There’s been plenty of speculation, but no one knew until now why the weight of dried distillers grains with solubles (DDGS) loaded onto rail cars varies so much, tons difference in some cases.

Blame it on particle size, says Klein Ileleji, a Purdue University assistant professor of ag and biological engineering. More specifically, variation in particle size is a primary driver of variation in load weight.

According to Ileleji, previous studies show that when poured, like-sized DDGS particles congregate, with the smallest particles –fines – accumulating in the center of the pile; the larger particles moving to the outside. Ileleji and doctoral student Clairmont Clementson hypothesized that this is what happens when DDGS are poured into railcar hoppers for transport.

To test the theory, Ileleji and Clementson created a scaled-down mechanism that mimicked the loading of railcar hoppers. When simulated, the first containers had mostly fines, with proceeding containers getting more of the larger particles. The containers with fines weighed more because they can be packed more densely.

“When we analyzed our data with what we would expect in theory, the weight pattern of our containers can be explained by particle segregation during filling of the hoppers, which is amplified during discharge, causing bulk density variation,” Ileleji says.

Ileleji explains the problem is exacerbated by the tendency of DDGS to discharge from hoppers in a funnel-like flow pattern in which the fines that have accumulated at the hopper's core are released first. Outside contents, which are primarily larger in size, are released last. Even mass-flow hoppers with steep sidewall angles designed to release their contents in layers caused weight irregularities because the DDGS continued to exhibit funnel-flow tendencies.

To mitigate the challenge, Ileleji suggests either finding a way to mix the contents better during loading or using a process he patented to create uniform-sized particles.

“The granules are larger, so they would not pack as well as fines. But granules are denser than the same amount of fines, so you get more in the same amount of space,” Ileleji said. “This would ensure that railcars would weigh about the same amount and help control the cost of shipping DDGS.”