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We are operating our NHT unit at 125% of its capacity. To maintain NHT reactor RIT, its heater firing is very high. As per our observation firing is very high due to heaters absorption heat duty is 55% only. we want to modify our furnace to increase its absorption heat duty, kindly suggest us what modification is required to increase the existing heaters efficiency?
 
Answers
27/10/2014 A: Vineet Singh, IOCL Panipat Refinery, SINGHVK007@GMAIL.COM
We have achieved NHT capacity up to 140% by utilizing available hydraulic capacity. Of course heater was the main debottleneck to increase CIT. You can try to reduce gas to oil ratio to reduce furnace firing. Also we are planning to install HP steam heater upstream of furnace.
22/10/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Apparently, you have the hydraulic capacity to operate at 125% of design and meet product specifications. Exchanger capacity can be added as suggested without increasing reactor section pressure drop. This is achieved by “restacking” the existing exchanges and changing the number of passes in a particular shell if helpful. Those changes will reduce the duty achieved by the existing exchanges. Simply add even more new surface to compensate. This is a very common and relatively inexpensive method for increasing throughput in a hydrotreater. Your current cold side approach temperature is probably 80 degrees F or more. You can go down to 25 degrees with significant results. The practical stopping point, as Keith has pointed out, is where the heater is supplying 15% of the total input to the feed. Below that can result in an unstable operation.
21/10/2014 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Your efficiency is going down because you need to fire harder and have a higher firebox temperature to get the necessary heat transfer into a limited number of tubes. Your options will depend on the type of furnace you have. If it's a horizontal cabin type heater, you might be able to make the heater taller and add some additional rows of tubes to get more heat transfer area.
For any type of heater, you could install an air preheater to recover heat from the flue gas. Economics will depend on the size of the furnace.
Sometimes furnaces lose efficiency due to external scale build up on the tubes. A short term fix if this is part of your problem can be to do a descaling type operation by injecting something like walnut hulls into the firebox to try and knock the scale off of the tubes.
20/10/2014 A: keith bowers, B and B Consulting, kebowers47@gmail.com
What you really need to do is increase heat recovery with much better/larger heat exchangers for feed vs RX effluent. One can safely go up to about 85% recovery of RX heat into feed before furnace turn down limits become problematic.
20/10/2014 A: Satyalal Chakravorty, Sr Consultant, satya1354@yahoo.co.in
The options are 1) improve the preheat thru heat integration to increase CIT of the Furnace. 2) Install APH to improve furnace efficiency 3) use catalyst which requires less operating temp.
Or combination of all.