Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel
Sanjana Mandrai 2011CH10113 CHL221
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 t o 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: 2. Water shift reaction: 3. Synthesis:
CH4 + H2O → CO + 3 H 2 CO + H2O → CO2 + H2 2 H2 + CO → CH3OH
ΔrH = +206 kJ mol -1 ΔrH = -41 kJ mol -1 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. 2. 3. 4.
Methanol synthesis Dimethyl ether synthesis Gasoline synthesis Gasoline treatment
Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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5. 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 reason s, 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 match the overall usage ratio of the FT reactions, which in turn depends on the product selectivity. Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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Fisher Tropsch process is known in two parts, first is High-temperature Fischer – Tropsch (HTFT) and second is Low-temperature Fischer–Tropsch (LTFT). Hightemperature 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. 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: Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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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.
Figure1 Sasol Slurry Reactor Courtesy: 1 Sasol/Sastech PT limited
Packed Bed Reactor: Synthesis gas is fed at the rate of 30,000 m 3/h (STP) at 240oC and 27 atm.
Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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Courtesy: 2 (Schematic and photo) Sasol/sastech P T limited
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 CH 4 selectivity and that of some selected hydrocarbon product. Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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For all Fisher Tropsch catalysis an increase in o perating 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 Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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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.
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 (7 th edition). Pearson Education Inc. Fogler, H. Scott (2006). Elements of Chemical Reaction Engineering (4 th edition). Pearson Education Inc.
Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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Material and energy balance: Conversion of 1 tonne of Natural gas to liquid fuel | Sanjana Mandrai
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