Gasoline blending is where streams of gasoline of different grades are blending into one mixture, this is a common process undertaken at refineries. The composition of the final blend determines the RVP (Reid Vapour Pressure) of the mixture, which must be set depending on where the fuel will be used, with colder climates requiring gasoline blends with a high RVP.
One of the key steps in the gasoline blending process is distillation. A study has been carried out to determine the performance of an on-line gasoline analyzer, the MicroDist, when compared to the Optidist analyzer which adheres to ASTM D86. The aim of the research was to see if the MicroDist can be used for on-line analysis of blending for the purpose of process control and optimization.
Testing across samples with a large range of vapor pressures demonstrated areas of improvement for the MicroDist such as enhanced sample cooling to reduce sample consumption and to allow more samples to be ran on small (less than 4 L) samples, revising the software to increase internal pressure to 300 kPa, and improvements in the handling of samples to prevent sample loss.
Following the addition of these improvements more research was carried out to determine the performance of the MicroDist in terms of precision and relative bias to the lab for gasoline samples with a wide range of vapor pressures, which correspond to different seasonal gas blends. The benchmark for success was a ≤3 °C bias (between the process and the lab) from IBP (initial boiling point) to FBP (final boiling point).
The MicroDist was configured to imitate a process condition analyzer with single sample injections. Sample cooling was also provided to reduce the temperature 15 °C below the IBP with all sample lines insulated.
An established refinery provided one gallon of eight EtOH free gasoline samples (labeled R1-R8) to act as unknowns and 20 L of a control (QC) EtOH free sample. The unknowns were base gasoline and also blended samples which covered the range of vapor pressures present in gasoline blending. An aliquot of all samples was provided to the refinery lab for D86 analysis on an OptiDist system for comparison.
Testing took place over four days, with site precision tests occurring over the first two. Replicates were analyzed at high volumes and sample analyses were separated from each other using a recirculating gasoline stream between tests, mimicking what happens in the blending process.
The results underwent statistical analysis to show that the deviation at each measurement point was either less than or equal to the lab test results for each boiling point (initial, 5%, 10%, upwards to final). Having passed the precision test, the unknown analysis could be carried out.
Figure 1: QC Sample Data Results Overlay. The first two days of quality control testing demonstrated a site precision for the MicroDist that was better than or equivalent to the refinery D86 laboratory instrument.
A test matric was produced which interspersed testing of the QC throughout the day to ensure no other variables influenced the study. The basis between MicroDist and lab D86 was determined to show the difference between lab and analyzer results. The MicroDist agreed well with lab results over the entire vapor pressure range.
Figure 2: T50 Agreement Between Lab and Process Met ≤3 °C Performance Specification
Table 1: T-50% Distillation Results MicroDist vs. Lab Result Agreement for Full-Range of Vapor Pressure
The MicroDist showed excellent agreement with lab results over the full range of sample vapor pressures. This testing shows that the on-line MicroDist system fits the criteria of ≤3 °C bias (between IBP and FBP) between the lab and process, validating that the MicroDist can be used for real-time analysis and
This information has been sourced, reviewed and adapted from materials provided by PAC L.P.
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