GEK Wiki / Gas to Liquids
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Gas to Liquids

Page history last edited by jay@... 8 years, 4 months ago

Gas to Liquids Page

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Intro

  • the reactions(exothermic, heterogeneous/homogeneous) Syngas Thermo-Intro
  • reactor designs (history, current, future)
  • catalysts (physical, chemical, attrition)

 

Reaction:

  • Overview
  • The Reactant- synthesis gas
    • coal gasification, natural gas (methane) reformation, wood gasificaiton
    • CO/H2 ratio

 

H2/CO preferred range between 1.7:1 and 3:1

-options for increasing hydrogen concentration: (1)water/gas shift or (2) burn methane (CH4) from FT process.

 

  • Contaminates (deactivation/recovery), dilutants
    • sulfur
    • cyanide
    • amonia
    • metallic carbonyls
    • soot
    • CO2
    • other acidic and basic components 
    • nitrogen
    • nitrous oxides
  • The Product- liquid fuel 

The Fischer-Tropsch reaction is the recombination of CO and H2 into hydrocarbon chains -CH2- over nickel, cobalt, iron and ruthenium catalysts. The temperature, pressure, and catalyst type determines the chain polymerization versus chain termination ratio. This ratio governs the average molecular weight of  the 'syn crude' produced. 

 

Chain Growth:

The Fischer-Tropsch reaction has the capability of producing a large range of hydrocarbons, which have to be separated using fractional distillation. The environment of the reactor can be controlled so that a smaller range of hydrocarbons can be selected. In a fancy equation that will be posted later, the Probability of Chain Growth (alpha) can be calculated for a given reactor environment and the product distribution are as follows:

 

Affecting alpha: Temperature, catalyst activity, pressure, etc.

 

Temperature:

There is either a low temperature Fischer-Tropsch process (LTFT) or high temperature Fischer-Tropsch process (HTFT).

LTFT: 200-240C (typically iron or cobalt catalyst)

HTFT: 300-350C (typically iron catalyst)

An increase in temperature leads to shorter chains (ex: at 330 C mostly gasoline and olefins are produced; at 180-250C mostly diesel and waxes are produced)

 

 

Exothermic Reaction:

This is a highly exothermic reaction and cooling the reactor is usually combined with steam production and electrical generation. This excess heat can also be used towards fractional distillation.

Typical hydrocarbon recombination reactions of Fischer-Tropsch:

Reaction                                   Reaction Enthalpy: deltaH(300K) kJ/mol

CO + 2H2   --> -CH2- + H2O          -165.0

2CO + H2   --> -CH2- + CO2          -204.7

CO + H2O --> H2 + CO2               -39.8

3CO + H2 --> -CH2- + 2CO2          -244.5

CO2 + 3H2 --> -CH2- +2H2O          -125.2

 

  • table of properties

Reactors

  • overview of possible physical characteristics and demands of catalysts
  • reactor designs (fixed, fluidized, slurry, recycling, microtube)

 

-Slurry-Phase Reactor: This reactor uses a wax support that is liquid at reaction temperatures (higher boiling point than FT product) which supports the metal oxide catalyst particles of which the syn gas is bubbled through the bottom. Because of the high exothermic reaction, the slurry acts as a heat sink which stabilizes the temperatures in the reactor. Because of the interface of mineral oil slurry with the metal oxide catalyst, the hydrocarbon is soluble in the slurry phase, pulling it away from the catalyst which increases catalyst activity, decreases oxidation of the catalyst, increases catalyst activity, and decreases/stabilizes chain-growth. Slurry-phase cobalt catalyst reactors are the most common theme for most companies, and they are typically the least expensive. (LTFT)

-Fixed-bed reactor: Typically these reactors have been made of many small tubes with the catalyst fixed in the inside of the tube where the syn gas flows through. On the outside, water is flushed over to stabilize the temperature of reaction. (LTFT)

(1)

 

  • heat transfer calculations
  • scaling (process heat loss or use)
  • handling of heterogeneous media
    • separation of catalyst from liquid product
    • downstream upgrade of product

Catalysts

  • elemental notes and mechanisms
    • transition metal oxides 

-Iron oxide- less expensive! (*)

 

-Product Distribution charts:

Iron catalyst: 30 bars, 280C (x-axis: chain length; y-axis: percentage on weight)

High selectivity of C10-C18 (high yield of diesel fuel)

(Source: Technical University of Vienna)

 

-Nickel oxide- high activity, more selective towards shorter chains, will tend to the production of methane, does not do well at higher pressures.

-Cobalt oxide- much more resistant to oxidation by oxygen and water giving it a higher activity and longer life than iron.

 

Cobalt catalyst: 30 bars, 240C (x-axis: chain length, y-axis: percentage on weight)

Wider distribution with a higher growth probability and heavier products produced,(1)

(Source: Technical University of Vienna)

Cobalt does not promote the water gas shift as much as iron catalysts typically, therefore less oxygenates, other than alcohols are created in the resulting liquid product.

-Ruthenium catalysts: The availability of Ru is limited, forcing high prices. Ru based catalysts have been more recently studied in their selectivity of the gasoline and jet fuel- C9-C16 hydrocarbons using a supported zeolite matrix.

 

    • aluminum and silicon oxides (promoters, supports)
    • K and Na promotor (bicarbonates)
    • carbies
    • raw
    • other
  • catalyst macro physical characteristics (solid, liquid, slurry, embedded, microtube, zeolite)
  • micro characeristics (surface area, active sites)
  • procedural methods to make catalysts
    • OMX, co-precipitate, fused, etc.
  • activation/deactivation/re-activation
  • recycle catalyst/haz mat concerns/economical impact

 


 References:

"Slurry Phase Fischer Tropsch Reactor"http://www.rodisyngas.com/rodi_syngas_inc/technology.html

-comparison of slurry and fixed bed reactors. equation references for effective reaction rate constants and syngas space time yield figures. 

"The Fischer Trposch (FT) Process" http://knol.google.com/k/the-fischer-tropsch-ft-process#

-very broad simple overview of FT

"Recent Research on the Fischer Tropsch synthesis" http://www.fischer-tropsch.org/primary_documents/presentations/recent_research/recent_report.htm

-brief summery of FT temperature and gas throughput relationships to space velocity yield.

Catalyst Manufacturers:

http://www.topsoe.com/Business_areas/Methanol/Processes/MethanolSynthesis.aspx/

 

 

 

Catalyst Notes

 

 

 

 

More articles to sift through:

More articles

 

 

 

Comments (5)

Dave S. said

at 11:00 pm on Apr 17, 2009

Methane would probably be the "low-hanging fruit" for a small gasifier using a catalyst - "synthetic" natural gas, eliminating the carbon monoxide. You do want to read about nickel carbonyl (a.k.a. "liquid death", I believe) and any other side reactions of catalysis though, before trying any experiments; some of the organometallics that can be produced by unwanted reactions with catalysts are really nasty stuff. Also removal of tars would probably be a prerequisite, because tars fouling the catalyst would, at the least, require a period of extremely high temperature operation to burn them out; catalysts are quite finicky, as their (clean) surface must be exposed to what they're catalyzing.

Dave S. said

at 11:05 pm on Apr 17, 2009

Note that I've never actually dealt with this stuff - I have no idea what I'm talking about. I've only read about it.

jay said

at 1:42 pm on Apr 18, 2009

Yes the nickel organometallics are bad news, I think iron and cobalt would be better to try noting this concern. The tars would kill most any catalyst and gunk up reactors. Tar free gas need for sure! Although I did find some article on some zeolites (posted on the methanol to gasoline page) that do get a build up from very small amounts of impurities, what is interesting about the zeolites is that you can heat them up and burn it off, and still maintain catalyst activity. Although there is a limit to the number of times that the catalyst can be 're-charged' so-to-say.

Dave S. said

at 6:42 pm on Apr 18, 2009

If we're going to go the route of Fischer-Tropsch, or methanol, then the idea of building a fluidized-bed gasifier becomes all the more important. Fluidized beds are really the key to an efficient F-T process, because of the issue of catalyst surface area...and gaining experience with the whole fluidized bed thing now may help down the road with that.

A fluidized bed is just fine sand with air blown through it, (like a popcorn popper with the kernels like the sand), just heat up the air blast with some nichrome wire or a glow plug, throw some wood on the top, once it's going, turn off the glow plug, switch the air flow to reheat and voila, gasification...maybe, if my understanding's correct.

Not to mention, the fluidized bed will take care of the problem of bridging, and may solve some tar problems. Plus, with a fluidized bed, if tar becomes an issue, you can always increase airflow and let the tars just oxidize right off the sand surface. You might even be able to dodge the whole fuel size uniformity issue, because the fuel's being continuously sandblasted into sawdust as it reacts. Check out this site, it has some interesting stuff about fluidized beds...http://www.energyproducts.com/, and their fluidized bed biomass gasifier page, http://www.energyproducts.com/fluidized_bed_gasifiers.htm.

Dave S. said

at 7:00 pm on Apr 18, 2009

These are some videos of the basic idea behind fluidized beds:
http://www.youtube.com/watch?v=MyQ25ME6J18
http://www.youtube.com/watch?v=Fb42X8fgVjk
http://www.youtube.com/watch?v=uLhblc48NIA
Here's one with a bit more gasification implication:
http://www.youtube.com/watch?v=EB0r6A5VxFU
And here's one with an actual fluidized bed gasifier:
http://www.youtube.com/watch?v=P-vt8K2YLGc

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