2 replies to this topic

[Travesh]

Hi All,

I am presently designing for an crude atmospheric tower:

1. Overheads condenser (shell & tube)
Hot side is overheads vapor at 146 degC. Cold side is crude feed at 40 degC.
Please advise what TEMA type of exchanger is used typically in industry. I am considering an BES type. Is this OK? What are the considerations in deciding the type?

2. Air cooler after shell & tube
Also, please advise typical design considerations (material of construction, pressure drops, velocity limitations etc)


Further, is a water wash necessary for the overheads line before the condensers? How does one determine the flow rate & necessity of this wash? I understand that NH3 injection is also a possibility.

Please also provide good literature references for all these issues.


THANK YOU

[Zauberberg]

I tend to agree that AES/BES is probably the best choice for CDU overhead condenser applications. What you need is really very low pressure drop on the shell-side of exchanger, so X-shell could be an option as well (normally used in vacuum tower overheads). However, your design poses some other concerns.

A huge problem when having two-staged condenser system in CDU's is: corrosion. Having 40C crude oil on the tube-side of this exchanger will generate local condensation points/spots where hydrochloric acid vapor will be absorbed almost instantaneously, creating low-pH regions and accelerated corrosion. We have seen these kind of exchangers completely fail in a matter of days. Proper desalting and anti-corrosion program will help you with that but it will not eliminate the problem. So instead of recovering energy from CDU overheads and having exchanger failures every once in a while, I would decide to go for lower energy efficiency and employ a single-stage, air-cooled only, overhead system. Also, there are some interesting operational issues with two-stage systems: if you finally decide to go for it, make sure that you can export a portion of condensed naphtha to debutanizer tower, and not only using it as reflux.

As far as fin-fans are concerned, nothing much to be said: it depends on H2S, Chlorides, NH3, and other aggresive agents you have in desalted crude oil. For majority of applications, CS would do just fine. You have to loook into the overall conditions/composition, include certain safety margin, and the choice is more or less straightforward.

Good luck,

[Art Montemayor]

Travesh:

Please refer to the attached Excel Workbook.

As you can see, there are various ways to configure a Shell & Tube heat exchanger. An AES TEMA type has its advantages but, like everything else in engineering, it also has its disadvantages - or tradeoffs. There are no free rides or lunches in engineering.

A lot of the basic data needed for a well, defined and logical decision has to be defined - which is missing from your post. Normally, I will never specify an internal floating head unless I am forced to do so by circumstances or by hard, factual basic data. In my experience, all internal floating head exchangers are not only very expensive, a virtual "hell" to do maintenance on, but also will eventually leak at the gasketed internal flange if left in service for a sustained time. There is no mechanism - other than a pre-determined torque - that will ensure that the internal flange bolts will maintain the proper tension and stress on the flange gasket. If you can't tolerate any leakage between tube and shell, then you have been warned.

If you have a large temperature differential between the shell side and the tube side, then you must provide some differential expansion ability in the exchanger. A floating head is just one gimmick to resolve that problem. But there are others as well. As you will note, there are these and other issues that you haven't even mentioned. Therefore, either you furnish ALL the pertinent and applicable basic data and scope of work or we simply are unable to make a specific recommendation. You are in control or possession of all the basic data and scope of work, so you are the best indicated individual to make the engineering decision.

Attached Files

•  Art\'s TEMA Designations.xls   84.5KB   99 downloads