High temperature Fischer–Tropsch synthesis in commercial practice
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)
References (17)
- et al.
Power Technol.
(1986) - et al.
Powder Technol.
(1978) - et al.
Powder Technol.
(1983) - et al.
Chem. Eng. Sci.
(1994) - et al.
Powder Technol.
(1988) - T. Shingles, A.F. McDonald, in: P. Basu, J.F. Large (Eds.),Circulating Fluidised Bed Technology II, Pergamon Press, New...
- T. Shingles, R.J. Dry, N.P. Cheremisinoff (Eds.), Encyclopedia of Fluid Mechanics, vol. 4, Gulf, New York, 1986, p....
- L.S. Leung, in: J.R. Grace, J.M. Matsen (Eds.), Fluidisation, Plenum, New York, 1980, p....