Ethylene (C2H4) reacts with water to form ethanol via a catalytic addition reaction. The yield of ethanol production is determined by the equilibrium of the reaction:

C2H4 (g) + H2O (g) — C2H5OH (g) AH = -43.4 kJ/mol

The catalyst used is phosphoric acid (silica gel based), which sets some demanding standards concerning corrosion of the equipment.

The equilibrium reaction is influenced by temperature, pressure and the ratio of water to ethylene. Normal process conditions are equimolar concentrations, 250-300°C and 5-8 MPa, resulting in a conversion degree of only 7-22%. A lower temperature favours the ethanol production, but also favours the side reaction to diethyl ether:

C2H5OH (g) + C2H4 (g) — C2H5OC2H5 (g)

Too high pressure is also not favourable because higher alcohols will be formed:

C2H4 (g) — C4H8 (g) + H2O (g) — C4HPH (g)

17.2.4 The indirect hydrolysis of ethylene

The indirect hydrolysis of ethylene takes place with the aid of sulphuric acid. Ethylene is dissolved in concentrated sulphuric acid. Addition of water leads to the production of ethanol and some diethyl ether as side product:

C2H4 + H2SO4 — C2H5OSO3H AH = -60 kJ/mol

C2H4 + C2H5OSO3H — c2h5oso2oc2h5

Hydrolysis after addition of water:

C2H5OSO3H + H2O — C2H5OH + H2SO4 C2H5OSO2OC2H5 + H2O — c2h5oh + c2h5oso3h

Side reaction with water to form diethyl ether:

Dependent on reaction conditions 5-10% diethyl ether is formed during reaction. The use of concentrated sulphuric acid sets high standards for the equipment due to the corrosive character.

Conclusion is that for the chemical routes the direct route is not economically attractive and that the indirect routes are the best up to now, although relatively cumbersome. This is the reason that the biological route of sugars to ethanol is economical, interesting and also remains to be a good alternative for ethanol production.

Newest developments focus on a combination of gasification and biological fermentation processes. After gasification, anaerobic bacteria such as Clostridium ljungdahlii are used to convert the CO, CO2 and H2 into ethanol. Higher rates are obtained because the process is limited by the transfer of gas into the liquid phase instead of the rate of substrate uptake by the bacteria.2

Subsequent conversion of the formed synthesis gas to ethanol brings the formation of ethanol from a diversity of biomass sources into reach. Two routes can be followed in this respect: catalytic conversion of the synthesis gas via the routes shown in Table 17.1 or biological route via direct fermentation. For smaller scale installations (<100 Kton) this last route seems to be interesting compared to the catalytic route. The catalytic route needs a catalyst which is always sensitive to deactivation via pollution in the feedstock. Also catalysts are relatively expensive. This leads to high investment cost for cleaning (on ppm level) of synthesis gas and thus economically attractive at large scale processes only. The direct biological route seems more promising for smaller scale systems because they can endure pollution of the synthesis gas and low cost fermenters can be used at ambient process conditions. Fermentation tests for ethanol production from synthesis gas have been done in various reactor types. Phillips and others (1994) used a stirred batch reactor. Klasson and others (1990) used several continuous reactors, namely a stirred-tank reactor, a packed bubble column and a trickle-bed reactor. The processes take place at 37°C, and the pH is controlled. A frequently used bacterium is Clostridium ljungdahlii. This bacterium produces acetic acid as a side-product.

In the ideal case (no side-products) this results in the following overall theoretical reaction when starting from pinewood:

CH134Oa66 + 0.17O2 + 0.17H2O ^ 0.28C2H5OH + 0.44CO2

AH° = -59 kJ/mole wood

Overall combustion energy efficiency in the ideal case is (LHVethanol/LHVwood) = 0.28×1233/410 = 84.2%. This means that this route has potential as a possible route for ethanol production from wood.

It has also been shown (Durre, 2007) that it is possible to produce butanol. Durre mentions that ‘butanol has advantages over ethanol, such as higher energy content, lower water absorption, better blending ability and use in conventional combustion engines without modification. Like ethanol, it can be produced

fermentatively or petrochemically. (……. ) The best-studied bacterium to perform

a butanol fermentation is Clostridium acetobutylicum. Its genome has been sequenced, and the regulation of solvent formation is under intensive investigation. This opens the possibility to engineer recombinant strains with superior biobutanol — producing ability’. It is also possible to produce butanol from grass and straw via an enzymatic way (www. biobutanol. nl as of January 2010). However, for this route the same disadvantages are valid as for the enzymatic route to ethanol: only the biodegradable fractions can be converted to alcohols.

The question is how far the ideal route can be approached and at what costs. For this reason the next section describes a conceptual design and simulation of a wood to ethanol plant via gasification and direct fermentation. The design is based on literature data and performed with the use of the software package Aspen Plus (Van Kasteren et al, 2005). In the following sections the design assumptions, the input composition, the reactor section and the purification section are described in detail. Other process components are discussed in the general process description.