The task of the separation section is to recover and purify the ethanol generated by the cyanobacteria. This process takes the ethanol present in the pond solution, which is very dilute (0.015 moles per liter or 0.027 mole percent), and produce ASTM fuel grade ethanol. Many different process technologies are employed and some of which lack empirical demonstration of their function. By drawing a stream of 14.8 liters per second of solution from the cyanobacteria ponds, the separation process employs centrifugal filtration, pressure filtration, microfiltration, ultrafiltration, pervaporation, distillation, and adsorption to produce fuel-grade ethanol.
The first step in the separation process is a centrifugal pusher filter which is designed to remove over 90% of the cyanobacteria and suspended solids from the stream. The solids from this process can be burned for fuel or sent to a landfill. The second step is to remove any remaining cyanobacteria, suspended solids, and large particles using a bronze mesh plate filter. From there, a microfiltration system removes cell fragments and macromolecules. The ultrafiltration system removes remaining proteins and any remaining large molecules. In the next step, the pervaporation system selectively separates ethanol and water. The distillation column continues to concentrate ethanol. Finally, the adsorption column removes residual water, producing fuel-grade ethanol.
The separation process is designed for high product recovery and because of the dehydration requirements of fuel grade ethanol, the separation process also provides high product purity. Since recovery, purity, and cost cannot be mutually minimized, this separation process uses some process steps that are expensive, because no other feasible solutions are present. Also, this process relies on separation technology that cannot be optimized without pilot plant data. Still, this process will recovers and purifies the available ethanol to produce 46,000 gallons of fuel-grade ethanol annually.
Separation Process Diagrams: