Saturated Brine

[ECD40]

Hello all,
I have an unusual problem regarding the properties of saturated brine containing both KCl and NaCl. This application is at a minesite, where a continuous flow of saturated brine has to be delivered down a 1,000m deep mine shaft from the surface. The fluid temperature at the surface will be 25 degrees C and the fluid will take about 3 minutes to traverse the shaft from top to bottom. The fluid will be subject to the pressure rise in the pipeline of about 100 bar, with no pressure relief at the shaft bottom. The literature that I find is plentiful regarding the effect of temperature on the equilibrium of the solubility of the salts in the fluid, but nowhere do I see what happens if the pressure is raised. My concern is the possibility of precipitation of any of the salts from the solution if the pressure is increased. I don't know why it would, but if it did, the project would have a 'fatal flaw', which obviously cannot be accepted.
Does anyone know of the effect of pressure (at constant temperature) on a saturated brine solution.
Thanks,
ECD40

 

[jari001]

Generally speaking pressure does not affect large changes in solubility since liquids and solids are considered incommpressible. Since you aren't approaching the bulk modulus of water, there shouldn't be gross percipitation. However, I think you must account for some amount because you will have hot spots and maybe preferential nucleation sites that would lead to percipitation.

 

[georgeverghese]

Agreed, temperature would have a large effect on the saturation solubility of KCl and NaCl at the operating temp at -1000m.
The operating pressure of 100barg is much less than the critical pressure of water at approx 230barg. There is some brief discussion of the behaviour of water as it approches supercritical conditions in Perry Chem Engg Handbook 7th edition pg 22-15 if it helps in any way.

 

[ECD40]

Thanks jari001 and georgeverghese. I agree with your comments, but the project will involve about $100 million to get up and running, so I cannot afford any 'fatal flaw'. I didn't expect there to be much, if any, information in the public domain on brine at 100 bar (in fact it's 124 bar because of the 1.24 SG). Are you aware of any lab that might be capable of testing saturated brine under this high pressure to ensure there will be no surprises later? We will be able to maintain constant brine temperature throughout the process, to maintain saturation and avoid precipitation.
Thanks again for your participation and any further ideas will be appreciated.
ECD40

 

[georgeverghese]

NaCl soluibility rises slightly with temp at const press, while that for KCl rises more with temp at const press. On reading through the narrative on pg 22-15, it does look like saturation solubility rises with press as we approach critical press, so we may then infer there shouldnt be a problem with saturation solubility salts as you increase press.

 

[georgeverghese]

There is a diagram Fig 18-57 in the 7th edition of Perry Chem Engg Handbook which shows NaCl - KCl mix aqueous solubility limit isotherms - the accompanying narrative may be of interest also.

 

[ECD40]

Thanks again georgeverghese for your added comments. I think that it's safe to say, that there will be no catastrophic precipitation caused by increased pressure alone. I'll go to the Perry reference that you supplied for some additional support and create an auditable paper trail.