Date of Award

2018

Document Type

Open Access Thesis

Department

Chemical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Edward P. Gatzke

Abstract

The Hybrid Sulfur process is a thermo-electrochemical cycle used to produce hydrogen from water. The process requires a high temperature energy source for H2SO4 decomposition with temperature reaching 800°C. This step is followed by SO2 - depolarized water electrolysis. Using solar energy as the high temperature energy source allows for efficient environmentally friendly production of hydrogen. This method is an alternative to traditional photovoltaic electrolysis for hydrogen production. Making the process economically competitive is a major challenge. Operating the process with changes in the availability of solar energy also increases process complexity. The dependence of the process on solar energy requires analysis of the electrolysis and decomposition sections separately.

The Hybrid Sulphur process was modelled in ASPEN Plus for a target production rate of 500 gram moles of H2 per second. The process simulation includes H2SO4 decomposition and O2 separation of the SO2/O2 product from the H2SO4 decomposition. Given the transient nature of solar energy utilized for the decomposition reaction, analysis of the dynamics of the separation section is of primary importance. A dynamic simulation was developed with control schemes to stabilize the process. This simulation was analyzed for step changes in feed flowrate corresponding to the target hydrogen production rate of 500 gram moles per second. With the proposed controller

configuration, the separation process exhibits time constants ranging from approximately 40 min for a step change in the overall production rate from 100% to 50%. The settling time for the same production rate change is approximately 60 min. The separation system can accommodate the system operating a 0% capacity by maintaining column flow with dilution water. At zero feed the process is functional but it just the recycles the water from the electrolyzer section through the system making it entirely redundant and uneconomical. To avoid shutdown of the separation section at low production rates, this work proposes to include holdup storage tanks for the product streams from the decomposition section. This will allow the distillation columns to run continuously, but the separation system must accommodate variable feed rates. Dynamic variation in the separation section caused by changes in the solar-powered decomposition reactor may thus be mitigated by use of gas and liquid holdup tanks. The results of these simulation prove vital in analysis of the viability of the future for large scale hydrogen production through high temperature Hybrid Sulphur process.

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