Measurement Automation Strategy: Key to Bio-Ethanol Refinery Efficiency

Transcription

1 7 Dearborn Road Peabody, MA (USA) / Measurement Automation Strategy: Key to Bio-Ethanol Refinery Efficiency, Industry Specialist February 2008 Copyright KROHNE, Inc.

2 Measurement Automation Strategy Key to Bio-Ethanol Refinery Efficiency Introduction The conversion of corn into fuel ethanol is once again gaining popularity and is an appealing business opportunity for farm communities and agribusinesses in the mid-west and other regions of the U.S. and Canada. Investors are attracted by the prospect of more than doubling the monetary value of a bushel of corn by converting it into 2.75 gallons of fuel ethanol, 17 pounds of animal feed, and other value added byproducts. In the U.S. alone, over 120 plants are now producing ethanol from corn feedstock, and another 76 are under construction. Dozens more are in various stages of planning, with total domestic production capacity expected to double during the next five years. The Des Moines Register recently reported that farmers planted more corn in 2007 than in any year since 1944, based on expectations of increased ethanol production. The rapid increase in production reflects the expanding market for bio-ethanol, driven by growing recognition of the economic, social and environmental benefits of biofuels. Ethanol is increasingly in demand as an octane-enhancing substitute for the additive MBTE, which is being banned in many states. The 2005 Energy Policy Act stimulated the growth of this industry by offering federal incentives and goals for replacing a portion of our nation s gasoline requirements with a renewable fuel source by Current estimates indicate we will surpass these benchmarks well before Fourteen billion gallons of ethanol would be required, if an E10 (10%) blend were to replace all 140 billion gallons of gasoline consumed in the U.S. annually. Add to this a strong push from state legislatures for the market adoption of E85 (85%) blends, and the growing interest in Canadian and overseas markets, and it is easy to understand the high level of investment in new production capacity. Thin Profit Margins The strong demand, however, represents only one part of the profitability equation. The whole proposition is not profitable if inefficiencies in the production process itself add significant costs, as a result of excess energy consumption, poor yields, wasteful use of raw materials, process chemicals and enzymes. Unfortunately, in many cases, that is exactly what s happening. The cost of natural gas and other utilities are especially problematic, as these items far exceed the other major cost components, including the cost of plant construction and labor. For example, in a typical plant, the natural gas that fuels boilers, evaporators, February 20, of 12

3 dryers and other equipment comprises 15% of the cost of producing a gallon of ethanol. For a 100MGY plant, that translates to millions of dollars in operating costs. Conserving energy by just 1% improves the bottom line by tens of thousands of dollars. The elevated price of natural gas is not the only financial challenge. When an ethanol production facility comes online, the increased local demand for corn puts upward price pressure on corn prices and poses problems with variability of feedstock. Some ethanol producers report that ethanol yields vary as much as 7 percent, depending on the variety of corn. Lower yields add considerable financial risk an anathema to investors. Need for Tight Process Control For these reasons, achieving consistent profitability can be a tough challenge for bio-ethanol producers. An individual ethanol producer has little influence on the market price it receives for the product it ships and little control over the price of feedstock and natural gas. What ethanol producers can do must do is tightly control their own manufacturing process, and thereby produce consistently high yield, while minimizing the consumption of energy and raw materials, such as yeast, enzymes, nutrients, and chemicals. As any business guru will tell you, process control depends on accurate and reliable measurement. Process improvement methodologies, such as Lean Manufacturing and Six Sigma, are fundamentally measurement-based strategies that seek to maximize productivity while eliminating variation. In fact, measurement is both the second step and the fifth step of the Six Sigma DMADV process (define, measure, analyze, design, verify). However implementing process control strategies requires a careful understanding of process needs, including identifying measurement challenges and applying an optimal, customized solution that takes advantage of the best available technology. Importance of Meter Selection and Placement Careful selection and installation of measurement devices is important for three reasons: Measurement Accuracy. First and foremost, measurement devices give visibility to the process, allowing plant operators to see what is going on inside the pipes and production systems and how full each tank is. This is actionable information that provides the basis for planning and management systems and for real-time process control. To be useful, this information must be accurate. February 20, of 12

4 Automation. Accurate and reliable measurement devices enable automation by programmable logic controllers and computer systems. Automation improves plant efficiency and production consistency. Measurement devices should have selfdiagnostics and alarming capabilities to automatically monitor conditions on a continual basis and to alert operators of problem states such as changes in flow caused by solids, entrained gas, or temperature changes, as well as problems with the functioning of the measurement device itself. Maintenance Costs. High performance measurement devices may have a higher initial purchase cost, but they will typically have lower total lifetime costs, because of the elimination of downtime for recalibration and repair. Some manufacturers now offer interchangeable components, such as a universal signal converter that is compatible with a variety of meters with different sensor technologies or different pipe diameters and flow rates. This flexibility greatly simplifies engineering, procurement, and parts inventory. Measurement Devices Measurement devices serve several purposes in an ethanol plant. Flowmeters Flowmeters measure the speed and volume (or, in some cases, the mass) of liquids and gases that move through a pipe, including beer, stillage, syrup, enzymes, water, steam, CO2 and natural gas, as well as methane used as an alternate fuel. A wide variety of flowmetering technology is available, including Coriolis, magnetic, ultrasonic, variable area, and vortex-based devices. Density meters Density meters measure the percent solids during feed stock preparation. Density meters also measure the final alcohol quality (proof) to monitor and control energy intensive processes and to satisfy ASTM documentation requirements for selling final products into the transport fuel distribution system. Level meter Level meters indicate the volume of solids or fluids in a tank, for process control and inventory management. Mechanical devices that utilize hydrostatic pressure are fast being replaced by more accurate and reliable direct level measuring technologies such as guided wave radars and noncontact methods that employ radar and ultrasonic waves. Other Measurements At various points in the production process, it is also necessary to measure February 20, of 12

5 and control temperature -- during cooking, fermentation and distillation, for example as well as other attributes such as pressure, ph, conductivity, and moisture. Achieving High Performance Every stage of production demands precision, and a plant s measuring device supplier must have a deep understanding of the entire ethanol production process, as well as the best technology available and suitable for each type of measurement required. Performance is not just a question of selecting the best technology and device for the particular application. Proper placement of the device in the pipeline or tank is also critically important to ensure not only accurate measurement but also to avoid distortions, clogging and damage. Achieving good performance and controlling total lifecycle costs also depends on a preventive maintenance and periodic calibration checks on some instruments. The Production Process This white paper provides examples of how accurate measurement improves efficiencies and contributes to profitability in a typical dry milling process during the five main parts of the production process: conversion, fermentation, distillation, dehydration, and recovery of byproducts (e.g., CO2, and DDGS). Conversion Conversion, the first step of the dry milling process, converts the starch in the feedstock to simple sugars in preparation for the subsequent fermentation process. Such conversion is not necessary for sugarcane and other sugar-rich feedstock sources, because they do not contain starches that need to be converted into fermentable sugars. Corn feedstock is processed using enzymatic conversion. The feedstock is ground into fine particles and mixed with water to prepare corn mash, a consistent slurry. The grinding process increases surface area and frees the starches from inside the protective cell walls. Alpha amylase enzymes are typically used for the liquefaction of the starch-rich grain. A second enzyme, glucoamylase, is commonly used to convert the liquefied starch into glucose (simple sugar) in a process called saccharification. Cellulosic feedstock requires more complex hydrolysis conversion to break down the closely-bonded cellulose and hemicellulose from the lignin and release the fermentable sugars. Once the feed stock has been converted to fermentable sugar, the downstream steps of the dry milling process fermentation, distillation and dehydration do not tend to vary by type of feedstock. February 20, of 12

6 Fermentation Fermentation employs yeast to convert the glucose into ethanol. Fermentation can be done in batches or continuously when combined with saccharification. Depending on yeast strain, fermentation can take one to five days. Yeast dosage is dependant on the sugar content and the desired percent of alcohol at the end of the fermentation process. Fermentation severely slows after reaching 14% alcohol, because alcohol denatures the process. However, some types of yeast can ferment up to 21%. Tightly controlling temperature is critical to maximizing efficiency, because fermentation will usually cease above 85 F. The fermentation process produces beer containing diluted alcohol and solids. Two downstream processes, distillation and dehydration, extract the liquid and purify it in steps to achieve 100% alcohol content (200 proof) as required for E100 fuel ethanol specification. Carbon dioxide (CO2) Carbon dioxide (CO2), a second co-product of the ethanol production process, is given off in large quantities during fermentation. As a gas, CO2 is colorless, odorless, and incombustible. In its liquid form, CO2 reaches an extreme -17 F, which makes it an excellent source for cooling and freezing applications. CO2 vapors released from fermentation are usually scrubbed to recover any alcohol vapors and cleaned. If sold to commercial users, the CO2 is liquefied. Otherwise, it is typically released back into atmosphere. Distillation Distillation is the process by which alcohol is separated from the mash and water. It exploits the difference in boiling points between alcohol and water. Distillation can only purify the ethanol to about 94% alcohol (6% water), because water and alcohol form an azeotrope at atmospheric pressure. Dehydration Dehydration A molecular sieve is the simplest method and most common method of purifying ethanol from 190 to 200 proof. Evaporation and Drying Evaporation and Drying The solids waste at the bottom of (a?) beer well is processed separately in a centrifuge/evaporation/drying process to produce distillers dried grains with solubles (DDGS). Distillers grains are rich in cereal proteins, fat, minerals, and vitamins, and they serve as an excellent source of digestible protein and energy. Each area of the plant described briefly above milling, slurry preparation, liquefaction, fermentation, distillation, dehydration, evaporation and drying requires a measurement solution that provides an optimal relationship between performance and purchase February 20, of 12

7 price. This performance includes not only accuracy, but also reliability and durability, as well as frequency and method of calibration, and lifetime maintenance costs. Optimizing Solids Measurement Most production operators understand the importance of percent solids control for an effective fermentation process, and measurements are routinely conducted at various points in the process. Unfortunately, most percent solids measurements are performed only periodically, by a laboratory moisture analyzer. Although the lab analyzer may be calibrated for high accuracies, this sampling and testing process usually proves unreliable in practice, and is extremely difficult to get repeatable results. As a result, an increasing number of plant operators are opting for real-time, online measurement to continuously monitor the contents of the mixing tank and make instantaneous changes to the process. Plants are thereby able to maintain process requirements within acceptable tolerances (typically within 0.5%). This real time measurement and control allows more consistency in the slurry mix solids and enables operators to push the solids percentage higher, thereby reducing flow problems and enzyme usage and other costs. Continuous monitoring of solids concentration facilitates the detection and prompt correction of any slow decrease in solids. By continually measuring percent solids, a record is established for each fermentation batch. The data facilitates realistic prediction of the outcome of the fermentation process, in order to optimize the overall process by reducing problems with previous known flow issues. The solids data can also be used to improve the beer well averages, a key factor in increasing alcohol yields. The newest generation of industrial grade Coriolis meters has proven to provide highly accurate and repeatable density measurements, as the basis of good solids measurement. Unfortunately, the bent design and internal flow splitters in previous generations of Coriolis meters frequently failed due to fouling and blockages. These problems have been eliminated by the single, straight tube OPTIMASS 7000 series coriolis meters developed by KROHNE, Inc. The single, straight tube design provides an affordable, accurate and highly reliable solids measurement solution. Optimizing Ethanol Rectification and Dehydration Rectification and dehydration provide a textbook case of the importance of accurate measurement for effective process control. The rectification and dehydration are common to any fuel ethanol process, whether wet milling, dry grind or February 20, of 12

8 cellulosic. The goal of the rectification process is to achieve maximum purification (up to 190 proof). Then the dehydration process employs molecular sieves to convert the 190 proof ethanol into 200 proof ethanol, going from 5% moisture content to 0%. If the process fluid moves too quickly through the molecular sieves and some moisture remains, then the entire batch must be run back through the dehydration system again a completely inefficient step that wastes energy and ties up production capacity and potentially causes a bottleneck for the entire plant. To avoid this problem, a plant could extend the dwell time in the molecular sieves to ensure that all moisture is removed. However, this margin of comfort comes at a cost in terms of productivity loss and unnecessary energy consumption. Alcohol proof measurement of rectifier output (190 proof) and dehydration output (200 proof) can dramatically improve process efficiency. Precise density measurements, detects exactly when the ethanol reaches the anhydrous threshold (zero moisture content), so that the dehydration process can meet its target without overreaching, thereby obviating the need for any wasteful comfort margin. As with the percent-solids measurement application discussed above, the new generation of Coriolis meters with single, straight tube design, has proven to be a highly accurate and reliable solution for realtime monitoring of alcohol proof during the rectification and dehydration processes. The continuous trends data allows for instantaneous correction of process upsets and ensures the consistent and tight control of proof values in final product. This process control is critical to meeting quality control specifications and to increasing throughput and profitability. An important advantage of the state-ofthe-art Coriolis meters, such as the KROHNE 7000 Series T80 OPTIMASS meter, is the ability to measure multiple parameters in a single device proof, density, temperature and flow. Previously, plant operators needed to purchase, install, calibrate and maintain several separate devices to perform all these functions: typically one meter would measure density and proof on a slipstream, and additional instruments would be installed in the main line to measure temperature and flow. More Efficient Dryer Operation Single straight tube Coriolis meters, with multiple-parameter capability, also provide high pay backs when installed on the evaporator syrup draw. Installations of an online flow/density meter at the intermediate and final stages of the evaporator process have successfully demonstrated significant reductions in energy consumption, by February 20, of 12

9 tightly controlling the syrup percent and flow rate through the evaporators. Running syrups at higher solids is desirable, because less moisture means less work for the dryers and therefore lower energy consumption in the drying process. Higher solids percentage (i.e., lower moisture content) can be achieved by reducing the flow rate through evaporators which increases retention time. Unfortunately, lower flow rates can cause extensive build up inside the lines and can also affect the evaporator efficiency, and line plugging can cause costly downtime. For these reasons, plant operators need to manage the process to maintain the optimal solids level at all times. As with other processes, the continual, automated measurement by coriolis meters facilitates better control than offline lab samples. The single, straight tube design of the KROHNE Coriolis meters mitigates the maintenance and reliability problems that can plague conventional "bent tube" Coriolis meters, especially given the high solids content and viscosity of the syrup. The KROHNE Coriolis meters employ adaptive sensing technologies (AST), a patented design makes accurate measurements even of mixtures with high viscosity or solid matter content and the straight-tube design causes less pressure drops than other flowmeters, resulting in lower energy costs. Volumetric Flow metering Upstream of fermentation, dry-milling ethanol plants make extensive use of inline electromagnetic flowmeters to measure flows containing high solids content, such as backset, corn slurry, mash, beer, as well as on whole, thick and thin stillage. Most magnetic flowmeters available in the market today use a pulsed DC technology which has replaced older AC technology that had inherent problems with drift and zero stability. However, pulsed DC magnetic flowmeters have performance limitations, especially with noisy applications such as slurry. These problems can be addressed by using the KROHNE IFC3000 signal converters with digital noise filtering and a low-noise electrode configuration. For demanding, highsolids applications, KROHNE s advanced magnetic flowmeters provide self-diagnostic capabilities for detecting sensor coating degradation and predicting electrode or liner failure, in order to minimize failures and expensive downtimes. The multiple-parameter capability of some magnetic flowmeters, such as the KROHNE OPTIFLUX 4300C, provides an added benefit during CIP procedures. The CIP wash includes flushing the lines and recirculation chemical in a specific sequence, such as acid-wateralkali-water. By using the conductivity measurement feature built into the flowmeter, the process can be February 20, of 12

10 controlled automatically and remotely to ensure a more efficient CIP cycle, facilitating chemical reuse and reducing waste. Applications downstream of the rectification process pose a challenge for magnetic flowmeters due to very low conductivities. In such cases, alternative technologies, including vortex shedding and ultrasonic, provide a better solution. Flowmeters with moving parts, such as turbines and paddle wheel, are prone to coating and mechanical failure. The KROHNE UFM3030 inline triple beam ultrasonic flow meters have demonstrated better value over competing vortex shedding technologies due to high accuracy and repeatability over a wider turndown. Controlling Steam Consumption Flowmeters also play a critical role in controlling energy consumption by measuring the flow of steam used extensively in cooking, dehydration and evaporation. Steam measurements are performed on a mass flow basis, even though the high moisture contents in saturated steam vapors causes failure of true mass flow sensing designs such as Coriolis and thermal mass meters. Consequently, volumetric devices such as orifice plates or vortex meters are extensively used. Unfortunately, most of these volumetric devices measure the velocity of steam in the pipeline and compute volumetric flow rate from the line size. When outputting mass flow, they use a fixed density correction to convert volumetric flow to mass flow, usually calculated around a fixed operating pressure. This fixed correction will cause significant errors in the final mass flow during inevitable changes of the operating pressure (density) of the steam. Field tests show that a 10% change in saturated steam line pressure can cause a fixed-density compensated meter to over/under read by up to 25%, even though the primary volumetric measurement from meter is well below 1%. The KROHNE OPTISWIRL 4070 multivariable steam meter was designed specifically to overcome these challenges. It precisely measures flow rate, pressure and temperature with integrated sensors and provides accurate density compensation for highly accurate mass flow measurement independent of operating conditions. The fully-welded stainless steel construction makes the measuring tube of the OPTISWIRL highly resistant to pressure, temperature, corrosion and aging and provides high immunity to water hammer even of wet steam applications. Accurate steam metering is critical not only for controlling the flow of steam and regulating temperature, but it also provides a dependable means for determining loss and wastage. February 20, of 12

11 Improved Inventory Management Level meters play an important role in improving the plant s bottom line by enabling tighter inventory management and process control. For example, accurate measurement allows close tracking of the use of the enzymes in the slurry tanks during the mash preparation process, and of the chemical consumption in the clean-inplace routines. Measurement of the level in fermentation tanks enables the regulation of foam, which can be a challenge when controlling a fermentation process. Level measurement is also critical to operating reboilers and distillation tanks, as well as final storage tanks. Simply put, you need to know exactly how much material you have, and how much you are using. Reboilers, Distillation Tanks and Final Storage For reboilers, distillation tanks and final storage tank farms, the KROHNE BM26A side mount level gauge offers high reliability over technologies that have problems measuring under tough conditions of vacuum, high temperatures, vapor phases and foaming. Some level measuring systems deployed use hydrostatic pressure to measure from the bottom of the tank. This is an indirect method of measurement that assumes a constant density to determine the actual surface level. However, when the temperature changes, the density of the medium will change, causing a change in pressure, even though the level of material in the tank has not changed. Pressurized tanks need additional compensation. An alternative approach is to use noncontact radar or guided radar level devices to directly detect the level from the top of the tank. These methods measure distance and they are unaffected by pressure changes, vapors or tank pressure. Top-of-tank placement also makes repair or replacement easier. Bottom-of-tank implementations can only be serviced when the tank is empty a rare occurrence in active, high-volume plants. Return on Investment Ethanol production holds great promise for our country s energy future and also provides an excellent opportunity to support our domestic agricultural economy and communities. Investments in plant infrastructure today will yield benefits for years to come. As more and more plants come online, competition naturally increases among producers. As the market matures, it will demand the highest possible quality at the lowest price. In this economical environment, producers will maintain profitability only by controlling production costs, reducing waste, and conserving energy wherever possible. February 20, of 12

12 Investing in the plant s measurement systems both during new construction or retrofit, is one area that has been proven to offer high returns in the form of increased productivity and lower costs. February 20, of 12