Q & A > Question Details
Issue : Since commissioning our coker naphtha yield remains always on higher side by 1 to 1.5 wt%. The quality of the Naphtha end point also remains on higher side 145-150 Deg C than the design value of 125-130 Deg C. We are operating our fractionator with top temperature 99 Deg C & pressure of 0.56 Kg/cm2 G. Top temperature, reflux flow rate & pressure are same as design conditions. We tried simulating the scenario but could not get any clues from that.
1. What may be the probable causes of deviation in Naptha end point from design?
2. To what extent can we reduce our top temperature, to drop heavy end of Naptha to LCGO cut below?
3. What are concerns foreseen for low fractionator top temperature operation?
4. To what extent Naptha quality degrades if section trays are damaged or reflux distributor is not working properly?
03/09/2013 A: Arvinder Singh Sahney, Indian Oil Corporation Ltd, sahneyas@yahoo.com
Point wise reply/ observations are as follows:
1. we have to look into the no. of theoretical stages available for actually allowing fractionationation in the zone between LCGO and Naphtha. Normally, we have a Naphtha splitter at downstream of the stabilizer/ debutanizer and there we can trim the light naphtha as per requirement. In order to achive light naphtha specifications (125 - 130 °C EP); in a conventinal delayed coker main fractionator would be quite difficult.
2. depends on the Dew point + 15°C
3. low fractionator top temperature will have corrosion issues duw to water condensation
4. in case of trays/ reflux distribtor suspected to be damaged; we expect spikes in Naphtha quality, inconsistent colour of Naphtha. Pressure drop in the zone would also give some indication
04/07/2013 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
Agree with Mr. Ragsdale on yield predictions. To move the cutpoint, increase your LCGO draw rate while reducing the overhead temperature. Check your water dewpoint in the column - you don't want to go so cold that you are condensing water on the trays - stay 15-20°C above calculated dewpoint. If you go too cold you may get salt deposition (and corrosion), or the potential for water boiling off to steam in a non-steady-state manner, causing tray damage. Well documented in Kister's book.
04/07/2013 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
(1) Yield predictions for delayed cokers are not very accurate.
(2) When comparing with the simulation, how does the 5-95 deg gap compare?
(3) As long as you are yielding full range naphtha overhead, as opposed to just light naphtha, top temperature should not affect corrosion rates.