pH control without using caustic 2

[Garethukuk2002]

I am currently working at a refinery were we have a problem with ammonia in our waste water this is due to partly to direct release to our holding basin and performance problems with slop water processing. we hae identified one of our problems as the presence of Organic acids that are trapping ammonia in the ammonium ion form. i have looked in to caustic but i doun't want to use it because of caustic cracking issues at process temps over 50 decC any suggestions on another way of neutralizing these organic acids

 

[StoneCold]

what about sodium carbonate?

 

[techv]

Hi Gare..

From what I know from refinery wastewater experience is that ammonia source is primarily stripped sour water. Ammonia levels in wastewater directly depend on the SWS efficiency for this pollutant. Normally, the wastewater is subjected to stripping by elevating stream pH. If you have any issues with caustic, you can try other bases too. Lime has been proven one but generates sludge.

Regards
TechV

 

[DedalusEnviro]

Either lime or magnesium hydroxide will generate voluminous
"lime soap" sludge. MgO will not raise the pH high enough to convert the ammonium salts into ammonia to any great extent.

I second the suggestion to consider sodium carbonate if caustic is unacceptable. There are also zeolites that selectively adsorb ammonium ion, such as clinoptilite.



Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[Garethukuk2002]

Thanks for the info guys ill have a quick look at the sodium carbonate and see if its okay to use. Waiting on some analysis of the feed do get exact concs of the organic acids ill write back with what i find

thanks

Gareth

 

[Jason5000]

Allow me to propose another alternative. If problems with your slop system are sending more sour water to your WWTP, would it be possible to instead error on the other side and push a little more oil to your sour water system to help keep the unstripped sour water stream out of your water plant?

If this is a possibility there are a few questions you should ask yourself. Does your sour water skimmer(s) provide adequate residence time for good oil recovery from their sour water stream? Do your sour water tanks, and the level you run them at, provide adequate residence time for additional oil water separation, and are you set up well to skim those tanks if necessary?

The potential consequences include upsetting your sour water strippers, hydrocarbon fouling in your sour water strippers, consuming sulfur plant capacity with excess hydrocarbon in acid gas, and maybe even elevated CO emissions from your final incinerator. So this option should not be considered lightly, but it may be worth looking into if your ammonia problems are bad enough.

Good luck.

 

[Garethukuk2002]

Sodium carbonate will still give me sodium hydroxide (caustic) in the line as the reaction with a weak acid produces sodium hydroxide

Na2CO3 + 2H2O à H2CO3 + 2 NaOH

and

pushing oil through would not reduce the amount of ammonia in the effluent which is the aim if increasing the stripper efficency. as iam trying to process crude water draw water which has been in contact with cude so im not sure werther Jason5000 what you proposed works.

thanks for posting guys its getting closer to a soulution

 

[bimr]

The neutralization reaction is sodium hydroxide + carbonic acid -> sodium carbonate + water:

2 Na(OH) + H2CO3 -> Na2CO3 + 2 H2O

This reaction does not reverse.

You are definitely not going to get caustic by neutralizing an organic acid with sodium carbonate.

 

[bimr]

Would you elaborate on the "cracking issues"?

 

[bimr]

The overall equation for the reaction between sodium carbonate solution and dilute hydrochloric acid for example is:


Na2CO3 (aq) + 2 HCl (aq) >>>> 2NaCl (aq) + H20 + CO2 (g)

 

[DedalusEnviro]

"Would you elaborate on the "cracking issues"?"

Thanks. I had figured it was some metallurgical issue beyond my understanding.

Perhaps, there are some kinds of stainless steel, in which intergranular corrosion will be promoted by sodium hydroxide. But, I don't know of any, and caustic soda is one of the friendliest materials to metal (if not to human flesh) that there is.

Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[Garethukuk2002]

Caustic Crackin / Corrosion = Localized corrosion due to the concentration of caustic that usually occurs under evaporative or high heat transfer conditions. It appears as localized metal loss such as grooves. it affects carbon steel and low alloy steel even with stress releaved welds.

The Feed contains mainly contains water, ammonia, H2S, Acetic acid, Propionic acid and Formic acid.

so im unsure as to how the Sodium carbonate will affect the system.

 

[DedalusEnviro]

Hmmm.

When you said "organic acids" I was thinking you meant fatty acids, like stearic, palmitic, etc.

The calcium salts of acetic, formic, and propionic acids are soluble in water. So, perhaps a lime slurry is a good choice for you. But, you need to have very good agitation in the reaction tank, lime is very prone to settle out in dead spots, unreacted.

Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.

 

[bimr]

If all that you are doing is neutralizing, then caustic should not be a concern. Mild steels (steels with low carbon and low alloy content) and stainless steels will crack if they are exposed to concentrated caustic (high pH) environments with the metal under a tensile stress.

Neutralization doesn't involve a concentrated caustic solution nor would it be occurring with a high pH. Neutralization involves taking the solution pH to a neutral pH of 7. In addition, if you had high pH, you would be experiencing scaling from the other components in the water.

Intercrystalline cracking of steel caused by exposure to caustic solutions above 70°C while under tensile stress; once common in riveted boilers. Also known as caustic cracking. Your temperature does not appear to be high enough either to be concerned with.

Caustic usage will allow you to use the least amount of chemicals. If you use lime, you will also get volumes of waste sludge. Sodium Carbonate will also give you fizzing due to the off gassing of CO2.



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[Garethukuk2002]

The Problem is i need the pH at 8-9 (sorry for the confusion)For efficent stripping in the towers.

The caustic problem is due to possible build up of caustic in heat exchangers and the bottom of the tower which are at 70-120 degC.

ive been told by our inspections department that 5wt% caustic would count as a high concentration in this process, which i think is wrong. So the problem still stands!

I think 5Wt% caustic would be perfectly safe with an inline mixer down stream of the injection point along with an anlyzer to stop over doseing and it would be perfectly safe.

 

[bimr]

A pH of 8-9 is not a high pH.

The 5% wt caustic is in the chemical feed line, not in the vessel itself.

The temperature is not that high either.

Don't see where you inspections department came up with this Caustic Crackin / Corrosion arguement.

 

[DedalusEnviro]

"A pH of 8-9 is not a high pH.

The 5% wt caustic is in the chemical feed line, not in the vessel itself.

The temperature is not that high either.

Don't see where you inspections department came up with this Caustic Crackin / Corrosion arguement."

I don't either. I've seen equipment made of similar materials give reliable service for many years, on electroplating lines.

I recall, for example, a mild steel tank used to contain a steel blackening bath, 50+% sodium hydroxide, at 150+ degrees C for at least three years.


Dave Wichern

http://www.dedalusenviro.com

Science is a business of empiricism.