7.8 ± 0.9

tive to distillation under 1.33 kPa, but both pressure levels shall not be applied consecutively in the same distillation run. The reflux ratio applied shall be 5:1 in all cases, including distillation at 0.266 kPa.

X2.7.3 Performance Verification—Determine the ECP and the efficiency Arminimum at three appropriate pressure levels, but only at the highest cut point usually attained at that pressure level.

X2.7.4 Data Interpretation-Efficiency:

X2.7.4.1 Regression of the efficiency (Aminimum) obtained against the AET should indicate a best fit for linear regression and produce a straight line more or less parallel to the lines given in Fig. X2.1. The regression line shall not exceed the upper or lower limits shown in Fig. X2.1. Individual points should be randomly spread along the regression line and be within 0.7 theoretical plates from the regression line. If one point is outside these limits, rerun the appropriate GCs, redo the efficiency calculation, and check again. If this check fails or more then one point fails this criterion, the performance of the column shall be considered suspect. Consult X2.7.4.2-X2.7.4.4 for possible causes. Take corrective action and repeat the whole procedure, including the distillation (see Note X2.6).

X2.7.4.2 If the regression line is curved, yielding relatively high efficiency (Aminimum) values at the lowest pressure level, it is an indication for unacceptable heat loss from the column. Check Annex A3. The reverse is true for an overhead column (see

X2.7.4.3 If the regression line is not parallel and yielding a steeper slope, it is an indication of unacceptable heat loss in the column (check Annex A3). The reverse is true for an overheated column (see Nonparallel regression lines can also be an indication of the application of a non-approved and non suitable packing (see 6.1.3 and Table 1).

X2.7.4.4 If the (parallel) regression line is located outside the upper or lower ^minimum limits, or both, the following causes might be applicable.

(a) Efficiency of the packing too high or too low. Check Table 1 or Annex A1, or both. If appropriate, correct the efficiency by adding or subtracting packing material from the column.

(b) Reflux ratio too high or too low. Check Annex A7.

(c) Incorrect distillation rate. Consult 10.3.2 and 10.4.5.

X2.7.4.5 For a (shortened) periodic verification of efficiency, the three values for efficiency (Aminimum) obtained shall be located inside the band for the regression line obtained from the full performance check and more or less parallel to that regression line. If one or both criteria are not met, it is an indication that the performance of the column has changed. For possible causes refer to X2.7.4.1-X2.7.4.4.

Note X2.6—It is recognized that the setting of acceptable tolerance limits to trigger corrective action is the responsibility of the laboratory. However, it is recommended that they do not exceed the limits as indicated.

X2.7.5 Data Interpretation-Cut Point:

X2.7.5.1 When using an average crude oil (30 to 40 API) distilled under TBP (15/5) conditions, it is expected that the difference between ECP and AET will not exceed 0.7 times the reproducibility of the test method. The reproducibility of the

Cutpoint (ECP), °C

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