Gas to Liquids

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on March 27, 2010 at 2:54:14 am

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

Catalysts

-Iron oxide- less expensive! (*)

-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.

-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.

Types of Reactors

-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)

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Carbon Monoxide and Hydrogen 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.

More graphs!

Source: Technical University of Vienna:

-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)

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 that can be 'easily' cracked (Jay- What does 'easy' mean here? and on what scale exactly?) (1)