Dryhouse technologies and DDGS production

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Reprinted from The Alcohol Textbook 5th Edition 2009 with permission from Lallemand Ethanol Technology and Nottingham University Press

D.A. Monceaux¹ and D. Kuehner²

1 AdvanceBio, LLC, 5405 Dupont Circle, Milford OH 45150 USA (monceaux@advancebiollc.com)
2 Barr-Rosin, Maidenhead, Berkshire, UK (dominique.kuehner@geagroup.com)

Introduction

The cereal grains used for starch-based ethanol production such as corn, wheat, barley, rye and grain sorghum (milo) typically contain from 50 to 75% w/w starch (dry solids) (Table 1). Between 25 and 50% w/w of the grain remain after fuel ethanol has been produced. The 'whole stillage', containing both dissolved and insoluble nonfermentables and nondistillable microbial by-products, is rich in nutrients, fibre, protein, lipids and yeast and has traditionally been incorporated into animal feed rations.

The dryhouse is where these valuable animal feed components are recovered from the ethanol production process. The sole purpose of dryhouse operations is to remove water from whole stillage in an efficient and mechanically reliable manner. This process yields a number of possible concentrated products - distillers' wet grains (only some water is removed by centrifugation), distillers' dried grains (DDG), distillers' dried grains with solubles (DDGS), thin stillage (backset) and/or syrup (concentrated thin stillage, which may be sold separately). Excepting wet grains and thin stillage that must be used near the site of production, these products can be cost-effectively stored and shipped to distant animal feed ration markets.

Dry-grind ethanol process overview

Historically, ethanol production from starch-bearing cereal grains used one of two general categories of process technologies: wet milling or dry grind. Recently, dry-grain fractionation processes, similar to technologies that were in practise before the advent of corn wet milling, have been re-introduced to the dry-grind fuel ethanol industry. Dry-grain fractionation processes enable the ethanol producer to extract additional value from the non-ethanol coproduct by separating it into a broader spectrum of products for both conventional and developing markets.

Currently, the majority of fuel ethanol production facilities not using wet milling employ conventional dry-grind technologies, with variations of the following major unit operations:

These eleven unit operations are typically grouped into the two general process areas, as shown in Figure 1. The 'front-end' processes convert the starch-rich cereal grain into ethanol as efficiently as possible. The ethanol-rich beer that is produced in the front-end operations is recovered in the 'back-end' distillation process and dehydrated to produce fuel ethanol. The residual suspension, called whole stillage, rich in nonfermentable dissolved and suspended solids, is processed in back-end dryhouse operations, usually producing distillers' dried grains with solubles (DDGS) and thin stillage/backset.

Figure 1. Dry-grind block flow diagram

Ethanol process unit operations are often managed as separate front-end and back-end processes, a practise derived from the industry's early association with the distilled spirits industry. Numerous process and energy-integration strategies continue to blur the distinction, as technology changes drive greater process efficiencies and capital cost reductions.

Dryhouse processes - overview »