Production of Single Cell Protein from Natural Gas

John Villadsen

Center for Biochemical Engineering

Technical University of Denmark

Genome of Methylococcus capsulatus

The bacteria with membrane bound Methane-monooxidase

Dividing M. capsulatus with clearly visible membranes

The key-enzyme Methane monooxygenase

Capture of CH4 by Methane monooxygenase

Further oxydation of methanol in the organism

Methane and Oxygen demand for SCP production

• From 1.25 kg methane one obtains 1 kg biomass*)

This corresponds to 1 kg biomass per 1.75 N m3 methane

or Ysx = 0.520 C-mole biomass per C-mole methane

• The O2 demand is (8 – 0.520٠4.20) / 4 = 1.45 mol O2 per C-mole CH4

or 2.53 N m3 O2 / kg biomass = 3.62 kg O2 / kg biomass.

Stoichiometry of methane conversion to biomass:

CH4 + 1.45 O2 + 0.104 NH3 → 0.52 CH1.8O0.5N0.2 + 0.48 CO2 + 1.69 H2O

*) Reference : Wendlandt, K.D, Jechorek, M, Brühl, E.

”The influence of Pressure on the growth of Methanotrophic Bacteria”

Acta Biotechnol. 13, 111-113 (1993)

and industrial experience: Dansk Bioprotein A/S 1992 - present.

Demand for heat removal

The reaction should take place at ≈ 45o C, the optimal temperature for

Methylococcus capsulatus fermentation.

Stoichiometry:

CH4 + 1.45 O2 + 0.104 NH3 → 0.52 CH1.8O0.5N0.2 + 0.48 CO2 + 1.69 H2O

Heat of reaction 460Yso kJ (C-mol carbon source)-1

or Q = 460٠ 1.45 = 667 kJ (mole CH4)-1 = 52 MJ (kg biomass)-1

This is an appreciable heat duty!

Demand for O2 and CH4 mass transfer

• The production rate depends on the rates of two separate processes

A. The reaction between bacteria and dissolved O2 + CH4

B. The rate of mass transfer from gas- to liquid phase.

The ”bio-chemical” reaction is limited by NH3 since we need to keep the

NH3 concentration below about 40 mg L-1 to avoid formation of NO2-

which is toxic to the bacteria. At 30 mg L-1 the rate is

qx = 0.21 X kg m-3 h-1

where X is the biomass concentration in kg m-3.

But qo2 = (1.45 / 0.52)(1000 / 24.6) qx = 113 qx mol m-3 h-1

= kl a (cO2* - cO2)

where cO2* and cO2 are respectively the saturation and the actual O2

concentrations in the liquid.

Factors that affect the mass transfer

The rate of mass transfer kl a (cO2* - cO2) (and kl a (cCH4* - cCH4))

depend on :

A. The mass transfer coefficient kl a

Maximum achievable kl a ≈ 1200 h-1

B. cO2*

C. cO2

cO2* is proportional with the partial pressure of O2 in the gas phase.

At 1 atm total pressure and pure O2 one obtains cO2* = 0.9 mM (45o C)

cO2 should be above about 20 μM to keep the organism healthy.

The switch from bioreaction control to mass transfer control

Assume that we wish to have X = 20 kg m-3 (qx = 4.2 kg m-3 h-1)

qO2 = 113 ٠ 4.2 = 475 mol m-3 h-1 = kl a (cO2* - 20) 10-3 mol m-3 h-1

For kl a = 1000 h-1 cO2* must be > 495 μM to obtain a gas transfer rate

that is higher than the rate of the liquid phase reaction 4.2 kg m-3 h-1.

For a total pressure of 1 atm and pure O2 (cO2* = 900 μM) about 50 %

of the oxygen is consumed before O2 limitation sets in.

With O2 extracted from air (21% O2, cO2* = 189 μM) oxygen limitation

prevails throughout the reactor.

With pure oxygen and 4 atm total pressure (cO2* = 3600 μM) O2

limitation occurs only in the last ≈ 14 % of the reactor.

Consequences of O2 limitation

The constant production rate qx = 4.2 kg m-3 h-1 can not be maintained

The production rate in the last part of the reactor is 1st order in cO2*

If we wish a high utilization of O2 (e.g. 95 %) the reactor volume may

increase beyond reasonable limits (or qx may decrease to an

unacceptably low level).

Reactor design

• A stirred tank reactor is hopeless: We wish the first order conversion of O2 in the last part of the reactor to proceed in plug-flow mode. In a CSTR cO2* would be 0.05 of inlet value.

• The large heat release dictates that external heat exchange is to be used.

• Liquid and gas is forced through a number of stationary mixer elements at a velocity of ≈ 1 m s -1. Gas is injected through an ejector. Ample allocation of head space assures gas/liquid separation. Holding time for liquid ≈ 5 h and for gas ≈ 60 s.

• Centrifuges (or drum filters) are used to separate biomass from liquid.

• Ultrafiltration gives ≈ 20 wt% biomass sludge. Spray drying gives the final powdery product

• Heat shock treatment (123 oC, 2-5 min) removes nucleic acids and gives a product suitable for direct human consumption.

500 L pilot plant loop-fermentor at DTU

6

11

4

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3

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89

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66

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1122

55

77

1010

33

1010

8899

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1111

Design of a 10 m3 loop reactor

A 10 m3 fermentor

250 m3 reactor (≈ 9000 t year-1 production) in Norway