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