Production of Single Cell Protein from Natural Gas John Villadsen Center for Biochemical Engineering...

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

12

5

7

10

3

10

89

11

11

66

1111

44

1122

55

77

1010

33

1010

8899

1111

1111

Design of a 10 m3 loop reactor

A 10 m3 fermentor

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