Systematic Design Procedures for Natural Gas Desulphurization

Natural gas is a mixture of variable hydrocarbons and many contain other
contaminants such as nitrogen, carbon dioxide and sulfur. The undesirable
compound such as hydrogen sulfide must be removed to prevent corrosion and
environmental problems. Sulfur compound also poisons the catalysts and
consequently disrupts reactor performance. Reactor systems that have been poisoned
by sulphur sees lower the conversion, lower selectivity and higher temperature
requirement for a particular lower conversion. While sulfur removal from natural
gas stream is necessary, there is insufficient framework to systematically design the
removal system. The objective of this project is therefore to develop a framework
for the systematic design of H2S adsorber. The desired design framework will be
able to predict the breakthrough curve of the chemisorption reaction and determine
the size of the adsorption column. The project will also study the interactions
between parameters that affect the system's design. The desulphurization system
selected in this project uses zinc oxide adsorbent. Additionally HiS is chosen as the
adsorbate in the natural gas stream. Desulphurization of natural gas is a two step
process; firstly the natural gas containing organic sulfur is catalytically
hydrogenated to H2S. Then the natural gas stream containing H2S is send to a
chemisorption column. Zinc oxide adsorbent is converted to zinc sulfide upon
contact with H2S. The shrinking core model is selected to describe the solid gas
reaction on the surface of the adsorbent. The model considers chemical reaction
coupled with diffusion as the rate limiting step. Solutions of the shrinking core
model enable the prediction of breakthrough curve. The shrinking core model was
found to give a good description of the sulfur removal process whereby it has been
found that the conversion of single solid sorbent increases continuously with time
until it completely converted in 8.32min. A single zinc oxide pellet able to adsorb
1.66E-03 moles ofH2Sper hour before it reaches its breakthrough limit. Further on,
sizing is done to calculate the amount ofadsorbent needed for a column. The results
obtained are 34,747 kg ZnO needed for an adsorption column with a service lifetime
of 6 months and the dimensions of column from calculation deviates less than 5%
then the actual industrial equipment. Therefore, the systematic design procedures
outlined are applicable for industrial use.
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