aqua-ammonia 5

[aquafied]

Is the vapour given off during the generation cycle of an absorption refrigeration system basically pure Ammonia and does the Ammonia conform to the the saturation/pressure charts on the high temperature side and low temperature side of these systems?

 

[Latexman]

Aqueous ammonia has not been used for absorption refrigeration systems for many years, so, yes it is pure ammonia, and it follows the equilibrium vapor pressure/temperature data of pure ammonia.

Good luck,
Latexman

 

[25362]

Latexman is right. However, sometimes the absorption system is justified, if heat is cheap and available.

The water content of various ammonia qualities can be seen in:

[highlight #204A87]

http://www.tannerind.com/anh-specifications.html

[/highlight]

 

[aquafied]

An article written by a Professor Bassam (University of Wisconsin) stated that a substantial fraction of ammonia dissolved in water remains in the molecular form in solution. I have learned from other sources that, at any temperature and pressures we have in a system, the vapour above the NH4OH liquid is 95% NH3 and 5% H2O. I don't understand if the vapour above the NH4OH is separate NH3 and H2O molecules or some other molecule.  If they exist as separate molecules, why don't they combine to form NH4OH?

Most if not all absorption refrigerators have a means of condensing water from the NH3 vapour (after bubble pump) before the NH3 is condensed how is this possible?

 

[Latexman]

There really is no such thing as a molecule of NH4OH, ammonium hydroxide. In a solution of ammonia and water, the ammonia deprotonates a small fraction of water to yield ammonium ions and hydroxide ions in the solution according to the equilibrium reaction:

NH3 + H2O <-----> NH4⁺ + OH^-

Ammonium hydroxide is just a word/name that was invented and is synonomous to aqueous ammonia, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, i.e. a solution of ammonia and water.

Good luck,
Latexman

 

[25362]

From Wikipedia:

Water is unique because its oxygen atom has two lone pairs and two hydrogen atoms, meaning that the total number of bonds of a water molecule is up to four. For example, hydrogen fluoride—which has three lone pairs on the F atom but only one H atom—can form only two bonds; (ammonia has the opposite problem: three hydrogen atoms but only one lone pair).

IMO, that's the apparent reason for water condensing before ammonia.

I think the professor is right. A 17% w/w (10 N) solution is ionized by much less than 0.5%. A very dilute solution can be ionized by more than 10%. In these cases ammonia molecules pick up H⁺ ion from neighboring water molecules to form ammonium and hydroxide ions.

NH₃(aq) + H₂O ⇌ NH₄⁺(aq) + OH^-(aq)
​

It is OH^- that turns phenolphthalein pink.

Both compounds in the ammonia-water mixture are very polar, making it a highly non-ideal solution.
Good examples of the non-linearity of mixture-functions with composition are an enthalpy-composition plot showing parabola-like isotherms with minima in the mid-range and solution dynamic viscosities showing peaks at about a 0.25 ammonia mass fraction.

The following link shows thermophysical properties:

http://www.mie.uth.gr/ekp_yliko/NH3_H2OProperties_1.pdf

 

[aquafied]

The Hydrogen reserve vessel (connect to the top of the condenser and evaporator) in an absorption refrigeration system is designed to raise the Hydrogen pressure in the evaporator to neutralise the increase in Ammonia condensing pressure as the ambient temperature rises. Obviously the Hydrogen and Ammonia in this vessel must be stratified for this system to work, doesn't this contradicts Boyle's law?

 

[aquafied]

Correction to my last post. I meant Dalton's law of partial pressures [individual gasses act as if they occupy the space alone]. The Ammonia and Hydrogen in the reserve vessel must be stratified so that Hydrogen only is forced into the evaporator. Thank you to all my contributors.

 

[25362]

Now, you're referring to a hydrogen-assisted ammonia absorption refrigeration system
As in your attached sketch, the reserve vessel acts as a "balance drum" receiving, as well as delivering, hydrogen, as the double-ended arrow shows. Hydrogen is coming in not only from the condenser but mainly from the absorber/regenerator after been cooled by heat exchange.

Therefore, there is no gas stratification effect. Am I right?

 

[Compositepro]

The constant pressure ammonia absorption cycle is what we are talking about. The best description that I could find (with difficulty) on the internet is here:

http://www.rvmobile.com/tech/trouble/cooldoc.htm.

It is an ingenious cycle and is not easy to understand without a good diagram. The "hydrogen reservoir" in your link is equivalent to the "vent tube" in my link, and is a small, but important, part of the entire cycle.

The vent tube keeps the condenser and evaporator at the same pressure, while the hydrogen keeps ammonia vapor from passing through the vent tube. The other and main purpose of the hydrogen is to cause ammonia to evaporate in the evaporator, just like blowing air over water causes cooling by evaporation. For more details study the link I provided. Bear in mind that there is a hydrogen flow loop, an ammonia flow loop, and a water flow loop.

 

[aquafied]

25362 (Chemical) CompositeprThank you "25362" (Chemical) and "Compositepro" (Chemical). I believe you are correct about the function of the “balance drum” or “Hydrogen reserve vessel “ but my problem is why it functions. According my interpretation of Dalton's Law when two or more gasses are contained within a vessel the mixture is homogeneous not stratified. It is obvious the 'balance drum” operates on the theory that the density of Ammonia is greater than Hydrogen and therefore floats the Hydrogen out as the Ammonia pressure rises or is forced back by the Hydrogen if the condensing pressure decreases. Note the height of the Hydrogen tube outlet in the balance drum.

Is the mixture homogeneous or stratified?o (Chemical)

 

[25362]

It appears that the hydrogen stream contains some ammonia vapor (no stratification). If temperatures in the reserve vessel drop, some ammonia would condense and be drained back to the condenser or to a liquid trap connected with the evaporator.

That is probably the reason for the protruding hydrogen pipe and the sloping of the reserve vessel towards the condenser.

 

[Compositepro]

Dalton's Law is a relationship that applies at equilibrium. The refrigeration cycle is a dynamic process and creates "stratification". Stop the power input to the cycle and the ammonia and hydrogen will mix uniformly.

 

[25362]

See fig. 13.4 in

http://www.parliament.vic.gov.au/im...supply/Submissions_57_110/102_Attachment3.pdf

 

[aquafied]

Thank you Compositepro you have no idea how many academics I have asked that same question and not received a satisfactory answer.

 

[25362]

This systen is called the Platen-Munters cycle. Hydrogen (or helium) is never pure. It contains various amounts of ammonia vapor depending on what part of the circuit is beeing considered.

The hydrogen reserve vessel acts as a presure equalizer by being connected to both the liquid coming out of the condenser and the gas returning from the absorber.

 

[NXTCOLD]

Ammonia vapor driven off in the generator or reboiler of the ammonia absorption refrigeration system contains more water vapor than one would anticipate. It is not yet refrigerant grade. Further rectification is required up the column through reflux injection or other suitable cooling means. At condensing pressure, a column vapor exit temperature within 6 or 8 F of pure NH3 condensing temperature must be reached for distillate to reach 99.95% purity. Desired column outlet temperature difference changes with various operating conditions. Without rectification after the generator, excessive water concentration builds up at the evaporator low point, and eventually causes significant capacity reduction. I designed, built, and operated two 300 ton AAR systems fired with engine-generator waste heat. More ammonia/water vapor than we could tolerate would find its way to the column outlet whenever reflux feed was insufficiently maintained.
NXTCOLD

 

[aquafied]

Compositepro I can't find info on dynamic process creating "stratification" within a mixed gas environment, can you advise a site for further reference please?

 

[25362]

The following link may be of interest:

http://www.thaiscience.info/Article...on of a diffusion absorption refrigerator.pdf

 

[Compositepro]

Thanks 25362. Yours is the first reference I've seen that gives a COP for this cycle, about 0.1. This explains why it is not more popular.