Introduction

Ethanol is an alcohol which can be produced either by petrochemical method though the hydration of ethylene or through biological processes like the fermentation of sugarcane, molasses, starch, wheat, corn etc. The biological process is considered to be world’s first sustainable biofuels, accounts for approximately 95 % of the ethanol production. Bio-ethanol is a leading renewable fuel used in the transportation sector due to it’s clean, affordable and low-carbon property. Ethanol alone is considered to be a low-cost alternative fuel for the environment than gasoline. This is because the bio-ethanol supplemented vehicles produce either lower carbon dioxide emissions or approximately the same or lower levels of hydrocarbon and oxides of nitrogen emissions. Hence, ethanol can be blended with gasoline and used in the automobile industry as a fuel.

Similar to the automobile industry, the aviation industry is also seeking technically viable and economical approaches to endowing sustainable alternatives to petroleum-based jet fuel. The major reasons for looking alternative jet fuel are cost in stability and climate change. There are two types of jet fuel such as Jet A or Jet A-1, which is most commonly used in commercial aviation industry. Other type is Jet B (i.e. naphtha-kerosene) which is commonly used in military aircrafts aviation flight. Mostly, Jet-A fuel is used (~22 billion-gallon per year) in the US, which has a freezing point of – 40 oC and Jet A-1 fuel is used (80 billion gallons per year) in the rest of the world, which has a lower freezing point of – 47 oC. Ethanol can also be converted into aviation turbine fuel (Jet A-1 fuel).

The alcohols cannot be directly used as aviation turbine fuels (called as jet fuel) due to its lower energy content (i.e. energy density of ethanol is 25 MJ/kg) and other properties such as flash point, freezing point, auto ignition temperature, when compared to the jet fuels namely Jet A and Jet A -1 (i.e. 43 – 48 MJ/kg).

There are many routes available for the production of jet fuel from renewable sources, such as

a) Hydroprocessed Esters & Fatty Acids (HEFA): The vegetable oils (triglyceride) and fatty acids are the most commonly used as feedstock.

b) Gasification and Fischer-Tropsch synthesis (FT): The syngas, which are derived from biomass, can be used as feedstock for the production of aviation turbine fuel.

c) Sugar to Jet fuel: It can be processed via two pathways such as (1) Catalytic upgrading of sugars and (2) Fermentation of sugars to produce liquid hydrocarbons products.

d) Alcohol to Jet fuel: Commonly, alcohol (e.g. ethanol / butanol) is used as feedstock.

This technical report discusses the Alcohol to Jet Fuel route and focuses on process description, opportunities, and challenges of jet fuel production from ethanol.

Ethanol to Jet fuel

Typically, alcohol to jet fuel conversion process includes the following process steps

• Dehydration

• Oligomerization

• Hydrogenation

The dehydration, oligomerization and hydrogenation process steps are not new for chemical industries and this route is designed to rely on established and well-proven technology as shown in the Figure 1.

Figure 1 Schematic diagram of ethanol to Jet fuel

Dehydration

In dehydration stage, water molecule is removed from ethanol molecule through an acid catalytic reaction to produce ethylene. The basic reaction equation is as follows,

C2H5OH + catalyst → C2H4 + H2O

The thermal decomposition of ethanol to produce ethylene and water, takes place at a temperature range of 400°C to 450°C and at a pressure of 11 bar (approximately) in presence of alumina or transition metal oxides or H-ZSM5 zeolite catalyst.                                                  Higher temperature is required in order to shift the equilibrium toward product formation. There are a few side reactions that produce by-products like diethyl ether, acetaldehyde, propane and butane. These by-products could be removed through the separation unit to obtain high purity of ethylene product (99.96%). The by-product formation could be minimised by maintaining an optimal reaction temperature (400°C to 450°C).

Oligomerization

Oligomerisation process is conversion of ethylene (short-chain molecules) into linear α-olefins (long-chain molecules) via a catalytic reaction. Recently, there are many new catalysts are developed, such as Ziegler Natta, chromium diphosphine and zeolites etc. which can be used in the oligomerization process. Typically, the reaction takes place in the presence of sulfonic acid resins, solid phosphoric acid or acidic zeolites at a temperature range of ~ 100 to 300°C and high pressure. . The operating condition depends on the type of catalyst used and feedstock. The oligomerization of ethylene is given as follows,

n [C2H4] → C2nH4n

There are two commercially available processes of ethylene oligomerization. One of the processes is Gulf process or Ethyl-process, which is based on aluminium catalysts and the second one is Shell Higher Olefin Process.

1) Typically, Gulf-process produces linear α-olefins at temperatures above 200 °C and at a pressure of up to 250 bar in presence of tri-ethyl-aluminum (TEA) catalyst. This process which produce by-products in the range of 1.4 to 1.5 % of alkanes and branched α-olefins.

2) Ethyl-process is similar to the Gulf-process. The product stream is fractionated from C4, C6 to C10 and C12 to C16. The C4-fraction is further subjected to trans-alkylation, which increase the yield in higher alkenes.

3) SHOP-process involves liquid ethene as feedstock. The reaction occurs a temperature range of 80 °C to 120 °C at a pressure between 70 to 140 bar in presence of a nickel catalyst with phosphine ligands.

The selection of appropriate technology depends on several economic and site specific factors.

Hydrogenation

The α- olefins (C9-C16 and >C16) are subjected to hydrogenation process by addition of hydrogen in presence of nickel or palladium or platinum on activated carbon catalyst or zeolite support, which operates at temperature range of 300-400°C.

Challenges for ethanol to Jet fuel production

Following are the challenges for commercial scale operation

• Selection of technology to suit project objective

• Techno commercial feasibility

• Impact of feedstock variation & availability

• Estimation of yields at larger scales

• Process flexibility

• Understanding stability of catalysts, catalyst lifetimes, on-stream factors, and regeneration protocols and other critical process components

• Optimizing capital costs, which are especially challenging when new technologies are being developed

• Selection and optimization of separation technologies required to meet fuel specifications

Opportunities

Generally, the bio-ethanol is mainly produced in three ways such as sugar, starch, cellulose & hemicelluloses etc, which have high starch contents and non-molasses sources such as agricultural residues also. The feedstock of ethanol changes from country to country due to their soil and climate condition. In India, sugarcane molasses and straw of rice is the most common feedstock for bio-ethanol production. Recently, India has focusing on the production ethanol from bamboo feedstock also, which is available in north-east region of