First Generation Biofuels

Although there are unlikely to be many new first generation ethanol plants built in the future, existing first gen plants can be beneficially retrofitted with PTD to increase throughput, to improve economics, or both.

One strategy is to fit each fermentation tank with a PTD evaporator/absorption unit (like the Drystill SAM) This would be piped in series with the existing cooling circuit. It would remove ethanol from the batch as it was being produced, limiting ethanol concentration to around 5%. This mode of operation is known as CARAF (concurrent alcohol removal and fermentation). CARAF is known to greatly increase volumetric productivity.

The PTD unit would require no heating or cooling services. They would only require a steady stream of concentrated absorption fluid to operate. Leaving the units would be the same stream diluted with water and ethanol. This diluted stream would be pumped to a central absorption fluid regenerator still. There it would be re-concentrated. The water and ethanol taken out of it would be sent to the beer well.

In each fermentation tank so equipped, batch time would fall from 55 hours to 20 hours, enabling a doubling of plant capacity without investing in new fermentation tanks.

Another area of application in a first generation plant is the evaporator section that concentrates stillage into syrup. PTD can add to evaporation capacity at less capital cost than the large stainless steel falling film units prevalent in the industry today.

Adding capacity to a first generation plant will make the beer column a bottleneck. Rather than duplicating the conventional beer column, a PTD unit can augment the capacity of the existing unit at a lower capital cost.

Most first generation plants feature batch fermentation; not because batch operation is more efficient (it is not) but because infections are catastrophic in continuous plants but little more than a nuissance in batch plants. Infections require time to develop. The residence time in a continuous fermentation plant operating in CARAF mode would have such a rapid fermentation rate that infections need never occur. It is foreseeable that one day all the huge fermentation tanks in first generation plants will be put to some other use while fermentation is taken over by smaller bioreactors using PTD to implement CARAF.