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Table 1 Typical pretreatment process conditions of pretreatment*

From: Impact of pretreatment and downstream processing technologies on economics and energy in cellulosic ethanol production

  Dilute Acid Dilute Alkali Hot Water Steam Explosion
Temperature (°C) 160-220 > 100 160-230 160-290
Pressure (MPa) Saturated vapor pressure Saturated vapor pressure Up to 5 0.69-4.83
Solution (acid/alkali) concentration (%) 0.05-5 0.5-3 - -
Residence Time 1-60 min -Few minutes to several hours 12-60 min Few seconds-several minutes
Mode of Action -Hydrolysis of hemicellulose to soluble sugars
-Alteration the lignin structure
-Solublization and extraction of lignin
-Swelling of cellulose which increases internal surface area and separation of linkages between lignin and carbohydrates
-Water acts as dilute acid at high temperatures
-Release of acetic acid and other acids from hemicellulose hydrolysis helps in further hydrolysis
-Hydrolysis of hemicellulose to oligomers
-Water acts as dilute acid at high temperatures
-Rapid release of pressure opens up the structure of biomass
-Removal of hemicellulose
Challenges -Process require Detoxification before fermentation
-Acid is corrosive and hazardous so reactor material should be corrosion resistant
-Cost of alkali is very high as compared to other reagents
-Longer residence times are required at low temperatures
-Process require high pressure to keep water in liquid state -Formation of inhibitory compounds due to hemicellulose degradation
  1. *Ref: [2, 3, 9, 10, 16, 22, 23, 60, 61]