[lullu]

What's the difference between  isothermal flash from adiabatic flash operation?

[RealEngineer]

The simple answer is that you will get a different product split between the liquid and vapour products (all other factors being equal).

As I mentioned in a previous post, a good introduction to flash calculations is given in Separation Processes by C.J. King (McGraw Hill).  This book covers flash calculations when you specify temperature and when you specify enthalpy.