Enriching Trace Impurities in Hydrogen

Sheldon Lee, Dennis Papadias, Shabbir Ahmed, and Romesh KumarArgonne National Laboratory, Argonne, IL

Presented at the NHA Hydrogen ConferenceLong Beach, CA, May 3-6, 2010

Gas suppliers are concerned about the cost of analysis and certification

Analysis at such low concentrations requires advanced analytical equipment and expertise

– 1GC-PDHID for CO, GC-SCD for S, GC-NCD for ammonia, etc.

ASTM is developing standardized methods

1J.P. Hsu, “H2 Gas Analysis Certification: Challenges and Options,” DOE H2 Quality Working Group Meeting, Oct.2006GC – Gas Chromatograph, PDHID – Pulse Discharge Helium Ionization Detector, SCD – Sulfur Chemiluminescence Detector, NCD – Nitrogen Chemiluminescence Detector

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Hydrogen, minimum 99.97 %

Impurities & Limits1 Max. ppm

Carbon Dioxide 2

Carbon Monoxide 0.2*

Ammonia 0.1

Sulfur (as H2S, COS, etc.) 0.004*

Proposed H2 Fuel Quality Guidelines

1Total of gaseous non-helium impurities: <100 ppm

* The allowable limits for CO and S have recently been revised to 0.1 ppm and 1 ppb, respectively.

Argonne is evaluating two sampling devices that incorporate enrichment of the impurities– Enable rapid on-site monitoring of one or more key impurity species– Enable use of simpler analytical devices

Method 1 – Permeate the hydrogen outRetain the impurities

Collect the sample at a high pressure, P1 (say, 1000 psia) H2 permeation lowers the pressure to P2 (say, 25 psia) Enrichment factor E ≈ P1/P2 (E=40) Hydrogen is removed through a heated Pd-alloy membrane

P2 ,TP1

To Analyzer / Vent

H2 Sample

Chamber 1 Heater Rod

Relief Valve

Membrane TubeChamber 2

Permeate H2

500 mL 300 mL

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The permeation rate is sensitive to the pressure and membrane temperature

The hydrogen permeation rate is a function of membrane temperature, surface area, and differential pressure across the membrane

Enrichment time of the device needs to balance between these parameters and the volume of enriched sample (in Chamber 2) needed for analysis

0

200

400

600

800

1000

0 1 2 3 4 5 6Time, hours

Pre

ssur

e (p

sig)

, T

empe

ratu

re (

°C)

0

5

10

15

20

25

Hyd

roge

n R

emov

ed (

L)

Membrane Temperature

P1

P2

H2 Removed (Calculated)

10/06/09

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Sulfur reduces membrane permeabilityHigher temperature accelerates permeation

5

0

5

10

15

20

25

0 200 400 600 800

Time, minutes

H2 P

erm

ea

ted

(L

)

250°C - 2 ppm H2S(10/13/09)

270°C - 2 ppm H2S(10/14/09)

250°C (No S)(10/06/09)

Tests validated theoretical predictions of enrichment with excellent reproducibility

Test (M1) 1 5Initial Pressure, psig 800 800Final Pressure, psig 62.6 62.6Theor. Enrichment, ET 15.0 14.9Membrane Temperature, °C 250 270

EA : N2

Species Balance Error15.8 + 5.3%

15.9+ 6.9%

EA : CH4

Species Balance Error

14.5- 3.3%

14.4- 3.4%

EA : COSpecies Balance Error

14.5- 3.4%

14.4- 3.6%

EA : CO2

Species Balance Error

14.8- 1.6%

14.5- 2.5%

EA : H2SSpecies Balance Error

6.8- 54.7%

13.1- 12.4%

ET : Theoretical Enrichment Factor EA : Analytical Enrichment Factor

Sample Gas CompositionCO2, CH4, N2 = ~0.1%CO = 100 ppmH2S = 2 ppm

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Enriched Gas CompositionCO2 = 1.502 ± 0.002 %CH4 = 1.510 ± 0.001 %N2 = 1.556 ± 0.003 %CO = 1410 ± 20 ppm H2S = 28 ± ? ppm

%,1001

T

A

E

ESpecies Balance Error

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The enrichment factor is affected by the pressures and the chamber volumes

50 100 150 200 250 300 350 400 4500

50

100

150

200

250

Sampling Vessel Pressure (atm)

Th

eore

tica

l E

nri

chm

ent

Fac

tor

(TE

F)

(P lo, Vr=1)

(60

00

ps

i)

(Plo, Vr=0.5)(0.5xPlo, Vr=1)

Sampling Vessel Volume = V1 ; Enrichment Vessel Volume = V2 ; Volume Ratio = Vr = V1/V2 ; Post Permeation Pressure, Plo = 3 atm

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Designing the membrane enrichment device will need to balance the constraints of the application

The design (size, membrane temperature) needs to balance the trade-offs– Enrichment factor

• Can be increased with higher pressure ratios– Enriched gas volume

• Can be increased with higher enriched gas pressure– Enrichment time

• Can be decreased with more membrane area or flux• Can be decreased if enriched gas volume need can be reduced

– Cost• Can be reduced with reduction in membrane surface area

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Enrichment by Adsorption

Method 2 – Trap the impurities on a sorbent Flow the analyte gas at high pressure P1 (say, 700 psig) over a

sorbent bed Reduce the sorbent chamber pressure to P2 (say 6 psig) to

desorb the impurities Analyze the released gas, which contains a higher concentration

of the impurities

Back-Pressure Regulator (BPR)

To GC

Vent

Mass Flow Meter

123

Relief Valve

Chamber 1(Sorbent)

Chamber 2(Empty)

H2 Sample

To GC

P1 P2

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A hydrogen gas with ~0.2% each of CO, CO2, CH4, and N2 was flowed through the sorbent device 700 psig pressure during adsorption 24 g of activated carbon Up to 900 ml/min of gas flow Total sorption time: 150 minutes

N2, CO breakthrough occurred in less than 4 minutes– Equilibrated within 70 liters of flow

CO2 adsorbs most strongly and was the last to breakthrough – Needed 150 liters to equilibrate 0.00

0.05

0.10

0.15

0.20

0.25

0 50 100 150 200Gas Flow, Liters

Exit G

as C

onc.

, % CO2

CH4CO

N2

1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.451

2

3

4

5

6

7

The model predictions approach the measured concentrations in Chamber 1

N2

CO

CH4

CO2

Pressure (atm)

Co

nce

ntr

ati

on

in

Ch

am

be

r 1

(%)

En

rich

men

t F

acto

r

35

30

25

20

15

10

5

12

Exptl. Data: 9/8/09

The chamber pressure decreased when the enriched gas sample was withdrawn for analysis

Lower pressure led to release of adsorbed gas The concentration of impurities in the gas phase increased

1st

ga

s a

na

lysi

s

2n

d g

as

an

aly

sis

Chamber 2 enrichment factors are lower but are not affected by sample withdrawal

Sorbent : Activated Carbon, 24 gChamber 1 (Sorbent) Vol. = 50 mLChamber 2 (Empty) Vol. = 10 mLTest (M2) 9/04/09 9/08/09

Initial Pressure, P1, psig 700 700Flow Rate, SLPM 1.1 1.1Total Flow at P1, Std. Liters 200 200Final Pressure, P2, psig 6 6Analytical Enrichment, EA : N2 5.51 5.53Analytical Enrichment, EA : CO 6.31 6.35Analytical Enrichment, EA : CH4 8.56 8.55Analytical Enrichment, EA : CO2 9.82 9.82Analytical Enrichment, EA : H2S -- --

The enrichment factors follow the adsorption energy for each species

CO2 concentration was enriched the most

The EA values are based on the average concentrations from five analyses

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0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Enri

ched

Con

cent

rati

on, %

N2 CH4 CO CO2

9/4

/20

09

9/8

/20

09

The pressure swing based enrichment device is simple

The reproducibility of the experiments is excellent– Standard deviation as % of average concentration

• N2: ±0.3%; CO: ±0.07%; CH4: ±0.2%; CO2: ±0.4%

The enrichment process can be accelerated– With a smaller sorbent chamber

The enrichment factors can be increased by– Using a combination of sorbents– Using a wider pressure swing

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The choice of enrichment device can be determined from the priorities

Membrane Adsorption

Sample Collection Time1 Faster Slower

Enrichment Time Slower Faster

Enrichment Factor Higher (Uniform)

Lower (Varies with Species)

Sample Gas Needed Less More

Hardware Simpler

Operation Simpler

Effect of Sulfur Slows permeation Adsorbs strongly

Temperature Cycle Larger2 Small

Cost Lower

1Based on size of the units in the laboratory, 2May not be necessary

Summary

Both systems take advantage of high pressure of the hydrogen The hydrogen permeating membrane system can easily enrich

the impurities by more than two orders of magnitude The pressure swing adsorption system is a very simple device The enrichment devices will enable analysis of trace

impurities using less expensive analytical instruments

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