Water-Splitting Cycles-Presentation

Water-Splitting Cycles-Presentation - Evaluation of...

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Unformatted text preview: Evaluation of Hydrogen Production Cycles Based on Efficiency on Ch E 4273 Senior Design Spring 2007 Purpose Purpose Develop and implement a method Develop of ranking water-splitting cycles of splitting Outline Outline Importance of Hydrogen Importance Thermochemical/Hybrid Cycles Current Methods of Evaluation Current Our Methodology based on Efficiency Our Results Results Hydrogen Economy Hydrogen Advantages Advantages Disadvantages Disadvantages Methods for producing hydrogen Methods Other suggested economy types Other Hydrogen Economy Hydrogen The current U.S. demand † – 11 MM tons/yr total Crude lightening Crude Produce cleaner-burning fuels Produce Methanol/Ammonia production Methanol/Ammonia – Steam reformation of methane Demand in Hydrogen Economy † – 200 MM tons/yr for transportation – 450 MM tons/yr for all other energy needs † K. R. Schultz 2003, General Atomics, DOE grant Water-Splitting Cycles Water 1 H 2O ⇔ H 2 + O2 2 Only products are hydrogen and oxygen Only reactant is water Other species form and breakdown in a cyclic manner, non-effluent Water Splitting Cycles Water Thermochemical Cycles Thermochemical – Thermochemical reactions only Hybrid Cycles Hybrid – Thermochemical reactions – Electrochemical reactions Some could benefit from nuclear plant heat sources sources Over 200 known cycles Over Water Splitting Cycles Water Thermochemical Reactions Thermochemical – Established equipment in industry – Often require high temperatures Electrochemical Reactions Electrochemical – More rare in industry – Difficult to implement – Occur at lower temperatures Water Splitting Cycle Examples Water Hallett Air Products (hybrid) Hallett 1 Cl2 + H 2O → 2 HCl + O2 2 2 HCl 298 K → Cl2 + H 2 1123 K US Chlorine (thermochemical) US 1 Cl2 + H 2O → 2 HCl + O2 2 1123 K 2CuCl2 → 2CuCl + Cl2 773 K 2CuCl + 2 HCl 473 K → 2CuCl2 + H 2 Water Splitting Cycle Example Water Hallett Air Products (hybrid) Water Splitting Cycle Example Water US Chlorine (thermochemical) Issues Issues Coupled reactions Coupled – Cyclic Process- Degrees of freedom Special reaction methods (high ∆G>0) – Electrolysis Electrolysis – Continuous product removal Reaction Equilibrium Reaction – Reversibility – Equilibrium conversion Separation Energy (heat and work) may complicate the flowsheet flowsheet Prior Evaluation Efforts Prior L.C. Brown (2000, DOE) scored cycles based on known characteristics based – Qualitative approach – Questionable evaluation technique Point system for process complexities Point Equal weighting assigned to all criteria Equal “Judgment call” basis “Judgment Heavily favors well-researched cycles Heavily Prior Evaluation Efforts Prior Prior Evaluation Efforts Prior Lewis (2005, DOE) evaluated cycles based on efficiency on – Quantitative approach – Considers equilibrium – Assumes 50% efficiency for all work processes Actual efficiency may be better or worse Actual Different processes have different efficiencies Different Our Proposed Solution Our Efficiency Based Ranking System: Find maximum possible efficiency Find – Pinch technology to determine hot utility Stream analysis Stream Heat of reaction Heat – Estimate work required Consider reaction equilibrium Consider DOF: optimize flow rates and temperatures temperatures Cycle Efficiency Cycle η=− ∆H H 2O °(298K ) Q +W Enthalpy of formation of water is minimum energy required for water-splitting energy Cycle Efficiency Cycle Only heat and work transferred across system boundary included system Heat term Heat – Hot utility – Heat for separations Work term – Pump/compressor work – Electrical work in electrolysis Equilibrium Equilibrium ∆G = ∑ν i (µi ° + RT ln ai ) Setting ∆GRXN equal to zero gives the following, K EQ ∆GRxn ° = exp − RT K EQ = ∏ ai νi Equilibrium Equilibrium Sulfur Iodine – Reaction 1 (1123 K) 1 H 2 SO4 (v) → SO2 (v) + H 2O(v) + O2 (v) 2 1123 K K1 (T1 ) = nSO2 n n ⋅P .5 H 2O O2 1.5 nH 2 SO4 (nH 2 SO4 + nSO2 + nH 2O + nO2 ) 1.5 Relate number of moles to conversion ni = ni,o + νiXi ⇒ K (T ) = f ( X ) P1.5 1 1 Excess Reactants Excess Two primary options of handling Recycle Recycle – Increases separation Increases requirement requirement – Strategic product Strategic placement placement No Recycle No – Increases heat Increases requirement requirement – 2 configurations for cycles configurations with 3 or more reactions with No Recycle Handling No Sulfur Iodine Cycle: 1 H 2SO 4 T =1123K → SO 2 + H 2 O + O 2 2 2HI T =723K → I 2 + H 2 I 2 + SO 2 + 2H 2 O T =393K → 2HI + H 2SO 4 Strategic Separation Minimum Separation Strategic Separation Minimum Separation Degrees of Freedom Degrees Caused by linearly dependent equations Caused Assume design parameters to define system Assume Hallett Air Products 1 Cl2 + H 2O → 2 HCl + O2 2 2 HCl 298 K → Cl2 + H 2 1123 K Degree of Freedom Degree Reaction #1: K1 (T1 ) = Mix Point #1: 2 .5 n HCl ,1nO2 ,1 nCl2 ,1n H 2O ,1[nCl2 ,1 + n H 2O ,1 + n HCl ,1 + nO2 ,1 ].5 nH 2O ,1 = nH 2O , 0 − ξ1 nHCl ,1 = 2ξ1 n H 2O , 0 = 1 + nH 2O ,1 nCl2 , 0 = nCl2 ,1 + nCl2 , 2 nO2 ,1 = 0.5ξ1 nCl2 ,1 = nCl2 , 0 − ξ1 P 0.5 Reaction #2 (electrolysis): nHCl , 0 = 2ξ 2 nCl2 , 2 = ξ 2 nH 2 , 2 = ξ 2 Mix Point #2: nHCl , 0 = nHCl ,1 Degree of Freedom Degree Example: Hallett Air Products Cycle Example: – 10 variables – 9 linearly independent equations – DOF = 10 – 9 = 1 DOF After substitutions: After – Define nCl2,0 Define K1 (T1 ) = nCl2 , 0 = nCl2 ,1 + 1 2 2 0.5 0.5 nCl2 ,1nH 2O ,1[nCl2 ,1 + nH 2O ,1 + 2 + 0.5].5 nH 2O , 0 = 1 + nH 2O ,1 P 0. 5 Optimizing Excess Optimizing – DOF = 1 – Define nCl2,0 Define 44% Efficiency Hallett Air Products Cycle Products 46% 42% 40% 38% 36% 1 1.4 1.8 2.2 Amount of Cl2 (mol) nCl2,0 = 1.49 mol 1.49 Cl2,0 Efficiency = 44.7% Efficiency 2.6 3 Minimum Utility Minimum Popular method of finding hot utility Popular Heat cascaded from high T low T low Pinch occurs at temperature where cumulative system heat is zero system No heat is transferred across the pinch No Minimum Utility Minimum Interval analysis Interval Single hot utility & cold utility Single Isothermal reaction Heat of reaction Heat Effect of ∆Tmin on Qhot on Influence of T on Qhot 750 Julich UT-3 Tokyo 650 Ispra Mark 9 Ispra Mark 4 Gaz de France 550 Qhot (kJ) Hallet Air Products Ispra Mark 13 Sulfur Iodine US Chlorine 450 Westinghouse 350 250 0 10 20 ∆ Tmin (K) 30 Electrical Work Electrical Electrical Work Electrical – Nernst equation for electrolytic cells – Assume steady-state operation – Assume cell efficiency of 90%† WElectric = −n ⋅ F ⋅ E ° † Millikan, Christopher E., DOE 2002 Separations Separations Separation Energy Separation – Minimum work found Estimate separation efficiencies as 50%† Estimate – Complete separation – Phase separation when possible – Isothermal Process – Real heat & work found † Michele A. Lewis, “FY 2005 Progress Report” Minimum Separation energy Minimum Separate species into streams Separate – Depends on excess handling configuration – Phase separation requires no energy Example: Sulfur Iodine RXN#1 Separation Example: Minimum Separation Work Minimum WSEP , Minimum = ∆GMixing = R ⋅ T ∑ ni ⋅ ln( xi ) i WSEP , Minimum = R ⋅ T ∑ ni ⋅ ln( xi ) − ∑ ni ⋅ ln( xi ) OUT i IN i Minimum Separation Work Minimum Results Results Degree of Freedom Degree Constant for all configurations investigated Constant – Does not include reaction temperatures Minimum Utility Criterion Cycle rankings based on Qhot (∆Tmin=10K) – Optimized feeds, equilibrium considered, reactants recycled Minimum Utility and Electrolysis Work Criterion Work Cycle rankings based on Qhot (∆Tmin=10K) and =10K) Welec only – Optimized feeds, equilibrium considered Minimum Utility, Electrolysis Work and (ideal) Separation Work (η=0.5) Criterion (ideal) Cycle rankings based on Qhot (∆Tmin=10K), Welec, =10K), and Wsep – Optimized feeds, equilibrium considered Temperature Optimization Temperature Limited by phase changes Limited – Optimized feeds, equilibrium considered Comparison of Results Comparison W is ideal/0.5 is All energy terms have been included All Different option is best depending on cycle Different Comparison with Brown’s Results Comparison Real Separation Energy Real Numerous recycle configurations Numerous Numerous separation techniques Numerous Difficult to achieve 100% separation Difficult Used membrane separators to estimate separation work for gas phase separations separation Membrane separators cannot operate at high temperatures temperatures Real Separation Energy Real Real Separation work estimated for top 4 cycles Real Estimated as compressor work for membrane separator separator Real Separation Work (Membranes Only) (Membranes Cycle Reaction 1 Reaction 2 Reaction 3 Wsep Ideal (kJ) Wsep Real (kJ) Efficiency w/ Wsep Real US Chlorine Membrane HCl,O2 HCl/O2 Phase CuCl2,H2 CuCl2/H2 Phase CuCl,Cl2 CuCl/Cl2 61.75 83.28 77.4% Sulfur Iodine Membrane H2O,SO2,O2 Phase HI,H2SO4 HI/H2SO4 Phase I2,H2 I2/H2 110.68 119.80 70.4% Phase H2SO4,H2 H2SO4/O2 N/A 23.36 32.5 79.0% Phase K,K2O2 K/K2O2 Phase KOH,O2 KOH/O2 0 0 74.6% H2O,SO2/O2 Westinghouse Membrane H2O,SO2,O2 H2O,SO2/O2 Gaz de France Phase K2O,H2 K2O/H2 Real Separation Energy Real Actual work for a given system can only be determined with a detailed flow sheet be Taken from Y.H. Jeong, M.S. Kazimi, K.J. Hohnholt, and B. Yildiz resource Real Separation Work Real Comparison of efficiency for Lewis separation and real separation for top 4 cycles and Screening Process Screening Over 200 documented cycles Over Find a quick method of finding efficiency Find Screening Process Screening Exclude process with high exothermic reaction(s) at low temperature reaction(s Conclusions Conclusions Efficiency based method can quickly rank hydrogen producing cycles hydrogen Best configuration of excess handling depends on cycle being considered depends Phase separation and good cascade properties benefit efficiency properties Questions Questions References References Aqueous Standard Reduction Potentials”. Physical Sciences Aqueous Information Gateway. 9 March 2006 < http://www.psigate.ac.uk >. http:// Bagajewicz, M. “Pinch and Minimum Utility Usage”. 11 March 2007 Bagajewicz, <http://www.ou.edu/class/che-design/a-design/ design/ Minimum%20Utility.pdf>. Minimum%20Utility.pdf Brown, L.C., S.K. Showalter, and J.F. Funk. Nuclear Production of Brown, Hydrogen Using Thermochemical Water-Splitting Cycles, 2000. Hydrogen JANAF Data. National Institute of Standards and Technology. 9 March arch 2006 <http://webbook.nist.gov>. 2006 Jeong Y.H., M. S. Kazimi, K.J. Hohnholt, and B. Yildiz. Optimization of Y.H., Kazimi K.J. Hohnholt and Yildiz the Hybrid Sulfur Cycle for Hydrogen Generation. MIT-NES-TR-004, MIT 004, 2005. 2005. Lewis, M. A.; High-Temperature Thermochemical Processes. DOE Lewis, High DOE Hydrogen Program, FY 2005 Progress Report, September 2005. Hydrogen Perry, Robert H. Perry’s Chemical Engineers’ Handbook, 7th ed. Perry, Perry’s McGraw-Hill, 1997. Winnick, Jack. Chemical Engineering Thermodynamics. John Wiley & Chemical Sons, Inc, 1997. Sons, ...
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