Dryhouse processes - overview

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

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Two basic processes (solids separation and dehydration) and three unit operations (centrifugation, evaporation and drying) make up the dryhouse in a modern, large-scale dry-grind fuel ethanol plant. Other unit operations have been used on a limited basis. The objective is to concentrate the solids fraction of whole stillage using a combination of mechanical separation and thermal processes. A wide variety of technologies are available. Those selected must deliver mechanical reliability and thermal efficiency at a moderate capital investment. The resultant product is 90% dry matter DDGS, made by maintaining properties of the original, approximately 15% dry matter, whole-stillage feedstock. Table 2 provides information on DDGS composition. In this discussion, DDGS is assumed to be the final product made in the dry mill.

Dryhouse technologies have changed little over nearly half a century because

• the fuel ethanol industry has experienced only two periods of significant growth between protracted periods of limited new plant construction (this has restricted technology development),
• alternative large-scale dryhouse processes have not been demonstrated on a commercial scale and
• project finance strategies have suppressed the implementation of new, innovative technologies.

Most improvements in dryhouse operations and energy efficiency have been the result of incorporating interprocess unit operations, energy-integration strategies and technology breakthroughs into front-end processes. Foremost has been the dramatic reduction in stillage volumes that has resulted from increasing beer ethanol content. Ongoing improvements in fermentation technology and yeast metabolism have brought about dramatic increases in per cent v/v ethanol content in fermentors. In the early 1980s, corn dry-grind fuel ethanol plants considered 8% v/v ethanol beer acceptable. Today, a number of plants claim consistent operation at ethanol concentrations above 20% v/v (Figure 2). The impact of this 'very high gravity fermentation' is a significant reduction in the related decanter centrifuge and evaporator duty and in plant investment.

Figure 2. Relationship between beer ethanol content and stillage flow rate

Dryhouse mass balance

The increased demand for renewable fuel has led to a corresponding increase in dry-grind fuel ethanol plant production capacity. While new plant annual production capacities averaged less than 30 million denatured gallons ten years ago, today most new dry-grind plant capacities range from 60 to 100 million US gallons per year, and above.

To successfully manage dryhouse operations, it is important to know the 'mass balance' (or 'material balance') - the quantities of materials that need to be processed. Process simulation modelling of a corn dry-grind, motor-fuel-grade ethanol plant that produces a nominal 100 million US gallons per year, generates the dryhouse material balance illustrated in Table 3. The stillage volumes and solids concentrations shown are representative of a plant producing a nominal 14% v/v ethanol beer and recycling approximately 30% of the thin stillage as dilution water (backset) in grain mashing. The evaporator condensate and dryer vent flow rates indicate the energy demand of the corresponding unit operations. Solids concentrations in the evaporator condensate and dryer vent are due to the presence of volatile organic compounds in the stillage.

Table 3. Dryhouse mass balance Please click here to enlarge this table

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