Method of modifying and controlling catalyst selectivity in...

Details Method of modifying and controlling catalyst selectivity in... Method of modifying and controlling catalyst selectivity in...

2000-10-04

2003-11-25

Parsa, J. (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Group viii metal containing catalyst utilized for the...

Group ia or iia light metal containing material utilized...

C518S715000, C518S721000

Reexamination Certificate

active

06653357

ABSTRACT:

This invention relates to a method of modifying and controlling the performance results of a Fischer-Tropsch Synthesis process. Particularly, this invention relates to a modified method of predicting, controlling and thus improving the product selectivity of the High Temperature Fischer-Tropsch synthesis process, and more specifically, the selectivity of the olefinic fraction of the product spectrum.
BACKGROUND OF THE INVENTION
Fischer-Tropsch processes are known to produce gaseous and liquid hydrocarbons as well as oxygenates containing, amongst others, paraffins, olefins, alcohols and aromatics, with a variety of carbon chain length ranges and isomers, which, in general, follow the well-known Anderson-Schulz-Flory distribution. Much emphasis has been placed on the modification endeavors, more particularly to improve, as well as maximize the selectivity of the unsaturated hydrocarbons, especially olefins in the C
2
-C
4
range. whilst maintaining high activity and stability under the normal Fischer-Tropsch synthesis conditions with an iron catalyst. Equation 1 is a general presentation of the Fischer-Tropsch reaction:
CO+(1
+x
)H
2
&tgr;CH
2x
+H
2
O  (1)
The reaction can be carried out in fixed, fluidized or slurry bed reactors. The production of olefins and petrol range products is most favoured by synthesis carried out in a two-phase fluidized bed reactor operating at ~350° C. and 20 bar or higher pressures and utilizing a fused promoted iron catalyst. The fused iron catalyst is typically promoted with alkali chemical and structural promoters. As a result of the high temperatures which are used in these reactors, they are known as High Temperature Fischer-Tropsch (HTFT) reactors, thus distinguishing them from fixed bed and slurry bed reactors (Low Temperature Fischer-Tropsch—LTFT), which operate at temperatures which are about 100-150° C. lower than the said HTFT process.
The HTFT process also utilizes a technique which facilitates online removal of spent catalyst and online addition of fresh catalyst to maintain catalyst activity and selectivity profiles at levels which are as favourable as possible. This technique is aimed at achieving an equilibrium performance and also inhibiting the occurrence of undesirable and negative sudden changes in synthesis performance; thus providing a means through which the product spectrum demands, as dictated by the market forces and downstream requirements, can be met.
The Fischer-Tropsch process is known to be directly influenced by process conditions, for example, feed composition, feedrate, conversion, reaction pressure and temperature. In addition, and particularly for the HTFT process, the chemical composition of the catalyst used in the synthesis process has been shown to have a direct influence on the said product spectrum. Thus the concentration level of the chemical components of the Synthol catalyst matrix, such as sodium, potassium, alumina, silica and the like, has been shown to have a direct correlation with yields and the selectivities of the olefins, paraffins, acids and the oils produced in the process. A number of reports have been published which claim that potassium increases the alkene content of the hydrocarbon products, increases the rate of the water-gas shift reaction and suppresses methane formation.
SUMMARY OF THE INVENTION
The applicant has surprisingly found a method of modifying and controlling, and thus improving, the selectivity profile in favour of the desired Fischer-Tropsch synthesis products. Particularly, the applicant has found a unique method of manipulating, and thus improving, the selectivity profile of the lower olefins produced by means of a High Temperature Fischer-Tropsch process.
The method is characterized in that predetermined amounts of promoter-carrying compound either dissolved in solution or in a powdered form are directly injected into the reactor medium, typically into the reactor feedstream. A typical chemical promoter for the HTFT process is potassium. The applicant has found that by adding or doping the reaction medium with the promoter-containing compound during the synthesis process, the promoter being potassium, the selectivity profile of the olefins and the paraffins in the product stream is significantly changed, with more olefins being formed whilst the level of paraffins is reduced.
Analysis of the iron catalyst sample has surprisingly shown that within the catalyst matrix, potassium is the most mobile component in the solid solution. The Scanning Electron Microscope (SEM), Energy Dispersing X-ray (EDX) and Secondary Iron Mass Spectrometry (SIMS) techniques have convincingly shown that, with time online, a fraction of the potassium promoter continuously migrates away from the iron metal nuclei, to an extent that it is eventually lost altogether from the matrix and is ultimately captured, for example, in the carbon mass deposit that is formed around the catalyst particle during the synthesis process. The applicant has found that the potassium promoter becomes diluted by migration into the mass of the continuously forming elemental carbon around the catalyst particle with time online.
Furthermore, the analysis has surprisingly revealed that the catalyst particles do not contain a homogeneous concentration of the potassium promoter i.e. the amount of potassium contained in the catalyst particles progressively follows a Gaussian trend. Surprisingly, this applies to catalyst particles of the same size. The effect hereof is that some particles have very low levels of K
2
O. This appears to be an inherent problem which originates from the procedure that is used in the preparation of the catalyst (fusion process).
The applicant has found that when physically adding potassium into a HTFT reactor during the synthesis process, the added potassium replenishes the ‘lost’ potassium in the catalyst matrix, and in the process the product spectrum becomes more olefinic. The potassium that is added online is in the form of a compound dissolved in solution or in a pulverized state, the compound selected from potassium carbonate and potassium silicate. This added potassium distributes itself homogeneously through all the catalyst particles inside the reactor, boosting those particles which initially contain very little K
2
O.
The applicant has further found that an expression which combines the concentrations of the previously mentioned catalyst components, known as the selectivity factor, can be successfully used in correlating the selectivities and the yields of the olefins, paraffins, and thus the olefin/paraffin ratios. Previously, such correlations could not be established, so that it was virtually impossible to predict the yield and the selectivity profiles of the Synthol product spectrum. The applicant has also shown that, to a reasonable degree of accuracy, the selectivity levels of the olefins, as compared to the paraffins in the product stream, may be sufficiently predicted based on the amount of potassium added in the solution prepared for injection.
Accordingly, according to a first embodiment of the invention there is provided a method for controlling a selectivity profile of products of a Fischer-Tropsch synthesis process, the method including the step of introducing into a Fisher-Tropsch reaction medium, during the synthesis process, a catalyst promoter or substance, composition or salt containing the catalyst promoter.
The Fischer-Tropsch process is preferably a High Temperature Fischer-Tropsch process, and the catalyst promoter may be introduced into a fluidized bed Fischer-Tropsch reactor feedstream or at any other suitable location.
The catalyst promoter may be a promoter for an iron catalyst. The catalyst promoter may be a Group I element, more particularly the Group I element may be potassium or a salt or compound thereof.
An alkali promoter-containing compound may be used to introduce the catalyst promoter into the feedstream, the promoter-containing compound including an oxide or salts thereof. Typically, the promoter-containing compound is a potas

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