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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,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 CH,4,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 m,3,methane,or Y,sx,= 0.520 C-mole biomass per C-mole methane,The O,2,demand is (8 0.520,4.20) / 4 = 1.45 mol O,2,per C-mole CH,4,or 2.53 N m,3,O,2,/ kg biomass = 3.62 kg O,2,/ kg biomass.,Stoichiometry,of,methane,conversion to biomass:,CH,4,+ 1.45 O,2,+ 0.104 NH,3, 0.52 CH,1.8,O,0.5,N,0.2,+ 0.48 CO,2,+ 1.69 H,2,O,*),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 45,o,C, the optimal temperature for,Methylococcus,capsulatus,fermentation.,Stoichiometry:,CH,4,+ 1.45 O,2,+ 0.104 NH,3, 0.52 CH,1.8,O,0.5,N,0.2,+ 0.48 CO,2,+ 1.69 H,2,O,Heat of,reaction 460Y,so,kJ (C-mol carbon source),-1,or Q = 460,1.45 = 667 kJ (mole CH,4,),-1,= 52 MJ (kg biomass),-1,This is an appreciable heat duty!,Demand for O,2,and CH,4,mass transfer,The,production rate depends,on,the rates of,two separate processes,A. The,reaction,between,bacteria and dissolved O,2,+ CH,4,B. The rate of,mass transfer from gas- to liquid,phase.,The ”bio-chemical” reaction is limited by NH,3,since,we,need to keep,the,NH,3,concentration,below,about 40 mg L,-1,to avoid formation of NO,2,-,which is toxic to the,bacteria. At 30 mg L,-1,the rate is,q,x,= 0.21 X kg m,-3,h,-1,where X is the,biomass,concentration in kg m,-3.,But q,o2,= (1.45 / 0.52)(1000 / 24.6) q,x,= 113 q,x,mol m,-3,h,-1,= k,l,a (c,O2,* - c,O2,),where c,O2,* and c,O2,are respectively the saturation and the actual O,2,concentrations,in,the,liquid,.,Factors that,affect,the,mass transfer,The rate of,mass transfer k,l,a (c,O2,* - c,O2,) (and,k,l,a (c,CH4,* - c,CH4,),depend,on :,The,mass transfer coefficient,k,l,a,Maximum,achievable,k,l,a 1200 h,-1,c,O2,*,c,O2,c,O2,* is proportional with the partial pressure of O,2,in the gas phase.,At 1,atm,total pressure and pure O,2,one obtains c,O2,* = 0.9,mM,(45,o,C),c,O2,should be above about 20,M 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,(q,x,= 4.2 kg m,-3,h,-1,),q,O2,= 113,4.2 = 475 mol m,-3,h,-1,= k,l,a (c,O2,* - 20),10,-3,mol m,-3,h,-1,For k,l,a = 1000 h,-1,c,O2,* must be > 495,M 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 O,2,(c,O2,* = 900,M) about 50 %,of,the oxygen is consumed,before O,2,limitation sets in.,With O,2,extracted from air (21% O,2, c,O2,* = 189,M) oxygen limitation,prevails,throughout,the,reactor.,With pure oxygen and 4 atm total pressure (c,O2,* = 3600,M) O,2,limitation,occurs,only in the last 14 %,of,the,reactor,.,Consequences,of O,2,limitation,The,constant,production rate q,x,= 4.2 kg m,-3,h,-1,can not be,maintained,The,production rate in the last part of,the,reactor is 1,st,order in c,O2,*,If,we,wish a high,utilization,of O,2,(,e.g.,95 %) the,reactor,volume,may,increase,beyond,reasonable limits (or,q,x,may,decrease to an,unacceptably,low,level).,Reactor design,A stirred tank reactor is hopeless:,We,wish,the,first,order,conversion,of O,2,in,the last part of,the,reactor,to proceed in plug-flow mode. In a CSTR c,O2,* 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,o,C, 2-5 min),removes,nucleic,acids,and gives a,product,suitable,for,direct,human,consumption,.,500 L pilot plant loop-fermentor at DTU,Design of a 10 m,3,loop reactor,A 10 m,3,fermentor,