Table 2.

Summary of relevant parameters of bioreactor operation and process performance for H₂/CO₂ and CO fermentations

Variable	Symbol	Unit	CO fermentation	H2/CO2 fermentation
Performance indicators
Ethanol volumetric productivity	$R_{\text{et}}$	g L−1 h−1	4.25	5.1
Gas utilization	$U_S$	%	17.1	22.9
Gas outflow composition
Hydrogen	$y_{{\text{H}}_2}$	mol mol−1	0.00	0.71
Carbon dioxide	$y_{{\text{CO}}_2}$	mol mol−1	0.11	0.24
Carbon monoxide	$y_{\text{CO}}$	mol mol−1	0.84	0.00
Ethanol	$y_{\text{et}}$	mol mol−1	0.01	0.01
Water	$y_w$	mol mol−1	0.04	0.04
Concentrations in the fermentation brotha,b
Hydrogen	$C_{{\text{H}}_2}$ ($C_{{\text{H}}_2}^{\ast }$)	mol m−3	0.00 (0.00)	0.025 {0.033; 0.018} (1.15 {1.63; 0.78})
Carbon dioxide	$C_{{\text{CO}}_2}$ ($C_{{\text{CO}}_2}^{\ast }$)	mol m−3	0.32 {0.00; 4.22}	12.46 {17.11; 8.94} (13.09 {18.51; 8.86})
Carbon monoxide	$C_{\text{CO}}$ ($C_{\text{CO}}^{\ast }$)	mol m−3	2.7 × 10−3 {3.6 × 10−3; 2.0 × 10−3} (1.62 {2.45; 1.01})	0.000 (0.000)
Ethanol	$C_{\mathit{et}}$	mol L−1 (g L−1)	0.96 (44.3)	0.98 (45.0)
Biomass	$C_x$	Cmol m−3 (g L−1)	395 (10.0)	399 (10.1)
Parameters estimated with thermodynamicsa
Catabolic energy production	$\mathrm{\Delta }{G}_{\text{cat}}^{0^{\prime }}$	kJ mol−1CS	− 29.2 {− 48.2; − 24.0}	− 19.9 {− 23.0 − 16.45}
Biomass yield	$Y_{x/CS}$	Cmolx mol−1CS	0.041	0.020
Biomass specific consumption/production rates (logarithmic mean)
Hydrogen	$q_{{\text{H}}_2}$	mol Cmol−1x h−1	0.00	− 1.67
Carbon dioxide	$q_{{\text{CO}}_2}$	mol Cmol−1x h−1	1.00	− 0.56
Carbon monoxide	$q_{\text{CO}}$	mol Cmol−1x h−1	− 1.52	0.00
Ethanol	$q_{\text{et}}$	mol Cmol−1x h−1	0.23	0.28
Water	$q_w$	mol Cmol−1x h−1	− 0.73	0.84
Cells	$\mu$	h−1	0.06	0.01
Non-dimensional electron donor uptake rate	$q_D^{\prime }$	–	0.35	0.38
Streams entering and leaving the bioreactor
Gas flow rate at the top	$F_{G,t}$ (${\dot{V}}_{G,t}$)	mol s−1 (m3 s−1)	462 (7.8)	418 (7.1)
Gas flow rate at the bottom	$F_{G,b}$ (${\dot{V}}_{G,b}$)	mol s−1 (m3 s−1)	479 (4.0)	528 (4.4)
Liquid outflow rate	${\dot{V}}_{L,o}$	m3 h−1	30.0	39.3
Fresh syngas	$F_S$	mol s−1	80.0	118
Parameters regarding gas and liquid flows and mixing (logarithmic mean)
Gas flow rate	$F_G$(${\dot{V}}_G$)	mol s−1 (m3 s−1)	471 (5.6)	471 (5.6)
Superficial gas velocity (pressure-corrected)	$v_{\mathit{sG}}^c$	m s−1	0.14	0.14
Liquid flow rate	${\dot{V}}_L$	m3 s−1	26.9	26.9
Mixing time	$t_m$	s	60.4c 54.3d	61.2c 54.3d
Mass transfer coefficients (logarithmic mean)
Hydrogen	$k_L{a}_{{\text{H}}_2}$	s−1	0.000	0.164
Carbon dioxide	$k_L{a}_{{\text{CO}}_2}$	s−1	0.000	0.098
Carbon monoxide	$k_L{a}_{\text{CO}}$	s−1	0.104	0.000

^aThe average value is shown first, followed by the values at the top and the bottom of the liquid column between curly brackets

^bThe values between round brackets represent the saturation concentrations of CO, H₂ and CO₂

^cSimulated using the 9 vertically stacked compartments model

^dCalculated with Eq. (21)