Selectivity of the Fischer–Tropsch process: deviations from single alpha product distribution explained by gradients in process conditions

Abstract

Non-ASF product distributions in Fischer-Tropsch synthesis can occur with a gradient in process conditions at the particle or reactor scale, leading to a gradient in chain growth probability a. Weighted summation of local product distributions gives the proper, non-ASF product distribution.The Fischer-Tropsch (FT) process, in which syngas is converted to hydrocarbons, is a key conversion step in one of the most important routes to alternative fuels and chemicals. Various models exist for the selectivity of this reaction. The simplest and most widely used is the Anderson-Schulz-Flory (ASF) model, 1 where the FT reaction is modeled as an addition polymerization reaction with chain growth probability a. The resulting product distribution, depicted as a plot of the logarithmic molar fraction versus the carbon number ('ASF-plot'), is a straight line.However, deviations from this ASF product distribution are regularly observed. Commonly reported deviations from the ASF model are the C1 (methane) and C2 (ethane/ethene) values. Typically, C1 is formed in a greater quantity than that predicted by the ASF distribution, while a much lower amount of C2 is usually measured. 2 Another deviation from the ASF product distribution shape that is encountered in practice is a chainlength-dependent chain growth probability. Instead of a linear trend in an 'ASF-plot' (cf. Fig. 1c), the product distribution shows a double chain growth probability or a curved distribution. 3,4 The fractions of heavier hydrocarbons are in some cases lower than expected based on the ASF model 5 but in most cases they are higher. 2,4,[6][7][8][9][10][11][12][13] The cause of these positive deviations from ASF product distributions is a disputed topic. It has been attributed to flaws in experimentation, 8-10 the existence of multiple chain growth mechanisms 11 or multiple different active sites on the catalyst. 4,12 However, more recent work assumes that olefin readsorption and incorporation is responsible. 2,6,7,13 In this communication we propose an alternative explanation for non-ASF product distributions which is nevertheless based on a single-chain-growth-probability model. Our explanation is based on the process condition dependence of a, where we assume a variable-a model that takes into account the local reactor temperature and the syngas ratio. 14 It has previously been demonstrated that gradients in the process conditions may lead to selectivity gradients. 3,8,14 Temperature and concentration gradients are the most likely variables that cause variations in selectivity. These gradients can take place on the particle scale due to diffusion limitations inside the catalyst particle, as well as on the reactor scale, e.g. due to heat transport limitations in a packed bed reactor. 8,15 We will show that these gradients in process variables can result in an apparent non-ASF Fischer-Tropsch product distribution.Gradients in FT process conditions and resulting chain growth probability and product distributions were modeled on the particle scale and the reac...

Accumulation of liquid hydrocarbons in catalyst pores during cobalt-catalyzed Fischer–Tropsch synthesis

“…21,[23][24][25] This is -for reasons of simplification -not considered here, but will be studied in future in more detail. 21,[23][24][25] This is -for reasons of simplification -not considered here, but will be studied in future in more detail.…”

Performance of diffusion-optimised Fischer–Tropsch catalyst layers in microchannel reactors at integral operation

“…Despite an abundance of simulation work on the FT process, to the authors' knowledge, there is no work satisfyingly describing and comparing the behaviour of catalysts under integral operation for different diffusion lengths or for employment of transport pores. Simulation work with differential reactors was either only focusing on pore optimisation in general 29,30 or elaborating specifically on the relation between diffusion, selectivity and pore filling degree 28,[31][32][33][34] or FT kinetics. 35,36 When complete reactors were considered often fluidized catalyst systems have been used, [37][38][39] which are useful for kinetic studies but not directly applicable to fixed bed reactors.…”

Six-flow operations for catalyst development in Fischer-Tropsch synthesis: Bridging the gap between high-throughput experimentation and extensive product evaluation

“…23 Although this assumption is fairly relevant for many catalytic systems, several reports deal with non-ASF product distributions. 24 Dissimilar sites or (parallel) growth mechanisms in one catalyst, 25 altered α values in different carbon number regions, 26 and the so-called "bifunctional catalysts" that represent a "break" in ASF plot 27 are among such exceptions. Moreover, the degree of saturation (and/or branching) may vary at different hydrocarbon ranges, even over an ASF-behaving catalyst.…”