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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