The Proof Is In the Profit

Ethanol research center investigates new technologies to benefit the biofuel industry
By Terry Lash | January 04, 2010
The National Corn-to-Ethanol Research Center in Edwardsville, Ill., operates a round-the-clock pilot plant facility for producing and analyzing biofuels and their coproducts. Public and private entities such as biofuel manufacturers, seed-trait producers, enzyme manufacturers, equipment providers, and other clients use the NCERC.

The Center's core mission is demonstration research for the validation of new process technologies, products, and equipment that could potentially improve biofuel production and optimize the value of coproducts such as distillers dried grains. The facility's clients use the data generated from the piloting studies to make their own processes more efficient and productive.

The NCERC's 36,000-square-foot facility contains research labs where ideas can be tested and perfected on a small scale before being scaled-up in the pilot plant. In addition the Center has provided laboratory and process training to more than 500 people. NCERC is continually searching for and implementing new state-of-the-art laboratory analytical methods and process measurement technologies to expand and improve the quality of the data the center provides the client.

Ethanol Distillation Process
Rapid feedback on changes in distillation performance is important for the successful operation of fuel ethanol plants. Any upset in the purity of the final product can be detrimental to the plant's smooth operation and its profits.

The distillation unit operators in fuel ethanol plants typically depend on a semi-hourly hydrometer to determine the proof or alcohol concentration of the final product. This method involves drawing an alcohol sample and using a hydrometer to determine the specific gravity of the fluid. To determine the ethanol concentration from a specific gravity measurement, the operators must measure the temperature and consult a table to determine the alcohol concentration.

On a more infrequent basis, laboratory personnel will use the Karl Fischer titration method to determine the alcohol concentration. The accuracy and precision of this laboratory bench method is excellent, but the time needed to collect and analyze the samples can be problematic when operators are troubleshooting distillation unit problems. The lag time between a change in the reported ethanol concentration and the plant operators being notified to make adjustments to bring the distillation unit under control can be substantial.

A drop in ethanol proof can occur for many reasons. For example, flooding of the rectifying column may cause material that is not fully distilled to be carried over into the plant's 190 proof tank or molecular sieves. The molecular sieves are designed to remove water from a 95% pure ethanol stream. So, a carryover of incompletely distilled ethanol can cause the molecular sieves to become overloaded and unable to absorb the excess water.

Lower ethanol proof can also result from a failure within the molecular sieve system itself. For instance, failure of a purge valve would prevent removal of the water collected in the sieve material during the regeneration cycle, which would inhibit the absorption of water when the next absorption cycle begins. Also over time, the molecular sieve's ability to absorb moisture can decline due to foreign material clogging the pores.

Economics
Another concern for ethanol producers is to manufacture ethanol with an acceptable water content, which is specified by the ethanol buyer. The situation of producing ethanol with a higher concentration of alcohol than required can be a major cost to the plant. For example, if a 50 MMgy plant sells ethanol at $1.75 a gallon and produces ethanol that is 0.1 proof (.05 percent) higher than required by their contract, the plant will lose about $0.000875 per gallon. This is approximately $44,000 per year or $88,000 per 0.1 proof in a 100 MMgy plant. This savings goes straight to the bottom line.

Distillation Unit Performance Monitoring
As a fermentation piloting facility, NCERC performs many dissimilar fermentations to optimize a process during a research trial. Typically the complete fermentation of a material takes place between 40 and 80 hours depending on the organism and the feedstock. Fermentation research can involve adjusting some parameter in each fermenter set to find out which process condition results in the best alcohol yield. The fermenter batches containing different levels of alcohol are pumped to a distillation unit to remove the alcohol and send the remaining solids and water for further processing into coproducts.

In one test, plant operators needed a method to continuously monitor the performance of the plant's distillation unit. NCERC chose a novel approach to continuously monitor alcohol concentration of its distillate using a Coriolis type mass flow meter. Providers of Coriolis flow measurement technology (density, mass flow) can provide an optional software programming function that allows the process engineer to program the instrument with data tables that relate temperature, density and concentration data. NCERC engineering wanted to validate and apply this technology to turn the instrument into a proof-meter to relieve the plant operations team of the responsibility of measuring the alcohol content using the manual hydrometer method and temperature reference chart.

An Endress+Hauser Promass 83F mass flow measuring system was supplied to NCERC with the "special density" calibration option, which optimizes the density measurement so that the flow meter can also be used as a densitometer. With this calibration, if the process fluid changes, the special density calibration compensates for the temperature effect on both meter and fluid. The instrument manufacturer then also programmed the unit to measure alcohol proof using ATF tables, and the flow meter was programmed to indicate and transmit concentration, density, and temperature.

Validating Method for Proof Measurement
NCERC installed the Promass instrument in a pumped recirculation loop on the plant's shift tank containing 200 proof alcohol. As the 200 proof alcohol circulated through the instrument, the displayed data indications for density, concentration and temperature were logged. At the same time, a sample was collected, and the ethanol concentration was measured by Karl Fischer titration and the density was measured using an Anton Paar densitometer.

The laboratory measurements of concentration and density were compared to the Promass values. The temperature of the 200 proof alcohol was varied over time and multiple data sets were collected from the Coriolis instrument, the Karl Fischer titration and Anton Paar laboratory density instrument. The alcohol was then diluted with water progressively over time, and the Coriolis meter temperature, concentration and density readings were compared with the laboratory measurements. Data was collected at eight concentrations ranging from 200 to 195 proof (100 percent to 97.5 percent) and a map relating ethanol concentration to temperature and density data was generated.

Analysis
In Figure 1, the pilot test showed that the Promass Coriolis meter achieved accurate and repeatable density readings compared to the Anton Parr laboratory instrument, with the original calibration coefficients entered by the instrument manufacturer.

The average offset between the two densities was 0.0004 gm/cc, which is within the Coriolis instrument manufactures specification of 0.0005 gm/cc maximum measuring error for density. This average offset was then used to adjust the density coefficient within the meter to obtain a more accurate reading.

Figure 2 compares the ethanol concentration displayed by the Promass meter to the concentration calculated using Karl Fischer titration, which is the laboratory standard for the industry. The average error was 0.12 percent (v/v), which corresponds to about 0.24 proof. This demonstrates that once properly calibrated, the Promass 83F is capable of determining ethanol proof with acceptable accuracy. The immediate feedback that the instrument can provide to operations staff should improve overall distillation performance in the typical ethanol plant.

Experiencing 199-Proof Results
These tests show that a Coriolis-based mass flow and density instrument equipped with special software has the accuracy and repeatability comparable to established laboratory methods. Therefore, this technology is a reliable solution for on-line proof measurement.

While this instrument will not replace established laboratory methods for determining ethanol proof, it is a valuable supplement to these methods. This instant on-line measurement will give the producer better information to control their alcohol content in the ethanol, which is especially important in light of tight profit margins. For the plant operations and maintenance staff, it will serve as an excellent troubleshooting tool by providing an earlier indication of process upsets than current industry methods. This technology also has the potential to cut production costs by identifying upsets sooner, thus saving time and money by reducing the amount of rework required to correct an off-spec ethanol stream.

Other applications of this measurement technology can be applied in an ethanol plant:
>Monitor the solids level of syrup produced in the evaporators
>Monitor the solids level of the corn slurry
>Monitor the sodium hydroxide concentration for clean in place fluids. EP

Terry Lash is a research engineer with the National Corn to Ethanol Research Center. Reach him at tlash@ethanolresearch.com