Transcript Slide 1

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
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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
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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 NO2which 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
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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.
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The large heat release dictates that external heat exchange is to be used.
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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.
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Centrifuges (or drum filters) are used to separate biomass from liquid.
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Ultrafiltration gives ≈ 20 wt% biomass sludge. Spray drying gives the final powdery product
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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
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Design of a 10 m3 loop reactor
A 10 m3 fermentor
250 m3 reactor (≈ 9000 t year-1 production) in Norway