ASTM D2892 for distillation of crude petroleum (15-Theoretical Plate Column)
7. Verification of Apparatus Performance
7.1 Test Method D2892 provides a standard framework for the laboratory distillation of crude oils in order to produce cuts of defined quality (for further testing) and the concurrent production of a boiling point curve. As the quantity requirements and cut points might be widely different between companies and application areas, this test method does not standardize on equipment design but on equipment performance.
7.2 The nature of the test method (the use of large sample quantities and very time consuming) and the nature of the product being tested (highly volatile and unstable material), precludes the use of standard statistical control techniques. Moreover, this test method does not produce a single result, nor is the series of results (the boiling point curve) derived under rigidly defined conditions (see 4.2).
7.3 Equipment performance in the context of Test Method D2892 consists of two elements; the efficiency of the column, defining cut quality, and the correctness of the cut point (AET), defining the boiling point curve.
7.4 The correctness of the AET is mainly, but not only, dependent on the accuracy of the (vapor) temperature and (operating) pressure sensors (Annex A6). Other factors affecting the accuracy and precision of the boiling point curve are:
7.4.1 The location of the temperature and pressure sensor (Annex A4).
7.4.2 The dynamic response of the sensors (Annex A5).
7.4.3 The correct operation of the reflux divider (Annex A7).
7.4.4 The heat loss from the column (Annex A3).
7.4.5 The efficiency of the column (Annex A1).
7.4.6 These factors are basically covered through the appropriate annexes. However, it should be realized that this takes only care of individual components and does not cover the combined effect of small deviations. Moreover, the aforementioned tests are all done under more or less static conditions, not necessarily representative for the behavior of the system under actual dynamic conditions.
7.5 Cut quality is mainly defined by the efficiency of the column (Annex A1), but is also affected by:
7.5.1 The correct operation of the reflux divider (reflux ratio) (Annex A7).
7.5.2 The heat loss from the column, that is, internal reflux (Annex A3).
7.5.3 The dynamic hold up of the column (Annex A2).
7.5.4 Column efficiency is covered in this test method through Table 1 and Annex A1. However, Table 1 only provides an assumption on efficiency and is not a guarantee. Annex A1 only provides a check under static conditions, infinite reflux ratio, rather low actual temperatures and a binary component system. Hence, although there is some safeguard on standard performance, through conformance to Annex A1, Annex A2, Annex A3, and Annex A7, again it remains questionable whether this is truly representative for columns under actual operating conditions.
7.6 Theoretically, an overall performance check, like the one described in Appendix X2, is capable of verifying the performance of a column and the correctness of the AET under actual operating conditions. Appendix X2, in principle, measures the combined effect of all factors affecting the results of Test Method D2892.
7.6.1 The minimum tray number as defined in Appendix X2 is a measure of overall cut quality, and the difference between nominal cut point (AET) and effective cut point (ECP as defined in Appendix X2) provides a measure for the correctness of the AET. However, insufficient data are available right now to define the allowable tolerances in a rigid statistical way. Moreover, the test method described is very labor intensive and precludes its use on a regular, short time interval basis and, therefore, its use as a mandatory statistical control technique.
7.6.2 Appendix X2, therefore, provides only recommended guidelines for statistical control on column performance, including both correctness of AET and column efficiency. It is the responsibility of the laboratory to provide for sufficient quality controls to guarantee conformance to the test method.
