To use a flash you need a compresion ratio that can give you a good separation, while in distillation you dont need that but a temperature gradient, in any case you need energy to complete both, check your energy consumption for example you may have a high temperature current that can be helpful...
It will depend on the vaporized flow in te separator, while vertical separators are more convinient for higher vaporization than horizontal, if your separator is handling from 60-85% liquid then you should use the horizontal if less then the vertical will be a lot more eficient. Efficency will...
