4 replies to this topic

[markymaark]

I am looking at a Mechanical Vapor Recompression unit to reuse "high quality" water from the wastewater. Our feed consists of ph~10.4 from ~200ppm alkalinity as CaCO3.

I am wondering what type of seperation would be attained from a standard packaged deaerator that is designed for O2 and CO2.

The vendor is getting back to me with specifics, but in your experience what type of seperation would be expected?

There is 180 ppm Ammonia at pH 10.4 giving about 90% of the Ammonia present as dissolved gas and 10% NH4+. The feed is coming in at 16,000 L/h.

It will be a steam stripper, since the MVR unit diverts a fraction of the steam to be used in the deaerator which is upstream of the MVR.

All I can think of is, the solubility that I found for ammonia/water at 100C water (assuming the 110C steam heats it that high) is 7g/100g water which is well above the feed. Therefore, is the ammonia being removed only based on the steam flowrate removing the ammonia present as vapor in equilibrium with the liquid? If so, this seems as though a very small fraction of ammonia will be removed.

[markymaark]

Most importantly, what set of equations describes this system since there is no boiling

[TS1979]

Experience shows that the efficiency of removing ammonia through normal deaerator is very low. Mostly the reason is that at low temperature, the dissolve ability of ammonia in water is so high or the ammonia vapor partial pressure is so low. Higher temperature is essential for ammonia being stripped out from water. If you use the water to create steam, the steam will create corrosion problem for all the components made of copper.

[Art Montemayor]

This query is related to two other recent threads in this Forum:
http://www.cheresour...ripping-column/

http://www.cheresour...-in-condensate/

One of the queries was posted by TS1979.

Ammonia (a weak base) reacts with water (a weak acid) to form ammonium hydroxide. In ammonia stripping, lime or caustic is added to the wastewater until the pH reaches 10.8 to approximately 12 standard units. This level of pH converts the ammonium hydroxide ions to dissolved ammonia gas according to the following reaction(s):

NH₄⁺ OH^- = NH3 + H₂O

Below pH 7, virtually all the ammonia will be soluble ammonia ions.
Above pH 12, virtually all the ammonia will be present as a dissolved gas in the water. In the range between pH 7 and 12, both the ammonium ions and dissolved ammonia gas co-exist in the aqueous solution.
The percentage of dissolved ammonia gas increases with temperature and pH. A higher temperature and pH favor the removal of ammonia (as a dissolved gas) from the aqueous solution.

If you can't tolerate caustic in your boiler feedwater, you can't "fix" the ammonium ions and consequently you can't strip out the ammonia as a gas.

[markymaark]

Yes, the problem seems to be the low vapor pressure. I mean, hell, Perry's has increments of 5 molal concentration. I'm at 0.1%. The solubility is like 7%.

The pH is 10.4 which is ~93% NH3 and 7% NH4+.

I was just hoping the deaerator would have a secondary helpful purpose with ammonia reduction, but all I came across, as you have too, is a very, very small reduction.

So let's take a step back,

The process is fed with 180ppm NH3 (water analysis), which by the pH is ~10 ppm NH4+. This goes to a softener to primarily remove the Ca and Mg to negate CaCO3 formation.

So,

Assumption 1) The affinity for the resin bed to remove NH4+ will not be high since it is not a multivalent cation. But maybe its affinity is higher than that of the Na+ in the bed.

Assumption 2) The equilibrium of NH4+ to NH3 will remain even though NH4+ will be removed. Thus, NH3 will be converted to NH4+ due to the pH.

Are these assumptions correct?

Edited by markymaark, 22 August 2012 - 12:26 PM.