Yields and Efficiency of Chemical Processing

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Due to the long history of the petrochemical industry, highly efficient process conditions, yields and selectivities are now commonly established. But, how do biotechnological processes work? In essence, the carbon source of biomass is innately associated with a high oxygen content due to the photosynthetic reaction.

As shown in Table 12.3, it is not surprising that significant quantities of CO 2 are emitted during the bacterial or yeast-based conversion of carbohydrate-derived renewable feedstocks, since the carbon is partially oxidized. An additional loss is consumed as an energy source for the metabolism of the cell. The overall stoichiometric yield is limited to around 50%. One of the most optimized and oldest large-scale biotechnological production processes worldwide-bioethanol-reveals a stoichiometric carbon efficiency of 51%. In reality 45-47% can be reached.

Therefore, to overcome this 'metabolic penalty' fermentation processes must be greatly simplified in comparison to existing petrochemical processes.

In order to use the synthetic potential of nature more efficiently both feedstock and product should be chemically closely related. Catalysis, traditional and in particular biocatalysis, can enable a faster and more efficient stepwise-based conversion compared with the fermentative processes. This and other options are described in detail in Section 12.3.2. Traditionally, in the petrochemical industry the desired product is directly purified after leaving the reactor. In the product stream, the desired product content is well above 10%-often higher than 95%. Aside from biocatalytic processes, fermentation processes operate at ambient temperatures and pressures in an aqueous medium. As a consequence, even if the metabolic carbon efficiency is high, the product concentration in the aqueous broth after fermentation is below 10%, only rarely somewhat above. An entirely different down-stream-processing is therefore required, which are associated with currently high specific investments and consumption of utilities.

Table 12.3 Carbon and oxygen content of different substances.

Fossil Renewable

Carbon and oxygen content in %

2

Bituminous coal

73

9

Wood

52

42

on

Lignite

56

18

Sugar (C12H22O11)

42

51

Starch (QH^^

44

49

Gasoline

87

<2.7

Biodiesel (RME)

77

13

'3 a

Diesel

86

~0

Bioethanol

52

35

3

Naphtha (CnH2n)

85

0

m rt

Natural gas

69

~0

Biogasa)

75

0

u

Ethylene

86

0

a) Pure methane.

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