Fischer-tropsch synthesis

Details Fischer-tropsch synthesis Fischer-tropsch synthesis

1999-11-23

2001-04-03

Richter, Johann (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Group viii metal containing catalyst utilized for the...

Reexamination Certificate

active

06211255

ABSTRACT:

The present invention relates to a Fischer-Tropsch synthesis.
Conventional methods of operating a low temperature Fischer-Tropsch synthesis employ fixed bed reactors. The catalytically active material is generally carried on relatively large carrier particles and this results in poor intraparticle mass transfer characteristics. Also, the catalyst particles are packed in the tubes of a shell and tube arrangement with coolant on the shell side. This means that the space/time yield is limited by the heat transfer in the catalyst bed.
An alternative is to operate the fixed bed as a trickle-bed reactor. This may have advantages from the point of view of heat transfer, however, the poor intraparticle mass transfer characteristics remain.
It has also been proposed to carry out an F-T synthesis in a slurry bubble column reactor. Here, the catalyst particles would be significantly smaller since they would be in suspension in the liquid product. This in turn gives rise to significantly improved mass transfer characteristics within the catalyst particles. However, this type of reaction system can be disadvantageous in that separation of the catalyst from the product can be troublesome. There is also back mixing, due to the nature of the reactor, which is less efficient in terms of reactor volume than a plug flow reactor.
It is an object of the present invention to provide a reaction system for an F-T synthesis which has high mass transfer characteristics at the catalyst and in which heat transfer is not a significantly limiting factor, without the drawback of a difficult catalyst separation.
As world oil resources diminish, natural gas is becoming more attractive as an energy source and methods of upgrading this to higher hydrocarbon fuels are increasing in importance. Thus, in one system, the methane (natural gas) is reformed to CO and H
2
and this synthesis gas is subjected to a Fischer-Tropsch reaction to form higher hydrocarbon products.
Gas fields are now being discovered at significant distances offshore and under certain circumstances it is not commercially viable to pipe the gas on-shore for processing. It would be advantageous, therefore, if the natural gas in these remote fields could be processed on board marine vessels at the fields and then taken to port by those vessels or by other transport vessels. Under these conditions, reactor size is crucial, and the existing fixed bed and slurry reactors are undesirably large. They are also sensitive to movement, particularly the slurry reactor which would be unstable in heavy seas.
It is therefore a further object of the invention to provide a reaction system for an F-T synthesis in which the reactor size is minimised and which is less sensitive to movement than existing systems.
Accordingly, the invention is directed to the use of a monolithic catalyst to conduct a Fischer-Tropsch synthesis, in which the monolith comprises a solid body defining a series of discrete and continuous channels extending from one end of the body to the other, the walls of the channels consisting of or containing a Fischer-Tropsch catalyst.
The invention also provides a method of conducting a Fischer-Tropsch synthesis reaction which comprises: passing synthesis gas comprising H
2
and CO through discrete and continuous channels in a monolithic catalyst, the walls of the channels consisting of or containing a Fischer-Tropsch catalyst; removing the liquid product from the monolith; and removing heat produced in the reaction in the liquid product.
Preferably heat from the reaction is removed from the liquid product stream outside the reactor and a portion of the liquid product stream is recycled to the reactor. Unreacted synthesis gas may be recycled from the reactor, for example to the synthesis gas production unit.
Preferably, the synthesis gas feed and the liquid product flow co-currently. Preferably the synthesis gas feed and liquid product travel along the channels in a slug flow or Taylor Flow regime. Taylor Flow of a gas and liquid in a channel is defined as periodic cylindrical gas bubbles in the liquid having almost the same diameter as the channel and without entrained gas bubbles between successive cylindrical bubbles. Preferably, the flow is downwards.
Alternatively, the gas/liquid flows could be counter-current.
In a preferred form, the invention provides a reaction system for a Fischer-Tropsch synthesis which comprises a reactor including a monolithic catalyst and having an inlet for synthesis gas comprising H
2
and CO and an outlet for liquid product, the monolithic catalyst comprising a solid body defining a series of discrete and continuous channels extending from one end of the body to the other, the walls of the channels consisting of or containing a Fischer-Tropsch catalyst, whereby the synthesis gas is supplied via the inlet and is passed through the channels where the synthesis takes place and liquid product is removed via the outlet, the heat produced by the reaction being removed from the system by the liquid product.
Mass transfer (diffusion) effects are also very important in determining selectivity. Diffusion rates in the liquid phase are typically 3 orders of magnitude slower than in the gas phase, meaning that even slow reactions may be diffusion limited in liquid phase. Recent investigations have shown that a moderate diffusion resistance within the catalyst pellet (intraparticle) can give a strong negative influence on C5+ selectivity. The key parameter is the characteristic diffusion distance, determined by the catalyst pellet size or the thickness of a catalytic layer. As a guideline, negative effects on selectivity are experienced for diffusion lengths above 0.1-0.4 mm, corresponding to 0.2-0.8 mm diameter spherical pellets (the exact value depending on catalyst properties and reaction conditions).
Conventional fixed bed reactors typically use 2-6 mm diameter pellets in order to avoid an unacceptable pressure drop through the catalyst bed. In a fixed bed reactor, the selectivity problem can be solved by using catalyst pellets where the catalytic material is deposited in a thin outer layer (“egg-shell” catalysts). However, this means that only a fraction of the catalyst present in the reactor is participating in the reaction, reducing the solid fraction from typically 60% to about 23% for a 2 mm diameter spherical particle with a 0.15 mm catalytic layer. In a slurry reactor, the selectivity problem is solved by using small catalyst particles, usually 0.1 mm or less.
In the proposed monolith reactor concept, a short diffusion distance (typically <0.15 mm) can be maintained without having to reduce the fraction of active material, as a result of the fact that the catalyst is located in the thin walls of the monolith structure.
The Fischer-Tropsch synthesis is a strongly exothermal reaction and effective heat transfer is a prerequisite for successful reactor operation. In a fixed bed reactor, the catalyst is located within tubes and the heat is removed by steam generation on the shell side. The space time yields are therefore limited by the heat transfer properties of the reactor. In addition, the maximum conversion per pass is limited by the high gas velocities necessary for achieving optimum heat transfer. The heat transfer properties can be improved by decreasing the tube diameter, but this increases the pressure drop. In addition, the cost and weight of the reactor will increase strongly with decreasing tube diameter. In practice, significant axial and radial temperature gradients are unavoidable in fixed bed reactors used for FT-synthesis.
In slurry reactors, the catalyst/slurry is located on the shell side and the heat of reaction is removed by steam generation on the tube side. Due to the turbulent motion of the slurry, the heat transfer properties are more favourable and the necessary heat transfer area is drastically reduced when compared to a fixed bed reactor.
In the proposed monolith reactor design, cooling is performed by direct heat removal by the production stream (preferably the heavy FT products) whic

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