An efficient evaporation technology has been developed for concentration of highly viscous fluids. The High Efficiency Stillage Concentration (HESC) system has been commercially proven for stillage concentration and is available in modular form to be easily integrated with an existing ethanol plant for energy reduction and/or efficient capacity expansion. 
 
 This technology is capable of producing syrup concentrations in excess of 50 percent total solids (TS) and can be used to offset or supplement evaporation requirements in the dryer or evaporators while recovering process water normally evaporated into the air. Energy savings result from reduced dryer duty, lower natural gas consumption and reduced steam consumption in the evaporation plant. Depending on the relative costs of power and natural gas, energy savings of 6 to 10 cents per gallon of ethanol produced can be achieved.
 
 Water Removal from Stillage: 

 Energy Requirements and Viscosity Challenges
 Evaporation of water is an energy-intensive process, requiring approximately 1,000 Btu per pound of water evaporated. Concentrating a process stream via evaporation generally causes the viscosity and fouling tendencies of the stream to increase, adding additional expense and difficulty to the process.
 
 Inside the ethanol plant, the evaporation of water from still bottoms is required to produce concentrated syrup to be sold as distillers wet grains (DWG). Syrup production from thin stillage is generally accomplished through multiple-effect evaporation using falling or rising film indirect heating evaporation technologies. This configuration minimizes overall energy requirements through multiple stages of heat reuse. Using two or three effects of evaporation, for example, can decrease energy consumption by 400 or 500 Btu/lb of water evaporated. Additional efficiency can be realized by integrating the evaporation plant with other large energy users inside the facility (e.g., distillation).
 
 Further evaporation of water from the stillage allows for production of distillers dried grains (DDG), a dry product with a longer shelf life. Because the stillage becomes quite viscous as it is concentrated, conventional evaporation technology becomes impractical at higher solids levels. The typical film-type evaporators begin to have difficulty at 25 percent-by-weight (wt%) TS and rarely operate consistently above 35 wt% TS. Heat transfer efficiency drops at higher viscosities, and the heat transfer surface experiences a higher rate of fouling. Final water removal occurs in drying equipment utilizing direct contact heating and evaporation. 
 
 Unfortunately, the direct contact dryer is the most inefficient of evaporation unit operations, requiring as much as 1,500 Btu/lb of water evaporated. The ethanol plant dryers, because they run the longest, are the highest capital cost unit operations in the facility.
 Maximizing water removal from syrup upstream of the dryer can result in substantial operating and capital cost savings for the modern ethanol plant. As increasing ethanol capacity comes on line and the marketplace becomes more competitive, it becomes increasingly important to reduce operating costs as much as possible. Energy efficiency has also become a larger issue as natural gas prices continue to increase.
 
 HPD, a Veolia Water Solutions & Technologies Company, developed the HESC system to bridge the technology gap between the evaporation equipment and drying systems that are typically supplied for the standard ethanol plant. The HESC system incorporates unique heat-transfer-enhancing technology that facilitates the processing of highly viscous fluids with minimal fouling. It allows syrup production to solids levels exceeding 50 wt% TS, offsetting energy requirements in the dryer and operational issues (e.g., fouling) in the conventional evaporation plant while decreasing makeup water requirements that are normally lost in standard drying systems. HESC can be provided on a stand-alone, modular basis for easy attachment to an existing ethanol facility, or it can be incorporated into a new facility where capital and operating savings can be realized from the outset.
 
 Enhanced Forced Circulative Evaporation 
 Although conventional forced circulation would seem to offer a viable alternative as a low-fouling approach, it has several drawbacks. The power consumption is very high, thus making it difficult to integrate with the rest of the plant when compared to falling film evaporators. The flooded tube design does not allow for turbulence of the process stream, thus it is unable to achieve the desired high-heat transfer rates and limiting product concentration.

 
 HPD has developed an enhanced heat transfer system that allows the non-fouling forced circulation technology to be applied without the high power consumption and poor heat transfer efficiency associated with a conventional unit. The requirements of the enhanced unit were to increase heat transfer efficiency, decrease power consumption and minimize the possibility of fouling while allowing full access to the tubes, in case boil-out or mechanical cleaning becomes necessary due to a severe upset which led to pluggage. An insert was developed for tubular heat exchangers with a proprietary spiral rib, and a patent is pending. This design results in a 50 percent to 75 percent increase in heat transfer coefficient and a 50 percent reduction in power when compared to the conventional forced circulation design.
 
 The enhanced forced circulation design is now HPD's standard design for concentrations of highly viscous fluids; it is in operation at over 35 commercial installations.
 
 HESC System Overview 
 The HESC system is a modular, enhanced forced-circulation evaporation system designed to further concentrate syrup from conventional stillage evaporators to greatly reduce dryer operating costs and recover valuable process condensate not possible in standard plant design. The HESC unit achieves very efficient evaporation through the use of mechanical recompression technology. The modular configuration and stand-alone operation allows for easy installation at an existing facility with minimal expense and little disruption to existing operations. The system may also be driven with waste heat or steam from the plant if the overall plant heat balance shows this to be beneficial. 
 
 HESC Operating Cost Analysis 
 HESC unit efficiency can be measured with a comparison to the process used in the conventional ethanol plant (see Figure 2). The example used is a 40 MMgy dry-grind facility where all the syrup is dried and sold as DDGS.
 
 The evaporation plant is a typical falling film system designed to produce 40% TS syrup but only operating near 30 percent TS due to difficulties with fouling and excessive, unplanned stoppages for cleaning. The syrup is fed to rotary dryers using air heated with natural gas to remove the remaining water and dry the DDGS to a final moisture content of 15 percent. 
 
 Assuming a DDGS production of approximately 260 million pounds per year, the required dryer water evaporation rate with 30 percent TS from the evaporator system is approximately 59,000 pounds per hour (lb/hr). With a heat requirement of 1,350 Btu/lb, the total dryer energy usage is 80 MMBtu per hour, which is equivalent to $5.6 million per year with a natural gas cost of $8/MMBtu.
 

 To remove evaporation duty from the dryer for more efficient unit operation, an HESC system may be installed as shown in Figure 3. Raising the syrup concentration to 50 wt% TS in the HESC unit will reduce the dryer evaporation by approximately 28,000 lb/h, saving $2.6 million per year in natural gas costs. The system will require 650 kilowatts (kW) of power, equivalent to a yearly operating cost of $227,000 at a power cost of 4 cents per kW. 
 
 The net yearly operating savings with the HESC system would be approximately $2.43 million or the equivalent to a savings of over 6 centsper gallon of ethanol produced. The total savings will obviously depend on the relative cost of natural gas and power, but payback for the HESC system will be quite short due to a more efficient means of evaporation. EP
 
 Jim Rieke is sales manager for the renewable fuels industry at HPD. Reach him at jim.rieke@veoliawater.com or (815) 609-2047. Adam Edwards is a process design engineer at HPD. Reach him at adam.edwards@veoliawater.com or (815) 609-2121.

With New Stillage Concentration System, Fouling Problems Evaporate

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Upcoming Events

2024 International Fuel Ethanol Workshop & Expo

Biodiesel Summit: Sustainable Aviation Fuel & Renewable Diesel

Carbon Capture & Storage Summit

North American SAF Conference & Expo

2025 International Biomass Conference & Expo

With New Stillage Concentration System, Fouling Problems Evaporate