Running a large ethanol plant is usually challenging because it needs constant supplies of variable feedstocks and must be managed carefully to maintain profit margins and economic viability. So attempting to upgrade it from grain ethanol to also making cellulosic ethanol for fuel and other products can be very difficult. However, operating and upgrading during a drought that kills most of your corn supply is almost impossible—unless you have some almost angelic assistance.
This was the grim scenario facing one of the Midwest's largest ethanol plants, Hankinson Renewable Energy LLC in Hankinson, N.D., when a crippling drought dragged on through much of 2012's spring and summer.
"Last year was very bad," says Gary Jubien, solutions consulting manager at Honeywell Process Solutions. "Much of the corn crop was lost, many supplies were unavailable, prices went up and the plant's operations were threatened. So assistance from something like advanced process control (APC) could really help them pull through." Jubien presented "APC Helps Ethanol Plant Through Tough Times" at Honeywell Users Group 2013 on June 18 in Phoenix, Ariz.
The plant was designed by ICM Inc. in Colwich, Kan. Its first-generation plants use corn or grain sorghum as feedstock, while its second-generation plants are rolling out cellulosic ethanol this year. Hankinson is at the generation-1.5 stage, which means using add-ons like kernel husks as feedstock. At full second-generation, the plant will use multi-cellulosic materials.
The objectives of Hankinson's APC project were to improve control and reduce variability, reduce energy use and increase dried distiller's grain with solubles (DDGS) moisture close to spec limits to increase profit. "The plant's dryers have four disturbance variables each, and PID controls poorly," explains Jubien. "Each set of three centrifuges has a dead-end feed header, so they load unevenly, and APC could adjust each feed while balancing amps. Also, molecular sieves at the end of the ethanol process cycle every four minutes, so steam demand changes frequently. And,the plant's thermal oxidizers (TOs) have no air-flow measurement, so we needed oxygen sensors for trim. The TOs and dryers also use natural gas, which is the plant's second highest direct cost after grain."
Before the drought, Honeywell began working with ICM and Hankinson to implement some APC solutions. Consequently, the plant adopted Honeywell's Profit Controller multivariable control software, Profit Stepper automatic step testing and identification software tools, and Profit Suite operator interfaces. As a result, several APC models were implemented, and generated some useful results.
For example, key control variables (CVs) for the TOs were furnace temperature, steam pressure and oxygen, while the multivariables (MVs) valve and damper positions, natural gas flows and other factors). Understanding these factors and their performance allowed Hankinson's operators to run the TOs closer to their limits, which saved gas and other costs.
Likewise, APC enabled the plant's dryers to more consistently control the moisture of its DDGS, increase its moisture by 0.67% and increase profits without going off-spec. Also, APC applied to the plant's centrifuges enabled them to balance their loads more efficiently.
Thanks to implementing APC, the gas/steam ratio of Hankinson's two TOs were reduced by 1% and 5.6%, respectively. Besides increasing product moisture, the dryers also reduced variability by 20%, reduced gas use by 4.45%, and increased production by 6.38%. Finally, the centrifuges reduced amps variability by 56%, and reduced centrate variability by 31%.
"The plant reduced its natural gas costs by more than $116,000 and increased its DDGS revenue by more than $675,000," adds Jubien. "The payback period of the APC project was eight months, and the plant also reduced gas use, but still increased production by 6.38%."