Jeff Jenneman James Phan Quang Nguyen Miguel Bagajewicz

Background Decreasing supply of fossil fuels. Increasing demand from large developing countries like China and India. Need for reduced CO 2 output and other greenhouse gases. Hydrogen Economy – Argues hydrogen fuel is the answer for the replacement of fossil fuels. Clean, environmentally friendly, form of energy

Background Water-Splitting Cycles Main overall reaction: H 2 O  H 2 + 1/2O 2 Thermal decomposition of water occurs at about 2500 K Electrolysis of water found to be only 30 – 45 % efficient Instead, to lower temperatures, a set of reactions with the net products of H 2 and O 2 Other materials are continually used and regenerated

Outline Introduction and Purpose Code for cycle generation Restraints Configurations Thermodynamic data Results Cycle generation Cycle analysis Separation Work Functional Group Method Summary

Introduction and Purpose Previous methods of hydrogen production employ High temperatures Nuclear reactors assumed as high temperature heat sources Metallic/inorganic reactants and products

Previous Work at OU “Nuclear Production of Hydrogen Using Thermochemical Water Splitting Cycles” by Brown et. al Over 100 potential water splitting cycles analyzed and reduced to 25 cycles for detailed study Cycle criteria included: 1.Number of reactions and/or separation steps in the cycle 2.Number of elements in the cycle 3.Are non-stationary solid reactants involved? 4.Corrosive properties of chemicals used, etc.

Previous Work at OU Most of the work done in previous water splitting projects has dealt with thermodynamic analysis and efficiency screening for currently developed cycles Analysis of 10 cycles compared cycle configurations, excess reactants, ideal separation work, pinch analysis, etc. First project aimed at developing new, unique, lower temperature water splitting cycles

Previous Work at OU Conclusions Hydrogen production cycles can quickly be ranked based on efficiency The best configuration depends on the considered cycle Phase separation and good cascade properties

Purpose of this Work Find unique cycles that operate at lower temperatures employing non-metallic/inorganic reactants Methods Use a pool of molecules and discover cycles by enumeration Use functional groups instead of a pool of initial molecules

Outline Introduction and Purpose Code for cycle generation Restraints Configurations Separation work Thermodynamic data Results Cycle generation Cycle analysis Functional Group Method Summary

Code for Cycle Generation Pool of 100 molecules Cyclic and non-cyclic compounds Choose 4 unique molecules Each must be unique H 2 O + aA + bB  cC + dD + H 2 / ½O 2 cC + dD  aA + bB + H 2 / ½O 2

Constraints Atomic balance Stoichiometric coefficients Coefficients are < 5 Gibbs Energy (< 50) Varying temperature ΔG < 0 – Reaction favorable 0 < ΔG < 50 – Reaction possibly favorable

Atomic Balance H 2 O + aA + bB  cC + dD +H 2 / ½O 2 cC + dD  aA + bB + H 2 / ½O 2 Oxygen Balance: O A + O B – O C – O D = 0 Hydrogen Balance: H A + H B – H C – H D + 2 = 0 Atom X balance: X A + X B – X C – X D = 0 Atom Y balance: Y A + Y B – Y C – Y D = 0 Coefficient Matrix Variable Matrix Solution Matrix

Kinetics The reaction kinetics were not directly studied in this project. Some constraints were built into the code to ensure the cycles generated had the greatest kinetic viability The coefficients in each reaction were limited to 5 Successfully generated cycles that required more than 5 total molecules in any one reaction were further eliminated due low kinetic feasibility.

Flow sheet for Code Populate Atom check of products and reactants If not satisfied  Next species Build matrix If determ = 0  Next species Calculate coefficients If Coeff >5 or <0.5  Next species Calculate Gibbs Energy If Gibbs > 0 and Temp < 1000  Change Temp If Gibbs > 50 and Temp > 1000  Next species Calculate equilibrium constant and heat of RXN Calculate efficiency Print results Atomic Balance Pinch analysis to obtain utilities

Pinch Analysis – Minimum Heat Duty No heat can transfer from cold streams to hot streams Minimum approach temperature of 10 K Divides streams in reactor network into sections in which heat can transfer.

∆T min T1 Reaction 2 ∆H r2 = E 2 kJ Reaction 1 ∆H r1 = E 1 kJ 298 ½O 2 H2H2 H2OH2O Interval Pinch Analysis 2 Reactant/2 Product 1 (Rxn 2) (Rxn 1) 5 6 T2 T1 + ∆T min T2 + ∆T min T2 - ∆T min T1 T1- ∆T min CD AB 7

Thermodynamic Data Sources Gibbs and Enthalpy of formation as well as heat capacity correlations Chemical Properties Handbook by C.L. Yaws JANAF Thermochemical Tables

Comparison of Yaws Handbook and JANAF Thermochemical Tables Some disagreement with Yaws correlations and JANAF data. In these cases curve fitting to JANAF data was done to develop accurate correlations in pinch calculations.

Equilibrium constant and Efficiency Equilibrium constant Efficiency = heat of formation of water at 298K Hot Utility is found from the heat cascade Equilibrium constant determines amount of conversion used for separation work calculations

Separation Work Addition of ideal separation work to adjust efficiency calculation Efficiency of 50% assumed Excess reactants - Increased reactant to water ratio to 3:1 from stoichiometric feed.

Electrolysis/Hybrid Cycles In cases where Gibbs energy greater than zero electrolysis possibility examined W electric = zF∆E (Nernst Equation) z is number of electrons transferred F is Faraday’s Constant ∆E is change in electric potential or electromotive force High Gibbs energy, electrolysis is used Cl 2 + 2e -  2Cl - ∆E 0 = V

Outline Introduction and Purpose Code for cycle generation Restraints Configurations Thermodynamic data Results Cycle Configurations New Cycles Separation Work Functional Group Method Summary

Cycle Configurations 1 Reactant/2 Products H 2 O + aA  bB + cC + H 2 / ½O 2 bB + cC  aA + H 2 / ½O 2 2 Reactants/1 Product H 2 O + aA + bB  cC + H 2 / ½O 2 cC  aA + bB + H 2 / ½O 2 2 Reactants/2 Products H 2 O + aA + bB  cC + dD + H 2 / ½O 2 cC + dD  aA + bB + H 2 / ½O 2 3 Reactants/2 Products H 2 O +aA +bB +cC  dD +eE + H 2 / ½O 2 dD +eE  aA + bB + cC + H 2 / ½O 2 3 Reactions H 2 O + aA + bB  cC + dD +½O 2 cC + dD  eE + fF eE + fF  aA + bB + H 2

Cycle Configurations 2 Reactants 2 Products 1 Reactant 2 Products 2 Reactants 1 Product 3 Reactants 2 Products 2 Reactants 2 Products (3 Reactions) 400 Unique Cycles Highest Efficiency 72.66% 0 Unique Cycles 160 Unique Cycles Highest Efficiency 50.40% 600 Unique Cycle Highest Efficiency 71.71% < 10 Cycles Possible Electrolysis

New Cycles 2 reactants/2 Products

New Cycles 3 reactants/2 Products This reaction still has 7 molecules reacting in the first reaction, so the kinetic feasibility is extremely low

Separation Work Separation work reduces efficiency about 10 %, excess reactant reduces 15 %; however, about 65 % increase in hydrogen conversion

Separation Work Separation work reduces efficiency about 7 %, excess reactant reduces 7 %; however, about 68 % increase in hydrogen conversion

3 Reaction Cycle with Electrolysis/Excess Reactants Best efficiency and lowest temperatures of any cycle found. Kinetically, the low number of reactants in each reaction looks more realistic. However, large excess reactants, 10 to 1 ratio to water feed, in reaction 1 needed for significant conversion.

Cycles From Functional Groups Generate molecules using a combination of functional groups The same algorithm is then used The generation of the molecules even for a small number of functional requires a large number of enumerations. Large amount of enumerations required very long computing times, and the scope of this method was limited to only one reaction configuration. Advantage is more elements can be integrated into search since molecules are not chosen by handpicked method

Functional Group Method Flow sheet for the functional group method Populate Check Constraints Check for Duplicates Print Results If not satisfied then next species If duplicate found then next species

Functional Group Method Set of 15 functional groups The first functional group is left blank to numerate molecules of less than 5 functional groups For example: CH 3 – F Functional Groups -CH 3 >CH 2 >CH->C< =CH 2 =CH-=C<=C==CH = C- -F-Cl-Br-O-

Functional Group Method Previous work by Joback and Stephanopoulos (1989) Used a variety of computational techniques to find new compounds possessing improved properties for refrigerants Evaluated a number of structural and chemical properties

Constraints Our code used some of Joback and Stephanopoulos’s structural constraints. Examples are: 1. The number of groups having an odd number of free bonds must be even 2. There are 3 types of bonds: single, double, and triple there must be an even number of each bond present in each molecule

Cycles produced using the functional group method 308 unique molecular formulas were found using combinations of the 15 functional groups Cycle were produced in the following configuration H 2 O + aA  bB + H 2 -½O 2 bB  aA + H 2 -½O 2

Cycles produced using the functional group method Cycle 1: H 2 O + C 2 H 6 O  C 2 H 6 O 2 + H 2 C 2 H 6 O 2  C 2 H 6 O + ½O 2 Both of the reactions in this cycle had highly positive Gibbs Energy of formation for the temperature range of 400 to 1000K Cycle 2: H 2 O + C 2 H 4  C 2 H 6 + ½O 2 C 2 H 6  C 2 H 4 + H 2 The second reaction has a negative value for Gibbs energy at a temperature of 700 K However, the first reaction has a highly positive Gibbs energy

Conclusions Successfully generated and thermodynamically evaluated new water splitting cycles for a variety of cycle configurations Efficiencies of non-electrolysis cycles reached a maximum of ~75% Ideal separation work on average reduced total efficiency about 10% Successful in identifying hybrid cycles and additional electric work. The most practical cycle found achieved 90% cycle efficiency