Enriching Trace Impurities in Hydrogen Sheldon Lee, Dennis Papadias, Shabbir Ahmed, and Romesh Kumar Argonne National Laboratory, Argonne, IL Presented at the NHA Hydrogen Conference Long 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 – 1 GC-PDHID for CO, GC-SCD for S, GC-NCD for ammonia, etc.  ASTM is developing standardized methods 1 J.P. Hsu, “H 2 Gas Analysis Certification: Challenges and Options,” DOE H 2 Quality Working Group Meeting, Oct.2006 GC – Gas Chromatograph, PDHID – Pulse Discharge Helium Ionization Detector, SCD – Sulfur Chemiluminescence Detector, NCD – Nitrogen Chemiluminescence Detector 2 Hydrogen, minimum99.97 % Impurities & Limits 1 Max. ppm Carbon Dioxide 2 Carbon Monoxide 0.2* Ammonia 0.1 Sulfur (as H 2 S, COS, etc.) 0.004* Proposed H 2 Fuel Quality Guidelines 1 Total 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 out Retain the impurities  Collect the sample at a high pressure, P 1 (say, 1000 psia)  H 2 permeation lowers the pressure to P 2 (say, 25 psia)  Enrichment factor E ≈ P 1 /P 2 (E=40)  Hydrogen is removed through a heated Pd-alloy membrane 500 mL300 mL 3

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 4

Sulfur reduces membrane permeability Higher temperature accelerates permeation 5

Tests validated theoretical predictions of enrichment with excellent reproducibility Test (M1) 15 Initial Pressure, psig800 Final Pressure, psig62.6 Theor. Enrichment, E T Membrane Temperature, °C E A : N 2 Species Balance Error % % E A : CH 4 Species Balance Error % % E A : CO Species Balance Error % % E A : CO 2 Species Balance Error % % E A : H 2 S Species Balance Error % % E T : Theoretical Enrichment Factor E A : Analytical Enrichment Factor Sample Gas Composition CO 2, CH 4, N 2 = ~0.1% CO = 100 ppm H 2 S = 2 ppm 6 Enriched Gas Composition CO 2 = ± % CH 4 = ± % N 2 = ± % CO = 1410 ± 20 ppm H 2 S = 28 ± ? ppm Species Balance Error

7 The enrichment factor is affected by the pressures and the chamber volumes Sampling Vessel Pressure (atm) Theoretical Enrichment Factor (TEF) (P lo, V r =1) (6000 psi) (P lo, V r =0.5) (0.5xP lo, V r =1) Sampling Vessel Volume = V 1 ; Enrichment Vessel Volume = V 2 ; Volume Ratio = V r = V 1 /V 2 ; Post Permeation Pressure, P lo = 3 atm

8 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

9 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 10

11 A hydrogen gas with ~0.2% each of CO, CO 2, CH 4, and N 2 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  N 2, CO breakthrough occurred in less than 4 minutes –Equilibrated within 70 liters of flow  CO 2 adsorbs most strongly and was the last to breakthrough –Needed 150 liters to equilibrate

The model predictions approach the measured concentrations in Chamber 1 N2N2 CO CH 4 CO 2 Pressure (atm) Concentration in Chamber 1 (%) Enrichment Factor 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 gas analysis 2nd gas analysis

Chamber 2 enrichment factors are lower but are not affected by sample withdrawal Sorbent : Activated Carbon, 24 g Chamber 1 (Sorbent) Vol. = 50 mL Chamber 2 (Empty) Vol. = 10 mL Test (M2) 9/04/099/08/09 Initial Pressure, P 1, psig 700 Flow Rate, SLPM 1.1 Total Flow at P 1, Std. Liters 200 Final Pressure, P 2, psig 66 Analytical Enrichment, E A : N Analytical Enrichment, E A : CO Analytical Enrichment, E A : CH Analytical Enrichment, E A : CO Analytical Enrichment, E A : H 2 S --  The enrichment factors follow the adsorption energy for each species  CO 2 concentration was enriched the most The E A values are based on the average concentrations from five analyses 13

The pressure swing based enrichment device is simple  The reproducibility of the experiments is excellent –Standard deviation as % of average concentration N 2 : ±0.3%; CO: ±0.07%; CH 4 : ±0.2%; CO 2 : ±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 14

15 The choice of enrichment device can be determined from the priorities MembraneAdsorption Sample Collection Time 1 FasterSlower Enrichment TimeSlowerFaster Enrichment Factor Higher (Uniform) Lower (Varies with Species) Sample Gas NeededLessMore HardwareSimpler OperationSimpler Effect of SulfurSlows permeationAdsorbs strongly Temperature CycleLarger 2 Small CostLower 1 Based on size of the units in the laboratory, 2 May 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 16