Downstream & purification
Purification step yields multiply; the selectivity of each step, not the speed of any single one, sets the cost of goods.
Purifying a biosynthesis product is a cascade of steps whose yields multiply: five steps at 90% leave 59%, so the selectivity of each step matters more than the speed of any single one. The cluster’s first recurring motif is choosing by the molecule’s coordinates: size, charge, hydrophobicity and affinity pick the unit operations, and the cascade is ordered by descending volume — get rid of water and biomass first, polish last.
The second motif is separation physics, honest and simple: a centrifuge lives by Stokes’ law and the rotor’s hoop-stress ceiling; crystallisation by the solubility curve and controlled supersaturation; extraction with compressed CO₂ by the fact that solvent power is a function of density. Each page takes one of these dependencies and shows where it breaks.
Start with bioseparation: the cascade logic there is derived, not enumerated.
Articles 4
| Downstream purification (bioseparation) Purification is a chain of compromises: every step that sees the product more precisely holds less of it, and the train's economics are decided at its dirtiest end. |
| Disc-stack centrifuges Gravity settling cannot separate microbes on a working-day timescale; a disc stack buys a few thousand gravities and then pays for them in shear, floc damage and lost seal liquid. |
| Crystallization of pharmaceutical APIs Supersaturation is the only lever crystallisation has, and it drives two rival processes at once — growth of the crystal you want and nucleation of everything you don't. |
| Green extraction: SC-CO2, DES, ultrasound Each green method replaces brute solvency with one tunable force — density, hydrogen bonding, or cavitation — and pays for the selectivity in pressure, viscosity, or scale. |