Modelling and Simulation Applied to Fischer- Tropsch Synthesis

Climate change is currently on the verge of becoming irreversible, mainly due to the high levels of Greenhouse Gas Emissions [1]. One of the major contributors to this catastrophe is the use of fossil fuels for the generation of energy, nevertheless the substitution of fossil fuels for more sustainable and clean forms of energy is currently at the utmost priority the reduce greenhouse gas emissions and keep global warming within the 1.5 °C temperature increase limit by the year of 2030 [2]. Fischer-Tropsch synthesis is one of the most promising technologies to produce fuels that can substitute fossil fuels as it can convert syngas obtained from the reform of biomasses to liquid hydrocarbons such as gasoline, diesel and SAF [3].

However, in order to become an economically feasible process, optimizations of the process are vital to determine the best operating conditions which will maximize the amount of hydrocarbons produced with desired selectivity and minimize the costs of production [4]. The optimization of a plant can become very costly if performed on the operating plant, as it consumes energy and material. Therefore, this analysis is more often performed on a process simulation software, which relies on material and energy balances to simulate the real conditions of a chemical plant. One of the main problems in using process simulation software is that some unit operations are not ideal or operate in very specific ways that cannot be simulated properly with the available generic blocks in order to obtain results that are realistic, and so, adjustments to the model must be made. This problem can be overcome by using CAPE-OPEN methodology, which permits the user to develop a custom-made model with an interface which is supported by different flowsheet applications [5]. The study of Fischer-Tropsch process using CAPE-OPEN methodology has been reported in literature. Ostadi et al. [6]. used CAPE-OPEN to study the effect of H₂/CO ratio on the production and power consumption of a FT reactor, results showed that for constant CO conversion, the reactor volume could be reduced by increasing the H2/CO ratio.

The Fischer-Tropsch process consists of a series of reactions which start with CO and H₂ and behave like a series of polymerization reaction to reach hydrocarbons of bigger carbon chains, this behavior can be described by the reactions shown in equations 1 to 4, which describe the formation of methane (1), paraffins (2), olefins (3) and alcohols (4) respectively [7].

In general, the FT reaction can be processed at high temperature, also called as HTFT, which uses iron-based catalysts and is processed at temperatures between 300 and 330 °C with pressures up to 25 atm, as well as at low temperature, known as LTFT, which is operated at temperatures between 180 and 230 °C and atmospheric pressure, and uses cobalt-based catalysts [8]. Usually, the choice between LTFT and HTFT depends on the quality of syngas feedstock, as some syngas feedstocks are more prone to poison the catalyst, iron-based catalysts are preferred to be used with these kinds of feedstock, as cobalt-based catalysts are more expensive [8]. The main challenge of simulating this reaction is to include the behavior of the polymerization reactions. From literature, it is known that the mass fractions of the polymerization reactions behave as the Anderson-Schulz-Flory distribution given in equation 5 [9].

where wα is the mass fraction of the hydrocarbon with k number of carbons in its structure, and α is the chain growth probability factor, which depends on temperature and H₂/CO ratio. An empirical equation for the calculation of α is given in equation 6 [9].

An example of a simulation flowsheet of the Fischer-Tropsch process from Aspen Hysys® is shown in (Figure 1). The flowsheet in (Figure 1) is comprised of two inlets, one containing an H2 stream and the other a CO stream. These two gases are heated up to the reaction temperature and then fed into the reactor, which in this case is a fixed bed reactor. After processing the reaction, the outlet of the reactor will be cooled down and fed into a separation drum, which separates the light hydrocarbon chains from the heavier fractions, then, both streams are cooled down to ambient temperature. Results reported in literature indicate that in order to obtain higher selectivity to C5+hydrocarbon chains, it is desirable to operate the process at 220 oC and a 2:1 H2/CO ratio. The work done by Amin et al. [10]. showed that it was possible to achieve up to 72.6% of gasoline fraction and up to 80% of diesel fraction selectivity using an iron-based catalyst with potassium promoter at these conditions. Other study by Boymans et al. [11]. It was shown that a selectivity of up to 78% C₅₊ could be achieved using a CO/SiO₂ catalyst, also in these conditions.

Figure 1:Example of a fischer-tropsch process simulation flowsheet.

The authors have given the key concepts for the process modelling of the Fischer Tropsch synthesis for the production of hydrocarbons, which is one of the most prominent candidates to produce e-fuels from clean sources using green H₂ ansd substitute fossil fuels, ie. gasoline, diesel and SAF.

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