(52e) 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 Si-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 be accurately determined because: 1) feed samples are not collected frequently enough to represent the feed and 2) the standard ICP test used by the industry does not accurately measure the silicon species present in the coker naphtha feed.  This makes the cycle life of coker naphtha hydrotreaters difficult to predict.  Hence, refiners often change out coker naphtha hydrotreater catalyst based on fixed cycle length or Si slippage.  This either underutilizes the Si 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) technique 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 indicated a combination of representative sampling and highly accurate Si analysis can improve the quality of the estimates of silicon accumulation in a catalyst system.