5 replies to this topic

[obsidian_dust]

Dear guys,

Is it normal for a refinery to select scale inhibitor chemical for sea water desalination application by doing some Lab Testing? If so, how should be the Lab test method?

Thank you in advance.

[kkala]

...Is it normal for a refinery to select scale inhibitor chemical for sea water desalination application by doing some Lab Testing? If so, how should be the Lab test method?..

I do not have experience on refineries (even though worked for refinery projects). Nevertheless I have worked in a fertilizer plant having a lot of scale problems. At that time (late 1970s) we did not test anti scaling agents in the laboratory, we applied them directly to the process. Probably it was not easy to obtain representative results from lab tests. Quantity and nature of anti scaling agent was specified by its supplier.
Note: Only flocculants were lab tested, when they were planned for use.
I imagine scale prevention of sea water consists of restraining the development of fungi, algae, etc that could even plug pipes and valves; may an experienced member give specific advice on this.

[obsidian_dust]

Dear kkala,

Thank you so much for your response. Our problem with scale is quiet severe and I'm worry that this problem could not be solved by just increasing the anti scale dosage since overdosing it could lead to more scale problem. That's what I heard. Please correct me if I'm wrong. Based on your experience, do you know any method you considered as effective for cleaning scale from SWD Unit (SWD box inner tube and brine heater inner tube)? We learnt that chemical cleaning could not be effective for some type of hard scale.

Best regards.

[kkala]

Unfortunately I can hardly have a (stable enough) web connection in the country; so please expect my answer later, even though my experience is limited. What about mechanical cleaning? This is usually the last resort.
Meanwhile, can you give clarifications on mentioned "SWD unit" and sea water temperatures? I doubt if it concerns "shortwave diathermy unit" found in web.

[kkala]

...Our problem with scale is quiet severe and I'm worry that this problem could not be solved by just increasing the anti scale dosage since overdosing it could lead to more scale problem. That's what I heard. Please correct me if I'm wrong.
Based on your experience, do you know any method you considered as effective for cleaning scale from SWD Unit (SWD box inner tube and brine heater inner tube)? We learnt that chemical cleaning could not be effective for some type of hard scale.

Apparently mentioned "SWD Unit" is a Sea Water Desalination Unit by evaporation (not using membranes).
Useful sites for the subject could be the following:
(Α) http://www.cheresour...h__1#entry43487 - General antiscaling practices. High liquid velocity in exchangers (> 1 m/s) can limit scalings (prevention). Scales already formed could be faced by Chemical Cleaning (e.g. caustic and/or acid cleaning), followed from time to time by mechanical cleaning.
(Β) http://www.merusonli...eneral/seawater- General info of sea water scales, mainly due to limescale and organisms (algae, bacteria, etc). Carbonates of Ca can be main source of scales, precipitated during heating. Hard deposits may indicate coprecipitation of other metals that can be specified by analysis of scales.
(Γ) http://cat.inist.fr/...cpsidt=19994804 - Sea water scales can be CaCO3, CaSO4, Mg(OH)2.
(Δ) http://sundoc.biblio...09H003/prom.pdf - Valuable scientific report. Solubility of CaCO3 decreases with temperature; main scaling of sea water may be due to CaCO3.
(Ε) http://www2.emersonp...ADS_4950-15.pdf - Scales of CaCO3, Mg(OH)2 can be minimized by acid cleaning (citric or H2SO4); keep CaSO4 below 2 g/l by purging heavy brine from evaporator.
1. Opinion that excess of antiscaling agent can increase scales may be theoretically right in few cases; e.g. if the agent is a week (organic) acid to act as a buffer solution, excess of this agent could promote precipitation of its Ca salt (due to anion increase in its solubility product). Moreover I have not heard of it, nor have I met it in design. In alumina plants, scales (mainly due to carbonates, SiO2, Al2O3) are treated by caustic or acid solutions (with corrosion inhibitors); the stronger the better, concerning their anti scaling ability.
Knowing antiscaling agent and its mechanism to dilute scales can result in a more precise view on this matter.
2. Anti scaling policy of alumina plants (consisting of frequent caustic cleaning, less frequent acid cleaning, mechanical cleaning every now and again, say once per year, on the same unit), could be close to the needs of the mentioned SWD plant. But caustic cleaning (mainly dissolving Al2O3) can be reasonably excluded, as not reported in web reference (Ε). So a scheme to look into could be acid cleaning every so often and mechanical cleaning whenever needed. We applied hot water cleaning in the H3PO4 evaporation unit of fertilizers before mechanical cleaning, but this would be of doubted efficiency (due to CaCO3 solubility decreasing with temperature).
Mechanical cleaning in pipes can be by pigging, if they have provision for it. Otherwise piping and equipment have to be cleaned by hand and mechanical tools. This seems unavoidable, to remove the hard scales; purpose of rest cleaning is to do "hand cleaning" as rarely as possible.
3. Hoping above "thoughts" on the subject could help in making an individual scheme for the specific plant, along with investigation and experimental work. I cannot be more specific due to lack of experience.

Edited by kkala, 12 August 2011 - 12:32 PM.

[breizh]

Hi ,
I would advise to talk to water treatment specialist like Nalco, GE betz, and others , they should be able to support , they are experts in the field
Hope this helps
Breizh