3 replies to this topic

[JayAlchemAPL]

Depending on the type of catalyst used, the Fischer Tropsch reaction usually operates in the range of 150-250C and 100-300psi to convert synthesis gas to a range of hyrdocarbon products including diesel and gasoline.

Synthesis gas is thermodynamically stable above 250-300C at ambient pressures. However, as the temperature of synthesis gas is lowered below 250C, the carbon, oxygen, and hydrogen favor more stable orientations such as methane, hydrocarbon chains (diesel, gasoline, even wax) and even methanol or longer chained alcohols are possible by varying the catalyst type. Characteristic of the Fischer Tropsch reaction, wax tends to form at lower temperatures and with higher temperatures methane is produced.

Because these reactions are thermodynamically favorable at this temperature range, they are exothermic, giving off heat during the reaction. But upon start up of the FT column, it does need to be brought up to its working temperature so that there is enough activation energy to start the reaction.

Large gas to liquid plants typically produce not only liquid fuel, but power as well. Because some of these larger plants have a great amount of thermal mass, large FT reactors are capable of creating power through steam by pumping water across the reactor. For small bench top reactors, this is not the case. Even though the reaction is exothermic, reactor designs of smaller scale and mass are more impacted by natural cooling which cools down the reaction faster than the exothermic capacity of the reactant gas's throughput capability. For instance I have built a 5' FT column. Even with the heat of reaction, the reactor column still losses about 300 watts due to natural cooling (at 250C in room temperature, deltaT=225C) including an 1'' of insulation surrounding the reactor column.
It makes me wonder if ten different 1' columns 3/4'' in diameter would do much in changing the heat transfer dynamics significantly.

I was curious if anyone has built or experimented with any small scale FT reactor designs. If so what was your motivation behind the design?

[HarryN]

Hi, you probably know this already, but here is a link to wikipedia on the FT process.

http://en.wikipedia....Tropsch_process

If you increase the surface area of the reactor wall, in general your thermal losses will increase, which seems to be counter to your needs in a small reactor.  This would indicate that a larger diameter system would be better than smaller pipes from a thermal perspective.

Since the reaction tends to form methane as a byproduct, in theory this can be reduced by providing a methane over pressure to force the reaction thermodynamically toward other products.  In practice, this means pumping natural gas into the reactor to enhance the desired conversions.

Since the goal is to make liquids, reactor pressure is going to be a big factor - similar to a Haber process.  The more pressure your reactor can withstand, the better.  Given that the practical pipe thickness that can be easily obtained is schedule 80, or perhaps oil well pipe, that is going to limit the reaction to perhaps 150  - 200 psi.  Even then, the thermodynamic benefit should be significant.

[HarryN]

The N2 in the gas from an air blown gasifier system is probably a detriment to the overall reaction, since it is not easy to remove from the reactor, but unless you have access to an O2 blown gasifier, there isn't much you can do.  

In theory, it is possible to cool the gas down and run it through a N2 membrane separator, but this would certainly pass the H2 as well.

Many of these kinds of setups are fluidized beds to help even out the temperature gradients.  Fluidized beds are neat, but sort of challenging to manage in a hobby setting, IMHO.  The bed itself is simple, but managing the sand and dust removal / return is not.

Home use of catalyst is tricky, but a mix of iron and something resembling activated carbon in a fixed bed might be useful.  The C bed will help absorb undesired products, and the Fe may or may not have useful catalyst properties, as you noted in other threads.

Given the challenges, it might make sense to treat a reactor as a mini batch rather than a continuous flow system.  A precurser charge of natural gas, the gasifier output, and a coarse iron / carbon mixed bed might help.  Perhaps run the reaction batch, drain off any liquids, vent off the gas to a flare, and refill.

[Ben Goldberg]

HarryN said:

The N2 in the gas from an air blown gasifier system is probably a detriment to the overall reaction, since it is not easy to remove from the reactor, but unless you have access to an O2 blown gasifier, there isn't much you can do.  

In theory, it is possible to cool the gas down and run it through a N2 membrane separator, but this would certainly pass the H2 as well.

You don't necessarily need an oxygen blown gasifier.  Consider using the blue water gas process, where the reduction zone of the gasifer alternately receives hot gasses from the combustion zone, and steam.

When steam is being applied, the resulting gas (the blue water gas) will have little or no nitrogen in it.  When gas from the combustion zone is being applied, the resulting gas will have N2 in it.