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Memo Report: Membrane Separator

TO: Professor Ravindra Datta

FROM: Martin Burkardt ____________________

Project Leader

Michael Bodanza ___________________

Brandon Clark _____________________

Sotirios Filippou ___________________

Marcus Lundgren __________________

DATE: September 9th, 2017

SUBJECT: Enriched Oxygen-Supply System Design Proposal

Introduction

Membrane separation processing has become a common, reliable practice for separating gases, especially in

industrial settings. In comparison with other equipment such as distillation, membrane separators require less

energy, minimal size, and mild conditions. Separation occurs in these units using a selective barrier that operates

based on different permeabilities through the membrane. In the case of nonporous membranes, such as the Pilot

Scale Prism© Membrane unit in Goddard Hall, the mechanism for separation is determined by the dissolution

and diffusion of each gas species within the membrane. The driving force for this separation is the partial pressure

difference of the two components on the two sides and not volatility differences of the feed. Permeance increases

proportionally with pressure.

The purpose of this experiment was to use the Monsanto Company's Pilot Scale Prism© gas permeation

unit to gather experimental data that can then be used to scale-up an oxygen enriching process. The U.O Lab

Supervisor for the Oxymoron Chemical Company has requested a method to produce a continuous supply of

oxygen-enriched air, at 250 SCFM of at least 43% oxygen, from clean dry air at ambient temperature and pressure

up to 110 psig, in order to renovate one of our processing plants. For our purposes, ideally, outlet pressure should

be high.

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The pilot unit in the lab is composed of four 1-inch (inner diameter) aluminum tubes mounted parallel in

series, and contain a proprietary design of hollow polysulfone fibers that acts as an ideal nonporous membrane

through which the feed will separate the oxygen by solubility-based diffusion (Figure 1a). In this scenario, there

are two streams: the permeate stream and the nonpermeate stream. The nonpermeate stream is located within the

shell of the aluminum tubes, and the hollow channels of the polysulfone fibers contain the permeate stream (Figure

1b). There are thousands of these hollow fibers placed in a compact module in order to provide a higher surface

area for a small volume unit. Under normal operating conditions, the permeate is oxygen-rich, while the

nonpermeate is nitrogen-rich.

Figure 1: Membrane Separator. The aluminum tubes are filled with thousands of fibers (left, a). The feed passes through four of these

tubes. Depending on which of the valves are opened in the permeate stream, will determine the direction of separation (countercurrent

or cocurrent).

Methodology

The oxygen analyzers were calibrated by allowing a feed of compressed air to bypass the separator and

enter the nonpermeate and permeate streams. The system was maintained at 40 psig and all instruments were

allowed to stabilize before corrections were made to the known oxygen concentration, 21%.

Following calibration, the system was operated separately with pure oxygen and pure nitrogen feeds under

various pressures, ranging from 40 to 100 psig, to calculate the membrane permeability of each species. The

selectivity is determined by the ratio of these permeabilities.

The system was run with a feed of air under various operating conditions, including various stage cuts,

pressures, and flow configurations. The flow can be altered between cocurrent and countercurrent flow by

selectively opening two of the valves from the permeate streams (Figure 1). For countercurrent flow, the feed

pressure was varied for four pressures, ranging from 40 to 100 psig. For each pressure, the nonpermeate needle

valve was opened at various levels to vary the stage cut over the range of 0.75 to 0.22 (Figure 2). Once the system

had equilibrated, the outlet flow rates, pressures and oxygen compositions for each stream were recorded. An

additional run was conducted under cocurrent flow at 100 psig with the same range of stage cuts.

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Figure 2: Pilot Scale P&ID. Control in this system is maintained by a single needle valve on the nonpermeate stream.

Results and Discussion

Our team conducted a feasibility study via COMSOL Multiphysics and Mathcad. We then collected

preliminary data for a scale-up via a pilot scale Prism membrane. We were able to determine the separation factor,

α, of the membrane-separation unit under these operating conditions by measuring the flux of oxygen and nitrogen

separately at several operating pressures (see Appendix A). The selectivity was found to be 6.05 by dividing the

flux trend line for oxygen by the trend line for nitrogen (Figure 3). We also studied the effect of operating pressure

and stage cut on the separation process. Using Fick’s law of diffusion, we are able to study concentration, velocity,

and pressure profiles.

Figure 3: Species Permeance. The slope of the curve plotting the driving force vs. the flux is the permeance of the individual species.

Data Collected from the Pilot Plant

When measuring oxygen mole percentage in the retentate (non-permeate) and permeate against stage cut, they

both tend to decrease as stage cut increases. A larger stage cut allows less air to release into the retentate stream,

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creating more time for oxygen to find the membrane and lowering the retentate oxygen mole percentage.

Conversely, the permeate stream begins at a high oxygen mole percentage because as the inlet stream is readily

pushed towards the retentate, the nitrogen can easily be swept past, and the membrane’s oxygen selectivity takes

over.

Figure 4: Effect of Stage Cut on Outlet Oxygen Percentage. Generally, as the stage cut decreases the outlet oxygen concentration

increases. This is especially so for the permeate stream. Additionally, as the feed pressure increases, the permeate oxygen concentration

increases.

When measuring oxygen mole percentage against inlet pressure, it seems that higher pressures force more

oxygen through the membrane compared to nitrogen, resulting in higher percentages in the permeate. Partial

pressure differences across the membrane are the driving force for mass transfer in this system. This difference

in effect on permeate percentage between low and high pressure becomes less pronounced as stage cut increases,

since less nitrogen can escape into the retentate.

Furthermore, it is interesting to note that at higher pressures such as 80 and 100 psig (countercurrent

operation) the oxygen mole percentage in the retentate increases sharply at high stage cuts. This might be due to

the fact that, at a certain turning point, a large stage cut and high partial pressure allows enough time and driving

force for nitrogen to permeate more through the membrane, thus counteracting the membrane selectivity exhibited

at lower stage cuts. Curiously, the co-current setup, operating at a high pressure of 100 psig, shows no sign of a

drastic oxygen mole percentage increase. This can either suggest a difference in permeability at high pressures

based upon stream orientation or a potential error in measurement, as this trend is not observed in any of our other

data.

Figure 5: Effect of Stage Cut on Outlet Flow Rates. Interestingly, the permeate flow rate is not affected by stage cut, but responds

more to feed pressure, while the nonpermeate responds more dramatically.

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When measuring flow rates compared to stage cuts, it seems that as the stage cut increases the retentate flow rate

decreases exponentially, as shown in Figure 5 (left). However, as the stage cut increases the permeate flow rate

sees little to no effect (Figure 5, right). This is because the rate-limiting step for mass transfer to the permeate is

the permeability of oxygen and nitrogen through the membrane, which is determined by the partial pressure

difference of these gas components on both sides. The retentate flow rate itself does not affect the permeate flow

rate; it only lowers the selectivity for oxygen through the membrane. The retentate flow rate mainly affects the

inlet flow rate. Therefore, by shutting the needle valve, the retentate flow decreases, forcing the inlet flow rate to

decrease, thus increasing the stage cut. This idea is why the permeate pressure is relatively constant with respect

to stage cut as well.

Figure 6: Effect of Stage Cut on Permeate Pressure. The stage cut is ineffective on the permeate pressure.

When measuring permeate pressure against inlet pressure, there seems to be no appreciable trend. Manometer

inches of water are a pretty small unit of measurement, so any trends that might occur would be hard to pick up.

Regardless, this data might be due to the membrane holding most of the inlet pressure on the retentate side.

Another curious observation was that the retentate flow meter sign said that it measured flow rate in SCFM

(standard cubic feet per minute), while the permeate flow meter measured flow in SCCM (standard cubic

centimeters per minute). The former unit is 28,317 times smaller than the latter, which is quite large of a difference

as shown in the Figure 6 (right). This is most likely a mistake on the retentate sign, since the two flows should be

a similar order of magnitude.

Verification of Data with Theory

After all data was collected, the selectivity calculated previously was used in partial differential equations that

modeled the pilot separation system for countercurrent and co-current flow (see Appendix B). These

calculations are the differential versions of the perfect mixing model, which uses two mass balances and two

flux expressions, as seen below. All calculations were performed in PTC Mathcad. This is necessary because

the permeability driving force changes continuously along the length of the membrane tube.

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Both theoretical models seem to match up very well with actual permeate flow rate data. What looks more

problematic is that both models also consistently predict higher permeate oxygen percentages than what was

measured. However, this can be explained by the oxygen meter performance in the lab. Since, they took an

extremely long time to equilibrate and come to a final reading, the team decided to record oxygen levels after a

period specified by the instructor in the interest of time. Since the oxygen percentage trend looks correct, it can

be safely assumed that both models predict percentages relatively well.

Figure 7: Comparison of Theoretical to Data Calculations. On comparison, the model accurately predicts the data collected. Some

deviations do occur, but are not appreciable.

Calculating Required Number of Membrane Tube Modules

From the determined correlation that a decrease in stage-cut is proportional to an increase in permeate

concentration, we decided to investigate the lowest stage cut at each operating pressure/system orientation. Of all

of the feed pressures, the 100 psig countercurrent set up provided above the required oxygen percentage (44%)

with the highest permeate flow rate (72.5 SCCM), which would minimize the number of tubes needed for the

final separation system design. Therefore, this set-up is recommended. The area of one pilot plant tube is smaller

than that of the ones specified by a factor of 4.7. However, increasing both the area and flow rate by the same

factor scales the permeate flow by that exact same factor, while preserving all outlet parameters in the Mathcad

countercurrent flow model. This means that scaling up the area and feed, and therefore the permeate flow to the

necessary 250 SCFM, doesn’t require complex calculation. Scaling up the two countercurrent pilot tubes by 4.7

creates two tubes that can provide 341 SCCM’s. Dividing this number by 28,316, the conversion factor mentioned

earlier, gives a SCFM value of 0.01204. Dividing 250 SCFM by 0.01204 SCFM gives the total number of pairs

of 12 cm, 1.5 m length tubes needed to provide 250 SCFM of flow. This number is quite large, 20,760 pairs, or

41,520 modules.

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Conclusion and Recommendations

Based upon the preliminary dataset collected through testing of the pilot scale membrane system, it is

recommended that 41,520 polysulfone membrane modules be used; where each module is 12 centimeters in

diameter and 1.5 meters in length. Operating at a system pressure of 100 psig would be adequate at producing

250 SCFM of 44% pure oxygen (1% above the design requirements). Under these conditions, we can produce the

highest flow and pressure of product, and the second-highest purity oxygen compared to the other potential

operating conditions that were investigated.

While this seems like a lot of modules, keep in mind that this is a scaled up, industrialized process that

requires an extremely large flow rate compared to that of the pilot system. Each module takes up only a volume

of 0.017 m2. Meaning that 41,520 would take up a minimum of 706 m3. This space can be provided at a large

scale, industrialized plant.

We always recommend that rapid changes to the system be avoided to prevent damage to the membrane.

High pressures can result in cracks or leaks in the system, or overload and damage the rotameters. Chemical

hazards such as flammability, explosions, and toxicity may be produced in the event that oxygen or nitrogen

escapes the system. Precautionary measures should be taken in order to make this process as safe for users as

possible.

References

Turton, R., et al. (2018). Analysis, Synthesis, and Design of Chemical Processes: Fourth Edition.

Prentice Hall. Print.

S. Weller, W.A. Steiner, Separation of gases by fractional permeation through membranes, Journal

of Applied Physics 21(4) (1950) 279–283.

C.T., Blaisdell, K. Kammermeyer, Counter-current and co-current gas separation, Chemical

Engineering Science 28(6) (1973) 1249-1255.

C.Y. Pan, H.W. Habgood, An analysis of the single-stage gaseous permeation process, Industrial

& Engineering Chemistry Fundamentals 13(4) (1974) 323–331.

W.P. Walawender, S.A. Stern, Analysis of membrane separation parameters. II. Counter-current

and cocurrent flow in a single permeation stage, Separation Science 7(5) (1972) 553-584.

C.J. Geankoplis, Transport Processes and Separation Process Principles (Includes Unit

Operations), Ch. 13, Prentice Hall Press, 2003.

R.A. Davis, O.C. Sandall, A simple analysis for gas separation membrane experiments,

Chemical Engineering Education 37(1) (2003) 74-80.

H. Binous, Gas permeation computations with Mathematica, Chemical Engineering Education,

40(2) (2006) 140-144.

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Appendix

Appendix A: Excel Data-Pilot Plant Data and Selectivity Calculations

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Appendix B: Mathcad Calculations on Countercurrent Model

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Appendix C: Mathcad Calculations on Co-current Model

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Appendix D: Nomenclature

Am - membrane surface area, Am,t = total membrane surface area

a - dimensionless membrane area, Am/Am,t

B - defined in equation 8

Lf - total inlet gas flow rate

Lo - total nonpermeate/retentate flow rate

l - dimensionless non-permeate flow rate, L/Lf

pf - nonpermeate/retentate pressure

pp - permeate pressure

Q’1 - ratio of oxygen permeability to membrane thickness

Q’2 - ratio of nitrogen permeability to membrane thickness

VP - total permeate flow

v - dimensionless permeate flow rate, V/Lf

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xf - mole fraction oxygen in feed

xo - mole fraction oxygen in nonpermeate/retentate

yP - mole fraction oxygen in permeate