Hydrogen Production using Water-splitting Cycles

1. Hydrogen Production using Water-splitting Cycles Generation of Hydrogen Production Cycles Through Water Splitting* Chemical, Biological, and Materials Engineering - University of Oklahoma Jeff Jenneman**, James Phan**, Quang Nguyen and Miguel Bagajewicz • Background • Thermodynamic Evaluation • Cycle Configurations • Results • Cycle Generation Code The minimum heat requirement for each generated cycle was determined from the pinch design method. The method identifies components that require heat addition and those that require cooling. A minimum approach temperature for any hot stream heat transfer to cold streams of 10 degrees Kelvin is assumed. The heat transfer between streams is divided into intervals in which it is allowable (i.e. 2nd Law of Thermodynamics) for heat transfer to occur. Included in this calculation is the heat supply or excess from the heat of reactions. The process is illustrated in the schematic below. The minimum heat requirement is determined by finding the change in enthalpy for each interval. This value for the highest temperature interval is added to the next lowest interval and so on. The value of these successive sums that is lowest (most negative) represents the maximum heat deficiency in the process and this value is each to the required minimum heat utility in order that thermodynamic laws are not violated. The code that was discussed in the previous section successfully generated a number of cycles for various configurations. There was one cycle configuration that did not yield any results, the 1 reactant – 2 products configuration. Despite the fact that there were cycles generated that satisfied the atomic balance, the thermodynamic analysis was cut short for a couple of different reasons. If the temperature of any one reaction is greater than 1000 K then it would be deemed unfit; additionally, if the Gibbs energy of formation does not satisfy the specified requirements it is also considered unfit and the code will move on to the next enumeration. The table below outlines the results of each cycle configuration including the number of cycles found and the highest efficiency. Additionally, two cycles are shown; one 2 reactant-2 product and one 3 reactant-2 product. Each of the cycles shown is the highest efficiency cycle for the specified configuration. • Introduction Water-splitting cycles are reactions, or sets of reactions, with net reactants of water and net products of molecular Hydrogen and Oxygen. Conventional thermal decomposition of water occurs at a temperature of 2500 K. However, the incorporation of two or more cycles can significantly reduce the temperatures of each reaction in the cycle. Brown conducted a study to evaluate and rank all the water-splitting cycles. A simple cycle that was included in Brown’s study was the Hallet Air Products cycle, shown below. The Hallet Air Products is a two reaction cycle and it is clearly shown that introduction of the second reaction dramatically reduces the temperature needed for the decomposition of water. Most of the cycles that were included Brown study involved temperatures in excess of 1000 K. The cycle configuration is a simple 1 reactant – 1 product cycle and it is clearly shown that the molecular Chlorine used in the first reaction is regenerated in the second reaction. This simple cycle configuration was a model that was used in the cycle generation code. • Abstract The cycle generation involved two different methods; a method that starts with an initial pool of molecules and discovers various cycles through the enumeration of each molecule and a method that combines functional groups to form molecules. For the functional group method, once the molecules have been generated , it can be placed in the molecule method and used in the same manner to generate cycles. The molecule method uses a pool of 100 molecules, both cyclic and non-cyclic, as well as organic and inorganic molecules. The flow sheets for the two methods are detailed below. • Purpose • Hydrogen fuel is viewed as a possible alternative energy source to fossil fuels and have minimal negative effects on the environment. Water splitting cycles produce hydrogen and oxygen from the decomposition of water. Multiple reactions, with continual regeneration and reuse of reactants, are the important attributes of water splitting cycles. In previous years, students have evaluated various potential water splitting cycles that were cited in literature as the most practically viable cycles. The evaluation included finding the minimum required heat utility, separation work, and electrical work for electrolysis reactions in a variety of cycle configurations. This work is the first at OU aimed at generating new water splitting cycles. Cycles were found by using a computer algorithm generated in VBA. The diminishing supply of hydrocarbons and their negative environmental impact has resulted in a great need to find alternative energy sources. Hydrogen production from the chemical decomposition of water using multiple reactions resulting in the net production of hydrogen and oxygen are termed water splitting cycles. Previous work at OU focused on developing good efficiency evaluation procedures. In this presentation we describe a computer algorithm that was implemented to find new potential water splitting cycles and evaluate their thermodynamic feasibility. The goal is to find cycles that are viable at low reaction temperatures. The purpose of this project is to discover unique water-splitting cycles that operate at lower temperatures which employ non-metallic reactants. Molecular Model Functional Group Method Atomic Balance The molecular method utilizes matrix multiplication and other operations to solve four atom balances simultaneously to solve for the stoichiometric coefficients. The molecular method code has built-in functions for calculation of Gibbs energy and Enthalpies of Formation for the pool of 100 molecules. Joback and Stephanopoulos conducted a study to find new molecules and evaluate them chemically and structurally. For the purpose of this project, only the structural constraints were considered when generating the pool of molecules. 2 Reactants – 2 Products 2 Reactants – 3 Products A number of different cycle configurations were considered in this project in order to provide the most greatest amount of variability . In addition to searching other cycle configurations for viability, the code has built-in routines to vary which reaction produces the hydrogen and oxygen. The naming convention for the cycle configurations in the code indicate the number of products and reactants present in the first reactions excluding water (H2O), Hydrogen (H2), and Oxygen (O2). The considered cycle configurations are shown below. 1 Reactants – 2 Products 2 Reactants – 1 Products H2O + A  B + C + H2/0.5O2 B + C  A + H2/0.5O2 H2O + A + B  C + H2/0.5O2 C  A + B + H2/0.5O2 Picture of the Heat Cascade The efficiencies listed above do not include separation work 3 Reactions - Electrolysis 2 Reactants – 2 Products 2 Reactants – 3 Products H2O + A + B  C + D + H2/0.5O2 C + D  A + B + H2/0.5O2 H2O + A + B  C + D + E + H2/0.5O2 C + D + E  A + B + H2/0.5O2 Separation and Electrical Work The ideal separation work and electrical work, if electrolysis reaction is in cycle, are included into the total calculation for the cycle efficiency. A 50 % efficiency is assumed for separation work, and 90% efficiency for electrical work. 2 Reactants – 2 Products (3 Reactions) Welectric= zF∆E (Nernst Equation) H2O + A + B  C + D + H2/0.5O2 C + D  E + F E + F  A + B + H2/0.5O2 Efficiency The cycle efficiencies, η, can be calculated according the equation below. Separation work reduces efficiency about 10 %, excess reactant reduces 15 %; however, about 65 % increase in hydrogen conversion 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. *This work was done as part of the Chemical Engineering Capstone at OU ** Undergraduate Capstone Students