Fischer–Tropsch synthesis: accounting for chain-length related phenomena

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Abstract

The attempts to explain the deviations from Anderson–Schulz–Flory (ASF) distribution, the strong dependency of olefin content on molecular size, and the negative deviations from a constant molar activity for an initiator in Fischer–Tropsch synthesis (FTS) have led to a two-active-site model, models related to olefin readsorption, and the vapor–liquid equilibrium based explainations. However, none of these theories is able to adequately explain all of these chain-length related problems. Based on the studies of accumulated products in FTS reactions conducted in a continuously stirred autoclave reactor, we proposed that the apparent products of the FTS reaction is a mixture of freshly produced FTS products (Φ) and the products left in the reactor (Δ), Sn = Δn + Φn. Using this concept, all of the chain-length related problems were accounted for. Based on the results of calculations, we predict that the correct α-value of hydrocarbons greater than C8 will be smaller than currently reported value, that the paraffin to olefin ratios will be much smaller than the values reported, and that the product distribution and the paraffin to olefin ratios of a FTS reaction may depend on the molecular size but to a much lesser degree than claimed. According to our model, we conclude that in order to obtain the correct product distribution and the paraffin to olefin ratio of a FTS reaction, it is necessary to find a way to evaluate or eliminate the term Δ, which requires conducting FTS research in a different way.

Introduction

As a practical way of converting coal to gasoline, diesel oil, wax, and alcohols, Fischer–Tropsch synthesis (FTS) has been studied for nearly 80 years [1]. Based on product analysis, Friedel and Anderson [2] in the 1950s found that the products obtained during synthesis followed a Schulz–Flory distribution (or Anderson–Schulz–Flory (ASF) distribution) as shown in Eq. (1):Wn=n(1+α)2α−1where n is the number of carbon atoms in products, Wn the weight fraction of products containing n carbon atoms, and α is the probability of chain growth. Around the same period, Emmett and coworkers [3], [4], [5] found that the C2 species derived from ethanol or ethylene acted as an initiator for the FTS reaction. This conclusion was based on the observation that the molar radioactivity of hydrocarbons from C3 to about C8 are the same when 14C labeled ethanol or ethylene were used as tracers. The ASF distribution equation and the conclusions obtained from 14C tracer studies provided the bases for understanding the mechanism of the FTS reaction.

However, the subsequent studies show that the measured product distribution from the FTS reactions seldom obeyed the ASF kinetics, especially when the carbon number of the hydrocarbons was greater than around 8, leading to negative [6], [7], [8], [9], [10] or positive deviations [11], [12], [13], [14], [15], [16], [17], [18] from the ASF distribution; the measured α values increase with decreasing syngas flow rate [17], [19]; the paraffin to olefin ratio increases exponentially with increasing the molecular size [20], [21], [22]; and the molar radioactivity or stable isotope content of hydrocarbons decrease with increasing molecular size for an initiator [23], [24], [25], [26], [27], [28].

To explain these chain-length related phenomena, theories proposed include: two-active-site model [14], [16], diffusion-enhanced olefin readsorption model [17], [29], [30], olefin readsorption product distribution model [31], different physisorption strength of the olefins [18], [19], [20], the greater solubility of larger olefins [19], [20], and a vapor–liquid equilibrium phenomena [10], [32], [33]. Some of the theories proposed over the past several decades can address some of the chain-length related problems, but none of them can answer all of the questions.

In this study, we propose a model that can explain all of the chain-length related questions. Based on this idea, correct product distributions and paraffin to olefin ratios of a FTS reaction are predicted.

Section snippets

Experimental

The general procedure for determining the accumulated products in FT synthesis is as follows: (1) the FT synthesis was started in a 1-L continuously stirred autoclave reactor (CSTR) using D2/CO as the synthesis gas as described previously [34]; (2) after several days, when the reaction has reached a stable CO conversion and/or at the point where the accumulation is to be evaluated, all the collection vessels are emptied and the feed is switched from CO/D2 to CO/H2; and (3) a sample is collected

Results and discussions

In adding a tracer to a reactor, one must consider the distribution time. Dankwerts [36] indicated that when a fluid flows through a vessel at a constant rate either piston-flow or perfect mixing is usually assumed. He indicated that these two idealized situations (a and c in Fig. 1) are frequently not encountered and developed F-diagrams for an added component as shown in Fig. 1 as well as equations to account for several types of flow. In the “dead water” situation (Fig. 1d) a considerable

ASF distribution equation

The ASF distribution equation was obtained based on earlier product analysis of the FTS reactions. At that time, the synthesis products from Co and Fe catalyzed reactions were well characterized up to C7 or C8 [11]. According to our model, if fn is close to 1, Eq. (6) becomes Eq. (7), which is equivalent to the ASF equation:S′n=Φ1αn−1

Negative deviation from the ASF product distribution

According to Eq. (6), at the beginning of a FTS reaction, Δn = 0, and Eq. (6) becomes S′n = fnΦ1αn−1. The measured α value is given by Eq. (8):α′=fn+1Φn+1fnΦn

Conclusions

The phrases “product accumulation” or “accumulated products” or “product holdup” have appeared in the literature during the past several decades to qualitatively explain the experimental results for FTS (e.g. [7], [10], [23], [24]). However, since the amount of accumulated products was not determined experimentally, the seriousness of accumulated products in FTS was underestimated by many researchers. For instance, the cause of strong dependency of paraffin to olefin ratio of heaver

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