Tamer, Like Anro, I also ...

Published April 6, 2017 01:19 by Matthew Brannock, Technical Director at Salt Water & AqMB Software

Tamer,

Like Anro, I also initially thought that the scale in the feed could be calcium sulphate due to the high levels of each in the feed. However, after looking more closely at the feed analysis and confirming via speciation/AqMB modelling (admittedly using a range of assumptions to complete the feed composition), calcium carbonate seems to form preferentially. Although CaSO4 only needs to be above 0.15-0.20wt% at 100C to form, if alkalinity is also present, CaCO3 will form preferentially and consume a lot of the calcium before CaSO4 gets a chance to take hold. A sample of the scale and XRD or FTIR analysis will confirm for sure.

In the down stream process though, calcium sulphate will likely form due to the concentrating up of these two species... and this is what occurs in a seeded process. Calcium sulphate is purposely precipitated in the thermal concentration unit (usually an FFE) to produce sites where other scale inducing species will preferentially form (instead of on heat exchange surfaces). There is a set level (i.e. TSS) of calcium sulphate solids that you want to be at though; this is dependent on the concentration factor and there still may not be enough in Tamer's process to run in full seeding mode.

A seeded CaSO4 process won't help the scaling problem in the feed though. On previous projects, where there is calcium and alkalinity in the feed, we have either dosed acid to prevent hardness formation or removed the hardness. It becomes very much an economic question (CAPEX vs OPEX) which is very much scenario specific.

For your scenario, you need to check that once you dose acid, that you don't inadvertently form CaSO4. If you are below ~0.15% CaSO4 in the mixed stream, you should be okay. Modelling will help confirm.

Cheers,

Matt.

matthew@saltwatersolutions.com.au