Q & A > Question Details
Would like to get expert advice on crude preheat train. We are not able to get the design preheat temp. In the preheat train the last two exchangers are in series. As per design tube side outlet of last exch. should enter at 323 temp. in the shell side of upstream exchanger with both exchangers operating at MTD of approx. 23-25 degC. However as per current operating conditions there is max. heat recovery in last exchangers with very high MTD and the outlet temp. is reduced to very low 290 degC. Due to which the upstream exchanger is operating at very low MTD 8 degC.
Based on this finding will it be advisable to partially bypass the last exchanger to achieve the tube side outlet design operating temperature or there is some other issue?

Additional Information:
Thank you for the replies. I have gone through detailed preheat train analysis and found that in one of the analysis where we were getting the desired preheat temp. there was no cross pinch in the same exchanger as the inlet temp. had sufficiently increased. And this was possible since the flow was meeting the design data. Thus we might have to increase the outlet temp. (keeping in mind the flowrate) from last exchanger but that again has to be analysed since there is one more exchanger in the upstream of train which is in series to these exchangers.
 
Answers
25/05/2015 A: Bruce Carr, Suncor Inc, matukaze1@hotmail.com
I don't think fouling is an issue... the last exchanger is exposed to the highest inlet heat so it looks like it is very efficient. I agree it would be better to have both exchangers share the load as the last exchanger will undoubtably foul quicker. I also agree that bypassing this exchanger would help share the load but would cauton that you need to ensure there is adequate flow to prevent coking the inside of the tubes... being in series that would cause a world of pain.
23/05/2015 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
I agree with those excellent answers. Do the calculated observed overall transfer rates check with the data sheets?
22/05/2015 A: Marcello Ferrara, ITW SrL, mferrara@itwtechnologies.com
If the problem is related to fouling you can consider Online Cleaning the entire preheat train in 24 hours and thereafter resume production under clean conditions.
21/05/2015 A: David Kujawski, Refinery Water Engineering & Associates Inc, dk@refinerywater.net
Exchanger Modeling Software not only will pinpoint the problem, but will prioritize and quantify the impact of every corrective action and process & design variable change. And, it will predict the end results with great accuracy.
21/05/2015 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
It's hard to tell for certain with the information provided. One possibility is that the exchanger(s) are fouled. This is common in the hot preheat train. You can calculate the actual heat transfer coefficient and compare with the design value to see if this is the case. Another possibility is that an operational change is causing this. A change in feed slate or tower operation can shift the heat balances and where they fall, which can lead to a temperature pinch. A third possibility related to the previous one is that you are starting to get vaporization in the preheat train. When this occurs the duty required per degree of preheat jumps, so you are more likely to get a temperature pinch.
Bypassing exchangers is normally more for relieving hydraulic constraints than for increasing heat recovery. If you bypass crude around the exchangers, the crude actually leaving the exchangers will get hotter, but the combined stream when after the bypass mixes back in will be lower in temperature. Bypassing exchangers can also be done to shift where the heat input is coming from to help meet other process objectives like increasing pumparound duty or relieving a temperature constraint on a product rundown stream.
21/05/2015 A: Mike Watson, Tube Tech International Ltd, mike.watson@tubetech.com
Firstly do you know if the shell side is clean?
Similarly the tubeside?