(705g) Selectivity of the Fischer-Tropsch Process: Deviations From Single Alpha Product Distribution Explained By Gradients in Process Conditions

The
Fischer-Tropsch (FT) process, which converts syngas to hydrocarbons, is a key
conversion step in one of the most important routes to alternative fuel. Various
models exist for the selectivity of this reaction. The simplest and most widely
used is the Anderson-Schulz-Flory (ASF) model, where the FT reaction is modeled
as an addition polymerization reaction with chain growth probability α.
The resulting product distribution, depicted as a plot of the logarithmic molar
fraction versus the carbon number, is a straight line. However, deviations from
this ASF product distribution are regularly observed.

In this
study, an explanation is given for the positive deviations from ASF product
distributions. This explanation is based on a variable-α model that takes into account the
local reactor temperature and syngas ratio [1]. Using this model, we aim to
demonstrate that gradients in process conditions can result in gradients in the
chain growth probability α and apparent non-ASF distributions.

Gradients in
process conditions can take place on the micro-scale due to diffusion
limitations in the catalyst particle, as well as on the meso-scale, e.g. due to
heat transfer limitations in a packed bed reactor. These two situations were
modeled, applying a process-condition-dependent α-model [1]. In the first case, a
reaction-diffusion model for a spherical catalyst particle was applied. In the
second case, a temperature gradient was imposed in a simple 1-D reactor model. The
product distribution was calculated in two different ways: using the average chain
growth probability, and by summating local product distributions weighed with
the reaction rate and incremental volume. 

In strongly
diffusion-limited systems, a significant difference between these two
distributions was found. The use of the average α gives a straight
distribution with a C₅₊ selectivity of 8.1%, while the weighted
summation of local distributions gives a curved plot with a positive deviation
in the higher carbon numbers and a C₅₊ selectivity of 32% (see
Figure 1).

The cause
of this effect is the occurrence of gradients in the local syngas ratio in the
catalyst particle due to diffusion limitations. Towards the inside of the
particle, CO becomes depleted, causing an increasing H₂/CO ratio and
a decreasing α. The prominence of this effect depends strongly on the
diffusion length, temperature, syngas ratio and catalyst activity. At H₂/CO
= 2, a popular choice for experimental conditions since it is close to the
overall stoichiometric consumption of the reactants, even catalyst particles as
small as 200 μm are  easily CO depleted.

On the
meso-scale, an imposed temperature gradient of 30˚C led to similar
α-gradients and curved product distributions.

We conclude
that curved (non-ASF) product distributions can occur when there is a gradient
in process conditions leading to a gradient in the chain growth probability
α. The origin of this effect can take place on the micro-scale, i.e.
concentration gradients caused by diffusion limitations in the catalyst particles,
or on the meso-scale, i.e. gradients in local conditions such as temperature. When
such gradients are modeled, the selectivity should be calculated by weighted
summation of local product distributions.

[1] Vervloet,
D., Kapteijn, F., Nijenhuis, J., van Ommen, J.R., Fischer-Tropsch reaction-diffusion in a cobalt catalyst particle:
aspects of activity and selectivity for a variable chain growth probability.
Catalysis Science & Technology, 2012. 2(6): p. 1221-1233.

[2] Kruit,
K.D., Vervloet, D., Kapteijn, F., van Ommen, J.R., Selectivity of the
Fischer-Tropsch process: Deviations from single alpha product
distribution explained by gradients in process conditions. Catalysis Science & Technology, 2013. DOI:10.1039/C3CY00080J

Figure 1. FTS product distribution results of
reaction-diffusion model for 2 example cases, calculated using the average
α as well as the weighted sum of product distributions from a gradient α:
(A) T=180˚C, d_cat=1mm, H₂/CO=2,
F_YS=2. Both curves
completely overlap. (B) T=230˚C,
d_cat=1mm,
H₂/CO=2, F_YS=2.