4 Experimental section
4.7 Gas separation experiments
All membrane separation experiments with gaseous mixtures were conducted in Zaragoza with a custom built gas separation device. Technical layout and measurement procedure of this mixed gas permeation equipment are explained in the following chapters. Also it is described how the experimental data was evaluated.
4.7.1 Equipment setup
The separation equipment used for mixed gas permeation experiments was constructed by the group of Prof. Coronas. A connection diagram is shown in figure 4.8. It can be seen easily that the equipment is divided into the feed and permeate side, separated by the membrane which was housed inside the central part, a stainless steel membrane cell. This cell is positioned inside an oven.
Figure 4.8: Schematic of the gas separation device used in this work. MC: membrane cell MFC #: mass flow controllers 1-3 V#: ball valves 1 & 2 mGC: micro gas chromatograph
On the feed side 50/50 vol.-% oxygen/nitrogen as well as hydrogen/methane mixtures were prepared using two Alicat Scientific mass flow controllers. The feed mixtures were created from pure gas streams by setting each mass flow controller to provide a flow of 25 cm3 (STP)/min, resulting in a global flow of the feed mixture of 50 cm3 (STP)/min. Using the valve V1 before the retentate outlet the pressure on the feed side of the membrane was set to 340 kPa. The membrane cell consisted of two stainless steel pieces from which the bottom part offered a cavity of 4 cm in diameter to hold the membrane on top of a porous metal disc (Mott Corporation 316LSS, 20 µm nominal pore size). Like in the pervaporation equipment a filter paper was positioned between the membrane and the metal support. Top and bottom part of the membrane cell were sealed using Viton® o-rings.
The permeate side of the membrane cell was purged by a sweep gas provided by another mass flow controller. A flow of 5 cm3 (STP)/min helium in oxygen/nitrogen separation experiments was applied. In hydrogen/methane experiments argon was used instead. The pressure on the permeate side had to be kept slightly above atmospheric to avoid gases from the outlet (e.g. air) to enter and contaminate the measurement. Therefore valve V2 was used to set the pressure on the permeate side to approximately 120 kPa. A computer controlled Agilent 3000A micro gas chromatograph
equipped with thermal conductivity detector was used to analyze the composition of the sweep gas stream containing the permeate.
4.7.2 Measurement procedure
In the following the operation of the mixed gas separation plant is explained. Generally all separation experiments were conducted at 35 °C. Prior to a new experiment a new membrane had to be inserted into the membrane cell. For this the module was placed in a small bench vice for easy removal of the eight hex head screws, used to tightly close the module. A fresh filter paper was placed on the porous metal support and on top of it the membrane which was to be analyzed. The membrane cell was then closed and mounted back into the separation plant. Now the mass flow controllers 1-3 were set to their according values. The controllers 1 and 2 were always opened and closed first. When opening the controllers this sequence allowed to immediately detect a defective membrane. In such a case no pressure rise on the feed side would be observed. Also when closing them this sequence avoids a possible pressure overload on the permeate side. This could let the membrane get out of place easily and the experiment would have to be aborted. After opening the mass flow controllers the feed and permeate pressures were adjusted using the valves V1 and V2. Now the program controlling the gas chromatograph was started and the device set to make one analysis every 10 minutes over a period of several hours. Usually after 2 hours equilibrium was achieved but the experiment was not finished until 5 to 6 hours had passed. Then the mass flow controllers were closed in the above mentioned order. When the feed and permeate pressure had declined back to atmospheric the module could be removed from the plant to allow insertion of a new membrane. If however the analysis of another gas mixture was planned, the module was left in place and instead the tubing and mass flow controllers were modified to operate under the desired gas mixture.
For some membranes a special preparation method was necessary to be able to conduct gas separation experiments. Occasionally a membrane was obtained that could not be inserted into the membrane cell as a whole. This could be necessary for two different reasons. First for some membranes strong reeling occurred during the drying process. In such cases only a part much smaller than the diameter of the membrane cell could be used. Secondly the incorporation of the titanosilicate lead to more brittle membranes, especially at higher filler loads. Sometimes such a membrane broke prior to analysis in gas separation or was so brittle that breaking could occur in the membrane cell only due to the feed pressure. In order to analyze these membranes in spite of their deficiencies they were laminated in a bigger piece of aluminium foil. Therefore a 10-15 cm long band was taken from a roll of adhesive aluminium tape (5 cm width). It was folded in the middle and a hole was cut into both layers using a circular cutter. In between these the membrane sample was placed and on the upper side the border between membrane and aluminium was sealed using epoxy resin. After drying of the resin the laminated membrane was cut to fit into the membrane cell. The membrane thickness was measured as the average over ten different spots using a micrometer screw. When the membrane was to be covered with aluminium this measurement had to be done prior to the lamination. Also this technique required a measurement of the membrane area. This was achieved by measuring the diameter of the uncovered membrane area ten times and taking the average.
4 Experimental section
4.7.3 Calculation of separation characteristics
Gas permeabilities were calculated on the basis of the peak areas detected by the gas chromatograph. A calibration was needed to convert peak areas into gas content in wt.-%. This calibration was checked and adjusted regularly. The individual calibration curves used in this work can be found in the appendix (see chapter 6.3). Using an excel sheet all individual analyzes of the gas chromatograph were evaluated to obtain gas permeabilities as described in the following. At first the flux of a gas component was calculated using the mass fraction obtained from the gas chromatograph. For this equation 4.10 was used and the flux and mass fraction of the sweep gas were needed.
(4.10)
Using this value the permeability of that component could be determined with aid of equation 4.11. Apart from the flux also the membrane thickness and area , the partial pressure difference between feed and permeate side
as well as the time were needed here. As the unit of permeability the unit Barrer was used.
(4.11)
Because the flux is calculated on basis of the sweep gas stream given in cm3 (STP)/min, as the time 60 seconds were used. The values for and were determined once for every membrane and