Mixing Fluids in Flow: How to Achieve a Fixed Concentration | Sanatan

[sanatan]

Hello,

I am trying to mix two fluids which are in flow. To be exact, I am trying to mix a concentrated mineral solution to an outlet of my RO filter.
Is there a method to mix the two fluids to get a fixed concentration of the mixed solution ?

Regards,
Sanatan

 

[boneh3ad]

A fixed concentration just depends on how much of each you inject. Getting them well-mixed is a separate issue and has many possible solutions depending on your constraints.

 

[sanatan]

Thanks boneh3ad. In this case I think the pressure from the RO output would be fixed, however my concentrated solution in a separate container will have varying pressure. So what can be done to control injected volumes of each solution. I am hesitant to use a constant pressure pump for the concentrated solution, especially when its flow rate is very small.

At this point I am not thinking about getting them well mixed. I think this would not be a problem

 

[jrmichler]

It sounds like you have two continuous flows that you want to mix, that the fluids are miscible, and that you want to do the mixing in the pipe that is conveying the fluids. If so:

1) A pipe will not mix longitudinally, so both flows need to be accurately metered. I once had a project where we conveyed a water solution through 4800 feet of 2" pipe. There was a concentration meter at each end. Step changes in concentration at the supply end came through as identical step changes at the receiving end an hour later.

2) If you have turbulent flow (Hint: calculate the Reynolds number), then you should get transverse mixing if the pipe is long enough. You can improve transverse mixing by injecting through a perpendicular nozzle sized to get reasonable velocity, adding some elbows, or by adding a static mixer.

3) Is your system open loop (set the flows, hope for the right ratio), or closed loop (measure the mixture with a concentration meter and feed back to the supply valves / pumps? If closed loop, make sure that the fluids are fully mixed before entering the concentration meter, and that the concentration meter is as close as possible to the supply valves/pumps. The flow time from the supply to the concentration meter is a delay, and delays make for really ugly control situations.

4) How to control flow is determined by viscosity, flow rate, pressures, necessary flow accuracy, line losses, and a few other variables. We need a lot more information to make recommendations on how to control flow. Start with a flow diagram that includes all available information.

 

[berkeman]

I only have a little experience with mixing (epoxy mixing for high-volume potted electronic products), but this combination can work well for some applications:

Use two peristaltic pumps to meter the flows of the two liquids, and feed the outputs into a helical mixer tube. Would that work for you?

Peristaltic pump: https://upload.wikimedia.org/wikipedia/commons/0/04/Peristaltic_pump_head.jpg

Helical Mixing Tube: https://i.ytimg.com/vi/JYW6QDldODk/hqdefault.jpg