4 replies to this topic

[drdave]

Ethylene glycol heater is specifically designed with steam ejected condensate circulation to heat up glycol solution from 6 deg_C to 16 deg_C for specified capacity of liquid ammonia(-33 deg_C) to be heated in Ammonia preheater before transfer upto +5 deg_C . In case, liquid ammonia supply quantity just doubles,it results into subzero temperature of liquid ammonia which not acceptable by down stream headers & road & rail tankers. How can we use same EG heater for higher load operation? Higher EG concentration will do?OR Should we provide another EG preheater to serve the purpose alongwith New Ammonia preheater? How can we ensure effectiveness of Anti-freeze solution?

[Art Montemayor]

drdave:

Your post is hard for me to interpret. Allow me to state what I believe you are describing and correct me if I have interpreted something wrong:

You are transferring liquid ammonia (which exists at -33 oC) to road and rail tankers for shipment. The tankers cannot tolerate the -33 oC temperature and you have to heat up the liquid ammonia to +5 oC prior to loading it into the tankers. You heat the liquid ammonia in a heat exchanger, using re-circulated hot (+16 oC) Ethylene Glycol (EG) as the heating medium. The EG is kept hot by heating it with steam. You are required to increase the loading operation and want to know how to increase the ammonia heating rate.

Before answering your query, I must clear up some items in your post:

1) You state that your EG heater works “with steam ejected condensate circulation”. This can’t be. Your description doesn’t make process sense. You can’t be “ejecting” steam. You normally eject the produced condensate – via a steam trap. I believe you are using a steam-heated EG exchanger. Confirm if this is correct.
2) The liquid ammonia that you are heating is SATURATED LIQUID at essentially atmospheric pressure. This would explain its temperature. However, since you don’t state the actual pressure of the liquid NH3, it is difficult to envision that you would be heating a saturated liquid and causing a vaporization of the same. You obviously want to transport LIQUID, so vapor production is out of the question. Additionally, why the need to heat up the saturated liquid NH3? I’ve handled and operated a lot of liquid NH3 equipment and I’ve found that all the steel employed in conventional NH3 processes (such as type 516-A) readily accepts the -33 oC temperature and is suitable for it. The practical way to effect the transfer is to pump the -33 oC liquid NH3 and load it in the transports on a MASS basis, not volume. Once the correct mass quantity is loaded, the saturated liquid will slowly warm up to ambient temperature and reach it’s saturated pressure at that temperature. For liquid NH3, it is 10.62 barg at 30 oC. In order for the liquid NH3 to be successfully heated from -33 to +5 oC, it must be pressurized (be in the supercooled state) to at least 4.23 barg.
3) Heating a contained liquid causes it to undergo hydraulic expansion – and it does this spontaneously, with a marked increase in pressure. This is why I consider the loading of the cold liquid, on a pre-determined mass basis, to be more efficient – but more importantly, SAFER. Any mix-up or loss of temperature controls on the outlet temperature of the liquid NH3 can cause a rapid rise in pressure – the corresponding saturated vapor pressure of the liquid NH3. Whereas, if I load a pre-determined mass quantity of the same liquid into a tank that is filled with a maximum of 85% of its volume and allow it to heat up gradually until it achieves ambient temperature, I am assured that the resulting tank pressure will not exceed the vapor pressure of the liquid and the operation will be safer. Please furnish us with a detailed explanation of how you are carrying out the liquid NH3 heat up.

You can use the same EG heater to increase the heat up rate by increasing the temperature of the inlet EG from +16 oC to a higher value – as high as you can tolerate the resulting vapor pressure of the saturated, liquid NH3. But again, I would caution you on this application of heating up a flowing supercooled liquid. There are inherent hazards involved in heating contained liquids and I would not employ this type of technology unless I could ensure that all the process (and instrumentation) would fail in the safe position each and every time.

You can certainly employ another, similar EG heater (either in series or parallel) for added loading capacity. I don’t know what you mean by “ensuring the effectiveness” of the EG solution.

I’ll await your response and comments on the above. Further, please furnish detailed and complete basic data on the operation in order to fully identify the conditions and properties of the fluids in question.

[Art Montemayor]

DrDave:

I have taken the liberty of copying your personal, edited, email to me and posting it here on our Forum. This is the normal, expected methodology of responding to queries. We initiate a thread with an original posting, and fellow Forum Members subsequently add their valued comments and ideas on the same thread. This allows for organized and shared information on the common subject theme. Please respond to the individual threads directly on the Forum, and not through personal email. Personal email does not allow all members to share in the same information and potential solutions and ideas are lost or never exploited.

You are mistaken about the ability of this Forum to transmit engineering diagrams. Please review the attached document as proof of what I state and feel free to generate your own, correct version - or edit mine to depict your true process and its conditions.

You can find ethylene glycol solution freezing points at:

http://www.lyondell....rotection.shtml

I see no reason why your EG solution concentration should decrease if you monitor it and maintain its concentration.


“Sir,

Thank you very much for your prompt reply on my query. Now, let me reply to your queries raised from my process inquiry on subject matter:

1) EG solution is steam heated one. Here, condensate gets circulated through injector wherein 13 Kg/cm2g steam is motive fluid to fulfill condensate circulation on shell side which is full of condensate. Steam injection equivalent condensate overflows to waste water basin (In our case, it goes to Cooling tower basin). Here heat duty is fixed to have just positive temperature & no more heating required & no cooling to subzero.

2) Liquid ammonia is pumped at 25 kg/cm2g from atmospheric ammonia storage tank (-33 oC) for captive consumers like urea production & shipments where material of construction is not LTCS. Hence, inter-granular cracking may be possible. Hence, it is not safe working practice.

3) This forum format is not accepting pictorial depiction of problem otherwise only one picture can explain whole problem effectively.

4) Normally, anti-freeze solution like ethylene glycol solution works effectively at 50:50 of EG: water by Wt. If concentration of EG depletes too much, it may cause frosting in shell side of the other NH3 preheater curtailing effective heat transfer area & higher ammonia loading may go cool below 0 oC. So, I want to know EG solution effectiveness up to what permissible concentration.

Thanks once again.”

Attached Files

•  Liquid_NH3_Loading.doc   31.5KB   78 downloads

[Art Montemayor]

DrDave:

I hope I have effectively depicted your problem in one "picture". You will note that I took the pains to itemize some of the more critical basic data. Perhaps you can correct any misconceptions I have about your described process.

At 25 kg/cm2g (355 psig), saturated liquid NH3 would have a temperature of approximately 135 oF (57 oC) - so we can easily deduce that the NH3 product of your heatup is still liquid at +5 oC - and supercooled.

The basis of effecting this type of liquid transfer is to establish and maintain a constant pressure on the NH3 being heated - and never go beyond the saturated temperature (57 oC in this case).

As I indicated, on the Lyondell website you can easily find the EG information you require to answer your question about the freezing point of EG solutions and their EG composition.

I think I already responded to your question on increasing the heatup capacity of the system.

[drdave]

Sir,
Thank you for your informative reply.
Loss of EG content at our end is most probably due to evaporation & minor leakages sometimes at pump gland. But its regular laboratory analysis is not available because of lower loading of system till date.Now it has picked its utility which has brought into new dimensions to think over.
In fact,Our system looks like a picture attached herewith.

Attached Files

•  Slide1.GIF   6.25KB   35 downloads