Bioethanol
Ethanol is produced by fermentation of sugars derived from the conversion of starches using enzymes. 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 and generally contain the following two or three columns: beer or mash column, rectifier or stripper, and scrubbers and strippers.
 

Beer or mash column
The beer column is either a stripper with a bottoms reboiler or a direct steam injection column that takes the product from the fermenters and strips out the ethanol overhead. The mash and residual water are removed in the bottoms.

This tower can be extremely fouling and requires trays that are resistant to plugging.

The beer column may operate at atmospheric pressure or at vacuum.
Often the beer column is operated under vacuum, because the fouling problem is reduced at the lower temperatures associated with operation at lower pressure.

Preferred internals are fixed valve trays.  Trays using VG-10 fixed valves or PROVALVE® fixed valves or SUPERFLUX® trays have all been used widely in this application.

Tray deck with VG-10 valves

Tray deck with VG-10 valves

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 to release dissolved CO2 in the feed that causes foaming and loss of capacity. This tower typically uses the same types of trays that are applied in the beer column, but sometimes uses simple baffle trays.


Rectifier/Stripper
The stripped ethanol from the beer column is concentrated in a distillation tower. Often two separate towers that perform rectification and stripping separately are used. The overhead product approaches the ethanol/water azeotropic concentration. Final dehydration beyond the azeotropic concentration is typically done using molecular sieves.

These towers have no special fouling tendencies. Depending on operating pressure, they may be equipped with either trays or structured packing. Koch-Glitsch has supplied the following trays for these columns: FLEXITRAY® valve trays, trays with VG-10 fixed valves, MINIVALVE® tray decks with VG-0 fixed valves, and SUPERFRAC® trays.

Structured packing is usually installed in the upper section of the rectifying column. 

FLEXIPAC® HC® structured packing

FLEXIPAC® HC® structured packing

Scrubbers and 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. Also any non-condensables in the rectifier overhead are similarly washed. In fuels ethanol plants, the final product is also typically passed through a packed stripping column to remove traces of dissolved CO2.

These towers may use metal or plastic random packing such as IMTP®, INTALOX® ULTRA, INTALOX® SNOWFLAKE®, FLEXIRING® or HY-PAK® random packing.  The scrubbers can also be equipped with FLEXIPAC® structured packing.