Most of the available tools and methods applied in the design of chemical
processes are not effective at the critical early stages of design when the process
data is very limited. Businesses are often under pressure to deliver products in
shorter times and this in turn prevents the evaluation of options. Early
identification of options will allow for the development of an experimental
program that will support the design process.
The main objective of this work is to apply the Process Synthesis approach to
develop a structured method of designing a process using mostly qualitative
information based on limited experimental data, prior experience, literature and
assumptions. Fischer-Tropsch (FT) synthesis of hydrocarbons from syngas
generated by reforming natural gas and/or coal has been used as a case study to
illustrate this method. Simple calculations based on experimental data and basic
thermodynamics have been used to generate some FT Synthesis flowsheet
models. The evaluation of different flowsheet models was done using carbon
efficiency as a measure of process efficiency.
It was established that when choosing the optimal region for the operation and
design of an FT Synthesis process, the influence of the system parameters must be
well understood. This is only possible if the kinetics, reactor, and process design
are done iteratively. It was recommend not to optimize the reactor independent of
the process in which it is going to be used without understanding the impact of its
operating conditions on the entire process. Operating an FT Synthesis process at
low CO per-pass conversions was found to be more beneficial as this will avoid
the generation of high amounts of methane which normally results in large
recycles and compression costs.
Whether the process is run as a once-through or recycle process, the trend should
be to minimize the formation of lighter gases by obtaining high Alpha values because carbon efficiency increases with the increase in value. Experiments should be
performed to obtain process operating conditions that will yield high values.
However, if the aim is to maximize diesel production by hydrocracking long chain
hydrocarbons (waxes), then an optimal value should be targeted to avoid the
cost of hydrocracking these very heavy waxes. The choice of the syngas
generation technology has a direct impact on the carbon efficiency of an FT
synthesis plant. This study has established that running an FT synthesis process
with syngas obtained by steam reforming of natural gas with CO2 addition can
yield high carbon efficiencies especially in situations were CO2 is readily
available. In FT synthesis, CO2 is normally produced during energy generation
and its emission into the environment can be minimized by using it as feed during
the steam reforming of natural gas to produce syngas.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5808 |
| Date | 23 October 2008 |
| Creators | Mukoma, Peter |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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