 1271-1279) value of the coefficient of water for the cross-linked poly(vinyl alcohol) membrane 35° C. from is dependent on the extent of crosslinkage, but it is mostly in the range of 2-100×10 −12 m 2 /sec. Therefore, the diffusion coefficient measured by the apparatus of this invention falls within the above range. 
     As mentioned above, permeation apparatus for on-line measurement of permeation characteristics of permeants through dense nonporous membrane, have the following advantages: the measuring time is much shorter comparing with the conventional method and permeation characteristics such as diffusion coefficient, solubility coefficient, and so on can be obtained easily by measuring the permeation rate change of the permeant. 
     Besides, the upper and lower condition of the permeation membrane can be kept constant during the process and the measurement is accurate and reliable by aid of the on-line measurement method of the permeation characteristics. Also, the liquid nitrogen, conventionally used to condense the permeant in measuring permeation characteristics, is unnecessary and the measurement is simple. 
     Consequently, the method and apparatus for measuring permeation characteristics of permeants through dense nonporous membrane presented by this invention enables to obtain the permeation rate of permeants rapidly and accurately. Thus, it can be very useful in membrane separation analysis and permeation behavior research.