Composite Mixed Ion-Electron Conducting (MIEC) Membranes for Hydrogen Generation and Separation

Transcription

1 Composite Mixed Ion-Electron Conducting (MIEC) Membranes or ydrogen Generation and Separation aibing Wang, Srianth Gopalan, and Uday B. al Division o Materials Science and Engineering & Department o Mechanical Engineering Boston University

2 utline o the resentation Motivations. Description o hydrogen generation and separation process. Fabrication and microstructure o the membranes. ydrogen generation experiments. Eect o the surace catalyst on hydrogen generation process. Conclusions. Suggested uture wor.

3 Motivations and Research bjectives Motivations: Currently, more than 600 billion cubic meters o hydrogen consumed each year. Simultaneous generation o pure hydrogen and syn-gas production at commercially attractive rates. Research bjective: Develop and analyze a new approach or hydrogen generation and separation rom steam employing a MIEC based membrane architecture.

4 Conventional Membranes or ydrogen Separation Feed side (igh ) > ermeate side (Low ) Feed side (igh ) > ermeate side (Low ) e + Mixed protonelectron conducting membrane d alloy X=0 X=L X=0 X=L Approach : (dense ceramics) ydrogen separation using mixed proton-electron conducting membrane ermeation mechanism: Coupled diusion o protons and electrons Drawbac: Low hydrogen production rate Approach : (dense metals and alloys) ydrogen separation using d and its alloys ermeation mechanism: Solution-diusion mechanism Drawbac: Unstable when exposed to sulur hydrogen

5 Continued: External potential e V Feed side (igh ) > ermeate side (Low ) Steam rganic waste xygen ionic conducting membrane X=0 X=L X=0 X=L Approach 3: (Ionic conducting membrane) ydrogen production rom organic waste using oxygen ionic conducting membrane ermeation mechanism: xygen ions transport through the dense membrane Drawbac: Complicated device Approach 4: (Microporous membrane) ermeation mechanism: ydrogen gas diuse through the pores o the membrane. Drawbac: Low selectivity

6 ur ydrogen Separation Approach (Simultaneous production o hydrogen and syn-gas ) I. Feed Side: + e = - + igh, Mixed conducting membrane - e II. ermeate Side:C 4 C = C + + e C, Low MIEC dense membrane Material Selection: Gd 0. Ce (GDC)/Gd 0.08 Sr 0.88 Ti 0.95 Al ±δ (GSTA) GDC: Ionic conducting phase Gd Ce 3 Gd Ce V GSTA: n-type electronic conducting phase SrTi 3 Gd 3 + Ti Gd Sr + Ti Ti (g) + V + e Ti Ti + e = Ti Ti

7 Critical Thicness o the Membrane Flux J Regime I: Bul diusion controlled Critical thicness Regime II: Surace exchange reaction controlled Surace exchange is the rate controlling step J = (/ K r ex L + + D K v ex / τl + D ) s - Resistance o surace exchange K ex reaction on side Resistance o surace exchange K ex reaction on side L D v Bul diusion resistance τls Lc Resistance o gas diusion D - through porous support Bul diusion is the rate controlling step /L Dv r J = ( / / ) L L: membrane thicness S. J. Xu and W. J. Thomson, Chem. Eng. Sci., 54, 3839 (999).

8 Derivation o the ermeation Equation (a) Five steps involved in the oxygen permeation through mixed ion-electron conducting membrane: (b) Two Steps not considered: Step : gas phase mass transer o gaseous species on side Step 5: gas phase mass transer o gaseous species on side (c) Three important steps are considered: Step : surace exchange reaction on side 0.5 J = C V - Step 3: oxygen bul diusion process J = J V = D V (C L Step 4: surace exchange reaction on side and D V r V 0.5 J = r - C r - C V V ) The orward and reverse reaction rate constants or oxygen incorporation reaction. Functions o gas compositions, microstructure and properties o the membrane surace xygen vacancy diusivity + V + Interace I e x + V + + e r r Step Step 4 Step 3 Step Step 5 Mixed conducting membrane Interace II x C V Feed side (igh ), C V S. J. Xu and W. J. Thomson, Chem. Eng. Sci., 54, 3839 (999). e V X=0 X=L > ermeate side (Low ) L is the membrane thicness xygen vacancy concentration at the low and high oxygen partial pressure sides o the membrane

9 Continued: Derivation o the ermeation Flux Equation xygen permeation lux equation: DV r ( ) = L + D ( + J 0. 5 V ) Equation (4) can be urther simpliied as: J = r ( L + D V ) 0.5

10 Continued: Derivation o the ermeation Flux Equation The area speciic hydrogen generation rate can be expressed as: J = J = r ( L + D V ) 0.5 = r ( L + D K V 0.5 ex 0.5 ) + K ex (both surace exchange reactions and bul diusion process are included) ( ) Driving orce or the permeation process 0.5 = Resistance on the side K 0.5 ex = Resistance on the side K ex L D V Bul diusion resistance For the bul diusion controlled permeation process with small surace exchange resistances: J DV r = J = ( ) (nly bul diusion process is included) L

11 Membranes Architectures Dense thin membrane orous support Sel-supported thic membrane orous supported dense thin membrane Thicness o the porous support: mm Thicness o the dense membrane: 0-45 microns

12 Fabrication rocess o the orous Supported Dense Thin Membranes Fabrication process o porous support The membrane was reduced at 900 o C or 4 hours (GDC+40 Vol. % GSTA)-0wt%C Ball-milled, dried and press into pellets samples Carbon burnout at 500 o C or 3hrs Sintering o the membrane at 50 o C or 6hrs; heating and cooling rates are 3 o C/min Drying o the coated membrane at 70 o C or about 3hrs Fabrication o the thin coating Using spin coating technique artial sintering at 80 o Cor 6hrs Mixing o GDC-40 vol.%gsta, alpha-terpineol, with binder Fabrication o dense thin membrane using dip-coating or spin-coating techniques

13 Typical Microstructure o the orous Supported Membrane 0µm Dense Membrane orous Substrate Micrograph o the dense membrane Thicness o the dense membrane can be decreased to 0 µmor even thinner using a spin coating technique. The porosity o the porous support is around 36%

14 Set-up or ermeation Experiment C 4 C, Stainless steel ermeate side Low eating element Gold gaset Dense membrane MIEC membrane Gold o-ring orous substrate igh Alumina tube Feed side Schematic o dense membrane supported on porous substrate, It is assumed that CSTR (continuous stirred-tan reactor) model may be applied to membrane reactor

15 Calculation o the Area Speciic ydrogen Generation Rate Feed side ermeate side S S Gas inlet Reactor at 900 o C ( g) = ( g) + / ( g) Gas outlet rich mixture gas S MIEC membrane J = F in [( S A [( ) S 0.5 ) ( 0.5 S + K ) eq 0.5 ] ] S S K eq A in F The oxygen partial pressure on the side inlet The oxygen partial pressure on the side exit The equilibrium constant o the steam dissociation reaction The area o the membrane surace ydrogen low rate in the side inlet.

16 Inlet Gas Compositions on the Feed Side and ermeate Side Exp. # Feed side low rate (cc/min) ermeate side low rate (cc/min) Ar-0.% Ar-0.% Ar-5% Variable steam content: 3% (5 o C), 0% (45 o C) and 5 % (65 o C) Water bubbler Feed side ermeate side MIEC membrane Water bubbler 3% steam (ixed) Ar-0.% Ar-0.% Ar-5% Gas cylinder Gas cylinders

17 Results or the. mm Sel-Supported Membrane Area speciic hydrog gen generation rate MIEC membrane.mm 5% steam on ees side 0% steam on side 3% steam on side ( / 9 / ) / 0 (Driving orce) Nonlinear itting equation: J r = (/ K ex L + + D K v / ex Resistance o surace exchange K ex reaction on side J K ex L D v Dvr = L Resistance o surace exchange Reaction on side Bul diusion resistance Linear itting equation: ( / / ) )

18 Results or the 45 µm orous Supported Membrane 45 µm Dense thin membrane mm orous support generation rate Area speciic hydrogen 5% steam on side 0% steam on side 3% steam on side ( / 9 / ) / 0 (Driving orce)

19 Results or the 5 µm orous Supported Membrane 5 µm Dense thin membrane Area speciic hydrogen generation rate ( / mm orous support 5% steam on side 0% steam on side 9 / ) / 0 (Driving orce) 3% steam on side Nonlinear itting equation: J r (/ = K L D v ex L + + D K v / ex Resistance o surace exchange K ex reaction on side Resistance o surace exchange K ex reaction on side Bul diusion resistance )

20 Fitted arameters or 5 µm orous Supported Membrane Fitted parameters 3% steam 0% steam 5% steam r (µmol/cm.s) (cm/atm 0.5.s) Calculated parameters 3% steam 0% steam 5% steam / K ex / K ex L/D V (s/cm) (s/cm) (s/cm) (g) (g) + / ( g) K = ex Resistance on the side = K eq K = ex Resistance on the side = K eq

21 An Example o the Measured xygen artial ressures Exp. # on side outlet on side outlet E-4.8E E-6.6E E-7 4.E-7.0E E-8.0E-8.E-8 5.8E-9.49E E-0 9.6E E-3.58E-3.74E E E-4.60E E E-5.47E E E-6.04E-6

22 Fabrication Architecture o orous Supported Membrane Coated with Surace Catalyst The membrane was reduced at 900 o C or 4 hours Sintering o the membrane at 300 o C or 6hrs; heating and cooling rates are 3 o C/min Drying o the coated membrane at 70 o C or about 3hrs Ni-GDC surace catalyst GDC-GSTA dense membrane GDC-GSTA orous support Schematic membrane architecture Fabrication o the surace catalyst using spin coating technique Mixing o Ni--60 vol.% GDC, alpha-terpineol, Ti and binder Fabrication process o the surace catalyst coated membrane Dense thin membrane orous support. Cui, A. Karthieyan, S. Gopalan, and U. B. al, Electrochemical and Solid-State Letters, 9, A79-A8 (006)

23 Microstructure o MIEC Membrane Coated with Surace Catalyst 0 µm Ni-GDC 00 µm Surace catalyst orous substrate Dense thin membrane Schematic o the surace catalyst coated membrane Gold gaset 0 µm Cross-section orous substrate Dense thin membrane Surace catalyst

24 ermeation Experiment or Membranes with Surace Catalyst Surace catalyst exposed to the side Surace catalyst exposed to the side C 4 C,, Stainless steel ermeate side Stainless steel Feed side Low eating element Low eating element MIEC membrane Gold o-ring MIEC membrane Gold o-ring igh Alumina tube igh Alumina tube Feed side ermeate side, C, C 4

25 Eect o the Surace Catalyst on the ydrogen Generation Rates Surace catalyst coated membrane ( / 9 / ) / 0 Steam content on the side o the membrane: 5%

26 Eect o Surace Catalyst on Feed Side and ermeate Side Surace catalyst exposed to the side Surace catalyst exposed to the side 5% steam on side.0 5% steam on side 9. 0% steam on side % steam on side % steam on side.54 3% steam on side ( / 9 / ) / 0 ( / 9 / ) / 0 Feed side catalyzed ermeate side catalyzed Thicness o the dense membrane: 35 µm Thicness o the surace catalyst : ~8 µm Thicness o the porous support: mm

27 Eect o Catalyst Thicness on ydrogen Generation Rate 8 µm 5 µm 8 µm 5 µm ( / / ) / / / ) / 0 ( Surace catalyst exposed to the side Surace catalyst exposed to the side Steam content on side: 5% Steam content on side : 5%

28 Comparison o the ydrogen Generation Rates using Dierent Membrane Technologies Materials ydrogen generation rate Membrane thicness GDC-GSTA µmol cm- s - coated with surace 35 µm membrane catalyst perating temperature 73 K SrCe 0.95 Yb α.5 µmol cm- s - µm membrane 950 K [a] [c] d-40cu 5 µmol cm- s -.3 µm membrane 638 K [b] In actual membrane reactor or hydrogen production using our approach, the steam content on the side will be increased to ~00% and a much higher hydrogen production rate is expected. [a]: Satoshi amaawa et al. Solid State Ionics, 48 (00) 7-8 [b]: aul M. Theon et al, Desalination, 93 (006) 4-9 [c]: S. Gopalan, A. V. Virar, Journal o The Electrochemical Society, 40, (993)

29 Conclusions and Future Wor Conclusions: orous supported dense membrane as thin as 0 µm was abricated using dipcoating and spin coating techniques. A mathematical model or the calculation o the area speciic hydrogen generation rates was derived based on the measured oxygen partial pressures, gas compositions, and gas low rates o the inlet and outlet gases on the side. ydrogen generation/oxygen permeation process or the sel standing thic membranes is mainly controlled by the oxygen bul diusion process, while the hydrogen generation or the porous supported dense thin membranes is controlled by both the surace exchange reactions and bul diusion process. The eect o the surace catalyst is more pronounced when it is exposed to the side rather than the side, and a high area speciic hydrogen generation rate (.0µmol.cm -.s - ) was obtained.

30 Wor In rogress: Catalyze both suraces o the membrane to get higher hydrogen generation rates. Investigate alternative surace catalyst or partial oxidation o hydrocarbons. Incorporation o pore diusion resistance in the transport model. J = r ( / / L τ L K s ex D V K ex D - ) τl D s s - is the resistance o the gas diusion through the porous substrate L is the thicness o the porous support is the porosity o the porous support is a parameter including the partial pressure o gas composition τ is the tortuosity o the porosity D - is the binary diusivity o the system