Balancing the Ethanol Formation in Continuous Bioreactors with Ethanol Stripping

Abstract

Ethanol microbially produced in continuously operated aerated bioreactors is partly discharged with the liquid flow and is partly stripped off with the gas phase. The calculation of ethanol formation rates – as required, for example, in data evaluation by Metabolic Flux Analysis – solely based on the liquid‐borne ethanol, while neglecting the ethanol stripping, inevitably results in defective data interpretation. The proportion of stripped ethanol can be measured or, alternatively, calculated. The developed structured model describes the ethanol stripping as a two‐step process while differentiating between the phase transition from the liquid into the gas and the discharge of the evaporated ethanol with the off‐gas. As shown by model analysis as well as stripping experiments, the stripping rate is mainly determined by the ethanol discharge rather than by the phase transition. Consequently, the ethanol‐stripping rate depends on the specific gas flow rate and the partition coefficient for ethanol (at 30 °C, KL/G = 3125 L/L), but not on the mass transfer coefficient. As shown by model simulations, the lower the dilution rate and the larger the gas flow rate of an aerated chemostat with a microbial ethanol formation, the higher the proportion of stripped ethanol. Under practically relevant conditions, more than 30 % of the produced ethanol may be stripped off. Exhaust‐gas coolers, actually used to reduce water losses by evaporation, do not prevent but slightly affect ethanol stripping.

Construction of a novel phenol synthetic pathway in Escherichia coli through 4-hydroxybenzoate decarboxylation

“…When the target product was toxic to the host cells, several approaches were used to reduce the toxicity, such as twophase extraction (Kim et al 2014;Malinowski 2001;Wierckx et al 2005), adsorption (Nielsen et al 2010), and gas stripping (Baez et al 2011;Löser et al 2005). Solvent extraction has been used to decrease phenol toxicity to microbial cells and increase the cell mass and phenol production during fermentation.…”

Formation of ethyl acetate by Kluyveromyces marxianus on whey: studies of the ester stripping

“…This simplification was true for ethanol because of its notably large partition coefficient [29], but it is yet questionable if this is also valid for ethyl acetate assuming appropriate k EA,L a and F G /V L values. Theoretical and experimental analyses were done to check out the validity of this simplification for ethyl acetate (see below).…”

“…It is based on a model formerly derived for describing the ethanol stripping in bioreactors [29]. Ethyl acetate exhibits a higher vapor pressure than ethanol and its water solubility is restricted which will result in a changed stripping behavior of this compound.…”

“…Ethyl acetate exhibits a higher vapor pressure than ethanol and its water solubility is restricted which will result in a changed stripping behavior of this compound. The basic equations from [29] were modified for a batch process in the stirred bioreactor without taking the retention of ethyl acetate by exhaust-gas cooling into consideration since this partial process is irrelevant for ethyl acetate (details see below):…”

“…This stripping effect has already been studied for ethanol [29,30] and needs attention as well when balancing the microbial formation of ethyl acetate. The stripping makes the quantification of ester formation difficult, but it also offers some advantages in large-scale processes: the stripping is a chance for process-integrated product recovery, it prevents inhibition of the yeasts by the formed ester, and a possible microbial assimilation of ethyl acetate is avoided (which has been observed in yeast cultures after depletion of the carbon source [17,25]).…”