MDMT when Blowing With N2 1

[sshep]

My Friends,

In several previous posts (example: thread124-181137) there is a question about how cold you can get blowing a hydrocabon system dry with N2. I haven't always felt we were that useful, but now that I was asked to specify some rail car MDMT for pentene service, the question was asked of me to calculate exactly that question: How cold can it get when purging with N2. Using Aspen plus I could get an answer by sensitivity analysis on N2 and Pentene, finding the minimum point (when the last liquid disappears). The temperatures were much colder than my peers realized, reaching (theoretically) below -30C. I knew the temperatures would be well below the boiling point, but didn't have reasoning that could be well communicated. Now I do.

Before I submit my conclusions I would like you to check my reasoning: The N2 and liquid hydrocarbon mix, some hydrocarbon evaporates and carries away heat from the remaining liquid. The minimum temperature can be calculated by energy balance. We could even calculate dT/dt, but we only care about the final temperature (dT/dt=0).

The enthalpy of incoming N2 is balanced by the heat being removed by evaporation:
Equation #1: N2flow*Cp*(Tsupply-T)=HCflow*HVap, where
T is the mixture temperature
N2flow is the nitrogen flow
Tsupply is the N2 supply temperature
HCflow is the hydrocabon flow leaving (evaporated)
HVap is the molar heat of vaporization
Cp is the molar N2 heat capacity

The HCflow depends on the vapor pressure, system pressure, and flow of N2
Equation #2: HCflow=N2flow * PL /(P-PL), where
PL is the vapor pressure which is a function of T from Antoine's equation

By reducing this down, the N2 flow drops out:
Cp*(Tsupply-T)=PL*HVap/(P-PL)
Since PL=f(T), this is most easily solved using the solver function in Excel.

The parameters that make the difference for a particular system are the N2 supply temperature, and the total system pressure (typically atmospheric worst case).

Does this seem right to you guys? Any comment is appreciated.

best wishes always,
sshep

 

[unclesyd]

Somewhere in this list papers and documents there is a requirement that specific impact properties of the MOC have to be met. Dont' get caught in violation of this requirement if an accident happens.

http://search.google.dot.gov/FRA/FR...=FORID:10&ie=UTF-8&q=material+of+construction

 

[dcasto]

I guess that in the uS we would have to throw away all our rail cars because they cannot be used in the winter time where temps drop below -30 reguarly.

why not pump in hot N2 and that will vaporize the pentanes quicker.

 

[sshep]

Hey Unclsyd, I haven't been able to find the reference in the link you sent me. I want to look more just to see, but I hope that my job ends when I give the data and that qualified people take it from there.

Hey DCasto, If the reference mimics the BPVC, ambient temperature does not count the same as process temperature. I am not sure why. In any event -29C (-20F) is allowed for SA516 Gr7 material up to 1" thick without any impact testing. Impact tested carbon steel can go lower. It would seem that since -29C is a freebee, that would be the typical specification, but I bet alot of guys blow it in estimating the temperature. As far as using hot N2 goes, you can't control the who, when, and where of purging, so that kind of admistrative control for a railcars doesn't seem practical.

I have seen ice on our flare lines when we make pentene runs (probably puking liquid into the flare from somewhere), so I really believe it can get pretty cold with N2 blowing past liquid.

best wishes to you both,
Sean Shepherd

 

[unclesyd]

I'm looking around for it, the only problem is that the few worked on it are not round anymore. At one time we had a fleet of of rail cars for chemical transport, mostly innocuous except the HNO3 and Anhydrous Ammonia. The problem was brought about by several accidents where temperature was the primary cause of the car failure. We had around 20 cars for transporting Anhydrous Ammonia that were required to have a {15 ft/lb Charpy @ -50°F ). Please don't quote me on this number. We had to take about half the cars out of Anhydrous Ammonia service. We inspected the rest with Acoustic Emission right and and every three years afterwards. Fortunately we got too big for rail transport of Anhydrous Ammonia and went to barges.

 

[sshep]

Hey Unclesyd,

This is a good story you have. My decision is basically to chose a temperature requiring charpy tested or not- I am within about 2degC such that assumptions on the N2 temperature used for purging will make the difference. Probably I just stay with a temperature that will ultimately end up with a charpy tested requirement, and be good to probably -50degF (my memory also) as you remember.

Your advice has always been excellent. If either of us finds a reference in those rail regulations, I will be more than glad to give you a star!

best wishes always,
sshep

 

[dcasto]

even charpy tested won't help you then as you said, you cannot control anything.

I've blown down carbon steel lines with ethane and ethylene in them, that is -120F and then purged them with N2 before repressuring them. and the lines are still in service at 2200 psig. look up cryogenic working of carbon steel.

Unless you use 304 SS, you won't be in compliance with your own rules that "you can't control the who, when, and where of purging, so that kind of admistrative control for a railcars doesn't seem practical."

I'd make it practcal or hire someone else that will.

 

[unclesyd]

I've read numerous papers hoping to find a definitive answer for
your OP concerning CVN testing. Trying to read into the papers there is continual mention of ASTM A515-70 Normalized, A516-70
Normalized, A212, along with some older steels.. New steels
being considered are TC128 B and HPS 70. It kinda hacks me
off when they state that CVN values were acceptable or better,
better than what.
The information is pretty convoluted as it uses cross references
to AAR, FRA, and DOT. You might contact AAR to get the
specification for rail car steel. I will keep looking as I recall seeing a graphic that covers some of the physical properties of steels.

Discusses the various steels but I see no definitive specification.

http://www.aar.org/~/media/aar/NewsFiles/2009 April Su

b%201%20Background.ashx

Concerns Weld and inspection. There may be lead to material
specifications
.

http://www.aar.org/~/media/aar/Events/FINAL 2009 Octo

ber%20Sub%202%20Background.ashx

See page 85 by Adobe

http://www.aar.org/~/media/aar/NewsFiles/2009 April Su

b%201%20Background.ashx

Here is an AAR search page.

http://www.aar.org/search.aspx?&lcid=1033&s=-7b0HYBxJliUmL2

3Ke39K9UrX4HShCIBgEyTYkEAQ7MGIzeaS7B1pRyMpqyqBymVWZ
V1mFkDM7Z28995777333nvvvfe6O51OJ%2fff%2fz9cZmQBbPbOSt
rJniGAqsgfP358Hz8ifvzHd3%2fXp%2fl5ti7b3%2fXHf%2fze7%2fu74
r%2b93%2fd33fl9f9fte5%2fSv7v4gFr9vr%2fv7%2fu7%2fqL6Gj9%2b
%2fMf3fl95qJn%2b0rR5XqbNKp8W58U0a4tq2Zjv7P8NHPcBdfC7rq
flt%2fNslte%2fG%2f32Os%2fq6fx3m7blzj3%2bd8f%2flwHgpf7%2fC
NDv%2bv8A

 

[sshep]

Hey dcasto,

I have as well worked in an ethylene plant some years back. Surely you have procedures to pressurize with ethylene vapor before introducing liquid, and also to drain and vaporize this liquid before depressuring to atmospheric or blowing with N2. Deliberately exposing carbon steel pipe and vessels to -120F does not seem right as the written procedure. It sounds like it would be like documenting a criminal act (but I'm an engineer not a lawyer). The chance of a crack from brittle fracture seems too high. I know the high quality of your posts and I think it would be out of character for you to do something like that deliberately, although we know it happens sometimes by accident.

My service is pentene. My calculated temperature is -30C and easily within the limit of impact tested steel. In fact, I'm only 1C away from no impact testing. I am looking for a purging N2 temperature standard (if there is one), I used 20C, but if it were 25C I would have a temperature above the threshold. In the end I will specify a MDMT that will result in impacted tested requirements.

best wishes always,
sshep

 

[dcasto]

Under controlled conditions, we always pressured the ethylene pipe with N2 to at least 400 psig, preferably 800 psig before we introduced the ethylene. Then we switched to a line heater system and only pressurized the line to 50 psig with N2 before we introduced the heated ethylene. but there were times when we just had to vent the the line and refill it at -120F. It was allowed by our procedures.

But what happens when a line is cut and the whole 20 mile section of line is blown down to atmosphere? How do you perform maintenance in the middle of nowhere? What happens when a interconnection has a relief valve failure?

NO, brittle fracture doesn't happen, because the stress on the line is low at the same time the temp is low.

what penetene has a -30F boiling point, all the ones I'm familiar with are closer to +30F and have mole wts closer to 68.

Finally, why do you use liquid N2, why not gaseous N2 for purging? Use gaseous and all your issues disappear.