Solids separation

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

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Dryhouse operations begin with partitioning the whole stillage suspension into fibre- and suspended-solids-rich wet cake and dissolved-solids-rich, thin-stillage fractions. A number of suspended-solids separation technologies have been used with varying degrees of success. These technologies include filtration systems that use inclined wedge wire or vibrating screens, or various types of belt or leaf filter presses. With whole-stillage substrates, filtration produces a low-solids cake as well as a filtrate with high suspended solids. Postfiltration presses improve the concentration of cake solids, but high sheer increases the concentration of suspended solids in the pressate. Applying organic polymers improves solids separation, but this process is generally not cost effective.

In the separation of liquid solids, the centrifugal force generated by the rotating assembly replaces the weaker force of gravity. High gravitational forces generated at very high rotational speeds efficiently separate suspended solids across a range of particle sizes in addition to dewatering the wet cake. The optimum technology is one that is capable of processing high hydraulic flow rates, requires moderate capital costs, operates with low maintenance and is robust while delivering high suspended-solids capture rates in addition to a high total-solids wet cake.

Centrifugation

Continuous decanter or solid-bowl centrifuges are generally used for whole-stillage solids separation, clarifying the centrate as well as dewatering the fibrous wet cake.

The eight major components of the decanter centrifuge are shown in Figure 3:

Figure 3. Decanter centrifuge components

The design of the decanter centrifuge, in conjunction with centrifuge settings, determines the settling velocity of particles as well as the particle critical diameter, which together affect the capture efficiency of the suspended solids. For whole-stillage solutions that contain suspended solids with a wide range of particle sizes, the settling velocity is defined by Stokes's law (Figure 4), where

Figure 4. Stokes's law equation for determining settling velocity

With a given decanter centrifuge and whole-stillage flow rate, particle critical diameter, or 'cut point', is defined by Figure 5, where

D_pc = particle critical diameter, or the smallest particle diameter capable of being separated,

Figure 5. 'Cut point' equation for determining particle critical diameter.

Decanter centrifuges provide three basic controls to the plant operator:

• Feed rate, which adjusts the effective residence time
• Weir height, which controls the liquid volume and pool depth
• Back-drive torque, which adjusts the differential speed between the solid bowl and the scroll conveyor

The effects of back-drive torque adjustments on cake solids and centrate clarity are illustrated in the following figures. In Figure 6, increasing torque by reducing the solid-bowl and scroll-conveyor differential speed results in an increased suspended-solids residence time in the dewatering beach as well as in higher wet-cake total solids. Figure 7 illustrates the capture efficiency of suspended solids as a function of wet-cake total solids. Together, Figures 6 and 7 show that centrifugation is a compromise between cake solids and centrate clarity.

Figure 6. Effects of back-drive torque adjustment of a decanter centrifuge on suspended solids.

Figure 7. Cake dryness versus the capture efficiency (recovery) of suspended solids.

Thermal processes

High-flow aqueous process solutions are found throughout the dry-grind fuel ethanol plant. Extensive movement of energy from streams needing to be heated or cooled in the various unit operations is needed - occurring by one of the following mechanisms:

• Conduction, which is heat transfer by means of molecular agitation within a material without any motion of the material as a whole
• Convection, which is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it
• Radiation, which is heat transfer by electromagnetic radiation

Dryhouse thermal processes remove water from various stillage streams, increasing whole-stillage solids from approximately 15% w/v total solids and producing a 90% total solids DDGS coproduct. The dryhouse of the nominal 100-million-US gallon-per-year facility represented in Table 3 requires the removal of 218 tons of water per hour.

Figure 8 presents the enthalpy (the quantity of heat contained in one kilogram of water at the selected temperature), or energy available in water as a saturated liquid and vapour, and the associated temperature at various pressures. The ability to use water as an energy-transport mechanism is critical to efficient dryhouse operations.

Figure 8. Water enthalpy chart.

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