Ideal Flow Reactors - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 190

R

R R

O

R H C O

C R C O CH 2

CO H 2

…

FIGURE 5.4 Proposed mechanism for Fischer e Tropsch polymerization of CO and H 2 to form alkane polymer.

depleted. Using coal (several hundred years supply in the United States alone) and natural
gas (more proven reserves than petroleum), we can be assured of sources of liquid fuels for
transportation through this technology long after supplies of crude oil and even natural gas
have been used up. The more prominent use of Fischer e Tropsch technology is the renewable
biomass feedstock, as petroleum, natural gas, and coal will eventually be depleted. Biomass
as we know it is renewable and will not be depleted as long as the Sun is sending out rays.
Fischer and Tropsch found that when a mixture of CO and H 2 was heated to about 250 C
at high pressures over an iron catalyst, polymer would form, and under suitable conditions
this had the appropriate molecular weight for gasoline and diesel fuel. Different metal cata-
lysts give different molecular weights (Ni produces CH 4 ) and different amounts of alkanes
(Fe and Re), olefins (Ru), and alcohols (Rh); so catalysts and process conditions can be altered
to produce a desired molecular weight and distribution.
The products are essentially all linear molecules, which for olefins and alcohols have the
double bond or the OH group on the end carbon (

a a

-alcohols). The mechanism
of this polymerization process is thought to be similar to (but also very different from) Ziegler
Natta (ZN) polymerization of ethylene and propylene on Ti. It is thought that CO adsorbs
and hydrogenates (perhaps to form the CH 2 group) on an adsorption site adjacent to an
adsorbed alkyl R, as shown in Fig. 5.4 . If the CH 2 inserts between the metal and the adsorbed
R, one obtains and adsorbed RCH 2 e , which can add another CH 2 to form RCH 2 CH 2 e , and
the chain repeats itself indefinitely until the adsorbed alkyl dehydrogenates to olefin, hydro-
genates to paraffin, or hydrates to

-olefins and

-alcohol.
The CO and H 2 (syngas) for FT synthesis were initially made by the gasification of coal,
which was plentiful in Germany. Coal contains various amounts of H depending on the sour-
ces (CH x , with 0

a

1), but hydrogen content is usually very low. For simplicity, let us
consider the gasification of carbon

< x <

C þ H 2 O / CO þ H 2 (5.27)

Starting from a stoichiometric feed of carbon and water, the gas-phase equilibriummixture is
dependent on the temperature. In the gas phase, various compounds can form. The most
common ones are CO, H 2 ,CO 2 , and CH 4 , and other larger molecules. For simplicity, let us
consider the four most abundant compounds only. The reactions involved are

CO þ H 2 O / CO 2 þ 2 H 2 (5.28)

2 CO þ 2 H 2 / CO 2 þ CH 4 (5.29)

Figure 5.5 shows the equilibrium compositions for the gasification of carbon as a function
of temperature. One can observe that the gas mixture consists of mostly CO and H 2 at 1:1 at
high temperatures (greater than 900 C). At lower temperatures, equilibrium favors the
formation of CO 2 and CH 4 in the gas mixture instead of H 2 O.