Ethanol from cellulose - an explanation of constraints on the
process
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"1. What is meant by mass transfer in a chemical engineering/transport
phenomenon sense at the microscopic level?"
Mass transfer refers to the selective movement of one species of molecules
or ions within a gas, liquid or solid. Mass transfer is caused by a
concentration gradient, just as heat transfer is caused by a temperature
difference. Mass transfer is necessary for a subsequent activity such as
evaporation, or chemical reaction. In terms of acid hydrolysis of
cellulose, the acid, hydrogen ions and sulfate ions, must move into the
"solid" wood or bagasse being hydrolyzed, and then the resulting sugars must
diffuse back in to the solution. In porous solids, such as biomass, these
mass transfer rates are inherently slow. The rate of mass transfer is
controlled by a physical constant, diffussivity, just a conductive heat
transfer rates are controlled by a physical constant the thermal conductivity.
"2. What is meant by diffusion rate and chemical kinetics constraints?
(The diffusion rate part of this question is discussed in the answer to the
previous question.) Chemical kinetics refers to the actual rate of a
chemical reaction, which may be rather rapid, once the reactants are made
available to each other as a result of mass transfer. It also refers to the
actual complexity of real chemical reactions. For simplicity we write
"oxygen plus hydrogen yields water" but an expert regarding combustion of
hydrogen would fill many pages with the exact way molecules, atoms, and
radicals combine to convert hydrogen and oxygen to water. Similarly you
would like to say "cellulose plus water yields sugars" but that too is a
very complex process. Unfortunately the reaction of water and cellulose
also yields "byproducts" that are not useful for fermentation. Many
scientists, such as Dr. Goldstein who participates in this group, have spent
a considerable portion of their career studying the cellulose/water reaction
and related ones. You can only study "reaction mechanisms" you do not
change them. The chemist/engineer can only change the external
circumstances to take best advantage of the existing reactions, by changing
temperature, pressure, catalysts, and concentrations.
"3. Why are these problems?"
These problems regarding rates of mass transfer and the complexity of actual
chemical reactions are an inherent part of the "real world." They are the
result of "physical chemistry" at the scale of molecules, atoms, and ions.
(Sometimes chemists derisively refer to chemical engineers as "applied
physical chemists." I consider the reference as almost a compliment. Some
chemical engineers spend their professional careers managing mass transfer
rates and chemical reactions in commercial-scale equipment. They know first
hand that liquids and gases are far easier to process than solids.)
"4. What are the possible solutions?"
Possible solutions to the problems caused by mass transfer rates and
complex chemical reactions are very limited, and they have already been
tried many times. In general, higher temperatures increase diffusion rates
and chemical reaction rates, but there are limits on increased temperatures.
The rate of undesirable reactions may increase faster than the desired
reaction rates, etc. You can wish for a "better" catalyst, meaning faster
and more selective. This wish is only a wish until it is demonstrated, and
then reported refereed journals, in patents or by the existence of a second
commercial plant. (The first commercial plant may have been a financial
mistake, but it may continue to operate.) The ideal catalyst will not solve
mass transfer problems.
Mass transfer problems can be minimized by grinding the solids to be reacted
to much smaller particles, so the there is more area for reaction and so
diffusion can take place over much smaller distances. Unfortunately, these
small particles still do not avoid mass transfer constraints at the
molecular level. Grinding to very small particle sizes becomes costly, and
actual processing of the small particles has its own set of mechanical
problems.
The history of cellulose hydrolysis is a story of inventors who think they
have solved these problems of mass transfer and complex reactions, perhaps
without even knowing the problems exist. The result is great disappointment
in the actual performance of the "new" process, and the wasting of financial
resources.
Harry W. Parker, Ph.D., P.E.
Professor of Chemical Engineering
Texas Tech University
Lubbock, TX 79409-3121
(806) 742-3553 Fax 742-3552
MIHWP@TTACS1.TTU.EDU
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