Scale-up and Demonstration of Fischer-Tropsch Technology

Scale-up and demonstration of Fischer-Tropsch technology 3
2.2. Technology and scale-up issues
The advantages described in Ch. 2.1. were all identified and formed the basis for
StatoilHydro to select the slurry bubble column reactor concept as the preferred FT
technology at an early stage. A small Fischer-Tropsch (FT) pilot plant was built and
successfully operated from 1987-1994 to demonstrate the basic viability of the slurry
reactor / cobalt catalyst concept.
However, the slurry bubble column concept presents some specific challenges in terms
of satisfying key reactor functions as well as scaling up to commercial size reactors:
• Separation of wax product from the slurry. A significant fraction of the product will
not vaporise from the reactor and therefore has to be removed from the reactor as
liquid wax from a slurry with relatively high solids concentration. No technology is
available in the open market for such applications.
• Hydrodynamics and scale-up. A commercial scale slurry reactor will typically have
a diameter of 5-10 m. While there is a vast body of data available for smaller
columns and simple air-water systems, little or no information is available in the
public domain regarding the hydrodynamics of very large diameter vessels
operating in the appropriate flow regime and at conditions relevant for FT
synthesis.
• Catalyst robustness. The mixing and turbulence of the 3 phase gas-liquid-solid
makes it necessary to use a catalyst which is resistant to attrition and break-up.
Formation of catalyst fines will not have any negative impact on the catalytic
reaction as such, but will cause problems for wax-slurry separation techniques such
as filtration, sedimentation or hydro-cyclones. Contamination of the wax product
with catalyst fines will also lead to increased catalyst consumption as well as
undesired downstream effects in the product upgrading section of a GTL plant.
The concept of operating a slurry reactor with cobalt catalyst and with continuous wax
separation was proven in small scale pilot plants as well as larger non-reacting mock-up
units at an early stage. Recognising the potential scale-up challenges of the technology,
the philosophy of the partners has therefore been to go beyond the normal scale-up
through an intermediate size pilot plant and rather build a demonstration plant of semicommercial
size in order to minimize further scale-up risks for a commercial plant.
3. Demonstration plant
Some key data for the semi-commercial demonstration plant are shown in Table 1. and
its integration within the main GTL complex in Mossel Bay is shown in Fig. 2.
The demonstration plant is using Syngas from the existing PetroSA GTL plant.
However, the Syngas produced for the high temperature Fe-based FT process in the
PetroSA plant has a higher H2/CO ratio than the optimum ratio employed by the low
temperature cobalt catalyzed process. The composition is therefore adjusted in a