Q & A > Question Details
In our CDU column we draw off naphtha (overhead) , kerosene , light diesel , heavy diesel and AGO fractions and 4 pumparound circuits (on kero, light diesel , heavy diesel and AGO sections). The top of the column is cooled by reflux (overhead –air coolers-receiver – column) . From a simulation it appears that approximately 55 % of heat from the atmospheric column is wasted in overhead line (air coolers) and the rest 45% is recovered in pumparounds heat-exchangers. We would like to introduce the additional pumparound (TPA) and recover some of the heat in new heat exchanger(s) upstream the desalter - of course the exact location of the added heat exchanger will be analyzed with pinch study. What do you think about the solution of introducing the additional pumparound in order to recover some of the heat which is currently wasted in air coolers? Maybe some other recommendations about recovering this heat to the process.
21/05/2013 A: Manas Banerjee, Centre For High Technology, manasumi@gmail.com
My experience is that despite the corrosion issues in an overhead low level heat recovery, this is an excellent method of improving your unit energy conservation. To take care of the corrosion issues the following points may be kept in mind:
1. The crude/ overhead exchanger may be prone to high rate of corrosion particularly if the desalter functioning is not perfect.
2. It may be preferable to design this exchanger with the overhead in tube side (which being more corrosive) which has lesser dead zones to accelarate corrosion.
3. Instead of bringing in crude at ambient temperature, the crude should be introduced at this exchanger at about 70 Deg C which will reduce corrosion rate due to higher metal temperature.
4. May consider not to re-use the wash water from the boot of the overhead drum if it contains higher chloride than normal.
5. A hot and cold separator system (two drum system) will improve the heat recovery and will also be less corrosive environment due to hot reflux used in fractionator.
6. It may be necessary to take a call on the metallurgy selection/ upgradation of this crude/ overhead heat exchanger to allow higher run length.
Apart from these following points should be in mind on any exchanger train modification:
1. maximise use of hot feed to the downstream units both in quantity as well as in temperature level- it can be a great heat saver.
2. If hot feed can be utilised in downstream unit then it becomes less important to recover the last bit of heat from the Run-down stream and then we can better concentrate in recovery of low level heat from the Circulating reflux streams without letting any of them be wasted in a Cooler.
09/05/2013 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The 'energy conservation' objective is really to reduce the amount of fuel required in the fired heater. The most direct manner of achieving this is pre-heating the combustion air. This may be significantly lower capital cost than revamping both the heat exchanger network and distillation column and internals. This also de-couples overhead flow enthalpy from the rest of the heat exchanger network, substantially improving overall stability.
07/02/2013 A: C S S Narayana, Engineers India Limited, cssnarayana@cht.in
I agree with the opinion of Alan that our enthusiasm to maximize recovery of ‘waste’ heat shouldn’t disturb the ‘distillation’ parameters for which the column has been designed for. While continuous efforts to ‘optimize’ waste heat recovery are very much appreciable, adding another exchanger in the o/h loop calls for careful consideration of pressure drop, etc.
Having said that, preheating crude or DM water or Boiler Feed Water (depending on the available o/h vapour flow and temperature) as the case may be is very much possible and same is being employed in many designs, to capture the waste heat to the extent feasible. However, some refiners have apprehensions about crude getting contaminated in case of any heat exchanger leakage or corrosion due to o/h vapour getting cooled below dew point) and hence do not prefer this option.
At the end of the day, it is ‘economic’ recovery of waste heat that matters. As pointed by Eric, it is often observed that crude columns normally continue to operate at design pressures even though present crude/feedstock blend is different from the design crudes. Definite scope exists to optimize the same and thus save energy. This perhaps is relatively one of the better options compared to ‘maximize’ (beyond certain stage) low level heat recovery from o/h vapours. I’m sure that Cast/Glass Air Pre-Heaters (APHs) or combination of these two, which is a very proven Energy Conservation (ENCON) measure, is already being incorporated at the design stage itself at many places.
02/01/2013 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Adding a TPA is one option to recover additional crude preheat. As others have indicated this can lower the overhead temperature and lead to corrosion issues in the column. If you limit the TPA duty so that it only replaces reflux then the overhead temperature is not impacted significantly and corrosion risk is reduced. The TPA will take up 3-5 fractionation trays so can hurt distillation if steps not taken to compensate (more trays or internal reflux). One advantage of a TPA is that it will offload the existing air coolers and reduce overhead system pressure drop. On some units this effect is significant, allowing the same fractionation to be achieved with a lower furnace outlet temperature. On other units the benefit is minor. When using a TPA you want to keep a high pumparound rate and maximize the return temperature to reduce the potential for shock condensation and resulting corrosion.
Installing a crude vs. overhead exchanger adds pressure drop to the system and creates its own set of corrosion issues, although they typically (But not always) are confined to the exchanger bundle and do not cause a unit shutdown.
The biggest drawback to either of these options is that the heat recovered is low level heat. The heat recovered in these exchangers will reduce the heat recovery in the rest of the preheat train, thus reducing the net benefit of the TPA or crude vs. overhead exchangers. In other words only a fraction of the recovered heat ends up as additional net preheat duty.
Other options for improving energy efficiency are to recover additional heat from product streams (look at temperature going into air or water coolers to assess potential for more heat recovery), to improve furnace efficiency with something like air preheat, or to reduce energy demand by decreasing operating pressure.
Which options work best typically depend on how old the unit is, how well it was designed in the first place, and what kind of crudes are being processed.
30/12/2012 A: S Banik, Centre for High Technology, sbdr@rediffmail.com
Heat recovery from overhead vapour by heating crude is gradually being practised. We have seen no of cases.After heating crude air cooler may be used for removing balance heat. Ofcourse you have to consider overhead corrosion aspects. Heat recovery by this method helps in reaching preheat of crude as high as 300 deg C plus.
28/12/2012 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Adding an overhead vapor to crude exchanger (free draining) ahead of the air cooler is the best solution, although elevating it is more costly than a grade mounted pumparound exchanger, for the following reasons:
(1) A pump-up pumparound design requires many tray spacings. Unless you add a section to the column, the fractionation would suffer.
(2) A pump-down pumparound requires fewer tray spacings, but still results in poorer fractionation as some spacing is required.
(3) Either pumparound design reduces the overhead vapor temperature, probably below the water dewpoint. That would mean severe corrosion and some column upsets due to liquid water being trapped in the column, for most of the world’s crudes.
As you add the new exchanger service to the train, the delta T’s of the other services worsen, so do not expect to recover too much of the apparent “wasted “ heat. Maybe you have already determined that with an exchanger train optimization run with the new service included.
My opinion is that every atmospheric crude section should include an overhead vapor to crude exchanger. Pipe the crude side to the tubes. That way, if the water condenses in this exchanger and results in corrosion, it is easier to plan for replacing a bundle during TAR than any other remediation task due to overhead corrosion.
28/12/2012 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Unfortunately, achieving fractionation which enhances value added generation for the refinery and within complex distillation towers requires expenditure of energy, so it is not unusual to need about half of the "encon" available Q to be expended in overhead condenser system(s). Parametric studies of CDU configured with four pump-arounds should be able to approach near maximum energy conservation heat recovery with respect to high level and medium level heat. Recovery of heat via energy conservation must not cause significant degradation of tails and front ends within TBP distillation curves for co-products as well as ensure locally relevant ASTM D86 distillation specifications are met.
There can be benefits and lower level encon heat recovery via upgrading to "two stage overhead condenser system"