Why Fixed Bed?
There are many types of reactors that have been used in commercial and demonstration of Fischer Tröpsch (FT) synthesis. These all have different risks and benefits and have to be weighed up in the context of the situation at hand. The two main types of reactors that are currently favoured in this application are fixed bed reactors and slurry phase reactors. The fixed bed reactors can be operated in either a “Dry Feed” or “Trickle Bed” mode.
Engineers at the University of the Witwatersrand, on the basis of their more than 15 years of experience with FT technology and their unique analytical methods, have proposed that the first incarnation of the technology to be implemented makes use of a dry feed fixed bed reactor.
Not only is this, in principle, the best reactor for the FT system, but its design concept and translation for scale-up is very well established. The implication of this for the investors in the project is that one can design such a reactor with a high degree of confidence that it will work as expected.
Over and above this there are a number of manufacturers that have experience in manufacturing this type of reactors for similar situations, further mitigating risk.
It should also be noted that when looking at the complete plant, the cost of the reactor module is a relatively small part of the total Capex, therefore one should not expend effort and resources on minimizing reactor costs at the expense of the associated increased risk with a reactor such as a slurry phase reactor. In principle, these slurry reactors might be smaller reactors for the same production rate but the scale-up and flexibility of operation of such units are highly problematic due to the current poor understanding and modeling competency of the hydraulics and associated vapour and liquid flows in these reactors.
Advantages of Fixed Bed Reactors
Fixed bed reactors are well understood and we have the capability to model these accurately. This permits the rapid assessment of piloting information. In addition, the ability to pilot on a small scale with a small number of the full size and length of the tubes permits a high degree of confidence when moving from the pilot to commercial design. This confidence cannot be duplicated by other reactor designs. In fact, the only difference between pilot fixed bed reactors and full-scale reactors is the number of tubes in the reactor shell.
In terms of heat removal, by controlling the size of the tubes and designing the shell side steam energy removal system appropriately, it is possible to ensure the necessary heat removal. For other reactor types, there is still a requirement for the usage of steam jacketing or coils and thus this requirement does not disadvantage the fixed bed reactors in any way.
The design of shell side steam-based heat removal systems is an established technology giving further confidence in the viability of the design.
For a fixed bed reactor, which functions as a plug flow reactor, the effect of contaminants in the feed gas and the subsequent poisoning of catalyst is mitigated. This mitigation comes about from the fact that any toxins entering the bed will only impact on the first areas of active catalyst they encounter. Therefore we get a moving wave of catalyst deactivation while the catalyst towards the end of the bed remains fully operational.
This should be compared to a slurry reactor where, due to the necessary good mixing, any breakthrough in toxins will affect catalyst throughout the reactor. We believe the moving wave deactivation experienced in a fixed bed reactor is infinitely more preferable from a monitoring and reactor operation perspective. Catalyst oxidation is once again a major issue for slurry and fluidised type reactors. In these reactors, the entire catalyst mass is exposed to the exit gas conditions of the reactor, completing with its increased water loading. In a fixed bed reactor however, only the back end of the reactor is exposed to this potential oxidant. This extends the life of the catalyst. Furthermore, the inclusion of intermediate knockouts of the liquid produced in our design permits a further enhancement of the catalyst lifetime over equivalent systems that lack this innovation.
Current technology implementation
Implementation of the offered technology is currently being explored with various feedstocks in a few locations, including Mozambique, Australia and North America, etc. Due to the technology’s modular fixed bed design, the scale up and down is a simple matter of reactor mechanical design unlike those faced with other available reactor designs.
The novelty in this technology lies in the use of a combined catalyst fixed bed system and an innovative interconnection of the process units. This offers significant benefits at a minimum of technology risk, as all of the major equipment items are based on proven or demonstrated technology. This risk has been further reduced by the successful operation of the pilot facilities applied with this reactor design concept in both China and Australia.
A world-leading technology that can
make a significant contribution
Clean Coal Technology (CCT), a South African FT technology provider, is at the forefront of providing a viable and proven environmentally greener coal to liquids technology.
CCT acts as project facilitator which sells technology and offers turnkey services
This technology can be applied to Gas to Liquids (GTL), Coal to Liquids (CTL), as well as a new combined feed process (CFTL). The technology offers reduced CO2 emission, reduced capital and operating costs, multiple product streams as well as simplicity of operation and ease of scalability for stranded, associated and main resource body resources. This scalability is achieved by a range of sizes in which the technology can be implemented.
Through the cooperation with international leading EPC contractors, a turn-key solution can be provided to the potential clients who plan to build commercial Coal to Liquids or Gas to Liquids plants.