8 replies to this topic

[kkala]

Having worked in a fertilizer factory, we had a phosphoric acid tank constructed (for 54% or 30% P2O5 acid) in ~1978, also used as settler of the phospho-gypsum. I cannot recall dimensions; but tank was "long" mainly due to lack of space, probably 12 m diameter and 25 m height. It was made of carbon steel, rubber lined after sand blasting by a local company specialized on rubber lining. Settled gypsum was "pushed" on the tank bottom (cladded with ~304 L steel) to a receiver by a slowly rotating vertical scraper (similar to agitator, but longer blades "touching" bottom), then removed as slurry by diaphragm pumps.
If the tank were not used as settler, we would install a vertical agitator, since experience of side agitators on ~400 m3 lead lined tanks (for phosphoric acid) was not favorable. They often leaked from the rotating axis, which had to be tolerated, otherwise tank had to be emptied to repair it. Observed spots of accumulated gypsum on tank bottom was another disadvantage, probably curable by increasing RPM (but not tried). In fact we replaced side agitators with a vertical one on one tank, with satisfactory results.
The smaller the tank diameter, the better for the vertical agitator, concerning agitator capital cost and power consumption. For long tanks like the first one, blades at different heights can be investigated.
Blades of said scraper had stainless steel (~304L) cladding, vertical agitator could be similar.

[ankur2061]

Attyub194,

Most large capacity tanks and specifically when you have side mounted agitators have very low L/D ratios, may be as low as 0.25-0.3. For a great discussion on tank L/D ratios refer the following link. Also the second last post in the link has an excel sheet for standard dimensions of vertical storage tanks.

http://www.cheresour...3969#entry33969

For knowing more about side-entry agitators for large storage tanks refer the free spreadsheet at:

http://www.cheresour...rs-large-tanks/

Regards,
Ankur.

[kkala]

Data by Ahmed, combined with Ankur's attachments, provide useful information on tank design. Not only total spent steel increases with excess height (after some optimum), but also soil loads require more sophisticated foundations (like piles to touch rock). Of course the above is not limited to API 650. The question is whether it is worth while leaving API 650 (if it does not have a "window") and design long tanks by another code, having a reason for it (e.g. space limitation).
Apart from mentioned phosphoric acid decanter, other long tanks are alumina plant precipitators (dia 14 x height 29.3 m, http://www.cheresour...h__1#entry49756). So long tanks with vertical agitator are used in Chemical Industries, probably because small diameters make vertical agitation easier (*).
One impeller per shaft height = tank diameter can be placed, but good dispersion of gypsum in the acid is needed, or avoidance of pipe clogging? In mentioned fertilizers, phosphoric acid feed was taken from those 400 m3 tanks, whose side propellers rather swept gypsum from bottom (in vicinity of pump suction) that created an overall suspension. In some fertilizer types extra gypsum slurry was added, while other types required decanted acid.
Use of carbon bricks to prevent side agitator leakage was not applied at that time, can Ahmet give some more details? Leaks come out from the "seal" (what kind?). Carbon (that is graphite?) bricks are placed where? It can be an interesting solution to a problem rather caused by suspended solids.

Side agitator leaks were not seen in Refineries. Tanks there seem to follow API 650 dimensions, no vertical agitators have been seen.

(*) This seems to be also true for modern low energy vertical agitators, such as those used in precipitators (EKATO, Robin, Lightling).

Edited by kkala, 10 December 2011 - 03:42 PM.

[kkala]

Thanks, Attyub194, for the information about phosphoric acid storage tanks, really interesting. Some relevant data remembered from the fertilizer factory is as follows.
1. Side agitators (on 400 m3 tanks) could not provide distribution of gypsum solids in the acid so good as the vertical agitator. The phosphoric acid reactor had a lot of small vertical agitators, resulting in a rather homogeneous slurry.
2. Vertical side agitators (Plenty) are used in a local refinery without leakage, so I suppose suspended gypsum particles destroy the side agitator seal causing leakage from phosphoric acid tank.
3. For vertical agitators, the series "Liquid agitation" of Chemical Engineering was of real help, referred in http://www.cheresou...reatment-plant/.
4. All local phosphoric tanks seen were without cover. The vertical agitator installed on 400 m3 tank was not supported on the tank, but on an independent frame. The scraper on the ~dia 12 m x H 25 m settler was probably independently supported too.
5. Concerning materials, mentioned 400 m3 tanks were internally covered with plumb (construction ~1962).
Scrapper blades were probably of 304 L, but this is not sure (hence written above ~ 304L).
A small phosphoric acid tank was supposed of 316 L for cost estimate, one year ago.
It is not easy to be certain about the material, since commercial phosphoric acid corrosion strength depends on its impurities (HF, SiF4, etc) varying according to phosphate used and the process.
Once we installed an NH3 evaporator above the phosphoric fertilizer reaction tanks, heated by the upcoming water vapor. The evaporator destroyed in few weeks, while a similar one had no problem in another factory. The reason was attributed to the HF content of local phosphoric acid, while that of the other factory was supposed to be in the form of SiF4.
All above concerns phosphoric acid production by dihydrate process (filter cake was gypsum). Hemihydrate process acid may theoretically contain less fluorides due to higher reaction temperature ( http://www.efma.org...ric%20Acid.pdf ), but I have no experience on it.

Edited by kkala, 11 February 2012 - 11:17 AM.