Downstream processing

When a fermenter at Kingfisher Bioenergy is dropped to the beer well, the work of the cells is done, but most of the plant's equipment and energy are still ahead. The beer is about 85 % water. The ethanol must be distilled and dried, the solids separated and dried into a feed product, the water recovered and reused, and the carbon dioxide cleaned. In high-value bioprocesses, downstream processing, the recovery and purification of products, can be half or more of the production cost. This lesson surveys the unit operations of downstream processing and applies them to the stillage train at Kingfisher Bioenergy, which Theo has not yet studied in detail. Ethanol recovery itself was covered in CHE-350.

The principles of recovery

Downstream processing usually follows a sequence: removal of insolubles, such as cells and solids; isolation, which concentrates the product and removes most of the water; purification, which removes impurities that resemble the product; and polishing, which brings it to final form and specification. Two principles guide the design. First, remove the largest component early: in most fermentations that is water, and every later step is cheaper on a smaller stream. Second, recover the product with as few steps as possible, because losses multiply: five steps each at 90 % recovery give only 59 % overall. Biological products bring an extra constraint: proteins and many metabolites are damaged by heat, extreme pH and shear, so the gentler separations of membranes and chromatography replace distillation for them.

Separating solids

Cells and other solids are removed by filtration or centrifugation. Their density is close to that of water, which makes them settle slowly. Stokes' law gives the settling velocity of a small sphere in a gravity field,

which for a yeast cell 6 µm across with a density difference of 80 kg/m³ is about 1.6 µm/s, or less than 6 mm an hour. Centrifuges multiply gravity by hundreds to thousands of times. Their capacity is described by sigma theory: a centrifuge's sigma factor, \(\Sigma\), is the area of a gravity settler that would perform the same separation, and the feed rate it can handle is \(Q = v_g \Sigma\). The code shows that a decanter handling 100 m³/h of stillage to capture such fine particles would need an equivalent area of nearly 18,000 m², an impossible settling pond but an ordinary decanter centrifuge. Disc-stack centrifuges, with many closely spaced cones, reach higher sigma values for fine cells; decanters, with a scroll that conveys solids out continuously, handle the heavy solid loads of whole stillage.

Filtration passes the suspension through a medium that retains the solids. Cake filtration, used in rotary vacuum filters and filter presses, builds a layer of solids that itself does the filtering; its rate falls as the cake grows and as compressible biological solids pack down. Filter aids such as diatomaceous earth keep the cake open. Cross-flow membrane filtration, where the suspension flows along the membrane surface, limits cake growth and is widely used to separate cells from enzyme-containing broths.

The stillage train at Kingfisher Bioenergy

Whole stillage from the bottom of the beer column contains about 13 % solids: fibre, protein, oil, dead yeast and dissolved solids. Decanter centrifuges split it into wet cake, about a third solids, and thin stillage, with about 7 % mostly dissolved solids. A third or so of the thin stillage returns to the front of the plant as backset, recycling water, nutrients and heat. The rest goes to multiple-effect evaporators, which concentrate it to syrup of about 30 to 35 % solids. Corn oil is skimmed or centrifuged from the syrup and sold separately. Wet cake and syrup are mixed and dried in rotary drum dryers to distillers grains at about 10 % moisture. The code carries out the balance.

import math
# Downstream processing of whole stillage: decanter, evaporators and the distillers grains dryer
ws = 340.0                # t/h whole stillage (illustrative)
ts_frac = 0.13            # total solids
cake_solids, thin_solids = 0.34, 0.07
# solids balance across the decanter: ws*ts = cake*0.34 + thin*0.07, ws = cake + thin
cake = ws*(ts_frac - thin_solids)/(cake_solids - thin_solids); thin = ws - cake
print(f"wet cake {cake:.1f} t/h, thin stillage {thin:.1f} t/h")
backset = 0.35*thin; to_evap = thin - backset
syrup_solids = 0.32
syrup = to_evap*thin_solids/syrup_solids; water_evap = to_evap - syrup
print(f"backset {backset:.1f} t/h, to evaporators {to_evap:.1f} t/h -> syrup {syrup:.1f} t/h, water removed {water_evap:.1f} t/h")
for effects in (1, 3, 5):
    steam = water_evap/(0.85*effects)
    print(f"  {effects} effects: steam about {steam:.1f} t/h")
# Dryer: cake + syrup dried to 10 % moisture
wet_in = cake + syrup; solids = cake*cake_solids + syrup*syrup_solids
product = solids/0.90; water_dryer = wet_in - product
print(f"dryer feed {wet_in:.1f} t/h, product {product:.1f} t/h, water evaporated {water_dryer:.1f} t/h")
print(f"dryer heat at 3.4 MJ per kg water: {water_dryer*1000*3.4/3600:.1f} MW")
# Sigma theory: equivalent settling area needed for a particle size
mu, drho, d = 1.0e-3, 80.0, 6e-6        # Pa s, kg/m3, m
vg = drho*9.81*d**2/(18*mu)
Qs = 100.0/3600                         # m3/s per decanter
print(f"settling velocity {vg*1e6:.2f} um/s, sigma needed {Qs/vg:,.0f} m2")

wet cake 75.6 t/h, thin stillage 264.4 t/h
backset 92.6 t/h, to evaporators 171.9 t/h -> syrup 37.6 t/h, water removed 134.3 t/h
  1 effects: steam about 158.0 t/h
  3 effects: steam about 52.7 t/h
  5 effects: steam about 31.6 t/h
dryer feed 113.2 t/h, product 41.9 t/h, water evaporated 71.2 t/h
dryer heat at 3.4 MJ per kg water: 67.3 MW
settling velocity 1.57 um/s, sigma needed 17,697 m2

The decanters make about 76 t/h of wet cake and 264 t/h of thin stillage. After backset, about 172 t/h goes to the evaporators, which remove about 134 t/h of water to make about 38 t/h of syrup. A single-effect evaporator would need about one tonne of steam per tonne of water evaporated; with five effects, each effect's vapour heating the next, steam use falls to about a fifth of that. The plant has five effects, some heated with vapour from the distillation system, which is one of the main ways dry-grind plants have cut their energy use. The dryers then evaporate about 71 t/h of water to make about 42 t/h of distillers grains, needing roughly 67 MW of heat from natural gas burners at about 3.4 MJ per kilogram of water, a large share of the plant's fuel. Theo notes that this is far more heat than all the plant's fermenters combined release, and that the dryers are where the plant's carbon intensity is decided, as lesson 12 will show.

Concentrating and purifying biological products

For products more delicate than ethanol, other operations take over. Ultrafiltration membranes, with pores of a few nanometres, retain proteins while letting water and small molecules through, and are the standard way to concentrate enzymes after fermentation; diafiltration washes out salts by adding water while filtering. Precipitation with salts such as ammonium sulphate, with organic solvents, or by pH adjustment brings proteins out of solution. Liquid-liquid extraction, including aqueous two-phase systems, separates products by their partition between phases, as Theo did with ethyl acetate and water in CHE-350. Adsorption and chromatography purify products by their binding to solid resins: ion-exchange chromatography by charge, affinity chromatography by specific binding, and size-exclusion by molecular size. Crystallization gives pure solid products such as citric acid and many amino acids. Drying, by spray drying, freeze drying or drum drying, gives a stable final form.

Industrial enzymes, like the plant's glucoamylase, are usually recovered by cell removal, ultrafiltration and formulation with stabilizers, without chromatography, because they are sold as technical products where activity matters and trace impurities do not. Pharmaceutical proteins, by contrast, go through several chromatography steps to reach very high purity, because impurities can be dangerous to patients. The cost of purification rises steeply with the purity required.

Distillers grains as a product

Distillers grains are worth a large share of the plant's revenue, often a fifth or more, and are sold on their protein, fat and fibre contents and their consistency. Over-drying darkens the grains and damages the protein's digestibility, and under-drying lets them mould in storage. The dryers are controlled on outlet temperature and product moisture, and the laboratory measures moisture, protein and fat on every shift's composite. Contaminants matter too: mycotoxins from the corn are concentrated about threefold in the grains, because the starch is removed and the toxin is not, so the plant tests incoming corn and rejects loads above its limits. Feed rules administered by the Canadian Food Inspection Agency, which regulates livestock feed ingredients under the Feeds Act apply to the product's labelling and to any additives carried over from fermentation, such as antimicrobials.

Dryers also bring hazards from CHE-380: they are large combustion systems handling an organic dust, and dryer fires are a known problem in ethanol plants. They are fitted with spark detection, deluge systems and temperature interlocks, and their exhaust passes through a thermal oxidiser to destroy volatile organic compounds and odours before release, under the plant's air permit, the ECA.

Water and energy integration

The stillage train and distillation together make the plant almost a closed water loop. Backset, evaporator condensate and distillation water are reused in the cook, so fresh water use is a few litres per litre of ethanol, much less than a few decades ago. Heat is integrated too: vapour from the distillation column drives the first evaporator effect, and the evaporator's last effect runs under vacuum. Wren Halvorsen, the environmental lead, tracks water use and the quality of the small discharge. Theo maps the energy flows and confirms that the dryers and distillation dominate: the fermentation itself, for all its biology, is a small part of the energy bill.

Recovering carbon dioxide

Fermentation carbon dioxide is a product too. The gas leaving the fermenters is nearly pure carbon dioxide, saturated with water and carrying ethanol, acetaldehyde and other volatiles. The vent scrubber Theo designed in CHE-314 recovers the ethanol. Beyond it, the gas can be released, sold, or captured for storage. To sell it for beverages or dry ice, a gas company compresses it, removes the remaining impurities with activated carbon and other adsorbents, dries it, and liquefies it by refrigeration; the purity requirements for food use are strict. Sales depend on there being a market nearby, because liquid carbon dioxide is costly to transport. Capture for geological storage, discussed in lesson 12, needs compression and drying but not food-grade purity, and the fermentation stream is one of the cheapest sources of carbon dioxide to capture because it is already concentrated.

Corn oil

Corn oil, recovered from the syrup by heating and centrifuging, is a modern addition to dry-grind plants. It is sold mostly as a feedstock for biodiesel and renewable diesel, or as an animal feed ingredient, and it lowers the fat content of the distillers grains. Recovery is typically a few kilograms per tonne of corn, and chemical aids called demulsifiers improve separation. Theo adds an oil recovery line to the balance: it removes oil that would otherwise have been dried with the grains, slightly reducing the dryer load and adding a revenue stream that is valued highly because of its low carbon intensity as a fuel feedstock.

Each of these co-products shows the same downstream principle: separate the valuable component where it is most concentrated and easiest to recover. Oil is easiest to recover from syrup, where it has been concentrated by evaporation; carbon dioxide is easiest to capture at the fermenter vent, before it is diluted with air.

Theo's map of the downstream section ends with a list of improvement options for the expansion: a sixth evaporator effect, more heat recovery from the dryer exhaust, and better decanter performance so that less water reaches the dryers. Each saves energy and carbon, and each will be weighed against its cost in lesson 12.