Materials selections for C3 Hydrogenation Reactor

[kissjxp]

Guys,

I have some questions on materials selection for a C3 Hydrogenation Reactor.

Design temperature (Normal operating): 250C
Design pressure (Normal operating): 3950 KPa
MDMT: -46C

Maximum operating Temp. for catalyst regeneration: 460C
Maximum design Temp. for catalyst regeneration: 490C
Design pressure for catalyst regeneration: 1000 KPa

The maximum operating temperature during regeneration is 460°C at reactor outlet during burning step. This regeneration step is short. There is no corrosive contaminant in this reactor.

My questions:

1) Can we use Low Temperature Carbon Steel for this case since the regeneration is short? What is the Graphite formation speed in short period of time at a temperature over 427C?

2) How long we can use carbon steel plate short-periodically exposure to a temperature above 427C (the total hours)?

3) Can we use 1.25Cr-0.5Mo at low temperature (-46C)?


Thanks!

 

[gr2vessels]

The nickel steels recommended for low temperature application, should not be normally used above say 200 F (some people recommend 32F the upper limit), nor is the PWHT recommended for these steels due to the deterioration of the impact properties.
The carbon steel and low alloys are recommended for use at your nominated MDMT - 50F as per Clause UCS-66 of ASME VIII Div 1, most of them also stabile up to 900 F as listed in ASME II, Table D, hence your 800 F is not really a concern for these materials (particularly materials less than 450 MPa tensile strenght). Obviously, you're looking for carefuly selected fabrication procedures, testing, heat treatment as required, etc. Other than that, it looks to me pretty standard application, no hassles.
gr2vessels

 

[kissjxp]

gr2vessels: thanks for your comments.

Acctually, I know that LTCS should be fine at the normal design temperature (427C). What I worried about is that the short term exposure to a temperature over 427C, up to 490C.

 

[gr2vessels]

kissjxp,
The actual limit for continuous use of these plates is 538 deg C (1000 F), per table D. Hence your "exposure" to 490 deg C is not a concern, no changes (other than strenght due to temperature) will take place in the material up to the Code allowable limit of 1000 deg F.
Best regards,
gr2vessels

 

[kissjxp]

gr2vessels,

Thanks for your comments.

Do you mean ASME II, PART D-TABLE 1A?

In TABLE 1A, the maximum temperature is 1000F for VIII. But for note G10:upon prolonged exposure to temperature above 800F, the carbide phase of carbon steel may be convert to graphite.

 

[gr2vessels]

kissjxp,
You are right in asking these questions, because you obviously noticed weaknesses in the code and normal practice.
I pesonally consider the G10 and similar notes in other standards only a#$e covering statements, because no qualification is given for this vague, generic comment. Maybe, possible and could if.. should not be part of engineering language. What I know of, is that two similar reactors to yours, are operating since early 1950's, with identical temperature excursions and are still just fine. Over 400,000 hours!
Speaking for myself, for low temp application and high temp excursions, I would use lower strenght carbon steel plate, less than grade 60. That would generally increase the plate thickness, welds, etc, making it more expensive. I also keep in the back of my mind the warning of bloomin' graphitization (which is a function of temperature and loooong process of difusion, before any noticeable loss of impact properties is detected). For this two reasons, I evaluate the design pressure to see if costwise, the carbon steel fabrication could be less expensive than using 1.25 Cr plate. If the high design pressure is pushing the carbon steel plate way too thick, then I'll use the 1.25Cr. Less thickness, less labour and no graphitization!
In short terms, I cannot see your design pressure qualifying for high pressure, thus making it worthwhile to use the 1.25Cr. Don't ask for reference, but I would quantify the "prolonged exposure" to weeks of continuous operation over the 800 F mark, few days only now and then every year, would not lead to graphitization ever. However, I would also like to hear the opinion of other people in the forum.
cheers,
gr2vessels

 

[kissjxp]

gr2vessels:

Thanks for your comments!

 

[kissjxp]

gr2vessels:

As you said, most of the carbon steels are stable up to 900F in ASME II, TABLE D (Acctally I can not find ASME II Table D and assumed to be ASME II, PART D). Could you please show me where you get this temperature? is it applicable for plate? Or is it for section VIII application?

 

[gr2vessels]

Actually, the referenced carbon steel plate material are listed in Table D up to 1000 F. However, most likely commercial considerations and possible liability complications brought the note G10 in the ASME Section II, as a disclaimer (American litigation paranoia), but if you take it as a warning rather than restriction and using your own engineering judgement, you still should be able to use carbon steel instead of 1.25Cr.
I've got in the front of me the ASME Section II, 2007 edition, 879 pages;- page 10 to 13 of Table 1A and other pages down the table are listing various carbon steel materials with allowable stresses up to 1000 F suitable for your application. Obviously, I cannot send you the book, but if you need, I could scan say, three pages for your reference and attach them to my post. Remember, for a documented selection, you would need more than three pages...
Howzat,
gr2vessels