Chemical process technology - How to make fuels and chemicals from syngas

6. What is ammonia mainly used for?

85% of the ammonia produced is used for nitrogen fertilizers (mainly urea). Ammonia is also used to produce intermediates such as amines, nitriles and organic nitrogen compounds, for use in the fine chemicals industry.

1. What process conditions are favorable in WGS reactor?

Allthough the reactions is only moderately exothermic, the temperature has a great impact on the equilibrium constant. The temperature has to be low! This is why two reactors are used, to keep the temperature down.

1. What is WGS used for?

WGS is Water gas shift reaction which is served to increase the H2/CO ratio of the syngas for use in e.g. production of various chemicals.

8. Discuss the reactor design in ammonia synthesis.

Ammonia synthesis reactors are classified by flow type (axial, radial, or cross flow) and cooling method. Temperature control is crucial: the exothermic reactions need heat removal. Cooling can be applied by either direct cooling or indirect cooling. Direct cooling: cold feed gas is added at different heights of the reactor (quench reactor). Indirect cooling: heat exchangers placed between the catalyst beds.

1. What is the main reactions in WGS?

An exothermic reaction where carbon monoxide reacting with water vapor to form carbon dioxide and hydrogen. CO+H2O->CO2+H2

5. What is the reaction that takes place for CO2 removal from syngas?

CO2+4H2<->CH4+2H2O (highly exothermic) H=-165kj/mol

16. Describe Fisher-Tropsch synthesis. Reactions and catalysts?

Highly exothermic reactions. Hydrocarbons created by chain-growth process which is very unselective and produces a wide veriety of hydrocarbons. Catalysts: Ni=hydrocarbons, Fe=Alcohols+Aldehydes. Co=Paraffins+Olefins. Co=Wax

1. What catalyst is used for WGS?

Iron-oxide based catalyst in high-temperature reactor. Copper-based catalyst in low-temperature reactor.

4. When synthesis gas is used for ammonia production it is necessary that all CO is removed. Why is this important?

It is important to remove all CO contents from synthesis gas for ammonia production because CO may poison the catalyst used for ammonia synthesis.

14. Describe the MTG process and draw principle scheme. Main chemical reactions? Process conditions? catalyst? Raw materials?

MTG Process: First DME reactor with acidic catalyst. The further dehydration in paralell reactors due to coke formation on catalyst for regeneration. Chemical reactions: Process conditions: Catalyst: Zeolites Raw materials:Crude methanol with (17%) water

11. Describe the industrial production of methanol from synthesis gas. Draw a principle scheme. What are the main chemical reactions, process conditions and catalyst?

Main chemical reactions: Process conditions: High temperature and high pressure in main reactor. Catalyst: Modern low-pressure process uses more active and selective copper-based catalyst. Catalysts are very sulfur sensitive. Reactro is usually quench or cooled multitubular reactor. Main reactions use hydrogen and CO or hydrogen and CO2 to produce methanol.

3. Why is mathanation important in commercial ammonia plants?

Methanation is the final purification step in ammonia plants. It is done to remove small residuals of CO and CO2 in gas. This is important because even small amounts of CO/CO2 can be poisonus to the iron-based catalysts used for ammonia synthesis.

10. What main reaction results in methanol formation from synthesis gas? What other reaction occurs parallel on the catalyst? What should the H2/CO ratio in an ideal syngas for methanol synthesis be?

Methanol formation: CO+2H2 <-> CH3OH. and CO2+3H2<->CH3OH+H2O. Parallel reaction is WGS reaction: CO+H2O<->CO2+H2. The ideal H2/CO ratio is 2mol H2/1mol CO.

13. Describe production of DME from methanol. Chemical reaction? Catalyst? Uses? Physical properties of DME?

Production: Produced by dehydration of methanol which is an equilibrium limited reaction so 100% conversion cannot be reached. Catalyst: Acid-based catalyst. Uses: DME is used as fuel, can also be further dehydrated to create lighter alkenes such as olefins. Physical properties: Has very low boiling point and can be stored under pressure.

12. Point out differences between the WGS reaction and methanol synthesis in terms of reactions, catalysts and feed composition.

Reactions: The side reactions of methanol synthesis happening on catalyst is the same as the main reaction in WGS: CO+H2O<->CO2+H2 Catalysts: Methanol synthesis uses copper-based catalyst due to selectivity while WGS uses iron-based catalyst due to sulphur sensitivity. Feed composition: Methanol synthesis requires a very specific H2/CO composition of 2mol/mol while WGS can use whatever composition.

4. Describe the purification step of syngas for removal of CO. What is it called? Conditions? Catalyst?

The final purification step for ammonia production is called methanation. The CO and CO2 is removed catalytically by reaction with hydrogen over nickel catalyst. The two main reactions that take place are highly exothermic and converts the CO and CO2 to methane (CH4) and water.

3. What is the relation between methanation and steam reforming?

The primary methanation reaction is the reverse of the steam reforming reaction. CO+3H2<->CH4+H2O

2. Which kind of reactor is commonly used for WGS? Describe how they work.

WGS uses two adiabatic fixed-bed reactors with cooling in between. First reactor is HT shift reactor where most of the CO is converted. The second is LT shift reactor where the CO content in gas is further reduced. In the LT reactor an excess of catalyst is needed, about 70% than what is used because of deactivation.

15. How could higher alcohols be synthesised from syngas?

higher alcohols can be produced from the methanol process by using different catalyst (Cu/ZnO/Al2O3) and using syngas with lower H2/CO ratio (more CO).

7. Describe the synthesis of ammonia. Describe i. Reaction formula. ii. Reaction conditions (pressure, temperature, and feed) iii. Catalyst.

i. N2 + H2 <-> 2NH3 ii. Low temperature, High pressure. Ideal feed is 3 parts hydrogen and 1 part nitrogen. iii. Iron-based Catalyst.