developing a generic approach for modelling production processes covered in brew morna isaac, martin...
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Developing a generic approach for modelling production processes
covered in BREW
Morna Isaac, Martin Patel
The aim: Analysis of the manufacturing process of products for which only basic data are available
Relevancy: Products in an early stage of development and/or with data limitations due to competitive sensitivity
Methodology: Available data will be used to develop generic elements of processes.
Focus: Energy use, as a first indication of environmental impact, and costs
The main stages of a process:
1. Feedstock treatment (activation): Depends mainly on the type of feedstock, not on the desired product
2. Biological conversion
3. Product separation
4. Waste treatment and utilities
Feedstock types and their treatment:
Feedstock Treatment Energy consumption
Sugar Extraction of juice
Starch Enzymatic saccharification
Corn wet milling 3.718-5.631 MJfinal/kg glucose1
Lignocellulosic
(woody)
Pretreatment, hydrolysis and saccarification
Corn stover: 1 kg steam/kg sugar + 0.25 MJe/kg sugar2
Plant oil Pyrolysis oil
Extraction Soybean milling: 1.8 MJfossil/kg oil
1Gerngross, 19992NREL, 2002. For comparison according to Lynd et al., 1996 for pretreatment of poplar feedstock: 0.69 kg steam/kg sugar + 0.55 MJe/kg sugar
Major energy uses, aerobic processes:
Item General data PHA fermentation (Gerngross, 1999)
Sterilization steam
Batch 0.2-0.4kg/l broth, continuous up to 75% less1
0.45 kg steam/kg PHA
Aeration 0.5-2.0 vvm, for 1.0 vvm: 5 kW/m3 1
4.57 MJe/kg PHA
Agitation 1-3 kW/m3 broth1 1.15 MJe/kg PHA
Cooling Heat produced: 15.7 MJ/kg-O2 consumed2
2.74 MJe/kg PHA
Nitrogen requirement
0.045 kg N/kg glucose3
0.109 kg NH3/kg PHA (4.03 MJ/kg) (0.033 NH3/glucose)
Biological Processing
1(Blanch & Clark, 1996) 2(Akiyama et al., 2003) 3(Lynd & Wang, in press)
Comparison of energy consumption values:
Item General data PHA fermentation (Gerngross, 1999)
Sterilization steam 50%*batch: 100-200 kg steam/m3 broth1
68 kg steam/m3 broth
Aeration 900 MJe/m3 for 50 hr1
690 MJe/m3 broth
Agitation 180-504MJe/m3 for 50hr1
170 MJe/m3 broth
Cooling 410 MJe/m3 broth
Nitrogen requirement
440 MJ/m3 broth2 600 MJ/m3 broth
1According to data from Blanch & Clark, 19962According to data from Lynd & Wang, in press
Energy uses in anaerobic fermentation:
Item Energy use
Sterilization 0.29 kg steam /kg EtOH
No aeration -
Agitation 0.75 MJe/ kg EtOH
Cooling is about 1/5 of aerobic processes (less heat is released)1
0.36 MJe / kg EtOH
Nitrogen requirement is about 1/5 of aerobic processes1
0.53 MJ / kg EtOH
1(Lynd and Wang, in press)
Calculation based on values for PHA, converted using 46 wt% yield for EtOH:
Total fermentation energy use:
Total energy use is related to broth volume and to fermentation time:
(1)
where E = energy use in absolute terms [GJ], VR = volume of reactor
vessel [m3] and = residence time [hr]
Specific energy use (per mass of product):
(2)
where
e = specific energy use per mass of product [GJ/kg] = mass flow of product [kg/hr] and A is a constant [GJ/(m3*hr)]rp = productivity [kg/m3/h], c = concentration of product in broth [kg/m3]
RVE
Ac
Ar
Am
VA
m
VmEe
PP
R
P
RP
1
/
Pm
Establishing parameter A
Process type Product/process SourceAheat
[MJ/(m3.h)]
Aelectr.
[kJ/(m3.h)]
1. Enzymatic pretreatment
1.1 Sugars from ignocellulosics NREL, 2002 1.48 114
2. Anaerobic fermentation
2.2 Sodium lactate from glucose PEP 188B (0) 527
2.3Representative best practice (yield of ethanol)
Lynd and Wang, forthcoming (based on Gerngross, 1999)
3.70 2,124
2.4Ethanol from sugars originating from lignocellulosics
NREL, 2002 0.00 225
2.6Succinic acid (as sodium succinate) from glucose/corn steep liquor/CO2
SRI PEP 236 2.01 <1,791
3. Aerobic 3.1 a PHA from starch Gerngross, 1999 3.94 26,455
3.2 a PHA from glucose (Case 9) Akiyama et al., 2003 7.55 52,500
3.2 bPHA from glucose (Case 10, based on Gerngross)
Akiyama et al., 2003 4.60 52,785
3.3 a PHA from soybean oil (Case 1) Akiyama et al., 2003 2.36 42,053
3.3 b (Case 2) Akiyama et al., 2003 1.89 42,028
3.3 c (Case 3) Akiyama et al., 2003 4.38 41,974
3.3 d (Case 4) Akiyama et al., 2003 1.74 41,853
3.3 e (Case 5) Akiyama et al., 2003 4.98 43,980
3.3 f (Case 6) Akiyama et al., 2003 2.13 46,767
3.3 g (Case 7) Akiyama et al., 2003 4.94 46,824
3.3 h (Case 8) Akiyama et al., 2003 3.95 46,716
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Yield and productivities1 :
Maximum product yield is 90% of stoichiometric yield, with the remaining 10% being needed for growth and maintenance of the organisms
Maximum productivity for anaerobic fermentation: 50-100 g/l/h
Maximum productivity for aerobic process: 20 g/l/h
Productivities reported for ethanol fermentation2:Simple, conventional batch process, usually: 1.8-2.5 g/l/hSimple CSTR up to: 6 g/l/hContinuous with cell recycle has achieved: 30-51 g/l/hFlocculating cells (internal recycle), continuous up to: 50 g/l/hFermentation vessel coupled to membrane filtration up to: 100 g/l/h
1These values were arrived at in a discussion with T. Nisbet and P. Nossin.2Ullman’s Encyclopedia
Yields of bioprocess and separation:
Stage Current yields
Future yields
Bio- conversion*
50% 90%
Separation 90-95% 95-98%
Total 45-48% 86-88%
*Relative to stoichiometric yield
These values were arrived at in a discussion with T. Nisbet and P. Nossin.
Separations:1. Separation of insolubles:
– Filtration – Centrifugation– Decantation– Sedimentation (Depending on the type of organism)
2. Primary isolation of product (separation of water): – Extraction* – Adsorption*
– Precipitation– Membrane filtration*
– Distillation*
Separations (2):
3. Purification: – Activated carbon treatment– Fractional precipitation – Fractional extraction– Chromatography
4. Final isolation of product: – Drying– Crystallization*
– Evaporation*
For intracellular products additional separation steps are needed:
After removal of insolubles, the product stream is the insoluble fraction. This undergoes:
1. Cell disruption
2. Removal of insoluble cell debris
Separation
Separation is a complex, multistep process, the details of which depend on the particular physico-chemical properties of the product
Separation is easier as• Boiling point of the product decreases• Aqueous solubility of the product decreases
Parameters giving an indication of the amount of energy needed for separation:• Concentration of product in broth• Heat of evaporation of product
Waste treatment
Microbial biomass and unconverted feedstock (primarily lignin) can be treated by:
Incineration with power generation Anaerobic digestion with power generation
from the produced gases and from incineration of solids
Acc. to literature: For lignocellulosic feedstocks this can supply more energy than the consumption in the plant.
Cost-determining parameters: Feedstock costs: About 2/3 of product value for mature
commodity products Costs for inoculum Reactor costs: depend on the reactor size and lifetime Downstream processing costs (Utilities)
In standard chemical plants the investment costs are usually 25% for reaction, 75% for product recovery. In biotechnology the ratio is currently closer to 50/50 (Nossin/Nisbet).
For competitive bulk chemicals the cost price needs to be at most $900/t