Process Water Specification

[marvalar]

Hello. This is my first post here.
Anyone familiar with steam crackers know that hidrocarbon feeds are diluted in steam prior to the cracking.
My question is. Where can I find some guidelines to control the quality of the water that is purged in the dilution steam boilers?

Thanks a lot!

 

[EMKWR]

Marvalar,
this strongly depends of the pressure and type of the boiler you are using. Based on this you can choose a conditioning regime which sets the quality of the feed water.
I expect that you are using a drum type boiler above 125 bar. For this type of boiler VGB (Germany) guidelines suggest the following:
Cation conductivity (conductivity measured after kation exchanger): < 0,20 uS/cm
pH > 9.0 (for drum type boilers solid alklines can be used, in case of a once through boiler you need all volatile treatment)
Oxygen: < 0,100 mg/l (personally I would go much lower, below 0,020 mg/l)
Silicate < 20 ppb
Copper < 3 ppb
Sodium < 10 ppb
Iron < 20 ppb
(No value for standard conductivity is presented. I would personally aim at a value below 0,1 uS/cm. )
Comparable guidelines (sometimes with a surprisingly different point of view) are issued by for instance EPRI, JIS and as British Standard


Also important is the quality of the boiler water. Do you need these guidelines as well ?

Edwin Muller
KW2 Burau Veritas

 

[marvalar]

Thanks a lot.

This steam is generated at 9 bar in a kettle hx by condensation of 14 bar steam.
Nowadays we control pH, conductivity, iron and oils & fat. I remember that this is steam to mix with hydrocarbons.

 

[EMKWR]

Marvalar,

a 9 bar boiler makes it a lot easier. Much less strict guidelines are needed. For a steam generator at 9 bar there are other guidelines available. The following values are issued by the company I work for:

pH minimum 7,0 but a bit higher would give better protection against corrosion
Hardness preferably below 0,05 degrees German Hardness
Oxygen maximum 50 ppb (do you use a deaerator or oxygen scavenger ?)
Oil : abundant if once through system is used, otherwise 1 mg/l is allowed
Total salt: 10 mg/l maximum in case a once through system without a water-purge is used
For the boiler water also guidelines are available but I need a bit more information on the exact type of boiler (once through, circulation)and your conditioning scheme (fosfate or caustic, sulfite or hydrazine).



Edwin Muller
KW2 Burau Veritas

http://www.kw2.nl

 

[marvalar]

First of all let me apologize for the delayed answer.

This is a rather strange steam cycle, I believe.

The process steam is mixed with naphtha and heated in cracking furnaces. The cracking furnaces effluent is then cooled by direct contact with water in a quench tower. In this quench tower, water is removed in the bottom along with c5+ hydrocarbons. Bear in mind that this water is the steam that was mixed with naphtha.
This water is boiled and the makeup to the system is made by live steam.
The boilers are kettle type heat exchangers. We use an anti-fouling in this boilers.

Sorry again for the delay

 

[TD2K]

Let me dig back through my old documents. I worked in an ethylene plant but we used ethane for a feedstock rather than naphtha. Dilution steam was essentially generated as you described, about 10 bar if I remember. Water for the dilution steam was excess quench tower water which was first steam stripped and then fed to the dilution steam generators.

We didn't have much of a spec on the water quality as I recall, it was pretty poor water and we pretty much cleaned the dilution steam generators (4 vertical thermosyphons) on an ongoing basis.

 

[marvalar]

That's it. We also have a stripper prior to the steam generators. My goal is to minimize the blow down from the generators.