In the production of biofuels through the fermentation of biomass, microorganisms such as yeast progressively convert sugars into other chemical compounds. As this takes place the concentration of the desired product steadily increases in the fermentation broth. From the viewpoint of the microorganism the “desired product” is not at all desirable; it is a toxic metabolic waste that inhibits its vital functions. Consequently the rate of sugars converted slows down greatly as the product concentration climb.
It has been proven in laboratories that if the product can be removed from the broth as the fermentation proceeds, the microorganisms remain healthy and highly productive until the sugar is almost fully converted. This technique, called CARAF (Concurrent Alcohol Removal And Fermentation), has many economic benefits including reduced capital cost of fermentors.
In laboratory studies distillation is the means used to remove the alcohol from the broth. But it is a low temperature distillation made possible by refrigerating the condenser. That technique is not economically feasible at plant scale. Distillation using evaporative cooling towers as the heat sink is a hot process that would kill the bugs. Consequently CARAF has not been implemented commercially.
Pass-through Distillation, however, is a cool, bug-friendly process that makes it possible to apply CARAF at commercial scale. PTD is the enabler of CARAF at commercial scale.
In a different section we observed that the capital cost of a complete distillation plant equipped with PTD is roughly comparable to one equipped with conventional distillation. However, if the plant is a biofuel plant using PTD to implement CARAF, then the PTD plant will be much less capital intensive. The fermentors will be smaller and less expensive by at least a factor of three. Moreover, The smaller footprint of the buildings housing the equipment will further improve the capital cost picture.