Systematic errors in data evaluation due to ethanol stripping and water vaporization

Systematic errors due to the neglect of water and/or ethanol partition between liq. and gaseous phases are discussed for bioreactors equipped with or without a condenser. Both water vapor and ethanol vapor are present in the off-gas leaving the condenser. Presence of residual water vapor largely influences the gas measurements by diln. As a consequence, the oxygen consumption rate can be overestimated by a factor of 3 if calcns. are not cor. for water vapor content or if no addnl. device is implemented.after the condenser to completely dry the off-gases. The mass balance and partition equations predict that the condenser has only a small effect on redn. of the ethanol vapor content of the off-gas. The reason is the high ethanol concn. of the condensate droplets on the condenser wall in contact with the off-gases. Model predictions as well as exptl. results show that ethanol evapn. represents a large fraction of the ethanol prodn. rate and influences greatly the elemental recoveries. For a reactor working at 30 DegC without condensation of the vapors and for a volumetric aeration rate of 0.63vvm, stripping of ethanol resulted in a gaseous diln. rate of 0.016 h-1 for ethanol. The diln. rate by stripping was reduced to 0.014 h-1 when a condenser at 12 DegC was implemented. The fraction of ethanol that is stripped is mainly dependent on the ratio D/vvm (liq. to gaseous flow rates), and the effect is only slightly influenced by low condenser temp. The evapn. of ethanol may account for more than 20% of the ethanol formation rate. Therefore, the condenser does not succeed to reflux all ethanol to the reactor broth. In terms of a unit operation, ethanol vapor can be efficiently reduced by absorption instead of condensation. To demonstrate the feasibility, a simple modification of the reactor was tested for continuous cultures: the feed port was changed from the top-plate to the top of the condenser, which was used as an absorption column. Ethanol stripping was reduced by a factor of 4 as compared to the condensation setup (at 12 DegC): it accounted for 2% of the ethanol prodn. rate as compared to 8.2% at D = 0.19 h-1 and 0.63vvm. [on SciFinder (R)]