2011CH10113
Introduction
Converting gas to liquid is a refinery process, through this refinery process natural
gas is converted into longer-chain hydrocarbon such as petrol or diesel. Various
methane rich gases into liquid either by direct conversion or through an
intermediate i.e., syngas. There are various process through which conversion of
natural gas to liquid fuel can be achieved for example, Fisher Tropsch process,
Methanol to gasoline process, Syngas to gasoline plus process. We will discuss
Fisher Tropsch process in detail later.
Methanol to gasoline process is also called as a Mobil process it starts by
conversion of the natural gas to syngas then conversion of the syngas to methanol
which is later polymerized into alkanes over a zeolite catalyst.
Methanol is made from natural gas in a series of three reactions:
1. Steam reforming: CH4 + H2O → CO + 3 H2 ΔrH = +206 kJ mol-1
2. Water shift reaction: CO + H2O → CO2 + H2 ΔrH = -41 kJ mol-1
3. Synthesis: 2 H2 + CO → CH3OH ΔrH = -92 kJ mol-1
Then methanol is dehydrated to give dimethyl ether:
2 CH3OH → CH3OCH3 + H2O
Dimethyl ether then further dehydrated over a zeolite crystal to give a gasoline with 80% C5+ hydrocarbon products.
Syngas to gasoline plus process is carried out via five stages:
1. Methanol synthesis2. Dimethyl ether synthesis3. Gasoline synthesis
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4. Gasoline treatment5. Separator
Converting Natural gas to liquid is an energy-intensive process hence, the number of commercial-sized plant are limited.
Objective
The objective of the term paper is to show material and energy balance of conversion of 1
ton of natural gas to liquid fuel. In order to get the result we will use Fisher Tropsch
process. Hence, we will understand more about Fisher Tropsch process and the role of
catalysis in it.
Analysis based on literature survey
Fisher Tropsch Process:
Fisher-Tropsch process takes the idea of converting natural gas to synthesis gases
which contains CO and H2. After that CO is passed over the metal catalysis to
produce aliphatic hydrocarbon.
CH4 + 1/2O2 → 2H2 + CO
(2n + 1) H2 + n CO → CnH(2n+2) + n H2O
Fisher-Tropsch process is said to be a risky process due to many reasons, one of the
reason includes the most expensive and complex section of Fisher-Tropsch
mechanism which is the production of purified syngas and so its composition should
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match the overall usage ratio of the FT reactions, which in turn depends on the
product selectivity.
Fisher Tropsch process is known in two parts, first is High-temperature Fischer–
Tropsch (HTFT) and second is Low-temperature Fischer–Tropsch (LTFT). High-
temperature Fischer–Tropsch (HTFT) uses iron catalyst while Low-temperature Fischer–
Tropsch (LTFT) uses cobalt catalyst.
Role of catalysis:
Fisher-Tropsch process is a catalyst based reaction, various catalyst is used in
reaction but the most common are the transition metals cobalt, iron and ruthenium.
CATALYST PRODUCT
Iron Linear alkenes
Cobalt Alkanes
Nickel Methane
Ruthenium High molecular weight hydrocarbon
Rhodium Large amounts of hydrocarbon
Cobalt catalysts are more active for Fischer–Tropsch synthesis when the feedstock is
natural gas. Natural gas has a high hydrogen to carbon ratio, so the water-gas-shift is not
needed for cobalt catalysts.
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Fischer–Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds. And
Cobalt-based catalysts are more sensitive towards such poisoning. Hence, purity of syngas is
an issue arising in Fisher-Tropsch process.
Reactors:
Generally, FT reaction is carried out in two types of reactor. First is a straight through
reactor which is also known as riser or circulating bed reactor and second is known as
packed bed reactor.
Riser:
Straight through reactor is used because the catalyst used in FT process decays rapidly at
high temperature.
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Figure1 Sasol Slurry Reactor
Courtesy: 1 Sasol/Sastech PT limited
Packed Bed Reactor:
Synthesis gas is fed at the rate of 30,000 m3/h (STP) at 240oC and 27 atm.
Courtesy: 2 (Schematic and photo) Sasol/sastech PT limited
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Critical comment
As we can see through material balance the final moles of hydrocarbon is 50.382/n. This
implies that as the number of carbon increases moles of hydrocarbon will decrease. Apart
from this the final product will depend on various factors like selectivity, temperature, feed
gas composition, pressure, catalyst type and promoters. According to the condition we will
get different and wide range of olefins, paraffin and oxygenated products (alcohols,
aldehydes, acids and ketones). Figure below illustrates the relationship between the CH4
selectivity and that of some selected hydrocarbon product.
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For all Fisher Tropsch catalysis an increase in operating temperature results in a shift in
selectivity towards lower carbon number products and to more hydrogenated products.
The degree of branching increases and the amount of secondary products formed such as
ketones and aromatics also increases as the temperature is raised.
Conclusion
Apart from material and energy balance the final product condition and amount depends on
many factors like catalyst and reactors has to be kept in mind. Different reactors has their
advantage and disadvantages which has to be calculated. Selectivity and choice of catalyst to
avoid poisoning is also an important factor determining the final output. Also, it is well
known that the economic viability of gas conversion is determined by capital costs and
average product price. In this respect it should be taken care that the manufacture of
synthesis gas is by far the most capital intensive part of a gas conversion plant. Hence, the
Fischer–Tropsch step should aim to utilize synthesis gas as efficiently as possible.
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References
Geerlings, J.J.C., Wilson, J.H., Kramer, G.J., Kuipers, H.P.C.E., Hoek, A., and Huisman, H.M., 1999, Fischer–Tropsch technology — from active site to commercial process, ScienceDirect, v. 186, p. 27-40.
Mark, E. Dry., 2002, Catalysis Today, ScienceDirect, v. 71, p. 227-241.
Wikipedia, http://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_process
Himmelblau, David M., Riggs James B., 2009, Basic Priciples and Calculation in Chemical Engineering (7th edition). Pearson Education Inc.
Fogler, H. Scott (2006). Elements of Chemical Reaction Engineering (4th edition). Pearson Education Inc.
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