Granulation: A Novel Method to Improve the Bulk Density and Flowability of Distillers Grains with Solubles (DDGS)

One of the options regarding ways to increase the bulk density of DDGS and improve its flowability is pelleting. Pelleting is a popular process used in the feed industry for blending and agglomerating feed ingredients in feed manufacturing. Added benefits include better product handling, palatability and flow. While pelleting is a favorable process for a lot of feed products, the high energy needed to extrude the feed material from the die and die wearing can make it a cost prohibitive process. A recent study indicates that the cost of pelleting declined with production scale, and pelleting DDGS is only feasible for larger plants (250 million gallons/year). This limits pelleting to a few large existing fuel ethanol plants in the United States. While there have been reported successes from research of pelleting DDGS, this practice might not be an economically feasible solution for tackling the logistics issue.

A recent invention at Purdue University titled Apparatus for Producing Biobased Carriers from ByProducts of Biomass Processing (Pending U.S. Patent 65095.00.US) adds value to DDGS by producing spherical granules capable of carrying and dispersing biologically and chemically active molecules such as fertilizers, pesticides and enzymes. Additionally, granulating DDGS increases the overall particle size and bulk density, thereby ultimately resolving the flowability and handling issues such as avoiding particle segregation which will reduce sampling error and improve product consistency.

This new invention was developed after preliminary studies were done producing DDGS granules using a lab-scale rotary drum dryer. Recently, extensive studies have been conducted at the lab-scale to understand the kinetics of granule growth. DDGS granules manufactured in a granulating drum are spherical pellets with improved handling properties and increased bulk density compared with DDGS produced using the traditional blending and rotary drum drying process common in dry-grind ethanol processing. However, the basic feed material needed for granulating DDGS is similar to what is needed in the traditional process. The granulation process involves the mixing of WDG with CDS, and granulating the mixture using a rotary drum granulator. The granulating drum provides the mechanical force that enables solid particles of WDG to collide, densify and agglomerate (grow in size), while CDS acts as a binder gluing colliding particles together. The wet distillers grains with soluble granules produced are dried to obtain DDGS granules. Unlike pelleting, this method of granulation involves less energy and operating cost than pelleting, and thus is a good alternative to be considered for use.

Figure 2 shows four possible particle size ranges which were produced during our lab-scale experiments, compared with DDGS without granulation, also produced in our experiments. The granules produced were targeted in the size range of 1.7 to 3.4 mm. The bulk density of granulated DDGS in Group size II and III were higher than the ungranulated DDGS by 17% and 20%, respectively. It should be noted that bulk density of granulated DDGS will depend on the solids content of the feed and so a fundamental understanding of the feed (WDG and CDS) is essential to obtaining granules of the desired characteristics.

In comparison to other particles such as pharmaceutical powders, detergents, etc., granulating DDGS is not a trivial task and involves skill and good understanding of the fundamental science of granulation. This is because of the very heterogeneous nature of the feed material which is a combination of particles of germ, protein, fiber, residual sugars, and tipcap with varying physical structure and chemical properties. Our study was a first important step to understanding the process variables and granulation kinetics of DDGS granulation. While more work is still needed in understanding some of the fundamentals of granulating a complex feed material such as WDG and CDS blends, our study has progressed to the point where some pilot-scale work can begin.

Just add nitrogen: DDGS as fertilizer

In another parallel study, DDGS granules of desired size characteristics was manufactured using nitrogen in the feed, with the attempt of producing a slow release fertilizer. In this study, DDGS was used as a carrier, while nitrogen was mixed within particles of DDGS and binder in one granule design and in a second granule design, nitrogen was used as the core, while DDGS and binder coated around the surface of the granule.

Our experiments were able to successfully granulate DDGS with the inclusion of nitrogen — thus proving its potential for use in a fertilizer matrix; however, the granules produced were not rated as slow release, and thus more work needs to be done in order to develop slow release fertilizers or other similar functions with DDGS. Two important granule characteristics that are important as fertilizer carriers are shown in Tables 2 and 3.