Hydrogen Production using Water-splitting Cycles

Hydrogen Production using Water-splitting Cycles

Generation
of
Hydrogen
Production
Hydrogen Production using Water-splitting Cycles
*
Cycles Through Water Splitting
Chemical, Biological, and Materials Engineering - University of Oklahoma

Jeff Jenneman , James Phan , Quang Nguyen and Miguel Bagajewicz
**

Abstract

**

Background

Cycle Generation Code

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 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.
Molecular Model

Functional Group Method

Atomic Balance

A simple cycle that was included in Browns study was the Hallet Air Products cycle,
shown below.

Introduction
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 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.
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.

Thermodynamic Evaluation
Results

Purpose
The purpose of this project is to discover unique water-splitting cycles that
operate at lower temperatures which employ non-metallic reactants.

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.
2 Reactants 2 Products

2 Reactants 3 Products

Cycle Configurations
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 (H 2O),
Hydrogen (H2), and Oxygen (O2). The considered cycle configurations are
shown below.
1 Reactants 2 Products
H2O + A B + C + H2/0.5O2

3 Reactions - Electrolysis

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. 2 nd 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.

Picture of the Heat Cascade

The efficiencies listed above do not include separation work

2 Reactants 1 Products
H2O + A + B C + H2/0.5O2

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.

Efficiency of 2 reactant/2 product
80
70

B + C A + H2/0.5O2
2 Reactants 2 Products
H2O + A + B C + D + H2/0.5O2

C A + B + H2/0.5O2

60

2 Reactants 3 Products

50
Cycle 1

H2O + A + B C + D + E + H2/0.5O2

40

Cycle 2
Cycle 3

30

C + D A + B + H2/0.5O2

C + D + E A + B + H2/0.5O2

20

2 Reactants 2 Products
(3 Reactions)

E + F A + B + H2/0.5O2

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.

10

Welectric = zFE (Nernst Equation)

0
w/o Wsep

H2O + A + B C + D + H2/0.5O2
C+D E+F

Separation and Electrical Work

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.

w/ Work sep

Excess Reactant

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

*This work was done as part of the Chemical Engineering Capstone at OU
** Undergraduate Capstone Students

Efficiency
The cycle efficiencies, , can be calculated according the equation below.

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