becomes erratic in many circumstances due to changes in crude composition. The refinery does not have an online tool to analyse the crude composition on a continuous basis and provide input to the inferential model to correct the prediction. Therefore, the APC inferential is not
AGO D95 % recovery temperature
300 310 320 340 330 360 370 380 350 390
11.9.21 31.10.21 20.12.21 8.2.22 30.3.22 19.5.22 8.7.22
27.8.22 16.10.22
5.12.22
Figure 2 AGO D95% recovery temperature
has an installed capacity of 4.3 MMTPA (with 10% overdesign). The unit is operated at 600 TPH (max) capacity. The feed stream mainly consists of delayed coker unit (DCU) light cycle gas oil (LCGO), petro fluid catalytic cracking unit (PFCCU), light cycle oil (LCO), CDU AGO and vacuum residue (VD), along with kerosene and naphtha streams from the CDU and DCU units. Due to the processing of various crude types and continuous changes in crude density, it is very difficult to maintain the FBP of AGO, VD, LCGO, and LCO at the desired level on a steady, continuous basis. Since laboratory analysis for these streams is performed only once a day or every other day, it is not possible to take timely action and maintain consistent product quality in terms of distillation. Limitation of advanced process control BPCL Kochi Refinery has an advanced process control (APC) controller for all process units across the refinery. For example, there is an APC inferential for the AGO D95% recovery temperature in CDU3, and the APC can take action to control this within the specified range. However, the APC inferential prediction
consistently helpful in predicting the DHDT feed qualities. As a result, feed quality fluctuations lead to product quality fluctuations, and when trying to bring the product on-grade by increasing the catalyst bed temperature, it results in catalyst deactivation. Figure 2 shows the trend of fluctuation of AGO D95% recovery temperature. Figure 3 shows the trend of fluctuation of the DHDT product diesel sulphur. Multiple incidents of product sulphur peak can be observed during the period, mainly because of fluctuation in the feed diesel distillation range. Discussion with the process licensor Various discussions were held with the process licensor to find a suitable solution for the intermittent sulphur peak issue. However, it was understood that there is no other way to control product sulphur effectively without proper control of feed quality. Otherwise, higher catalyst bed temperatures would be required, and sometimes over-treating would also be necessary. This leads to faster catalyst deactivation and ultimately shortens the catalyst cycle length. Table 1 shows the various factors that impact on the BPCL KR DHDT unit’s product diesel sulphur content.
Investigation by internal committee recommendations The following actions were suggested by the internal committee after a long investigation of the sulphur peak issue: u Kerosene content (designed for DHDT feedstock) should not be
Product diesel sulphur (ppmw)
20
15
10
5
0
26.9.20 15.11.20
4.1.21
23.2.21
14.4.21 3.6.21
23.7.21
11.9.21
31.10.21
Figure 3 Product diesel sulphur (ppmw)
Refining India
54
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