Short term contact with long chain hydrocarbons is not harmful to algae. This has been well proven. A few years ago it was discovered that if a solution of living algae was stirred with some decane or similar solvent, the solvent would extract a fraction of the lipids through the cell walls, without harming it. The original research was on extractind beta carrotene from the algae traditionally used to manufacture it. They found about a 20 fold increase in productivity per square meter of pond. They found that survival was very high unless the algae was extracted more often than about once a day. They found that the specific solvent was unimportant, as long as the polarity was in the right range as measured by the octanol/water coefficient. They call it "milking" the algae.
Obviously the productivity of lipids could not be increased 20 fold. But I can image it would still be quite an increase, up to doubling the output possibly. Even if total lipid production stayed the same, it would still be very worthwile. The process could eliminate the problems associated with species selection, harvesting, drying, and extracting the oil all at once.
The system could be exceedingly simple, and able to run continously with little maintainance. A fraction of the the total circulation of a photobioreactor would be diverted into a small extractor where solvent and algae are mechanically and/or ultrasonically mixed for a period of seconds or minutes. Next the two layers would be allowed seperate, algae returned to photobioreactor, solvent layer would run through a filter/drier, then into a (preferably continuous) transesterifier. How do you seperate the newly made biodiesel and solvent you may ask? That's where my own personal idea (AFAIK) comes in. You simply use biodiesel as the solvent. It is nonpolar enough, chances are very high it would work. That allows your reservoir of biodiesel to act as the solvent, no need for any potentially expensive/polluting alkanes.
Check this out: http://www.isaaa.org/kc/cropbi.../default.asp?ID=2198
The technology for selectively killing of algae predators also mentioned on this page could be built resonably easily too, I believe: http://www.originoil.com/techn...live-extraction.html
There are a few patents about these processes too. Just because they make it sound high tech and call it proprietary technology does not mean it is tough to build yourself on a small/medium scale. There is no need to use expensive high tech to improve the yields slightly on a small scale. As far as I can tell, if people ever want truely low cost energy, the entire infrastructure must be decentralized, community based and operated by the people who are using the energy. Otherwise we will continue being robbed blind by corporations, regardless of the energy source.
For a while I was planning on trying this out myself, but I have come to the realization that it may be a year or three till I have the chance. I decided I needed to throw it out there for others to try in the mean time. I am willing to try to consult anyone who attempts this. Hope you like it.
My group and startup company here at Penn State are going to be setting up some continuous extractors similar to the one you have outlined. We are probably going to focus more-so on separating long chain hydrocarbons from Botryococcus braunii, but we also grow triglyceride bearing algae such as chlorella, chlamy, nannochloropsis etc.
We will be setting up a hexane extractor and try many solvents, contact times, mixing times etc. I am also familiar with ultrasound transducer design for mixing. The use of biodiesel interests me a bit, so maybe I can try that as well. I did not think of using biodiesel as a triglyceride extraction solvent, but sounds like a quick experiment. Please feel free to contact me to collaborate, plan on starting around June.
Chemical & Energy Engineering -- The Pennsylvania State University
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