Novel Design of an Integrated Pulp Mill Biorefinery for the ...

3.3.2.4 CO conversion and efficiency
Fig. 15. shows the effect of H2/CO ratio on syngas conversion and product selectivity.
The dependence of one-through conversion on the reaction pressure and temperature are shown
in Fig.6[48]. From Fig.3.3.4, when H2/CO ratio is 0.8, the DME selectivity is more than 95% and
syngas conversion is more than 50%. From Fig.16., at 270 o C and 50 bar, the CO conversion is
about 60%. Therefore, we suppose our CO conversion is 60%, and hence the syngas to DME
efficiency is about 51%.
Figure 16: CO conversion as a function of temperature and pressure.
3.3.3 Product separation and purification
The separation scheme design is based on the invention of Peng. et al[105]. The DME
reactor effluent, which is a mixture of DME, methanol, CO2 and water and unconverted syngas
will be cooled and fed to a high-pressure flash column. The high-pressure flash column will
separate the polar molecules and non-polar molecules. The vapor phase containing DME, CO2
and unconverted syngas (non-polar molecules) will pass through a scrubbing column. DME is
used as the scrubbing solvent to remove DME and CO2 and methanol from the unconverted
syngas. The mixture of DME and CO2 from the bottom of the scrubbing column is fed to a
DME-CO2 distillation column to separate CO2 from the product DME.
3.4 Fischer-Tropsch synthesis
This section will describe and elaborate on a design for the production FTD, based on
black liquor and coal feed stocks. Company such as Sasol and Texaco, which produce large
amount of FTD, do not share the specifics of there plant designs. For this reason, scraps of
information that are found in literature must be pieced together with assumptions to create a
design.
Figure 28 list all of the major parameters of the FTD production design. LTFT was used
because it produces a greater yield of FTD then HTFT. A slurry-phase bubbling-bed reactor was
chosen because of its higher efficiency compared to a fixed bed reactor. Slurry-phase bubblingbed
reactors also have responsive temperature control and low down time in the case of catalyst
poisoning. An iron based catalyst was chosen because of its low H2/CO ratio requirement, so that
an external water gas shift was not necessary. An α of 0.94 was used to maximize wax
production and minimize light hydrocarbon production.
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