A Case Study of East Kansas Agri-Energy Introduction A Case Study of East Kansas Agri-Energy East Kansas Agri-Energy (EKAE) is one of seven ethanol production plants in the state of Kansas as of 2007. Located at the end of the Corn Belt in Garnett, Kansas, EKAE has been producing ethanol from corn since June 2005 using a dry mill process, where the starch portion of the corn is fermented into sugar and then distilled into alcohol. In addition to ethanol, EKAE produces distiller grains, a high value product of the dry mill process. Application of Advanced Process Control to an Ethanol Dry Mill Process: East Kansas Agri-Energy EKAE produces approximately 35 million gallons of 200 proof ethanol, 68,000 tons of dried distiller s grains with solubles (DDGS) and 110,000 tons of wet distiller s grains with solubles (WDGS). The production process consists of six steps: grinding, cooking, fermentation, distillation, dehydration and nonfermentables. After a detailed evaluation of many options for capital expenditure, EKAE engaged Pavilion Technologies, a Authors: division of Rockwell Automation to provide an advanced Doug Sommer: process control Plant (APC) Manager solution. East The Kansas APC solution Agri-Energy focused on the milling, cooking and stillage dehydration Maina Macharia: sections of Manager the plant Project (Table 1). Engineering EKAE set the Pavilion following main objectives for the APC project: Technologies, Rockwell Automation Celso Axelrud: Senior Applications Engineer Pavilion Increase dryer capacity by 4-6% Reduce Energy utilization (MMBTU Gas/gal EtOH) by 2.5-3 % Technologies, Rockwell Automation Increase ethanol production through more Lina stable Rueda: plant Applications control Engineer Pavilion Technologies, Rockwell Automation Ethanol Production Process Description Figure 1 shows a schematic overview of the process units on which the APC strategy was implemented. The cooking part of the dry mill process is a challenging control problem, dealing with the disturbances introduced by water recycling from the back end of the plant. The cooking process prepares the corn starch for fermentation and starts with the hammer mills pulverizing the corn kernels into small particles. After the corn is milled, the coarse flour is mixed with water to form a substance known as slurry. The water used in this section of the process is part fresh water and part recycled water from the back-end of the plant.
Hammer Mills Alpha Amylase Cook Water Tank Slurry Blender Plant Steam Cook Tube Flash LIQ. TANK 1 LIQ. TANK 2 Ferm Tanks Beer Well Distillation Process Whole Stillage Tank Centrifuges Backset to Slurry Tank 1 st Effect Evaporators 2 nd Effect Evaporators 200 Proof Day Tank Plant Steam Syrup Tank Centrate Tank Thin Stillage Tank Wet Cake Wet Cake to Wet Pad Syrup to Wet Pad Syrup to Dryer Dryer A DDGs Steam Cook Water Tank Natural Gas To HRSG Stack Combustion Burner Cook Water Economizer Heat Recovery Steam Generator Boiler Feed Dearator Softeners Figure 1. Process Schematic. At EKAE, the water is recycled from the side stripper bottoms and backset from the thin stillage tank, and then mixed with the milled corn in the slurry blender, where the gelatinization process begins. Gelatinization describes the water s penetration through the starch to facilitate the enzyme reaction. After fermentation, unfermented components are collected from the distillation process as whole stillage and centrifuged to generate thin stillage and wet grains. Some of the wet grains are sold as wet distiller grains with solubles (WDGS) and the rest are dried in a natural gas fired drum dryer. The product from the dryer, with a moisture content of around 10 percent, is sold as dried distiller grains (DDGs). The exhaust gases from the dryer are circulated through a natural gas fired thermal oxidizer (TO) to destroy volatile organics. The hot gases from the TO are then used in a heat recovery steam generator (HRSG) to generate steam for process use. Unrecycled thin stillage from the centrifuge is thickened in a series of eight evaporators to generate syrup, which is added to the wet grain feed in the dryer. Residual heat from the evaporators is used by the beer distillation column. There are a number of concerns associated with DDGs. DDG moisture should be kept as high as possible to increase product throughput and minimize energy use, but it should not be too high, as the product can plug the conveyers and downstream transportation equipment. There also is a maximum moisture specification for DDG. The syrup solids concentration should be controlled to help minimize disturbances to the dryers and reduce 2
energy usage. The syrup solids concentration should be as high as possible to help reduce energy load on the dryers, but the concentration that can be handled by the syrup pumps is limited, as gradual plugging of the syrup system will occur if the viscosity is too high. Production Challenges East Kansa Agri Energy has a unique operation: DDGS dryers are run only 4 days in a week and shutdown for 3 days on a weekly cycle. This is governed by local market demands for DDGS and WDGS in this location. When the dryers were taken offline or placed online, the plant had to be run conservatively until the entire process steadies out. Not only does this present a large disturbance to the plant, it also impacts EKAE s ability to maintain waterbalance of the fermentation inventories; these inventories significantly fluctuated and adds to the overall operation challenge. Because of the market incentive to be efficient and meet ethanol demand, East Kansas then made it a priority to maximize the plant s operating potential. East Kansas quickly realized that traditional engineering studies and operational testing required to attain this goal took many months. Pavilion approached East Kansas Agri-Energy with the proposal to use APC technology and achieve the goals with improved process control. Pavilion also had a proven track record in achieving significant debottleneck using APC technology; East Kansas then decided to implement the APC technology with Pavilion s APC methodology. Advanced Process Control Application Pavilion installed the leading APC solution for the ethanol industry at EKAE. The Pavilion solution is based on a nonlinear model predictive control (NMPC) methodology, a control algorithm that, based on the dynamic model of the process, predicts and optimizes the future response of the process. The model predictive control problem is formulated by solving online a finite horizon open loop optimal control problem taking process dynamics, interactions and plant limits into account. One of the key parts of the NMPC application is the model used for process move prediction and optimization. The models developed for the EKAE application used a combination of fundamental and empirical models identified from plant measurements. These models were created with state-of-the-art Pavilion modeling technology. The use of nonlinear models in the controller helps EKEA manage the process within a wide range of operating conditions. Often solutions only employ empirical modeling in NMPC, because it can be infeasible to rapidly solve a set of complex differential equations describing first principle models for calculating the optimal solution. However, first principle models add great value to the application by extending understanding of the process behavior. The Pavilion solution for the ethanol industry combines feasible empirical models and comprehensible first principle models with a constraint-handling, multi-variable control algorithm. The first principle models are 3
developed based on heat and mass transfer balances and validated with operating data and plant tests to develop the process control models. The APC solution developed for EKAE consists of two controllers. One controller was configured for the milling and cooking section and a separate controller was developed for the stillage dehydration section. The application was designed to accomplish the following objectives for each section: Milling/Cooking: Balance load and energy between mills to achieve efficient milling Increase feed to beer column to increase ethanol production Manage the fermentation inventory with the water balance Control liquefaction and slurry solids percentages to reduce disturbance to the fermenters Control backset percentage to ensure a constant mineral feed to the fermentation process to maintain consistent water quality in the fermentation feed Stillage Dehydration (Centrifuges/Evaporators/Dryer): Balance load distribution between centrifuges Decrease energy use in ethanol production Control dryer moisture for consistent DDG product quality Reduce average TO hotbox temperature while maintaining the temperature within environmental limits Stabilizing steam pressure within TO operating limits To accomplish these objectives, liquefaction, evaporator solids and dryer moisture were controlled to an operator determined target using values predicted in real-time from virtual online analyzers (VOA) developed by Pavilion as part of the solution. The backset flow was manipulated by the water balance controller and the trajectories sent to the slurry/stillage controller. Slurry solids were controlled by manipulating the quantities of corn and water to the slurry tank. The operator decided on a target value for liquefaction solids. The application calculated the slurry solids target value required to achieve the desired consistency of the liquefaction solids. The application included virtual online analyzers for slurry, liquefaction and syrup solids, as well as dryer product moisture. These analyzers in turn provided real time calculations to the controller. Process Performance Figure 2 illustrates a comparison between the 200 Proof flow rate before and after the APC application was installed. Figure 3 presents a comparison between the total natural gas usage (TO plus Dryer A) in BTU/min before and after the APC application was installed. 4
Figure 2. Process Performance 200 Proof Flow Rate. NG to EtOH Ratio 40000 35000 30000 Ratio [BTU/Gal] 25000 20000 15000 EtOH to NG Ratio After APC EtOH to NG Ratio Before APC Mean Before APC= 27439.5 BTU/Gal Mean After APC= 24710.7 BTU/Ga 10000 0 1000 2000 3000 4000 5000 6000 7000 Data Point Figure 3. Process Performance Natural Gas Usage. 5
Before APC implementation the beer feed flow rate average was roughly 511 GPM; it increased 3.6 percent to an average of 529.3 GPM. 190 proof production was 91.4 GPM on average before APC implementation and increased to 104.5 GPM. Most importantly, the average 200 proof flow to storage increases from 74.6 to 84.2 GPM which represents a 12.8 percent increase in ethanol production. The energy utilization of steam in lbs/gal of ethanol improved approximately 9.7 percent. Quality Variable Analysis Climatic conditions, e.g. ambient humidity, required a de facto reduction in the dryer moisture target from 13 percent before APC to 11.5 percent after APC. The standard deviation in the dryer moisture reduced 3.34 percent, which allows tighter control around the desired target. We observed a 5.3 percent dryer capacity increase as indicated in the total centrifuge flow. Figure 4. Syrup Solids Before and After APC Histogram Plots. Syrup solids variability was reduced by 36 percent. Figure 4 illustrates the histogram plots for the syrup solids before and after APC implementation; control over slurry solids regulates liquefaction solids. The APC application reduced the liquefaction solids variability, stabilizing the fermentation feed quality and this allowed for an increase in the slurry solids which results in an overall higher fermentation yield of ethanol. Figure 5 presents the histogram plots for the slurry solids, which were reduced in variability by 4.07 percent. 6
Benefits of the APC solution Figure 5. Slurry Solids Before and After APC Histogram Plots. Tables1 and 2summarize the major results obtained at EKAE with the APC application. Pavilion s MPC solution helped East Kansas Agri-Energy to achieve the following key benefits: 12.8 percent increase in ethanol production 9.9 percent increase in energy efficiency An overall reduction in key process and quality variables Table 1. Process Performance Summary After APC Before APC Variable Mean Std Dev Mean Std Dev Mean % Change Rectifier 190 product rate GPM 104.53 6.14 91.39 3.91 14.38% Sieve 200 production rate GPM 84.15 4.49 74.60 2.92 12.80% Beer column feed GPM 529.29 8.79 511.00 5.05 3.58% 7
Total Centrifuge feed GPM Steam to Ethanol Ratio (lb/gal) NG to EtOH Ratio (BTU/Gal) 505.48 26.28 480.13 67.28 5.28% 14.65 16.07-8.83% 24710.70 27439.47-9.94% Table 2. Quality Variables Summary After APC Before APC Variable Mean Std Dev Mean Std Dev Std Dev % Change HRSG inlet temp 1509.40 9.01 1495.42 13.07-31.04% Syrup Solids 39.88 0.87 39.00 1.36-35.97% Dryer Moisture 11.74 0.29 12.95 0.30-3.34% Slurry Solids 34.16 0.036 32.82 0.038-4.07% In addition to the quantitative benefits, Pavilion s NMPC technology also provided EKAE an automation application with the following advantages: - Models do not require any major adjustment unless major modifications to the process are carried out. - The process is kept stable during fluctuating conditions such as changes in the weather or upsets in the distillation section. - Better control of the fermentation/beer-well inventories makes it easier to manage the milling rate with the beer feed target rates. This stability increased the plant s ability to push production rates to capacity or production permit limit. Application Potential The installed control system is designed to accept operator set points for the dryer product s moisture target and to hold this stable at the target value. As DDGS specification changes, the target can readily be changed, but the above benefits are consistent over a variety of moisture targets through the tighter target control. 1. Increasing the dryer moisture target as permitted by customer specification will further reduce energy costs. 2. Syrup solids control is highly dependent on stillage level control; the APC application uses thin stillage and whole stillage levels to help reduce disturbances in the quality variables. Increasing the limits in the level control will allow better syrup solids control. Conclusions The APC project allowed EKAE achieve record production rates by finding optimal process operating conditions. This reduced the amount of energy needed per gallon of ethanol produced. With the APC application, EKAE was able to overcome operational constraints that limited plant production efficiency. Pavilion provided EKAE with a tool that helps the company makes full use of the plant s operating potential. 8
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