Accurate Estimation of Si Accumulation in Coker Naphtha Hydrotreaters Using an Improved Direct Injection ICP (ICPDIN) Analysis and Hot Loop Feed Sampling

Delayed coking processes typically use silicon containing oils, polydimethylsiloxane (PDMS), to suppress foaming in the coker drums. Inside the coker, these high molecular weight oils crack into fragments (e.g. dimers and trimers of the dimethylsiloxane). The majority of these fragments boil in the naphtha range and are processed in the coker naphtha hydrotreater where they absorb onto the catalyst surface causing irreversible loss of catalytic activity.

The cycle life of a coker naphtha hydrotreater is normally dictated by: 1) the silicon capacity of the selected catalyst system and 2) the amount of silicon entering with the coker naphtha feed. The silicon capacity of a selected catalyst system is known. However, the amount of silicon accumulated on a catalyst system cannot usually be accurately determined from feed properties because: 1) feed samples are not collected frequently enough to represent the feed and 2) the standard Inductively Coupled Plasma (ICP) test used by the industry does not accurately measure the silicon concentration in the coker naphtha feed. This makes the cycle life of coker naphtha hydrotreaters difficult to predict and therefore often results in refiners changing out coker naphtha hydrotreater catalyst based on fixed cycle length or silicon slippage. This either underutilizes the silicon capacity of the catalyst system or results in an unplanned shutdown.

This paper focuses on a case study in which a hot loop feed sampling station is used in combination with an improved silicon analysis applying Direct Injection ICP (ICPDIN) to estimate the amount of silicon accumulated in a naphtha hydrotreater as the cycle progressed. At the end of the cycle, spent catalysts from the unit were analyzed to determine the actual accumulated silicon. Results demonstrate that the silicon deposition estimated using hot loop sampling and ICPDIN was within 10% of the silicon deposition determined from spent catalyst analysis. The improved accuracy of the estimate would have allowed the catalyst cycle life of the unit to be extended up to 4.5 months beyond the scheduled 12 month cycle length.