Page 51 - Elements of Chemical Reaction Engineering Ebook

22 Mole Balances Chap. 1

sometimes offset by the high cost of the reactor and catalyst regeneration
equipment.

Example 1-5 Gas-Phase Industrial Reactor/Process

Synthesis gas contains a mixture of carbon monoxide and hydrogen and can be
obtained from the combustion of coal or natural gas. This gas can be used to produce
synthetic crude by the Fischer-Tropsch reaction. Describe two industrial reactors used
to convert synthesis gas to a mixture of hydrocarbons by the Fischer-Tropsch process.

Solution
Reactions. The Fischer-Tropsch reaction converts synthesis gas into a mix-
ture of alkanes and alkenes over a solid catalyst Usually containing iron. The basic
reaction for paraffin formation is as fo1:ows
nCO+(2n+1)H2 __j C,H,,+,+nH,O (El-5.1)
Making Gasoline For example, when octane, a component of gasoline, is formed, Equation (El-5.1)
becomes
8CO + 17H, --+ C, H,, + 8H,O (El-5.2)
Similarly, for the formation of olefins,

nCO + 2nH, ---+ C, H,, + nH,O (El-5.3)
For ethylene formation, Equation (El-5.3) becomes

2CO+4H, ---+ C,H,+2H,O (E 1 -5.4)
The other type of main reaction that occurs in this process is the water-gas-
shift reaction
H,O + CO e (El -5.5)
+
H,
CO,
In addition to the simultaneous formation of paraffins and olefins, side reac-
tions also take place to produce small quantities of acids and nonacids (e.g., ethanol).
Reactors. Two types of reactors will be discussed, a straight-through trans-
port reactor, which is also referred to as a riser or circulatingjuidized bed, and a
packed-bed reactor (PBR), which is also referred to as ajxed-bed reactor.
Riser. Because the catalyst used in the process decays rapidly at high temper-
atures (e.g., 35OoC), a straight-through transport reactor (STTR) (Chapter 10) is
used. This type of reactor is also called a riser andor a circulating bed. A schematic
diagram is shown in Figure El-5.1. Here the catalyst particles are fed to the bottom
of the reactor and are shot ap through the reactor together with the entering reactant
gas mixture and then separated from the gas in a settling hopper. The volumetric gas
feed rate of 3 X lo5 m3/h is roughly equivalent to feeding the volume of gas con-
tained in the University of Michigan football stadium to the reactor each hour.
A schematic and photo of an industrial straight-through transport reactor
used at Sasol are shown in Figure El-5.2 together with the composition of the feed
and product streams. The products that are condensed out of the product stream