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Hydrocarbon Production

PFD 3: hydrocarbon generating section (P300)

Clean syngas is converted into a range of hydrocarbon compounds in the

Fischer-Tropsch (FT) reactors via the generic reaction:

where , , and are the number of carbon, hydrogen, and oxygen atoms,

respectively, in a given hydrocarbon compound. The distribution of the

hydrocarbon products formed in the reactors can be assumed to follow the

theoretical Anderson-Schulz-Flory (ASF) distribution based on the chain

growth probability values.

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Wn is the mass fraction of the species with carbon number and is the

chain growth probability.

The type of FT reactor in use is the one operating at high temperature

(P301A, T = 320C). The high-temperature process has a lower chain growth

probability ( = 0.65) that favors the formation of gasoline-length

hydrocarbons, while the low-temperature process ( = 0.73) form heavier

hydrocarbons and waxes. The syngas is compressed and preheated to the

corresponding FT operating temperatures.

The conversion of CO in the FT reactor is assumed to be 80 mol%. This high

conversion can be achieved in a slurry-phase system due to the high syngas-

catalyst contact and mixing in the reactor. Oxygenated compounds formed

in the reactors are represented by vapor phase, aqueous phase, and organic

phase pseudo-components. The total converted carbon present in each

pseudo-component is 0.1%, 1.0%, and 0.4%, respectively.

The distribution of the remaining carbon follows a slightly modified ASF

distribution described in the Process Modeling section to account for

increased formation of light hydrocarbons.

The FT effluent stream is treated in a series of product separations and

catalyst recovery process, following a Bechtel design. The streams are mixed

and passed through a wax separation unit (P302). The vapor is cooled, sent

to an aqueous oxygenate separator (P303), flashed to remove entrained

water (P304), and passed through a vapor oxygenate separator (P307). The

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knocked out water and oxygenates are sent to the knockout mixer (M303)

while the vapor and organic liquids are sent to the first hydrocarbon mixer

(M306). The wax from P302 is cooled to 150C before being sent to an

entrained vapor removal unit (P305). The wax is sent to the second

hydrocarbon mixer (M304) and the vapor is further cooled to 40C and sent

to a flash unit (P306) for water knockout. The vapor is sent to M306, the

organic liquid is sent to M304, and the knockout water is sent to M303. All

hydrocarbons are directed to M401 before being sent to the upgrading

section.