(42f) Reaching Higher Productivity In a Fixed-Bed Fischer-Tropsch Reactor at Laboratory Scale and Pilot Scale

It is commonly accepted that the
productivity of Fischer-Tropsh reaction is one of the keys to economic
efficiency of the whole XTL process. The expression for calculation of
productivity is shown below in equation (1) as mass of liquid product per
volume of catalytic bed in period of time (kg/(m³´hr).

                                            
                                                                              (1)

where  P_FT is
productivity of the bed; M_liq is weight of liquid
Fischer-Tropsch product released from the reactor bed (usually accounted as sum
of hydrocarbon from pentane and heavier or C₅₊); V_bed
is the volume of the catalytic  bed; and t stands for the period of time
necessary for the release of M_liq .

Although there are many other types of productivity
definition in literature, the one shown in equation (1) is the best for
understanding relation between catalyst properties and reactor performance
parameters. The productivity is closely related with the catalyst activity but
not equal to it. Due to severe diffusion limitations and heat transfer
complications, which are typical for Fisher-Tropsch reaction, catalysts with
very high activity may show quite low productivity if placed into a catalytic
bed. The above-mentioned limitations and complications can be relatively easily
removed if one uses a catalyst in the form of small micrometric grains diluted
with thermally conductive neutral agent such as quartz sand or silicon carbide
powder. The active centers of the catalyst would release a large amount of
product under such conditions. However, the volume of such diluted bed V_bed 
would be too large and hence P_FT would not reach any
significant level as is obvious from equation (1). So this work does not
consider experiments with diluted catalytic beds.

Higher productivity means smaller reactor
size and, hence, lower capital investment in XTL plant. Also higher
productivity means that smaller amount of the catalyst needs to be exchanged
for fresh one during regular reloading.

This work reports the results of the study devoted
to the development of a fixed bed with significantly increased productivity 240
to 400 kg/(m³´hr). The research includes:

a)     
The mathematical modeling of the
bed;

b)     
Formulation of pre-conditions for
higher productivity, including requirements to the catalyst, requirements to
the reactor and requirements to the temperature and pressure of the process;

c)     
The results of developing and
laboratory testing pelletized catalyst capable of manifesting productivity
higher than 240 kg/(m³´hr)  in a fixed bed;

d)    
The results of designing, building
and starting up a scaled-up pilot GTL unit ( ¼ bbld capacity) with a
highly productive Fischer-Tropsch reactor.

The mathematical model describes stationary
process occurring in a fixed pelletized bed of a Fischer-Tropsch reactor. The
model integrates the following contributing parts:

·        
Chemical kinetics of paraffine and
olefine synthesis from CO and H2 at the surface of a Co-based catalyst;

·        
Diffusion model of heat and mass
transfer inside quasi-spherical catalytic particles including capillary
condensation of water inside pores of the particles;

·        
Heat and mass transfer model of
inter-particle medium of the bed;

·        
The pressure drop correlation of
vapor-liquid flow inside porous medium;

·        
model for Mass transfer of vapor
phase components through the liquid film, which is formed at the surface of the
catalytic particles and at the points of interconnection between particles
during the process;

·        
Modern equation of state (PC-SAFT)
for estimation of thermodynamic properties of vapor and liquid phases in
three-phase systems (gas -water-rich phase ? hydrocarbon-rich phase).

 The model allows estimation of thermal
stability of spherical catalytic particle with internal heat generation and
diffusion resistance of gaseous component.

This model was integrated into a dialogue
computer software, which allows calculation of temperature profile along and
across the reactor as well as pressure gradient, velocity vector field and the
map of concentration distribution of components in the reactor.

The analysis of the calculations showed
that a highly productive catalytic bed should include a catalyst with high
thermal conductivity and advanced pore system. It was also shown that a chosen
highly productive catalyst can manifest high productivity if aspect ratio of a
fixed bed is high enough. The value of aspect ratio determines an optimal value
of the diameter of a fixed bed.   Another important result of the calculations
was a conclusion that high productivity can be reached at gas hour space
velocity (GHSV) exceeding certain limit, which most usually varies between 2000
hr^-1 and 4000 hr^-1.

We developed a catalyst in compliance with
recommendations of the mathematical model. This is a cobalt-based catalyst,
which is manufactured by impregnation of a pelletized composite thermally
conductive support. The catalyst composition and manufacturing procedure are
described in the patent applications PCT/RU2010/000323 and PCTRU2010/000429.

The catalyst was tested at a laboratory
test rig in a fixed pelletized bed (pellet size 2.5 mm) at the pressure of 21 bar. The testing procedure included activation by hydrogen at 400°Ñ and consequent measurement of catalytic properties in a stream of
synthesis gas at the temperatures 150 to 250°Ñ and GHSV
from 1000 hr^-1 to 5000 hr^-1. An extract from experimental
results is shown in Table 1.

Table 1. Extract from experimental records
of testing the highly productive palletized catalyst.

(Conditions: H₂/CO ratio in feedstock
1.98, P = 21 bar)

It can be seen from Table 1 that the
catalyst is indeed capable of producing more than 240 kg/(m³´hr)  of
liquid Fischer-Tropsch product in a fixed bed. The data of Table 1 suggest that
optimal conditions for operation of Fischer-Tropsch process with this catalyst
are as follows:

• GHSV 3000 hr^-1
• Temperature of flow 233°Ñ

The results of testing the developed
catalyst allowed us to design, build and start up a scaled-up pilot unit for
modeling a complete XTL process. The pilot unit with capacity of ¼ bbld models
a full cycle of XTL process with natural gas as a feedstock. The unit includes
a reactor of desulfurization of feedstock; a reactor for steam-CO₂
reforming, heating-cooling station, compressor station and two parallel highly
productive Fischer-Tropsch reactors. The unit in the form of a
truck-transportable block was complete and commissioned in October 2010.