cellulase---literature-review-2 by xiagong0815


									“Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute
Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover”
By A. Aden, M. Ruth, K. Ibsen, J. Jechura, K. Neeves, J. Sheehan, B. Wallace, L. Montague, A.
Slayton and J. Lukas
National Renewable Energy Laboratory – June 2002
        The US Department of Energy has been promoting the development of process to
convert lignocellulosic biomass to ethanol. This report discusses the process including co-
current dilute prehydrolysis with simultaneous saccharification (enzymatic) and co-
fermentation. The production of the cellulase enzymes as well as feed handling and product
recovery and purification were included in the development of the process design and the
economic model. It has been recommended that for the process to be economically feasible for
a 2010 start-up date the selling price of ethanol must be $1.07 per gallon.
        A PFD has been developed at the recommendations found from this research using
ASPEN software. This consists of 164 unit operations and 457 streams. This PFD could be used
as a base case and simplified for the purposes of our project. Sizes and specifications of the
equipment were determined using mass and energy balances in the ASPEN software. Then
direct quotes were obtained from vendors to determine capital costs. When unable to obtain
direct quotes, the capital costs were determined using cost correlations. Similar cost
correlations could be found in “Product and Process Design: Synthesis, Analysis and Evalution”
for the purposes of our project.
        Details of feed handling and pretreatment have been excluded from this summary as
those parts of the design will be handled by other group members. Also, this report has
selected to buy cellulase enzymes developed by an outside source, so the extraction,
modification and production of the enzymes was not covered in-depth.
        The purchased cellulase enzyme was used for continuous enzymatic hydrolysis
(saccharification) of the detoxified hydrosylate slurry, which was followed in series by an
anaerobic fermentation tank. Z. mobilis was chosen as the fermenting organism. The cellulose
will be hydrolyzed at a slow rate and at low temperatures for several days before most of the
sugars have been converted to ethanol and the beer can then be sent to product recovery.
        Distillation is used to separate the ethanol and water to its azeotropic composition.
Solids from the bottoms were sent to a boiler. The water phase was treated by aerobic and
anaerobic digestion and recycled back to the process.
        The location of a plant and the distance the feedstocks must be shipped should be
considered in the economic feasibility. As a result the plant should be built in Iowa to minimize
transportation costs. Additionally, the “economics of scale” were assumed to have an exponent
of 0.6, meaning that the capital costs do not scale linearly with the size of the operation and
savings can result from designing a larger process. This analysis assumes 2000 metric tons of
corn stover were to be processed each day.
        An in-depth discussion of transportation and other costs associated with the corn stover
feed stock was included in the article. This was not summarized as we have not yet declared
that corn stover will be our feedstock. If this is to be used, this article can be revisited later. A
good approximation can be $38 to $55 per metric ton of corn stover delivered.
Process Design and Cost Estimating
       Feed Stock Storage and Handling
       Pretreatment and Hydrosylate Conditioning
       Saccharification and Co-Fermentation
        o      Saccharification – Occurs at elevated temperature to take advantage of
               enzymatic activity to reduce time and amount of enzyme required. Cellulase
               enzyme is actually a mixture of endoglucanases and exoglucanases and β-
                glucosidase. Cellulase enzyme and detoxified, diluted hydrosylate are processed
                in a train of five 1 million gallon tanks. The total residence time is assumed to be
                around 36 hours.
       o        Fermentation - Z. mobilis is used as biocatalyst. It will ferment both glucose and
                xylose and is assumed to also ferment mannose and galactose. First grown in a
                series of “train” vessels. Then inoculated in a small vessel with the saccharified
                slurry and nutrients, this then progressed through a series of vessels increasing
                in size each time.
       o        Design – Tables are included listing process conditions and reactions occurring
                during saccharification as well as the seed train development and co-
                fermentation. Both predicted and lab-scale experimental conversions are
      Distillation, Dehydration, Evaporation and Solid-Liquid Separation - Two distillation
columns. First (called the beer column) removes CO2 and most of the water and the second
concentrates the ethanol near to the azeotropic mixture. Water from the azeotropic mixture is
removed using molecular sieve adsorption. Fermentation vents remove CO2 but also some
ethanol and are therefore fed to a water scrubber and the recovered ethanol is fed to the beer
column. Estimates include column efficiencies and the number of trays required.
      Waste Water Treatment
      Product and Feed Chemical Storage
      Combustor, Boiler and Turbogenerator
      Utilities
      Water and Carbon Balances and Energy Analysis – A loss of 784 kg/hr of water is
experienced. The carbon balance closes to 99%, with only 0.1% being lost to the atmosphere.
Based on energy balances, the 77% of the energy in the carbohydrates of the feedstock is
recovered as ethanol.

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