caustic soda dilution

[wwtp2014]

Dears

please why always we should add caustic soda solution be diluted to water not vice versa?

Thank you

 

[EmmanuelTop]

The reason for boiling and splashing is due to the large amount of heat released in the hydration reaction of Caustic OH- ions, when water is introduced in the concentrated caustic (or acid) solution. The release of heat can be quite substantial.

If water is added to Caustic, a layer of hot weak caustic solution is formed at the surface, which boils and sprays around due to hydration energy of OH- ions. When Caustic is poured into water, it flows down and mixes much better, so no boiling/splashing occurs.

Dejan IVANOVIC
Process Engineer, MSChE

 

[itdepends]

Somewhat related- I've seen rubber lining in acid tanks fail when water is added to concentrated sulphuric acid due to the formation of a hot zone at the interface. The other reason for adding caustic to water is that under most circumstances the water is the larger volume of the two (with respect to mix ratio) and having it in place first provides you with the bulk of the mass/heat capacity to minimise the temperature increase.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[bimr]

The addition of caustic soda solution to water will cause a rise in temperature of the solution. If caustic soda solution becomes concentrated in one area, is added too rapidly, or is added to hot or cold water, a rapid temperature increase can result in dangerous mists, boiling and spattering, or even a violent eruption. Remember to always add caustic soda solution to water with constant agitation.

http://dowac.custhelp.com/app/answers/detail/a_id/2600/~/caustic-soda-solution-dilution