9 replies to this topic

[proinwv]

I need the latent heat of vaporization of 28-30% NH4OH but cannot find any of the physical properties in my references or online.

Any help?

Thanks!

[Art Montemayor]

Paul:

I don't think you can find the Latent Heat of Evaporation for Ammonium Hydroxide. I remember this compound well - it was the cause of a bad chemistry lab experiment in college for me.

As I can recall, it is a misnomer of sorts because it is just an aqueous solution of Ammonia. It is a substance that easily dissociates and which can only exist in a free state at very low temperatures.

Ammonia dissolves extremely well in water, but it does so without extensive ionization. Ammonia aqueous solutions are often labeled as “ammonium hydroxide”, but are actually best described as ammonia molecules dissolved in water, with only a few percent being converted to ammonium and hydroxide ions.

This sort of "compound" can give a lot of problems if one is dealing with a tank filled with it and the tank is subjected to an external pool fire. The immediate design criteria would be to apply API Standard 2000 which has a formula depending on the latent heat of evaporation of the stored liquid and the area that it "wets". In this specific example, the ammonium hydroxide would start to liberate its dissolved ammonia gas before the solvent water boiled. This means that a sudden liberation of gas would occur first and could overload a pressure relief device.

I don't know your reason for needing a Latent Heat value, but I don't think you will find one that really represents what we normally interpret as the amount of heat required to evaporate a unit of mass of the compound.

[proinwv]

Art, that is exactly what I am trying to do is size emergency venting for a NH4OH tank for fire exposure using API 2000. As you know I am a ME and I came to you ChE's for the information because I couldn't locate it in reference texts.

I was tempted to use the latent heats for both NH3 and H2O and proportion it based on concentration, but from what you said, as I understand it, the NH3 will boil off first. Then wouldn't it be best to simply use the value for NH3 since that will be the first vapors we would see and have to handle?

I certainly need to do something and I don't have any problem oversizing the vent, as opposed to undersizing. Certainly cost is not even a consideration, versus the possibility of a vessel rupture.

I again appreciate your help in these matters.

Paul

[djack77494]

Paul,
Accepting the scenario as it's been presented, you would want to use the heat of solution rather than the latent heat. I have never done a calculation like this, and so can't be of very much assistance. I suggest that your best bet would be to get someone with a process simulator who could generate a heating curve for your situation. It's really not very difficult - ask your friendly neighborhood process engineer if he/she has the proper tools available to do this job. It is imperative, however, that a good (for this situation) thermodynamic package be used. An aqueous ammonium hydroxide solution is far, far from an ideal system. You cannot take your "typical" general purpose process simulator and use it.

From the heating curve, you get the relationship between enthalpy, temperature, and composition. Changes in composition equate to releases of ammonia gas, which you must provide for.

I hope this is helpful and that when you've discussed this with your process engineer, you don't look over and see someone shaking their head with wide open eyes and mouth. If you do, you may need to think about a consultant.

Good luck,
Doug

[Zauberberg]

Mr Montemayor gave the full basis for accurate calculation of NH3 release: it is a question of ammonia solubility in water at different temperatures. Ideality and non-ideality of the system is certainly a (general) issue, but without any serious impact on total venting load - the highest percentage of vapor will consist of NH3, in any case. Experimentally validated charts for NH3 solubility in water are available in many engineering textbooks.

Don't use latent heat of H2O and NH3 - it will give you wrong conclusions. What I would do in your place, is to employ a process engineer to calculate NH3 evaporation rate (kg/h) as a function of tank temperature - based on solubility data and total system inventory, starting from normal/equilibrium conditions at given storage temperature. This is the basis for emergency venting load calculations.

Best regards,

[proinwv]

All of this information has been very helpful and enlightening and I thank all of you.

I will be able to employ your thoughts properly. This old mechanical thanks you chems.




Paul

[Art Montemayor]

Paul:

First, and foremost, I strongly believe you are doing the right thing – and in the right order. You are definitely correct in prioritizing the protection of the NH4OH storage tank at all costs. By protecting the tank you are protecting the personnel around the tank and its environs – and this, in my opinion, is the most serious hazard. Ammonium Hydroxide is a basic solution; it acts like a strong lye (caustic) and will quickly attack human membranes – especially the eyes, nose, lips, and all exposed skin. Its effects destroy the human mucous cells and recovery is often not possible. In the event of a spill, any unprotected human in the surroundings would be immediately overcome by the caustic ammonium liquid and fumes. Blindness, pain, and loss of consciousness would be rapid, in my opinion, and there couldn’t be much hope for help from anywhere without further fatalities. You are doing the right thing in challenging the selection of the proper and safe pressure-relieving flow rate. This is not our normal API Standard 2000 relief scenario. In addition, I’m not even addressing the safe piping design for the venting yet.

I think I now have a good idea of the design stance that you are taking because you seem to sense already that there is more danger here than one would normally presume – and you are right. I believe you are planning on designing the relief situation (in the event of a pool fire around the tank) such that the (ammonia + ammonium) vapor which will be released first (prior to any boiling) due to a rapid over-pressurization from the surrounding fire, will be safely vented away from the area and allowed to dissipate into the atmosphere. Additionally, the vapor + gas + evaporated water relief will safely go on while the fire continues to evaporate essentially all of the remaining water in the tank. If the fire proceeds this far, it is obvious that the tank will subsequently fail structurally – but there will be no toxic or hazardous contents spilled should this occur. As you correctly state, cost is secondary at this stage of the scope of work and the incremental price involved in ensuring a conservative design is more than justified. However, we need an engineered estimate of the maximum flow rate amount of gas + vapor generated by the pool fire.

Estimating the vapor release prior to normal water evaporation is difficult for an ammonium hydroxide solution – but not impossible. Many times those operators storing the solution will have empirical test data (sometimes proprietary) that predicts the vapor release with regards to temperature increase (or heat-up rate). If this is not available, then this leaves us with the theoretical, thermodynamic approach. I believe this is the method that Doug is mentioning. This is an analytical method whereby we presume that the heat of reaction (the heat generated in “dissolving” the ammonia into the water) is the same as the heat up of the tank causing the release of the toxic vapors. As Doug correctly points out, this analytical calculation for the heat of reaction is easier said than done. It just so happens that the reaction between the ammonia gas and the water to form the unstable hydroxide is very “non-ideal”. In other words, it doesn’t follow our “ideal” equation of state that defines, among other factors, the enthalpy of the resulting solution. As a result, there are various different equations of state that are known to produce credible – if not, acceptable – resultant values for the released vapor flow rate. And it is here, that the success of predicting a conservative (but close) estimate lies. I believe the highest vapor release flow rate (the desired vent sizing flow rate) for this application will take place as the tank is heated below its boiling point. This must be shown in order to justify the correct, conservative vent rate design. It takes an experienced Process Engineer with a good simulation background to select and apply the appropriate equation of state. The non-ideality of the ammonium solution and its behavior is the main problem. The “ideal” answer, as you probably already know, would be to have accurate, recognized, empirical data. I do not believe that it is logical to apply a multiplication factor to the API Standard 2000 calculated value and use this as a relief design flow rate. The two values are not related in this application. If, indeed, the released ammonium gas + vapor flow rate is larger than the API Standard 2000 calculated value, this must be established and designed for.

This is such a special, important, and unique pressure relief application that I am hoping that our Phil Leckner will join in on this thread and contribute his valued opinion. Doug has already contributed a valuable input and hopefully can continue to follow this thread. Others – such a Joe Wong, etc. – are certainly welcomed to add their contributions. With luck, we may hit on someone who has battled this application already – and won.

I am presently on my way to Sunday church, but I will continue to follow this important thread hoping that our valued members continue to contribute.

[Art Montemayor]

Paul:

I am now back from church and have had a chance to do some research on the subject. Please go to the following Websites and download the related information you will find there on Ammonium Hydroxide (a.k.a. "aqua ammonia"):

http://www.airgasspe.../AAPhysical.pdf

http://www.rmtech.net/Aqua%20Ammonia.htm


http://www.rmtech.net/technical_information.htm


http://www.ammoniapro.com/Ammonia%20Library/Tech%20Data%20Booklets/Aqua%20S%20&%20H%20-%20Tanner.pdf


http://www.ammoniapro.com/Ammonia%20Library/Tech%20Data%20Booklets/Aqua%20Ammonia%20Info%20Manual%20-LaRoche.pdf

Some very interesting information starts to make itself apparent in this information – particularly the Tanner Industries and the LaRoche booklets. Note that what is being recommended is NOT an API atmospheric storage tank. An ASME Section VIII pressure vessel is being specified – a low-pressure (25 or 30 psig) horizontal "bullet".
LaRoche specifies the sizing formula for the recommended pressure relief device and it appears to be a variation of the equation given in API RP 520 (Part 1):

CFM (air) = (22.11) (A)^0.82

Where,

A = Tank outer area (I believe the "wetted" area is meant here);

I believe the majority of the information in this literature is specific enough for your use and/or reference.

I hope this helps you out.

[latexman]

Paul,

In my Chemical Engineers' Handbook 5th Edition on page 3-158 there is an enthalpy-concentration diagram for aqueous ammonia. The diagram may be in previous and subsequent editions. If you can access this diagram, it explains how to use it. It can be used to obtain enthalpy values for liquid and vapor in equilibrium at various pressures. I think it's exactly what is needed by a process engineer to calculate your scenario.

[proinwv]

All,

This has been an extraordinary forum today. I am overwhelmed, in the most positive sense of the word and thank all of you for your input.

My role, with the client, at this time is to review their proposed system and provide them with an opinion on the entire matter which includes other elements beyond this emergency venting. This has provided myself, and by extension them with the tools to properly proceed with the work. I most likely will not be involved with the actual design work.

I may add to this particular thread later, but for now, I must go and digest this material and prepare for the best advice for the client. I hope that advice I may have provided others, in the past, on these forums may have been as informative as you have been.

Paul