Gas fermentation reactors

In a conventional fermentation, carbon arrives dissolved and only oxygen has to be transferred. In gas fermentation both arrive as gas. Acetogens running the Wood–Ljungdahl pathway reduce carbon monoxide and carbon dioxide to acetyl-CoA and onward to ethanol or acetate, using carbon monoxide or hydrogen as the electron donor; hydrogen-oxidising bacteria do something comparable aerobically. In every case the substrate must dissolve before a cell can touch it, so the constraint set out in high cell density culture systems applies to the carbon source as well as to oxygen, and applies to a far larger molar flux.

The solubility problem, one step worse

Henry’s law constants for carbon monoxide and hydrogen are lower than for oxygen — hydrogen’s by roughly half — so the equilibrium concentration driving transfer is smaller still. That alone would be manageable. What is not is that these are the substrate. An aerobic yeast fermentation transfers oxygen for respiration while carbon walks in dissolved at gram-per-litre concentrations; a gas fermenter must transfer every carbon atom of the product across the same thin film. Rate of production is therefore rate of transfer, almost tautologically, and reported productivities track kLa more closely than they track any property of the strain.

The energetics compound it. Acetogenesis conserves very little energy per mole of substrate, so biomass yields are low and most of the transferred carbon has to go to product rather than to cells. A slow-growing culture cannot simply be run denser to compensate; it has to be retained, which is why membrane cell retention appears in these systems for the same reason it appears in perfusion.

Why the geometry is a column

The answer is gas hold-up and contact time, not agitation. Bubble columns and gas-lift loop reactors are tall, so a bubble spends a long time in the liquid, and hydrostatic head at depth raises the partial pressure and hence the local driving force. Circulation is driven by the gas itself through a density difference between riser and downcomer, which avoids the power draw and the seal of a large impeller. Height buys utilisation, and utilisation per pass is what the economics turn on: unconsumed carbon monoxide or hydrogen leaving the top is either vented as loss or recompressed and recycled, and compression of a poorly soluble gas is expensive. A design is judged on how much of the feed gas is converted in one pass, not on titre alone.

The constraint that is not biological

Aerobic gas fermentation on hydrogen has to supply hydrogen and oxygen to the same broth. Hydrogen is flammable in air across an unusually wide composition range and detonable across much of it, so the working gas mixture must be held outside the flammability envelope at every point in the system — typically by keeping oxygen well below the lower limit for the mixture, and by ensuring the headspace, the recycle loop and any dead leg stay there too. That ceiling on oxygen fraction directly caps the oxygen driving force, which caps the growth rate. Here the safety envelope, not the organism, sets the achievable productivity — an unusual situation, and the reason these processes stay less common than their anaerobic counterparts.