Original, to the best of my knowledge, as of March 2011. Idea first conceived roughly 2006.
EXPOSITION
Some time back, I had read an article or two on the process of thermal depolymerization[1], and, hey! I had an idea.
This process is designed to turn organic material into what is essentially crude oil. The story highlighted a pilot plant using the process, turning castoff turkey guts from a turkey-processing plant into moderately useful amounts of rather heavy crude.
The basic idea is that, when placed under high pressure and heated, carbon chains inside the organic goo rearrange themselves into the sorts of chains one ordinarily finds in petroleum products. These can then be refined to produce more useful products, such as gasoline and lubricating oil.
One major problem in industrial processes is that pressure is typically not cheap; it is economically and energetically expensive. Heat is likewise expensive, unless the heat exhaust can be harvested from another industrial process.
Another major contributor to this idea was the story of iron fertilization[2]. Apparently, oceanographer John Martin discovered that iron, when added to the surface layers of ocean water, significantly increased the growth rate of plankton. Plankton is a wonderful source of biomass; many algae-based biofuels processes are currently in development.[3]
The keystone of the idea was my discovery that there were such things as dead hydrothermal vents on the seafloor, i.e. vents around which a highly-localized biosphere either does not exist or has already disappeared. Dead vents are key to this process, since they do not have an existing biosphere to disrupt; most vents have sulfide-based biocommunities surrounding them, and any disruption of flow from the hydrothermal vent would presumably be destructive to these unusual creatures.
DESCRIPTION
The basic idea is this:
After finding a dead hydrothermal vent system, preferably with "smoker" vents, an converted offshore oil platform can be positioned over it, and the vent tapped to feed the process. A reactor column would be sunk to the sea floor; this column could be relatively thin compared to the requirements for a land-based reactor column, as the static pressure within the column would be opposed by the water pressure outside it. Hot, sulfide-laden water would be extracted from the vent and brought up through the column to power the thermal-depolymerization process, and if any excess heat was available, to power refining and/or electricity-generation processes on the surface.
Water from hydrothermal vents is typically rich in iron sulfide. This can be oxidized to iron sulfate, which, when added to the surface water surrounding the platform, should stimulate phytoplankton growth and thereby biomass generation. A collection system, possibly using converted tanker vessels or flexible suction pipelines, would strain the plankton from the water and move it to to the column top, where it would enter the depolymerization process. Since plankton are ubiquitous in water, this is essentially an algae-farming operation in the open ocean. Specific natural algae that produce higher-quality lipids can be "seeded" in the area; if the process is carefully tuned, they may naturally dominate the farming area.
The column, when full, should achieve a "steady-state" thermal and reaction distribution; withdrawal of synthetic crude from the column to the surface can take place from an experimentally-observed, optimal depth. This can then be fed to a surface refining process - possible powered by residual heat from the vents, from heat supplied by other vents, or from burning fractions of the synthetic crude - to yield useful fractions, which can then be offloaded to tankers, disposing of the need to transport raw crude to centralized refineries elsewhere.
BENEFITS
- The process is carbon-neutral. The biomass to be refined is produced from CO2 in the atmosphere, effectively "recycling" CO2 released from burning fossil fuels.
- It is, at least notionally, more efficient in several ways. It allows for selective tanker transport to market of finished products instead of crude oil, allowing lower transport cost per unit. It uses currently-unused waste heat from naturally-occurring processes, and has naturally-occurring feedstocks, thereby reducing the lower bound for pollution to zero.
- Since vent fields can be found in international waters, production can take place in less-politicized, secure, and royalty-free environments.
- Since the upper bound of crude oil storage is the volume of the subsurface column plus any external storage tanks, any leaks are automatically limited, unlike deepwater drilling of oil reservoirs, where the upper bound of the potential leak is the size of the reservoir.
- Any organic material is theoretically a feedstock for this process; carbonaceous waste such as plastics (a major component of the Great Pacific Garbage Patch[4]), waste biomass, fish-processing residues, &c., can theoretically be processed as well. While these would require transport to the column, it may be economically feasible, depending on the capital costs of construction.
- The refining process, generally considered to be a source of pollution and unpleasant odor, is by definition remotely located, away from human population centers.
PROBLEMS
- It requires a particular oceanic feature - a hydrothermal vent field - to work as designed. There are only some charted vent fields in the world.
- As described, it requires vents that are "dead", i.e. have no biospheres surrounding them; these comprise an unknown fraction of vent fields, but apparently a minority of them. While a "live" vent field is technically as usable for this purpose, there are potential political and scientific objections to appropriation of the critical materials (heat and sulfides) of a functioning biosphere for industrial processes.
- There may be political or ecological objections to the use of oxidized iron sulfides to "fertilize" stretches of open ocean, regardless of the natural origin of the sulfides.
- Since this is essentially a permanent placement, the construction must be robust to weather extended exposure to normal marine-environment conditions, including storms, rogue waves, tsunami, corrosion, &c.
DRAWINGS
CALCULATIONS
To be added.
REFERENCES
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