While planning the agenda for the upcoming RefComm CatCracking meeting, we came across an old question which still provokes a lot of interest; “What can we do to reduce coking in the FCC reactor overhead line, especially near the fractionator inlet?” This question has been asked and answered numerous times. Why is it of interest now? The answer seems to be that, for some at least, the old solutions aren’t working so well anymore.
A quick review of the literature reveals that interest in this issue comes and goes in waves. The first wave appeared as the industry transitioned from bed cracking to riser cracking. Subsequent surges and ebbs appear to correlate with later industry trends such as the introduction of rare earth exchanged catalysts, the introduction of USY catalysts, resid cracking, feed hydrotreating or high severity operation. All of these effect the chemistry and operations in different ways, aggravating or alleviating the formation of extraneous coke but a simple explanation can help us predict these effects.
Overhead line coke forms when heavy molecules condense along the pipe walls. Anything that inhibits condensation will reduce coke formation. Anything that promotes condensation, before the effluent vapor is safely inside the fractionator quench zone, will promote coke formation. The section near the fractionator inlet is particularly vulnerable for several reasons:
- Being at the end of the line allows the most heat transfer, cooling the effluent vapor below its dew point.
- The final horizontal section provides a nice area for liquid to accumulate.
- There is usually an elbow near the fractionator inlet, which can encourage condensed droplets to accumulate along the wall.
- The large flange that connects the reactor line to the fractionator is often left uninsulated to facilitate inspection of the flange bolts.
- High velocity flow patterns at the fractionator entrance may entrain liquid from the fractionator bottom pool.
So, the first line of defense against effluent line coking is to keep the effluent above its dew point and to eliminate horizontal legs where liquid might accumulate. Increased vapor velocity should also help, but could contribute to line pressure drop. Adding steam to increase velocity or turbulence is often a mistake since the steam also cools the effluent vapor. More insulation and careful line design are usually the answer.
But then why does the issue suddenly reappear on units that have operated well for years? Most likely, the answer is that the dew point of the effluent vapor has dropped – more heavy molecules are present. This could be caused by anything that reduces conversion, increased recycle rates, or the removal of light feeds. In this case, more insulation might do the trick since the vapor was at or above its dew point when it left the reactor. However, it could also be due to an increased presence of heavy reactive molecules. Such molecules could polymerize in the effluent line, decreasing the dew point after the vapor has left the reactor. In this case, changes to feed quality, reaction conditions or catalyst formulation might have tipped the balance.
In the past few years we have seen a radical shift in the types of feeds available to the FCC, as well as new catalyst formulations and increased interest in diesel mode operation. Join in the discussion at next RefComm® to investigate which trends seem to correlate with increased coking.