Ethanol is produced synthetically from fossil fuels or by the fermentation of renewable agricultural products.
Primarily produced by catalytic hydration of ethylene, the crude ethanol product from the reaction step is concentrated and purified, typically in two distillation columns. The first tower, commonly called the topping column, removes light ends, while the second tower, the rectifying column, removes intermediate components and water.
Trays are commonly used in these towers because the service is clean and they can meet the capacity and flexibility requirements of the application.
Ethanol is produced by fermentation of sugars derived from sugarcane directly or by using enzymes in the conversion of starches from grains such as corn, barley, sorghum or wheat.
After fermentation, the resulting "beer,” a dilute mixture of ethanol, water, residual sugars, proteins and mash solids is concentrated via distillation. Several flow schemes exist, generally containing beer or mash columns and rectifier/stripper columns.
Beer or Mash Column
The beer column is a reboiled stripper that takes the product from the fermenters and strips out the ethanol overhead. This tower can be extremely fouling and requires trays that are resistant to plugging. The beer column may operate at atmospheric pressure or vacuum. (The fouling problem is significantly reduced at the lower temperatures associated with operation under vacuum; however, the required tower diameters are greater.)
Preferred internals are fixed valve trays. PROVALVE® trays, VG-10 trays, or SUPERFLUX® trays have all been widely used in this application.
Some plant designs include a separate degassing tower prior to the beer column, or the beer column may have a section of degassing trays at the top of the beer column to release dissolved CO2 in the feed, which causes foaming and loss of capacity.
Rectifier / Stripper
The stripped ethanol from the beer column is concentrated in a distillation tower (or often two separate towers that perform rectification and stripping separately). The overhead product approaches the ethanol/water azeotrope concentration, with approximately 5% water. Final dehydration beyond the azeotropic concentration is typically done using molecular sieves.
The rectifier/stripper towers have no special fouling tendencies and, depending on operating pressure, may be equipped with either trays or structured packing (usually in the upper section of the rectifying column).
Scrubbers + Strippers
In addition to the distillation equipment, there are usually several packed scrubbers. The CO2 gas produced by fermentation is washed to remove volatile organics. Any non-condensable gases in the rectifier overhead are similarly washed. The final ethanol product may also pass through a packed stripper column to remove traces of dissolved CO2.
These towers may use metal or plastic random packing or FLEXIPAC® structured packing (scrubbers only).
Current bioethanol process development is focused on improving the economics by using lower cost lignocellulosic biomass feedstocks (corn stover, sugarcane bagasse, wheat, barley or rice straw, perennial grasses, and wood byproducts). The distillation equipment is largely similar to first-generation bioethanol plants; however, depending on the nature of the feedstock, the trays in the beer mash stripper column may be more susceptible to fouling in some of these processes.
Other processes are being developed to produce ethanol by gasification of biomass followed by conversion to synthesis gas and then to alcohols by the same processes as outlined under synthetic ethanol above. Distillation equipment is similar to that used in synthetic ethanol plants.