Membrane Separation Final Report PDF

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Membrane Air Separation

Final Lab Report Unit Operations Lab 1 September 19th, 2018

Group 3 Raymond Trelka Reshma Ravi Ekaterina Ivanov

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Executive Summary: This experiment’s main objective was to investigate the effect of pressure and series and parallel set ups on the separation of air. We were able to calculate the transport coefficients and recoveries and learn under which conditions one can successfully execute gas membrane separation. The two separator modules are used to separate the Oxygen from an organized stream of air. Using variables such as pressure and side flow rates, we were able to calculate the transport coefficients which helped us identify the set up for better separation of O2 from the stream After conducting 15 runs and constantly changing operating pressure with respect to set up (series or parallel), it was evident that separation of air was more efficient in series. Pressure and selectivity for oxygen are inversely proportional in series whereas in parallel, are directly proportional.The coefficients and recoveries were higher for series than parallel.For example, for trial 1 with operating pressure set to 20 psig, O2 recovery values for series and parallel were 0.984 and 0.548. With both setups, we achieved membrane gas separation successfully but with series, a much better separation was executed. Introduction : A membrane is a thin layer of semi-permeable material that is used for solute separation as transmembrane pressure is applied across the membrane.Membrane separations are commonly used in chemical industries that include desalination and component gas separation. In the following experiment we use a membrane separation process using a nonporous semi-permeable membrane. Membrane separations mainly focus on the principles of mass transfer. The experiment also focuses on rearranging the two membrane modules in series and in parallel to determine the optimal configuration for better separation.We adjusted the apparatus configuration such as inlet pressure,flow rate several times in order to get the most accurate results for run in parallel and series reaction. We used 20, 40, 60, and 80 psig for parallel and series run and calculated transport coefficients and recoveries with varying mass flows as well. Theory: Gas permeation is an important term that we use to describe a process using a nonporous semi-permeable membrane. Gas permeation is a technique for fractionating gas mixtures by using non-porous polymer membranes having a selective permeability to gas according to a dissolution–diffusion mechanism. The membrane gas separation process is driven by a pressure difference across the membrane. The gaseous feed stream is fractionated into permeate and non-permeate streams mechanism.

Membrane separation is primarily operated on the principle of different rates of mass transfer. In this lab, nitrogen and oxygen will be separated by passing through a semi permeable, non-porous membrane. The separation of the two in the membrane depends on the characteristic

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permeation rate of each, which is a function of the diffusivity and solubility. They can be described by equation 1, Fick’s Law, and Equation 2, Henry’s Law as follows: C −C J A = D A a1 Z a2 (Equation 1)

(

)

where, J A = flux of component A [mol/m2 /s] D A = diffusivity of component A [m2 /s] Z = thickness of membrane [m] C a1 = concentration of component A inside membrane wall on feed side [mol/m3] C a2 = concentration of component A outside membrane wall on permeate (shell side) [mol/m3] C A = pA S A

(Equation 2)

where, C A = concentration of component A inside membrane wall [mol/m3 /P a] S A = solubility constant for component A in the membrane [mol/m3 /P a] pA = partial pressure of component A in the gas phase [P a] Results:

Data was gathered for 15 consecutive runs for both series and parallel configurations.Upon setting the desired operating pressure from the air cylinder (in psi), the desired tube flow rate was selected in mL/min. A conversion from mL/min to standard liters per minute (SPLM) was needed to be compatible with mass flow meter. With these two parameters set, the system was allowed to achieve steady state after two minutes and readings were taken from tube and shell O2 analyzers, and the shell side mass flow meter. After collecting O2 data and steady state mass flow rates, mass transfer coefficients were calculated and compared to determine effectiveness of configurations and operating conditions. Air valves were arranged to allow for the parallel and series configuration. Four different operating pressure settings were tested, with varying mass flow rates at each of these pressures. By keeping pressure constant, we were able to identify the effects of mass flow rate on O2 levels in both tube and shell side, and subsequently, the effects on mass transfer coefficients KO2 and KN2. The following data was gathered for 15 runs in parallel configuration.

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Figure 1. Parallel configuration data.

Figure 2. Series configuration data.

Figures 3 & 4 indicates several trends. As operating pressure is increased, tube side oxygen concentration decreases with each increase in pressure, while shell side oxygen concentration increases. As the operating pressure is increased, more oxygen will permeate through the membrane into the shell side effluent, resulting in a greater oxygen transfer. Thus, Figure 3 indicates the transport coefficient of oxygen as a function of operating pressure. The comparison between series and parallel configurations can be seen Figure 4 . Refer to tables listed in the appendix for exact selectivity values.

Figure 3. O2 vs N2 Selectivity.

 Figure 4. Overall Transport Coefficient (Ratio of O2/N2).

From Figure 3, the data indicates a correlation between the oxygen selectivity and tube operating pressure, with flow rate as a parameter. As inlet flow rate is increased for a given pressure, it can be seen that a larger resultant selectivity coefficient will be produced. For a series module configuration, as operating pressure is increased, oxygen selectivity will decrease. The trend for a parallel configuration is not so clear, but does indicate the slight decrease in

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selectivity with increasing pressure. Conversely, as tube flow rate is increased, oxygen selectivity is proportional and will also increase for both configurations. From Figure 5, it is seen that a series configuration offers higher oxygen selectivity than parallel.

Figure 5. Membrane Selectivity of Oxygen

Discussion: To further investigate the effects on separation of nitrogen and oxygen with respect to varying operating pressures and flow rates, transport coefficients and recoveries were calculated and graphs plotted representing the relationship with pressure as well. As the graphs indicate, with an increase in operating pressure and flow rate, a steady increase in transport coefficients is also seen. Figure 2 and Figure 3 support this statement and exhibit the effect on values of transport coefficients with respect to series and parallel set up. As data indicates, recoveries are also higher in series than parallel which accounts for better separation with the former. With the transport coefficients for nitrogen, it is shown to be vice versa. With increase in flow rate and constant pressure, nitrogen transport coefficients show a steady decline. While calculating oxygen and nitrogen recoveries and plotting respective graphs to see how the values are affected by operating pressure. As the graphs indicate, higher the pressure and lower the flow rate, higher the recovery. Calculated O2 recovery is higher in series than in parallel indicating better air separation once again.

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Figure 6. Oxygen Recovery.

Conclusion: Data discussed above in this report strongly validates that a series configuration achieves better air separation than parallel therefore, producing higher transport coefficients. With higher operating pressures, recovery of the gas also increases and also the speed at which the gas flows through the membrane. Calculations also show that recovery and transport coefficients increase with increasing operating pressure exhibiting a directly proportional relationship. On the other hand, increasing flow rate results in a higher transport coefficient for oxygen and lower transport coefficient for nitrogen.

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References:

1. Zdunek, Alan, “Membrane Air Separation”, Student lab manual 2. McCabe, Smith, Harriot. “Membrane  Separation Processes.”  Unit Operations of Chemical Engineering. McGraw-Hill, New York: 2005. 3. R. H. Perry and D. W. Green, Eds., Perry’s Chemical Engineer’s Handbook, 7th ed. McGraw-Hill, 1997. 4. Mohanty, K., and M.K.Purkait,eds., “Membrane technologies and applications,” CRC Press, Boca Raton, FL(2011) 5.

Coker, David T., Rajeev Prabhakar, and Benny D. Freeman. "Gas Separation Using Polymers." Membranes in ChE Education  (2013). Print.

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Appendix II: Error Analysis The common error that our group noticed is the pressure was decreasing during parallel and series runs and we were adjusting it. This factor might affected our data collection. Another common mistake is that we didn’t calibrate it well to let battery-powered oxygen analyzer to reach its steady state. Appendix III: Sample Calculations Overall Transport Coefficients ( K O 2 and KN 2 ) For Run No. 1, in parallel operation, oxygen mole fraction of 0.139 fed, the operating pressure 20 psig, tube side flowrate 130 mL/min, the tube side oxygen level at 14%, the shell side flowrate 1.5 SLPM (standard liter per minute), and shell side oxygen level of 28.6%. F = tube side flowrate (gmol/min) = 500/22400 = 0.0223 gmol/min G = shell side flowrate (gmol/min) = (1.32*1000)/22400 = 0.0589 gmol/min Pt = tube side pressure (torr) = (20+14.7)/14.7 * 760 = 1,794.01 torr p1a = O 2 p artial pressure at tube inlet (torr) = 0.139 * 1,794.01 = 249.36 torr p1b = O 2 partial pressure at tube outlet (torr) = (13.9/100) * 1,794.01 = 249 torr p1t = O 2 average partial pressure in tube (torr) = (249.36 + 249)/2 = 249.18 torr p1s = O 2 p artial pressure in shell = (28.6*.01) * 760 =217.3 torr g1 = O 2 p ermeation rate (gmol/min) = (1.49*1000) * (28.6/100)/22400 = 0.0190 gmol/min KO 2 = 0.0190/(307-217) = 0.00003626 g mol O 2 /min torr Similarly, for N 2 where the tube side nitrogen level and the shell side nitrogen level: K N 2 = 0.00004665 gmol N 2 /min torr The ratio of the transport coefficients is 3.703 Separation Coefficients, Separation Factors, and Recovery (using the same conditions above for Run no. 1)  ole fraction at tube inlet = 0.209 x1a = O 2 m  ole fraction tube outlet = 13.9*.01 = 0.139 x1t = O 2 m x1s = O 2 mole fraction in shell = 0.286  ole fraction at tube inlet = 0.714 x2a = N 2 m  ole fraction tube outlet = 0.861 x2t = N 2 m x2s = N 2 mole fraction in shell = 0.714 α(AB) = separation coefficient = KO 2 / K N 2 = 3.70 α ′ (AB) = separation factor based on retentate composition = (0.286/0.714)/(0.139/0.861) = 2.48 α ′′ (AB) = separation factor based on feed composition = Unit Operations ChE-381 Group No. 3

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(0.286/0.714)/(0.209/0.809) = 1.49 O2 R  ecovery = (0.286 * 1.5)/(0.209 * (500/1000)) = 0.5480 N2 R  ecovery = (0.67 * 1.5)/(0.714 * (500/1000)) = 0.277

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Team Member Individual Contributions NAME: Raymond Trelka

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NAME: Reshma Ravi T  IME ( H  OURS)

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NAME: EKATERINA IVANOV

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