High temperature Fischer–Tropsch synthesis in commercial practice

https://doi.org/10.1016/S0926-860X(99)00163-5Get rights and content

Abstract

The commercial application of high temperature Fischer–Tropsch (HTFT) technology is described. The types of reactor used are discussed with emphasis on the advantages of using the Sasol Advanced Synthol (SAS) reactor. A reactor replacement project at Secunda, South Africa will result in a total installed capacity of 124 000 barrel per day of products from the new SAS reactors. The preparation of the catalyst used in these reactors is described. The changes to the catalyst structure during synthesis are illustrated, making it clear that the actual catalyst consists of small carbide/magnetite particles embedded in a continuous carbon phase. The changes in catalyst morphology which occur during synthesis are discussed with reference to the effect on the observed post sulphur poisoning behaviour. The SAS process can be considered to be a mature technology which has been applied in large-scale commercial plants. Future applications should take advantage of the considerable potential to recover chemical feedstocks from the primary Synthol products. The potential amounts of these chemical feedstocks are quantified.

Introduction

The high temperature Fischer–Tropsch (HTFT) technology applied by Sasol in the Synthol process in South Africa is the largest commercial scale application of the Fischer–Tropsch (FT) technology. The most recent version of this mature technology is the Sasol Advanced Synthol (SAS) process. The SAS process uses conventional fluidised bed reactor technology rather than the more complex circulating fluidised bed (CFB) reactor originally used for the Synthol process.

The Sasol plant located in Secunda, South Africa, was constructed with a total of 16 Synthol CFB reactors, each with a capacity of 7500 bbl per day. Construction is in progress, at the time of writing, to replace all the Synthol CFB reactors at Secunda with SAS reactors, following the successful commercial demonstration of the 8 m diameter SAS reactor commissioned in June 1995. On completion of this replacement project, there will be four 8 m diameter reactors, each with a capacity of 11 000 bbl per day and four 10.7 m diameter SAS reactors, each with a capacity of 20 000 bbl per day in operation at Secunda. There are also three Synthol CFB reactors, each with a capacity of 8000 bbl per day installed at the natural gas based Mossgas plant in Mossel Bay, South Africa.

Section snippets

The CFB Synthol reactor

The Synthol CFB reactors used commercially in South Africa have had a long history of continuous development and improvement, from the original Synthol reactors installed in Sasolburg, to the most modern Synthol CFB reactors now operating at the Mossgas plant in Mossel Bay. The most significant developmental challenge was the scale-up of these relatively complicated reactors from the Sasolburg experience to the approximately three times larger capacity reactors used in the Secunda Plant.

The SAS reactor

The SAS reactor is a conventional fluidised bed that may be designed to operate at pressures ranging from 20 to 40 bar and it typically operates at a temperature of around 340°C using an iron catalyst similar to that used for the Synthol CFB reactors. A sketch of the reactor is shown in Fig. 2. The reactor consists of a vessel with a gas distributor; a fluidised bed containing the catalyst; cooling coils in the bed; and cyclones to separate entrained catalyst from the gaseous product stream.

Bed

Advantages of the SAS reactor compared to the Synthol CFB reactor

The advantages of the SAS reactor compared to the CFB reactor have been well documented [15], [16]. The main factor, which determines the relative conversion performance of the two types of Synthol reactors, is the quantity of catalyst which comes into contact with the feedgas in the reactor. The catalyst/gas ratio in the reaction zone for the SAS reactor is about twice that for the CFB reactor. This is due to the fact that, although both reactors contain about the same quantity of catalyst

Product selectivities for the SAS reactor

Operating data from a SAS reactor at the Secunda site gives the product spectrum as shown in Table 1, with further breakdown of the components in the main liquid cuts shown in Table 2. It can be seen that there is considerable opportunity for producing chemical products in addition to the hydrocarbon fuels.

A more detailed breakdown of the SAS selectivity for oxygenate components is shown in Table 3.

Economics

Fig. 5 helps to illustrate why the capital cost of the SAS reactor is less than that of the Synthol CFB reactor. This is a scale representation for equal capacity CFB and SAS Synthol reactors.

In a study for a 50 000 barrels per day synfuels plant based on natural gas, it was shown that an 18% saving in the total plant cost is achievable using SAS reactor technology. Cost estimates indicate a capital cost reduction of 50% for the Synthol reactor train as shown in Fig. 4.

For the SAS reactor,

Catalyst preparation

At the high temperatures used for the Synthol process, iron is the only practical FT catalyst. Other metals would give very high methane selectivities. The iron material employed for the preparation of the Synthol catalyst should be as free of impurities as possible. Impurities like SiO2, Al2O3, MgO, TiO2, etc. may be detrimental during the catalyst preparation and FT synthesis. The influence of the promoters on the physical and catalytic properties of the high temperature FT catalyst has been

Conclusion

The SAS process can be considered to be a mature technology that has been applied in large commercial scale plants with no significant risk for further commercial applications. Furthermore, the process can be modelled accurately to provide an optimum design for each potential application scenario. This modelling is based on a thorough understanding of the reactor hydrodynamics and the catalyst kinetic and selectivity performance as well as the understanding of the effects of changes in the

List of symbols

    dB

    average bubble diameter

    DT

    column diameter (m)

    g

    acceleration due to gravity (9.81 m/s2)

    h*

    height above the gas distributor where the bubbles reach their equilibrium size (m)

    h0

    parameter determining the initial bubble size at the gas distributor (m)

    H

    height of expanded bed (m)

    U

    superficial gas velocity (m/s)

    Udf

    superficial gas velocity through the dense phase (m/s)

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