1. I have a nitrogen gas stream that is wetted using water at 50°C in a bubbler tank , after that I need to calculate how much dry nitrogen gas has to be added to get a dew point of -20°C.

I have found that the vapor pressure of water at 50°C is 123mbara. When the tank is at 1200mbara the fraction of water vapor should be 123/1200, is this correct ?

How do I calculated the dew point of the wetted stream when I have only the fraction ?  2.

3. Ok I'm a bit rusty but I think what you need is as folliws:

The 123/1200 s the relative humidity at 50 C (10.25 %)

Next you need the saturation vapour pressure at -20 C. (0.1 kPa using Tetens formula)

Calculate the relative humidity this vapour pressure would correspond to at 50 C. (0.1/12.36) = 0.81%
[12.36 is the saturated vapour pressure of water at 50 C in kPa]
This assumes the final mixture will also be at 50 C.

Calculate the volumes of wet and dry nitrogen.
Flow rate(wet) x 10.25 = 0.81 x Flow rate (wet + dry)

So the ratio of wet and dry flow rates is 0.086.  dew, nitrogen, point, wetting 