Sulfuric Acid dilution, H-X material of construction?

[tulane21]

I'm looking for a heat exchanger that can handle 78% sulfuric acid at 320oF. I am diluting 98% acid to 78%, therefore generating a great deal of heat. I've looked into several options inlcuding: Hastelloy B, C and D, teflon/graphite, fluoropolymer, etc. Does anyone have experience with these materials - expected life, durability, etc.?

 

[kenvlach]

I am pretty sure that zirconium is the best material:

http://www.alleghenytechnologies.com/wahchang/pages/products/data/zirconium/zrsulf.htm

I have had HXers made by Astro Cosmos

http://www.astrocosmos.com/

and
Titan Metal Fabricators

http://www.titanmf.com/

Both have chemical resistance data for various materials on their sites.

 

[kenvlach]

Sorry, zirconium is not suitable for your conditions – guess I was in a hurry to leave on a Friday afternoon.

Metals Handbook, 9th/10th Edn., vol. 13 Corrosion, pages 1148-1152 has information on materials for handling sulfuric acid:

PTFE, PFA or glass coatings should work. Graphite can be used if impregnated with PTFE.

Corrosion curves for most of the metals used with sulfuric do not extend to 300[sup]o[/sup]F; there are some cast alloys such as Alloy 55 (Ni-Cr-Fe-Co-Si) and Illium B (Ni-Cr-Mo-Cu) which look pretty good (data extend to 284[sup]o[/sup]F) but may not be available as HXers.

Anodic protection is commonly used for HXers in sulfuric service. It is also mentioned that a wrought, Si-containing SS alloy A-611 “has useful corrosion resistance in 99% H[sub]2[/sub]SO[sub]4[/sub] up to 120[sup]o[/sup]C (250[sup]o[/sup]F) without anodic protection” [p. 1151], so presumably anodic protection would extend its range.

 

[moltenmetal]

Too hot for fluoropolymer, possibly too hot for graphite block exchangers too. You might be in tantalum territory- very expensive...

The problem is the dilution. Conc sulphuric can be handled by a variety of materials, but the range of dilution may eliminate many from consideration. I have no experience with the electrochemical protection methods so can't comment- perhaps somebody in the acid plant business could.

 

[tulane21]

Tantalum seems very pricey, I don't think that's an option. Anodic protection looks interesting, does anyone have direct experience with how long it lasts in a similar environment?

Another option may be Ametek exchangers that have Teflon based tubes. Does anyone have experience with these? Apparently, they have replaced many metal based acid coolers.

 

[kenvlach]

Corrosion (cited above), p. 1154 shows a maximum temperature rating for PTFE & PFA liners of 500[sup]o[/sup]F for sulfuric acid 0-100%.

 

[moltenmetal]

kenvlach has cited the max temperature for a teflon "liner", which is not necessarily the same as the safe service temperature of a fluoropolymer exchanger. The temperature limit of the actual exchanger will be lower than this. How much so depends on the exchanger design, the pressures you intend to operate the exchanger at, and the consequences of a leak etc.

500 F is an absolute maximum service temperature for PTFE and PFA- both materials become soft and creep readily at these temperatures unless they are entrapped. Most people use 450 F as a safe upper temperature for PTFE as a material of construction (even in well entrapped service)- PFA may perhaps be stretched the extra 50 F depending on who you talk to.

I personally would be very hesitant to use an exchanger with PFA tubes at these temperatures, unless there was a way to ensure that the shell and tube side pressures were well matched all the time so that the tubes experienced very little pressure driving force to creep. The tubes must also be very thoroughly mechanically supported, and there was a thorough plan in place to handle leakage when it occurs. Perhaps at the larger sizes there's a way to use PFA-sleeved metal tubes to provide better support, but I can't see how such an exchanger could be fabricated in a way that I'd personally trust.

And I'm assuming shell and tube, which are the only fluoropolymer units I've seen- perhaps another design like a plate coil can be fitted with a thick and durable liner. Perhaps a fluoropolymer exchanger manufacturer could comment.

This is of course assuming that the relatively poor thermal conductivity of the PFA itself does not render the exchanger too large for your liking.

 

[kenvlach]

Agree with molternmetal re liners cf. unsupported fluoropolymer. For the latter, Ametek's fluoropolymer HX shows a steep drop in allowable internal and external pressures with increasing T, to 40 & 20 psig, respectively, at 300[sup]o[/sup]F.

http://www.process-heating.com/CDA/...rstory/BNPCoverStoryItem/0,3154,22072,00.html

http://preview.ametek.com/content-manager/files/HAV/040048.pdf

Alfa Laval's HXer page

http://www.alfalaval.com/scripts/We...591&siteNodeID=1944&contentID=-1&languageID=1

specifically mentions "Cooling of concentrated sulfuric acid..."
so definitely contact them.

 

[tulane21]

Ametek's heat exchanger looks okay, but we'd prefer an exchanger with higher thermal conductivity, so the size is not too large. Also, we'd like to operate at higher presures, up to 100 psig if possible. Does anyone have experience with a silicon carbide exchanger, info found at:

http://www.ptfe-lining.com/usa/flurosicusa.htm

Thanks!

 

[bkmuduli]

Consult NIDI at

http://www.nidi.org.

They can recommend you a suitable Ni based material.

 

[moltenmetal]

Cooling conc. sulphuric is not as big a problem- the problem here is the dilution, the concentration range and the temperature.

Is silicon carbide an option? From a corrosion standpoint, I believe so. It's not cheap (we were quoted $400 US for a small thermowell tube, and small magdrive pump bushings are usually $250 or so, so I'd hate to see what a SiC exchanger would cost...). Even the best grades are sensitive to thermal shock, particularly to rapid cooling- it is a ceramic after all. The tubes can have quite high thermal conductivities, and the material is surprisingly strong- but the thermal shock and brittle fracture concerns tend to require that the tubes be made quite thick. Thermal resistance includes both thickness AND thermal conductivity, so you may not get the area reduction you want. And I have no idea how the tubes are mated to a tubesheet- if it's with fluoropolymer ferrules, you'll be in the same boat as you would be with the fluoropolymer exchanger.

You might get away with a lower alloy exchanger with anodic protection, but this method is for corrosion mitigation and control rather than elimination. Do it wrong and the unit may dissolve, rapidly. The same goes for spray-applied thin fluoropolymer coatings etc.- a pinhole through the coating will rapidly mean a pinhole through the metal.

The only metal I know will survive these conditions is tantalum. It's used for the tubes which add acid into hydrometallurgical autoclaves (i.e. units which dissolve metals like nickel and chromium...). Very, very expensive- 1/4"- 0.035" wall tubing is hundreds of dollars per foot. But also thermally conductive, metallic (i.e. not brittle), and possible to build an exchanger out of which will withstand 100 psig at these conditions. It can also be applied as a vapour deposited coating on other metals- with the same problems as other coatings wrt pinholes and scratches etc.

I think you either have to compromise on pressure handling, max temperature, size and/or cost. You're up against some hard physical limitations here...If you can compromise on your pressure and/or temperature a bit, you might get away with the Ametek unit- but make sure you baby the thing, because the fluoropolymer tubes will be like cooked spaghetti at these temperatures. The key is to find out what the expected temperature of the tubes themselves will be- they'll be somewhere between the shell and tubeside temperatures, and just where they end up will depend on the heat transfer conditions on each side. Careful design may yield an exchanger which, during normal operating conditions, keeps the tubes cool enough to be safe- but an upset may rupture the spaghetti, resulting in an acid-water dilution event that nobody will enjoy...

Agree that the Nickel Development Institute is a good place to go for advice in selecting a nickel superalloy for this service. I also suggest you speak with somebody in the acid plant business to see what they recommend- they have lots of contacts in various industries and they have a pretty good idea what works and what doesn't- plus their advice is generally free.

 

[TAHX]

Tantalum does not have to be expensive. Most designs that utilize it's properties well put the final pricing about the same as impregnated graphite. How much pressure drop do you have to work with? How long do you want the exchanger to last? I can give you quick budget pricing with more details. I work at TITAN Metal Fabricators. Tantalum exchangers are our business.

 

[patrickraj]

Someone could help me on the following:

What material is suitable for 70% Conc Sulphuric Acid.Pump(Impeller) and Piping.
Temp is 40°C.
Suction is from road tanker(open to atomosphere), it is to be transferred to storage tank of about 10 Mts Height.

Thanks in advance

 

[moltenmetal]

40 C and 70 % sulphuric: definitely polymer-lined equipment will be the cheapest and most resistant option. You might get away with PP lining, but fluoropolymer lining will last forever and will only be a bit more money. Any resistant metal/alloy you may choose will cost MUCH more, installed.

Never choose a corrosion-resistant material when a corrosion-IMMUNE material is suitable for the service AND cheaper!

 

[venoma]

hi Tulane,

I am interested what material selection you finally made for this problem and what your experiences are so far...we are concentrating sulphuric acid up to 78%, so we do not have this heat development problem..nevertheless it is an aggressive corrosion environment and the material selection is limited ..we use graphite/epoxy heat exchangers with mixed success/ a few years lifetime...always looking for improvements !

thanks in advance

 

[b36]

Could someone help me on the following.
What is the heat of solution in diluting 98% sulfuric acid to 10% sulfuric acid at 25 °C.
Also let me know the suitable material of construction for this process

Regds,
Bharathi

 

[kenvlach]

b36,
It is better to start a new thread for a question like this, but OK.

Resistoflex has all the info you need.

http://www.resistoflex.com/sulfuric.htm