Evolving Processes at Ethanol Plants and New, High-Protein Feed Products

Traditional Ethanol Dry Grind Processing
The dry grind approach to ethanol processing (Figure 1 above) has been the predominant method for ethanol production in the United States due to lower investment costs and operational requirements than corn wet mill processing, including energy and water use.

The entire corn kernel is used during the fermentation process (Liu and Rosentrater, 2011). Corn is screened, then hammermilled to a medium- to fine-grind meal/ flour. The resulting flour is combined with water to form a slurry, and enzymes (alpha amylase) are added to begin to break down the starch into shorter glucose chains.

The resulting mash is normally cooked and sterilized to kill non-desirable bacteria and to gelatinize the starch (although some companies do not have a cooking step).

Once cooled, more enzymes (including alpha amylase and others) are added to continue to break down the starch. Yeast (Saccharomyces cerevisiae) is added to the mash in the fermentor, where the glucose is then metabolized into ethanol and carbon dioxide during anaerobic fermentation. After fermentation, ethanol is separated from the water and non-fermentable solids (both dissolved and suspended) in the distillation column. The remaining water and solids are collected (this is known as whole stillage). Whole stillage can be pressed, but it is more commonly centrifuged to separate the coarse solids from the dissolved solids; both streams will still contain a large quantity of water.

The liquid containing the dissolved solids is often referred to as thin stillage. Thin stillage is frequently concentrated in evaporators to become condensed distillers solubles (CDS; Figure 2 above), also referred to as “syrup.” The wet coarse solids, or wet cake, which are collected from the centrifuge, are commonly known as distillers wet grains (DWG; Figure 3 above) and are generally 30–35% dry matter.

DWG and CDS can be combined and partially dried to form distillers wet grains with solubles (DWGS) with moisture contents generally between 45–50% (wet basis), or they can then be dried to about 10% (wet basis) moisture content to produce DDGS (Figure 4).

As a traditional rule of thumb, each bushel of corn (about 56 pounds/25.4 kg) will be converted into approximately 2.8–2.9 gallons (10.6–11. L) of ethanol, 17–18 pounds (7.7–8.2 kg) of carbon dioxide, and 17–18 pounds (7.7–8.2 kg) of nonfermentable solids (which include non-fermentable corn kernel components and byproducts produced by the yeast during fermentation), all of which are then sold as feed coproducts – either in wet or dry form.

Put another way, each kernel of corn will result in about one-third each of the resulting fermentation products, respectively.

Numerous companies have been testing and commercializing modifications to the traditional dry grind process over the past few decades. Many of these have resulted in the development of new feed products beyond traditional distillers grains, and these alterations will shift the traditional one-third, one-third, one-third paradigm.

New Processes
As discussed in Liu and Rosentrater (2011), more than a dozen different technologies have been explored over the years that could be implemented in dry grind ethanol processing to concentrate or fractionate various nutrients from the cornkernel – especially proteins, lipids, and fibers – and to develop targeted, higher-value uses for these components.

At the time of this publication, there are six key approaches that have commercial viability. They are briefly summarized below. All of them implement a variety of physical and/or chemical separation techniques which are unique to each specific approach; some include pre-fermentation component separations; others use postfermentation segregations; and some use various combinations.

For more in-depth information, however, the reader is referred to each company’s website. There are additional companies working on various ideas and technologies for nutrient concentration, but those have not been covered here.

1. FluidQuip Method

Figure 5 above illustrates the FluidQuip approach to nutrient concentration. This is known as the Maximized Stillage CoProducts (MSC) process.

This process is installed after fermentation and separates components from the whole stillage. It will result in a high protein stream that contains corn proteins, spent yeast cells, fine fiber, and other soluble materials (examples are shown in Figures 6 and 7).

This higher-protein stream is dried to produce a feed product with a minimum of 48% (as-is, or wet basis) protein content. This process impacts the thin stillage, which is then sent to the facility’s existing evaporator system. This system utilizes the existing whole stillage decanters for dewatering of fiber and other suspended solids.

Specifically, the MSC process will feed whole stillage to paddle screens as the first of several fiber separation stages, where the stillage is separated into a solids (i.e., cake) stream and a centrate (i.e., liquid – process water with dissolved solids) stream. The protein, yeast, fine fiber, oil, and solubles are washed through the screen while the coarse fiber and other suspended solids are carried over to the cake discharge. The cake from the paddle screens is then rehydrated and passed through another separation process that further separates the coarse and fine fiber from the protein centrate.

The second separation process consists of a pressure screen system, which uses high-pressure water to feed the fiber and protein stream over a concave screen. The screen allows the protein and soluble particles to pass through the screen while retaining the cake (or concentrated fiber) on the outside of the screen. The fiber cake is then hydrated again and fed to a perforated horizontal decanter; the remaining cake is then sent to the plant’s existing drier system to be dried into DDGS.

The protein centrate (sold as Still Pro 50) with the dissolved solids (including the oil and other minerals) is collected from the previous fiber washing steps where the stream has been processed through a high-speed centrifuge. The protein clarifier centrifuges are high-speed disc nozzle centrifuges designed to concentrate fine solids (especially proteins) and separate them from the centrate stream.

From the clarifier centrifuges, proteins, fine fiber, and water exit as a “heavy” phase. Water, oil, and lighter particles will exit as a “light” phase. The concentrated protein from the clarifier underflow is then collected and sent to a dewatering step via horizontal high-speed decanters. The dewatered protein cake is then sent to a ring drier to dry the product prior to storage and loadout.

The final product is partially recycled to the paddle mixer, while the remaining product is pneumatically conveyed through the protein cooling system to product storage.

2. ICM Method

ICM, Inc. uses a unit operations-based approach that can be integrated to achieve nutrient concentrations (Figure 8) and a variety of resulting feed products. These include pre-fermentation and/or post-fermentation modifications that systematically separate and de-water process streams and can be used in different combinations. ICM’s Selective Milling Technology (SMT) combined with Fiber Separation Technology (FST) can be used to produce high-protein DDGs (HP-DDG) and bran with syrup, or it can produce traditional DDGS. The Advanced Processing Package (APP) utilizes four separate technologies – SMT, FST, Feed Optimization Technology (FOT), and Thin Stillage Solids Separation System (TS4) – to produce corn fermented protein, bran with syrup, and DDGS simultaneously.

Prior to fermentation, corn is subjected to coarse dry milling by roller mills, followed by a secondary wet grinding integrated into the cook process. This sequence of operations is known as SMT. It allows corn fiber to be removed downstream via FST as the fiber particles are not pulverized, which is the case with traditional hammermilling.

FST utilizes counter-flow washing to separate the fiber, which is then dewatered by a rotary press. The FST process reduces the fiber load going to the fermenter and subsequent downstream processes and thus increases overall plant throughputs and efficiencies.

After SMT and FST, facilities utilizing APP further separate and concentrate protein post fermentation using FOT and TS4. FOT separates the whole stillage stream using two stages of separation, resulting in a liquid stream and a concentrated suspended solids stream containing concentrated proteins and yeast.TS4 removes suspended solids from the FOT liquid stream to concentrate the corn proteins and spent yeast cells. The resulting thin stillage has lower suspended solids, which allows for higher evaporative concentrations due to reduced syrup viscosity, which also improves corn oil recovery.

Protein products recovered from the FOT and TS4 processes are combined to produce a feed product known as “corn fermented protein,” which can be dried in the plant’s existing rotary dryers. This product is targeted at monogastric markets, while the fiber (i.e., bran) and syrup streams are combined to produce a feed product targeted at ruminant markets since it is higher in energy.

3. Marquis Method

Marquis uses flocculation technology to facilitate the separation of proteins, oils, and fibers. As shown in Figure 9, a flocculant is added after fermentation to the centrate (i.e., process water and dissolved solids, which result from centrifugation of the whole stillage). This stream is then subjected to dissolved air flotation (known as DAF), where fiber particles are removed from a stream rich in lipids (oils) and proteins (i.e., the flocs that float on the surface vs. the fiber particles which sink).

Oil can be concentrated using traditional centrifugation, whereas the fiber is concentrated via de-watering and drying. Ultimately, the benefit of this approach to nutrient concentration is that only two additional unit operations are required – the DAF process and de-watering the proteins. The resulting distillers grains (sold as ProCap) are substantially higher in protein content than traditional DDGS.