Appendix l-D—The Impact of Genetics on Ethanol — A Case Study • 299 
related to operating costs and the second is related to 
energ\ et'ticiencv . It' coal is used to [)ro\ ide energ\' 
tor distillation, and it is valued at S30/ton, with 
10,51)0 Btu Ih or S 1 .50/million Htu, then the energy 
cost for distillation (optimistically assuming 40,000 
Btu gal) is SO gal. If lignin from cellulosic hiomass is 
used as a fuel, the cost is reduced further. On the 
other hand, if oil at $40,1)hl (130,000 Btu/gal and 42 
gall)!)!) or S7 million Btu is used, then the energv' 
cost is 28 cents gal of ethanol. 
From a common sense, economic, and political 
point of view, it does tiot seem reasonable to utilize 
liquid fuel to produce liciuid fuel from hiomass. 
rherefore, it w ill he assumed that petroleum will not 
he used for distillation and that either coal or bio- 
mass will lie employed. 
In order to assess the impact of process improve- 
ments on the energv demand, it is necessary to look 
at an o\ erall material balance. This is summarized in 
figure I-I)-5. Only a portion of the entering biomass 
feedstock is fermented to ethanol and there are two 
product streams, one containing ethanol and the 
other solids, both must he separated from water. It is 
important to note that as the ethanol concentration is 
increased, the energv requirement for both ethanol 
recovery from the water and for drying will de- 
crease. Therefore, the impact of developing ethanol 
tolerant micro-organisms is seen as a reduction in 
energv’ cost. 
Figure l-D-5.— Process Schematic for Material and 
Energy Balance 
Biomass 
Solids 
SOURCE; Massachusetts Institute of Technology. 
The third major cost for ethanol manufacturing is 
the capital investment, which represents about 4 to 
12 percent of the manufacturing cost. The capital in- 
vestment is determined by the complexity of the 
processes and the volumetric productivity of ethanol 
production. Thus, the development of a micro-orga- 
nism that will require a minimum amount of feed- 
stock pretreatment and will produce ethanol at a 
higher rate will reduce the net capital investment. 
The volumetric productivity (Q^) for ethanol pro- 
duction is given by: 
Qe = 
where q^ is the specific productivity expressed in g 
ethanol per g cell hr, and X is the culture density. 
Therefore, there are two approaches to obtain high 
productivity; first, to choose or create an organism 
with a high specific rate of ethanol production and 
second, to design a process with high cell density. 
The application of genetics can be used to enhance 
the intracellular enzyme activity of the enzymes 
used for ethanol production. The resulting increase 
in Qp will result in reduced capital investment re- 
quirements. 
There are four types of ethanol processes based 
on different organisms; they are: 
1. Saccharomyces cerevisiae and related yeast, 
2. Saccharomyces cerevisiae/T richoderma reesei, 
3. Zymomonas mobilis, and 
4. Clostridium thermocellum/thermosaccharolyti- 
cum, or thermohydrosulfuricum. 
The first is the traditional yeast based process using 
S. cerevisiae to ferment soluble hexose sugar to eth- 
anol. In the second, the substrate range is extended 
to cellulose by the use of cellulase produced by T. 
reesei. The third approach utilizes Z. mobilis; this 
organism is a particularly fast and high ethanol yield- 
ing one. Its range of fermentable substrates, how- 
ever, is limited to soluble hexose sugars. 
In many tropical areas of the Americas, Africa, 
and Asia, alcoholic bev^erages prepared from a mixed 
fermentation of plant steeps are popular. Bacteria 
from the genus Zymomonas are commonly em- 
ployed. In the early 1950's, the genus Zymononas ac- 
quired a certain fame among biochemists by the dis- 
covery that the anaerobic catabolism of glucose 
follows the Enter-Doudoroff mechanism.^ This was 
very surprising, since Zymomonas was the first ex- 
ample of an anaerobic organism using a pathway 
mainly in strictly aerobic bateria.® 
In spite of its extensive use in many parts of the 
world, its great social implications as an ethanol pro- 
=M. Gibbs and R. D. de Moss, "Ethanol Formation, in Psuedomonas 
Undneri," Arch. Biochem. Biophys.. 34:478-479, 1951. 
®J. Swings and J. DeLey, "T he Biology of Zymomonas," Bacteriological Re- 
views 41:1-46, 1977. 
