Table 2 Summary of relevant parameters of bioreactor operation and process performance for H₂/CO₂ and CO fermentations

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 broth^a,b

 Hydrogen

\(C_{{{\text{H}}_{ 2} }}\)

(\(C_{{{\text{H}}_{ 2} }}^{*}\))

mol m⁻³

0.00 (0.00)

0.025 {0.033; 0.018}

(1.15 {1.63; 0.78})

 Carbon dioxide

\(C_{{{\text{CO}}_{ 2} }}\)

(\(C_{{{\text{CO}}_{2} }}^{*}\))

mol m⁻³

0.32 {0.00; 4.22}

12.46 {17.11; 8.94}

(13.09 {18.51; 8.86})

 Carbon monoxide

\(C_{\text{CO}}\)

(\(C_{\text{CO}}^{ *}\))

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

\(C_{et}\)

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 thermodynamics^a

 Catabolic energy production

\(\Delta G_{\text{cat}}^{{0^{\prime}}}\)

kJ mol ⁻¹_CS

− 29.2 {− 48.2; − 24.0}

− 19.9 {− 23.0 − 16.45}

 Biomass yield

\(Y_{x/CS}\)

Cmol_x mol ⁻¹_CS

0.041

0.020

Biomass specific consumption/production rates (logarithmic mean)

 Hydrogen

\(q_{{{\text{H}}_{ 2} }}\)

mol Cmol ⁻¹_x h⁻¹

0.00

− 1.67

 Carbon dioxide

\(q_{{{\text{CO}}_{2} }}\)

mol Cmol ⁻¹_x h⁻¹

1.00

− 0.56

 Carbon monoxide

\(q_{\text{CO}}\)

mol Cmol ⁻¹_x h⁻¹

− 1.52

0.00

 Ethanol

\(q_{\text{et}}\)

mol Cmol ⁻¹_x h⁻¹

0.23

0.28

 Water

\(q_{w}\)

mol Cmol ⁻¹_x h⁻¹

− 0.73

0.84

 Cells

\(\mu\)

h⁻¹

0.06

0.01

 Non-dimensional electron donor uptake rate

\(q^{\prime}_{D}\)

–

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)

\(v_{sG}^{c}\)

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

60.4^c

54.3^d

61.2^c

54.3^d

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