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Applying a Measurement Automation Strategy

By Hemant Narayan | November 13, 2007
The conversion of corn into fuel ethanol is gaining popularity and is an appealing business opportunity for farm communities and agribusinesses. Investors are attracted by the prospect of more than doubling the monetary value of a bushel of corn by converting it into ethanol, animal feed and other value-added coproducts.

In the United States alone, more than 135 plants are producing ethanol and another 63 are under construction. Dozens more are in various stages of planning, with total domestic production capacity expected to double during the next five years. Based on expectations of increased ethanol production, farmers planted more corn in 2007 than in any year since 1944.

The rapid increase in production reflects the expanding market for ethanol, driven by growing recognition of its economic, social and environmental benefits. The Energy Policy Act of 2005 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 2012. Current estimates indicate we will surpass these benchmarks well before then.

Fourteen billion gallons of ethanol per year would be required if an E10 blend was instituted in the United States. Include a strong push from state legislatures for the market adoption of E85 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.

Profit Margins, Process Control
However, strong demand represents only one part of the profitability equation, which is negatively impacted by inefficiencies in the production process, such as excess energy consumption, poor yields, wasteful use of raw materials, process chemicals and enzymes.

The cost of natural gas and other utilities is especially problematic, as these items far exceed the other major cost components, including plant construction and labor. For example, in a typical plant, the natural gas that fuels boilers, evaporators, dryers and other equipment comprises 15 percent of the cost of producing a gallon of ethanol. For a 100 MMgy plant, that translates to millions of dollars in operating costs. Conserving energy by just 1 percent can result in tens of thousands of dollars in profits.

The elevated price of natural gas is not the only financial challenge. When an ethanol production facility comes on line, the increased local demand for corn puts upward pressure on corn prices and poses problems with feedstock variability. Some ethanol producers report that ethanol yields vary as much as 7 percent, depending on the variety of corn. Lower yields add considerable financial riskan anathema to investors.

For these reasons, achieving consistent profitability can be a tough challenge for ethanol producers, who have little influence on the market price for their product and the price of feedstock and natural gas. Ethanol producers can tightly control their own manufacturing process, and thereby produce a consistently high yield, while minimizing the consumption of energy and raw materials.

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 and fifth step of the Six Sigma DMADV (define, measure, analyze, design and verify) process. 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.

Meter Selection, Placement
Careful selection and installation of measurement devices is important for three reasons. The first is measurement accuracy. Measurement devices give visibility to the process, allowing plant operators to "see" what is going on inside the pipes and production systems, and the fill level of each tank. 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.

The second reason is 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 self-diagnostics and alarm 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.

The final reason is maintenance costs. High-performance measurement devices may have a higher initial purchase cost, but they typically have lower total lifetime costs because of the elimination of downtime for recalibration and repair. Some manufacturers 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. Flow meters measure the speed, volume and, in some cases, mass of liquids and gases that move through a pipe, including beer, stillage, syrup, enzymes, water, steam, carbon dioxide and natural gas, as well as methane used as an alternative fuel. A wide variety of flow metering technology is available, including Coriolis, magnetic, ultrasonic, variable area and vortex-based devices.

Density meters measure the percent solids during feedstock preparation. They 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 meters indicate the volume of solids or fluids in a tank for process control and inventory management. Mechanical devices that use hydrostatic pressure are being replaced by more accurate and reliable direct-level measuring technologies such as guided wave radars and non-contact methods that employ radar and ultrasonic waves.

At various points in the production process (e.g., cooking, fermentation and distillation), it is also necessary to measure and control temperature, pressure, pH, conductivity and moisture.

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 available technology suitable for each type of required measurement.

Performance is not just a question of selecting the best technology and device for the particular application. Proper placement in the pipeline or tank is also critically important to ensure accurate measurement and avoid distortions, clogging and damage. Achieving good performance and controlling total lifecycle costs also depends on preventive maintenance and periodic calibration checks on some instruments.

This article provides examples of how accurate measurement improves efficiencies and contributes to profitability in a typical dry-grind process during the five main parts of the production process: conversion, fermentation, distillation, dehydration and recovery of coproducts (e.g., carbon dioxide and distillers grains).

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 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, on line 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 percent). 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, 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 previously 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. 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, Dehydration
Rectification and dehydration provide a textbook case of the importance of accurate measurement for effective process control. Rectification and dehydration are common to any fuel ethanol process, whether wet milling, dry grind or 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, reducing moisture content from 5 percent to 0 percent.

If the process fluid moves too quickly through the molecular sieves and some moisture remains, then the entire batch must be run through the dehydration system againan inefficient step that wastes energy, 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 detect 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, the new generation of Coriolis meters with single, straight-tube design has proven to be a highly accurate and reliable solution for real-time monitoring of alcohol proof during the rectification and dehydration processes. The continuous trends data allow for instantaneous correction of process upsets and ensures the consistent, tight control of proof values in final product. This process control is critical to meeting quality control specifications and increasing throughput and profitability.

An important advantage of the state-of-the-art Coriolis meters is the ability to measure multiple parametersproof, density, temperature and flowin a single device. 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 paybacks when installed on the evaporator syrup draw. Installations of an on line flow/density meter at the intermediate and final stages of the evaporator process have successfully demonstrated significant reductions in energy consumption by 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 also affect the evaporator efficiency. 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 off line lab samples. The single, straight-tube design of the newest generation of 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 straight-tube design also causes less pressure drops than other flow meters, resulting in lower energy costs.

Volumetric Flow Metering
Upstream of fermentation, dry-grind ethanol plants make extensive use of in line electromagnetic flow meters to measure flows containing high solids content, such as backset, corn slurry, mash, beer, and thick and thin stillage. Most magnetic flow meters available in the market today use a pulsed, direct current technology which has replaced older alternating current technology that had inherent problems with drift and zero stability. However, pulsed, direct current magnetic flow meters have performance limitations, especially with noisy applications such as slurry.

These problems can be addressed by using signal converters with digital noise filtering and a low-noise electrode configuration. For these demanding, high-solids applications, advanced magnetic flow meters 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 flow meters provides an added benefit during clean-in-place (CIP) procedures. The CIP wash includes flushing the lines and recirculation chemical in a specific sequence, such as acid-water-alkali-water. By using the flow meter's built-in conductivity measurement feature, the process can be 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 flow meters due to very low conductivities. In such cases, alternative technologies, including vortex shedding and ultrasonic, provide a better solution. Flow meters with moving parts, such as turbines and a paddle wheel, are prone to coating and mechanical failure.

Controlling Steam Consumption
Flow meters 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 cause 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 used extensively. 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 percent change in saturated steam line pressure can cause a fixed-density compensated meter to over/under read by up to 25 percent, even though the primary volumetric measurement from the meter is well below 1 percent.

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 an accurate mass flow independent of operating conditions. The fully-welded stainless steel construction makes the measuring tube 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.

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 CIP routines. Measurement of the level in fermentation tanks enables the regulation of foam, which can be a challenge when controlling a fermentation process. Level measurements are also critical to operating reboilers and distillation tanks, as well as final storage tanks. Simply put, producers need to know exactly how much material they have, and how much they are using.

Some level measuring systems 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 non-contact radar or guided radar level devices to directly detect the level from the top of the tank. These methods measure distance and 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 emptya rare occurrence in active, high-volume plants.

Ethanol production holds great promise for this country's energy future and also provides an excellent opportunity to support the domestic agricultural economy and communities. Investments in plant infrastructure will yield benefits for years to come.

As more plants come on line, 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.

Hemant Narayan is the industry manager for biofuels and energy for Krohne Inc., a privately owned flow meter manufacturer headquartered in Germany. Reach him at narayan@krohne.com or (800) 356-9464.

The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of
Ethanol Producer Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).
 

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