4 replies to this topic

[chem101]

Hi:
Our company design Amine plants. we use shell and tube exchanger as a lean/ Rich Exchanger. I have talked to my Manager that we should use Plate exchnager instead of shell and tube.My reason for
Plate Echanger are:
(1) it takes less area.
(2) less cost as compare to shell and tube
(3)no leakage problem
but my manger does not agree with me. His point is that CO2 will eat up the gasket in plate exchanger.
whereas, i say that it can also cause erosion on tube side.
what are your thoughts? do you think My manager is making right decision.

[Art Montemayor]

Chem101:

I believe that plate heat exchangers are a great heat transfer device that give a process engineer another option and a possible economic and efficiency tool.

However, I would never employ them in an amine acid –sour gas removal process. I believe you have some built-in flaws in your 3 assertions:

1. PHEs don’t necessarily take less area. In fact, if you consider the structural steel and rack that they require, some of them require more space volume.

2. While they cost less than a shell & tube unit, this is valid only when you apply them to a low-pressure process where leaks can be tolerated. (here I am talking about gasketed plates – not seal-welded plates; and you should state which type you mean).

3. Contrary to what you state, the gaskets WILL leak – depending on the process and the temperature differences.

Additionally, as amine heat exchangers placed directly under the amine reboiler, you will find that you will have a pronounced pressure drop and this starts to affect your lean amine pump NPSHa if you locate this pump directly after the exchangers and before the amine cooler.

And then you have the pressure problem in the amine cooler if your absorber is running at relatively high pressures and you are using normal cooling water. The pressure difference across the plates start to give design problems and the cost go up.

As a condenser, the PHEs contribute a configuration problem and you inherit a larger pressure drop. This pressure drop imposes a higher-than-normal pressure on the amine reboiler and stripper.

I do not agree with your manager that CO2 will eat up the gasket. But I also do not agree with your assertion that CO2 can cause erosion on tube side. I have never seen this effect in the overhead condensers that I have built and operated and those of others. I also don’t see why it should.

I am presently using PHEs in a TEG dehydrating unit – but they are all seal-welded 100%. Any fouling or plugging of the PHEs means that the entire unit has to scrapped and a new one replacing it. The client made that decsion and I have to implement it. The TEG process was bought under a world tender so the supplier employed the cheapest way to heat and cool the TEG to obtain the bid and also ensure a profit. This is OK, if you are willing to live with the tradeoffs - and our client is.

[ankur2061]

chem101,

Some guidelines based on experiences of renowned international companies for PHE's is as follows:

1. Exxon guidelines say that gasketed PHE's should be restricted to maximum pressure and temperature conditions of 450 deg F and 350 psia (230 deg C and 24 bar (abs)). They also say that best results are obtained with conditions not exceeding 300 deg F and 250 psia (150 deg C and 17 bar (abs)). To prevent particulate fouling usage of wide-gap PHEs having thicker gaskets is recommended. Mueller, GEA and Alfa-Laval all manufacture wide-gap gasketed PHEs.

2. Gasketing material can be customized to prevent / minimize corrosion related failures due to sour service applications. Consult the PHE vendor to find the suitable gasketing material. However, as suggested by Art, full welded PHEs are a better solution in terms of uninterrupted and trouble free operations. However, usage of exotic gasketing material may push the PHE cost up, to the extent that a S&T heat exchanger proves more economical then a PHE.

All these factors need to be taken into account before you decide.

Have a look at the link below which actually describes and provides references for gasketed PHEs in lean-rich amine service:

http://www.calwestma...-Lean Amine.pdf

Hope this helps.

Regards,
Ankur.

[Zauberberg]

This will probably bring a little confusion into the topic, but Plate & Frame exchangers DO work in Amine units. And they are employed not only as Lean/Rich exchangers, but also as Regenerator reboiler(s).

I have never liked the idea of having PFE in a dirty, corrosive service such as Amine treating units - whether the exchangers are gasketed or welded. There's so many ways to plug those exchangers with particulates, that it seems better to avoid headache in the first place by selecting another type of exchanger, e.g. Shell & Tube being the most robust solution. But I have seen Plate & Frame exchangers working trouble-free in several Amine units, and achieving 10 degC temperature approach on both the hot-end and the cold-end of exchanger (Lean/Rich). And the exchangers used were of both types, i.e. gasketed and seal-welded.

Additionally, several trains in the Middle East do have these exchangers upstream of the Lean solvent Booster pump, and the end result is that subcooling of lean amine brings more benefits to the NPSHA, than the harm done by pressure drop across the exchanger. Obviously, if units are designed properly and carefully, the thing should work.

One thing that is common for all these units worldwide, is the presence of inlet strainers/bypass around the exchanger, on both sides, Mesh size ranges from 10-40. Additionally, the column bottoms (Regenerator/Flash Drum) are also equipped with inline strainers - all these being in function of preventing particulates entering the exchanger, as this would be the scenario that leads you to shutting down and cleaning the unit.

And finally - as far as cost is concerned - keep in mind that PFE have to be made of Stainless Steel, whereas S&T exchangers can be fabricated of Carbon Steel. Depending on size of the unit, S&T exchangers may be a more economical option.

Good luck,

[Propacket]

No doubt, PHE has a big disadvantage of fouling but we still prefer it due to its low cost. Headache of fouling can be encountered by using two PHEs, one in operation and one in standby. It may be surprising for you but even having two PHEs is far cheaper than employing one shell and tube exchanger. Let me give you a comparison for a 7 MMBTU/hr lean rich amine exchanger.
PHE:
Surface Area=525 ft2
Cost (Per Unit) =$10,000
S&T (Carbon Steel Shell):
Surface Area=925 ft2
Cost (Per Unit) =$75,000

P.S.
Above costs are for vendor standard items.
Thanks

Edited by P.Engr, 10 October 2010 - 10:54 PM.