Lactic Acid Fermentation Endothermic but should be Exothermic. Bug or limit of biosteam?

[iilard]

Hi, I will post this again because the other entry remained inactive sofar. I know you only have limited time for BioSTEAM and this github, but I would really appreciate your support on this issue, as it's the main unit for the process and of integral importance for my thesis, in which I evaluate BioSTEAM as a tool for LCI creation.

My fermentor is endothermic, and I have tried various fixes but nothing works. The only "success" I had, was arbitrarily adjusting the Hf value of Calcium Lactate, from literature backed value -1686100 J/mol to ex. -1930000 J/mol. This "knob" makes the fermentor unit exothermic. I was not able to spot any other setting to make it work.

**

• Can this be fixed or is it a bug?
• Is the issue that salts can't be properly simulated in Thermosteam? (dissociated species and such)
• Is there any way to "workaround"?**

Below my code (separate py files):

_settings.py:

import thermosteam as tmo
import biosteam as bst
from _chemicals import chemicals 
from thermosteam import settings

#Activate chemicals list & register them in bio/thermosteam
tmo.settings.set_thermo(chemicals) 
bst.settings.set_thermo(chemicals)

_chemicals.py:

import thermosteam as tmo
import biosteam as bst
from thermosteam import Chemicals, Chemical

def create_chemicals():
    chemicals = Chemicals([])
    def add_chemical(ID, ref=None, **data):
        chemical = Chemical(ID, **data) if ref is None else ref.copy(ID, **data)
        chemicals.append(chemical)
        return chemical
    
    Water = add_chemical('H2O')
    Glucose = add_chemical('Glucose', phase='l', rho=1560, Cp=1.213)
    CaCO3 = add_chemical('CaCO3', phase='s', rho=2710, Cp=0.834, default=True)
    CalciumLactate = add_chemical('CalciumLactate', phase='l', Hf=-1930000, rho=1494) 
    lla = add_chemical('L-LacticAcid', phase='l', Hf=-686300, Cp=2.109, rho=1206)
    SulfuricAcid = add_chemical('H2SO4', phase='l', rho=1840, Cp=1.38)
    CarbonDioxide = add_chemical('CO2', phase='g')
    Gypsum = add_chemical('CaSO4_2H2O', phase='s', rho=2320, MW=172.18, Hf=-2023000, Cp=1.081, CAS='10101-41-4', mu = 0.00095, search_db=False)
    Biomass_l = add_chemical('Biomass_l', phase='l', formula='CH1.8O0.5N0.2', MW=24.626, rho=1050, Hf=-106800, Cp=1.25, search_db=False)
    Biomass_s = add_chemical('Biomass_s', phase='s', formula='CH1.8O0.5N0.2', MW=24.626, rho=1093, Hf=-130412, Cp=1.25, search_db=False) # Hf value taken from bioindustrial park, no literature given
    nutrient_N = add_chemical('NH3', phase='l')
    Octanol = add_chemical('Octanol', phase='l')
    Trioctylamine = add_chemical('Trioctylamine', phase='l')
    SodiumHydroxide = add_chemical('NaOH', phase='l') 
    SodiumSulfate = add_chemical('Na2SO4', phase='l', rho=1200, Cp=3.5, mu = 0.001, search_db=True)

    chemicals.compile()
    return chemicals

chemicals = create_chemicals()

# show chemical properties
for chemical in chemicals:
    chemical.show()

_reactions.py:

import thermosteam as tmo

# Fermentor Unit Parallel Reactions
def fermentation_reaction():
    fermentation = tmo.Reaction(reaction='Glucose -> 1.84 L-LacticAcid + 0.512 Biomass_s', reactant='Glucose', X=1, basis='mol', correct_atomic_balance=False) 
    return fermentation
# NOTE: Homo-lactic fermentation should theoretically yield 2 moles of lactic acid per mole of consumed glucose 
# with a theoretical yield of 1 g of product per g of substrate, but the experimental yields are usually  
# lower (0.74–0.99 g/g) because a portion of the carbon source is used for biomass production (0.07–0.22 g/g) [AIMS Microbiology, 4(4): 665–684.]
# --> This was used to calculate adjusted coefficients that already implement the empirical yields!


# Separate Neutralization for correct mass balance (has to happen when lla alreaedy formed)
def neutralization_reaction():
    neutralization = tmo.Reaction(reaction='2 L-LacticAcid + CaCO3 -> CalciumLactate + CO2 + H2O', reactant='L-LacticAcid', X=1, basis='mol')
    return neutralization

# Acidification Tank Unit
def acidification_reaction():
    acidification = tmo.Reaction(reaction='CalciumLactate + H2SO4 -> 2 L-LacticAcid + CaSO4_2H2O', reactant='CalciumLactate', X=1, basis='mol')
    return acidification

# Caustic Wash
def caustic_wash_reaction():
    caustic_wash = tmo.Reaction(reaction='2 NaOH + H2SO4 -> Na2SO4 + 2 H2O', reactant='H2SO4', X=1, basis='mol')
    return caustic_wash

_units.py:

import biosteam as bst
import thermosteam as tmo
from biosteam import CSTR
import numpy as np
from _reactions import fermentation_reaction, neutralization_reaction, acidification_reaction

# -----------------------------------CUSTOM CLASSES----------------------------------------

# Custom Fermentor based on CSTR for cont. ferm.
class Fermentor(bst.CSTR):
    _N_ins = 1
    _N_outs = 2
    def __init__(self, ID='Fermentor', ins=None, outs=(), thermo=None, *, T=323.15, **kwargs):
        if outs == ():
            outs = [None, None]  # liquid, gas
        super().__init__(ID, ins, outs, thermo=thermo, **kwargs)
        self.T=T
        self.fermentation_rxn = fermentation_reaction()
        self.neutralization_rxn = neutralization_reaction()

    def _run(self):
        feed = self.ins[0]
        liquid_effluent, gas_vent = self.outs
        liquid_effluent.copy_like(feed)
        # Apply reactions using the class-defined reactions, first parallel fermentation, then neutralization
        self.fermentation_rxn(liquid_effluent.mol)
        self.neutralization_rxn(liquid_effluent.mol)
        # Handle gas separation - remove CO2 from liquid and send to gas vent
        gas_vent.copy_flow(liquid_effluent, 'CO2', remove=True)
        liquid_effluent.phase = 'l'
        gas_vent.phase = 'g'
        liquid_effluent.T = gas_vent.T = self.T


# Custom Acidification Reactor
class AcidificationReactor(bst.CSTR):
    _N_ins = 2
    _N_outs = 1

    def __init__(self, ID='', ins=None, outs=(), P=101325, T=298.15, tau=1.0, V=1.0):
        bst.CSTR.__init__(self, ID, ins, outs, tau=tau, T=T, P=P)
        self.acidification_rxn = acidification_reaction()
        
    def _run(self):
        feed, acid = self.ins
        effluent = self.outs[0]

        # Acid dosing
        n_CL = feed.imol['CalciumLactate']
        acid.imol['H2SO4'] = n_CL * 1.1
        acid.imass['H2O'] = acid.imass['H2SO4'] / 0.93 * 0.07

        # Mix and pre‑instantiate phases
        effluent.mix_from([feed, acid])
        effluent.phases = ('l', 's')
        effluent.T = self.T
        effluent.P = self.P

        self.acidification_rxn(effluent)

        # Force gypsum into solid phase (safety net)
        n_total = effluent.imol['CaSO4_2H2O']
        if n_total:
            effluent.imol['l', 'CaSO4_2H2O'] = 0
            effluent.imol['s', 'CaSO4_2H2O'] = n_total

        # Keep outlet as slurry/liquid
        effluent.phase = 'l'

_system.py:

import biosteam as bst
from _units import Fermentor, AcidificationReactor  # custom classes
from _reactions import fermentation_reaction, acidification_reaction, caustic_wash_reaction  # reaction functions
from _settings import chemicals  # compiled thermo package

# -------------------- FEED STREAMS FOR ALL UNITS --------------------
glucose_feed = bst.Stream('glucose_feed', Glucose=80, Water=920, units='kg/hr') # 80 g/L
CaCO3_feed = bst.Stream('CaCO3_feed', CaCO3=44.5, Water=82.6, units='kg/hr') # Based on mol calc. from reaction; 1mol CaCO3 per 1mol Glucose
Nutrient_feed = bst.Stream('Nutrient_feed', NH3 = 1.34, H2O = 3.13, units='kg/hr')
H2SO4_feed = bst.Stream('H2SO4_feed', units='kg/hr')
Organic_Solvent = bst.Stream('Organic_Solvent', Octanol=172.5, Trioctylamine=172.5, units='kg/hr') # 1.5 : 1 ratio -> org : aq phase
Extr_Water_1 = bst.Stream('Extraction_water_1', H2O=100, units='kg/hr') # NOTE How much?
Extr_Water_2 = bst.Stream('Extraction_water_2', H2O=100, units='kg/hr') # NOTE How much?
Caustic_Sol = bst.Stream('Caustic_Solution', NaOH=4, H2O=50, units='kg/hr')
OrgSolvent_Makeup = bst.Stream('Solvent_Makeup', Octanol=1.72, Trioctylamine=1.73, units='kg/hr') # Add the 1% of solvent lost in caustic wash
Recycled_Solvent = bst.Stream('Recycled_Solvent')

# -------------------- SUBSYSTEM 1: CONVERSION --------------------
# Unit and stream definitions
with bst.System('conversion_sys') as conversion_sys:
    feedstock_mixer = bst.Mixer('feedstock_mixer', ins=[glucose_feed, CaCO3_feed, Nutrient_feed], outs=('crude_feed'))
    feed_pump = bst.Pump('feed_pump', feedstock_mixer.outs[0], outs=('crude_feed'), P=110000)  # NOTE: Provisional Pump; missing correct settings
    preheater = bst.HXutility('preheater', feed_pump.outs[0], outs=('heated_feed'), T=323.15) # NOTE: Placeholder 50°C
    fermentor = Fermentor('fermentor', preheater.outs[0], outs=['fermented_broth', 'CO2'], P=110000, T=323.15, tau=72, V_wf=0.8, kW_per_m3=0.05, dT_hx_loop=35)    # NOTE find source for possible kW_per_m3 value for anaerobic ferm.
    ferm_cooler = bst.HXutility('fermentor_cooler', fermentor.outs[0], outs=('cooled_broth'), T=298.15)


# -------------------- SUBSYSTEM 2.1: SEPARATION --------------------
# Biomass Removal --> NOTE: Fine tuning of splits, realism vs. idealism
# Acidification & Gypsum Filtration

with bst.System('separation_sys') as separation_sys:
    gravity_decanter = bst.units.SolidsSeparator('GravityDecanter',
                                            ins=ferm_cooler.outs[0], outs=('decanted_cake', 'decanted_broth'),
                                            split={'Biomass_s': 1, 'CaCO3': 0.85, 'CalciumLactate': 0.05}, # NOTE: FINE TUNING, HOW MUCH PRODUCT LOST?
                                            moisture_content=0.35)
    bm_Filter = bst.units.SolidsSeparator('BM_Filter',
                                          ins=gravity_decanter.outs[0], outs=('BM_cake', 'BM_filtrate'),
                                          split={'Biomass_s': 1, 'CaCO3': 0.75},
                                          moisture_content=0.08)
    filtrate_mixer = bst.Mixer('filtrate_mixer', ins=(gravity_decanter.outs[1], bm_Filter.outs[1]), outs='clarified_broth')
    acidification_reactor = AcidificationReactor('AcidificationReactor', ins=[filtrate_mixer.outs[0], 'H2SO4_feed'], outs=('acidified_slurry'))
    gypsum_filter = bst.units.SolidsSeparator('GypsumFilter', ins=acidification_reactor.outs[0], outs=('gypsum_cake', 'lactic_acid_solution'), 
                                          split={'CaSO4_2H2O': 0.99}, # Fine particles stay, removed by extraction
                                          moisture_content=0.2)


# -------------------- SUBSYSTEM 2.2A: PURIFICATION --------------------
# MEE 1 to conc. extraction feed
# Extraction with organic amine solvent
# 2 Stage Back-Extraction into fresh water
# MEE 2 to 88wt% lla

with bst.System('purification_sys') as purification_sys:
    Evaporator_1 = bst.units.MultiEffectEvaporator(ID='MEE_pre_extr', ins=[gypsum_filter.outs[1]], outs=('conc_la_sol', 'condensate'), 
                                                   V=0.85, V_definition='Overall', # NOTE adjust to 30wt% lactic acid conc. before extraction for eff.
                                                   P=(101325, 90000, 70000)) 


    ExtrMixer1 = bst.LiquidsMixingTank(ID='Extraction_M1', ins=[Evaporator_1.outs[0], Organic_Solvent], outs=['mixed_phase_1'])
    PreExtr_Cooler = bst.HXutility('extraction_cooler', ins=ExtrMixer1.outs[0], T=313.15)  # 40°C extr feed for optimal eff.
    ExtrSettler1 = bst.LiquidsSplitSettler(ID='Extraction_S1', ins=[PreExtr_Cooler.outs[0]], outs=['organic_phase_1', 'aq_phase_1'],
                                    split={'L-LacticAcid': 0.99, # Simulate two stage extraction in one
                                            'H2SO4': 0.98,
                                            'Octanol': 1,
                                            'Trioctylamine': 1,
                                            'H2O': 0.01, # some water carry-over
                                            'CaCO3': 0,
                                            'CaSO4_2H2O': 0,
                                            'NH3' : 0}) # NOTE "realistic" assumptions of split

    BackExtrMixer1 = bst.LiquidsMixingTank('BackExtr_Mixer1', ins=[ExtrSettler1.outs[0], Extr_Water_1])
    PreBackExtr_Heater1 = bst.HXutility('backextr_heater1', ins=BackExtrMixer1.outs[0], T=348.15) # ca. 20-30°C higher T for Back Extraction
    BackExtrSettler1 = bst.LiquidsSplitSettler('BackExtr_Settler1',
                                               ins=PreBackExtr_Heater1.outs[0],
                                               outs=('back_extr_aq1', 'back_extr_org1'),
                                               split={'L-LacticAcid': 0.85,
                                                   'H2SO4': 0,
                                                   'H2O': 0.99,
                                                   'Octanol': 0,
                                                   'Trioctylamine': 0})

    BackExtrMixer2 = bst.LiquidsMixingTank('BackExtr_Mixer2', ins=[BackExtrSettler1.outs[1], Extr_Water_2])
    PreBackExtr_Heater2 = bst.HXutility('backextr_heater2', ins=BackExtrMixer2.outs[0], T=348.15) # ca. 20-30°C higher T for Back Extraction
    BackExtrSettler2 = bst.LiquidsSplitSettler('BackExtr_Settler2',
                                               ins=PreBackExtr_Heater2.outs[0],
                                               outs=('back_extr_aq2', 'back_extr_org2'),
                                               split={'L-LacticAcid': 0.8, # 80% of remaining LLA (total 97%)
                                                   'H2SO4': 0,
                                                   'H2O': 0.99,
                                                   'Octanol': 0,
                                                   'Trioctylamine': 0})
    AqPhaseMixer = bst.MixTank('PostExtraction_AqPhase', ins=[BackExtrSettler1.outs[0], BackExtrSettler2.outs[0]], outs='pure_lactic_acid_solution')

# -------------------- SUBSYSTEM 2.2B: SOLVENT RECOVERY & RECYCLING --------------------
#Organic solvent washed (neutralized) with aq. NaOH solution

    Caustic_Mixer = bst.LiquidsMixingTank('Caustic_Mixer', ins=[BackExtrSettler2.outs[1], Caustic_Sol], outs=('caustic_mix'))
    CausticWash_Reactor = bst.SinglePhaseReactor('Caustic_Wash', ins=Caustic_Mixer.outs[0], outs=('neutralized_solvent'),
                                                 T=323.15, P=101325, V_wf=0.8,
                                                 tau=0.5, 
                                                 reaction=caustic_wash_reaction())
    Caustic_Settler = bst.LiquidsSplitSettler('Caustic_Settler', 
                                              ins=CausticWash_Reactor.outs[0], 
                                              outs=('caustic_w_waste_brine', 'recovered_solvent'),
                                              split={
                                                 'Octanol': 0.01,
                                                 'Trioctylamine': 0.01,
                                                 'NaOH': 1,
                                                 'Na2SO4': 1,
                                                 'H2O': 1,
                                                 'L-LacticAcid': 1})
    OrgSolvent_Recycler = bst.Mixer('Solvent_Mixer', ins=[Caustic_Settler.outs[1], OrgSolvent_Makeup], outs=Organic_Solvent)

# MEE 2
    Evaporator_2 = bst.MultiEffectEvaporator(ID='MEE_post_extr', ins=[AqPhaseMixer.outs[0]], outs=('88wt_product', 'condensate'),
    V=0.95, V_definition='Overall', P=(101325, 80000, 60000, 30000)) # NOTE More details required, what pressure? PEP not detailed enough... how many effects? only one flash drum?
    Evaporator_2.split={
        'H2O': 1,
        'LacticAcid': 0,
        'H2SO4': 0,
        'Glucose': 0}

# -------------------- MAIN SYSTEM --------------------
# Combine subsystems into the main system
lactic_acid_system = bst.System('lactic_acid_system',
                                path=[conversion_sys, separation_sys, purification_sys],
                                recycle=[Organic_Solvent])


# SIMULATION 
if __name__ == '__main__':
    lactic_acid_system.simulate()
    print("Simulation completed!")

# ------KPI -------
glucose_feed.show()
CaCO3_feed.show()

feedstock_mixer.outs[0].show()
fermentor.ins[0].show()
fermentor.ins[0].T
fermentor.outs[0].show()  # Liquid
fermentor.outs[1].show()  # Gas

print("--Fermentor PU & HU--")
fermentor.power_utility.show()
fermentor.heat_utilities[0].show()

The output I get for the fermentor duty with different Calcium Lactate Hf values:
Hf = -1930000 --> Chosen arbitrarily to illustrate.
PowerUtility:
consumption: 34.6 kW
production: 0 kW
power: 34.6 kW
cost: 2.71 USD/hr
HeatUtility: chilled_water
duty:-4.57e+04 kJ/hr
flow: 26.7 kmol/hr
cost: 0.202 USD/hr

Hf = -1686100
PowerUtility:
consumption: 46.2 kW
production: 0 kW
power: 46.2 kW
cost: 3.61 USD/hr
HeatUtility: low_pressure_steam
duty: 5.68e+04 kJ/hr
flow: 1.61 kmol/hr
cost: 0.384 USD/hr

As you can see the shift in Hf for CalciumLactate has a big influence.

EDIT: Also note that the duty numbers are quite small, although that could be because of streams being rather small.

EDIT 2: Biomass Hf value is also a knob to make the fermentation more exothermic. I adjusted the Hf value of Biomass_s to match your bioindustrial park lactic acid simulation (from -106800 to -130412 J/mol). That makes the fermentation reaction more exothermic, but not enough to make the fermentor exothermic. I also checked the two reactions above separately for their reaction enthalpies (using rxn.dH) and got the follwing:
Fermentation --> H = -78395.9
Neutralization --> H = 107400.5 (per mole of LA)

Additional thought:
Could this be a limitation on reaction realism in biosteam? As I've read that electrolyte speciation does affect some thermo properties and can distort enthalpies (even to this degree?).

Id really appreciate any support here...

[yoelcortes]

@iilard, happy to help. I'll have a look soon on Friday (Jan 9)

[iilard]

@yoelcortes Much appreciated, looking forward!

I also went ahead and found some interesting literature
QUOTE
“The observed enthalpy change during glucose fermentation was -30.8 cal per mmole of glucose fermented. When corrected for heat of neutralization of lactic acid, the average obtained from 12 experiments was -29.1 cal per mmole of glucose.”
“The heat of neutralization of lactic acid by phosphate buffer at 37°C was found to bbe -0.85 cal per mmole.”
END QUOTE

This translates to ca. -121 kJ/mol glucose (fermentation) and ca. -3.5 kJ/mol LA (neutralization, although with phosphate buffer).
Comparing this to my values I feel like something can't be right, as the order of magnitude is very different.

Thought this could maybe help with finding the issue, having a realistic range in mind.

[yoelcortes]

@iilard, I thought I would have time today, but some urgent things came up. So sorry!! My next availability is Friday, Jan 16. I have you marked on my calendar, rest assured I will address your questions/issues.

Thanks,

[iilard]

@yoelcortes Thanks for letting me know, appreciate the communication!

[yoelcortes]

@iilard,

I believe main issue is the heat of cell mass formation. The enthalpy of formation for cell mass which you are using makes it super endothermic. As a conservative assumption, it is common to assume the heat of cell mass formation is negligible; this is equivalent to assuming cell mass has the same enthalpy as glucose if your reaction is "glucose -> cellmass".

Did you know you can also directly set the duty of the reactor? This is a common to do for aerobic bioreactors (which result in greater cell mass). You can do this by overriding the _set_duty method in CSTR.

Regarding the pump and viscosity issue, CalciumLactate was missing the viscosity. You can set it with CalciumLactate.mu.add_method(f=lambda *args, **kwargs: 0.00091272) or simply let biosteam default values to that of water with bst.Chemical(..., default=True).

Here is the working code with minor changes here and there based on my personal preference (e.g., I set the first fermentor stream as the gas, second liquid).

import biosteam as bst
    
# %% Thermo package
Glucose = bst.Chemical('Glucose', phase='l', rho=1560, Cp=1.213)
bst.settings.set_thermo([
    'H2O',
    Glucose,
    bst.Chemical('CaCO3', phase='s', rho=2710, Cp=0.834, default=True),
    bst.Chemical('CalciumLactate', phase='l', Hf=-1930000, rho=1494, default=True),
    bst.Chemical('L-LacticAcid', phase='l', Hf=-686300, Cp=2.109, rho=1206),
    bst.Chemical('H2SO4', phase='l', rho=1840, Cp=1.38),
    bst.Chemical('CaSO4_2H2O', phase='s', rho=2320, MW=172.18, Hf=-2023000, Cp=1.081, CAS='10101-41-4', mu = 0.00095, search_db=False),
    bst.Chemical('Biomass_l', phase='l', formula='CH1.8O0.5N0.2', MW=24.626, rho=1050, Hf=-106800, Cp=1.25, search_db=False),
    Glucose.copy(ID='Biomass_s'), 
    bst.Chemical('NH3', phase='l'),
    bst.Chemical('Octanol', phase='l'),
    bst.Chemical('Trioctylamine', phase='l'),
    bst.Chemical('NaOH', phase='l') ,
    bst.Chemical('Na2SO4', phase='l', rho=1200, Cp=3.5, mu = 0.001, search_db=True),
    bst.Chemical('CO2', phase='g'),
])

# %% Reactions

# Fermentor reactions
production = bst.Reaction(
    'Glucose -> 2 L-LacticAcid', 
    reactant='Glucose', X=0.92, basis='mol', 
    correct_atomic_balance=False
) 
cell_growth = bst.Reaction(
    'Glucose -> Biomass_s', 
    reactant='Glucose', X=1, basis='mol', 
    correct_mass_balance=True,
) 
fermentation = bst.SeriesReaction([
    production, cell_growth
])
neutralization = bst.Reaction(
    '2 L-LacticAcid + CaCO3 -> CalciumLactate + CO2 + H2O', 
    reactant='L-LacticAcid', X=1, basis='mol'
)
acidification = bst.Reaction(
    'CalciumLactate + H2SO4 -> 2 L-LacticAcid + CaSO4_2H2O', 
    reactant='CalciumLactate', X=1, basis='mol'
)
caustic_wash = bst.Reaction(
    '2 NaOH + H2SO4 -> Na2SO4 + 2 H2O', 
    reactant='H2SO4', X=1, basis='mol'
)

# %% Unit operations

# Custom Fermentor based on CSTR for cont. ferm.
class Fermentor(bst.CSTR):
    _N_ins = 1
    _N_outs = 2
    T_default = 323.15
    dH_glucose = -121 * 1000 # kJ / kmol
    dH_neutralization = -3.5 * 1000 # kJ / kmol

    def _run(self):
        feed = self.ins[0]
        gas_vent, liquid_effluent = self.outs
        liquid_effluent.copy_like(feed)
        # Apply reactions using the class-defined reactions, first parallel fermentation, then neutralization
        fermentation(liquid_effluent.mol)
        neutralization(liquid_effluent.mol)
        gas_vent.copy_flow(liquid_effluent, 'CO2', remove=True)
        liquid_effluent.phase = 'l'
        gas_vent.phase = 'g'
        liquid_effluent.T = gas_vent.T = self.T
        
    def _get_duty(self):
        # return self.Hnet
        return (
            self.dH_glucose * self.ins[0].imol['Glucose']
            + self.dH_neutralization * self.outs[1].imol['CalciumLactate']
        )

# Custom Acidification Reactor
class AcidificationReactor(bst.CSTR):
    _N_ins = 2
    _N_outs = 1
    T_default = 298.15
    P_default = 101325
    tau_default = 1.0
        
    def _run(self):
        feed, acid = self.ins
        effluent = self.outs[0]

        # Acid dosing
        n_CL = feed.imol['CalciumLactate']
        acid.imol['H2SO4'] = n_CL * 1.1
        acid.imass['H2O'] = acid.imass['H2SO4'] / 0.93 * 0.07

        # Mix and pre‑instantiate phases
        effluent.mix_from([feed, acid])
        effluent.phases = ('l', 's')
        effluent.T = self.T
        effluent.P = self.P

        acidification(effluent)

        # Force gypsum into solid phase (safety net)
        n_total = effluent.imol['CaSO4_2H2O']
        if n_total:
            effluent.imol['l', 'CaSO4_2H2O'] = 0
            effluent.imol['s', 'CaSO4_2H2O'] = n_total

        # Keep outlet as slurry/liquid
        effluent.phase = 'l'
        
# %% System

# -------------------- FEED STREAMS FOR ALL UNITS --------------------
glucose_feed = bst.Stream('glucose_feed', Glucose=80, Water=920, units='kg/hr') # 80 g/L
CaCO3_feed = bst.Stream('CaCO3_feed', CaCO3=44.5, Water=82.6, units='kg/hr') # Based on mol calc. from reaction; 1mol CaCO3 per 1mol Glucose
Nutrient_feed = bst.Stream('Nutrient_feed', NH3 = 1.34, H2O = 3.13, units='kg/hr')
H2SO4_feed = bst.Stream('H2SO4_feed', units='kg/hr')
Organic_Solvent = bst.Stream('Organic_Solvent', Octanol=172.5, Trioctylamine=172.5, units='kg/hr') # 1.5 : 1 ratio -> org : aq phase
Extr_Water_1 = bst.Stream('Extraction_water_1', H2O=100, units='kg/hr') # NOTE How much?
Extr_Water_2 = bst.Stream('Extraction_water_2', H2O=100, units='kg/hr') # NOTE How much?
Caustic_Sol = bst.Stream('Caustic_Solution', NaOH=4, H2O=50, units='kg/hr')
OrgSolvent_Makeup = bst.Stream('Solvent_Makeup', Octanol=1.72, Trioctylamine=1.73, units='kg/hr') # Add the 1% of solvent lost in caustic wash
Recycled_Solvent = bst.Stream('Recycled_Solvent')

with bst.System('lactic_acid_sys') as lactic_acid_sys:
    # Conversion
    feedstock_mixer = bst.Mixer('feedstock_mixer', ins=[glucose_feed, CaCO3_feed, Nutrient_feed], outs=('crude_feed'))
    feed_pump = bst.Pump('feed_pump', feedstock_mixer.outs[0], outs=('crude_feed'), P=110000)  # NOTE: Provisional Pump; missing correct settings
    preheater = bst.HXutility('preheater', feed_pump.outs[0], outs=('heated_feed'), T=323.15) # NOTE: Placeholder 50°C
    fermentor = Fermentor('fermentor', preheater.outs[0], outs=['CO2', 'fermented_broth'], P=110000, T=323.15, tau=72, V_wf=0.8, kW_per_m3=0.05, dT_hx_loop=35)    # NOTE find source for possible kW_per_m3 value for anaerobic ferm.
    ferm_cooler = bst.HXutility('fermentor_cooler', fermentor-1, outs='cooled_broth', T=298.15)

    # Separation
    gravity_decanter = bst.units.SolidsSeparator('GravityDecanter',
                                            ins=ferm_cooler.outs[0], outs=('decanted_cake', 'decanted_broth'),
                                            split={'Biomass_s': 1, 'CaCO3': 0.85, 'CalciumLactate': 0.05}, # NOTE: FINE TUNING, HOW MUCH PRODUCT LOST?
                                            moisture_content=0.35)
    bm_Filter = bst.units.SolidsSeparator('BM_Filter',
                                          ins=gravity_decanter.outs[0], outs=('BM_cake', 'BM_filtrate'),
                                          split={'Biomass_s': 1, 'CaCO3': 0.75},
                                          moisture_content=0.08)
    filtrate_mixer = bst.Mixer('filtrate_mixer', ins=(gravity_decanter.outs[1], bm_Filter.outs[1]), outs='clarified_broth')
    acidification_reactor = AcidificationReactor('AcidificationReactor', ins=[filtrate_mixer.outs[0], 'H2SO4_feed'], outs=('acidified_slurry'))
    gypsum_filter = bst.units.SolidsSeparator('GypsumFilter', ins=acidification_reactor.outs[0], outs=('gypsum_cake', 'lactic_acid_solution'), 
                                          split={'CaSO4_2H2O': 0.99}, # Fine particles stay, removed by extraction
                                          moisture_content=0.2)
    
    # Purification
    Evaporator_1 = bst.units.MultiEffectEvaporator(ID='MEE_pre_extr', ins=[gypsum_filter.outs[1]], outs=('conc_la_sol', 'condensate'), 
                                                   V=0.85, V_definition='Overall', # NOTE adjust to 30wt% lactic acid conc. before extraction for eff.
                                                   P=(101325, 90000, 70000)) 
    ExtrMixer1 = bst.LiquidsMixingTank(ID='Extraction_M1', ins=[Evaporator_1.outs[0], Organic_Solvent], outs=['mixed_phase_1'])
    PreExtr_Cooler = bst.HXutility('extraction_cooler', ins=ExtrMixer1.outs[0], T=313.15)  # 40°C extr feed for optimal eff.
    ExtrSettler1 = bst.LiquidsSplitSettler(ID='Extraction_S1', ins=[PreExtr_Cooler.outs[0]], outs=['organic_phase_1', 'aq_phase_1'],
                                    split={'L-LacticAcid': 0.99, # Simulate two stage extraction in one
                                            'H2SO4': 0.98,
                                            'Octanol': 1,
                                            'Trioctylamine': 1,
                                            'H2O': 0.01, # some water carry-over
                                            'CaCO3': 0,
                                            'CaSO4_2H2O': 0,
                                            'NH3' : 0}) # NOTE "realistic" assumptions of split
    BackExtrMixer1 = bst.LiquidsMixingTank('BackExtr_Mixer1', ins=[ExtrSettler1.outs[0], Extr_Water_1])
    PreBackExtr_Heater1 = bst.HXutility('backextr_heater1', ins=BackExtrMixer1.outs[0], T=348.15) # ca. 20-30°C higher T for Back Extraction
    BackExtrSettler1 = bst.LiquidsSplitSettler('BackExtr_Settler1',
                                               ins=PreBackExtr_Heater1.outs[0],
                                               outs=('back_extr_aq1', 'back_extr_org1'),
                                               split={'L-LacticAcid': 0.85,
                                                   'H2SO4': 0,
                                                   'H2O': 0.99,
                                                   'Octanol': 0,
                                                   'Trioctylamine': 0})
    BackExtrMixer2 = bst.LiquidsMixingTank('BackExtr_Mixer2', ins=[BackExtrSettler1.outs[1], Extr_Water_2])
    PreBackExtr_Heater2 = bst.HXutility('backextr_heater2', ins=BackExtrMixer2.outs[0], T=348.15) # ca. 20-30°C higher T for Back Extraction
    BackExtrSettler2 = bst.LiquidsSplitSettler('BackExtr_Settler2',
                                               ins=PreBackExtr_Heater2.outs[0],
                                               outs=('back_extr_aq2', 'back_extr_org2'),
                                               split={'L-LacticAcid': 0.8, # 80% of remaining LLA (total 97%)
                                                   'H2SO4': 0,
                                                   'H2O': 0.99,
                                                   'Octanol': 0,
                                                   'Trioctylamine': 0})
    AqPhaseMixer = bst.MixTank('PostExtraction_AqPhase', ins=[BackExtrSettler1.outs[0], BackExtrSettler2.outs[0]], outs='pure_lactic_acid_solution')
    # Solvent recovery
    Caustic_Mixer = bst.LiquidsMixingTank('Caustic_Mixer', ins=[BackExtrSettler2.outs[1], Caustic_Sol], outs=('caustic_mix'))
    CausticWash_Reactor = bst.SinglePhaseReactor('Caustic_Wash', ins=Caustic_Mixer.outs[0], outs=('neutralized_solvent'),
                                                 T=323.15, P=101325, V_wf=0.8,
                                                 tau=0.5, 
                                                 reaction=caustic_wash)
    Caustic_Settler = bst.LiquidsSplitSettler('Caustic_Settler', 
                                              ins=CausticWash_Reactor.outs[0], 
                                              outs=('caustic_w_waste_brine', 'recovered_solvent'),
                                              split={
                                                 'Octanol': 0.01,
                                                 'Trioctylamine': 0.01,
                                                 'NaOH': 1,
                                                 'Na2SO4': 1,
                                                 'H2O': 1,
                                                 'L-LacticAcid': 1})
    OrgSolvent_Recycler = bst.Mixer('Solvent_Mixer', ins=[Caustic_Settler.outs[1], OrgSolvent_Makeup], outs=Organic_Solvent)
    # MEE 2
    Evaporator_2 = bst.MultiEffectEvaporator(ID='MEE_post_extr', ins=[AqPhaseMixer.outs[0]], outs=('88wt_product', 'condensate'),
    V=0.95, V_definition='Overall', P=(101325, 80000, 60000, 30000)) # NOTE More details required, what pressure? PEP not detailed enough... how many effects? only one flash drum?
    Evaporator_2.split={
        'H2O': 1,
        'LacticAcid': 0,
        'H2SO4': 0,
        'Glucose': 0}

lactic_acid_sys.simulate()
print('actual duty:', fermentor.Hnet / 0.444 / 1000) # kJ / mol
print('user-specified duty:', fermentor.heat_utilities[0].duty / 0.444 / 1000)

Output

actual duty: -136.17598465824216
user-specified duty: -124.23675531424321

[iilard]

Hi @yoelcortes

Thank you for the in-depth answer.
I understand the conservative assumption you make with biomass and setting it equal to glucose.
However, in my example the fermentation reaction (althogh altered, not the typical theoretical 1 Glucose --> 2 LA) is exothermic already. Only the neutralization reaction seems to stay endothermic. Thus, I would argue, the biomass enthalpy is not the cause of the problem. In the neutralization reaction, there is no biomass.
Additionally, if I use the glucose enthalpy for my biomass definition and adjust the reactions according to the "conservative assumption" the fermentor is still endothermic, although a little less.

With my script the duty is:
1.8e5 kJ/hr
With biomass enthalpy assumed as glucose:
1.4e5 kJ/hr

In your solution the Calcium Lactate Hf value is the arbitrarily chosen one (as stated in my initial question). That value makes the neutralization exothermic, but that is "cherry picked" and not backed by any literature.
Thus, your actual duty is:actual duty: -136.17598465824216

Could you please share your thoughts/reasoning on this? It don't need the simulation to work 100%, but I would like to understand the limit BioSTEAM faces here.

Again, thank you for your input and the work-around!